Preparation And Electrochemical Properties Of Ti₃SiC₂ Modified Li₃V₂?PO₄?₃ Cathode Materials For Lithium-ion Batteries

As an important member of phosphate system of lithium ion battery cathode materials,the monoclinic lithium vanadium phosphate?Li₃V₂?PO₄?₃?has been a focus for researchers due to the advantages of stable structure,high safety,environmental friendly and excellent cycling performance and so on.However,its low electronic conductivity and high rate capacity fading are the main obstacles to the practic al applications.Although the modification of carbon coating can improve the electronic conductivity of Li3 V2?PO₄?₃,carbon layer can hinder the diffusion of lithium ions,and shows insignificant improvement of high rate capacity.With the rapid progress in the development of lithium ion battery at low temperature,it is meaningful to improve the poor performance of Li3 V2?PO₄?₃ at low temperature.In this work,Li3 V2?PO₄?₃/C is mixed with Ti₃SiC₂ via a convenient suspension mixing method.The contents of Ti₃SiC₂ are 0 wt.%,3 wt.%,4 wt.% and 5 wt.%.The chemica l composition and microstructure of these composites were studied.The electrochemical performances of all the samples at different temperatures?room temperature,0? and-20??were conducted.Revealed by high resolution transmission electron microscopy?HRTEM?and scanning electron microscopy?SEM?,Ti₃SiC₂ is helpful to constituting continuous electron pathways via a “plane-to-point” conducting mode,which accompanied with improvement of electronic conductivity.The 4 wt% Ti₃SiC₂-modified Li3 V2?PO₄?₃/C sample exhibits the best electrochemical performances in the voltage range of 3.0–4.8 V,especially at high rates among four samples.The 4 wt% Ti₃SiC₂-modified Li₃V₂?PO₄?₃/C sample shows initial discharge capacity of 114 mAh g^-1 at 10 C at room temperature,while the discharge capacity of the pristine sample is only 37 mAh g^-1 at the same rate.The 4 wt% Ti₃SiC₂-modified Li₃V₂?PO₄?₃/C sample holds capacity retention of 88 % at 10 C after 200 cycles,shows improved cycle performances.The 4 wt% Ti₃SiC₂-modified Li₃V₂?PO₄?₃/C sample also shows improved initial discharge capacity of 130 mAh g^-1 at 1 C at 0?,but the pristine sample is only 118 mAh g^-1.When cycled from 0.1C to 5C,then back to 0.2C at-20?,the 4 wt% Ti₃SiC₂-modified Li3 V2?PO₄?₃/C sample shows an improved cycle reversibility.It is found that the electronic and ionic conductivities are increased simultaneously with one single addition due to the nanolaminated structure of Ti₃SiC₂.Ti₃SiC₂-modified samples exhibit improved initial discharge capacity at high rates,cyclic stability and rate performance.As a result of Ti₃SiC₂ addition,an extremely low Rct?5.57 ??and significantly reduced polarization are obtained.A model of double-decker bus is proposed to understand the significant improvements of both electronic and ionic conductivities.