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This section of the website provides information regarding the chemicals involved in teh differnt types of batteryavailable. Descriptions of what is inside the batteries, the chemical reactions that go on and other background information about the batteries is discussed. Primary Batteries Alkaline (Zinc Manganese Dioxide) Anode: Zinc Powder Cathode: Manganese dioxide (MnO2) powder Electrolyte: Potassium hydroxide (KOH) First introduced in the early 60’s as an improvement to the existing zinc-carbon cells; advantages include: higher energy density, longer shelf life, superior leakage resistance, better performance in both continuous and intermittent duty cycles, and lower internal resistance, which allows it to operate at high discharge rates over a wider temperature range. Zinc in a powdered form increases the surface area of the anode, allowing more particle interaction. This lowers the internal resistance and increases the power density. The cathode, MnO2, is synthetically produced because of its superiority to naturally occurring MnO2. This increases the energy density. Just as in the zinc carbon cell, graphite is added to the cathode to increase conductivity. The electrolyte, KOH, allows high ionic conductivity. Zinc oxide is often added to slow down corrosion of the zinc anode. A cellulose derivative is thrown in as well as a gelling agent. The half-reactions are: Zn + 2OH—> ZnO + H2O + 2e 2MnO2 + H2O + 2e—>Mn2O3 + 2OH The overall reaction is: Zn + 2MnO2 —> ZnO + Mn2O3 E = 1.5 V Zinc Carbon Anode: Zinc can Cathode: Manganese Dioxide and Carbon Black mix Electrolyte: Ammonium Chloride and Zinc Chloride paste Developed in 1866 by George Lenlanch and was the first type of cylindrical cell design that we see today. The development of these batteries paved the way for the introduction of the Alkaline battery, possibly the most common battery technology today. Zinc Carbon batteries are still in use today, but most have been replaced by Alkaline cells. Zinc carbon batteries are the cheapest of all battery technologies, but their performance relates to this price difference. They have a short shelf life and deteriorate rapidly in cold temperatures. Oxidation of the Zinc can eventually cause the contents to leak out, so they cannot be used in devices where it is expected that the battery is going to last for a long time (e.g. remote controls, clocks). They are best suited to devices requiring light to moderate drain; their service capacity (Ampere-hour) is not fixed because the battery will function will vary depending upon the conditions imposed upon it. They are much more sensitive than Alkaline batteries to variants such as current drain, operating schedule, cut-off voltage and temperature. The half reactions are: Anode: Zn(s) -> Zn2(aq) + 2e Cathode: 2NH4(aq) + 2MnO2(s) + 2e -> Mn2O3(s) + H2O(l) + 2NH3(aq) The overall reaction is: 2MnO2 + 2NH4Cl + Zn --> ZnCL2.2NH3 + H2O + MN2O3 Zinc Chloride Anode: Pure Zinc can Cathode: Mix of Manganese Dioxide, Carbon Black and Zinc Chloride electrolyte Electrolyte: Zinc Chloride The Zinc Chloride battery is a more robust version of the Zinc Carbon battery. The chemical mixture of the battery increases its capacity by 50%. They are also less prone to leakage than Zinc Carbon batteries. They cannot withstand high temperatures (above 70°C) but do perform well at low temperatures (down to -17°C). The chemistry behind Zinc Chloride batteries had been known about since the 1890’s, however cells made this was did not provide much service or life after storage with the materials available until around 1960. During this time, Zinc Chloride batteries provided numerous advantages over Alkaline cells such as price, availability and size range. However since then, advances in other battery technologies and manufacturing techniques have eliminated these advantages making Alkaline batteries the most effective of the two technologies. However Zinc Chloride cells are still used today, mainly to power torches and some radios. The overall reaction is: 8MnO2 + 4Zn + ZnCl2 + 9H2O --> 8MnOOH + ZnCl2.4ZnO.5H2O Zinc Silver Oxide Anode: A gelled mixture of amalgamated zinc powder (a mixture of zinc and mercury) and the electrolyte Cathode: Mixture of Ag2O (silver oxide) Electrolyte: 1. Sodium Hydroxide (NaOH) or 2. Potassium Hydroxide (KOH) Silver Oxide is the nickname for a silver oxide-alkaline-zinc primary battery. The two different types of electrolyte available offer slightly differing characteristics and are therefore used in differing applications. 1. Sodium hydroxide types last two to three years making
them highly suitable for quartz analog digital watches or digital watches
