Silicon is one of the most promising anode materials because of its high theoretical capacity of 3579 mAh/g at room temperature which is ten times higher than graphite, a common anode currently used in commercial lithium ion batteries.¹ However,  Si  has poor cycle life because of the massive volume change during cycling followed by the pulverization of active particles and the loss of electrical contact within electrode components. ¹ Binder, usually long-chain polymers, are important electrode components that help to hold particles together and to current collector. Poly(vinylidene fluoride) (PVdF) has been widely used as binder in lithium ion batteries due to its good electrochemical and thermal stability and good adhesion to electrode material and current collector.² The performance of Si with PVdF, however, has been reported to be poor.² Recently, the cycle life of Si was greatly improved by using binders which contains a  large quantity of  –OH and –COOH such as sodium carboxymethylcellulose (CMC),²  poly(acrylic acid) (PAA).³ The formation of covalent bond and/or strong interaction of hydrogen bonds between silicon particles and binders are believed the reason for the improvement.^4,5 However, there is no report on using small molecule carboxylic acids as binders in lithium ion batteries.

In this study, Si electrode has been prepared with silicon nanoparticles (≤50nm), conductive carbon and carboxylic acids with ratio of 50:25:25. The Si electrode with PVDF binder was also prepared for comparison. The Si anodes were then evaluated in half cells with lithium metal as current collector and reference electrodes in 1.2M LiPF₆/Ethylene carbonate (EC) : Dimethyl carbonate (DEC) without and with 10 wt.% additive.

In the absence of additive, Si electrode with carboxylic acids show very high first cycle specific capacity and efficiency, ~3500 mAg/g and 79%, respectively. After 30 cycles, the electrode still maintains a capacity > 1500 mAh/g. On contrary, Si electrode with PVDF binder shows a low first discharge capacity and efficiency: 1700 mAh/g and ~60%, respectively. Furthermore, the capacity fades rapidly after first cycle. The dramatic improvement cycling performance of silicon NP electrodes with carboxylic acids over PVdF are believed due to the interaction of carboxylic acids with silicon particles. We will further discuss effects of carboxylic acids for enhancing cyclability of silicion nanoparticle anode in the meeting.

Acknowledgement

The authors gratefully acknowledge funding from Department of Energy Office of Basic Energy Sciences EPSCoR Implementation award (DE-SC0007074)

Reference

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