Toward safer lithium metal batteries: a review

Lithium dendrite

Unrefined lithium metal is highly susceptible to dendritic growth when deposited unevenly on the current collector. Figure 2 illustrates each phase of dendritic growth. Once these dendrites reach a certain length, they can pierce through the separator, resulting in a direct connection between the positive and negative electrodes and ultimately leading to a short circuit [Figure 3E]. Furthermore, if a small branch of dendrites carries a substantial amount of current, an extraordinary amount of heat can be generated within the battery. In severe cases, these conditions may even cause explosions. These concerns regarding the lack of guaranteed safety present significant obstacles against the successful commercialization of these batteries.

Figure 2. Schematic illustration of Li dendrite growth. Li: Lithium.

Figure 3. (A-E) Schematic illustration of lithium anode failure process.

The primary cause of mechanical failure in solid-state electrolytes (SSEs) is typically the high-stress field that arises from the electroplating of Li within pre-existing defects. This stress field drives crack propagation throughout the electrolyte and leads to the growth of internal Li filaments. In severe instances, the filament may reach another electrode, resulting in an internal short circuit. To understand the role of interfacial defects on mechanical failures of SSEs, Xu et al. constructed an electro-chemo-mechanical model, which provides visual insights into the distribution of stress, relative damage, and crack formation during the electrochemical plating of Li in defects^[4]. Geometry of interfacial defects was found as a dominating factor for a concentration of the local stress field, while semi-sphere defects delivered less damage accumulation at the initial stage and the longest failure time for disintegration of the electrolyte. The aspect ratio, a key geometric parameter of defects, was investigated to reveal its impact on electrolyte failure. In particular, pyramidic defects with low aspect ratios from 0.2 to 0.5 exhibit branched regions of damage that extend near the interface, leading to surface pulverization of the electrolyte. Conversely, for defects possessing a high aspect ratio exceeding 3.0, damage accumulation occurs primarily in the bulk electrolyte. The elucidation of such a correlation between interfacial defects and electro-chemo-mechanical failures in SSEs may offer invaluable guidelines for the design of interfaces in high-power-density solid-state lithium metal batteries. Clearly, mechanics play a more central role in solid-state batteries than in their liquid counterparts because of their rigid solid-solid interfaces^[5]. The application of stresses during both the manufacturing and cycling of these batteries may give rise to substantial and often adverse effects on cell performance. During Li plating and stripping cycles, the manifestation of Li-induced stress at the anode/electrolyte interface is heavily influenced by the non-uniform morphology that encompasses heterogeneous nucleation and growth. Notably, each cycle inevitably induces a level of stress upon the battery, thereby causing irreparable damage to the solid electrolyte interphase (SEI) layer^[6]. Furthermore, relative damage that arises from local high-stress events preferentially transpires within the region of significant electrolyte/Li interface fluctuations^[7]. This damage is characterized by the formation of cracks [Figure 3A and B], which become the site for dendritic material growth [Figure 3C]. Inhomogeneous deposition of Li-ions tends to exacerbate the emergence of dendritic structures, and such behavior can be elucidated from both thermodynamic and kinetic perspectives^[8]. Thermodynamically, metallic lithium undergoes repeated electroplating and stripping cycles during battery operation, inevitably leading to heat generation. However, the uneven distribution of heat within the electrode compromises its interfacial uniformity and exacerbates dendrite formation^[3]. Consequently, dendrite growth is nearly unavoidable. From a kinetic standpoint, dendrite formation in lithium is caused by many factors, including the surface status of lithium and the anodic electric field. The presence of surface defects in Li₂O, Li₂CO₃, and pristine lithium makes them more susceptible to uneven deposition^[9]. Typically, when lithium metal with surface defects is immersed into an electrolyte, rapid reactions occur on the metal surface, forming an uneven SEI layer. The unevenness of the SEI layer leads to an uneven current distribution, causing non-uniform deposition of lithium after multiple battery cycles^[10]. The uneven current distribution and uneven SEI surface layer promote mutually in a vicious cycle, resulting in a significant decrease in cell Coulomb efficiency and difficulty in ensuring battery safety.

Apart from the above scenario, non-uniform electric fields caused by ion concentration gradients or unstable electrolyte concentrations can also lead to dendrite growth. Reportedly, the transport of Li⁺ is more challenging than that of anions due to the abundant solvent molecules in the solvent sheath surrounding Li^+[11]. As a result, the concentration of anions adjacent to the cathode quickly decreases to zero, forming a space-positive charge region. This space charge domain generates a local electric field that triggers dendrite growth. To counteract this phenomenon, electrolytes with better ionic conductivity and reduced anion mobility are employed to mitigate anion depletion near the electrode/electrolyte interface^[12], consequently suppressing the nucleation of dendrites. Through live scanning electron microscope observations, Dollé et al. made the astute observation that the evolution of lithium from a mossy to a needle-like structure was contingent on the increase in current density^[6]. Furthermore, based on meticulous calculations and rooted in the mechano-electrochemical phase field model, it was established that Li deposition on the anode was smoother within a particular range when the battery underwent heightened pressure^[3]. Specifically, the lower pressure endured by the lithium anode of the pouch battery ultimately determined its inferior performance. Yet, the exertion of excessive pressure ran the risk of fracturing dendrites at their roots^[13], which caused a corresponding loss of capacity. While the appropriateness of pressure levels plays a crucial role in attaining optimal performance in lithium metal batteries, the underlying mechanics are multifaceted and nuanced.

As reported, lithium dendrites are highly susceptible to formation under low surface energy and high migration energy. High temperatures can reduce surface migration barriers and promote ion diffusion, effectively reducing nucleation density and forming smoother Li deposition. Consequently, adjusting temperature can alter the nucleation and growth behavior of lithium. The ultimate state of Li deposition is highly dependent on its initial shape. The most common morphologies are needle-like, moss-like, and dendritic shapes, and lithium dendrite is widely used to describe the above uneven deposition morphology^[14]. Dendrite nucleation involves multiple disciplines, including chemistry, electrochemistry, crystallography, molecular dynamics, and thermophysics, which render a single model insufficient in explaining this phenomenon. While many high-potential metals, such as Zn, Cu, and Ni, also produce dendrites during electrochemical deposition, their dendrite growth mechanism has been thoroughly studied. However, the excessively low reduction potential of metal Li-ions leads to a dendrite growth mechanism that is fundamentally different from that of other metals. As a result, the problem of lithium dendrites remains unresolved, and the theory of dendrite growth requires further investigation. This review will follow various schemes for the suppression of lithium dendrites.

Although various strategies are committed to inhibiting the growth of lithium dendrites, dendrite growth is still inevitable thermodynamically. Also, lithium dendrites grow to a certain extent and then spontaneously fall off to form ineffective metal lithium, which can no longer participate in subsequent battery reactions [Figure 3D]. The invalid lithium metal is customarily called “dead lithium”.

Dead lithium

Dead lithium refers to the irreversible capacity loss experienced by batteries^[15]. Recent studies suggest that the CE of Li deposition/dissolution in nonaqueous electrolytes is significantly lower than 99.2%^[16], primarily due to the uncontrolled formation of dendrites and subsequent accumulation of dead lithium. Therefore, a thorough comprehension of the underlying mechanisms behind the formation of dead lithium is necessary. Figure 3A-E illustrates the detailed formation mechanism of dead lithium and the consequent failure process of lithium batteries. Notably, dendrite necks with larger curvature tend to accumulate at a higher electron density and exhibit faster rates of lithium dissolution^[17]. This often results in dendrite fractures at these sites. Subsequent exposure of the newly-exposed lithium to the surrounding electrolyte initiates repeated reactions that continuously consume it, reducing battery capacity. Empirical studies demonstrate that dead lithium is directly attributed to internal stress and current distribution within the battery. For instance, uneven current distribution in the microporous collector can lead to detachment of the lithium metal located in the center of the pore channel from the porous skeleton. Scholars have proposed to adjust porous collector pore sizes and current distributions to minimize the incidence of “dead lithium”^[18]. Notably, among the myriad of challenges identified over the years, high chemical reactivity and volume fluctuations are often regarded as two fundamental hurdles. These issues are intensified by SEI failures, which exacerbate electrochemical side reactions, resulting in the formation of abundant dendrites and dead lithium. This ultimately leads to significant volume expansion within the anode, posing considerable safety concerns and contributing to a marked decline in CEs.

