Development And Performance Of Electrolyte Additives For Aqueous Zinc Ion Batteries

Lithium-ion batteries possess high energy density,long life,and have dominated the energy storage market for many years.However,the shortcomings of high price,uneven distribution of lithium resources,flammability and toxicity of organic electrolytes have seriously hindered the wide application of lithium-ion batteries.In recent years,aqueous zinc-ion batteries（AZBs）have attracted considerable attention due to their low cost,environmental friendliness,abundant raw materials,high theoretical specific capacity(820 m Ah g^-1),low overpotential（-0.76 V vs.standard hydrogen electrodes）and extremely high volum energy density（about three times that of lithium metal）.However,severe dendrite growth and side reactions on the surface of zinc metal anodes will greatly shorten the cycle life of AZBs.In the past decades,the works for inhibiting the growth of zinc dendrites put their eyes on the zinc anode and electrolyte.Surface modification and three-dimensional collectors,which are considered as the effective solutions,lead to time wasting and high cost in practicle applications.As reported,electrolyte additives have a significant effect on inhibiting zinc dendrite growth with appealing convenience and cost effective.So,it can be deduced that the electrolyte enginnering by additive possess the potential of extensive development and application prospect.The focus of this study is to design additives that inhibit zinc dendrite growth and restrict side reactions on anode surface under faintly acid electrolyte systems.Therefore,two zinc-ion battery electrolyte additives were designed to inhibit the growth of zinc dendrites on the zinc anode surface of AZBs.The details are displayed as follows:（1）Sn Cl₂accompanyed with C₆H₅Na₃O₇·2H₂O were used as the additives in 2 M Zn SO₄electrotlytes.To our knowledge,strategies to inhibit zinc dendrite by regulating the surface energy of zinc anodes have rarely been reported.It has been shown that doping atoms in the main lattice helps to reduce its surface free energy.Co-deposition is a practical method to achieve in-situ doping of heteroatoms in the metal zinc lattice.Therefore,we propose a new strategy to obtain in situ Sn heteroatomic doped Zn anode by adding a small amount of economical Sn Cl₂additive to the Zn SO₄electrolyte,thereby reducing the surface energy of the zinc anode and inhibiting the growth of zinc dendrites.Our strategy is to electrolyte modify AZBs in a very simple way,ie.add a small amount of Sn Cl₂and C₆H₅Na₃O₇·2H₂O additives to the Zn SO₄solution without further processing.In addition,with this very simple strategy,we can also successfully suppress the formation,corrosion and hydrogen evolution of zinc dendrites during charge and discharge.According to the calculation results of density functional theory（DFT）,the doping of Sn atoms in the lattice of zinc can significantly reduce the surface free energy of the low-index zinc crystal surface.Experiments have shown that under the condition of adding Sn Cl₂additives,the overpotential of zinc plating/stripping is significantly reduced,and the life of Zn∥Zn symmetrical batteries is greatly extended to 2000 h.In particular,the Zn/HATN full battery containing Sn Cl₂additive has good stability and shows a long cycle life（～10,000 cycles）,which indicates the application potential of Sn Cl₂electrolyte additive.（2）D-sorbitol was further investigated as the electrolyte additive in AZBs.In this work,due to the exsitance of hydroxyl groups,D-sorbitol adsorbs on the one hand accumulate near the the tips of the zinc anode surface before the zinc ions under the driven force of electrolytic polarization to re-regulate the Zn²⁺distribution,and on the other hand reduce the activity of water molecules in the zinc ion solvating sheath layer by hydrogen bonding,inhibiting the formation of zinc dendrites.Studies have shown that Zn∥Zn symmetrical batteries assembled with an electrolyte with D-sorbitol as an additive can be stably cycled for more than 400 h under experimental conditions of 3m A cm^-2.Moreover,at different current densities,Zn∥Zn symmetrical batteries still maintain good battery performance,which means that the addition of additives makes the battery system very stable.After assembling a full battery with HATN,after 1000cycles at a current density of 5 A g^-1,the specific capacity is 169 m Ah g^-1.We intuitively demonstrated the inhibition effect of D-sorbitol additive on zinc dendrite growth through in situ galvanizing observation experiments.