Synthesis And Electrochemical Properties Of Rechargeable Magnesium Battery Electrolytes

Because of its low density (1.74g·cm-3), low electrode potential(-2.37V (in acid)，-2.69V (in alkali)), high theoretical capacity (2205Ah·Kg-1), low price, high security, rechargeable magnesium battery systemwith Mg as the anode has become the spot of the study. Although it cannotcompare with the lithium battery used in small devices, it has become acompetable candidate in heavy load applications. Since the reactivity ofmagnesium, a dense passivation film is formed on the surface of themagnesiumin in many solutions, which prevents the free access ofmagnesium ions. It is very important to persue the solutions which do notform the dense passivation. Although several kinds of solutions, in whichmagnesium can be reversibly deposited and dissolved, have been used asthe electrolyte for rechargeable magniusium batteries, there are still someproblems such as the narrow electrochemical window, complicatedpreparation process, air sensitivity, strong volatility, corrosivity to thecurrent collectors.In this paper, we studied two kinds of electrolytes, they areethylenethiourea-Grignard reagent/THF solutions and benzenethiol-basedsolutions. Compared with the well-known―the first generation‖and―thesecond generation‖electrolytes, ethylenethiourea-Grignard reagent/THFelectrolytes improve the corrosion resistance of the commercial nickelcurrent collector, enzenethiol-based electrolytes enhance the airinsensitivity.Three kinds of ethylenethiourea-Grignard reagent/THF electrolyteswere prepared by reacting ethylenethiourea with different Grignard reagents such as EtMgBr/THF, PhMgBr/THF and PhMgCl/THF. Cyclicvoltammetry tests on Pt disk electrode show that the electrolytes have goodperformance of magnesium deposition-dissolution as Grignard reagents andhigher anodic stabilities than Grignard reagents. The oxidativedecomposition potential of ethylenethiourea-PhMgBr/THF on Pt electrodecan reach to2.3V (vs. Mg/Mg2+). The conductivity ofethylenethiourea-PhMgBr/THF electrolyte initially increases and thendecreases with the increase of the concentration and reaches to the highestvalue615μS·cm-1at0.9mol·L-1. Cyclic voltammetry comparison of0.9mol·L-1ethlenethiourea-PhMgBr/THF electrolyte on Pt, Ni, Cu and Alelectrodes demonstrates the oxidative decomposition potential on Nielectrode is the highest (2.4V vs. Mg/Mg2+, higher than2.17V for―the firstgeneration‖electrolyte and2.2V for―the second generation‖electrolyte).Constant-current discharge and charge tests of coin cells further show thatmagnesium deposition-dissolution potential on Ni substrate is low and thestable cycle efficiency can reach to92%. For ethlenethiourea-PhMgBr/THFelectrolyte, nickel is suitable to be used as a practical current collector.Benzenethiol-based solutions (RSMgCl)n-AlCl3/THF(R=4-methylbenzene,4-isopropylbenzene,4-methoxybenzene; n=1,1.5,2)were obtained by a simple reaction of benzenethiol compounds with aGrignard reagent C2H5MgCl/THF and AlCl3in THF and theelectrochemical performance for rechargeable magnesium batteryelectrolytes were firstly reported. At first, RSMgCl (labeled as MBMC,IPBMC, MOBMC, respectively)/THF solutions were synthesized by thereaction of4-methylbenzenethiol,4-isopropylbenzenethiol, and4-methoxybenzenethiol compounds with C2H5MgCl/THF via a hydrogenmetal exchange with rapid evolution of ethane gas, respectively.Furthermore,(RSMgCl)n-AlCl3/THF solutions were obtained by thereaction of RSMgCl/THF with AlCl3/THF at the different molar ratios ofRSMgCl with AlCl3. The benzenethiol-based solutions as electrolytes forrechargeable magnesium batteries were characterized in term of anodic stability and reversibility of magnesium deposition-dissolution using cyclicvoltammetry and galvanostatic charge/discharge techniques. Furthermore,the compatibility of the solutions with Mo6S8cathode material was verifiedusing coin cells with Mo6S8cathode, Mg anode and benzenethiol-basedelectrolyte. It is concluded that both substituents on benzenethiol and theratios of RSMgCl with AlCl3have effects on the electrochemicalperformance.0.5mol·L-1(IPBMC)1.5-AlCl3/THF shows the bestelectrochemical performance with2.4V (vs. Mg RE) anodic stability, a lowvoltage for magnesium deposition-dissolution, a high cycling efficiency of98.9%. The coin cell constructed with the Mo6S8as cathode and0.5mol·L-1(IPBMC)1.5-AlCl3/THF as electrolyte shows good charge-dischargeperformance. XRD pattern further proved the deposition of Mg in thesolution. The room temperature ionic conductivity of0.5mol·L-1(IPBMC)1.5-AlCl3/THF is2.48mS·cm-1. Moreover, the air insensitivecharacter and easy preparation make it a promising candidate forrechargeable battery electrolyte.