Plating/stripping Property Investigation Of High Area Capacity Magnesium Metal Anodes And Their Electrodeposition Substrate Design

As the lithium-ion batteries are gradually approaching their energy density limits,batteries with multivalent metals such as magnesium,aluminum,and zinc have gradually received attention.The Mg metal battery with potential high energy density and volumetric capacity(3877 m Ah cm^-3)has been considered by researchers as a promising battery system in recent years.However,the scale application of magnesium anode still faces many challenges:On the one hand,the deposition/dissolution performance and the structure of the electrode electrolyte interface at actual current density and area capacity are still unclear;on the other hand,the controllable regulation and related mechanism of the electrodeposition/dissolution process of magnesium anode at high current density and area capacity are rarely studied.The above two problems seriously affect the reversible capacity and cycle life of magnesium metal battery.In order to solve these problems,the deposition/dissolution process of magnesium metal in different electrolyte and composition of electrode electrolyte interface.Furthermore,a three-dimensional nitrogen/oxygen doped carbon nanofiber array structure was constructed to realize the microchannel-filling growth of magnesium and low electrochemical barrier.The specific research contents are as follows:（1）Insight into interfacial speciation and deposition morphology evolution at Mg-electrolyte interfaces under practical conditions.The electrochemistry of Mg stripping/plating processes within four distinctive Mg-ion electrolytes and the Mg anode electrolyte interfacial chemistry were systematically investigated under practical conditions.Electrochemical results show that the cycle life of Mg//Cu asymmetric cells using these above electrolytes is significantly shortened（less than 10 cycles）.Further optical and electron microscopic analyses reveal that large porous arrays and hemispherical corrosion pits were formed during the stripping process,and the crystal embryos formed preferentially would grow into loosely connected large aggregates instead of uniform and strong film-like deposits.In spite of showing an interconnected particle-like morphology,the Mg deposits could easily penetrate the porous separator,leading to cell failure.The co-deposition of metallic Al is revealed from surface region to bulk,while the Cl-containing species exist in the near surface of Mg deposits.（2）Uniform magnesium electrodeposition via synergistic coupling of current homogenization,geometric confinement and chemisorption effect.The experimental results of the first chapter show that under high current density,uneven distribution of magnesium electrodeposition is an important factor leading to short-circuit of magnesium metal batteries under high areal capacity conditions.In this chapter,a three-dimensional（3D）magnesium affinity array structure electrode is designed,which can regulate the electrodeposition/dissolution process of magnesium through the synergistic coupling of homogenization of electric field distribution,geometric confinement and chemical adsorption.First,electrodeposition-high temperature calcination method was used to prepare a vertical array of nitrogen/oxygen co-doped carbon nanofiber arrays（called"VNCA@C"）electrodes.The experimental results and theoretical calculations show that the uniform short nano arrays help to homogenize the surface current density,and the microchannels in the 3D array structure can promote the preferential nucleation of metal Mg through the geometric confinement effect due to their concave surface characteristics.In addition,the nitrogen/oxygen doped carbon defects show strong chemisorption on Mg atoms,and the abundant defect sites are for the deposition of magnesium.The preferential nucleation sites were provided to promote the uniform deposition.The results show that magnesium can be reversibly deposited/dissolved in a unique and highly reversible microchannel filling mode.The VNCA@C supports exhibit a low nucleation overpotential(429 m V at 10.0 m A cm^–2,while 718 m V and 610 m V at the same current density for carbon cloth and copper foil electrodes)and a highly reversible mg deposition/dissolution cycle.