The role played by the binder is often underrated in electrochemical storage devices such as batteries and EDLC. Electrochemically inactive, and usually introduced in minor amounts (<10% wt.), the binder is therefore merely considered as “dead weight”, with the only function of holding together the electrode components. This is even more true for EDLCs in which the absence of strain associated to faradaic reactions (e.g., alloying and conversion reactions in LIBs) makes the binder less relevant, thus partially justifying the limited attention paid to the binder in EDLCs-related scientific literature. Nevertheless, using natural polymers as binders (employing water as the only solvent) can provide benefits in terms of cost and the environment. Actual electrode fabrication technologies involve the casting (or spraying) of slurries constituted by a mixture of the electroactive components dispersed in a binder solution/dispersion. Hence, the binder determines the solvent needed for the electrode production and, in turn, the overall process' sustainability. Polyvinylidene fluoride (PVdF) and Polytetrafluorethylene (PTFE) have represented for decades the state-of-the-art binders for both LIBs and EDLCs. Nowadays, however, with the growing market of such devices, the employment of fluoropolymers is being questioned for several reasons. Besides containing fluorine, which makes them difficult to dispose at the end-of-life, PVdF, for example, requires the use of expensive and toxic solvents (e.g., N-methyl-2- pyrrolidone) that need to be properly handled to avoid health hazards, thus increasing the production costs. For these reasons, many efforts are put into developing greener alternatives.
This paper will provide a comprehensive overview of fluorine-free and water-processable binders. Advantages and open challenges of natural polymers (and their derivatives) such as cellulose [1], starch [2] and casein [3] will be thoroughly discussed.
The use of environmentally friendly binders for the realization of high power batteries employing, will be presented, also. The combination of, e.g., carbon-coated ZnFe2O4 nanoparticles and LiFePO4-MWCNT composite processed using Na-carboxymethyl cellulose as aqueous binder, yields to Li-ion full cell with exceptional rate capabilities. The resulting cell, in fact, delivers 50% of its nominal capacity (limited by the cathode) at currents corresponding to ≈20C. The required pre-lithiation of the negative electrode offers the possibility of storing excess lithium in the cell and tuning the cell potential, achieving remarkable gravimetric energy and power density values of 202 Wh kg−1 and 3.72 W kg−1, respectively. The high reversibility of the electrochemical processes enables the system to perform more than 10,000 cycles at elevated C-rates (≈10C) while retaining up to 85% of its initial capacity.
References
[1] A. Varzi, A. Balducci, S. Passerini, J. Electrochem. Soc. 161 (2014) A368.
[2] A. Varzi, S. Passerini, J. Power Sources 300 (2015) 216-222.
[3] A. Varzi, R. Raccichini, M. Marinaro, M. Wohlfahrt-Mehrens, S. Passerini, J. Power Sources 326 (2016) 672-679.
[4] A. Varzi, D. Bresser, J. von Zamory, F. Müller, S. Passerini, Adv. Energy Mater. (2014) DOI: 10.1002/aenm.201400054.
