郭再萍团队EcoMat：钾硫电池现状与展望
The following article is from EcoMat Author EcoMat
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研究背景
基于硫化学的金属硫电池是有望替代当前锂离子电池技术、满足人类社会快速增长的能源需求的潜在候选者。其中，对锂硫电池的研究在过去几十年中已经取得了显着进展。但是由于锂资源稀缺、价格昂贵，锂硫电池难以满足大规模储能应用的需求。钾硫（K-S）电池由于钾还原电位低（−2.93 V vs SHE）、路易斯酸性较弱、去溶剂化能低、储量丰富、成本低而吸引了科研界的众多关注。最重要的是，K-S的理论能量密度高达914 Wh kg−1，远高于商用锂离子电池。然而，K-S储能体系仍处于发展的初级阶段，目前面临着一系列挑战。
成果简介
卧龙岗大学郭再萍教授团队与深圳大学王任衡在EcoMat发表了题为“Potassium-sulfur batteries: Status and perspectives”的综述性文章。旨在深入了解钾硫（K-S）储能体系的研究现状，介绍该技术面临的挑战，并提出实现其实际应用的可能研究方向。作者首先介绍了K-S电池的基本电化学原理，并强调了K-S储能体系与目前较成熟的锂硫(Li-S)储能体系之间的区别。随后，作者从正极、K负极、各种电解质体系方面重点关注K-S电池所涉及的关键材料的开发。最后，作者提供了几个可能推动K-S电池走向实际应用的研究方向。作者认为，重点是尝试通过采用当前Li-S体系的关键指标来构建K-S电池的实用参数。
内容详情
Figure 1 A, Comparison of the properties and performances of Li-S, Na-S and K-S batteries. B, Percentage of numbers of publications on different K-S battery topics. C, Strategies developed to improve the electrochemical performance of K-S batteries each year since the invention of the first K-S battery in 2014
Figure 2 A, Schematic illustration of the configuration of a K-S battery. B, Structure transformation of elemental sulfur at various temperatures. C, The sulfur allotropes from S2 to S8 based on density functional theory (DFT) calculations.
Figure 3 A, Typical voltage profiles illustrating the sulfur conversion reactions for a Li-S battery. B, Typical voltage profile for a Na-S battery. C, Representative voltage profiles for a K-S battery. D, An assessed K-S phase diagram. E, Schematic illustration of the reaction mechanism for K-S batteries during cycling: the solid conversion reaction in discharging and the solution pathway reaction in charging.
Figure 4 Challenges of potassium-sulfur batteries
Figure 5 A, Electrochemical reactions for discharge, charge, and overall processes of the K-S battery. B, Schematic diagram of the electrode reactions of rechargeable K-S batteries. C, X-ray diffraction (XRD) patterns with selected states after the first charge and the first discharge in comparison with the standard JCPDS card of K2S3. D, Raman spectra for selected processes after the first charge and the first discharge in comparison with the pristine cathode. E, Cycling performance and Coulombic efficiency at a current rate of 50 mA g−1. F, XRD patterns of pure sulfur, porous carbon, and the microporous C/S composite. G, Raman spectra of pure sulfur, porous carbon, and the microporous C/S composite. H, The time-of-flight secondary ion mass spectrometry (TOF-SIMS) data of the microporous C/S composite. I, Schematic illustration of the existing forms of sulfur in the porous carbon matrix: S2 (purple) and S3 (green). J, Transmission electron microscope (TEM) image and K, the high-resolution S 2p X-ray photoelectron spectroscopy (XPS) spectrum of the microporous C/S composite electrode in the fully discharged state. L, TEM image and (M) the high-resolution S 2p XPS spectrum of the microporous C/S composite electrode in the fully charged state. N, Fourier transform infrared spectroscopy (FTIR) analysis of polyacrylonitrile (PAN) and sulfurized PAN (SPAN). O, Cyclic voltammograms of the SPAN electrode with the PAA binder. P, Cyclic voltammograms of the SPAN electrode with the polyvinylidene difluoride (PVDF) binder. Q, Rate capability test (sulfur loading amount in the SPAN electrode: 0.8 mg cm−2). R, Long-term cycling performance with a relatively high sulfur loading of 1.5 mg cm−2 in the SPAN electrode.
