Elsevier

Polymer

Volume 42, Issue 9, April 2001, Pages 4181-4188
Polymer

Novel interpenetrating polymer network electrolytes

https://doi.org/10.1016/S0032-3861(00)00818-1Get rights and content

Abstract

A new type of solid polymer electrolyte (SPE) was prepared by sequential interpenetration of cross-linked methoxyoligo (oxyethylene) methacrylate (MOEnM) and poly(methyl methacrylate) (PMMA). When the interpenetrating polymer networks (IPNs) were swollen with liquid electrolyte solutions, they showed structures that were between the chemically cross-linked gel SPEs and the porous SPE. They could swell/hold much more electrolyte solution than the porous SPE. The electrolytes exhibited ionic conductivities in the order of 10−3 S cm−1 at 25°C, and could be useful in many practical electrochemical devices.

Introduction

Since the discovery by Wright [1] of ionic conduction in polymers containing inorganic salts, and the suggestion by Armand [2] in 1978 that polymer ionic conductors could be used as electrolytes in practical electrochemical devices, much effort has been dedicated to exploring solid polymer electrolytes (SPE) [3], [4], [5] for their practical application. Among the various potential applications, the use of SPE in lithium batteries has been most widely studied and is probably the most important and promising application of SPE. In a lithium secondary battery, a polymer electrolyte will function as a separator as well as an electrolyte. The requirements for polymer electrolytes in lithium secondary batteries are, therefore, good mechanical strength (for their function as separators) and favorable electrochemical properties including high ionic conductivity and interfacial compatibility (for their function as electrolytes).

Initial work on polymer electrolytes was mostly based on the complexes of poly(ethylene oxide) (PEO) with inorganic lithium salts. This kind of polymer electrolyte is usually referred to as the first-generation SPE. However, the main drawback of this first-generation SPE is the high degree of crystallization of PEO. This restricts the application of the polymer electrolytes in lithium batteries to temperatures above the melting points of the crystalline PEO, normally around 60°C. At temperatures below the melting points, the conductivities of the polymer electrolytes are just too low to be of practical use, owing to the low segmental movement of polymer chains that restricts the mobility of Li+ ions.

One of the approaches to overcome the above drawback is to use low molecular weight aprotic plasticizers having high dielectric constant (ε) and low vapor pressure such as propylene carbonate (PC) (ε=64.4) and ethylene carbonate (EC) (ε=89.6) [6]. The polymer electrolytes with these plasticizers are known as the second-generation SPE. The plasticizers impart salt-solvating power and high ion mobility to the polymer electrolytes. However, the use of plasticizers tends to decrease the mechanical strength of the electrolytes, particularly at a high degree of plasticization. At the same time, polar solvents are generally reactive towards lithium electrodes, causing lithium batteries with such polymer electrolytes to be plagued often by electrode/electrolyte incompatibilities.

Composite polymer electrolytes with inorganic fillers can be considered the third-generation SPE. Inorganic fillers are used to improve the electrochemical and mechanical characteristics of gel polymer electrolytes (the second-generation SPE) [7], [8], [9], [10], [11], [12]. This approach, however, is rather limited because of the small number of inorganic fillers that can be used.

Another approach towards producing polymer electrolytes that have high ionic conductivity (like the gel SPE) and strong mechanical strength would be to use polymers in modified forms. Such modified polymers as copolymers, cross-linked polymers [13], [14], [15] and polymer blends [16], [17], have shown great potential in overcoming the drawbacks of polymer electrolytes mentioned above.

An interpenetrating polymer network (IPN) is a special kind of polymer blend. An IPN can be defined as a combination of two polymers in network form, one of which is synthesized and/or cross-linked in the immediate presence of the other [18], [19], [20]. It can be distinguished from other types of polymer blends in two ways: (i) an IPN swells, but does not dissolve in solvents; and (ii) creep and flow are suppressed in an IPN [21]. An IPN may be made from (i) polymer I, which can complex or hold lithium salts effectively and thus exhibits high ionic conductivity, and (ii) polymer II, which is mechanically and electrochemically stable to lithium electrodes. Such an IPN system would, in principle, make an ideal polymer electrolyte that not only satisfies the dual-function requirement for its use in secondary lithium batteries, but also accommodates the thermal and volumetric changes occasioned by cycling of the lithium anode.

Poly(methyl methacrylate) (PMMA) has been blended with PEO to impart good adhesiveness to solid electrolytes while making them stable to atmospheric moisture [14], [22], [23]. Scrosati and co-workers [24] showed that PMMA-based electrolytes were either less reactive towards lithium electrode, or able to induce a more favorable passivation film on the electrode surface. Therefore, it is reasonable to consider PMMA as one of the two polymers in an IPN electrolyte system. The present paper reports the synthesis and ionic conductivity study of an IPN electrolyte composed of cross-linked methoxyoligo (oxyethylene) methacrylate (abbreviated as Cr-MOEnM for convenience, where n represents the number of –CH2CH2O– units), referred to as polymer I, and PMMA, referred to as polymer II.

Section snippets

Materials

Oligo (ethylene glycol) monomethyl ethers (HO–(CH2CH2O)n–CH3) with average molecular weights of 350, 550 and 750 (i.e. n=8, 12 and 16, respectively) were obtained from Fluka. They were azeotropically distilled with benzene to remove residual moisture before use. Methacryloyl chloride (from TCI) was distilled under nitrogen atmosphere to remove the inhibitor immediately before use. Triethylamine (J.T. Baker) was purified by refluxing with anhydrous potassium hydroxide followed by distillation

Synthesis of IPNs

The gel fraction values of cross-linked MOEnM networks and Cr-MOEnM/PMMA IPNs are shown in Table 1. The data show that the gel fraction values of cross-linked MOEnM networks are all above 95%. It is worth noting that the soluble fraction in the polymer networks can affect not only the extent to which the networks may be swollen (with the mixture consisting of MMA, 5 wt% of 1,4-butanediol dimethacrylate (cross-linker) and 1 mol% AIBN (initiator)), but also the mechanical strength and ionic

Conclusions

A new type of cross-linked methoxyoligo (oxyethylene) methacrylate (Cr-MOEnM)/PMMA interpenetrating polymer network (IPN) electrolyte was synthesized by swelling the IPNs with liquid electrolyte solutions. Thermal analysis showed that interpenetration eliminated the crystallization in Cr-MOEnM. The IPNs with 35 wt% PMMA and above had two Tgs, showing that phase separation had taken place in them. The morphology of the IPNs was studied with SEM, confirming the DSC results. It is also shown that

Acknowledgements

The authors are grateful to the National University of Singapore for a research grant for this work.

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