Sodium contamination in electrolyte: Consequences for high power NMC831–graphite lithium-ion cells

Sodium (Na + ) is a common trace impurity in batteries originating from raw materials, manufacturing processes, and/or recycling pathways. In this study, the impact of Na + contamination in LP572 electrolyte on the electrochemical performance of high-power NMC 831 –graphite full cells was systematically investigated. Even at low concentrations (as low as 3 μg/mL), Na + led to a noticeable decrease in discharge capacity and a significant increase in cell impedance compared with cells containing Na + -free electrolyte. The presence of Na + , a larger alkali-metal cation, adversely affects interfacial stability at both electrodes. Degradation phenomena such as intraparticle cracking and graphite exfoliation are observed, likely arising from increased mechanical stress associated with enhanced volume changes during cycling. Although Na + can, in principle, insert into graphite under certain conditions, its larger ionic radius alters both interfacial chemistry and structural evolution within the electrodes. The degradation effects induced by Na + contamination were further analyzed through post-mortem characterization, including morphological analysis and Raman spectroscopy of aged graphite anodes and NMC 831 cathodes.