Normal batteries like the AA — and AAA-size ones we use everyday are sandwiches of three materials made into long sheets, as shown in Fig. 3.55a. The anode and cathode materials are separated by a thin insulating sheet, and all three are made as thin as possible and rolled up tightly to fit the largest area into the smallest space. The anode and cathode materials have a chemical potential between them such that the anode is negative and the cathode is positive. They are connected to the contacts at the bottom and top of the battery, respectively. When a light bulb is connected to the contacts, an electric current flows, lighting the bulb, and discharging the built-up charges between the sheets. The chemical potential sets the voltage of the battery, typically 1.5 V, and the area of the sheets determines how much charge they can hold, and therefore the “life” of the battery. Most batteries are not rechargeable.

Lithium-ion batteries are rechargeable. How they work is illustrated in Fig. 3.55b, where the anode and cathode layers are represented by shelves holding Li ions. The anode material is usually graphite (loosely packed carbon) holding some positive lithium ions. The cathode can be made of any of a number of materials, including proprietary ones, which largely determine the performance of the battery. Before the two electrodes are connected together, the chemical potential between them

Fig. 3.55 (a) Construction of a battery; (b) Layers of a lithium-ion battery [33] draws the lithium ions from the anode to the cathode until the extra positive charge added to the cathode cancels out the chemical potential. The ions travel through an electrolyte, which is a conducting liquid like salt water, only thicker. It is the gooey stuff that leaks out of an old battery. A thin plastic sheet, the separator, prevents the electrodes from touching each other. The separator is thin enough to allow the ions to pass through. A short circuit develops if there is a hole in the separator. Now if the battery is connected to a load, electrons which are attracted by the extra positive charge on the cathode can flow through the load to do useful work. As shown, the electric current is in the opposite direction to the electron motion because the elec­trons carry negative charge. To recharge, a negative voltage is applied to the anode to draw the lithium ions back. This is what takes hours. A large battery pack could consist of 100 cells, each 5 cm in diameter and 20 cm long (4 x 8 in.), divided up into modules so that overheating in one module does not spread to others.

As for cathode materials, cobalt-containing compounds such as cobalt dioxide have high-energy density and are commonly used for small Li-I cells, but they are not suitable for cars because of a tendency toward thermal runaway. The best found for cars so far is iron phosphate, which is more stable and less likely to overheat. It gives lower voltage, so that chains of batteries have to be longer to provide a high output voltage. Higher power and longer life are claimed if the cathode is made with nano-sized divots to increase surface area [33]. More on this will come in the next section. The race to make the best iron phosphate battery has already led to patent fights among battery companies.

The long charging times for Li-I batteries have been overcome by Ceder et al. [34] working with LiFePO4 (lithium-iron-phosphate) cathode material.

A123 Systems, a company started in Boston, has expanded into a $91M business in Asia using this material in small batteries for power tools and hobbyists.59 Employing techniques from ultracapacitors (next section), Ceder et al. form the cathode in such a way that it has large surface area with channels aligned so that Li ions can get in and out of the cathode rapidly. In small samples, discharge times of the order of seconds were observed, more than ten times faster than normal. Critics, including J. Goodenough, an inventor of LiFePO4 cathodes, doubted that charging times could be as short as discharging times.60 However, Ceder claims that the rates are for both charging and discharging. If we accept that, there is still a problem with charging a car, even a hybrid, in 10 minutes. It requires a lot of power. A plug-in hybrid using 0.24 kWh/mile can go 40 miles (64 km) on about 10 kWh of electric­ity. To put that much energy into a battery in 10 minutes would require 60 kW of power, enough to run an office building. Charging at home would have to be sched­uled so that not everyone on a grid line plugs in at once. However, there is no need to charge that fast at home; overnight will do. Where fast charging is needed is in filling stations en route. To charge nine cars at once would require half a megawatt of power. Probably high-voltage lines and a small substation would be required at each “gas” station. Some people suggest that such stations should have large battery banks to store the energy slowly and continuously so that not so much instanta­neous power is needed. In any case, building the infrastructure to support electric cars is worthwhile for saving oil and cleaning up the environment. Ultimately, when oil runs out and fission and fusion plants generate most of the energy for transporta­tion, the electric grid will have to handle the power for all vehicles.