BU-104b: Battery Building Blocks

Learn about the composition of the three most common batteries and how they serve our society.

An electrochemical battery consists of a cathode, an anode and electrolyte that acts as a catalyst. When charging, a buildup of positive ions forms at cathode/electrolyte interface. This leads electrons moving towards the cathode, creating a voltage potential between the cathode and the anode. Release is by a passing current from the positive cathode through an external load and back to the negative anode. On charge, the current flows in the other direction.

A battery has two separate pathways; one is the electric circuit through which electrons flow, feeding the load, and the other is the path where ions move between the electrodes though the separator that acts as an insulator for electrons. Ions are atoms that have lost or gained electrons and have become electrically charged. The separator electrically isolates the electrodes but allows the movement of ions.
 

Anode and Cathode

The electrode of a battery that releases electrons during discharge is called the anode; the electrode that absorbs the electrons is the cathode.

In a discharging battery, the anode is always negative and the cathode is positive. A vacuum tube, diode or a battery on charge anode is the positive electrode; however, taking power away from a battery on discharge turns the anode into negative. Since the battery is an electric storage device providing energy, the designation does not change between charging and discharging. The battery anode in discharging mode is always negative.
 

Battery symbol
The cathode of a battery is positive; the anode is negative.


Tables 1a, b, c and d summarize the composition of lead-, nickel- and lithium-based secondary batteries, including primary alkaline.
 

Lead acid Cathode (positive) Anode (negative) Electrolyte
Material Lead dioxide (chocolate brown) Gray lead, (spongy when formed) Sulfuric acid
Full charge Lead oxide (PbO2), electrons added to  positive plate Lead (Pb), electrons removed from plate Strong sulfuric acid
Discharged Lead turns into lead sulfate at the negative electrode, electrons driven from positive plate to negative plate Weak sulfuric acid (water-like)

Table 1a: Composition of lead acid.
 

NiMH, NiCd Cathode (positive) Anode (negative) Electrolyte
Material Nickel 
oxyhydroxide
NiMH: hydrogen-absorbing alloy
NiCd: Cadmium
Potassium hydroxide

Table 1b: Composition of NiMH and NiCd.
 

Lithium-ion Cathode (positive)
on copper foil
Anode (negative)
on aluminum foil
Electrolyte
Material Metal oxides derived from cobalt, nickel, manganese, iron, aluminum Carbon based Lithium salt in an organic solvent
Full charge Metal oxide with intercalation structure Lithium ions migrated to anode.
Discharged Lithium ions move back to the positive electrode Mainly carbon
 

Table 1c: Composition of Li-ion.
 

Alkaline Cathode (positive) Anode (negative) Electrolyte
Material Manganese dioxide Zinc Aqueous alkaline

Table 1d: Composition of primary alkaline battery.

 

Electrolyte and Separator

Ion flow is made possible with an activator called the electrolyte. In a flooded battery system, the electrolyte moves freely between the inserted electrodes; in a sealed cell, the electrolyte is normally added to the separator in a moistened form. The separator segregates the anode from the cathode, forming an isolator for electrons but allowing ions to pass through. (See BU-306: Separator and BU-307: Electrolyte)

Last Updated 2017-07-31
 

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Comments

On January 28, 2015 at 5:45am
Bonani wrote:

You guys are baddasses, super knowledge you giving me. I am new in this field of battery doing my PhD, I have been struggling for sometime to understand the basic concepts.

Thank..

On May 13, 2015 at 9:45am
Rich wrote:

FYI, there is a typo in BU-104b:  “For the cathode, Li-ion uses metal oxide derived from cobalt, manages and/or, nickel and other metals;...”
Should be “...cobalt, manganese and/or…”  Otto correct strikes again!

On October 17, 2015 at 2:04am
hrncirik wrote:

must be /inserted/ in fig. 1a /lithium atoms reduced from/ lithium ions ...
                1c njckel /III. oxide/

On May 10, 2016 at 3:36pm
Fred wrote:

Every physics text and electrical engineering text I have uses the opposite of this convention. They show the large plate marked positive and as the anode, the other end is marked negative and as the cathode, it shows the electrons flowing from the Positive toward the Negative end through the external load. I suspect the difference is that this written from the perspective of chemists where the anode is marked negative as it is at a more negative Reduction Potential than the cathode. In my chemistry texts, it explains in detail that the reaction leaves electrons behind on the terminal is the anode, the other is the cathode. However, the sign convention for chemists is different from that of physicists.  My chemistry text states clearly, that for the given example of a copper-zinc battery, “Electrons flow externally from the Zn electrode (anode) to the Cu electrode (cathode).”  Physicists, use the + sign to designate a higher electrical potential, i.e. the ability to do work, hence the electrons carry the energy to the external device, dissipate their energy. Every battery I have shows the anode end marked with a plus sign, every battery I have put in my truck shows the + sign and it obviously the source of the high potential as that is where the corrosion appears and seldom does corrosion appear at the negative terminal (provided the battery is not leaking).

I suspect that this article has a simple typo. Here is what it says: “When charging, a buildup of electrons forms on the anode, creating a voltage potential between the anode and the cathode. Release is by a passing current from the positive cathode through an external load and back to the negative anode. On charge, the current flows in the other direction.”

Here is what it should say: “When DIScharging, the internal chemical reaction supplies high potential electrons to the anode, creating a voltage potential between the anode and the cathode. On charge, the current flows in the other direction.”

Unfortunately, chemists and physicists would mark the terminals in the opposite sense.

On May 14, 2017 at 6:07am
Mark du Preez wrote:

I think the reason that we get confused about which is anode and cathode is that different devices are marked differently. A diode has its anode marked with a plus (+) whereas a cell (or battery of cells) has its anode marked with a minus (-). This is because an anode is defined as an electrode *into* which conventional current flows.

Consider a simple circuit with a battery, an LED (Light Emitting Diode) and a resistor to limit current, all connected in series. Depending on which side you put the resistor, you would either have the “+” terminal of the battery connected to the “+” terminal of the LED with the resistor between the “-” terminals or vice versa. Now although both terminals are marked “+”, they are connected together so obviously the current must be coming out of one and into the other. Therefore, by definition, one must be an anode and one must a cathode.

Remember it like this:

“ACID” - Anode: Current Into Device

(Electron flow is obviously in the other direction.)

On June 13, 2017 at 12:42am
Lary Anoes wrote:

Very good.

On July 25, 2017 at 7:11am
Slavko wrote:

Is this correct: “When charging, a buildup of electrons forms on the cathode”? I think it should be “When charging, a buildup of electrons forms on the anode”.

Look at this picture:
BU-306: What is the Function of the Separator?

Source Wikipedia:

In a discharging battery or a galvanic cell, the cathode is the positive terminal since that is where the current flows out of the device (see drawing). This outward current is carried internally by positive ions moving from the electrolyte to the positive cathode (chemical energy is responsible for this “uphill” motion). It is continued externally by electrons moving inwards, this negative charge moving inwards constituting positive current flowing outwards. For example, the Daniell galvanic cell’s copper electrode is the positive terminal and the cathode.
In a recharging battery, or an electrolytic cell performing electrolysis, the cathode is the negative terminal, from which current exits the device and returns to the external generator. For example, reversing the current direction in a Daniell galvanic cell would produce an electrolytic cell,[1] where the copper electrode is the positive terminal and the anode.

I might misunderstand that, so I apologise in advance.