The batteries that power our devices are largely based on lithium-ions because of their potency. Not all lithium-ion batteries are equal though as different internal designs and materials affect their performance, and one of those materials is nickel. Nickel has the ability to improve the energy density of a battery but also can impair its long-term performance, and researchers at the Pacific Northwest National Laboratory and Berkeley Lab have discovered why.
Within a battery, ions move around and their placement determines how much energy is being stored. The more easily the ions can move, and more places they can connect to the electrodes, the better the battery will perform. It turns out though that the nickel in lithium-ion batteries can prevent the movement of the ions by clumping together in solid blocks in the ions' channels. This is why a battery will seem to hold less and less of a charge over time, because fewer and fewer ions can connect with the electrode.
The good news is that because we now know what is going on, the PNNL researchers and others can find a way to address this issue. It may be awhile before a fix is found, but now we know the direction it is in, albeit roughly.