Volume expansion of lithium anode

In the realm of electrochemistry, it is a widely accepted fact that volume fluctuations are ubiquitous in electrode materials during charge and discharge cycles. Even commercial graphite electrodes, which are commonly used in batteries, experience up to 10% volume fluctuations. This phenomenon becomes more pronounced in the case of lithium metal. Notably, a commercially viable single-sided electrode must possess an area capacity of 3 mAh·cm^-2, which, for lithium, will result in a volume change of 14.6 μm^[6]. This value is bound to escalate in the future, implying that the lithium interface will move by tens of microns during cycling.

The non-uniform distribution of internal stress generates cracks in the SEI layer, leading to the growth of dendrites^[19]. These loose dendritic structures exacerbate the volume expansion of the lithium metal anode^[20] [Figure 3E]. Dendrites and volume expansion jointly contribute to the formation of a vicious cycle that further disrupts the electrode/electrolyte interface. After several battery cycles, a substantial amount of dead lithium builds up on the electrode, and this accumulation increases the anode thickness, thus intensifying battery resistance.

In the realm of applied lithium battery technology, anode expansion can be attributed to the insult and pulverization of LiH. Xu et al. studied a battery featuring an ultra-thin lithium anode and a high-load cathode (LiCoO₂) and found that the proportion of LiH by-products on the anode surface rose dramatically from 0.74% to 16.55% after 20 cycles^[4]. The content of LiH on the anode surface is reportedly inversely related to battery life, and the volume of lithium metal anodes without any supporting structure significantly changes during the electrochemical process. There is no experimental evidence to confirm that high current density (1.5 mA·cm^-2) directly causes battery failures^[21]. However, at high current density, the expansion of the lithium/electrolyte interface region leads to the formation of more SEI components, which curtail contact between lithium particles and bestow a porous structure. This porous structure has a larger specific surface area and reduces electrolyte wettability at the interface, resulting in greater interfacial impedance. Accordingly, this increase in the interstitial resistance of the porous SEI is considered as the actual cause of ultimate failure in lithium metal batteries.

Corrosion of lithium anode

Due to its high negative redox potential, lithium spontaneously reacts with the electrolyte to form a detrimental SEI layer^[22]. During cycling, this unstable SEI continuously fractures, giving rise to freshly-exposed lithium, which repeatedly reacts with the electrolyte. Continuous consumption of the electrolyte and electrochemical corrosion of metallic lithium lead to a low CE and gradual capacity decay^[23,24]. The three primary factors affecting the reactivity of lithium and electrolytes are the electronic structure, the strength of the electrolyte, and the stability of the SEI. Moreover, the viscosity of the electrolyte can affect the corrosion rate of lithium. Reports suggest that low-viscosity dimethoxyethane and tetrahydrofuran perform poorly in lithium stability compared to propylene carbonate and ethylene carbonate. A robust SEI can prevent fresh lithium exposure, which is the primary factor influencing lithium corrosion. The composition and properties of the SEI are closely related to the electrolyte composition. For instance, an unstable SEI is formed on the surface of a lithium metal battery electrode with a conventional ethylene carbonate electrolyte. In contrast, a fluorine-substituted cyclic carbonate electrolyte yields a chemically stable LiF-enriched SEI on the surface of a lithium anode^[25]. Therefore, adjusting the electrolyte composition to enhance the stability of the SEI can mitigate lithium corrosion.

The causes of battery failures have not been fully explained, which is not conducive to subsequent battery optimization. Hence, understanding the underlying mechanism of battery failures must be pursued relentlessly to improve battery performance. An exhaustive array of factors is thought to contribute to the failure of lithium metal batteries. Figure 4 depicts the current state of battery technology, highlighting the inherent complex relationships therein. This article delves into the specific strategies that can be employed to address these issues.

Figure 4. The correlations among anode issues.

Polymer electrolytes

Metallic lithium batteries, known for their high energy density, are promising for a wide range of applications. However, due to the complexity of multiple reactions inside the battery and the impact of factors such as high temperature, overcharging, and discharging, metallic lithium batteries are prone to thermal runaway, explosions, and other safety issues. In this regard, the application of polymer electrolytes can significantly improve the safety of metallic lithium batteries. Firstly, compared with traditional liquid electrolytes, polymer electrolytes have lower volatility and higher melting points, thereby reducing the risk of thermal runaway inside the battery. Secondly, polymer electrolytes have higher chemical stability, which prevents harmful substances generated by internal reactions from damaging the electrolyte, thereby reducing the production of gas and sediment in the battery and lowering pressure accumulation. Additionally, polymer electrolytes can achieve fast ion transfer and low resistance by changing their molecular structure, further improving battery performance and safety.

Polymer electrolytes have been identified as a promising solution to the mechanical suppression of lithium dendrites. Various polymer electrolytes, such as copolymers, cross-linked polymers, and polymer/nanoparticle composites, have been developed to integrate high ionic conductivity with sufficient shear modulus to prevent the formation of lithium dendrites^[30]. Polymer electrolytes have been widely studied as they maintain high electrochemical stability, flexibility, and mechanical durability, specifically focusing on solid polymer electrolytes (SPEs) in this discussion.

Among SPEs, the polyethylene oxide (PEO)-based electrolyte has garnered significant attention due to its remarkable solubility of Li-ions and high chain flexibility^[31]. The ethylene oxide (EO) functional group can coordinate with Li⁺ to form an EO-Li⁺ complex, enabling the Li-ions to traverse along the long polymer chain of PEO. However, the crystalline structure of PEO impedes ionic conductivity at room temperature, although this obstacle can be overcome by raising the temperature. Nevertheless, this approach can also reduce the shear modulus of the electrolyte. Studies show that the growth of lithium dendrites can be entirely prevented when the shear modulus of the electrolyte is twice that of lithium metal^[26]. Consequently, there is a contradiction between shear modulus and ionic conductivity. The shear modulus of PEO homopolymer is about four orders of magnitude lower than that of lithium metal, which is insufficient to entirely prevent dendritic growth. In pure PEO, the rate of lithium dendrite growth is ten times faster than that in block copolymers. These findings suggest that the shear modulus of the electrolyte influences Li deposition.

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) is a popular polymer matrix for gel polymer electrolytes, but its stiffness is negligible. To address this issue, Tu et al. developed a laminated electrolyte/separator material by placing a nano-porous Al₂O₃ sheet between PVDF-HFP membranes^[32]. The resulting electrolyte/separator has an ionic conductivity of over 1 mS·cm^-1 and high mechanical strength at room temperature. This design effectively stabilizes lithium metal batteries and prevents cell short circuits. Incorporating polymer electrolytes with nanostructured ceramics is a common method to enhance the stiffness of the electrolytes for dendrite suppression. For instance, SiO₂ hollow nanospheres were utilized to provide a mechanically strong scaffold for the gel polymer electrolyte as a composite solid electrolyte with a high ionic conductivity of 1.74 mS·cm^-1 and enhanced safety features such as non-leakage, low volatility, flammability, and dendrite resistance^[33].

In the pursuit of developing lithium metal batteries, researchers have endeavored to improve the mechanical properties of SPEs to hinder the formation of lithium dendrites. A theoretical prediction suggests that the growth of lithium dendrites can be prevented by ensuring that the shear modulus (G) of the separator/electrolyte is twice that of lithium metal (G > 2G_Li, or G > 6G_Pa at room temperature)^[26]. Recently, a photopolymerization technique was employed to create an interpenetrating network of poly(ether-acrylate) SPE, which combines impressive mechanical strength (≈12 G_Pa) and high ionic conductivity (2.2 × 10^-1 mS·cm^-1 at room temperature), effectively preventing dendrite growth and facilitating Li-ion migration for efficient lithium metal plating/stripping. Nevertheless, Tikekar et al. disputed the necessity of the G > 2G_Li criterion to inhibit lithium dendrite growth^[34]. They developed an SPE utilizing polyethylene (PE) cross-linked with PEO segments^[12]. The incorporation of PE backbones decreased the crystallinity of PEO, thus enhancing ionic conductivity. The addition of polyethylene glycol (PEG) allowed for tuning of the SPE, resulting in a plasticized cross-linked polymer with a high ionic conductivity of 2.0 × 10^-1 mS·cm^-1 at 25 °C. The dendrite-resisting properties of the SPE were demonstrated through testing with a charge passed through the cell at various current densities. The cross-linked SPEs increase the lifetime of the Li cells by over an order of magnitude that of PEO, which indicates their exceptional dendrite-resisting capability despite their modest shear moduli.

