Controllable Synthes Is Of Ni₃FeN And Its Catalytic Performance In Lithium-Sulfur Batteries

As the name suggests,lithium-sulfur batteries（LSBs）are energy storage systems which metal lithium（Li）is used as the negative electrode and elemental sulfur（S）is used as the positive electrode.Compare with most of energy storage systems,lithium-sulfur batteries have higher theoretical specific capacity and theoretical specific energy,which is 1675 m Ah g^-1 and 2600 Wh kg^-1 respectively.Also,the elemental sulfur has abundant resources on the earth and is environmental-friendly.However,some disadvantages exist in lithium-sulfur batteries,such as low utilization rates of sulfur,volume expansion,poor electronic and ionic conductivity of lithium sulfide（Li₂S）and sulfur,the slow kinetics in the redox process of lithium polysulfides（Li PS）,these may lead to continuous loss of active material,anode passivation and low coulombic efficiency.Based on this,this thesis introduces Ni₃FeN nanoparticles,combined with multi-cavity carbon spheres（MCC）and carbon nanofibers（CNFs）carriers to construct a special nano-micro composite which directly used in S positive electrod.It can enhance physical adsorption and relieve volume expansion.At the same time,strengthen the redox reaction kinetics of polysulfides,improve the"shuttle effect"of lithium-sulfur batteries.This paper also conducts research on the catalytic process and mechanism of Ni₃FeN through adsorption experiments,DFT calculations,XPS,and in-situ infrared tests.The specific research contents are as follows:（1）The sol-gel method combines surface free energy induction to assemble grid-like multi-cavity carbon spheres（MCC）,and then construct a special nano-micro composite material（MCC@Ni₃FeN）.When used as the positive electrode of lithium-sulfur batteries,the multi-cavity interconnected by the porous walls can fully contact the sulfur species,prevent the soluble long-chain lithium polysulfide from migrating into the electrolyte during the charging/discharging process,thereby ensuring the excellent cycle stability of the lithium-sulfur batteries.Ni₃FeN nano-particles are used as the catalyst,the iron ions which are more positive will enhance the adsorption of terminal sulfur（S_T）in polysulfides.This speeds up the conversion of polysulfides and further improves the kinetics of the lithium-sulfur batteries.MCC@Ni₃FeN@S can provide a high initial discharge capacity of 1223 m Ah g^-1,and after 100 cycles,it can still maintain a high reversible capacity of 837 m Ah g^-1,while the capacity of MCC@S composite material is attenuated after 100 cycles up to 460 m Ah g^-1.The MCC@Ni₃FeN@S positive electrode can achieve an initial reversible capacity of 759m Ah g^-1 even at a high current rate of 2 C,showing a good capacity retention rate.（2）The carbon nanofibers（CNFs）and Ni₃FeN composite material（CNFs@Ni₃FeN）are synthesized by electrospinning,which provide the large specific surface area and pores,resulting in ideal contact between the electrolyte and the electrode material,supporting the transmission of electrons and shorting the transmission path of ions.At the same time,Ni₃FeN’s more positive iron ions will enhance the adsorption of terminal sulfur（S_T）in polysulfides.The rapid oxidation-reduction reaction kinetics and good structural stability contribute to the improvement of electrochemical performance together.The battery using CNFs@Ni₃FeN@S as the positive electrode was cycled 500times at 0.5 C,which shows about 64%of the initial capacity(812 m Ah g^-1),and a low decay rate of 0.072%per cycle.When change the current density from 0.1 C to 2 C and finally restore to 0.2 C,a reversible capacity of 1129.5 m Ah g^-1 can be achieved.