Constructing three-dimensional architectures to design advanced anodes materials for sodium-ion batteries: from nanoscale to microscale
Abstract
Sodium-ion batteries (SIBs) are emerging as a possible substitute for lithium-ion batteries (LIBs) in low-cost and large-scale electrochemical energy storage systems owing to the lack of lithium resources. The properties of SIBs are correlated to the electrode materials, while the performance of electrode materials is significantly affected by the morphologies. In recent years, several kinds of anode materials involving carbon-based anodes, titanium-based anodes, conversion anodes, alloy-based anodes, and organic anodes have been systematically researched to develop high-performance SIBs. Nanostructures have huge specific surface areas and short ion diffusion pathways. However, the excessive solid electrolyte interface film and worse thermodynamic stability hinder the application of nanomaterials in SIBs. Thus, the strategies for constructing three-dimensional (3D) architectures have been developed to compensate for the flaws of nanomaterials. This review summarizes recent achievements in 3D architectures, including hollow structures, core-shell structures, yolk-shell structures, porous structures, and self-assembled nano/micro-structures, and discusses the relationship between the 3D architectures and sodium storage properties. Notably, the intention of constructing 3D architectures is to improve materials performance by integrating the benefits of various structures and components. The development of 3D architecture construction strategies will be essential to future SIB applications.
Keywords
INTRODUCTION
As the mainstay of contemporary industrialization, electricity boasts numerous applications, including heating, transportation, and telecommunication[1]. At present, most of the electricity is generated by the burning of nonrenewable fossil fuels, which raises problems with energy depletion and global warming[2-4]. Additionally, these questions lead to the redistribution of precipitation, the melting of glaciers and permafrost, and the rise in sea levels, which endangers human life and disturbs the ecosystem’s delicate balance. In recent years, some renewable green power resources, including solar power and water power, have been used as supplementary or substitute resources for fossil energy[5]. However, limited by the natural environment, renewable energy sources have the characteristics of randomness, volatility, and instability. Once the electricity power is directly output, it can have a huge impact on the power grid, accompanied by large power loss. Thus, the expansion of sustainable energy technologies depends on the progress of large-scale energy storage systems[6]. Lithium-ion batteries (LIBs), a typical electrochemical storage system, have been widely applied in many types of portative electronics because of the less pollution and excellent energy density. However, the consumption of lithium resources is increasing, paralleled by the growing global demand for them. The rising price of lithium resources will prompt people to find new types of battery systems due to the unequal distribution of lithium sources in the crust of the Earth[7-9].
Compared to lithium, sodium is abundant and distributed across the Earth. Hence, extensive research has gradually shifted focus toward sodium-ion batteries (SIBs) since both sodium and lithium belong to an identical main group and have similar chemical and physical characteristics[10]. Although these two energy storage systems have parallel electrochemical reaction principles, the larger ionic radius of sodium
As is well known, the morphologies of materials should significantly affect their electrochemical performance because various microstructures show distinct surface and internal features. Due to the advancement of nanotechnology, nano-scaled materials in batteries have been massively fabricated to elevate the specific capacity and rate performance[16]. Nevertheless, nanomaterials have a higher specific surface area, making them more susceptible to side effects with electrolytes, leading to terrible electrochemical performance. While having shorter ion diffusion distances, nanomaterials also face worse thermodynamic stability, which will cause nanoparticles to agglomerate and battery life to be reduced. However, micro-materials with larger particle sizes exhibit inferior kinetics and lower energy density[17-19]. To deal with the constraints of nano- and micro-materials, the strategy of constructing three-dimensional (3D) architectures has been proposed[20]. Over the past few years, advances in 3D architecture construction have clearly demonstrated the indispensable position of this strategy in synthesizing and eventually optimizing battery materials[21-23]. Three-dimensional architectures are mainly composed of nanoscale building blocks (such as nanospheres, nanosheets, and nanoshells), which could reduce the ion/electron migration pathways and further return favorable kinetics and high capacities. Additionally, the 3D architectures can provide multiple transport channels, which will facilitate the transmission of electrons/ions[24]. Besides, the well-designed 3D electrode materials with excellent structural stability could also improve the cycling performance and increase volume energy density. As illustrated in Figure 1, 3D architectures can be classified into five types, depending on their morphological characteristics: hollow structures, core-shell structures, yolk-shell structures, porous structures, and self-assembled nano/micro-structures. Hollow structures have a large specific surface area and great deformation resistance, which can improve rate performance and cycle life[25-27]. Core-shell structures are constructed by tightly combined shell and core, whereas yolk-shell structures are made up of separated core and shell. They offer distinct advantages in terms of enhancing anode material electrochemical performance. Core-shell structures have a high volume energy density and rich heterogeneous interface, while yolk-shell structures reduce volume expansion during charge and discharge[28-30]. Hollow, core-shell, and yolk-shell structures can be further classified into single, multi-core/shell, and hierarchical structures, which have different construction strategies and unique performance advantages. Besides, porous structures, which can be categorized into microporous, mesoporous, and macroporous structures according to the pore size, provide a large number of sodium storage sites and ultrafast sodium ion pathways[31,32]. Self-assembled nano/micro-structures are a combination of nanostructures and microstructures. Assembling nanostructures into microstructures can improve the ion/electron exchangeability and structural stability of anode materials. Generally, self-assembled nano/micro-structures include thousands of morphological features that can typically be classified into two types by different self-assembly ways: (i) several nanostructures self-assemble to create microstructures; and (ii) nanostructures are embedded within another structure to form
Figure 1. Different types of three-dimensional architectures of anode materials for SIBs.
In this review, we attempt to comprehensively summarize the latest progress in 3D architectures in anode materials for SIBs. We describe the definition, advantages, and disadvantages of 3D architectures, including hollow structures, core-shell structures, yolk-shell structures, porous structures, and self-assembled nano/micro-structures. Additionally, the design thought, synthesis method, electrochemical performance, and sodium storage mechanism of each unique morphology are summarized by a detailed analysis of several examples. Prior to this, we briefly introduce the typical SIB anode families, including carbon-based anodes, titanium-based anodes, conversion anodes, alloy-based anodes, and organic anodes, so we can comprehend the advantages of each type of anode material and the dilemma they suffered.
TYPICAL FAMILIES OF ANODES FOR SIBS
Carbon-based anodes
Based on the reversible deintercalation reaction of lithium ions, graphite materials have been successfully applied to the anode for LIBs. In fact, carbon-based anode materials have long played a vital role in electrochemical energy storage (EES), so researchers pay special attention to the application of carbon-based anodes for SIBs[35]. Although sodium ions have similar chemical properties to lithium ions, graphite and sodium cannot form thermodynamically stable compounds in traditional ester-based electrolytes, leading to the fact that commercial graphite anodes cannot be used as anodes for SIBs[36]. At present, the developed carbonaceous materials mainly include graphene, graphite sheets, carbon spheres, carbon fibers, carbon tubes, porous carbon, and amorphous carbon. Generally, the specific capacity of carbon-based anodes is 200~500 mAh g-1[37]. Additionally, because of the advantages such as a wide range of sources, non-toxicity, air stability, and low cost, carbon-based materials are considered as suitable anodes for SIBs. Generally, the widely studied carbon-based anodes for SIBs can be divided into four categories: graphite, graphene, hard carbon, and soft carbon materials[38].
