To enable wider use of lithium ion batteries, great efforts have been made to increase their energy and power density with novel active materials and nanostructures. The electrode structure in conventional cells is shown in Fig. 1a, which involves active material coated on metal foil with polymer binder and conductive additive. Minimizing the use of such non-capacitive materials is another approach for improving energy density. “Carbon nanotube (CNT) three-dimensional (3D) current collectors” show promise for the replacement of heavy current collectors as well as polymer binders with a light-weight freestanding CNT network as shown in Fig. 1b. We have realized semi-continuous production of sub-millimeter-long few-wall CNTs (FWCNTs) by fluidized bed chemical vapor deposition (FBCVD) [1, 2], and applied them to electrochemical devices. The long and flexible FWCNTs obtainable easily form 3D networks with submicron-sized pores and reasonable conductivity (~100 S cm−1). The freestanding composite electrodes made of 90 wt% activated carbon and 10 wt% FWCNTs operated as electrochemical capacitors with fair in-plane conductivity, making them suitable as 3D current collectors [3]. In this work, we investigated the applicability of a “CNT 3D current collector” in fabricating lithium ion full cells without polymer binder nor metal foils. Both cathode and anode were prepared with 99 wt% active materials and 1 wt% CNTs only; conventional LiCoO2and natural graphite were used as model electrodes. After basic coin cell tests with full cells, we fabricated laminate cells with 50 mm × 50 mm electrodes. By comparing the cells with and without metal foils, we demonstrate the operation of the proposed CNT 3D current collector in practical sized lithium ion batteries. LiCoO2-CNT and graphite-CNT composite sheets were prepared by ultrasonication and filtration. FWCNTs prepared by FBCVD (11 nm in diameter, 370 µm long on average) [2] were added to 2-propanol at 0.02 mg mL−1 concentration and then dispersed for 30 min with a bath-type sonicator. Next, 2.0 mg mL−1 LiCoO2 or natural graphite powder was added, followed by dispersion for an additional 10 min, filtrated on a membrane filter, mechanically peeled off, dried and pressed. Coin cells of lithium ion full cells with 8 mmΦ LiCoO2/graphite were fabricated and charge-discharge measurements were carried out to evaluate the performance of the electrodes. Laminate cells with 50 mm × 50 mm electrodes were also prepared with three types of current collector using 20-µm-thick Al and Cu foils for the cathode and anode, respectively; with metal foils in full contact with the electrodes (Fig. 2a), with comb structured foils (1-mm-wide wires spaced 10 mm apart) for partial contact (Fig. 2b), and with wires sewn into one of the edges of the electrodes (Fig. 2c). Flexible and freestanding sheets with fair electric conductivities were formed with both 99 wt% LiCoO2-1 wt% CNTs and 99 wt% graphite-1 wt % CNTs, which were easily handled by hand without suffering damage. In a series of coin cell tests using LiCoO2/graphite full cells, electrodes containing 99 wt% active materials and 1 wt% CNTs not only showed fine performance in LIBs but also exhibited positive effects on both rate performance and cycle stability. A discharge capacity of 353 and 306 mAh ggraphite −1based on the anode weight were achieved at 0.1C and 1C, respectively, with a capacity retention of 65% even at the 500th cycle. The electrode with 10 wt% CNTs showed smaller capacity than those with 1−3 wt% CNTs because of the additional side reaction owing to their large surface area, so a CNT content of less than 3 wt% is preferable. By the laminate cell test using 50 mm × 50 mm electrodes, the cell with a comb-shaped metal current collector with 10 mm-wide spaces showed good rate performance and cycle stability, whereas that with a metal line at one of the electrode edges did not charge and discharge at 0.3C. Thus, the combination of CNT 3D current corrector and metal wires with ~1 cm spacing were effective in improving the cell-base gravimetric energy density while avoiding resistivity-based power loss. The CNT 3D collectors are applicable not only to commercially-used active materials but also to novel active materials under development, and will contribute to future cells with breakthrough performances. [1] D.Y. Kim et al., Carbon 49 (2011) 1972–1979. [2] Z. Chen et al., Carbon 80 (2014) 339–350. [3] R. Quintero et al., RSC Adv. 4 (2014) 8230–8237. Figure 1
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Aug 21, 2024
Teng Zhang + 11
Open Access