The Effect of Volume Change on the Accessible Capacities of Porous Silicon-Graphite Composite Anodes

Abstract

Silicon-graphite (Si/C) composite anodes are used to increase total anode capacity while maintaining a tolerable degree of active material volume expansion. However, increasing the Si/C ratio does not directly lead to an increase in the accessible capacity because excessive volume expansion can lead to unacceptable cell pressure or electrode porosity. To predict the accessible capacity as a function of Si/C ratio, we integrated mechanical behavior for individual cell components into our previous battery model that couples mechanical and electrochemical phenomena and then simulated a full charge of a pouch cell with foam packing. The simulations were used to determine the anode accessible capacity as a function of Si/C ratio, based on practical pressure and porosity design limitations. The resulting predictions illustrate the tradeoff between the capacity gained by increasing the Si/C ratio and the accessible capacity lost based on the pressures that build up in the cell due to the interconnected mechanical and electrochemical phenomena. For a given set of cell and pack design requirements, battery designers can use these types of simulations as a tool to maximize accessible capacity of an operating cell by selecting appropriate cell/pack materials and identifying optimal Si/C ratios.