Synthesis And Modification Of FeS₂ Cathode Material For Lithium Batteries

《The 14th Five-Year Plan for National Economic and Social Development and the Long-Range Objectives Through the Year 2035》emphasized the strategic significance of new energy in green development.The general expectation of new energy vehicles will accelerate the development of high-energy-density lithium-ion batteries.Li-free cathode of FeS₂,as an advanced material that can realize a theoretical four-electron reduction reaction（FeS₂+4Li⁺+4e^-→Fe+2Li₂S）,provides an ultrahigh theoretical specific capacity of 894 m Ah g^-1 and a high-energy-density of 1671 Wh kg^-1.As a result,it is regarded as one of the most promising cathode materials for next-generation high-energy-density batteries.However,it still faces a series of key challenges,including irreversible losing of active materials,big volume variation during cycling process（a volume expansion of～159%at full discharge depth）and shuttling effect of polysulfide.To improve the electrochemical performance of FeS₂,this study concentrates on the influences of synthesis methods,Co_xFe_1-xS₂（x=0,0.1,0.5,1）solid solution and prelithiation on the electrochemical performance of FeS₂.The main contents include the following aspects:1.This study explores the influence of synthesis methods on the morphology and cycling performance of FeS₂,and compares the morphology,reaction kinetics,cyclestability and rate performance between FeS₂ microspheres synthesized by hydrothermal,nano FeS₂@carbon fiber synthesized by electrospinning,irregular FeS₂ synthesized by the high temperature solid-state reaction and FeS₂ microcube.The FeS₂ microcube and irregular FeS₂show a better cycling performance at a current of 0.5C than others.2.On the basis of studing the influence of synthesis methods on the performance of FeS₂,the high temperature solid-state method is used to introduce Co atom and synthesize Co_xFe_1-xS₂（x=0,0.1,0.5,1）solid solution.Due to the catalysis of Co for the reversible conversion reaction of sulfides and adsorption of polysulfides,the cycling stability of Co_0.1Fe_0.9S₂ has been greatly improved.After 200 cycles,the capacity retention rate keeps at 47.9%,whereas the pristine FeS₂ only shows a low-capacity retention of 22.8%.Moreover,the Co_0.1Fe_0.9S₂ also demonstrates a superior cycling performance at 2C with a discharge specific capacity of 264 m Ah g^-1,comparing with the 56 m Ah g^-1of pristine FeS₂.3.This study proposes a facile and low-cost chemical prelithiation for FeS₂ to produce the Li_xFeS₂ for pre-expanding volume,which reserves the space for volume variation during cycling process.As a result,the Li_0.3FeS₂ cathode delivers a longitudinal expansion rate of 36%,comparing with that of FeS₂electrode about 100%.Therefore,the Li_xFeS₂（x=0.2,0.3,0.4）deliver an improved capacity retention.The Li_0.3FeS₂ delivers the highest specific capacity of 419 m Ah g^-1 compared to the pristine FeS₂(70 m Ah g^-1)due to the stable structure and suitable coating of Li₂S.Moreover,it also shows a superior discharge specific capacity of 140 m Ah g^-1 at a rate of 2C,compared to the 0.5 m Ah g^-1 of pristine FeS₂.Therefore,the prelithiation strategy improves the electrical contact between the active material and the conductive additive caused by the severe volume expansion.