There are many types of batteries in the world. Primary ones are dry-cell batteries are used once and discarded. Secondary battery is lithium-ion battery can be recharged and repeatedly. Solar cells represent anther type of battery. Conventionally, button batteries contain mercury to prevent the generation of hydrogen gas. However, the use of mercury is not without risks. The improper use or disposal of mercury-based batteries carries adverse risks for both the environment and human health. Yet, developing technology necessary to create mercury-free button batteries was an extremely difficult challenge. Sony's commitment to reducing its environmental impact is a reflection of its unrelenting efforts to meet that challenge, and in 2004 it succeeded in developing the world's first mercury-free silver oxide battery. In 2009, Sony achieved what was regarded as an even more difficult task: the development of technology leading to the world's first mercury-free alkaline button battery.