3D solid-state batteries use vertical topography to increase both energy and power density relative to batteries with planar geometry. Here we investigate, using finite element simulations, a battery that incorporates a 3D cathode consisting of a vertical array of microcolumns, and a planar 2D anode, which we call a 2.5D battery. By confining the 3D structure to the cathode we greatly simplify the battery fabrication process (i.e., 3D structure only needs to be introduced into one electrode) at the cost of introducing a non-uniform electrolyte thickness in which the anode-cathode distance varies significantly across different parts of the battery. We use simulations to show that a non-uniform electrolyte thickness results in inhomogeneous lithiation and subsequent underutilization of the cathode. The underutilization of the cathode can be avoided by using an electrolyte with sufficient ionic conductivity, which suggests that non-uniform electrolyte thickness can be tolerated in high ionic conductivity electrolytes but should be avoided in low ionic conductivity electrolytes. (C) The Author(s) 2017. Published by ECS. All rights reserved.