Intergranular degradation of secondary NMC particles in liquid and solid-state environments

Abstract

The growing demand for high-performance and reliable energy storage systems is driving advancements in cathode optimization in lithium-ion battery technologies. This study investigates particle-scale intergranular mechanical degradation of secondary NMC particles in both liquid and solid-state environments, highlighting the critical role of microstructural evolution, electrolyte medium, and fabrication conditions in influencing different cathode environments. Single-particle electrochemical cycling experiments, single-particle SEM-nanoindentation tests, and continuum modeling were employed to investigate how different cycling conditions impact mechanical integrity and fracture behavior. Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) were employed to numerically assess the mechanical degradation of secondary NMC particles under electrochemical cycling in liquid electrolyte and solid electrolyte environments. Our findings on liquid environments pinpoint the first charge as the critical point of mechanical failure with a complex and non-linear degradation pathway. The particle fracture strength (σ_T) degrades dramatically from a pristine value of ∼220 MPa to ∼50 MPa (about 80%) after the first charge. This is followed by a significant mechanical recovery after the first discharge, where the strength is partially restored to ∼100 MPa. After the first cycle, the particle strength continues to decrease more gradually, reaching about 90 MPa after 10 cycles. Conversely, solid-contact environments demonstrate much better mechanical stability. Despite a lower fracture strength of ∼102 MPa compared to liquid environments after the first cycle, they show a much slower decay, retaining a strength of approximately 95 MPa after 10 cycles. It was also revealed that a composite cathode made with excessive fabrication pressures above 500 MPa may cause secondary NMC particle fragmentation, leading to reduced ionic conductivity and capacity retention. Overall, this work provides a particle-resolved mechanical framework—supported by simulations—for interpreting and mitigating intergranular degradation in secondary NMC across liquid and solid-state environments.