TRANSITION METAL CARBODIIMIDES, A NEW CLASS OF ANODE MATERIALS

Battery technology is central to the development of improved transportation and communication systems. The market is hungry for improved energy storage devices which motivate sustained efforts in finding new anode, cathode, or electrolyte materials. In this context it is thus surprising to stumble across entirely unchartered territory as happened with our work on iron carbodiimide, FeNCN. Transition metal carbodiimides have been investigated only in the last fifteen years, although the material class is known since CaNCN was first synthesized in 1877. CaNCN is still the best known family member and has been used intensively as fertilizer. The transition metal carbodiimides can be thought of as equivalent to transition metal oxides, where the O2- is replaced by the (–N=C=N–)2- moiety. The rigid character of this carbodiimide group leads to unique structural properties, and, amazingly, for some transition metals, magnetism survives.The intriguing magnetic properties of FeNCN led us to investigate this material by iron-57 Mössbauer spectroscopy [1]. A quite complex spectrum is observed below the Néel temperature of 345 K. The spectrum consists of 5 visible lines and simplifies to an asymmetric doublet upon further cooling to 6 K, see Fig. 1, left. An in depth analysis reveals that the magnetic hyperfine field is perpendicular to the principal axis of the electric field gradient and decreases with temperature because the orbital contribution is reduced through thermal depopulation. The interesting rather open framework of the structure, of NiAs type with As replaced by NCN, with prominent diffusive channels prompted us to investigate electrochemical properties. The result of the electrochemical testing with combined in-situ XRD and Mössbauer spectroscopy reveals an outstanding reversible capacities of 600 and 400 mAh/g against Li and Na respectively and excellent resilience against high current rate cycling (9 A/g at 32 C) [2]. The scenario that emerges from combining XRD, Mössbauer and infrared spectral data is that the material undergoes a conversion mechanism that can be formulated as
FeNCN(s) + 2 Li(s) ⇌ Fe(s) + Li2NCN(s).
Interestingly, although the diffractograms after cycling do not exhibit any peak of the starting FeNCN material, Mössbauer spectral data confirms that a magnetic FeNCN spectrum is recovered. These observations indicate nanometric crystalline FeNCN as the endpoint Several Mx(NCN)y carbodiimides with M = Mn, Cr, Zn where investigated electrochemically and exhibit similar promising performance and cyclability. This investigation thus opens the way to the design of a whole novel family of anode materials where O is replaced by NCN and a corresponding patent[3] has been filed.
References
[1] M. Herlitschke, et al., New J. Chem. 2014, 38, 4670–4677.
[2] M. T. Sougrati, et al., Angew. Chemie Int. Ed. 2016, 55, 5090-5095.
[3] M. T. Sougrati, et al., Metal Carbodiimides and Metal Cyanamides as New Active Electrode Materials, EP15305888 (2015), European Patent pending.