As a compound analogue of black phosphorus, a new 2D semiconductor of SnS layers is proposed. Based on state-of-the-art theoretical calculations, we confirm that such 2D SnS layers are thermally and dynamically stable and can be mechanically exoliated from α-phase SnS bulk materials. The 2D SnS layer has an indirect band gap that can be tuned from 1.96 eV for the monolayer to 1.44 eV for a six-layer structure. Interestingly, the decrease of the band gap with increasing number of layers is not monotonic but shows an odd−even quantum confinement effect, because the interplay of spin−orbit coupling and lack of inversion symmetry in odd-numbered layer structures results in anisotropic spin splitting of the energy bands. It was also found that such 2D SnS layers show high in-plane anisotropy and high carrier mobility (tens of thousands of cm 2 V −1 s −1 ) even superior to that of black phosphorus, which is dominated by electrons. With these intriguing electronic properties, such 2D SnS layers are expected to have great potential for application in future nanoelectronics.

Ni/Li exchange (disordering) usually happens in layered Li(NiMnCo)O (NMC) materials and affects the performance of the material in lithium-ion batteries. Most of previous studies attributed this phenomenon to the similar size of Ni and Li, which implies that Ni should be more favorable than Ni to be located at Li 3b sites in the Li slab. However, this theory cannot explain why in Ni-rich NMC materials where most Ni cations are Ni, Ni/Li exchange happens even more frequently. Using extensive ab initio calculations combined with experiments, here we report that a superexchange interaction between transition metals plays a dominating role in tuning the Ni/Li disordering in NMC materials. Under this scheme, we also propose a new charge compensation mechanism that describes that after Ni/Li exchange the nearest Co transforms to Co in Ni-rich NMC materials. On the basis of this theory, the existence of Co in the initial Ni-rich NMC samples was predicted for the first time, which was further confirmed by our synchrotron-based soft X-ray absorption spectroscopy.

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

Nickel-rich transition-metal (TM) layered oxides, particularly those with high Ni content, attract worldwide interest for potential use as high-capacity cathodes in next-generation Li-ion batteries. However, as Ni loading increases, Li and Ni sitting at octahedra tend to mix, resulting in reduced electrochemical activity, which has been one major obstacle to their practical applications. Herein, we investigate the kinetic and thermodynamic aspects of Li/Ni mixing in LiNi0.7Mn x Co0.3–x O2 (0 ≤ x ≤ 0.3) as they are synthesized, through quantitative determination of structural ordering and comparison to ab initio calculations. Results from this study elucidate the role of Co/Mn-substitution in tuning Li/Ni ordering, intrinsically through local magnetic interaction. Specifically, Co substitution facilitates Li/Ni ordering by relieving the intra-plane magnetic frustration and reducing the inter-plane super-exchange (SE) interaction; in contrast, Mn exacerbates magnetic frustration and strengthens SE, thereby aggravating Li/Ni mixing. These findings highlight the interplay between local magnetic interaction and cationic ordering, which has yet to be fully investigated for the needs of designing high-Ni layered cathodes and, broadly, TM-based oxides for various applications.

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