Rechargeable lithium-ion batteries are now commonplace in the electronics community and are found in many devices in the common household. Despite their widespread commercial use, the manufacturing process and the fabrication of the electrodes within these batteries still suffers from many drawbacks.
Building on previous work, a team of Researchers from China and Hong Kong have developed an elastic nitrogen-doped carbon foam (ECF) as a free-standing flexible anode material for high-performance lithium ion batteries through a pressure-assisted pyrolysis method that shows better properties compared to other anodic materials.
Despite being the main power source for many household devices, the slurry and current collection production process of many conventional carbon electrodes within rechargeable lithium ion battery power systems suffers from multiple issues.
These range from some of the internal components being electronically inactive and therefore not contributing to the lithium storage, to the polymeric binders reducing the electronic conductivity of the electrode, to the produced electrodes possessing a high fragility, inflexibility and high microcracking possibility.
In a different approach, the Researchers previously produced an elastic nitrogen-doped carbon foam (ECF) through the direct pyrolysis of melamine foam. The Researchers found that the resulting material possessed a high specific surface area, a low density and a high electrically conductivity.
As similar foam-based materials have been used to produce anodes, albeit with inefficient stabilities and capacities, the Researchers decided in their new work to test their new material as a free-standing flexible anode material for high-performance lithium ion batteries.
To produce the material for electrode purposes, and to increase the properties of the material itself, the Researchers used the same type of pyrolysis method, but an improved version where a constant compression was imposed on the melamine foam during pyrolysis. Upon completion of the pyrolysis process, a 150 mm × 25 mm piece of material was obtained.
The Researchers characterized the morphologies and microstructures of the carbon material using scanning electron microscopy (SEM, Vega 3, TESCAN; JSM-6335F, JEOL), transmission electron microscope (TEM, JEM-2100, JEOL). X-ray diffraction spectroscopy (XRD, D5000, Siemens), Raman spectroscopy (LabRam HR-800, Horiba), X-ray photoelectron spectroscopy (XPS, ESCaLAB 250, Thermo Scientific), Brunauer-Emmett-Teller (BET) measurements (ASAP 2020, Micromeritics Instrument Corporation) and pore size distribution measurements (PSD).
The Researchers also tested the 3-point bending capacity of the material using a Microforce Testing System (Tytron 250, MTS) and the electrochemical performance of the anodes were characterized using CR2025-type coin cells, galvanostatic charge-discharge measurements (LAND-CT2001A, Wuhan Jinnuo) and Electrochemical impedance spectroscopy (EIS, VMP3, Bio-Logic).
The Researchers have developed the material, which they coin as a nitrogen-doped carbon paper (NCP), which is composed of a highly dense three-dimensional (3D) hierarchical cellular structure that possess a high bending flexibility.
The Researchers have utilized the material as a free-standing anode for lithium-ion batteries. In this form, the carbon material was found to exhibit an initial reversible capacity of 480 mAh g-1 at a current density of 0.5 Ag-1, with a stable capacity of 329.8 mAh g-1 at a current density of 0.5 Ag-1 after 200 cycles.
Even at a high current density of 8 Ag-1, the Researchers found that the material possessed a steady capacity of 126.5 mAh g-1 after 500 cycles at room temperature. This performance was also found to be an enhancement of previous carbon materials developed by these Researchers which did not use compression methods.
Even at a high mass loading of 3.2 mg cm-2, the carbon paper still maintained a high flexibility and a good electrochemical performance.
The increased electrochemical performance of the material has been attributed to the highly dense interconnected 3D network facilitating an efficient electron transfer, and short lithium ion transport paths, through multiple junctions; a high mechanical flexibility and an ability to be free standing which does not require binding materials, conductive additives or current collectors. All of which remove conductivity from the electrode.
Aside from their own research, the electrode performs better than electrodes constructed from both graphene and carbon nanotube-based paper materials, and possesses more benefits than other carbon electrodes.
The Researchers have produced a material with great commercial potential, as the manufacturing process is more economical and easier for scaling up than other electrode fabrication methods.
As such, the carbon paper has great commercial potential to be implemented as carbon anodes in high-performance Li-ion batteries for flexible and wearable electronics applications.
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“Nitrogen-doped carbon paper with 3D porous structure as a flexible free-standing anode for lithium-ion batteries”- Zhang H., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-07345-y