A new and promising P2‐type layered oxide, Na5/6[Li1/4Mn3/4]O2 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 Na5/6[Li1/4Mn3/4]O2 consists of two MeO2 layers with partial in‐plane √3a × √3a‐type Li/Mn ordering. Na/Li ion‐exchange in a molten salt results in a phase transition accompanied with glide of [Li1/4Mn3/4]O2 layers without the destruction of in‐plane cation ordering. P2‐type Na5/6[Li1/4Mn3/4]O2 translates into an O2‐type layered structure with staking faults as the result of ion‐exchange. Electrode performance of P2‐type Na5/6[Li1/4Mn3/4]O2 and O2‐type Lix[Li1/4Mn3/4]O2 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 Li2MnO3‐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.

A new high-capacity electrode material made from only earth-abundant elements.

O4-type LiCoO2 as a third polymorph of LiCoO2 is prepared by an ion-exchange method in aqueous media from OP4-[Li, Na]CoO2, which has an intergrowth structure of O3-LiCoO2 and P2-Na0.7CoO2. O4-type LiCoO2 is characterized by synchrotron X-ray diffraction, neutron diffraction, and X-ray absorption spectroscopy. Structural characterization reveals that O4-type LiCoO2 has an intergrowth structure of O3- and O2-LiCoO2 with stacking faulted domains. Three LiCoO2 polymorphs are formed from the close-packed CoO2 layers, which consist of edge-shared CoO6 octahedra, whereas the oxide-ion stacking is different: cubic in the O3-phase, cubic/hexagonal in the O2-phase, and alternate O3 and O2 in the O4-phase. Structural analysis using the DIFFaX program suggests that the O4-phase consists of approximately 30% of O12-domains, while stacking faults are not evidenced for O2-phase. The results suggest that a nucleation process for Na/Li ion-exchange kinetically dominates a growth process of ideal O4-domains because the presence of CoO2-Li-CoO2 blocks as O3-domains could be expected to prevent through-plane interaction of Na layers. Electrochemical behavior and structural transition processes for three LiCoO2 polymorphs are compared in Li cells. A new phase, OT(#)4-type Li0.5CoO2, is first isolated as an intergrowth phase of O3- and T(#)2-Li0.5CoO2. However, some deviations from ideal behavior as the O2/O3-intergrowth phase are also noted, presumably because of the existence of stacking faults.

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