Flame-resistant and fluorine-free electrolytes based on (combining) the salts lithium saccharinate (LiSac) and lithium bis(oxalato)borate (LiBOB) in a single solvent triethyl phosphate (TEP) solvent and vinylene carbonate (VC) additive are presented and evaluated for lithium metal battery application. The dual salt electrolyte, 1.5 M LiSac + 0.2 M LiBOB in TEP w. 2 % VC, clearly outperforms the single salt ones in terms of electrochemical performance, especially vs. LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes, properties that originate in a Li+ cation first solvation shell mainly composed of Sac and BOB anions, promoting formation of a mechanically stable, inorganic-rich cathode electrolyte interphase layer, which by X-ray photoelectron spectroscopy was revealed to comprise Li3N, BxOy and SO32− species. Overall, this also results in stable cycling, and a capacity retention of 86 % in both Li||LiFePO4 and Li||NMC811 cells after 500 cycles at 1C rate – hence offering an intrinsically safer electrolyte that also enables the use of both lithium metal anodes and medium-to-high-voltage cathodes. 

Herein, we introduce a composite membrane comprising polyvinylidene fluoride/graphene nanosheets (PVDF/graphene) for applications in piezoelectric nanogenerators (PENGs) and lithium-ion batteries (LIBs), where the graphene nanosheets play a vital role in enhancing the piezoelectric properties, surface energy, and porosity. A comparative analysis of the pure PVDF and the PVDF/graphene is conducted to evaluate their piezoelectric performance and suitability as separators in LIBs. The PVDF/graphene composite membrane produced a significantly improved piezoelectric output of ∼10.8 V under a force of 75 N, while the pure PVDF membrane exhibited only ∼3.7 V under the same conditions. Additionally, the Li//PVDF/graphene//graphite half-cell retained ∼81.3% of its specific capacity and maintained a coulombic efficiency of over 99.2% after 100 cycles at a 0.2 C rate. In contrast, the Li//pure PVDF//graphite half-cell retained only ∼48.6% specific capacity. Furthermore, in a full-cell configuration, the graphite//PVDF/graphene//LCO cell demonstrated excellent stability, retaining ∼88% of its specific capacity after 50 cycles, whereas the cell with pure PVDF membrane retained only 38%. Therefore, the PVDF/graphene nanosheet composite membrane has the potential to be used as a piezoelectric membrane in PENGs and as a separator in LIBs.

Here, we introduce three new halogen-free salts based on the green, sustainable, and hydrolytically stable saccharinate (Sac) anion, and their deep eutectic solvents (DESs) with ethylene glycol (EG). All the three salts exhibit distinct and well-defined thermal behaviors, ranging from ionic plastic crystals (IPCs) to supercooled liquids and classical ionic liquids (ILs). In contrast, their corresponding DESs display no detectable thermal events, a clear indication of successful DES formation, which is well supported by FTIR spectroscopy and suggests that EG interacts with the −CO and −SO2 groups of the Sac anion. DES with [EMPip][Sac] offers superior ion transport and electrochemical properties, supporting a voltage range up to 5.7 V, and as an electrolyte in a symmetric supercapacitor, a specific capacitance of 46.5 F g⁻¹ at 5 mV s⁻¹, an energy density of 9.9 Wh kg⁻¹, and power density of 1022 W kg⁻¹, at a current density of 0.2 A g⁻¹. The capacitor retained 99 % of its initial capacitance after 20,000 cycles at ambient temperature. Altogether, these halogen-free DES electrolytes offer promising electrochemical properties, making them ideal electrolytes for supercapacitors operating at ambient temperatures over a wide potential range.

Despite their widespread adoption, LIBs are still facing several challenges, mainly related with safety concerns of conventional electrolytes that are currently limiting their practical use. Solid-state polymer electrolytes (SPEs) represent a promising alternative to produce safer devices, as they offer higher flexibility and energy density, easy processability, non-flammability and improved mechanical strength, even if they often suffer from low ionic conductivity at room temperature. To address the latter issue, the development of novel SPEs based on a blend of polyethylene oxide (PEO) and Xanthan gum (XG), a natural polysaccharide with notable mechanical and rheological properties, is proposed. In this study, the thermal, physical, and electrochemical properties of the PEO-XG blends were investigated, aiming to assess their potential as electrolyte for all solid-state lithium metal batteries. Improved ionic conductivity, electrochemical stability window and cycling stability are achieved, confirming the effectiveness of XG incorporation. The most promising electrolyte formulation was studied using self-diffusion 7Li pulse-field-gradient (PFG) NMR measurements and different temperatures and further evaluated in full-cell configurations employing two olivine-type cathodes (LFP and LMFP). When paired with a lithium manganese iron phosphate (LMFP) cathode and cycled over an extended voltage window of 2.5–4.5 V, the cell demonstrates high specific capacity and excellent capacity retention, maintaining stable performance for at least 750 cycles.

