Secondary cell

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A secondary cell is any kind of electrolytic cell in which the electrochemical reaction that releases energy is reversible. Widespread examples are rechargeable batteries found in portable consumer electronics such as notebook computers and cell phones, and car batteries.

Unlike primary cells, secondary cells must be charged before use. Once used, the batteries can be recharged by using an external electric source that reverses the cell reaction and creates a non-equilibrium mixture of reactants. Commonly used secondary cell chemistries are nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion (Li-ion), and lithium ion polymer (Li-ion polymer).

1 Charging
2 Active Components
3 Energy density
4 Advantages/disadvantages
5 See also
6 External links

[edit] Charging

During charging, electrons are taken from the cathode and given to a chemical species in the electrolyte. At the opposite side of the battery, electrolyte ions transfer an electron to the anode. This results in a net flow of electrons from the cathode to the anode. Energy is stored by this process when the electron transfer processes require the input of energy. This energy is released when the battery is operated in the reverse direction.

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Diagrams of the charging and discharging of a secondary cell battery.

Diagrams and equations of the charging and discharging of a lead-acid cell.

[edit] Active Components

The active components in a secondary cell are the chemicals that make up the anode, cathode, and electrolyte. The anode and cathode are made up of different materials, both of which can undergo an electrochemical reaction with the electrolyte, causing oxidation and reduction.

Example: Nickel Metal Hydride

Nickel oxyhydroxide (NiOOH) is the active component in the cathode, while the anode is composed of hydrogen in the form of metal hydride. The electrolyte of this secondary cell is an aqueous form of potassium hydroxide.

In the discharge process, the nickel oxyhydroxide is reduced to nickel hydroxide and the metal hydride is reduced to an alloy.

Nickel-Metal Hydride

Location	Reactions	Voltage
Anode	MH + OH^- —> M + H₂O + e^-	0.83
Cathode	NiOOH + H₂O + e^- —> Ni(OH)₂ + OH^-	0.52
Overall	NiOOH + MH —> Ni(OH)₂ + M	1.35

[edit] Energy density

Graph of energy densities of several secondary cells

The comparison of energy output of the active components to the overall size of a cell and those components is known as energy density. Many of today’s secondary cells have high energy densities, with the exception of the lead cell battery. Small size and high energy output, make these batteries perfect for small portable objects.

[edit] Advantages/disadvantages

Each secondary cell has its own advantages and disadvantages when compared to other secondary cells and primary cells. Only time and advances in science will perfect the use of the secondary cell.

Cell Type	Advantages	Disadvantages
Lead acid battery	Inexpensive and simple to make; Well understood; Dependable; Low maintenance.	Low energy density; Environmentally unfriendly; Can't be stored in discharged condition.
NiCd	Fast charge; Long life; High number of charge/discharge cycles.	Low energy density; Environmentally unfriendly; Significant self-discharge.
NiMH	High capacity; High energy density.	Significant cost; High self-discharge.
Li-ion	Low maintenance; Relatively low self-discharge; High energy density.	Subject to aging; High cost.
Li-ion polymer	Can be very small High energy density; Safe.	Not yet widely available; High cost.