Jennifer M. Pringle

Fluorine-free ionic liquids and salts are attracting growing interest as safer and more sustainable electrolyte components for Li-ion batteries, addressing both the flammability of conventional carbonate-based electrolytes and the environmental persistence of fluorinated anions. This motivates the use of the nonfluorinated acesulfame (ACE–) anion, derived from potassium acesulfame, a toxicologically evaluated tabletop sweetener. Further, ether-functionalized cations may facilitate lithium-ion transport by promoting favorable lithium coordination environments. Here, two ionic electrolytes were prepared by combining the ACE– anion with either the cyclic-ether N-ethyl-N-methyloxazolidinium (C2moxa+) cation or the N-methoxy-N,N,N-trimethylammonium (N111(1O1)+) cation, where the ether is functionalized on the side chain, and lithium acesulfame (Li-ACE) to evaluate their promise as battery electrolytes. The thermal and transport properties and the ion dynamics were studied and compared at various Li-ACE concentrations. Among all the lithium-based electrolyte systems, [N111(1O1)][ACE] containing 10 mol % Li-ACE exhibited the highest conductivity, reaching 5.4 × 10–4 S cm–1 at 50 °C. However, [C2moxa][ACE] exhibited a higher Li+ diffusivity ratio at both 20 and 50 mol % Li-ACE content, indicating that the cyclic ether oxygen may create a distinct local chemical environment that facilitates greater relative mobility of Li+. Additionally, the Li+ solvation environment in acesulfame-based electrolytes was elucidated using combined experimental and simulation approaches. In both electrolytes, Li+ was found to coordinate with ACE- through the nitrogen atom as well as the sulfonyl and carbonyl oxygens. Moreover, molecular dynamics simulations suggest that smaller Li–Li clusters were formed in [C2moxa][ACE] electrolytes compared to [N111(1O1)][ACE], which may contribute to the enhanced Li+ mobility compared with ACE– in [C2moxa][ACE] electrolytes at 20 and 50 mol %. These results indicate that cation chemistry plays an important role in tuning Li+ solvation structure, ion clustering, and ion transport.

Barocaloric (BC) materials represent a highly promising family of solid-state refrigerants that offer real potential to replace environmentally damaging gas refrigerants. The rational design of BC materials relies on understanding solid–solid phase transitions (SSTs) under pressure. Organic ionic plastic crystals (OIPCs) emerge as promising BC candidates, but direct characterization of their pressure-dependent phase behavior remains a challenge. Herein, we employ high-pressure broadband dielectric spectroscopy (HP-BDS) as a tool to map the phase diagrams of three OIPCs: 4-ethyl-4-methylmorpholinium bis(fluorosulfonyl)imide ([C₂mmor][FSI]), tetraethylammonium bis(fluorosulfonyl)imide ([N₂₂₂₂][FSI]), and tetraethylammonium bis(trifluoromethanesulfonyl)imide ([N₂₂₂₂][TFSI]). We demonstrate that the real part of the complex dielectric permittivity (ε′) is uniquely sensitive to SSTs in OIPCs, which are driven by changes in dipolar reorientation. By performing both isobaric cooling/heating and isothermal compression/decompression experiments, we directly determine the pressure sensitivity (dT_SS/dP) of the SSTs and observe the transition hysteresis in terms of temperature and pressure, finding both properties to be strongly ion-dependent. Furthermore, hysteretic behavior at high pressure is found to be unique to each OIPC, despite similarities in ion structure. In particular, the pressure hysteresis directly observed for [C₂mmor][FSI] is lower than that predicted from quasi-direct (isobaric) measurements, indicating the kinetic delay is smaller for a pressure change than a temperature change, which would further improve the already excellent BC metrics of this OIPC.

Barocaloric (BC) materials, being those in which a thermal change is driven by the application and removal of hydrostatic pressure, offer an energy efficient, solid-state alternative to ubiquitous but problematic volatile hydrofluorocarbon refrigerants. However, finding BC materials with high entropy transitions occurring within the required sub-ambient temperature regions remains a challenge. Here, we report a new family of barocaloric organic ionic plastic crystals (OIPCs) based on seldom-explored sulfonium cations that demonstrate solid–solid phase transitions exhibiting strikingly high entropy changes, up to 375 J kg⁻¹ K⁻¹. These transitions are pressure-sensitive (up to 14.5 K kbar⁻¹) and occur at temperatures suited to refrigeration and freezing technologies (−20 °C to 4 °C), with low transition hysteresis (111][FSI]). This combination of properties leads to excellent performance over a range of BC metrics, with pressure-normalised refrigeration capacities among the highest to-date and as high as 2.9 J kg⁻¹ bar⁻¹. For the first time, we also demonstrate the pressure cyclability of BC OIPCs, finding [S₁₁₁][FSI] retains its exceptional entropy change and low hysteresis over 100 DTA pressure cycles. The nature of the molecular structure and dynamics introduced at the transitions, which underpins their large entropy changes, is investigated using solid-state nuclear magnetic resonance spectroscopy and positron annihilation lifetime spectroscopy, the latter being a novel technique in the BC context. We elucidate structural features of the constituent ions that benefit the BC performance of OIPCs, demonstrating their tunability and as-yet untapped potential for application in sustainable cooling technologies.

