天津大学钟澄课题组：锌空电池循环充放电过程中的失效机制探究

【研究背景】

现代社会可穿戴和小型化电子产品的蓬勃发展，刺激了对灵活、轻便、无毒的储能装置的强烈需求。金属空气电池由于其独特的半开放式电池结构，具有较高的理论能量密度，因此被认为是最佳的候选者之一。其中柔性锌空气电池由于其低成本、环境友好性和高安全性而受到越来越多的关注。然而，可充电锌空气电池的大规模开发受到其较差循环寿命的严重限制。在过去的几十年里，通过开发和优化电极、电解质和电催化剂的材料以及电池配置的设计，大量的研究工作致力于提高FZABs的循环寿命。然而，很少有研究关注电池的失效机制。此外，研究学者们对于电池在充放电循环过程中的失效原因众说纷纭。具体而言，一些学者提出空气电极对锌空气电池的循环寿命有显着影响。相反，其他一些研究人员认为凝胶电解质在影响锌空气电池的循环寿命方面起着至关重要的作用。除此之外，一些学者表明锌电极是影响电池寿命的重要因素。但制约锌空气电池循环寿命的瓶颈因素仍不清楚，这严重阻碍了电池的商业化。因此，非常迫切需要研究FZAB在充放电循环过程中的失效机制。

【工作介绍】

本工作通过分析循环失效前后电池的各个组成部分，系统地研究了锌空气电池在不同电流密度下充放电循环过程中的失效机制。本文通过商业Co₃O₄和炭黑的混合物作为空气电极催化剂，聚丙烯酸（PAA）基碱性凝胶聚合物作为电解质，锌箔作为锌电极，通过逐层法制备了一种三明治结构的锌空气电池。首先，对组装的锌空气电池在室温下进行充放电循环测试，直到它们失效。其次，将失效的电池进行拆解，将拆开的空气电极、电解质和锌电极分别重新组装成新电池，并进行循环充放电测试以研究锌空气电池的失效机理。这项工作为延长下一代电池的循环寿命提供了重要指导。相关工作以“Investigation of Failure Mechanism of Rechargeable Zinc–Air Batteries with Poly(acrylic acid) Alkaline Gel Electrolyte during Discharge–Charge Cycles at Different Current Densities”为题发表在Chemical Engineering Journal上，宋志双博士为本文第一作者。

【内容表述】

本文所制备的电池的结构示意图以及实物图如图1所示。通过对失效后的电池进行组分替换，可以看出在2 mA cm^–2电流密度下，电池失效的主要原因是电解质性能较差导致。在10 mA cm^–2电流密度下，电池失效的主要原因是空气电极性能较差导致。

Fig. 1. (a) The schematic image and (b) the optical image of the prepared FZAB.

Fig. 2. Galvanostatic discharge–charge cycling performance of (a) FZABs in the first discharge–charge test, (b) reassembled FZABs using the disassembled Zn electrode with the replacement of a fresh GPE and a new air electrode, (c) reassembled FZABs using the disassembled air electrode with the replacement of a fresh GPE and a new Zn electrode, (d) as well as reassembled FZABs using the disassembled GPE with the replacement of a fresh Zn electrode and a new air electrode at a current density of 2 mA cm⁻². Galvanostatic discharge–charge cycling performance of (e) FZABs in the first discharge–charge test, (f) reassembled FZABs using the disassembled Zn electrode with the replacement of a fresh GPE and a new air electrode, (g) reassembled FZABs using the disassembled air electrode with the replacement of a fresh GPE and a new Zn electrode, (h) as well as reassembled FZABs using the disassembled GPE with the replacement of a fresh Zn electrode and a new air electrode at a current density of 10 mA cm⁻².

通过光学照片可以看出，失效后的锌电极表面会形成一层灰色的物质。XRD表征得出该灰色物质是ZnO。通过SEM可以看出失效后的锌电极表面形成了棒状的枝晶。通过扫描的元素分布可以看出失效后的锌电极表面有Zn和O元素的存在。

Fig. 3. Analysis of Zn electrodes disassembled from failed FZABs after discharge–charge cycles at current densities of 2 and 10 mA cm⁻², respectively. Optical images of (a) pristine Zn electrode, disassembled Zn electrode after (b) 2 and (c) 10 mA cm⁻² cycling. (d) XRD patterns of pristine Zn, and disassembled Zn electrodes. (e) SEM image, (f) optical image, (g-i) elemental mapping of the cross-section of disassembled Zn electrodes after 2 mA cm⁻² cycling. (j) SEM image, (k) optical image, (l-n) elemental mapping of the cross-section of disassembled Zn electrodes after 10 mA cm⁻² cycling.

