Recent Advances in Anode-free Solid-state Batteries: A Review

Article information

J. Electrochem. Sci. Technol. 2024;.jecst.2024.00759
Publication date (electronic) : 2024 August 12
doi : https://doi.org/10.33961/jecst.2024.00759
1School of Chemical Engineering, Yeungnam University, Gyeongsan, 38541, Republic of Korea
2Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States
3School of Chemical, Biological and Battery Engineering, Gachon University, Seongnam-si, Gyeonggi-do 13120, the Republic of Korea
*CORRESPONDENCE T: +82-31-750-5599 C: +82-10-3303-6658 E: dslee9117@gachon.ac.kr (D. Lee) junghchoi@gachon.ac.kr (J. Choi)
Received 2024 July 18; Accepted 2024 August 9.

Abstract

Solid-state batteries (SSBs) have been intensively studied due to their high energy density and improved safety for nextgeneration energy storage systems. Anode-free SSBs (AFSSBs) maximize the energy density with zero-excess lithium and enable simplified manufacturing processes. However, the absence of lithium and interfacial irreversible reactions lead to fast capacity fading of AFSSBs. Therefore, examining the difficulties and challenges encountered by AFSSBs can pave the way for enhancing electrochemical stability with prolonged cycle life. In this review, the key issues affecting the electrochemical stability of AFSSBs are described, focusing on the stability of inorganic solid electrolytes (ISEs), kinetic limitations, and interfacial diffusion. Furthermore, we introduce recent strategies to improve the interfacial stability and electrochemical properties of AFSSBs. Finally, prospects and potential pathways for AFSSBs are stated, aiming to encourage the exploration of the emerging field.

INTRODUCTION

Since the commercialization of lithium-ion batteries (LIBs) in 1991, they have revolutionized daily life from consumer electronics to electric vehicles (EVs) [13]. Over the past three decades, the energy density of LIBs has been continuously improved with the development of new electrode materials and the discovery of battery chemistry [4,5]. However, the energy density of LIBs is close to the theoretical limit of 280 Wh kg–1 for the current system employing high-nickel layered oxide cathodes (LiNixCoyMn1–x–yO2) and graphite anodes (Fig. 1) [68]. In this regard, Li metal batteries (LMBs) are attracting increasing attention employing Li metal as an anode, which is regarded as a ‘holy grail’ anode due to the extremely high theoretical capacity of 3860 mAh g–1 and low electrochemical potential of –3.04 V (vs. standard hydrogen electrode, SHE) [9,10]. However, the localized current density associated with concentration polarizations in LMBs with liquid electrolytes leads to poor cycling stability and uncontrollable Li dendrite growth susceptible to short circuits, implying potential safety hazards [11,12]. With the emerging nonflammable inorganic solid electrolytes (ISEs) with excellent Li+ conductivity, solid-state Li metal batteries (SSLMBs) have been intensively studied for high energy density and high safety batteries [1315].

Fig. 1.

Schematic illustration of LIBs, SSLMBs, and AFSSBs and their specific energy densities (Wh kg–1 and Wh L–1).

SSLMBs with ISEs hold promise with advantages such as high modulus of ISEs, high thermal stability, and extended cycle performances than LMBs with liquid electrolytes [16,17]. A variety of ISEs with high Li+ ion conductivity have been developed with promising materials such as oxide-based ISEs (Li7La3Zr2O12 (LLZO), Li3xLa2/3–xTiO3 (LLTO), Li1+xAlxGe2–x(PO4)3 (LAGP)), sulfide-based ISEs (Li10GeP2S12 (LGPS), Li2S–P2S5, Li6PS5Cl (LPSCl), etc.), and halide-based ISEs (LixMCl6, M=Y, Zr, In, Sc, Yb, etc.) [1826]. However, significant challenges arise from the electrochemical and chemomechanical properties of SSLMBs, which require an in-depth understanding of fundamental interfacial chemistry [27,28]. The charge transfer at the interface directly influences the crystal growth or dissolution of Li metal. In LMBs with liquid electrolytes, the heterogeneous kinetics is critically influenced by nano- and microstructure at the electrode interface with growing and dissolving Li metal [29]. Furthermore, Liquid electrolytes easily infiltrate electrodes, allowing Li-ions can transport throughout the cell. In contrast, charge transfer is restricted to physically adjoining particles and is constrained at their interfaces in SSLMBs. The mechanism of Li metal deposition and dissolution are not as thoroughly studied in SSLMBs, despite substantial insights gained from numerous investigations involving various ISEs with Li metal anodes. Furthermore, most of research on SSLMBs employ thick Li metal anodes which are not suitable for increasing the energy density of batteries. It is difficult to modify Li metal anodes with a protective layer due to their high reactivity. Therefore, comprehensive understanding is required on interfacial kinetics to develop high energy density SSLMBs.

