A structural and electrochemical study of lithium-ion battery electrolytes using an ethylene sulfite solvent: from dilute to concentrated solutions.

We report the structural and electrochemical characteristics of lithium (Li)-ion battery (LIB) electrolyte solutions using an ethylene sulfite (ES) solvent that is used as an electrolyte additive for LIBs. From dilute to highly concentrated ES solutions with lithium bis(fluorosulfonyl)amide (LiFSA), the formation of Li-ion complexes was investigated using a combined Raman and infrared spectroscopy study with the aid of density functional theory (DFT) calculations to quantitatively determine their solvation and ion-pair structures depending on the Li salt concentration (cLi). The results reveal that, in the dilute solutions (<1.0 mol dm-3), Li-ions are fully solvated with ES molecules to form a tetrahedral-like [Li(ES)4]+ complex; however, with the increasing cLi (up to 2.5 mol dm-3), the Li-ion complex changes in structure to form contact ion-pairs coordinated with both ES and FSA anions. It also reveals that further increasing cLi to approximately 3.0 mol dm-3 leads to the ionic aggregate formation, i.e., multiple Li-ion complexes linked via several FSA anions. LiFSA/ES electrolyte solutions exhibited a reversible Li-ion insertion/deinsertion reaction into/from the graphite anode irrespective of cLi. This is due to the high-grade ES-derived passivation films on the electrode as a result of the preferential reductive decomposition of the ES molecules trapped within the Li-ion coordination sphere. According to the charge-discharge test, the concentrated LiFSA/ES solutions exhibited the high C-rate performance, which is superior to the concentrated electrolyte solutions using conventional organic solvents.

Effect of Alkyl Side Chain Length on the Lithium-Ion Conductivity for Polyether Electrolytes.

The design guidelines of polymer structure to effectively promote lithium-ion conduction within the polymer electrolytes (PEs) are crucial for its practical use. In this study, the electrolyte properties of a simple polyether having alkyl side chains with varied lengths (-(CH2)m-H, m = 1, 2, 4, 6, 8, and 12) were compared and established a valid design strategy based on the properties of the alkyl side chain. Various spectro-electrochemical measurements successfully connected the electrolyte properties and the alkyl side chain length. Steric hindrance of the alkyl side chain effectively suppressed the interaction between ether oxygen and lithium-ion (m ≥ 2), decreasing the glass transition temperature and the activation energy of lithium-ion transfer at the electrode-electrolyte interface. The strong hydrophobic interactions aligned and/or aggregated the extended alkyl group (m ≥ 8), creating a rapid lithium-ion transport pathway and enhancing lithium-ion conductivity. A clear trend was observed for the following three crucial factors determining bulk lithium-ion transport properties along with the extension of the alkyl side chain: 1) salt dissociability decreased due to the non-polarity of the alkyl side chain, 2) segmental mobility of polymer chains increased due to the internal plasticizing effect, and 3) lithium-ion transference number increased due to the inhibition of the bulky anion transport by its steric hindrance. The highest lithium-ion conductivity was confirmed for the PEs with an alkyl side chain of moderate length (m = 4) at 70°C, indicating the optimized balance between salt dissociability, polymer segmental mobility, and selective lithium-ion transfer. The length of an alkyl side chain can thus be a critical factor in improving the performance of PEs, including thermal stability and lithium-ion conductivity. Precise tuning of the alkyl side chain-related parameters such as steric hindrance, polarity, internal plasticizing effect, and self-alignment optimizes the polymer segmental mobility and salt dissociability, which is crucial for realizing high lithium-ion conductivity for PEs.

Improved ionic conductivity for amide-containing electrolytes by tuning intermolecular interaction: the effect of branched side-chains with cyanoethoxy groups.

Polymeric materials are considered as promising electrolytes for all-solid-state secondary lithium batteries with superior energy and power densities, long cycle lives, and high safety. To further improve the ionic conductivity of polymer electrolytes, the development of a simple and efficient method that enables precise tuning of the three key factors, polymer segmental dynamics, Li+ coordination structure, and salt dissociability, is desired. In this study, we focus on an amidation reaction, which is a simple reaction with broad applicability, to explore the impact of the side-chain structure on the intermolecular interactions within the polymer, which dictates the aforementioned key factors. We synthesized a series of polyoxetane-based polymers having different branched side-chains, i.e., methyl (PtBuOA) and bulky cyanoethoxy (P3CEOA) groups, via amidation reaction. Spectro(electro)chemical analysis verified that the large steric hindrance of the cyanoethoxy side-chain effectively breaks the hydrogen bond network and dipole interaction within the polymer, both of which decrease the polymer segmental mobility, leading to better long-range Li+ conduction. Furthermore, the unique Li+ coordination structure consisting of a cyano group, ether/carboxyl oxygen, and TFSA anion in P3CEOA electrolytes has moderate stability, which effectively promotes the short-range Li+ conduction. The amide group, with a relatively high dielectric constant, improves the dissociability of lithium salt. We confirmed a more than three orders of magnitude improvement in ionic conductivity by introducing the cyanoethoxy side-chain, than that obtained by introducing the PtBuOA electrolyte with a methyl side-chain. This work provides a holistic picture of the effect of the side-chain structure on the intermolecular interaction and establishes the new design strategy for polymer electrolytes, which enables the precise tuning of the molecular interaction using the side-chain structure.

Role of polar side chains in Li+ coordination and transport properties of polyoxetane-based polymer electrolytes.

Lithium ion conducting polymer electrolytes (PEs) have been the subject of intense research for lithium metal battery applications. Here, we investigate the effects of polar side chains on Li+ coordination and ionic transport properties to gain insights for improving the insufficient conductivity of traditional ether-based solid PEs. Poly(trimethyleneoxide)-based (or polyoxetane-based) polymers with ether or nitrile groups were synthesized by ring-opening polymerization. The thermal, ionic transport, and electrochemical properties and the local structure of Li+ coordination were studied in the presence of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA). The glass transition temperature (Tg) of the PEs with ether side chains increased with increasing LiTFSA content, whereas the PEs with the nitrile functionality showed the opposite trend at higher salt concentrations. In addition to the unique trend for the Tg values of the PEs in the presence of LiTFSA, the nitrile groups played pivotal roles as coordination sites for Li+ ions in the first coordination shell and as a polar medium to increase the permittivity of the PEs. These characteristics of the nitrile groups can endow PEs with improved ionic transport properties.