Research on carbon-based and metal-based negative electrode materials via DFT calculation for high potassium storage performance: a review

Hard carbon-based anode for KIBs

Given its affordable price, eco-friendliness, and high cycle stability, non-graphite carbon, which includes HC, soft carbon, and amorphous carbon, has become one of the potential candidate anodes in KIBs. Even if it is carbonized at an annealing temperature higher than 3,000 °C, HC is difficult to be graphitized and is typically produced from a precursor with a strong cross-linked structure. The distinctive “pseudo graphite”^[36] HC structure featured by disordered nanostructure with turbostratic nanodomains increases the structural stability by adding extra rigidity and buffer room to prevent volume expansion. The larger (002) peaks, increased interlayer spacing, and smaller grains that are provided by this disordered structure in the c-direction assist in preserving the structural integrity of materials^[37]. Additionally, HC has a high standard potential of K⁺/K, which helps to prevent the growth of potassium dendrites and enhance safety.

There is no denying that SIBs are developing quickly, and several HC materials have been presented with distinctive structures and outstanding performance. In 2016, Jian et al. successfully added HC microspheres (HCS) to KIBs for the first time by pyrolyzing the hydrothermally treated sucrose^[38]. With remarkable long-term stability, the typical HCS voltage curves showed a sloping form in the high potential area and a quasi-plateau region in the low potential area. In 2018, He et al. created highly disordered HC from degreased cotton, which displayed high initial coulomb efficiency (73%)^[39]. This explains the resultant distinctive porous structure, huge specific surface area, and extremely high electrochemical performance of the disordered HC^[40].

Furthermore, the electrochemical energy storage mechanism of HC as KIB anode material has been widely discussed. The unique structure of HC at the molecular level provides many active sites for the storage of Na⁺/K⁺, which makes its real capacity limit as an electrode material immeasurable. Based on this, different storage mechanisms have emerged. In 2000, the “insertion absorption” concept, additionally referred to as the “house of cards model,” was initially reported by Stevens et al. after they researched the storage mechanism of Na⁺ in nanoporous HC^[41]. In this model, Na⁺ can be inserted into the interlayer of carbon microdomains at high potential and filled with pores at low potential. Since then, a lot of research has been conducted on the Na⁺ storage mechanism. Kim et al. developed HC with two distinct crystallinities as anode materials for SIBs^[42]. Microcrystalline cellulose (MCC) with more micropores formed by removing amorphous regions exhibits higher specific capacity (300 mAh/g) and rate performance. The impact of amorphous pore structure on the reversible storage ability of sodium was then further investigated on the basis of Na MAS NMR analysis, as shown in Figure 5A. The Na ions can be stored more efficiently in the nanopores created by larger and more open pores, which also increases the pace at which ions diffuse. In 2015, Bommier et al. proposed a novel Na⁺ storage mechanism: the “intercalation filling” mechanism [Figure 5B]^[43]. In the platform area, Na⁺ is filled into the nanopores in addition to being embedded in the graphite layer and adsorbed in the defective carbon materials in the slope area.

Figure 5. (A) Schematic diagram of the adsorption and pore filling mechanism of sodium ions in nanopores based on NMR analysis. Copyright 2021, The Journal of Physical Chemistry C^[42]. (B) Visualization of the “intercalation pore filling mechanism” of sodium ion storage in HC. Copyright 2015, American Chemical Society^[43]. (C) Ion storage mechanisms of Li⁺ and K⁺ in LPG materials. Copyright 2019, Electrochimica Acta^[45].

Recently, the adsorption mechanism in the high potential zone and the embedding mechanism in the low potential region were the two separate K⁺ storage mechanisms that Li et al. discovered in porous HC (p-HC)^[44]. Among this, a sufficient number of them serve as an ion trap and ion buffer reservoir in the porous structure, which improves K⁺ storage and reduces volume expansion. Similar results were also obtained by Wu et al. in their study on the synthesis of luffa-derived pseudo-graphite (LPG) through alkali treatment and one-step pyrolysis^[45]. The K⁺ storage mechanism of KIBs is primarily responsible for their strong performance. As shown in Figure 5C, the slope area during the initial discharge process (Li⁺ is 0-0.17 V; K⁺ is 0-0.56 V) is linked to the adsorption of alkali metal ions in the near-surface area of HC activated by heteroatoms and various defects, and the platform area during the subsequent discharge process (Li⁺ is 0.17-3 V; K⁺ is 0.56-3 V) is traced back to the ion insertion into the graphite layer and the formation of ACx (A = alkali metal ions, LiC₆ and KC₈). However, K ions are thought to be more challenging to enter into layered graphite due to the differing atomic radius.

To further interpret the K-storage mechanism in HC, Yuan et al. has successfully created a number of chemically active HC spheres (AHCS) with adjustable micro/mesoporous structures to investigate the connection between K migration behavior and micro/mesoporous structure^[46]. They also revealed that the storage mechanism of K ions in the HC system can be divided into two stages: adsorption and embedding. There is still much to learn about the fundamental K⁺ storage process in HCs and the internal factors impacting K⁺ storage behavior, despite the fact that research on HC materials as KIB anodes has advanced greatly over the years.

Due to its inherent characteristics, HC provides good bearing capacity and structural integrity^[47]. Among them, Bin et al. prepared functional porous hollow carbon materials using molecular/environmental variable-controlled growth processes [Figure 6A]^[48]. And their extremely low density and low ICE, which results in a low bulk density, restrict their usefulness. In order to overcome the poor diffusion dynamics in KIBs, Chen et al. developed a graded N-doped HC microsphere (NCS) composite material with high porosity^[49]. The unique porous structure provides more buffer space for volume expansion in the electrochemical process [Figure 6B]. The charge distribution was examined, as shown in Figure 6C-E, in order to systematically analyze the impact of N doping on the K⁺ storage performance in NCS materials. The nearly full transfer of Valence electron density from the carbon atom to K⁺ demonstrates its ionic characteristics. Among them, the density of states (DOS) diagram in Figure 6F shows that more electrons in the carbon materials doped with graphitic N (GN) and pyridinic N (PN) are conducive to improving the DOS value at the Fermi level (EF), thereby increasing the electronic conductivity. Controlling the microstructure of HC (interlayer, defect, heteroatom doping) is thought to be a successful method for enhancing K⁺ storage performance. Zhang et al. successfully prepared N-doped CoSe/C composite materials with egg yolk-shell structure, as shown in Figure 7A^[50]. CoSe nanoparticles are completely surrounded by a carbon skeleton with high N content and graphitization degree, exhibiting high capacity and excellent rate performance. Lu et al. prepared porous N-doped CNF aerogel from chitin as the anode material of SIBs, which shows a high reversible capacity of 282 mAh/g at 0.1 A/g and excellent cycle stability^[51]. DFT calculations were used to investigate the effect of N-doped on the electrochemical performance of SIB anodes. Figure 7B shows different types of N-doped graphene groups and high electron density regions, mainly including defect pyridine-N (N-6), defect N-6, pyrrole-N (N-5), and large defect (pyridine/pyrrole/graphite-N) (N6-N5-NG). Among them, the binding energy of Na⁺ adsorption on the corresponding structure is -0.89 eV, -0.94 eV, and -1.67 eV, respectively, and the distance between Na⁺ and the surface of graphene sheet is 2.05 Å, 0.99 Å, and 0.89 Å [Figure 7C], respectively, indicating that the N6-N5-NG-1 structure has a higher electronic conductivity, which is more conducive to the rapid (de-)insertion of Na⁺. In summary, the synergistic effect of N doping and defects provides a theoretical basis for the sodium storage performance of porous CNFs.

Figure 6. (A) Schematic illustration of the different shape evolution paths for hollow structures. Grey, yellow, and blue represent carbon, insoluble component, and dissolvable component, respectively. Copyright 2017, Journal of the American Chemical Society^[48]. (B) Schematic illustration for the large-scale fabrication of hierarchically porous nitrogen-doped carbon microspheres. (C-E) The electronic density differences of K⁺ ion adsorbed on pristine carbon, GN-doped carbon, and PN-doped carbon, respectively. (F) The density of states of corresponding carbon materials. Copyright 2017, Energy Storage Materials^[49].

Figure 7. (A) Schematic illustration of the preparation of the nitrogen-doped CoSe/C composites. Copyright 2017, ACS Applied Materials and Interfaces^[50]. (B) The DFT optimized structure of N6, N6-N5-1, and N6-N5-NG-1 N-doped graphene and the corresponding electron density. The blue and red regions correspond to the electron-rich and electron-deficient regions, respectively. (C) Optimized top and side views of Na⁺ adsorption on corresponding structures. Copyright 2019, Applied Surface Science^[51]. Top views of the K-absorption models (D) of defect C, P-3, P-2, pyrrolic N, graphite N, and pyridinic N frameworks, the Ead, and the differential charge density distribution maps. (E-G) The TDOS and the PDOS before and after K adsorption. Copyright 2021, Applied Surface Science^[52].

