物理化学学报 >> 2019, Vol. 35 >> Issue (8): 868-875.doi: 10.3866/PKU.WHXB201811033

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氟代碳酸乙烯酯添加剂对钠离子电池正极的影响

程振杰1,2,毛亚云2,3,董庆雨1,2,金锋1,2,沈炎宾2,*(),陈立桅2   

  1. 1 中国科学技术大学纳米技术与纳米仿生学院，合肥 230026
    2 中国科学院苏州纳米技术与纳米仿生研究所国际实验室卓越纳米科学中心，江苏 苏州 215123
    3 上海大学材料科学与工程学院，上海 200444
  • 收稿日期:2018-11-20 录用日期:2018-12-10 发布日期:2018-12-12
  • 通讯作者: 沈炎宾 E-mail:ybshen2017@sinano.ac.cn
  • 基金资助:
    国家科技部(2016YFB0100102);中国科学院战略性先导专项(XDA09010600, XDA09010303);中国科学院战略性先导专项(XDA09010303);和国家自然科学基金(21625304);和国家自然科学基金(21733012)

Fluoroethylene Carbonate as an Additive for Sodium-Ion Batteries: Effect on the Sodium Cathode

Zhenjie CHENG1,2,Yayun MAO2,3,Qingyu DONG1,2,Feng JIN1,2,Yanbin SHEN2,*(),Liwei CHEN2   

  1. 1 School of Nano Technology and Nano Bionics, University of Science and Technology of China, Hefei 230026, P. R. China
    2 i-Lab, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, Jiangsu Province, P. R. China
    3 School of Materials Science and Engineering, Shanghai University, Shanghai 200444, P. R. China
  • Received:2018-11-20 Accepted:2018-12-10 Published:2018-12-12
  • Contact: Yanbin SHEN E-mail:ybshen2017@sinano.ac.cn
  • Supported by:
    the Ministry of Science and Technology, China(2016YFB0100102);the "Strategic Priority Research Program" of the CAS(XDA09010600, XDA09010303);the "Strategic Priority Research Program" of the CAS(XDA09010303);the National Natural Science Foundation of China(21625304);the National Natural Science Foundation of China(21733012)

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摘要：

使用电解液成膜添加剂是一种简单高效的提高电池循环稳定性的方法。氟代碳酸乙烯酯(FEC)的最低未被占据分子轨道(LUMO)能量较低，易被还原，通常被认为是很好的负极成膜添加剂，但因其最高占据分子轨道(HOMO)能量也较低，抗氧化性较好，故其被认为不在正极上发生作用。本工作结合电化学，形貌分析，化学成分表征，原位结构分析等方法研究了FEC添加剂在钠离子电池中的作用。我们发现适量的FEC添加剂不仅可以显著抑制电解液溶剂碳酸丙烯酯(PC)的分解，而且会在正极上形成一层富NaF的保护层，提高循环过程中正极晶格结构稳定性，从而提高电池的循环稳定性。密度泛函理论(DFT)计算表明，FEC之所以能在正极上形成保护层，可能与其容易在正极界面与钠盐阴离子ClO4-结合反应有关。

关键词: 电解液添加剂, 氟代碳酸乙烯酯, 钠离子电池, 正极材料, 原位XRD

Abstract:

Driven by the wide-scale implementation of intermittent renewable energy generating technologies, such as wind and solar, sodium-ion batteries have recently attracted attention as an inexpensive energy storage system due to the abundance, low cost, and relatively low redox potential of sodium. However, in comparison with lithium-ion batteries, which are known for long cycle life, sodium-ion batteries usually suffer from significant capacity fading during long-term cycling due to the large volume expansion/contraction of the electrode active materials caused by insertion/extraction of the large sodium ion. In recent years, intense effort has been focused on the search for high performance electrode materials and electrolytes to improve the cyclability of sodium-ion batteries, and some progress has been achieved. The incorporation of additives into the electrolyte is a simple and efficient method of improving the cycle stability of sodium-ion batteries. Fluoroethylene carbonate (FEC) is generally considered to be a suitable additive for the formation of the anode solid electrolyte interphase (SEI), due to a relatively low-lying lowest unoccupied molecular orbital (LUMO). However, it is suggested that FEC it will not be oxidized on the cathode since it also has a relatively low highest occupied molecular orbital (HOMO). In this study, we investigated the effect of FEC as an additive on the cycle life of a sodium-ion battery with a P2-NaxCo0.7Mn0.3O2 (x ≈ 1) layered sodium transition metal oxide as the cathode active material, a sodium metal foil anode, a glass fiber separator, and an electrolyte composed of NaClO4 and a varying mass content of FEC dissolved in propylene carbonate (PC). We analyzed the effect of the FEC additive on the morphology and chemical composition of the separator and cathode electrode surface using scanning electron microscopy (SEM), transmission electron microscopy (TEM), infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS), and studied the evolution of the crystalline structure of the cathode active material during charge and discharge using in situ X-ray diffraction (XRD). We found that an appropriate amount of FEC additive significantly suppressed the decomposition of the PC solvent, and assisted the formation of a NaF-rich protective layer on the cathode surface, which helped to maintain the structural stability of the cathode material, thereby improving the cycle stability of the sodium-ion battery. Density functional theory (DFT) calculations showed that FEC coordinates more readily with the ClO4- anion on the cathode surface than does the PC solvent. This drives the formation of the NaF-rich protective layer on the cathode surface. We believe these results could provide inspiration in the design of electrolyte additives for protection of the sodium cathode during cycling, thus improving the cycling performance of sodium-ion batteries.

Key words: Electrolyte additive, Fluoroethylene carbonate, Sodium ion battery, Cathode materials, In situ XRD