Preparation And Modification Of The Li₂FeSiO₄ Cathode For Lithium Ion Batteries

The portable electronic devices have been involved in almost every aspect of the social life and production,and many application scenarios have asked for higher energy density and longer life time of the lithium ion batteries.Currently,the conventional cathode materials are the primary bottleneck that limits the performance of lithium-ion batteries.The conventional layered oxide cathode materials and polyanionic lithium iron phosphate cathode materials are suffered from safety issue and low specific capacity,respectively.The lithium-rich polyanion Li2FeSiO4 has attracted attention due to its structural safety,much higher theoretical capacity and cheap chemical sources.It has been considered as one of the new cathode materials that have great potentials in this field.However,the intrinsic conductivity of Li2FeSiO4 is extremely low and serious phase transitions would take place in the de-lithiation process,so that its accessible capacity and cyclic performance are unsatisfactory.In this work,the synthesis of Li2FeSiO4 and Li2FeSiO4/C through the solid-state calcination and the sol-gel method has been investigated;and the nanoscale Fe-catalyzed carbon composite Li2FeSiO4/C/Fe0.1 has been developed.The key results are listed as below:1.The Li2FeSiO4 and Li2FeSiO4/C composite materials were prepared by mechanical ball milling assisted high-temperature solid-state calcination method.The influence of chemical sources on the purity of final product was investigated.The results show that using the Li2SiO3 and SiO2 as co-sources for Si would yield high purity Li2FeSiO4.Moreover,high-purity Li2FeSiO4 can be prepared by heating at 15℃·min-1 to 850℃ for 10 h;The carbon coated Li2FeSiO4/C can release 161.5 mAh·g’1 in the 1st cycle under 0.2 C(1 C=166 mA·g-1)at 55℃.2.The Li2FeSiO4/C composite was prepared by citric acid-assisted sol-gel method.The purity of the product and the size of the particle size were focused on.The results showed that when the sintering temperature was 750℃ the product had the highest purity(100%),and the particle size was small and the distribution was uniform.The Li2FeSiO4/C can offer 202 mAh-g-1 in the 1st cycle under 0.2 C at 55 ℃,which shows significantly improved electrochemical performances.3.The Li2FeSiO4/C/Fe0.1 composite was also prepared by sol-gel method with introducing Fe nanoparticles as catalyst.The effect of nanoscale Fe on the graphitization degree of the carbon was studied.The results show that the graphitization degree and conductivity of the Li2FeSiO4/C/Fe0.i composite have been greatly improved.An ultrahigh capacity of 312.5 mAh·g-1 is achieved at 55℃under the 0.2 C rate.