Graphite and graphene are typical two-dimensional (2D) layered materials. Graphite, which is a widely used anode in LIBs, offers an excellent specific capacity (372 mAh g-1), but its performance in SIBs is not satisfactory. In 1988, Ge and Fouletier found that graphite would form NaC64 compounds with sodium, and the specific capacity of sodium storage was only 35 mAh g-1[39]. Therefore, over the past few decades, researchers determined that graphite was not suitable for SIB anodes. Until 2014, when Jache et al. applied ether electrolytes in SIBs, graphite anodes showed excellent sodium storage performance, attracting significant attention from numerous researchers[40]. In particular, hard carbon is characterized by its resistance to graphitization above 2,500 °C, making it a crucial material in the development of SIBs. Dahn first introduced glucose-based hard carbon as anode materials into SIBs and found that hard carbon had a high specific capacity (300 mAh g-1) and an ideal sodium storage platform (~0.1 V)[41]. This discovery has aroused widespread research interest. In the next 20 years, a variety of biomass precursors have been used to prepare hard carbon, such as grapefruit peel, peat moss, banana peel, seaweed, ramie, and natural polymer phenolic resin[42-45]. Soft carbon is a unique type of carbon material that has disordered nanostructures and ordered carbon layers. Heat treatment can change the degree of graphitization of soft carbon. Different from hard carbon, it will transform into graphite when heated above 2,500 °C. The carbon layer of soft carbon is more ordered and longer, and the conductivity is higher, resulting in a relatively good rate performance. Besides, the low defect concentration and small specific surface area of soft carbon could weaken the decomposition degree of electrolytes and help to improve the Coulombic efficiency of the first cycle. It is worth noting that the voltage range of soft carbon sodium storage usually falls between 0.2 and 1.2 V, higher than the voltage of sodium deposition, contributing to improved safety. However, a high voltage range will reduce the voltage window of the whole battery, which is not conducive to the energy density of the cell. In terms of synthetic precursors, soft carbon can usually be prepared by conversion of polypyrrole (PPy), asphalt, tar, and hydrocarbon-based materials. From a practical point of view, soft carbon has the potential for practical applications due to its low price, high carbon production rate, excellent safety, and good sodium storage performance.
Titanium-based anodes
Although the comprehensive performance of hard carbon is quite eminent, the capacity contribution potential of hard carbon materials is mainly at 0~0.1 V, which is close to the sodium plating potential. Therefore, there is a risk of sodium precipitation that leads to safety problems[46]. In SIBs, the redox potential of Ti3+/Ti4+ is usually between 0.5 and 1.0 V, which effectively avoids the dangerous sodium deposition behavior and ensures strong stability during large-scale energy storage. In addition, titanium-based anodes can provide long cycle life in SIBs due to their low lattice stress. Therefore, titanium-based anodes are regarded as potential substitutes for SIBs, which can embed/remove sodium ions reversibly while maintaining structural integrity. During the past few decades, many different kinds of titanium-based compounds, such as titanium dioxide, spinel lithium titanate, and sodium titanate, have drawn the interest of scientists in the light of the obvious benefits of titanium-based compounds: inexpensive, nontoxic, appropriate working voltage, small strain, and excellent cycle stability[47]. At present, most of the research on titanium-based compounds focuses on optimizing electrochemical performance and exploring electrochemical mechanism[48].
Conversion anodes
Binary metal compounds, which can store sodium ions through the conversion reaction mechanism, are expected to become the alternative anodes for SIBs owing to the great abundant species and excellent theoretical specific capacity[49]. Nevertheless, binary metal compounds in the conversion reaction mechanism usually face problems such as poor reversibility, significant voltage hysteresis, severe volume expansion, and poor initial Coulombic efficiency (ICE) and cycle performance. To overcome these challenges, several new strategies have been proposed, including material structure design, nanocrystallization, surface engineering, and electrolyte optimization[50]. The following reaction equation can be used to represent the conversion reaction between binary metal compounds and sodium:
In general, M can be one of the transition metal elements, mainly including Fe, Mn, Ni, and so on. On the contrary, X is one of the nonmetallic elements, including O, S, Se, P, and so on. For some special conversion anodes, the conversion reaction and alloying reaction will both occur during the sodium storage process. This occurs because some M elements will continue to react with sodium to form alloys, indicating that the capacity contribution is derived from two aspects of alloying and conversion reaction mechanism. Therefore, MaXb anodes often have a larger theoretical specific capacity than pure M metal anodes[51]. Besides, the conversion anodes usually transform into nanoparticles via phase separation during charge and discharge. In theory, nanoparticles have a higher specific surface area and shorter ion diffusion distances, which facilitates the reversibility of e-/Na+ transmission and conversion reactions. The interface of the electronic conductive phase (M) and ionic conductive phase (NazX) can store more Na+ through the “sharing” mechanism to generate additional capacity[52]. However, a larger specific surface area means that nanoparticles are more susceptible to side effects with electrolytes, leading to terrible electrochemical performance. Besides, the crushed anode particles are more prone to fall off from the electrode, resulting in battery failure.
Conversion anodes, depending on their thermodynamic properties, can be employed as both positive and negative electrodes in batteries[53]. Although the conversion reaction can be carried out through the intermediate phase, which determines the actual voltage capacity curve, it has little influence on the theoretical specific capacity and average potential. Besides, thermodynamic data can be used to contrast the main characteristics and distinctions of Li and Na conversion reactions. By using the Gibbs free energy of the conversion reaction (∆rG), Klein et al. revealed the potential difference of the conversion reaction between MaXb and Li or Na[54]. Compared with the conversion anodes of LIBs, MaHb, MaOb, MaSb, and MaFbfor SIBs show lower redox potentials, while MaBrb and MaIb provide higher redox potentials.
Alloy-based anodes
The main group elements of IV and V (P, Si, Bi, Sn, Sb, Ge, etc.) can form binary alloys to store sodium by alloying with Na+, providing high sodium storage capacity, which can serve as an anode for the development of high-capacity SIBs[55]. Different from the reversible intercalation-deintercalation mechanism of carbon and titanium-based anode materials, alloying reaction anode materials are carried out through a series of chemical bond formation or fracture and structural evolution. The sodium reaction of alloyed anode materials occurs at low potential, which can produce high energy density[56]. Alloying reactions can usually be expressed as:
Many alloy anode materials may have multiple intermediate phases before the alloying reaction reaches the final state. Although the phase diagram can be used to predict the intermediates formed in the sodiation/desodiation process, there are still some situations where the experimental results do not match the theoretical predictions. The resulting alloying products determine the volume change and the specific capacity. Therefore, the alloying reaction mechanism still needs further study.
The anodes based on alloying reactions have excellent specific capacity and rate performance. Among them, the black phosphorus exhibits an outstanding sodium storage capacity (> 2,000 mAh g-1), which is considered to be highly suitable for the development of anodes for high-capacity SIBs. Although significant research improvement has been made in alloy-based anodes during the past few years, and numerous alloy-based anodes with high capacity have been developed, the alloy reaction usually causes huge volume changes, resulting in the pulverization of active substances. In addition, the stress and strain generated by the huge volume change have a chain reaction, which will cause a series of adverse effects. For example, the unstable solid electrolyte interface (SEI) film, the shedding of active substances from the collector, and the large voltage lag caused by deformation will aggravate the decline of electrode performance. Sn[57], Ge[58], Sb[59], and Bi[60] exhibit great rate performance despite having low specific capacities due to their excellent electronic conductivity. Although P[61] and Si[62] have high theoretical specific capacity, their poor electrical conductivity leads to slow reaction kinetics. Some common methods can improve the defects of alloy anodes to some extent, such as material nanocrystallization, surface coating, building composite electrodes, and optimizing binders and electrolytes[63]. At present, alloying anode materials remain on the laboratory scale, and there is no successful case of mass production, especially nanostructured alloy anode materials, so more efforts are still needed to make alloying anode materials practical.
Organic anode materials
Due to the sodium storage mechanism of multi-electron reversible redox reactions, organic electrode materials have abundant natural resources, diverse structures, and high electrochemical performance, which provide a possible choice for the design of green SIBs with high capacity, low cost, sustainability, and flexibility. The application of organic electrodes in SIBs is not a new technology. As early as the 1980s, studies have explored the possibility of inserting Na+ into organic electrodes. After that, polyacetylene and polyphenylene oxides have been successfully used as polymer-based anodes for non-aqueous rechargeable batteries, such as SIBs, according to Shacklette[64]. During the past few years, many research groups have begun to conduct in-depth studies on organic compounds to explore their potential sodium storage characteristics. At present, it is found that organic materials that can be used as anodes of SIBs should be roughly divided into two categories: polymers and organic small molecules. Among them, Schiff base polymers, polyquinones/polyamides and conductive polymers are several typical kinds of polymers for SIBs anodes[65].
