Study of High-Ni Positive Electrode Materials for Li-ion Batteries

Abstract

Replacing the combustion engine by electric vehicles powered by lithium-ion batteries (LIBs) is a crucial part of the current energy revolution. The advantages of LIBs, long cycle life and high energy density, make them ideal for use in both energy storage and electric vehicles (EVs). Applying a more electrochemically stable coating layer on the surface of positive electrode particles is one way to mitigate the degradation rate of positive electrode materials. In Chapter 3, we introduce a low-cost dry particle fusion instrument built in house. This is an instrument that applies coatings on materials by mechanical force. Suitable coating materials, applied by dry particle fusion at the laboratory scale using this instrument, are effective in improving capacity retention. Chapter 4 reports the successful coating of Al2O3 on a Ni(OH)2 precursor by dry particle fusion followed by heating with LiOH•H2O. This work suggests that coating desired materials on precursors by dry particle fusion is an attractive approach for synthesizing next generation positive electrode materials. Tungsten has been shown to be an effective dopant to improve capacity retention in LiNiO2, and the mechanisms for this effect were studies in Chapter 5. Tungsten doped LiNiO2 was prepared by both dry particle fusion and coprecipitation, in both cases followed by heating with a lithium source. Tungsten was shown for the first time to exist primarily in the grain boundaries between adjacent primary particles within a secondary particle. The tungsten was incorporated in LixWyOz amorphous phases which wet the surfaces of the LiNiO2 grains well and act as a “glue” to improve the mechanical strength of the secondary particles, thus improving their resistance to fracture during calendaring or charge-discharge cycling. Similar studies of tantalum as a dopant were carried out in Chapter 6. It is our hope that this work can provide some helpful information to both industry and academia on how to improve the performance of NMC and NCA materials with high nickel content.