Bezoquinone Dimer Fused By the Tetrathiafulvalene Unit As an Orgunic Positive Electlode Material for Rechargeable Batteries

Abstract

Recently, the development of a high performance rechargeable battery system without using any minor metal-based materials, especially in the positive-electrode is desired due to the increasing concern about the limited resources. One of the candidate categories is a series of redox active organic materials that contain no scarce metal resources. Organic materials are flexible and light-weight, and have a high degree of freedom in molecular design. Due to these unique characteristics, several types of organic compounds have already been proposed so far as positive electrode active materials. Among some categories, 1,4-benzoquinone (BQ) derivatives are reported to exhibit high capacities [1] since these derivative undergo a two-electron transfer type redox reaction per skeleton. However, their discharge potentials are usually lower than those of the inorganic competitors. This is related to the redox mechanism of the BQs, in which the electron accepter type carbonyl group receives an electron and a lithium cation during the discharge process. On the other hand, electron donor type tetrathiafulvalene (TTF) analogues, which release electrons and receive anions during the charge process, are known to exhibit a relatively high discharge potential [2], although their discharge capacities are not high enough. To satisfy the both high capacity and high discharge potential, we have focused on a fused structure (1) in which two BQ skeletons are crosslinked by one TTF unit. A high capacity of 441 mAh/g is expected for this compound, 1. In this paper, we synthesized 1 to evaluate its battery performance. The synthetic route for 1 is described in Fig. 1a. First, 4 was prepared in 32% yield by the P(OEt)3-mediated cross coupling of 2 [3] with 1.5 equiv. mol. of 3 [4] (lit. 45%, in the case of homo-coupling of 3 [3]. Then, 1 was prepared by a literature procedure [3] in 53% yield. An IEC R2032 coin-type cell composed of a positive electrode of 1 and a lithium metal sheet as a negative electrode was prepared to examine the battery performance. The positive electrode was prepared by mixing the powder of 1 with acetylene black as the conductive additive, and poly(tetrafluoroethylene) as the binder in the weight ratio of 1: 8: 1. The initial five cycles of charge-discharge curves of the prepared cell is shown in Fig. 1b. The observed initial discharge capacity of 259 mAh/g corresponds to 88% of the theoretical value of 294 mAh/g under the assumption of only the redox of the BQ moieties. The discharge capacities of second cycle was increased to 310 mAh/g. This capacity is 70% of the theoretical capacity in which six-electron transfer reaction per molecule is considered. This result undoubtedly indicates that both BQ and TTF moieties in the skeleton of 1 participate in the discharge process. The average potential of 2.84 V vs. Li/Li+ is higher than those of reported normal quinone derivatives [5] and the energy density was calculated to be 880 mWh/g. As shown in Fig. 1c, the electrode of 1 had a relatively good cycle stability: it maintained 206 mAh/g after 25 cycles. The results obtained in this paper suggests that fusing a donor and acceptor type redox units can be an effective strategy for designing a high energy density positive electrode material.