Study On Synthesis And Electrochemical Properties Of Maricite NaFePo₄ Materials

NaFePO₄ has attracted much attention due to its low cost,high theoretical capacity and environmental friendliness.However,the poor rate performance and cycle stability of the material greatly hindered its practical application.In order to overcome these shortcomings,it is necessary to take effective strategies to improve the electrochemical performance.In this paper,NaFePO₄/C was synthesized by solid-state method,optimizing synthesis conditions,the properties of NaFePO₄ were modified by doping metal ions in Fe site and controlling the non stoichiometric ratio of Na:Fe.（1）In the aspect of preparation of NaFePO₄/C materials:NaFePO₄/C materials were prepared by solid state method.To explore the difference of electrochemical performance of NaFePO₄ samples with different carbon content.The addition of carbon can improve the electron mobility of the material.In order to further optimize the performance of the electrode,the calcination temperature and time were optimized.Among all the carbon coated samples,NaFePO₄/C samples sintered for 4 h at 300℃and further calcined at 600℃for 14 h have higher discharge specific capacity and high rate performance.The results show that the NaFePO₄/C composite prepared under the optimal conditions has a discharge capacity of 75.4m Ah·g^-1 at 0.1 C rate in the voltage range of 1.5-4.5 V,till maintains 53.9 m Ah·g^-1 after 50cycles,and the discharge capacity is 27.9 m Ah·g^-1 at 2C rate（2）In the aspect of doping modification of NaFePO₄/C materials:NaFePO₄ was doped to improve the bulk phase electronic conductivity,further then improve the properties of the material.Na Fe_1-xM_xPO₄/C（M=Mg,Cu,Zn;x=0.00,0.01,0.03,0.05）materials were prepared by solid state method.All the samples show the pure phase structure and good crystallinity of NaFePO₄.The results show that a small amount of cation doping does not destroy the structure of NaFePO₄.The electrochemical performance test data show that the electrochemical performance of the doped samples is significantly improved compared with that of the pure phase.The best electrochemical performance of the doped sample with x=0.03;In Na Fe_0.97M_0.03PO₄/C（M=Zn,Cu,Mg）samples,Na Fe_0.97Zn_0.03PO₄/C composite exhibits better ratio performance and cyclic stability,Na Fe_0.97Zn_0.03PO₄/C composite showed the good rate performance and cycle stability,the discharge capacity is 113.5 m Ah·g^-1 at 0.1C,and the capacity retention rate is 92%after 50 cycles.Especially at 2C,the discharge capacity can reach 50.6 m Ah·g^-1.（3）In the aspect of non metrological ratio modification of NaFePO₄/C materials:In order to solve the problem of poor rate performance and cycle stability of NaFePO₄,nonstoichiometric NaFePO₄ was successfully synthesized by solid phase method by directly controlling the molar ratio of Na:Fe in the starting material.The effect of nonstoichiometry on the structure and electrochemical performance of NaFePO₄/C cathode material was analyzed to determine the optimal amount of Na.Among all the samples,Na_1.04Fe PO₄/C shows the highest discharge capacity of 121 m Ah·g^-1,which is 60.5%higher than that of the sample with x=1.00.It has excellent rate performance of discharge capacity of 64.5 m Ah·g^-1at 2C and stable capacity retention of 95.4%at 0.1C,It was proved that non stoichiometric modification is an effective and simple strategy to improve the performance of electrode materials.