Zhi Lu, Shiqiang Hao, Ziliang Wang, Hyungjun Kim and Christopher Wolverton*,
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引用次数: 0
Abstract
Li-rich layered transition metal oxides (Li1+xM1–xO2 or mLi2MnO3–nLiMO2) have been widely studied as cathode materials for Li-ion batteries recently due to their enhanced capacity of larger than 250 mAh g–1. However, even the qualitative nature of the phase stability of these materials, whether they form a solid solution or are phase separated, has been the subject of intense debate. In this work, we use density functional theory calculations to investigate the phase stability of these Li-rich layered transition metal oxides (Li2MnO3–LiMO2, M = Co, Ni, Mn). We calculate the mixing enthalpy and coherency strain energy between Li2MnO3 and LiMO2 for two distinct cases: (1) mixing of M on the Li and Mn sites respectively in the transition metal layer of Li2MnO3, resulting in a solid solution with C2/m symmetry, and (2) mixing of Li and Mn on the M sites of LiMO2, resulting in a solid solution with R3̅m symmetry. We show that phase separation is energetically preferred relative to a solid solution at T = 0 K, and the coherency strain energy has little influence on phase stability. Results also display that a solid solution with R3̅m symmetry has a larger mixing enthalpy than that with C2/m symmetry at T = 0 K. Furthermore, we use the mixing enthalpies along with mean-field mixing entropies to calculate free energies and phase diagrams. At low temperature, the system exhibits phase separation between the C2/m and R3̅m phases, with appreciable solubility in each phase, and at high temperature, there is a transformation to the single-phase R3̅m solid solution. For high Li content compositions, the phase diagram shows a region of stability for the single phase C2/m solid solution. Our calculations support one possible explanation for the discrepancies between various reports of the structure of these Li-rich layered materials; the compositions and temperatures of these synthesized materials could be close to phase boundaries separating the regions of solid solution vs phase-separation. The calculated phase diagrams also indicate that the phase stability of Li-rich layered materials largely depends on the synthesis temperature, the amount of excess Li, and the combination of transition metals.
期刊介绍:
The journal Chemistry of Materials focuses on publishing original research at the intersection of materials science and chemistry. The studies published in the journal involve chemistry as a prominent component and explore topics such as the design, synthesis, characterization, processing, understanding, and application of functional or potentially functional materials. The journal covers various areas of interest, including inorganic and organic solid-state chemistry, nanomaterials, biomaterials, thin films and polymers, and composite/hybrid materials. The journal particularly seeks papers that highlight the creation or development of innovative materials with novel optical, electrical, magnetic, catalytic, or mechanical properties. It is essential that manuscripts on these topics have a primary focus on the chemistry of materials and represent a significant advancement compared to prior research. Before external reviews are sought, submitted manuscripts undergo a review process by a minimum of two editors to ensure their appropriateness for the journal and the presence of sufficient evidence of a significant advance that will be of broad interest to the materials chemistry community.