Banding Structure Optimization Of Cobalt (Iron) Diselenides Enhancing Potassium Storage

Band structure engineering is acknowledged as an efficient approach to develop advanced potassium ion batteries(PIBs)with superior electrochemical performance by the microstructure regulation.Specifically,the ellectronic conductivity and ion diffusivity of an odes are two key parameters of the reaction kinetics.Focused on the poor cycling stability of the transition metal selenides(TMSs),the large volume expansion and sluggish ionic/electronic diffusion in the charging and discharging processes,which not only cause the crack of active materials,but also result in adversely effects on the rate performance.Hence,based on the comprehensive and indepth exploration of the structure change,including phase evolution,interface behaviors,and K+ diffusion during the electrochemical reaction processes,various strategies in achieving high-rate PIBs are elucidated in detail,including heterojunction adjusting,heteroatom doping,vacancy inducing,and composite optimizing.Besides,the advent of advanced characterization techniques enables dynamic observation and monitoring at atom level,thereby gaining extensive insights into the intricate mechanism of capacity degradation and interface kinetics.By coupling with the powerful measurements,it provides constructive insights into the application of PIBs.(1)CoSe2@NC nanocomposite particles were prepared by a simple process using ZIF-67 precursors as the template.The controllable substitution of Ni for Co atoms significantly improves the cyclability and reinforces the reversible capacity of NiCoSe2@NC-Ⅱ.The doping samples with more electron delocalized sites show optimized energy-level distribution,enhanced reaction kinetics,and improved potassium storage.In-depth structural analysis of the possible chemical coordination environment is confirmed via HADDF scanning transmission electron microscopy and the X-ray absorption fine structure spectra.The doping effect on the energy band structure and charge/atomic state were investigated in detail,and the doping mechanism based on the intercalation-conversion mechanism was deeply analyzed.When the K+ions insert into the anodes,the discharge intermediate(Ni-KxCoSe2@NC)with expanded crystal planes Ni-KxCoSe2@NC was confirmed.With continuous K+intercalation the final discharge product K2Se/Co(Ni 10%)with superior conductivity effectively boosts the K+ diffusion and reversible intercalation.The average diffusion coefficient of Ni-CoSe2@NC-Ⅱ was higher than that of the other samples,which can be attributed to the controllable Ni-doping strategy.As a result,Ni-CoSe2@NC exhibits improved potassium storage with a high reversible capacity of 400.7 mAh/g after 100 cycles,and a superior rate capability of 284.0 mAh/g at 2 A/g.(2)By controlling the reaction temperature,Se vacancy(Vse)was successfully introduced into the FeSe2@C structure during the seleinization process by taking mil88A as the precursor.Defect engineering is a promising approach for optimizing the energy storage performance of TMSs due to the unique properties of vacancies in manipulating the electronic structure and active sites.Ni doping FeSe2@C with Se vacancy(N i-FeSe2@C-Vse)was achieved by a simple coprecipitation deposition.The synergistic effect of heteroatoms and vacancy endows the electrode materials higher electronic conductivity and alleviates structural variation during the potassium storage process.In this work,the operating mechanism insights into the Ni-FeSe2@C-Vse anodes in PlBs,especially the potential effect of heteroatoms and vacancies on the reaction kinetics were researched systematically.It demonstrated that the doping samples with more stable structures and higher conductivity exhibit superior cycling;while the Se vacancy additionally offered more active sites and enhanced K+adsorption energy for Ni-FeSe2@C-Vse.Due to the prominent surface-controlled behaviors,it delivers improved rate capability,further illustrating the advantages of defect engineering in the optimization in electrochemical performance.Consequently,a high surface-dominated contribution(79.4%)of the as-prepared Ni-FeSe2@C-Vse electrode can be obtained at 0.1 mV/s;and it exhibited an excellent cyclability after 500 cycles(338.29 mAh/g at the current density of 0.1 A/g).(3)Taking MIL-88a as the precursor,a simplified method was applied to synthesize the CoSe2/FeSe2@C heterojunction.By modulating crystal orientation,the semi coherent phase with rich vacancies can be simultaneously obtained,which not only provides a more reasonable energy level of the electrode,but also favors the potassiation process in the heterojunction.Besides,the K+storage mechanism of the heterojunction was clarified by DFT calculation.Due to the well-designed structure,more active storage sites were activated and higher electron states at Fermi level were confirmed.The localized electric field effectively accelerates ion transfer and electron diffusion,leading to an enhanced adsorption energy to potassium ions.In addition,by taking carbon layers as protection and buffer space,the heterostructure with secondary morphological advantages would function much better in the de-/potassiation process.Wh en employed as anodes for PIBs,CoSe2/FeSe2@C-Ⅱ displays a reversible potassium storage of 401.1 mAh/g at 100 mA/g and even 275 mAh/g at 2 A/g.