Dynamic Electrochemical Processes at Solid-solid Interfaces in Alkaline Ion Batteries

Abstract

This dissertation presents the theoretical, experimental and computational results of studies on the dynamic electrochemical processes at solid-solid interfaces in lithium solid state batteries and in layered sodium transition-metal oxide (NaTMO2) cathodes. More specifically, the term “dynamic” refers to the phenomenon that the kinetics interrupts the thermodynamic reaction pathway herein. For lithium solid state batteries, the study is on the dynamic evolution of the inter-material solid-solid interfaces from a thermodynamically unstable state to a kinetically stable state. For the sodium ion cathodes, the thermodynamic phases and redox evolutions are affected by the intra-particle solid-solid phase boundary kinetics. My contribution to the solid state battery theory follows the mechanically induced metastability and kinetic stability formulated previously in the Li group, which added an energy penalty to the electrochemical decomposition, thus broadening the operating voltage window. Importantly, atomic diffusion is suppressed by the local strain field in the decomposition front and the decomposition is kinetically prevented by the diffusion limiting process. Within this framework, a dynamic voltage stability picture for lithium metal-solid electrolyte interface is depicted by a two-parameter space consisting of reaction energy and critical effective modulus to classify different electrolytes for lithium metal dendrite constriction for long cycling without battery short. The electrolyte needs to have sufficient initial thermodynamic reaction energy with lithium metal, and a low critical modulus to stop the decomposition without cracking the electrolyte. A high throughput calculation-machine learning prediction-experimental synthesis and characterization methodology is used to successfully obtain new materials in the two-parameters space, which is a proof of concept of the dynamic voltage stability. Next, silicon anode with much smaller lithiation capacity in solid electrolyte than in liquid electrolyte is used as a model system for dynamic voltage stability study. Silicon lattice shrinkage and shear during lithiation due to inhomogeneous strain field in solid state battery can shut down lithium diffusion pathway. The constriction sensitivity of lithium capacity and the anodic voltage together guided the search for anode materials to suppress lithium dendrite. Alongside these new understandings, it was realized that the sulfide solid electrolyte we’ve been working on is compliant yet brittle, so that it would have cracked before reaching the GPa level local stress, therefore to keep the local stress small, the kinetic stability should contribute a significant portion to the dynamic voltage stability. The meaning of effective modulus is then broadened to include the kinetic stability. The constrained ensemble computational approach is applied across most types of solid-state electrolytes to systematically evaluate and compare their dynamic stability voltage windows in response to the mechanical constriction effect. High-throughput calculations are used to search for coating materials for different interfaces between sulfide, halide, and oxide electrolytes and typical cathode materials with enhanced dynamic voltage stability. A comparison with experiment is given to highlight the value of these computational predictions. This work sums up the solid state battery part in the thesis. Lastly, the hysteresis of charge-discharge in P2-Na2/3Mg0.205Ni0.1Fe0.05Mn0.645O2 is studied. It is found that the thermodynamic two-phase reaction during charge (sodium de-intercalation) forms a P2-O2 solid-solid two-phase boundary with Schottky barrier to impede electron and Na diffusions in discharge only, thus making the kinetically preferred P2 solid solution phase abnormally coexist with the thermodynamic two-phase region during discharge, which increased the discharge voltage comparing to P2-Na2/3Mg0.28Mn0.72O2. A more general description of comparison between working voltage of thermodynamically or kinetically preferred electrochemical process is given as a criterion for whether the hysteresis will happen. Other two kinetics related cathode studies follow as the end of this thesis. In P2-Na0.75Mn0.6Fe0.2(CuxNi0.2-x)‌O2, better electron conductivity at high voltage phase and better Na ion conductivity at low voltage phase benefits the cycling stability. In LixNa2-y¬TMF7, room temperature Li/Na ion-exchange enables new intercalation Li metal fluoride that is hard to achieve by high temperature sintered thermodynamic phases.