Rechargeable Batteries

Batteries are electrochemical devices that convert chemical energy into electrical energy. Their major components include an anode and a cathode that are separated by a non-conductive separator that will allow the flow of ions but not the flow of electrons, a case and an electrolyte (Figure 1). In battery terminology, the cathode is the electrode through which the electrons enter a cell and the anode is the electrode through which they leave the cell. When the battery is discharged, electrons move from the anode to the cathode as ions move from the cathode to the anode.

Batteries can be divided into two types – primary and secondary. In primary batteries, the chemical energy of its constituents is changed when the current is allowed to flow and this type cannot be recharged because the chemical reactions are irreversible.

In secondary batteries, the application of an electrical current brings about chemical changes which are reversed as the cell discharges. Such batteries can be recharged hundreds of times before degradation occurs.

Since the mid 1980’s, the reduced size and portable nature of electronic devices such as camcorders, portable telephones and laptop computers has generated enormous demand for high capacity, rechargeable batteries to power these devices. For instance, in the developing countries, there has been little interest in establishing a hard-wired communication infrastructure and portable telephones are being used to meet communications needs.

This trend has been particularly advantageous to cobalt in that the three high energy density batteries best suited to portable devices use substantial amounts of cobalt.

In the last few years, demand for portable rechargeable electronic devices has grown rapidly, such that the use of cobalt in these applications has more than doubled. Basically, there are three technologies which are in order of increasing cobalt content and growth opportunity:

•  Nickel-Cadmium (Ni-Cd)
•  Nickel-Metal Hydride (Ni-MH)
•   Lithium ion (Li-ion)

Figure 2 summarises the major applications where these batteries are used and the various materials used in each.

Nickel-Cadmium Batteries

In the late 1980s and early 1990s, Ni-Cd batteries were the most common rechargeable batteries used in portable electronic devices as a result of their low cost, ready availability and established technology. In Ni-Cd cells, cobalt is used only in the positive electrode (cathode) where it enhances performance.

The amount of cobalt used is usually about 1% by weight of the nickel hydroxide but can be up to 5% in high performance batteries. The cathode is either a nickel foam filled with spherical nickel hydroxide or a sintered nickel substrate impregnated with nickel hydroxide. Cobalt, in the form of fine powder, oxide or hydroxide, is used as additive in these electrodes for the following reasons:

• It drastically improves the conductivity of the nickel electrode
• It mechanically stabilises the electrode by inhibiting the formation of y-NiOOH and reduces the rigidity of the nickel hydroxide deposit
• It increases the potential for electrolyte decomposition

The ability to deliver high currents makes them particularly suitable for portable power tools. However, there are a number of problems associated with these batteries which means that little future growth is anticipated. The major problems are:

• The so-called memory effect whereby loss of battery capacity occurs as a result of recharging the battery before it is fully discharged
• Over-discharging which causes cells to develop internal short circuits and cause the battery to run down prematurely and eventually take no charge at all

The specific energy output is about 50% less than the nickel-metal hydride and lithium ion cells.

Nickel-Metal Hydride Batteries

The advent of Ni-MH rechargeable batteries can be attributed to Phillips Electronics in 1969 who were carrying out research to develop improved powerful magnets based on SmCo5. Related studies showed that the compound LaNi5 could store large amounts of hydrogen in a highly reversible manner at room temperature. The significance of this discovery led to their use as rechargeable battery negative electrodes. Since 1988, metal hydride (MH) the so-called “hydrogen storage alloys” have been commercialised in several applications.

Early alloys used were of the CaCu5 type, most notable LaNi5. Such alloys suffered from poor cycle life, internal cell pressure and corrosion as a result of the simple single-phase natures of these hydride alloys. The development of multi-component multiphase alloys overcame the problems. It was also found that the addition of cobalt to these rare-earth/Ni alloys substantially enhanced the cells’ cycle life. The addition of cobalt also tends to increase hydride thermodynamic stability and inhibits corrosion. Today’s alloys from the LaNi5 family are generally complex materials containing six to eight elements with complex phase structures.

