Change in the half-cell open-circuit potential curves of silicon–graphite and nickel-rich lithium nickel manganese cobalt oxide during cycle aging

The relationship between the degree of lithiation and open-circuit potential (OCP) of the half-cells of lithium-ion batteries is mostly regarded to be invariant during battery aging. In electrical cell modeling, the OCP curve of aged half-cells is therefore usually obtained by linear scaling of OCP curves measured for pristine electrodes. In this study, the aging invariance of the shape of both half-cell OCP curves of a commercial NMC-811/silicon–graphite cell is investigated experimentally. Full-cells are cycled until different degradation levels are reached. Subsequently, several electrode samples are extracted and the OCP of both electrodes is measured using coin-cells containing electrode samples as working electrode and lithium metal foil as counter electrode. Changes in half-cell OCP are analyzed using differential voltage analysis and incremental capacity analysis. The OCP of the NMC-811 does not change with aging, while the OCP of silicon–graphite exhibits changes which are mainly due to a decrease in the relative capacity contribution of the silicon. The main consequence of our findings is, that changes in the shape of the OCP curve of silicon–graphite during cycle aging should be considered in electrical battery models which are used for full-cell aging diagnostics and state estimation algorithms in battery management systems.