Morphologies And Electronic Modulation Of Cobalt Phosphide-Based Electrocatalysts For High-Efficiency Lithium-Sulfur Batteries

In the research of new energy storage systems,lithium-sulfur batteries（LSBs）with high theoretical energy density,low cost and environmental friend have attracted the attention of researchers.However,the poor cycling stability caused by the"shuttle effect"of lithium polysulfides（Li₂S_n,4≤9）≤8)still limits the commercial progress of LSBs.To achieve the goal of high energy density of 400 Wh kg^-1 for the practical application of LSBs,except as the in-depth study of materials,sulfur cathode with high sulfur areal loading and sulfur content,electrolyte/sulfur（E/S）ratio,and cathode compacted density are also the focus of the study.As more Li₂S_n is produced during the discharge due to the higher sulfur loading and higher sulfur content,the viscosity of the electrolyte becomes larger and the ion conductivity decreases,thus aggravating the"shuttle effect"of polysulfides and the polarization of the electrode.Besides,insoluble and electrochemically insulate discharge products（Li₂S）are unevenly deposited on that electrode surface due to the slow oxidation-reduction kinetics,resulting in incomplete conversion of the sulfur species,and severely hampering the development of high-volumetric energy-density LSBs.To solve these issues,based on the transition metal phosphide（TMP）with wide application and excellent catalytic performance,the cobalt phosphide electrocatalyst（CoP,with hollow cube and two-dimensional nano-sheet）with strong adsorption ability and high catalytic activity was designed through regulating the morphology and electronic structure,and which was applied in the functionalized separator coating and sulfur host,respectively,thus enhancing the adsorption of Li₂Sn and promoting the Li₂S_n-Li₂S conversion.The concentration of Li₂S_n in electrolyte is effectively reduced,and the oxidation-reduction kinetics of Li₂S are promoted at the same time,so that the Li₂S are prevented from gathering on the surface of the electrode.And finally,the dense thick electrode with high sulfur content and sulfur areal loading is prepared through the dense engineering,and the LSBs with long cycle stability and high volumetric capacity are realized.The research contents are as follows:（1）CoO@CoP of hollow box was prepared by a series of step-by-step low-temperature calcination of Co₂（CN）₅NH₂ precursors.The mechanism of blocking the shuttle of Li₂S_nbetween the cathode and the anode by using CoO@CoP-Box as the functional coating of the separator was explored and analyzed,and its electrochemical performance were studied.Due to the strong adsorption ability of CoO,the high catalytic activity of CoP,and the large specific surface area exposing a large number of active centers,Li₂S_n in the electrolyte is effectively adsorbed on the cathode side of the separator and is rapidly converted into Li₂S on the surface of the separator coating catalyst,the conversion rate and the percent conversion of Li₂S_n to Li₂S are improved.The LSBs based on CoO@CoP-Box separator exhibited significantly improved cycle stability,maintaining a capacity of 654.5 m Ah g^-1 after 300 cycles,and exhibited low polarization at least 139 m V under different current densities.This work has provided ideas for the design of efficient catalysts with synergistic effects from the point of view of structural adjustment and surface composition adjustment.（2）2D Electron-donor Cu-doped CoP/MXene nanosheet bi-directional catalyst with rich active sites and high specific surface area was prepared via hydrothermal method followed by low-temperature phosphating method.Cu CoP/MXene bidirectional catalyst was used as sulfur host,which effectively accelerated the conversion of polysulfide ions and the redox kinetics of Li₂S.Systematical investigations of density functional theory（DFT）calculation,kinetics,and thermodynamics confirm the Cu_0.1Co_0.9P/MXene（the doping content of Cu is 10at%）had the lowest activation energy and higher conversion rate in the Li₂S oxidation-reduction process,which stem from the following advantages:（1）Electron-donor Cu doping can incuce more defects and vacancies of CoP,thus exposing more active sites for the catalytic conversion of sulfur species（Li2S nucleation and decomposition）,thereby facilitating to enhance the Li2S redox kinetics.（2）Electron-donor Cu doping can easily make the strongly electronegative Co³⁺in CoP converted into weakly electronegative Co²⁺,so the bond length of the formed Co-S bond（bonding with polysulfides）is lengthened（lengthening from 2.150（?）to 2.168（?））,thereby facilitating the diffusion of polysulfides and Li₂S on the electrocatalyst surface,further decreasing the diffusion energy barrier and activation energy of Li₂S nucleation and decomposition,thus intrinsically boosting the Li₂S redox kinetics.（3）The synergistic effect between the modulated CoP and Ti₃C₂T_x-MXene can effectively enhance the overall electron conduction,active area,and structural stability of the hybrid catalyst,thus endowing the catalyst satisfactory electrocatalytic activity and capture capability for polysulfides.Therefore,the initial discharge capacity of LSBs based on Cu CoP/MXene are as high as 1475 m Ah g^-1,and the capacity loss per cycle is only 0.1%in the process with 500cycles of a 0.1 C.The remarkable improvement of the catalyst performance by the electron-donor doping engineering of electron donor was illustrated,which provided the ideas for the design of electrocatalyst for LSBs.（3）To obtain the sulfur cathode with high volumetric capacity and high areal capacity,a small amount of graphene oxide（GO）（15 wt.%）as a self-assembly agent was introduced to assemble the as-obtained S/Cu_0.1Co_0.9P/MXene（S content of 78.6%）into a three-dimensional structure by a mixed solvothermal method,and then the three-dimensional structure was naturally dried and shrunk into a dense S/Cu_0.1Co_0.9P/MXene monolith.The dense S/Cu_0.1Co_0.9P/MXene monolith exhibits a high density of up to 1.95 g cm^-3 and a high conductivity of 283 S m^-1.The dense thick S/Cu_0.1Co_0.9P/MXene cathode with a high sulfur loading(5.1 mg cm^-2)displays an areal capacity of 5.66 m Ah cm^-2 and a volumetric capacity of 1280 Ah L^-1 under a low-dosage electrolyte(E/S of 5μL mg^-1).And a thicker dense S/Cu_0.1Co_0.9P/MXene cathode gives an areal capacity of 8.3 m Ah cm^-2,with a capacity retention of 76%after 50 cycles.The result mainly originate form the highly dense and conductive network structure of 3D graphene and MXene in the dense S/Cu_0.1Co_0.9P/MXene cathode.This is of great significance for the development of dense sulfur cathode materials and high-energy-density LSBs.