Ultrasonic Study of Lithium-Ion Battery Degradation Induced by Transition Metal Dissolution

Transition metal (TM) dissolution is a key degradation pathway in lithium-ion batteries. However, the roles of individual ions in gas evolution and interfacial degradation remain unclear. Here we developed an operando system coupling ultrasound imaging, differential electrochemical mass spectrometry, and electrochemical impedance spectroscopy to simultaneously visualize gas accumulation, identify gas species, and track impedance growth in NMC||graphite pouch cells. Mn2+ markedly lowers the onset potential for electrolyte reduction and shifts gaseous products from C2H4/CO toward H2/CO2, leading to persistent gassing and rapid performance decay. Co2+ causes delayed but substantial gassing, whereas Ni2+ generates little gas despite pronounced impedance growth. We also observe gas reabsorption during rest. However, it does not recover capacity and instead increases the charge-transfer resistance. Overall, the detrimental impact of TM ions follows Mn2+ > Co2+ > Ni2+. The operando methodology developed in this work is broadly applicable to investigating other gas-related battery degradation mechanisms.