| This paper aims to improve the conductivity and lithium ion diffusion rate of lithium iron phosphate battery（LFP）and improve its application value in the field of new energy.Li-Fe,Li-P and Li-O doped LiFePO4,Li Fe7/8Mn1/8PO4,Li Fe P0.97Si0.03O4,LiFePO3.97F0.03,Li0.97Na0.03Fe7/8Mn1/8PO4,Li0.97Na0.03Fe P0.97Si0.03O4,Li0.97Na0.03Fe PO3.97F0.03 were prepared by ion doping method.（LFP,LMP,LFS,LPF,LNMP,LNFS,LNPF）,The electrochemical properties of the materials were studied by means of XRD,SEM and electrochemical tests.The structure,energy band,density of states and lithium ion diffusion were simulated by MS,and compared with the experimental results.The conclusions are as follows:Li-Fe site doping:The lattice volume of LNMP increases from 291.09（?）3 to 292.65（?）3,and the Li+migration channel is broadened to facilitate the deintercalation of Li+.During charging and discharging,the discharge voltage platform of LNMP increases,the charge-discharge polarization decreases,and the performance at high rate is effectively improved.The capacity at 5 C is 80.4 mAh·g-1,which is higher than 72.1 mAh·g-1 of LFP.The long cycle stability of LNMP is greatly improved compared with LFP,and the capacity retention rate is 98.9%.CV and impedance tests show that LNMP has the best electronic conductivity(Rct 125.3Ω)and better lithium ion mobility(8.79×10-15),and the redox potential difference（0.69 V）is also significantly reduced,with good reversibility and reaction kinetics.The simulated crystal structure changes are consistent with the experimental results.The band gaps of LFP and Li7/8Na1/8Fe7/8Mn1/8PO4 are 0.628 e V and 0.314 e V,respectively.The band gap decreases and the conductivity of the material is improved.Na-Mn doping can improve the cycle performance and discharge capacity of LiFePO4 at high current.Li-P site doping:The lattice volume of LNFS increases from 291.09（?）3 to 291.69（?）3,and the Li-O bond length in the lattice is elongated to facilitate Li deintercalation in the lattice.The charge-discharge voltage platform of LNFS is improved,the polarization effect is weakened,and the performance at high rate is obviously improved.The capacity i s 92.1mAh·g-1 at 5 C.The long cycle test showed excellent cycle life with capacity retention of96.3%.CV and impedance tests show that LNFS has the best electronic conductivity(Rct312.1Ω),the best lithium ion mobility(2.7×10-14),the smallest redox potential difference（0.56 V）,and good kinetics provides LNFS with good electrochemical performance.The simulated crystal structure changes are consistent with the experimental results.The band gap of Li7/8Na1/8Fe P7/8Si1/8O4 is 0.512 e V,and the band gap conductivity is the best,which is consistent with the experiment.Na-Si doping improves the electronic conductivity and ion mobility of LiFePO4,and has a great improvement in high rate and cycle performance.Li-O site doping:The lattice volume of LNPF increases from 291.09（?）3 to 291.68（?）3,and the extension of Li-O bond length in the unit cell is beneficial to the deintercalation of Li.When LNPF discharges,the discharge platform is smoother,the spacing is smaller,the polarization effect is weakened,and the rate performance is significantly improved.The capacity of LNPF is 160.2 mAh·g-1 at 0.1 C and 111.4 mAh·g-1 at 5 C.The capacity of LNPF does not change much after 100 cycles,and it still has 98.5%of the initial capacity.CV and impedance tests show that LNPF has the best electronic conductivity(Rct 194.9Ω)and lithium ion mobility(2.7×10-14),and the redox potential difference（0.59 V）is also the smallest,and the conductivity and lithium ion diffusion are significantly improved.The simulated crystal structure change is consistent with the experimental results.The band gap of Li7/8Na1/8Fe PO31/8F1/8 is 0.538 e V,which is significantly reduced compared with LFP,so the conductivity of LNPF is greatly improved.The electrochemical performance of LNPF is better than that of LFP,so Na-F co-doping is a very effective optimization method. |
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