Capacity-limited cycling constitutes a practical approach to improve the cycling stability of silicon-based electrodes in Li-ion batteries.^1–3 As the volume expansion of silicon increases with the uptake of lithium, the degree of lithiation (Li_xSi) displays a trade-off between delivered capacity and detrimental volume changes.⁴ Composite electrodes, consisting of silicon and graphite active materials, are considered as a compromise between the high capacity of silicon and excellent cycling performance of graphite.⁵ However, decreasing the amount of silicon in these electrodes also increases the need for a higher degree of lithiation to obtain a commercially reasonable electrode specific capacity in the order of 1000 mAh g^‑1_el.⁶

In this study, we investigate silicon-graphite composite electrodes with different active material ratios (20-60 wt% silicon) in regard to their degradation during capacity-limited cycling. Besides silicon nanoparticles (~200 nm diameter) and graphite (~20 µm diameter) as active materials, the electrodes comprise conductive carbon fibers and lithiated poly(acrylic acid) as binder. The areal capacity of all electrodes was set to 2.1 ± 0.2 mAh cm^-2. As electrolyte solution, 1 M LiPF₆dissolved in a 3:7 (w:w) mixture of ethylene carbonate:ethyl methyl carbonate (LP57), and 5 wt% of fluoroethylene carbonate (FEC) was used.

To investigate the degradation behavior, the electrodes are always fully lithiated to 0.01 V vs. Li/Li⁺ and only partially delithiated until a capacity-limit of 800 mAh g^‑1_el. As the theoretical capacity of these electrodes strongly depends on the silicon content and increases from ~960 to ~2200 mAh g^-1_el, the practical capacity utilization of these electrodes to deliver 800 mAh g^‑1_el consequently behaves inversely to the silicon content and thus decreases from ~90 to ~40%. By evaluating the cycling performance vs. capacitively oversized LiFePO₄ electrodes, we demonstrate that none of the investigated electrodes shows a visible capacity fade within 120 cycles. However, by detailed analysis of the coulombic efficiency and voltage profiles, we show that in spite of a decreasing silicon content the irreversible capacity loss and electrode polarization increase with the capacity utilization of the electrode. In accordance with that, we demonstrate through ¹⁹F-NMR analysis of the electrolyte solutions after different number of cycles that the FEC consumption also decreases with the capacity utilization. Supported by theoretical considerations of the surface area changes of silicon particles, we finally compare capacity- and voltage-limited cycling procedures and conclude that the degree of lithiation, and thus the capacity utilization, is a key parameter in the design of commercial silicon-based composite electrodes. Furthermore, we present a practical set of analyses to differentiate the cycling performance of silicon-based electrodes and to quantify the underlying degradation mechanisms.

References:

(1) Gauthier, M.; Mazouzi, D.; Reyter, D.; Lestriez, B.; Moreau, P.; Guyomard, D.; Roué, L.; Roue, L. Energy Environ. Sci. 2013, 6(7), 2145–2155.

(2) Markevich, E.; Fridman, K.; Sharabi, R.; Elazari, R.; Salitra, G.; Gottlieb, H. E.; Gershinsky, G.; Garsuch, A.; Semrau, G.; Schmidt, M. A.; Aurbach, D. J. Electrochem. Soc. 2013, 160(10), A1824–A1833.

(3) Obrovac, M. N.; Krause, L. J. J. Electrochem. Soc. 2007, 154(2), 103–108.

(4) Obrovac, M. N.; Chevrier, V. L. Chem. Rev. 2014, 114, 11444–11502.

(5) Wetjen, M.; Jung, R.; Pritzl, D.; Gasteiger, H. A. ECS Meet. 230 2016, Abstr. #87585.

(6) Gröger, O.; Gasteiger, H. A.; Suchsland, J.-P. J. Electrochem. Soc. 2015, 162(14), A2605–A2622.

 

Acknowledgements:

The German Federal Ministry for Economic Affairs and Energy is kindly acknowledged for funding (funding number 03ET6045D).