Quantitative Analysis Of Cation Selectivity Of Electrodes Based On 2H-MOS₂ In Multi-ion Electrolytes

The world is increasingly demands for renewable and clean sources of energy,such as solar,wind and hydro,due to the environment pollution and energy shortage.However,the intermittency of these renewable energy sources prevents them from meeting the demand for energy.Therefore,sustainable energy production/storage systems and technological developments are critical to meeting energy needs and reducing the impact of energy intermittences.Rechargeable batteries and supercapacitors have more comprehensive advantages in energy density and power density than other traditional energy storage devices,but multi-ion batteries and super capacitors with higher potential window and higher energy density are more promising energy storage devices.However,at present,the intercalation/deintercalation mechanism of different cations on the electrode in the mixed electrolyte is not very clear,so it is necessary to study the selectivity of the electrode to different cations（K⁺,Na⁺,Li⁺）in the mixed electrolyte.In this paper,a quantitative method based on first principles is proposed to quantitatively study the cation selectivity in hybrid electrolytes.The results from the first principles show that the final cations intercalating into the MoS₂ interlayer space mainly depend on the competitive reactions of various cations in the hybrid electrolyte to the electrode and the chemical potentials related to the molar concentration ratio,and the binding energies of different cations intercalating into the electrode play a key role in the cationic selectivity mechanism.In this paper,layered two-dimensional graphene-like material MoS₂ is selected as the electrode material for the cation intercalation/deintercation verification experiment,and its weak interlayer Van der Waals force provids convenience for the ion intercalation/deintercalation.Employing a straightforward hydrothermal technique in in-situ growth of MoS₂ nano flowers without any binder to directly farbricate MoS₂@ACC electrode eliminates the influence of the traditional electrode additive such as binder on the result of the experiment.Compared with the electrochemical performance of the traditional electrode with adding binder in the fabrication process,the electrodes with one-step fabrication process present better electrochemical performance.Moreover,the experimental results based on 2H-MoS₂@ACC electrode further verifiy the cation selectivity predicted by the theoretical calculation.When K⁺and Li⁺coexist in the electrolyte,even a small amount of K⁺will dominate the process of ion intercalation and deintercalation and Li⁺will be suppressed by K⁺in competitive reaction,resulting in the cationic K⁺intercalating into the MoS₂ interlayer space.Similarly,when Na⁺and Li⁺coexist,a small amount of Na⁺dominates the process.On the other hand,when the K⁺and Na⁺coexist,the existence of critical value of molar concentration ratio ultimately determines what kind of cation eventually intercalates into MoS₂ interlayer space.If the molar ratio of K⁺and Na⁺is lower than the critical molar concentration（1:10）,Na⁺can intercalate into MoS₂ layer space,and K⁺can intercalate into MoS₂ layer space if higher than the critical molar concentration.In summary,this study provides a basic for understanding the electrochemical storage behavior between electrolytes and electrodes in multi-ion capacitors.