Design of Fast-Charging SiO
x
/Graphite Composite Anodes Via Three-Dimensional Electrochemo-Mechanical Simulations
Authors: Yang Soo Young , Shin Hong Rim , Kim Siwon , Lee Jong-Won 
Published: December 2024

Enhancing the fast-charging capability of high-energy density batteries is an urgent need for the evolving electric vehicle market. Owing to high theoretical capacities of SiO
x
, SiO
x
/graphite composite anodes have garnered great attention for anodes in Li-ion batteries. Higher lithiation potentials of SiO
x
than that of graphite are beneficial for mitigating Li plating, which is the primary problem under fast-charging conditions. Furthermore, the SiO
x
/graphite composite anode can reduce cell polarizations due to its reduced thickness compared to that of the graphite anode. However, the significant volume expansion of SiO
x
upon lithiation poses a challenge to electrode integrity, thereby inhibiting stable cycling. During fast-charging, particularly, the inhomogeneous reaction in the composite anode results in non-uniform volume changes of SiO
x
particles, causing the development of mechanical stress and the rapid capacity fading.

In this study, the reaction kinetics of SiO
x
/graphite is investigated to design the composite anode with mechanical stability and high electrochemical performance through the three-dimensional (3D) electrochemo-mechanical simulation. We perform simulations on SiO
x
/graphite composites with various compositions and SiO
x
distributions to determine the amount of Li plating, anode potentials, and local current densities. In addition to the electrochemical properties, the distributions of mechanical stress and strain within the composite electrode are calculated.

The 3D model geometry for the composite anode is stochastically generated based on the result of microstructural analyses. 3D electrochemo-mechanical models are constructed to predict the reaction kinetics in the composite anode. Simulation results show that SiO
x
particles in the composite anode preferentially react with Li, mitigating the anode potential drop during fast-charging. Also, the composite anode exhibits a more homogeneous distribution of polarization, compared to that of the graphite anode. The mechanical simulation demonstrates that difference in the volume expansion along the thickness increases as the charging proceeds. The predicted significant displacement (strain) at the separator/anode interface is attributed to the preferential reaction of SiO
x
particles near the separator. This leads to localized mechanical degradation of the composite anode.

Furthermore, a bi-layer electrode design with different concentrations, porosity, and particle size of SiO
x
on top and bottom is suggested through 3D simulation to mitigate mechanical fatigue while maintaining high energy density during fast-charging. The improvement in fast-charging performance is confirmed through electrochemical experiments with the proposed bi-layer composite anode. Both the prevention of Li plating and the reduction of stress distribution are achieved using the bi-layer design. This study offers strategic insights into fast-charging, energy-dense anodes for the development of advanced Li-ion batteries.