Understanding Different Battery Chemistry

Lithium-ion cells , in their most common form, consist of a graphite anode, a lithium metal oxide cathode, and an electrolyte of a lithium salt and an organic solvent. Lithium is a good choice for an electrochemical cell due to its large standard electrode potential (–3.04 V) resulting in a high operating voltage (which helps both power and energy) and the fact that it is the metal with the lowest density (which reduces weight).

The construction of a typical cylindrical cell is shown in Figure 2, while Figure 3 shows a typical pouch cell. Such cells provide a relatively light and small source of energy and are now manufactured in very large quantities (>1 billion cells per year) (2). In an automotive application, a lithium-ion battery consists of tens to thousands of individual cells packaged together to provide the required voltage, power, and energy. Individual cells are normally mounted into a number of modules, which are then assembled into the complete battery pack as shown in Figure 4.

Fig. 3.Internal construction of a typical pouch cell

Many countries have now put in place binding carbon dioxide emissions targets for cars, for example in Europe the requirements are for fleet average CO₂ emissions of 130 g km^–1 by 2015 and 95 g km^–1 by 2021 (3). It will be shown in Section 2 of this paper that by using a (lithium-ion) battery it is possible to significantly reduce a car's CO₂ emissions. More lithium-ion batteries are now being used in automotive applications for this reason.

The structure of this paper is as follows. In Section 2 a number of automotive passenger car applications for lithium-ion batteries are presented and their key requirements listed. Section 3 will give a brief overview of the capabilities of a number of lithium-ion chemistries currently in use for automotive applications and Section 4 will compare the requirements (from Section 2) with the capabilities listed in Section 3. Section 5 will look at future developments, while Section 6 will offer some conclusions.

There is a range of applications for batteries in passenger cars (4). The ones that will be considered here were selected either because they already use lithium-ion batteries or because they could potentially do so in the future. Note that there are a number of standard automotive requirements that all lithium-ion batteries used in cars need to meet: these include life (8–15 years are typical requirements), temperature range (–40°C to at least +60°C, ideally 80°C), and vibration resistance (at least 4.5 root-mean-square-acceleration (grams)) (5). Each application will now be briefly described.