Thermal Stability of LixCoO2 Electrodes for All-Solid-State Secondary Batteries Operating at High-Temperature

Abstract

Over the last decades, the development and the miniaturization of portable electronic devices have stimulated many researches in the field of micro power sources. In this context, all-solid-state microbatteries have been developed and are currently commercialized for room temperature applications [1]. The thermal stability of the LiPON ceramic electrolyte used in these microbatteries suggests that they could be used for powering autonomous sensors located in harsh environment, and particularly those exposed to high temperatures (typically 200 °C) [2]. In order to estimate the sustainability of standard microbatteries LiCoO2/LiPON/Li at high temperature, the thermal stability of the positive electrode material and its compatibility with the LiPON electrolyte have been studied.Lithium cobalt oxide is the most used material for positive electrode in microbatteries. Its thermal stability was investigated for the first time by Dahn et al. [3] who showed that LixCoO2 materials start to decompose above 200 °C. Ever since, only few studies have evaluated their intrinsic thermal stability, i.e. without electrolyte and in airtight conditions, but never over extended periods of time and at lower temperatures. Here, LixCoO2 compounds with various compositions (0.45<x<1) were prepared by chemical oxidation of a commercial LiCoO2 powder using a NO2BF4 [4]. ICP-OES and XPS were used to determine the Li/Co ratio and the oxidation state of cobalt respectively. XRD and Raman spectroscopy analyses, conducted in air-tight cells, were achieved to characterize the structural evolution of the pristine delithiated samples. Their thermal stability was firstly evaluated by DSC in sealed crucible. For all the delithiated phases, an exothermic peak linked to the decomposition of the structure accompanied by oxygen loss is observed above 200 °C [3,5]. Long term thermal stability under argon atmosphere was assessed for temperatures ranging from 100 °C to 200 °C, by XRD and Raman spectroscopy. LixCoO2 compounds start to evolve from 100 °C to form a HT-LiCoO2 phase and a spinel Co3O4-like one. In situ XRD temperature analyses were achieved under helium atmosphere to study the decomposition mechanism of the LixCoO2 compounds. The kinetics of the decomposition reaction was followed by in situ XRD isothermal measurements. Kinetics parameters and model function were determined by conventional [6] and Coats-Redfern methods [7]. Identification of decomposition products at 200 °C was carried out by combining XRD, TEM, Raman, XPS and 7Li NMR spectroscopies. All these results suggest that the spinel phase is partially lithiated in this temperature range (100-200°C), whereas HT-LiCoO2 and Co3O4 were clearly identified as decomposition products at higher temperature (600 °C) in agreement with prior studies [3]. The decomposition mechanism appears therefore more complex than the previously reported one. The presence of lithium vacancies destabilizes to the layered structure which collapses locally to form a spinel-like phase due to cobalt moving into the vacancies. Given the ionic radius of cobalt, its presence in the interslab space traps some lithium in the spinel-like phase. At the same time, the major part of remaining lithium tends to segregate in other parts of the crystals where it re-stabilizes the layered scaffold.Raman spectroscopy analyses carried out on LixCoO2 films prepared by magnetron sputtering and delithiated in coin cells, showed that thin films exhibit the same thermal behaviour as bulk compounds.In parallel, LiCoO2/LiPON interface was studied before and after annealing at 200 °C. Thin film bilayers of LiCoO2 (5 µm)/LiPON (3 µm) were prepared by magnetron sputtering. GD-OES and ToF-SIMS analyses were achieved post deposition and after high temperature annealing under argon atmosphere. For pristine bilayers, a 500 nm lithium-rich interphase is observed [8,9]. A marked growth of this interphase, accompanied by phosphorous diffusion into the latter is highlighted for annealed samples.