Theories and Experiments on the Electro-Chemo-Mechanics of Battery Materials

Abstract

Li-ion batteries is a system that dynamically couples electrochemistry and mechanics. The electrochemical processes occurring during battery operation induces a wealth of elemental mechanics such as deformation, plasticity, and fracture. Likewise, mechanics influences the electrochemical processes via modulating the thermodynamics of Li reactions and kinetics of ionic transport. These complex interrelated phenomena are far from being well understood and need to be further explored. This thesis studies the couplings between the mechanical phenomena and electrochemical processes in Li-ion batteries using integrated theories and experiments. A continuum model coupling the kinetics of Li diffusion and kinematics of large elasto-plastic deformation is established to investigate the coupling between Li transport and stress evolution in electrodes of Li-ion batteries. Co-evolutions of Li distribution, stress field and deformation in the electrodes with multiple components are obtained. It is found that the Li profile and stress state in a composite electrode are significantly different from that in a free-standing configuration, mainly due to the regulation from the mechanical interactions between different components. Chemomechanical behaviors of the heterogeneous electrodes in real batteries are further explored. Three-dimensional reconstructed models are employed to investigate the mechanical interactions of the constituents and their influence on the accessible capacity of batteries. Structural disintegration of the state-of-art cathode materials LiNixMnyCozO2 (x+y+z=1, NMC) during electrochemical cycling is experimentally revealed. Microstructural evolution of different marked regimes in electrodes are tracked before and after lithiation cycles. It is found that the decohesion of primary particles constitutes the major mechanical degradation in the NMC materials. Electrochemical impedance spectroscopy (EIS) measurement confirms that the mechanical disintegration of NMC secondary particle causes the electrochemical degradation of the battery. To reveal the reasons for particle disintegration, the dynamic evolution of mechanical properties of NMC during electrochemical cycling is explored by using instrumented nanoindentation. It is found that the elastic modulus, hardness, and interfacial fracture strength of NMC secondary particle significantly depend on the lithiation state and degrade as the electrochemical cycles proceed, which may cause the damage accumulation during battery cycling. Corrosive fracture of electrodes in Li-ion batteries is investigated. Li reaction causes embrittlement of the host material and typically results in a decrease of fracture toughness. The dynamics of crack growth depends on the chemomechanical load, kinetics of Li transport, and the Li embrittlement effect. A theory of coupled diffusion, large deformation, and crack growth is implemented into finite element program and the corrosive fracture of electrodes under concurrent mechanical and chemical load is simulated. The competition between energy release rate and fracture resistance as crack grows during both Li insertion and extraction is examined in detail, and it is found that the corrosive fracture behaviors of the electrodes rely on the chemomechanical load and the supply of Li to the crack tip. The theory is further applied to model corrosive behavior of intergranular cracks in NMC upon Li cycles. The evolving interfacial strength at different states of charge and different cycle numbers measured by in-situ nanoindentation is implemented in the numerical simulation.