Attributed to the introduction of N doping has a favorable impact on the Na ion absorption and diffusion, Gao et al. further produced a novel KBT-7 electrode with exceptional electrochemical performance from kitchen biological waste of 700 °C carbonized waste black tea by layered porous structure and co-doping effect^[52]. The impact of defects and N, P co-doping on the kinetics of K adsorption capability was further explained using DFT analysis. The differential charge density distribution and adsorption energies of the systems with and without the carbon defect, the carbon defect, the N-doped NC₃ (graphite N), the NC₂ (pyrrole N and pyridine N), the P-doped PC₃ (P-3) and the P-doped PC₂ (P-2), are each shown in Figure 7D. The charge transfer between the K atom and the carbon-based framework is what causes the charge density at C to fall when the K atom is absorbed by the surface, and electrons gather on the carbon atom nearby. Additionally, the variation in charge densities at various sites following K adsorption suggests that the inclusion of N, P doping or defects will dramatically increase the adsorption energy of the structure and intensify the electron transfer event. Figure 7E and F displays the total density of states (TDOS) before and after K adsorption for the systems with defect C, defect C, P-3, P-2, pyrrole N, graphite N, and pyridine N (designated as a-TDOS and b-TDOS, respectively), as well as the corresponding partial density of states (PDOS) of K 4s and K 3p in the K adsorption model. By comparing the TDOS before and after K adsorption, it can be shown that the addition of K⁺ will trigger the DOS value guide band at the EF to shift in a certain direction, increasing the relative electronic conductivity, which is mostly caused by the contribution of K atomic orbital. The pyridine N model also exhibits the largest TDOS at the EF, which is further subdivided into two peaks, as seen in Figure 7F and G. The findings demonstrate that the sample with a moderate amount of pyridine nitrogen has improved conductivity when compared to other heteroatom doping models. Different N doping strategies can provide different electrochemical properties for HC materials. Thus, Yang et al. created a hollow biomass carbon sphere (NOP-PB), where N/O/P ternary doping increased the interlayer distance on the graphite surface and added more defect sites^[4]. The special solid hollow porous structure can buffer the volume expansion of the potassium insertion process, promote the charge transfer of K ions and electrons, and greatly improve the electrochemical performance of the material. The adsorption and diffusion mechanism of K⁺ in HC and NOP-PB electrodes can be better understood by simulating the effects of N, O, and P doping on K⁺ adsorption behavior via DFT simulations. N-5 and P/O doping are the most effective at enhancing the electronic conductivity of electrode materials, according to a number of calculations and analyses. As a result, NOP-PB electrodes perform better in terms of potassium storage.

HC exhibits good speed capabilities and excellent structural integrity owing to its loose structure^[53]. It should be underlined that HC produced during pyrolysis always performs poorly for K ion storage in carbon-based materials. Their extremely low density and low ICE result in low bulk density, limiting their usefulness. As a result, designing a carbon-based anode with a decent degree of graphitization, a finely tuned pore structure, and a chosen surface function is extremely optimal. Surprisingly, not as many individuals are aware of how the solid electrolyte interface (SEI) film impacts the electrode reactivity and changes during the cycle. The commercialization of HC as a high-performance KIB anode will be greatly influenced by this.

Nanoporous carbon composite anode for KIBs

Compared with traditional carbon materials, nanoscale carbon-based materials with high conductivity and specific surface area, such as carbon nanotubes (CNTs), carbon nanospheres, and CNFs^[54], can store more charges at the interface and improve the specific capacity and rate performance of KIBs^[55,56]. Due to the restrictive C@C, It has incredible elasticity, which increases its mechanical properties of strength, stiffness, and durability^[57]. CNTs become one of the most viable carbon materials among potential KIB anodes because of their inherent mechanical properties and connected conductive network, which makes them become a crucial part in sustaining electrode integrity during the (de-)potassium process for alkali metal ion batteries^[58]. The cautious alteration of the shape and size of nanoscale carbon materials is a rational and efficient modification method since nanomaterials with various geometric configurations can enhance the ion storage performance of electrode materials to diverse degrees. Given its established use in LIBs, some typical examples of success may be applied to KIBs. In 2017, freestanding porous CNF paper was used as an anode for KIBs by Zhao et al., which showed excellent reversible capacity (211 mAh/g after 1,200 cycles at 0.2 A/g) and rate performance^[59]. As said by Cao et al., a carbon nanocage (CNC) can greatly lessen anisotropy and, as a result, prevent layer slippage^[60]. Based on their distinctive cage structure, CNCs have outstanding discharge capacity and capacity retention as an anode material for KIBs.

Numerous modification techniques of carbon fiber materials have been documented in an effort to further enhance the efficacy of K ion storage in CNFs. High-performance KIBs utilized unique anodes made by Wang et al., which consisted of highly active hollow carbon nanosphere composite materials (AHCS) with large interlayer spacing, high specific surface area, and surface oxygen-containing functional groups (OCFGs)^[61]. The introduction of OCFGs further improves the pseudo-capacitive behavior and specific capacity. After chemical activation, the interlayer gap (0.408 nm) of AHCS is significantly larger than that of hollow carbon nanospheres (HCSs), which can accommodate more reversible K⁺ insertion and removal to adapt to larger volume expansion during the (de-)potassium process. Moreover, OCFGs offer additional active sites, which can increase the capacity of reversible K storing. As the anode of the KIB, AHCS exhibits exceptional initial specific capacity, ultra-long cycle life, and extraordinary magnification performance as a result of the synergistic impact of structural properties. The interlayer spacing, surface oxygen functionalization, and layered porosity of carbon-based anodes are highlighted in this investigation as crucial factors in potassium storage. Furthermore, Figure 8A illustrates the porous carbon fibers (PCFs) that Sun et al. created utilizing the electrospinning technique as independent electrode materials for high K ion storage^[62]. These PCFs have a high reversible capacity of 256 mAh/g under circumstances of 50 mA/g. Corresponding to the potassium storage process [Figure 8B], larger holes increase the surface area, enabling PCFs to absorb and hold more K⁺. Therefore, the synergistic effect of K embedding and K clustering is responsible for the extremely reversible potassium storage performance. And Wu et al. further fabricated macroporous honeycomb CNFs (MHCNFs), where poly(tetrafluoroethylene) emulsion (PTFE) is primarily involved in the creation of porous materials^[63]. Extremely high potassium storage capacity and competitive rate capacity are displayed by the adhesive-free electrode [Figure 8C]. Additionally, a porous structure can offer enough gaps to offset a significant volume shift while attaining prolonged cycling stability^[64,65]. Besides, Wen et al. prepared PNF-Co₃O₄ as a KIB anode via a solutions-combustion process and proposed a “bubble gum fracture effect” as its main growth mechanism [Figure 8D]^[66]. The excellent electrochemical performance of PNF-Co₃O₄ is mainly attributed to the fact that the lower discharge platform (0.3 V) can generate a higher working voltage while having the discharge platform slightly higher than the potassium plating potential can partially reduce the formation of dendrites and improve the safety. Compared with bulk materials, porous structures can withstand larger volume changes and shorten the diffusion path of K⁺, thus improving electrochemical performance. These findings provide a better understanding of the electrochemical Li storage behavior of PNF-Co₃O₄. In 2020, highly graphitized CNFs (HG-CNFs) were created by Tian et al. by electrospinning, preoxidation, and carbonization^[67]. As seen in Figure 8E, the discharge voltage distribution of the third cycle of HG-CNFs exhibits several platforms in the low potential area, indicating a phase shift in the K-GIC system. The potential distribution exhibits a consistent phase shift from KC₆₀, KC₃₆, KC₂₄, and KC₁₆ to KC₈ in the dilution stage, in accordance with the trend of voltage decrease. HG-CNFs electrodes may gain high energy density, high reversible capacity (200 mAh/g), and stable structure owing to the independent fiber network and highly graphitized structure. In 2022, Zhao et al. prepared KIBs with significant K ion storage performance by porous CNFs, including excellent reversible specific capacity(270 mAh/g) and ion mobility^[59]. The exceptional performance is attributable to the distinctive structure of CNFs, where the porous design may successfully reduce the volume expansion brought on by the injection of massive K ions.

Figure 8. (A) Schematic illustration of the synthesis process of PCF. (B) schematic of K-ion storage mechanism for CF and PCF. Copyright 2022, Chemical Engineering Journal^[62]. (C) Schematic illustration of the synthesis process of MHCNFs. Copyright 2021, ACS Applied Materials & Interfaces^[63]. (D) Schematic presentation of proposed growth mechanism of porous nanoflaked Co₃O₄. Copyright 2020, Journal of Power Sources^[66]. (E) The discharge curve (solid line) of the third cycle of HG-CNFs and the theoretical capacity contribution of each K-GIC transition reaction marked by dotted lines. Copyright 2020, Journal of Power Sources^[67].

