Theory And Research On Anionic Redox Cathode Material For Batteries

With many issues such as the energy crisis,environmental pollution and new energy put forward,various new energy storage materials and devices have ushered in vigorous development.Among them,lithium-ion secondary batteries have attracted much attention as a representative of new energy devices.Among the many studies related to lithium-ion secondary batteries,the development of new cathode materials is crucial,because its performance is directly related to the energy density,cycle life,and safety performance of lithium-ion secondary batteries.With the increasing demand for energy density,the urgent task of battery material research is to improve the reversible specific capacity of cathode materials.In many schemes,it is a more effective way to use the anion redox of the cathode material to realize the occurrence of multi-electron reactions during the charge and discharge process,thereby increasing the specific capacity.In order to make good use of the redox capacity of anions,it is necessary to understand the redox mechanism and propose a modification scheme for related problems.Li_1+xM_1-xO₂（M is a transition metal or non-transition metals such as Al,Sn or Mg）is a lithium-rich layered oxide with anionic activity that has been studied more at this stage.However,the anionic redox of Li-rich layered oxides suffers from voltage plateau decay,polarization hysteresis,and slow kinetics.Corresponding to the redox of oxygen ions,sulfur has less electronegativity and is able to form more covalent bonds with transition metals,forming S^--S^-dimers and S^2-ions during charge and discharge processes,which is expected to provide more stable cycling performance than Li-rich metal oxides.Therefore,based on the understanding of lithium-rich metal oxides,this paper discusses the lithium-rich sulfides and selenides,and conducts material synthesis and lithium-ion battery performance tests to explore the contribution of anionic redox to battery capacity.1.Lithium-rich metal sulfides（Li₂Fe S₂）with a layered structure were synthesized,and the redox capacity of sulfur anions was investigated.Li₂Fe_1-xMo_xS₂ was synthesized by high-energy ball milling and high-temperature solid-phase method,and Mo was doped to obtain Li₂Fe_1-xMo_xS₂.The d electrons of Mo were used to excite the p electrons of sulfur to release the sulfur anion capacity.After Mo doping,the initial capacity of Li₂Fe_1-xMo_xS₂ material was increased from 213 m Ah/g to 338 m Ah/g,realizing the release of sulfide anion capacity.The doped sample improved the ionic and electronic conductivity of the material and accelerated the kinetic process.Further computational simulations based on density functional theory（DFT）were used to explore the energy barrier changes of lithium ion migration paths in different environments.It was found that after Mo doping,the energy barrier of lithium ion migrationwas reduced,which was beneficial to the deintercalation and charge transfer of lithium ions during the charging and discharging process,thereby realizing the effective display of anion redox capacity.2.The transition metal selenide Ni Se was synthesized to explore the contribution of selenium anion to the capacity during charge and discharge.Carbon nanofibers were introduced as substrates,and Ni Se@CF materials were synthesized using metal-organic frameworks（MOFs）as precursors.The introduction of carbon material could improve the overall conductivity of the material,and meanwhile could avoid the agglomeration of nano-metal selenide particles,which effectively improved the rate performance of battery materials.In comparison,the initial capacity of Ni Se@CF increased from 350 m Ah/g of Ni Se to 450 m Ah/g,and the specific capacity could still reach 200 m Ah/g after100 cycles at 1C,which proved proved that carbon fiber can improve the discharge capacity and cycle stability of Ni Se.Combined with elemental analysis and ex-situ XRD studies,it was demonstrated that Ni Se had multiple reaction pathways for conversion and intercalation reactions during cycling.In summary,starting from the use of anionic redox capacity,this paper explores the synthesis methods of lithium-rich metal sulfides(Li₂Fe_1-xMo_xS₂)and selenides（Ni Se@CF）and their applications in lithium-ion secondary batteries applications,obtaining direct evidence for the redox reaction of sulfide and selenium ions.Studies have shown that heterovalent element doping and carbon recombination are effective means of modifying cathode materials,improving the electrochemical activity of materials and promoting the release of sulfur ion and selenium ion capacity.This paper explores the anionic redox reactions in lithium-rich metal sulfides and selenides,and provides relevant theoretical references and synthetic modification strategies for the next step to improve the capacity of battery cathode materials.