Effect of magnesium on the electrochemical behavior of lithium anode in LiOH aqueous solution used for lithium–water battery

Abstract

The effect of adding magnesium to lithium on lithium electrochemical behavior in 4 mol L⁻¹ LiOH is studied using electrochemical techniques. The results show that the hydrogen evolution rate is decreased with increasing Mg content. Through theoretical analysis and X-ray (XRD) investigation, MgH₂ and Mg(OH)₂ are created on the surface film of lithium–magnesium alloys after discharge. The porosity of the lithium surface film is decreased by MgH₂, Mg(OH)₂ combined with LiOH and LiOH·H₂O and the hydrogen evolution rate is decreased effectively. Magnesium addition to lithium reduces the extent of hydrogen evolution by alteration of the hydride–hydroxide layer also reduces the extent of anodic dissolution without a significant change in the system efficiency.

Introduction

Lithium is a promising anode material due to its more negative standard electrochemical potential (−3.05 V/SHE) and higher unit mass electrochemical equivalence (3.86 Ah g⁻¹) than any other metals [1], [2], [3]. The theoretical specific energy could reach 8450 Wh kg⁻¹ when lithium is used as the anode in a lithium water battery [4]. It is vitally important to study the electrochemistry behavior of lithium in alkaline aqueous solution. The parasitic corrosion reaction between lithium and water occurs when lithium discharges: 2Li + 2H₂O = 2LiOH + H₂. Lithium is consumed without creating current due to this reaction. Measurement must be taken to reduce the parasitic corrosion reaction in order to enhance the current efficiency. Study on the electrochemical behaviors of lithium in alkaline aqueous solutions has been carried out by several research groups [4], [5], [6], [7], [8], [9]. It was reported that bilayer passive films were formed on the lithium surface in alkaline aqueous solutions [6]. The passive film on lithium comprises a thin lithium hydride film in contact with the metal, and a thicker hydrated lithium hydroxide film [10]. The property and integrity of the surface film are affected by many factors such as temperature, concentration and flow-rate of solutions. In consequence, the parasitic corrosion reaction is affected. The organic corrosion inhibitor in the solution could reduce hydrogen evolution by adsorption or chemical reaction but its effect is limited at high temperature [11]. A polymer membrane and ionic liquid have been used on the lithium anode surface to hinder the direct reaction between lithium and water. It might reduce hydrogen evolution but the current density is very low [12], [13], [14]. The properties of the film can also be altered by alloying the metal [15], [16], [17]. Aluminum was utilized as an alloying element added to the lithium anode [16], [17]. However, the higher aluminum content in the alloy anode caused a lower activity in current density of the Li–Al alloy. Magnesium is an alkaline-earth metal with a comparatively negative standard electrochemical potential [−2.69 V vs. standard hydrogen electrode (SHE)] in alkaline solution. Lithium and magnesium have similar physical and chemical properties. In the Li–Mg phase diagram [18], the Li-rich solid solution phase (β) covers almost 90% of the phase diagram. The solubility of magnesium hydroxide in alkaline aqueous solution is comparatively lower, and more stable than that of lithium hydroxide. The purpose of the present project is to investigate the effect of addition of magnesium to lithium on lithium anode electrochemical behavior in 4 mol L⁻¹ LiOH aqueous solution.