Intercalated Metal-Organic Framework Electrode Materials for High-Voltage Stacked Batteries

The research and development of rechargeable batteries is of particular importance because of their promise as large-scale energy storage devices in automotive applications and stationary power-storage systems. By stacking multiple positive and negative electrode pairs described as bipolar electrodes within a single cell, design of more powerful and useful stacked rechargeable batteries for the large-scale applications is possible. While Al and Cu are typically used as current collectors for positive and negative electrodes, respectively, the bipolar electrodes for the proposed battery use a single Al foil coated with both positive and negative electrode materials. When designing a stacked cell with bipolar electrodes based on Al current collectors, unfortunately, conventional negative electrode material of graphite carbon cannot be used because Al reacts with Li at a potential (0.4 V (vs. Li/Li⁺)) higher than that of the Li intercalation of graphite (0.1). When t lithium titanate spinel (Li₄Ti₅O₁₂) operating at a potential of 1.55 V is used, on the other hand, the cell voltage becomes considerably low and total energy density decreases drastically as a result. Therefore, reversible Li intercalation materials which operate from 0.5 to 1.0 V are essential for the negative electrode. So, we focus on redox systems of π-conjugated dicarboxylate materials ^1,2 and found that some of the π-conjugated dicarboxylate materials had a characteristic molecular self-assembly which provided reversibility. In the presentation, we introduce the intercalated metal-organic framework (iMOF), 2,6-naphthalene dicarboxylate dilithium (2,6-Naph(COOLi)₂) as the target negative electrode material (Fig. 1).^{3, 4}

This material has repeating organic and inorganic units comprising π-stacked naphthalene packing and tetrahedral LiO₄ units. The naphthalene units interact with each other through π-stacking interactions. The Li atoms form slightly distorted LiO₄ tetrahedral structures, which are connected by an extensive network of one edge-sharing and two corner-sharing tetrahedral LiO₄units that are framed by four O atoms of different naphthalene dicarboxylate units. Hence, such molecular self-assembly ensures high structural stability.

The 2,6-Naph(COOLi)₂ electrode shows a reversible two-electron-transfer reaction (230 mAh g^–1 per active material) at a flat potential plateau of 0.8 V with narrow polarization in in LiPF₆/EC+DMC+EMC. This material exhibits an intercalation reaction of Li into the tetrahedral LiO₄ layer by redox systems of the π-stacked naphthalene packing layer. Its volumetric change was ca. 10 % while the framework remains constant during charging and discharging. This value is approximately the same volumetric change as that observed for graphite carbons. Intercalated Li⁺ is stabilized as a tetrahedral LiO₃C structure composed of three O atoms of different dicarboxylate units and naphthalene C atom covalently-bonded carboxylate groups indicating Li⁺transport channel. The organic-inorganic interlayer distance remains constant while π-stacking interaction for naphthalene packing slightly increases, which contributes to electron-transfer channel. Such molecular self-assembly having two-dimensional pathways for efficient electron and ion transports provides the observed electrochemical reversibility.

As a preliminary step for the high-voltage stacked Li-ion batteries, a 4-V Li-ion cell was fabricated with 2,6-Naph(COOLi)₂ negative and high potential-operating LiNi_0.5Mn_1.5O₄spinel positive electrodes in the available potential range of the Al current collector. The resulting charge–discharge profile reveals a cell voltage of 3.9 V (Fig. 2) and the same reproducible reversible capacity. In addition, the operating potential of the negative electrode is expected to suppress the deposition of metallic Li (0 V). Thus, our results show that the use of iMOFs will aid in designing high-energy-density batteries with improved safety.

References

1. M. Armand, S. Grugeon, H. Vezin, S. Laruelle, P. Ribière, P. Poizot, J.-M. Tarascon, Nature Mater. 8, 120 (2009).
2. W. Walker, S. Grugeon, H. Vezin, S. Laruelle, M. Armand, F. Wudl, J.-M. Tarascon, J. Mater. Chem. 21, 1615 (2011)
3. N. Ogihara. Japanese Patent, JP2011-074054.
4. N. Ogihara. PTC Patent, WO2012-053553.