Controlled synthesis of concentration gradient LiNi0.84Co0.10Mn0.04Al0.02O1.90F0.10 with improved electrochemical properties in Li-ion batteries

Abstract

For Ni-based core–shell materials, a separation may occur between the coating layers and bulk material in the charge–discharge process because of the phase difference. Therefore, concentration gradient materials have higher cycling stability than core–shell materials. In this study, a concentration gradient material, LiNi_0.84Co_0.10Mn_0.04Al_0.02O_1.90F_0.10, was achieved by an ion exchange method. In the synthesized LiNi_0.84Co_0.10Mn_0.04Al_0.02O_1.90F_0.10, the distribution of the metal elements Al, Ni and Mn is a radial gradient, as confirmed by line scanning on the cross section of a single particle of the sample, whereas Co, O and F show almost uniform distribution. As a cathode material of Li-ion batteries, the capacity retention after 40 cycles of the synthesized LiNi_0.84Co_0.10Mn_0.04Al_0.02O_1.90F_0.10 with low cost is 1.38 times that of LiNi_0.83Co_0.10Mn_0.07O₂. Furthermore, the differential scanning calorimetry result shows that LiNi_0.84Co_0.10Mn_0.04Al_0.02O_1.90F_0.10 demonstrates a much lower amount of exothermic heat release than LiNi_0.83Co_0.10Mn_0.07O₂. In addition, the cycling stability of LiNi_0.84Co_0.10Mn_0.04Al_0.02O_1.90F_0.10 is better than those of LiNi_0.83Co_0.10Mn_0.05Al_0.02O₂ and LiNi_0.85Co_0.10Mn_0.05O_1.97F_0.03 prepared via the same method. All of the abovementioned materials are calcined in air with low cost and show somewhat low discharge capacity. However, for the materials obtained from calcining Ni_0.84Co_0.10Mn_0.04Al_0.02(OH)_1.9F_0.10 in O₂ with higher cost, the discharge capacity increases from 140.3 mA h g⁻¹ for LiNi_0.84Co_0.10Mn_0.04Al_0.02O_1.90F_0.10 calcined in air to 177.5 mA h g⁻¹. To the best of our knowledge, this is the first time that cation Al³⁺ and anion F⁻ have been introduced via an ion exchange method to improve the electrochemical properties of high Ni content cathode materials. This study provides a simple way to synthesize hierarchical micro-spherical high Ni content cathode with a gradient distribution of elements in air.