without backlights. Full chemical reaction: 1. Zn + Ag2O NaOH --> ZnO + 2Ag Zinc Air Anode: Amalgamated zinc powder and electrolyte Cathode: Oxygen (O2) Electrolyte: Potassium hydroxide (KOH) Zinc air cell fits into the alkaline cell category because of its electrolyte; it also acts as a partial fuel cell because it uses the oxygen from air as the cathode. Oxygen is let in to the cathode through a hole in the battery and is reduced on a carbon surface. The metal oxide reduces, the zinc becomes oxidized, and electric current results. The half reactions are: Anode: Zn2+ + 2OH—> Zn(OH)2: 1.25v Cathode: 1/2O2 + H2O + 2e —> 2OH:0.4v The overall reaction is: 2Zn + O2 + 2H2O —> 2Zn(OH)2 : 1.65 The electrolyte is an alkali hydroxide in 20-40% weight solution with water. One disadvantage is that since these hydroxides are hygroscopic, they will pick up or lose water from the air depending on the humidity. Both too little and too much humidity reduces the life of the cell. Selective membranes can help. Oxygen from the air dissolves in the electrolyte through a porous, hydrophobic electrode—a carbon-polymer or metal-polymer composite. there is no need to carry around the cathode, the energy density of these batteries can be quite high, between 220–300 Wh/kg (compared to 99–123 Wh/kg with a HgO cathode), although the power density remains low. However, the use of potassium or sodium hydroxides as the electrolyte is a problem, since these can react with carbon dioxide in the air to form alkali carbonates. For this reason large zinc air batteries usually contain a higher volume of CO2 absorbing material (calcium oxide flake) than battery components. This can cancel out the huge increase in energy density gained by using the air electrode. Lithium Manganese Dioxide Anode: Lithium (Li) Cathode: Manganese-dioxide (MnO2) Electrolyte: Organic solvent mixture into which alkali metal salt is dissolved. The major advantages of lithium manganese batteries over alkaline batteries are their high energy and power density, good storage life and discharge performance. Lithium Manganese Dioxide is one of many Lithium battery types, and it is recommended for outdoor use (requiring a low temperature range) and for high-discharge devices, e.g. digital cameras, portable power tools and other portable digital devices. Lithium-manganese dioxide cells are very stable meaning these batteries can be stored for several years and operating temperatures have little effect on operating characteristics because the cell is so efficient. They can also offer twice the voltage of other cells (nominally 3v per cell). Battery technologists have long been aware that lithium has the highest potential on the emf-scale as well as a low equivalent weight. This makes it a good anode candidate for a high-density battery, but it was not until a manufacturing process was developed to electroplate lithium for use as an anode that the lithium battery became a commercial product. Full Chemical Reaction: Li + MnO2 --> LiMnO2 Secondary Batteries Nickel Metal Hydride Anode: Rare-earth or nickel alloys with many metals Cathode: Nickel oxyhydroxide Electrolyte: Potassium hydroxide This sealed cell is a hybrid of the NiCd and NiH2 cells. Previously, this battery was not available for commercial use because, although hydrogen has wonderful anodic qualities, it requires cell pressurization. Fortunately, in the late 1960s scientists discovered that some metal alloys (hydrides such as LiNi5 or ZrNi2) could store hydrogen atoms, which then could participate in reversible chemical reactions. In modern NiMH batteries, the anode consists of many metals alloys, including V, Ti, Zr, Ni, Cr, Co, and Fe. Except for the anode, the NiMH cell very closely resembles the NiCd cell in construction. Even the voltage is virtually identical, at 1.2 volts, making the cells interchangeable in many applications. The half reactions are: Anode: MH + OH—> M + H2O + e: 0.83v Cathode: NiOOH + H2O + e—> Ni(OH)2 + OH: 0.52v The overall reaction is: NiOOH + MH —> Ni(OH)2 + M: 1.35v The anodes used in these cells are complex alloys containing many metals, such as an alloy of V, Ti, Zr, Ni, Cr, Co, and (!) Fe. Some metals absorb heat, and some give off heat when absorbing hydrogen. Both of these are bad for a battery, since we would like the hydrogen to move easily in and out without any energy transfer, the alloys used are mixtures of exothermic and endothermic metals to achieve a balance and eliminate energy transfer. Nickel Cadmium Anode: Cadmium Cathode: Nickel oxyhydroxide Ni(OH)2 Electrolyte: Aqueous potassium hydroxide (KOH) The cathode is nickel-plated, woven mesh, and the anode is a cadmium-plated net. Since the cadmium is just a coating, this cell's negative environmental impact is often exaggerated. The electrolyte, KOH, acts only as an ion conductor and does not contribute significantly to the cell's reaction. That's why not much electrolyte is needed, so this keeps the weight down. The half reactions are: Cd + 2OH—> Cd(OH)2 + 2e: 0.81v NiO2 + 2H2O + 2e—> Ni(OH)2 + 2OH: 0.49v The overall reaction is: Cd +NiO2 + 2H2O —> Cd(OH)2 + Ni(OH)2: 