From: ptimmerm@kathy.jpl.nasa.gov (Paul Timmerman)
Subject: Nickel-Cadmium Chemistry
Reply-To: ptimmerm@kathy.jpl.nasa.gov (Paul Timmerman)
Organization: Jpl.Nasa.Gov
Date: Mon, 14 Sep 1992 21:16:06 GMT

	Howard Clase states:	The reactions in a NiCad are:

>2Ni(OH)3(s) + 2e-     =  2Ni(OH)2(s) + 2(OH)-(aq)
>Cd(s) + 2(OH)-        =  Cd(OH)2(s) + 2e-
>--------------------------------------------------
>2Ni(OH)3(s) + Cd(s)   =  2Ni(OH)2(s) + Cd(OH)2(s)

Which ws modified by Antoni S. Gozdz:

>>Just a few additions/corrections on the chemistry and
>>failure mechanism of NiCd batteries:
>>
>>1. modern NiCd batteries use nickel oxyhydride NiO(OH) for
                                     ^^^^^^^^^^
>self-correction: of course, it should be oxyhydroxide

>>
>2 NiO(OH) + 2 Cd + 2 H2) <====> 2 Ni(OH)2 + 2 Cd(OH)2
>
	Well were finally strting to get close. However there is not,
under normal conditions hydrogen present in the cell.  Thermodynamics 
prevents this.  If you balance the equation with water and OH- you will get
the overall reaction.  It also helps to know how many electrons are involved in
each reaction (1 for the positive and 2 for the negative).

2 ( NiOOH  + H20 + e-  =  Ni(OH)2 +  OH-)
       Cd(s) + 2(OH)-  =  Cd(OH)2(s) + 2e-
--------------------------------------------------
   2NiOOH + Cd + 2H20  =  2Ni(OH)2 + Cd(OH)2


   The positive electrode active material can be seen as an itercaltion material 
with incorperates protons, which are  formed electrochemically at the surface
of the active material and have to diffuse into the fixed nickel hydroxide/ 
oxyhydroxide lattice structure.  At the negative electrode, there is a solution/
dissolution reaction involving a soluble intermeadiary, Cd(OH)3-: Cadmate.  This
reaction is typical of metal/sparingly salt couples, similar to the zinc electrode.
It too can be looked at as seperate steps, inluding at least one chemical, and 
one electro-chemical.  This soluble species is very important to battery life.

	John Whitmore wrote:

>	Whiskers are burned out by this procedure; if you discharge
>QUICKLY, the whiskers don't grow long (but blunt instead), so some
>people recommend regular heavy-discharge cycles (NOT the same
>as DEEP-discharge).

	The wiskers are cadmium dendrites, formed due to electrochemical deposition 
of the soluble species.  Cadmium normally forms hexagonal plates, which comprise the
blunt crystals John mentioned, but this happens prferentially at LOW rates.  The
dedritic grpwth occurs due to localized HIGH current desnity and large resulting 
concentration gradients, which act like an electrochemcial black hole sucking in
the soluble species, as quickly as possible.  The confusion is that high rates can
also cause some phases of the cadmium crystals to fracture and spall, increasing 
surface area and utilization.  

	Practically, there is another way you may break free the wiskers, it 
is rather crude, but probably as effect as big spark type fixes.  Slam your cell
down rudely on a hard surface, or mabey a nice cinder block walll.  If it dosent
work, at least you had fun!  As a regular maintanance item, a brief high rate 
charge and or discharge may help keep the negative electrode material utilization
high.  The cadmium electrode invariable is the cause of failure. So looking after
it is the most important thing.  This is why you must be careful with storage.
If done improperly, the cadmium electrode passivates, reducing performance.  
Cold is best. Discharged and shorted is good, but beware short down procedure, it 
must be individual cell based.  Charged storage may be ok.  Discharge, open circuit
is bad.  Trickle charge is pretty good.  But hey they are replaceable, right!

Paul Timmerman:  Battery Systems Group, Jet Propulsion Laboratories



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