Methacrylic-based solid polymer electrolyte membranes for lithium-based batteries by a rapid UV-curing process
Introduction
Nowadays, Li-ion batteries are compact and lightweight rechargeable power sources, stable to over 1000 cycles, operating at about 4 V with energy densities ranging between 200 and 300 Wh kg−1 [1]. The lithium-based battery technology is continuously evolving and has gained an unprecedented significance since the demand for portable electronic devices, such as cellular phones, notebooks and Mp3 players, has been continuously increasing [2], [3], [4]. In the last few years, Li-based polymer batteries have been identified as a very promising technology to meet the requirements of upcoming applications, such as standby power and electric vehicles; much work has been done to discard the unsafe, conventional liquid organic electrolyte solutions made of a lithium salt (e.g., LiClO4 or LiPF6) dissolved in organic carbonates. In fact, inert and solvent-free lithium ions conducting membranes are ideally needed for the development of all-solid state batteries [5], [6], [7], [8]. The benefits are substantial: besides excellent processability and flexibility, switching to a fully solid configuration gives concrete promise of higher safety, due to the absence of flammable organic solvents, possible prevention of short circuits due to the growth of lithium dendrite crystals upon cycling, large modularity in design and ease of handling [9].
Solid polymer electrolyte (SPE) membranes formed by the dissolution of suitable lithium salts in suitable polymer matrixes were first studied by Wright [10], but it was the work of Armand et al. [11] that focussed on polyethers, namely poly(ethylene oxide) (PEO) which has been the subject of lithium-based electrolyte research for more than a decade. SPEs hold a potential to be used in high energy density batteries, to replace the liquid and gel-based electrolytes of today and boost the performances even further [5], [6], [12], [13]. They serve two principal roles in rechargeable Li-based batteries: not only do they function as the traditional electrolyte (i.e., the medium for ionic transport), but also as the separator which insulates the positive electrode from the negative. Consequently, they are requested to have certain level of physical properties (e.g., sufficient mechanical strength) in order to withstand the electrode stack pressure and stresses caused by dimensional changes which the rechargeable electrodes undergo during charge/discharge cycling, as well as elasticity and thermal stability [14], [15].
At present, the main limits of SPEs are the low ionic conductivity at ambient temperature and the low Li+ transference number under equilibrium conditions, which is due to the low segmental mobility of the polymer chains complexed with the lithium salt. Typically, the conductivity of bulk polymeric systems used nowadays, independently of the chemical nature of the polymer matrix, remains in the range of 10−7–10−5 S cm−1 at ambient temperature, whereas a conductivity of the order of 10−3 S cm−1 or higher is considered as the target for a commercial applicability: appreciable conductivities can be achieved only at temperatures above 70 °C [16], [17], [18], [19]. However, if applications of SPEs in electrochemical devices are remote, excluding the automotive fields where temperature is not a crucial parameter, the concept of a fully solid lithium metal polymer battery is still very appealing and is presently being considered in industrial laboratories involved in electric transportation [9]. Therefore, the study of novel yet efficient solid polymer electrolytes is important. Another drawback of present polymer electrolytes is the time requested for preparing the membrane, while fast, easy and efficient processing in batteries production are key factors for their development.
In this respect, the present work describes the preparation of SPEs by free radical photo-polymerisation (UV-curing) of multi-functional methacrylic oligomers containing polyethylene oxide chains, in the presence of a lithium salt. The process is fast and takes few seconds for producing fully-solid polymer electrolyte membranes without the use of solvents and volatiles [20], [21], [22], [23], [24]. The process is also versatile: in this paper we show that by appropriately choosing the oligomers to modulate the concentration of ethoxy units and changing the average methacrylic functionality of the systems one can vary the SPE performances and guarantee an appreciable ionic conductivity along with a wide electrochemical stability window. This approach could lead to solid polymer electrolytes which can be applied for high temperature applications.
Section snippets
Experimental section
Unless otherwise stated, all starting materials and reagents were purchased from commercial suppliers and used without any further purification. Experiments were repeated two to three times and were found to be reproducible. All samples were stored in ambient laboratory conditions.
Characterisation of the prepared membranes
Different kinds of variables (i.e., reactants ratio, their molecular weight, chain length, lithium salt concentration) were considered in the different stages of this work. Firstly, various salt-free solid polymer membranes (SPMs) were prepared by changing the ratio of the di-functional methacrylic oligomer BEMA to the mono-functional PEGMA and changing the molecular weight of PEGMA. Table 1 shows the composition of the various salt-free solid polymer membranes. Sample SPM-1 is made of BEMA:
Conclusions
Solid polymer electrolyte membranes were prepared by a solvent-free rapid photo-polymerisation procedure and thoroughly characterised in terms of kinetics, thermal and morphological properties, ionic conductivity and electrochemical stability. The reported findings show that lithium salt doped highly cross-linked methacrylates can be used as solid polymer electrolytes in lithium-based rechargeable batteries for high temperature applications. Their ionic conductivity (>10−4 S cm−1) at above 60 °C
References (42)
- et al.
Polymer
(2006) - et al.
Polymer
(2000) - et al.
J. Power Sources
(2010) - et al.
Polymer
(2004) - et al.
Polymer
(2005) Solid State Ion.
(1994)- et al.
J. Power Sources
(1999) - et al.
J. Power Sources
(2008) - et al.
Electrochem. Commun.
(2009) - et al.
J. Power Sources
(2010)
Electrochim. Acta
Polymer
Polymer
Polymer
Electrochim. Acta
Electrochim. Acta
Polymer
Solid Polymer Electrolytes: Fundamentals and Technological Applications
Macromolecules
Cited by (69)
Roles of metal element substitutions from the bimetallic solid state electrolytes in lithium batteries
2022, ParticuologyCitation Excerpt :The crystal structures of ferroelectric materials can realize spontaneous polarization, greatly improving the ionic conductivity of the composite electrolytes (Itoh et al., 2003; Sun et al., 2000). The migrations of Li+ in the polymer can be achieved along the molecular chain and in the amorphous phase (Nair et al., 2011; Robitaille & Fauteux, 1986). The inorganic fillers effectively weakened the interaction between the polymer chains, which increased the amount of amorphous region and enhanced segmental dynamics.
Chitosan as a paradigm for biopolymer electrolytes in solid-state dye-sensitised solar cells
2021, PolymerCitation Excerpt :Thus, intermolecular interactions between these functional groups and alkali metal ions are created, leading to the solvation of salt and mimicking a solid-state version of a LE system. To avoid the use of synthetic polymers, which are not biodegradable and derive from production processes that could not be seen as compatible with the sustainable requirements of DSSCs, the researchers started to explore materials from the agriculture industry when producing novel polymer electrolytes, such as cellulose [25], agar [26,27], starch [28], alginate [29], carrageenan [30] and chitosan [31]. The use of these biopolymeric materials contributes to minimizing environmental issues, as well as lowering the cost of solar cells [32].