Optimization of Si/Gr Based Anode Formulation for High Energy Density Li-Ion Batteries

Abstract

In need of increasing automotive applications, it is necessary to find both higher capacity and higher-capacity retention electrode materials for Li-ion batteries. Among the candidates for negative electrode [1], silicon appears as an attractive alternative to graphite due to its natural abundance, high specific gravimetric capacity (3579 mAh.g-1 vs. 372mAh.g-1 for graphite) and a large volumetric capacity (2081 mAh.cm-3 vs. 779 mAh.cm-3). Silicon based electrodes nevertheless suffer from poor cyclability usually due to dramatic volume expansion of the electrode during the cycling and electrolyte reduction [2]. As a consequence, irreversible capacity losses are not limited to the first few cycles, but may continue to deteriorate cell performance and life [3, 4]. On one hand, previous studies demonstrated that the formulation [5], the processing [6] of the composite electrode and the adhesion to the current collector are critical for Si with respect to the electrochemical performance. The design of a flexible network of shaped conductive additives bonded with an aqueous binder to silicon particles through an optimized ratio of covalent and hydrogen bonds greatly help in facilitating a homogeneous functioning of the electrode that minimizes electrolyte reduction and keep the mechanical integrity of the electrode. On the other hand, using a combined silicon/graphite material also help in mitigating volume variations expected to occur upon cycling, especially for high mass loading electrodes (>3-4 mg). In this study, we use both approaches by tuning the binder incorporation in the case of a 50/50 %wt Silicon/Graphite composite electrodes. We focus here in particular on the electrode formulation, using a polyacrylic acid polymer binder thanks to its grafting mechanism to silicon particles enabling a strong mechanical cohesion even in the case of high-loaded electrodes. [7] pH and viscosity adjustments of the electrode slurry will be discussed, as they seem to have a remarkable effect at the interface between electrode and current collector. In fact, the longer drying process (due to thicker coating) exacerbate different corrosion mechanisms and migration phenomena that were not observed previously for thin electrodes usually studied. Characterizations on this interface were carried out, combining XPS and SEM, in order to offer a further comprehension of Si/Gr cycling performance as a function of electrode preparation. Reference : 1) Kasavajjula, U., Wang, C. & Appleby, A. J. Nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells. 163, 1003–1039 (2007). 2) Obrovac, M. N. & Chevrier, V. L. Alloy negative electrodes for Li-ion batteries. Chem. Rev. 114, 11444–11502 (2014). 3) Ryu, J. H., Kim, J. W., Sung, Y. & Oh, S. M. Failure Modes of Silicon Powder Negative Electrode in Lithium Secondary Batteries. Electrochem. Solid-State Lett. 7, 306–309 (2004). 4) Beaulieu, L. Y. et al. Colossal Reversible Volume Changes in Lithium Alloys service Colossal Reversible Volume Changes in Lithium Alloys. Electrochem. Solid-State Lett. 4, A137 (2001). 5) Mazouzi, D. et al. Critical roles of binders and formulation at multiscales of silicon-based composite electrodes. J. Power Sources 280, 533–549 (2015). 6) Nguyen, B. P. N., Chazelle, S., Cerbelaud, M., Porcher, W. & Lestriez, B. Manufacturing of industry-relevant silicon negative composite electrodes for lithium ion-cells. J. Power Sources 262, 112–122 (2014). 7) Mazouzi, D., Lestriez, B., Roué, L. & Guyomard, D. Silicon Composite Electrode with High Capacity and Long Cycle Life. Electrochem. Solid-State Lett. 12, A215 (2009).