Effect of Soft Carbon Coating on Electrochemical Performance of Graphite As Negative Electrode Materials for Lithium-Ion Batteries
Authors: Gao Xinli , Tsujimoto Shota , Sakamoto Ryo , Lee Changhee , Miyahara Yuto , Miyazaki Kohei , Abe Takeshi 
Published: December 2024

Introduction

In the past decades, lithium-ion batteries have become the primary power source for high-capacity, high-power mobile devices and electric vehicles. The demand for lithium-ion batteries with higher capacity and faster charge/discharge rates has emphasized the need for electrodes with improved structural stability and cycling performance. Graphite anodes are currently preferred for mainstream lithium-ion batteries due to their stability and cost-effectiveness. However, they suffer from surface degradation during high-speed discharges, leading to suboptimal long-term performance.
To address these challenges, there is growing interest in surface modifications of carbon materials, particularly through carbon coatings, to enhance their performance under high-speed charge/discharge conditions. Carbon coatings strengthen the graphite surface, reduce contact between mesopores and the electrolyte, and significantly reduce irreversible capacity loss during the initial charge/discharge cycle.[1] While carbon coatings have shown promising results in improving cycling stability and suppressing irreversible electrolyte decomposition, the design and implementation of carbon coating technologies are still developing. Understanding the mechanisms by which thermally decomposed carbon coatings affect SEI formation processes and their post-formation effects is crucial. This uncertainty not only adds to the cost but also obscures the process's impact. Consequently, there is an urgent need to develop a principled approach for enhancing graphite performance through carbon coatings. Previous studies have explored graphite surface modification through experimental means[2], often involving high-temperature heat treatments, but significant progress in understanding the underlying mechanisms has been limited.

Objective

This study reverts to fundamental principles by utilizing highly oriented pyrolytic graphite (HOPG) to construct model electrodes. This approach enables the clear distinction of the two distinct crystalline faces of graphite. The study delves into the transport mechanisms at the interface of carbon materials and graphite, which are formed during heat treatment. Building upon this understanding, the study seeks to establish design guidelines for thermal decomposition carbon coatings.

Experimental

In this study, two types of carbon-coated highly oriented pyrolytic graphite (HOPG) samples were prepared at different stages. One sample was coated with soft carbon layer derived from the precursor of perylenetetracarboxylic dianhydride (PTCDA) and heated to the decomposition temperature (700 °C). The other sample was coated with an intermediate product layer from thermal decomposition and heated to a temperature below the decomposition temperature (550 °C).
To prepare the model electrode, 2.0 g of PTCDA was added to HOPG and subjected to heating at 500 °C under a nitrogen atmosphere for 5 hours. The successful modification process was confirmed by Raman spectroscopy, which detected the presence of the PTCDA coating on the surface of HOPG.
A three-electrode cell was constructed, using the PTCDA@HOPG electrode as the working electrode, lithium metal as the counter electrode, and lithium metal as the reference electrode. The potential was referenced against a lithium metal electrode. A solution of 1 mol dm-3 LiClO4/EC+DEC (1:1 by vol.) electrolyte was used. Cyclic voltammetry was conducted with a voltage range from 0.005 to 3.0 V at a scan rate of 0.1 mV s-1 for 3 cycles.

Results and discussion

Figure 1 illustrates cyclic voltammogram (CV) of the PTCDA@HOPG electrode. Compared to the untreated HOPG, the oxidation-reduction currents of the model electrodes covered with thermal decomposition intermediates and thermal decomposition carbon show an overall increase, exhibiting a trend of initially increasing and then decreasing. This indicates that partially thermally decomposed coatings can also enhance the reaction rate, possibly due to the increased active surface area provided by the coatings.
The results of electrochemical impedance spectroscopy are presented in Figure 2. Measurements at 0.5 and 0.2 V indicate that as thermal decomposition progresses, the resistance of the solid electrolyte interphase (R
SEI) on the electrode surface gradually increases, leading to the gradual formation of a more stable SEI. Meanwhile, the resistances of the electrolyte and electrode surface show a trend of decreasing first and then increasing. This suggests that using fully decomposed material as a coating has a beneficial effect on reducing resistance to the electrolyte.

Conclusion

This study systematically elucidates the pivotal role of soft carbon coatings on the graphite negative electrode surface, offering promising strategies for understanding and designing the graphite/electrolyte interface. Furthermore, the utilization of HOPG has provided deeper insights into the reaction mechanisms and intricacies occurring at the graphite negative electrode surface. These findings lay a solid foundation for future investigations into innovative coating materials and technologies, with the aim of further enhancing battery performance and longevity.

References

[1] J. Lee, et al. J. Power Sources., 166(1), 250-254 (2007).
[2] Li, F.-S, et al. Adv. Mater., 27(1), 130-137 (2015).

Figure 1