title: Interface Simulation of All-Solid-State Lithium-ion Thin Film Battery
Creators: name: Liu, Yao
Publisher: name: UNSPECIFIED
Year: 2022

description: Lithium-ion batteries have attracted extensive research attention in the past decades, and have become the premier energy storage technology due to their high energy density and long cycling life. Up to date, commercial lithium-ion batteries heavily rely on liquid organic carbonate electrolytes. Nevertheless, liquid electrolytes can trigger the explosion after the thermal runaway in batteries, and are electrochemical unstable at high voltage. To address these issues, all-solid-state batteries (ASSBs) are widely researched as promising alternatives. Even though ASSBs show impressive merits in comparison to the conventional LIBs, the solid/solid interface remains one of the main bottlenecks that limit currently its application. Particularly, the influence of the space charge layer on the total interface impedance remains controversial in the community. Different theoretical electrochemical models have been employed to help understand the solid/solid interface in ASSBs. The electrochemical models in the literature have paved a solid fundament and have helped to gain important insights on the electrochemical behavior of the interface, but can be improved in different aspects. For instance, the exchange current is assumed to be an important input parameter in these models and is widely employed in the dynamic study of ASSBs. But it is difficult to experimentally determine the exchange current. The first contribution of this thesis is devoted to intrinsic interface equilibrium study, which allows both the determination of exchange current and the interface resistance from fundamental material properties. Thereby, an advanced electrochemical model was proposed on the basis of the Planck-Nernst-Poisson (PNP) and the Frumkin-Butler-Volmer (FBV) theories. In particular, it takes the electrical double layer (EDL) structure and the unoccupied regular lattice sites (vacancies) into account, as lithium-ion migration at the solid/solid interface is limited by the available lattice sites. The model is implemented using the finite element method and applied to simulate a model thin-film half-cell consisting of LiCoO2 as cathode and LiPON as solid electrolyte. Numerical results based on this model have demonstrated its capability and are verified well against theoretical and experimental results. They show that vacancies play an important role in the concentration and the electrostatic potential distributions in the space charge layer region. The influence on concentration and electrostatic potential by the different EDL structures, the state of charge (SOC), and the diffusivity are also investigated through a comprehensive parameter study. A few conclusions can be drawn, e.g., the total electrostatic potential drop is only related to the free enthalpy difference of materials, even with the different electrical double layer structures. Furthermore, the charge transfer resistance with the diffuse double layer structure is higher than that with the compact double layer. Besides the interface equilibrium, the thesis also delivers a comprehensive ASSBs interface impedance study based on the proposed electrochemical model. One important new aspect is thereby the consideration of the activation energy of materials in the reaction kinetics. It allows the subsequent results for both electrolytes and battery half-cells, such as new equivalent circuit models and extended analytical results directly linked to material properties. Owing to the advantages of the proposed modified Planck-Nernst-Poisson (MPNP) model, the space charge layer impedance investigation has been carried out here. Compared to the conventional model, we provide a new analytical solution for the space charge layer capacitance because of the vacancy effect. Moreover, due to the charge accumulation or depletion in the space charge layer, a constant resistance has been considered in some equivalent circuit models. Nevertheless, the charge density in the space charge layer region should be frequency-dependent with the perturbation potential. Consequently, a new space charge layer resistance is introduced in the corresponding equivalent circuit model. Results indicate that our new model can explain well the experimentally observed impedance tail at the low-frequency region. Additionally, the quantifications of the circuit elements are presented based on material properties. Thereafter, the proposed model has been employed to investigate ASSBs impedance with consideration of the reaction kinetics determined by the free enthalpy difference. Additionally, we derive from the electrochemical model a comprehensive equivalent circuit model with all elements are quantified from material properties. Results show that the high-frequency semicircle in the impedance spectroscopy attributes to the bulk impedance and is associated with ion migration. Moreover, the plots at low and medium frequencies are assigned to the charge transfer resistance and the space charge layer capacitance. Moreover, batteries with a higher free enthalpy difference lead to a significant decrease of the charge transfer resistance, but, increase the total electrostatic potential drop across the interface. This thesis provides not only an advanced electrochemical model for ASSBs, but also an in-depth understanding of the space charge layer and the interface impedances. The knowledge obtained is general and can be applied for high-performance batteries investigation.