Synthesis And Performance Of Spherical Spinel LiNi_0.5Mn_1.5O₄ As High Voltage Cathode Materials For Lithium-ion Batteries

Operating voltage and discharge capacity of the cathode material are two important factors effecting the energy density of lithium-ion batteries. Spinel LiNi0.5Mn1.5O4 is one of the most promising cathode materials due to various advantages, such as its low cost, environmental friendliness, unique 3-dimensional lithium-ion diffusion channels, and relatively high working voltage(ca. 4.7 V). In addition,Recent studies also have shown the great promise of Li-ion batteries based on this material. Although it possesses many advantages, LiNi0.5Mn1.5O4 still encounters many obstacles for commercial applications including low conductivity and poor cyclic performance due mainly to reaction with electrolyte and existence of LixNi1-xO impurity. In order to improve and make use of the spinel LiNi0.5Mn1.5O4, in this dissertation, we focus on the following three contents. Firstly, the spherical LiNi0.5Mn1.5O4 was synthesized via co-precipitation method, and then the as-prepared LiNi0.5Mn1.5O4 was modified by nano-Y2O3 coating through heterogeneous nucleation route. Secondly, the spherical LiMg0.056Ni0.444Mn1.5O4 material with a core-shell structure was successfully synthesized via co-precipitation process. Finally, spherical LiMn2O4 with a novel concentration-gradient shell was design and prepared. The main works are as follows:1. The nano-Y2O3 coated LiNi0.5Mn1.5O4 was successfully synthesized via heterogeneous nucleation route. The effects of nano-Y2O3 coating on structure and electrochemical performance of LiNi0.5Mn1.5O4 are systematically investigated. The results of TEM and XRD demonstrate that the surface of LiNi0.5Mn1.5O4 particle is uniformly encapsulated by nano-Y2O3 coating. The nano-Y2O3 coated Li Ni0.5Mn1.5O4 sample can deliver an initial discharge capacity of 126.1 mAh/g with the capacity retention of 97.7% after 300 cycles at current rate of 1C operated between 3.0 V and 4.9 V, while the pristine LiNi0.5Mn1.5O4 had only 66.5% capacity retention during the same cycles. In addition, the nano-Y2O3 coated LiNi0.5Mn1.5O4 sample exhibits still excellent capacity retention of 91.6% after 100 cycles even at elevated temperature and a rate of 2 C.2. A novel high-voltage spinel cathode material Li Mg0.056Ni0.444Mn1.5O4 with concentration-gradient shell was reasonably designed and successfully synthesized. The electrochemical properties of the LiMg0.056Ni0.444Mn1.5O4 compared to the LiNi0.5Mn1.5O4 are carefully investigated by electrochemical measurements. It has been found that the electrochemical properties of the LiMg0.056Ni0.444Mn1.5O4 material are far superior to those of the LiNi0.5Mn1.5O4 material. The LiMg0.056Ni0.444Mn1.5O4 sample exhibits excellent cycling performance. In the voltage range of 3.0~4.9 V, it delivers a discharge capacity of 126 mAh/g with a retention of 98.9% over 200 cycles at a rate of 1 C(147 mA/g),while the pristine Li Ni0.5Mn1.5O4 had only 85% capacity retention during the same cycles. At 55 ℃, it delivers a discharge capacity of 127 mAh/g with a retention of 99% after 100 cycles in the same condition of charge-discharge. Besides, the LiMg0.056Ni0.444Mn1.5O4 shows a good rate capability, it can still deliver a high discharge capacity of 116 mAh/g even at a rate of 10 C, while the LiNi0.5Mn1.5O4 sample only offers 96 mAh/g at the same conditions.3. Spherical LiMn1.912Ni0.072Co0.016O4 with a novel concentration-gradient shell was successfully synthesized by a co-precipitation route. The electrochemical properties of the LiMn1.912Ni0.072Co0.016O4 compared to the LiMn2O4 are carefully investigated. Electrochemical testing showed that the electrochemical properties of the LiMn1.912Ni0.072Co0.016O4 material are far superior to those of the alone LiMn2O4 core material. The Li Mn1.912Ni0.072Co0.016O4 sample exhibits excellent cycling performance at elevated temperature. It delivers a discharge capacity of 118 mAh/g between 3.0 and 4.4 V vs. Li/Li+ with a retention of 96% over 200 cycles at a rate of 1 C(148 mA/g) at 55 ℃,while the pristine LiMn2O4 had only 72% capacity retention during the same cycles.