Study on Li-Rich Layered Cathode Material for Li-Ion Batteries

The high-energy-density Li-rich layered materials are promising cathode materials for the next-generation high-performance lithium-ion batteries^[1]. 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 voltage decay caused by an undesired phase transformation during cycling and poor rate capability^[2]. To conquer these issues, the authors applied F doping and surface modification on the pristine Li_1.2Mn_0.54Ni_0.13Co_0.13O₂.

To conquer the poor cycling stability, fluorine doping was conducted on Li_1.2Mn_0.54Ni_0.13Co_0.13O₂. Except for the enhanced capacity retention, F doping shows remarkable effect to stabilize the layered structure by retarding the undesired layered-to-spinel phase transition. As shown in Figure 1, Due to the severe layered-to-spinel phase transformation in the pristine material^[3], the cathodic peak for Mn reduction decays from 3.38V to 2.78V in 100 cycles, accompanied by severe capacity decrease. However, the F doped materials exhibit more stable discharge voltage in 100 cycles. Surprisingly, the Li_1.12Mn_0.54Ni_0.13Co_0.13O_1.92F_0.08 material does not show any new cathodic peak before 50 cycles and even in the 100^thcycle, only a small hump below 3V could be identified, regarded as the emergence of spinel phase. This retarded phase transformation could be validated by the Ex situ TEM analysis of the four electrodes after 100 cycles in Figure 2. By retarding the undesired phase transformation, the lattice distortion and amorphourization of the original layered structure is successfully suppressed as well. The stabilized structure benefits the cycling stability of F doped materials.

To improve the rate capability of the pristine Li-rich layered cathode material, surface modification was 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 from layered phase to spinel phase at the particle surface, as shown in TEM images and FFT results (Figure 3). Figure 4 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 is about 109 mAh·g^-1 at 1C, which is only 43% of the capacity at 0.1C. Compared with the pristine material, the materials modified by NH₄F could exhibit greatly improved rate capability. Particularly, the material modified by 20 wt.% NH₄F has a discharge capacity as high as 172 mAh·g^-1 at 1C, which is over 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 the discharge capacity of the 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 material, which can be attributed to the modified particle surface. Firstly, the spinel shell of the particle provides three-dimensional Li⁺ ion diffusion paths^[4], 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.

In both works, the enhancement in the electrochemical properties of modified materials are comprehensively investigated using Power X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, electrochemical impedance spectroscopy and electrochemical tests.

References:

[1]  M. S. Whittingham, "Lithium batteries and cathode materials," Chemical Reviews, vol. 104, pp. 4271-4301, 2004.

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

[3]  J. W. Min, J. Gim, J. Song, W.-H. Ryu, J.-W. Lee, Y.-I. Kim, et al., "Simple, robust metal fluoride coating on layered Li_1.23Ni_0.13Co_0.14Mn_0.56O₂ and its effects on enhanced electrochemical properties," Electrochimica Acta, vol. 100, pp. 10-17, 2013.

[4]  B. Song, H. Liu, Z. Liu, P. Xiao, M. O. Lai, and L. Lu, "High rate capability caused by surface cubic spinels in Li-rich layer-structured cathodes for Li-ion batteries," Scientific Reports, vol. 3, 2013.