Operando X-Ray Analyses of NMC811/Li Batteries: Progress Towards Understanding Capacity Fading Mechanisms at High Voltage and Fast Charge

Abstract

LiNi0.8Mn0.1Co0.1O2 (NMC811) is a commercially successful Li-ion battery cathode due to its high energy density and cyclability. Utilizing this system under fast charge and high potential charge conditions is of great interest as this will help facilitate widespread electric vehicle adoption; however, these aggressive charging conditions lead to enhanced capacity fade. To design next-generation NMC811 batteries with longer life and higher capacity the origins of the capacity fade must be understood. Operando X-ray characterization techniques are critical for this endeavor as they allow the acquisition of information about the evolution of structure, oxidation state, and coordination environment as the material (de)lithiates in a functional battery. Charging to high potential (4.7 V) leads to greater delivered initial capacity but much more significant fading; while fast charging (4C) leads to slightly reduced capacity but with similar fading. Operando XRD and SEM results indicated that secondary particle fracture from increased structural distortions at >4.3 V was a contributor to capacity fade. On the other hand, fast charging led to a slight reduction in structural distortions consistent with the reduced capacity fade. Operando hard XAS revealed significant Ni and Co redox during cycling as well as a Jahn-Teller distortion at the discharged state (Ni3+). Greater bulk redox was observed when charged to 4.7 V while less bulk redox was observed at 4C, consistent with the different delivered capacities. Soft XAS analyses revealed significant surface reconstruction after cycling to 4.7 V. Overall, charging to 4.7 V leads to more structural distortions and particle fracture, as well as surface reconstruction that lead to increased capacity fading. Fast charge does not induce additional structural distortion or surface reconstruction as compared to the slower, moderate charging potential cycling suggesting that the phenomena at the anode may be more critical under fast charge.