MadSci Network: Chemistry
Query:

Re: How does the recharging process take place in rechargeable batteries?

Date: Sun Dec 12 00:16:23 1999
Posted By: Martin Thomas, Post-doc/Fellow, Phyiscal Chemistry, Quantachrome Corporation
Area of science: Chemistry
ID: 944440354.Ch
Message:

Dear Jennifer,

The clue to a rechargeable battery is that the redox chemical reactions which produce electricity (flow of electrons during discharge) are reversible. That is, during a charge cycle electrons are forced to flow in the opposite direction as that during discharge. A special charger is used to contol the charge rate and time. Not all reactions are easily reversible, and not all battery construction allow it either. Single use "one-way" batteries are called primary cells. Rechargeable ones are called secondary cells.

There are a number of secondary cells in use, most notably

The NICAD serves to illustrate the reversible nature of a secondary cell well:

At the first electrode

DISCHARGE CYCLE     Cd + 2OH- => Cd (OH)2 + 2e-
CHARGE CYCLE     Cd + 2OH- <= Cd(OH)2 + 2e-
and at the other
DISCHARGE CYCLE     NiOOH + H2O + e- => Ni(OH)2 + OH-
CHARGE CYCLE     NiOOH + H2O + e- <= Ni(OH)2 + OH-

You might prefer to look at at it like this:

DISCHARGE CYCLE
Cd(0) => Cd(II) + 2e-
Ni(III) +2e- => Ni(II)

CHARGE CYCLE
Cd(II) + 2e- => Cd(0)
Ni(II) => Ni(III) +2e-

The efficiency of these processes can be enhanced by various additives such as lithium ions to the aqueous potassium hydroxide electrolyte, and cobalt hydroxide to the nickel hydroxide(see References).

This whole question of rechargeable batteries was actually addressed a little while back on this network. I have included some WEBlinks from that answer and provided a few more in the WEBOGRAPHY table below:

Title/SubjectAuthor
Ni-Cd seminar"Red" Scholefield
Nickel-based CellsChris Dreyer
Nickel- metal hydride rechargeable batteries, Structure and Chemistry   Maxell Batteries
NiCd Battery FAQKen A. Nishimura
What makes some cells rechargeable and others notMichael Lerner & Frank McLarnon
New Technology Batteries Guide National Institute of Standards and Technology (NIST)

References
"The Nickel Hydroxide Electrode: Structural and Thermodynamic Aspects" C.Greaves, M.A.Thomas and M.Turner, Power Sources 9 (1983) 153-182
"Structural and Electrochemical Characteristics of Plastic (PTFE) Bonded Nickel Hydroxide Electrodes" C.Greaves, M.A.Thomas and M.Turner, Journal of Power Sources 12 (1984) 195-202

Thanks for your question to MadSci Network!


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