2 mA cm^–2下电池循环失效后的电解质中形成了K₂CO₃·1.5H₂O和ZnO。碳酸盐的形成跟空气中的CO₂和空气电极的碳腐蚀有关。通过研究电池循环性能的影响因素，可以得出电解质中OH^–的减少是电池失效的主要原因。

Fig. 4. Analysis of GPEs disassembled from failed FZABs after discharge–charge cycles at a current density of 2 mA cm⁻². Optical images of the disassembled GPE (a) with the contact surface between the GPE and the air electrode, (b) as well as the contact surface between the GPE and the Zn electrode. (c) XRD patterns of the disassembled GPE from the failed FZAB. (d) Galvanostatic discharge–charge cycling performance of the FZAB using the disassembled GPE after removing the ZnO layer on the surface. Optical images of (e) the pristine GPE and (f) the GPE after storage in the air for 133 h. (g) XRD patterns of the pristine GPE and stored GPE. (h) Galvanostatic discharge–charge cycling performance of the FZAB using the stored GPE. Optical images of the GPE disassembled from the failed FZAB cycling in the pure O₂ (i) with the contact surface between the GPE and the air electrode, (j) as well as the contact surface between the GPE and the Zn electrode. (k) XRD patterns of the disassembled GPE from the FZAB cycling in the pure O₂. (l) Galvanostatic discharge–charge cycling performance of FZABs using the disassembled GPE after immersing in 0.8 mL of 6 M KOH + 0.2 M Zn(AC)₂ electrolyte and H₂O for 24 h, respectively. The galvanostatic discharge–charge measurements are performed at a current density of 2 mA cm⁻².

10 mA cm^–2下电池循环失效后的电解质从无色透明状变为了棕色，并且在电解质中形成了K₂CO₃·1.5H₂O和ZnO。碳酸盐的形成跟空气中的CO₂和空气电极的碳腐蚀有关。

Fig. 5. Analysis of GPEs disassembled from failed FZABs after discharge–charge cycles at a current density of 10 mA cm⁻². Optical images of the disassembled GPE (a) with the contact surface between the GPE and the air electrode, (b) as well as with the contact surface between the GPE and the Zn electrode. (c) XRD patterns of the disassembled GPE from the failed FZAB. Optical images of (d) the pristine GPE and (e) the GPE after storage in the air for 50 h. (f) XRD patterns of the stored GPE. Optical images of the GPE disassembled from the failed FZAB cycling in the pure O₂ (g) with the contact surface between the GPE and the air electrode, (h) as well as the contact surface between the GPE and the Zn electrode. (i) XRD patterns of the disassembled GPE from the FZAB cycling in the pure O₂.

2 mA cm^–2下电池循环失效后的催化剂表面形成了棒状的ZnO，ZnO的形成会阻塞空气电极的活性位点，从而降低空气电极的电化学性能。

Fig. 6. Analysis of the air electrode disassembled from the failed FZAB after discharge–charge cycles at a current density of 2 mA cm⁻². (a) XRD patterns of the pristine air electrode and the disassembled air electrode. (b) SEM image of the pristine air electrode. (c) SEM image, (d) EDS spectrum, (e-i) element mapping of C, Co, Zn, O of the disassembled air electrode.

10 mA cm^–2下电池循环失效后的催化剂表面形成了裂痕，裂痕的形成会破坏催化剂的整体性，降低空气电极的离子电导率，进而降低空气电极的电化学性能，导致电池过早的失效。

Fig. 7. Analysis of the air electrode disassembled from the failed FZAB after discharge–charge cycles at a current density of 10 mA cm⁻². (a) XRD patterns of the pristine air electrode and the disassembled air electrode. (b-c) SEM images, (d) EDS spectrum, (e-h) element mapping of C, Co, O of the disassembled air electrode.

失效后的催化剂的XPS表明循环后的催化剂的含氧官能团的含量比循环前的催化剂的含量高，证明了循环后的空气电极发生了碳氧化。碳氧化的发生降低了循环后的空气电极的OER/ORR性能。

Fig. 8. Analysis of the air electrode disassembled from the failed FZAB after discharge–charge cycles at current densities of 2 and 10 mA cm⁻², respectively. (a) XPS survey spectra, (b) high-resolution XPS spectra of C 1 s, (c) ORR performance, and (d) OER performance of the pristine electrode and disassembled air electrodes.

【结论】

本文通过分析循环失效前后电池的各个组成部分，系统地研究了锌空气电池在不同电流密度下充放电循环过程中的失效机制。结果表明，在2 mA cm⁻²的低电流密度下，电解质中的OH⁻含量决定了锌空气电池的循环寿命。在电池的充放电循环过程中，ZnO和碳酸盐的形成以及空气电极的碳氧化会降低电解质中OH⁻的含量，导致电池过早失效。此外，在10 mA cm⁻²的高电流密度下，由于充电电位高，空气电极中的碳在碱性电解质中容易氧化，导致空气电极电化学性能严重下降，从而导致到电池故障。此外，在20 mA cm⁻²的较高电流密度下，电池失效的原因跟锌电极钝化、锌电极与电解质之间的界面电阻增加以及空气电极的电化学性能降低有关。为了延长电池循环寿命，建议在低电流密度下循环时，减少电解质中OH⁻的消耗应该是最重要的考虑因素。此外，在高电流密度下，降低电池的充电电位或优化空气电极的组成和结构对于长寿命的锌空气电池的开发具有重要意义。在较高的电流密度下，除了保证空气电极良好的电化学性能外，抑制锌电极钝化和降低锌电极与电解质之间的界面电阻是延长电池寿命的两种有效措施。本文的研究内容不仅对于从根本上理解电池的失效机制很有重要的价值，而且对未来设计和构建长寿命锌空气电池具有深远的指导意义。

Zhishuang Song, Jia Ding, Bin Liu, Yuanhao Shen, Jie Liu, Xiaopeng Han, Yida Deng, Cheng Zhong, Wenbin Hu, Investigation of failure mechanism of rechargeable zinc–air batteries with poly(acrylic acid) alkaline gel electrolyte during discharge–charge cycles at different current densities, Chemical Engineering Journal, 2021, https://doi.org/10.1016/j.cej.2021.132331