Figure 6 A, Voltage profiles of repeated K plating-stripping in various electrolytes with different solvents, and different concentrations and additives in K//K symmetrical cells with photographs of K metal discs in different electrolytes after 14 days. B-F, First-principles DFT calculations on the surface diffusion characteristics of Li and K metal: Adsorption energy landscape for (B) Li adatom on Li (001) and (C) K adatom on K (001); D, Snapshots of the atomic configuration along the minimum energy path (MEP) for self-diffusion, with the adatom in a 4-fold hollow in the exchange mechanism; E, The calculated activation energy barrier via the MEP method for the diffusion by exchange mechanism for Li and K; F, The calculated variation of the diffusion rate constant with temperature for both Li and K. G-M, Morphology and XPS spectra of SEI layers formed on the surfaces of CNTs: TEM images of (G) pristine CNT, and CNT lifted from the surface of the K foil after resting in 0.5 m KPF6 EC/DEC electrolyte for (H) 0.5 hour and (I) 40 hours, and (J) after 2000 hours of cycling in a coin-type K/CNT|K/CNT symmetric cell. K, TEM image and L, the corresponding schematic illustration of the observed mosaic structure of the SEI. Scale bar, 5 nm in (G)-(K). M, XPS spectra of the SEI layers formed on the surfaces of CNTs after 40 hours rest (upper) and 100 hours of cycling (bottom). N-Q, Electrochemical performance of symmetric cells with bare K or K-aligned carbon nanotube membrane (K-ACM) electrodes: Galvanostatic cycling tests at different current densities of (N) 1 mA cm−2, (P) 2 mA cm−2, and (Q) 5 mA cm−2 with a stripping-plating time of 2 hours and capacity fixed at 1 mAh cm−2; and (O) selected specific voltage profiles at 1 mA cm−2.
Figure 7 A, Images of K2Sx (x = 1-3, 5) dissolved in DME. B, UV-Vis spectra for K2Sx (x = 1-3, 5) in different solutions. C, Schematic illustration of polymer electrolyte with the structure of the star polymer. D, Wettability of liquid K on untreated K-BASE. E, Cycling performance of a K-S cell at 150°C
结论与展望
总体而言，K-S电池因其高能量密度和低成本引起越来越多的关注。虽然科研界在正极设计、负极保护、电解液优化等方面都已取得一些进展，但K-S电池的研究仍处于起步阶段。本文强调了深入理解硫的钾化机理、增强反应动力学、缓解穿梭效应和解决安全隐患的重要性。此外，构建适用于工业应用的评估参数对于K-S技术的发展与未来应用至关重要。作者对于推进K-S电池的发展，给出如下建议：
1. 开发先进正极材料
K-S电池对理想正极的要求包括高电导率、高硫负载和利用率、对多硫化物的良好捕获能力以及从K2S3到K2S转变的快速还原动力学。
2. 构建稳定钾负极
稳定钾负极的先决条件是在其表面形成稳定的SEI层，可以阻止钾与电解质之间的进一步反应，同时避免枝晶生长。目前亟需深入探究钾SEI的结构和化学性质，了解诸如SEI层的具体成分、随着工作电压变化下的结构演变、钝化金属的核心成分等问题。基于对KS电池中SEI层的基本理解，可以设计有效可行的改性策略。此外，可以利用多尺度建模解释实验结果并进行预测和优化。
3. 电解质优化
目前已有包括液态电解液、聚合物电解质、无机固态电解质等各种电解质应用于KS电池中。电解质在克服多硫化物溶解和穿梭问题方面起着至关重要的作用，应给与更多的关注。未来应更多地致力于设计具有阻燃溶剂的低成本和不可燃电解质体系，以推动K-S体系的实际应用。
4. 实用的电池参数
为实现可供实际应用的金属硫电池，需要实现高硫负载、低电解质/硫 (E/S)比、超薄金属负极等。
文章信息
Xinyu Zhao, Yan Lu,* Zhengfang Qian, Renheng Wang,* Zaiping Guo,* Potassium-sulfur batteries: Status and perspectives, EcoMat. 2020;2:e12038.
原文链接：https://doi.org/10.1002/eom2.12038
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