into and out of the structure, with a minimum of disruption to that structure. This structural integrity is important in maintaining a long cycle life. The negative electrode is made of various types of carbon and graphite (the original Sony cell used LiCoO2). The CoO2 has a layered structure that readily accommodates the Li without the formation of a new structure (or new phase).
Although Li ion cells have the best performance of any available rechargeable battery, they have a number of problems that are currently being addressed by the R&D community. Overcharge or overdischarge can lead to capacity loss and even cell failure in the form of thermal runaway and fire, so each cell has a protective microcircuit that controls the voltage limits of the cell and the recharge process. The solvents for the electrolyte are flammable organic liquids (such as ethylene carbonate and dimethyl carbonate), so there is research on flame-retardant additives. Also, because the cobalt oxide positive electrodes are expensive, alternative low-cost, high-capacity positive electrode materials are being explored, including LiNiO2-based, LiMnO2-based, Li(Mn,Ni,Co)O2-based, and LiFePO4-based materials. Some nickel-containing materials are close to commercialization. Performance can also degrade by spontaneous film formation on the electrodes, so there are efforts to find additives for the electrolyte that control film formation and film properties.
Li polymer cells are derivatives of the Li ion cells. They have the same electrochemistry, but the liquid electrolyte is gelled with a polymer such as polyvinylidene fluoride (PVdF) or polyethylene oxide so that it is immobilized and behaves like a polymer. The gel offers flexibility in the shape of the cell and eliminates any free-flowing liquid. Li polymer cells have performance similar to that of the Li ion cell, with specific energy values up to about 150 Wh/kg and 300 Wh/L for −20°C to +60°C, and have been recently introduced to the commercial market.5
The nickel metal hydride, or MH/NiOOH, cell has become very popular for many consumer applications, including portable electronics and power tools. It has largely replaced the Ni-Cd (Cd/NiOOH) cell in the consumer market, because of concern about the environmental impact of cadmium. The MH/NiOOH cell has an aqueous electrolyte of potassium hydroxide, which offers a much higher conductivity than the nonaqueous electrolytes used in lithium cells, so it can be discharged at high power. Both of the electrode reactions in Table D-3 are reversible and have rapid reaction rates, so high specific power values can be achieved, but their specific energy is less than 100 Wh/kg, which limits its usefulness. Other problems with this system include its low cell voltage (~1.2 V), limited temperature range for reasonable operation, and the need for charging at a relatively low temperature (<45°C).
Li/S cells offer the opportunity for very high specific energy (theoretical value = 2,600 Wh/kg) and low cost, using environmentally benign materials. Their characteristics are summarized in Table D-3. The drawback of this battery system is its short cycle-life, which is due to the sulfur electrode. During operation of the cell, polysulfides of several stoichiometries form and dissolve in liquid electrolytes, allowing them to migrate throughout the cell. This stability issue has been addressed by using gel and polymer electrolytes that prevent migration of the sulfur species. Sion Power Corporation6 is striving to introduce commercial lithium/sulfur batteries in 2004 with 1-Ah pouch-style cells.
Metal/air cells comprise a cathode that uses oxygen in the air as an oxidant and a solid fuel as the anode. They are different from fuel cells and other batteries in that the anode is consumed during operation. Often, metal/air cells are described as semi-fuel cells. Metal/air cells are being studied because they have the advantage of using air as an inexhaustible cathode reactant, leading to compact, anode-limited cells with high energy density. Carbon/air batteries are grouped with this class of power sources even though they operate at elevated temperatures.
The properties of metal/air and carbon/air electrochemical couples are summarized in Table D-4. The total metal/air reaction is the sum of the reaction of the oxidation at the metal anode and the reduction of oxygen at the air cathode:
4M + nO2 + 2nH2O → 4M(OH)n
M + nO2 → MO2n
where M is the metal and n depends on the valence change for the oxidation of the metal. Most metal/air cells do not have a long shelf life once they are activated with electrolyte and exposed to air, because the metal anode tends to react with water in the aqueous electrolyte or moisture in the air to generate hydrogen:
M + nH2O → M(OH)n + n/2 H2
Moisture in the air is a big factor in the performance of metal/air cells. Too much moisture causes flooding of the air electrodes, while insufficient moisture causes water to evaporate from the cells and dries out the electrolyte. In addition, metal/air cells that use alkaline electrolyte also suffer from the buildup of carbonates in the electrolyte from the reaction with CO2 in the air. Finally, the slow gas-solid