Pursuing Better Rate Performance of Li-Rich Layered Cathode Materials for Li-Ion Batteries

The high-energy-density Li-rich layered materials are promising cathode materials for the next-generation high-performance lithium-ion batteries¹. They have attracted a lot of attentions due mainly to their high reversible capacity of more than 250 mAh·g^-1 at low charge-discharge current. However several drawbacks still hinder their applications, such as the poor rate capability². 

To conquer this critical issue, the present study is focused on surface modification of Li-rich layered cathode materials to improve their rate capability as well as maintain the high capacity retention of the pristine material. Surface treatment is conducted on Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ using NH₄F by thermal annealing at low temperature. Material characterizations reveal that the modification process triggers fluorine doping and phase transition (Figure 1) from the layered phase to a spinel phase at the particle surface. Figure 2 shows the rate performances of the pristine Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ and the modified materials. The pristine material could deliver a high reversible capacity of about 250 mAh·g^-1 at 0.1C (25 mA·g^-1). However, its discharge capacity decreases to 109 mAh·g^-1 at 1C, which is only 43% of the capacity at 0.1C. Compared with the pristine material, both the materials modified by 5 wt.% and 10 wt.% NH₄F can deliver a discharge capacity over 140 mAh·g^-1at 1C, which is more than 70% of their discharge capacities at 0.1C. Particularly, the material modified by 20 wt.% NH₄F has a discharge capacity as high as 172 mAh·g^-1 at 1C, which is about 87% of its capacity at 0.1C.  Moreover at even higher rate like 5C, the discharge capacity of the material modified by 20 wt.% NH₄F still can reach 126 mAh·g^-1 while that of pristine one is only 41 mAh·g^-1. Generally, the NH₄F modified Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ exhibits greatly improved rate performance and satisfactory cycling stability compared to the pristine one, which can be attributed to the modified particle surface. Firstly, the spinel shell of the particle provides three-dimensional Li⁺ ion diffusion paths³, which creates fully opened surface, enabling fast Li⁺ ion transfer at the electrode/electrolyte interface. Secondly, the formation of spinel shell prevents the Ni segregation at the surface, thus suppressing its negative effect on Li⁺ ion diffusion. Finally, the fluorine doped spinel surface improves the surface stability during wide-voltage-range charge-discharge process, resulting in improved cycling stability.

The enhancement in the electrochemical properties of the modified materials as a function of the NH₄F amount is comprehensively investigated using powder X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and electrochemical tests.

References:

1. Whittingham, M. S., Lithium batteries and cathode materials. Chemical Reviews 2004, 104(10), 4271-4301.

2. Thackeray, M. M.; Kang, S.-H.; Johnson, C. S.; Vaughey, J. T.; Benedek, R.; Hackney, S. A., Li₂MnO₃-stabilized LiMO₂ (M = Mn, Ni, Co) electrodes for lithium-ion batteries. Journal of Materials Chemistry 2007, 17(30), 3112.

3. Song, B.; Liu, H.; Liu, Z.; Xiao, P.; Lai, M. O.; Lu, L., High rate capability caused by surface cubic spinels in Li-rich layer-structured cathodes for Li-ion batteries. Scientific reports 2013, 3.