Pan et al. recently introduced a new cross-linked SPE that utilized polyhedral oligomeric silsesquioxane (POSS) and PEG to address lithium dendrite formation [Figure 6A and B]^[35]. The researchers found that POSS particles provided the necessary mechanical strength for the cross-linked polymer electrolyte, while PEG chains facilitated ion conduction. The mechanical properties of the cross-linked SPEs were measured via dynamic mechanical analysis (DMA) using a three-point bending mode [Figure 6C]. By varying the molar ratio of POSS to PEG and the molecular weight of PEG in POSS-nPEGm, the researchers can optimize the shear modulus and ionic conductivity of the polymer electrolyte. Results showed the ionic conductivity increased with the POSS/PEG ratio [Figure 6D]. At 105 °C, the POSS-4PEG2K electrolyte had a high conductivity of > 1 mS·cm^-1 and maintained a shear modulus of 33.6 MPa. While the moduli of the POSS-nPEGm electrolytes did not meet the modulus criterion to mechanically prevent lithium dendrite growth, the cross-linked polymer electrolytes were effective in blocking lithium dendrites and improving cell life. The symmetric cell using POSS-4PEG2K [Figure 6E] was tested at different current densities. The cell was still stable after 2,600 h of cycling at 0.3 mA·cm^-2 for POSS-4PEG2K [Figure 6F]. These findings demonstrate the promising potential of the cross-linked polymer electroly

Figure 6. (A) Schematic illustration of LMB with a cross-linked POSS-PEO SPE as the separator to block the growth of lithium dendrites. (B) Schematic presentation of the cross-linked POSS-PEG hybrid polymer electrolyte on Li-metal anode to block Li dendrites; (C) The storage modulus G’ and (D) ionic conductivity of the POSS-PEO solid polymer electrolytes as functions of temperature; (E) Configuration of the lithium symmetric cell; (F) Charge passing the lithium symmetric cell (Cd) in the galvanostatic cycling tests at different current densities^[35]. Copyright 2015, WILEY.

The inorganic solid electrolyte boasts exceptional ionic conductivity and mechanical properties. However, its high brittleness, complex processing, and elevated interfacial impedance impede its practical applicability. In contrast, the polymer electrolyte offers superior flexibility and machinability, enabling its seamless connection with lithium metal. Nonetheless, the ionic conductivity of the polymer electrolyte is suboptimal, which dampens its practicability. To exploit the benefits of both electrolyte types, researchers have devised a novel polymer-inorganic composite electrolyte. The interaction between inorganic particles and polymer electrolytes can mitigate polymer crystallization and enhance ionic conductivity. The incorporation of polymers likewise reduces the interfacial resistance between lithium metal and the solid electrolyte. Zhao et al. probed into the ternary sulfide Li₁₀GeP₂S₁₂ (LGPS) and a PEO lithium salt system to assemble a composite electrolyte with high Li⁺ conductivity (1.18 × 10^-2 mS·cm^-1)^[36]. The effect of the solid plasticizer succinonitrile (SN) on the ionic conductivity of PEO electrolytes has been widely examined, culminating in the successful preparation of a new SPE (PEO₁₈-LiTFSi-1%LGPS-10%SN) via conventional solution casting. Its maximum ionic conductivity reaches 9.1 × 10^-2 mS·cm^-1 at 25 °C, 15 times larger than that of PEO-LiTFSI^[37].

The Li/PEO₁₈-LITFSi-1%LGPS-10%SN/LiFePO₄ battery demonstrates impressive cycling and capacity results at 40 °C. The battery reaches a maximum discharge capacity of 138.4 mAh·g^-1 at 0.5 °C, and the capacity retention rate is nearly 90% even after 100 cycles. The newly-developed SPE effectively inhibits dendrite growth, which contributes to the outstanding electrochemical performance of the battery. The research group incorporated Li_1.5Al_0.5Ge_1.5(PO₄)₃(LAGP) active particles of varying sizes and ratios into PEO to produce various composite electrolytes (PEO-LAGP)^[38]. It was found the electrolyte with the smallest particle size had the highest conductivity of 0.676 mS·cm^-1 at 60 °C. By using composite electrolytes (PEO-20%LAGP-l), the researchers assembled Li/PEO-20%LAGP-l/LiFePO₄ cells, which achieved a capacity of 100 mAh·g^-1 with a capacity retention rate of nearly 90% after 50 weeks of cycling at 1 and 60 °C. Notably, the capacity retention rate for Li/PEO/LiFePO₄ batteries under the same conditions is only 44% after 50 cycles. The addition of LAGP-I particles to PEO significantly improves the interface stability and compatibility, thereby optimizing the battery performance. Furthermore, the researchers designed a novel composite electrolyte with layered structures to address the different needs of positive and negative interfaces. The Goodenough Research Group developed a polymer/ceramic/ polymer sandwich structure composite electrolyte (PCPSE)^[39]. In this design, the inorganic layer blocks the transportation of lithium salt anions, reduces the influence of space charge on the lithium and electrolyte interface, and thus relieves the interface impedance. The Li/PCPSE/LiFePO₄ battery demonstrated excellent performance and achieved a specific capacity of ~100 m·Ah·g^-1 after 640 cycles of charging and discharging at a current density of 0.51 mA·cm^-2.

The primary objective of optimizing polymer electrolytes, as mentioned above, lies in elevating their mechanical hardness and enhancing their electrical conductivity, which effectively prevents dendritic growth, assures lithium battery safety, and concurrently maximizes the CE. In addition to the polymer electrolyte modification strategies mentioned previously, hybrid electrolytes comprising polymers and nanomaterials offer significant potential for electrolyte optimization. The rapid development of nanotechnology presents endless opportunities for enhancing electrolytes, significantly improving ion flow within the electrolyte. Detailed investigations of polymer-nanomaterial electrolytes are discussed below.

Polymer-nanomaterial composite electrolytes

In general, the composite electrolytes of polymers and nanomaterials significantly enhance the safety and performance of metal lithium batteries. Firstly, utilizing composite electrolytes can significantly improve the mechanical strength and tensile resistance of the electrolyte, affording it higher stability and reliability. Compared with traditional electrolytes, polymer-nanomaterial composite electrolytes effectively reduce safety issues, such as thermal runaway and electrolyte leakage. Secondly, owing to their large specific surface area and unique conductivity, nanomaterials, when combined with polymer electrolytes, enable an enhanced ion transmission in the electrolyte, improving the electrochemical performance and cycle stability of the battery and prolonging its lifespan. Moreover, polymer-nanomaterial composite electrolytes can delay the formation and growth of lithium dendrites, thereby reducing the risk of short circuits within the battery. Several types of polymer-nanomaterial composite electrolytes will be specifically introduced in the following section.

The integration of nanofillers into polymer electrolytes can yield a higher elastic modulus and an expanded electrochemical stability window. Lin et al. reported a high-ionic-conductivity PEO electrolyte fortified with monodispersed ultrafine SiO₂ nanospheres^[40]. The incorporation of nanoparticles corresponded with an impressive 1-2 orders of magnitude increase in ionic conductivity in comparison to the pure polymer matrix, which was attributed to the structural rearrangement of the interface around the nanoparticles. Polymers and active fillers share a common component, which elicits a driving force and balances the lithium chemical potential between the two phases^[41].

The incorporation of active nanowires or nanofiber networks into polymer matrices can significantly enhance ionic conductivity by offering long ion transport pathways in the polymer matrix [Figure 7A]. Liu et al. recently demonstrated the potential of one-dimensional Li_0.33La_0.557TiO₃ nanowires as excellent fillers in composite electrolytes, resulting in enhanced ion conduction^[42]. When the Li_0.33La_0.557TiO₃ nanowire content was at 15 wt.%, the ionic conductivity of the electrolyte reached 0.24 mS·cm^-1 at ambient temperature. The increase in ionic conductivity was due to the strong complexation between ClO₄^- and the acidic groups on Li_0.33La_0.557TiO₃, resulting in the dissociation of LiClO₄ and the generation of a fast Li-ion diffusion channel [Figure 7B]. Similarly, the utilization of Y₂O₃^- and ZrO₂ nanowires with positive oxygen vacancies on their surface can interact with salt anions, releasing Li-ions and resulting in the formation of a continuous fast conduction path^[43]. This interaction improves the ionic conductivity of composite polymer electrolytes, offering a greater advantage than a single nanoparticle [Figure 7B].

