*Article* **Investigation of a Novel Ecofriendly Electrolyte-Solvent for Lithium-Ion Batteries with Increased Thermal Stability**

**Marco Ströbel \*, Larissa Kiefer and Kai Peter Birke**

Electrical Energy Storage Systems, Institute for Photovoltaics, University of Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany; st142567@stud.uni-stuttgart.de (L.K.), Peter.Birke@ipv.uni-stuttgart.de (K.P.B.) **\*** Correspondence: Marco.Stroebel@ipv.uni-stuttgart.de

**Abstract:** This study presents tributyl acetylcitrate (TBAC) as a novel ecofriendly high flash point and high boiling point solvent for electrolytes in lithium-ion batteries. The flash point (*T*FP = 217 ◦C) and the boiling point (*T*BP = 331 ◦C) of TBAC are approximately 200 K greater than that of conventional linear carbonate components, such as ethyl methyl carbonate (EMC) or diethyl carbonate (DEC). The melting point (*T*MP = −80 ◦C) is more than 100 K lower than that of ethylene carbonate (EC). Furthermore, TBAC is known as an ecofriendly solvent from other industrial sectors. A life cycle test of a graphite/NCM cell with 1 M lithium hexafluorophosphate (LiPF6) in TBAC:EC:EMC:DEC (60:15:5:20 wt) achieved a coulombic efficiency of above 99% and the remaining capacity resulted in 90 percent after 100 cycles (*C*/4) of testing. As a result, TBAC is considered a viable option for improving the thermal stability of lithium-ion batteries.

**Keywords:** ecofriendly electrolyte for lithium-ion batteries; increased thermal stability of electrolytes; enhanced electrolyte safety based on high flash point; tributylacetylcitrate; acetyltributylcitrate
