Diffusion is modeled in a storage particle where ions and electrons are inserted at non-coincident surfaces. Under a set of simplified but reasonable assumptions, a design rule based on transport properties is derived for the optimal geometry of both the particle and its interfaces with the ion- and electron-providing contact phases. This result can guide the rational design of storage particle dimensions and solid-solid contact geometry in a battery electrode. It can also be used to estimate to what extent the inactive solid mass loading (e.g., carbon or solid electrolyte coating) in an electrode can be reduced without slowing the discharging. However, the guideline should be applied with care, as material-specific factors can cause the discharge behavior to deviate from the model presented here.