Abstract
The electrochemical properties of 
and 
were studied for different conditions of sample preparation and different degrees of cation substitution 
. In the voltage range 3.5–4.5 V, cells of either spinel 
or 
(made by leaching the Li from 
) reversibly insert 0.4 Li per Mn at an average voltage of 4.1 V, leading to an energy density of 480 Wh/kg of cathode. Cells cycled 50 times lost less than 10% of their initial capacity, suggesting that this material could be used instead of 
or 
as the cathode in the new generation of "rocking chair batteries." Replacing Mn with cations of valence 2 (Ni, Zn) or 3 (Fe) reduces the amount of Mn+3 and correspondingly reduces the capacity of the cells at 4.1 V, but does not affect their cycling performance.