New O2/P2‐type Li‐Excess Layered Manganese Oxides as Promising Multi‐Functional Electrode Materials for Rechargeable Li/Na Batteries

Abstract

A new and promising P2‐type layered oxide, Na 5/6[Li 1/4 Mn 3/4]O 2 is prepared using a solid‐state method. Detailed crystal structures of the sample are analyzed by synchrotron X‐ray diffraction combined with high‐resolution neutron diffraction. P2‐type Na 5/6[Li 1/4 Mn 3/4]O 2 consists of two MeO 2 layers with partial in‐plane √3 a × √3 a‐type Li/Mn ordering. Na/Li ion‐exchange in a molten salt results in a phase transition accompanied with glide of [Li 1/4 Mn 3/4]O 2 layers without the destruction of in‐plane cation ordering. P2‐type Na 5/6[Li 1/4 Mn 3/4]O 2 translates into an O2‐type layered structure with staking faults as the result of ion‐exchange. Electrode performance of P2‐type Na 5/6[Li 1/4 Mn 3/4]O 2 and O2‐type Lix[Li 1/4 Mn 3/4]O 2 is examined and compared in Na and Li cells, respectively. Both samples show large reversible capacity, ca. 200 mA h g−1, after charge to high voltage regardless of the difference in charge carriers. Structural analysis suggests that in‐plane structural rearrangements, presumably associated with partial oxygen loss, occur in both samples after charge to a high‐voltage region. Such structural activation process significantly influences electrode performance of the P2/O2‐type phases, similar to O3‐type Li 2 MnO 3‐based materials. Crystal structures, phase‐transition mechanisms, and the possibility of the P2/O2‐type phases as high‐capacity and long‐cycle‐life electrode materials with the multi‐functionality for both rechargeable Li/Na batteries are discussed in detail.