Analysis of the Active Material Microstructure Constituting the Positive Electrode in Lithium-Ion Batteries Application

Abstract

Our project aims to complete an experimental study supported by computer modeling to bring a better understanding of the impact of the microstructure on electrochemical behavior. Research has shown in particular that the material microstructure is of tremendous importance to fully exploit its electrochemical capabilities. Indeed, a same chemical and crystallographic phase can exhibit vastly different energy densities, power capabilities and capacity retention depending on its microstructure. LiNi1/3Mn1/3Co1/3O2materials (NMC) consisting of different microstructure are used as a model compounds. NMC synthesis is set up via co-precipitation route which allows the adjustment of different synthesis parameters such as the pH, concentrations of reactants and feeding and stirring rates, to obtain different morphologies. It gives the possibility to tune the primary particle crystallinity and morphology from thin nanosized pellets to large micrometric cuboids. The differences in the resulting electrochemical performances are discussed regarding the diffusion of lithium in both solid and liquid phases, the wettability and the electronic conductivity of the material by By coupling electroanalytical techniques, microscopy and computer simulations, we show that the microstructure induces a fundamental difference in the rate-limiting step of the electrochemical process. The microstructure is first qualitatively and quantitatively investigated using mercury porosimetry, helium pycnometry helium pycnometry and FIB-SEM reconstruction. Cyclic voltammetry shows that porous materials behaviour is indeed limited by charge-transfer over a wide range of scan rates whereas dense materials are rapidly limited by solid-state diffusion. The support of modelling allows us to quantify important kinetics parameters such as diffusion coefficients, diffusion length and exchange surface areas and currents. The electronic conductivity investigated using broadband dielectric spectroscopy. This technique allows to separate and treat individually the different relaxations arising from the different length scales: crystallite, nanostructure, and microstructure. It is shown that the electronic conductivity is highly dependent on the microstructure of the primary particles. Flake-shape particles exhibit a higher conductivity, which has an effect at all larger scales. This study shows that the application of electroanalytical techniques must take into account carefully the microstructure of the studied materials and need the support of modelling to fully understand the electrochemical process. Figure 1