钠离子电池HOPG负极固体电解质界面膜的AFM研究

doi:10.15541/jim20160472

摘要/Abstract

摘要：

固体电解质界面膜(Solid Electrolyte Interphase, SEI)在钠离子电池(Sodium Ion Battery, NIB)中扮演着重要作用。迄今为止, 对于钠离子电池SEI膜的探索仍然十分有限。本研究利用电化学原子力显微镜(Electrochemical AFM, EC-AFM), 通过循环伏安法研究了钠离子电池负极材料高定向热解石墨(Highly Oriented Pyrolytic Graphite, HOPG), 在碳酸乙烯酯(Ethylene Carbonate, EC)和氟代碳酸乙烯酯(Fluoroethylene Carbonate, FEC)电解液中首次充放电过程SEI膜的结构变化。通过纳米刻蚀的方法, 进一步获得首次充放电结束后SEI的厚度。结合X射线光电子能谱(X-ray Photoelectron Spectroscopy, XPS)分析了HOPG在EC和FEC电解液中所形成的SEI膜的化学组成区别。研究结果表明, 在EC电解液中, 所生成的SEI膜在HOPG表面非台阶处较薄, 但在HOPG的台阶处较厚; 在FEC电解液中, 所生成的SEI膜很厚, 具有明显的双层结构。其中上层是由体积较大的颗粒组成, 下层则由致密的小颗粒组成。

关键词: 钠离子电池, 固体电解质界面膜, 电化学原子力显微镜, 电解液

Abstract:

Chemical and morphological structures of solid electrolyte interphase (SEI) play a vital role in sodium-ion battery (NIB). Up to date, SEI remains the least understood component in NIB due to its trace presence, delicate chemical nature, heterogeneity in morphology, elusive formation mechanism, and lack of reliable in situ quantitative characterization tools. SEI morphological evolution during the first cyclic voltammetry and the thickness of SEI after first discharging-charging cycle on highly oriented pyrolytic graphite (HOPG) surface were investigated by using in situ electrochemical atomic force microscopy (EC-AFM). Complemented by an ex situ XPS analysis, the differences in interfacial features and fundamental constitution of SEI formation in ethylene carbonate (EC) and fluoroethylene carbonate (FEC)-based electrolytes on HOPG electrode surface were revealed. In 1 mol/L NaClO₄/EC/DMC electrolyte, dense films were formed at the step edges, while thinner films were observed on the basal plane. In 1 mol/L NaClO₄/EC/DMC electrolyte, stable films with doubled layer structures were formed on HOPG electrode after initial electrochemical cycling. The upper layer was composed of large particles, which was soft and easy to be scrapped off by AFM tip while the under layer was composed of dense small particles.

Key words: sodium ion battery, solid electrolyte interphase, electrochemical atomic force microscopy, electrolyte

中图分类号:

TQ174

王舒玮, 胡和丰, 王德宇, 沈彩. 钠离子电池HOPG负极固体电解质界面膜的AFM研究[J]. 无机材料学报, 2017, 32(6): 596-602.

WANG Shu-Wei, HU He-Feng, WANG De-Yu, SHEN Cai. AFM Investigation of Solid Electrolyte Interphase on HOPG Anode in Sodium Ion Battery[J]. Journal of Inorganic Materials, 2017, 32(6): 596-602.

