First-principles Study On Doped Sulfur Cathode Materials To Improve The Performance For Lithium-sulfur Batteries

As the development of science and technology, the demand for energy is increasing.Sustainable energy becomes the focus of attention. Secondary battery as sustainable a form ofenergy, having a wide range of applications. Lithium-sulfur battery as a member of thesecondary batteries’ family, because of its incomparable properties, as high theory specificcapacity, which is1675mA/g and high energy capacity, which is2600Wh/kg, and at thesame time, because of the widespread in nature and friendly to environment of low costadvantages, becomes the focus of the researchers.But because of the sulfur under atmosphericfor insulation, thus led to the use of lithium battery sulfur electrode sulfur the conductivity ofthe performance and circulation are quite poor. In experiment studies, the researchers usuallyuse four kinds of methods to improve the problem: compositing with the carbon; compositingwith the carbon nanotube materials; compositing with the polymer materials; compositingwith the metal oxide compound materials. But in the theoretical calculation, this field arerarely involved. Therefore this paper in order to solve this problem, the research are on thefollowing.This paper uses CASTEP software and VASP software, on the basics of the densityfunctional theory, to calculate the sulfur on first principle calculation. First of all, on the basics of testing the parameters in convergence forcalculation,optimize the crystal structure of sulfur,which is from the experiment data. Afterthis finish the further study on dopping. The crystal structure of sulfur is forming of a ring ofS8. According to the lowest energy is the most stable system, the principle of selection is tochoose the high symmetry position to dope, which is in the body heart. We will dope sometransition metal atoms in this position. After the optimization of the doped structure, caculatethe density of states. Compared with the results, we found that doping will really improve theconductivity of the sulfur. According to the formula of energy of forming defects, calculatethe energy for different dopping atoms. By comparing the results, we found that Pd atom isthe one which is the easier than other atoms to dope. Further calculations on the concentrationof dopping atoms shows the concentration of Pd atom, which is the easiest dopping one, onlycan achieve10ten power orders of magnitude, which means the atoms dopping in sulfurcathode, can’t exist as a point defect but sulfide. The calculation of enthalpy of thecompounds also is a proof of that. This shows that metal atoms and sulfur electrode willproduce chemical reaction, and generate metal sulfide.Such a result, is not the originalintention.Other factors which can influence the performance are considered. According to theliterature investigations, the sulfur will become amorphous with the rise of pressure, and alsowill appear a metallization phenomenon. Another way to get the amorphous sulfur isquenching except the pressure. This paper uses VASP software, simulation the temperature,hoping to get the same or similar results with the pressure increases. The results shows,crystal sulfur appears liquefaction phenomenon and its structure changes with the rise of temperature. The structure of S8ring will be broken, different length of chain will form. Inthis process, the conductivity increases. Quenching keeps the chains, which leads the sulfurchanging from crystal to amorphous. The results of the DOS of the amorphous sulfur showsthat the gap of amorphous is smaller than that of crystal, which means amorphous sulfur isbetter than crystal sulfur for electronic transition.Finally, to improve the performance of the sulfur electrode, this paper uses moleculardynamics method to simulate amorphous sulfur dopping. The doping position for amorphousis the same as the crystals ones, still in the heart body position. After molecular dynamicssimulation, we get the amorphous sulfur with impurities. The calculation results of DOSshows that the gap of amorphous sulfur with impurities is obvious narrow, which is far lessthan that of amorphous sulfur and crystal sulfur. Calculate the energy of difference betweenthe top of valence band and the bottom of conduction band. The results shows the idealdopping atoms are Y、Au and Mn elements, which can achieve the purpose of improvingthe conductivity of the sulfur electrode.