Room‐Temperature Sintering of Amorphous Thiophosphate Solid Electrolyte (Li₃PS₄): Coupling Morphological Evolution to Electrochemical Properties

Abstract

Thiophosphate solid electrolytes (Li3PS4, hereafter denoted LPS) have the advantage of presenting a reasonable ionic conductivity at room temperature (≈ 0.3 mS cm−1) and an easy manufacturing, meaning that they can be sintered at room temperature. Unfortunately, during cycling, several chemo‐mechanical degradations quite often attributed to the electrochemical activities occur, but they could also be linked to the sintering process. To date, a fundamental understanding of room‐temperature sintering and its impact on the microstructure, the ionic conductivity, and the link between electrochemistry and structure/morphology remains imprecise. In this study, a comprehensive study of homemade amorphous 75% Li2S – 25% P2S5 (Li3PS4) is presented, investigating the influence of pressure and time of room temperature sintering. Focused ion beam‐scanning electron microscopy coupled to electrochemical techniques such as electrochemical impedance spectroscopy, Li plating/ stripping and coupled to structural techniques such as wide‐angle X‐ray scattering are used to establish the link between structure, morphology, and electrochemical properties. It is demonstrated that the room temperature sintering of solid electrolytes is not that trivial and that the commonly accepted rule “less porosity = better ionic conductivity” is not always true and that many additional parameters should be considered to properly sinter the solid electrolyte.

Heteroionic Interfaces in Hybrid Solid-State Batteries─Current Constriction at the Interface between Different Solid Electrolytes

“…The microstructure of garnet-type oxides, such as grain size distribution and (residual) porosity, is highly dependent on the sintering protocol. − Similarly, the manufacturing conditions (e.g., pelletizing pressure or time) of cold-pressed thiophosphate ceramics have a major influence on the formation of intergranular contacts . When two solid electrolytes are combined in a layered stack, the resulting heteroionic interface morphology introduces an additional geometric degree of freedom into the system.…”

Composite Electrode (LiNi0.6Mn0.2Co0.2O2) Engineering for Thiophosphate Solid-State Batteries: Morphological Evolution and Electrochemical Properties

“…Based on these results, we look at two different times for the room temperature sintering of the composite electrode at 255 MPa for 10 min and 15 h to optimize the sintering without damaging too much the NMC particles. In addition, from a previous study on the LPS morphology carried in our laboratory, we expect a longer sintering to slightly reduce the porosity as well as improves the contact between NMC and LPS. For sake of simplicity, we will refer to the two samples as 255–10m and 255–15h, respectively.…”

“…Indeed, with longer compressed electrode, we assume to get greater surface contact between particles (NMC and LPS) and thus a greater chemical decomposition. Here, we believe that the decompositions observed in the first cycle are mostly dominated by the chemical degradation, as witnessed by the poor electrochemical stability windows of LPS solid electrolyte . Looking now at the other cycles, the tendency for the Coulombic efficiency is drastically different for both samples: (i) for the 255–10m, the CE remains poor for several cycles, indicating additional chemical decomposition within the cell, whereas (ii) for the 255–15h, the CE increases directly to reach 97–98% for the next 10 cycles, indicating that the decomposition processes are drastically reduced.…”

“…Here, we believe that the decompositions observed in the first cycle are mostly dominated by the chemical degradation, as witnessed by the poor electrochemical stability windows of LPS solid electrolyte. 37 Looking now at the other cycles, the tendency for the Coulombic efficiency is drastically different for both samples: (i) for the 255−10m, the CE remains poor for several cycles, indicating additional chemical decomposition within the cell, whereas (ii) for the 255−15h, the CE increases directly to reach 97−98% for the next 10 cycles, indicating that the decomposition processes are drastically reduced. This behavior is counterintuitive and cannot be solely explained by the chemical degradation, since if the chemical degradation was the main cause of fading, the cell with supposedly better contact would have the worse electrochemical signature, which is not the case, indicating that the mechanical stability is also playing a key role in improving the overall electrochemical performance.…”

Nonintuitive Role of Solid Electrolyte Porosity on Failure

“…High density (low porosity) solid electrolytes can result in higher ionic conductivity, while low density (high porosity) often results in lower ionic conductivity. − While there is a clear relationship between porosity and solid electrolyte ionic conductivity, the role that microstructure plays on degradation is less understood. Recently, filament formation and growth in solid electrolytes has been described by a range of solid electrolyte properties including the solid electrolyte electronic conductivity, solid electrolyte pore connectivity, − and interface discontinuities and defects . In reality, there is likely an ensemble of electro–chemo–mechanical phenomenon, driven by external operating conditions and microstructure, which contribute to solid electrolyte failure.…”