Development of operando diagnostics for Li-ion cathodes by Raman spectroscopy

Abstract

The ever increasing demand for better-performing Li-ion batteries (LIBs) motivates substantial research efforts into the understanding of the working principles of every battery component, particularly of the cathode because it is the performance-limiting component in LIBs. The lithium-transition-metal oxides (LiMO2) with layered structure are the state-of-the-art cathode active materials for LIBs but many aspects of their operation are not fundamentally understood and, thus, leave a room for significant improvements. From the large portfolio of techniques available for characterizing LIB electrodes, only a handful are capable of investigating the materials at the individual particle level and operando, i.e. within their working environment and while they operate. One of them is Raman spectroscopy, a versatile technique for studying condensed phases based on their intrinsic normal modes of vibration. Despite the advantages of this technique, it has been rarely applied to LiMO2 due to multiple challenges related to the limitations of the operando measurements: the cell designs, the inherently weak Raman signals of the oxides, the limited time resolution and the lack of fundamental understanding about the origin of the spectral features. This work reports the development of a new spectro-electrochemical cell and multiple data-analysis tools for recording and analysing the operando Raman spectra of electrode materials. A cell design, mindful of multiple optical and electrochemical constraints, yields a device enabling superior spectral quality, time resolution, and electrochemical performance comparable to commercial-like cells. Instead of analysing only few sample spectra, several Matlab-based routines have been developed for the automated analysis of all hundreds of spectra resulting from an operando experiment. The resulting trends are analysed as a function of the electrode potential and the state of lithiation (SOL) of the oxide during cycling. The developed methodology is applied for the investigation of several commercially-relevant LiMO2 cathode materials, which are currently used in portable and automotive applications: LiCoO2 (portable electronics), LiNi0.33Co0.33Mn0.33O2 (BMW electric vehicles), LiNi0.6Co0.2Mn0.2O2, LiNi0.8Co0.1Mn0.1O2 and LiNi0.8Co0.15Mn0.05O2 (Tesla electric vehicles). The relationships between spectral features and material properties are established by comparing the experimental findings to various models, formulated based on crystallographic symmetry, classical electrodynamics, DFT calculations and complementary experiments. These spectrum–property relationships are utilized for supporting the accurate interpretation of the Raman spectra and their evolution during cycling, and in turn enable the identification of features intrinsic to the structure and dynamics of LiMO2. The spectral trends reveal the occurrence and nature of structural and electronic phase transitions, surface reactions and degradation processes limiting the electrochemical performance of the oxide-based cathodes. This work demonstrates the diagnostic capability of operando Raman spectroscopy for elucidating physical and electrochemical phenomena of LIB electrodes. The holistic approach towards cell development, data analysis and spectrum interpretation had enabled establishing a methodology able to elucidate the properties and dynamics of electrode materials, which carries great potential for further investigating complex processes within the batteries, and for eventually formulating design principles and identifying new strategies for improving LIB performance.