**2.2 Boron-based ionic liquids in advanced battery technologies (Lithium, sodium and magnesium ion batteries)**

The properties of ILs, such as inflammability, extremely wide liquid range, and especially the stability of some types of cations and anions at high anodic potentials, make them suitable for lithium batteries. ILs are mixed with appropriate lithium salts to make electrolytes for lithium-ion batteries. The choice of electrolyte used for lithium battery production is critical and determined by a variety of factors, including safety and "greenness." Because RTILs are non-volatile and non-flammable, they are more appealing as lithium battery electrolytes than conventional organic liquid solvents. The structural chemistry of boron and oxygen compounds, the building blocks of orthoborates, is characterized by extraordinary complexity and diversity. Lithium borate (see **Figure 8**), one of the metal borates, is one of the building blocks of IL, which is used as a lithium-ion battery electrolyte. Orthoborate-based ILs have been reported to be more efficient than conventional salts such as LiPF6 as they offer several advantages such as halogen-free, non-toxic, good thermal stability, and high compatibility with cathode materials [59–62].

Faiz Ullah Shah et al. [61] described the ion transport mechanism of a ternary combination of phosphonium bis(salicylato)borate IL, diethylene glycol dibutyl ether, and a lithium bis(salicylato)borate (Li[BScB]) salt for lithium-ion batteries. The ion transport properties and viscosity of the orthoborate-based halogen-free ionic liquid hybrid electrolytes were investigated. In the investigation, the lithium bis(salicylato)borate salt was dissolved in a combination of IL and diethylene glycol dibutyl ether. Diethylene glycol dibutyl ether has a flashpoint at 118°C and is miscible with ILs. This research is the first to look at the ion transport processes of orthoborate-based ionic liquid hybrid electrolytes. As a consequence, the maximal solubility of Li [BScB] salt in a combination of ILs and diethylene glycol dibutyl ether at room temperature was determined to be 1.0 mol kg−1. The viscosity of the combination was 1000 times lower than that of the neat phosphonium bis(salicylato)borate ionic liquid. However, no differences in ionic conductivity were observed between the combination and the neat phosphonium bis(salicylato)borate ionic liquid.

Liang, Fuxiao et al. [62] have investigated a novel BBIL electrolyte for high voltage lithium-ion batteries with outstanding cyclic stability. According to the study, adding an appropriate amount of N-propyl-N methylpiperidinium difluoro(oxalate)borate (PP13DFOB) to an electrolyte containing lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) results in a high discharge capacity. In addition, a promising strategy is presented to mitigate aluminum corrosion of boron-based ionic liquid electrolytes and further improve the cycling performance of lithium-ion batteries at high cut-off voltages in the paper.

The construction of electrochemical cells requires the use of target carrier ions such as lithium cations, protons, or iodides. That is, a matrix that transports these target ions is essential for such applications. Several solutions have been suggested

**Figure 8.** *Lithium orthoborate.*

#### *Investigation of Boron-Based Ionic Liquids for Energy Applications DOI: http://dx.doi.org/10.5772/intechopen.105970*

to facilitate selective lithium-ion transport in ILs. Several techniques have been described to accomplish selective lithium-ion transport in ILs. For instance, zwitter ions, which have a cation and an anion in the same molecule, have been proposed to inhibit IL component ion movement under an electrical potential. An IL with alkyl borane units has been synthesized to trap anions by the interaction between boron and the anion [63].

A few publications have subsequently explored variations of IL cation (dominated by small aliphatic and cyclic ammonium cations) and anion (dominated by bis- (trifluoromethanesulfonyl)-imide [TFSI<sup>−</sup> ] or [BF4] − combinations with a range of lithium battery anodes and cathodes.

Clarke-Hannaford et al. [64] have conducted enhanced cycling performance in lithium metal batteries of boronium-cation-based ILs. They investigated using a combination of density functional theory calculations and ab initio molecular dynamics simulations, the chemical stability and reaction mechanisms between the Li surface and [NNBH2][TFSI] and dihydroborate ([NNBH2] + ) to understand the existence of the solid electrolyte interphase layer formed using boronium-cation-based ILs. Simulations showed that the surface interaction with the [NNBH2] + cation is weak, and its anions easily dissociated to create numerous chemical species (LiF, Li2O, and Li2S). The results provided evidence that the [NNBH2] + cation is stable against a lithium metal surface and that self-dissociation of the cation is unlikely to occur. Enhanced cycling performance in lithium metal batteries, a stable solid electrolyte interphase [NNBH2] + cation, usually formed on the Li surface, has been shown to have similar properties compared to commonly used cations, helping to explain the positive performance of boronium cation-based ILs.

