Toward Optimization of the Chemical/Electrochemical Compatibility of Halide Solid Electrolytes in All-Solid-State Batteries

Abstract

All-solid-state batteries (ASSBs) that rely on the use of solid electrolytes (SEs) with high ionic conductivity are the holy grail for future battery technology, since it could enable both greater energy density and safety. However, practical application of ASSBs is still being plagued by difficulties in mastering the SE–electrode interphases. This calls for a wide exploration of electrolyte candidates, among which halide-based Li+ conductors show promise despite being not stable against Li or Li x In y negative electrodes, hence the need to assemble cells with a dual SE design. In the work described herein, we studied the electrochemical/chemical compatibility of Li3InCl6 against layered oxide positive electrode (LiNi0.6Mn0.2­Co0.2O2, NMC622), carbon additive, and Li6PS5Cl under both cycling and aging conditions. Combining electroanalytical and spectroscopic techniques, we provide evidence for the onset of electrochemically driven parasitic decomposition reactions between Li3InCl6 and NMC622/carbon at lower potentials (3.3 V vs LiIn/In) than theoretically predicted in the literature. Moreover, to combat chemical incompatibility between dual SEs, we propose a new strategy that consists of depositing a nanometer-thick (1 or 2 nm) surface protective layer of Li3PO4 made by atomic layer deposition between Li3InCl6 and Li6PS5Cl. Through this surface engineering process with highly conformal and pinhole-free thin films, halide-based solid-state cells showing spectacular capacity retention over 400 cycles were successfully assembled. Altogether, these findings position halide SEs as serious contenders for the development of ASSBs.

Visualizing the Chemical Incompatibility of Halide and Sulfide‐Based Electrolytes in Solid‐State Batteries

“…[ 33 ] The high resistance and particularly the increase upon cell cycling appears to be caused by the recently reported chemical incompatibility between Li 3 InCl 6 and Li 6 PS 5 Cl, which results in the formation of an interphase and thus to a higher resistance (see discussion section below). [ 34,35 ]…”

“…This in turn implies a reaction of Li 3 InCl 6 and Li 6 PS 5 Cl and thus a thermodynamic instability of both materials in contact with each other, which has been suggested recently. [ 34,35 ]…”

“…In addition, a separation of the sulfide and halide interphase with a thin Li 3 PO 4 coating by atomic layer deposition is possible and can increase the cycle stability. [ 35 ] As recent work has shown that the chemical stability of halides against CAM centers around the metal ion in Li‐M‐Cl, the chemical stability against sulfides may also be a function of the halide solid electrolyte composition, indicating the need to search for more stable combinations at the triple phase boundary. [ 33 ] This is further substantiated as the Li 3 InCl 6 ‐based cell, with Li 3 InCl 6 in the composite and double separator layer, showed growth in the R separator resistance as well, whereas the other two tested single‐layer separators did not.…”

“…[30,31] Nevertheless, recent work shows that while the halides may show high electrochemical stability (stability against a certain oxidation potential), [32] chemical stability (stability of the different cell components against side reactions) is not necessarily given. [33][34][35] Li 3 InCl 6 is suggested to have both a high electrochemical stability window [30] and reasonable conductivity of greater than 1 mS cm −1 . [25,26] However, Li 3 InCl 6 is not stable against lithium metal and decomposes, inhibiting its use as separator in Halide-based solid electrolytes are currently growing in interest in solidstate batteries due to their high electrochemical stability window compared to sulfide electrolytes.…”

“…It is also in stark contrast to simple processing on large scale. In addition, it was recently found that the incompatibility of Li 3 InCl 6 and Li 6 PS 5 Cl in LiNi 0.6 Co 0.2 Mn 0.2 O 2 -based cells lead to decomposition products and higher interfacial resistance, [34,35] suggesting possible issues with this approach. Nevertheless, no direct analyses of any ongoing chemical and electrochemical processes have been established to explain the chemical processes occurring at the respective interfaces.…”