图/表 8

图1 在1 mol/L NaClO4/EC/DMC电解液中和1 mol/L NaClO4/FEC/DMC电解液中的第一圈循环伏安曲线(电压范围: 3.0~0 V, 速率: 0.5 mV/s)

Fig. 1 Cyclic voltammogram of HOPG electrode in 1 mol/L NaClO4/EC/DMC and 1 mol/L NaClO4/FEC/DMC at scan rate of 0.5 mV/s

图2 在1 mol/L NaClO4/EC/DMC电解液中首次电化学循环伏安的原位AFM图片

Fig. 2 In situ AFM images of HOPG electrode cycled at a scanning rate of 0.5 mV/s between 3.0 and 0 V in 1 mol/L NaClO4/EC/DMC(a) During discharging, potential range of 3.0-2.84 V; (b) During discharging, potential range of 0.24-0.04 V; (c) The potential is sweeped from 0.04 V to 0 V then rised to 0.13 V; (d) During charging, potential range of 2.98-3.0 V. The arrow indicates AFM scanning direction

图3 (a)HOPG负极在1 mol/L NaClO4/EC/DMC电解液中首次循环伏安结束后的图像; (b)中间SEI膜被刮除后HOPG负极的图像; (c)蓝色实线所示的高度剖面图

Fig. 3 (a) AFM image of HOPG electrode after first CV cycle in the 1 mol/L NaClO4/EC/DMC; (b) AFM image of the HOPG anode after the SEI in the middle of the image being scraped off and (c) cross-section analysis of the location marked by blue line

图 4 在1 mol/L NaClO4/FEC/DMC电解液中进行首次循环伏安测试的原位AFM图片

Fig. 4 In situ AFM images of HOPG electrode cycled at a scanning rate of 0.5 mV/s between 3.0 and 0.0 V in 1 mol/L NaClO4/FEC/DMC(a) During discharging, potential range of 3.0-2.85 V; (b) During discharging, potential range of 1.06-0.93 V; (c) During discharging, potential range: of 0.93-0.80 V; (d) During discharging, the potential range of 0.66-0.52 V; (e) The potential is sweeped from 0.01 V to 0 V then rised to 0.17 V; (f) During charging, the potential range of 2.90-3.0 V; The arrow indicates AFM scanning direction

图5 (a) HOPG负极在1 mol/L NaClO4/FEC/DMC电解液中首次循环伏安结束后的图像; (b) 中间SEI膜被刮除后HOPG负极的图像; (c) 蓝色实线所示的高度剖面图

Fig. 5 (a) AFM image of HOPG electrode after first CV cycle in the 1 mol/L NaClO4/FEC/DMC; (b) AFM image of the HOPG anode after the SEI in the middle of the image being scraped off; and (c) cross-section analysis of the location marked by blue line

图6 (a) HOPG负极在1 mol/L NaClO4/FEC/DMC电解液中首次循环伏安结束后下层SEI膜的图像; (b) 中间SEI膜被刮除后HOPG负极的图像; (c) 蓝色实线所示的高度剖面图

Fig. 6 (a) AFM image of under SEI layer on HOPG electrode after first CV cycle in the 1 mol/L NaClO4/FEC/DMC; (b) AFM image of the HOPG anode after the SEI in the middle of the image being scraped off; (c) cross-section analysis of the location marked by blue line

图7 在EC/DMC和FEC/DMC电解液中进行电化学循环伏安(电压范围: 3.0~0 V; 速率: 0.5 mV/s)后, HOPG负极上SEI膜的XPS图谱

Fig. 7 XPS spectra of the SEI on HOPG electrode cycled at a scanning rate of 0.5 mV/s between 3.0 and 0 V in the EC/DMC and FEC/DMC electrolytes

表1 XPS测试结果C、O、Na、Cl和F的原子百分比

Table 1 Atomic concentrations of C, O, Na, Cl and F obtained from XPS

HOPG		Atomic concentration /%
HOPG		C	O	Na	Cl	F
1 mol/L NaClO₄ /EC/DMC	Before etching	80.22	16.63	2.56	0.59	-
1 mol/L NaClO₄ /EC/DMC	After etching	83.16	11.62	3.42	1.80	-
1 mol/L NaClO₄ /FEC/DMC	Before etching	77.90	12.36	4.18	0.87	4.69
1 mol/L NaClO₄ /FEC/DMC	After etching	78.02	5.21	6.95	2.03	7.79

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