Strain Effects on Redox Reaction in Li-Rich Layered Oxide Electrode

Abstract

It has been conventionally understood that constituent transition metals mainly contribute to the charge compensation that accompanies Li insertion/extraction in oxide-electrode materials for lithium ion batteries (LIBs). However, such understanding is no longer sufficient in Li-rich layered electrode materials, which is a quasi solid solution of Li2 M 1O3 and LiM2 O2 (M 1 = Mn, Ru, Sn, M 2 = Mn, Co, Ni) because their practical capacities are larger than the capacities predicted from the valence changes of the constituent transition metals. This electrode family frequently exhibits the domain structures of Li2 M 1O3 and LiM2 O2 phases1. It would induce significant strain in the structure due to a lattice mismatch of each phase during the Li insertion/extraction, which facilitates an orbital hybridization between oxygen and transition metal. Such strong hybridization is expected to eventually unlock the limited capacity beyond that predicted by the valence changes of the constituent transition metals. It is, therefore, of great importance to reveal the local strain in the domain structures of the electrode during Li insertion/extraction in order to expedite the development of this electrode family. In this study, the local structure of transition metal and its evolution in a Li-rich layered oxide electrode upon charge/discharge were investigated using a combination of X-ray diffraction spectroscopy (XDS)2,3, with which phase-selective X-ray absorption fine structure (XAFS) can be obtained, and X-ray absorption spectroscopy (XAS). The active material of Li[Li0.2Ni0.2Mn0.6]O2 (LNMO) was synthesized by well-established procedure of the solid-reaction method followed by the coprecipitation technique4. Charged and discharged samples in the first two cycles for the X-ray analyses were prepared by the electrochemical Li insertion/extraction at 50 °C using a plastic bag cell. Both XDS and XAS measurements around Mn and Ni K-edges were performed at BL28XU in SPring-8. For the phase-selective XAFS analysis, resonant X-ray diffraction intensities were measured at the superlattice-diffraction peak near 003 diffraction of the layered rock-salt structure and its fundamental diffraction peaks, i.e., 003 and 104. The XDS study on superlattice diffraction of LNMO revealed that this material exhibits composite structure of Mn- and Ni-rich domains. Long-range order of the Mn-rich domain vanishes during the first charge. However, the XAFS analyses indicates that the local honeycomb ordering of Mn in this domain, which is also seen in Li2MnO3, remains in subsequent charge/discharge cycles. Little change in Mn-O bond length indicates little contribution to the charge compensation of this domain accompanies with Li insertion/extraction. In contrast, the drastic change in Ni-O bond length indicates the intensive redox reaction of Ni-rich domain, the magnitude of which is beyond that expected by the valence change of Ni from +II to +IV. The crystalline structure of this domain is constrained within the transition-metal layer by electrochemically inactive Mn-rich domain, which induces the trigonal distortion of the octahedral NiO6cluster by the in-plane strain when shortening the bond length of Ni-O that accompanies Li extraction. Such distortion would facilitate the orbital hybridization between an atomic d-orbital of transition metal and p-orbital of oxygen, and eventually allow us to utilize the deeper redox reaction of the transition-metal oxide electrodes. Evaluation and design of the domain structure of the electrode will pave the way to bring out the unseen potentials of the layered oxide electrodes.