The Significant Influence of the Conductive Carbon Additive on the Performance of High Areal Capacity Silicon Electrodes

The constant demand for lithium-ion batteries with higher energy density requires finding new electrode materials. Silicon-based electrodes are particularly attractive due to the higher gravimetric capacity of Si (3579 mAh g-1) compared to conventionally used graphite (372 mAh g-1). However, during the process of lithiation/delithiation, the silicon material suffers from a huge volume change, leading to the fracturing of the silicon particles, an unstable solid electrolyte interphase layer (SEI) and the disconnection of inter-particle contacts, which all have a negative repercussion on the electrode cycle life. Another serious challenge for commercializing silicon electrodes is to reach a high areal capacity of more than 6 mAh cm-2, in order to achieve an energy density improvement over the use of conventional graphite-based anodes. Silicon electrodes with such high areal capacity require very careful design of their formulation at different scales. In particular, a special attention must be paid to creating durable intimate contacts between the active material particles and the conductive additive network, so that sufficient electron transfer could be achieved throughout the electrode from the copper current collector while good mechanical stability of the electrode coating is still maintained.1 Our group has recently shown that high performance silicon-based anodes can be achieved by combining (i) the use of high-energy ball-milling as a cheap and easy process to produce nanostructured silicon powder, (ii) the processing of the electrode with carboxymethylcellulose (CMC) binder at pH 3 condition, which promotes the covalent grafting of the CMC to the Si particles; (iii) the use of fluoroethylene and vinylene carbonates (FEC/VC) electrolyte additives resulting in a more stable SEI.2 In the present work, silicon-based electrodes of various areal capacities were prepared by using either carbon black, vapor grown carbon nanofibers, or graphite nanoplatelets as conductive additive.3 It was observed that the electrical conductivity, capacity retention, and coulombic efficiency of the silicon electrode are significantly affected by the morphological characteristics of the used carbon additives. Spherical-shaped carbon black particles have a strong tendency to agglomerate, and thus fail in creating a conductive network resilient to the silicon particle volume change. In contrast, vapor grown carbon nanofibers maintain more durable contacts with silicon particles, compared to carbon black, by forming a more resilient conductive network due to their wire-like structure.4 Graphite nanoplatelets also create a continuous conductive network, which limits the mechanical degradation of the electrode coating, likely by playing the role of electrically conducting lubricant.5These results demonstrate that the choice of the conductive additive is of crucial importance for the optimization of silicon negative electrodes with commercially relevant areal capacities. References (1) Mazouzi, D.; Karkar, Z.; Reale Hernandez, C.; Jimenez Manero, P.; Guyomard, D.; Roué, L.; Lestriez, B. J. Power Sources 2015, 280, 533–549. (2) Gauthier, M.; Mazouzi, D.; Reyter, D.; Lestriez, B.; Moreau, P.; Guyomard, D.; Roué, L. Energy Environ. Sci. 2013, 6(7), 2145. (3) Karkar, Z.; Mazouzi, D.; Reale Hernandez, C.; Guyomard, D.; Roué, L.; Lestriez, B., Submitted (4) Lestriez, B.; Desaever, S.; Danet, J.; Moreau, P.; Plée, D.; Guyomard, D. Electrochem. Solid-State Lett. 2009, 12(4), A76. (5) Nguyen, B. P. N.; Gaubicher, J.; Lestriez, B. Electrochim. Acta 2014, 120, 319–326. Figure 1

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