How Many Lithium Ions Can Be Inserted onto Fused C6 Aromatic Ring Systems?

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A fundamental and persistent problem in the study of carbonbased electrode materials for lithium ion batteries is the question of how many lithium ions can be inserted onto a C6 aromatic ring. Although different empirical models of Lix/C6 (x< 3) have been proposed, the question remains unresolved. Herein we employ 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), an aromatic compound containing a naphthalene ring system (fused C6 aromatic rings), to demonstrate that each carbon in a C6 ring can accept a Li ion to form a Li6/C6 additive complex through a reversible electrochemical lithium addition reaction. This process results in Li ion insertion capacities of up to nearly 2000 mAhg , depending on the exact molecular structure. This value is several times higher than any other organic electrode material previously reported and can be fully released under certain conditions. Our experiments and theoretical calculations indicate that the anhydride groups on the sides of the aromatic system are crucial for this process, which provides a promising strategy for the design of novel high-performance organic electrode materials. Organic molecules are intriguing candidates for electrode materials for use in rechargeable Li ion batteries. The application of such species has aroused much interest recently, owing to the obvious advantages of such a system: no need for rare metals, low safety risks compared to transition metal oxides, and design flexibility at the molecular level. However, organic molecules are usually considered to possess relatively poor specific energies and cycling properties, as compared to those of inorganic materials, and these factors greatly limit their practical application. Recently, studies on aromatic carbonyl derivatives showed that organic materials can possess outstanding electrochemical performance comparable to, or even superior to, inorganic materials. Furthermore, the wide diversity of organic redox systems, as well as the excellent flexibility in their molecular design, suggest even greater prospects for these materials, and this has inspired the exploration of new organic Li ion insertion systems with improved performance. Aromatic C6 rings are the basic structural units of graphite and other carbon-based electrode materials, which are the most commonly used anodes in commercial Li ion batteries owing to their high electric conductivity and low cost. It has traditionally been believed that each C6 ring can accept one Li ion to form an intercalated Li/C6 complex, giving a relatively low theoretical capacity of 372 mAhg . Recently, studies on graphene, nanographene, and their derivatives reveal that, through the reduction of size and dimensionality, these materials exhibit unique electric and electrochemical properties superior to those of conventional graphitic materials; thus, these materials are currently a hot research topic. In studies of electrode materials for Li ion batteries, these derivatives also exhibit high reversible capacities of up to almost twice the theoretical value of graphite, although the detailed mechanism is still unclear. This leads to a fundamental question in the study of carbonbased electrode materials: How many Li ions can actually be inserted onto each C6 aromatic ring? Multi-ring aromatics (for example, naphthalene, NTCDA, perylene, etc.) and their derivatives have planar C6 ring structures similar to graphene or nanographene. NTCDA is a typical example; it has a naphthalene-like ring structure consisting of two C6 rings fused together along with two cyclic anhydride groups (Figure 1a). NTCDA is a well-known organic semiconductor with good crystallinity and has been extensively studied for use in molecular electric devices. It provides an ideal model to study Li ion insertion onto C6 rings owing to the minimal number of C6 rings it possesses, which guarantees the necessary insolubility of the electrode materials in the commonly used electrolyte solution (ethylene carbonate/dimethyl carbonate/LiPF6) for Li ion batteries. NTCDA also possesses the necessary degree of conductivity for electron transport among molecules. We investigated the electrochemical Li ion insertion/deinsertion properties of NTCDA using model test cells with Li metal as the counter electrode. The working electrode consisted of NTCDA, acetylene black (AB), and polytetrafluoroethylene binders in a weight ratio of about 60:35:5. The cells were initially cycled by discharging (Li ion insertion) and charging (Li ion deinsertion) repeatedly in a potential range of 0.001–3.0 V vs. Li/Li at a moderate current rate of 100 mAg . Figure 1b shows selected discharge/charge curves (the 1st, 2nd, 3rd, and 8th cycles) for NTCDA. Figure 1c shows the corresponding discharge and charge capacities of NTCDA versus the cycle number. The first discharge and charge capacities are 1273 and 724 mAhg , respectively, showing a coulombic efficiency [*] X. Han, G. Qing, T. Sun State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology Wuhan, 430070 (China) E-mail: suntaolei@iccas.ac.cn