Several modification strategies have been constructed so as to utilize nanocarbon materials as an alternative alkaline metal ion insertion, adsorption, and diffusion electrode material. Doping heteroatoms [N, O, sulfur (S), P, etc.] in nano carbon materials have also been shown to be a successful method to enhance the efficacy of K ion storage for the following reasons: (1) Heteroatom doping technology can accumulate charges by altering the local electronic structure on the surface of carbon lattice, thereby raising the electronic conductivity of carbon nanomaterials^[68]; (2) Heteroatom doping can provide a large number of active sites for K⁺ adsorption^[69] and; (3) Due to the large covalent radius of nano carbon materials, heteroatom doping can regulate its interlayer distance^[70]. The carbon source generally originates from polyacrylonitrile (PAN) carrying N element, which is the most frequent doped element in CNFs. Thereby N-doped CNFs are often synthesized in situ during carbonization. It has been demonstrated that adding more defects due to N doping increases reactivity and that adding better local electronic configurations due to N doping increases electronic conductivity^[71]. In 2019, as separate anodes for high-performance potassium-based dual-ion batteries (KDIBs), Zhang et al. created graded porous N-doped carbon fibers (HPNCF) with distinct hierarchical architectures (micro, medium, large pores, and nanochannels)^[72]. The distinctive structure offers the HPNCF not only an excellent ion transport channel and an intrinsic electronic channel but also adequate buffer space to survive the volume shift of the cycle, which favors enhancing electrochemical efficiency. Moreover, calculations based on the DFT theory are commonly used to examine how N-doped porous carbon materials affect the performance of potassium storage. Through a single carbonization of N-rich energetic metal-organic frameworks (EMOFs), Tong et al. created N-doped porous carbon frameworks (NPCF) with a large N content (MET-6)^[73]. The greater porosity and specific surface area are responsible for the optimized potassium storage ability (327 mAh/g). To further research the effect of pyrrole and pyridine N-doped porous carbon structures on the K⁺ adsorption mechanism, the K ion adsorption abilities and the relative adsorption energy (ΔE_a) at three different models were constructed and calculated by DFT, including pristine C-doping, graphitic N-doping, pyrrolic N-doping. It is evident that the endothermic reaction occurs in the graphite N and C co-doped system, and the extremely unstable potassium insertion reaction may be a result of the electronic abundance of graphite N doping^[71]. Whereas, the adsorption energy of pyrrole N-doped C and pyridine N-doped C is much higher than that of the original C, which demonstrates why the K adsorption abilities and specific capacity are made possible by the pyrrolic and pyridinic N-doping. These findings show that improving the kinetics and, consequently, the electrochemical performance of KIBs through the doping of pyrrolic and pyridinic N are effective techniques. Chen et al. also prepared a KIB cathode material with a multi-stage structure (N-doped cluster composite hollow carbon fiber) and high reversible capacity (NHCF@NCC), which can effectively reduce the distance that K ions must diffuse before coming into contact with the electrode material^[74]. More reactive sites can also be provided by asymmetric N-doped clusters attached to hollow carbon fibers. Benefiting from its high aspect ratio, NHCF@NCC additionally enhances its electrochemical performance as a KIB anode, which can create a self-supporting structure with a three-dimensional (3D) connected conducting network. In order to obtain higher reversible potassium storage capacity and outstanding multiplying performance, Zhao et al. produced N-doped sea-urchin-structured porous carbon (N-SPC) with a porous siphon effect and increased kinetics and effectively incorporated it into KIBs, taking cues from the framework of sea urchins and their ability to transfer and absorb K^+[75]. Large specific surface areas, hierarchical micro/mesopores, moderate N-doped defect sites, and appropriate K⁺ diffusion barrier energy are all factors of the biomimetic N-SPC that help it achieve amazing cyclability (296 mAh/g at 1 A/g after 2,000 cycles) and rate performance (116 mAh/g at 20 A/g) for KIBs. Moreover, the adsorption energy and DOS of different defect structures were calculated by DFT to investigate the relationship between the adsorption/diffusion behavior of K⁺ and N-doped carbon. Previous research^[4] indicates that the porous structure is more favorable to the adsorption energy of K⁺ than that of the dense structure; thus, diverse N-doping is used for atomic doping in the porous graphene layer, including quaternary-N (N-Q), N-5, and N-6. The lowest adsorption energy (-4.40 eV) reveals that K⁺ is significantly easier to absorb by N-6 than N-Q or N-5, resulting in a significantly higher reversible capacity and superior rate performance [Figure 9A]. As a result, several N-6 dosages were used to compute the change in K⁺ storage with various N-doping contents. The adsorption energy rises as the concentration of N-6 climbs, as seen in Figure 9B. Unexpectedly, the DOS of the associated carbon structure exhibits the same pattern. The addition of more N-6 doped graphene layers demonstrates an improved DOS around the EF in comparison to a single N-6 doped, leading to increased electrical conductivity [Figure 9C]. But as the amount of N doping grows, the barrier energy (E) for K⁺ diffusion likewise rises (from 3.67 eV to 3.86 eV and 4.10 eV), showing that the migration energy needed for K⁺ diffusion doubles [Figure 9D]. In order to further develop and build enhanced KIB anodes, it is, therefore, possible to demonstrate the largest tilt capacity and quick K⁺ diffusion capacity using optimized N-SPC materials with more K⁺ adsorption sites and smaller K⁺ diffusion barriers.

Figure 9. (A) Top view of a single K atom absorbed in quaternary-N (N-Q), pyrrolic-N (N-5), and pyridinic-N (N-6). (B) The adsorption energy of different amounts of one N-6, two N-6, and three N-6. (C) The DOS of the defect site without N-doping, one, two, and three N-6 doping sites. (D) The barrier energy of one N-6, two N-6, and three N-6 doping sites. Copyright 2021, Electrochimica Acta^[75]. (E) The interlayer distances of different carbon structures before insertion of K⁺: pristine carbon, N-6 doped carbon, N-6/P-C co-doped carbon, and N-6/P-O co-doped carbon. (F) The layer spacing and adsorption energy of K⁺adsorbed by corresponding structures. (G) The DOS of different carbon structures. Copyright 2020, ACS Nano^[76].

Due to the extensive study that has been done on the diatomic co-doping modification technology in CNFs, it is anticipated that high-performance CNF anodes will be produced thanks to the synergistic interaction of two distinct heteroatoms. In 2019, Ma et al. created sodium polyacrylate as the carbon precursor for graded N, S co-doped porous carbon (NSPC)^[68]. The structure of nitro and S co-3D doping makes it easier to inject K ions, increases layer gap, and hence enhances electronic conductivity. NSPC has exceptionally high reversible capacity (289.7 mAh/g after 70 cycles at 50 mA/g) and outstanding cycling performance as the anode material for KIBs. Besides, several studies showed that the introduction of P improved the electrochemical performance of the carbon anode of KIBs. Thus, Gong et al. successfully constructed an independent P/N co-doped porous carbon monolithic (PN-PCM) anode for KIBs using supercritical CO₂ foaming technology and a number of pretreatment steps to increase the potassium storage capacity (including amidation, phosphorylation, and heat treatment)^[76]. The PN-PCM anode has good reversible specific capacity (396 mAh/g at 0.1 A/g after 300 cycles), initial coulomb efficiency (63.6%), and magnification performance owing to the 3D macroporous open structure and the heterodimer of high P/N ratios. According to theoretical calculations, P-C bonds are more useful for enhancing potassium storage and improving carbon conductivity through adsorption in P/N co-doped carbon, whereas P-O bonds are more beneficial for extending the distance between carbon layers and lowering the ion diffusion barrier. Based on this, further research has been done to determine how P-C and P-O functional groups affect the electrochemical performance of P/N co-doped carbon materials (the most typical N-6 bond is used to represent N doping). The layer spacing of original carbon, N-6 doped carbon, N-6/P-C co-doped carbon, and N-6/P-O co-doped carbon is depicted in Figure 9E. This graph demonstrates how the layer spacing of carbon may be greatly increased by the addition of P, particularly for the P-O form. Additionally, the inclusion of N-6, P-C, and P-O bonds increases the E_ad of carbon, as seen by the corresponding adsorption energies (E_ad) of each structure following K⁺ intercalation [Figure 9F]. N-6/P-C co-doped carbon exhibits a lower value than N-6/P-O co-doped carbon (-0.97 eV), indicating that the P-C bond contributes most to the capacitance of P/N co-doped carbon. Then again, Figure 9G displays the DOS of four carbon structures. N-6/P-C co-doped carbon and N-6/P-O co-doped carbon both exhibit higher DOS around EFs than original carbon and N-6 doped carbon, with N-6/P-C co-doped carbon exhibiting the greatest DOS. Further analysis proved that the addition of P significantly increases the electrical conductivity of carbon and that the P-C bond contributes a greater amount of electron density to the EF than the P-O bond. Therefore, it is thought that carbon with an optimal P-O/P-C ratio will still have great electrochemical performance.

Consequently, N doping is the correct strategy since it may efficiently increase the adsorption energy toward K⁺ ions^[77]. To achieve high magnification capacity, the creation of KIBs will also need appropriate nanocarbon materials^[78,79]. The design of nanostructures is typically accompanied by an increase in the conductivity of the material, which can significantly shorten the electron transmission path and speed up the diffusion of K⁺ in the bulk phase^[80,81]. One of the most successful methods for enhancing the electrochemical performance of carbon materials is the production of CNTs, carbon nanowires, and other nanomaterials. The high throughput electrochemical performance may also be directly predicted by DFT calculations, accelerating the screening and practical design of KIBs anode materials. At the same time, it can give us fresh perspectives on how various anode materials behave from a molecular to a macro level. Additionally, it offers new insights into how to create high-performance KIBs.

