Stable Li Ion Batteries Anode By Inducing the Covalent Bond between Active Material and Binder By Esterification Reaction

Abstract

Lithium ion batteries (LIBs) has been the main energy storage medium for portable electronics since the commercialization by Sony Corporation since 1991. Ever growing demand for large-scale energy storage devices such as electrical vehicles requires the high energy density and long cycle life LIBs that could not be attained by the currently used battery. Graphite is the most common anode material in commercial LIBs, but its specific capacity is relatively low (372 mAh g-1) with a poor rate capability, which does not meet the high demand for large-scale applications. Silicon (Si) is one of the most promising anode candidates due to its high theoretical specific capacity (3579 mAh g-1 for Li15Si4), relatively low discharge potential (~0.4V vs Li+/Li), and abundance in nature. However, Si goes through a dramatic volume change during cycling and thus undergoes a drastic structural deformation, which induces loss of electrical contact between Si and current collector, breakdown of solid electrolyte interphase (SEI) layer, and Si aggregation during cycling, eventually resulting in failure of cyclic performance. Although design a various nanostructure forms or introduction of secondary phase layers has been realized as a method to achieve high-performance Si anode, these methods require complex reactions such as hydrothermal or template-assisted synthesis and usually involve hazardous reagents like SiH4 or HF, which eventually result in cost and barrier to scale up. From the practical view, use of commercial Si is the most promising, where the commercial production of Si has been benefitted from the development in semi-conductor industry. Conventionally, battery is prepared by mixing active materials and binder with conducting agent in organic solvent to produce viscous slurry and blading onto copper current collector. Recently, the selection of binder has turned out to play critical role in the battery performance as maintaining the structural integrity of electrode film. It has been revealed that carboxylic acid or hydroxyl functional groups terminated binders which is commercially available, such as polyacrylic acid (PAA), carboxymethyl cellulose (CMC or Na-CMC) or alginate (Alg or Na-Alg) binder, can result in enhanced cycling performance utilizing the hydrogen bonding interactions with the native oxide on Si, compared to that of conventional PVDF binder which has no functional group and bonds weekly with Si through a week Van der Waals interaction. Nonetheless, due to the linear chain characteristic of these binders, susceptible sliding upon the continual volume change of Si results in the contact loss between binder and Si, which eventually results in the irreversible capacity failure. Herein, we propose a novel concept that can achieve stable Si anode by transitioning the bonding nature between binder and Si from hydrogen bond to mechanically robust covalent bond, which can effectively restrain any large movement of Si, and eventually prevent the destruction of the electrical network during cycling. Inducing the esterification reaction between binder and Si can be an efficient method to form a covalent bond between those two materials. (eq. (1)) R-COOH+HO-R’ -> R-COO-R’+ H2O -------- (1) To induce an esterification reaction between binder and Si, three crucial points have to be addressed. First, as carboxylate (–COO-) functional group terminated binder such as Na-CMC can hardly react with the hydroxyl functional group (-OH), PAA binder which is terminated with the carboxylic acid functional group (-COOH) has been selected in our system. Second, while commercially purchased Si has a –OH functional group by a partially hydrolyzed SiO2 layer covering the Si particles, its functional group density is insufficient for an esterification reaction to take place. Therefore, terminating –OH functional group at the surface of Si is crucial factor to increase the site to react with PAA binder. Third, due to kinetically sluggish characteristic of esterification reaction, proper usage of esterification catalyst is required. Experimental procedure is as follows; firstly, as-received Si was pretreated in piranha solution to terminate –OH. Then, –OH terminated Si was prepared into slurry by mixing with Super-P and PAA binder, casted onto copper foil, dried overnight and punched for a coin-type cell. To induce the esterification reaction, punched cell was dipped into sodium hypophosphite catalyst solution and then annealed in 180 °C for 1 h. Three counter groups are denoted as HB-Cell (punched cell with no annealing process), PE-Cell (annealed cell without usage of sodium hypophosphite), and FE-Cell (annealed cell with addition of sodium hypophosphite.) The fabricated FE-Cell exhibits superior cycle stability (2200 mAh g-1 at 100 cycles) compare to HB-Cell or PE-Cell, demonstrating the effect of covalent bond on obtaining the high-performance Si anode. Figure 1