Study On Synthesis And Modification Of 5V Cathode Material LiNi_0.5Mn_1.5O₄ For Lithium Ion Battery

5V cathode material LiNi_0.5Mn_1.5O)₄ is considered as one of the most promising cathode materials for next generation of advanced lithium ion battery due to its large capacity and high discharge plateau at 4.7V. These distinctive characters can not only meet the demands of novel power sources from individual electronic consumer products and large-scale power-driven devices, but also accommodate the anode materials with high working voltage for enhancing the safety property of batteries. The key problems which are desiderated to be solved for LiNi_0.5Mn_1.5O)₄ are toimprove the cycling stability and the rate capability. Therefore, this study is focused on preparation and modification of LiNi_0.5Mn_1.5O)₄. Through investigating the influence of preparation technologies on the physicochemical properties and electrochemical performances of the as-prepared material, the growth of LiNi_0.5Mn_1.5O)₄ and the relationship between the structure and morphology of material and its electrochemical performances are explored. By the comparisons on physicochemical properties and electrochemical performances of the materials before and after cycling, the reasons of capacity fading are discussed. Besides, the electrochemical performances of the LiNi_0.5Mn_1.5O)₄ cathode material are enhanced by the new methods of co-doping, carbon coating and diffusion modifications, respectively.The selection of lithium source can give rise to the differences in physicochemical and electrochemical performances of LiNi_0.5Mn_1.5O)₄ material. The product prepared by lithium acetate has smaller particle size and lower crystallinity. In contrast, the cathode material prepared by lithium hydroxide possesses higher crystallinity and better electrochemical performance. Raising the calcination temperature or prolonging holding time results in the increasing tendency of particle size, the enhancement in crystallinity and the increment of Mn³⁺ ion. The results show that the material with high crystallinity has good cyclability, the material with small particle size delivers larger initial discharge capacity while its capacity fading is serious, and the material with higher Mn³⁺ content presents better rate capability. However, it is found that over-high calcination temperature or overlong calcination time results in the new observation of LiNi_0.5Mn_1.5O)₄ decomposition. A decomposition mechanism is proposed.The method of Fe³⁺ and F- ions co-doping is used to modify the LiNi_0.5Mn_1.5O)₄ cathode material. Not only the LiNi_0.4Mn_1.5Fe_0.1O_3.95F_0.05 and LiNi_0.475Mn_1.425Fe_0.1O_3.95F_0.05 materials are prepared, but also the new modification strategies by the syntheses of the Mn-substituted LiNi0.5Mn_1.4Fe_0.1O_3.95F_0.05 and deficiency-introduced LiNi_0.325Mn_1.5Fe_0.1O_3.95F_0.05 modified materials are attempted. The co-doping modifications do not change the crystal structure type of LiNi_0.5Mn_1.5O)₄, but can improve the structural stability and the purity of products. The results show that the co-doping modifications can enhance the capacity retention and rate capability of LiNi_0.5Mn_1.5O)₄ cathode material more or less. The substitutes with Fe³⁺ and F- ions result in the changes of Mn³⁺ content. On the one hand, the Mn³⁺ ion can raise the electronic conductivity, which facilitates the enhancement in electrochemical reactivity of material. On the other hand, the Mn³⁺ ion aggravates the side reaction between the cathode material and the electrolyte, which stimulates the formation and development of the solid electrolyte interfacial (SEI) film as well as consequently hinders the electronic and ionic transfers. Therefore, the modification effect on electrochemical performance of material is attributed to the combination of above two factors. Overall, the modification pattern resulting in the decrease of Mn³⁺ ion in material facilitates the enhancement of capacity retention, and the modification pattern resulting in the increase of Mn³⁺ ion in material facilitates the enhancement of rate capability. LiNi0.5Mn_1.4Fe_0.1O_3.95F_0.05 has the best cycling stability, and its capacity retention is as high as 95.1% after 100 cycles. The discharge capacity at 5C rate of LiNi0.4Mn1.5Fe0.1O_3.95F_0.05 is 110.4mAh g^-1. The LiNi0.475Mn1.425Fe0.1O_3.95F_0.05 presents the outstanding combination property, its capacity retention is 92% after 100 cycles and the discharge capacity at 5C rate is 111.4mAh g^-1. Besides, it is found that the existence of deficiency can increase the diffusion rate of lithium ion in bulk material and the Mn³⁺ ion amount, therefore the material with deficiency displays the best rate capability. LiNi_0.325Mn_1.5Fe_0.1O_3.95F_0.05 can deliver a high discharge capacity of 125mAh g^-1 at 10C rate with the capacity retention of 90.7% after 40 cycles.The carbon coating method is applied to modify the LiNi_0.5Mn_1.5O)₄ cathode material through the thermal decomposition of sucrose, and the influence of sucrose amount on physicochemical property and electrochemical performance of material is systematically investigated for LiNi_0.5Mn_1.5O)₄. It is found that the carbon coating modification does not change the structure of materials and cause the reduction of Mn⁴⁺ ion. The increment of sucrose increases the carbon amount in modified materials, thickens the carbon layer, accelerates the transfer rate of electron and ion as well as increases the agglomeration degree of particle. The carbon coating modification can significantly enhance the cycling and rate performances of cathode material without the adverse effect on discharge behavior. The modified material by 1mass% sucrose exhibits the best electrochemical performance, the discharge capacity of 129.8mAh g^-1 at 1C rate with high capacity retention of 92.8% after 100 cycles and the discharge capacity of 114.2mAh g^-1 at 5C rate are obtained. Through the analysis of EIS characterization, the enhanced electrochemical performance of the modified material is caused by the remarkable suppression of side reactions between the cathode material and the electrolyte as well as the enhancements of electronic kinetics and lithium ion kinetics. The differences of electrochemical improvement originated from the carbon coating with different carbon contents are attributed to the different improvement degrees of the electronic conductivity and the lithium diffusion capacity as well as the different agglomeration degrees of particles.The diffusion method is proposed to modify the LiNi_0.5Mn_1.5O)₄ material. It is found that the diffusion modification allows the chromium oxide to enter into the lattice of material and causes the enrichment of Cr³⁺ ion at the surface of material to form the LiNi0.5-xMn1.5-yCrx+yO4 solid solution. Therefore, the surface structural stability is enhanced. The results confirm that the diffusion modification can significantly promote the cycling and rate stabilities of material, even though the application amount of Cr2O3 is quite low. The diffusion modified material delivers a discharge capacity of 130.6mAh g^-1 at 0.2C rate with high capacity retention of 96.2% after 100 cycles. In addition, after a series of discharge tests at different rates, this modified material can still exhibits 100%, 99.8%, 100%, 99.7% and 99.6% of its initial capacities at corresponding 0.2C, 0.5C, 1C, 3C and 5C rates, respectively. The mechanism of the enhancement in electrochemical performance is investigated through the EIS characterization. Results show that the diffusion modification effectively suppresses the SEI film resistance, indicating that the dramatically improved electrochemical stability is caused by the reduction of reactivity at the interface of active material and the electrolyte...