Sb and its derivative anode for KIBs

With its abundant resources, low price, excellent theoretical capacity (660 mA h/g), and adequate voltage window, Sb has drawn a lot of interest as an alternative anode for Li⁺ and Na⁺ storage^[13]. Then, Han et al. described the binary phase diagram of the K-Sb system^[10]. The phase diagram demonstrates that Sb will go through a series of potassium alloying transformations, including KSb₂, KSb, and K₅Sb₄, before the development of the K₃Sb phase. The thermodynamic stability of the K-Sb phase can also be investigated using the binary K-Sb phase diagram. In an attempt to shorten the ion diffusion path, 1D nanowires, nanorods, and nanotubes have all been investigated extensively as part of the nano-engineering profile strategy of alloy-based anode materials. As a KIB anode, Yi et al. created nanoscale Sb particles with high reversible capacity (381 mAh/g at 100 mA/g) and capacity retention rate (210 mAh/g at 500 mA/g after 200 cycles)^[109]. A series of related characterization and in situ tests show that the typical peaks of the cubic K₃Sb phase disappear completely after the complete removal of potassium and eventually form amorphous Sb, while the cubic K₃Sb phase forms again after the complete embedding of potassium. It is further proved that the storage mechanism of potassium is an alloying mechanism. Then, Gabaudan et al. explored the K⁺ storage mechanism in Bi/K and Sb/k systems, respectively^[110]. Figure 10A demonstrates the electrochemical activity of Sb and Bi electrodes in KIBs. At the conclusion of the first discharge, cubic K₃Sb is formed after the kaliation of Sb through the creation of amorphous intermediates, with minimal hexagonal K₃Sb contribution. However, the potassification/depotassification process of Bi first forms crystals K₃Bi₂ and KBi₂ and finally forms hexagonal K₃Bi. The data indicate that the potassium storage mechanism of the two systems in KIBs is an alloying mechanism, and the stability of different phases of electrochemical reaction needs to be further investigated.

Figure 10. (A) Schematic diagram of potassium mechanism of Bi/K and Sb/k systems in KIBs. Copyright 2018, The Journal of Physical Chemistry C^[110]. (B) The XRD patterns for electrodes at various charge/discharge states. Copyright 2022, Applied Surface Science^[111]. (C) Schematic presentation of the formation and (de-)potassiation process of Sb-C-rGO composite. (D) Crystal structure of cubic K₃Sb (c-K₃Sb) and hexagonal K₃Sb (h-K₃Sb). (E) Formation energies of KxSb obtained by the ionic substitution method. Phase stability is a relative energy of a given phase compared to all other materials with different compositions. (F) Atomic migration barriers in the cubic A₃Sb phase (A = K, Na, and Li). Copyright 2019, ACS Applied Materials & Interfaces^[117].

Besides, Shi et al. successfully prepared 3D flowered antimony oxychloride (Sb₄O₅Cl₂) as the anode material of PIBs by a hydrothermal method^[111]. As depicted in Figure 10B, the XRD results of Sb₄O₅Cl₂ electrodes show that the typical peak of Sb₄O₅Cl₂ weakens with the formation of KCl, KSb, and K₃Sb products and reversibly recovers from 0.01 V to the original strength at 3.0 V during (de-)potassification process. The presence of KCl and K-Sb alloys further confirms the synergistic effect of conversion type and alloying reaction. Therefore, the reversible electrochemical mechanism of storing K⁺ ions by Sb₄O₅Cl₂ can be established: Sb₄O₅Cl₂ + k⁺ + e^- ⇔ KCl + K₃Sb + K₂O.

Unfortunately, the unavoidable high volume strain of the cyclic operation causes the rupture of the active material and insufficient electrical contact, which immediately causes a quick attenuation of capacity and prevents the practical use of KIBs^[112]. Thus, numerous modification methods have been developed till now with a view of balancing the (de-)alloying process of Sb-based materials, which include nanoparticles^[113], the inclusion of carbon components^[114], the improvement of adhesives^[115], and the creation of open structures^[116]. Surprisingly, mixing alloy anodes with carbon materials may dramatically boost their cycling stability. The investigation on the electrochemical behavior of Sb-C composite electrodes as KIB anodes, which deliver 650 mAh/g high reversible capacity and good coulomb efficiency, was initially reported by McCulloch et al.^[5]. According to in-situ XRD and electrochemical analysis, the discharge result is the final cubic K₃Sb phase. Additionally, SEM inspection at various voltage cut-off points revealed that the Sb-C electrode underwent significant expansion during the potassium alloying event. Therefore, carbonaceous materials are frequently utilized in Sb-based systems as a volume change buffer to conduct ion conduction in the full potassium alloying/dealloying reaction. In the research of Ko et al. the multi-component electrode [Sb-C-rGO (reduced graphene oxide)] with uniform and fine Sb particles was prepared from Sb-based composite materials containing Sb nanoparticles, amorphous carbon, and rGO [Figure 10C]^[117]. And the side reactions in potassium metal and the electrolyte, not the degradation of Sb nanoparticles, are to blame for the capacity decline in the Sb-C-rGO cell. And three intermediate phases, K_0.5Sb, K_1.0Sb, and K_1.25Sb, which are congruent with the experimental phase diagram of the K-Sb system, exist between the two reaction end members (Sb and K₃Sb). Among the stable phases, cubic and hexagonal K₃Sb polycrystalline phases are more stable than other crystalline phases. As can be shown in Figure 10D, the two polymorphic forms of K₃Sb are structurally identical. The hexagonal system (h-K₃Sb) is more stable than the cubic system between the two polymorphs [Figure 10E]. The relative stability of K₃Sb polymorphs is the same at a higher theoretical calculation level. This finding demonstrates that the electrochemically produced phase of c-K₃Sb is metastable. One intriguing aspect is the ability of the final phase, the c-K₃Sb phase, to change into a stable h-K₃Sb phase after forming. However, no phase change was noticed while the battery was in use, indicating that dynamics are hampered during the phase transition from c-K₃Sb to h-K₃Sb. This cubic to hexagonal phase shift may necessitate both the vertical migration of K and the lateral sliding of the K-Sb layer. In actuality, the c-K₃Sb before h-K₃Sb creation is still present. After several hours of heating at the samples between 160 °C and 170 °C, the resultant c-K₃Sb can likewise be changed into h-K₃Sb. According to the aforementioned findings, the c-K₃Sb to the h-K₃Sb phase transition is dynamically non-spontaneous under environmental conditions, and the h-K₃Sb phase is more stable. The migration potential barrier of each component atom can be a suitable signal to estimate the phase transition kinetics since the phase transition necessitates the reorganization of atoms. The migration barriers for Li, Na, and K in each cubic A₃Sb phase are 0.09 eV, 0.16 eV, and 0.26 eV, respectively, as shown in Figure 10F. The occurrence of the metastable phase in the K-Sb system and its simple nucleation properties are explained by the higher K atom migration barrier. The inclusion of carbon substrates and nanoscale materials still lowers their energy density despite the tremendous progress that has been accomplished thus far; therefore, in-depth modification exploration continues to be the main focus of research. For example, Ge et al. employed Sb@CNFs as the alloy anode to investigate the dynamic volume expansion during the charging and discharging process by in-situ transmission electron microscopy (TEM) in order to explore the structural changes of carbon-coated conversion anode materials^[118]. Studies indicate that Sb@CNF compound materials can successfully buffer the volume growth of Sb nanoparticles during the K ion insertion/extraction process, which enhances the electrochemical performance^[90]. Owing to the aforementioned study, porous structures may successfully prevent the accumulation of Sb nanoparticles and respond to the significant volume variations of Sb during charging and discharging while preserving structural stability. The K⁺ diffusion distance can also be minimized via porous structures, which also enable ions to transfer charges quickly along 3D skeletal structures.

