Synthesis And Electrochemical Performance Of Li?Mn,Ni?₂O₄-based Spinel Cathode Materials For Lithium Ion Batteries

With the increasing demand for the energy storage in portable electronics,electric vehicles and hybrid electric vehicles,lithium-ion batteries?LIBs?have attracted considerable attention as the most viable and eco-friendly energy storage devices.At present,the current commercial cathode materials,layered LiCoO₂ and polyanion materials LiFePO₄,are still unsatisfactory for the requirements of high energy density.Thus,there is an urgent need to develop new cathode materials for LIBs with high energy density and high capacity.Due to its high working potential?4.7V vs.Li/Li⁺?,high energy density and low cost,spinel LiM_xMn_2-xO₄ material has attracted great attention in recent years.However,a series of side reactions between active materials and electrolyte easily lead to the destruction of crystal structure during high current density and high temperature cycling,and cause poor electrochemical properties.In this thesis,sol-gel and conventional solid-state reaction method have been chosen to synthesize spinel LiM_xMn_2-xO₄ materials,and the effects of the doping of Ni ion,the coating of LiCo O₂,and complex modification of Li₂SiO₃ on the phase structure,microstructure and electrochemical performance of the materials have been detailedly investigated.The main content and results are listed as follows:?1?High-voltage and high-performance Ni-doped LiMn₂O₄-based composite cathodes have been obtained from the nominal formula of Li₂Mn_1-xNi_xSiO₄ via a citric acid-assisted sol-gel method.The spinel Li?Mn,Ni?₂O₄ and layered Li₂SiO₃ coexist in the composites with x=0 and 0.05,while the materials with x=0.15 and 0.25 are consisted of Li?Mn,Ni?₂O₄,Li₂SiO₃ and layered LiNiO₂;at x?0.35,the impurity phases of NiO and Ni₆MnO₈ are detected.A significant improvement in discharge capacity and rate performance of the materials has been caused by the simultaneous existence of LiNiO₂ and Li₂SiO₃.The composite with x=0.25 gives a very high initial discharge capacity of 168 mAh g^-1 in the potential range of 3-5 V and exhibits the excellent rate performance.Our study shows that the composites consisted of Li?Mn,Ni?₂O₄,Li₂SiO₃ and LiNiO₂ may be promising candidates for high-voltage and high-performance lithium-ion batteries.?2?Li₂SiO₃,a layer-structured fast Li⁺-ion conductor with three-dimensional Li⁺paths,has been firstly introduced into LiNi_0.5Mn_1.5O₄ to form novel cathode composites of?1-x?LiNi_0.5Mn_1.5O₄·xLi₂SiO₃ via a citric acid-assisted sol-gel method.The effects of Li₂SiO₃ on the phase structure,morphology and electrochemical performance of the materials were investigated.As x increases,the structure and electrochemical properties of the materials are tailored.The cathode material with x=0.10 delivers ultrahigh initial discharge capacity¹50.3 mAh g^-1,which is 24.4%larger than that of the pristine LiNi_0.5Mn_1.5O₄(120.8 mAh g^-1).The Li₂SiO₃-composited LiNi_0.5Mn_1.5O₄ materials present an enhanced cycling stability,better rate performance and lower charge transfer resistance.These excellent electrochemical properties indicate that the compositing of fast Li⁺-ion conductor Li₂SiO₃ is an effective method to enhance the electrochemical performance of LiNi_0.5Mn_1.5O₄-based cathode materials.?3?A simple solid state method was utilized to synthesize excellent spinel Li₂SiO₃-modified LiNi_0.5Mn_1.5O₄ cathodes.Dual-modification of coating and doping of fast-ion conductor Li₂SiO₃ leads to stable crystal structure and additional three-dimensional?3-D?channels for Li⁺-ion diffusion,and protects the electrode surface from electrolyte attack.The Li₂SiO₃-modified material possesses coexistence of ordered P4₃32 and disordered Fd-3m phases.As a result of improved structure,morphology and conductivity,the Li₂SiO₃-modified LiNi_0.5Mn_1.5O₄ shows more superior cyclic performance,rate capability and high temperature stability than the pristine:capacity retention of 95.17%at 0.1 C after 100 cycles vs.that of 79.23%;rate capability of 130-89.1 mAh g^-1 at 0.1-10 vs.that of 120-57.6 mAh g^-1;and high temperature capacity retention of 99.42%at 0.1 C from 6 to 50 cycles vs.that of79.59%.Our study provides a facile approach to stabilize crystal structure,optimize morphology and conductivity,and thus enhance electrochemical performance of LiNi_0.5Mn_1.5O₄-based materials.?4?A core-shell structured LiNi_0.5Mn_1.5O₄@LiCoO₂ cathode material has been successfully synthesized by the combination of sol-gel and solid state methods.The coating of LiCoO₂ has a significant effect on the electrochemical performance of spinel LiNi_0.5Mn_1.5O₄-based cathode material,especially cycling stability at high temperature and rate capability.After modification,the ionic conductivity of the material is greatly improved due to the high ion conductivity of LiCoO₂.The LiNi_0.5Mn_1.5O₄@LiCoO₂with 1%LiCoO₂ presents the optimal rate capability and devilers a relatively high discharge capacity of 122 mAh g^-1 at 10 C.On the other hand,the surface coating of LiCoO₂ can effectively facilitate Li⁺interfacial diffusion,and alleviate the side reactions between active material and the electrolyte;as a result,the capacity retention of 96.17%for LiNi_0.5Mn_1.5O₄@LiCoO₂ electrode with 1%LiCoO₂ is much higher than that for the bare LiNi_0.5Mn_1.5O₄?74.93%?after 100 cycles at elevated temperature.