Interviews with Engineers - Mercury-free Alkaline Button Battery A Goal Others Dismissed as Impossible

In 2004, Sony succeeded in developing a mercury-free silver oxide button battery, a task that other manufacturers had dismissed as impossible. Five years later in 2009, Sony also announced the development of a mercury-free alkaline button battery. We asked Masatsugu Shiota---a Sony engineer involved in these initiatives---to talk about his experiences in developing these batteries. 

Mercury as a Panacea

Silver oxide batteries and alkaline button batteries have anodes that contain zinc. Corrosive reactions affecting this zinc produce hydrogen gas. In button batteries, this not only reduces output capacity, but also causes pressure to build up within the battery, which can lead to swelling, leakage and other problems. Traditionally, the production of hydrogen gas was suppressed through the use of mercury, which is highly effective in preventing zinc corrosion. Mercury was a panacea that maintained both the performance and safety of batteries.

For this reason, the development of mercury-free batteries was not simply a matter of removing the mercury. Without an alternative "panacea" there would be a heightened risk of hydrogen production. In 2004, Sony was able to create a mercury-free silver oxide battery by developing new technologies to curb corrosive reactions affecting the zinc. Sony also took advantage of the fact that the silver oxide used in the cathode had the capacity to absorb hydrogen. In the mercury-free silver oxide battery, the amount of hydrogen gas produced was dramatically reduced by improving the ability of the zinc to resist corrosion, and any minute amounts of hydrogen gas that were still generated were absorbed by the silver oxide. However, the cathode in an alkaline button battery is made from manganese dioxide. Unlike silver oxide, this material lacks the capacity to absorb hydrogen. This meant that it was impossible to eliminate the mercury from alkaline button batteries.

Adhoc Research

Our efforts to develop a mercury-free silver oxide battery were initially prompted by growing international concern about the environmental effects of mercury, and the tightening of environmental protection regulations. Everyone thought that button batteries couldn't be made without mercury, and thus they were exempt from regulations. However, we staked Sony's reputation on the early development of a mercury-free silver oxide battery. We couldn't use the same technology to produce a mercury-free alkaline button battery because the cathode wouldn't absorb hydrogen, with the result that there would be an increased risk of swelling and leakage. At the time, even members of the development team were convinced that the development of a mercury-free alkaline button battery was impossible.
Yet the only challenge facing the team was the lack of a substance to absorb any hydrogen gas produced. There was a nagging feeling that somehow this problem could be solved. In addition to our normal work, we began to carry out adhoc research and experiments in our spare time in the hope of discovering a way to address the hydrogen gas issue. The most difficult challenge was finding a suitable material to absorb the gas. We weren't even sure how much hydrogen needed to be absorbed to make the battery safe. So we simply continued to experiment with substances that could absorb hydrogen. In addition to checking individual substances, we also tried combining them to create new substances. 

Safety First

To meet our quality standards, the battery had to remain safe even when subjected to moderate misuse. Safety was especially important for alkaline button batteries, which are used in toys and other portable electronic items. However, safety wasn't the only priority. It might be possible to build a car with a body so robust that the passengers remain totally unharmed in the event of a collision. However, extremely poor fuel efficiency would halve the value of such a car. The same applies to batteries. It would be meaningless to improve the safety of a battery unless we also improved its value as a commercial product. The goal that we set for ourselves was to develop a battery that would be at least as good as an alkaline button battery containing mercury in terms of both safety and performance. After numerous experiments, we eventually succeeded in discovering an effective substance and the right mixture. Once we had combined this discovery with the technology we had developed for the mercury-free silver oxide battery, we knew we were on the right track.

Manufacturing Stage Challenges

We added a new hydrogen-absorbing material to the cathode leading to the development of a mercury-free alkaline button battery. Because this material had never been used before, we encountered a number of mass-production challenges. One of these was a decline in productivity when existing production methods were used. We were able to solve this problem by modifying the processes with the dedicated assistance of the production team. We had to set up a mass-production system to manufacture batteries in the same way as before using a material that we'd never used before. It was extremely difficult. However, we were eventually able to address all the problems thanks to the combined efforts of the design and production staff.

A Pass, but not 100%

The development of the mercury-free alkaline button battery was not the result of a totally different approach based on completely new ideas. The foundation for our success was technology accumulated through the development of various types of batteries since 1977. That gave us a base from which we were able to move forward.
Rather than a "eureka" experience, our sense of achievement when we succeeded in creating a mercury-free battery was more a feeling of relief that we had at last developed a product we could supply to users with confidence and pride. Yet while our product is safe, problems can arise if used improperly. We believe that while every product may receive a passing grade, no product, including this one, has achieved a score of 100%. That's why we'll continue to enhance our ability to develop new technology.

Posted in: technology