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Abstract
In this paper we investigated the calculation of the anodic limit of two anions of ionic liquids, largely used as electrolyte of lithium batteries. Starting from a model based on calculations performed on single ions at the MP2 level of theory, we showed that the matching between calculation and experiments decreases while using more expanded basis set with respect to 6-31G**, possibly because of the destabilization of the neutral species when larger basis sets are considered. Additionally, in order to decrease the computational time, the performances for the calculation of the anodic limit obtained by means of a series of DFT functionals with increasing level of complexity (from the Generalized Gradient Approximation to the Range Separated Hybrid meta-Generalized Gradient Approximation) were compared. Overall, the best performing functionals are BMK, ωB97M-V and MN12-SX, while acceptable results can be obtained by M06-2X, M11, M08-HX and M11-L. Some less computationally expensive functionals, like CAM-B3LYP and ωB97X-D, also provide reasonable values of the anodic limit.
Highlights
Electrochemical energy storage devices are nowadays ubiquitous [1]
Independently of the particular choice of the functional and basis set, in order to calculate the anodic stability of bis-fluorosulfonyl imide (FSI) and bis-trifluoromethyl sulfonyl imide (TFSI), we followed the procedure previously indicated as that giving the best agreement [21] with the experimental data obtained on five different ionic liquids: EMIFSI, EMITFSI, N1114FSI, N1114TFSI and N122(2O1)TFSI [23, 24]
3.1 Impact of the basis set on the accuracy of MP2 predictions In the first part of this study, we investigated the influence of the basis set extension on the predicted oxidation potential of FSI and TFSI anions, calculated at the MP2 level of theory
Summary
Electrochemical energy storage devices are nowadays ubiquitous [1]. A massive injection of batteries in all-day life occurred with the advent of portable phones and the need for high energy density batteries to power them. Simulated solvent unphysically stabilize charged species leading to unrealistic large ESWs. Overall, the best agreement between computations and experiments is obtained when the oxidation or reduction process are modelled without a structural relaxation (vertical transitions, purely electronic) in vacuum with a MP2 level of theory, whereas the typical B3LYP hybrid-DFT functional is much less accurate [21].
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