Investigations Of Zn Metal Anode In Intrinsic-safe Organic Electrolytes For Rechargeable Zn Batteries

Zinc battery,as an oldest one,recently revives as a hot topics in the battery community due to its low cost,great eco-friendliness and high safety.However,there are several great challenges,particularly Zn metal anode cycle life,which hamper the development and wide applications of Zinc ion rechargeable batteries?ZIB?.Zn metal anode in traditional alkaline electrolytes suffers dendrite formation,passivation,self-corrosion and shape change during repeated charging/discharging that remarkably shorten the battery cycle life.The main scientific problem lies in the thermodynamic instability of aqueous electrolyte with Zn metal.Numerous strategies have been reported to resolve the aforementioned problems,including the separator modifications,incorporation of additives into electrolytes or electrodes,changing the electrode architecture,however no breakthrough has been made on the basis of conventional alkaline electrolytes.Recently,thermodynamically stable non-aqueous electrolytes with organic solvents or ionic liquid,even polymer have been investigated to enhance the electrochemical performances of Zn batteries,but the Zn dendrite and additional safety issue are still unsolved.Therefore,in this thesis,we proposed the concept to adopting intrinsic safe organic electrolyte to obtain dendrite-free and high Coulombic efficiencies?CEs?Zn anode,leading to safe and long-cycle-life Zn batteries.The main research content and results are listed the following sections:1.TEP-based Electrolytes for Zn anodeFor the first time we report the use of Triethyl Phosphate?TEP?as a sole and co-solvent to gain highly stable and dendrite-free performance of Zn-anode.Stable plating/stripping of Zn-anode in Zn/Zn symmetric cells for more than 1500 cycles?3000hours.?was obtained at a current density of 0.1,0.25 and 0.5 mAcm^-2.Very high Zn-deposition/stripping average CE of 99.68%was afforded by TEP electrolyte in a Zn/SS cell.Moreover,the SEM images of cycled Zn-anodes uncovered highly porous interconnected Zn-deposits free from dendrites.Surface characterizations of the cycled Zn-anodes in TEP electrolyte revealed the presence of Zinc phosphate/or its derivatives that served as molecular template for guided nucleation of Zn resulted in highly porous networking which has decreased the local current density that paved way for long-term stable cycling of the Zn/Zn symmetric cells.The electrolyte displayed good compatibility with both Zn-anode and Copper Hexacyanoferrate?CuHCf?cathode when employed in CuHCf/Zn battery.The full cell showed 74%capacity retention for over 1000 cycles at a rate of 1C and high rate capability.The performance of the CuHCf/Zn cell was found superior to previous reports in literature using Zn²⁺ions as intercalating ions.The present work is supposed to be a contribution towards cost-effective and safe battery systems owing to the economical friendliness of the materials used.2.Intrinsically Safe TMP Solvent for Zn anodeBased on the results of TEP,the maximum current density for long cycling obtained is only 0.5mA cm^-2 that is insufficient,to improve the current density further we have adopted TMP as it has high dielectric constant,more solubility of Zn-salt that will increase the ionic conductivity and in turn decrease the deposition/stripping overpotential and afford Zn-anode stable at high current densities.Dendrite and interfacial reactions have puzzled zinc?Zn?metal anodes for rechargeable batteries more than decades of years.Here we have tackled this serious issue by the introduction of non-flammable Trimethyl Phosphate?TMP?based safe organic electrolyte replacing traditional alkaline aqueous electrolyte for Zinc anode.Excellently stabilized and reversible deposition stripping of zinc is obtained for more than 2000 hours at current densities of 0.1,0.25,0.5 and 1mA cm^-2 without any dendrite formation.The graphene-analogous dendrite-free Zn deposit was gained via TMP surfactant and Zn phosphate molecular template.A remarkably high average Coulombic efficiency?CE?of 99.57%for Zn-plating/stripping on Stainless Steel?SS?was obtained for over 500 cycles in our non-flammable TMP electrolyte by using a Zn/SS cell.The practical viability of our newly explored electrolyte was investigated by coupling Na₂MnHCF cathode with Zn negative electrode in?0.5 M Zn?OTf?