Final answer:
Lithium is the lightweight alkali metal used in high-energy density batteries, such as lithium-ion batteries, due to its light weight and high reactivity. It's different from alkaline batteries, which use potassium hydroxide as an electrolyte.
Step-by-step explanation:
The lightweight alkali metal mentioned in the question is lithium (Li). Lithium is known for its role in producing efficient, high-energy density batteries, notably lithium-ion batteries. These batteries make good use of lithium's properties, such as its light weight and high reactivity, to provide a portable power source for a wide range of devices.
Alkaline batteries, which were developed in the 1950s as replacements for zinc-carbon dry cells, utilize alkaline electrolytes, commonly potassium hydroxide. However, lithium batteries and alkaline batteries are different; lithium batteries provide higher voltage and longer-lasting performance. Lithium, being an alkali metal, is part of Group 1 in the periodic table, which includes other elements like sodium and potassium.
Despite its reactivity and softness, which make lithium unsuitable for structural applications, its properties are highly advantageous when it comes to battery manufacture. Lithium's high reactivity plays a key role in its ability to store and release energy efficiently in batteries.