First-principles Calculation And Energy Storage Application Of Cobalt Sulfide

Li-ion batteries have been widely used in electronic equipment and electric vehicles due to their advantages such as high power density,high specific capacity and excellent cycle stability.In order to improve the performance of Li-ion batteries,researchers are constantly exploring and developing new electrode materials and electrolytes.After decades of research,people have developed various types of cathode materials for Li-ion batteries,such as lithium cobalt oxide,lithium manganate,and lithium iron phosphate.However,anode materials are still dominated by graphite-like carbon materials.Therefore,it is particularly important to find and develop new anode materials for the development of Li-ion batteries.Transition metal sulfides(TMSs)have good electrical conductivity,rapid conversion reaction and high specific capacity,making them one of the ideal anode materials.Most of the researches focused on improving the electrochemical performances of TMSs by controlling their particle size and combining TMSs with nano-carbon materials.The lithiation of TMSs inevitably produces structural distortion and volume expansion,which will cause changes in mechanical properties.Therefore,revealing the lithiation process from a mechanical point of view will help us understand the energy storage mechanism of TMSs.In this paper,as the representative of transition metal sulfides,cobalt sulfide is selected as the research object.Based on the first principles,the mechanical properties(such as bulk modulus,shear modulus,and Poisson's ratio)of four cobalt sulfide materials(Co S,Co S2,Co3S4,Co₉S₈)are calculated.The results show that Co₉S₈ is the most stable structure among the above materials.Its mechanical properties are also significantly better than the products after lithiation(or sodiumization)(Co₉S₈>Li3Co>Na3Co).The results show that the Li⁺intercalation will cause the structural deterioration of Co₉S₈.Finally,we synthesized Co₉S₈ and Co₉S₈/C composite materials through experiments,and used them as anode materials for lithium/sodium ion batteries.The electrochemical results show that the porous carbon nanofibers provide Co₉S₈ with a buffer zone for the volume changes.When Co₉S₈ composite material is used as the anode material for Li-ion battery,after 700 and 200 cycles at 1.0 Ag^-1 and 0.3 Ag^-1current density respectively.the specific discharge capacity of Co₉S₈/C is 680 m Ahg^-1and 1063.4 m Ahg^-1,which is much higher than that of the raw Co₉S₈.