Accelerated Short Circuiting in Anode-Free Solid-State Batteries Driven by Local Lithium Depletion

Abstract

“Anode-free” solid-state batteries (SSBs), which have no active material at the anode and undergo in situ lithium plating during the first charge, can exhibit extremely high energy density (~1500 Wh L-1). However, there is a lack of understanding of lithium plating/stripping mechanisms at bare solid-state electrolyte (SSE) interfaces since excess lithium is often used. Here, we demonstrate that commercially relevant quantities of lithium (> 5 mAh cm-2) can be reliably plated at relatively high current densities (1 mA cm-2) using the sulfide SSE Li6PS5Cl. Investigations of lithium plating/stripping mechanisms, in conjunction with cryo-focused ion beam (FIB) and ex situ synchrotron tomography, reveal that the cycling stability of these cells is intrinsically limited by spatially uneven plating/stripping. Local lithium depletion toward the end of stripping decreases electrochemically active area, which results in high local current densities and void formation, accelerating subsequent filament growth and short circuiting compared to lithium-excess cells. Despite this governing degradation mode, we show that anode-free cells exhibit comparable Coulombic efficiencies to lithium-excess cells before short circuiting, and improved resistance to short-circuiting is achieved by avoiding local lithium depletion through retention of lithium at the interface. These new insights provide a foundation for engineering future high-energy anode-free SSBs.

Structural and Electrochemical Evolution of Alloy Interfacial Layers in Anode-Free Solid-State Batteries

“…The increased impedance driven by contact loss at the end of stripping of the bare Cu electrode in later cycles is direct evidence for the growth of voids at the working electrode interface. As we have recently described, 35 near the end of stripping, Li will be present across only part of the interface, as many regions will have been fully depleted of Li. The remaining pockets of Li still undergoing stripping will experience high local current densities and current constriction effects because of the minimal electrochemically active interfacial area remaining.…”

“…The bare Cu cell also shows a sharp voltage increase at the end of stripping due to exhaustion of available Li, potentially correlating to current constrictions/void growth at remaining Li islands. 35 In contrast, the Agmodified cell shows a sloping potential from 7.8 h due to extraction of Li from the Li-Ag alloy.…”

“…Thickness variations across the electrode have previously been observed when using bare Cu in anode-free cells. 35 There are regions of porosity within the Li layer, and the Li also clearly grows into pores of the SSE (red arrow in Fig. 2a).…”

“…This process is "intrinsic" in that it occurs due to the geometric depletion of Li even at low global current densities, in contrast to voiding which occurs across a uniform Li-excess interface when high current densities are applied. 26,35 The Ag-and Au-modified electrodes exhibit distinct EIS evolution compared to the bare Cu case. During the first stripping half-cycle of the Ag-modified electrode (Fig.…”

“…34 Our recent work has revealed that localized Li depletion and island formation near the end of stripping play a critical role in limiting the performance of anode-free SSBs. 35 During stripping from anode-free cells with argyrodite SSEs, slightly nonuniform Li thicknesses led to some Li regions being fully stripped before others, locally exposing the electrochemically-inactive current collector and leading to current constriction, void growth, and rapid short circuiting upon cycling. This mechanism is unique to anode-free cells since it only occurs when attempting to strip all Li, and it significantly reduces cell stability compared to Liexcess cells.…”