Scribable CNT/Si Heterostructured Anodes for Lithium-Ion Batteries

Abstract

Silicon, with a theoretical capacity of 4200 mAh/g has attracted tremendous interest to replace graphite as the high energy density anode to be used in the next generation of lithium ion batteries. Silicon however is known to undergo colossal volume expansion (>300%) during lithium cycling leading to pulverization of the active material resulting in loss of electrical contact with the current collector causing rapid decrease in capacity and consequent failure of the battery. Several approaches have been developed to address this problem which involve the use of nano-sized silicon particles [1], active-inactive matrices [2], amorphous silicon composites [3] and strain engineered 1-D dimensional nanostructures [4-6].Our previous work on CNT/Si core-shell heterostructures exhibited capacities >2000 mAh/g with very good cyclability and rate capability [5-7]. This was possible due to excellent mechanical properties of CNTs which act as good mechanical supports to prevent mechanical failure upon colossal volume changes. In this work, we report silicon coated vertically aligned CNTs as writable or scribable electrodes as a high capacity and stable anode system. The CNT/Si nanostructures were first synthesized using a simple CVD approach, similar to our previous published work and were later processed into a ‘Writing-Scribe’. This scribe was manually scribed onto copper foils to serve directly as the anode. The scribed electrodes were tested in a half-cell configuration against lithium metal between the voltage range 0.01 to 1 V vs. Li+/Li in the 1M LiPF6(dissolved in EC:DEC:FEC=45:45:10) electrolyte. No binders and conductive additives were however used to make the composite electrode.The Raman spectra of the silicon coated on the CNTs showed a broad peak ~ 480 cm-1 which indicates the silicon obtained by the CVD process is amorphous. Fig-1 (a) & (b) show the electrochemical cycling results of the scribed CNT/Si core-shell heterostructures. They exhibit a high first discharge capacity of 3100 mAh/g at a current density of 300 mA/g between the voltage range 0.01-1 V vs. Li+/Li (Fig-1a). A low first cycle irreversible loss of only 19% was observed due to the possible SEI formation. The first cycle lithiation appears to occur at 0.2 V vs. Li+/Li and is consistent with our previous reports indicating the lithiation of amorphous silicon [7, 8]. Following the first cycle, the scribed CNT/Si electrodes exhibit capacities in the range 2000-2500 mAh/g with capacity retention of 76% at the end of 50 cycles corresponding to a capacity fade of 0.47% loss per cycle (Fig-1b). This method eliminates the use of conductive additives or binders which add to dead weight to the electrode. Also, no slurries were used which further reduces the manufacturing time, cost and labor. The scribed electrode system offers a unique, simple and cost effective approach for generating high capacity and stable anode to replace graphite in the next generation of lithium ion batteries. Acknowledgements: The authors gratefully acknowledge financial support of the DOE-BATT (DE-AC02-05CHl1231) and NSF (CBET-0933141) programs. The authors also acknowledge the Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials (CCEMM) for support of this research.