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, 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.

Understanding sodium-ion diffusion in layered P2 and P3 oxides via experiments and first-principles calculations: a bridge between crystal structure and electrochemical performance

“…16 They have been extensively studied as electrode materials for rechargeable sodium-ion batteries. 17,18 Owing to direct Na-ion diffusion and a more open framework, the original structure of the P2 phases is more easily preserved during the sodium extraction process, [19][20][21][22] although a partial P2-O2 phase transition is found in the literature. 23,24 In contrast, the O3 phase always transforms to the P3 phase via gliding of the MeO 2 slabs (1/3, 2/3, 0) that results from the presence of intermediate tetrahedral sites between two octahedral sites ( Figure 1).…”

Layered Oxide Cathodes for Sodium‐Ion Batteries: Phase Transition, Air Stability, and Performance

“…[97] The addition of lithium partially suppresses the detrimental phase transition observed upon operating Na-ion insertion cathodes up to high potentials and reduces cooperative Jahn-Teller distortions of Mn 3+ . [98] On the other hand, Mg-substituted P2-Na 2/3 Mn 1−x Mg x O 2 (0 ≤ x ≤ 0.2) demonstrated smoother electrochemistry, improved capacity retention, and better rate performance with a fewer distinct electrochemical processes. [44,99,100] The presence of Mg on the transition metal lattice leads to fewer JahnTeller distorted Mn 3+ ions as well as disrupts potential Mn 3+ / Mn 4+ ordering.…”

Review—Manganese-Based P2-Type Transition Metal Oxides as Sodium-Ion Battery Cathode Materials

“…when 1/3 of the Na sites are vacant. 13 It is highly unfavorable for the Na content to exceed 0.9, as this necessarily results in simultaneous occupation of nearest-neighbor Na sites. 67,69 Instead, O3-type structures are typically obtained at high Na contents.…”

“…4,5,9,12,13,85 The role that oxide anions play in the redox reaction is still a subject of much debate. 47 Covalency effects control the redox couples in a well established manner, for example, replacing an oxide anion by a polyanion (such as a phosphate) raises the redox couple of the metal ion, due to an increase in the ionicity of the metal (the socalled inductive effect).…”