Anode-free solid-state batteries (AFSSBs) are similar to SSLMBs in that the negative (anode) to positive (cathode) capacity ratio (N/P ratio) is exactly zero [30]. In other words, Li metal is not employed in AFSSBs, as shown in Fig. 1. Regarding this configuration, AFSSBs have unique advantages over conventional LIBs by maximizing gravimetric and volumetric energy densities and simplifying manufacturing processes [31]. These advantages maximize the industrial value of AFSSBs. By eliminating the cost-intensive Li foil in AFSSBs, concerns about excessive Li production are alleviated. By 2030, the cost of sulfide-based SSLMBs is expected to be approximately $116 $ (kWh)–1 at the pack level, which is about 20% cheaper than LIBs using NCM811, estimated at 145 $ (kWh)–1 [32]. Considering the cost of Li foil, AFSSBs are expected to achieve an additional 10% reduction in production cost [33]. However, AFSSBs show a more severe deterioration in electrochemical performances than SSLMBs because of the zero Li reservoir. Especially, the interfacial degradation is one of the most critical factors in AFSSBs because Li metal is deposited and stripped during cycling [34]. First of all, the chemical and electrochemical instability at the anode interface, associated with the inherent chemical potential incompatibility between Li metal and ISEs, causes spontaneous side reactions with severe interfacial resistance evolution [35,36]. In addition, the morphology of the Li metal during electrochemical plating and stripping processes governs the stability of AFSSBs, which is related to the various factors such as microstructure, critical current density (CCD), electronic conductivity, Li+ ion conductivity, Li+ ion diffusion rate, and kinetic limitations. Because there are a lot of critical factors affecting the stability of AFSSB, a comprehensive understanding of the fundamental chemistry is required [3740].

Since the research interest in high energy density SSBs is exponentially increasing, this review seeks to focus on the interfacial challenges and recent advances in AFSSBs with ISEs. First, we discuss fundamental understanding regarding the chemical and electrochemical incompatibility of the Li metal and ISEs, chemomechanical degradation with Li dendrite growth, and kinetic limitations for AFSSBs. Then, we present recent efforts to create novel interlayers for the improved cycle and rate performances for high energy density AFSSBs. This review provides valuable insights from fundamental understanding to technological innovation. We also provide an outlook on the emerging and promising AFSSBs, aiming to provide inspiration for the development of high-energy-density batteries.

CHALLENGES FOR DEVELOPING AFSSBs

Achieving high-performance AFSSBs critically hinges on the reversible Li plating and stripping processes, indicated by the Coulombic efficiency (CE). To retain a capacity retention of 80% after 1000 cycles, an average CE during cycling should exceed 99.98% [41,42]. With an average CE of 99.9%, the number of cycles until the capacity retention reaches 80% is just 224 cycles. The high demand for the CEs exceeding 99.98% represents the difficulty in developing practical AFSSBs with stable cycling performances over hundreds of cycles because of the zero excess Li reservoir. The irreversible Li loss associated with Li plating and stripping in AFSSBs largely depends on the interfacial reactions on a heterogeneous substrate at the anode side. To enhance the performance of the AFSSBs, it is necessary to gain insights into the fundamental interfacial chemistry of the Li metal and ISEs, Li plating and stripping behaviors, and the interaction between them in AFSSBs.

Stability of ISEs in contact with Li metal anodes

The stability of the anode interface between Li metal and ISEs significantly influences the CEs and the overpotentials in AFSSBs. Due to the lowest electrochemical potential of Li, most of the ISEs are reduced at the anode interface, forming a solid electrolyte interphase (SEI) layer including lithium compounds such as LiF, LiCl, Li2O, and Li2S (Fig. 2a,(b) [35,43]. The density functional theory (DFT) computational calculation can predict electrochemical oxidation and reduction of ISEs from their electrochemical stability windows, as shown in Fig. 2a. Especially, sulfide-based ISEs have narrow electrochemical stability windows, resulting in their thermodynamic incompatibility against Li metal [44]. From a thermodynamic point of view, a wide electrochemical stability window of ISEs is one of the key parameters for achieving stable interfaces in AFSSBs.

Fig. 2.

(a) Electrochemical stability windows of various ISEs classified by fluorides, chlorides, bromides, oxides, sulfides, and Li compounds. (b) Electrochemical stability windows of various SEI components reduced at the anode interface [43]. (c) Schematic diagram of the change of chemical potentials at the anode interface [35].

