A little less than a decade after their quantum-chemical prediction and eventual synthesis, solid-state transition-metal carbodiimides and closely related compounds have somewhat unexpectedly emerged as energy materials. In these carbodiimides, the O2- oxide dianion has been replaced by the complex NCN2- dianion, and the outstanding properties of such materials are likely related to their metastability and their higher amount of covalency compared to related oxides. When used as anode materials in rechargeable Li- and Na-ion batteries, one finds a conversion reaction, and further improving their performance will likely involve studying the redox behavior of NCN2-, the synthesis of novel ternary carbodiimides, in particular those with redox-active transition metals, and controlling their morphology. At present, such materials serve as catalysts in photochemical water oxidation, where they outperform their oxide cousins.

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.