Mechanics-based Design and Realization of Electrodes for Electrochemical Energy Storages

Abstract

With the development of nano-sized electrode materials with complex physical and chemical phenomena from atomic scale to macroscale, multiscale mechanics analysis have occupied an indispensable manner for the design and development of energy-related materials. For electrochemical energy storages, the electrodes consist of nano-sized cathode and anode active materials, polymer binder, and conducting agent, which are operated by electrochemical reactions and by involving severe structural distortion, phase transformation, mechanical deformation, and crack generation due to relatively large size of charge carrier (i.e. Li+ and Na+). For these reasons, the migration of the charge carrier affects qualitative and quantitative changes of electrode materials in atomic scale, which inevitably has an effect on the electrochemical performance for the entire system. This is why mechanics-based multiscale investigation and design is indispensable for the development of electrodes in electrochemical energy storages. We present the mechanics-based multiscale framework from first-principles calculations, statistical thermodynamics, surface dissolution, phase field model, finite element method to phase field crack model, which deals with the atomic physics and chemistry including electronic structures, nanoscale thermodynamics and kinetics, particle-level phase transformation, stress generation and mechanical deformation, and finally generation and propagation of cracks. Specifically, this methodology was developed to describe complex phase transformations, which is frequently observed from the electrodes during electrochemical operations, by defining the total free energy functional for the combined-phase reactions. Most of electrodes involving ionic migrations during operations suffer from complicated phase transformations with the combined-phase reactions. By establishing governing equations for phase transformation, mechanical equilibrium, and fracture based on this combined-phase free energy functional, diversely derived formulations can appropriately reflect these combined-phase transformations. Based on this mechanics-based multiscale framework, we investigated and designed major cathode materials for Li-ion battery. For high energy materials with large capacity and high operating voltage, Li-rich oxides were fundamentally investigated, and on the basis of this understanding, O-defected and Cu-doped Li-rich oxides were theoretically designed and investigated, and experimentally realized and validated. For high power materials, conventional spinel and high-voltage spinel oxides were fundamentally understood and modified through bulk and surface doping approaches using Al and Ti in the both calculation and experiments. To unfold the relationship between electrochemical characteristics, and phase transformation, we applied our multiscale method to olivine cathode, and figured out different electrochemical states with respect to degrees of phase separation. For high capacity cathodes, Ni-rich layered oxides were investigated in terms of anisotropic structural distortion, heterogeneous phase transformation, and intrinsic crack generation. Further, Ti doping was suggested to suppress the fracture. Above theoretically designed and experimentally realized cathodes have a meaning of not only the effectiveness of as-developed methodology, but also a foundation of experimental realizations from theoretical design without empirical approaches. The combined-phase free energy functional is determined by thermodynamic calculations from first-principles, and this free energy functional governing fundamental physics and chemistry of the multi-physics system has a various applicability to many levels of computational theories such as the phase field model, electrochemical kinetics, micro-mechanics models, and continuum models for the experimentally undescribed cases due to the complex phase behaviors. Further, the present model could be applied to various energy-related systems such as batteries, capacities, fuel cells, solar cells, and even catalyst for improving their performances and designing modified and new materials.