Comprehensively-modified polymer electrolyte membranes with multifunctional PMIA for highly-stable all-solid-state lithium-ion batteries

Highlights

• Multifunctional PMIA is employed to improve the overall properties of PEO-based electrolytes.

• PMIA-based electrolytes have higher ion conductance, mechanical strength and thermostability.

• PMIA-based electrolytes show greatly-enhanced interface resistance against Li dendrites.

• Composite electrolyte-based solid-state Li metal batteries have superior cycling performance.

Abstract

Polyethylene oxide (PEO)-based electrolytes have obvious merits such as strong ability to dissolve salts (e.g., LiTFSI) and high flexibility, but their applications in solid-state batteries is hindered by the low ion conductance and poor mechanical and thermal properties. Herein, poly(m-phenylene isophthalamide) (PMIA) is employed as a multifunctional additive to improve the overall properties of the PEO-based electrolytes. The hydrogen-bond interactions between PMIA and PEO/TFSI⁻ can effectively prevent the PEO crystallization and meanwhile facilitate the LiTFSI dissociation, and thus greatly improve the ionic conductivity (two times that of the pristine electrolyte at room temperature). With the incorporation of the high-strength PMIA with tough amide-benzene backbones, the PMIA/PEO-LiTFSI composite polymer electrolyte (CPE) membranes also show much higher mechanical strength (2.96 MPa), thermostability (419 °C) and interfacial stability against Li dendrites (468 h at 0.10 mA cm^‒2) than the pristine electrolyte (0.32 MPa, 364 °C and short circuit after 246 h). Furthermore, the CPE-based LiFePO₄/Li cells exhibit superior cycling stability (137 mAh g⁻¹ with 93% retention after 100 cycles at 0.5 C) and rate performance (123 mAh g⁻¹ at 1.0 C). This work provides a novel and effective CPE structure design strategy to achieve comprehensively-upgraded electrolytes for promising solid-state battery applications.

Introduction

The utilization of organic liquid electrolytes in lithium-ion batteries (LIBs) usually results in the safety problems such as leakage, flammability and side reactions [1,2]. The replacement of the organic liquid electrolytes by solid-state-electrolytes (SSEs) is expected to eliminate these safety issues and meanwhile achieve higher energy density by applying Li metal anodes and conversion-typed oxygen/sulfur electrodes [3], [4], [5]. The SSEs can also work as separators and thereby offer the possibility of simplifying the battery preparation process [6].

Compared to inorganic SSEs, solid polymer electrolytes (SPEs) comprised of polymer matrices and Li salts have obvious advantages including low density, good film-formation ability, high flexibility and easy fabrication process [2,[7], [8], [9], [10]. Particularly, polyethylene oxide (PEO) is considered as an important polymer matrix for SPEs, because of its excellent dissociation ability of Li salts, relatively high ion conductance at high temperature and low cost [11], [12], [13]. The SPEs also exhibit lower interfacial resistance with the electrodes than the stiff/brittle ceramic electrolytes [14,15]. Li⁺ ions can easily transport in the free volume of the polymer matrices through the intra-/inter-polymer chain hopping, however, the high crystallization degree of the polymer matrices and the slow-down dynamics of the polymer chains limit the ionic conductivity (10⁻⁸‒10⁻⁶ S cm⁻¹) at room temperature [16], [17], [18], [19], [20], [21]. Increasing the operation temperature can elevate the ionic conductivity, but the SPEs lose the dimensional stability being in the molten state [16,22,23]. Besides, the inadequate mechanical strength causes the penetration of the SPEs by Li dendrites especially at high current densities [16,[24], [25], [26]. Thus, it needs to improve the ion conductance, mechanical strength and thermostability of the SPEs for their practical applications in all-solid-state LIBs.

Constructing co-block/cross-linking polymers and adding organic molecule plasticizers can increase the ion conductivity of the SPEs, but this causes other issues such as the incompatibility with electrodes and the mechanical property deterioration [25,[27], [28], [29]. Fabricating composite polymer electrolytes (CPEs) by adding inorganic nanoparticles (NPs) into the polymer matrices can simultaneously enhance the ionic conductivity, mechanical strength and thermostability [30], [31], [32], [33]; however, this method cannot greatly improve the ion conductance and mechanical properties, due to the poor dispersity of the large-surface-area nanofillers [34], [35], [36], [37], [38] and the failure to form interconnected reinforcements in the matrices [4,24,39,40], respectively. Besides, the synthesis of these nanofillers is of low efficiency and high cost [11,15,41].

