Novel Mn[N(CN)2]2 Anode with Cationic and Anionic Redox Chemistry for Li‐Ion Batteries

Abstractα‐Mn[N(CN)2]2 (Pnnm) with divalent Mn2+ being octahedrally coordinated by six [N(CN)2]− dicyanamide (dca‒) complex anions is known since 1999. A novel β polymorph is now prepared and its structure characterized via powder X‐ray diffraction. β‐Mn[N(CN)2]2 also crystallizes orthorhombically (Cmcm) but with Mn2+ tetrahedrally coordinated to four [N(CN)2]− anions. Property‐wise, α‐Mn[N(CN)2]2 can be efficiently used as a negative electrode material for lithium‐ion batteries, maintaining a large reversible capacity of more than 600 mAh g−1 for 250 cycles tested at 0.5 C, comparing favorably to well‐established negative electrode references such as graphite (≈372 mAh g−1). The electrochemical lithiation/delithiation mechanism of α‐Mn[N(CN)2]2 is investigated using advanced characterization techniques and theoretical calculations. Upon lithiation, α‐Mn[N(CN)2]2 undergoes a reversible conversion reaction, forming LiN(CN)2 and metallic manganese, which are transformed back into α‐Mn[N(CN)2]2 upon delithiation. Further, there is evidence for reversible and additional charge/discharge processes on the dca‒ anion throughout the entire discharge/charge process in α‐Mn[N(CN)2]2, reflecting an anionic charge compensation. Moreover, density‐functional (DFT) and chemical‐bonding theory are employed to investigate the detailed anodic behavior of α‐Mn[N(CN)2]2 via conversion reaction during (de‐)lithiation processes. This mechanism, evidenced for the first time in transition metal dicyanamides, is likely behind its outstanding electrochemical properties.