Sb also displays considerable volume variations during the alloying/dealloying process, similar to other alloy-based anode materials^[119]. Severe battery polarization issues can also be caused by the poor K⁺ diffusion rate within Sb crystals^[120]. To address this concern, Luo et al. created a novel form of N-doped carbon hollow nanotube-wrapped Sb nanorod composite (Sb@N-C) via the nanostructure technique^[121]. The hollow structure of 1D conductive carbon covering takes advantage of its advanced structural advantages, including the high conductivity brought on by heteroatom doping, and makes room for the superior ion transfer capacity Sb@N-C, which performs electrochemically quite well. In 2018, An et al. reported a novel 3D nanoporous Sb material (NP-Sb) which is created by vacuum distilling the zinc (Zn) atoms out of a commercial Zn-Sb alloy^[116]. It is also discovered that altering the Zn-Sb ratio and distillation temperature can change the shape and porosity of NP-Sb materials. As shown in Figure 11A, the unique nanoporous structure can effectively reduce the volume expansion caused by electrochemical cycling, accelerate the diffusion of K⁺, and achieve high electrochemical performance. In the same year, by perfecting the crystal structure and ion storage mechanism, Huang et al. reported the heterostructure of In₂S₃-Sb₂S₃ formic acid microspheres and enhanced their electrochemical performance^[122]. After a series of characterization methods, it is known that when the voltage drops to 0.9 V, the conversion reaction occurs and generates metal Sb, metal In, and sodium sulfide. Strong pseudocapacitive effects can be produced because In₂S₃-Sb₂S₃ is highly reversible during the charging process, and the InSb phase retained in the Sb₂S₃ gap leaves enough room for later conversion and alloy processes. The synergy of In₂S₃ and Sb₂S₃ also maintains the primary structure and improves electronic conductivity throughout the cycle, reducing the likelihood of a devastating collapse brought on by the structural expansion zone. A CNT-coated electrode is also advantageous for enhancing cycle and rate performance. Therefore, a hollow material that was created using the self-template procedure Sb₂Se₃@C microtubules was reported by Yi et al.^[123]. The type of alloying in the microtubule is conventional, as confirmed through Sb₂Se₃@C electrochemical behavior and in-situ Raman spectroscopy. The original electrode exhibits significant Raman signals at 187 cm^-1 and 253 cm^-1, which are closely related to the production of Sb₂Se₃. These peaks gradually diminish and eventually vanish with the discharge. The rapidly growing signals of the newly discovered peaks at 112 cm^-1, 148 cm^-1, and 250 cm^-1 are indicative of the formation of hexagonal Sb. Further discharge causes the peaks at 112 cm^-1 and 250 cm^-1 to vanish while the peak centered at 148 cm^-1 shifts left to 145 cm^-1 and weakly expands, possibly indicating the transition from hexagonal Sb to cubic K₃Sb phase. According to the aforementioned findings, K₂Se is produced during the first stage of the conventional reaction, and K₃Sb is generated during the second stage of the alloying process. Due to mechanical protection and enough reverse space, Sb₂Se₃@C microtubules displayed cycle stability and a reversible capacity of 312.8 mAh/g at 0.1 A/g after 40 cycles. The carbon-supported hollow structure is advantageous for improving potassium storage in light of the aforementioned findings. Additionally, Xiong et al. effectively generated composite nanosheet anodes (BiSb@C) by embedding Bi, Sb alloy nanoparticles into porous carbon matrix through freeze-drying and carbonization procedures^[124]. The addition of carbon and Bi significantly reduced the stress and strain brought on by the volume change that occurs during charging and discharging, and as a result, the electrochemical performance was outstanding. The XRD technique was used to properly study the potassium storage mechanism of BiSb@C for further discussion [Figure 11B]. The Bi and Sb phase peaks gradually vanished during the first discharge, and a new peak, K₃Bi and K₃Sb, formed at 0.2 V, indicating that (Bi,Sb) had interacted with potassium to generate K₃(Bi,Sb), respectively. Peaks are seen at 0.7 V during the charging process, indexed to K(Bi,Sb), an intermediate phase during the transition from K₃(Bi,Sb) to (Bi,Sb). In contrast to the first discharging process, the second discharging process yields first K(Bi,Sb) and subsequently K₃ from the potassiation reaction of (Bi,Sb). The identical two-step dealloying process as in the first cycle, K₃(Bi,Sb)- K(Bi,Sb)- (Bi,Sb), is seen in the subsequent charging, indicating a reversible potassium reaction. According to the aforementioned research, the BiSb@C composite materials initially experience an irreversible potassium reaction in the (Bi,Sb)-K₃(Bi,Sb) discharge process, which is then followed by a reversible K⁺ (de-)intercalation reaction of K₃(Bi,Sb)-K(Bi,Sb)-(Bi,Sb) [Figure 11C]. These findings demonstrate that the potassization/depotassization process of the BiSb alloy is comparable to that of metallic Bi and Sb. The Bi and Sb in the BiSb@C alloy composite share similar physical physiochemical characteristics and miscibility. This coincident process may help reduce volumetric expansion and improve cycling stability. Moreover, by using a solvothermal reaction and calcination, Wang et al. created self-assembled Sb₂S₃ nanoflowers on the surface of an MXene (Ti₃C₂) sheet [Figure 11D]^[125]. The highly conductive two-dimensional (2D) Ti₃C₂ soft substrate can effectively buffer the volume growth of Sb₂S₃ while simultaneously promoting charge transport dynamics. Additionally, the structural stability of Sb₂S₃ nanoflowers is improved by their in situ growth on the Ti₃C₂ sheet due to strong interface interaction. As a consequence, improving the capacity utilization of KIBs for potassium storage by modifying the binding structure of electrodes is thought to be one of the most adept modification tactics.

Figure 11. (A) Schematic of potassiation process for bulk-Sb and NP-Sb anodes. Copyright 2018, ACS Nano^[116]. (B) XRD diagram of the first two cycles for BiSb@C composite anode. The corresponding CV curve (10 mA/g) is shown on the right. (C) Schematic diagram of potassium/dipotassium process of BiSb@C composite anode. Copyright 2019, ACS Nano^[124]. (D) Schematic illustration of Ti₃C₂-Sb₂S₃ during cycling. Copyright 2020, ACS Applied Materials & Interfaces^[125]. (E) Schematic illustration of the potassium storage mechanism for the in situ XRD results. Copyright 2020, ACS Nano^[94].

What is more, following previous research, double heteroatom doping is a common modification method for Sb-based compounds, which improves their electrical activity. Han et al. employed a method of co-encapsulating N and P in carbon nanorods (Sb@NPMC) to create a unique Sb nanoparticle anode material^[6]. Due to the collaborative impact of N/P co-doping, SSb@NPMC possesses excellent electrochemical reactivity and electrical conductivity as an anode electrode. Through coordination and spatial co-refinement methodology, Yi et al. successfully rGO matrix (SbSA/C) based on S-rich amorphous carbon covering to get highly loaded and well-dispersed Sb atoms^[126]. The rGO substrate can be utilized to load atomically distributed Sb species, and the pyrolytic Sb chelate can be used to coordinate certain heteroatoms (S, O) owing to the open 2D space framework. Also, the special preparation method can minimize migration and agglomeration during the electrochemical potassium storage cycle. In fact, the K ion diffusion barrier was dramatically lowered, as demonstrated by DFT calculations and electrochemical analysis. Given the active Sb centers and coordination heteroatoms, SbSA/C electrodes provide excellent rate capability and lengthy duration capability for KIBs. In 2020, an ultra-small Bi, Sb nanocrystalline material (Bi_xSb_1-x@P) was synthesized, as reported by Chen et al. Bi_xSb_1-x nanocrystals with the greatest performance can be obtained through careful management of the ratio of Bi/Sb reactants, which can effectively reduce volume change and electrode collapse during the cycle^[94]. Likewise, the P matrix can carry out quick charging and discharging processes because it is a suitable medium for electron and K⁺ ion transmission. As shown in Figure 11E, the in situ XRD analysis applied to further explain the potassium storage mechanism following K⁺ ion insertion and extraction indicates that the chemical pathway: (Bi,Sb) ↔ K(Bi,Sb) ↔ K3(Bi,Sb) is reversible (Bi,Sb). In conclusion, the optimized composition of Bi_0.5Sb_0.5@P composite electrodes demonstrated outstanding electrochemical performance as KIB anodes. Following the aforementioned findings, Bi_xSb_1-x@P nanocomposite materials can improve the potassium reaction with K⁺ ions. These physical design techniques wide up a new avenue for the study of Sb-based anode materials in light of the electrochemical properties mentioned above.

Nevertheless, certain theories and experiments have confirmed that stable rare earth-free MnBi materials can generate a ferromagnetic low-temperature phase employing Sb dopant. Thereby, Nguyen et al. adopted DFT theoretical calculations to carry out phonon dispersion and DOS analysis of the doping system in order to further clarify the implications of Sb doping on the structural stability, internal magnetism, and electronic structure of MnBi components^[127]. Figure 12A-C displays the phonon dispersion and phonon DOS of MnBi, MnBi_0.5Sb_0.5, and MnSb along the high symmetry direction. This figure demonstrates the instability of the MnBi material, which is indicated by the abundance of imaginary frequencies. The number of imaginary frequencies in the system diminishes when Sb is introduced. The imaginary frequencies in the MnSb system vanish, and the system energy becomes more stable when Sb totally replaces Bi. By lowering the magnetic moment, it is possible to stabilize MnBi while Bi is exchanged by Sb. The calculation of the DOS reveals that doping causes a decline in the DOS at the EF. And hence, to investigate how Sb substitution doping influences the electronic structure of MnBi, DOS calculations of spin and lower spin states are employed, along with calculations of energy band structure, which is shown in Figure 12D. Due to their shared crystal structure and the isomorphism of Bi and Sb, the energy band structures of MnBi and MnSb and the related DOS-s data exhibit the same trend. Subsequently, the energy band structure of doped materials splits along some k-paths, and the band within the energy range is divided into two separate bands [Figure 12E]. Along Г MK Г k-path, the dispersion curve of the k-path becomes flatter. In contrast, the dispersion curve along the ALHA k-path is split into a unique double dispersion curve, meaning that there is a linkage between the split band and the s orbital, p orbital, and d orbital of Mn and between the split band and the non-split band. Analysis of the DOS concerning MnBi_1-xSb_x demonstrated that the reduction of DOS through the substitution doping from MnBi to MnSb was another effect on the band structure (see Figure 12F). According to the data, this reduction gradually lowers the magnetic moments of Mn and MnBi1-xSbx in contrast to pure MnBi. The study of PDOS for MnBi_0.5Sb_0.5 at the EF (E > -5 eV) reflects that there is a notable orbital hybridization between the Bi/Sb-p orbit and the Mn-d orbit, with the electron contribution from the Mn-d orbit being essential but the energy value near -10 eV almost entirely arising from the Bi/Sb-s orbit [Figure 12G]. This observation and the dual line splitting effect of doping, which varies along distinct k-pathways, point to a significant connection between the Mn-d orbitals, ALHA k-paths, and Bi/Sb-p orbitals. The aforementioned authors concluded that in the situation of substitution doping, Sb dopant can be employed as a candidate for the stable phase. Only a tiny quantity of Sb can be doped to the gap position in gap-doped MnBi due to the possibility of increased hybridization between Bi/Sb-p orbital and Mn-d orbital.

