The high power and energy density of lithium ion batteries have made them one of the most attractive energy storage systems. Spinel type LiMn₂O₄ is considered for a promising material of Lithium ion batteries due to good safety, low cost, abundance, nontoxicity, and high-rate capability. However, poor cycle stability during repeated cycling occurs at room temperature and is more severe at high temperature, originating from the interplay of Jahn-Teller distortion of the lattice ^[1] and manganese dissolution ^[2], and retarding its implementation in commercial products. Lithium-rich layered oxide Li[Li_0.167Ni_0.225Co_0.005Mn_0.558]O₂ possesses larger specific capacity and excellent cycle life at 2.75 ~ 4.2V, but slightly lower in high-rate discharge capability compared to LiMn₂O₄. Therefore, positive active material is the key in Lithium ion batteries for different application, and using mixed positive active material is an effective way to deal with the balance between energy density, power density and electrochemical performance for energy storage systems application.

18650 cells are subjected to evaluation tests using Li₄Ti₅O₁₂ as the anode material, the mixture of LiMn₂O₄ and Li[Li_0.167Ni_0.225Co_0.005Mn_0.558]O₂ as positive active material, 1.2 mol L^-1 LiPF₆ in mixture of EC:DMC:EMC (1:2:2 in weight) as electrolyte. The  rate capability and cycling performance at different temperatures of the cells are investigated. For comparison, the 18650 LiMn₂O₄ / Li₄Ti₅O₁₂ and Li[Li_0.167Ni_0.225Co_0.005Mn_0.558]O₂ / Li₄Ti₅O₁₂ cells are fabricated.

The capacity plots of the three kinds of cells are shown in Fig.1. The voltage platform of Li[Li_0.167Ni_0.225Co_0.005Mn_0.558]O₂ / Li₄Ti₅O₁₂ cell is at around 2.2 V and 2.35 V, be lower than that of LiMn₂O₄ / Li₄Ti₅O₁₂ cell of 2.5 V and 2.55 V, so the voltage platform of the cell using the mixture cathode material (LiMn₂O₄ : Li[Li_0.167Ni_0.225Co_0.005Mn_0.558]O₂ =7:3 in weight) is slightly lower. This system with the mixture cathode material shows excellent capacity retention of 90% after 5000 cycles at 1C and room temperature. In particularly, the cyclic performance of the cells with the mixture cathode material is obviously improved with the capacity retention of 87% after 4000 cycles at 1C and 55 ◦C, compared with that of the LiMn₂O₄ / Li₄Ti₅O₁₂ cell. As shown in Fig.2, the capacity of LiMn₂O₄ / Li₄Ti₅O₁₂ cell is markedly fading before 100 cycles, owing to the manganese dissolution of  LiMn₂O₄ at high temperature. But the fading trend is slower between 1000 cycles an 2700 cycles, and it may be due to the saturation of manganese dissolution, resulting from the high voltage platform of Li₄Ti₅O₁₂ anode material with 1.55 V, unlike the carbon anode, for which significant deposition of  manganese. In contrary, the capacity fading of the cell with the mixture cathode material is gentle, due to the introduction of Li[Li_0.167Ni_0.225Co_0.005Mn_0.558]O₂. Furthermore,  the improved rate performance of the cells with the mixture cathode material also is observed with capacity retention of 85 % at 5 C (0.3C = 100%), compared with that of Li[Li_0.167Ni_0.225Co_0.005Mn_0.558]O₂ / Li₄Ti₅O₁₂ cell. Therefore, the mixture of LiMn₂O₄ and Li[Li_0.167Ni_0.225Co_0.005Mn_0.558]O₂ is a promising alternative material to LiMn₂O₄for lithium ion battery. This work was supported by National 863 Program (No. 2013AA050902) and Shanghai technological innovation (No.14JC1491800).

References

[1] K.Y. Chung, K. Kim, Electrochim. Acta 49 (2004) 3327-3337.

[2] L. Yang, M. Takahashi, B.F. Wang, Electrochim. Acta 51 (2006) 3228-3234.