Research On The Electrochemical Behavior Of Impurities In Lithium And Magnesium Electrolysis

Metal magnesium and lithium are key alloying elements of high-strength and ultra-light metal structural materials.Lithium metal is a national strategic metal,widely used in new energy,new materials,new medicine and other fields,known as "industrial monosodium glutamate" and "new energy metal".Lithium metal and its products are the key negative electrode materials for high specific energy solid-state lithium batteries.With the continuous application of solid-state lithium batteries in the field of new energy vehicles,the development prospects and demand space for metal lithium will continue to expand.Molten salt electrolysis is the main method for producing metallic lithium and magnesium.In actual production,it is inevitable that raw materials,equipment and tools bring or produce impurities into the electrolyte.The impurities will generate slag,accelerate anode consumption,affect electrolyte circulation,increase current consumption,reduce current efficiency,and increase energy consumption.Some impurities precipitation at the cathode and reduce the purity of the product,resulting in a decline in product quality.Studying the electrochemical behavior of impurities in the electrolysis of metallic lithium(magnesium)molten salt has important theoretical and practical significance for revealing the electrochemical microscopic mechanism,optimizing the electrolysis process,improving current efficiency,ensuring product quality,and energy saving and consumption reduction.According to the technical requirements of battery-grade lithium metal products,this article has investigated the electrochemical behavior of light metal impurity ions(Mg(?),Al(?)),transitional impurity ions(Mn(?),Ni(?),Fe(?))and oxygen compound impurities H2O,LiOH,Li2O and SO42-in the lithium electrolysis process,and the cathode electrochemical behavior of Mn(?),Ni(?),Fe(?)impurities in the magnesium electrolysis process.The results showed that in the process of lithium electrolysis,the reaction of impurity ions Mg(?),Al(?),Mn(?),Ni(?)and Fe(?)on the electrode surface was a one-step reduction process controlled by diffusion and had quasi-reversible characteristics.Impurity ions would be precipitated on the electrode surface prior to the target ions,consuming part of the power and reducing the purity of the product.The experimental results of chronoamperometry showed that the deposition of these impurities on the surface of the tungsten electrode was an instantaneous nucleation process.The research results of cyclic voltammetry and square wave voltammetry showed that Fe(?)cannot exist stably in molten salt system and would be further decomposed into Fe(?).The study of the electrochemical process of oxygen compound impurities such as H2O,LiOH,Li2O and SO42-in the lithium electrolysis system showed that the H2O in the air would diffuse into the electrolyte and participate in the electrochemical reaction,and it was calculated that the electrochemical reaction exchanged electron was 1,indicating that H2O reacted with LiCl to form LiOH.OH-underwent two electrochemical reductions.The calculated number of electron transfer in the first electrochemical reaction was 1,indicating that the first step was the process of OH-reduction to generate H,and the second reaction represented the reduction of H adsorbed on the electrode surface and interacted with the Li+forming LiH.In the lithium electrolysis system,Li2O could easily react with H2O to generate LiOH impurities,which would change the electrode process.The results of chronoamperometry showed that the addition of Li2O changed the nucleation mode of metallic lithium from instantaneous nucleation to progressive nucleation.The presence of H2O,LiOH and Li2O in the electrolyte would consume part of the electricity,affect the deposition of metallic lithium,accelerate electrode wear,and reduce electrode service life.Cyclic voltammetry research results showed that SO42-would migrate to the cathode and participate in the cathode process.The reaction path of SO42-at the cathode was proposed.It was speculated that SO42-can undergo a four-step reduction reaction on the surface of the cathode to generate a variety of products,consumed electricity,and caused cathodic passivation which inhibited the normal deposition of lithium.In the process of magnesium electrolysis,the reduction of impurity ions Mn(?),Ni(?)and Fe(?)on the electrode surface was a one-step two-electron quasi-reversible process controlled by diffusion.The diffusion coefficients at different temperatures were calculated by the semi-integral method.Based on the diffusion coefficients,the activation energies of different ion diffusion were determined as 30.60 kJ mol-1,45.81 kJ mol-1 and 25.12 kJ mol-1,respectively.Nicholson method calculated the standard rate of Mn(?)/Mn(0),Ni(?)/Ni(0)and Fe(?)/Fe(0)systems when the scan rate of cyclic voltammetry was 0.5 V/s.The constant k0 was 0.72×10-3 cm/s,1.68×10-3 cm/s and 2.32×10-3cm/s.The ratio of intrinsic reaction rate to mass transfer rate ? value was 0.0314,0.0785,0.0886.The experimental results of chronoamperometry showed that the deposition of impurities on the surface of the tungsten electrode was instantaneous nucleation.