Free-standing polydimethylsiloxane-based cross-linked network solid polymer electrolytes for future lithium ion battery applications

Highlights

• Network solid polymer electrolytes based on polydimethylsiloxane were fabricated.

• Polydimethylsiloxane usage enhanced ionic conductivity of the electrolytes.

• Mechanical and thermal properties were improved by the network structures.

• The solid electrolytes showed favorable electrochemical properties for batteries.

Abstract

A series of cross-linked network solid polymer electrolytes based on dual end-functionalized polydimethylsiloxane bearing reactive thiol terminal groups with poly(ethylene glycol) diglycidyl ether and pentaerythritol tetrakis(3-mercaptopropionate) have been synthesized and characterized. The obtained results can be briefed as: ionic conductivity in the range of 1.5 × 10⁻⁶ S cm⁻¹ at room temperature to 1.6 × 10⁻⁴ S cm⁻¹ at 90 °C, lithium ion transference number ranged from 0.15 to 0.20, electrochemical window up to 5.3 V (vs. Li⁺/Li), thermal resistance (T_d5 ca. 254.5 °C), and tensile strength (ca. 1.6 MPa at max.). The interplay taking place between structure and physico-electrochemical properties of in these systems based on merits of poly(ethylene glycol) moieties as high solvation matrix for Li⁺ ions flexible along with smooth solid support from polydimethylsiloxane allow the tailoring of the design of polymer electrolytes for future all-solid-state lithium batteries.

Introduction

For the past several decades, both academic and industrial researches have stimulated by the search for safer and lower-cost batteries [[1], [2], [3]]. In the present time, rechargeable lithium ion batteries (LIBs) are the key components in of a variety of portable electronic devices such as watches, calculators, digital cameras, computers, laptops, mobile phones, or for implantable medical devices; and hence, largely dominate the consumer electronics [[4], [5], [6]]. LIBs are attractive because they offer the high energy density [1] based on the lowest electrochemical potential (−3.04 V vs. standard hydrogen electrode) and lightest Li metal (equivalent weight M = 6.94 g mol⁻¹, and specific gravity ρ = 0.53 g cm⁻³), flexible and lightweight design, and longer lifespan than comparable present battery technologies [[7], [8], [9]]. Despite the impressive potential as the energy storage device, the lithium ion batteries are often criticized for their slow advancement owing to the performance decay, the safety issues of inflammable liquid electrolytes, and the fabrication costs [[10], [11], [12], [13]].

As one of the critical components, electrolytes are very important for preparing safe and high-performance LIBs. In 1973, the groundbreaking article on ion conduction properties of poly(ethylene glycol) (PEG) by Wright and co-workers [14] opened up the possibility of solid-state polymer electrolytes (SPEs) [[15], [16], [17], [18]]. SPEs provide an important alternative to conventional liquid electrolytes by enhancing the mechanical integrity whilst retaining the high electrochemical performance [[19], [20], [21]]. Although various polymer-based SPEs have been examined, [[22], [23], [24]] PEG remains one of the best lithium ion conductors in these SPEs. Therefore, PEG and PEG-based electrolytes have been widely used in energy conversion devices such as dye-sensitized solar cells, electrochemical capacitors, and secondary batteries [25,26]. However, several issues, including its crystallinity, low ionic conductivity, and low lithium ion transference numbers, have to be solved although PEG-based solid electrolytes offer the safety, no leakage of electrolyte, high thermal resistance, and mechanical robustness over conventional liquid electrolytes [27,28].

It has been reported that SPEs with lower glass transition temperatures (T_g) have better ionic conductive characteristics [[29], [30], [31]]. One of such interesting and promising materials is poly(dimethylsiloxane) (PDMS) that has a very low T_g of −125 °C. PDMS is a well-known silicon-based inorganic polymer that has unique rheological (or flow) characteristics; and therefore, PDMS has been popularly used for a large variety of industrial applications [32,33]. The high thermal and mechanical stability of PDMS make it an intriguing candidate for a large number of applications ranging from medical devices to architectural components [34,35]. As an electrolyte material, PDMS has been attracted attention with its high Li⁺ ion transport properties due to the high flexibility of the polymer chains and large free volume [[36], [37], [38], [39], [40]]. Furthermore, the dimensional stability of these polymer electrolytes can also be enhanced by crosslinking or combining other components.

These above-mentioned unique features of PDMS led us to the study of cross-linked network polymers comprising of PDMS with flexible chains attached to ethylene glycol units and a small cross-linker (Fig. 1). It is envisaged that this unique chemical platform can provide higher lithium ions conduction owing to the flexible PDMS units compared to the PEG units. In addition, the lower interaction between lithium cations and the PDMS units than the PEG units, which strongly coordinate to the lithium cations, presumably contributes higher lithium transference numbers. Furthermore, the incorporation of PDMS in the network structures entirely disrupts the crystallization of the PEG moieties. In this study, mercapto-terminated PDMS, PEG with epoxy terminal groups (PEGDGE: poly(ethylene glycol) diglycidyl ether), and a tetra-functional thiol cross-linker (PEMP: pentaerythritol tetrakis (3-mercaptopropionate)) were selected to provide cross-linked network structures via the highly efficient “thiol-epoxy” click chemistry [41]. Overall, the present polymer network system serves as a good competing electrolyte coupled with notable mechanical integrity in the form of free-standing solid membranes for LIBs. The thermal, mechanical and electrochemical properties of resultant cross-linked network polymers were investigated. Most importantly, an effective integrated cross-linked network polymer electrolyte is proposed here based on the interplay between network structure, and their electrochemical, thermal and mechanical properties.