In addition, the conductive properties of SEI components govern the interphase evolution with the further reduction of ISEs. Theoretically, high Li+ ion conducting and electronically insulating SEI components can protect the anode interfaces from the spontaneous reduction of the ISEs, enabling stable cycling of batteries [45]. Electronically insulating interlayers can reduce the electrochemical potential for electron transfer across the SEI, thus lowering the high electrochemical potential of Li atoms within the electrochemical stability window of ISEs, preventing further reduction (Fig. 2c) [35]. Conversely, an electronically conductive interlayer enables electron transfer through the interlayer to the ISEs, promoting the continuous reduction of ISEs. In this regard, the stable cycling performance of SSLMBs and AFSSBs critically depends on the stability of the SEI.

Among the promising ISEs, the perovskite-type LLTO, Li–NASICON-type LAGP, and Li superionic conductor LGPS contain reducible elements such as Ti, Al, or Ge within their structures [1921]. Those reducible elements have an alloying reaction with Li metal and become electronic conductive Li metal alloys, which cannot protect the anode interfaces leading to the continuous reduction of ISEs [46,47]. ab initio MD simulations and energy calculations regarding the LGPS ISE demonstrated the formation of the SEI interlayer with lithium germanium alloy phase (Li15Ge4), which cannot prevent the continuous decomposition of the ISE due to the mixed electronic and ionic conductivities of the SEI interlayer [48]. In a similar vein, the NASICON-type LATP has been observed to form mixed conducting interphase (MCl) containing the Ti and Al alloys [49].

In addition to the ISEs mentioned above, sulfide argyrodite LPSCl, garnet-type LLZO, and lithium phosphorus oxynitride (LiPON) exhibit distinct reduction behaviors at the anode interfaces [18,23,50]. The SEI interlayers associated with their decomposition have low electronic conductivity, which can mitigate further interfacial degradation. However, their limited ionic conductivities lead to the interfacial resistance evolution, reducing interfacial Li+ kinetics [51]. For example, the bridging sulfur species such as P–S–P and Li–S–P in Li3PS4 (LPS) can be reduced to poor ionic compounds such as Li2S and Li3P, resulting in severe interfacial resistance evolution [52]. LiPON also has a similar interfacial issue with Li metal, indicating the formation of Li3P, Li2O, and Li3N as a SEI interlayer [53]. In addition, LLZO was once considered highly stable against Li metal anodes due to its wide electrochemical stability window. However, it is revealed that the interfacial stability of LLZO with Li metal anodes is critically dependent on the doping elements, showing the reduction of Zr4+ for all doped samples [54]. In addition, the oxide-based ISEs have higher shear modulus, which leads to mechanical failures and fractures due to strain during repetitive Li plating and stripping cycles [55]. The low shear modulus of polymeric solid electrolytes makes them unable to withstand the mechanical stress associated with Li plating, ultimately leading to cell failure. The sulfide-based ISEs have a relatively lower shear modulus than that of oxide-based ISEs [56]. In these regards, the sulfide ISEs are considered to be more effective in reducing mechanical failures and inhibiting Li dendrite formation compared to the others.

The interfacial reactions in AFSSBs can occur only for the physically contacted ISEs and current collector, which is less compared to those in LMBs with liquid electrolytes [57]. However, significant interfacial degradations exist between the deposited Li and ISEs because of the inhomogeneous current densities on the interlayer or current collectors. Furthermore, once the deposited Li reacts with the ISEs, AFSSBs present fast capacity fading due to the zero excess Li reservoir to compensate for the Li loss [58]. Consequently, AFSSBs demand a more rational design for the interfacial composition and structure between the current collector and ISE to achieve stable cycling performances.

Kinetic limitations and interfacial diffusion

Ideally, during the Li plating process, Li-ions should uniformly nucleate on the anode surface despite significant impediments, a process that reverses during stripping. However, in practice, the surface of Li metal anodes or current collectors has nonuniform microstructures with defects, which interface evolution can intensify after repeated Li plating and stripping cycles [59]. The inhomogeneous interface evolution leads to a current constriction phenomenon [60]. The line current is distorted near the interface in contact with the ISE layer and it is contradicted to pass through the ruptured contact points [34]. This issue primarily stems from nonuniform interfacial contacts and varied distribution of active sites. Existing defects and surface impurities reduce the actual contact area between the anode and ISE layer at the atomic level. The current collector is one of the most distinct components of AFSSBs compared to conventional batteries [61].