Figure 7. Composite electrolytes using inorganic fillers and SPEs. (A) Schematic of nanowire and nanoparticle fillers in the polymer matrix^[42]. Copyright 2015, American Chemical Society; (B) Mechanism of ion transport in nanowire or nanoparticle-filled polymer electrolytes^[43]. Copyright 2016, American Chemical Society; (C) Schematic of nanofiber network in the polymer electrolyte; (D) Schematic of preparation of fiber-reinforced polymer composite electrolyte; (E) SEM images of garnet nanofibers and (F) cross-section of the electrolyte composites^[44]. Copyright 2017, WILEY. SEM: scanning electron microscopy. SPEs: solid polymer electrolytes.

Fu et al. have explored the potential benefits of using 3D garnet-type Li_6.4La₃Zr₂Al_0.2O₁₂ (LLZO) nanofiber networks to enhance the ionic conductivity of dry polymer electrolytes^[44]. The LLZO nanofiber network acts as an effective medium for the continuous transportation of Li-ions in the electrolyte system [Figure 7C]. Figure 7D illustrates the preparation process of a fiber-reinforced polymer composite electrolyte, which involves the utilization of electrospinning technology. Initially, a combination of garnet LLZO salt and the sacrificial polymer was spun into nanofiber membranes and subsequently sintered at 800 °C in the air to form a garnet ceramic nanofiber network. SEM images reveal that these garnet nanofibers possess strong interconnectivity [Figure 7E] and have a membrane thickness ranging from 40-50 microns [Figure 7F].

Solid electrolytes based on garnet nanofibers exhibit high thermal stability and mechanical strength, thereby mitigating the safety hazards (e.g., battery short circuits) that may arise from polymer electrolyte softening or liquefaction at elevated temperatures. Notably, the ionic conductivity of garnet nanofiber solid electrolytes is 0.25 mS·cm^-1, which shows high electrochemical stability even under high current densities of up to 0.5 mA·cm^-2[44]. Enhancing ion conduction remains a crucial research direction in resolving lithium battery issues. The integration of liquid and solid electrolytes, known as “liquid-solid hybrid electrolytes”, has demonstrated the potential to improve ionic conductivity by filling the gap between the solid electrolyte and the electrode.

Liquid-solid hybrid electrolytes

Liquid-solid hybrid electrolytes possess remarkable transport capability, chemical stability, and safety. In the context of metal lithium batteries, this type of electrolyte shows great promise as a potential substitute for traditional electrolytes. In terms of transport capability, liquid-solid hybrid electrolytes offer a more uniform distribution compared to their conventional counterparts, which allows for efficient transfer of Li-ions within the electrolyte. This high-performance transport capability enhances the CE, simultaneously reducing the need for cooling measures and minimizing the risk of thermal runaway. Firstly, liquid-solid hybrid electrolytes contain a greater proportion of flowable media that can maintain excellent flowability even under high-temperature conditions. Secondly, due to the higher concentration of solvents and additives present within the electrolyte system, liquid-solid hybrid electrolytes are better equipped to curtail adverse phenomena, such as lithium dendrite growth or precipitates. Additionally, these electrolytes can effectively reduce the production of gas within the battery, further enhancing the overall safety performance of these batteries, particularly in extreme operating environments characterized by prolonged usage, rapid charge-discharge cycles, or high/low-temperature conditions. The subsequent portion of this exposition shall explicate in meticulous detail several distinct breeds of liquid-solid hybrid electrolytes.

Researchers have extensively studied the structural composition and compatibility of battery interfaces. Among binary liquid-solid electrolyte hybrids, LAGP, Li_1.3Ti_1.7Al_0.3(PO₄)₃ (LATP), and lithium super-ionic conductor (LISICON) are commonly used as electrolytes. Hagen et al. employed a polymer interlayer that was saturated with liquid organic solvents as interlayers to optimize the contact between electrodes and solid electrolytes^[45]. Additionally, Wang et al. developed Li-S cells that utilized Li_1.5Al_0.5Ge_1.5(PO₄)₃ as a diaphragm and conventional Li-S electrolyte (LiTFSI in DME/DOL) was filled in the space between solid electrolyte and electrodes^[46]. Figure 8A provides a visualization of this battery configuration. Later, Wang et al. further extended these advancements by creating a hybrid battery founded on a LISICON-type Li_1+x+yAl_xTi_2-xSi_yP_3-yO₁₂ (LATP) solid electrolyte [Figure 8B]^[47]. Moreover, Yu et al. constructed a hybrid battery using a LISICON-type Li_1.3Al_0.3Ti_1.7(PO₄)₃ electrolyte, with the dissolved Li₂S₆ catholyte solution acting as a cathode [Figure 8D]^[48]. The research endeavors undertaken within this area of study provide compelling evidence that indicates a significant improvement in both the CE and capacity of Li-S batteries. However, there is still a lack of thorough understanding regarding interface stability between solid electrolytes and liquid organic electrolytes.

Figure 8. Liquid-solid hybrid electrolytes for Li-metal batteries. Schematics of (A) a hybrid Li-S cell using LAGP solid electrolyte, (B) a hybrid Li-Li₂S cell using LISICON-type LATP solid electrolyte, (C) liquid electrolyte and solid electrolyte^[49], Copyright 2016, Macmillan Publishers Limited. (D) a hybrid Li-Li₂S₆ cell using LISICON-type LATP solid electrolyte.

In the realm of lithium battery research, distinguished scholars Busche et al. recently investigated the interface between liquid electrolytes and solid electrolytes and revealed the presence of a resistive solid-liquid electrolyte interface (SLEI) at the boundary. Figure 8C delineates a comprehensive schematic diagram of ion transport and resistance contribution at the solid-liquid phase boundary within the battery^[49]. The movement of ions across this boundary makes a noteworthy contribution to the total impedance. At the junction between solid and liquid electrolytes, the mechanism underlying ion conduction will shift from the solvation ion diffusion in liquid electrolytes to the hopping mechanism in solid electrolytes. Furthermore, investigations confirm that SLEI encompasses various decomposition products, including inorganic compounds, organic compounds, and polymer compounds. As the thickness of SLEI increases to its maximum, its consequent resistance contribution will eventually reach a threshold. Impedance-wise, the large impedance engendered by the electrolyte-electrode gap must be taken into account, thus making interface engineering an essential consideration.

SSE surface coating modification

Tsai et al. introduced the Au layer on the surface of Li₇La₃Zr₂O₁₂ (LLZO) and achieved good results, improving the Li/SSE interface contact^[50]. The interfacial resistance was reduced, and no lithium dendrite formation was evident after long-time cycling. Inspired by the unique H⁺/Li⁺ exchange reaction of garnet-type LLZO, Cai et al. directly dropped an AgNO₃ aqueous solution onto Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) surfaces to construct a lithium-philic layer^[51]. This layer swiftly generates a uniformly dispersed layer of Ag nanoparticles, which subsequently react with molten Li to form Li-Ag alloy. Consequently, interfacial wettability between Li and LLZTO is considerably enhanced, with interfacial resistance reduced to about 4.5 Ω·cm^-2. The modified LLZTO was used to assemble a fully solid state. The Li/LLZTO/LiFePO₄ cell was cycled at 60 °C for 200 cycles. The cell has good cycling stability and effectively inhibits dendrite growth^[52].

Huo et al. etched dense LLZTO ceramic sheets in hydrochloric acid for 1 h before fabricating 30 μm 3D-porous LLZTO (3D-LLZTO)^[30]. Three different interfaces (unmodified Li/LLZTO, lithium-philic Li/LLZTO@ ZnO(zinc oxide), and Li/3D-LLZTO@ZnO) are constructed, and their deposition behaviors are displayed in Figure 9A-C. Different from conventional coating techniques, this method involves pretreatment of the SSE prior to coating and the construction of a 3D porous surface through acid-etching technology. The 3D-LLZTO@ZnO interface sufficiently reduces local current density and volume fluctuations, thereby effectively curbing dendritic growth. Assembled Li/3D-LLZTO@ZnO/Li symmetric batteries demonstrate a stable cycle lifespan exceeding 600 h at a current density of 0.5 mA·cm^-2. However, lithium dendrites are clearly visible in both unmodified Li/LLZTO and Li/LLZTO@ZnO lithium symmetric batteries. Sheng et al. added Li₂S additive to modify the PEO-LiTFSI interface in developing PEO-LiTFSI-Li₂S solid electrolyte^[53]. Modified interface impedance was reduced from 557 to 374 Ω·cm^-2. Cryogenic transmission electron microscopy (Cryo-TEM) and molecular dynamics simulations reveal that Li₂S accelerates the decomposition of LiTFSI by constructing in-situ interfacial layers rich in highly conductive LiF. The generated LiF effectively inhibits the fracture of C-O in PEO chains and stabilizes the Li/PEO interface during cycling. The Li/PEO-LiTFSI-Li₂S/LiFePO₄ battery exhibits a specific capacity of 140 mAh·g^-1 with a CE of 99% at 50 °C and 0.5 C, retaining 85% capacity after 1,000 cycles, significantly better than PEO-LiTFSI batteries (77%, 150 cycles). Table 1 briefly summarizes the functions and mechanisms of the various electrolytes.

