Isocyanate compounds as electrolyte additives for lithium-ion batteries
Introduction
For operation of graphite anodes in liquid organic and inorganic electrolytes of lithium-ion batteries, formation of a protective, but at the same time still lithium cation conductive film – the so-called solid electrolyte interphase (SEI) – is necessary. Film forming electrolyte additives are able to establish and/or enhance the SEI. Prominent additive examples include CO2, N2O, Sx2−, SO2, chloroethylene carbonate, fluoroethylene carbonate, vinylpropylene carbonate, vinylene carbonate, catechol carbonates, 12-crown-4, ethylene sulfite [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], and many others. There is intense search for new electrolyte additives; often this is done by a try-and-error approach.
Here we present our recent studies on isocyanate electrolyte additives. The isocyanate functional group –NCO is known to be reactive in general. Several applications as electrolyte additive in LIBs are self-suggesting:
- (i)
Monomers with –NCO functional groups can be electrochemically polymerized. The reductive polymerization to polyimides is well known from early investigations in organic electrochemistry [14], [15]. Electrochemical oxidative polymerization is also possible.
- (ii)
Isocyanates can act as anion receptors to improve the electrochemical and chemical stability of the electrolyte. For instance, PF6− anions are known to be thermally unstable and to decompose in the electrolyte according to:
Isocyanates acting as anion receptors form complexes with the anion and thereby can inhibit the decomposition reaction in the electrolyte. By reducing thus ion-pairing in the electrolyte, a positive side-effect is the increase of both lithium-ion diffusion and lithium transport number.
- (iii)
Thermal, i.e., temperature induced, reactions also could yield polymer films. This mechanism could be used as shutdown procedure during thermal runaway, thus providing a non-reversible safety feature.
- (iv)
SEI decomposition at elevated temperatures is the first reaction in a series of exothermal reactions taking place during thermal runaway. Considering that this SEI decomposition mechanism might take (partially) place via radical disintegration, the –NCO group may slow down SEI decomposition by functioning as radical trapper.
In summary, studies on isocyanate compounds are motivated by several possible applications in LIBs. Here we report on screening studies of a number of isocyanates. Among hundreds of isocyanates we concentrated on compounds with (i) benzyl, (ii) phenyl, and (iii) linear structure with electron-donating or electron-withdrawing substituents.
Section snippets
Experimental
Propylene carbonate (Honeywell, battery grade), LiClO4 (Mitsubishi Chemical Corp., battery grade) and isocyanate compounds (all from ALDRICH) have been used as received without further purification. The “standard electrolyte” was 1 M LiPF6/EC:DMC (v/v, 1:1, Merck). The electrode materials included mesocarbon microbeads (MCMB 1028, Osaka Gas), LiCoO2 (Nippon Chemical, Japan), KS6 graphite (TIMCAL) and poly(vinylidenefluoride)/hexa-fluoropropylene (PVDF/HFP; Kureha Chemical). Carbon electrodes (on
Electrochemical characterization
Isocyanate additives should be reduced at potentials well-positive to PC co-intercalation potentials. PC co-intercalation and subsequent reduction of the co-intercalated PC results in strong gassing and thus finally graphite electrodes exfoliate in PC [17], [18]. Cyclic voltammograms of these compounds on MCMB graphite anodes in 1 M LiClO4/PC are shown in Fig. 1. The onset potentials of reduction are displayed in Table 1. The cyclic voltammograms of isocyanate compounds on LiCoO2 cathodes are
Conclusion
As like vinylene compounds, isocyanate compounds are an interesting family of SEI-forming electrolyte additives for lithium-ion batteries; and they also function according to an electrochemical polymerization mechanism. The final performance, however, strongly depends on the (i) chemical structure, (ii) steric similarities/differences, (iii) nature and position of potential electron withdrawing group(s), and (iv) the number and nature of heteroatoms within the molecule. The chances to identify
Acknowledgements
We thank Honeywell (Seelze, Germany), Merck (Darmstadt, Germany) and TIMCAL Group (Bodio, Switzerland) for supplying samples.
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2019, Journal of Power SourcesCitation Excerpt :For graphite cycled with FI, the N 1s spectra (Fig. 6b) show two peaks, corresponding to CN bond (398.4 eV) and CN (400.2 eV) bond, respectively. CN comes from the NCO groups of FI and the CN bond results from the electrochemical polymerization of the NCO group, which is reported to form polyamide-like SEI components [5,6]. This suggests that FI helps to form polymeric components inside the SEI, which favors a stable and passivating SEI film.
A propylene carbonate based gel polymer electrolyte for extended cycle life and improved safety performance of lithium ion batteries
2018, Journal of Power SourcesCitation Excerpt :For this reason, SEI forming additives/co-solvents are indispensable for the application of PC as the main solvent. With this in line, various additives/co-solvents were reported for PC, such as, ethylene sulfite (ES) and propylene sulfite [19–22], vinylene carbonate (VC) [23], vinyl ethylene carbonate (VEC) [24], fluoroethylene carbonate (FEC) [25], methyl tetrafluoro-2-(methoxy) propionate (MTFMP) [15], vinylene and vinyl compounds [26–30], isocyanates [31–33], etc. However, in literature, PC was employed to evaluate the effectiveness of the SEI forming additives on graphite anodes, rather than formulating a PC-based electrolyte that enables long cycle life in LIBs [34].
Shutdown potential adjustment of modified carbene adducts as additives for lithium ion battery electrolytes
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