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. 

Two fluorine-free phosphonium-based ionic liquids (ILs), (P₄₄₄₄)(CH) and (P₄₄₄₄)(CP), incorporating cyclohexane- and cyclopentane-carboxylate anions, are synthesized to elucidate the role of ring geometry on their properties. The (P₄₄₄₄)(CH) forms a deeply amorphous system with a low glass-transition temperature (T_g) at −55 °C) and a single-step thermal degradation onset at 244 °C, indicating a disordered yet thermally stable liquid phase. In contrast, the (P₄₄₄₄)(CP) exhibits a higher T_g at −49 °C with a higher decomposition onset of 286 °C. The (P₄₄₄₄)(CH) displays an increase in ionic conductivities from 0.058 30 °C to 1.367 mS cm^–1 100 °C, whereas (P₄₄₄₄)(CP) consistently exhibits higher values, rising from 0.078 to 2.618 mS cm^–1 at the same temperatures. When employed as electrolytes in activated-carbon supercapacitors (SCs) tested over 30–90 °C and 0.5–4.0 V, the (P₄₄₄₄)(CP) enables markedly enhanced capacitive performance. The (P₄₄₄₄)(CP)-based device delivers 140.75 mF cm^–2 (at 1 mV s^–1) and 117.51 mF cm^–2 (at 0.195 mA cm^–2), along with an energy density of 65.29 μWh cm^–2, substantially exceeding the (P₄₄₄₄)(CH)-based system (85.36 mF cm–2, 47.42 μWh cm^–2). The devices with both ILs reveal low interfacial resistance and excellent cycling stability, maintaining Coulombic efficiencies exceeding 99% over 10,000 charge–discharge cycles.

This study introduces two new fluorine-free ionic liquids (ILs) produced by coupling biomass-derived heterocyclic anions, i.e., tetrahydro-2H-pyran-4-carboxylate (THP) and furan-3-carboxylate (3-FuA), and tetrahydroxyphosphonium cation (P₄₄₄₄). The (P₄₄₄₄)(3-FuA) IL exhibits slightly higher thermal stability, displays a lower glass-transition temperature and significantly higher ionic conductivity than (P₄₄₄₄)(THP). This improvement arises from π-electron delocalization in the (3-FuA) anion, by dispersing the negative charge over the ring, weakening the cation–anion attractions, and thus enhancing the ion mobility. Owing to the favorable ion transport characteristics, (P₄₄₄₄)(3-FuA) performs exceptionally well as a supercapacitor electrolyte. When paired with multiwalled carbon nanotubes (MWCNT)-based electrodes, (P₄₄₄₄)(3-FuA) delivers an areal capacitance of 430 mF cm⁻² at 2 mV s⁻¹, an energy density of 86 µWh cm⁻² at 0.298 mA cm⁻², and a power density of 1492 µW cm⁻² at 0.995 mA cm⁻², while maintaining 97% Coulombic efficiency after 6 000 cycles at 60°C. In comparison, the (P₄₄₄₄)(THP) IL demonstrate a lower capacitance performance, albeit with robust long-term stability. Overall, both the ILs display enhanced capacitance with increasing temperature, underscoring their potential as fluorine-free electrolytes for supercapacitors operating under elevated thermal conditions.

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.

Here, we report novel electrolytes based on fluorine-free (F-free) salt zinc methanesulfonate (ZMS) in N,N-dimethylpropyleneurea (DMPU) and triethyl phosphate (TEP). While the single-solvent DMPU-based electrolyte exhibited suboptimal performance at 0.5 mA cm–2, the addition of TEP as a dual solvent significantly enhanced the electrochemical performance for over 1000 h at 1 mA cm–2 and up to 2.5 mAh cm–2. TEP modulated the solvation structure of the electrolyte, which facilitated Zn2+ desolvation and promoted uniform zinc plating/stripping, while its partial reduction during initial cycling formed a robust solid electrolyte interphase (SEI) composed of zinc phosphate and organic phosphate species, effectively suppressing the dendrite growth and minimizing side reactions. The dual-solvent electrolyte demonstrated an excellent cycling stability and a high Coulombic efficiency, while the polyaniline (PANI)-based full cell achieved 88% capacity retention at 0.1 A g–1, and ∼80% retention at 1 A g–1 over 500 cycles. Besides, the full cell based on activated carbon (AC) also maintained 76% retention at 0.1 A g–1. This study manifests the potential of the F-free dual-solvent electrolyte for applications in both zinc-ion supercapacitors (ZICs) and zinc-ion batteries (ZIBs).

Energy storage is a crucial element of green transition that drives the growth of technology, and its importance is progressively increasing. The expanding demand for reliable power sources necessitates the development of innovative methods to balance energy production and consumption. Among energy storage devices (ESDs), supercapacitors are considered superior due to their outstanding power, swift charging, long lifespan, and sustainability. The ion mobilities in supercapacitors are mainly determined by the electrolyte, which further influences the charge transfer, voltage, and total power holding capacity. Due to the critical role of electrolytes in supercapacitors, several advancements are made in the design of the electrolyte to increase voltage strength and energy density. The electrolytes for supercapacitors include aqueous-based, organic solvent-based, ionic liquids (ILs) and solid-state electrolytes. Among them, IL-based electrolytes are the most suitable electrolytes due to their combination of beneficial physical and electrochemical properties. However, more than 95 % of the IL-based electrolytes are heavily based on fluorinated compounds, which are creating serious problems not only in the synthesis and implementation levels but also at the recycling stage. There is an urge to develop fluorine-free and non-flammable functional electrolytes for enabling next-generation supercapacitors.

This thesis is an attempt to elevate energy storage technology by providing economically and environmentally efficient solutions in terms of fluorine-free IL-based electrolytes for next-generation supercapacitors. The main aim is to design and identify Fluorine-Free Ionic Liquids (FFILs) as electrolytes for efficient next-generation supercapacitors. The first part of this thesis (Paper I) is focused on the synthesis, physical characterization, and transport as well as electrochemical properties of a novel class of ten FFILs derived from biomass. The 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 FFILs, for which the nature of cation controlled their properties. The second part (Paper II) is dedicated to the synthesis and characterization of dialkylphosphate-based FFILs, that offered high thermal decomposition and low glass transition temperatures, and high ionic conductivities as well as high electrochemical stabilities. Further, their performance as electrolytes in symmetric supercapacitors showed high coulombic efficiency and capacity retention even after long charge-discharge cycles. The part three (Paper III) presents the synthesis, physical and electrochemical characterization of two novel fluorine-free zinc salts and their electrolytes derived from artificial sweeteners. The three-component zinc electrolytes are composed of either zinc saccharinate (Zn(Sac)2 or zinc acesulfamate Zn(Asf)2 salt, tetrabutylphosphonium saccharinate [P4444][Sac] IL, and γ-valarolactone (VL). These electrolytes demonstrated significantly long cycle life in Zn||Zn symmetric cells, while maintaining high capacity retention after extended cycles at a lower current density in an asymmetric hybrid supercapacitors (Zn||Ac).

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.

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.