New opportunities from the microstructural and electrochemical modelling of lithium-ion battery electrodes

Introduction Automotive batteries for electric vehicles require fast discharge capability to guarantee sufficient driving range under complex driving cycles. Understanding the intricate physical and chemical processes occurring within battery electrodes across multiple length-scales is critical to assist the design of electrodes for improved performance. Although excellent modelling studies have been published in recent years, an integrated multi-scale modelling framework capable to assist the design from particle level to cell scale is still missing. Material and Methods Both continuum-based and particle-resolved electrochemical models, fed by microstructural information coming from X-ray nano-computed tomography, are developed, describing transport of charges and species, as well as electrochemical reactions, within active material particles, electrolyte and carbon-binder domain. The models use concentrated solution theory, Butler-Volmer kinetics and mass balance equations, implemented in Comsol Multiphysics across a range of length scales, from pseudo-2D up to 3D geometries. Results At the particle scale, simulations show that crystal orientation and cracks reduce the specific accessible capacity of cathode active materials up to ca. 20 % at C-rates higher than 2. A core-shell double-layer particle design, with primary particles oriented along the radial direction in the outer layer of secondary particles, along with a core of active material with higher specific energy density, is proved to be an ideal particle design for fast discharge capability. At the electrode level, simulations show that the heterogeneous distribution of the carbon-binder domain, which is rarely resolved in 3D models, critically affects the distribution of charges in the electrode. Grading the particle size and the porosity in the through-thickness direction, with smaller particles and larger porosity at the separator interface, enables for both increased accessible capacity beyond 1C and reduces the heterogeneous lithiation of particles, arguably reducing mechanical stresses. Discussion The study shows that coupling microstructural and electrochemical modelling can provide useful guidelines for optimizing the design of lithium-ion batteries. In particular, simulations must coherently and simultaneously capture both particle-level and electrode-level phenomena, paying attention to the heterogeneous distribution of lithiation.