BU-104b: Battery Building Blocks

An electrochemical battery consists of a cathode, an anode and electrolyte that act 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 anode; the electrode that absorbs the electrons is the cathode.

The battery anode is always negative and the cathode positive. This appears to violate the convention as the anode is the terminal into which current flows. A vacuum tube, diode or a battery on charge follows this order; however taking power away from a battery on discharge turns the anode negative. Since the battery is an electric storage device providing energy, the battery anode is always negative.

The anode of Li-ion is carbon (See BU-204: How do Lithium Batteries Work?) but the order is reversed with lithium-metal batteries. Here the cathode is carbon and the anode metallic lithium. (See BU-212: Future Batteries) With few exceptions, lithium-metal batteries are non-rechargeable.

Battery Symbol
Figure 1: Battery Symbol.
The cathode of a battery is positive and the anode is negative.

Tables 2a, 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 2a: Composition of lead acid.


NiMH, NiCd
Cathode (positive)Anode (negative)Electrolyte
MaterialNickel OxyhydroxideNiMH: hydrogen-absorbing alloy
NiCd: Cadmium
Potassium Hydroxide
Table 2b: Composition of NiMH and NiCd.


Lithium-ionCathode (positive)
on aluminum foil
Anode (negative)
on copper foil
Electrolyte
MaterialMetal oxides derived from cobalt, nickel, manganese, iron, aluminumCarbon basedLithium salt in an organic solvent
Full chargeMetal oxide with intercalation structureLithium ions migrated to anode.
DischargedLithium ions move back to the positive electrodeMainly carbon
Table 2c: Composition of Li-ion.


AlkalineCathode (positive)Anode (negative)Electrolyte
MaterialManganese dioxideZincAqueous alkaline
Table 2d: 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: 20-Oct-2021
Batteries In A Portable World
Batteries In A Portable World

The material on Battery University is based on the indispensable new 4th edition of "Batteries in a Portable World - A Handbook on Rechargeable Batteries for Non-Engineers" which is available for order through Amazon.com.

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On November 26, 2018, BK wrote:
It is highly appreciated to give a lot of good information in here. Regarding the 1-c table, it is noted that " Cathode on copper foil" and "Anode on aluminum foil". Is it correct ? The book which I read indicates opposite as "Cathode on aluminum foil" and "Anode on copper foil". Your help to clarify this would be much appreciated.
On September 10, 2018, Tom Szabo wrote:
Can one turn a Negative plate to a positive plate in the lead acid battery? In example if I have 2 good negative plates out of an old battery, could one be turned into a positive plate so a battery could be made again ? If so what would be the best process? TIA Tom
On April 2, 2018, Mehdi Karamad wrote:
We should consider that the atom that has the least elctron on its outer orbit will loss its electron and became positive charge and the one who receives electron wiil have negtive charge so upon charging i am agree that all the positive ions will be collecting on cathod and are ready to loss their electron through external load to anode. This part is easily understandable but in the second part when you say the electrod that releases electrons is called anode and the one which attract elctrons is call cathod seemes to be against in your article and i can.t figure it out why suddenly everything changed?
On July 25, 2017, 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.
On June 13, 2017, Lary Anoes wrote:
Very good.
On May 14, 2017, 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 May 10, 2016, 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 October 17, 2015, hrncirik wrote:
must be /inserted/ in fig. 1a /lithium atoms reduced from/ lithium ions ... 1c njckel /III. oxide/
On May 13, 2015, 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 January 28, 2015, 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..