Structure And Electrochemical Properties Of LiSn₂?PO₄?₃ Compound

Nasicon negative electrode materials have recently attracted much attention due to their large capacity,low working voltage and high safety.Among them,LiSn₂?PO₄?₃compound presents high theoretical capacity.However,poor cycling performance greatly limits its commercial application.In this paper,Cr³⁺and VO₄^3-doped LiSn₂?PO₄?₃ anode materials were prepared by sol-gel method for the purpose of improving their electrochemical properties.Our results proved that:Substitution of Sn⁴⁺in LiSn₂?PO₄?₃by Cr³⁺resulted in a pure?-LiSn₂?PO₄?₃ phase in the Cr³⁺contents of 0.1 to 0.3 after heating at 900?for 6 h by sol-gel process.Further increase in the Cr³⁺content led to precipitation of some SnO₂,in addition to the?-LiSn₂?PO₄?₃.Discharging capacity of the Cr³⁺substituted materials varied in a parabolic form with an increase in the Cr³⁺contents.Among them,the sample with the Cr³⁺content of 0.3presented the best electrochemical performance.Its second discharge capacity was about574.6 mAh/g,this value was about 58.6%higher than that of the undoped sample.Its capacity retention efficiency was about 70.2%after 25 charging-discharging cycles,which was2.1 times as high as that of the undoped sample.When the discharging rate was increased to 2C,its capacity retention efficiency was about 64.0%after the same cycles.This value wasabout2.0 times as large as that of the undoped sample.By gradually increasing discharging rates from 0.25C,0.5C,1C to 2C and then decreasing back to 0.25C again,at a condition of keeping cycling at each rate for 5 times,we found that its discharging capacity was about 78.2%of the second capacity at 0.25C.The above results should be attributable to decrease in charge transfer resistance and increase in lithium ion diffusion coefficient in batteries caused by Cr³⁺doping.Similarly,VO₄^3--doped LiSn₂?PO₄?_3-y?VO₄?_y compounds could be prepared by sol-gel method.The VO₄^3--doped LiSn₂?PO₄?_3-y?VO₄?_y compounds was mainly composed of?-LiSn₂?PO₄?₃,together with secondary phases like SnP₂O₇ and SnO₂.Discharge capacity of the VO₄^3--doped samples initially increased in the VO₄^3-content lower than 0.5,reached the maximum at the 0.5,and then decreased when larger than 0.5.The LiSn₂?PO₄?_2.5?VO₄?_0.5 sample delivered the second capacity of 541.7 mAh/g at a rate of0.25C,this value was 49.6%higher than that of the undoped sample.After 25charging-discharging cycles,its capacity efficiency was about 82.9%,which was 2.5 times higher than that of the undoped sample.Improvement in electrochemical properties for the VO₄^3--doped samples should be attributable to the decrease in charge transfer resistance and increase in the lithium ion diffusion coefficientIn conclusion,Cr³⁺cation or VO₄^3-anion substitution was an effective method of improving electrochemical properties of the Nasicon compound LiSn₂?PO₄?₃.