Direct Observation of Overpotential in Li(Ni,Mn,Co)O2 Cathode materials at Single- and Poly-crystalline Levels under Various Discharge Rates

The degradation of capacity in Li-ion batteries during charging and discharging cycles has been a persistent challenge in advancing battery lifespan. Among the commonly accepted causes of capacity degradation is the formation of cracking in NMC cathode materials. These cracks are known to occur along the grain boundaries of polycrystalline Li(Ni,Mn,Co)O2 (NMC) secondary cathode particles from the anisotropic expansion and contraction of the crystal lattice induced by the (de)intercalation of lithium.

To address this issue, single-crystalline NMC particles were introduced, lacking grain boundaries and seemingly less prone to developing cracks. batteries utilizing single-crystalline NMC particles exhibited improved capacity retention over multiple (dis)charging cycles. However, in terms of rapid (dis)charging, these single-crystalline NMC batteries suffered a sudden increase in overpotential compared to poly-crystalline particle batteries. However, a clear explanation for this abrupt rise in overpotential during rapid (dis)charging in single-crystalline particle batteries has remained elusive.

In this study, we examined the overpotential of more than 20 individual single- and poly-crystalline NMC particles under various charging rates using an innovative high-throughput single-particle electrochemistry platform. We observed that larger single-crystalline particles experienced a severe increase in overpotential, unlike poly-crystalline particles. To understand the increase in overpotential in larger single-crystalline particles, we suggest that the formation of cracks in poly-crystalline particles, which were previously considered detrimental to capacity, actually facilitate electrolyte penetration along the grain boundaries and enhance the reaction surface area. Consequently, we propose that these cracks play an essential role in enabling rapid charging and discharging in NMC cathode particles.