Understanding Criteria of Cross-Linking Polymer As Effective Binder for Si-Based Lithium-Ion Battery Anodes

Silicon is a promising anode material for lithium-ion battery because of its high theoretical capacity of 3500 mAh g-1 (formation of Li15Si4) and low operating potential (0.4 V vs Li/Li+) against lithium. However, particle cracking and electrode delamination induced by the large volume change during lithiation and delithiation degrade the cycle performance significantly. Binder in the electrode thus plays an important role in holding the active materials and carbon together. So far, polymers such as alginate and polyimide have been most commonly studied as alternative binder for Si electrodes. While cycle performance is improved, so far there has not been any guiding principles based on the properties of the polymers that can tell a “good” binder from a “bad” one without physically testing it. In this study, we aim at developing a basic understanding on how the mechanical and chemical properties of binders affect cycle performance of Si-based electrodes. We start with polymers with different functional groups such as polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC) and polyacrylic acid (PAA) and add different amount of borax as cross-linker into the electrodes. By tuning the amount of borax, we can tune the mechanical properties of the binder. We found that amongst the different binder combinations, PAA-borax gives the best electrochemical property in terms of capacity and cycle retention. In addition, the performances are highly dependent on the amount of borax. Figure 1 shows the cycle performance and rate performance of Si electrodes with PAA-borax as the binder. While the capacity fading is significant without borax cross-linker, addition of a small amount of borax improves cycle stability. In particular, we observe that there is an optimal amount of borax of 5% that gives excellent cycle performance. More mechanical characterizations of the electrodes with methods such as in-situ dilatometry and indentation, as well as chemical characterizations such as Fourier Transfer Infrared Spectroscopy (FTIR) are underway to understand the reason for the observed performance, and the results will be shown during the meeting. Figure 1