Ether-functionalized ionic liquid electrolytes (ILEs) with fluorinated anions enhance Li⁺ mobility through the cation’s influence on the Li solvation. Herein, we examine how the cationic oxygen position affects its interactions with Li⁺ at high concentrations of lithium bis(fluorosulfonyl)imide. Advanced nuclear magnetic resonance spectroscopy techniques are employed to probe Li–cation interactions, local oxygen environments, and ion mobilities. In acyclic ether cations, the N-(3-methoxypropyl)-N-methylpyrrolidinium [C_3O1mpyr] exhibits more selective coordination and enhanced Li⁺ diffusivity and transport number relative to 1-methyl-1-(2-methoxyethyl)pyrrolidinium. On the other hand, cyclic ether cations show that Li⁺ remains closest to the ring oxygen, with 3-ethyl-3-methyl-oxazolidinium [C₂moxa] displaying interactions across both the cyclic ether and alkyl chain, whereas 4-ethyl-4-methylmorpholinium presents more localized interactions. Faster Li⁺ transport was observed in [C_3O1mpyr] and [C₂moxa], consistent with stronger Li–cation coordination. These results demonstrate that variations in cation structure can have a strong impact on Li⁺ interactions and ion transport in ILEs.

Ionic liquids and salts are of interest as safer next‐generation electrolytes for energy storage applications due to beneficial properties that can include nonvolatility and nonflammability. However, to overcome environmental concerns associated with perfluorinated anions prevalent in these materials, it is important to develop alternative, fluorine‐free anions. Here, we report the synthesis and characterization of five nonfluorinated salts by pairing the acesulfame anion (ACE ^‐ ) with a range of cations: N‐methoxymethyl‐N,N,N‐trimethylammonium (N _111(1O1) ⁺ ), N‐methoxyethyl‐N,N,N‐trimethylammonium (N _111(2O1) ⁺ ), N‐ethyl‐N‐methyloxazolidinium (C ₂ moxa ⁺ ), N‐ethyl‐N‐methylmorpholinium (C ₂ mmor ⁺ ), and N‐methyl‐N‐propylpyrrolidinium (C ₃ mpyr ⁺ ). The thermal, structural, transport and electrical properties of these materials were studied using techniques including differential scanning calorimetry, single crystal X‐ray diffraction, electrochemical impedance spectroscopy, pulsed‐field gradient nuclear magnetic resonance and cyclic voltammetry. Discussion of the structure–property relationships in acesulfame‐based salts and their comparison with fluorinated salts with the same cations is reported for the first time, revealing distinct trends that are concluded to be largely a result of the size, conformational freedom, charge delocalisation and the intermolecular interactions of the ACE ^‐ anion. All acesulfame‐based salts displayed thermal stabilities above battery operational range (>180°C) and wide electrochemical stability windows exceeding 4.9 V, demonstrating their potential as electrolytes for lithium and sodium batteries.

Sodium conducting ionic materials employing single and mixed-anions were synthesised and characterised to bring insights into the anion properties driving phase formation and ion mobility.

The development of non-flammable, non-volatile electrolytes is important for safer lithium and sodium batteries, to facilitate our transition to a net zero economy, but the reliance on fluorine-containing anions brings significant environmental concerns. Here, acesulfamate based ionic liquids (ILs), sodium acesulfamate (Na[ace]) and lithium acesulfamate (Li[ace]) are introduced as promising new fluorine free materials as a more sustainable alternative to the perfluorinated anions (BF₄, PF₆, FSI and TFSI) currently utilised in energy storage devices. The resulting ammonium, phosphonium and pyrrolidinium acesulfame ILs showed promising physicochemical properties with a relative high conductivity (e.g. 1.4 × 10⁻⁴ S cm⁻¹ at 30 °C for [N₁₂₂₂][ace]) and a wide electrochemical stability window (∼4 V). Lithium and sodium acesulfamate salts were synthesised from potassium acesulfamate (ACE-K) via an acid-base reaction. The use of acesulfamate ILs as electrolytes was investigated by mixing with sodium or lithium acesulfamate salts and characterisation of their physicochemical and electrochemical properties. Polyethylene oxide based free standing membranes composed of [N₂₂₂₂][ace] with Na or Li [ace] were fabricated and tested in symmetrical Li or Na metal cells, demonstrating good electrochemical performance in both variable current density tests and longer-term cycling at elevated temperatures. Thus, the new Li and acesulfamate salts, the ILs and their mixtures, represent valuable new materials for the development of fluorine free electrolytes for energy storage devices.

The thermal, transport and electrochemical performance of zwitterion-based electrolytes for Li and Na batteries is reported.