Elsevier

Electrochimica Acta

Volume 319, 1 October 2019, Pages 189-200
Electrochimica Acta

Influence of MOF ligands on the electrochemical and interfacial properties of PEO-based electrolytes for all-solid- state lithium batteries

https://doi.org/10.1016/j.electacta.2019.06.157Get rights and content

Abstract

Magnesium-1,4-benzenedicarboxylic acid (Mg-TPA) and magnesium- 1,3,5-benzene tricarboxylic acid (Mg-TMA) MOFs were synthesized and successfully incorporated in a poly (ethylene oxide) (PEO) matrix as filler for different proportions of LiN(CF3SO2)2 (LiTFSI) as salt. The membranes prepared were thermally stable up to 360 ̊°C. The ionic conductivity of the polymer electrolytes was enhanced upon addition of MOF and a maximum conductivity of 7.02 × 10−4 S cm−1 was achieved for CPE containing 10 wt % of Mg-TPA as filler. The interfacial properties of the CPEs with lithium metal anode were analysed by compatibility, Fourier transform infrared (FT-IR) and XPS analyses. Li/NCPE/Li symmetric cells were assembled and the dendrite growth was also studied. The lithium transference numbers (t Li+) was measured as 0.58 and 0.52 for the CPE containing Mg-TPA and Mg-TMA, respectively which is appreciable for battery applications. The influence of different organic ligands on the electrochemical and interfacial properties of solid polymer electrolytes was investigated and discussed.

Introduction

Although lithium-ion batteries (LIBs) were commercialized in 1991 by Sony, Japan for consumer electronics (e.g. laptop, mobile) their applications in hybrid electric vehicles still require improvements in terms of energy density, cycle life and better safety [1,2]. LIBs also face critical safety issues since it contains organic liquid electrolytes which have low thermal stability and flash point that lead to poor safety issues [3]. In order to overcome these challenges, new types of solid electrolytes with better safety, excellent flexibility and outstanding electrochemical properties are being explored [4].

Advancement in lithium battery technology in terms of energy density can be achieved only by replacing the conventional liquid electrolyte by solid polymer electrolytes (SPE) in conjunction with Li- metal anode [4]. Although inorganic solid electrolyte possesses high thermal stability the poor electrode/electrolyte contact hampers it from practical application [5,6]. Obviously, by virtue of its advantages, SPEs have been identified as a potential candidate for all-solid-state lithium batteries. The basic requirements for solid polymer electrolyte are; (i) high ionic conductivity (10−4 S cm−1 at ambient temperature) with negligible electronic conductivity, (ii) appreciable mechanical integrity and compatibility with electrodes and (iii) chemical and electrochemical stability. However, identifying a SPE with appreciable ionic conductivity at ambient temperature remains as a great challenge [7].

Although low molecular weight liquid plasticizers such as ethylene carbonate, propylene carbonate, diethyl carbonate etc. added to PEO- based electrolytes increase the ionic conductivity, it adversely affects the mechanical integrity and safety of the electrolyte membrane. Composite polymer electrolyte (CPE), on the other hand, is a sub- set of polymer electrolytes for electrochemical devices in which nanofiller is introduced to promote the ionic conductivity and interfacial properties. So far, several polymeric hosts have been explored. However, poly (ethylene oxide) (PEO) has been extensively studied due its advantages such as high salt content/complexation, acceptable cost, mechanical integrity and good corrosion resistance [8].

The crystalline structure plays a key role on the ionic conductivity of PEO which can be modified by adding ceramic additives [8,9]. Additionally, the electrochemical and interfacial properties of polymer electrolytes are mainly influenced by the nature of ceramic fillers added and are broadly classified into two main categories as active (LiAl2O3) and passive (SiO2, TiO2) [10]. Recently, incorporation of superionic conductors such as Li1.4Al0.4Ti1.6 (PO4)3 (LATP) [11,12], Li7La3Zr2O12 (LLZO) [13], Li6.75La3Zr1.75Ta0.25O12 (LLZTO) [14], Li1.4Al0.4Ge1.6 (PO4)3 (LAGP) [15] in a PEO matrix has been considered as an efficient way to enhance the ionic conductivity of polymer electrolytes. Nan et al. have added Li0.3La0.557TiO3 (LLTO) in a PEO matrix and achieved an ionic conductivity as high as 1.8 × 10−4 S cm−1 at room temperature [16].

MOFs are class of crystalline porous materials having metal ions and clusters linked by organic units [17] through metal- ligand coordination bonds. Recently, MOFs are widely used in diverse technological and industrial applications in areas such as catalysis [18], sensors [19], proton conduction [20], gas storage, purification, separation [21,22] and sequestration [23,24] due to its outstanding structural properties. It also finds applications in emerging technologies such as drug delivery [25], light harvesting [26] and nanofluids [27]. Very recently Wu et al. [28] reported the enhanced performance of PEO based polymer electrolytes with the addition of MOF-derived nanoporous multifunctional fillers. The PEO-n-UIO solid electrolytes showed an increased conductivity by a factor of ∼37 to1.3 × 10−4 S cm−1 at 30 °C.

The lithium ion migration in a SPE is generally influenced by the environment of polymeric host which can be tailored by the added fillers. Numerous reports describe the electrochemical and interfacial properties of CPE with inorganic fillers [29]. However, reports on CPE with MOFs having different organic ligands are scanty. Therefore, in the present work the influence of organic ligands that present in Mg-TPA and Mg-TMA on the electrochemical and interfacial properties of PEO containing LiTFSI as salt is studied and compared.

Section snippets

Materials

Materials PEO (Mw = 3 × 105, Aldrich, USA) and lithium bis(trifluoromethane)sulfonimide, LiTFSI (Aldrich, USA), were dried under vacuum for 48 h at 45 and 90 °C respectively. The MOF magnesium 1,4-benzenedicarboxylic acid (Terephthalic acid- TPA) and magnesium 1,3,5-benzene tricarboxylic acid (Trimesic acid- TMA) were also vacuum dried at 80 °C for 3 days prior to use. The synthesis and characterization of MOFs are discussed in the supporting information (SI-1).

Preparation of composite polymer electrolyte

Composite polymer electrolytes

Results and discussion

Fig. 1 (a and b) depicts the surface morphology of composite polymer electrolyte (Sample S4) containing Mg-TPA and Mg-TMA respectively. The SEM images show a smooth surface morphology and a uniform distribution of MOF particles throughout the membrane. However, small pores are also observed on its surface and are attributed to non- uniform evaporation of solvent at the time of casting the membrane [33]. The size distributions of MOFs were calculated by considering the diameter of more than 100

Conclusions

PEO based composite polymer electrolytes were prepared by the incorporation of Mg-TPA and Mg-TMA MOFs independently as filler and their electrochemical and interfacial properties were analysed. The CPE containing MOF filler (Mg-TPA/Mg-TMA) and LiTFSI salt above 10 wt% was found to be optimal in terms of ionic conductivity and compatibility points of view. Comparing the ionic conductivity of samples, Mg-TPA (Sample S4) offered highest ionic conductivity and was attributed to the lesser amount of

Acknowledgement

The authors gratefully acknowledge the UGC, New Delhi for the financial support.

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