First Author: Weng Chaocang
Contact:HUANG Su-mei*,PAN Li-kun*,LI Jin-liang**
Affiliation: East China Normal University, Jinan University
【Background】
Bis(trifluoromethanesulfonyl)imide ionic liquid (TFSI-IL) electrolytes are expected to achieve higher voltages (>5 V), a wider temperature range (>80°C), and non-flammability in lithium metal batteries (LMBs). However, its lethal lithium compatibility severely limits the cycle life of the battery. The traditional solution is to introduce co-solvents (esters or ethers) into IL to extend the life of LMB. However, the electrochemical stability of these co-solvents is limited by the voltage range (1.5 ~ 4.7 V vs. Li+/Li) and operating temperature (below 50°C). This impairs the intrinsic properties of TFSI-based ILs and imposes limits on the voltage and operating temperature of the battery. This work proposes a single-solvent dual-anionic ionic liquid electrolyte strategy that improves the compatibility of ionic liquid lithium while retaining its intrinsic properties. This paper opens up new possibilities for the design of electrolytes under extreme conditions in energy storage systems.
【Introduction】
近日，来自华东师范大学的潘丽坤教授&黄素梅教授与暨南大学黎晋良副研究员合作，在国际知名期刊Energy Storage Materials上发表题为“Single-solvent ionic liquid strategy achieving wide-temperature and ultra-high cut-off voltage for lithium metal batteries”的研究成果。 该文章利用离子液体本征的宽电压窗口和热稳定性优势来开发适用极端条件下的锂金属电池电解液。 使用双阴离子(FSI-和TFSI-)策略协同构建坚固的SEI，克服了TFSI基IL电解质不稳定的锂沉积。 作为结果， 离子液体电解质(HCILE)组装的Li//Li电池以0.5 mA cm−2/0.5 mAh cm−2能够稳定循环1900 h。 此外， HCILE保留了TFSI基ILs的固有特性，具有5.4 V的极高氧化电位和250 °C的高温耐受性。 在高压条件下(2.5~4.95 V)， Li//LiFePO4电池也表现出可逆的阴极电化学特性。
【Main points of the text】
1. Analysis of the physicochemical properties and solvation structure of IL electrolyte
The imidazole cation weakens its interaction with the anion due to the charge delocalization property on the imidazole ring. As a result, EMIMTFSI exhibits a higher ionic conductivity than other ILs. The physicochemical properties of ionic liquids are sensitive to temperature, and as the temperature increases, the ionic conductivity of IL electrolytes increases significantly and the viscosity decreases significantly, showing potential advantages for high-temperature applications. In addition, compared to the commercial electrolyte LP30, HCILE has excellent thermal stability and almost no mass loss up to 200°C. The flammability test showed that LP30 is highly flammable, while HCILE is non-flammable, indicating that LMB using HCILE has higher safety performance. Raman spectroscopy data showed that with the increase of LiFSI, the intensity of free anion FSI and TFSI in the electrolyte decreased, and more anions formed coordination with Li ions, which promoted the formation of LiF-rich SEI layer.
Figure 1. Characteristics of new ionic liquid electrolytes
2. The dual anion strategy enables dendrilite-free lithium deposition and reversible cathode electrochemistry
HCILE has a dissolved structure with double anion coordination. Among them, the introduction of LiFSI (containing S-F, which is prone to breakage) effectively improved the poor compatibility of TFSI-based ionic liquids (derived from robust C-F), resulting in the transformation of the solvation structure dominated by TFSI to the solvation structure dominated by FSI. The Li//Li battery using HCILE can be stably cycled for 1900 h at 0.5 mA cm−2/0.5 mAh cm−2, overcoming the limitation that traditional TFSI-based IL electrolytes can only operate at limited current densities and have a short lifespan. In addition, the introduction of FSI significantly inhibited the growth of lithium dendrites and promoted the formation of a smoother and denser lithium deposit. The ionic liquid electrolyte using a single solvent has a high oxidation potential of 5.4 V, and the matched Li//LiFePO4 battery retains a specific capacity of 143 mAh g-1 after 300 cycles at 2.5-4.95 V.
Figure 2. LMB performance using IL electrolytes at room temperature
Figure 3. Topography and schematic diagram of lithium deposition behavior
Figure 4. High-pressure performance of IL electrolytes
3. LiF-rich solid-state electrolyte interface
With the increase of LiFSI concentration, the formation of LiF-rich SEI is more favorable. Deep XPS shows that HCILE has an inner LiF-rich distribution. This robust SEI layer derived from the double anion FSI and TFSI has a three-dimensional gradient distribution structure and contains inorganic and organic species such as LiF, Li2S, Li2SO4, Li2SO3, Li2O, and Li3N. This high-quality SEI layer is able to effectively cope with severe volume expansion and the decomposition of EMIM+ cations during cycling. HCILE is endowed with excellent Li dendrite inhibition ability, which overcomes the compatibility challenge of TFSI-based IL electrolyte with Li metal.
Figure 5. Analysis of the SEI layer on a lithium metal anode
【Conclusion】
In this work, the lithium compatibility of ionic liquid electrolytes was improved while retaining the intrinsic properties of ionic liquids by using a single-solvent and double-anion policy. Due to their low volatility, non-flammability, wide voltage window, and thermal stability, ionic liquids have great potential under extreme conditions. This strategy provides insights into improving the poor lithium compatibility of ionic liquid electrolytes and facilitates their application in high-temperature and high-pressure applications, advancing the development of electrolytes under extreme conditions.
Chaocang Weng, Liang Ma, Bingfang Wang, Fanyue Meng, Jiaqi Yang, Yingying Ji, Botian Liu, Wenjie Mai, Sumei Huang, Likun Pan, Jinliang Li, Single-solvent ionic liquid strategy achieving wide-temperature and ultra-high cut-off voltage for lithium metal batteries, Energy Storage Mater.
https://doi.org/10.1016/j.ensm.2024.103584
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