Non-flammable liquid polymer-in-salt electrolyte enabling secure and dendrite-free lithium metal battery

Highlights

• A novel concept of liquid “polymer-in-salt” electrolyte is proposed.

• Nonflammable electrolyte with fast Li⁺ mobility is achieved.

• A robust organic/inorganic dual layer solid electrolyte interface is constructed.

• Excellent electrochemical performance is presented in lithium metal battery.

Abstract

Lithium-metal batterie (LMB) is regarded as one the most promising high energy battery systems, however the safety concern from lithium dendrites and organic liquid electrolytes seriously impedes its widely application. Herein, a non-flammable liquid “polymer-in-salt” composite electrolyte with superior dendrite-suppression capability is well designed and achieved by using lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, polyethylene glycol dimethyl ether (PEGDME) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE). Non-flammable TTE used here is on one hand to reduce the high viscosity of this “PEGDME polymer in LiTFSI salt” electrolyte (PISE), and thus enhance its wettability; on the other hand, to maintain the merits of highly contented salt. By taking advantage of these localized highly concentrated lithium salts, aggregated ion clusters are generated in this PISE-TTE system, which construct fast ionic transportation channels, and thus accelerates Li⁺ transportation. Furthermore, this PISE-TTE electrolyte system interacts with Li anode forming a robust organic/inorganic dual layer solid electrolyte interphase (SEI) layer. With the assistance of this tough SEI, the optimized PISE_0.7-TTE₉₀ exhibits outstanding anti-dendrite growth behavior in Li/Li and Li/Cu cells. Moreover, PISE_0.7-TTE₉₀ also delivers an impressive performance in both Li/LiFePO₄ (LFP, a capacity of 150 mAh g⁻¹ at 0.5C after 150 cycles) and Li@Cu/LFP (electrochemically deposited Li on Cu current collector, a capacity of 150 mAh g⁻¹ at 0.5C after 50 cycles) cells.

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 (PISE_0.7-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 PISE_0.7-TTE₉₀ electrolyte is promising for the practical application of LMBs.