Modification Of Spinel LiNi_0.5Mn_1.5O₄ As Cathode Materialfor 5V Lithium-ion Battery

Spinel LiNi_0.5Mn_1.5O₄ has the advantages of high operating voltage（⁴.7 V）,high energy density（⁶58 Wh/kg）,eco-friendliness,abundant supply and low cost.It is considered as one of the most promising cathode materials for the next generation high energy density lithium ion batteries.However,there are three important challenges hindering the commercial application of LiNi0.5Mn1.5O4 spinel.Firstly,small amount of Mn³⁺appears in the crystal during the high temperature calcination process.Its Jahn-Teller effect and disproportionation severely accelerate the capacity fade of the electrode.Secondly,poor rate capability of the LiNi_0.5Mn_1.5O₄ cathode is aroused from low lithium ion diffusion coefficient.Meanwhile,serious oxidation of electrolyte presents a great challenge for successful use of the spinel LiNi_0.5Mn_1.5O₄.To solve the problems above,the dissertation attempts to improve the electrochemical performance from two aspects.One is to prepare superionic conductor/LiNi0.5Mn1.5O4spinel composite cathode material by co-doping of metal cations.The stablility of the crytal structure can be enhanced and Mn³⁺interference inside the material can be eliminated.Meanwhile,the doping elements are designed to produce superionic conductors with co-doping metal cations dispersed in the cathode material,thereby enhancing the diffusion rate of lithium-ion within the lattice,realizing the bi-functional effects of doping cations.The onter one is to surface modification using PVDF as a coating layer on the surface of LiNi_0.5Mn_1.5O₄.PVDF coating layer is able to protect the active material from the erosion of electrolyte.Meanwhile,it can suppress the capacity fade resulted from Mn dissolution.The main contents of this dissertation are as follows:Superionic conductor Li₇La₃Zr₂O₁₂/LiNi_0.5Mn_1.5O₄ spinel composite cathode material was prepared by spray-drying method through La and Zr co-doping and the electrochemical performance of different composite proporions was researched.FTIR,CV and initial charge discharge curves demostrated that La and Zr co-doping can enhance the cation ordering and suppress Mn³⁺.XRD,STEM,line-scan and mapping showed the presence of Li₇La₃Zr₂O₁₂ inside LiNi_0.5Mn_1.5O₄.Among all the composite materials,sample LNMO-50 exhibited the best electrochemical performance.After discharging at 1C for 300 cycles,the retention was 95.9%.When discharged at the rate of 20 C,LNMO-50still remained 84 mAh/g,owning to the high lithium ion diffusion coefficient calculated as1.83×10^-10 cm²/s,which is three times as large as that of pristine LNMO.Superionic conductor Li_0.5La_0.5TiO₃/LiNi_0.5Mn_1.5O₄ spinel composite cathode material was prepared by solid method.Composite proportion with the best performance under this condition was studied.Series of characterizations demostrate that co-doping can enhance the cation ordering and suppress Mn³⁺.Meanwhile,the prescence of Li_0.5La_0.5TiO₃inside LiNi0.5Mn1.5O4 is cearly observed.By comparison,sample LNMO-T20 exhibited the best electrochemical performance.After discharging at 1 C for 300 cycles,the capacity retention was 91.0%.At the rate of 20 C,LNMO-T20 remained 87.2 mAh/g.The improvement of rate performance is due to the high lithium ion diffusion coefficient of9.05×10^-11 cm²/s.At last,we compared composite material with Ti dopped LiNi_0.5Mn_1.5O₄material.Although Ti dopping can stabilize the crytalline structure,it cannot improve the lithium ion diffusion coefficient,manifesting limited effect of single dopping element.To stabilize the pinle LiNi_0.5Mn_1.5O₄ cathode in organic electrolytes,we coat an electrochemically inert PVDF layer on the surface of LiNi_0.5Mn_1.5O_4.PVDF owns the advantage of high thermostability and high anti-oxidation properties,which is able to protect LiNi0.5Mn1.5O4 from the erosion of electrolyte and surpress the decomposition of electrolyte components on the electrode surface.Meanwhile,the coating layer can reduce Mn dissolution into the electrolyte,which benefits to an improvement of the electrochemical performance.It was found the LNMO-P20 showed the best performance.At room temperature and high temperature（55℃）,the capacity retentions were 97.8%and86.1%at 1 C rate,respectively.At 20 C,LNMO-T20 remained 106.4 mAh/g.ICP determination confirmed that PVDF coating layer can effectively reduce the Mn dissolution into electrolyte.The crystal and surficial stability of the LiNi_0.5Mn_1.5O₄ cathode is simultaneously improved.