Increasing the energy density of Li-ion batteries to push further their application for powering electric vehicle implies, at the material level, either increasing the electrodes capacity or the battery (i.e. cathode) operating voltage. However, the limiting factor for operating cathodes at high voltage, be them NMC above 4.3V¹, or LiMn_1.5Ni_0.5O₄  up to 4.8V - 5.0V, is the limited anodic stability of state-of-the-art alkyl carbonates-based electrolytes. Thus, the use of alternative solvents such as alkyl-sulfone, ionic liquids, fluorinated alkylcarbonates has been proposed².

Among them, aliphatic alkyl dinitrile (CN(CH₂)_nCN, n = 3-8), and adiponitrile (ADN, n = 4) in particular, offer high anodic stabilities^3,4. However, the preparation of high energy Li-ion cells requires, in most cases, the use of graphite-based anodes. Thus, solutions had to be found for the operation of graphite electrodes, given their insufficient cathodic stability and poor solid electrolyte interphase (SEI) forming ability on graphite.

In fact, using EC as co-solvent allowed the cycling of either full graphite/LiCoO₂ cells or graphite half-cells with either LiTFSI ³ or LiBF₄⁵_. However, EC is considered responsible for both the poor low temperature performance of the electrolytes⁶ and their failure at high voltage¹ and efforts have thus been directed toward EC-free electrolytes as well. Gmitter et al.⁶, in particular, demonstrated successful cycling of MCMB/LiCoO₂ cells, by using VC or FEC as additives and LiBF₄ as a co-salt for a LiTFSI/ADN-based electrolyte. However, as MCMB are known for allowing the use of PC-based electrolytes, which are usually incompatible with graphite⁷, the possibility of operating graphite anodes with high voltage, EC-free electrolytes had not been demonstrated up to now.

In fact, the low solubility of typical inorganic salts such as LiPF₆ and LiBF₄ in pure alkyl dinitriles initially led toward the use of LiTFSI, a salt with high thermal and electrochemical stability and low lattice energy but which possesses poor SEI forming ability and induces Al current collector corrosion. However, other salts, such as lithium difluorooxalatoborate (LiDFOB) and lithium bis(fluorosulfonyl)imide (LiFSI) are good candidates for substituting LiPF₆as they provide enhanced SEI forming ability in various type of electrolytes. In addition, they are usually more soluble in organic solvents, including those with lower dissociating properties than typical EC mixtures.

Thus, we report here on the electrochemical performance of EC-free electrolytes based on ADN:DMC mixtures with LiDFOB or LiFSI, alone or combined with fluoroethylene carbonate (FEC) as additive for a use in high energy Li-ion cells. As an example, the cycling results of a 7 mg cm^-2graphite electrode in 1 M LiDFOB ADN/DMC (1:1, wt) is shown in Figure 1.

Acknowledgement:

The research presented is part of the ‘SPICY’ project funded by the European Union’s Horizon 2020 research and innovation program under grant agreement N° 653373.

References:

1. Xia, J., Petibon, R., Xiong, D., Ma, L. & Dahn, J. R. Enabling linear alkyl carbonate electrolytes for high voltage Li-ion cells. J. Power Sources 328,124–135 (2016).

2. Xu, K. Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chem. Rev. 114,11503–11618 (2014).

3. Abu-Lebdeh, Y. & Davidson, I. High-Voltage Electrolytes Based on Adiponitrile for Li-Ion Batteries. J. Electrochem. Soc. 156,A60 (2009).

4. Duncan, H., Salem, N. & Abu-Lebdeh, Y. Electrolyte Formulations Based on Dinitrile Solvents for High Voltage Li-Ion Batteries. J. Electrochem. Soc. 160,A838–A848 (2013).

5. Isken, P. et al. High flash point electrolyte for use in lithium-ion batteries. Electrochim. Acta 56,7530–7535 (2011).

6. Gmitter, A. J., Plitz, I. & Amatucci, G. G. High Concentration Dinitrile, 3-Alkoxypropionitrile, and Linear Carbonate Electrolytes Enabled by Vinylene and Monofluoroethylene Carbonate Additives. J. Electrochem. Soc. 159,A370 (2012).

7. Aurbach, D., Zinigrad, E., Cohen, Y. & Teller, H. A short review of failure mechanisms of lithium metal and lithiated graphite anodes in liquid electrolyte solutions. Solid State Ionics 148, 405–416 (2002).