Silicon is one of the promising active materials for negative electrodes of Li-ion batteries (LIB) because of its larger reversible capacity (~3600 mAh g^-1) through lithiation/delithiation process, compared with that of conventional graphite active material (~370 mAh g^-1).  However, it is difficult to maintain its superior discharge capacity for long-term cycles because large volume change of Si occurs during cycles, which triggers intensive electrode degradation.  To overcome this drawback, binder polymer is thought to be a key factor for improvement of the performance.  In particular, polyacrylate binder^1,2) and natural polymer binder³⁾ effectively suppress the degradation due to their superior mechanical strength, coatability for active materials, and effects of pre-formed solid electrolyte interphase (SEI)⁴⁾.  Recently, we have focused on polyglutamate (PGlu) as an efficent binder for the Si/graphite composite electrode of LIB.  PGlu is known to be contained in Japanese traditional food “Natto,” which is fermented soy beans.  In this study, effects of discharge cut-off voltage in the cycles and electrolyte additive on the performance of Si/graphite electrodes with PGlu binder are investigated from the viewpoint of surface chemistry.

  Figure 1 shows cycle performance of the Si/graphite electrodes with different discharge cut-off voltage.  For additive-free cells, capacity retention was highly improved under the cycles with lower discharge cut-off voltage (1.0 V vs. Li/Li⁺).  Furthermore, addition of fluoroethylene carbonate (FEC) to the electrolyte effectively suppressed deterioration of the performance and brought the best capacity retention, leading to discharge capacity of ~800 mAh g^-1 even after 100 cycles.  To clarify details of the effects, chemical components and thickness of the SEI formed on the electrode surface were examined by hard X-ray photoelectron spectroscopy (HAXPES, hν = 7.94 keV).  Figure 2 shows the HAXPES C 1s and Si 1s spectra of Si/graphite electrodes before and after the first cycle with different discharge cut-off voltage.  For the electrode with cut-off voltage of 1.0 V, the peak intensity at 284.6 eV is relatively lower than that of the 2.0 V cut-off electrode, which is assigned to sp² carbon bonding in graphite of the composite.  Similarly, the peaks at 1839.5 eV and 1844 eV in the Si 1s spectra, which are assigned to Si⁰ and silicon oxide, respectively, are smaller for the case of lower cut-off voltage.  These results indicate that thickness of SEI is affected by the discharge cut-off voltage. We presume that moderately-low discharge cut-off voltage of 1.0 V effectively brings thicker and stable SEI on the electrode surface by suppression of oxidative decomposition and dissolution of SEI, which probably occur at higher potential condition in delithiation process.  In this presentation, other electrochemical and morphological characterizations of the electrode and superior effects of FEC for the cyclability will be also discussed.

References:

1) Z.-J. Han, S. Komaba et al., Energy Environ. Sci., 5, 9014 (2012).

2) Z.-J. Han, K. Yamagiwa, S. Komaba et al., Phys. Chem. Chem. Phys.,, 17, 3783 (2015).

3) M. Murase, S. Komaba et al., ChemSusChem, 5, 2307 (2012).

4) S. Komaba, T. Ozaki et al., J. Power Sources, 189, 197 (2009).