| What's
Li-ion and Li-Po? Lithium battery
can be classified into primary(nonrechargeable) lithium battery
and secondary (rechargeable) lithium battery. Lithium ion (Li-Ion)
and lithium polymer (Li-Po) are a secondary lithium battery composed
of lithium metallic oxide in its positive electrode and carbon material
in its negative electrode. Lithium, however, is an active metal.
The lithium ions inside the battery transfer between the two electrodes
during charge or discharge.
The characteristics and performance of the commercial
polymer and traditional lithium ion cells are very similar. Their
main difference is packaging. Unlike traditional lithium ion technology
using rigid metal case to tie the chemical materials together, lithium
polymer uses flexible and aluminum foil type case, by which the
cells can be made typically lighter, wafer-thinner and easily shaped
to fit the device it will power.
Rechargeable lithium battery is widely used in the
portable equipment market. Because of its lightness and high energy
density, they are ideal for portable devices such as mobile phone,
portable media player, laptop computers, PDA, portable lighting,
digital camera, camcorder, portable DVD player and etc. In addition,
it is free from memory effect and poisonous metals, friendly to
the environment.
Comparison table of VRLA,
Ni-Cd, Ni-Mh, Li-ion and Li-Po
| Items |
VRLA |
Ni-Cd |
Ni-Mh |
Li-ion |
Li-Po |
Nominal Voltage
(V) |
2.0 |
1.2 |
1.2 |
3.7 |
3.7 |
Energy Density
(Wh/Kg) |
30 |
50 |
65 |
100~160 |
200 |
Energy Desnsity(Wh/L) |
40 |
150 |
200 |
250~300 |
400 |
Self-Discharge
Rate (%/month) |
3~5 |
25~30 |
30~35 |
6~9 |
6~9 |
Cycle Life (Times) |
200~400 |
500 |
500 |
500 |
500 |
Memory Effect |
Yes |
Yes |
Yes |
No |
No |
Enviroment Friendly |
Medium |
Low |
Medium |
High |
High |
The electrochemistry
principle of rechargeable lithium battery
1) Charge:
Positive Pole: LiCoO2 -> Li1-xCoO2 + xLi+ + xe-
Negative Pole: C + xLi+ + xe- -> CLix
Total Reaction: LiCoO2 + C -> Li1-xCoO2 + CLix
2) Discharge:
Positive Pole: Li1-xCoO2 + xLi+ + xe- -> LiCoO2
Negative Pole: CLix -> C + xLi+ + xe-
Total Reaction: Li1-xCoO2 + CLix -> LiCoO2 + C
Why do rechargeable lithium
batteries need protections?
The negative electrode of li-ion/li-po batteries consists
of, mainly, carbon or graphite that can store or release Li ions.
Once there's an over discharge voltage, the Li ions turn to be Li
metals and deposit on the carbon or graphite surfaces. Because the
Li metal is very active, it can burn by any flush or other reasons.
The electrolyte contains flammable organic solvent.
Therefore, once the used charger goes wrong and there is an overcharge
voltage existing, the voltage goes above 4.7V, the Li-ion batteries
will be overheated and then to be burned.
In order to prevent deep discharge that leads to shorten
the cycle life of the battery, the Li-ion batteries need an over
discharge protection. |
