Study On The Uniform Deposition Strategy Of Lithium/Zinc Metal Anode

Li and Zn metal anodes have extremely high theoretical specific capacities(Li:3860 mA h g^-1;Zn:820 mA h g^-1)and low electrode potentials.The organic Li-ion batteries with metallic Li as the anode and aqueous Zn-ion batteries with metallic Zn as the anode have high energy densities,making them the research hotspots for nextgeneration high-performance rechargeable batteries.However,Li/Zn metal anodes face the challenge of nonuniform and uneven deposition during cycling,which can exacerbate side reactions between the electrode and the electrolyte,thus reducing the coulomb efficiency as well as increasing the internal resistance of the batteries,more seriously,can induce Li/Zn dendrites growth thus causing battery short circuit.In a word,the nonuniform and uneven deposition of Li/Zn metal anodes significantly limits the service life and commercialization of Li/Zn metal batteries.Aiming at the problem of uneven deposition of Li/Zn metal anodes,in this paper,with the goal of regulating uniform electron distribution on the electrode surface and adjusting uniform metal ions dispersion at the electrode interface,a rational design of 3D current collector for Li metal anode has been conducted,and optimization for Zn foil metal anode has been carried out from three dimensions:from surface to interface,and then to electrolyte.The modification strategies were demonstrated to effectively induce uniform Li/Zn deposition,achieving stable and high-performance Li/Zn metal anodes.The research contents are as follows:1.To address the problem of Li accumulation on the top surface of carbon cloth collector during cycling due to the lack of electronic conductivity difference in its 3D structure,a uniform ZIF-8 insulation layer with rich cavities was designed to coat the carbon fibers,which guided multi-channel Li-ion diffusion inside the carbon cloth collector and enabled multi-site Li nucleation on the 3D carbon fiber skeleton,thus achieving uniform Li deposition on the 3D bulk structure of carbon cloth collector.With the high specific surface area advantage,3D carbon cloth collector is theoretically able to provide sufficient sites for Li deposition.However,it was found in studies that,since the pristine carbon cloth collector shows no electronic conductivity difference over the 3D structure,during deposition,Li-ions tend to be reduced at the top surface of carbon cloth due to lower migration energy barriers,which results in rapid blockage of the top surface due to large amounts of Li accumulation,limiting the fully utilization of its 3D structure advantages.In this work,electronic insulated ZIF-8 is used to coat the carbon fibers to limit Li deposition within the interlayer composed of carbon fiber and the ZIF8 layer,which effectively inhibited the localized Li accumulation on the carbon cloth and promoted Li-ion diffusion to carbon cloth collector inner as well as uniform Li nucleation over the whole 3D carbon fiber skeleton,achieving uniform Li deposition within the ZIF-8 layer on the 3D bulk structure of carbon cloth collector.Moreover,the rich cavities design on the ZIF-8 layer ensure high Li capacity of the interlayer（8 mA h cm-2）.Such uniform and spatially limited deposition route inhibited Li dendrite growth and controled the volume expansion of metallic Li,significantly improving the cycling stability of the carbon cloth collector.A full cell assembled with a commercial LiFePO₄ cathode and the ZIF-8 coated carbon cloth Li anode has achieved a stable cycle of over 1200 cycles at a high current density of 6 C.2.To address the problem of the severe Zn dendrite growth caused by the surface machining defects such as scratches and cracks on Zn foil anode,a kinetically slow SnF₂+Zn→ZnF₂+Sn reaction was designed to achieve targeted chemical polishing of the defect sites,which improved the uniformity of the electric field distribution on the electrode,significantly suppressing the growth of Zn dendrite on Zn foil.It was observed in studies that the machining defects on Zn foil are often with sharp Zn burrs and can lead to the localized high electric field intensity thus inducing severe Zn dendrite growth.In view of this phenomenon,taking advantage of the slow kinetic of the SnF₂+Zn→ZnF₂+Sn reaction in DMF solvent,the Zn burrs with high reactivity were specifically reacted,achieving the targeted chemical polishing of the Zn foil anode.Electrodeposition experiments and COMSOL electric field simulations showed that such targeted polishing significantly improved the electric field uniformity thus inhibiting Zn dendrite growth at the defects and extending cycle life of the symmetric cells by nearly 3 times.By additionally adding PVDF to the polishing reaction solution,an anti-corrosion layer with the resulted ZnF₂ and Sn was synchronously constructed on Zn foil,further improving the cycling stability of the electrode,the final anode achieved a stable cycle of 1500 hours in symmetric cell.In a Zn‖CNT@MnO₂ full cell,the capacity retention for 1600 cycles was as high as 98.6%.3.An ultrathin（1 μm）quinone carbonyl rich polymer protective layer with strong zincophilicity was in situ constructed on Zn foil anode by the polymerization of oxidized tannic acid,silane coupling agent and metallic Zn,which induced highly dispersed Zn-ion flux,achieving ultra uniform and flat Zn deposition morphology.In this work,it was demonstrated that the spontaneous oxidation of hydroxyl groups on tannic acid molecules to quinone carbonyl groups under alkaline conditions can effectively activated the Zn-ion conductivity of tannic acid based polymers.Basing on it,a quinone carbonyl rich tannic acid-organosilicon polymer interphase was constructed on Zn foil through in-situ polymerization reaction,which was found to induced uniform and flat zinc deposition over the entire Zn foil surface.The electrochemical analysis and density functional theory calculations revealed that the quinone carbonyl on the oxidized tannic acid molecule has a strong adsorption effect on Zn-ion,which can promote the rapid kinetic processes of Zn-ion capture,transfer and deposition on Zn foil,thus realizing uniform Zn nucleation and growth at multiple sites,also alleviating the Zn dendrite growth caused by concentration polarization of the electrolyte,finally significantly increasing the uniformity of Zn deposition.Such protected Zn anode obtained a stable cycle of 2800 hours in symmetric cell.Matching with CNT@MnO₂ cathode,after 1500 cycles at 1 A g^-1,the full cell achieved a capacity retention up to 99.2%.4.A polyvinyl alcohol-agarose（PVA-AG）bimolecular gel electrolyte with hierarchical structure was prepared by a low-pressure freezing method.When PVA-AG gel electrolyte was applied to aqueous Zn metal batteries,it was demonstrated to induce the（002）crystal face preferred Zn deposition and obtained a flat and dense deposition morphology.In this work,the room temperature formability of AG gel and the function of a freeze dryer to quickly freeze water under low pressure condition were ingeniously united to greatly shorten the preparation time of PVA gel.In addition,the prepared PVAAG gel presented a hierarchical structure,that is,it has rich pores inside,which can accommodate a large amount of electrolyte,and obtain high Zn-ion conductivity,while the outer surface presents a uniform,dense and smooth film shape,showing excellent surface adhesion with the Zn foil anode.The electrodeposition experiments proved that the outer film of PVA-AG effectively homogenized the Zn-ion flux in the gel pores and guided the preferential oriented Zn deposition along the（002）crystal plane with uniform and flat deposition morphology.The Zn symmetric cell assembled with PVAAG electrolyte achieves a stable cycle of nearly 3000 hours under a large current density of 5 mA cm^-2.Matching with CNT@MnO₂ cathode,the full cell can stably cycle for nearly 600 cycles at a current density of 1 A g^-1.