Modification Of FeS₂ Cathode Material For Lithium Batteries

Compared to other rechargeable batteries,lithium-ion batteries have a higher energy density.However,it is difficult for commercial lithium-ion batteries to break through the energy density limit of 300 Wh kg^-1.For long-range electric vehicles,advanced portable electronic devices and many other applications,there is a great need to explore energy storage devices with energy densities higher than 300 Wh kg^-1.FeS₂ has been extensively studied due to its theoretical specific capacity of 894 m Ah g^-1 and a theoretical energy density of 1671 Wh kg^-1,which is considered to be one of the best candidates for the next generation of high-energy density battery cathode materials.However,due to the“shuttle effect”of lithium polysulfide and the volume change of the active material during the conversion reaction,the practical application of Li-FeS₂ rechargeable batteries is limited.Therefore,in order to solve the above problems,this thesis uses lithium polysulfide adsorption material composite,prelithiation and other modification methods to modify the pristine FeS₂ materials.Mainly include the following:First,our pristine FeS₂ has been verified that it’s a pure phase with a micron level and uneven size dispersion.The electrochemical performance of pristine FeS₂ is tested in ether electrolyte,ester electrolyte,and high concentration electrolyte.The results show that all of the three electrolytes can provide a specific capacity of about 750 m Ah g^-1,but the cycle stability of conventional ether electrolytes is the most outstanding.Next,by using the lithium polysulfide adsorbent CoS to inhabit the“shuttle effect”,the electrochemical cycle stability of the FeS₂ is improved.The cobalt-based zeolite imidazolate framework material（ZIF-67）is used to react with pristine FeS₂ to generate CoS in situ at high temperature,then successfully prepare n%-FeS_x@CoS（n%=1,4,20wt%）.Through testing and analysis,the 1%-FeS_x@CoS has more advantages than the pristine FeS₂ in terms of rate performance and cycle performance.DFT theoretical calculations show that CoS and lithium polysulfide have a binding energy of up to 3.78e V,further confirming that the in-situ CoS can inhibit the“shuttle effect”of lithium polysulfide to improve the cycle stability of FeS₂.In addition,in order to further solve the problem of FeS₂ cycle stability,the prelithiation is performed for the serious volume expansion.By pre-intercalating lithium with binary inorganic lithium salt compounds（Li₃N）and pristine FeS₂ at high temperature,n%Li₃N-FeS₂（n%=1,2,5,20,33.3,66.7wt%）with Li-Fe-S prelithiation product and Li₂CO₃ passivation layer are successfully synthesized.During the electrochemical cycling test,2%Li₃N-FeS₂ shows the best electrochemical performance.Its retention rate after 100 cycles at 0.5C rate is 68.9%,while the pristine FeS₂ retention rate is only 51.7%.The volume expansion test of 2%Li₃N-FeS₂ shows that there is almost no volume expansion or even shrinkage,while the pristine FeS₂ test expansion rate is as high as 188.1%.Through a series of ex-situ test analysis,it is confirmed that the Li₂CO₃layer on the surface of the pre-intercalated lithium compound is stable,which is an important factor in alleviating volume expansion and even inhibiting the“shuttle effect”of lithium polysulfide.Finally,another binary inorganic lithium salt,Li₂S,is used to perform a lithium pre-insertion test on FeS₂.However,it does not show excellent electrochemical performance,and it even exacerbates the“shuttle effect”of lithium polysulfide.Therefore,not only the volume expansion problem,but also the“shuttle effect”must be considered through the prelithiation method.