Invited Presentation: Polymers, Still Solid after 30 Years

Introduction:

The 70’s have seen the generalization of the concept of electrochemical intercalation, leading in principle to immortal electrodes. The lithium-ion battery had later an early demonstration ^[1], but gained accep­tance after the dangerous failures of lithium–metal batteries using liquid electrolytes had become a liability.

The shortcomings of the Li-ion are, in terms of energy density, the weight devoted to the negative electrode is accentuated by the necessity of a copper current collector.. Two strategies are presently followed: i) the use of high voltage positives (LiNi_0.5Mn_1.5O₂, “lithium-rich”) though no electrolyte seems still satisfactory to span a 5 V stability window especially at T > 40°C; ii) the re-emergence of the Li° electrode. I

The 70’s saw also the emergence of the polymer electrolytes, after Wright ^[2] found appreciable conductivity in PEO-NaI adducts, conditional to heating to ≥ 60°C. After the polyethers comp­lexing ability was extended to lithium salts, the field became very active and showing much ingenuity. However the main effort towards scaling-up to practical batteries was sustained by Hydro-Québec from 1978 to 2000 totalling close to one billion $.

Polymer electrolytes:

The carbon-carbon-oxygen sequence in polyethers corresponds to the optimal spacing to allow wrapping of the backbone around the cations. However, in practice, even the first homologue, with a CH₃ (PO) side group introduces already some steric hindrance with much decreased cation sheathing ability. Defects like PO units or keeping through the design of the polymer architecture a M_w of the sequential PEO segments < 600 resolve the problem of crystallinity of the homopolymer and its stoichiometric complexes. The inclusion of nanoparticles (SiO₂, Al₂O₃, BaTiO₃…) increases the amorphous domains and had been widely used to create polymer electrolyte from commercial PEO without resorting to complex macromolecular synthesis.

The next problem is the exponential decrease of the conductivity starting from the glass transition temperature, which it has not been possible to bring lower than ≈ ‑60 °C, resulting in practice to an operating temperature close to + 60 °C. The only practical way to lower T_g, i.e. reduce the volumetric density of chemical bonds is to add plasticizers, including ionic liquids, the drawback being then the risk of chemical cross-talk between the electrodes, as in liquid electrolytes.  The callenge of breaking the T_g barrier in a mostly organic based matrix, has yet to be solved. 

Though, the Li° batteries operating with polymer electrolytes are relatively long-lived with minor dendrite problems.

There is now an emphasis on unipolar conductivity polymers, to totally suppress the source of dendrites, the salt depletion at the Li° electrode during charging. This is obtained by including the anion is a backbone, either copolymerized ^[3] or separated ^[4] from the solvating chain. Another options are the surface coating of nano-particles with the salt ^[5] or in block polymers ^[6]. There is now convincing evidence that the lithium-metal electrode can function safely, the only restriction being the acceptability, for polymer electrolytes, of a “warm” battery (EVs, busses).

References

[1]      Lazzari, M & Scrosati B, J. Electrochem. Soc, 127, 773, (1980).

[2]      Wright P.V. Br. Polym. J.,7,319-327 (1975).

[3]      Feng et al., Electrochimica Acta, 93, 254-263, (2013)

[4]      Meziane et al. Electrochimica Acta, 57, 14-19, (2011).

[5]      Villaluenga et al.,J. Mater. Chem. A, 1, 8348, (2013).

[6]      Bouchet et al., Nature Materials 12, 452–457 (2013)