(Digital Presentation) Fabrication of Conductive Carbon Composite Films for Freestanding Lib Anodes Using Cellulose Nanofiber Binder
Authors: Honda Shiho , Hara Masanori , Yoshimura Masamichi 
Published: July 2022

Fig.1 Cycle performance and Coulomb efficiency of graphite/CNT/CNF composite electrodes

Introduction

Currently, lithium-ion batteries (LIBs) have been the primary power source for electric vehicles and portable devices. LIBs consist of anodes (e.g. graphite), cathodes (e.g. LiCoO2: LCO), and electrolytes,[1] and are dis-/ charged as Li+ de-/ intercalate between the electrodes. Since LIBs require expensive fluorinated polymer as binders (e.g. poly (vinylidene fluoride): PVDF) and a volatile and toxic organic solvent such as N-methyl pyrrolidone (NMP), cheaper and easier-to-handle materials are necessary. Furthermore, the metal substrates (Cu, Al foil, etc.) as the current collectors are heavy and reduce the weight energy density of electrodes. Therefore, providing alternatives to the conventional components of LIB electrodes is a key challenge. Developing functional nanocomposites using sustainable natural resources is one of the most importance strategies.[2] Conductive thin films of cellulose nanofibers (CNFs) combined with carbon nanotubes (CNTs) have been reported as novel carbon composites.[3]

Environmentally friendly and only carbonaceous electrodes without PVDF and organic solvents, and graphite are normally used for anodes. As an allotrope of graphite, CNTs have been approved to be a suitable anode material due to their unique structure (one-dimensional cylindrical tubule of graphite sheet), high conductivity (106 Sm-1 at 300 K), low density, high rigidity (Young’s modulus 1 TPa), and high tensile strength (up to 60 GPa).[4]

In this study, the free-standing carbon composite anodes were successfully fabricated by using ultrasonication of CNF/ CNT or graphite mixture and vacuum filtering. Graphite slurry as the active material layer was dispersed in DI-water as the solvent containing CNF binder. This composite film has a dual-layer structure consisting of graphite/CNF active materials layer and CNT/CNF layer; here, the CNT/CNF layer with a thickness of about 10 μm is utilized as a current collector replacing the conventional metal substrate. These fabricated electrodes were evaluated by structural evaluation and dis-/ charge measurements.

Experimental method

CNF gel (2 wt%, Rheocrysta, DKS Co. Ltd.) and functionalized (carboxylic acid) multi-walled CNT (f-MWCNT, Sigma-Aldrich) were suspended in DI-water and sonicated for 2 h. Synthetic graphite was dispersed in DI-water with 5 wt% CNF and 10 wt% Carbon Black (CB) as a conductive material and sonicated for 30 min. Nanocomposite dual-layer films were formed by vacuum-filtering the dispersion using PVDF (pore size: 0.1 μm) membrane filters. These films were characterized by scanning electron microscope (SEM), atomic force microscope (AFM), and Raman spectroscopy. The conductivity was measured by four probe method.
The electrochemical properties of the free-standing electrode were measured using coin cells at room temperature. The coin cells were assembled in an argon-filled glovebox using Li as a counter electrode, glass fiber filter as a separator, and 1 M LiPF6 in a 1:1 (volume) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as the electrolyte. Charge-discharge characteristics were recorded from 0.01 to 3 V at charging speeds of 0.1 – 2 C. The electrochemical impedance spectra (EIS) were measured in the frequency range from 100 mHz to 100 kHz with a potential perturbation at 5 mV.

Results

CNF combined with CNT constructs a robust conductive fibrous network, and CNT linked with graphite builds electronic conductive paths to improve the electronic conductivity. Specific capacities of the electrode were 44 mAhg-1 at 2 C and 272 mAhg-1 at 0.5 C, respectively (Fig.1). The discharge capacity of the first cycle was 363 mAhg-1 at 0.1 C. After 125 cycles, the discharge capacity was 255 mAhg-1 at 0.5 C. The ratio of the charge and discharge capacities (Coulombic efficiency) suggests that the charge-discharge behavior is stable in the cycle measurement, indicating that the irreversible capacity was small. Other information obtained by cyclic voltammetry (CV), and EIS measurement and further results will be shown in the presentation.
In summary, we successfully fabricated the graphite/CNT/CNF composite film as environmentally friendly and lightweight anodes for the first time. The present results indicate that graphite/CNT/CNF composite film is a good candidate of flexible free-standing anodes. This research focused on CNFs as a promising alternative to conventional battery materials, and CNFs were successfully used to fabricate electrodes without using expensive PVDF binders and toxic organic solvents. Moreover, the gravimetric energy density was improved by replacing the conventional heavy Cu foil with a lightweight and thin composite film of CNT/CNF.
References
[1] M. Weiss, et al., Adv. Energy Mater. 2021, 11, 2101126.
[2] C. Chen, et al., Comp. Sci. and Technol. 2018, 156, 103-108.
[3] S. Cao, et al., ACS Appl. Mater. Interfaces 2015, 7, 10695-10701.
[4] C. de las Casas, et al., J. Power Sources 2012, 208, 74-85.

Figure 1