Performance Optimization for Nanoscale Nickel Manganese Cobalt Oxide (NMC) Li-Ion Battery Cathode

Cathode materials with high energy storage capacity, thermal stability and extended cycle life are remaining challenges in Li-ion batteries[1]. Materials that are considered for Li-ion battery cathodes include Li-containing layered oxides, spinel oxides, phosphates, fluorophosphates, and silicates. Requirements are that they must be capable of dedoping lithium during charge, and undergo lithium doping during discharge. Layered LiCoO₂ has been used in the first generation of commercial cathode material introduced by Sony Corporation in 1991. It was selected due to its reversibility, dischargecapacity, charge/discharge efficiency, discharge curve and other properties. The theoretical capacity of LiCoO₂ is 274 mAh/g, however, practically attainable capacity is only 120-130 mAh/g in the potential window of 2.8 to 4.2 V[2]. Other layered materials such as LiNiO₂ and LiMn₂O₄ and their doped derivatives (LiNi_xCo_yO₂ and LiMn_1-xCo_xO₂ are also being investigated because of relatively high capacity (180-200 mAh/g), however the capacity fading and conversion from layered to spinel structure under cycling remain challenging[3].

The mixed oxide LiNi_0.33Mn_0.33Co_0.33 (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[4]. 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[5]. 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.

References:

1. Croy, Jason R., et al. "Designing high-capacity, lithium-ion cathodes using X-ray absorption spectroscopy." Chemistry of Materials 23.24 (2011): 5415-5424.

2. Deb, Aniruddha, et al. "In situ x-ray absorption spectroscopic study of the Li [Ni1∕ 3Co1∕ 3Mn1∕ 3] O2 cathode material." Journal of applied physics 97.11 (2005): 113523.

3.  Armstrong, A. R., A. D. Robertson, and P. G. Bruce. "Structural transformation on cycling layered Li (Mn< sub> 1− y</sub> Co< sub> y</sub>) O< sub> 2</sub> cathode materials." Electrochimica Acta45.1 (1999): 285-294.

4. Matsui, Yukiko, et al. "Charge-Discharge Characteristics of a LiNi 1/3 Mn 1/3 Co 1/3 O 2 Cathode in FSI-based Ionic Liquids." Electrochemistry 80.10 (2012): 808-811.

5. Meethong, Nonglak, et al. "Size-dependent lithium miscibility gap in nanoscale Li1− x FePO4." Electrochemical and Solid-State Letters 10.5 (2007): A134-A138.