Portable electronics and electric vehicle applications require high energy density cells with long cycle life.  Much attention has been given to Si and Si-based alloy negative electrodes because of their high theoretical volumetric capacity.  These benefits are tempered by the large volume changes during charge/discharge cycling that can lead to electrode failure [1].  To deal with this problem, many researchers are developing advanced binders.  It has been shown that good binders for alloy materials provide good adhesion to the active materials and to the current collector, and complete coverage of the alloy particles [2].  It is suspected that by completely covering the surface of the alloy particles, binders can form an "artificial SEI" layer to reduce electrolyte decomposition reactions [2].  Other studies have shown that conductive polymers can be used as excellent binders for alloy negative electrodes [3, 4].

Aromatic polyimides (PI) have been shown to work well as binders for alloy negative electrodes [5].  PI provides excellent surface coverage of the alloy particles and adheres strongly to the alloy and the current collector.  However, PI has first cycle irreversible capacity.  Recent work by Wilkes et al. [6] argues that this first cycle irreversible capacity is due to the carbonization of the polyimide during the first lithiation (charge) of the anode, during which the polyimide undergoes a 34-electron reduction.  The reduction product is thought to be hydrogen-containing -carbon, which serves as a high performance conductive binder for subsequent cycles.

Here, it will be shown that the thermal reduction of binders to form hydrogen-containing carbons also results in electrodes with excellent cycling performance.  Figure 1 shows a comparison of the differential capacity curves for a Si-based alloy electrode employing a polyimide binder that has been cured at 300 °C to a similar electrode that has been heated to 600 °C before cell construction.  The cells perform very similarly, indicating that binder reduction by electrochemical lithiation can be thought of as equivalent to a thermal reduction of the PI binder.  The resulting products are similar to a low temperature, hydrogen-containing carbon that provides a conductive framework and continuous coating for the alloy particles and allows excellent charge/discharge cycling.

Conductive polymers also exhibit this high irreversible capacity [4, 7].  We suspect that they are also undergoing carbonization during their first lithiation.  If carbonization of the binder is key to good cycling properties, the utility of using expensive conductive binders is questionable, when other more inexpensive polymers exist that undergo reduction during lithiation or can be thermally decomposed to produce conductive species.

The most inexpensive polymers that we could think of that would likely undergo reduction during lithiation are aromatic phenolic resins.  Phenolic resins are widely used as inexpensive binders in applications such as sandpaper, roofing materials and wood adhesives.  Figure 2 shows the cycle life and coulombic efficiency of an electrode similar to that of Figure 1, except utilizing an inexpensive phenolic resin binder.  Excellent cycling performance can be achieved by either electrochemical or thermal reduction of the phenolic resin binder.

This demonstrates that binder reduction, either thermally or electrochemically, provides a pathway towards achieving excellent cycle life with alloy electrodes.

References.

[1] M.N. Obrovac, L. Christensen, Dinh Ba Le, and J.R. Dahn, J. Electrochem. Soc., 154, A849 (2007).

[2] M.N. Obrovac, V.L. Chevrier, Chemical Reviews, 114 (23) , 11444 (2014).

[3] G. Liu , S. Xun , N. Vukmirovic , X. Song , P. Olalde-Velasco , H. Zheng, V. S. Battaglia , L. Wang and W. Yang, Adv. Mater., 23, 4679 (2011).

[4] S.P. Xun, X. Song, V. Battaglia and G. Liu, J. Electrochem. Soc., 160 (6) A849 (2013).

[5] J.S. Kim, W. Choi, K.Y. Cho, D. Byun, J. Lim, J.K. Lee, Journal of Power Sources, 244 , 521 (2013).

[6] B.N. Wilkes, Z. L. Brown, L.J. Krause, M. Triemert, and M.N. Obrovac, J. Electrochem. Soc. 163 (3), A364 (2016).

[7] H. Wu, G. Yu, L. Pan, N. Liu, M.T. McDowell, Z. Bao, Y. Cui, Nature Communications, 4 , 1943 (2013).