According to species and sodium storage mechanism, the anode materials of SIBs may generally be categorized into five types. Among them, carbon-based anodes and titanium-based anodes store sodium through an insertion reaction. The carbon-based anode has been widely used in commercial SIBs due to their abundant raw material sources and low cost. Ti-based anodes have excellent structural stability but also suffer high operating voltage and low theoretical specific capacity. Alloy-based anodes store sodium by alloying reactions, which can form a binary alloy between metal and sodium. The sodium storage mechanism of conversion anodes is the conversion reaction, which is similar to the substitution reaction. In fact, both alloy anodes and conversion anodes could provide ultra-high theoretical specific capacity. However, their challenge lies in poor cycle stability due to the severe volume expansion during sodium storage. Organic anodes can store sodium through multi-electron redox reactions, but the poor conductivity of organic materials and the solubility of organic molecules in electrolytes hinder the development of organic anodes.
STRATEGIES FOR CONSTRUCTING 3D ARCHITECTURES
Hollow structure
Hollow structures are one of the most common 3D architectures, which are widely used in the EES field. Generally, an architecture containing a shell and a cavity can be defined as a hollow structure. However, to summarize the application of hollow anodes in SIBs, we classify and define three kinds of hollow structures to distinguish them from other complex 3D architectures. Hollow anodes can be divided into single hollow structures, double/multilayer hollow structures, and hierarchical hollow structures. In fact, these hollow structures have long been studied in LIBs[24]. As a potential substitute for LIBs, SIBs have similar research ideas, including material selection and structure design. However, the SIB anode faces a more serious volume expansion problem, which means that the structure of the anode material is more likely to collapse and lead to failure. At the same time, the reaction kinetics of sodium ions are slower because of their larger ionic radius. Therefore, more attention needs to be paid to the 3D architecture construction from the nanoscale to the microscale while designing hollow structures. Here, we summarize the research results of these structures for SIBs in recent years.
Single hollow structure
Single hollow structures consist solely of a single shell and a cavity. Moreover, the shell of single hollow structures is uniformly constructed by nanoparticles or rolled by 2D layered materials, and there are no more complex building blocks. Single hollow structures mainly include hollow spheres and hollow tubes. Among these, carbon-based anode materials are the most studied materials with hollow sphere structures. In fact, hollow carbon materials are an important member of the carbon material families and are generally used in EES fields, such as supercapacitors, SIBs, and lithium sulfur (S) batteries. With the general advantages of hollow structural materials, they also have low specific density, good mechanical strength, and excellent thermal stability. Tang et al. reported hollow carbon nanospheres applied to the anodes of SIBs for the first time[66]. They designed a simple hydrothermal carbonization approach to prepare hollow carbon spheres by using latex as the template and obtained hollow carbon nanospheres after removing the template. The thickness of a carbon shell is about 12 nm, mainly composed of disordered graphite-like structures. At 50 mA g-1, the initial discharge capacity of prepared hollow carbon nanospheres is up to
Figure 2. (A) TEM, (B) HR-TEM images, and (C) cycling performance of hollow carbon nanospheres. (D) Schemes of the electrochemical reaction process and (E) rate performance of hollow carbon nanospheres and carbon nanospheres electrode[66]. (F) SEM and (G) TEM images and (H) cycling performance of hollow carbon spheres[68].
Comparable to hollow spheres, hollow tubes are also a kind of common hollow structure. Usually, the size of hollow tubes in one direction is much larger than in other directions. Hollow tubes have good buffer capacity to effectively resist the volume expansion of anodes during sodiation. Additionally, the winding among hollow tubes can build a large conductive framework, connecting with each other to provide a good electronic transmission path. Han et al. used the electrospinning method to synthesize hollow carbon nanofibers (HCNFs), and PMMA is the template [Figure 3A][69]. The carbon nanofibers (CNFs) have a length of tens of microns, a diameter of about 1 μm, a wall thickness of about 130 nm, and a low Brunauer-Emmett-Teller surface area of 21.8 m2 g-1. Compared with ordinary CNFs, hollow nanofibers have higher charge capacity (326 mAh g-1) and higher ICE (70.4%). In the rate test, the reversible capacity of HCNFs at different current densities is higher than that of ordinary CNFs. After 450 cycles, the capacity retention of common carbon nanotubes decreased to 35.5%, while the HCNFs reached 70% after 5,000 cycles at 1.6 A g-1 [Figure 3B-D]. These excellent performance outcomes benefit from larger inner diameters, appropriate wall thicknesses, and interconnected 3D frameworks. To significantly enhance the performance of hollow carbon nanotubes, Zhong et al. reported a high-performance nitrogen (N)-rich hollow carbon nanotubes (NCNTs) for LIBs and SIBs [Figure 3E and F][70]. The nitrogen content reached 15.7%, and the aspect ratio was 12 μm/200 nm. NCNTs showed excellent reversible capacity, maintaining 198 mAh g-1 after 3,000 cycles at 1 A g-1. Meanwhile, they also have reliable cycling performance and enhanced rate performance
Figure 3. (A) Schematic of the synthesis, (B) SEM and (C) TEM images, and (D) cycling performance of hollow carbon nanofibers (HCNF)[69]. (E) SEM and TEM images, (F) cycling performance of nitrogen-rich hollow carbon nanotubes (NCNT)[70]. (G) SEM and (H) TEM images, (I) rate performance, and (J) cycling performance of SnS hollow nanofibers (SnS HNFs)[71].
The above studies have shown that the anode materials with hollow structures often show better electrochemical performance than the solid materials, which is due to the low BET surface area, thin shell, and open interior. However, single hollow structures also have many limitations. Researchers should develop many complex hollow structures in continuous exploration, which can often provide better electrochemical performance.
Double/multi-shell hollow structure
A double-layer hollow structure consists of two layers of shells and cavities, usually comprising distinct materials or phases in each of the shell layers. Carbon layers usually play an important role in double-layer hollow structures. Thin carbon coating can limit the inner material, which will effectively resist the volume expansion during sodiation. Meanwhile, the carbon layer can also increase the conductivity. Hou et al. prepared a double-layer NaTi2(PO4)3@C hollow nanocubes (NTP@C) for aqueous SIBs by hydrothermal methods[72]. As shown in Figure 4A-D, NTP@C is composed of an inner layer of 15 nm (NaTi2(PO4)3) and an outer layer of 5 nm (carbon layer). The NTP@C//Na0.44MnO2 aqueous SIB assembled with deep eutectic electrolytes showed an ultra-high energy density (50.0 Wh kg-1, according to the overall cathode and anode weight) and ultra-long cycle life (3,500 cycles, capacity retention rate of 90%). Benefiting from the carefully designed negative structure, this battery will hopefully become a candidate for large-scale energy storage. Zhang et al. synthesized a double-layer ZnS-SnS@C hollow nanobox (H-ZSS@C@G) for LIB and SIB anodes and used graphene for encapsulation [Figure 4E-H][73]. The outer layer of the nanobox is a carbon layer, and the inner layer is composed of ZnS-SnS heterostructures. At 10 A g-1, H-ZSS@C@G still has a reversible capacity of 347 mAh g-1 and excellent cycling performance (247 mAh g-1 after 1,000 cycles at
Figure 4. (A) SEM, (B) TEM, and (C) HR-TEM images and (D) cycling performance of NaTi2(PO4)3@C hollow nanocubes (NTP@C)[72]. (E) SEM, (F) TEM, and (G) HR-TEM images and (H) rate performance of ZnS-SnS@C hollow nanobox [(H)-ZSS@C@G][73]. (I) SEM, (J) TEM, and (K) HR-TEM images and (L) rate performance of Sb@C coaxial hollow nanotubes[74].
