(Invited) Role of Ag in Ag-C Interlayer for Reversible Plating/Dissolution of Li-Metal for Anode-Free Li-Metal Solid-State Batteries
Authors: Asano Tetsuya , Suzuki Shinya , Sugata Shoichi , Yashiro Nobuyoshi , Kim Sewon , Kim Jusik 
Published: December 2024

Reversible lithium metal plating/striping is a vital requirement to realize anode-free configuration of lithium-metal solid-state battery (LMSSB), and has remained as a challenge for long years. To improve uniformity and reversibility of lithium metal plating, interlayer formation on solid-state electrolyte has been proven to be an effective strategy. While various types of interlayers have been studied so far, Ag-C composite interlayer has attained excellent cycle performance (89 % after 1,000 cycles) and high loading capacities (> 6 mAh/cm2) simultaneously[Ref]. The mechanism of Ag-C composite interlayer is, however, not clear yet, due partly to lack of the most fundamental characteristics of Li-Ag alloys such as phase diagram and their electrochemical characteristics.
Here, in this study, we have clarified the fundamental Li-Ag properties and their influence on the Ag-C composite interlayer in anode-free LMSSB configuration. We, first, unambiguously identified phase diagram of Li-Ag, their crystal structures and their thermodynamic and kinetic electrochemical characteristics. Next, we revealed phase transformations of Ag-C interlayer during charging and discharging via synchrotron operand XRD. Then, we clarified that the role of Ag in Ag-C interlayer for anode-free LMSSB is to maintain interfacial intimacy between the Ag-C interlayer and current collector, rather than their influence on the bulk properties of Li-metal.
First, we renewed Li-Ag phase diagram and revealed their crystal structures by systematic studies of structural, thermal, and electrochemical characterizations. The newly identified phase diagram is different from conventionally recognized phase diagram in lithium-rich region. No Li9Ag phase, usually referred as γ
1 phase with incongruent decomposition, was absent. But, instead, Li3Ag and Li83Ag17 phases, respectively are present and their crystal structures were identified by analysis of XRD spectrum using data-science approach. Furthermore, electrochemical characterization revealed that the open-circuit potential of Li3Ag phases is only 4 meV above Li/Li+ equilibrium potential, very close to the lithium metal. Thus, the whole map of Li-Ag phase diagram together with electrochemical properties around room temperature was clarified.
Having correctly identified phase diagram and crystal structures of Li-Ag alloys, we performed operand XRD of Ag-C interlayer during charging/discharging processes using synchrotron X-ray radiation source. It was revealed that during charging, phase transformations of Li-Ag nearly follows the equilibrium conditions. Slight deviation from quilibrium is that lithium-metal nucleation overpotential was observed before most Li-rich phase (Li83Ag17) appeared. This is due to very low open-circuit potential of Li3Ag, which leads to non-equilibrium Li metal plating with slight overpotential. During discharging, the thermodynamically unstable phase appeared at the end of discharging, due to slow kinetics of Li-Ag alloy in the Li-rich region.
Given that the Li-Ag materials characteristics and Ag-C interlayer charge/discharge phase transformation behavior was clarified, we then characterized charge/discharge performance of anode-free LMSSB with Ag-C interlayer. For comparison, the same battery configuration but the Ag-C interlayer replaced with carbon interlayer was also characterized. Discharge rate-performance of Ag-C interlayer shows nearly constant discharge capacity with increasing current for higher than 3 mA/cm2, while that of carbon interlayer abruptly dropped within 1 mA/cm2, when the discharge current was increased with cycles. This result seemingly showed the improved diffusion of lithium in Li-Ag alloy, which, however turned out to be not right. This drop of rate performance in carbon interlayer was observed only in the 2nd or latter cycles but not in the first cycle, which indicate that the effect of Ag in Ag-C interlayer for high cycle capability is its enhanced adhesion between Ag-C interlayer and current collector, but not the bulk diffusivity in Li-Ag layer. This was further verified with cross-sectional SEM images.
This study reveals the role of Ag in Ag-C composite interlayer, and provides insight about the design principles of the interlayer towards realization of anode-free lithium-metal solid-state battery.

[Ref] Y. Lee et al. Nat. Energy 5, 299 (2020).