Coatings and New Generations of Massive All-Solid-State Batteries

The growing interest in hybrid and electric vehicles calls for the development of new safer batteries structures with little or no maintenance. Furthermore, specific requirements such as high temperature (> 100°C) stability in applications in measurement while drilling tools or during medical tools sterilization, force us to avoid the use of liquid electrolyte and to turn to all-solid-state systems. The major advantages of these all-solid-state massive batteries, and thus without solvent, are the thermal stability, the absence of leakage and pollution and the possibility to use high potential electrode materials. These batteries, shaped by Spark Plasma Sintering (SPS) technique, which allows the preparation of dense materials (without additives) at lower temperature, particularly in shorter time (few minutes) than by conventional technique, open the way to an industrial breakthrough.

Recent works, particularly in the framework of ANR Ceralion and now within the framework of ANR Solibat including different French laboratories such as LRCS, CEMES, ICMCB, LCP and the SAFT company, showed the feasibility of such batteries with specific systems LiFePO₄ (LFP) / Li_1.5Al_1.5Ge_0.5(PO₄)₃ / Li₃V₂(PO₄)₃ (LVP) or LVP / LAGP / LVP. The use of other commercial electrode materials, like LiCoO₂ (positive electrode LCO) or Li₄Ti₅O₁₂ (negative electrode LTO), which is interesting due to its good cyclability at 1.5V vs Li⁺/Li and its small volume variation during the electrochemical process, proved complex because of their reactivity with the LAGP solid electrolyte. Moreover, the ANR Ceralion study has shown that the performances of these massive all-solid-state batteries are not only governed by the solid electrolyte electrical properties, but also by the properties of the electrode/electrolyte interfaces which can be modified during chemical and/or electrochemical reactions. Indeed, the need to activate the sintering process, despite short heat treatment, causes a detrimental reactivity between materials, as during the sintering of LTO with LAGP. To solve these problems and in particular to use the LTO as active material in the negative composite electrode, it is intended to stabilize the interfaces by limiting the incompatibility between electrode and electrolyte and/or implementation of coating with active materials selected to be chemically inert without introducing other limitations.

A dedicated study of coating based insulating phosphate materials such as AlPO₄ or Li₃PO₄ was undertaken as far as these materials result from the electrolyte decomposition with the electrode materials. As these decomposition materials used as coating agent are more stable than the initial materials, it is then possible to avoid the decomposition of certain materials such as LTO and the generation of detrimental interfaces. Different coating routes were tested such as hydrothermal or sol-gel route. Coated materials were characterized by XRD and TEM. Their impact on the active materials cyclability and performances were first tested in liquid electrolyte to optimize them before their integration in all-solid-state batteries. The first promising obtained results will be presented, opening the way for new generations of positive/negative composites combinations and so of all-solid-state batteries with commercial materials allowing the improvement of energy density as well as potential, and thus of aimed applications.