Übergangsmetallcarbodiimide als molekulare negative Elektroden‐ materialien für Li‐ und Na‐Ionenbatterien mit hervorragendem Zyklisierungsverhalten

14N, 13C, and 119Sn solid-state NMR characterization of tin(II) carbodiimide Sn(NCN)

We report the first magic-angle spinning (MAS) nuclear magnetic resonance (NMR) study on Sn(NCN). In this compound the spatially elongated (NCN)2− ion is assumed to develop two distinct forms: either cyanamide (N≡C–N2−) or carbodiimide (−N=C=N−). Our 14N MAS NMR results reveal that in Sn(NCN) the (NCN)2− groups exist exclusively in the form of symmetric carbodiimide ions with two equivalent nitrogen sites, which is in agreement with the X-ray diffraction data. The 14N quadrupolar coupling constant | C Q | $\vert {C}_{\text{Q}}\vert $  ≈ 1.1 MHz for the −N=C=N− ion in Sn(NCN) is low when compared to those observed in molecular compounds that comprise cyano-type N≡C– moieties ( | C Q | $\vert {C}_{\text{Q}}\vert $  > 3.5 MHz). This together with the information from 14N and 13C chemical shifts indicates that solid-state NMR is a powerful tool for providing atomic-level insights into anion species present in these compounds. The experimental NMR results are corroborated by high-level calculations with quantum chemistry methods.

Flexible yet Robust Framework of Tin(II) Oxide Carbodiimide for Reversible Lithium Storage

Metal-organic frameworks (MOFs) can become promising electrode materials for advanced lithium-ion batteries (LIBs), because their loosely packed porous structures may mitigate volume expansion and metal atom aggregation, which occur at the respective metal oxides. However, they suffer from poor electrical conductivity and irreversible structural degradation upon charge/discharge processes, which impede their practical utilization. Herein, we investigate MOF-like Sn₂ O(CN₂ ) as a new electrode material. The conductive yet flexible [N=C=N] linkers are tilted between [Sn₄ O] nodes and cross-linked into a porous quasi-layered structure. Such structure offers abundant channels for fast Li-ion transport and tolerance of enormous volume expansion. Notably, anisotropic [N=C=N]^2- arrays hardly migrate so that Sn⁰ nanodots are physically separated via robust [N=C=N]^2- framework during discharge, thereby effectively preventing the formation of large Sn islands. Owing to the structural advantage, the Sn₂ O(CN₂ ) electrode exhibits an initial Coulombic efficiency as high as ∼80 %. With the addition of graphite as conductive supporter, the electrode provides 978 mAh g^-1 at 1.0 A g^-1 even after 300 cycles. Such MOF-like carbodiimides hold potential for the advanced electrodes in LIBs and other battery systems.

Quantum-Chemical Study of the FeNCN Conversion-Reaction Mechanism in Lithium- and Sodium-Ion Batteries

We report a computational study on 3d transition-metal (Cr, Mn, Fe, and Co) carbodiimides in Li- and Na-ion batteries. The obtained cell voltages semi-quantitatively fit the experiments, highlighting the practicality of PBE+U as an approach for modeling the conversion-reaction mechanism of the FeNCN archetype with lithium and sodium. Also, the calculated voltage profiles agree satisfactorily with experiment both for full (Li-ion battery) and partial (Na-ion battery) discharge, even though experimental atomistic knowledge is missing up to now. Moreover, we rationalize the structural preference of intermediate ternaries and their characteristic lowering in the voltage profile using chemical-bonding and Mulliken-charge analysis. The formation of such ternary intermediates for the lithiation of FeNCN and the contribution of at least one ternary intermediate is also confirmed experimentally. This theoretical approach, aided by experimental findings, supports the atomistic exploration of electrode materials governed by conversion reactions.

Chloride carbodiimide K12Pb51(CN2)30Cl54 with an unprecedented 45 Å unit cell axis and a large birefringence

K₁₂Pb₅₁(CN₂)₃₀Cl₅₄ exhibits a giant birefringence of 0.094 at 1064 nm and a large cell axis due to the rotation of Pb²⁺ cations.