【研究背景】

水系锌离子电池在大规模储能器件中很大的应用潜力。然而，正极材料限制着水系锌离子电池的能量密度和循环寿命进一步提升。相比于无机材料，有机材料是水系锌离子电池正极材料最具有竞争力的选择之一。例如，有机材料的电活性位点通常暴露在其表面，因此表现出类电容的电荷储存行为。另外，有机化合物也表现出灵活的结构设计性，电压可调性及持续性的优势，有机材料的发展仍处于初期，越来越受到研究人员的关注。不可忽视的是，有机材料也面临着溶解性高和导电性差的问题，其表现出差的循环寿命和低的容量利用率。目前也报道了很多策略来解决这些问题。例如，将有机小分子聚合成线性或二维结构来抑制活性单元的溶解。但是这种策略不可避免的引入大量非活性单元，导致聚合物表现出低的容量和差的导电性。除了聚合，也可以考虑采用分子工程设计策略设计低分子量的有机化合物。例如，通过引入杂原子和拓展π共轭平面，可以兼顾有机材料的导电性和溶解性。另外，低的分子量和拓展的π共轭平面也可以提升有机材料的容量。因此，这种策略是有效的并且值得去尝试。

【工作介绍】

考虑到苯醌和萘醌具有高理论比容量，但溶解性高、导电性差的特点，近日，南开大学陶占良教授课题组将共轭硫醚键（-S-）作为连接单元将苯醌和萘醌连接起来分别合成了6a,16adihydrobenzo[b]naphtho[2′,3′:5,6][1,4]dithiino[2,3-i]thianthrene-5,7,9,14,16,18-hexaone (4S6Q) 及 benzo[b]naphtho[2′,3′:5,6][1,4]dithiino[2,3-i]thianthrene-5,9,14,18-tetraone (4S4Q) 低分子量的有机化合物。引入的硫醚键：（1）拓展了分子的π共轭平面；(2) 提升了材料的导电性；（3）赋予有机分子骨架高的柔性；（4）显著抑制了材料的溶解。另外，丰富的羰基官能团和低的分子量使合成的材料具有高的理论比容量。使用3.5 M Zn(ClO4)2 电解液和4S6Q正极组装的电池表现出240 mAh g-1 的高放电容量，在150 mA g-1的小电流密度下循环500圈没有容量衰减，库仑效率接近100%。该材料也表现出出色的倍率性能，在30 A g-1的大电流密度下也能维持208.6 mAh g-1的放电容量（150 mA g-1下容量的86.9%）。值得注意的是，得益于电解液的低凝固点，该体系在-60°C也可以稳定运行，在150 mA g-1的电流密度下仍能获得201.7 mAh g-1的放电容量。此外，作者通过综合的表征手段证实了4S6Q材料中H+的嵌入/脱出的反应机理。该文章发表在国际期刊Nano-Micro Letters上。孙田将博士为本文第一作者。

【内容表述】

1. 4S4Q 和4S6Q的合成及表征

Fig. 1 Synthesis and characterizations. a The synthesis processes of 4S4Q and 4S6Q compounds. b The FTIR spectra of 4S4Q and 4S6Q compounds. c The TEM image of 4S6Q sample. d The TEM-mapping images of 4S6Q sample.

2. DFT计算

Fig. 2 The density functional theory (DFT) calculations. a The MESP image of 4S4Q and 4S6Q molecules. b The calculated LUMO and HOMO plots of 4S4Q and 4S6Q molecules. c The calculated HOMO plots of 4S4Q, 4S4Q4-, 4S6Q, and 4S6Q6- molecules. d The calculated HOMA values of 4S6Q and corresponded reduction states. e The calculated HOMA values of 4S4Q and corresponded reduction states. f The energy level of neutral and reduced 4S6Q and 4S4Q molecules (the inserted images reflect the simulative LOL-π).

3. 电化学性能

Fig. 3 Electrochemical performance. a The GCD curves of Zn//4S6Q battery at 150 mA g-1. b The cycling stability of Zn//4S6Q battery at 150 mA g-1. c The self-discharge performance of Zn//4S6Q battery. d The GCD curves of Zn//4S6Q battery at different current densities. e The rate performance of Zn//4S6Q battery. f The cycling stability of Zn//4S6Q battery at 3 A g-1.

4. 电荷储存动力学

Fig. 4 The studies of reaction kinetics. a The CV curves of Zn//4S6Q battery at different scan rates. b The calculated b values for different peaks. c The calculated capacitance contribution ratios at different scan rates. d The charge-discharge curves of Zn//4S6Q battery for GITT test. e The calculated Dions by GITT test; f) the EIS of 4S4Q and 4S6Q.

5. 反应机理研究

Fig. 5 The charge storage mechanism of Zn//4S6Q. a The ex-situ FTIR of 4S6Q electrodes at different states. b The ex-situ XRD patterns of 4S6Q electrodes at different states. c The ex-situ SEM images of 4S6Q electrodes at different states. d The ex-situ XPS of O 1s. e The TEM-mapping images of 4S6Q material and by-product at discharged state.

6. 低温电化学性能

Fig. 6 The low-temperature electrochemical performance of Zn//4S6Q. a The discharge capacity of Zn//4S6Q battery at different temperatures (at 150 mA g-1). b The GCD curves of Zn//4S6Q battery at different current densities under -40℃. c The GCD curves of Zn//4S6Q battery at different current densities under -60 ℃ (1 C = 300 mA g-1). d The cycling stability of Zn//4S6Q battery at 3 A g-1 under -60 ℃. f Optical image of LED powered by two soft-packaged batteries in series at -60 ℃.