Polycrystalline and Single Crystalline NCM Cathode Materials—Quantifying Particle Cracking, Active Surface Area, and Lithium Diffusion

Representatives of the LixNi1−y−zCoyMnzO2 (NCM) family of cathode active materials (CAMs) with high nickel content are becoming the CAM of choice for high performance lithium-ion batteries. In addition to high specific capacities, these layered oxides offer high specific energy, power, and long cycle life. Recently, the development of single crystalline particles of NCM has enabled even longer lifetimes due to achieving higher Coulomb efficiencies. In this work, the performance of NCM materials with different particle size and morphology is explored in terms of key parameters such as the charge-transfer resistance and the chemical diffusion coefficient of lithium. Cracking of secondary particles leads to liquid electrolyte infiltration in the CAM, lowering the charge-transfer resistance and increasing the apparent diffusion coefficient by more than one order of magnitude. In contrast, these effects are not observed with single-crystalline NCM, which is mostly free of cracks after cycling. Consequently, severe kinetic limitations are observed when cycling large "uncracked" secondary particles at low potential and capacity. These results demonstrate that cracking of polycrystalline particles of NCM is not solely detrimental but helps to achieve high reversible capacities and rate capability. Thus, optimization of CAMs size and morphology is decisive to achieve good rate capability with high-nickel NCMs.