Implications of the Hypothesis
Pressing process is just the physical combination of electrospun membrane and Cu foil. When pressing, the solvent-containing electrospun fibers are similar to the binder and adheres strongly to the current collector. The pressing process did not damage the porous structure of the materials (Figure 2). After carbonized, the Cu foil will form a strong connection with the polymer. It is worth noting that this method is suitable for a variety of electrospun fibers, and here only three representative materials are demonstrated, namely: pure polymer (Figure 2a), polymeric composite (Figure 2b), and inorganic and polymeric composite (Figure 2c)
PMMA@PAN membrane is selected as the example for the stability study of the carbonized membrane because the PAN membrane has relatively good film formation, while PMMA@PAN and oxides@PMMA@PAN membranes have poor stability and similar structures. As can be seen in Figure 3a, PMMA@PAN membrane becomes brittle after carbonization, and cracks can be obviously observed. In contrast, the PMMA@PAN@Cu is very smooth with no cracks (Figure 3b). This method enables the high-quality binder-free electrodes in large-scale production (about 5 cm × 10 cm) in the laboratory. To further demonstrate the structural stability of materials, the PMMA@PAN and PMMA@PAN@Cu are placed in ethanol solution for ultrasonic treatment for 30 min to test the strength of the membrane. It shows that PMMA@PAN starts to break at the beginning of the treatment, and is completely destroyed and dispersed in ethanol after about 5 min, whereas the PMMA@PAN@Cu remains intact after 30 min where there are no visible cracks (Figure 3c, b). Moreover, PMMA@PAN powder is ball-milled and coated onto the Cu foil with PVDF as binder to test the adhesion as shown in Figure 3e. PMMA@PAN is easily aggregated during milling process. In addition, the surface of the fabricated electrode is quite rough and the active materials can be entirely peeled. However, a large amount of PMMA@PAN@Cu material smoothly remains on the Cu foil after the same testing process (Figure 3e, f). The ultrasonic treatment and adhesion test clearly demonstrate that the carbon material of the PMMA@PAN@Cu has a strong adhesion to the Cu foil [12].
The crystal structure of PMMA@PAN and PMMA@PAN@Cu is characterized by Raman spectroscopy and XRD to observe the differences after pressing the polymer fibers onto the Cu foil (Figure 3g, h). The first peak of Raman spectra at about 1350 cm-1 and the second at 1590 cm-1 corresponds to the D band of defect-induced mode and the G-band of E2g graphitic mode, respectively [13]. The intensity ratios between the D and G band indicating the disorder degree of carbon materials. It shows the same value of 1.2 demonstrating the negligible impact after pressing the polymer fibers onto the Cu foil. Moreover, the disorder feature may be caused by the PPMA, which leads to the uneven carbonization of PAN and brittle property of the material. PMMA@PAN and PMMA@PAN@Cu have similar XRD pattern where both show strong diffraction peaks of 2θ value at 25.0o. This featured peak is corresponding to layers of the graphite structure [14]. In short, the carbonization process of the electrospun membrane has not changed after being composited with Cu foil.
Electrochemical performance
The electrochemical performances of various binder-free electrodes are examined using a CR2032 coin-type half-cells. The rate performances at current densities ranging from 250 to 2500 mA g-1 are displayed in Figure 4a. The discharge capacity of ZnO@PMMA@PAN@Cu, ZnO@PMMA@PAN, PMMA@PAN@Cu, PMMA@PAN, PAN@Cu, and PAN can remain at 260, 248, 202, 163, 174, and 162 mAh g-1 at the current density of 2500 mA g-1, respectively. However, the capacity retention with the increasing of current density is generally lower after pressing the polymer fibers onto the Cu foil. It is mainly because that the pressed electrodes show less porosity and some fibers are crushed together, limiting the Li ions transfer from electrolyte into the carbon materials. After 300 cycles, the discharge capacity remains at 219, 178, 165, 137, 130, and 124 mAh g-1 for ZnO@PMMA@PAN@Cu, ZnO@PMMA@PAN, PMMA@PAN@Cu, PMMA@PAN, PAN@Cu, and PAN, respectively. The capacity retention of the electrodes prepared by pressing the polymer fibers onto the Cu foil and carbonization keeps almost 100% from the 50th cycle while the membrane without Cu foil supporting show poor retention, namely, about 71%, 89%, and 81% for ZnO@PMMA@PAN, PMMA@PAN, and PAN, respectively. The cycle life of ZnO@PMMA@PAN@Cu and ZnO@PMMA@PAN are evaluated at a current density of 2500 mA g-1 (Figure 4b). ZnO@PMMA@PAN@Cu and ZnO@PMMA@PAN show the reversible capacities of 180 and 96 mA h g-1 and the capacity retention of 82% and 55% after 2000 cycles, respectively. It demonstrates the excellent cycling performance after pressing the polymer fibers onto the Cu foil.