Capture And Inhibition Mechanism Of Lithium Polysulfides By First-Principle Calculations

With the rapid development of portable electronic device and electric vehicle,the demand for battery system with high energy density and long cycle life is ever-growing.Compared to the convintional lithim-ion batteries(LIB),the lithium-sulfur(Li-S)battery has been considered as promising candidates for next-generation rechargeable batteries due to the high theoretical capacity of 1675 mA h g-1 of elemental sulfur and the high energy density of 2600 Wh kg-1 of the cell weight.Moreover,sulfur has prominent advantages of nontoxicity,low cost and abundance of the raw materials in application.However,the commercialization of Li-S batteries is still hindered by several obstacles,such as low sulfur utilization,fast capacity fading,poor coulombic efficiency,and self-discharge.Essentially,these issues are originated from the intrinsic electrically insulating nature of sulfur and the migration of the dissoluble long-chain lithium polysulfides(LiPS)intermediate between anode and cathode during the charge-discharge cycles.In order to depress the LiPS induced shuttle effect,we designed various sulfur hosts for the LiPS retaining via chemical adsorption or for the long-chain LiPS inhibiting via small sulfur molecules.Using first-principles calculations,various vacancies,N doping,and B,N co-doping in graphene sheets have been systematically explored for LiPS entrapping.The chemical affinity of vacancies to the LiPS varies with the size of the vacancies.Also,N doping induces strong N-Li interaction in the defective graphene systems,in which the pyrrolic N rather than the pyridinic N plays a dominant role for LiPS trapping.Furthermore,the shuttle effect can be effectively depressed via B,N co-doped defective graphene materials,in which the synergetic N-Li and B-S interactions contributed to the strong adsorption of LiPS.Moreover,the charge transfer between Li2S6 molecule and the defective graphene systems have been analyzed via the Hirshfeld charge to understand the intrinsic mechanism of lithium polysulfides retaining in these systems.Based on the two-dimensional(2D)defective graphene for chemical adsorption of LiPS,a series of three-dimensional porous SiC(3D-porous SiC)materials with active sp2 Si atoms have been designed for LiPS entrapping.The ZGM-SiC-1 and AGM-SiC-3 have been confirmed to be thermodynamically and dynamically stable and meanwhile showing good mechanical properties.The 3D porous ZGM-SiC-1 and AGM-SiC-3 display strong affinity to S8 and LiPS with the direct Si-S and Li-C interactions,which are comparable to the case of N doped carbon host.As compared to the 3D porous carbon and 2D SiC nanosheet,the entrapping of LiPS in the 3D porous SiC host is much stronger(ca.2.5 eV to 3.5 eV),which is accounted for the nano-confining of LiPS and the strong bonding between sp2 Si and S atoms.Except for the chemical adsorption of LiPS,the utilization of small sulfur molecules to avoid the formation of long-chain LiPS is also an effective strategy to depress the shuttle effect.To effectively stabilize the small sulfur molecules,we adopt the active silicene,phosphorene and borophene as sulfur hosts.Meanwhile,these 2D hosts with high surface area can achieve the high surfiur loading and also accelerate the lithiation process by affording the open channel for the transport of Li+.By realizing the coverage of S2 molecules on silicene,the dissolution and migration of the intermediate LiPS has been avoid,leading to a high theoretical capacity of 891 mA h g-1.There is good retention of the capacity since all S atoms have been firmly anchored on the surface of silicene without the loss the active materials.Moreover,the discharge products of atomic layer of lithium sulfides on silicene surface exhibit completely different behavior with the traditional discharge products of solid Li2S,which can accelerate the the conversion of LiPS from long-chain to short-chain.Based on the strategy of anchoring small sulfur molecules on silicene,we propose another feasible way to stabilize the small sulfur species to avoid the loss of active materials.The aminomethyl functionalized carbon nanotube(AM-CNT)has been adopted to anchor the small sulfur molecules,which is distinctly different from the previously reported N-Li interaction induced chemical sequestering of the lithium polysulfides in the amino groups functionalized carbon.By comparing the different ways of combining the small sulfur molecules and aminomethyl group,we found that the small sulfur molecules prefer to catenate the dehydrogenated amino groups(-NH)with strong covalent N-S bond rather than to catenate the dehydrogenated methyl groups(-CH)with covalent C-S bond.In spite of the minor compromise to the capacity with 1.66-electron charge transfer redox reaction,the integrated AM-CNT-Sn composite exhibits superior stability during the discharging process.Also,the capacity fading issue can be effectively depressed by avoiding the formation of the long-chain LiPS.