Non-flammable liquid polymer-in-salt electrolyte enabling secure and dendrite-free lithium metal battery
Introduction
With the rapid development of portable devices and electric vehicles, rechargeable batteries with higher energy density are highly demanded [1], [2]. Lithium-metal batteries (LMBs) have been regarded as the most promising next generation high-energy secondary batteries due to the high theoretical specific capacity (3860 mAh g−1) and the lowest potential (-3.04 V vs the standard hydrogen electrode) of metallic lithium [3]. However, safety issue severely limits the practical application of LMBs [4]. Apart from battery short circuit caused by lithium dendrite, flammable electrolyte systems (either carbonate nor ether) may bring huge disaster in case of leaking [5]. Therefore, exploration and development of safe and efficient electrolyte for LMBs are quite important [6], [7].
Recently, polymer electrolyte has attracted great interest because of its superior safety feature [8]. Wang et al developed a heteroatom-based all-fluorinated gel polymer electrolyte showing excellent non-flammability [9]. Liu et al designed a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)-based polymer-in-salt electrolyte (PISE) [10]. Although solid polymer electrolytes well address the safety concern, the sluggish ionic diffusion behavior (low ionic conductivity) severely limits their practical application. Researchers make great endeavor to enhance ionic conductivity of polymer electrolyte, such as investigating different polymer chains like polyacrylonitrile (PAN), polyethylene oxide (PEO) to construct PISE [11], [12]. Unfortunately, the ionic conductivity of these solid PISE are still unsatisfied, which are much lower than liquid electrolyte systems [11], [13]. Considering ionic diffusion via solid polymer could not compete with liquid solvent, the development of nonflammable liquid polymer-based PISE may well address the sluggish kinetics but also maintain superior safety. In 2015, Carbone et al. demonstrated that polyethylene glycol dimethyl ether (PEGDME) with excellent chemical stability, superior thermal stability, environmental benign (non-toxic, non-corrosive) and low cost was a good solvent in LMB system for the improved electrochemical performance [14]. But this PEGDME-based PISE still faces wettability problem caused by superhigh viscosity. Recently, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE) has been intensively studied as an impressive cosolvent for its unique features, including low viscosity, non-flammability, and non-interaction with Li cations [15], [16], [17]. The introduction of the TTE diluent not only endows favorable properties with respect to viscosity and wettability but also enables highly effective interphases on the cathode and anode to suppress undesirable side reactions [18]. Consequently, a PEGDME-based PISE assisted by TTE is expected to be an efficient and safe electrolyte in the application of LMBs.
Herein, a liquid polymer-in-salt electrolyte (PISE0.7-TTE90) is designed and fabricated with highly concentrated lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as salt, PEGDME as solvent and TTE as diluent cosolvent. By taking the advantages of highly concentrated LiTFSI in PEGDME and non-flammable TTE, this PISE-TTE electrolyte exhibits superior properties, such as nonflammability, low viscosity, high ionic conductivity and excellent compability with separator. Moreover, a robust solid electrolyte interface (SEI) composed by organic/inorganic components was constructed, which effectively inhibits the growth of lithium dendrites. As a result, with the assistance of PISE-TTE electrolyte, the coulombic efficiency (CE) of Li/Cu cell is around 96.7% and Li/Li cell can stabilize over 500 h. A high-capacity retention of 150 mAh g−1 after 150 cycles can be achieved in Li/LiFePO4 (LFP) cell. What’s more, even using a self-deposited Li@Cu as the anode, Li@Cu/LiFePO4 cell can still deliver a capacity of 150 mAh g−1 and remain stably around 50 cycles. Both experimental results and computer simulations well demonstrate that this safe and effective PISE0.7-TTE90 electrolyte is promising for the practical application of LMBs.
Section snippets
Preparation of electrolytes
Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (99.9%, Monils Chem Co., Ltd.) was dried at 80 °C for more than 12 h. Molecular sieves were activated at 300 °C for more than 6 h. Polyethylene glycol dimethyl ether (PEGDME, Mn = 400) (99%, Aladdin) and 1,1,2,2,-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE) (98%, Shanghai Jinghui Industrial Co., Ltd) were dried with the activated molecular sieves for three days before use. Then, LiTFSI with different mass was dissolved in 2.5 mL
Results and discussion
High concentrated lithium salt has been confirmed to improve the safety of LMBs [24], [25]. Herein, a series of Li(PEGDME)xTFSI (x = 0.7, 0.8, 0.9, 1.0, 1.1) composite electrolytes with high concentration of LiTFSI have been successfully synthesized. Further increasing the concentration of LiTFSI to x = 0.6 causes undissolved LiTFSI in PEGDME solvent due to its oversaturation (Figure S1), therefore, the following discussion about composite electrolytes focuses on x = 0.7–1.1. As listed in Table
Conclusion
In summary, a novel liquid PEGDME-TTE electrolyte with high ionic conductivity, good flame retardancy and wide oxidative stability window, has been well designed. This PEGDME-TTE electrolyte greatly improves the performance of LMBs from the following aspects: 1) outstanding thermal stability, the addition of non-flammable TTE effectively enhances the safety of LMBs; 2) wide electrochemical stability voltage window, this PEGDME-TTE system widens the voltage window from 4.2 V to 5.1 V, which
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The financial support from National Natural Science of China (22075313), the Science and Technology Project of Jiangxi Province (20192BCD40017), Outstanding Youth Fund of Jiangxi Province (20192BCB23028), Jiangxi Double Thousand Talent Program (JXSQ2019101072) and Suzhou Livelihood Science and Technology Program (No. SS2019024) are acknowledged. The technical support from Nano-X, Suzhou Institute of Nano-Tech and Nano-Bionics is also acknowledged.
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These authors contributed equally to this work.