Electrical Conduction in Metals and Alloys (Electrical Properties of Materials) Part 4

Secondary Cells

Rechargeable batteries are called secondary batteries. The price per KW-h is spread over several use cycles, for example $5 per KW-h for a Nickel-Cadmium battery (see below). In principle, the chemical reaction which is used for providing energy can be reversed in these devices. Important parameters for rechargeable batteries are the specific charge given in A-h per kg, the attainable specific energy (in W-h/kg) and the efficiency which is in the neighborhood of about 50 to 80% due to heat and chemical losses. Assuming an efficiency per charge cycle of 99%, the available stored energy after 100 cycles is reduced to 37% of the original value, and to less than 1% after 500 cycles! Let us look now at a few important secondary battery types.

The lead storage battery, invented in 1859 by Gaston Plante, is still the dominant workhorse for starting automobiles, power supplies in cars, and many emergency lights. In principle, it can be built using two identical lead electrodes which are immersed in dilute sulfuric acid (H2SO4). In this case, it needs to be electrically "charged" before usage. In practice, however, the negative electrode utilizes a grid made of a lead-antimony alloy whose open spaces are filled with lead in spongy form. The positive electrode consists of lead dioxide (PbO2). The chemical reaction during discharge on the negative electrode is

whereas on the positive terminal the reaction is

During discharge, the cell produces water (see the equation above) and thus dilutes the sulfuric acid eventually to a point that does not allow any further chemical reactions. At that time, the battery needs to be connected to an external power supply (of the same polarity!) which forces the above reactions to occur in the reverse direction. In practice, however, a lead-acid cell is often trickle charged, that is, a voltage is almost constantly provided to the battery by the alternator. The lead cell provides about 2 V at open circuit. A number of these cells are commonly connected in series to yield a higher voltage. The specific energy of the lead cell is at most 30-40 W-h per kg, (energy density 120-170 W-h/L), and the efficiency is near 50%. Corrosion of the grid and the development of hydrogen and oxygen gases lead eventually to a loss of water which needs to be occasionally resupplied. This disadvantage can be largely reduced by a closed battery system which provides, however, a slightly reduced voltage.

Rechargeable Alkaline Manganese (RAM) cells are similar in construction to the above-mentioned Zn-MnO2 primary batteries. RAMs are quite popular in many countries and are thus, among the most-sold, small rechargeables. Special features are: (1) a micro-porous separator which prohibits shorts between the electrodes by Zn-dendrites, (2) an addition of Ag2O to the positive mass which fosters the oxidation of hydrogen, and (3) addition of BaSO4 to MnO2 which enhances the recharging properties of MnO2. Several 100 recharge cycles are possible; however, this will eventually yield only 30 to 40% of the original capacity. The specific energy is about 90 to 100 W-h/kg. RAMs can be considered as environmentally safe if no Hg is used in the Zn (see above).

The nickel-cadmium storage battery (invented 1899 by Waldemar Jun-ger) utilizes Cd as the negative electrode and nickel hydroxide (NiOOH) at the positive terminal. The electrolyte consists of an alkaline solution of potassium hydroxide (KOH) in water. Ni-Cd’s provide an open circuit voltage of 1.35 V, have a specific energy of about 50 W-h/kg, (and an energy density of 120-170 W-h/L).The reaction equation on the negative electrode during discharge is

At the positive terminal we observe:

The above reactions occur in the opposite directions during charging. Ni-Cd’s can be manufactured air-tight but corrosion reactions in the cells eventually lead to self-discharging. Ni-Cd batteries are mainly used in cell phones, laptops, tooth brushes, cordless power drills, garden tools, and other portable equipment, e.g. for airplanes and space applications. One of their main characteristics is the "memory effect", that is the battery remembers the voltage in the charge cycle where previous rechargings began. It is assumed that under partial discharge/recharge condition the intermetallic compound Ni5Cd21 is formed on the Cd electrode. Reducing the cell voltage by at least 0.2 V or better occasional complete discharge alleviates this effect. Even though Ni-Cd’s are widely used, they will be eventually phased out because of their toxic cadmium content, which is an environmental handicap, preventing disposal in land-fills. They should be recycled. Ni-Cd’s will probably be replaced by nickel-metal hydride batteries; see next paragraph. However, Ni seems to cause some environmental concern in land-fills too.

