  For understanding the stability of Co-free positive electrode material,
LiNi0.5Mn0.5O2 was synthesized with different addition amount of lithium during
calcination. The valence states of transition metal in the prepared samples
were determined by combining accurate stoichiometry analysis via ICP, magnetic
moment measurement via SQUID, and element valence analysis via XPS with Ar ion
etching. Unexpectedly, the transition metals, Ni and Mn, at interior and
surface of LiNi0.5Mn0.5O2 particles show different electrochemical properties.
This answer lingering questions of Li de-intercalation mechanism in
LiNi0.5Mn0.5O2.
