Study On The Dual Ion Battery Comprised Of Zinc Anode And Graphite Cathode

The development of large-scale electrochemical energy storage technology with low cost,safety,long life,high energy conversion efficiency and high energy/power density is one of the keys to overcoming the intermittent problem of renewable energy generation.Zinc anode electrode has the characteristics of high volume specific capacity,low cost and can be operated in air atmosphere.Graphite is a kind of redox-amphoteric intercalation host which can be intercalated by anions.As a cathode material,it has the advantages of lower cost,superior electrochemical stability,higher electrical conductivity,and higher reaction potential over metal oxide cathodes.Herein,by taking their advantages,we constructed a new kind of dual ion battery comprised of a zinc anode and graphite cathode and studied its energy storage mechanism and electrochemical performance.To improve the Coulombic efficiency and cycle life of this battery,we also developed a high-voltage nitriles electrolyte.HFE featuring low viscosity and high anodic stability was added as a diluent to improve the Zn²⁺transport ability while nocompromisingse its oxidation stability.The introduced HFE co-solvent interactions with Pyr⁺and thus weakens the extensive ionic interactions among different species confined in ionic aggregate clusters,which results in the breaking down of the larger ionic aggregates in ionic liquid.Consequently,the viscosity is greatly reduced,which leads to the increase the of diffusion coefficient of Zn²⁺up to～30 times.（1）The charge storage mechanism in Zn/graphite dual ion battery involves Zn plating/stripping on the anode and TFSI^-（de）intercalation into graphite cathode.Due to the high potential of TFSI^-（de）intercalation into graphite cathode,the output voltage of Zn/graphite battery reaches 2.2 V,which is much higher than those based on metal oxide cathodes.（2）The Zn/graphite dual ion battery based on Zn（TFSI）₂/AN electrolyte has an ultrafast charging rate of up to 200 C（to be fully charged in only 18 s）and delivers a high power density of 16.3 k W kg^-1 based on the mass of graphite.（3）To develop high-voltage nitriles electrolytes for Zn/graphite batteries,we modified the anion solvation structure by altering the cation-coordination domains,since multivalent Zn²⁺solutions exhibit a strong tendency for cation-anion association.After introducing a strong electron-donating solvent,namely trimethyl phosphate（TMP）into Zn（TFSI）₂/AN electrolyte,both Zn²⁺and TFSI^-are preferentially confined in the TMP solvation regime in a way of TMP-solvated ionic pairs and aggregates.Thus the complexation of Zn²⁺with AN at high potential is avoided,and the neighboring methyl hydrogen is activated and extricated as protons,the formation of unstable negative carbon ions.（4）HFE was added as diluents to weakens the extensive ionic interactions among different species confined in ionic aggregate clusters,which results in the breaking down of the larger ionic aggregates in ionic liquid.Consequently,the viscosity is greatly reduced,which leads to the increase of diffusion coefficient of Zn²⁺up to～30 times.The high reversibility of Zn plating/stripping at a high areal capacity also enables the operation of an“anode-free”Zn/graphite dual ion batteries cell,which maintains 90%capacity after 100 cycles.To the best of our knowledge,this is a successful attempt to improve the mass transfer of active species in electrolytes for Zn/graphite dual ion batteries while not sacrifice their oxidation stability.