Experimental Assessment of the Practical Oxidative Stability of Lithium Thiophosphate Solid Electrolytes

Abstract

All-solid-state batteries are often expected to replace conventional lithium-ion batteries in the future. However, the practical electrochemical and cycling stability of the best-conducting solid electrolytes, i.e. lithium thiophosphates, is still a critical issue that prevents long-term stable high-energy cells. In this study, we apply a stepwise cyclic voltammetry approach to obtain information on the practical oxidative stability limit of Li 10 GeP 2 S 12 , two different Li 2 S−P 2 S 5 glasses, as well as the argyrodite Li 6 PS 5 Cl solid electrolytes. We employ indium metal and carbon black as the counter and working electrodes, respectively, the latter to increase the interfacial contact area to the electrolyte as compared to the commonly used planar steel electrodes. Using a stepwise increase in the reversal potentials, the onset potential of oxidative decomposition at the electrode−electrolyte interface at 25 °C is identified. X-ray photoelectron spectroscopy is used to investigate the oxidation of sulfur(-II) in the thiophosphate polyanions to sulfur(0) as the dominant redox process in all electrolytes tested. Our results suggest that in later cycles the crystalline solid electrolyte itself is not the major redox active phase, but rather that only after the formation of such electrolyte decomposition products is significant redox behavior observed. Indeed, the redox behavior of the decomposition products is an additional contributor to the overall cell capacity of solid-state batteries. The stepwise cyclic voltammetry approach presented here shows that the practical oxidative stability at 25 °C of thiophosphate solid electrolytes against carbon is kinetically higher than predicted by thermodynamic calculations and that the decomposition products dominate the redox behavior of cathode composites. The method serves as an efficient guideline for the determination of practical, kinetic stability limits of solid electrolytes with respect to the employed electrode materials.

Toward Higher Voltage Solid‐State Batteries by Metastability and Kinetic Stability Design

“…In contrast, the pressurized cell almost avoids these peaks. The peaks were also reported in other works, [ 24 ] which represent the decomposition of small fraction of LGPS that resides on the surface of voids remaining in the pellet after the initial press. At these surfaces, mechanical constriction is weak, thus the LGPS can show certain limited decomposition, expanding into the void region, without causing any local pressure to form.…”

“…Such lattice vector compression results in peak shifting caused by the externally applied uniaxial pressure and/or uniform (de)lithiation. Thus, the presence of strain broadening suggests that in addition to the expected peak shifting, due to (de)lithiation and uniaxial compression at certain experimental conditions, [ 12,14,24 ] there also exists sources of localized strains that distort particular unit cells away from normal position. The experimental step of discharging the high voltage scanned LGPS back to 2.5 V before taking SXRD was designed to maintain a constant Li composition in LGPS, so the effect of Li composition induced lattice parameter change and Bragg peak shift can be minimized, while the irreversible strain effect induced at high voltage can be most clearly observed.…”

“…The results show direct evidences of LGPS straining during these electrochemical processes. In addition to LGPS straining due to (de)lithiation, which shifts XRD peaks, [ 12,14,24 ] strain broadening suggests the existence of small localized domains of highly strained regions. These small strained pockets inside LGPS particles are evidences of decomposed inclusions within the solid‐electrolyte, for the first time providing direct experimental evidences to prior predictions of nucleated decay morphologies.…”

Mechanochemical Synthesis: A Tool to Tune Cation Site Disorder and Ionic Transport Properties of Li3MCl6 (M = Y, Er) Superionic Conductors

“…While these solid electrolytes are being used in solid‐state batteries, the interface behavior in contact to different electrode materials and the need for large‐scale processability remain a challenge . Nevertheless, despite their higher sensitivity to decomposition in atmosphere and at higher oxidative potentials, thiophosphates have shown promising solid‐state battery performance because of their mechanically soft nature and improved processability …”

Direct Observation of Interfacial Mechanical Failure in Thiophosphate Solid Electrolytes with Operando X‐Ray Tomography

“…[1] Despite the successful implementation of LIBs in portable electronics, proliferation of Li + -based energy storage SEs results in decomposition of the SE at both the anode and cathode surfaces. [13][14][15][16][17][18] While progress has been made in the implementation of low voltage cathodes, the reductive instability of LGPS at lithium anodes remains a considerable issue. Previous investigations of the LGPS/Li 0 interface have identified Li 2 S, Li 3 P, Li-Ge alloys, and other PS and SS containing species as some of the most common reductive decomposition products formed during lithium plating.…”