Study On The High-energy Density Fluorophosphates Cathode Materials Based On Alkali Metal Ion Insertion/Extraction

Nowadays,the fluorophosphates materials?Li₂MPO₄F,M=Fe,Co,Mn?based on alkali metal ion insertion/extraction due to its structure stability,safety,higher working voltage are throught to be able to meet the ever-increasing requirements for high-energy and high-power density cathode materials.Among them,Li₂CoPO₄F is one of the most promising candidates due its high operating voltage?4.8 V vs.Li/Li⁺?and high theoretical capacity of 287 mAh g^-1.In this work,some measures have been taken to improve the performance of Li₂CoPO₄F material,such as selection and optimiziation of synthetic methods,metal ion-doping,coating,synergistic modification and compositing modification.Firstly,the Li₂CoPO₄F material was prepared by hydrothermal method and solvothermal method,then time of hydrothermal treatment and subsequent calcining temperature were further optimized.The studies showed that when the hydrothermal time was 12 h and the calcining temperature was 600?,the synthesized materials harvested higher phase purity and better electrochemical performance.Secondly,the modifications of Li₂CoPO₄F were carried out under the best synthetic methods and technological conditions.?1?Doping modification:The study of Li₂Co_1-x-x Ni_xPO₄F showed that all the intense diffraction peaks of the Ni doped Li₂CoPO₄F materials could be well indexed to the standard map PDF#56-1493,the FTIR result exhibited the stretching vibration peaks of Ni-O and O-Ni-O.The EDS mappings showed that the Ni element was uniformLy distributed in the bulk structure.When the doping amount was x=0.07,the obtained material had better structural stability and reaction reversibility,and had higher discharge specific capacity(133.8mAh g^-1).For Li₂Co_1-x-x Cr_xPO₄F and Li₂Co_1-xV_xPO₄F,the experimental results showed that the modified Li₂CoPO₄F materials produced the characteristic diffraction peaks of the PDF#56-1493.Among them,when the Cr doping amount of x=0.04,the material had a good reaction reversibility in the process of Li⁺ions reversible extraction/insertion.When the doping amount of V was 0.01,the initial discharge capacities were 116.5?67.6?60.7?50.6and 30.2 mAh g^-1 at the rate of 0.1,0.5,1.0,2.0 and 5.0 C,respectively.The discharge capacity was maintained above 96 mAh g^-1 when it came back to 0.1 C after different high discharge cycling.?2?Coating modification:nano-SiO₂ targeted and partial surface modified high voltage cathode material Li₂CoPO₄F had been successfully fabricated via a facile self-assembly process in silica dispersion under ambient temperature.With the aid of polar-OH groups adsorbed on the surface of SiO₂ micelles,the nano-SiO₂ preferentially anchored along the borders and boundaries of Li₂CoPO₄F particles,where protection should be deployed to resist the undesirable interactions between materials and electrolyte.Compared with pristine Li₂CoPO₄F,the SiO₂ selectively modified Li₂CoPO₄F cathode materials,especially LCPF-3S,exhibited desirable electrochemical performance with higher discharge capacity,more outstanding cycling stability and favorable rate capability without any additional carbon involved.NCNF coated Li₂CoPO₄F study exhibited that the bulk structure and purity of the coated materials had almost unchanged and presented two discharge plateaus.The electrochemical properties of the coated materials were better than the pristine material,especially LCPF-5.0NCNF.The discharge capacity of the LCPF-5.0NCNF reached up to 117 mAh g^-1 at 0.1 C after 16different high rate discharge cycles and the capacity retention rate was as high as 90%of its initial capacity.Study of ZnCo₂O₄-coated Li₂CoPO₄F found that the purity of the coated materials had not charged.From the investigation of electrochemical properties,we can see the discharge plateaus of coated materials were obviously widened and showed superior electrochemical performance,such as LCPF-3ZnCo₂O₄ material.The LCPF-3ZnCo₂O₄exhibited higher specific discharge capacity(¹00 mAh g^-1)and more outstanding cycle stability after several different high rate discharge cycling and then backed to 0.1 C.But excessive amounts of modification could bring about unduly thick accumulation of ZnCo₂O₄over the surface of core particle,leading to barely satisfying performance instead.?3?Synergistic modification:Nickel doped Li₂CoPO₄F was successfully synthesized by one-step hydrothermal process,and then the material was modified by in situ NCNF coating.Finally,nano-SiO₂ particles targeted and partial surface modified were introduced into above-mentioned material through a facile self-assembly process.The prepared material had smaller particle size and higher purity.By doping the Ni element which possessed a similar radius and energy level to Co,we could not only improve its intrinsic conductivity,but also elevate the capacity of high-voltage region.Besides,the in situ-carbon coating contributed to the electron conductivity between the material particles,meanwhile,nano-SiO₂ particles targeted partial surface modified can effectively block the vicious interaction between the electrode material and the electrolyte,reduced the occurrence of side reactions with electrolyte in the micro-region of the core material,and improved the stability of the core material.Moreover,the ion adsorption property of nano-SiO₂ facilitated the Li⁺transportation between cathode particles and electrolyte,which formed a synergistic effect with carbon coating,complementary advantages,comprehensively improved the ionic and electronic conductivity,especially Li₂Co_0.93Ni_0.07PO₄F/C/2%SiO₂ material.Finally,LiNaCo_0.5Ni_0.5PO₄F was successfully prepared by hydrothermal method.The study found that material of Li-site composited with Na had poor discharge capacity at different discharge rates.This may be explained by the fact that the LiNaCo_0.5Ni_0.5PO₄F material was unfavorable for the ion extraction/insertion under 1.0 mol·L^-1 LiPF₆/?EC+DMC?electrolyte.However,such composite materials can maintain excellent stability in the latter part of cycling.The result showed that Na-composition Li-site contributed to the enhanced structure stability and improved the irreversibility during charge and discharge cycling.