Synthesis And Modification Of NASICON-type Sodium Manganese Titanium Phosphate Cathode Materials

The charge-discharge characteristic and mechanism are similar between sodium-ion batteries（SIBs）and lithium-ion batteries（LIBs）.Furthermore,due to the advantages of low cost and widely distribution of sodium,SIBs are regarded as a hopeful alternative to energy storage equipments.Among a variety of cathode materials,Na super ionic conductor（NASICON）structural materials have been widely studied because of the stable three-dimensional frame and open sodium ion transport channels,resulting in a high ion diffusion rate.However,the inherent isolation properties and large size of the PO₄^3-groups result in low electron conductivity and a moderate capacity,limiting the further application.Therefore,it is necessary to further study on modification of the materials to improve the electrochemical performance.In this paper,the object is NASICON-type Na₃MnTi（PO₄）₃ material（NMTP）and it is synthesized by a simple sol-gel method.We carried out a variety of strategies to optimize the synthesis process and further modification by surface carbon coating and bulk doping.XRD,EDS,TEM and other characterization methods are used to analyze the crystal structure,composition and microscopic morphology of the material,and the electrochemical performance is tested by constant current charge and discharge test,cyclic voltammetry test and other methods.The NASICON-type Na₃MnTi（PO₄）₃ cathode material is prepared by the sol-gel method,and sodium source,titanium source in the process of the material preparation,the calcination temperature,time and heating rate during high-temperature sintering are optimized.The optimal preparation conditions are as follows:the source of sodium and titanium are Na OH and C₁₂H₂₈O₄Ti,respectively.The calcination temperature is 650°C,the heating rate is 3°C/min,and the calcination time is 12-14h.After detailed comparison,When the calcination time is 12h,the specific discharge capacity of the sample at a current density of 10C is equivalent to 49.3%of 0.1C,indicating the better rate performance.When the calcination time is 14 h,the sample shows the best cycle stability.After 100 cycles at 0.2C,the capacity retention rate is97.5%.In order to improve the conductivity of the material,we select different carbon sources to perform the double carbon coating for NMTP materials.The carbon source is added during the preparation of the precursor,which affects the crystal structure and crystallinity of the material.It is found that the effect of coating with PVP as the carbon source on improving the cycle stability of the NMTP material is positive,and the double-coated carbon layer can effectively reduce the particle size and improve the conductivity.It is showed that the crystal structure of the NMTP material does not change after coating by PVP through XRD,EDS,Raman,TG and other characterization methods.The material exhibits the best sodium storage performance.The capacity retention is as high as 97.3%at 0.2C over 100 cycles.At a higher current density of 1C,it retains 92.0%capacity after 500 cycles.On the basis of using PVP as the double-coated carbon source,we further optimize the amount of carbon coating.Through HRTEM and other characterization methods,the existence of double-layer carbon coating is testified,which effectively promotes the improvement of material conductivity and achieves the better electrochemical performance.The sample with 10%carbon coating delivers the highest initial discharge specific capacity and performs the best rate performance.At current densities of 3C,5C and 10C,the material can deliver high specific capacities of 89.2,84.5 and 71.2 m Ah g^-1,respectively.The discharge specific capacity of 10C accounts for 65.3%of 0.1C.The sample with 7.5%carbon coating has the best cycle stability,and the capacity retention is 86.5%at 1C over 500 cycles.In order to further improve the electrochemical performance of the NMTP material,a small amount of V is doped on the Mn site of the NMTP material.It is found that the small amount of V doping can significantly reduce the particle size and reduced the agglomeration of NMTP particles.Besides,it enhances the conductivity and increases the diffusion rate of sodium ions.The best NMTP-V5 sample achieves the high discharge specific capacities,the discharge specific capacity is reduced from116.5 m Ah g^-1 to 109.1 m Ah g^-1 at 0.2C after 100 cycles,equivalenting to the capacity retention rate is 93.6%,which is significantly higher than the pristine sample.The rate performance is also significantly improved after V doping.At a current density of 10C,the specific discharge capacity of the NMTP-V5 sample can reach 69.1 m Ah g^-1.Finally,Na₃Mn_0.5V_0.5Ti（PO₄）₃/C material is prepared by V substitution.It was found that the particle size was significantly reduced,and the electrochemical performance was significantly improved.After 500 cycles at 1C,the capacity retention rate is 76.2%and the rate performance is also considerable.