Similar to the single hollow structure, the double-layer hollow tube is also a typical double-layer hollow structure. Liu et al. designed a coaxial double hollow nanotube structure [Figure 4I-L][74]. This kind of nanotube is formed by coating the carbon layer evenly on the Sb hollow tube, and the hollow structure can be changed by adjusting the annealing time. Sb is a type of high theoretical capacity anode (660 mAh g-1), but it will lead to rapid capacity attenuation due to the volume change during sodiation and desodiation. The results showed that the coaxial double-layer hollow structure greatly improves the rate and cycling performance of Sb, which has the following reasons: (i) The large cavity in the nanotubes allows the Sb nanotubes to expand; and (ii) The coated carbon layer prevented the formation of unstable SEI film by isolating Sb from the electrolyte.
A multilayer hollow structure is mainly composed of three or more shells and cavities. Bin et al. constructed multi-shell hollow carbon nanospheres (MS-HCNs), each layer of which is carbon material[75]. By controlling the polymerization process of 3-aminophenol (3-AP)/formaldehyde (3-AF) resin, they realized the accurate regulation of the shell number of hollow nanospheres [Figure 5A and B]. Studies have shown that the reversible capacity of MS-HCNs is enhanced as the shell number increases. Among them, the four-layer shell HCNs have the best reversible capacity (360 mAh g-1 at 30 mA g-1) and excellent rate performance [Figure 5C and D]. Wang et al. prepared two-to-five-shell cobalt sulfide hollow nanoboxes by an “ion conversion and exchange” strategy based on metal-organic frameworks (MOF), as shown in
Figure 5. (A) Schematic of the synthesis, (B) SEM and TEM images, (C) rate performance, and (D) cycling performance of multi-shell hollow carbon nanospheres (MS-HCNs)[75]. (E) rate performance, (F) schematic of the synthesis, and (G) TEM images of two-to-five-shell cobalt sulfide hollow nanoboxes[76].
Hierarchical hollow structure
Hierarchical hollow structure is usually composed of complex nanostructure, among which hollow nanospheres composed of 2D layered nanosheets are more common. This structure presents a hollow structure at the micron scale and is composed of many nanosheets at the nanoscale. As a piece of evidence, Yang et al. reported a hierarchical hollow structured vanadium tetrasulfide (VS4) for SIBs [Figure 6A-D][77]. The shell of hollow VS4 microspheres is composed of numerous ultrathin nanosheets, which can reduce the stress caused by volume expansion [Figure 6E and F]. The reversible capacity is about 378 mA hg-1 at
Figure 6. (A and B) SEM, (C) TEM, and (D) HR-TEM images of VS4 hierarchical hollow spheres, (E) FE models and (F) the maximum stress of three different structures, (G) rate performance of VS4[77]. (H) SEM, (I) TEM, and (J) HR-TEM images and (K) rate performance of Na2Ti3O7 hollow spheres[78]. (L) SEM and (M) HR-TEM images and (N) rate performance of MoS2/SnS hollow superassemblies (HSs)[79].
Core-shell structure
The core-shell structure is a 3D architecture consisting of several shells and cores, and there is almost no void between the core and shell. Compared with hollow structures, core-shell structures usually have good resistance to volume expansion. During charge and discharge, the core and shell support each other and limit volume changes to each other. In addition, since there is no cavity, the core-shell structure usually has a larger density than the hollow structure, which also means higher volume capacity. There are three typical kinds of core-shell structure:
Single Core/shell structure
A single core-shell structure usually consists of a shell and a core. This structure can maintain a stable microstructure while having both core and shell electrochemical properties. As common coatings, carbon materials also play an important role in forming core-shell structures. The research showed that[80] carbon-based core-shell structures have a series of advantages, and one of the most beneficial approaches to enhance the synergistic effect and reaction kinetics is to build core-shell structures: (i) For the shell, carbon is a suitable choice to achieve fast electron and ion transfer; (ii) Core-shell structures can significantly reduce the distance of ion diffusion and provide rich active sites; (iii) Synergy between shells and cores results in excellent durability for long cycle performance; and (iv) Rich heterogeneous interfaces result in lattice defects and mismatches, which are needed to increase the active sites, specific surface area, and porosity.
Figure 7. (A) SEM, (B) TEM, and (C) HR-TEM images and (D) rate performance of core-shell V2O5@C[80]. (E) SEM, (F) TEM, and (G) HR-TEM images and (H) rate performance of carbon-coated MnS (MnS@NSC). (I) Schematics comparing the sodiation/desodiation process in core-shell MnS@NSC and bare MnS, (J) cycling performance of MnS@NSC[82].
Lim et al. reported a core-shell MoS2/silicon carbide (SiOC) anode material[83]. As shown in Figure 8A-C, a SiOC coating (~10 nm) is homogeneously coated on the MoS2 sphere, forming a typical core-shell structure. In the experiment, the N-MoS2/C@SiOC anode showed much higher reversible capacity, cycling ability, and rate performance than other MoS2-based anodes [Figure 8D]. The capacity retention is 128% after 200 cycles at 0.1 A g-1. Studies have shown that the SiOC shell successfully inhibits the volume expansion of MoS2 and keeps its effect during the long cycle. In addition, SiOC acts as a channel between the core and the shell, which enables rapid ion and electron exchange and prevents excessive contact between the electrolyte and the core. As mentioned above, one of the advantages of core-shell structures is their higher density. Tellurium-based materials are typical high-density anode materials widely used in LIBs and SIBs.
Figure 8. (A) SEM, (B) TEM, and (C) HR-TEM images and (D) rate performance of core-shell MoS2/SiOC[83]. (E) SEM, (F) TEM, and (G) HR-TEM images and (H) rate performance of core-shell ZnTe@N doped carbon nanowire(ZnTe@C)[84]. (I) SEM, (J) TEM, and (K) HR-TEM images and (L) rate performance of core-shell TiC/C[85].
Multi-core/shell structure
Multi-core/shell structures consist of multilayer shells or multiple cores, usually with more complex 3D configurations such as multi-shell or multi-core. Liu et al. synthesized a multilayer core-shell Sn nanorod for SIBs using a tobacco mosaic virus (TMV) as a biological template [Figure 9A-C][86]. As one of the alloy-based anodes, Sn offers an ultra-high theoretical specific capacity (847 mAh g-1). Nevertheless, the volume change of Sn during the alloying process is large, and the material particles are easily pulverized, resulting in poor cycling performance of Sn. The authors uniformly deposited Sn nanoparticles on the Ni coating and then wrapped a thin carbon layer outside to obtain multilayer core-shell nanorods composed of TMV-Ni-Sn-C. Meanwhile, they vertically arranged nanorods in an orderly fashion on a stainless steel substrate, and the gap between the nanorods will effectively relieve the strain caused by volume expansion. Compared with the ordinary 2D Sn film, this nanorod provides great electrochemical properties as an anode for SIBs. As shown in Figure 9D, the ordinary 2D Sn thin film rapidly decays to zero after ten cycles, while the core-shell Sn nanorods have a stable capacity of 400 mAh g-1 after 150 cycles. Additionally, they also explored the influence of carbon coating on Sn nanorods. The experimental results showed that the capacity of Sn nanorods without carbon coating decreases to about 100 mAh g-1 after 50 cycles because the carbon coating can efficiently inhibit the reunion of Sn. Li et al. designed a 3D core-shell structure BTO@SnO2@P-C composite composed of a barium titanate core, tin dioxide shell, and phosphorus-doped carbon shell
Figure 9. (A) SEM, (B) TEM, and (C) HR-TEM images and (D) cycling performance of multi-shell Sn nanorods (TMV-Ni-Sn-C)[86]. (E) SEM, (F) TEM, (G) HR-TEM images and (H) cycling performance of multi-shell BTO@SnO2@P-C, (I) schematic illustration of the sodiation mechanism of BTO@SnO2@P-C[87]. (J) SEM and (K and L) TEM images and (M) cycling performance of multi-core-shell
Hierarchical core-shell structure
As mentioned earlier, hierarchical structures refer to a complex 3D arrangement, usually constructed from low-dimensional simple structures. For core-shell structures, the grading usually appears as the shell consists of low-dimensional or more complex nanostructures. Because the core and shell are tightly bonded in the core-shell structure, the shell structure is usually easier to control. For example, the uniform growth of a 3D architecture material on the surface of the substrate material is a common strategy for constructing a hierarchical core-shell structure. In recent years, MOF materials have become an expected precursor for SIB electrode materials due to their excellent modification ability, regular polyhedron morphology, and simple synthesis methods. Generally, MOF can be synthesized in a simple solution and uniformly wrapped on the surface of the matrix material. Not only that, MOF can maintain its original skeleton and morphology and even selectively leave elements during subsequent processing. Therefore, MOF plays an important role in the strategy of designing complex core-shell structures.
