Synthesis,Surface Modification And Electrochemical Performance Of Spinel LiNi_0.5Mn_1.5O₄ Cathode Materials For Lithium Ion Batteries

Lithium-ion batteries?LIBs?have been considered as one of the most important energy storage devices due to their long circle-life,low cost and environmental benignancy and wide application in smart portable devices,such as mobile phones,laptops and digital cameras.The current commercial cathode materials for LIBs?e.g.,LiCoO₂,LiMn₂O₄ and LiFePO₄,etc.?possess low energy density and thus cannot meet the requirements of high energy density and power density for electric vehicles?EVs?and hybrid electric vehicles?HEVs?.Among numerous cathode materials,LiNi_0.5Mn_1.5O₄?LNMO?is one of the most promising cathode candidates for LIBs because of its high operating voltage?4.7 V?,excellent energy density?650Wh/kg?and inherent three-dimensional diffusion channel for Lithium ions.However,the working voltage of LNMO cathode exceeds the voltage window of organic liquid electrolyte,causing the oxidation of electrolyte.The decomposition of organic electrolyte generates diverse organic and inorganic byproducts,which may adhere to the surface of LNMO cathode and yield solid electrolyte interface?SEI?film,leading to the capacity fading.Additionally,HF originated from the decomposition of organic electrolyte will induce the erosion of active material and consequently dissolve transition metals during cycling,resulting in fast degradation of cycling performance,especially at elevated temperature.After surface modification,a separator layer can be formed at the surface of cathode material,which can suppress side reactions by mitigating the corrosion of HF and the dissolution of transition metals,and thus lead to improvement in electrochemical performance.In this thesis,BiFeO₃,LaFeO₃ and Li₂SnO₃ were selected as coating modifiers for Li Ni_0.5Mn_1.5O₄.The LiNi_0.5Mn_1.5O₄@BiFeO₃,LiNi_0.5Mn_1.5O₄@LaFeO₃ and LiNi_0.5Mn_1.5O₄@Li₂SnO₃cathode materials have been prepared and the structure,morphology and electrochemical performance of the materials were studied in detail.The main research contents and results are listed as follows:?1?BiFe O₃-coated LiNi_0.5Mn_1.5O₄ materials were synthesized via a combined co-precipitation and wet chemical method,and the effects of various coating amounts on the structure,morphology and electrochemical properties were studied.All as-prepared materials possess cubic spinel structure with the space group of Fd3m.The coating of BiFeO₃ has no significantly effect on the crystal structure of LiNi_0.5Mn_1.5O₄.A thin BiFeO₃ layers were successfully coated on the surface of LiNi_0.5Mn_1.5O₄particles.The coating of 1.0 wt%BiFeO₃ on the surface of LiNi_0.5Mn_1.5O₄ exhibits a considerable enhancement in specific capacity,cyclic stability and rate performance.The initial discharge capacity of 118.5 mAh g^-1 is obtained for 1.0 wt%BiFeO₃-coated LiNi_0.5Mn_1.5O₄ with very high capacity retention of 89.1%at 0.1 C after 100 cycles.Meanwhile,1.0 wt%BiFeO₃-coated LiNi_0.5Mn_1.5O₄ electrode shows excellent rate performance with discharge capacities of 85.8 and 74.8 mAh g^-1 at 5 and 10 C,respectively,which is higher than that of LiNi_0.5Mn_1.5O₄(77.5 and 60.9 mAh g^-1,respectively).The surface coating of BiFeO₃ effectively decreases charge transfer resistance and inhibits side reactions between active materials and electrolyte and thus induces the improved of electrochemical properties for LiNi_0.5Mn_1.5O₄.?2?LaFe O₃-coated LiNi_0.5Mn_1.5O₄ materials were synthesized by a combined method of sol-gel and wet chemical processes.Structural analysis indicates that all samples are indexed to cubic spinel structure with space group of Fd3m,and the coating of LaFeO₃ has no significant impact on the structure of the materials.The surface modification of LaFeO₃ significantly protects the cathode from the corrosion of HF and alleviates the dissolution of Mn ions into organic liquid electrolyte during?dis?charge processes.The 2.0 wt%La Fe O₃-coated Li Ni_0.5Mn_1.5O₄ cathode exhibits much better cycling stability,rate capability,and elevated temperature stability than the pristine.The capacity retention of 2.0 wt%LaFeO₃-coated material is 97.71%at 1 C after 100 cycles.2.0 wt%LaFeO₃-coated LiNi_0.5Mn_1.5O₄ electrode exhibits excellent rate capability of 111.9 and 99.6 mAh g^-1 at high C-rates of 5 C and 10 C,respectively,which is higher than that of LiNi_0.5Mn_1.5O₄(90.6 and 76.4 mAh g^-1,respectively).Additionally,the LiNi_0.5Mn_1.5O₄ coated with 2.0 wt%LaFeO₃ shows the superior high-temperature cyclability with the capacity retention of 93.29%at 1 C after 100cycles.Present study provides a facile method to mitigate the dissolution of Mn ions into electrolyte and side reactions,resulting in excellent cycling performance and rate capability.?3?A sol-gel method was used to synthesize LiNi_0.5Mn_1.5O₄,and the Li₂SnO₃ was used as coating material.A serial of Li₂SnO₃-coated LiNi_0.5Mn_1.5O₄ cathode materials were synthesized and the effects of Li₂SnO₃ coating on the structure,morphology and electrochemical performance were investigated.Structural analysis indicates that all samples are indexed to cubic spinel structure.After appropriate coating of Li₂SnO₃,the cyclic stability and rate performance have been remarkably improved in the Li₂SnO₃-coated LiNi_0.5Mn_1.5O₄.The 1.0 wt%Li₂SnO₃-coated LiNi_0.5Mn_1.5O₄ cathode exhibits excellent cycling stability with a capacity retention of 88.2%after 150 cycles?0.1 C?and rate capability at high discharge rates of 5 and 10 C,giving discharge capacities of 119.5 and 112.2 mAh g^-1,respectively.Additionally,a significant improvement in cyclic stability at 55 ^oC is obtained after the coating of 1.0 wt%Li₂SnO₃,giving the capacity retention of 86.8%after 150 cycles at 55 ^oC?1 C?.The coating of Li₂SnO₃ obviously protects the cathode from the corrosion of HF and alleviates the dissolution of Mn ions,and thus inhibits side reactions between active materials and electrolyte,leading to the enhancement of electrochemical performance in Li₂SnO₃-coated LiNi_0.5Mn_1.5O₄ cathode materials.