Nickel-metal hydride (Ni-MH) batteries have a storage capacity which is roughly twice as large as the above-discussed Ni-Cd’s. The specific energy is about 80 W-h/kg and the open circuit voltage of one cell is 1.35 V. The positive electrode consists, as above, of NiOOH. The negative electrode material of the form AB5 or AB2 can store and release large amounts of hydrogen ions (oxidation and reduction respectively) per volume. In essence, the H+-ions are transferred through the alkaline electrolyte from one electrode to the other. The A-metal in AB5 alloys are for example pure or blends of La, Ce, Nd, Pr, or other lanthanides, whereas the B-metal consists of Ni, or Co (with small additions of Al, Si, and Mn).The A-metals in AB2 are V, Ti, or Zr, whereas B stands for Ni, Co, Mn, Al, or Cr. The listed additive elements are used for different purposes, for example for increasing the hydrogen storage capability, corrosion resistance, or reversibility of the reactions. The "classic memory effect" (see above) does not occur in Ni-MH’s.

The Lithium-ion cell (invented in 1912 by G.N. Lewis and developed by M.S. Whittingham in the 1970s) is one of the most important batteries from a commercial point of view because of its light weight, high open circuit voltage (around 4 V), high specific energy (about 130 W-h/kg), non-aqueous chemistry, lack of memory effect (see above), small self-discharge rate (about 5-10% per month compared to 30% in Ni-MH batteries and 10% in Ni-Cd’s), and environmental safety when disposed. Indeed, Li-ion cells are now the most-sold batteries in Japan. Thus, Li-ions are the subject of, as of this writing, a stormy research activity in particular with respect to new materials.7 The main concerns are, however, some safety problems (fire or explosion) which have lead to recalls because of thermal runaway and cell rupture when overheated, overcharged, or mistreated (e.g. shorts). Specific built-in circuits prevent charging at excessive high voltages or charging below a threshold voltage. Li-ion batteries are typically used at present for laptops, power tools, electric cars, camcorders, etc. The materials used in lithium-ion batteries are so manifold that a complete list would only confuse the reader. Instead, a characteristic example (not necessarily the most efficient one) is given for demonstrating the typical electrochemistry involved. The negative electrode (anode) of this storage device is often made from porous carbon (graphite), whereas the positive electrode (cathode) consists of a metal oxide, such as lithium cobalt oxide (LiCoO2) or a spinel such as lithium manganese oxide. The electrolyte is a lithium salt in an organic solvent such as organic carbonates containing complexes of lithium ions. (Since pure Li reacts violently with water a non-aqueous electrolyte and hermetic sealing against external moisture is imperative.) The principal mechanism during discharge is as follows: positive lithium ions are extracted from the negative electrode (leaving electrons behind) and inserted into the positive electrode (called intercalation). During charging the reverse process takes place. Specifically, on the positive electrode (LiCoO2), xLi-ions are extracted:

and inserted into the negative (carbon) electrode:

In other words, the current in both directions within the battery is carried by the movement of lithium ions (and outside, of course, by electrons), once a load is connected to the terminals.

Li-ion batteries are not without flaws. For one, they have a somewhat poor life cycle (300 to 500 charges/discharges). Specifically, the Li+ transport is increasingly impeded by deposits which form in the electrolyte and which gradually decrease the conductivity and thus, the current which can be drawn. This requires more frequent charging. Second, the shelf life, i.e. without use, is said to be 3-5 years. Third, elevated temperatures (storage of laptops in hot cars and poorly ventilated laptops!) diminish permanently the storage capacity. Specifically, Li-ion batteries lose irreversibly 20% to 30% of their capacity per year, depending on their storage temperature. (They should be stored in a cool place at 40% charge.) Fourth, power tools which require large currents are better served with Ni-Cd’s or Ni-MH batteries. Fifth, they are expensive to manufacture. Finally, safety concerns, particularly for batteries with high energy density (electric cars) and after mechanical abuse, have been mentioned already at the beginning. It is, however, anticipated that the above-explained problems will be solved over time.

