Functionalized Cyclic Phosphonium (CylP+ nA-) Ionic Liquids As Candidate Materials for Li-Ion Battery Electrolytes

Abstract

Realizing high energy density Li-ion and beyond Li-ion batteries requires transition from traditionally used graphite anode to Li alloys or Li metal anodes. In addition to electrodes, newer and better electrolytes with improved electrochemical and thermal stability are also required for developing batteries that are safer and have longer cycle life. Room temperature ionic liquids (RTIL) are potential alternatives to currently used organic electrolytes due to their intrinsic properties—wider electrochemical windows, lower vapor pressures and greater thermal stabilities—compared to organic electrolytes. Here we present a systematic investigation of electrochemical stability of barely researched cyclic phosphonium (or phospholanium) cation, CylP+ n (n = 5 or 6) based ionic liquids. CylP+ n (n = 5 and 6) are phosphorous analogues of pyrrolidinium and piperidinium cations. We explore the chemical space spanned by of linear alkane-, cyclic alkene-, ether- and amine- functionalized ILs to identify the relevant chemical structures that are potential alternatives to currently used organic electrolytes. We use quantum chemistry calculations to calculate redox free energies and electrochemical stabilities of functionalized CylP+ 5 based ionic liquids. In the past, Density Functional theory (DFT) has been extensively used by multiple research groups to provide invaluable insights into electrolyte decomposition pathways that determine the inherent redox stability of electrolytes in the bulk and at the electrode/electrolyte interfaces. All functionalized CylP+ 5 cations demonstrate excellent oxidation stability against currently preferred cathode materials and are marginally unstable at the Li interfaces. Linear alkyls, cyclic alkenes, and primary amines show good oxidative stability among the variants in the combinatorial library and hence can be potential alternatives to currently preferred electrolytes. Linear alkyl functionalized phospholanium cations and other variants that are marginally unstable at Li interface can become potential alternatives to organic electrolytes when used in combination with graphite anode or Li anode with SEI forming additives, such as Fluoroethylene carbonate (FEC). Besides the electrochemical stability of base phospholanium cations, chemical stability of the functionalities calculated from free energies of lithiation prior to reduction show that the functional substituents are stable and do not breakdown at the Li interface. Overall, we find that alkyls feature good electrochemical stabilities compared to ethers, and it is further possible to design functionalized CylP+ 5 cations with longer alkyl chains terminated by alkoxy groups to achieve better electrochemical and thermo-physical properties. We further explore CylP+ 5 and Pyr+ (pyrrolidinium) cations with alkoxy and alkyl functionalization since the addition of oxygen units to side chains of base cations was reported to improve physico-chemical properties of similar nitrogen based ILs. Redox stability, inter-molecular interactions (of ILs) and electrostatic interactions between Li ions and functionalized cations were calculated to demonstrate the potential of CylP+ 5 cation based ILs as battery electrolytes.