Figure 12. (A-C) Phonon dispersion, (D) phonon density of state, (E) up-spin state band structure, (F) density of states of MnBi, MnBi_0.5Sb_0.5, and MnSb, and (G) Partial density of state of MnBi_0.5Sb_0.5. Copyright2021, Solid State Communications^[127].

In accordance with the studies mentioned above and accounts, quite a few solutions have been implemented to lower the volume expansion of Sb-based anode materials and strengthen their potential for potassium storage: (1) To minimize volume changes and shorten the ion diffusion distance, alter the morphology and structure at the nanoscale; (2) To stop the accumulation of Sb nanoparticles, contain volume variations throughout the cycle, improve electrical conductivity, and utilize carbon materials for buffer layers or conductive structures; (3) The addition of heteroatom doping increases the number of active sites on the surface of carbon-based materials and promotes electrical activity. As a consequence, choosing the proper electrolyte component in accordance with the unique properties of the materials will also aid in increasing the ability of Sb-based anode materials to store potassium. And the practical research on the future large-scale production of Sb-based anode KIBs is intimately tied to the selection and design of appropriate electrolyte components.

Sn and its derivative anode for KIBs

Alternative metallic elements that can be alloyed with K have also been investigated as potential KIB anode materials, except that metal Sb. They appear to be a potential option to meet the requirements of the upcoming large-scale energy storage devices due to their low cost and high natural abundance. As a desirable anode material for energy storage devices, tin-based materials offer an extremely specific capacity, a substantial relative concentration, and are environmentally friendly as well^[100,128]. In addition, the theoretical capacity of Sn for the LIB and NIB anodes can be up to 990 mAh/g and 847 mAh/g, respectively^[129,130]. It is thought that this phenomenon is caused by the contribution of high-density property to the volume energy density of the battery. Sn is, therefore, essential to the investigation of metal-based anode materials for KIBs. Pure Sn also experiences enormous volume changes throughout battery cycling, which may result in electrode material separation and poor battery capacity^[131]. Basically, metallic Sn reacts with K through alloying reaction (Sn + K⁺ + e⁻ ↔ KSn) with a theoretical specific capacity of 225 mAh/g. The first study on the electrochemical behavior of a Sn-based anode in a potassium battery was given by Sultana et al.^[132]. The material has a promising capacity of roughly 150 mAh/g and is active at low potential in comparison to K/K⁺. According to experimental data, Sn can be reversibly alloyed or dealloyed with potassium. Combining a metal foil anode (Sn, Sb, K, or Na) and an EG cathode (EC+DMC+EMC), Ji et al. developed a unique K double-ion battery^[133]. Its special storage mechanism, which offers exceptional capacity retention (93%), is formed up of the PF₆ anion insertion/removal procedure on the graphite cathode and the K⁺ alloying/dealloying event on the anode. Yet, the amount of Sn grows significantly after (de-)lithification or (de-)solidification (260% for Li₂₂Sn₅ and 420% for Na₁₅Sn₄)^[134,135]. The volume change of Sn in KIBs is more severe than that of LIBs and SIBs owing to increased K^+[136].

Ramireddy et al. conducted a thorough investigation of the K⁺ alloying reaction mechanism of Sn thin film electrodes following a range of in-situ characterization techniques^[137]. After an electrode was fully potassiated, the KSn alloy phase was discovered. Till the initial cycle of potassization/depotassization, a reversible capacity of up to 245 mAh/g was noted. It is interesting that just the second potassium reaction stage revealed the creation of SEI membranes. Besides that, in-situ studies of stress variations in Sn film electrodes during potassium/potassium removal cycles prove that mechanical instability happens both during surface reactions and the phase shift caused by potassium alloying and dealloying. Also, research on the cyclic stability of electrode materials at various operating voltage ranges has found that the ideal value is between 0.01 V and 1.2 V. At the same time, Wang et al. reported a study of the K ions storage mechanism of Sn nanoparticles in KIBs using in situ TEM and XRD testing^[93]. Sn anodes have a 197 mAh/g capacity; however, they are accompanied by serious capacity decay. The results from TEM and XRD analysis reveal that the final alloy in the production of potassium is KSn, which was in accordance with the work of Ramireddy et al.^[137]. According to in-situ TEM, a two-step potassium mechanism with volume changes of 101% and 180% occurred when K₄Sn₉ and KSn alloys were formed, and the development of reversible nanopores during the cycle of Sn nanoparticles precluded the complete recovery of Sn particles. The intense crushing of Sn particles is responsible for the capacity decline that occurs quickly after several cycles. As an example, the metals Sn and K can be alloyed at room temperature to produce a variety of intermetallic compounds, including K₂Sn, KSn, K₂Sn₃, K₂Sn₅, KSn₂, and K₄Sn₂₃^[138]. Other charge storage mechanisms have also been proposed in recent years. Despite the fact that these studies highlight some benefits, the Sn element is prone to large volume expansion during the potassium process, which will inevitably result in material crushing and stripping and lower cycling performance and make it more challenging for KIBs to achieve high energy density. Consequently, the Sn-based material modification technique for LIBs and SIBs can also be employed for the K ion system, even though research into Sn-based materials for KIBs has only started. Other approaches to optimizing the capacity of alloy anodes for maintaining potassium, including carbon-based nano-engineering, heteroatom doping, profitable adhesives, and electrolytes, have also made considerable strides.

To further safeguard the active components, graphene can be distributed in CNFs and used as a soft buffer substrate. Additionally, it can improve the conductivity and magnification of materials. Therefore, one of the methods to address this issue is to limit Sn-based alloys to a carbon matrix and introduce the method of dealloying to create porous metal particles by alloy corrosion^[112,139]. Ge et al. expressed Sn nanoparticles in the non-graphitized carbon in the Sn-C composite made by mechanical ball milling for the first time, and they have a capacity of 150 mA h/g as the anode of NIBs^[140]. The KIB anode is, therefore, predicted to have a reversible capacity of between 150 mA h/g and 190 mA h/g when the carbon content of an electrode is taken into account. Qin et al. developed a 3D porous graphene network using 3D self-assembly as a template, encasing Sn nanoparticles inside a graphene shell (Sn@G-PGNWs), as shown in Figure 13A^[141]. In addition to successfully preventing Sn from coming into direct contact with the electrolyte, the highly elastic graphene shell created by Chemical Vapor Deposition (CVD) also helps to preserve the structure and interface stability of Sn nanoparticles. Sn@G is a tightly fixed 3D porous graphene network with great mechanical flexibility, huge surface area, and good conductivity. It is possible to considerably increase the conductivity and structural integrity of the entire electrode. As such, porous carbon materials that serve as the buffer layer may assist in maintaining the integral design of composite electrodes. Huang et al. built a 3D carbon/Sn porous polymer (3D-HPCS) anode utilizing a NaCl template^[142]. The performance of potassium storage is improved by the enormous surface area of Sn nanoparticles and composites uniformly placed in 3D porous carbon. This can somewhat reduce the volume expansion during the (de-)potassium process. The initial Coulombic efficiency (ICE) exceeding 96% and the reversible capacity of up to 276.4 mAh/g imply that non-uniform structures are also vital for the efficacy of potassium storage. Via the sol-gel technique, Yang et al. created N-doped porous carbon materials (Sn/NPC)^[143]. Given their distinctive shape, strong conductivity, and substantial surface area, Sn/NPC electrodes have an amazing ability to store potassium. As illustrated in Figure 13B, Li et al. described the hierarchically porous carbon-supported Sn₄P₃@C composite as an anode material for KIBs first^[144]. According to a number of characterization techniques and electrochemical performance research, Sn₄P₃@C undergoes a three-phase transformation into Sn, KSn, and K₄Sn₂₃ during the whole discharge process. And then, due to the fact that graphene sheets may successfully prevent Sn₄P₃ nanoparticle aggregation and enhance electronic conductivity, Du et al. created Sn₄P₃/multilayer graphene sheets (Sn₄P₃/MGS) composites by impregnating Sn₄P₃ nanoparticles into graphene sheets [Figure 13C]^[145]. Additionally, nanoparticle impregnation increases the surface area of contact of the electrolyte, reducing the diffusion distance of K⁺. With regard to this, the improved Sn₄P₃/MGS has a high reversible capacity and magnification performance of 378.2 mAh/g at 0.1 A/g. During the discharged process [Figure 13D], the peak of Sn₄P₃ drops significantly, and the Sn phase, K₃P phase, K₄Sn₂₃ phase, and KSn phase appear once. Later in the charging process, there is a gradual regeneration of the corresponding peak Sn₄P₃. The peak is still related to some discharge products, suggesting that the conversion and alloy reaction of potassium/potassium in Sn₄P₃/MGS materials are partially reversible. Then, applying electrospinning technology, Li et al. accurately constructed N-doped porous Sn nanosphere composites (Sn/N-CNFs)^[146]. In comparison to other Sn-based materials, Sn/N-CNFs have superior cycling stability as KIB anodes (198.0 mAh/g at 1 A/g after 3,000 cycles), with a corresponding capacity retention rate of 88.4%. The mechanism underlying the electrochemical performance of Sn/N-CNFs in KIBs is revealed as follows: To begin with, the interaction between extremely small nanoparticles within Sn nanospheres simplifies the ion migration path and yields a buffer space for volume expansion and contraction. The presence of a carbon framework expands electronic conductivity by preventing the agglomeration of internal Sn nanoparticles. The notably linked CNFs also support a significant improvement in the reversible and rate capacity of Sn/N-CNFS as KIB anodes by offering a viable transmission channel for electrons while conserving structural integrity.

