First Principles Study Of Li⁺/β-Li₃PS₄|CuS Surface/Interface Structure And Electronic Properties

All-solid-state lithium-sulfur batteries（ASSLBs）have received wide-spread attention because of their high energy density and high theoretical capacity as well as their outstanding advantages in safety.However,the point-to-point interfacial contact between the solid electrolyte and the cathode material makes the interfacial impedance increase,which limits the development of all-solid-state lithium-sulfur batteries.Sulfide solid electrolytes have been shown to provide good interfacial compatibility with sulfur cathodes,and transition metal sulfides have also been shown to improve the performance of lithium-sulfur batteries as cathode materials.In this work,an all-solid-state lithium-sulfur battery withβ-Li₃PS₄（LPS）as the electrolyte and CuS as the cathode material was investigated using the first-principles approach.The electronic structures ofβ-Li₃PS₄ and CuS bulk phases and their electronic properties on the surface,the adsorption behavior of Li⁺on the cathode surface,and the structure and electronic properties ofβ-LPS|CuS at the interface between the solid-state electrolyte and the cathode material were studied to provide theoretical support for the design of the all-solid-state lithium-sulfur battery,and the main results were obtained as follows:（1）By calculating the energy band structure and Projected Density of States（PDOS）of the electrolyteβ-LPS,the indirect band gap is 2.78 e V,which is an insulator,and the cell can work stably at an operating voltage up to 2.78 e V.The electronic states in the valence band at the Fermi energy level of LPS are mainly provided by the electrons in the p-orbital of the S atom.The electronic states at the bottom of the Conduction Band Minimum（CBM）are mainly contributed by S and P atoms,where the electronic states are mainly from the p orbitals of S atoms and the s orbitals of P atoms.The calculated energy band structure of CuS is a typical P-type semiconductor with electronic states at the Fermi energy level,which is a promising anode material with excellent electrical conductivity.（2）The most stable structure of the LPS（100）surface is the LPS（100）-S surface configuration with S as the terminal,its surface free energy is 0.24 J/m²,the work function of the LPS（100）surface is 3.16 e V,and electrons can escape from its surface relatively easily.By calculating its surface electrostatic potential and Electron Localization Function（ELF）,it is found that the LPS（100）-S surface can easily attract positively charged particles to aggregate on its surface,and Li exists mainly as Li⁺on its surface.The most stable surface structure of CuS（001）is the Cu-terminated surface structure with a surface free energy of 0.35 J/m²,and the work function of CuS（001）surface is 5.13 e V,which makes it relatively difficult for electrons to escape from the surface and shows a bound state for them.The surface electrons are localized around the S atom,and the surface electrostatic potential is positive,which easily attracts the negatively charged particles to gather.（3）The adsorption energy of Li⁺at the T₁ site（Top1 site of S atom）on the surface of CuS（001）is-0.42 e V and at the H₁ site（Hollow1 site of Cu/S hexameric ring）is-0.40 e V,both of which are less than zero,and the adsorption structure can exist stably after the occurrence of adsorption;the position of the surrounding atoms and the bond angle/length between the atoms change after the occurrence of adsorption of Li⁺at the T₁ site,and a chemical bond is formed with the S atom;in addition,the position of Li⁺changed after the adsorption at the H₁ site,and formed a chemical bond with the S atoms around H₁.The migration potential barriers of Li⁺on the CuS（001）surface were calculated by NEB and found that the migration potential barriers on the T₁-H₁,H₁-T₂（Top2 site of S atom）and T₂-H₂（Hollow2 site of Cu/S hexameric ring）paths were 0.28 e V,0.39e V and 0.47 e V,respectively,and the migration potential barrier of Li⁺on the T₁-H₁path was the lowest,so the T₁-H₁ path on the CuS（001）surface is more favorable for the migration of Li⁺.（4）The interfacial binding energy of LPS（100）|CuS（001）,the interfacial structure formed between LPS and CuS,is-0.57 J/m²,so the interfacial structure is thermodynamically stable.Calculation of the density of states at the interface from the interface reveals that the interfacial structure has the appearance of electronic states at the Fermi energy level,mainly contributed by S-s,p and Cu-d orbital electrons in CuS and Li-s orbital electrons in LPS.There is also the appearance of electronic states at the CuS side of the interface,which can accelerate the kinetic processes in the cathode material.In the LPS（100）|CuS（001）interface,charge transfer between Li and S in the interfacial layer occurs with overlapping electron clouds and chemical bonding,so that the interfacial contact is stable and the interfacial impedance is effectively reduced.Since the work function of the LPS（100）surface is lower than that of CuS（001）,electrons at the interface migrate from the LPS（100）side to the CuS（001）side,and a Space Charge Layer（SCL）is formed at the interface.The space charge effect leads to the generation of a built-in electric field at the interface in the direction of the electric field from LPS（100）to CuS（001）,and the formation of the built-in electric field accelerates the Li⁺transport at the LPS|CuS interface,which will accelerate the discharge rate of the cell.