An Air‐Stable and Dendrite‐Free Li Anode for Highly Stable All‐Solid‐State Sulfide‐Based Li Batteries

Abstract

Li metal is an attractive anode material for all-solid-state lithium batteries (ASSLBs) because of its high specific capacity (3860 mAh g−1) and low electrochemical potential (−3.04 V vs the standard hydrogen electrode).[1–4] Moreover, the use of intrinsically nonflammable inorganic solid-state electrolytes (SSEs) can theoretically solve the safety issue associated with thermal runaway.[3,4] Various SSEs have been recently developed, including oxide-, sulfide-, halide-, and borohydride-based SSEs. Sulfide-based SSEs have emerged as one of the most promising candidates for use in ASSLBs due to their mechanical properties and ultrahigh ionic conductivity which can reach as high as 10−2 S cm−1 at room temperature.[5–8] However, pure Li metal is still unsuitable for realapplication in sulfide-based ASSLBs due to the side reactions at the interface. Similar to conventional liquid-based lithium ion batteries, the first major obstacle for the use of Li metal anodes is the penetration of Li dendrites through the sulfide-based SSEs, which raises safety concerns and often results in the decay of battery performance or even short circuit.[9,10] Among the typical sulfide-based SSEs, such as Li10GePS12, Li3PS4 (glass or ceramic),[13–17] and Li6PS5X (X = Cl, Br, I),[18,19] the formation of lithium dendrites in Li3PS4 system is well-recognized.[13–16,20,21] It is proposed that the voids and grain boundaries within the SSEs, as well as the insufficient interfacial contact between Li and SSEs are the two main reasons for lithium dendrite formation in sulfide-based ASSLBs.[16,22] Thus, there should be a critical current density at which short-circuiting of the cell occurs in the Li2S-P2S5 solid electrolyte system.[14,15] However, recent reports indicate that the Li dendrites can be formed in Li4Ti5O12/Li2S-P2S5/ Li[20] and Se/Li3PS4/Li ASSLBs using bare Li as the anode, even at a low current density of 50 mA g−1. Another possible reason proposed by Wang and co-workers is the relatively high electronic conductivity (10−9–10−8 S cm−1) of Li3PS4. Though not fully understood yet, the introduction of additives into SSEs similar to Li protection in liquid batteries have been developed. For example, LiI,[16] LiF,[23] and P2O5 have been introduced into Li3PS4 and proven to be effective suppressing lithium Li metal is a promising anode material for all-solid-state batteries, owing to its high specific capacity and low electrochemical potential. However, direct contact of Li metal with most solid-state electrolytes induces severe side reactions that can lead to dendrite formation and short circuits. Moreover, Li metal is unstable when exposed to air, leading to stringent processing requirements. Herein, it is reported that the Li3PS4/Li interface in all-solid-state batteries can be stabilized by an air-stable LixSiSy protection layer that is formed in situ on the surface of Li metal through a solution-based method. Highly stable Li cycling for over 2000 h in symmetrical cells and a lifetime of over 100 cycles can be achieved for an all-solid-state LiCoO2/Li3PS4/Li cell. Synchrotron-based high energy X-ray photoelectron spectroscopy in-depth analysis demonstrates the distribution of different components within the protection layer. The in situ formation of an electronically insulating LixSiSy protection layer with highly ionic conductivity provides an effective way to prevent Li dendrite formation in high-energy all-solid-state Li metal batteries.