Studying the Effect of Ca Doping and Li3PO4 Coating on Ni-Rich LiNi0.91Co0.06Mn0.03O2 Ncm Cathode Material for Lithium Ion Batteries
Authors: Sattar Tahir , Jin Bong-Soo , Kim Hyun-Soo 
Published: December 2020

Both environmental and energy concerns have shifted our dependence from fossil fuels to develop advanced renewable energy resources and energy storage devices for the portable electronic devices and electric vehicles [1]. Improvements in energy density, cost and safety are prime requirement for enabling the long range electrical vehicles [1]. Lithium ion batteries got huge attention due to their high discharge capacity and excellent capacity retention. Currently, Li[NixCoyMnz]O2 (NCM) is a promising candidate which has shown great potential in hybrid vehicles and electric vehicles. NCM is consists on three transition metals; Ni provides high discharge capacity but suffers from thermal instability, Co offers better electronic conductivity but at high cost and Mn provides stabilizes the host structure and enhance cycling at the cost of low capacity. Ni-rich LiNi1-x-yCoxMnyO2 (x + y ≤ 0.4 ) cathode materials have garnered a lot of attention for EV applications due to their higher discharge capacity and power density [2]. However, the major problem of Ni-rich NCM is their structural instability which leads to phase transition from layered to spinel to rock-salt structure. Another major issue is the cation mixing; unstable Ni3+ reduce to Ni2+ (0.69 Å) spontaneously, which migrates from transition metal slab (TM-slab) to Lithium slab (Li+; 0.76 Å) due to similarity in their ionic sizes. The dissolution of transition metal ions in the electrolyte (LiPF6-based) is another critical problem [3].
To solve these problems, we exploit CaHPO4 to provide enhance structural stability and protects the surface from side reaction with electrolyte as well. The precursor Ni0.91Co0.06Mn0.03(OH)2 and LiOH were mixed in 1:1.05 ratio and pre-heated at 500 °C - 5 h followed by sintering at 740 °C - 15 h. The modified samples were prepared by mixing Ni0.91Co0.06Mn0.03(OH)2 and LiOH (1:1.05) along with CaHPO4 (0, 0.5, 1, 3 and 5 wt%). The samples were investigated with XRD and SEM. Later on, the electrodes were fabricated and their electrochemical properties were measured.
XRD results confirm the Ca doping by shifting of (003) diffraction peak to the low angle and additional peaks were detected which belongs to the Li3PO4 phase. SEM results show that the particles have sustained their spherical morphology which means that calcium is doped successfully. The 0.5 wt% sample resulted in improved electrochemical studies as compared to the rest. The enhanced properties are originated from optimized amount of doping and coating which results in stabilizing the host structure and protecting the surface.
Ca2+ enhanced the structural stability while Li3PO4 acts as Li-reactive coating to suppress the reaction between the active material and electrolyte. The modified sample offers superior cyclability and enhanced electrochemical performance.

References:

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