Lithium-rich nickel manganese oxide battery cathode materials and methods

A method of forming an active material for a positive electrode of a lithium-ion battery, the method comprising:

• quenching a powder of the active material in water,
• wherein the water is at room temperature prior to the quenching, and
• the powder of the active material is quenched at a rate of at least 1750° C./second, and
• wherein the active material comprises a layered lithium-rich nickel manganese oxide, and
• wherein after the step of quenching but before the active material is charged for a first charge, the layered lithium-rich nickel manganese oxide comprises a first crystal phase and a second crystal phase different than the first crystal phase, wherein the first crystal phase is a monoclinic crystal phase.