Theoretical Calculations Based On Stable Polysulfides Guided Material Modifications For Lithium-Sulfur Battery

Lithium-sulfur batteries（LSBs）with high energy density have great potential for applications in electric vehicles,smart grids,and mobile electronics,but the practicality of LSBs requires further solutions to the shuttling of lithium polysulfide,the slow reaction kinetics,and the dendrite of lithium metal anode.As first-principles calculations become more widely used,it will be more efficient to combine theoretical calculations and experiments to understand and solve the problems of LSB.Based on this,this dissertation screened the stable molecular configurations of lithium polysulfides,aiming at unifying the molecular configurations of lithium polysulfides for theoretical calculations of LSB.Further,first-principles calculations based on stable molecules of lithium polysulfide were carried out,which guided the improvement of the performance of TiO₂as cathode host materials for the immobilization and reduction of sulfur,as well as led the design and preparation of the orderly porous carbon composited MXene with the ability for immobilizing,catalyzing the reduction of sulfur,and homogenizing the deposition of lithium.The main research contents and results are as follows.（1）Ab initio molecular dynamics（AIMD,Ab initio molecular dynamics）simulations and high-precision CCSD（T）-F12b method,WMS/W3X-L functional method with aug-cc-pVDZ basis set were used to obtain the Li₂S_n（n=2,3,4,5,6,7,8）/S₈molecular configuration space of the lowest energies,as well as their highly accurate atomization energy benchmark database LiSAE38 and relative energies.The results provide reliable input structures and benchmark level energy data for Li₂S_n/S₈-related theoretical simulations for LSB.（2）Based on the obtained accurate atomization energy and relative energy of Li₂S_n/S₈,the accuracy of 39 commonly used DFT functionals was evaluated.It was proved that the HSE06 and PW6B95 hybrid functionals are of high accuracy for calculating the atomization energies（mean unsigned deviation,AMUE,less than 2.5kcal/mol）and the relative energies（AMUE less than 0.5 kcal/mol）of Li₂S_n/S₈.Mn15-L and rev M06-L DFT functionals have the best comprehensive performance in the local functionals with small computational cost.It provides foundations for selecting functionals for DFT simulations of LSB.（3）Based on the obtained most stable molecular configurations of lithium polysulfides,first-principles calculations were used to explore the microscopic active sites of TiO₂and oxygen defect TiO₂（D-TiO₂）to capture lithium polysulfide molecules.The Ti and O on the surface of the polar TiO₂will form Ti-S or Li-O bonds with the S and Li atoms from Li₂S_n/S₈,respectively.The uneven distribution of local charges triggered by the oxygen defects can enhance the bonding and thus increase the adsorption capability of TiO₂to lithium polysulfide.The comparison of the free energies of the transformation reaction of lithium polysulfides on the surfaces of TiO₂and D-TiO₂to those of free lithium polysulfides shows that both TiO₂and D-TiO₂accelerate the conversion reaction of lithium polysulfides,but the promotion effect of D-TiO₂is more prominent.（4）Based on the results of theoretical calculations,D-TiO₂of the corresponding phases were synthesized and composited with three-dimensional graphene through experiments,which were used as the sulfur cathode host to assemble LSB.The electrochemical performance tests including cycling performance and rate performance tests,etc.were carried out to prove the correctness of theoretical calculation results.The experimentally assembled battery achieves excellent electrochemical performance.When the sulfur loading capacity is 3.2 mg/cm²in the cathode,the capacity of the battery after cycling 100 cycles at the current density of 1C（1675 mA/g）is as high as 1049.3 mAh/g,and the average capacity decay rate is only 0.094%per cycle.Moreover,the LSB also shows a high areal specific capacity of 13.8 mAh/cm²and stable cycling performance under a high sulfur loading of 13.6mg/cm².（5）M-NC@MXene nanosheets with ordered nanochannel structure were designed,and the first principles theory was used to demonstrate that the oxygen-passivated MXene surface,pyrrole nitrogen and pyridine nitrogen sites contained in this structure have high binding energy for lithium polysulfide,i.e.,0.96–2.71,0.62–1.70 and 0.65–1.56 eV,which can effectively adsorb lithium polysulfides.These active sites can also promote the conversion reaction of lithium polysulfide.The binding energy of the active sites contained in the structure to Li is above 1.73 eV,which is higher than the binding energy of Li（110）surface to Li and can guide the uniform deposition of Li.（6）The M-NC@MXene-PP composite separator can significantly improve the electrochemical performance of LSB,enhancing the capacity of the LSB with sulfur loading of 6.4 mg/cm²up to 1175.7 mAh/g after cycling for 100 cycles at the current density of 0.2 C,with an average capacity decay rate of only 0.14%per cycle.The Li₂S₆molecule barely permeates through the M-NC@MXene-PP separator in the lithium polysulfide shuttling experiment for 10 h,and the post-cycling structure of the M-NC@MXene nanosheet contains N-S and S-Ti-C bonds,indicating the effective adsorption of lithium polysulfide on M-NC@MXene nanosheets.The M-NC@MXene-PP composite separator is also able to improve the Coulombic efficiency of Li|Cu cells to 99.39%at 1 mA/cm²,10 mAh/cm²;stabilize the polarization voltage of cycling Li|Li symmetric cells in the range of 1.7–1.9 mV at 1mA/cm²,10 mAh/cm²for 1400 h.Structural characterization after Li metal deposition also showed that the M-NC@MXene can homogenize lithium deposition with remarkable effect.