Study On The Cathode Material Of Fluorine/Magnesium/Copper Ion Modified Lithium/Sodium Ion Battery

As a safe and reliable energy storage device,lithium/sodium ion batteries have been widely known in our daily life.However,the traditional lithium/sodium layered cathode materials are still suffering form the problems of insufficient cycle life,poor rate performance and low energy storage density.In view of the above shortcomings,this dissertation mainly focuses on the research of F^-,Cu²⁺,Mg²⁺doping and fluoride coating to improve the cycle period of lithium/sodium ion layered cathode materials under high voltage and large rate conditions.The research results includ:(1)In this paper,the commercial LiNi_0.5Co_0.2Mn_0.3O₂(NCM)is used as the matrix and modification of NCM by hydrolysis of Li PF₆.By optimizing the content of Li PF₆,the 0.07%modified NCM showed the best electrochemical performance,which can discharge capacity of 179.4 m Ah g^-1after 100 cycles at 0.5 C with the voltage range from 3.0 to 4.5 V and high capacity retention ratio up to 93.7%.When the current density increases to 5 C,the capacity retention ratio can reach 81.1%after 300 cycles,which is much higer than the unmodified samples of 35.1%.When it is applied to the self-made high-voltage electrolyte based on succinonitrile,the material shows better cycle stability and the capacity retention ratio is as high as90.0%and the coulomb efficiency is higher than 99.5%at 5 C after 300 cycles.Besides,the material also shows excellent cycle performance at 3.0-4.7 V.(2)The F^-doping effect on P2 type sodium ion layered cathode materials(Na_2/3Ni_1/3Mn_2/3O₂)is explored.Neutron diffraction(ND)results show that F^-doping induces the redistribution of Ni and Mn ions and promotes the disorder arrangement of Ni and Mn ions.Synchrotron radiation X-ray near edge absorption spectroscopy(XANES)and FIB-assisted electron energy loss spectroscopy(EELS)show that the part of Mn⁴⁺is reduced to Mn³⁺by F-doping.At the same time,the ex-EELS results show that not only Ni²⁺but also Mn³⁺are involved in the electrochemical reaction.In addition,in-situ X-ray spectroscopy(operando XRD)shows that F^-doping effectively suppresses the Jahn-Teller effect of Mn³⁺under low voltage.0.05 mol Fluorine substitution shows the best cycle and rate performance in the voltage range of 2.0-4.0 V.This materials display the high discharge capacity up to 95.4 mAh g^-1at 2 C and the capacity retention ratio is up to 89.5%after 400 cycles.Meanwhile,the modifided samples delivers an extraordinary cycling stability with a capacity retention of 75.6%after 2000 cycles at 10 C and at 55?.When the battery is assembled into a full cell(hard carbon as the negative electrode),this material presents the excellent capacity retention rate as high as 89.5%after 300 cycles at 1 C in 1 M Na PF₆EC/DMC(v:v=1:2)electrolyte with 2 v%FEC additive.(3)The Cu²⁺/Mg²⁺codoped is used to effectively inhibit the phase transition of the material at high voltage of 4.2 V and the relevant research mechanism shows that Mg²⁺doping can increase the oxygen vacancy and Cu²⁺improve the electron-conductive of materials which decrease the overvoltage of battery and enhance the voltage platform.Ex-XRD result display Cu²⁺/Mg²⁺codoped can effectively inhibit the phase transition from P2 to O2,and improve the structural stability of the material at 2.5-4.4 V.This modified samples extraordinary cycle stability with the capacity retention up to 79.4%at 0.5 C after 100 cycles and 68.6%at 1 C after 200 cycles as well as 82.6%at 10 C after 2000 cycles.Besides the material also shows excellent rate performance and the discharge specific capacity at10 C accounts for 58%of 0.1 C which is higher than that of unmodified samples.