CNT Paper as Anode for Flexible Lithium-Ion Battery
Fig. 4. BET test for the AMP and the CMP.
Fig. 6. The EIS of the CMP and the AMP battery after being fully lithiated.
frequency range from 10 000 Hz to 0.01 Hz. Figure 6 shows the EIS graphs of the two electrodes. Obvi- ously, only one evident semicircle appeared in the middle frequency area which presents charge transfer resistance ( R ct Þ . And the radius of the CMP was smaller than the AMP, which demonstrated that the R ct of the battery with CMP is smaller compared to the battery with AMP. It might be the result of that the insulated cellulose turned to elec- tric carbon by the carbonization treatment. And the di®usion of Li ions became easier as the impedance of an electrode decreased, thus the electrochemical performance be improved. Figure 7 shows the galvanostatic charge – dis- charge curves of batteries with MWCNT paper as negative electrode. There was a discharge voltage plateau at 1.2 V in the ¯rst time, which is the
characteristic plateau of lithium ion inserting into CNTs. The AMP and CMP electrodes exhibited the ¯rst speci¯c capacity of 379 mAh/g and 875 mAh/g, respectively, and a reversible speci¯c capacity of nearly 220 mAh/g and 500 mAh/g, respectively. It was considered that lithium ions can intercalate into the MWCNTs bundle staying either inside the tube of the MWCNTs or in the space between MWCNTs. Li ions between neighboring MWCNTs have strong adsorption potential. The potential is four times larger than that for the Li ions located along the central axis of the MWCNT. This sug- gests that Li ions located among neighboring MWCNTs would be very di±cult to deintercalate, thus resulted in a considerable irreversible capaci- ty. 30 Via the process of heat treatment, the paper ¯bers turned to amorphous carbon. Though the amorphous carbon do not make contribution to the storage of lithium ion, the amorphous carbon make the CNT paper more conductive. Therefore, the lithium ion will be easier to be di®used and inserted into the MWCNTs. That is the reason why the battery with CMP had a higher ¯rst speci¯c ca- pacity and a higher irreversible speci¯c capacity than the AMP battery. Although the CMP battery had larger irreversible speci¯c capacity, its revers- ible speci¯c capacity still reached about 500 mAh/g, which was 50% higher than commercial graphite anode (about 330 mAh/g). As we know, the high irreversible capacity will consume a large amount of lithium ion from the cathode material. Pre-lithia- tion of MWCNT method can be employed as a valid
Fig. 5. TGA curves of AMP and CMP.
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