Functionalized gel polymer electrolyte membrane for high performance Li metal batteries

Highlights

• A “ceramic-in-polymer” membrane is fabricated with nanosized Li_6.4La₃Zr_1.4Ta_0.6O₁₂ particles being filled into the porous PVDF-HFP as gel polymer electrolyte (GPE).

• This “solid-in-soft” membrane displays a high ionic conductivity of 3.57 × 10⁻⁴ S cm⁻¹, large Li-ion transference number of 0.62, good mechanical strength and thermal stability.

• The fabricated LiFePO₄|GPE|Li coin cell delivers a high Coulombic efficiency of 98%, a good rate capability at 2C (about 120 mAh g⁻¹) and capacity retention of 92% after 110 cycles (150 mAh g⁻¹, 0.4C) at room temperature.

Abstract

Inorganic solid-state electrolytes have high ionic conductivity and stability, which demonstrate the promising application in building solid batteries excepting their poor interface compatibility. Hence, making the solid electrolytes flexible, such as the combination with polymer is an important strategy in mitigating such problem. In this context, a robust membrane is fabricated with nanosized Li_6.4La₃Zr_1.4Ta_0.6O₁₂ particles being filled into the porous PVDF-HFP as gel polymer electrolyte (GPE). This membrane displays a high ionic conductivity of 3.57 × 10⁻⁴ S cm⁻¹, a large Li-ion transference number of 0.62, as well as the good mechanical strength and thermal stability. The as prepared Li/GPE/Li symmetric cell demonstrates an underlying electrochemical activation in stabilizing the membrane-electrode interface, which seems to be balancing the trade-off between the Li diffusion and side reactions at the interface. The fabricated LiFePO₄|GPE|Li coin cell delivers a high Coulombic efficiency of 98%, a good rate capability at 2C (about 120 mAh g⁻¹) and capacity retention of 92% after 110 cycles (150 mAh g⁻¹, 0.4C) at room temperature. This important design provides a rewarding avenue to address the interface compatibility and develop composite electrolytes for high performance Li metal batteries.

Introduction

Rechargeable lithium-ion batteries (LIBs) with increased energy density are widely used in electric vehicles and portable electronic devices [[1], [2], [3], [4], [5]]. However, commercial LIBs suffer from a major safety problem due to the organic liquid electrolytes [[6], [7], [8], [9]], which have obviously irresolvable disadvantages, such as the leakage, flammability [2,10], corrosivity and thermal instability [11], significantly limiting their prevailing applications. Hence, solid-state LIBs return back again to the center stage of developing better LIBs. Replacing liquid electrolytes with solid-state electrolytes (SSEs) is considered an effective strategy to achieve the practical solid-state Li-metal batteries free of the aforementioned issues [[11], [12], [13], [14]]. In terms of SSEs, their good chemical/electrochemical stabilities make solid state batteries feasible working in high voltages with desired high energy density, long cycling life, and good reliability [12]. Moreover, with the rigid solid electrolytes, the utility of Li metal with the most negative electrochemical potential (−3.040 V vs. SHE) [15] becomes highly likely to promote the battery energy density, which renders an ultra-high theoretical capacity (3860 vs. 372 mAhg⁻¹ of graphite anode).

There are mainly two families of SSEs, namely the polymer and inorganic electrolytes [16] employed at present to fabricate solid batteries. Inorganic solid electrolytes have advantages of excellent ionic conductivity and high electrochemical windows at room temperature, such as the 0.1–1 mS cm⁻¹ of NASICON type SSEs at 25 °C [14,17,18] and the much-anticipated Garnet type SSEs with the ionic conductivity of >1 mS cm⁻¹ at 25 °C, comparable to organic electrolytes. Of special interest is that Garnet SSEs are thought to be chemical stable with Li anode and their electrochemical window of ~9 V (vs. Li/Li⁺) [19] is experimentally reached, which is significantly higher than that of liquid electrolytes (usually lower than 4.5 V). However, the Garnet SSEs suffer from the poor interface compatibility with electrolytes and high interface resistance [[20], [21], [22], [23], [24]] that makes it difficult in fast charging owing to their fragility and brittleness [8,22,25]. While polymer-based electrolytes demonstrate the enough flexibility to improve the interface compatibility in solid LIBs [26], although they have some drawbacks including the poor Li conductivity of 10⁻⁶ to 10⁻⁸ S cm⁻¹ at room temperature [27,28], small ionic transference number (t⁺ < 0.5), and lower thermal and electrochemical stabilities as well [29]. Apparently, the combination of Garnet and polymer electrolytes, as the “ceramic in polymer” strategy, could play a multiple role in making full advantages of both polymer and inorganic electrolytes to build a consecutive and stable SSE-electrode interface, as well as transport the Li ions and electrons quickly enough [[30], [31], [32], [33], [34], [35], [36], [37], [38], [39]].

In polymer electrolytes, the polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) demonstrates better thermal stability [40], lower crystallinity and higher ionic conductivity with respect to other polyethylene oxide (PEO) based electrolytes, which has drawn increasingly more attention [41] in battery applications. However, the intrinsic inferior mechanical strength of PVDF-HFP electrolyte restricts their further applications because they fail to sustain the stresses generated by the electrode materials during cycling [42]. As a result, such a combination of PVDF-HFP polymer and inorganic electrolytes as gel polymer electrolytes (GPEs) is somewhat compromise, but effective way in utilizing the high ionic conductivity of inorganic electrolytes and good flexibility of polymer electrolytes [43] for the promising solid LIBs. In this “ceramic in polymer” electrolyte, the PVDF-HFP has good thermoplasticity and easily become colloid as the structural skeleton. To increase the ionic conductivity, inorganic electrolyte can be supplemented into the polymer matrix by many methods, such as the cross-linking, forming block copolymer, adding plasticizers, and introducing ceramic fillers [28,44]. Among these attempts, dispersing ceramic fillers in polymer matrix has attracted great interest because it can enhance the ionic conductivity efficiently, as well as the mechanical strength and thermal stability of polymer electrolytes [45], such as the recent result by Huo et al. using Garnet or Al doped LiGe₂(PO₄)₃ SSEs to enhance the ionic conductivity [46].

In this context, a new GPE membrane is prepared as the PVDF-HFP-LLZTO-LiFSI (PL_xL-GPE, x corresponds the mass ratio of LLZTO vs. PVDF-HFP) composite, in which the Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) and LiFSI are filled as the ion-conductive agents to improve the ionic conductivity of porous PVDF-HFP electrolyte. The PL_xL-GPE membrane exhibited superior electrochemical performance, thermal stability, and mechanical strength of 9.42 MPa. With a higher transference number of 0.62 in Li metal battery tests, both symmetric Li/PL₃₀L-GPE/Li and the LiFePO₄|PL₃₀L-GPE|Li cells exhibit excellent electrochemical performance, especially the rate capability (118 mAh g⁻¹ for LiFePO₄ at 2C) which rewards from the construction of stable electrolyte/electrolyte interfaces.