Swelling and Elastic Deformation of Lithium-Silicon Electrode Materials

Lithium-battery electrodes made from silicon increase volume by a factor of four when charging from a de-lithiated to fully lithiated state, but by charging and discharging slowly enough, stresses can be reduced, which avoids stress damage to the electrode. We provide a mathematical description comprehending the relevant solid mechanics (i.e., stress and strain) and diffusion processes so as to describe highly expanding battery electrode materials and determine the maximum current at which lithium-silicon can be charged and avoid exceeding the material yield stress. An approximate metric for acceptable stress levels is that they are less than 1% of the elastic modulus. At these low stress levels, infinitesimal strain theory can be used for the computations. The rate of charge and discharge that keeps stress below this level depends on values for the elastic modulus, the Poisson ratio, the lithium diffusion coefficient, the particle size, the open-circuit potential of the material, and several other parameters whose values are generally concentration dependent. The derived formulas are analytic and straightforward to apply (e.g., they can be employed in conventional spreadsheet software) and can be used to assess potential new materials, design better electrodes, and improve operating strategies. (C) 2016 The Electrochemical Society. All rights reserved.