Performance Optimization for Nanoscale Nickel Manganese Cobalt Oxide (NMC) Li-Ion Battery Cathode
The mixed oxide LiNi0.33Mn0.33Co0.33 (NMC) has received much attention in recent years because of its higher charging voltage up to 4.7 V, lower cost, low toxicity, good thermal stability and most importantly attainable specific capacity of 200 mAh/g. However, this complex material also suffers from capacity fading during cycling. The capacity reduces from 175 mAh/g to 140 mAh/g after 50 cycles discharge at 1C rate when cycled to 4.5 V. The fading is typically prescribed to either (a) the formation of cathode electrolyte interface (CEI) layer due to the decomposition of electrolyte or (b) the loss oftransition metal ions from the cathode during charging and discharging at high voltage, or both. For LiFePO4 cathode materials it has been demonstrated that smaller particle sizes can improve performance due to shorter diffusion paths and higher surface-to-volume ratios. However several concerns arise when shifting from micron sized to nanoscale battery materials. One is the electrical connectivity of each nanoparticle to the current collector and another is higher electrolyte/particle contact area resulting in increased CEI formation. In this work we explore the effects of particle size(micron and nanoscale) of the NMC cathode materials, the effects of charging potential limits and the effects of the electrolyte additives on the performance and cycle life. Charge/discharge cycling, electrochemical impedance spectroscopy (EIS), x-ray diffractometry (XRD), scanning electron microscopy (SEM), Raman spectroscopy and in-situ x-ray absorption spectroscopy (XAS) techniques are used to understand the lithiation/delithiation mechanisms and capacity fading phenomena in those cathodes with the goal of finding reasonable mitigation approaches to challenges with nanoscale battery materials. The presentation will summarize the results of this investigation and will suggest performance optimization strategy for improved capacity retention innanoscale NMC cathodes.
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