Why can’t alkaline batteries be recharged?
D1B is a high intensity powder diffractometer with an 80° PSD (position sensitive detector). One of its major use is for kinetic and real time experiments like the one shown here. This video was recorder in January 1995 by M6, the French TV company. It shows the installation on the instrument of a sample alkaline battery and explains that in situ neutron study of the charge/discharge process of alkaline batteries made it possible to understand why alkaline batteries are not rechargeable. To summarise: when an alkaline battery discharges, electrolyte water decomposes into H+ and OH- ions. The H+ ion inserts itself into MnO2 forming an interstitial compound, transferring an electron at the same time (reduction).
MnO2 + xH+ + xe- <=> HxMnO2
The OH- hydroxyl ion oxides zinc into zinc hydroxide, Zn(OH)2 and loses one electron e-.
Zn+2OH- <=> Zn(OH)2 + 2e-
At the start of the discharge, when less than 40% has reacted (x<0.4), the intermediate component HxMnO2 has the same structure (ramsdellite) as MnO2. As the discharging continues, a new compound appears. It is MnOOH (groutite), whose structure is very different and which does not re-transform back into ramsdellite if H+ is taken out. This is why, beyond x = 0.4 discharge, the reaction is no longer reversible and the battery cannot be recharged.