Graphene-like Graphite as a High-Performance Cathode Material for Dual-Ion Batteries

Dual-ion batteries are rechargeable battery technology that differs from traditional lithium-ion batteries in their electrode materials and ion transport mechanisms. In a dual-ion battery, two different types of ions are involved in the charge and discharge processes, typically using an anion a cation rather than just lithium ions as in conventional lithium-ion batteries. Traditional cathode materials in dual-ion batteries have primarily relied on high crystallized graphite due to its inherent stability and low cost. However, these materials face critical challenges, primarily related to the high potential required (up to 5.0 V vs. Li⁺/Li) for the oxidation of graphite to form stage 1 type intercalation compounds of anions, such as PF₆^– and AlCl₄^–. The high potential will lead to the decomposition of the electrolyte solution and the degradation of the graphite cathode over cycling, leading to a decline in capacity. Furthermore, the capacity of graphite as a cathode material is limited, with a maximum capacity of up to 110 mAhg^-1 which is lower than that of transition metal oxides used in lithium-ion batteries. Therefore, there is an urgent need for alternative cathode materials that can overcome these limitations and provide higher performance and stability. In a new research study published in the peer-reviewed Journal ChemSusChem by Professor Yoshiaki Matsuo, Kazuhiro Sekito, Yusuke Ashida, Assistant Professor Junichi Inamoto from the University of Hyogo, and Dr. Noriyuki Tamura from Energywith Co., Ltd., presented the use of graphene-like graphite (GLG) as a cathode material for dual-ion batteries.

The research team synthesized GLG through the thermal treatment of graphite oxide (GO) and was originally developed for use as an anode material in lithium-ion batteries. GLG exhibits a morphology that closely resembles that of starting graphite, but it contains a significant number of oxygen atoms and nanopores within its graphene layers, which are arranged with three-dimensional regularity. Notably, the onset potential for the intercalation of PF₆^– ions into GLG is lower than that of graphite. This lower onset potential is advantageous in preventing the oxidative decomposition of the electrolyte solution. Moreover, GLG exhibits a higher discharge capacity, reaching up to 130 mAhg^-1, compared to graphite. These results represent a stark contrast to previous reports on the intercalation behaviors of anions into various carbon materials, including graphite and reduced graphene oxides.

While GLG shows significant potential as a cathode material, the authors conducted a series of detailed experiments to understand the factors influencing its electrochemical behavior. They investigated GLG samples prepared from GO obtained through different methods. The properties of GO samples can vary significantly depending on the preparation methods, making it crucial to analyze how these differences affect GLG’s performance.

When the authors conducted X-ray diffraction (XRD) patterns of GLG samples it revealed substantial variations based on the preparation method of GO and the heat treatment temperatures. GLG samples obtained from GO with higher oxygen contents exhibited shifts in peak positions and increased peak widths in XRD patterns. This suggests that the oxygen content in GO has a substantial impact on the resulting GLG’s structure. The authors then performed X-ray photoelectron spectroscopy (XPS) analysis of the surface region of GLG samples which showed no significant differences in the C-O bonding between samples. This suggests that the differences in GLG’s electrochemical behavior are primarily related to the internal structure, such as the presence of nanopores.

Professor Yoshiaki Matsuo and colleagues thoroughly investigated charge-discharge behavior of GLG samples in two different electrolyte solutions: 3m LiPF6-ethyl methyl carbonate (EMC) and 3 mol cm^-3 lithium bis(fluorosulfonyl)amide (LiFSA)-EMC. The findings provided valuable insights into how GLG’s structural parameters, such as interlayer spacing and oxygen content, influence its electrochemical performance. They also demonstrated that the interlayer spacing of GLG is a critical parameter affecting its electrochemical performance. As the interlayer spacing increased, the onset potential for anion intercalation decreased, leading to higher discharge capacities. This finding is consistent with a simple thermodynamic model of the Born–Haber cycle, which includes layer separation and lattice stabilization processes. The larger interlayer spacing in GLG reduces the layer separation energy and results in lower onset potentials.

The authors presented a comprehensive examination of GLG as a cathode material for dual-ion batteries. Their findings highlight the potential of GLG to address the limitations associated with traditional cathode materials, such as high crystallized graphite, by offering higher discharge capacities and improved electrochemical performance.

The study underscores the significance of structural parameters, including interlayer spacing and oxygen content, in influencing GLG’s electrochemical behavior. It emphasizes the importance of understanding and manipulating the layer separation energy and lattice stabilization energy to optimize GLG’s performance further.

According to Professor Yoshiaki Matsuo, the first and corresponding author: GLG holds promise as a viable cathode material for dual-ion batteries and offers potential applications in storing renewable intermittent energy. Further research into the effect of various oxygen-containing functional groups on GLG’s intercalation potential is ongoing and promises to enhance our understanding of this promising material’s capabilities. In conclusion, the new study demonstrated the potential of GLG as a game-changing cathode material, offering hope for more efficient and sustainable energy storage solutions in the near future.