Ethylene Carbonate-Free, Adiponitrile-Based Electrolytes Compatible with Graphite Anodes
Authors: Ehteshami Niloofar , Paillard Elie 
Published: February 2020

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 LiMn1.5Ni0.5O4  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 stabilities3,4. 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 LiBF4
5
. 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:

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Figure 1