Copper (Cu) has been widely employed as an anode current collector. However, its structural incompatibility with Li metal anodes results in a high overpotential in the Li plating process [62]. The initially plated Li becomes the point of the high current density with the concentrated line current, resulting in vertical Li growth and contact losses in other areas. Consequently, Li plating becomes inhomogeneous, leading to the continuous growth of Li dendrites and whiskers deteriorating battery performance [63]. In addition, the Cu current collector reacts with sulfide-based ISEs such as LPSCl forming the Cu2S interphase hindering Li-ion transport, which further reduces the actual contact area and induces Li dendrite growth [64]. Similarly, isolated Li plating was observed on stainless steel (SS) current collectors [65]. The microstructural evolution and the resulting increase in overpotential are more affected by the Li stripping process during cycling. When the current density during a Li stripping process exceeds the Li replenishing rate at the interface, voids develop at the interface between the Li metal anode and the ISE layer [66]. Kasemchainan et al. revealed that the critical current density (CCD) on stripping is a major factor limiting the high rate capability of SSLMBs [38]. The stripping polarization increases during cycling because not all voids created during stripping are replenished in subsequent plating cycles (Fig. 3a). Remaining voids become enclosed and exposed again to the interface contacted with the ISE layer on the next stripping cycle. Consequently, more voids are generated and growing during the Li stripping process. In this regard, when the Li is plated, the localized current density becomes elevated due to the diminished contact area between the anode and the ISE layer, leading to Li dendrite growth.

Fig. 3.

(a) Schematic illustration of the interfacial microstructure evolution between Li metal anode and ISE layer. (b–e) Schematic illustration of the Li–ion and vacancy diffusion at the interface under anodic polarization [71].

Pressure is one of the critical factors governing the interface morphologies during Li plating and stripping [67]. The physical and mechanical properties of SSBs feature the Li metal deformation and diffusion characteristics. The ductile nature of Li metal allows it to deform under external pressure. Notably, significant creep occurs across a broad range of applied pressure at high operating temperatures. Understanding the Li metal deformation and diffusion characteristics is crucial for the successful integration of Li metal anodes in SSLMBs and AFSSBs. Masias et al. reported a comprehensive understanding of Li metal deformation with respect to Young’s modulus, shear modulus, time-dependent deformation (creep), and stress-dependent deformation [68]. Without hydrostatic pressure, the deformed Li gradually migrates to the grain boundaries of the ISE layer, resulting in short circuits. Applying external pressure on the SSBs helps prevent the undesirable migration of Li metal. In addition, the Li metal deformation leads to microstructural changes that mitigate interfacial issues and improve CCD in the Li plating and stripping processes. Due to the pressure-sensitive nature of the Li metal deformation, increasing the external pressure enhances the intimate interfacial contact. Due to the roughness at the interface, the effective contact area is generally much smaller than the actual contact area. However, when hydrostatic pressure is applied, discrete contact points undergo plastic deformation with the increased effective contact area, preventing short circuits from the subsequent creep. Since the Li metal is soft compared to the most ISEs, its deformation significantly governs the reduction of the deformation-dependent impedance [69,70].

The mechanism of the Li metal diffusion at the interface is illustrated for the comprehensive understanding of the dynamic behavior of the Li metal dissolution (Fig. 3be) [71]. The fundamental anodic polarization at the interface can be represented with Kröger–Vink notation as (Eq. 1)

LiLiLi+x(ISE)VLi+'(ISE)+e'(Li)+VLix(Li)

When a Li-ion passes the Li/ISE interface, an electron e'(Li) and a vacant site VLix (Li) are generated at the Li metal surface, while the Li ion occupies a vacant site VLi+' (ISE) [71]. Every stripped Li-ion from the Li metal anode leaves a vacant site, which can be either annihilated during repeated Li plating or diffuse away from the interface into the bulk of the Li metal anode [72]. Since the charge transfer is not a limiting factor and the bulk Li-ion transport in the ISEs is significantly high, the vacancy diffusion will primarily govern the interface dynamics disregarding the vacancy annihilation for simplicity [73]. With the high diffusion coefficient of vacancies, the local current density will not reach the diffusion limit of Li metal dissolution, leading to a stationary vacancy concentration at the interface with stable interfacial morphologies (Fig. 3b). If the externally applied current density surpasses the limit for vacancy diffusion, the interfacial morphologies will be ruptured due to the supersaturation and accumulation of vacancies, resulting in the severe contact losses (Fig. 3c,d) [74]. Since adatom diffusion along pore surfaces occurs faster than bulk vacancy diffusion, the voids will develop three-dimensionally with unstable interfacial contact points. Due to the surface tension of Li metal, voids will continue to accumulate, driven by electrochemical Ostwald ripening [75]. On the other hand, if external pressure is applied enough to replenish voids, the plastic deformation and creep of the Li metal anode can prevent further pore generation (Fig. 3e) [68,76].

