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

• Published in: Journal of power sources 2021-09, Vol.506, p.230240, Article 230240
• Main Authors: Schmitt, Julius, Schindler, Markus, Jossen, Andreas
• Format: Article
• Language: English
• Subjects: Cycle aging; Half-cell open-circuit potential; Lithium-ion battery; NMC-811; silicon–graphite
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2021.230240

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. •Half-cell OCP of Si-graphite and NMC-811 is measured at different aging stages.•Several samples per electrode enable OCP averaging and quantification of deviations.•The shape of the Si-graphite OCP curve changes during cycle aging.•Change in Si-graphite OCP is due to reduced contribution of Si to electrode capacity.•The shape of the NMC-811 OCP curve stays similar during cycle aging.