Structure and Electrochemical Properties of Li1 − x  ( Ni y Co1 − y  ) 1 + x  O 2 : Effect of Chemical Delithiation at 0°C

Acid treatment at 0°C of Li 1 − x ( Ni y Co 1 − y ) 1 + x O 2 solid solutions yields delithiated well crystalline oxides Li 0.5 ( Ni y Co 1 − y ) 1 + x O 2 which do not contain protons. For the nearly stoichiometric nickel‐rich oxides, removal of more than 20% of the lithium leads to a monoclinic distortion of the rhombohedral unit cell. Electron paramagnetic resonance (EPR) of low‐spin Ni 3+ ions shows that for up to 0.2 extracted lithium, the generated Ni 4+ ions are statistically distributed in the NiO 2 layers while above 0.2 extracted lithium the Ni 4+ ions trend to order. During acid delithiation the oxidation of Ni 3+ to Ni 4+ ions proceeds prior to the oxidation of Co 3+ to Co 4+ as for a nonaqueous electrolyte. Discharge‐charge cycles of lithium cells using acid‐delithiated oxides as cathode materials show a significant improvement of reversibility compared with the charge‐discharge cycles of the corresponding pristine samples.