Cobalt-containing alloys are of the type V-Ti-Zr-Ni and contain up to 15% cobalt. However, alloy development continues and new magnesium based hydrogen storage alloys are currently under development for a generation of cheaper lighter nickel-metal hydride cells.

The first Ni-MH batteries used the same nickel electrode as that of the positive electrode in Ni-Cd cells.

The addition of fine cobalt powder or cobalt oxide to the pasted nickel hydroxide electrode serves to provide some reserve capacity in these electrodes (to prevent gas evolution). The fine cobalt is oxidised to CoOOH during charge and remains in the cobalt(III) form during discharge thus providing reserve capacity to the MH electrode.

The combination of a rechargeable nickel electrode and a metal hydride electrode results in a battery system that is superior to Ni-Cd. It has greater specific energy – i.e. a lighter battery, greater volumetric energy density – i.e. a smaller battery with less environmental impact because of the absence of cadmium and it does not exhibit the memory effect which can reduce the life of Ni-Cd batteries.

Ni-MH batteries operate at 1.2 volts, the same as Ni-Cd types, but possess a much higher capacity. They are used extensively in portable computers, mobile phones and camcorders and have largely displaced Ni-Cd batteries in these applications.

Lithium-Ion Batteries

Rechargeable batteries based on a metallic lithium anode have many theoretical advantages over other systems but early designs failed commercially as a result of the reactivity of lithium metal which resulted in a number of battery fires. The problem has been overcome by replacing the lithium anode with a non-metal such as LiC6 which is capable of storing and reversibly exchanging a large quantity of lithium ions.

In this way, rather than lithium plating and stripping as in conventional lithium batteries, the electro-chemical process at the anode is the uptake of lithium ions during charge and their release during discharge. If the cathode is also non-metallic such as LiCoO2, capable of reversibly exchanging lithium ions, the entire battery process involves the shifting of lithium ions back and forth between electrodes. The lithium-ion rechargeable battery is also called the swing cell because of this action.

The Li-ion battery is the most advanced of the three systems. Unlike the 1.2V Ni electrode systems, Li-ion cells operate at about 3.7V and rely on lithium ions moving through organic solvents rather than protons in water to balance external charge transfer. A single lithium-ion cell replaces three Ni-Cd or Ni-MH cells in most applications. The much higher voltage and very light negative electrode (LiC6) explain why this is the most advanced rechargeable system and the one preferred for high power applications such as portable computers which often use more cells per device.

Of the three systems, the Li-ion battery contains by far the greatest amount of cobalt per cell. The cathode active material contains 60% cobalt rather than the 5-15% of Ni electrode cells and accounts for about 50% of the weight of the cathode.

LiCoO2 is the preferred materials but both LiNiO2 and LiMn2O4 can also be used. All three systems have advantages and disadvantages and all have been used in commercial applications.

Market

The use of cobalt in rechargeable batteries grew enormously between 1995 and 2000. Estimates suggest growth went from about 700 tonnes/annum to nearly 5,000 tonnes/annum during the period. However, the severe worldwide economic recession beginning towards the end of 2001 resulted in a massive drop in demand in the telecommunications industry. The fall in demand for portable telephones resulted in up to a 20% fall in the demand for cobalt in these applications in 2002.

The surge in demand for portable telephones, particularly in China, in the last two years has resulted in a massive increase in cobalt demand. On the other hand, the dramatic increase in the cobalt price since late 2003 has strengthened efforts to substitute with alternative materials. Latest industry predictions indicate that many of the disadvantages of alternative materials have been overcome and although rechargeable battery demand is expected to increase rapidly in the next few years, cobalt demand in this application could remain stable or even decrease slightly.

However, the emerging use of cobalt in rechargeable batteries for electric vehicle applications is expected to increase cobalt demand dramatically over the next ten years.