Modification And Interface Optimization Of High Voltage Spinel LiNi_0.5Mn_1.5O₄ Cathode Material

High-voltage spinel LiNi_0.5Mn_1.5O₄ material with the operation voltage of 4.7 V,can effectively improve the energy density of battery.However,it also raised concerns on the stability of the electrolyte,the collector and other components when operating at a high voltage.In the process of charging,high concentration and strong oxidation of Mn⁴⁺ and Ni⁴⁺ on the surface of LiNi_0.5Mn_1.5O₄ material will aggravate the interfacial reaction at the electrode surface,which affects the electrochemical properties of the material.Therefore,in order to improve the electrochemical properties of LiNi_0.5Mn_1.5O₄ material and promote the commercial applications,doping strategy and morphology-controlled methodes were employed to enhance the performance of LiNi_0.5Mn_1.5O₄.Morever,optimization of electrode / electrolyte and active material / current collector interfaces were carried out.LiNi_0.5Mn_1.5O₄ materials doped with different types of elements were synthesized using hydroxide as the precursor.When doped with fluorine,the ordering degree and the particle size of the material were increased,and the polyhedral morphology was formed.However,when doped with aluminum,the ordering degree and the particle size of the material was decreased,and the octahedral morphology was formed.Among them,the aluminum doping could significantly improve the rate performance of the material.Its discharge capacity reached 109.8 mAh�g^-1 at 40 C with 82% of 0.5C capacity.In addition,the aluminum and fluorine co-doping could effectively inhibite the decomposition of electrolyte on the surface of the electrode,and reduce the thickness of the interfacial film and the content of LiF,showing the best cyclic performance at 55�C.It was found that the reaction of the electrode under charge and discharge at a rate of 1C proceeded with the coexistence of Li_1.0Ni_0.5Mn_1.5O₄?Li1? and Li_0.5Ni_0.5Mn_1.5O₄?Li0.5?in the low potential plateau,and of Li_0.5Ni_0.5Mn_1.5O₄?Li0.5?and Li₀Ni_0.5Mn_1.5O₄?Li0?in the high potential plateau,as shown by synchrotron radiation in-situ XRD results.Fluoride doping slowed down the kinetics of phase transformation,showing a lower cyclic and rate performance at room temperature.After aging at room temperature,the coexistence of Li1,Li0.5 and Li0 phases in the discharge processwas observed for the fluorine-doped materials.Aluminum-doped material exhibited excellent rate and cyclic performance at room temperature,due to the high rate of Li_0.5Ni_0.5Mn_1.5O₄?Li0.5?phase transformation.LiNi_0.5Mn_1.5O₄ materials with octahedral morphology and porous spherical morphology were synthesized by using hydroxide or carbonate as precursor.Due to the presence of nano particle,the diffusion path of lithium ion for the porous spherical material was shortened,and its rate performance was significantly improved.Among them,the discharge specific capacity of the aluminum and fluoride co-doped porous spherical LiNi_0.5Mn_1.5O₄ material could still reach 119.1 mAh�g^-1 at 40 C.Compared with the porous spherical shape,the material with octahedral shape showed a high degree of cation ordering and low specific surface area,reduced the thickness of the interfacial film and the content of Li F after cycled at 55 �C and exhibited a good cyclic performance at 55 �C.It was found that the reaction of the electrode under charge and discharge at a rate of 1C proceeded with the coexistence of Li_1.0Ni_0.5Mn_1.5O₄?Li1?and Li_0.5Ni_0.5Mn_1.5O₄?Li0.5?inthe low potential plateau,and of Li_0.5Ni_0.5Mn_1.5O₄?Li0.5?and Li₀Ni_0.5Mn_1.5O₄?Li0?in the high potential plateau,as shown by synchrotron radiation in-situ XRD results.Compared with the octahedral material,the porous spherical material exhibited a higher phase transformation rate,resulting ina better cyclicand rate performance at room temperature.In LiNi_0.5Mn_1.5O₄ cell,the corrosion of aluminum foil was obvious after 200 cycles at 55 �C.The graphene coated and carbon coated aluminum foil effectively inhibited the aluminum corrosion.Among them,LiNi_0.5Mn_1.5O₄ electrode using carbon coated aluminum foil exhibited the highest rate performance and cyclic performance.At 40 C,the discharge capacity of LiNi_0.5Mn_1.5O₄ electrode using carbon coated aluminum foil was 76.5 mAh�g^-1 with the platform of 4.1 V.Because carbon coated aluminum foil increased the contact area,while decreased the impedance between the active material and current collector,thus,alleviating the decomposition of electrolyte in a certain extent and improving the cyclic performance of LiNi_0.5Mn_1.5O₄ electrode.3-?1,1,2,2-tetrafluoroethoxy?-1,1,2,2-tetrafluoropropane?F-EPE?and fluoroethylene carbonate?FEC?were added as co-solvent into high voltage electrolyte.The electrolyte mixed with FEC and F-EPE had a high oxidation resistance.After the addition of fluorine containing solvent,the discharge specific capacity of LiNi_0.5Mn_1.5O₄ was reduced,but the cyclic performance and the coulombic efficiency were improved.The capacity retention was 95.0% and coulombic efficiency was 99.1% when using the electrolyte mixed with FEC and F-EPE after 200 cycles at 55 �C and 1C.These results showed that the electrolyte with F-EPE and FEC was helpful to reduce the side effect of the electrode / electrolyte,and form a stable interfacial film,which is of significance to improve the coulombic efficiency during charge-discharge process.