STRATEGIES FOR STABLE CYCLING PERFORMANCE OF AFSSBs

The chemical and electrochemical stability at the anode interface between the ISE and current collector (or Li metal) plays a crucial role in enhancing the efficiency of the AFSSBs. Unfortunately, the aforementioned inherent reactivity of Li metal leads to the formation of interphases during cycling, resulting in poor cycle stability. Therefore, it is necessary to build stable interphases to secure stable cycle performances in AFSSBs, which possess 1) high chemical and electrochemical stability, 2) robust mechanical strength against Li deposition, 3) high ionic conductivity, 4) thermal stability over a wide temperature range, and 5) uniform current distribution (Fig. 4). Furthermore, a high pressure exceeding 50 MPa is applied in accordance with Li metal growth in the Li plating process. Therefore, the interfacial stability critically influences the Li metal growth. Employing lithiophilic substrates or interlayers with good Li wetting characteristics can lead to uniform current distribution with minimized localized current density, resulting in a uniform and dense Li plating [77,78]. Additionally, novel interlayers enabling Li creep can control the Li plating underneath the interlayer [79]. This section aims to introduce strategies for controlling interfacial reactions through materials and compositional modifications. Specifically, the discussion will focus on recent research on mitigating Li dendrite growth and controlling Li plating behaviors by employing novel interlayers.

Fig. 4.

The major challenges on an anode-free ASSBs regarding current collector and interlayer.

Lithiophilic interface engineering on a current collector

As mentioned in Section 2.1, the interphase is generated at the anode interface due to the high reactivity of Li metal. The repetitive inhomogeneous Li plating and stripping cause the exposure of bare Li metal interfaces, resulting in continuous interphase evolution from side reactions that negatively impact the CEs and resulting in poor cycling performances of AFSSBs. Furthermore, the Li plating and stripping behaviors are governed by various factors such as external pressure, current density, chemical and electrochemical properties of the interphase, and Li diffusion kinetics. Therefore, the formation of a stable interphase is essential to control the Li metal deposition with higher CEs. The Li plating and stripping behaviors can be greatly affected by the lithiophilicity of the interlayer or interphase, facilitating homogeneous Li nucleation. The lithiophilicity of substrates and their electrochemical properties regarding Li metal nucleation and deposition have been reported [80]. It has been revealed that lithiophilicity is critically governed by Gibbs formation energy and the formation of new chemical bonds. Consequently, elements such as Au, Ag, Al, Mg, Pt, Sn, and Zn have been identified as key elements that can promote the nucleation of Li [81].

Silver is a well-known lithiophilic material to regulate uniform Li nucleation and deposition. Pyo et al. reported that silver nanoparticles incorporated p-doped conjugated polymer (Ag–PCP) induce the rapid formation of the LiF-rich SEI layer, resulting in the improved capacity retention of 72% after 200 cycles at 1C in anode-free batteries with liquid electrolytes (Fig. 5a) [82]. Sandoval et al. reported that lithiophilic alloying layers, such as Ag and Au, play a crucial role in improving the cycling performance of AFSSBs with uniform lithium plating and stripping (Fig. 5b) [83]. These findings provide valuable insights into the mechanisms of Li plating and stripping behavior in anode-free batteries and suggest that interfacial engineering with Li alloy interlayers is essential for the development of high-performance AFSSBs. Although Ag and Au are highly useful in anode-free batteries, they have limitations from a cost perspective. Lu et al. proposed the Zn and carbon black dual-layer coated current collector [84]. The dual-layered current collector significantly improved the cycling performance of AFSSBs. They suggest that optimizing the thickness of the zinc layer and combining it with conductive carbon black is a promising strategy for AFSSBs (Fig. 5c). The dual-layered interlayer prepared with optimized zinc and carbon black could play a crucial role in the formation of the lithiophilic interphase with cost effective materials, resulting in outstanding cycling performances in AFSSBs. Müller et al. reported on the Li deposition behavior and Li plating/stripping efficiency of bare Cu, Au, Pt, and amorphous carbon seed layers [85]. They demonstrated the critical role of seed layers in enhancing the efficiency and stability of AFSSBs.

Fig. 5.

(a) Schematic illustration of silver nano-particle induced LiF-rich SEI layer formation and Li plating behavior mechanism[82]. (b) Lithiophilic metal alloying reaction based Li plating and stripping [83]. (c) Schematic illustration of working mechanism of Zn – carbon black dual layer coated current collector [84]. (d) Cross-sectional SEM evaluation according to lithiophilic materials (bare Cu, Au, Pt, C) of the current collector–solid electrolyte interface after 0.2 mAh cm−2 (1 μm) plated lithium [85]. (e) Schematic illustration of the fabrication process of the Cu–CNT current collector and proposed mechanism of the stable Li plating [86]. (f) Schematic diagrams of 3D interconnected carbon paper current collectors for AFSSLBs [87].