Figure 9. Three different interfacial behaviors of lithium deposition: (A) LLZTO/Li interface; (B) LLZTO@ZnO/Li interface; (C) 3D-LLZTO@ZnO/Li interface^[30]. Copyright 2020, American Chemical Society.

The oxide-garnet type SSE, Li₇La₃Zr₂O₁₂, exhibits exceptional ionic conductivity, making it an auspicious candidate for solid lithium metal batteries. Despite intensive investigations to enhance chemical stability and ionic conductivity, the poor wettability between lithium metal and garnet electrolyte still engenders many microporous structures and microscopic gaps at the interface[Figure 10A]^[61]. High interfacial impedance impedes the further development of solid lithium metal batteries; the introduction of an alumina coating can reduce the interfacial impedance by forming a Li-Al-O layer with Li-ion conductivity and electronically insulating properties^[61]. The nanoscaled alumina coating ensures optimal wettability and intimate surface contact. In the absence of the ALD coating, a vast physical gap is clearly delineated [Figure 10B], leading directly to interfacial resistance up to 10³ Ω·cm^-2. In contrast, with the presence of the ALD-Al₂O₃ coating, the reaction of Al₂O₃ with molten Li provides the driving force for mutual contact, leading to a marked reduction in total impedance. The interfacial resistance decreases from 1,710 to 34 Ω·cm^-2 with the addition of the Al₂O₃ layer [Figure 10C]. Moreover, the Li-symmetric garnet cell containing Al₂O₃ manifests a stable potential of about 13 mV at a current density of 0.1 mA·cm^-2[Figure 10D]. Conversely, the Li-symmetric garnet cell lacking Al₂O₃ demonstrates a noisy potential. The successful assembly of a battery comprising Li/Al₂O₃ garnet electrolyte and Li₂FeMn₃O₈ cathode that powers a light-emitting diode (LED) highlights this potential. Further advancements in SSE engineering and interface modification will undoubtedly pave the way for more stable and efficient lithium metal batteries.

Figure 10. Interface of garnet SSE and Li-metal anode. (A) Schematic of the interface between of garnet surface and metallic Li; (B) SEM images of the garnet solid electrolyte/Li-metal interface. Insets are photos of a drop of molten on a garnet disk; (C) EIS profiles of the symmetric Li/garnet cells with and without ALD coating. Inset shows the enlarged impedance curve of the ALD-treated garnet cell; (D) DC cycling of Li/garnet symmetric cells at a current density of 0.1 mA·cm^-2. The inset is the magnified curve of the ALD-garnet cell^[61]. Copyright 2017 Macmillan Publishers Limited. ALD: Atomic layer deposition; EIS: electrochemical impedance spectroscopy; SEM: scanning electron microscopy; SSE:solid-state electrolyte.

SSE porous optimization

Constructing an engineered interface (e.g., active material embedded in a porous structure) is another means of reducing interfacial resistance between the electrode and solid electrolyte^[62]. Figure 11A compares planar interfaces with porous interfaces. Sacrificial organic templates are added onto high-density solid electrolyte micelles, which are promptly removed by sintering for the creation of the porous interface. A slurry consisting of Li₄Ti₅O₁₂, conductive carbon, and the polymer is cast on a planar or porous structure [Figure 11A]. Cross-sectional SEM images of both interfaces are compared [Figure 10B]. In the non-modified cell, a distinct poor interface occurs, leading to extremely high interfacial impedance [Figure 11C]. However, after interface engineering modification, the electrode layer is tightly embedded in the porous solid electrolyte layer, with almost no gap or splitting between electrolyte and electrode. To make further strides toward improving the efficiency and efficacy of SSEs, it is recommended that more attention shall be devoted to the development of interface-engineered structures as well as the construction of sturdier SSE frameworks. Significant progress will be made in lithium batteries by optimizing the contact area between electrodes and SSEs via these techniques.

Figure 11. Nanoengineering on SSE for interface contact. (A) Schematic of non-modified or interface-engineered solid-state battery; (B) SEM images of cross-sectional interfaces of the electrolyte and electrode for non-modified or interface-engineered pellets; (C) Nyquist plots of non-modified or interface-engineered batteries^[62]. Copyright 2016, WILEY. SEM: scanning electron microscopy.

Lithium anode surface coating modification

In recent years, researchers have explored the application of Au, Si, and ZnO on Li surfaces. Tsai et al. utilized a thin Au buffer layer to enhance contact and minimize lithium dendrite formation [Figure 12A]^[50]. The Au buffer layer significantly reduced the total resistance of the Li-symmetric solid electrolyte cell from 3,000 to 380 Ω·cm^-2. At room temperature, Au and Li combine to create a unique contact with the solid electrolyte, enabling the redistribution of Li dissolution and deposition. After the groundbreaking research, subsequent studies have delved into the application of Si and ZnO. More specifically, an ultrathin layer (about 10 nm) of amorphous Si was conformally coated onto a garnet electrolyte through chemical vapor deposition^[63]. ZnO has been identified as yet another active material that can react with molten Li at the interface, eventually leading to the formation of a Li-Zn alloy [Figure 12B]^[64]. The coating layer of ZnO can facilitate high-speed diffusion of molten Li, thereby ensuring the optimal contact between Li and the garnet solid electrolyte while effectively suppressing interfacial resistance [Figure 12C].

Figure 12. Li-SSE interface wettability improvement via Li alloys. (A) Garnet/Li interface with an Au buffer layer to improve its interfacial contact; (B) Schematics, digital photos, and SEM images showing the wettability of garnet-Li interface with a ZnO coating layer; (C) EIS of the Li symmetric cells with or without ZnO^[64]. Copyright 2016, American Chemical Society. EIS: electrochemical impedance spectroscopy; Li: Lithium; SEM: scanning electron microscopy. EIS: electrochemical impedance spectroscopy; SEM: scanning electron microscopy.

Interface engineering technology has demonstrated a significant improvement in battery performance. Additionally, optimizing the physical properties of the electrodes presents a viable strategy for enhancing battery efficiency. The next section of this review will delve into a common electrode modification technique - designing an electrode framework.

3D framework lithiophilicity optimization

The role of lithiophilicity in achieving uniform Li-ion deposition is of utmost importance. Due to the typically high nucleation barriers of 3D frameworks and poor affinity with Li-ions, it is customary to deposit a lithiophilic transition layer on the surface of the substrate to enhance its lithiophilicity^[67,68]. Recently, Wang et al. reported a method to modify a collector framework (NRA-CC) based on a 3D carbon cloth with lithiophilic metal-organic framework (MOF) nanorod arrays^[69]. The schematic illustration of a synthesis process of the NRA-CC is displayed in Figure 13A. The NRA-CC was immersed in a solution of nitric acid and dimethylimidazolidinone precursor to generate Co-based MOF nanorod arrays in situ on the surface of the 3D carbon cloth, which was then carbonized to form a C-N-Co three-phase nanoscale modified layer. This structure can significantly enhance the lithiophilicity of the 3D framework surface.

Figure 13. (A) Schematic illustration of the synthesis procedures of NRA-CC; (B) Galvanostatic discharge/charge profiles of the Li@NRA-CC|NCM battery at 1 C (1st-200th cycle); (C) Long-term cycling performance of the Li@NRA-CC|NCM and Li|NCM batteries at 1 C; (D) The charge-discharge curves of Li@NRA-CC|NCM battery at different rates; (E) Rate performance of Li@NRA-CC|NCM and Li|NCM batteries^[69]. Copyright 2020, WILEY.

