Solid-state Graft Copolymer Electrolytes for Lithium Battery Applications

Summary

Lithium ion batteries employ flammable and volatile organic electrolytes that are suitable for ambient temperature applications. A safer alternative to organic electrolytes are solid polymer batteries. Solid polymer batteries operate safely at high temperatures (>120 °C), thus making them applicable to high temperature applications such as deep oil drilling and hybrid electric vehicles. This paper will discuss (a) the polymer synthesis, (b) the polymer conduction mechanism, and (c) provide temperature cycling for both solid polymer and organic electrolytes.

Abstract

Battery safety has been a very important research area over the past decade. Commercially available lithium ion batteries employ low flash point (<80 °C), flammable, and volatile organic electrolytes. These organic based electrolyte systems are viable at ambient temperatures, but require a cooling system to ensure that temperatures do not exceed 80 °C. These cooling systems tend to increase battery costs and can malfunction which can lead to battery malfunction and explosions, thus endangering human life. Increases in petroleum prices lead to a huge demand for safe, electric hybrid vehicles that are more economically viable to operate as oil prices continue to rise. Existing organic based electrolytes used in lithium ion batteries are not applicable to high temperature automotive applications. A safer alternative to organic electrolytes is solid polymer electrolytes. This work will highlight the synthesis for a graft copolymer electrolyte (GCE) poly(oxyethylene) methacrylate (POEM) to a block with a lower glass transition temperature (T_g) poly(oxyethylene) acrylate (POEA). The conduction mechanism has been discussed and it has been demonstrated the relationship between polymer segmental motion and ionic conductivity indeed has a Vogel-Tammann-Fulcher (VTF) dependence. Batteries containing commercially available LP30 organic (LiPF₆ in ethylene carbonate (EC):dimethyl carbonate (DMC) at a 1:1 ratio) and GCE were cycled at ambient temperature. It was found that at ambient temperature, the batteries containing GCE showed a greater overpotential when compared to LP30 electrolyte. However at temperatures greater than 60 °C, the GCE cell exhibited much lower overpotential due to fast polymer electrolyte conductivity and nearly the full theoretical specific capacity of 170 mAh/g was accessed.

Introduction

Lithium (Li) is a highly electropositive metal (-3.04 V relative to standard hydrogen electrode), and the lightest metal (equivalent weight of 6.94 g/mol and specific gravity of 0.53 g/cm³). This makes it attractive as a choice for the active material in the negative electrode and ideal for portable energy storage devices where size and weight matter. Figure 1 shows that lithium-based batteries (Li ion, PLiON, and Li metal) have higher energy densities than lead-acid, nickel-cadmium, and nickel-metal-hydride batteries ¹.

A full lithium-ion battery consists of a cathode (positive), an anode (negative), ....

Protocol

1. Graft Copolymer Synthesis^18-19

1. Synthesize the graft copolymers (POEM-g-PDMS and POEA-g-PDMS at 70:30 weight ratio) using a free radical polymerization approach by mixing 26 ml of POEM (or POEA) monomers (Figure 3), 12 ml of PDMS macromonomers, and 12 mg of 2,2'-Azobis(2-methylpropionitrile) (AIBN) (monomer:initiator [825:1]) in 160 ml of EA.
2. Seal the flask containing the clear solution with a rubber septum and purge with ultra high purity argon for 45 min.
3. Heat the solution to 72 °C (AIBN has a decomposition rate of 3.2 x 10^-5 sec^-1) in an oil bath under constant stirring for 24 h....

Results

The room temperature cell cycling performance is shown in Figure 8. The left plot shows the charge and discharge profiles of cells with conventional liquid electrolyte (LP30) at 15 mA/g, and GCE/binder at 10 mA/g. Figure 9 shows the discharge voltage profiles of the solid polymer cells at room temperature, 60 °C, and 120 °C using a low current of 0.05 C. The discharge voltage profiles as functions of specific capacity are shown in Figure 10,where the discharge current is.......

Discussion

The LiFePO₄/GCE/Li curves show greater overpotential than the LiFePO₄/LP30/Li curves on both charge and discharge. Since the GCE is used as both electrolyte and binder, ion conduction is provided to all of the cathode particles, and nearly the entire practical specific capacity (150 mAh/g) was accessible. The theoretical specific capacity of 170 mAh/g is not achieved since it is limited by lithium diffusion within LiFePO₄ particles, which is low at room temperatures. The cycling capacitie.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
POEM	Sigma Aldrich	26915-72-0
POEA	Sigma Aldrich	32171-39-4
LiTFSI	Sigma Aldrich	90076-65-6
AIBN	Sigma Aldrich	78-67-1
EA	Sigma Aldrich	141-78-6
THF	Sigma Aldrich	109-99-9
PDMS	Gelest	146632-07-7
Argon Gas	Air Gas		Ultra high purity (Grade 5)
PE	Sigma Aldrich	8032-32-4
LiFePO4	Gelon
Carbon black	SuperP		Super P
Lithium metal	Alfa Aesar	7439-93-2
PVDF binder resin	Kynar		Kynar
PVDF Separator	Celgard
LP30	Merck		LiPF6 in EC:DMC
MACCOR battery tester	MACCOR
El-Cut	EL-CELL