乙二醇甲醚电解液添加剂对水系锌离子电池电化学性能的影响

Effect of Ethylene Glycol Methyl Ether on Electrochemical Performance of Aqueous Zinc-Ion Battery

水系锌离子电池(AZIBs)由于其高理论容量、高储量、高安全性等优点,在大规模储能系统中极具应用前景。然而,锌负极界面上锌枝晶和副反应等问题严重制约了AZIBs的发展。本工作通过使用乙二醇甲醚(MECS)作为电解质添加剂来增强锌离子电镀/剥离的高度可逆性。实验以及理论计算结果表明:MECS可以减少配位H_(2)O分子的数量,打破水的氢键网络并重构Zn^(2+)的溶剂化结构,从而抑制副产物和锌枝晶的生成。因此,含有MECS的混合电解液可以使Zn//Zn对称电池在10 mA/cm^(2)以及15 mA·h/cm^(2)的测试条件下稳定循环450 h,并且放电深度(DOD_(Zn))达到了54%。即使在高电流密度的测试条件下(10 m A/cm^(2),1 m A·h/cm^(2)),Zn//Ti半电池的平均Coulomb效率(CE)也达到了99.39%。此外,与纯ZnSO_(4)电解液相比,利用MECS电解液添加剂所组装的Zn//V_(6)O_(13)·H_(2)O全电池表现出优异的倍率性能和循环性能。这项工作为稳定锌金属负极提供了一种可行的方法。

Introduction Aqueous zinc-ion batteries (AZIBs) as one of the highly anticipated areas in the new generation of battery technologies have broad application prospects and potential.Compared to conventional lithium-ion batteries,AZIBs have higher safety,cost-effectiveness,environmental friendliness,and higher energy density,making them highly sought after in energy storage and renewable energy.Nevertheless,a high concentration of water molecules in the AZIBs can result in the decomposition of water into H^(+) and OH^(–) during the charging/discharging process,which triggers significant hydrogen evolution reactions and corrosion problems,ultimately compromising the stability of the metal zinc electrode.Meanwhile,zinc dendrites are easily formed due to the uneven zinc deposition.To address these issues,this paper innovatively proposed an electrolyte additive of ethylene glycol methyl ether (MECS) to enhance the stability of the zinc electrode.The impact of MECS electrolyte additives on the AZIBs performance was systematically investigated via the experiments and theoretical calculations.Methods An electrolyte was prepared via adding different concentrations of MECS into 2 mol/L ZnSO_(4) solution.The concentrations of MECS additives used were 2%,4%,and 9%(in volume fraction,referred to as‘2%MECS’,‘4%MECS’,and‘9%MECS’).V_(6)O_(13)·H_(2)O was prepared.Firstly,2.73 g of V_(2)O_(5) and 4.52 g of H_(2)C_(2)O_(4) were added to 40 mL of deionized water (referred to as“solution A”) and stirred at 90℃for 1 h.Subsequently,10 mL of H_(2)O_(2) and 30 mL of ethanol were added to solution A,which was then transferred to a high-pressure vessel lined with PTFE.The vessel was heated at 180℃for 3 h.Finally,the product was filtered,washed with ethanol and deionized water for at least 3 times,and then the washed material was dried in vacuum at 60℃for 24 h.The preparation steps for the cathode plate were as follows:V_(6)O_(13)·H_(2)O,conductive agent (acetylene black),and PVDF were mixed at a weight ratio of 7:2:1,with NMP used as a solvent,and stirred evenly.The obtained slurry was coated on Ti foil,and dried in vacuum at 80℃for 24 h.The samples were characterized by nuclear magnetic resonance (NMR),Fouier transform infrared spectroscopy(FTIR) and Raman spectroscopy (RS).Also,the binding energy was calculated based on the density functional theory (DFT).Results and discussion Based on the results by BMR,FTIR and RS,the shift of ^(1)H peaks and the stretching vibrations of O—H both indicate a change in the solvation structure of Zn^(2+)in the solution with the MECS additives.The analysis of the DFT calculation results reveals that the binding energy between MECS and Zn^(2+)is significantly higher than that between Zn^(2+)and H_(2)O,indicating the preference of MECS to coordinate with Zn^(2+)and replace the positions of water molecules in the solvation structure of Zn^(2+).The restructured solvation structure of Zn^(2+)reduces the number of active water molecules in the electrolyte,thereby lowering the activity of parasitic reactions and enhancing the stability of the metal zinc anode.The effect of MECS additives on the electrochemical performance of batteries was thoroughly investigated via assembling Zn//Zn symmetrical cells,Zn//Ti cells,and full cells.Under relatively mild testing conditions (i.e.,0.5 mA/cm^(2) and 0.5 mA·h/cm^(2)),Zn//Zn symmetrical cells with MECS additives can cycle stably for 1 250 h,while those without MECS additives only cycle for about250 h before short-circuiting occurs.The batteries with MECS additives demonstrate an excellent performance even under different testing conditions due to the regulation effect of MECS at the zinc anode/electrolyte interface.Conclusions A novel electrolyte additive (MECS) was developed to stabilize zinc metal anode.Based on experimental and theoretical calculations,MECS molecules could participate in altering the solvation structure of Zn^(2+),reduce the quantity of active water molecules at the zinc anode interface through strong coordination with Zn^(2+),resulting in a smooth zinc deposition and a decreased by-product formation.Consequently,Zn//Zn symmetrical cells assembled with the improved electrolyte demonstrated stable cycling for over 1 250 h.In addition,the assembled Zn//V_(6)O_(13)·H_(2)O full cell also showed an excellent performance,maintaining a high capacity even after 800 cycles at a current density of 1 A/g.This study could present a simple,effective,and economic electrolyte modification approach to achieve effective utilization of zinc in aqueous zinc-ion batteries,providing innovative pathways for the development of next-generation secondary batteries.