Electrochemical Properties Of The Nasicon Type Compounds

Nasicon compounds were paid much attention due to its stable three dimension framework,in which structure of the compounds couldn't be changed by substituting the transition metal by another elements.Therefore,the compound can be used not only as a solid electrolyte but also as an electrode materials of the cell.Li₃Sc₂?PO₄?₃ and Li₃Fe₂?PO₄?₃ were used as typical Nasicon solid electrolyte and cathode material,respectively.Although many researches had been done on those two compounds,it was difficult to meet the performance of commercial lithium batteries because of their poor electrochemical properties.In this thesis,anions and/or anions simultaneous substitutions were tried in the LiHf₂?PO₄?₃ and Li₃Fe₂?PO₄?₃ compounds by mechanochemical synthesis and sol-gel method,and our purpose was to further improve their electrochemical properties.Based on our researches on SiO₄^4-and VO₄^3-substitution for PO₄^3-in the Li₃Sc₂?PO₄?₃,partial substitution of VO₄^3-for PO₄^3-in the LiHf₂?PO₄?₃ was initially used.Compared with the undoped sample,the VO₄^3-doped LiHf₂?PO₄?₃ presented a marked increase in ionic conductivity,commitant with an obvious increase in electronic conductivity.Therefore the VO₄^3-doped LiHf₂?PO₄?₃ compounds cannot be used as solid electrolytes.Then,partial substitution of SiO₄^4-for PO₄^3-in the LiHf₂?PO₄?₃ compound was tried to prepare Li_1+xHf₂?PO₄?_3-x?SiO₄?_x?x=0⁰.5?series compound by using Li₄SiO₄ as the starting material,Our results proved that the ionic conductivity of the samples varied in parabolic curve with an increase in the SiO₄^4-content,and the compound with the SiO₄^4-conten of 0.1presented the maximum conductivity of 4.0×10^-55 Scm^-1 at room temperature,and the minimum activation energy for ion conduction of 40.8 kJ/mol.The Li_3-xTi_xFe_2-x?PO₄?_3-y?VO₄?_y?x=0⁰.3,y=0,0.45?compounds were prepared by heat treatment to the xerogels at 600?.It was proved that the sample with VO₄^3-content of 0.45,together woth the Ti⁴⁺content of 0.2 presented the best electrochemical performance.Its initial discharging capacity was about 125.4mAh/g at a rate of 0.5C,this value was about 98%of the theory capacity for the Li3Fe2?PO4?3,and was about 29.3%and 10.6%higher than the undoped and only VO₄^3-doping samples,respectively.After 60galvanostatic charge-discharge cycles,its capacity efficiency was about 92.5%,which was much higher than 76%of the undoped sample.By gradually increasing discharging rates from 0.5C,1C,2C,5C to 10C and then decreasing the discharging rate to 0.5C again,cycling at each rate for 10 times,we found that its discharging capacity was about 99%of the initial capacity at 0.5C.On the other hand,cyclic voltammetry showed that the cation and anion simultaneous substitution not only increased intensities of the redox peaks but also decreased their potential differences in the CV curves.Those results indicated that the internal polarization of the batteries was reduced and the electrochemical reaction rate was accelerated.And the sample with VO₄^3-content of 0.45,together woth the Ti⁴⁺content of0.2,showed the minimum charge transfer resistance?Rct?,which was only 30%of that of the undoped sample,and the maximum lithium ion diffusion coefficient of 5.3×10^-1010 cm²/S,which was two orders of magnitude higher than that of the undoped sample.In conclusion,anion and/or cation substitution was an effective method to improve electrochemical properties of the Nasicon structural compounds.