A new type of storage device, namely a high-temperature battery, based on sodium exists, such as the Na-NiCl2 cell, and the Na-S cell in which liquid sodium is separated by solid electrolytes made from ceramic materials (sodium-b-alumina) that have an exceptionally high ionic conductivity. These devices are still in development and will not be discussed further here.

Finally, another type of "rechargeable" storage cell, called the flow battery has been developed, in which the electrolyte is pumped from an external tank through the device and this way converts chemical energy directly to electricity. The process is reversible and thus, can be utilized to store wind, solar and cheap night-time electricity. The flow battery functions on the fuel-cell principle whereas a fuel (e.g. hydrogen or methanol) and an oxidant (e.g. oxygen or air) undergo electron transfer reactions at the anode and cathode respectively. The anode and cathode are separated by an ion exchange membrane. Among the various types of flow batteries are the redox (reduction-oxidation) device in which the electroactive components are dissolved in an electrolyte. Examples are the polysulfide bromide battery, and the uranium redox flow battery. In hybrid flow batteries, the electroactive components are deposited as a solid layer. Examples are the zinc-bromide, cerium-zinc, and the all-lead flow batteries.

Closing Remarks

The development of new, stronger, and improved rechargeable battery systems is to a large extent politically motivated and driven. Governments in many industrialized countries and regions (particularly in California) are interested in "pollution free", or at least pollution-reduced vehicles. Rechargeable batteries as a sole energy source for propelling cars are often considered to be the solution. However, one has to take into consideration that the electricity required for charging the batteries need to be produced first; this occurs generally in power plants which utilize mostly coal, petroleum, or natural gas as fuels. (50% of the electricity in the USA and Germany is produced by coal, in contrast to 81% in China, 68% in India, and 5% in France, amounting to 41% as world average). These power plants also emit CO2 and other pollutants (sulfur, mercury, nitrogen) into the atmosphere. One should also keep in mind that transmission losses to the consumer, losses when charging batteries (about 50%), and consumed energy to manufacture batteries may contribute likewise, in an indirect way, to energy consumption and pollution. The same is true for solar energy systems and atomic reactors (with all their problems) which need years to reach the break-even point at which energy consumption for production and emission of pollutants are compensated. In this respect, the specific energy contained in gasoline or Diesel oil (about 12,000 W-h/kg) does not look too bad. The cost per mile derived from electricity stemming from a household outlet is higher than that obtained using a Diesel engine. Moreover, Diesel engines can be made so energy efficient and pollution-reduced today that they compare quite favorably with (more expensive) hybrid automobiles (that utilize a gasoline back-up engine for recharging the battery and capture the energy, evolved from braking. This makes particularly sense for inner city traffic with frequent stop and go maneuvers). Some consideration should also be given to the distance one can drive before recharging of the battery is required which for lead-batteries is about 60 km (37 miles), and about 40-80 km (25-50 miles) for a lithium-ion battery, weighing each between 300 and 400 kg. These values are reduced in cold weather when electric heating is required. In short, matters do not look as favorable for battery-propelled cars as some proponents want us to believe (except for niche markets such as intercity delivery and utility repair trucks). This does not mean that new energy sources will not be found and used in the future (e.g. fusion). Conservation of energy and electrically propelled public transportation systems seem to be among the better alternatives. Finally, batteries are not the only available storage devices for energy, particular for smoothing out energy peaks. Among the alternative storage devices are super-capacitors (10 W-h/kg), flywheels, superconducting magnetic energy storage systems, electrolysis of water in combination with hydrogen fuel cells (1,100 W-h/kg), flow batteries, and reservoirs in which water is pumped up during off-peak hours.