Research On Synthesis And Modification Of High-voltage Spinel LiNi_0.5Mn_1.5O₄ Cathode Materials

Spinel LiNi_0.5Mn_1.5O₄ cathode material is one of the ideal candidates of layered Li Co O2, due to its high working voltage plateau, high capacity, good cycling life, abundant nickel and manganese sources, eco-friendliness, etc. However, the large-scale commercialized production of this material is restricted by some factors, such as the chemical dissolution of Mn, the effect of Lix Ni1-x O impurities, the decomposition of electrolyte, and so on.In order to improve the electrochemical performance of spinel LiNi_0.5Mn_1.5O₄, we have done some research works. Firstly, we synthesized the LiNi_0.5Mn_1.5O₄ cathode materials via a citric acid-assisted sol-gel method. Then doping technique was used to modify the electrochemical performance of spinel LiNi_0.5Mn_1.5O₄. XRD、SEM、EDS、CV、EIS、galvanostatic charge/discharge tests were used to characterize the physical and electrochemical performances of synthesized materials, respectively. The main content is as follows:（1） The spinel LiNi_0.5Mn_1.5O₄ materials were prepared by a sol-gel method using citric acid as the chelating agent. The effect of the molar ratio of citric acid and lithium, the existence of nitrate, the temperature and time of sintering, the annealing process, the speed of dropping ammonia water on the synthetic process and electrochemical performance of this material were investigated in detail. When the molar ratio of citric acid and lithium was 3:1, we could not achieve the gel but white paste with bad electrochemical performance. However, we could prepare the LiNi_0.5Mn_1.5O₄ with good electrochemical performance via a modified sol-gel method. The LiNi_0.5Mn_1.5O₄ delivered the discharge capacity of 122.7 m Ah g-1 at 0.5 C with good capacity retention of 93.2% after 20 cycles, which was synthesized by sintering at 800℃ for 18 h in the furnace. The speed of dropping ammonia water is one of the key factors to achieving the stable gel system. When conducted at the optimal sintering condition, spinel material with regular octahedral morphology and smooth surfaces could be synthesized.（2） The Li Ni0.5Mn1.5-x Erx O4（0 ≤ x ≤ 0.02）cathode materials were produced by a sol-gel method. The addition of Er3+ would decrease the lattice parameter of spinel LiNi_0.5Mn_1.5O₄ so the crystal structure was accordingly stabilized. In addition, the crystallinity of LiNi_0.5Mn_1.5O₄ was also improved. The addition of appropriate amount of Er3+ could increase the discharge capacity of the spinel and greatly improve the cycling stability. Among these erbium-doped compounds, the spinel Li Ni0.5Mn1.4925Er0.0075O4 presented the best electrochemical performance. When cycled at the rate of 0.5 C, it delivered the initial discharge capacity of 120.6 m Ah g-1 with good capacity retention of 93.0% after 100 cycles. When tested at 0.2 C and 2.0 C, the discharge capacities of the Li Ni0.5Mn1.4925Er0.0075O4 compound were 123.9 m Ah g-1 and 104.7 m Ah g-1, respectively. As decreasing the rate of 2.0 C to 0.2 C, it still achieved 123.9 m Ah g-1 of discharge capacity with excellent recovery rate of 100%. The analysis result of EIS showed that the addition of Er3+ greatly decreased the charge transfer resistance, which made it much easier for lithium ion diffusion in the particles, contributed to excellent electrochemical behaviors.