Boron-based cations and anions as ionic liquid electrolyte components for rechargeable lithium metal batteries
Creators: name: Clarke-Hannaford, Jonathan
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340601
subject: Catalysis and mechanisms of reactions
classificationCode: 340607
subject: Reaction kinetics and dynamics
classificationCode: 340704
subject: Theoretical quantum chemistry

description: The development of next generation rechargeable battery technology is required to enable further electrification of our transport and to satisfy the ever-increasing energy storage demands of batteries in stationary and portable applications. Lithium (Li) metal batteries (LMBs), which use a pure lithium metal anode, could provide incredibly lightweight and energy dense rechargeable batteries, as lithium metal possesses the highest theoretical capacity (3860 mA h g-1) and the lowest density (0.59 g cm-3) among all known anode materials. However, rechargeable LMBs have serious safety concerns due to the highly reactive nature of lithium metal and the volatility of traditional organic solvent-based electrolytes. The low-flammability, high thermal stability and wide electrochemical stability windows (ESWs) of ionic liquids (ILs) establishes them as a safer alternative electrolyte solvent that could enable LMBs to operate safely at higher temperatures. Moreover, a select few ILs have shown that they can stabilise the Li metal anode through forming a stable solid electrolyte interphase (SEI) layer on the Li anode surface. This SEI layer consists of electrolyte and anode reaction products that cover the surface in a thin film which can act as a protective layer that prevents unwanted side-reactions and degradation of the anode, therefore prolonging the cycle-life of the battery. Hence, determining the chemical reactions and species that form the SEI layer can enable the design and formulation of safer and more compatible LMB electrolytes.

High thermal stabilities and wide ESWs have been previously demonstrated for boron-based anions and cations when employed as part of an IL or Li-salt. However, their reaction with the Li metal anode, ability to form a SEI layer and interaction with Li-ions is unknown. Hence, the scientific research presented in this thesis will focus on: (i) the fundamental properties and stability of boron-based anion and cation electrolyte components, (ii) the reaction mechanisms and chemical species leading to the initial stages of SEI layer formation at different temperatures, (iii) the effect of cyclisation on the stability of the boronium cation, and (iv) elucidating the ion-ion interactions between the IL anions and cations, and the Li-ion charge carrier. To investigate these electrolyte components individually and at the Li metal surface, a computational approach was used that includes density functional theory calculations (DFT) and ab initio molecular dynamics (AIMD) simulations. Fourier transform infrared (FTIR) spectroscopy experiments were also performed to assist with the characterisation of the electrolytes.

The key reaction processes and composition of the initial SEI layer formed on the Li(001) surface were determined for the ILs based on the boronium cation (trimethylamine)(dimethylethylamine)dihydroborate [NNBH2]+ and bis(trifluoromethanesulfonyl)imide ([TFSI]-) and bis(fluorosulfonyl)imide ([FSI]-) anions using AIMD simulations. Decomposition fragments from the anions that include Li2O and LiF were responsible for the formation of an inner inorganic layer while the [NNBH2]+ cation remained intact and is more likely to be involved in the formation of the outer organic layer of the SEI. The weak interaction of [NNBH2]+ with the Li metal surface indicates unwanted side reactions originating from the cation are unlikely to occur and can partly explain the enhanced cycling stability previously shown for IL electrolytes based on [NNBH2]+.

Cyclisation of the (open-chain) [NNBH2]+ cation to the (cyclic) N,N,N',N'-tetramethylethylenediamine)dihydroborate ([(TMEDA)BH2]+) cation did not change the composition of the inner-organic layer formed on the Li surface, which is still dominated by the [TFSI]- or [FSI]- decomposition species. Interestingly, the cyclic cation is predicted to be more prone to decomposition on the Li(001) surface via B-H and C-H bond dissociation than [NNBH2]+. However, the [(TMEDA)BH2]+ cation is found to be remarkably stable when in contact with the Li(001) surface based on the absence of any decomposition during room and elevated temperature AIMD simulations. The ability of the boronium cations to remain intact after contact with the Li metal anode suggests they could enable stable cycling of LMBs when used as part of the electrolyte. 

The key vibrational frequencies characteristic of [NNBH2]+ and [(TMEDA)BH2]+ based-ILs, electrolytes and IL/Li(001) systems were determined using Fourier transform infrared (FTIR) spectroscopy and DFT calculations. The FTIR spectra provided a fundamental insight into the Li+ coordination environment for the respective electrolyte formulations of the boronium cation-based ILs, such as Li+ coordination with [FSI]- and [TFSI]-, leading to hypsochromic shifts of key vibrational modes assigned to the anions. The calculated vibrational frequencies for the IL/Li(001) systems provide critical information for future in-situ spectroscopic studies that analyse the IL | Li interface.

The boron-based anions tetrakis(trifluoromethyl)borate ([B(CF3)4]-), pentafluoroethyltrifluoroborate ([(C2F5)BF3]-) and pentafluoroethyldifluorocyanoborate ([(C2F5)BF2(CN)]-) were found to form a LiF-rich SEI layer when modelled as Li-salts on the Li(001) surface which may help stabilise the Li anode. Additionally, superior (theoretical) electrochemical properties were determined for all three borate anions when compared to the established [TFSI]- and [FSI]- anions, with [B(CF3)4]- having the highest oxidative stability limit. The formation of a LiF-rich SEI layer and the superior electrochemical stability limits of the boron-based anions indicate they could improve the cycling stability and longevity of LMBs when used as electrolyte components.

To conclude, electrolyte components with high thermal stability and that can stabilise the lithium anode surface are needed to enhance the safety and longevity of next generation LMBs. The findings presented in this thesis have shown the boron-based ions to be suitable components of ionic liquid electrolytes that may provide a protective SEI layer on the Li metal anode surface as well as superior electrochemical and thermal stability, which can enable safer and more reliable LMBs.