Research has been done on Na metal to make second-generation batteries a safer, lower-cost option for energy storage. Regarding sodium batteries, Nikitina et al. [49] [bmim] showed that sodium salt, unlike lithium, has only a minor effect on the conductivity, dielectric properties, and viscosity of sodium tetrafluoroborate (NaBF4) in ILs. Electrical conductivities, densities, viscosities, and molar conductivities of [bmim][NaBF4] ionic liquid were measured in the wide temperature range of (278.15–358.15) K and (238.15–458.15) K, respectively. The values for viscosities and conductivities were described by the Vogel-Fulcher-Tammann equation. The Walden plots, log(Λ) vs. log(η–1), for the NaBF4 solutions, coincide with the straight line found for neat [bmim][BF4], indicating that the solute had only a limited impact on the structure of the ionic liquid. Also, they found that the dielectric properties of the most concentrated NaBF4 solution (0.1739 mol·kg–1) are identical to those of pure [bmim][BF4].

Basile et al. [21] reported on room temperature ionic liquid comprising the dicyanamide anion as a successful electrolyte system for sodium metal batteries that do not contain expensive fluorinated species. At a current density of 10 μA cm−2, the effects of sodium plating and stripping from Na metal electrodes were examined in a symmetrical Na|electrolyte|Naconfiguration. The presence of residual water molecules in the ionic liquid electrolyte was seen to have a significant impact on the surface film and plating/stripping behavior. The increase in moisture content from 90 to 400 ppm has hampered both electrodeposition and electrodissolution of the Na+ /Na. They also used cyclic voltammetry on Ni electrodes at various Na salt concentrations to further understand the mechanism. As a result, the water concentration in this pyrrolidinium ionic liquid alters the Na electrochemistry.

For rechargeable magnesium batteries, Guo and colleagues [18] have developed a boron-based electrolyte system with outstanding electrochemical performance,

formed through the reaction of tri (3,5-dimethylphenyl)borane (Mes3B) and PhMgCl in tetrahydrofuran. In the study, the structure-function correlations of the novel electrolyte were investigated, as well as the identification of the equilibrium types in the solution using NMR, single-crystal XRD, fluorescence spectra, and Raman spectroscopy. Moreover, the electrochemical stability of various current collectors, air sensitivity, and charge-discharge performance of a Mg-Mo6S8 battery in the electrolyte are analyzed. As a consequence, fluorescence and Raman spectroscopy investigations revealed that the Mes3B–(PhMgCl)2 electrolyte's strong anodic stability (about 3.5 V vs. Mg reference electrode) is due to non-covalent interactions between the anion [Mes3BPh]− and Ph2Mg. Motivated by this finding, the researchers proposed a reversible electrochemical technique of Mg intercalation into a Mo6S8 cathode, indicating that the novel boron electrolyte may be used in rechargeable Mg battery systems.

Carter TJ et al. [19] studied a combined carboranyl magnesium halide and closo-borane electrolyte for unconventional electrolyte system optimization in Mg batteries. In the results, they found that closo-borane compounds can function as high-oxidative-stability magnesium-battery electrolytes while maintaining compatibility with magnesium-metal anodes. Also, the carboranyl magnesium halide demonstrated compatibility with magnesium-metal anodes and outstanding oxidative stability (3.2 V vs. Mg) on non-noble-metal electrodes in the study.

## **2.3 Hypergolic fuels**

The concept of hypergolicity is that one chemical (fuel) reacts spontaneously when it comes into contact with another (oxidizer). Hypergolic ILs tend to have low volatility and high thermal and chemical stability that could allow the utilization of these substances as bipropellant fuels under different conditions. Meanwhile, the adjustment of the oxidizer/fuel ratio, the order of adding the fuel and oxidizer, and the ignition temperature should be considered when evaluating new hypergolic fuels.

