Theoretical And Experimental Investigation For Electron/ion Conduction Mechanism Of Inorganic Electroactive Ion Exchange Materials

Electroactive ion exchange materials（EIXMs）are a class of functional materials which can generate electronic response by electrochemical redox reactions.The EIXMs can transform between oxidation and reduction state by modulating the electrochemical potential.So the charging and discharging of the EIXMs were achieved with the adsorption and desorption of ions from electrolyte solution.Based on this unique electrochemically switched ion exchange performance,the EIXMs have been wildly applied in new energy storage,toxic ion separation,high value-added ion recovery and other fields,which could solve the energy crisis brought by the industrialization.The performance of EIXMs is related to electrons conduction and ions diffusion,and it is difficult to study from electronic and molecular scale through experiments.So,theoretical calculation is important for the design of EIXMs.In this work,theoretical and experimental study has been used to investigate the electron/ion conduction mechanism of two types of inorganic EIXMs,i.e.,layer double hydroxides（LDHs）andλ-MnO₂.Transition metal-based LDHs are a family of inorganic EXIMs of the layer structure,wildly used in supercapacitor.Moreover,the ratio of metal cations plays an important role to dominate the electrochemical performance.Ni_3-xCo_xAl-LDHs（x=0-3）were synthesized on carbon paper by unipolar pulse electrodeposition（UPED）.The structures were characterized by X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）.The electrochemical performance was evaluated by cyclic voltammetry（CV）and electrochemical impedance spectroscopy（EIS）.The mechanism on the different electrochemical performance with various Ni/Co ratio was investigated by density functional theory（DFT）plus U method and molecular dynamic（MD）simulations,which was verified by experiments.The results of DFT calculation demonstrated that the delocalization of electrons was enhanced with Co-doping,resulting in high electronic conductivity.The deprotonation of NiAl-LDHs were also improved by doping Co species.The results of MD simulations indicated that the diffusion of OH^-was enhanced by doping Co species,and the incorporation of Co and Ni cations made LDHs have larger interlayer spacing which can facilitate the diffusion of OH^-ions,resulting that NiCo₂Al-LDHs had the highest specific capacitance.The demand for lithium has increased in various countries and regions with the development of new energy vehicles.Therefore it is promising to extract lithium from seawater and salt lake brine.λ-MnO₂ is one of the most promising lithium adsorbents due to its high selectivity to Li⁺based on its special structure.In this study,the performances of rGO dopedλ-MnO₂ for extracting lithium electrochemically have been investigated systematically by theory and experiment from three aspects:electronic conductivity,ionic conductivity and ionic selectivity.Based on density functional theory（DFT）plus U method,the band gaps of electrode materials has been decreased and electron delocalization increased by doping rGO which enhanced the electronic conductivity.The diffusion energy barrier of Li⁺has been decreased and the diffusivity bas been increased by doping r GO using the nudged elastic band（NEB）method and molecular dynamic（MD）simulations,indicated that the ionic conductivity was improved.The ionic selectivity ofλ-MnO₂/rGO for Li⁺has been investigated by selecting Na⁺and Mg²⁺as competitive ions.Based on NEB method,the size of diffusion energy barrier are in the order of Mg²⁺>Na⁺>Li⁺.And Li exist inλ-MnO₂ as pure ionization by DFT calculation which facilitates the reversible Li electrochemical adsorption and desorption.