Amorphous carbon presents a cost-effective alternative to lithiophilic noble metals such as Au and Pt. This study opens new avenues for simplified manufacturing and improved electrochemical performances of AFSSBs (Fig. 5d). Drawing from the key insights provided by Shan et al. on the fabrication of Cu–carbon nanotube (Cu–CNT) current collectors, it is evident that carbon-based materials significantly enhance the electrochemical performances of AFSSBs [86]. The researchers demonstrated that the incorporation of multi-walled CNTs into Cu matrix composites via deformation-driven metallurgy (DDM) greatly improves the lithiophilicity of current collectors. This method results in fine-grained Cu–CNTs with numerous grain boundaries that serve as favorable nucleation sites for lithium ions. The homogeneous dispersion of broken CNTs further contributes to uniform lithium deposition, reducing the risk of dendrite formation and enhancing cycling stability (Fig. 5e). Additionally, Huang et al. showed that a 3D interconnected carbon paper (CP) structure creates a highly efficient ionic-electronic composite layer [87]. The interconnected CP enabled uniform Li nucleation and deposition by providing numerous lithiophilic sites and scalable spaces, thus achieving long cycle life and high areal capacity (Fig. 5f). These studies demonstrate that incorporating an interphase layer is an effective strategy to stabilize the interface between Li metal anode and ISE.

Interlayer between a current collector and ISEs separator

One of the most monumental studies is reported by Lee et al., featuring the silver–carbon (Ag–C) interlayer in AFSSBs [79]. The Ag–C composite was introduced at the interface between the anode and the ISE, facilitating uniform Li nucleation and deposition by reducing nucleation energy (Fig. 6a). The AFSSBs with the Ag–C composite anodes exhibited lower overpotentials with ineligible Li reduction and less interphase formation, enabling stable Li metal deposition underneath the composite interlayer. They demonstrated the prototype AFSSBs with the pouch cells employing the Ag–C composite anodes with a high energy density of 900 Wh L–1. The AFSSBs with the Ag–C composite anodes exhibited outstanding cycle life over 1,000 cycles with excellent Coulombic efficiency (>99.8%) compared to conventional stainless steel (SS) current collector (Fig. 6b). The uniform Li plating and stripping behaviors could be achieved by the Ag–C composite interlayer. This research underscores the potential of Ag–C composite anodes in enhancing the cycling performance of ASSBs, addressing key issues such as Li dendrite formation and interfacial resistance. Subsequent studies on the Ag–C layer have explored its mechanisms and limitations extensively [92].

Fig. 6.

(a) Schematic of an ASSB composed of a NMC cathode, SSE and a Ag–C nanocomposite anode layer. (b) Cycling performance and Coulombic efficiency of the Ag–C|SSE|NMC prototype pouch cell [79]. (c) Schematic illustration of Li plating by MFx-based conversion reaction [88]. (d) Schematic illustration (cross-section) of Li3.75Si-CNT interlayer in terms of LiBCC deposition mechanism [89]. (e) Illustration of the cell incorporated hydride-based interlayer and voltage-time plot of the cell [90]. (f) Schematic illustration of the Li plating behavior of TiN NT-incorporated AFSSB during the charging/discharging process [91].

Recently, Lee et al. reported on the use of conversion reaction to form an in-situ composite of Ag nanoparticles and LiF, which offers excellent mechanical strength and surface Li+-ion diffusivity [88]. The study highlighted the potential use of conversion-type metal fluorides to achieve high energy density and stable cycling performances in AFSSBs. The AgF-based AFSSBs demonstrated stable cycling performances at room temperature, achieving high Coulombic efficiency (>99.5%) over extended cycles. Furthermore, the AgF-based anode achieved an exceptionally high areal capacity of 9.7 mAh cm−2, maintaining 85.4% of the original capacity after 50 cycles. However, from a commercial perspective, the use of Ag is still challenging in terms of cost effectiveness. Consequently, numerous studies have explored replacing Ag with more affordable metals. Jun et al. investigated the potential use of Mg nanoparticles as a substitute for Ag [93]. The use of Mg nanoparticles resulted in stable Li nucleation and deposition behaviors. They proposed that the unique Li deposition between Mg particles is driven by the Li concentration gradient (LCG) from the surface to the core of the Mg particle, creating a driving force that attracts Li toward the low-concentration side, leading to Li accumulation and subsequent deposition on the Mg surface. The study underscores the importance of understanding Li-alloying kinetics and diffusion properties in designing advanced anode materials. Sung et al. reported a Li silicide (Li3.75Si)–CNT composite as an alternative to the Ag–C composite (Fig. 6d) [89]. The Li3.75Si–CNT interlayer features a nano-porous structure with pores less than 100 nm, which helps confine Li metal deposition to nanoscale sizes where it exhibits softer mechanical properties, reducing the risk of penetration into the SE. Additionally, the composite acts as a mixed-ionic and electronic conductor, providing long-range transport pathways for both ions and electrons. Due to the commercial viability of silicide and CNT, which have been extensively studied, this approach has high applicability. However, a notable limitation is that it operates under a higher-pressure environment of 65 MPa compared to the low external pressure AFSSBs (5 MPa). Recently, Kim et al. reported the implementation of a mixed ionic-electronic conductor (MIEC) interlayer in oxide-based solid electrolyte environments to provide space for lithium deposition [91]. This approach enables the realization of zero-strain anode-free ASSBs (Fig. 6f). The researchers propose using titanium nitride (TiN) nanotubes (NTs) and a silver–carbon (Ag–C) interlayer to address the anisotropic stress caused by recurring Li deposition layers during cycling. The TiN NTs, which are mixed ionic-electronic conductors, facilitate the entry of reduced Li into their structure through interfacial diffusion creep. This results in near-strain-free operation with significantly enhanced volume suppression compared to conventional copper anodes (Fig. 6f). The performances and research strategies for recent development of anode-free batteries are summarized in Table 1.

