try to keep the explanation as simple as possible .. wikipedia killed me x.x
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Its easy for Lithium to give up electrons. Electricity is the transfer of electrons.
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Lithium metal is VERY easy to oxidize. The reaction Li ==> Li2+ + 2e- has an oxidation potential of +3.05 V! So Li metal easily produces a lot of electrons, which is what a battery must do.
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Because its a good conductor of electricity