This study sought to explore the practical feasibility of a Li@NRA-CC hybrid anode by incorporating it into full batteries equipped with high-voltage LiNi_0.5Co_0.2Mn_0.3O₂ (NCM) cathodes. The battery operated with the Li@NRA-CC anode delivered outstanding cyclic stability, retaining about 75% of its initial capacity after 200 cycles with minimal voltage hysteresis fluctuations [Figure 13B]. In sharp contrast, the performance of the full battery containing Li foil as an anode degraded drastically within only 60 cycles and exhibited fluctuating CEs, indicating poor electrode-electrolyte interface stability [Figure 13C]. These two different battery configurations displayed analogous trends in rate performance when the current density did not exceed 1 C [Figure 13D and E]. However, upon increasing the rate to 5 C, the capacity of the battery utilizing Li foil decreased rapidly, while the battery oriented as a Li@NRA-CC anode yielded resilient capacity values coupled with relatively stable cyclic stability. Therefore, the application of a 3D porous carbon cloth framework has enhanced the cycling performance of the battery, effectively guiding lithium-ion deposition uniformly on the 3D framework surface and suppressing negative electrode volume expansion and dendrite growth. The pore size of the 3D framework shall also be appropriate, as excessively large or small pores may not be conducive to Li deposition.

Innovative 3D fluid collections

Porous 3D lithium substrates can effectively adjust to volume fluctuations during cycling. This review presents a variety of innovative fluid collections. Among the various substrate materials, carbon-based materials or metal frames are widely utilized as current collectors. These 3D nanostructured conductive substrates offer a network of free electron pathways and ensure full contact with liquid electrolytes, facilitating rapid charge transfer and uniform lithium nucleation. Among the carbon-based host materials, Ji et al. reported an inhomogeneous 3D current collector with modified carbon paper to prohibit Li deposition onto the separator. A SiO₂ or SiC insulating layer was applied to the line-of-sight side (facing toward the separator) of the pristine carbon paper [Figure 14B]^[70]. The spatially heterogeneous insulating property from SiO₂ or SiC coating restricts Li-ions from depositing on the front side of the carbon paper, resulting in a surface dendrite-free Li deposition and inhibition of short circuit even at 4 mA·cm^-2 after a deep Li deposition of 28.8 C·cm^-2[70]. Many other promising carbon hosts that can accommodate lithium metal have been proposed.

Figure 14. Li-metal anode with a 3D substrate. (A) Schematic illustration of unstacked graphene electrode before (left) and after Li plating (middle) and after stripping (right); (B) Structure illustrations of pristine carbon paper, spatially SiC or SiO₂^- coated carbon paper, and Li deposition on this modified carbon paper; (C) Voltage profiles of Li electrodes on the unstacked graphene or Cu foil at 2.0 mA·cm^-2 with a capacity of 0.1 mAh·cm^-2[72]. Copyright 2016, WILEY; (D) Illustration of electrochemical Li plating on planar Cu and 3D Cu^[73]. Copyright 2016 WILEY; (E) the corresponding cycling stability of Li on planar Cu and Li in 3D Cu electrodes at 0.2 mA·cm^-2[74]. Copyright 2015 Macmillan Publishers Limited; (F) Schematic diagrams for comparison of Li deposition within hierarchical porous carbon scaffold with and without decoration of ZnO nanoparticles; (G) The comparison of the Coulombic efficiency of Li deposition on pristine HPC and various ZnO@HPC^[71]. Copyright 2017, Elsevier.

Jin et al. described a 3D-layered porous carbon material that promotes uniform stripping/plating of lithium metal [Figure 14F]^[71]. The ZnO@HPC (hierarchical porous carbon) scaffold was synthesized through a precipitation reaction of zinc acetate dihydrate (Zn(OAc)₂·2H₂O) and lithium hydroxide monohydrate (LiOH·H₂O) in dilute ethanol solution. The content of ZnO can be easily controlled by varying the amounts of Zn(OAc)₂·2H₂O and LiOH·H₂O. Samples with 5%, 15%, 25%, and 50% ZnO by weight were prepared, and the particle size of ZnO was kept similar for each sample by controlling the concentrations of the LiOH·H₂O and Zn(OAc)₂·2H₂O solutions. However, differences still existed in the number and distribution of ZnO particles. In comparison to the 15% ZnO@HPC, too little ZnO in the 5% ZnO@HPC induced non-uniform deposition of lithium. Additionally, severe aggregation occurred in both the 25% ZnO particles and the 50% ZnO@HPC samples. Excessive ZnO particles may impair the uniform deposition of Li because they may occupy too much pore space, resulting in excessive discharge products of lithium oxide. Figure 14G compares the CE of Li deposition on different ZnO@HPC samples. Evidently, the CE is correlated with the content of ZnO. Lithium metal up to 131 mAh·cm^-2 can be confined within ZnO@HPC, achieving acceptable volume expansion, considerable reduction in overpotential, and effective dendrite suppression.

Zhang et al. recently presented a remarkable non-stacked graphene framework with nanoscale hexagonal drums that facilitate desirable sites for Li deposition^[72]. As illustrated in Figure 14A, the top layer comprises the SEI layer, followed by the deposited lithium metal in the middle and, finally, the graphene framework at the bottom. This innovative anode structure adopts a sandwich-like configuration. The larger specific surface area of nanodrums orchestrates uniform current dispersion and local current density reduction, enabling uniform Li deposition to take place beneath the SEI layer. Li deposition on nanostructured graphite or graphene electrodes manifests better cycling stability. In comparison to planar copper electrodes, the graphene-based anode exhibits enhanced stability over 800 cycles with a negligible hysteresis of 150 mV at 2 mA·cm^-2 and an excellent capacity of 0.5 mAh·cm^-2 [Figure 14C].

A simple approach is to increase the surface area of the electrodes to reduce the current density. Along this line, various current collectors, including 3D current collectors with submicron skeletons, 3D porous copper current collectors with vertically aligned microchannels, and independent copper nanowire (CuNW) networks, are used to accommodate volume expansion and inhibit lithium dendrite growth^[73]. For planar copper collectors, lithium growth is accelerated in a limited area due to the high local electric field strength [Figure 14D]. In contrast, interconnected CuNWs with a high surface area can significantly reduce the Li-ion flux and thus achieve more uniform Li deposition at the macroscopic scale. In particular, once some dendrites appear, they are confined to the interconnected network, forming lumpy lithium instead of needle-like lithium. Using a stand-alone CuNW network collector, the lithium metal can work stably for 200 cycles, maintaining an average CE of up to ≈98.6%^[73]. Yang et al. developed a novel 3D copper electrode with submicron holes to optimize lithium metal deposition. Compared with planar copper foil, the battery utilizing 3D copper as an electrode demonstrated excellent cycle stability, lasting for 600 h [Figure 14E]^[74]. Lu et al. discovered that forming 3D porous copper electrodes can effectively strip Zn from Cu-Zn alloys^[73]. The pore sizes of 3D porous copper electrodes range from 200 nm to 2 μm, with many protrusions on their inner surfaces providing charge centers and nucleation sites for Li deposition. Self-evaporation of CuNW suspension can synthesize CuNW membranes. After 200 cycles, the cycle efficiency of the 3D copper anode exceeds 97%^[74]. The optimized cell performance can be attributed to the significantly reduced local current density resulting from the 3D copper pores, allowing Li deposition to transpire within them and restraining the growth of lithium dendrites.

The carbon nanotube sponge boasts a sizeable specific surface area (300-400 m²·g^-1) and exhibits high porosity (> 99%), making it an ideal 3D host material for lithium metal anodes^[75], which is unattainable using traditional planar collectors. Consequently, Zhang et al. created a 3D current collector to serve as a host for Li deposition by growing a conformal layer of Al₂O₃ on the carbon nanotube sponge via ALD^[76]. The Al₂O₃ layer provides excellent chemical stability and high mechanical strength, providing robust protection to enhance the interface between the 3D electrode and electrolyte. In carbonate electrolytes without additives, the modified electrodes exhibit stable cycling with improved CEs^[76].

Molten lithium pre-injected 3D skeleton

The encapsulation of lithium metal within the intricate 3D structure remains a daunting task on account of the exceptional reactivity exhibited by lithium, thereby rendering it technically challenging. Consequently, the current approach utilized for this purpose involves pre-depositing Li on the 3D substrate through electrochemical treatment. Unfortunately, pre-deposition is an arduous and costly procedure.