In the formation of hypergolic boron-based ILs, boron compounds act as triggers, while cations (such as imidazolium) promote hypergolic firing. Recently BBILs frequently have been investigated as components in hypergolic fuel for rocket applications. In order to improve ignition performance and hydrolytic stability, electronwithdrawing moieties such as -CN group and nitrogen heterocyclic ring are often used to tune anions. A strategy of bridging another BH3 moiety or adding phosphorus atoms to anions can also improve the physico-chemical properties of hypergolic ILs.

According to studies in the literature, [BH4 − ] and [B(CN)H−3] were commonly used as anions.

In the design of hypergolic [65–70], hypergolic fuels with [BH4] − and [B(CN)H]−3 (ultra-fast spontaneous combustion with HNO3 oxidizers) and amines with boranebased ILs (low ignition delays) are promising.

Zhang et al. [67] reported the synthesis of water-stable hypergolic ILs ((B(CN)<sup>−</sup> based) in aqueous media. The B-H bond is unquestionably accountable for the hypergolic nature of compounds based on borohydride and cyanoborate. They stated that dicianoborate-based ILs exhibited similar phase transition temperatures, analogous thermal breakdown temperatures, and lower densities when compared to other hypergolic ILs containing amine anions. As a result, the novel boron-containing anionic ILs had substantially lower viscosities (12.4 mPa s) and ignition delays (4 ms) than nitrocyanamide and dicyanamide hypergolic ionic liquids.

*Investigation of Boron-Based Ionic Liquids for Energy Applications DOI: http://dx.doi.org/10.5772/intechopen.105970*

Bhosale et al. [69] synthesized boron-based B-H single bond-rich ILs and investigated hypergolic reactivity. Notably, 1-ethyl-3-methyl imidazolium borohydride ([EMIM][BH4] − IL) distinguished itself by having a short ignition delay time of 18.5 ms. The ignition delay time of [EMIM] [BH4] − and methyl imidazolium borane (1:1, w/w) as a combination of IL and fuel was determined to be around 35 ms. In addition, two algorithms were utilized to calculate the gas-phase heat of formation and the specific impact. One of the most promising possibilities for next-generation green hypergolic fluid and hybrid rocket propulsion has been identified as ILs rich in B-H single bonds.

Another hypergolic fuel study belongs to Li et al. [70]. They employed borane derivatives as additives to enhance the ignition delay of borohydride-based hypergolic ILs. First, they synthesized borohydride-based ILs (Amim-BH4 and Bmim-BH4) and then investigated the ignition delay time of these ILs with borane additives. The synthesized borohydride-based hypergolic ILs exhibited the shortest ignition delay time compared to any known hypergolic IL in the study. Also, they discovered that the triethylamine-borane combination is the most effective hypergolic additive studied to date. While the ID times of borohydride-based ILs are as short as 2 ms, IL solutions of borane-based additives have the lowest ID of 3 ms.

Based on the above explanations, borohydrides and boranes have great potential not only for ignition acceleration but also for alternative fuels due to their higher densities, lower vapor pressures, and adjustable heats of formation and viscosities. Although hydrazine and its derivatives are poisonous and carcinogenic, this new IL family offers a more environmentally friendly option. The increasing use of boron and its analogs to treat hypergolic ILs has resulted in more cost-effective solutions. However, in propellant systems, hydrazine and its derivatives remain the fuel of choice, despite containing a class of acute carcinogens and toxicants that exhibit extremely high vapor pressures and require expensive handling procedures and costly safety precautions.

#### **2.4 BBILs for hydrogen storage and production**

Having excellent hydrogen densities, safety, and release rate characteristics, many boron molecules can retent substantially more releasable hydrogen by weight and volume than pure liquid hydrogen [22]. Boron compounds are used as anions in ILs for hydrogen-containing applications, along with cations such as N,N′ dialkylimidazolium, N-alkylpyridinium, tetraalkylammonium, and tetraalkylphosphonium [35]. Particularly for commercially accessible sodium borohydride (NaBH4), hydrogen technologies provide easy and compact power for portable devices and backup power systems.

