Li-Ion Batteries

LITERATURE PROJECT CHEM3004

LITHIUM-ION BATTERIES

Abstract

Technology is striving to keep up with today's society. We continuously require smaller, lighter and more autonomous equipment. The rechargeable lithium ion battery is the most important factor in determining the rate of development of modern day consumer electronics. Perhaps not surprisingly the transport of lithium ions (Li+) remains the preferred technology to achieve this. As a result and for the purpose of this review, only the Li-ion battery (rather than the Li-metal battery) will be considered. This contribution is interested in presenting the issues and challenges this technology has and will encounter. Given the huge volume of literature published on this subject, this report cannot aim to cover everything, for example, experimentals are not a point of focus. As a result, an introduction describing the way these batteries generically function will be presented and thereof the individual components of a Li-ion battery will be considered; the positive and negative electrodes and the electrolyte. Additionally the importance of this field of research given today's world will be described alongside some crucial applications. Whatever the measures for the performance of a battery, they will always be related to the inherent properties of the materials used to construct it. As an example, the life of a cell is dependent upon the interaction of the electrodes and the electrolyte and the safety is a measure of the stability of the electrode materials and interfaces. Traditionally, and bearing in mind that only Li-ion batteries are considered, the positive electrode must act as source of lithium ions, requiring the used of air-stable Li-based intercalation compounds . These are used because the lithium can reversibly de-intercalate at high potential differences. The first commercial Li-ion batteries, introduced in 1990 by the Sony Corporation, used LiCoO2. In fact, this compound is still used in more than 90% of lithium-ion batteries . Since then a lot of research has been done to circumvent the safety and capacity issues. Different transition metal elements have been used as dopants such as chromium, and spinels such as LiMnO4 have also been used. The field has been given a real boost when considering using conductive oxide aerogels (e.g. V2O5) and also atmospheric oxygen as an active cathode material. Perhaps receiving slightly less attention are the anode materials since the already existing carbon anode has been reliable. Research effort has been focused on finding carbon alternatives hoping to find materials with larger capacities and more intercalating voltages in comparison to Li/Li+ to increase the safety1. The main point of focus in this paper will be the chemistry of the polymer electrolyte. Whilst the role of the electrolyte is often perceived as minor, its choice is in fact crucial. It depends on whether the battery is liquid or polymer-based1, . Liquid electrolytes have been the obvious choice and are still used in the abundant "rocking-chair" type battery due to their high ionic conductivity. However, parallel research in solid electrolytes based on organic polymer chains has shown that they can provide an excellent support for lithium ion transport. The mechanisms for this transport are only being fully understood recently with the advancement in the techniques used to characterise them.

Table of Contents

Introduction 5

The "rocking-chair" battery 6

Positive Electrodes 8

Negative Electrodes 17

Electrolytes 23

Liquid electrolytes 23

Polymer Electrolytes 25

Conclusion 39

Introduction

In this report it is the rechargeable lithium ion battery that is of interest. Already by 1992, primary lithium-ion batteries occupied an established role in the market whilst not even one rechargeable lithium ion battery had reached a wide commercial market . In 1997, it was predicted that the market share of rechargeable lithium-ion batteries would be 10% by 2000 (Figure 1).

Figure 1: Market projections for consumer electronics batteries in 1997 (ref 27).

In fact, by 2001, the rechargeable lithium ion battery occupied a 63% share in worldwide sales of all batteries which only re-enforces the demand for such a technology1. This illustrates the demand and the extensive development for such power sources.

Figure 2 illustrates some of the reasons why.

Figure 2: Comparison of the different battery technologies in terms of volumetric and gravimetric energy density in 2001 (ref 1).

Global warming and the end of fossil fuels looming conspire to present the greatest threat to today's world. A realistic solution to curb the use of fossil fuels is the use of clean energy sources. Transportation accounts for 30% of CO2 emissions, it follows then that the use of batteries to power such fields would be a great benefit to the planet. Already used in consumer electronics such as laptops and mobile phones, the rechargeable lithium ion battery offers the highest energy storage per unit weight and volume (Figure 2) and therefore the most appealing for use in applications such as electric vehicles48.

The "rocking-chair" battery

The rocking-chair battery is more commonly known as the rechargeable lithium-ion battery. As it will be described in this report, the materials used to constitute these cells have been extensively researched and have known many variations and changes over time.

Figure 3 depicts