Degradation Mechanisms at the Li₁₀GeP₂S₁₂/LiCoO₂ Cathode Interface in an All-Solid-State Lithium-Ion Battery

Abstract

All-solid-state batteries (ASSBs) show great potential for providing high power and energy densities with enhanced battery safety. While new solid electrolytes (SEs) have been developed with high enough ionic conductivities, SSBs with long operational life are still rarely reported. Therefore, on the way to high-performance and long-life ASSBs, a better understanding of the complex degradation mechanisms, occurring at the electrode/electrolyte interfaces is pivotal. While the lithium metal/solid electrolyte interface is receiving considerable attention due to the quest for high energy density, the interface between the active material and solid electrolyte particles within the composite cathode is arguably the most difficult to solve and study. In this work, multiple characterization methods are combined to better understand the processes that occur at the LiCoO cathode and the LiGePS solid electrolyte interface. Indium and LiTiO are used as anode materials to avoid the instability problems associated with Li-metal anodes. Capacity fading and increased impedances are observed during long-term cycling. Postmortem analysis with scanning transmission electron microscopy, electron energy loss spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy show that electrochemically driven mechanical failure and degradation at the cathode/solid electrolyte interface contribute to the increase in internal resistance and the resulting capacity fading. These results suggest that the development of electrochemically more stable SEs and the engineering of cathode/SE interfaces are crucial for achieving reliable SSB performance.

On the Functionality of Coatings for Cathode Active Materials in Thiophosphate‐Based All‐Solid‐State Batteries

“…A simple estimate shows that typical volume changes of a few percent cause less than 1% of lateral strain along the coating. [26,42,43] Interestingly, interdiffusion from the coating to the CAM could be potentially beneficial for the stability and operation of the CAM (e.g., Al 3+ doping from an aluminum-containing coating [44] ); however, this hypothesis would need to be systematically investigated before any claims could be made. [35] Whether a coating survives these strains without cracking depends on the mechanical properties of the coating material and its adhesion to the CAM.…”

“…However, it may be that in some cases, interdiffusion between the CAM and the coating can enhance the overall stability of the CAM. [26] Ultimately, the ideal coating material will behave like a solid electrolyte with low electronic partial conductivity (low redox activity of the cation(s)), be sufficiently soft/brittle to allow direct contact of CAM particles by mechanical deformation, and be sufficiently elastic to alleviate strain during volume changes of the CAM particles (Figure 6). Data for the diffusion of impurity ions are scarce, but are highly desirable to better judge the temperature stability of coated CAMs.…”

“…[19,20] Unfortunately, thiophosphates also have a rather narrow electrochemical stability window, i.e., the onset of oxidative decomposition begins even before 3 V versus Li + /Li due to S(0)/S(−2) redox reactions, while reductive decomposition is theoretically expected at potentials of about 1.7 V versus Li + /Li due to P(+5)/P(−3) redox reactions. [2,[26][27][28] Several years ago, Takada summarized the early work on SSBs and described the need for coatings against the formation of space charge layers at the interface between high-voltage cathodes and thiophosphate SEs. The perfect SE would combine the ionic transport properties of thiophosphates, the mechanical properties of polymers, and the oxidation stability of oxides.…”

“…[2,[26][27][28] Several years ago, Takada summarized the early work on SSBs and described the need for coatings against the formation of space charge layers at the interface between high-voltage cathodes and thiophosphate SEs. [2,[26][27][28] Several years ago, Takada summarized the early work on SSBs and described the need for coatings against the formation of space charge layers at the interface between high-voltage cathodes and thiophosphate SEs.…”

Interfacial Incompatibility and Internal Stresses in All‐Solid‐State Lithium Ion Batteries

“…However, significant interfacial impedance still occurs at the sulfide SSE/cathode material interfaces, being primarily ascribed to electrochemical reaction. Like the Li 2 S-P 2 S 5 and Li 3 PS 4 , the LGPS was also unstable against cathode materials (LiCoO 2 [248] and LiNi 0.5 Mn 1.5 O 4 [249] ). The first-principles calculation also predicted cation intermixing upon charging such as Co ↔P exchange in the LiCoO 2 /β-Li 3 PS 4 system.…”