| After nearly three decades of development,the energy density of lithium-ion batteries is close to the theoretical value and it cannot meet the requirements of the fast-growing electric vehicle industry.Therefore,there is an urgent need to develop new battery systems with lower cost and higher energy density.As a new secondary battery system,lithium-sulfur batteries have received widespread attention because of their high theoretical energy density(2600 Wh kg-1).At the same time,elemental sulfur has the advantages of abundant resources and low cost.However,the conventional sulfur cathode operates via a“dissolution-deposition”mechanism,and the intermediate polysulfides generated during the charge/discharge process can dissolve into the organic electrolyte and cause serious“shuttle”effect,leading to the quick loss of active material and low Coulombic efficiency.To solve these problems,this thesis will optimize the cathode material to realize a novel“solid-solid”sulfur conversion with high sulfur content and high rate capability.The main results are summarized as follow:（1）The“solid-solid”conversion of sulfur can be realized via the nucleophilic reaction of polysulfide with carbonate,which can in-situ form a SEI film on the surface of the cathode and prevent further contact between polysulfide and the electrolyte.However,the reaction kinetics of the“solid-solid”conversion is slow and it is difficult to obtain better rate performance.Here,selenium was introduced as a eutectic accelerator into the sulfur cathode(CMK-3/S1-xSex),thus accelerating the reaction kinetics of the“solid-solid”conversion.The results show that trace selenium can significantly reduce the interfacial impedance and charge transfer impedance of the cathode.In addition,the contents of alkyl lithium carbonate,lithium carbonate,and fluorine elements are enhanced in the SEI on the selenium-doped cathode,which has better lithium ion conductivity and chemical stability.The electrical performance results demonstrate that selenium greatly improves the rate performance of the cell:at a high current density of 5 C,the CMK-3/S0.94Se0.06 cathode still remains a specific capacity of 458 m Ah g-1.This new carbon/sulfur-selenium composite cathode with selenium as the eutectic accelerator provides a new idea for the performance improvement of the sulfur cathode based the“solid-solid”reaction mechanism.（2）The content of sulfur in the cathode via“solid-solid”conversion mechanism is always low,and the side reactions between the polysulfides and carbonate molecules also consume sulfur,therefore,the specific capacity of the whole cathode is low.In this work,a hollow carbon sphere was designed which containing uniform mesoporous pores as an active material carbon matrix,aiming to enhance the sulfur loading capability.During the electrode process,the carbon sphere matrix acts as a conductive network and lithium ion is conducted through the SEI film formed on the surface of the pores.Carbon sphere/sulfur cathodes with sulfur content of 25%and 75%were successfully synthesized by the sealing tube and the dissolution-diffusion methods,respectively.Compared with the above CMK-3/S cathode,the cycling stability is significantly improved and the polarization is significantly reduced,but the capacity is limited and needs to be further optimized in the subsequent work. |
