Characterization of composite electrolytes based on a hyperbranched polymer

doi:10.1016/S0378-7753(00)00442-0

Journal of Power Sources

Volume 90, Issue 1, September 2000, Pages 20-26

https://doi.org/10.1016/S0378-7753(00)00442-0 Get rights and content

Abstract

Composite polymer electrolytes composed of a hyperbranched poly[bis(triethylene glycol)benzoate] with terminal acetyl groups, LiN(CF₃SO₂)₂ as a lithium salt, and an inert ceramic filler such as α-LiAlO₂ or γ-LiAlO₂ were prepared by solvent casting method. Addition of an appropriate amount of the fillers to fully amorphous pristine polymer electrolytes led to an increase in ionic conductivities and lithium ion transference numbers. All composite polymer electrolytes exhibited good compatibility with a lithium metal electrode, and also, addition of fillers improved their mechanical performance. The α-LiAlO₂ filler was effective for improving the electrochemical compatibility with a lithium metal electrode, and the γ-LiAlO₂ filler was effective for enhancing the mechanical properties of the pristine polymer electrolytes.

Introduction

Application of polymer electrolytes to rechargeable lithium or lithium ion batteries nowadays has attracted much attention. Polymer electrolytes have been proposed for the realization of plastic-like, thin-layer type of batteries owing to their unique characteristics and promising performances [1], [2]. At the beginning of this area, most efforts were made to investigate lithium ion conducting polymer electrolytes based on a linear poly(ethylene oxide) (PEO) because of its easy coordination with alkali metal ions. However, such problems as the unsatisfied morphological characteristics and insufficient mechanical property of linear PEO polymer electrolytes in high-temperature regions have been proposed for practical applications of the polymer electrolytes. To overcome these problems, composite polymer electrolytes with inert ceramic fillers, initially explored by Weston and Steele [3], followed by many researchers, have been demonstrated as one of the effective candidates for fully solid lithium batteries, and recently it has been greatly developed [4], [5], [6]. Such composite polymer electrolytes showed improvements of interfacial stability with electrodes and mechanical properties and even an increase in lithium ionic conductivity.

It has been well established that, in solvent-free dry polymer electrolytes, the ionic conduction takes place predominantly in the amorphous region and the glass transition temperature of the polymer electrolytes might determine the ion mobility in the polymer to a great extent [7]. Recently, in addition to composite polymer electrolytes based on linear PEO, hyperbranched ion conductive polymeric materials have been prepared in order to find polymer electrolytes with novel physical and/or mechanical properties applicable for lithium batteries. One of interesting examples is a dendritic macromolecule with a large number of branching points, which results in a three-dimensional shape. Cloutet et al. [8], [9] has reported the stepwise synthesis of comburst macromolecules containing a central polystyrene layer surrounded by a PEO layer. Moreover, Hawker et al. [10] and us [11] have prepared hyperbranched poly(ethylene glycol) derivatives containing diethylene and triethylene glycols and 3,5-dioxybenzonate branching units. All these hyperbranched polymers were demonstrated to be useful as polymer electrolytes because they have glass transition temperatures below room temperature and do not show melting points, that is, no detrimental effect due to crystallization. Up to now, investigations have been focused mainly to thermal and electrochemical characterizations in these materials [10], [11], [12], [13], but not to their mechanical performances. For the purpose of practical applications of these hyperbranched polymer electrolytes for lithium batteries, it is also essential to improve their mechanical properties.

In this work, were investigated preparation of composite polymer electrolytes composed of hyperbranched poly[bis(triethylene glycol)benzoate] with terminal acetyl groups, LiN(CF₃SO₂)₂, and a lithium aluminate with alpha or gamma phase (α-LiAlO₂ or γ-LiAlO₂) in various particle sizes, and effect of ceramic fillers on thermal property, ion conduction, and electrochemical and mechanical performances in the composite polymer electrolytes.

Section snippets

Materials

A hyperbranched polymer, poly[bis(triethylene glycol)benzoate] with terminal acetyl groups, was prepared according to the method reported previously [11] and its chemical structure is shown below, where a unit inside the bracket represents the repeat unit of the polymer that contains nine oxygen atoms.

Molecular weight of the hyperbranched polymer was determined by gel permeation chromatography to be 25000. α-LiAlO₂ (10–20 nm in size) and γ-LiAlO₂ (0.1–0.2 μm in size), prepared by solid state

Bulk characteristics

Temperature dependence of the ionic conductivity for composite polymer electrolytes based on LiN(CF₃SO₂)₂ at a Li/O ratio of 1/9 was measured at α-LiAlO₂ (10–20 nm) contents of 5, 10, 20, 30, and 40 wt.% and at γ-LiAlO₂ (0.1–0.2 μm) contents of 10, 20, 30, and 40 wt.%, and the results are shown in Fig. 1, Fig. 2, respectively. The ionic conductivities of all samples increase in a monotonic fashion with increasing temperature and obey the Vogel–Tammann–Fulcher relationship proposed for amorphous

Conclusions

Both nanosized α-LiAlO₂ and γ-LiAlO₂ fillers improved mechanical properties of hyperbranched polymer electrolytes based on LiN(CF₃SO₂)₂. The γ-LiAlO₂ filler is more effective for an improvement of the mechanical strength compared to the α-LiAlO₂ one. However, the composite polymer electrolyte with the α-LiAlO₂ filler exhibited much better compatibility with a lithium metal electrode. Ceramic fillers could influence the conductivity of the fully amorphous pristine polymer electrolyte, and

Acknowledgements

We greatly acknowledge the financial support of Genesis Research Institute, Inc.

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¹: Visiting Professor at Mie University from Shanghai Institute of Ceramics, Chinese Academy of Sciences.

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Characterization of composite electrolytes based on a hyperbranched polymer

Abstract

Introduction

Section snippets

Materials

Bulk characteristics

Conclusions

Acknowledgements

Solid State Ionics

J. Power Sources

J. Power Sources

J. Electroanal. Chem.

Polymer

J. Electrochem. Soc.

Polym. Int.

J. Chem. Soc., Chem. Commun.