Preparation and Properties of Lithium(trifluoromethanesulfonyl)-(trifluoroethoxysulfonyl)imide as Conducting Salt for Nonaqueous Electrolyte Solutions†

doi:10.7503/cjcu20131260

Abstract

Abstract:

A novel lithium salt, lithium(trifluoromethanesulfonyl)(trifluoroethoxysulfonyl)imide{Li[(CF₃SO₂)(CF₃CH₂OSO₂)N], Li[TFO-TFSI]}, and the corresponding nonaqueous electrolytes of 1.0 mol/L Li[TFO-TFSI] in a mixture of ethylene carbonate/ethyl methyl carbonate(EC/EMC, 3∶7, volume ratio) were prepared. The structure, composition and purity of the as-prepared Li[TFO-TFSI] were confirmed by nuclear magnetic resonance spectroscopy(NMR), infrared spectroscopy(IR), mass spectrometry(MS), elemental analysis(EA), and ion chromatography(IC). The thermal properties of Li[TFO-TFSI] and its nonaqueous electrolytes, 1.0 mol/L Li[TFO-TFSI]-EC/EMC(3∶7), were characterized by differential scanning calorimetry(DSC) and thermogravimetric analysis(TGA). The fundamental properties of nonaqueous electrolyte solution of Li[TFO-TFSI] were characterized by electrochemical impedance spectroscopy(EIS), cyclic voltammetry(CV), chronoamperometry and scanning electron microscope(SEM). The nonaqueous electrolytes of Li[TFO-TFSI] shows excellent electrochemical stability, and does not corrode Al below 4.2 V. Both Li/synthetic graphite and synthetic graphite/LiCoO₂ cells using Li[TFO-TFSI] show good cycling performances at room temperature, and the graphite/LiCoO₂ cells using Li[TFO-TFSI] shows much better capacity retention after 100 cycles than those using LiPF₆.

Key words: Lithium-ion battery, Nonaqueous electrolyte, Lithium(trifluoromethanesulfonyl)(trifluoroethoxysulfonyl)imide{Li[TFO-TFSI]}, Al foil corrosion

CLC Number:

O646

TrendMD:

LIU Chengyong, ZHANG Heng, ZHENG Liping, XU Fei, FENG Wenfang, NIE Jin, HUANG Xuejie, ZHOU Zhibin. Preparation and Properties of Lithium(trifluoromethanesulfonyl)-(trifluoroethoxysulfonyl)imide as Conducting Salt for Nonaqueous Electrolyte Solutions^†[J]. Chem. J. Chinese Universities, 2014, 35(8): 1771.

Figures/Tables 15

Fig.1 1H NMR(A), 19F NMR(B), MS(C) and IR(D) spectra of Li[TFO-TFSI] salt

Fig.2 DSC(A) and TG(B) curves for Li[TFO-TFSI] salt

Table 1 Density(ρ), viscosity(η), ionic conductivity(κ) and lithium-ion transference number(tLi+) at 25 ℃, van der Waals volume(Va) of anion[10], glass transition temperature(Tg) and the parameters of VTF equation for various electrolytes

Electrolyte	ρ/(g·mL^-1)	η/(mPa·s^-1)	κ/(mS·cm^-1)	$t L i +$	V_a/nm³	T_g/K	k(T)=AT^-1/2exp[-B/(T-T₀)]
Electrolyte	ρ/(g·mL^-1)	η/(mPa·s^-1)	κ/(mS·cm^-1)	$t L i +$	V_a/nm³	T_g/K	T₀/K	A/(S·m^-1·K^-1/2)	B/K	R²
Li[TFO-TFSI]	1.26	3.78	4.60	0.37	0.176	171	130	2.32	565	0.9999
LiBF₄	1.16	2.23	3.72	0.31	0.049	154	127	1.55	543	0.9999
LiClO₄	1.18	2.77	6.26	0.40	0.055	152	137	2.54	509	0.9999
LiPF₆	1.21	3.00	9.33	0.48	0.069	193	153	2.86	418	0.9999
LiTFSI	1.23	3.40	7.57	0.55	0.147	156	151	2.10	408	0.9999

