Using a Zero‐Strain Reference Electrode to Distinguish Anode and Cathode Volume Changes in a Solid‐State Battery

Abstract Solid‐state battery electrodes expand and contract during cycling (“cell breathing”), which can cause cracking of the active material particles. This lowers their electronic conductivity, leading to inactive regions, and is the main cause of capacity fading for many electrode chemistries, including nickel‐rich LiNi_1‐x‐yMn_xCo_yO₂ (NMC). The active material volume change because of (de)lithiation is known as the reaction volume and can be calculated from the change in open‐circuit potential when varying the applied pressure. In order to determine the reaction volume in this way for both anode and cathode simultaneously, a “zero‐strain” reference electrode (i.e., without volume changes) must be used, as its potential remains constant with varying pressure. Herein, the study uses a reference electrode based on Li₄Ti₅O₁₂/Li₇Ti₅O₁₂ to measure the reaction volume for both electrodes in solid‐state battery cells. For a graphite||NMC cell, the total pressure change measured during cycling can be separated into the individual contributions of graphite and NMC. Graphite is responsible for most of the cell breathing, except at the end of charge, where the NMC contraction counteracted the graphite expansion, leading to a pressure plateau. This method reveals detailed information about the breathing of the individual electrodes without the need for complementary characterization techniques.