Researchers at the Georgia Institute for Technology have developed a lithium-ion battery with a few key chemical changes that allow it to charge itself when compressed or flexed. By substituting the plastic barrier that contains the battery electrolyte (a gelatinous goo that chemically stores electricity) with a layer of piezoelectric material called PVDF (the part that converts motion into energy), the team has created a hybrid battery that converts movement into both electric and chemical energy.
Normally, piezoelectric devices like this energy generating knee brace convert movement into electricity which is then converted into chemical energy by the charging circuit within a standard lithium-ion battery. By creating a hybrid battery that can directly influence the flow of ions within itself, the researchers can skip the charging circuit entirely, creating a far more efficient means of capturing mechanical energy. There's quite a bit of physics involved, but the smaller, less obtrusive biomechanical devices that could result from this research may be one answer the ever increasing power needs of mobile devices.