Among all potential active materials for the anodes of Li-ion batteries, silicon is considered as one of the most promising candidate because of its high specific and volumetric capacities, about 3600 mAh.g^-1 and 2200 mAh.cm^-3 respectively,¹ as well as its low operation voltage (0.4 versus Li/Li⁺), abundant resources (the second largest in the earth’s crust), and environmental benignity (non-toxic). However, during the lithiation (formation of LixSi, x ~ 3.75) the silicon undergoes an expansion of about 280% of its initial volume² which induces numerous damages to the electrode. The silicon micrometer particles tend to be pulverized when the size of the particles are upper than 150 nm.³ However, nanosizing the materials leads to more electrolyte degradation due to a higher specific surface developed and requires more binder and eventually more conducting additive in the electrode formulation. The pulverized Si particles are scattered and become electrically isolated and the adhesion of the Si electrode to the current collector are also damaged by these volume variations⁴. This effects drastically reduces the electrochemical performance and lifetime of the Si electrode. As a result, these challenges have seriously restricted the commercialization of Si anodes in high energy LIBs.⁵

To deal with these problems many researches are developing advanced binders. This later has a crucial role since it reinforces the mechanical strength of the electrode and thus helps to preserve the electrode architecture upon cycling against the large Si volume change.⁶ It is generally accepted that binders of very high molar masses (polymers) are necessary to allow cycling of silicon-based electrodes, as they favor the formation of more robust molecular bridges and therefore are a priori more capable of maintaining particle-to-particle contacts and therefore cohesion in the electrode. Quite surprisingly, we have discovered that it is possible to obtain good cyclability of silicon-based electrodes by using an organic binder of low molar mass (molecule). This calls into question the understanding of the mechanism by which the binder operates in silicon-based electrodes.

This low molecular weight binder is a natural polyphenol, namely tannic acid. Here we will highlight that Silicon-based composite electrodes of high areal capacity (~7 mAh.cm^-2) when prepared with tannic acid as small binder show a stable cycling like the one obtained with one state-of-the-art binder such as carboxymethyl cellulose. We will report in-depth characterization of the interactions between tannic acid and the silicon particles surface as well as of the rheological behavior of the electrode slurries and the electrodes properties and electrochemical performances.

References

(1) Obrovac, M. N.; Chevrier, V. L. Alloy Negative Electrodes for Li-Ion Batteries. Chem. Rev. 2014, 114 (23), 11444–11502.

(2) Jung, S. C.; Choi, J. W.; Han, Y.-K. Anisotropic Volume Expansion of Crystalline Silicon during Electrochemical Lithium Insertion: An Atomic Level Rationale. Nano Lett. 2012, 12 (10), 5342–5347.

(3) Liu, X. H.; Zhong, L.; Huang, S.; Mao, S. X.; Zhu, T.; Huang, J. Y. Size-Dependent Fracture of Silicon Nanoparticles During Lithiation. ACS Nano 2012, 6 (2), 1522–1531.

(4) Hernandez, C. R.; Etiemble, A.; Douillard, T.; Mazouzi, D.; Karkar, Z.; Maire, E.; Guyomard, D.; Lestriez, B.; Roué, L. A Facile and Very Effective Method to Enhance the Mechanical Strength and the Cyclability of Si-Based Electrodes for Li-Ion Batteries. Adv. Energy Mater. 2018, 8 (6), 1701787.

(5) Ko, M.; Chae, S.; Cho, J. Challenges in Accommodating Volume Change of Si Anodes for Li-Ion Batteries. ChemElectroChem 2015, 2 (11), 1645–1651.

(6) Eshetu, G. G.; Figgemeier, E. Confronting the Challenges of Next-Generation Silicon Anode-Based Lithium-Ion Batteries: Role of Designer Electrolyte Additives and Polymeric Binders. ChemSusChem 2019, 12 (12), 2515–2539.