Electrolyte	ρ/(g·mL^-1)	η/(mPa·s^-1)	κ/(mS·cm^-1)	$t L i +$	V_a/nm³	T_g/K	k(T)=AT^-1/2exp[-B/(T-T₀)]
Electrolyte	ρ/(g·mL^-1)	η/(mPa·s^-1)	κ/(mS·cm^-1)	$t L i +$	V_a/nm³	T_g/K	T₀/K	A/(S·m^-1·K^-1/2)	B/K	R²
Li[TFO-TFSI]	1.26	3.78	4.60	0.37	0.176	171	130	2.32	565	0.9999
LiBF₄	1.16	2.23	3.72	0.31	0.049	154	127	1.55	543	0.9999
LiClO₄	1.18	2.77	6.26	0.40	0.055	152	137	2.54	509	0.9999
LiPF₆	1.21	3.00	9.33	0.48	0.069	193	153	2.86	418	0.9999
LiTFSI	1.23	3.40	7.57	0.55	0.147	156	151	2.10	408	0.9999

Fig.3 DSC curves for electrolyte solutions and various lithium salts[1.0 mol/L lithium salts in EC/EMC(3∶7, volume ratio)] a. EC/EMC(3∶7); b. Li[TFO-TFSI]; c. LiBF4; d. LiClO4; e. LiPF6; f. LiTFSI.

Fig.4 1H NMR(A) and 19F NMR(B) spectra of fresh solution of 1.0 mol/L Li[TFO-TFSI]-EC/EMC(3∶7)

Fig.5 1H NMR(A) and 19F NMR(B) spectra of electrolyte solutions of 1.0 mol/L Li[TFO-TFSI]-EC/EMC(3∶7) after aging at 85 ℃ for 14 d

Fig.6 Vogel-Tammann-Fulcher plots of electrolyte solutions of 1.0 mol/L Li[TFO-TFSI], LiBF4, LiClO4, LiPF6 and LiTFSI in EC/EMC(3∶7)

Fig.7 Impedance spectra of electrolyte solutions of 1.0 mol/L Li[TFO-TFSI]-EC/EMC(3∶7) before polarization(A) and time-dependence response of dc polarization of 1.0 mol/L Li[TFO-TFSI]-EC/EMC(3∶7) with an applied voltage of 10 mV(B)

Fig.8 Cyclic voltammogram of electrolyte solutions of 1.0 mol/L Li[TFO-TFSI]-EC/EMC(3∶7) Working electrode: Pt; reference and counter electrode: Li; scan rate: 0.5 mV/s.

Fig.9 Cyclic voltammograms of electrolyte solutions of 1.0 mol/L Li[TFO-TFSI]-(A) and LiTFSI(B) in EC/EMC(3∶7) Working electrode: Al; reference and counter electrode: Li; scan rate: 1.0 mV/s.

Fig.10 SEM images of Al foil after testing of cyclic voltammograms in electrolyte solutions of 1.0 mol/L Li[TFO-TFSI](A) and LiTFSI(B) in EC/EMC(3∶7)

Fig.11 Chronoamperometric profiles of 1.0 mol/L lithium salts in a mixture of EC/EMC(3∶7) (A) Li[TFO-TFSI] recorded at 4.2 V(a) and 4.5 V(b); (B) LiTFSI recorded at 4.2 V.

Fig.12 Cyclic voltammograms of electrolyte solutions of 1.0 mol/L Li[TFO-TFSI]-EC/EMC(3∶7) Working electrode: synthetic graphite; reference and counter electrode: Li; scan rate: 0.01 mV/s.

Fig.13 Specific capacity and columbic efficiency as a function of cycle number for Li/graphite half cells using 1.0 mol/L Li[TFO-TFSI] and LiPF6 in EC/EMC(3∶7) as electrolyte solutions

Fig.14 Specific capacity(A) and columbic efficiency(B) as a function of cycle number for graphite/LiCoO2 using 1.0 mol/L Li[TFO-TFSI] and LiPF6 in EC/EMC(3∶7) as electrolyte solutions

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