Ionic liquids as electrolyte additives for high-voltage lithium-ion batteries

doi:10.11949/j.issn.0438-1157.20181175

Abstract

Abstract:

The functionalized ionic liquids 1-butyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)imide (BMIMTFSI) was synthesized as a high-pressure lithium ion battery electrolyte additive for inhibiting the oxidation of organic solvents to enhance carbonic acid. The electrochemical behaviors of LiNi_0.5Mn_1.5O₄/Li batteries and the surface morphology of LiNi_0.5Mn_1.5O₄ electrode were studied by charge-discharge test, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and scanning electron microscopy (SEM). The results show that when 20% (vol) BMIMTFSI is added to carbonate-based electrolyte, the highest discharge capacity of LiNi_0.5Mn_1.5O₄/Li cells is 126.81 mA·h·g^-1 at 0.2C rate, and the discharge capacity is 109.36 mA·h·g^-1 at high rate of 5C, which is 91.7% higher than that in 1.0 mol·L^-1 LiPF₆-EC/DMC electrolyte. And the discharge capacity retention rate of LiNi_0.5Mn_1.5O₄/Li cells reaches about 95% after 50 cycles at 0.2C rate, which is nearly 10% higher than that with blank electrolytes. The results of SEM showed that a uniform and compact solid electrolyte interface (SEI) film was attached to the surface of LiNi_0.5Mn_1.5O₄ electrode after adding BMIMTFSI to carbonate-based electrolyte.

Key words: ionic liquids, electrochemistry, oxidation, electrolytes, high voltage, additive, lithium-ion battery

摘要：

合成了功能化离子液体1-丁基-3-甲基咪唑双（三氟甲磺酰）亚胺盐（BMIMTFSI）作为高压锂离子电池电解液添加剂，用于抑制有机溶剂的氧化，以提高碳酸酯类电解液的耐高压性。分别采用充放电测试、电化学交流阻抗（EIS）、循环伏安法（CV）和扫描电子显微镜（SEM）等研究了LiNi_0.5Mn_1.5O₄/Li电池的电化学行为和LiNi_0.5Mn_1.5O₄材料表面形貌。结果表明，当在电解液中添加20% （体积分数）BMIMTFSI时，LiNi_0.5Mn_1.5O₄/Li电池在室温、0.2C下的最高放电比容量是126.81 mA·h·g^-1，5C下的放电比容量为109.36 mA·h·g^-1，比在1 mol·L^-1 LiPF₆-EC/DMC电解液中的放电比容量提高了91.7%；且该电池在0.2C下循环50圈后的放电比容量保持率在95%左右，比用碳酸酯类电解液提高了近10%。SEM结果表明，在碳酸酯类电解液中加入BMIMTFSI后，LiNi_0.5Mn_1.5O₄电极表面附着了一层均匀且致密的固态电解质界面（SEI）膜。

关键词: 离子液体, 电化学, 氧化, 电解液, 高压, 添加剂, 锂离子电池

CLC Number:

TQ 152

Wenlin ZHANG, Yu HUO, Gongwei LI, Tengfei SUN, Yongqi ZHAO, Chunli LI. Ionic liquids as electrolyte additives for high-voltage lithium-ion batteries[J]. CIESC Journal, 2019, 70(6): 2334-2342.

张文林, 霍宇, 李功伟, 孙腾飞, 赵勇琪, 李春利. 离子液体作为电解液添加剂用于高压锂离子电池[J]. 化工学报, 2019, 70(6): 2334-2342.

Figures/Tables 13

Fig.1 Synthesis route of BMIMTFSI

Fig.2 Infrared spectra of BMIMTFSI

Table 1 Frontier molecular orbital energy of EC, DMC and BMINTFSI

材料	HOMO/eV	LUMO/eV
EC	-8.4669	-0.2838
DMC	-8.1591	0.1298
BMIMTFSI	-6.4328	-1.4517

Fig.3 Viscosity and conductivity

Fig.4 Initial charge-discharge curves of LiNi0.5Mn1.5O4 /Li cells using additives with different concentration of BMIMTFSI at 0.2 C

Fig.5 Cycle performances of LiNi0.5Mn1.5O4/Li cells using different electrolytes at 0.2 C

Fig.6 Rate properties of LiNi0.5Mn1.5O4 /Li cells at room temperature

Fig.7 Equivalent circuit of LiNi0.5Mn1.5O4/Li cells

Fig.8 Nyquist plots of LiNi0.5Mn1.5O4/Li cells at 0.2 C

Table 2 Fitting values of EIS

电解液		循环5圈后			循环30圈后
电解液		R _s/Ω	R _f/Ω	R _ct/Ω	R _s/Ω	R _f/Ω	R _ct/Ω
空白电解液	4.43		49.03	61.08	3.41	69.03	77.18
15% (vol)BMIMTFSI	3.62		30.80	48.16	2.98	47.62	63.53
20% (vol)BMIMTFSI	3.70		19.17	68.31	3.22	25.17	56.75

Fig.9 Cyclic voltammograms of LiNi0.5Mn1.5O4/Li cells after 30 cycles at 0.2 C

Fig.10 SEM images of LiNi0.5Mn1.5O4 cathode material before and after cycling

Fig.11 XPS spectra of LiNi0.5Mn1.5O4 cathode material using blank electrolyte and 20% (vol) BMIMTFSI electrolyte after cycling

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