Theoretical Calculations And Experimental Modification Of Doped Lithium Iron Phosphate

With the growing energy demands and shortage of natural energy,Lithium-ion batteries have been widely used in electronic devices,electric vehicles and wind energy storage power plants.Olivine LiFePO₄ has been one of the promising power battery cathode materials for large-scale applications due to the high theoretical capacity of 170 mAh/g,high operating voltage?vs Li/Li+?,excellent thermal stability and environmental friendly.But the poor conductivity and lithium ion diffusion coefficient limit its high power applications.Therefore,Li FePO₄ cathode materials have been synthesized by solid-state method in this paper.In order to obtain optimal process conditions,the effects of synthesis temperature and sintering time on the structure and electrochemical performance of Li FePO₄ cathode materials have been explored.Based on this,we have researched the effects of doping on the LiFePO₄ cathode materials by experiments combined with the first-principle calculations.In this paper,the effects of synthesis temperature and sintering time on the structure and electrochemical performance of Li FePO₄ cathode materials have been explored.The results show that the Li FePO₄ cathode material with smaller particle size and more uniform particle size distribution has been synthesized under 400? for 6h and then 750? for 15 h,which exhibits excellent electrochemical performance.With the initial discharge capacity of 153.2 mAh/g at 0.1C in the voltage of 2.5-4.1V.Cation doping is considered as one of the most effective ways to improve conductivity and lithium ion diffusion coefficient.Herein,in this paper,the effects of doping on the lattice structure,electronic structure and electrochemical performance of Li FePO₄ cathode materials have been researched based on the first-principle calculations.The calculation results indicate that the band gap of Li FePO₄ has been decreased through doping?Na,K,Mn,V?,which improves the conductivity and lithium ion diffusion coefficient of the materials.Anhydrous sodium carbonate,anhydrous potassium carbonate,C4H6MnO₄·4H2O and V₂O₅ used as doped raw materials respectively,the doped Li FePO₄ cathode materials were prepared successfully via solid-state method.The better electrochemical performance of Li0.99Na0.01FePO₄ further demonstrates that an appropriate amount of Na doping could be beneficial to improve the cycling stability and rate capacity.The electrochemical impedance spectroscopy?EIS?results indicate that the charge transfer resistance of the sample decreases,corresponding to enhanced electrochemical active.Among the Li_1-xK_xFePO₄?x=0,0.005,0.01,0.02?samples,Li0.995K0.005FePO₄ shows the higher rate capacity and capacity retention of 96.4% after 100 cycles at 1C rate.The calculation and experimental results prove that the improved cycling stability and rate capacity are ascribed to expanded Li ion diffusion pathway and conductivity by K doping.The results of Mn-doped Li FePO₄ demonstrate that the higher discharge voltage plateau could be attributed to introduction of Mn.And the Li Fe0.5Mn0.5PO₄ exhibits the best cycling stability,corresponding 95.6% capacity retention after 100 cycles at 1C.Among V-doped LiFePO₄ samples,the sample doped with 2% V₂O₅ shows superior cycling stability and rate performance,which can be owing to increased conductivity by V.Based on the above studies,the electronic structure of LiFePO₄ has been discussed via the first-principles.Meanwhile,1% Na and 50% Mn co-doped Li_0.99Na_0.01Fe_0.5Mn_0.5PO₄ cathode material were synthesized successfully by solid-state method.Meanwhile,the calculation and experimental results confirm that Na and Mn co-doping improve rate performance and cycling stability of Li FePO₄ with higher discharge voltage plateau.The percentages of initial discharge capacity of Li_0.99Na_0.01Fe_0.5Mn_0.5PO₄ cathode material at 0.2C,0.5C,1C,3C relative to 0.1C are 89.3%,86.4%,80.6%,64.4%,respectively,higher than the un-doped sample.In addition,the capacity of Li_0.99Na_0.01Fe_0.5Mn_0.5PO₄ cathode material is almost no fading after 100 cycles at 1C rate.