Amir Doroodian et al. [71] conducted one of these investigations. They presented the first ionic liquid based on methylguanidinium borohydride ((N3H8C)C + BH4) with effective hydrogen storage capacity. It is stated in the study that a liquid electrolyte is an ionic liquid that releases 9.0% by weight H2 under both thermal and catalytic conditions.

Developing technologies based on metal borohydrides (M(BH4)n) and amine boranes (AB, NH3BH3) on the other hand, have the potential to produce greater power densities than established sodium borohydride systems [72]. AB (see **Figure 9**) is a class of borohydride and one of the leading candidates for chemical hydrogen storage, which can release 19.6 wt% H2 when heated (at 85°C) due to its high hydrogen

**Figure 9.**

*Structure of aminoborane and diamine bisborane.*

**Figure 10.** *Aminoborane and diamine bisborane mix ILs for degeneration of hydrogen.*

content [73]. A feasible AB regeneration technique, as well as a quick and regulated H2 release rate are required for AB mix ILs to be suitable for hydrogen storage. Several approaches can accelerate the release of AB H2 mechanically, including activation with transition metal catalysts. **Figure 10** depicts the systematic figure of AB and diamine bisborane mix ILs for the degeneration of hydrogen.

Daniel et al. [74, 75] showed that base-supported AB increases the rate of H2 release in ILs by promoting the anionic dehydropolymerization mechanism. They reported AB reactions in 1-butyl-3-methylimidazolium chloride that took 171 min at 85°C and only 9 min at 110°C to produce equivalents of H2. Moreover, ionic-liquid solvents were shown to be more beneficial than other solvents because they minimized the development of unwanted compounds like borazine.

The effect of ethylenediamine bisborane (EDAB), one of the AB derivatives, on the hydrogen release rate in combination with ILs has been studied in the literature. Many research have been conducted to improve dehydrogenation, reduce dehydrogenation temperatures, and enhance equivalent H2 production [75–77]. In one of these studies, Debashis Kundu et al. [77] reported thermal dehydrogenation of EDAB in [BMIM] sulfate-based ILs. The time-resolved and temperature-resolved dehydrogenation of EDAB/IL systems have been carried out and characterized by NMR characterization in the study. Also, the equal quantity of equivalent hydrogen created per mole of EDAB injected into the system was computed. The results showed that IL-facilitated dehydrogenation released a higher amount of equivalent hydrogen than dehydrogenation with pure EDAB at 120°C. At 100°C, the EDAB/[BMIM] [HSO4] system released 3.92 cumulative equivalent hydrogen.

## *2.4.1 Hydrogen generation by water splitting*

A significant trend in the creation of hydrogen by electrolysis from a mixture of ILs and water, a renewable resource, has lately emerged. Water splitting includes the simultaneous oxidation and reduction of water [78–80]. Water ILs serve as an electrolyte and a solvent in the water-splitting process reactant. Water is merely one of several solutes present in the reaction mixture. When used in water separation, IL electrolytes have produced unique and surprising outcomes.

Roberto F. de Souza et al. [80] tested different electrocatalysts (molybdenum, nickel, and chromium) using aqueous ILs such as [BMIMBF4] for hydrogen production by water electrolysis. The hydrogen evolution reaction (HER) was carried out at ambient temperature with a potential of −1.7 V. For the Mo electrode, a Hoffman cell apparatus with a current density value of 77.5 mA/cm−2 in water electrolysis was used. The system efficiency for all tested electrocatalysts was found to be very high, ranging between 97.0 and 99.2%. The findings indicate that hydrogen production in BMIM[BF4] − aqueous solution can be performed with inexpensive materials at room temperature, making this process economically viable.

Hydrogen bonds are more readily broken when water molecules are dissolved in a suitable IL, thus increasing the free energy and resulting in a lower energy input required in the water-splitting reaction. The free energy can approach that of gaseous water, which requires a lower free energy input of around 10 kJ mol−1. Such a process must include a thermodynamically endothermic water dissolution process. As an example, [C2mim] [B(CN)4] based ILs have been proposed. However, no data on the thermodynamic process in BBILs is yet available. Also, the four-electron water oxidation reaction (see **Figure 11**) is much heavier than the water reduction reaction. That is why there has recently been a significant effort focused on developing and understanding water oxidation electrocatalysts.
