(Invited) Heterogeneous Intercalated Metal-Organic Frameworks for Fast-Charging Electrodes
Authors: Ogihara Nobuhiro , Hasegawa Masaki , Kumagai Hitoshi , Mikita Riho , Nagasako Naoyuki 
Published: December 2024

Significant progress has been made in recent years with metal-organic frameworks (MOFs) being used as electrode materials for energy storage devices.1, 2 These frameworks comprise organic molecules and metal ions that form self-assembled structures. Intercalated MOFs (iMOFs) based on aromatic dicarboxylate are particularly appealing as negative electrode active materials for Li-based batteries3, 4 and capacitors.5, 6 Such iMOFs store Li ions at approximately 0.8 V vs. Li/Li+ and hence avoid Li metal plating during cell operation. The conductive mechanisms7 and phase transition reactions8 have been investigated, and the relationship between these properties and structural factors has been clarified.

Our proposal involves using machine learning9 to synthesize iMOFs with multi-aromatic units via solution spray drying,10 resulting in fast-charging electrodes (Fig. 1).11 The naphthalene-based multivariate material has been proposed, which has a nanometric thickness and allows for reversible storage of Li-ions in a non-aqueous Li metal cell configuration. This active material can retain up to 85% of its capacity at a discharge rate of 400 mA g−1, which means that it takes only 30 minutes to fully charge at 20°C, compared to 10 hours for a full charge when cycling at 20 mA g−1. When this material was tested as negative electrode with an activated carbon-based positive electrode, it showed a discharge capacity retention of about 91% after 1000 cycles at 0.15 mA cm−2, which takes 2 hours for a full charge at 20°C. It also exhibited improved high-temperature stability at 60°C.

Through our research, we have analyzed the changes in the XRD patterns and confirmed the vibration modes by Raman spectra and first-principles phonon calculations during Li intercalation. As a result, we have discovered that samples with fast charging performance have a distorted structure that restrains the bending vibration perpendicular to the naphthalene plane. This structure promotes the delocalization of electrons within the crystal, leading to a phase transition that suppresses phase separation. Consequently, the electrode shows better performance during fast charging.

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Figure 1