Here we present the synthesis, physical characterization, and transport as well as electrochemical properties of a novel class of ten ionic liquids (ILs) derived from biomass. Two biomass derived anions such as furan-2-carboxylate [FuA] and tetrahydrofuran-2-carboxylate [HFuA] are coupled to a range of nitrogen heterocyclic cations to create the ILs, for which the nature of cation controlled their properties. For instance, the thermal decomposition temperature ranges from 183 to 259 °C, the glass transition temperature from − 47 to − 70 °C, and the ionic conductivity from 0.002 to 1.4 mS cm⁻¹ at 20 °C. The supercapacitors prepared using [EPy][FuA] and [EMPip][FuA] exhibited specific capacitances of 99 F g⁻¹ and 70 F g⁻¹ at 0.2 A g⁻¹, respectively. The [EPy][FuA]-based supercapacitor achieved an energy density of 56 Wh kg⁻¹ as well as a power density of 410 W kg⁻¹ at 0.2 A g⁻¹, while the [EMPip][FuA]-based supercapacitor achieved an energy density of 36 Wh kg⁻¹ and a power density of 360 W kg⁻¹ at 0.2 A g⁻¹. In addition, the supercapacitors retained 98% and 94% of their initial capacitances after 6000 cycles, for [EPy][FuA] and [EMPip][FuA] electrolytes, respectively.

A number of bis(glycolato)borate (BGB) anion-based salts, comprising Li+, Na+, K+, Mg2+ and Ca2+ cations, has been synthesized and characterized. Fluorine-free electrolytes based on LiBGB and organic solvents, such as dimethyl sulfoxide (DMSO), triethyl phosphate (TEP), and trimethyl phosphate (TMP) have been created and their transport properties, thermal and electrochemical stabilities, and lithium compatibility examined. The ionic conductivities of the 1 M LiBGB-TEP and 1 M LiBGB-TMP electrolytes are ca. 2-3 times lower than for the 1 M LiBGB-DMSO electrolytes (2.05, 2.65 vs. 5.70 mS cm-1 at 25 °C), and as compared to the state-of-the-art 1 M lithium hexafluorophosphate (LiPF6) in EC:DEC (EC:DEC=1:1 in vol., LP40) they display lower ionic conductivities, but the formers’ redox stability on aluminum (Al) and glassy carbon electrodes are much better. Concentrated (>1 M) LiBGB-DMSO electrolytes display enhanced redox stability, but worse Al passivation. Among the electrolytes, 1 M LiBGB-TMP achieves the best long-term stability over 300 h at 0.1 mA/cm2 for Li plating-stripping while the Li compatibility needs to be further improved. Overall, this study introduces a family of versatile fluorine-free orthoborate salts and electrolytes for mono- and divalent batteries, and a fundamental understanding of their transport and electrochemical properties, aiming towards battery applications.

Synthesis, physicochemical, and electrochemical properties are presented for a number of fluorine-free ionic liquids (ILs) comprising dialkylphosphate anions coupled to several N-heterocyclic cations such as pyrrolidinium (Pyrr), piperidinium (Pip) and pyridinium (Py). All the ILs are synthesized in a single-step by reacting trialkyl phosphate with pyrrolidine, piperidine or pyridine. The ILs exhibit thermal decomposition temperatures in the range from 183 to 259 oC, ionic conductivities from 0.07 to 0.57 mS cm−1 at 20 oC and reaches 3.25 mS cm−1 at 60 oC, and electrochemical stability window (ESW) up to 6.8 V on glassy carbon (GC) electrode. The symmetric supercapacitors (SCs) based on multiwalled carbon nanotubes (MWCNTs) using [EMPyrr][DEP] and [BMPyrr][DBP] electrolytes are investigated. The SC based on [EMPyrr][DEP] reveals higher capacity retention, a power density of 1050 W kg−1 and an energy density of 68 Wh kg−1 using 0.5 A g−1 at 60 °C. This paves the way for developing fluorine-free and high-performant electrolytes for supercapacitors operating at elevated temperatures.