Chen et al. achieved stable lithium metal cycling by designing a novel type of 3D current collector, which uncovered a new mechanism for the deposition/stripping of lithium metal^[77]. They combined two mixed-ion-electron conductors (MIECs) and ion-electron-insulated (ELI) materials to stabilize the lithium metal electrochemically, ultimately constructing a 3D tubular lithium metal negative electrode composed of MIEC as the primary channel and ELI for sealing [Figure 15A]. Lithiated carbon tubules with a width of about 100 nm were employed as the MIEC material. The researchers demonstrated the plating and stripping of lithium or sodium inside individual carbon tubules. A solid electrolyte consisting of a PEO-based polymer with a thickness of around 50 μm was utilized in this experiment. On the opposite side of the solid electrolyte, a Li counter-electrode was coated and connected to a scanning tunneling microscope (STM)/transmission electron microscope (TEM) manipulator. The TEM copper grid served as the current collector attached to the carbon tubules on the other end. Notably, the inner diameter of the carbon tubule is around 100 nm, while its walls, with a width of 20 nm, are also nano-porous. In-situ TEM showed that lithium metal can advance and contract in a single-crystal form within the MIEC small tube and reversibly deposit/strip on a scale of tens of micrometers, preserving structural integrity within 100 cycles^[78]. The Coble creep mechanism along the MIEC metal lithium interface prevents the formation of dead lithium. Therefore, this structure solves dynamic issues since most deposition/stripping of lithium metal occurs within the electrochemically stable MIEC small tube, thus reducing direct contact between lithium metal and SSEs to produce SEI, ultimately achieving high reversibility of lithium metal negative electrode-SSE interface. This 3D design strategy provides a new approach for constructing high-performance full solid-state lithium metal batteries based on the Coble creep mechanism. MIEC materials shall exhibit good Li-ion and electron conductivity, be absolutely stable to lithium metal electrochemically, and have lithium affinity, which is conducive to pre-lithiation in the battery assembly stage and helps maintain contact between lithium metal and MIEC wall, enabling it to spread along the MIEC wall with zero contact angle. MIEC materials include lithiated anode materials, unordered solid solutions with certain lithium atom mutual solubility, or metals that do not mix with lithium metal body phases but contain lithium diffusion phase boundary channels. ELI materials act as “inert adhesives” positioned at the interface between MIEC and SSE, maintaining contact between the SSE and MIEC. Without ELI, lithium metal deposition will commence from the root of MIEC, potentially leading to mechanical separation between the root of MIECs and SSEs.

Figure 15. (A) Schematic process of creep-enabled Li deposition/stripping in an MIEC tubular matrix, where Coble creep dominates via interfacial diffusion along the MIEC/Libcc incoherent interface; (B) Charge/discharge profiles at 0.1 C; (C) cycling life of the all-solid-state Li-pre-deposited MIEC/SE/ LiFePO₄ batteries. The magenta (capacity) and blue (CE) colors indicate the use of 3D MIEC tubules on Pt foil as a Li host, the discharge capacity of which reaches 164 mA h g^-1 at 0.1 C and 157 mA·h·g^-1 at 0.2 C, while the green color indicates the use of 2D carbon-coated Cu foil as a Li host^[77]. Copyright 2020, Springer Nature.

In this study, an all-solid-state full cell was evaluated in terms of its performance with the MIEC tubules containing a minimal quantity of pre-deposited Li^[77]. At 0.1 C [Figure 15B], this cell exhibited superior performance, exhibiting a lower overpotential (0.25 vs. 0.45 V), much higher discharge capacity (164 vs.123 mA·h·g^-1), and significantly higher CE (99.83% vs. 82.22%). Furthermore, this full cell endured almost no deterioration throughout more than 50 cycles (in conformity with Figure 15C), and the specific gravimetric capacity of the Li/MIEC composite anode reached an exceptionally high value of approximately 900 mA·h·g^-1. Thus, these results validate the practical use of the MIEC architecture for applications in all-solid-state metal battery systems, which evolved from mechanistic notions to quantitative theory and ultimately to effective design application.

Despite the significant improvements achieved in enhancing stability and safety of lithium metal anodes by employing 3D nanostructures, the electrochemical Li deposition remains the primary source of Li in these anodes. Li-free substrates provoke Li depletion issues when combined with intercalation cathode materials, such as LiCoO₂ and LiFePO₄. Moreover, 3D substrates without preloaded Li cannot couple with high-capacity S and O₂ cathodes. To address these concerns, recent studies propose that utilizing preloaded lithium metal anodes created through a thermal infusion of Li into 3D nanostructured host materials^[79], exhibiting greater attraction and efficacy for use in batteries, particularly those featuring non-lithiated cathode materials (S and O₂).

The utilization of a lithiophilic Si coating on a porous scaffold permits uniform coating of molten lithium metal on the surface of the matrix or encapsulation inside the 3D structure [Figure 16A and B], resulting in a considerable specific capacity of up to 2,300 mAh·g^-1 as a stable lithium metal anode. Due to its reduced effective current density and ability to effectively accommodate Li volume variation, this groundbreaking electrode exhibits stable cycling performance over 80 cycles with lower overpotential (< 90 mV) at 3 mA·cm^-2, which stands in stark contrast to bare Li foil^[79].

Figure 16. Design of 3D host structure with preloaded Li. Schematics and digital images of (A) Li infusion into the 3D porous host; (B) SEM image of Li infused in carbon nanofibers^[79], Copyright 2017, Elsevier; (C) fabrication process from C-Wood (left) to ZnO-coated C-Wood (middle), and finally to Li/C-wood composite (right)^[81]. Copyright 2017, National Academy of Science; (D) schematic illustration of the infusion process for impregnating the nanocrevasses of the carbon scaffold with Li. SEM: scanning electron microscopy.

A well-designed channel can guide Li-ion flux to move vertically along the electrode instead of parallelly, preventing uncontrolled Li deposition since the flux in each channel is mutually isolated^[80]. An anode was proposed by Zhang et al. by adopting carbide wood (C-wood) as the substrate with preloaded lithium metal^[81]. Low-pitched wood with channel structures ranging from 15 to 100 μm was employed, and its unique channel structure was well preserved after carbonization. The wetting between carbon and lithium metals is crucial for preparing lithium/carbon composite materials. To achieve this, Zhang et al. utilized a low-cost and simple method to cover a thin layer of ZnO on the channel walls of C-wood^[81]. ZnO particles were formed and uniformly dispersed inside the channels. As expected, molten Li quickly rushed into the ZnO-coated channels, forming a brilliant lithium/C-wood composite material. This quick process was mainly due to the excellent reaction between lithium metal and ZnO and capillary forces generated by the hydrophilic surface. SEM was performed to investigate the morphological changes of the ZnO-coated-C-wood composite material. Figure 16C presents its typical channel structure, with channel sizes ranging from 5 to 60 μm and a groove wall thickness of ~1 μm. Such a formation resulted in a porous conductive framework capable of accommodating lithium metal (with a porosity rate of 73%) and maintaining optimal stability even after 230 cycles. This channel construction scheme holds immense potential in regulating lithium stripping/plating behavior.

Apart from conductive substrates, Liu et al. discovered that porous PI films, an electrically insulating material, can also accommodate molten Li injection^[82]. This injected lithium metal serves as a conductive medium for electron transfer, and the exposed insulating surface following lithium peeling effectively prevents further Li deposition and dendrite formation. Experimental outcomes indicate that preloaded lithium in PI electrodes still exhibits a flat voltage profile, even at a high current density of 5 mA·cm^-2 after 100 cycles^[82].

The large specific surface area, mechanical strength, and lightweightness of carbon fibers render them exceptionally appealing for energy storage applications. Regretfully, their poor affinity for lithium metal poses a serious impediment to their viability as Li hosts. Consequently, initiatives have been instituted to enhance the surface features of carbon fibers. Go et al. uncovered that carbon fibers heated to 500 °C exhibited a wealth of nanocrystals, which furnished substantial capillary force to expedite the injection of molten Li into the carbon scaffold [Figure 16D], thus accelerating the construction of the 3D skeleton anodes^[83].

A superb 3D composite anode

Recently, Lin et al. engineered an ultra-thin (≈50 µm) superb composite lithium anode through a spark reaction between Li and a layered reduced graphene oxide (rGO) film [Figure 17A]^[84]. Figure 17C compares the voltage distribution of a symmetric Li-rGO electrode cell and a bare Li foil cell after more than 100 cycles at 1 mA·cm^-2. Li-rGO exhibits a stable voltage curve with less hysteresis, whereas the Li foil experiences increasingly greater lag in the course of cycling. Further tests were conducted on the first, tenth, fiftieth, and hundredth cycles of the layered Li-rGO cell [Figure 17B]. Compared with the bare lithium electrode, there was no obvious increase in the flat voltage platform during charging and discharging states for layered Li-rGO.

