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Rechargeable Batteries

Rechargeable Batteries

Introduction

“What is a battery?” A battery is a device that creates electric energy through the conversion of chemical energy. “How?” Through oxidation-reduction reactions, that’s how! “What’s that?” An oxidation-reduction reaction (also known as a redox reaction) is a reaction in which electrons are passed from one object to another. 1 Although we commonly refer to these power-packs as batteries, the more scientifically appropriate terms for these electrochemical units would be the cell. 3 “Why?” In the chemistry world, a cell is defined as the most generic unit of electrochemistry that takes chemical energy and transforms it into electrical energy. Sounds a lot like a battery, doesn’t it? Batteries are typically comprised of at least one or more electrochemical cells that are electrically linked and assorted in a parallel fashion in order to supply the necessary functional voltage and circuit level. 3

Parts of a Battery

Every battery is composed of 3 major parts: The anode (negative electrode), the cathode (positive electrode) and the electrolyte. 2 An electrode is a type of conductor, through which electricity will enter or leave an object. 3 Electricity travels or flows through a circuit. The anode relinquishes its electrons to the external circuit, making it the reducing agent. 2 It is the one that gets oxidized during the electrochemical reaction. The cathode accepts the electrons from the external circuit, making it the oxidizing agent. 2 It gets reduced during the electrochemical reaction. The electrolyte (usually a liquid) serves as the medium in which electrical charge flows between the cathode and anode. 1

What makes a battery rechargeable? Non-rechargeable?

Batteries and electrochemical cells are distinguished as either primary cells or secondary cells. Primary cells are batteries that do not readily or efficiently recharge.  In primary cells, the electrochemical reaction that takes places in them is an irreversible process, meaning that the active material cannot be restored to its original state. 3 Because of their inability to recharge, these batteries are good for only one time use and are disposed of thereafter. 3 Primary cells are thus deemed as handy and low-cost batteries. An example of an everyday primary cell is an Alkaline Battery (there are also rechargeable alkaline batteries, but we will only discuss the non-rechargeable ones!).

Secondary cells are batteries that have the ability to recharge to its original state after being used. Just as in primary cells, secondary cells produce current (the rate at which electrical charge passes through a circuit) in a similar fashion. 1 They undergo an electrochemical reaction via an anode, cathode, and electrolyte. The difference, however, is that in a secondary cell, this process is reversible. 3 The cells are able to reverse the discharge flow of electrons from the anode to the cathode when electrical energy is introduced, thus replenishing the cell’s charge. 1 Lithium-ion batteries are one of the most popular rechargeable batteries.

Alkaline Battery

Alkaline batteries are a great example of primary cells. These batteries come in a variety of forms, such as AA, AAA, D or C . 3 In this type of battery, zinc powder is used to create the anode. This allows for more surface area which in turn increases the current. 1 The cathode is constructed of manganese dioxide. Aqueous potassium hydroxide is the electrolyte. Ions will flow from the zinc powder to the manganese dioxide, which will also cause electrons to be released from the electrode. These electrons will then gather inside the anode, resulting in a difference of charges between the two electrodes. The half reactions are: 1

Lithium Ion Battery

In a lithium ion battery, the lithium ions migrate back and forth from the anode to the cathode. 2 These batteries are extremely popular in consumer electronics. They contain one of the finest energy densities and experience a minor loss of charge while not in use. 2 These batteries are becoming more desired in electric vehicles, aerospace, and military uses. A carbonaceous sort of material (typically graphite) is what makes up the anode. 1 This carbonaceous material contains small regions in which lithium atoms and lithium ion reside. 1 When the intercalation method begins, the lithium migrates into the electrode. 2 Lithium moves out of the electrode during the extraction process. When a battery is expending energy, the lithium is removed from the anode and injected in the cathode. 2 A transition metal oxide, CoO 2, which is what the cathode is composed of, can also hold lithium ions. This metal has a high reactivity and therefore, a non-aqueous electrolyte must be used. 1 When the battery is set to recharge, the reverse reaction takes place. During the discharge of the battery, half reactions are: 1

 

Anode (oxidation):Li(s) --> Li+(aq) + e-

Cathode (reduction):Li+(aq) + CoO2(s) + e- --> LiCoO2(s)

Overall:Li(s) + CoO2(s) --> LiCoO 2(s)

References

  • Laird, Brian B. University Chemistry. Dubuque, IA: McGraw-Hill, 2009. Print. p 690-702.
  • Goodenough, John B., and Park, Kyu-Sung. "The Li-Ion Rechargeable Battery: A Perspective." Journal of The American Chemical Society 135.4 (2013): 1167-1176.
  • Smith, Michael J., and Colin A. Vincent. "Structure and Content of Some Primary Batteries." Journal of The American Chemical Society 78.4 (2001): 519-521.