Insight into the origin of lithium/nickel ions exchange in layered Li(NixMnyCoz)O2 cathode materials

Abstract

In layered LiNixMnyCozO2 cathode material for lithium-ion batteries, the spins of transition metal (TM) ions construct a two-dimensional triangular networks, which can be considered as a simple case of geometrical frustration. By performing neutron powder diffraction experiments and magnetization measurements, we find that long-range magnetic order cannot be established in LiNixMnyCozO2 even at low temperature of 3 K. Remarkably, the frustration parameters of these compounds are estimated to be larger than 30, indicating the existence of strongly frustrated magnetic interactions between spins of TM ions. As frustration will inevitably give rise to lattice instability, the formation of Li/Ni exchange in LiNixMnyCozO2 will help to partially relieve the degeneracy of the frustrated magnetic lattice by forming a stable antiferromagnetic state in hexagonal sublattice with nonmagnetic ions located in centers of the hexagons. Moreover, Li/Ni exchange will introduce 180°s uperexchange interaction, which further relieves the magnetic frustration through bringing in new exchange paths. Thus, the variation of Li/Ni exchange ratio vs. TM mole fraction in

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“…Consequently, the e g ‐electrons with higher energy lie close to the Fermi level and have a relatively strong hybridization with neighboring oxygen. Although the t 2g ‐electrons are more localized, they can have a small overlap with the oxygen 2 p‐states, consequently resulting in the super‐exchange interaction . The undesirable cation mixing leads to the transfer of TM ions into Li + diffusion channels.…”

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“…Zheng et al found that these defects benefit the thermal stability for Ni-rich NCM materials, because the Ni in the Li layer would form 180°Ni−O −Ni super exchange chains [17]. Moreover, it can partially relieve the magnetic frustration by forming a stable antiferromagnetic state in hexagonal sublattice with nonmagnetic ions located in centers of the hexagons [18]. Lee et al have proved that a little amount of Ni 2+ ions occupying Li + (3b) sites is not a major obstacle to the diffusion of Li + ions in the lithium layer [19].…”

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