Figure 17. (A) rGO (left), sparked rGO (middle), and Li-rGO composite (right); (B) Voltage profile comparison of the LCO/Li-rGO cells and the LCO/Li foil cells at rates of 1 C; (C) Galvanostatic cycling of a symmetric Li-rGO electrode (blue) and bare Li foil (red); (D) Rate capability of the LCO/Li-rGO and LCO/Li foil cells (E) Nyquist plots of the symmetric cells after ten galvanostatic cycles; (F) Full Li stripping curve of the Li-rGO electrode; (G) Layered Li-rGO electrodes with different thicknesses^[84]. Copyright 2016 Macmillan Publishers Limited.

The highly reduced polarization and stable cycling can be further supported by the electrochemical impedance spectroscopy (EIS) conducted on symmetric cells before cycling and after ten cycles [Figure 17E]. On the corresponding Nyquist plots, the semicircle at the high-frequency range is a good indicator of the interfacial resistance at SEI and the charge transfer resistance at the Li surface. Before cycling, the Li foil exhibited a high interfacial impedance (650 Ω), which decreased to 85 Ω. In contrast, the interfacial resistance of layered Li-rGO continuously dropped from 30 Ω before cycling to 25 Ω after ten cycles. This phenomenon suggests that layered Li-rGO can achieve better electrode stability and more favorable lithium stripping/plating kinetics. Notably, layered Li-rGO not only exhibits excellent electrochemical performance but also retains most of its capacity. An extractable capacity of 3390 mAh g^-1 can be obtained when charged to 1 V [Figure 17F], which is very close to the theoretical capacity of pure lithium. The high specific capacity can be attributed to the lightweight and porocity of the rGO framework induced by the reduction, where the mass of the framework only accounts for 7 wt% of the entire electrode. SEM images of the Li-rGO electrodes with thicknesses of 50, 80, and 200 μm are shown in Figure 17G. The magnified SEM images indicate a consistent layered structure with similar spacing, despite the electrode thickness.

The present study reports the performance of the superb layered Li-rGO anodes in combination with lithium cobalt oxide (LCO) and lithium titanium oxide (LTO). Notably, the utilization of the Li-rGO anode design led to significantly improved rate capability and lower hysteresis compared to conventional Li foil cells. A higher LCO capacity was retained when utilizing Li-rGO anodes, particularly at high rates (110 mAh·g^-1 at 4 C and 70 mAh·g^-1 at 10 C, Figure 17D). In comparison, Li foil cells only offered a capacity of 95 mAh·g^-1 and 5 mAh·g^-1 at 4 C and 10 C, respectively. Interestingly, Li foil cells displayed consistently higher overpotential during the initial stage of charging, resulting in a ‘bump’ shape [Figure 17B], and this effect becomes more pronounced at increased rates. Notably, this is consistent with the behavior observed in symmetric Li foil cycling experiments. Collectively, these results demonstrate the improved power and stability inherent in utilizing layered Li-rGO designs^[84]. The layering mechanism is not only found in electrode design but is also very common in battery separator modification.

Inorganic material/polymer hybrid-coated separators

Hybrid coatings composed of inorganic material and polymers are highly sought after for their unique advantages that arise from the combination of properties intrinsic to each component. It is widely anticipated that hybrid coatings will provide enhanced mechanical stability at high temperatures and improved electrolyte affinity in separators. One promising approach to accomplish this goal involves the coating of ordered poly(methyl methacrylate) (PMMA) and SiO₂ nanoparticles onto both sides of a PE separator^[85]. The improved mechanical stability is attributed to the reinforcing effects of SiO₂, while the augmented wettability arises from the higher affinity of PMMA toward the electrolyte compared with the pure PE separator. As a result, the SiO₂@PMMA-coated PE separator exhibits superior rate capability and safety performance relative to its uncoated counterpart.

A ZrO₂/poly (acrylic acid) (PAA) coating was constructed on a plasma-activated PE separator with a layer-by-layer approach^[86]. The PE substrate was carbodiimide-crosslinked with the ZrO₂/PAA coating. The thickness of the coating can be easily adjusted by altering the number of cycles within the layer-by-layer process. This composite separator integrates the advantages of functional polymers and inorganic nanoparticles and exhibits superior electrochemical and safety performance.

A thin poly(3,4-ethylenedioxythiophene)-co-poly(ethylene glycol) (PEDOT-PEG)/AlF₃ hybrid coating was applied onto a PE separator^[87]. The ionic conductivity of the PEDOT-PEG phase in the coating provides practical benefits for the electrochemical performance of the separator. Incorporation of AlF₃ into the coating can aid in enhancing the thermal stability of the separator and stabilizing the electrolyte. Through a synergistic effect arising from the combination of these two components, the coated PE separator showed reduced thermal shrinkage and improved discharge capacity and cycling performance. To further optimize the separator performance, a SiO₂/PVDF hybrid nanofiber coating was applied to a PP non-woven separator^[88]. Electrospinning a mixture containing PI and the corresponding filler followed by imidization was employed to develop SiO₂/PI and Al₂O₃/PI separators^[89]. The composite separators exhibited superior thermal stability and mechanical strength.

Separators based on other polymers

PVDF can adsorb a substantial amount of electrolyte, which significantly contributes to battery performance^[90]. The hydrophilic polymer HFP can be copolymerized with PVDF, further amplifying its affinity to electrolytes^[91]. However, PVDF separators exhibit intrinsic weaknesses in tensile strength and thermal stability. To address these drawbacks, poly(m-phenylene isophthalamide) (PMIA), possessing both remarkable mechanical properties and thermal stability, was implemented to create a tri-layer PVDF/PMIA/PVDF separator^[92]. The battery using the PVDF/PMIA/PVDF separator showcased better discharge capacity and cycling stability compared with the cell utilizing the Celgard 2320 separator.

Polyvinyl alcohol (PVA), an immensely hydrophilic polymer, demonstrates strong adsorption characteristics towards electrolytes. A microporous PVA separator was meticulously produced by leveraging non-solvent-induced phase separation^[93]. The PVA separator exhibits improved thermal stability relative to the PP separator. These features highlight the immense potential of the PVA separator in high-performance lithium batteries.

The performance of the separators with regard to the electrolyte was studied by examining their uptake and retention behaviors. The data presented in Figure 18C compare the electrolyte uptakes of the two separators. It was observed that the PVA separator displayed an electrolyte uptake of 170% after 15 min-almost double that of the PP separator (90%). Upon further inspection, both separators reached an equilibrium state after 2 h. At this time, the PVA separator demonstrated an electrolyte uptake of ~230%, outperforming its counterpart by a substantial 125%.

Figure 18. Surface (A) and cross-section (B) SEM images of the PVA separator after the cycling test. Electrolyte uptake (C) and retention (D) of PP separator and PVA separator. (E) Photograph showing liquid electrolyte wettability of different separators and the corresponding contact angle test results^[93]. Copyright 2015, Elsevier. SEM: Scanning electron microscopy; PVA: polyvinyl alcohol.

The current investigation provides a comprehensive analysis of the cycling performance of cells utilizing PVA and PP separators. Both cells demonstrated highly favorable cycling performances, with CEs remaining close to 100% after many cycles. Additionally, the PVA cell exhibited a higher discharge capacity than its PP counterpart. Furthermore, the capacity retention ratio of the PVA cell was determined to be ~97%, significantly greater than the PP cell (94%) after 50 cycles [Figure 18D]. To further characterize the stability of the PVA separator, SEM was conducted on the microstructure of the separator after cycling testing [Figure 18A and B]. Notably, minimal changes in separator morphology were observed before and after testing, which indicates the remarkable stability inherent to the use of the PVA separator design.

Figure 18E illustrates the evaluation of the wettability of the PP separator and the PVA separator via the liquid electrolyte contact angle. Results show the electrolyte droplets easily spread over a wide area of the PVA separator, demonstrating its quick wetness with the electrolyte. The PVA separator exhibits a contact angle of only 8.5º. Conversely, the PP separator is less wetted, with only an electrolyte droplet observed on its surface and exhibiting a contact angle of 42.4º. This significant discrepancy in electrolyte wettability verifies that the PVA porous separator is more electrolyte-philic than the PP separator, promoting the transport of Li⁺.

Thermal shrinkage is another significant consideration for battery safety. The images of the PVA separator and the PP separator before and after exposure to 160 °C for 1 h are highly different. After thermal treatment, the PVA separator demonstrated superior thermal stability, showing virtually no dimensional change, while the PP separator underwent substantial thermal shrinkage of ~85%.

In summary, addressing the imperative necessity of improving safety performance in metal lithium batteries, separator modification has emerged as one salient research area gaining broad attention across the industry. Multiple strategies for separator modification will undoubtedly contribute to further enhancing safety and stability in metal lithium batteries.