  Despite significant potential as energy storage materials for electric
vehicles due to their combination of high energy density per unit cost and
reduced environmental and ethical concerns, Co-free lithium ion batteries based
off layered Mn oxides presently lack the longevity and stability of their
Co-containing counterparts. Here, we demonstrate a reduction in this
performance gap via chemical doping, with Li$_{1.1}$Ni$_{0.35}$Mn$_{0.55}$O$_2$
achieving an initial discharge capacity of 159 mAhg$^{-1}$ at C/3 rate and a
corresponding capacity retention of 94.3% after 150 cycles. We subsequently
explore the nanoscale origins of this improvement through a combination of
advanced diffraction, spectroscopy, and electron microscopy techniques, finding
that optimized doping profiles lead to an improved structural and chemical
compatibility between the two constituent sub-phases that characterize the
layered Mn oxide system, resulting in the formation of unobstructed lithium ion
pathways between them. We also directly observe a structural stabilization
effect of the host compound near the surface using aberration corrected
scanning transmission electron microscopy and integrated differential phase
contrast imaging.
