**1. Introduction**

Lithium-ion cells are the technological standard for portable devices such as smartphones, notebooks, and electric vehicles, and as a result, they are viewed as a key for the global transition to electro-mobility. In terms of thermal stability, the electrolyte of a lithium-ion cell is considered a critical component, which is responsible for ionic conductivity. The electrolyte is mainly composed of solvents, lithium conducting salts, and various additives [1,2]. The usage of conventional solvents for electrolytes with low boiling points and flash points (*T*FP) like dimethyl carbonate (DMC, *<sup>T</sup>*FP < <sup>20</sup> ◦C [3]), EMC (*T*FP ≈ 22 ◦C [4]), or DEC (*T*FP ≈ 25 ◦C [5]) bear an increased risk to ignite lithium-ion cells [6–8]. The low boiling point generates high pressure gradients at moderate temperatures (<100◦C), which can lead to the explosion of the cell. The chemical products of burned fluorine-containing electrolytes are highly toxic [9–12]. Therefore, an expensive thermal management system and a massive casing for lithium-ion batteries are required. This heavy casing for battery packs for electric vehicles lowers the gravimetric density of the pack and increases the weight of the vehicles. Increasing the intrinsic thermal stability is a key factor to lower costs for cell protection and increase the gravimetric density of the battery pack. Investigations on new electrolyte formulations have been considered before, for example by using flame retardant additives like organic phosphates [13] or phosphonates [14]. However, using these additives to improve the thermal stability reduces the cell performance [15]. Investigations show that the use of ionic liquids can increase the flash point of electrolytes however, these ionic liquids are linked with high costs [16–19]. Another promising approach is to investigate co-solvents with higher flash points like adiponitrile (ADN, *T*FP ≈ 163 ◦C [20]) by Isken et al [21]. Co-solvents were able to increase the flash point significantly from *T*FP,EC:DEC ≈ 36 ◦C of the EC:DEC (3:7 wt) mixture to *T*FP,EC:ADN ≈ 149 ◦C of the EC:ADN (1:1 wt) mixture. This indicates that it is possible to formulate electrolytes with higher flash points by replacing volatile carbonates. However,

**Citation:** Ströbel, M.; Kiefer L.; Birke, K.P. Investigation of a Novel Ecofriendly Electrolyte-Solvent for Lithium-Ion Batteries with Increased Thermal Stability. *Batteries* **2021**, *7*, 72. https://doi.org/10.3390/ batteries7040072

Academic Editor: Catia Arbizzani

Received: 28 July 2021 Accepted: 12 October 2021 Published: 28 October 2021

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the melting points (MP) of EC *T*MP,EC ≈ 36 ◦C and ADN *T*MP,ADN ≈ 2 ◦C are extremely high for low temperature applications. To lower the working temperature applicability *T* < 0 ◦C research has shown many different classes of solvents like sulfones [22,23]. Unfortunately, most of these solvents are ecologically harmful.

Therefore, we investigated tributyl acetylcitrate as a solvent to formulate an electrolyte composition with a high flash point and a wide operating range from very low to high temperature. TBAC has a high flash point of *T*FP ≈ 217 ◦C [24], its melting point is very low with *T*MP ≈ −80 ◦C and the boiling point is at *T*BP ≈ 330 ◦C. Another noteworthy advantage of TBAC is that it is ecofriendly [25], nontoxic [26], and therefore safe to handle. For example, it is well known as a plasticizer in the nail polish industry [27].

This study presents tributyl acetylcitrate as a novel solvent for lithium-ion cells. The combination of conventional solvents like EC and DEC with TBAC creates an electrolyte with improved thermal stability.
