Regulating Single-Crystal LiNiO₂ Size and Surface Coating toward a High-Capacity Cathode for Lithium-Ion Batteries

Single crystals recently received a great deal of attention because the stabilities of cathode materials are improved. One of the major drawbacks of the single-crystal cathodes is that their achievable capacity is lower than that of the same composition polycrystalline cathodes. Although it is widely accepted that the large crystal size of single-crystal cathodes might be the main reason for their low capacity, a systematic study to verify all possible rationales is absent. In this work, we regulated the crystal size of a single-crystal LiNiO2 to investigate its relation to capacity for the first time. It was established that among the sizes studied, a 400 nm-sized single crystal LiNiO2 achieved high capacity, ∼240 mA h/g at 0.1 C, which is comparable to that of its polycrystalline counterpart. It is the first report that such a high capacity is obtained in a single crystal. Also, in our results, with increasing crystal size, a capacity decline was recorded as expected. Interestingly, it is first found that capacity loss occurs only in the high-lithium-composition region (x > 0.8 in LixNiO2), and polarization becomes high only in the same region upon increasing crystal size. This implies that kinetics of the region is significantly affected by the crystal size. Also, high capacity can be achieved in large single-crystal LixNiO2 once the region's kinetics is optimized. In terms of capacity retention, large single-crystal LiNiO2 exhibits the highest stability. Accordingly, high capacity can be achieved when the crystal size is reduced by trading-off its cycling stability. In order to achieve both high capacity and stability, LiF surface coating was conducted on the small single-crystal LiNiO2. It was shown that the LiF coating can effectively protect against capacity degradation, and the capacity retention by such small single-crystal LiNiO2 can be made even better than that of large crystal LiNiO2. Therefore, both high capacity and cycle retention were achieved in single-crystal LiNiO2 by reducing its crystal size and LiF surface coating.