Investigating the Electrochemical Effect of Core-Multi Shell Structured Nickel-Rich NMC Cathode Materials

Abstract

Nickel-rich nickel manganese cobalt (NMC) oxide is a high energy density cathode material for lithium-ion batteries. However, surface-related fast capacity fading has been a major obstacle to its practical application. In this work, to address this issue, nickel-rich core-multi shell structured NMC oxide Li[Ni0.9-x-yMn0.05+xCo0.05+y]O2 (x and y ≤0.15) materials with multiple outer layers of stable NMC compositions were synthesized via co-precipitation method using a specially developed continuous stirred-tank reactor (CSTR). We have used scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) with cross-sections of the grided particle samples, and inductively coupled plasma mass spectrometry (ICP-MS) to understand the contribution of core and outer shell layers and total composition of the synthesized core-multi shell structured NMC materials. The synthesized cathodes consisted of high-nickel NMC90/5/5 (LiNi0.9Mn0.05Co0.05O2) core and NMC333, NMC622, and NMC811 outer shells have an overall composition of NMC811 or NMC622 depending on outer shell ratio. The analyzed concentration distribution inside the particles indicates that the nickel composition decreases linearly from the center of the particle to the surface and that the manganese and cobalt concentrations increase. The NMC811 and NMC622 of the core-multi-shell structure with optimized outer shell layer improve cycling capacity, capacity retention (92% after 100 cycles) and rate capability as compared to commercial materials having the same overall composition due to the unique microstructure of the stabilized particle surface by increasing the stable Mn4+ ratio at the shell. Electrochemical impedance spectroscopy (EIS) and differential scanning calorimetry (DSC) results showed that the high stability of core-multi shell structured NMC materials is responsible for the reduction in surface and charge transfer resistance by decreasing the reaction with the electrolyte.