₂+1 M NaFSI?in TMP to form a dual ion(Na¹⁺-Zn²⁺)battery.The dual ion(Na¹⁺-Zn²⁺)battery provided a reversible capacity value of 89.3mAh g^-1 at a C-rate of 1(1 C=160 mAh g^-1)possessing a capacity retention of 77%over the span of 1000 cycles.3.TMP-DMC Based Electrolyte for Zn anode and batteriesFor the search of extended cycle life and high stability of Zn-anode,both TEP and TMP non-aqueous electrolytes have shown excellent results.However,the maximum current density achieved is 1.0 mA cm^-2.For practical applications high current densities are required and the critical issue of both TEP and TMP is the high viscosity and low ionic conductivity.In this work,we have adopted the incorporation of additives in TMP-electrolyte for achieving stable performance of Zn-anode at high current densities without sacrificing the CE of Zn plating/stripping.The addition of DMC in TMP to form TMP-DMC?1-1?electrolyte have enabled the Zn/Zn symmetric cell cycling at 2.0 mA cm^-2 along with extending the cycle life of Zn/Zn symmetric cells from 2000 hours to over 5000 hours at a current density of 1.0 mA cm^-2.Moreover,affording stable rate performance of symmetric cells up to 3.0 mA cm^-2.The addition of DMC has not sacrificed the CE by affording an average CE of 99.15%for over 300 deposition/stripping cycles.The morphology of the cycled Zn-anodes revealed dendrite-free smooth surfaces that has enabled stable cycling.Moreover,as a proof of concept,non-aqueous Zn-VS₂ battery delivered a specific capacity of 146.6 mAh g^-1 having a capacity retention of 94.38%over 500 cycles.4.Co-solvents for TMP based electrolyteIn very recent research findings,the presence of water as additive has been found critical for the operation of Zn batteries utilizing organic electrolytes.In the organic electrolytes,the dry cathode interface presents high interfacial resistance that restrits the diffusion and intercalation of Zn-ions,the presence of water as additive wets the surface thus facilitating the migration and intercalation of intercalating ions in to the cathode lattice.Moreover,the presence of water will improve the ionic conductivity values of the electrolyte and could afford cycling at high current densities as well.Considering this,we have formulated the TMP-based electrolytes with water as a co-solvent in a volumetric ratio of 80-20 for TMP and water respectively.The Zn/Zn symmetric cell cycling performance at high current density values of 3.0 mA cm^-2 afforded stable cycle performance for over 1000 deposition/stripping cycles,whereas in purely aqueous electrolytes the cycling stability of symmetric cells can last only for several hours even at a moderate current density of 0.5 mA cm^-2.Dendrite-free morphologies of the Zn-anode after cycling reveals the highly compatible nature of the electrolyte towards Zn-anode.Acetonitrile?AN?and Dimethylformamide?DMF?were also investigated as co-solvents with TMP electrolyte for obtaining highly stable Zn-anode at much high current densities without sacrificing the CE of Zn-deposition/stripping.In a word,a highly stable electrolyte for dendrite-free and highly reversible Zn-anode has been proposed in TEP and the stable cycling was obtained at current density of 0.5 mA cm^-2,following that TMP with high Zn-salt dissolution abilities have afforded high current density of 1.0 mA cm^-2.After this work DMC,DMF,H₂O and AN as co-solvents incorporated in TMP raised the current density to 5 mA cm^-2.Moreover,various cathode materials were adopted to investigate the compatibility with TEP or TMP based electrolytes.We think the excellent anodic stability,high CE and full cell cycle stability will accelerate the progress of low cost and intrinsic safe non-aqueous Zn rechargeable batteries.