Study On Surface Modification And Additives Optimization For Modulating Zn Metal Anode

Rechargeable aqueous Zn ion battery is regarded as the most promising candidates for lithium-ion battery,due to the intrinsic safety of aqueous electrolyte and the unique merits of Zn anode,such as low price,proper redox potential（-0.76 V vs SHE）and high theoretical specific capacity(5855 m Ah cm^-3,820 m Ah g^-1).Despite many advantages,Zn anode still suffers from some problems during the cyclic process of battery,including dendrites formation,hydrogen evolution reaction,electrochemical corrosion and passivation.These issues will destroy the stability of electrode-electrolyte interface and the consistency of Zn ions deposition,eventually weakening the electrochemical performance of aqueous Zn-ion batteries.Therefore,this work mainly carries out the researches of modifying Zn anode from the perspective of surface modification and additives optimization.The primary study process is as follows:（1）We first in-situ synthesize a 3D porous layer of Bi/Bi₂O₃ on Zn anode by the self-assembling method.Based on the own structure and multifunctional properties of composite layer,an innovative synergistic modulation mechanism is proposed:thermodynamically-favorable cluster-dissociation mechanism inhibiting dendrites production and chemically-inert interface protection mechanism preventing side reactions.With the aid of dynamic stimulation,theoretical calculation and numerous characterization techniques,we discuss these two mechanisms in depth.In terms of dendrites restriction,the zincophilic hetero-metallic Bi phase not only can speedy the ion-desolvation kinetics in electrons double layer,but also directionally capture Zn ions and induce the uniform nucleation at these Bi sites.Further,the molecular dynamics stimulation and corresponding charge density difference plots indicate that there exists the low dissociation energy and weaker charge interaction between large Zn atomic-clusters and Bi phase.Meanwhile,the atomic clusters themselves are thermodynamically-unstable and more easily decomposed on the surface of both Bi and Bi₂O₃ phase.Consequently,the Bi/Bi₂O₃ layer enable eliminate the atom accumulation caused by tip effects at harsh test conditions.Scanning electron microscope images show that even ions deposition is occurred in the 3D network of Bi/Bi₂O₃.In the matter of side reactions alleviation,the hydrogen evolution is greatly inhibited relying on the much high Gibbs free energy of hydrogen adsorption（ΔG_H）and low onset hydrogen evolution overpotential mainly contributed by the Bi₂O₃ phase.Meanwhile,the electrochemical corrosion is suppressed,due to appropriate hydrophobicity and high corrosion potential of Bi/Bi₂O₃ layer.Consequently,the electrode-electrolyte interface is protected by such chemically-inert characteristics of Bi/Bi₂O₃ interphase.Benefiting from the above cooperative modulation process,the Zn@Bi/Bi₂O₃|Zn@Bi/Bi₂O₃ symmetry-cell manifests an ultralong cyclic lifetimes of 3120 h at 1 m A cm^-2.In addition,even at a lean electrolyte environment(E/C:45μL m Ah^-1)and limited Zn supplement（N/P:6.3）,Zn@Bi/Bi₂O₃|Mn O₂ full-cell still shows excellent cyclic performance of 500 cycles at 1 A g^-1.（2）Apart from the surface modification,additives optimization is a good strategy.We add a well water-soluble and environmentally-friendly organic additive into the Zn SO₄ electrolyte,where the inositol molecule contains six hydroxyl groups with intense electron density.The polar polyhydroxy groups in inositol endow it with strong electronic interaction ability and good zincophilicity.On the one hand,the fourier transform infrared spectroscopy measurement show that inositol additives enable re-modulate the hydrogen bond network between hydrated Zn ions and sulfate anions in the original Zn SO₄electrolyte.Furthermore,some related electrochemical characterizations further confirm that inositol molecules can not only weaken the solvation structure of hydrated Zn ions,thereby accelerating the Zn ions transport kinetics,but also can directionally immobilize free water molecules and sulfate anions,thus avoiding the side reactions of electrochemical corrosion and hydrogen evolution involved with anions and active water molecules.On the other hand,when the electrostatic adsorption of inositol molecules on the surface of Zn electrode,the zincophilic hydroxyl groups in the inositol can provide a large number of nucleation sites,which prevent the horizontal diffusion of abundant Zn ions and uniform nucleation on the zinc electrode.Therefore,the surface of Zn electrode regulated by the inositol additives exhibit a flat and dense morphology without any Zn dendrites and inert by-products after the repeated galvanostatic charging-discharging cycles at high current densities.At 2 m A cm^-2 and 1 m Ah cm^-2,the Zn||Zn symmetry-cells maintain a stable cyclic lifespan of 1800 h.The Zn||Cu half-cells keep the high coulombic efficiency of 98.5%during the 250 cycles.Moreover,the Zn||NH₄V₄O₁₀ full-cells exhibit the long-term cycling time of 850 cycles with a superior capacity retention of 92.8%at 5 A g^-1.