Huang et al. designed a hierarchical core-shell Bi2S3@Co9S8 composite for SIB anodes. Bi2S3 has a high theoretical specific capacity but poor cycling performance[89]. However, the commonly used carbon coating method has little improvement in the capacity decay of Bi2S3. Therefore, the authors used Co-based MOF (ZIF-67) to coat BiOBr spheres and obtained Bi2S3@Co9S8 composites after vulcanization and sintering. As shown in Figure 10A-E, the hierarchical core-shell structured material with Bi2S3 spheres as the core and multi-cavity Co9S8 as the shell was successfully prepared and exhibited good cycling and rate performance (595 mAh g-1 at 0.5 A g-1 after 800 cycles). Due to the regular morphology and stable skeleton structure of MOF, ZIF-67 uniformly covered the surface of BiOBr spheres and formed a unique 3D architecture after sintering. Thanks to this hierarchical core-shell structure, Bi2S3@Co9S8 composites have good buffering capacity for volume changes. In addition, the multi-cavity structure of Co9S8 provides a larger specific surface area and a shorter ion diffusion path. Xie et al. reported a hierarchical core-shell structure of carbon and MoSe2 composites[90]. Unlike the usual carbon coating, their strategy to realize the core-shell structure is to allow the lamellar MoSe2 to grow in situ on the carbon skeleton, which is obtained by sintering the MOF. ZIF-8 with a larger size (~800 nm) was prepared in this paper and maintained a regular polyhedron morphology after sintering. Subsequently, MoSe2 is uniformly grown on the carbon polyhedron by a hydrothermal method and exhibits a hierarchical structure [Figure 10F]. The lamellar structure of MoSe2 effectively alleviates the volume expansion during sodium storage and provides more active sites. In addition, the carbon skeleton obtained by sintering the MOF brings good electrical conductivity and porosity and prevents the aggregation and dissolution of selenides. Therefore, compared with pure MoSe2, the hierarchical core-shell structured MoSe2/C anode material exhibits excellent rate performance and cycle stability in SIBs (436 mAh g-1 after 500 cycles) [Figure 10G]. This new carbon composite strategy based on MOF opens the way for SIB conversion anode materials. Hierarchical core-shell structures also have great potential in sodium-ion hybrid capacitors (SIHCs). SIHCs are a special energy storage system that combines the high energy density of batteries and the high power density of supercapacitors. However, it also faces problems such as dynamic imbalance, difficulty in matching positive and negative poles, and poor rate performance. In order to construct flexible electrode materials with stable structures and excellent electrochemical performance, Wang et al. prepared core-shell MoS2/porous carbon nanofiber (PCNF) materials [Figure 10H-K][91]. In this structure, the porous carbon nanotube core can provide a certain flexibility and resist the mechanical stress during charge and discharge, while the MoS2 shell assembled by 2D nanosheets exhibits good sodium storage performance (~400 mAh g-1 after 5,000 cycles in 2 A g-1).
Figure 10. (A) Schematic of the synthesis, (B) SEM, (C) TEM, and (D) HR-TEM images, and (E) cycling performance of hierarchical core-shell Bi2S3@Co9S8[89]. (F) Schematic of the synthesis and (G) cycling performance of hierarchical core-shell MoSe2/N-PCD[90]. (H) SEM, (I) TEM, and (J) HR-TEM images and (K) cycling performance of hierarchical core-shell MoS2/PCNF[91].
Yolk-shell structure
The yolk-shell structure has the following advantages: (i) Core-shell separation: the cavity has a good buffering effect on the volume expansion of the core during the electrochemical process; (ii) thin shell and small core: shorten the ion diffusion distance, increase the specific surface area, which is conducive to rapid sodiation/desodiation; and (iii) Confinement effect: The shell has a limiting effect on the active area of the nucleus, effectively preventing the pulverization and agglomeration of the nucleus and enhancing the cycle stability.
Single yolk/shell structure
The simplest yolk-shell structure consists of a shell and a core, similar to an egg. The cavity between the shell and the core resembles the protein between the eggshell and the yolk, so it is called the yolk-shell structure. Compared with core-shell structures, the construction strategy of yolk-shell structures is more complex, which usually includes coating, etching, and sintering.
Wang et al. reported a yolk-shell FeS@C anode material for high-performance SIBs[92]. FeS is a common LIB and SIB anode material due to its high theoretical capacity, low cost, and controllable microstructure. However, the conductivity of FeS and the volume expansion during sodium storage prevent its further application. Wang and others have chosen classic carbon-coating methods to solve these problems, but their strategy is different from others. As shown in Figure 11A-C, they obtained yolk-shell structured Fe3O4@C by etching the SiO2 template and maintained the morphology during subsequent vulcanization and high temperature sintering. It is worth mentioning that the yolk-shell FeS@C exhibits superior rate performance and cycle life than both pure FeS and core-shell FeS/C (488 mAh g-1 after 300 cycles) [Figure 11D]. In fact, the performance advantage of the yolk-shell FeS@C is due to its unique cavity that can accommodate a volume-expanding FeS sphere. Their work demonstrated that the yolk-shell structure has a unique advantage over the core-shell structure and also provides a new strategy for constructing 3D architectures for conversion and alloy-based anode materials. Sun et al. prepared a yolk-shell SnS2@C anode material for SIBs and PIBs using a green and simple method[93]. In this paper, the sodium storage mechanism of SnS2 is studied by in situ Raman technique. The results successfully confirmed that the two steps of conversion reactions and alloying reactions exist simultaneously. Therefore, SnS2 exhibits considerable theoretical specific capacity, but its unique sodium storage process also brings huge volume expansion. The yolk-shell SnS2@C was prepared by three steps: polydopamine (PDA) coating and high temperature carbonization and vulcanization, and the SnS2 spheres were perfectly confined in the carbon box [Figure 11E and F]. Compared to SnS2/C, yolk-shell SnS2@C exhibits better rate performance (362 mAh g-1 at 5 A g-1) and cycle stability (~300 mAh g-1 after 500 cycles at 2 A g-1) [Figure11I]. These results indicate that yolk-shell structures are one of the best strategies for solving volume expansion and polysulfide dissolution. Doping shell is a realizable and efficient modification method to improve sodium storage performance. Xiao et al. prepared a yolk-shell SnSe2@C anode material with Se doping in the shell for SIBs[94]. Firstly, the yolk-shell SnO2@C was obtained by the classical SiO2 template method, and then the doping of Se in the carbon shell was realized by high temperature selenization [Figure 11G and H]. Density functional theory (DFT) proves that the incorporation of Se forms a Se-C bond in the carbon shell, which successfully improves the binding ability between the SnSe2 core and the carbon shell and improves the conductivity of the carbon shell. Therefore, the prepared yolk-shell SnSe2@Se-C material exhibits superior cycle stability and sodium storage kinetics. The ratio of Se doping is directly related to the reaction time. SnSe2@Se-C, with the optimal doping ratio, exhibits a specific capacity of about 400 mAh g-1 at 5 A g-1 after 2,000 cycles [Figure 11J]. This work provides useful strategies and unique insights for doping modification of single yolk-shell structures.
