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Lithium-Ion Battery Reactions

The basic electrochemistry of the cell involves only the transfer of lithium ions between the two insertion electrodes. Due to the high cell voltage of up to 4 V, the specific energy of this battery system is very favorable in comparison to the other known and commercialized secondary battery systems; however, an organic electrolyte solution must be used in the case of the lithium-ion battery.

A cell of a lithium-ion battery consists of a carbon-based negative electrode and a lithium transition metal oxide positive electrode. Upon charging, lithium ions are extracted from the positive electrode material and inserted into the negative electrode material. Upon discharging, the reverse process is taking place. (reference)

As long as the charge and discharge currents and battery temperature are controlled, Li-ion batteries are safe. Overcharging can convert the lithium oxide to metallic lithium, though, with the associated danger of explosion. Because of this sensitivity to charging conditions, notebook manufacturers use 'intelligent' batteries to power their products. These incorporate simple sensors and electronics to monitor cell voltage, temperature and charge or discharge current.

Intelligent charging systems protect the battery from harmful or potentially dangerous conditions, either by temporary shutdown, which can be reset by removing and then re-installing the battery, or by a permanent 'fuse' that renders the battery useless. Most manufacturers warn against operation at temperatures in excess of 60C. Usually, intelligent batteries can only be charged while they're installed in the notebook they're ed to drive. Charging them in any other way can cause temporary battery shutdown or long-term failure.

Li-ion batteries should be stored charged. If they're stored for over three months with a cell voltage of less than 2.5V, unrecoverable capacity loss will occur. Also, leakage and degradation are more likely. Some Li-ion battery packs won't allow a recharge if individual cell voltages fall below 2.5V, because at this point cells will have chemically altered and recharging could be hazardous. It's best to store Li-ion batteries charged at between 70 and 90 per cent of full capacity.

All rechargeable batteries suffer from self-discharge when stored or not in use. Li-ion batteries self-discharge by three to five per cent in the first 30 days of storage, then settle down to one to two per cent per month. With intelligent batteries, the control circuits packaged with the battery can consume as much as three per cent of the charge per month. This means that a 90 per cent charged Li-ion intelligent battery can be stored for around 18 months before falling below the minimum charge limit. Shorting out, piercing, crushing, applying a reverse current or heating a Li-ion battery can lead to very high case temperatures or even battery explosion. (reference 66)