Summary of strategies, performances, and operating conditions in recent anode-free batteries and AFSSBs

SUMMARY AND PERSPECTIVES

The desire for higher energy density has spurred the interest in the use of LMBs. However, there are critical challenges of LMBs with liquid electrolytes associated with the uncontrollable Li dendrite growth and regarding safety hazards. Although SSLMBs have great attention employing nonflammable ISEs, they require high external pressure for the intimate solid-to-solid contacts, which is not suitable for soft Li metal. Furthermore, the high reactivity and potential explosiveness of Li metal anodes make them challenging to modify and (or) coat the protective layer. In these regards, research interest in AFSSBs has greatly increased. AFSSBs represent the ultimate batteries capable of maximizing energy density as they can offer the highest energy density for any given cathode system. In addition, the elimination of Li metal in AFSSBs simplifies the manufacturing processes such as modifying current collectors, designing anode interlayers, and assembling batteries.

Regarding AFSSB design strategies, there has been a focus on current collectors in terms of lithiophilicity. The concept of lithiophilicity, explored initially in LMBs with liquid electrolytes, has led to numerous significant studies on current collectors and substrates. On the other hand, the interlayer concept differs slightly. To minimize interfacial reactions between the electrolyte and the generated Li metal and to ensure electrochemical stability, a stable interlayer between Li metal and the ISEs separator should be introduced. Starting with the Ag–C composite interlayer, several studies have been published. Despite these advancements and achievements, many challenges remain to be addressed for commercialization. In summary, the following points should be considered for future research on AFSSBs:

1. The structure, composition, and coating layer of the current collectors significantly influence Li deposition behavior. Although many lithiophilic materials, such as Ag, Au, and Pt, are highly reported, they are expensive metals and thus these strategies are difficult to commercialize. Therefore, it is necessary to develop cost-effective lithiophilic current collectors.

2. An interlayer between the current collector and ISEs separator should be employed. The interlayer should enable stable Li plating underneath the interlayer through Li creep behavior. While various MIEC and lithiophilic metals have been studied, finding commercially affordable materials is still challenging. Furthermore, in-depth understandings are required regarding the mechanism and role of the interlayer due to the complex interplay of variables such as the interlayer’s pore structure, surface characteristics, and Li reaction properties of metals or MIECs.

3. The huge volume changes of the anode can cause crack generation and propagation, resulting in short circuits under the high external pressure environment of AFSSBs. Therefore, to realize practical AFSSBs, research on the anodes must be accompanied by the analysis of the volume changes in both cathodes and anodes, optimized external pressure and the novel electrode or cell design to buffer the volume changes.

4. A better understanding of Li plating and stripping behavior through theoretical simulation and in-situ advanced analysis is required. Especially for AFSSBs, it is necessary to microscopically analyze the Li plating and stripping behaviors in real-time. Developing advanced evaluation methods and continuous innovation could lead to significant breakthroughs.

Acknowledgements

This research was supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIT) (No. GTL24011-000) and the 2023 Yeungnam university research grant.

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Article information Continued

Fig. 1.

Schematic illustration of LIBs, SSLMBs, and AFSSBs and their specific energy densities (Wh kg–1 and Wh L–1).

Fig. 2.

