All-solid-state batteries: The interface between Li-metal and the solid state electrolyte Li6PS5Cl

Abstract

Lithium ion batteries are currently the most attractive choice for mobile energy storage and power sources [4] in terms of energy density. However, it is unlikely that current battery chemistries will reach the energy density desired for future applications [8]. Li-metal, which has the highest reduction potential of all metals and the highest achievable capacity per weight unit, could significantly increase the energy density when used as an Anode. Unfortunately, the dendritic growth of Li-metal is a safety hazard, and the low electrochemical potential of Li-metal
is outside the stability window of many electrolytes leading to capacity degradation.
It has been suggested that safe Li-metal Anodes may be possible in combination with solid electrolytes, as the higher shear modulo of solid over liquid electrolytes may be able to suppress dendrite formation [21].
To study the stability of the interface between Li-metal and the solid electrolyte Li6PS5Cl, electrochemical measurements of the cells, ex-situ x-ray diffraction, and solid state Nuclear Magnetic Resonance measurements were made. It was not possible to confirm the decomposition products that were proposed in literature by density functional theory calculations and in-situ XPS. The XRD-pattern of the cycled electrolyte did not show additional phases. The NMR spectra developed broader peaks and one additional phosphorous environment upon cycling. As a tool to study dendritic growth through Li-ion concentration profiles, a cell for in-situ neutron depth profiling (NDP) was developed. The electrochemical performance is comparable to regular cells, except for a voltage drop due to contact problems during the break after Li-metal stripping. A thin layer of Li-metal between the window and the electrolyte pellet, as well as a collimator plate to put some pressure on the window, could be the solution
to go towards operando measurements.
The current standard data analysis method was implemented and its sensitivity towards gaussian broadening, due to energy straggling (from small angle scattering in the material and the stochastic nature of energy loss), was
investigated. It is shown that the standard deviation of energy straggling is larger than that of energy broadening due to the detector resolution, and that the error on the depth scale is in the order of micrometers.