Abstract:

Objective As a flexible anode material, carbon nanofibers membrane is used in lithium-ion batteries, which has the advantages of high electron transfer rate and large electrolyte contact area. The search for novel carbon fibers precursors to prepare carbon fibers electrodes with good performance for flexible high-performance lithium-ion batteries calls for further research. Being an aromatic heterocyclic polymer, polypyrrilone (BBB) has a high carbon residue rate after carbonization, and is a preferred material as a precursor to new carbon fibers.

Method In order to investigate the electrochemical performance of polypyrrilone carbon fibers as self-supported anode materials, a mixture of polycarboxylic acid ammonium salt (PCAAS) and different mass fractions of poly(ethylene oxide) (PEO) was prepared into precursor nanofibers by electrostatic spinning technique. The precursor nanofibers were imidated and cyclized to form polypyrrilone fibers at 500 ℃, and then charred at 800 ℃ to form polypyrrilone carbon fibers (BCF), and BCF was assembled into lithium-ion batteries as an anode material, to investigate the morphology and electrochemical performance of BCF with different mass fractions of PEO, which were denoted as BCF-3, BCF-6, and BCF-9, respectively, based on the mass fraction of PEOs. BCF was used as an anode material for lithium-ion batteries, and the morphology and electrochemical properties of BCF were investigated under different mass fractions of PEO.

Results The surface of the precursor fibers is smooth, and the fiber diameter increases significantly when PEO concentration increased. It was found that the diameters of BCF-3, BCF-6 and BCF-9 are 31.2%, 24.2% and 3.2% lower than their precursors, respectively, and the decrease in fiber diameter was due to the pyrolysis of PEO as a sacrificial template. In addition, the fiber agglomeration of BCF-6 precursors decreased after carbonization. The FT-IR spectra and the XPS spectra showed that the residue of pyrrole N and imidazole N was present in all three samples, while the residual N in BCF-6 was the most. All three samples showed an amorphous carbon structure, and BCF-6 had the largest diffraction peak of 2θ=24°, and the carbon crystalline diameter corresponding to the carbon (002) surface and the width of the microcrystalline along the fiber radial direction were the largest, indicating that BCF-6 had a better carbon structure. Through thermogravimetric analysis, the carbon content of the three samples was estimated to be 94.9%, 97.5% and 96.4%, respectively. Cyclic voltammetry testing was essential for the study of electrode reaction processes and reversibility, and BCF was used as the anode material to assemble a button half-cell for the test. One oxidation peak appeared in the curves of all three samples, and the oxidation peak of BCF-6 was the most obvious. The high residual N in BCF-6 gave it a high electrochemical activity, and the higher carbon content of BCF-6 gave it a high specific capacitance. The Nyquist curve semicircle of BCF-6 depicted the smallest semicircle and the largest slope, indicating that it has the smallest charge transfer resistance and the fastest Li⁺ diffusion rate. The specific discharge capacity of the first cycle of the BCF-6 lithium-ion battery was 841.4 mA·h/g. After 100 cycles of charge-discharge cycles, the discharge specific capacity was 422.3 mA·h/g, and the coulombic efficiency was 98.71%. When the current density of rate test was 100, 200, 300, 400, 500 mA/g, the specific discharge capacity of BCF-6 was as high as 380.7 mA·h/g, which was 1.3% higher than the initial value. As a flexible self-supporting anode material, BCF was subject to the necessary flexibility tests. BCF-6 did not break and remained smooth and flat after folding and curling tests from 0° to 180°.

Conclusion The results show that the self-supporting structure of BCF is conducive to improving the structural stability and cycle stability, and BCF has residual N of pyrrole and imidazole after carbonization, which provides more active sites for Li⁺and improves the specific capacity of BCF. When the PEO concentration is 6%, the specific discharge capacity of the first cycle of the BCF-6 lithium-ion battery is 841.4 mA·h/g, and after 100 cycles, there is still a discharge specific capacity of 422.3 mA·h/g. In the rate performance test, when the current density returns to 0.1 A/g again, the specific capacity of BCF-6 discharge is 1.3% higher than the specific capacity of the first turn, showing excellent rate performance. BCF not only meets the demand for flexible energy storage, but also offers stable electrochemical properties. Its flexible self-supporting skeleton and high electronic conductivity loading substrate can effectively improve the mass specific capacity of anode materials, while reducing the problem of lithium dendrite growth. The selection of synthetic monomers for BCF can be further optimised by adopting tetramine and dianhydride monomers containing flexible groups in the monomers, in order to reduce the rigidity of the polymer chain segments and thus make BCF more flexible. In view of the excellent electrochemical properties and certain flexibility of BCF anode materials, their application in the field of wearable energy storage can be further studied.

Key words: electrospinning, polypyrrilone, carbon nanofiber, flexible self-supporting, electrochemical property, anode material