Why Li as a solid has high electical and thermal conductivity?

Like the other alkali metals, lithium has a single valence election that is easily given up to form a cation Because of this, lithium is a good conductor of heat and electricity as well as a highly reactive element, though it is the least reactive of the alkali metals. Lithium's low reactivity is due to the proximity of its valence electron to its nucleus (the remaining two electrons are in the 1s orbital, much lower in energy, and do not participate in chemical bonds)

Lithium conducts electricity in its solid and molten states. Lithium exhibits metallic bonding and as such its valence electrons are delocalised in the liquid and solid state allowing electricty to be conducted. ... However, when molten the ionic bonds are broken and the ions are mobile allowing electricty to be conducted