(a) Electrochemical stability windows of various ISEs classified by fluorides, chlorides, bromides, oxides, sulfides, and Li compounds. (b) Electrochemical stability windows of various SEI components reduced at the anode interface [43]. (c) Schematic diagram of the change of chemical potentials at the anode interface [35].

Fig. 3.

(a) Schematic illustration of the interfacial microstructure evolution between Li metal anode and ISE layer. (b–e) Schematic illustration of the Li–ion and vacancy diffusion at the interface under anodic polarization [71].

Fig. 4.

The major challenges on an anode-free ASSBs regarding current collector and interlayer.

Fig. 5.

(a) Schematic illustration of silver nano-particle induced LiF-rich SEI layer formation and Li plating behavior mechanism[82]. (b) Lithiophilic metal alloying reaction based Li plating and stripping [83]. (c) Schematic illustration of working mechanism of Zn – carbon black dual layer coated current collector [84]. (d) Cross-sectional SEM evaluation according to lithiophilic materials (bare Cu, Au, Pt, C) of the current collector–solid electrolyte interface after 0.2 mAh cm−2 (1 μm) plated lithium [85]. (e) Schematic illustration of the fabrication process of the Cu–CNT current collector and proposed mechanism of the stable Li plating [86]. (f) Schematic diagrams of 3D interconnected carbon paper current collectors for AFSSLBs [87].

Fig. 6.

(a) Schematic of an ASSB composed of a NMC cathode, SSE and a Ag–C nanocomposite anode layer. (b) Cycling performance and Coulombic efficiency of the Ag–C|SSE|NMC prototype pouch cell [79]. (c) Schematic illustration of Li plating by MFx-based conversion reaction [88]. (d) Schematic illustration (cross-section) of Li3.75Si-CNT interlayer in terms of LiBCC deposition mechanism [89]. (e) Illustration of the cell incorporated hydride-based interlayer and voltage-time plot of the cell [90]. (f) Schematic illustration of the Li plating behavior of TiN NT-incorporated AFSSB during the charging/discharging process [91].

Table 1.

Summary of strategies, performances, and operating conditions in recent anode-free batteries and AFSSBs

Composition and construction
Electrochemical performance
Ref.
Electrolyte Anode Cathode Interlayer Current collector Cycle number Capacity retention Coulombic efficiency
LPSCl anode less or anode free NCM Ag–C SUS 1000 90% (1000 cycle, 0.5C/0.5C, full cell) 99.8% (1000 cycle average, 0.5C/0.5C, full cell) [79]
Liquid electrolyte M–Li (M=Ag, Au) LFP -- -- 200 -- -- [81]
Liquid electrolyte anode less or anode free LFP@NC@rGO -- Ag–PCP/Cu 200 72% (200 cycle, 1C, full cell) 85.94% (initial coulombic efficiency) [82]
LPSCl Li metal NCM622 -- 100 nm Ag, Au, or Ni coating on a Cu foil -- -- 97.5% (Ag, 8 cycle, 0.25 mA cm–2, full cell) [83]
LPSCl anode less or anode free NCM532 Carbon black layer zinc layer SUS 100 68% (100 cycle average, 0.5C/0.5C, full cell) 99.7% (0.5C/0.5C, full cell) [84]
Liquid electrolyte anode less or anode free LFP CNT/Cu 500 69.4% (after 100 cycles, 0.5C, Full cell) 97.8% (after 500 cycle, 1.0C) [86]
LPSClBr LiNbO3 coated NCM:Li3InCl6=70:30 Carbon papers/Cu 5000 95% (3.0 mAh cm–2 including the capacity of the lithium intercalation, full cell) [87]
LPSClBr anode less or anode free NCM811 SUS 50 -- 99.5% (0.1C/0.1C AgF based full-cell) [88]
LPSCl/PEO hybrid metallic lithium NCM811 Li3.75Si–CNT (MIEC) SUS 200 88.9% (200 cycle, 0.35C, full cell) 99.9% (after 10 of cycle, 0.35C, full cell) [89]
LPSCl anode less or anode free NCA 3LiBH4–LiI (LBHI) Cu -- '-- 94.7% (initial coulombic efficiency, 1 cycle, 0.05 mA cm–2, half cell) [90]
LLZTO anode-free NCM111 Ag–C Li–Cu foil 600 78.3% (discharge capacity retention, after 600 cycle, 1 mA cm–2, full cell) 99.8% (after 600 cycle, 1 mA cm–2, full cell) [91]
LPSCl anode-free Li counter electrode carbon Li–Cu foil '-- '-- '-- [92]
LPSClBr Mg particle NCM811 Ag SUS 1000 80% (500 cycle and stably oprated over 1000 cycles, 0.5 mA cm–2, full cell) 99.9% (for 1000 cycles, 0.5 mA cm–2, full cell) [93]