1.30v Rechargeable Alkaline Anode: Zinc Powder Cathode: Manganese dioxide (MnO2) powder Electrolyte: Aqueous Potassium hydroxide (KOH) Designed for repeated recharge, however it loses charge acceptance with each recharge. The longevity of the reuseable alkaline is a direct function of the depth of discharge; the deeper the discharge, the fewer cycles the battery can endure. Like standard Alkaline batteries, they have a low load current capability so is not suitable to high demand devices such as camcorders and other portable digital equipment. The initial cost is low but the cost per cycle is high compared to nickel-based natteries however for many applications this cost is economical when compared with standard Alkaline cells. Care must be taken not to overcharge to prevent electrolysis of the KOH solution electrolyte, or to charge at voltages higher than 1.65 V (depending on temperature) to avoid the formation of higher oxides of manganese. They have been in commertial use since 1992 The half-reactions are: Zn + 2OH = Zn(OH)2 + 2e 2MnO2 + H2O + 2e = Mn2O3 + 2OH Lithium Ion Anode: Lithium Colbalt oxide (LiCoO2) or Lithium Manganese oxide (LiMn2O4) Cathode: Graphitic carbon Electrolyte: Vary, examples: LiPF6 or LiBF4. During the charge and discharge processes, lithium
ions are inserted or extracted The half reactions are: Anode: LiXXO2 Li1 -> xXXO2 + Li +xe- Cathode: C + xLi + xe -> LixC The overall reaction is: LiXXO2 + C LixC + Li 1 -> xXXO2 XX= Various Combining elements including Cobalt and Manganese Lead Acid Anode: Spongy metallic Lead Cathode: Lead Dioxide Electrolyte: Sulphuric Acid Lead acid batteries have developed into various different forms using different chemicals for the electrolyte, you may have come across Flooded Lead Acid, Sealed Lead Acid, VRLA, AGM and Gel. These different types have various different characteristics but the underlying chemistry is the same. Here is a brief description of each, and following will be a description of the chemical reactions that go on in the lead acid battery. Flooded Lead Acid: Has a liquid electrolyte which is free to move in the battery container and water can be added to the individual cells of the battery throughout its life. These types of battery are the traditional engine start style battery. Sealed Lead Acid: The term itself is quite broad and can apply to a number of different battery styles. In this type of battery, the manufacturer seals the battery and nothing can be added to the battery during its use. These batteries are most commonly used in standby power applications. VRLA: Valve Regulated Lead Acid battery; this is also sealed but have an inbuilt valve regulating system which allows for safe escape of hydrogen and oxygen gasses during the charging process. This is an added safety feature to the standard Sealed Lead Acid battery. AGM: Absorbed Glass Matte construction, the bulk of the electrolyte is Absorbed in the separator material; this allows the electrolyte to be suspended in close proximity with the plate’s active material. In theory, this enhances both the discharge and recharge efficiency. It is a variant on VRLA batteries and is becoming very popular in engine start and power sports applications. Gel: Similar to AGM because the electrolyte is suspended but the electrolyte in has a silica additive that causes it to stiffen initially into a gel; after numerous charge, discharge cycles it starts to become more brittle. Micro cracks form in the gelled electrolyte that provide paths for the oxygen recombination reactions between the positive and negative plates. The recharge voltages on this type of cell are lower than the other styles of lead acid battery. This is probably the most sensitive cell in terms of adverse reactions to over-voltage charging. Maintenance Free: This term is very generic and refers to basically all of the battery types except flooded batteries that have accessible individual cells so that the end user can add water. Since any sealed construction prevents the user from adding water to the individual cells, then by default it becomes maintenance free. Now lets take a look at the chemical reactions that take place inside a Lead Acid battery. The half-reactions are: Anode: Pb + HSO¯4 -> PbSO4 + H + 2e¯ Cathode: PbO2 + HSO¯4 + 3H + e¯ -> PbSO4 + 2H2O The overall reaction is: Pb + PbO2 + 2HSO¯4 + 2H -> 2PbSO4 + 2H2O + energy Additional Information: Half-reaction: Refers to the chemical processes occurring at each electrode. The potential of the two half-reactions add to give us the overall cell potential. EMF Scale: EMF stands for ElectroMotive Force, and is the maximum potential difference between two electrodes of a voltaic cell (a primary cell having two unlike electrodes immersed in a solution that chemically interacts to produce a voltage). So basically the EMF scale corresponds to the scale of chemicals in order of their levels of potential. |
