Abstract:
Polymer electrolytes have been an active field of research since the late 70's. This has culminated in the commercial launching of lithium metal polymer electrolytes batteries powering a fleet of cars since 2011 in different cities in France [1]. The poly(ethylene oxide) - PEO-based as 'solvent' for a low lattice energy salt has been the key to obtain decent conductivities. The operational temperature, however, is a ≈ 70°C, i.e. 10°C above the melting point of crystalline PEO and 100°C above the Tg of the resulting melt. These systems are coupled (diffusion with segmental motion) and can be called "fragile" according to Angell definition [2]. Besides, the fraction of the current carried by the cations (Li+, Na+), the only important species in the electrodes processes, expressed as T+, is only 0.2 to 0.3.
We will discuss here the strategies to improve the performances of such ionic conductors in the hopes to meet the requirement for the dearly sought after high energy density battery operating at/close to room temperature, and safer and longer cycling than the present ones using flammable liquid electrolytes technology.
Without resorting to modify PEO, the modification of the solute (salt) is one fruitful strategy. The introduction of the extensively delocalized fluorinated imides (RfSO2)2N- (Rf = F, CF3) anions where the charge is spread on 5 centers and importantly possess a a"hinge", with the flexible S-N-S "pia" bonds lowering the Tg of the resulting solid solution with PEO, have revolutionized the field. This is also true for ionic liquids, in majority based on these anions.
The concept can be pushed further with the "super imide" family, where the charge is further delocalized and the number of "hinges" extended. The first example is [(CF3SO2N)2S(O)(CF3)] with two S-N-S "pi" bonds. The Tg for the polymer-salt complex is thus further lowered (Fox equation). Besides, when this salt is tethered to a polymer to immobilize the anion, this results in the highest conductivities reported for a single-ion conductor (T+ = 1) [3].
Manipulation of the simple anions, while still keeping the flexible imide linkage, allows the increase of T+ to ≈ 0.5 by simply removing one fluorine from (CF3SO2)2N to (CF2HSO2)(CF3SO2)N , resulting in H bond formation with the ether oxygens, slowing the negative charge correspondingly [4]. Alternatively, the (CF3SO2)N( )SO2- moiety can be kept, attached to either long alkyl chains or short EO units. The former anions result in nanophase separation with the formation of micelles; for the latter, the CH2CH2O units attached to the imide center plasticize the polymeric chains without participating in the solvation. Both systems results in much decreased anion mobility, keeping the Li+ diffusion at a high value. Both salts seem to, for the first time, exhibit some decoupling.
The salt aspect as well as that of new alternatives to PEO will be discussed.
References:[1] http://www.bollore.com/en-us/activities/electricity-storage-and-solutions/electric-vehicles-solutions.
[2] Angell, C. A. (1995). "Formation of Glasses from Liquids and Biopolymers". Science. 267: 1924 -1935.
[3] Ma et al. https://doi.org/10.1002/anie.201509299
[4] Zhang et al., DOI: 10.1002/anie.201813700
|