Figure 11. (A) Schematic of the synthesis, (B) SEM and (C) TEM images, and (D) rate performance of yolk-shell FeS@C[92]. (E) Schematic of the synthesis, (F) TEM images, and (I) cycling performance of yolk-shell SnS2@C[93]. (G) Schematic of the synthesis, (H) TEM images, and (J) cycling performance of yolk-shell SnSe2@C[94].
Multi yolk/shell structure
Multiple yolk/shell structures have multiple yolks or multi-shell structures, which can bring different performances. Li et al. reported a yolk-shell V2O3/C composite with multiple shells for LIBs and SIBs[95]. Transition metal oxide (TMO) anodes usually improve their poor conductivity and reaction kinetics by optimizing electrode structures, coupling with other conductive materials or other methods. Based on that, the authors designed a simple solvothermal route to prepare multilayer yolk-shell V2O3/C, and the number of shells can be controlled by reaction time [Figure 12A]. The prepared V2O3/C composite consists of several V2O3 shells and a V2O3 core, and the V2O3 shell is uniformly covered with a 2 nm thick carbon coating [Figure 12B and C]. In this unique multilayer yolk-shell structure, a thin and porous V2O3 layer allows sodium ions to pass quickly, a carbon coating enhances the conductivity of the overall skeleton, and a stable yolk-shell structure effectively relieves volume expansion. As an anode material for SIBs, such a delicate structure brings a reversible capacity of 173 mA h g-1 at 1 A g-1 after 2,000 cycles and enhanced rate performance [Figure 12D]. The success of V2O3/C composites provides a strategy for the preparation of multilayer yolk-shell structured anode materials and reveals the great potential of multilayer structures in improving reaction kinetics.
Figure 12. (A) Schematic of the synthesis, (B) SEM and (C) TEM images, and (D) rate performance of yolk-multi-shell V2O3/C[95]. (E) Schematic illustration of the sodiation process in core-shell Sn4P3@C and multiyolk-shell Sn4P3@C. (F) TEM and (G) SEM images (H) cycling performance and (I) rate performance of multiyolk-shell Sn4P3@C[96]. (J) Schematic illustration, (K-M) TEM images, and (N-P) electrochemical performance of Bi@Void@C with different void sizes[98].
Liu et al. reported a multiyolk-shell Sn4P3@C nanosphere for SIBs [Figure 12E-G][96]. As an intermetallic compound anode material, Sn4P3 has higher theoretical volume capacity and conductivity than Sn and P. However, Sn4P3 suffers from the same volume expansion problem. The ingenious design of the yolk-shell structured Sn4P3@C alleviates the volume expansion to some extent. In the typical synthesis process,
For the yolk-shell structure, the size of the cavity has an important influence on the electrochemical performance of the material because the cavity determines its ability to alleviate the volume expansion. Yang et al. proved this view by adjusting the cavity volume between the Bi yolk and the C shell in the yolk-shell Bi@C structure[98]. By adjusting the amount of tetraethoxysilane (TEOS) and phenolic resin (RF), three yolk-shell Bi@C with cavity volume ratios of 1, 10, and 20 were prepared [Figure 12J]. The results showed that the Bi@C-2 anode material [Figure 12K] with medium cavity volume exhibits the best sodium storage performance (198 mAh g-1 after 10,000 cycles at 20 A g-1). Bi@C-1 with a smaller cavity volume [Figure 12L] exhibits a faster capacity decay in cycling tests (146 mAh g-1 after 100 cycles at 1 A g-1) due to the continuous expansion of the volume of Bi during sodiation, which breaks the carbon shell and eventually collapses the structure. On the other hand, Bi@C-3, with a larger cavity volume [Figure 12M], shows good cycling performance, which is similar to Bi@C-1. However, the rate performance of Bi@C-3 is much worse than that of Bi@C-1 because the cavity volume is too large and the sodium ion diffusion distance is greatly increased [Figure 12N-P]. The results prove the necessity of regulating the cavity volume of yolk-shell structures and provide a valuable scheme for its optimized design strategy.
Hierarchical yolk/shell structure
The graded yolk-shell structure is usually composed of a graded shell or a graded core. Unlike the core-shell structure, the shell and core of the yolk-shell structure are separate, so the two can form a hierarchical structure.
Chen et al. reported a yolk-shell Fe7Se8@C@MoSe2 with hierarchical shell structures for SIBs[99]. The problems of large volume expansion and low conductivity of iron selenide anode materials are usually solved by carbon coating, but more progress is still needed. The authors proposed a strategy to further release the volume expansion stress by in-situ growth of lamellar MoSe2 on the basis of iron selenide/carbon composites. In a typical synthetic route [Figure 13A], yolk-shell structured Fe3O4@C was first prepared, and then, MoS2 nanosheets were uniformly grown on its surface by a solvothermal method and finally transformed into Fe7Se8@C@MoSe2 composites by high temperature selenization. As shown in
Figure 13. (A) Schematic of the synthesis, (B) SEM and (C) TEM images, and (D) rate performance of hierarchical yolk-shell
In addition to 2D nanosheets, other types of complex structures can also be used to modify the surface of yolk-shell structures. For example, Zou et al. reported a hierarchical yolk-shell NiO/Ni/graphene composite[100]. Firstly, Ni-MOF with hierarchical yolk-shell structures was successfully prepared by adjusting the amount of polyvinyl pyrrolidone (PVP). Subsequently, the initial morphology was maintained during two annealings and core-shell NiO/Ni/graphene nanocrystals were formed on the shell surface [Figure 13E and F]. In fact, the formation of yolk-shell Ni-MOF is a typical Ostwald ripening process. It is called a hierarchical structure because the surface of the yolk shell is uniformly distributed with nanocrystals that are smaller than 5 nm. These nanocrystals are composed of ultrathin graphene shells and Ni/NiO cores. Graphene-coated nanocrystals enhance the conductivity of NiO and accelerate the transport rate of Na+, and the well-graded yolk-shell structure resists volume expansion during cycling. The results of electrochemical tests showed that the hierarchical yolk-shell NiO/Ni/graphene composite provides excellent rate performance
In the graded yolk-shell structure, the graded core structure also occupies a place. Liang et al. reported a bio-inspired hierarchical yolk-shell Bi2S3-PPy composite for LIBs and SIBs[101]. As an anode for conversion reactions, Bi2S3 cannot exhibit high reversible capacity due to its huge volume expansion. Carbon coating is a common solution, but it usually requires a high temperature pyrolysis process, which sometimes leads to the collapse of the designed structure or the reduction of the active material. Therefore, the authors proposed a strategy for PPy coating of hierarchical Bi2S3 materials without high temperature heat treatment. The hierarchical Bi2S3 was obtained by a simple solvothermal method, and then, PPy films were formed outside the Bi2S3 core by vapor phase polymerization. As shown in Figure 13H and I), the Bi2S3 core is assembled by ultrathin nanowires with a diameter of 5-10 nm, and the PPy film is uniformly covered on its surface. In the electrochemical performance test, Bi2S3-PPy exhibits better cycling performance (about
The terms “hollow”, “core-shell”, and “yolk-shell” are frequently interchanged and misused in many research papers. In this review, we attempt to provide clear definitions of these three structures. It is necessary to distinguish them because they exhibit different properties. Only by deeply understanding the differences between these three materials can we better design and utilize them. Compared with core-shell structures, hollow structures and yolk-shell structures both have unique advantages in alleviating volumetric expansion, but their energy density is generally lower than core-shell structures, which limits their commercial application. Although the theoretical energy density of the core-shell structure is high, the utilization rate of the core is not satisfactory due to slow kinetics. The yolk-shell structure has comprehensive advantages, but the construction method of yolk-shell structures is usually tedious. In the future, finding simple methods for constructing 3D architectures may become a research focus.
Porous structure
Porous structures are quite common 3D architectures and often coexist with other types of 3D architectures in a material. For example, hollow porous carbon nanospheres and core-shell structures with porous shells are common. However, when the materials exhibit other more concrete 3D structure types, they are usually not classified as porous structures. In this chapter, we will briefly introduce some anode materials dominated by porous structures. Porous materials can be divided into three categories by pore size: microporous materials, mesoporous materials, and macroporous materials.