Figure 13. (A) Schematic illustration of 3D Sn@G-PGNWs. Copyright 2014, ACS Nano^[141]. (B) Schematic illustration of the synthesis of Sn₄P₃ and Sn4P3@C. Copyright 2019, Energy Storage Materials^[144]. (C) Schematic illustration of the synthesis of Sn4P3/MGS-80. (D) The ex-situ XRD patterns of the Sn₄P₃/MGS electrode at different discharge/charge states. Copyright 2021, Journal of Energy Chemistry^[145].

For designing ideal high-capacity Sn-based KIBs, effective procedures, such as structural modification, nanomaterialization, combining conductive materials, and interface management, have a major practical value. In spite of several studies on tin-based anodes in recent years, research into the development of tin-based anodes in KIBs, which include intricate reaction mechanisms, potassium storage mechanisms, suitable electrode materials, electrolytes, etc., is still in its early phases. In 2020, Yamamoto et al. manufactured KIBs utilizing liquid electrolytes containing bis (fluorosulfonyl) amide ions to match submicron-sized Sn particles, increasing the capacity retention rate of the Sn negative electrode over 170 mAh/g^[147]. This paved the way for a thorough investigation into how various electrolytes can improve the capacity of KIBs for potassium storage.

P and its derivative anode for KIBs

P compounds, such as white, red, and BP, have also been investigated as prospective anode materials for KIBs, with a theoretical capacity of up to 843 mAh/g, as evidenced by LIBs and SIBs^[148,149]. Among them, white P cannot be utilized as an electrode material for scientific study since it is poisonous, volatile, unstable, and combustible in the air. Non-toxic and comparatively stable, amorphous RP can store ions (Li⁺, Na⁺) in an amorphous form. However, its poor conductivity and significant volume growth restrict its practicality^[150]. Lastly, during the high-temperature and high-pressure transition of RP into white P, an occurrence of BP was observed. Its exceptional 2D layered structure and superior conductivity render it perfect for the creation of high-performance KIBs. However, the harsh synthetic conditions for obtaining BP are difficult problems that must be solved^[151]. Likewise to SIB and LIB, the alloying reaction of manufacturing K_xP is thought to be the basic mechanism of RP in KIBs based on the existing three-electron mechanism^[152]. Although K₃P theoretically holds a very high capacity of 2,596 mAh/g, no empirical findings have proven this claim^[153,154]. On the other hand, the potassium method caused the production of KP and K₄P₃ alloys with corresponding capacities of 865 and 1,154 mAh/g^[155]. Hence, the latest objective of the studies of scientists is to figure out the development direction and response mechanism of the P anode in KIBs^[156,157]. With the support of a template method and a unique alloying mechanism (4K + 3P K₄P₃), Wu et al. created RP nanoparticles by encapsulating them in N-doped porous hollow CNFs [Figure 14A]^[92]. This results in RP possessing an excellent theoretical capacity of 1,154 mAh/g. Also, they noticed that the synthesis of potassium phosphate compounds and the following alloying processes caused the generation of K₄P₃, as shown in Figure 14B, which was supported by in-situ Raman and non-in situ XRD data. Lastly, Xiong et al. confirmed the creation of KP through research of potassium storage mechanism on RP@carbon nanosheet composite^[158]. The DFT evaluation of the K-P phase diagram implies that KP may possess stable thermodynamic properties with the lowest formation energy. Moreover, within high-performance KIB anode materials, the single-electron reaction (P + K⁺ + e KP) featuring a theoretical capacity of 843 mAh/g is thought to be the greatest and most realistic potassium reaction. In addition, using DFT calculations, Kim et al. computed the energetics of K and P alloying^[99]. The most reliable alloying product configurations, which equate to 865 mAh/g and 1,154 mAh/g in theoretical capacity, are KP and K₄P₃. Since alloying voltage of K₄P₃ is lower than that of KP, the matching KIB system will have a larger energy density. Meanwhile, a mechanism to ascribe potassium products to partially crystalline or amorphous K_3-xP was put out by Zhang et al.^[159]. As observed in Figure 14C, the binary K-P system exhibits the stages KP, K₄P₃, K₃P₁₁, and K₃P when the cyclic P/C electrode is discharged to 0.5 V or 0.01 V. Then, a recent DFT calculation by Yang et al. also postulated the alloying reaction mechanism of BP in KIBs^[160]. Figure 14D illustrates how the formation energies of many potential K-P alloy phases, including K₃P₁₁, K₃P₇, K₂P₃, KP, K₄P₃, and K₃P, were calculated during the potassization process. Since the K₃P₇ point separates from the energy curve for fairly low formation energy, this phase really is not generated during the potassification process. Additionally, due to the extremely negative potential of KIBs, K₃P cannot exist, which is in contrast to the three-electron alloying mechanisms in LIBs and SIBs. This will restrict the capacity of KIB BP to take in potassium. Graphite will be considered in future calculations for the KIB since the graphene layer in the P/G heterostructure can serve as a buffer to help adapt to the volume expansion of BP during the sodium reaction. By encasing BP particles within sizable graphite particles, Yang et al. successfully created a BP/carbon composite anode for KIBs^[160]. Nevertheless, a number of electrochemical characterization methods reveal that even in ideal circumstances, only 500 mAh/g of reversible capacity is observable. The capacity loss rate reaches 40% after 50 cycles. BP as the KIB anode continues to be difficult to employ at this time. The exorbitant cost of BP may also make it difficult to use. One of the major future prospects is the development of low-cost synthetic BP techniques.

Figure 14. (A) Schematic illustration of the (de-)Potassiation process of red P particles, hollow carbon fibers coated with red P, and nanostructured red P confined in the N-PHCNF matrix in KIBs. (B) In-situ XRD patterns of the red P/N-HCNFs-5 composite when discharged to 0.01 V. Copyright 2019, Nano Letters^[92]. (C) P/C electrodes in KIBs at different potentials. Copyright 2017, Journal of the American Chemical Society^[159]. (D) Formation energies of possible phase during potassiation for BP. Copyright 2019, Electronic Materials Letters^[160].

Despite having a large theoretical specific capacity (2,596 mAh/g), pure P exhibited a very poor specific charge capacity and CE, which severely restricts its broad use in the filed of KIBs. In particular, due to significant volume growth during the charge-discharge operation, the pure RP and BP anodes deliver very low reversible capacity. According to weak electrochemical involvement and strong polarization of RP, active materials for ion storage have limited electrical conductivity^[154]. Low redox kinetics can then impair cycle performance, especially when there is a high current density. Higher battery polarization can cause the formation of uneven SEI films and the excessive decomposition of electrolytes, resulting in low ICE and significant capacity degradation as electrode materials^[149]. In brief, the development of P anodes for KIBs has advanced significantly, but there are still numerous obstacles to overcome, including poor electronic conductivity^[92], significant volume expansion^[161], and an unstable SEI. These challenges will also cause capacity decay and poor coulomb efficiency^[162]. Researchers are now looking into the optimization of modification techniques for high-performance anode materials in an attempt to eliminate the barrier witnessed by the aforementioned KIB utilizing P anode materials. Recent studies have mainly focused on the combination of carbon-based materials to establish heterostructure, heteroatom doption, and electrolyte composition optimization. In 2019, Chang et al. used a straightforward one-pot wet-ball milling (WBM) technique to effectively create a KIB anode with excellent performance RP^[163]. The RP/C electrode has a high reversible specific capacity (750 mAh/g) and excellent rate capacity, which can be attributed to the assistance of the conductive network made of multi-walled CNTs and the equally dispersed nanoscale RP particles, making RP an appealing anode material for K ion storage. In particular, it is hypothesized that the absence of a P-C bond facilitates the efficient reaction of the K⁺ with the RP, which is based on the examination of electrochemical measurement. However, Wu et al. manufactured phosphorous/carbon composites using a straightforward mechanical ball milling procedure and investigated them as KIB anode materials^[157]. The findings indicate that the P/C electrode has a high capacity of 323.5 mAh/g, medium cycle stability, and rate performance. This is primarily attributable to the high theoretical specific capacity of P and the homogeneous distribution of amorphous BP in the carbon matrix by ball milling, which forms a steady P-carbon bond and effectively inhibits the volume expansion in the process of potassium and potassium removal [Figure 15A]. Ball milling is useful for promoting intimate contact between primary particles, conductive carbon matrix and P, and the viability of P/C composites with rich P-C or P-O-C chemical bonds, but it is still unable to further reduce the particle size of P to the nanoscale level. Since there was insufficient electrical contact between the collector and the active material and the limited carbon covering, which was unable to react to the significant volume shift of the P anode, performance eventually suffered. As a result, it needs to be used in conjunction with other synthesis techniques in order to significantly raise the electrochemical performance of P anodes. And then, the growing trend of the P anode is still limited by its intrinsic insulating properties and significant volume fluctuations. To overcome this situation, Liu et al. combined RP with mesoporous carbon (TBMC) and CNTs to create a special P@TBMC Composite material^[164]. Multi-walled CNTs can hasten electron transmission because they contain a lot of sp² carbon, while the mesoporous carbon layer can be used to load the right amount of P and act as a buffer layer to reduce the extreme volume expansion. Thus the P@TBMC composite performs admirably as a KIB anode in terms of reversible specific capacity, cycle performance, and rate capacity. The creation of high-performance electrode materials is facilitated by the design that restricts the active component to hybrid carbon nanostructures combined with highly conductive sp² carbon and porous carbon.

