Multiscale Modeling of Li-Ion Cells: Mechanics, Heat Generation and Electrochemical Kinetics.

Abstract

To assists implementing Li-ion battery technology in automotive drivetrain electrification, this study focuses on improving calendar life by reducing degradation due to stress-induced electrode particle fracture and heat generation, and creating models for computer simulations that can lead to optimizing battery design.

To improve the calendar life of Li-ion batteries, capacity degradation during battery cycling has to be understood and minimized. One of the degradation mechanisms is fracture of electrode particles due to intercalation-induced stress. A model with the analogy to thermal stress modeling is proposed to determine localized intercalation-induced stress in electrode particles. Intercalation-induced stress is calculated within ellipsoidal electrode particles with a constant diffusion flux assumed at the particle surface. It is found that internal stress gradients significantly enhance diffusion. Simulation results suggest that it is desirable to synthesize electrode particles with smaller sizes and larger aspect ratios, to reduce intercalation-induced stress during cycling of lithium-ion batteries.

Thermal runaway caused by excessive heat generation can lead to catastrophic failure of Li-ion batteries. Stress and heat generation are calculated for single ellipsoidal particles under potentiodynamic control. To systematically investigate how stress and heat generation are affected by electrode particle shape and cycling rate, a surrogate-based analysis is conducted. It is shown that smaller sizes and larger aspect ratios of (prolate) particles reduce the heat and stress generation inside electrode particles.

Battery scale modeling is required for optimizing battery design through computer simulations. To include the electrode microstructure information in battery scale modeling, a multiscale framework is proposed. The resulting closure terms for macroscopic scale governing equations derived from the volume averaging technique are calculated directly from 3D microscopic scale simulations of microstructure consisting of multiple solid electrode particles and liquid electrolyte. It is shown that 3D microscopic simulations give different values for closure terms from the traditional pseudo 2D treatment. To efficiently exchange the information between microscopic and macroscopic scales, a surrogate-based approach is proposed for scale bridging. The surrogate model characterizes the interplay between geometric and physical parameters, and is shown to be able to significantly enhance the macroscopic model.