Porous materials with an average pore size of less than 2 nm are called microporous materials. Microporous materials have received extensive attention in the field of EES due to their abundant and ultra-small pores, the most typical of which is microporous carbon materials. However, porous carbon materials usually face the problem of low ICE due to the uncontrollable formation of SEI films. In order to improve the ICE and sodium storage capacity of porous carbon materials in SIBs, Feng et al. proposed a strategy of sulfur coating and N, S co-doping[102]. In a typical synthetic route, N-doped porous carbon (NPC) precursors are synthesized by MOF-based methods. Subsequently, the precursor was calcined with sulfur to achieve sulfur filling in the micropores and boundary S doping [Figure 14A-C]. Compared with NPC, the ICE of N, S
Figure 14. (A) SEM, (B) TEM, and (C) HR-TEM images and (D) rate performance of N, S co-doped microporous carbon (NSPC)[102]. (E) Schematic of the synthesis, (F) TEM and (G) HR-TEM images, and (H) electrochemical performance of S and N co-doped mesoporous carbon spheres (SNMHCSs)[103]. (I) Schematic of the synthesis, (J) SEM and (K) TEM images, and (L) cycling performance of N and S co-doped 3D macroporous carbon (Sb@S, N-3DPC)[104].
The pore size of mesoporous materials is usually between 2 nm and 50 nm. In the case of the same pore volume, the specific surface area of the material will decrease with the increase of pore sizes, which will bring some different characteristics. Ni et al. reported a S and N co-doped mesoporous carbon sphere for high-performance SIB anode materials[103]. Mesoporous carbon spheres are synthesized using PDA as carbon sources, calcium carbonate as a template, and pore-forming agents. Calcium carbonate decomposes at high temperatures to form carbon dioxide, and then carbon dioxide will break through the carbon wall to form mesopores [Figure 14E-G]. The mesoporous structure can provide a large specific surface area, allowing rapid exchange of Na+ and reducing the volume expansion stress to some extent. In addition, N and S are doped at the same time as high temperature sintering. The results showed that the doping of N and S brings larger interlayer spacing and more functional groups and lattice defects to the mesoporous carbon anode while maintaining the initial mesoporous carbon morphology. Taken together, the S and N co-doped mesoporous carbon anode exhibits excellent cycling and rate performance, providing a specific capacity of 180 mAh g-1 even after 7,000 cycles at 20 A g-1 [Figure 14H]. This strategy of constructing mesoporous structures is unique and has guiding significance for the modification of 3D carbon materials.
Macroporous structures have large pore sizes (> 50 nm), which are common on larger substrates rather than nanomaterials. For example, Yang et al. prepared a macroporous carbon material for SIBs using SiO2 as a template[104]. As shown in Figure 14I-K, this carbon material is no longer composed of dispersed individual micro/nanoparticles but consists of a carbon matrix and uniform macropores, resembling the structure of a sponge. Under the action of SiO2 spheres, the macropores appear as quasi-spherical pores (about 200 nm) in the carbon matrix, thus forming a 3D interconnected macroporous carbon skeleton. Then, numerous Sb nanoparticles are encapsulated in the carbon skeleton, which can greatly alleviate the volume expansion of Sb and enhance the conductivity. Finally, N and S co-doped 3D macroporous carbon materials (Sb@S,
The above studies showed that the construction of porous structures is one of the effective ways to improve the anode of carbon-based materials. In general, the porous structure has the following advantages: (i) Large electrode/electrolyte interface: huge specific surface area and high porosity can increase active sites; (ii) Fast ion/electron exchange rate: porous structures accelerate sodium storage reaction kinetics and improve rate performance; and (iii) Excellent structural stability: stable porous structures can effectively buffer the volume expansion. However, the significant disadvantage of porous structures is that it is easy to form excessive SEI films. During the cycles, the pores are in contact with the electrolyte to form an irreversible SEI film, which will cause the pores to clog and affect performance.
Self-assembled nano/micro-structure
In many 3D structure types, hollow structures, core-shell structures, yolk-shell structures, and porous structures are easily identified and classified because of their strong morphological characteristics. However, there are still many reports on the morphology of electrode materials that cannot be strictly classified into one structural type. Those special structures can be called 3D self-assembled nano/microstructures. Three-dimensional self-assembled nano/microstructures possess both the fast ion/electron exchangeability of nanostructures and the strong stability of microstructure and can be divided into two categories according to the way they are assembled: (i) Multiple homogeneous nanostructures self-assemble into microstructures (AA type); and (ii) nanostructures are embedded into another structure to form microstructures (AB type).
AA-type 3D self-assembled nano/micro-structures are very common in SIB anode materials such as flower-like, thread-like, urchin-like, and dandelion-like structures. These structures are usually self-assembled by low-dimensional structures. For example, flower-like structures are self-assembled by 2D nanosheets, and clew-like structures are self-assembled by 1D nanowires. Bai et al. reported a self-assembled flower-like MoTe2/C anode material for SIBs[105]. As a Mo-based chalcogenide, MoTe2 usually exhibits the morphology of 2D nanosheets, which is similar to MoS2 and MoSe2. However, the direct use of lamellar nanomaterials as electrode materials will lead to the accumulation of nanosheets and the collapse of the structure. Self-assembly of nanosheets into flower-like microstructures is an efficient approach to solving the problem. The chelation method was used to prepare flower-like Mo-PDA, and the morphology was maintained during the subsequent oxidation and tellurization processes. As shown in Figure 15A and B, a large number of ultrathin MoTe2/C nanosheets self-assembled into flower-like structures. This structure retains the large specific surface area and fast electron/ion exchange rate of the nanosheets while also improving the stability of the structure. The results showed that the flower-like MoTe2/C exhibits excellent rates and cycling performance (190 mAh g-1 at 1 A g-1 after 3,000 cycles) [Figure 15C]. In addition, the pH level and telluride temperature have an influence on the electrochemical properties of flower-like MoTe2/C. This work proves the great effect of self-assembly strategies on improving cycle stability and provides ideas for the modification of self-assembled structures. Opra et al. prepared a self-assembled moss-like Na2Ti3O7 for SIB anodes by hydrothermal and annealing methods[106]. Na2Ti3O7 mossy clusters are composed of numerous small hollow nanotubes with only several nanometers in diameter [Figure 15D and E]. The special structure brings a huge specific surface area and lots of pores, which is conducive to increasing the ion channels and active sites. Additionally, the stability of this self-assembled structure was proved by high temperature experiments (350 °C). The self-assembled moss-like Na2Ti3O7 exhibits good cycling and rate performance (95 mAh g-1 at 350 mA g-1 after 200 cycles) [Figure 15F]. The self-assembly strategy may help Na2Ti3O7 show greater potential in the fields of energy, biomedicine, catalysis, and so on. In fact, these examples demonstrate that the self-assembled structure is not a simple mixture of inferiority building blocks but a more stable 3D structure that takes advantage of both nano and micro-structures.
Unlike AA-type structures, AB-type 3D self-assembled nano/microstructures usually contain two or more phases. In addition, the A structure can be not only a single-phase simple structure but also a complex structure such as the core-shell or yolk-shell structure. For example, Li et al. embedded yolk-shell Sn@C nanospheres into a carbon matrix to obtain an Eggette-like Sn@C self-assembled anode material for LIB and SIBs[107]. In a typical synthesis process, yolk-shell SnO2@C nanospheres were first prepared, then SnO2@C nanospheres and glucose were simultaneously covered on sodium chloride crystals by evaporation, and finally, Sn@C and carbon films were formed by high temperature calcination. The yolk-shell structured Sn@C has a diameter of about 300 nm; the distance between the Sn core and the carbon shell is about
All in all, a single structural type or material type faces various problems. The above work showed that 3D self-assembled micro/nanostructures can make up for the defects of a single structure and combine the advantages of multiple materials. Therefore, 3D self-assembled nano/micro-structured anode materials exhibit enhanced capacity and cycling performance. Thus, the construction of 3D self-assembled nano/micro-structures will be one of the future directions of anodes for SIBs.
