Electrodeposition Of Li-Cu Alloy From Ionic Liquid For Li/S Battery Anode Material

Lithium has been known as an attractive anode material for Li/S batteries because of its high theoretical capacity and high negative potential. Because of the excellent stability and electrical conductivity, Cu is commonly used as the collector of Li-ion batteries. Formations of Cu frameworks of Li-Cu alloys by dissolving Li during discharging process may inhibit dendrite formation of Li metal during charging. Li-Cu alloy with high atom content of Li is expected to be an active anode material to improve the safety of Li/S batteries. However, it is difficult to prepare Li-Cu alloys from aqueous systems because of negative deposition potential of Li. Currently, Room temperature ionic liquids have attracted much attention in electrodeposition of metal and alloys for the wide electrochemical window and thermal stability. In this work, Li-Cu alloy was obtained by electrodeposition from ionic liquids. The relationship between performance and structure was studied to provide theoretical basis for electrodeposition of other new functional materials.Electrodeposition behavior of metallic Cu from1-hexyl-3-methylimidazolium trifluoromethanesulfonate (HMIMOTF) ionic liquid was investigated. The optimal electrolyte composition and process conditions were as follows: Cu(OTF)20.06~0.12mol/L, depostion potential of-0.48~-0.52V and electrolyte temperature of60~100℃. The nucleation mechanism of Cu2+in HMIMOTF on a Cu electrode was studied. Cyclic voltammetry and chronoamperometry showed that the deposition of Cu2+was an irreversible electrode process controlled by the diffusion step and followed the mechanism of progressive nucleation and three-dimensional growth.Li-Cu alloys were electrodeposited from HMIMOTF ionic liquid by the potentiostatic method. The optimal electrolyte composition and process conditions were as follows: Li(OTF)0.75~1.00mol/L, Cu(OTF)20.10mol/L, electrolyte temperature of60~100℃, depostion potential of-3.0~-4.0V. The atom content of Li in Li-Cu alloy varied from20%to80%. X-ray photoelectron spectroscopy (XPS) and X-ray diffractometry (XRD) indicated that Li-Cu alloy existed as mechanical mixture alloy and no crystal Li was observed. Scanning electron microscope (SEM) results showed that uniform and dense Li-Cu alloy was obtained with higher atom content of Li. Cyclic voltammogram and potential~time curves indicated that Li-Cu alloy was obtained by an induced codeposition mechanism through an insoluble film which was not completely covering the surface of the working electrode.Effect of different kinds of addtives on deposition of Cu and Li-Cu alloy was studied. Cyclic voltammogram result showed that the addition of2-butyne-1,4-diol (BDO) promoted the reduction of Cu2+and Li+. Inductively coupled plasma (ICP) indicated that the addition of BDO reduced the atom content of Li in Li-Cu alloy. Complex of Cu2+with positive charge was formed by the addition of ethylenediamine (EDA), which fovred the reduction of Cu2+. Thus the atom content of Li was reduced. With vinylene carbonate as additives, reduction of Cu2+was inhibited while the reduction of Li+was promoted, so that the atom content of Li in Li-Cu alloy was increased.Li-Cu alloys with different atom content of Li were used as anode for Li/S batteries. Charge-discharge tests indicated that Li-Cu alloy with76.81at%Li exhibited a slower decay rate of battery capacity (1.05%per cycle) compared to that of commercial Li (2.69%per cycle). Cyclic voltammetry tests showed that Li-Cu alloy performed the same mechanism as commercial Li during charge-discharge process. During discharge process, Li in Li-Cu reacted with S as acitive material, leaving Cu framework with mesh porous structure. SEM results indicated that Cu framework with mesh porous structure inhibited the dendrite formation of Li during charge process, thus reduce the decay rate of battery capacity and improve the cycling performance of Li/S battery. Electrochemical impedance tests showed that a stable SEI layer was formed on the surface of Li-Cu alloy anode after the first charge-discharge process and this SEI film was permanently present on the surface of Li-Cu anode since the formation.