Fe₇s₈ Cathode Material For Lithium Metal Batteries

Currently,the development of high energy density and inexpensive lithium-ion battery materials has become an inevitable trend in the industry.Conversion cathode materials are considered as one of the candidates for next-generation battery materials due to their high energy density and low cost.Among them,Fe S₂ has been widely studied due to its high theoretical energy density,however,its subsequent development is limited by the"shuttle"effect of lithium polysulfide.In this thesis,Fe₇S₈ is a low-cost,environmentally friendly,high-energy-density conversion cathode material with a theoretical specific capacity of 663 m Ah·g^-1 and a better cycling stability compared with Fe S₂.On this basis,Fe₇S₈-Li pouch cell is prepared in this thesis,but problems such as large volume expansion during cycling led to overcharge failure of the pouch cell.Therefore,the failure mechanism of Fe₇S₈-Li system is firstly studied.Secondly,in order to alleviate the problems such as volume expansion of Fe₇S₈-Li system,this thesis investigates the performance of different binders related to this system.Mainly include the following:First,the Fe₇S₈ material is analyzed in terms of phase and morphology,and it’s confirmed that the prepared material is pure Fe₇S₈ of micron size and relatively aggregated dispersion.The electrochemical properties of Fe₇S₈ material is compared under different electrolytes and particle sizes,and it is found that the Fe₇S₈ material with micron size has the best cycling stability under conventional ether electrolyte system.Then,Fe₇S₈-Li pouch cell is prepared on this basis and the causes of failure of Fe₇S₈-Li system is investigated.Through electrochemical testing of the components of the failed coin cell,it’s found that the failure of the Fe₇S₈ system battery is the result of the combined action of several components.Further analysis using relevant characterization tools confirmed that the main cause of the Fe₇S₈-Li system battery failure is the growth of long-chain lithium polysulfide Li₂S₄ during cycling,and further conversion of long-chain lithium polysulfide Li₂S₄ into Li₂SO₃,which eventually led to the occurrence of the system overcharge phenomenon.And also the Fe₇S₈ system in the conversion process of the relevant reaction equation is refined.Finally,in order to alleviate the Fe₇S₈-Li system from the more serious volume expansion and other problems during the cycling process,four binders PVDF,PAN,SBR/CMC,and PAA/CMC are selected in this thesis to study the influence of binder on the electrochemical performance of Fe₇S₈,and it’s found that PAN binder has the best overall performance in Fe₇S₈ system.The two main reasons are as follows:1)the-C≡N group of PAN binder has a greater binding energy with Li-S bond than the side chain groups of the remaining three binders,which is more favorable to stabilize the sulfide produced in the system;2)the dipole interaction between the electronegative-C≡N group and the collector fluid makes the active material Fe₇S₈ can be firmly attached to the collector fluid,so it is more favorable to suppress the volume expansion of Fe₇S₈ system.