**1. Introduction**

Lithium-ion batteries (LIBs) are widely used in electronic devices ever since their successful commercialization by Sony in 1991 and Asahi Kasei and Toshiba in 1992 [1,2]. The conventional electrolyte used in LIBs is based on lithium hexafluorophosphate (LiPF6) salt dissolved in volatile organic solvents; typically, these are mixtures of carbonates such as ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) [3,4]. These combinations enhance desired properties in electrolytes. For example, EC has a high dielectric constant that promotes salt dissolution, and the addition of DMC to these organic solvents can lower the melting point and viscosity of the combined EC/DMC organic liquid electrolyte (OLE) [5]. Despite the high ionic conductivity and Li-ion di ffusivity of OLEs during the charge/discharge cycles in LIBs, they face serious safety concerns due to their high flammability and volatility [6]. Such safety hazards can lead to thermal runaway and serious consequences [7,8]. Recently, incidents involving violent battery

ignitions have caught the general public's attention, and this concern has increased even more with the application of lithium-ion batteries in electric vehicles and power grid storage devices [9]. Because ionic liquid electrolytes (ILEs) are non-flammable, non-volatile, and conductive, they possess safety advantages over OLEs and have been studied as electrolytes in rechargeable LIBs [10,11]. ILEs tend to be electrochemically and thermally stable, potentially allowing use of high voltage cathodes and safe operation at high temperatures. Nonetheless, ILEs face many challenges including larger viscosity, which increases significantly with decreasing temperature, crystallization at low temperatures, and large, strongly temperature-dependent interfacial impedance at both the cathode and anode. All of these issues are tied to the fact that the ILE-based Li-ion cells that have been developed to date operate only at elevated temperatures and at relatively low charging/discharging rates [10]. The high viscosity of ILEs typically results in a decreased total ionic conductivity at room temperature. Most important for electrolyte performance is to maximize the conductivity carried by the Li+ (cation), i.e., the product of total conductivity and transference number [12]. Additionally, it has been observed in both experiments and simulations that Li+ mobility increases more rapidly with dilution with an organic solvent than do the mobilities of the RTIL anion and especially cation, resulting in a higher transference number [13,14]. We note the further complication that the ability of a low viscosity solvent to dissolve lithium salt and transport Li<sup>+</sup> does not necessarily imply improved electrolyte performance [15,16].

A novel approach based on mixing organic solvents with ILEs has been used to improve the ionic conductivity of ILEs in Li-ion cells with the aim to offer combined advantages of OLs and ILEs such as decreased viscosity, higher Li+ diffusion/mobility, (i.e., improved conductivity), and, under appropriate volume ratios, better safety factors [6]. An increased tolerance to higher operational temperatures is beneficial not only because batteries are less likely to experience thermal runaway, but also because it enables specialty applications that require very low (−80 ◦C to −40 ◦C) or high (100 ◦C) temperatures; electrolyte crystallization can be prevented at lower working temperature conditions by modifying the IL solvent, while battery performance at higher temperature can be achieved by adding different lithium salts to ILs [17]. Several IL/Li salt systems have been employed as electrolytes in Li-ion batteries, but the number of ILEs that have been demonstrated as effective in operating cells is limited. Since demonstrating reasonable conductivity of an ILE (commonly done) and reasonable electrochemical stability (less commonly undertaken) are not sufficient to ensure reliable operation of a battery, a number of other issues such as interfacial defects must be considered. As a result, the number of operating batteries based on ILEs is less extensive than might be expected, given the huge number of anode/cathode and electrolyte combinations possible [11,18–20].

The most common anion investigated for ILEs is bis(trifluoromethanesulfonyl)-imide (TFSI), while the most common cations are alkylimidazoliums, tetraalkylammoniums, and alkylpryrrolidiniums (e.g., pyr13 and pyr14) [11]. 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-imide (EMI-TFSI) has been used with Li salt as an electrolyte for LIBs due to its low viscosity compared to other ILEs [21]. For example, a LiCoO2 cathode in a (EMI/TFSI + LiTFSI) electrolyte delivered higher discharge capacity at room temperature than in the EMIBF4 + LiBF4 system [21]. EMI-TSFI is employed here as an IL due to its good conductivity (8.7–9.1 mS·cm<sup>−</sup><sup>1</sup> at 25 ◦C), low viscosity (33–34 cP), and low melting point (−15 ◦C) [17]. Additionally, it has been found that EMI-TSFI with LiTSFI can increase the ionic conductivity and decrease the viscosity of the electrolyte, while no flammability was observed for compositions with IL (EMI-TSFI) wt% of 40% or more in EC–DEC–VC–1M LiPF6 electrolytes at increased temperatures [21,22].

Reversible capacities of up to 155 mAh g<sup>−</sup><sup>1</sup> and 128 mAh g<sup>−</sup><sup>1</sup> have been reported for Li-ion full cells with EMI-TSFI and EC/DMC MOILEs using LiFePO4 as the cathode with a graphite anode, and a LiFePO4 cathode with a Li4Ti5O12 anode, respectively [22]. Nonetheless, the addition of non-ionic organic additives such as succinonitrile (SN) to the electrolyte can improve the ionic conductivity of ILEs and the overall electrochemical performance of the Li-ion cell [1,23]. SN can dissolve different types of salts such as LiTFSI, LiBF4, LiPF6, LiN(CN)2, Ba(TFSI)2, Pb(TSFI)2, La(TSFI)2, Ag(CF3SO3), and Cu(CF3SO3) [24]. SN has been frequently used as a solid electrolyte in LIBs [24–28] but limited

results have been reported on the use of SN with OLEs and ILEs in LIBs [29]. For example, the addition of SN to a polymer electrolyte (PEO-LiTFSI, P(VDFHFP)– LiTFSI, and P(VDF-HFP)–LiBETI) resulted in improved ionic conductivity of the polymer electrolyte and favorable mechanical properties [30]. SN has been recently used as a functional additive to improve the thermal stability and broaden the oxidation electrochemical window of an OLE in lithium cathode half-cells containing a LNMO cathode. The results showed that the addition of SN to the electrolyte solution lead to a remarkably improved cycling stability, which was due to the formation of an electronically conductive film on the cathode [29,30]. SN was also used as an additive to improve the thermal stability of ethylene carbonate (EC)-based electrolytes in LIBs. This work showed that SN can suppress parasitic reactions between the positive electrode (LiCoO2) and the organic liquid electrolyte, because the nitrogen ion in the nitrile functional group (–CN) in SN has a lone pair of electrons leading to a strong bond with the transition metal ions on the cathode. It was also reported that the addition of SN resulted in a suppression of electrolyte decomposition in commercial cells [30]. The authors also suggested that SN can react with transition metal ions in the electrolyte to form metal ion compounds preventing their reduction on the negative electrode surface, which would compromise the SEI surface. This work focuses on the investigation of the electrochemical properties of EMI-TFSI-LiTFSI electrolytes in lithium anode (or cathode) half-cells using either a cathode or an anode [31]. OLE (EC/DMC 1:1 *v*/*v*), ILE (EMI-TFSI), and MOILE were used with a commercial cathode, LiCoO2, and a SnO2/C composite-fiber anode in lithium anode half-cells to investigate the effect of electrolyte type on the electrochemical performance. The effects of temperature and SN additive on the ionic conductivity and electrochemical performance of MOILEs were investigated by conducting charge/discharge and impedance measurements on the lithium anode (or cathode) half-cells with commercial cathode materials.