By contrast, polymer blending is regarded as a simple, cheap and scalable method to fabricate high-performance CPEs by integrating PEO/Li salts with polymer molecule additives [11,[42], [43], [44], and the electrolyte properties can be optimized by combining the advantages of several kinds of polymers [31,45]. A few researchers have reported that the utilization of poly(propylene carbonate) (PPC) [46], poly(4-vinylphenol-co-2-hydroxyethyl methacrylate) (PVPh) [47] and poly(methyl methacrylate) (PMMA) [48] can greatly increase the ionic conductivity of the PEO-based electrolytes by inhibiting the PEO crystallization through the interactions between PEO and the additives, however, they did not investigate the impact of the polymer additives on the mechanical properties. Other researchers even did not compare the ionic conductivity of the CPEs containing poly(methyl hydrosiloxane) (PMHS) [44] and poly(vinyl pyrrolidone) (PVP) [49] additives with that of the additive-free electrolytes. On the other hand, it is reported that the addition of polyurethane (PU) [11,43], PMMA [50], poly (dimethyl siloxane) (PDMS) [51] and poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) [52] can enhance the mechanical strength, but would lower the ionic conductivity. To the best of our knowledge, there are few reports about the comprehensive improvement of the PEO-based electrolyte properties of ionic conductivity, mechanical strength and thermostability by the polymer blending method. Besides, most of these researches have not given the electrochemical properties of the polymer blend electrolyte-based all-solid-state LIBs. So it is necessary to develop novel polymer additives with unique molecule structures or multiple functionalities to enhance the overall properties of the PEO-based SPEs for their practical applications in LIBs.

Similar to poly(p-phenylene terephthalamide) (PPTA), poly(m-phenylene isophthalamide) (PMIA) with meta-type amide-benzene linkages in its skeletal chains has high mechanical strength (3.8 GPa), thermal resistance (400°C), low electro-conductivity and good chemical stability, and is regarded as an important membrane material [53], [54], [55]. Recently aromatic polyamides are employed to prepare various nanocomposite membranes as separators in the conventional liquid electrolyte-based LIBs [55], [56], [57], [58]. The polar carbonyl groups in the polyamides can greatly increase the electrolyte wettability of the separators for higher ionic conductivity [55,58]. Moreover, the hydrogen-bond interactions between PEO and the amide groups of the polyamides can effectively reduce the PEO crystallization in the PEO/PPTA nanocomposite [56]. With the incorporation of the high-strength aromatic polyamide components, the nanocomposite membranes also exhibit both greatly-enhanced mechanical strength to suppress the Li dendrite growth [56,57] and thermal stability (up to 240 °C) [58]. Thus, PMIA may be a proper multifunctional additive for fabricating CPE membranes with comprehensively-updated properties such as ionic conductivity, mechanical strength and thermostability.

In this study, novel CPE membranes based on PMIA, PEO and lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) were facilely prepared by the polymer blending technology and subsequent casting process (Fig. 1a). The utilization of the PMIA additive was expected to improve the overall properties of the PEO-based electrolyte, based on these considerations: (1) The hydrogen-bond interactions between the −NH groups in PMIA and −O− groups in PEO can suppress the PEO crystallization and meanwhile reduce the coordination interaction between −O− groups in PEO and Li⁺ cations, thus increasing the ionic conductivity of the SPEs (Fig. 1b and c); (2) the hydrogen-bond interactions between the ‒NH groups in PMIA and TFSI^‒ anions can facilitate the dissociation of LiTFSI to offer more free Li⁺ ions for fast ion transport; (3) apart from the improvement in Li⁺ ion conductivity, the incorporation of the high-strength PMIA additive with benzene-amide backbones would greatly enhance the mechanical properties and thermostability.

Various measurements such as Fourier transform infrared spectroscopy, thermogravimetric analysis and electrochemical impedance spectroscopy were further used to investigate the impact of the PMIA additive on the PEO-based electrolyte. It was found that the addition of PMIA greatly enhanced the overall properties such as ion conductance (6.0 × 10⁻⁶ S cm⁻¹ at ambient temperature), mechanical strength (2.96 MPa), thermo-decomposition stability (419 °C), and interfacial stability against Li dendrites (468 h at 0.10 mA cm⁻², Tables S1‒S3). The PMIA/PEO-LiTFSI CPE-based all-solid-state LiFePO₄/Li cells were also assembled and tested. It was demonstrated that the CPE-based cell exhibited better cycling performance (e.g., 137 mAh g⁻¹ after 100 cycles at 0.5 C, and 123 mAh g⁻¹ at 1.0 C) than the pristine PEO-LiTFSI electrolyte-based cell and other CPE-based cells reported recently (Table S4). Hence, this work offers a novel and effective CPE structure design method to comprehensively upgrade the SPEs for promising all-solid-state battery applications.