Study On The Electrochemical Performance Of Li₂FeSiO₄/C Doped With Transition Metals As Cathode Materials For Lithium-ion Batteries

Cathode material is a key factor for the specific capacities enhancement of lithium ion batteries?LIBs?.More and more attentions paid to high capacity and high voltage cathode materials.Nowadays,Li₂MSiO₄?M=Fe,Mn,Co,Ni?are most popular cathode materials considering the high capacity,voltage,safety,thermal stability,cost and environmental friendliness.In these cathode materials,Li₂FeSiO₄shows stable cycling performance as Li Fe PO₄ due to the stable polyanionic structure,and with high theoretical capacity about 332 m A?h?g^-1 due to the reversible extraction/insertion of two lithium ions.However,the low electronic conductivity and lithium-ion diffusion efficiency still restrict its application.Carbon coating and element doping are effective methods to modify its electrochemical performance.The effect of ion doping on the physical and electrochemical properties of Li₂FeSiO₄composites was studied on the basis of carbon coating modification.The main research contents are concluded as following:?1?Li₂Fe_1-xMn_xSiO₄/C?x=0,0.02,0.04,0.06?composites were synthesized via a tartaric acid assisted sol-gel method.Mn-doped composites show similar structure with Li₂FeSiO₄/C cathode materials,according to the XRD results.FESEM,HRTEM and EDX results indicated that all samples maintain an initial spherical shape,and the particle size decreased as the manganese content increasing.Li₂Fe_0.96Mn_0.04SiO₄/C shows the best electrochemical performance,with the smallest polarization,highest discharge capacity and the best rate performance.The discharge capacities stables at 170 m Ah g^-1after 150 cycles at 0.2C,and shows 232.9,188.6,176.5,and 140.3 m Ah g^-1 at 0.1C,0.5C,1C,3C,respectively.?2?The effect of Mn-Sn co-doping on the physical and electrochemical properties of Li₂FeSiO₄/C composites was studied on the basis of 4%Mn doping.via a tartaric acid assisted sol-gel method.The effects of Mn-Sn co-doping on the chemical composition,structure characteristics,morphology and elemental distribution of the as-prepared polyanionic cathode materials are investigated by XRD,SEM,HRTEM,EDX,BET,Raman,FTIR,ICP,and XPS methods.The results showed that the Mn-Sn co-doping does not change the structure of Li₂FeSiO₄,and maintained the original spherical shape.Electrochemical tests show that the Mn-Sn co-doped Li₂FeSiO₄/C exhibits a higher reversible capacity than the original Li₂FeSiO₄/C and Mn doped Li₂FeSiO₄/C at different rates.Li₂Fe_0.95Mn_0.04Sn_0.01SiO₄/C shows the best electrochemical performance,with 327.3,283.1,256,220.4 and 168 m A h g^-1 at 0.1,0.2,0.5,1 and 3C,respectively.