Research On Electrolyte Systems For Rechargeable Magnesium Batteries

The demand for green secondary batteries，which possess high energydensity, high safety and low cost, is continuously increasing because of fossilenergy shortage and environmental issues in21^st century. Mg is an attractiveanode material for its high theoretical specific capacity (2205mAh g^-1,3832mAh cm^-3), and very negative standard electrode potential （-2.37V vs. SHE）.In addition, compared with lithium metal, Mg is much cheaper （1/25of thelithium price）, more easily processed, safer and not plagued by dendriticformation, etc. Hence, rechargeable Mg battery using Mg as anode hasacquired more and more attention as one of next-generation energy storagedevices given its potential advantages in safety and cost.However, the research of rechargeable Mg battery is still at the primarystage and its development was restricted by the two major bottlenecks: first,owing to the chemical activity of Mg, the passivating surface films formed onMg in most polar aprotic electrolytes do not conduct Mg²⁺, which result in thedifficulty to finding suitable electrolyte solutions with wide electrochemical window for high-energy rechargeable Mg battery. Second, the choice ofcathode materials has been limited by the difficulty of intercalating Mg ionsin many hosts. In this dissertation, aiming at the subsistent problems ofpresent electrolytes such as narrow electrochemical window, air sensitive andcorrosion to current collectors, etc. we synthesized a series of organicmagnesium salts or magnesium based compounds which were dissolved inproper polar and aprotic solvents, and finally developed new rechargeable Mgbattery electrolyte systems with high conductivity, wide electrochemicalwindow and excellent electrochemical reversibility for Mgdeposition-dissolution process. Furthermore, we assembled rechargeable Mgbatteries using the home-made electrolyte and cathode material， andinvestigated their electrochemical performance. The major research contentsare presented as follows:1. A series of phenyl based Grignard reagents with different substituentgroups or different substituent positions were synthesized via the reaction ofan aryl halide （RX） with Mg in high pure THF solvent under standard schlenkconditions, and their electrochemical performances were investigatedsystematically. The results indicated that electronic effect, differentsubstituent positions and substituent groups on phenyl ring can exert a strongeffect on electrochemical Mg deposition reversibility，anodic stability and ion conductivity of phenyl based Grignard electrolytes.2. On the basis of in-depth understanding of Grignard reagent electrolytes,we designed and synthesized a high-performance electrolyte system based onorganic boron Mg complex salts, which presented the highest electrochemicalstability window （3.5V vs. Mg RE） known to date, high ionic conductivity(2×10^-3S cm^-1) and the excellent reversibility for Mg deposition-dissolution（coulomb efficiency¹00%）. The main components and equilibriums of theunique boron based electrolyte solutions were identified by NMR,single-crystal XRD, etc., the correlation between the electrochemical anodicstability and the identified solution equilibrium species was also investigated.This boron based electrolyte system had recovered ability after partiallylosing its performance when exposed to air and less corrosive nature.Furthermore, the reversible electrochemical process of Mg intercalation intoMo₆S₈chevrel phase cathode confirmed that the boron based electrolyte couldbe practically used in rechargeable Mg battery systems.3. We designed and synthesized an air-stable electrolyte system based onthe novel Lewis acid-base complex via a reaction between AlCl₃andorganic Mg salt （ROMgCl）. The （BMPMC）₂-AlCl₃/THF electrolyte solutionpresented high ion conductivity (2.5610^-3S cm^-1), high reversibility of Mgdeposition-dissolution, and good anodic stability （2.65V vs. Mg RE）. The phenolate based electrolyte showed good compatibility with Mo₆S₈intercalation cathode, indicating that it could be practically used inrechargeable Mg battery systems.4. A new type of halide-free boron based electrolyte （Mg[Mes₃BPh]₂/THF）was designed and prepared. Electrochemical tests revealed its high ionconductivity (1.510^-3S cm^-1), high Mg deposition-dissolution reversibility,and wide electrochemical window （2.6V vs. Mg RE）. The electrochemicalproperties of the electrolyte on different metal current collectors weresystematically studied. The results showed that halide-free electrolyte indeedlessened the corrosion to conventional metal current collectors. This workprovided a stepping stone for developing new halide-free electrcolyte systemsfor rechargeable Mg batteries.