BRIEF SUMMARIZATION OF METHODS AND PERFORMANCE
In this review, we have mentioned many kinds of anode materials and classified them according to their different structures. As shown in Table 1, varying 3D architectures and materials exhibit distinct performance. In addition, we also pay extra attention to their synthesis methods. Among them, the most commonly used method for constructing 3D structures is the templating method, which includes hard template, soft template, and self-template methods. The hard template method is a simple and stable strategy that can obtain the product with uniform particle size. Meanwhile, the corresponding product structure can be obtained by customizing various template shapes. However, the removal of the hard template requires additional steps, such as acid/alkali etching. These additional steps will lead to higher costs and environmental pollution. The soft template method can remove or transform the template by heating or extraction, while the self-template method does not require additional templates. Under certain conditions, the reactants can be used as their own templates to construct a specific structure. The soft template and self-template methods are more environmentally friendly and cost-effective, but they are usually difficult to accurately control the morphology of products. Besides the template method, hydrothermal, vapor deposition, and calcination methods are also commonly used to construct 3D architectures. It is beneficial to construct complex multi-level structures by using hydrothermal methods, but they are not suitable for large-scale applications. The vapor deposition is commonly used to produce thin-film materials. It is a green method that does not require additional media and high temperature and pressure conditions. Calcination is a kind of high temperature treatment method, such as carbonization and vulcanization. The calcined material often exhibits higher crystallinity, density, and structural stability. Therefore, calcination is always used together with other methods.
Electrochemical performance and synthesis methods of different 3D architectures
| Architectures | Typical samples | Methods | Performance | Ref. |
| Hollow | Carbon | Hard template | 50 mA g-1, 310 mAh g-1 | [68] |
| Carbon | Self-template | 30 mA g-1, 360 mAh g-1 | [75] | |
| MoS2/SnS | Hard template | 10 A g-1, 368 mAh g-1 | [79] | |
| Core-shell | MnS/C | Self-template | 100 mA g-1, 594 mAh g-1 | [82] |
| Sn/C | Hard template | 50 mA g-1, 400 mAh g-1 | [86] | |
| MoSe2/C | Hydrothermal | 200 mA g-1, 464 mAh g-1 | [90] | |
| Yolk-shell | SnS2/C | Soft template | 100 mA g-1, 660 mAh g-1 | [93] |
| V2O3/C | Hydrothermal | 50 mA g-1, 258 mAh g-1 | [95] | |
| Bi2S3/PPy | Vapor deposition | 125 mA g-1, 591 mAh g-1 | [101] | |
| Porous | Carbon/S | Calcination | 500 mA g-1, 592 mAh g-1 | [102] |
| Carbon | Soft template | 20 A g-1, 180 mAh g-1 | [103] | |
| Sb/C | Hard template | 100 mA g-1, 482 mAh g-1 | [104] | |
| Self-assembled | MoTe2/C | Calcination | 250 mA g-1, 580 mAh g-1 | [105] |
| Sn/C | Self-template | 100 mA g-1, 400 mAh g-1 | [107] |
CONCLUSION AND OUTLOOK
The composition, structure, and process of materials have a decisive influence on the performance of materials [Figure 16]. In this paper, we summarize the application of 3D architectures in anode materials for SIBs. Many cases have proved that the construction of 3D architectures has a significant impact on improving the sodium storage capacity of anodes. Hollow structures have a large cavity, thin shell, and especially large specific surface area, which improve the rate and cycling performance. Core-shell structures can provide high volumetric energy density and reversible capacity due to the synergistic effect between different materials and abundant heterogeneous interfaces. Yolk-shell structures have a prominent contribution in alleviating volume expansion due to the confinement effect and maintain a good tap density, which can effectively balance the high reversible capacity and long cycle life. Porous structures offer lots of fast sodium ion channels and sodium storage sites, often in conjunction with other structures to improve battery performance. Three-dimensional self-assembled nano/micro-structures can improve the electron/ion exchange capability and structural stability of the anodes by assembling nanostructures into microstructures. While providing unique advantages, each 3D architecture also confronts different problems. For example, hollow structures have a low energy density due to the large cavity, and core-shell structures are not conducive to resisting volume expansion. Yolk-shell structures and porous structures have a large specific surface area, but the larger specific surface area necessitates a greater amount of electrolytes, which will also lead to lower Coulombic efficiency. In addition, a key attribute of high-energy-density batteries is the minimal use of electrolytes. The 3D self-assembly structure faces problems such as cumbersome synthesis routes and uncontrollable assembly processes. Therefore, while benefitting from the performance improvement brought by the 3D architectures, we must also face the various defects they bring. Comprehensively utilizing the advantages of these structures and compensating for their disadvantages will be a research focus in the future. In fact, 3D architectures are constructed to compensate for the defects of simple structures (such as nanospheres, nanowires, and nanosheets) and combine their advantages to enhance material properties. Therefore, the development of SIBs can be effectively promoted by designing 3D architectures of anode materials and utilizing the compatibility of multiple materials or structures.
Figure 16. Advantages of different types of three-dimensional architecture anodes for SIBs.
Compared with the commercial LIBs, the energy density of SIBs is still lower, but their overall cost is higher. Therefore, it is urgent to develop low-cost and high-capacity anodes to improve the competitiveness of SIBs. Although various 3D architecture anode materials have many application examples in the field of SIBs, more efforts are still needed. At present, the primary difficulty is how to optimize the synthesis route. Most of the 3D architectures have complex construction strategies and high energy consumption, which seriously hinders the commercialization of 3D architecture anode materials. At present, most of the construction strategies of 3D architectures are only in the laboratory stage. The development of simple, efficient, and high-yield processes is crucial for the promotion of SIBs. For the commercialized hard carbon anode, poor rate performance and ICE are urgent problems to be solved. Constructing a reasonable 3D architecture may be the future direction of hard carbon anodes. Due to the large specific surface area of the 3D structure anode, irreversible reactions will occur at the interface between the electrode and the electrolyte, resulting in rapid consumption of the electrolyte. In addition, the study of the electrode/electrolyte interface will also be a long-term topic. Another challenge is how to increase the volume energy density. Three-dimensional architectures with a large number of cavities or voids face the problem of low volume energy density exemplified by hollow structures and yolk-shell structures. As one of the potential candidates in the field of energy storage devices and electric vehicles, SIBs must consider the volume energy density of electrode materials more. Therefore, adjusting the ratio of cavity and solid in 3D architecture will be a significant research focus in the coming decades. In addition, due to the ionic radius of sodium, the electrode materials of SIBs have larger volume expansion stress and lower ion transport rates, which means that the electrode materials for SIBs need more effort in structural design. In the future, the development of strategies to construct 3D architectures will be of great significance for the practical application of SIBs.
DECLARATIONS
Authors’ contributions
Proposed the topic of this review; reviewed the manuscript: Wu C, Li Y
Wrote the manuscript: Sun YF
Discussed and revised the manuscript: Gong YT, Qiu ZX, Qian J, Wang ZL, Zhang RP, Bai Y
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by the National Natural Science Foundation of China (Grant Nos. 22279009, 22005033), the National Postdoctoral Program for Innovative Talents (No. BX20180037), and the Beijing Institute of Technology Research Fund Program for Young Scholars (No. XSQD-202108005).
Conflicts of interest
Not applicable.
Ethical approval and consent to participate
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Consent for publication
Not applicable.
Copyright
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Sun, Y. F.; Li, Y.; Gong, Y. T.; Qiu, Z. X.; Qian, J.; Bai, Y.; Wang, Z. L.; Zhang, R. P.; Wu, C. Constructing three-dimensional architectures to design advanced anodes materials for sodium-ion batteries: from nanoscale to microscale. Energy Mater. 2024, 4, 400002. http://dx.doi.org/10.20517/energymater.2023.63
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