Improved Electrochemical Performance of Multi-Phase Layered-Spinel Cathodes for Li-Ion Batteries

Li-ion batteries are implemented as the major power source for portable electronic devices because of their high energy density and long cycle-life. High capacity cathode materials are essential to increase the energy density of Li-ion batteries within the stability of standard electrolyte solutions. Layered LiNi_1/3Mn_1/3Co_1/3O₂ and high voltage spinel LiNi_0.5Mn_1.5O₄ are promising cathode materials as these can provide specific capacities of 160 and 130 mAh g^-1, respectively. It is known that LiNi_1/3Mn_1/3Co_1/3O₂ exhibits good cycling stability when cycled in the potential range of 2.5-4.3 V vs. Li/Li⁺ [1-3] and undergoes severe capacity fading upon cycling to potentials ≥ 4.5 V [4, 5]. Similarly, spinel LiNi_0.5Mn_1.5O₄ is usually cycled in the potential range of 3.5-4.9 V [6, 7]. When cycled to potential lower than 3.5 V in order to increase the specific capacity, it also undergoes capacity fading upon cycling, due to structural instability arising from a well-known Jahn-Teller effect [8, 9]. Recently, Li and Mn-rich layered compounds with a general composition xLi₂MnO₃.(1-x)LiMO₂ (M=Mn, Co and Ni) are considered as attractive cathode materials for high energy Li-ion batteries as these materials can provide capacities ≥ 250 mAh g^-1. Li-rich layered-spinel composites are also explored as high capacity cathodes which can exhibit capacities ≥ 200 mAh g^-1 within a wide potential range of 2.0-5.0 V [10-12]. Thus, multiphase cathodes may be found advantageous as compared to single phase, i.e., either layered or spinel cathode materials.

  In the present study, we have synthesized layered-spinel composite cathode materials LiNi_1/3Mn_2/3O₂ and LiNi_0.33Mn_0.54Co_0.13O₂ involving Li₂MnO₃ (monoclinic), LiNiO₂ (rhombohedral) and LiNi_0.5Mn_1.5O₄ (spinel) by self-combustion reaction (SCR). The Reitveld analysis and TEM study clearly indicates the presence of these phases. Interestingly, these cathode materials exhibited superior cycling stability when cycled in a wide potential range of 2.3-4.9 V vs. Li (Fig. 1). LiNi_1/3Mn_2/3O₂ exhibited an initial specific capacity of 80 mAh g^-1 which increased to about 220 mAh g^-1 after 20 cycles and then a stable capacity is observed even after 100 cycles. On the other hand, the specific capacity decreases from 190 to 150 mAh g^-1 with 79 % capacity retention for the spinel LiNi_0.5Mn_1.5O₄. Also, LiNi_0.33Mn_0.54Co_0.13O₂ exhibited a stable specific capacity of about 170 mAh g^-1 after 100 cycles when cycled in the potential range of 2.3-4.9 V. On the other hand, the specific capacity of LiNi_0.33Mn_0.33Co_0.33O₂ decreased from 208 mAh g^-1 to a value of 130 mAh g^-1 after only 50 cycles. The structural studies of cycled electrodes indicate that the spinel content in the active mass increases upon cycling due to structural layered-to-spinel transformation. However, the presence of untransformed Li₂MnO₃ in the active mass stabilizes the structure even after cycling in a wide potential range. These results indicate that neither layered nor spinel can be cycled in a too wide potential range while multiphase layered-spinel cathode materials can be cycled in a wide potential range with a stable high specific capacity in Li-ion batteries. Thus, the order of stability of these cathode materials can be presented as layered-spinel> spinel > layered. 

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