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

ABSTR A C T 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.

Automated summary

The study experimentally investigated the aging invariance of the shape of both half-cell OCP curves of a commercial NMC-811/silicon-graphite cell, revealing that the OCP of NMC-811 remains unchanged while the OCP of silicon-graphite undergoes changes mainly due to a decrease in the relative capacity contribution of silicon. These findings suggest that changes in the shape of the OCP curve of silicon-graphite during cycle aging should be taken into consideration in electrical battery models used for full-cell aging diagnostics and state estimation algorithms in battery management systems.