Figure 15. (A) Schematic diagram of the synthesis of the P/C composite. Copyright 2018, Journal of Power Sources^[157]. (B) Schematic illustration of the fabrication process of red P@AC and red P@AC@PPy composite. Copyright 2020, Nano energy^[165]. (C) DFT computations of P atoms adsorbed on nondefective or nitrogen-doped carbon surface and corresponding adsorption energy (E_ad). The brown, blue, and red balls represent C, N, and P atoms, respectively. Copyright 2019, Nano Letters^[92]. (D) The corresponding three-dimensional charge density and two-dimensional charge density maps in V₃Se₄/CNF system and V₃Se₄/NPCNF system, respectively. The three-dimensional charge density par value is set at 0.14 e/bohr³. Saturation values are marked on the two-dimensional charge density scale, and high values correspond to electron abundance. Copyright 2021, Chemical Engineering Journal^[168].

The introduction of carbon materials into P-based anode materials may significantly increase rate performance and structural stability owing to their larger surface area and powerful conductivity. For instance, Fang et al. synthesized an air-stabilized RP anode that improves electronic conductivity by isolating the active substance from direct contact with the electrolyte [Figure 15B]^[165]. The composite as a potassium electric anode presents reversible capacity and long cycle life, which is mainly attributed to the RP-activated carbon (P@AC) composite surface coated conductive polypyrrole (PPy) layer can prevent air oxidation, protect the RP material, and improve the structural stability of active materials. Additionally, Wu et al. added RP to N-doped porous hollow CNFs (RP@N-PHCNFs) to keep P particles from being crushed and aggregated^[92]. Since the establishment of P-C chemical bonds in the carbon matrix and N doping, the composite fibers exhibit good structural stability during the potassium process, according to electrochemical measurements. A theoretical P-adsorption ability of the N-doped carbon matrix is estimated using DFT calculation to further highlight how N doping on the N-PHCNF matrix improves the potassium storage efficiency of the RP@N-PHCNF anode. Figure 15C displays the equivalent undoped and N-doped carbon defect models. The calculations indicate that the bridge site-the midpoint of a C-C bond for undoped graphene and the midpoint of a C-C bond next to a N atom for N-doped graphene - is the most stable location for P atom adsorption sites. Additionally, the predicted adsorption energies for a single P atom in undoped and N-doped graphene are 1.52 eV and 2.22 eV, respectively. These findings show that the N-doped carbon matrix has a substantially higher attraction for adsorption than that of the undoped one, which results in a strong and close bond between RP and the N-PHCNF carbon matrix. The strong ability of RP@N-PHCNF anodes to store potassium is a result of their chemical adsorption function. Furthermore, through the one-step synthesis of self-template and recrystallization-self-assembly technique, Bai et al. first investigated the potassium storage performance of the core-shell (several nanometers) CoP anode embedded in the N, P co-doped porous carbon sheet matrix (CoP ⊂ NPPCS)^[166]. A single precursor can be created by utilizing the anomalous coordination properties of melamine and metal ions as well as the hydrogen bonding synergy between melamine and phytic acid to create a 2D network. In addition, Wang et al. prepared RP@rGO composite material by vaporization–condensation method^[167]. RP particles can be anchored on the rGO plate to increase both its structural stability and conductivity during the alloying and dealloying process. The superior conductivity and solid structure of the composite matrix, which can significantly increase the electronic conductivity of the electrode, are primarily responsible for good electrochemical performance. Additionally, through an alloying mechanism, P particles can contribute highly. Moreover, Xu et al. successfully embedded V₃Se₄ nanoparticles into N/P co-doped CNFs by biomass algae^[168]. DFT calculations were used to further explore the effect of N/P co-doping on electrochemical properties. As shown in Figure 15D, highly electronegative P atoms break the electric neutrality of the material, and charge tends to accumulate at the P doping site, which makes V₃Se₄/NPCNF composite materials possess stronger ion adsorption capacity and cycling stability. A selection of observations proves that N/P co-doping may accelerate the ion reaction kinetics and regulate the spacing of electrons in KIBs, boosting its capacity for potassium storage.

In addition, the use of P in electronic equipment is also constrained by various flaws. High carrier mobility and magnetism are vital for spintronic devices, which motivates researchers to keep investigating the electrical structure and magnetic properties of materials. Doping is a dependable and effective way to change the electrical structure of 2D materials and introduce magnetism. Nowadays, a novel 2D material known as a phosphor that possesses higher ranges of magnetic and electron mobility at ambient temperature has been successfully extracted from black phosphor block by mechanical stripping^[169]. Currently, Te-doped phosphors have been made by Zhang et al. using chemical vapor transfer, while Viti et al. have obtained Se-doped black phosphors for 2D photodetectors^[170,171]. In order to examine how Ce and Ti substitution doping affect the magnetic moment change in non-magnetic phosphors, Hussain et al. applied by the DFT theory based on the generalized gradient approximation approach to obtain the DOS of an impurity-doped phosphorene of different electronic configurations (P, Ti, and Ce) [Figure 16A]^[172]. It is evident that the systems with Ti-up, Ti-dn, and Ce-Ti pair doping exhibit weak magnetic activity. Ce-up and Ce-dn really do not behave magnetically or metallically halfway. Nevertheless, Ce-Ce couple-doped systems and Ti-Ti couple-doped systems exhibit magnetic and semi-metallic behavior since the spin polarization of majority and minority spin channels crosses the EF. The valence and conduction bands of atomic orbitals react with magnetic dopants to generate magnetic and semi-metallic characteristics. These findings show that each impurity doping has unique bonding properties. What is more, Duan et al. successfully reported the geometrical, electronic, magnetic, and optical properties of blue phosphorene doped with yttrium (Y), zirconium (Zr), niobium (Nb), and molybdenum (Mo) can be improved via DFT calculations^[173]. The computed binding energy of Y-, Zr-, Nb-, and Mo-doped blue phosphors is positive (8.1 eV, 9.04 eV, 11.34 eV, and 10.28 eV, respectively), indicating that the doping procedure is favorable energetically. Consequently, the binding energy is higher than that of atomic doping on blue phosphor, which is in the 8.10-11.34eV range. By definition, a more stable doping structure corresponds to a greater absolute value. Figure 16B displays the spin-polarized structure that was created through substitution doping. Since the spin-up and spin-down curves for the Y-doped system are consistent, induced spin polarization is not present. The indirect band gap of the Y-doped system is 1.23 eV, and it still exhibits non-magnetic semiconductor properties. Moreover, the system doped with Zr and Nb exhibits semiconductor properties in the spin-down channel while passing through the EF in the spin-up channel. The data demonstrate that Zr and Nb-doped systems exhibit semi-metallic properties. Additionally, they exhibit 100% spin polarization close to the EF, indicating that they are excellent spintronics candidates. And then, the TDOS and PDOS of Y-, Zr-, Nb- and Mo-doped systems are plotted in Figure 16C-F. The spin-up and spin-down channels in the Y-doped system are entirely symmetrical, which suggests that the system is not magnetic, according to the TDOS. The spin-up and spin-down channels, however, are asymmetric for single-layer phosphors doped with Zr, Nb, and Mo, indicating that they are magnetic. The spin-up channel for Zr-doped and Nb-doped systems crosses the EF, while the EF is found in the band gap of the spin-down channel, indicating that these systems have semi-metallic properties. It is critical to remember that the 4d orbitals of the Zr and Nb atoms are the primary source of spin polarization, which means that the unpaired 4d electrons of these atoms are the primary source of magnetism. The spin-up and spin-down channels are asymmetric in the Mo-doped system, and a diluted magnetic semiconductor can be seen in the open band gap. They discover that the Mod_xy, Mod_x2-y2, and Mod_z orbits around the EF also significantly contribute to the spin polarization from the PDOS curve of the Mo-doped system. The aforementioned findings demonstrate that blue phosphors doped with Y, Zr, Nb, and Mo can be utilized as thin film materials in the fields of nano-spintronics and optoelectronics and also offer a fresh concept for the future magnetic research of P-based materials.

Figure 16. (A)The spin-polarized DOS of Ce-Ce, Ti-Ti, Ce-Ti, Ce-up, Ti-up, Ce- dn, and Ti-dn, respectively. Copyright 2019, Nano Structures^[172]. (B) Band structures of Y-, Zr-, Nb-, and Mo-doped blue phosphorene by substitutional doping method. The blue lines (red lines) represent the spin-up (spin-down) channels. The Fermi level (EF) represented by a dotted line is set at E = 0 eV. The TDOS and PDOS of (C) Y-, (D) Zr-, (E) Nb-, and (F) Mo-doped blue phosphorene by substitutional doping method. The EF is set at zero energy and indicated by the vertical black dashed line. Copyright 2021, Thin Solid Films^[173].