This study explores the synthesis and electrochemical performance of graphene oxide co-doped with sodium and potassium (Na–K–GO) as electrode materials for supercapacitors (SCs) designed to operate at 60°C over an extended voltage window. The Na–K–GO is employed as the electrode material, while a fluorine-free ionic liquid (IL), [P4444][MEEA]—comprising a tetrabutylphosphonium cation and a 2-2-(2-methoxyethoxy)ethoxy anion—served as the electrolyte, enabling stable operation over a wide voltage window at elevated temperatures. Using this combination, three coin-cell SCs are fabricated: two symmetric devices (SC-1 and SC-2) and one asymmetric device (SC-3). All the three exhibited remarkable charge storage abilities, a retaining performance over 10 000 charge–discharge cycles at 60°C. Among the three devices, SC-3 exhibited the best overall electrochemical performance, delivering a high specific capacitance of 47.01 F g−1 and an energy density of 27.77 Wh kg−1 at 0.5 A g−1. Even at a higher current density of 1 A g−1, SC-3 maintained a maximum power density of 1000 W kg−1 while sustaining an energy density of 14.21 Wh kg−1, reflecting its strong rate capability. Moreover, the long-term cycling tests at 2 A g−1 demonstrated an outstanding durability of SC-3, which retained 99% coulombic efficiency after 10 000 cycles, significantly outperforming the SC-2 (90%) and SC-1 (79%).

Multinuclear (1H, 31P, and 7Li) NMR was applied to understand the ion dynamics in silica-based iongels with a fluorine-free ionic liquid (IL), tetrabutylphosphonium 2-2-(2-methoxyethoxy)ethoxy acetate, [P4,4,4,4][MEEA], doped with 10 and 30 mol % of LiMEEA. The results were compared with bulk [P4,4,4,4][MEEA]/LiMEEA electrolytes and those confined in the “hard” silica matrix of a porous glass. It was found that lithium ion (Li+) local dynamics and Li+ diffusion coefficients are strongly affected by confinements in an iongel and in the porous glass, as was revealed from the analysis of NMR parameters, such as diffusion decays (DDs) in 7Li PFG NMR spectra, broadening of the 7Li NMR resonance lines and variations in the 31P and 7Li chemical shifts. However, NMR diffusometry data does suggest that the studied electrolytes in the iongel confinement yet have properties like bulk electrolytes: (i) high ion diffusivities, (ii) weak alterations of Vogel-Fulcher-Tammann (VFT) parameters for diffusion; and (iii) high transport numbers of ions. The diffusion coefficients of the [MEEA]- anion and the [P4,4,4,4]+ cation are comparable in the bulk, while they are significantly different in the iongels: The specific interactions of the [P4,4,4,4]+ cations with the negatively charged silica matrix slowed down diffusivities of the cations, while almost no effect of the matrix on diffusivities of the [MEEA]- anions was noticed. It was also found that the tortuosity of the iongel channels has a negligible effect on diffusivities of ions. The lithium complexation or/and solvation shells of Li+ ions remained unaffected. Thus, the ionic liquid-based iongel electrolyte acquired the advantages of a semi-solid phase and offered transport properties of a liquid electrolyte. 

Driven by the potential applications of ionic liquid (IL) flow for charging graphene-based surfaces in many emerging technologies, recent research efforts have focused on understanding ion dynamics and structuring at IL–graphene interfaces. Here, graphene colloid probe (GrP) atomic force microscopy (AFM) was used to probe the dynamics and ion structuring of 1-butyl-3-methylimidazolium tetrafluoroborate at graphene surfaces under various bias voltages. In particular, the AFM-measured nanofriction provides a good measure of the dynamic properties of the ILs at graphene surfaces. Compared with the IL at the unbiased graphene surface (0 V), the charged graphene surfaces with either negative (–1, –2 V) or positive (+1, +2 V) voltages favor a reduction in the friction coefficient by the IL. A higher magnitude of the bias voltage applied on the graphene surface with either sign (–2 or +2 V) results in a smaller friction coefficient than that at –1 and +1 V. In combination with the AFM-probed contact stiffness, adhesion forces, and ion structuring force curves with an ion orientational distribution according to molecular dynamics (MD) simulations, we discovered that the unbiased graphene surface (0 V) possesses randomly structured IL ions and that the graphene colloid probe is more likely to become stuck, resulting in more energy dissipation to contribute to a larger friction coefficient. Biasing of the graphene surface under either negative or positive voltages resulted in uniformly arranged ions, which produced a more ordered ion structure and, thus, a smoother sliding plane to reduce the friction coefficient. Electrochemical impedance spectroscopy (EIS) for the IL with graphene as an electrode demonstrated a greater ionic conductivity in the IL paired with the biased graphene than in the unbiased one, implying faster ion movement at the charged graphene, which is beneficial for reducing the friction coefficient.