Electrochemical Evaluation of Pb, Ag, and Zn Cyanamides/Carbodiimides

PbNCN, Ag2NCN, and ZnNCN were tested as negative electrode materials for Li-ion batteries. A thorough analysis of the electrochemical mechanism by X-ray diffraction and X-ray absorption spectroscopy showed that, unlike transition metal carbodiimides, these compounds react with lithium via a two-step reaction, starting with conversion followed by alloying. The conversion reaction is highly irreversible for the three compounds, whereas the reversibility of the alloying reaction depends on the metal, that is, highly irreversible for PbNCN and Ag2NCN which contain the cyanamide group (NC−N2−) and more reversible for ZnNCN containing carbodiimide (−NCN−). In the case of the more covalent, cyanamide-type PbNCN and Ag2NCN, the conversion reaction occurs at a higher voltage compared to the more ionic, carbodiimide-type ZnNCN, correlated with the nature of bonding in the NCN group and in the phases themselves. Compared to transition metal carbodiimides, these materials show rather low performance, with no improvement in capacity as it would have been expected from the combination of conversion and alloying

Les carbodiimides et cyanamides, une nouvelle famille de matériaux d'électrodes pour batteries Li-ion

Li-ion batteries are currently the most common choice for all portable electronic devices but also for hybrid electric vehicles and renewable energy sectors. At present, graphite is routinely employed as the anode material for Li-ion-batteries due to its excellent attributes such as long cycle life, abundance, and relatively cost effective. However, the disadvantages of graphitic anode include low energy density and safety concerns. As a consequence, alternative cost effective anode materials with high energy density and long cycle life have been widely explored. Among this transition metal based compounds are an exciting and reasonable alternative for graphite owing to their high specific capacity. Compounds with the formula MX where M is a divalent metal and X = O, S, PO4, and CO3 have been reported to be electrochemically active at average voltages around 1 volts. In spite of their high theoretical specific capacities, high irreversible capacity in the first lithiation and the weak cycling life prevent the practical use of these materials. Since 2015, the possibility of using transition metal carbodiimides (MNCN, with M = Fe, Mn, Co, Cu, Zn, Ni) have been reported, and some of them have shown promising electrochemical performance as anode materials for both Li and Na ion batteries. Like all divalent metal based electrode materials, carbodiimides have been found to suffer from high initial irreversible capacity and high operating voltage, however they show a better cycle life. The application of transition metal carbodiimides in the field of energy storage (and conversion) is still in its early stages and despite progress in electrochemical evaluation much remains to be done in order to establish the reaction mechanisms that govern the reported promising performances. Besides the transition metal carbodiimides there are still many other inorganic cyanamides and carbodiimides materials to explore. Therefore the main targets of this PhD work are (i) to assess the properties of new carbodiimides/cyanamides as electrode materials for LiBs and (ii) to establish their electrochemical reaction mechanisms via advanced operando techniques and DFT calculations. Concerning the electrochemical performance, Cr2(NCN)3 turned out to be by the far the best carbodiimide anode material with stable specific capacity of more than 600 mAh.g-1 for more than 900 cycles at 2C rate. CoNCN and FeNCN have also shown excellent electrochemical properties since they can sustain a specific capacity higher than 500 mAh.g-1 for more than 100 cycles at 2C rate. Poor performance was observed for PbNCN, Ag2NCN and ZnNCN since the practical capacities are well below the theoretical ones. These phase show also fast capacity fading during the first 20 cycles. These three performance categories correlate well with the three different reaction mechanisms established for the investigated phases. Up to now, three types of reaction mechanism have been identified including (i) Combined intercalation and conversion processes in the case of Cr2(NCN)3 as evidenced by both theoretical and experimental methods, (ii) pure conversion reaction in the case of CoNCN and finally (iii) a combined conversion and alloying mechanism in the case of Pb, Zn and Ag compounds. It is worth noting that whatever the reaction pathway, all the carbodiimide/cyanamide anode materials face the limitation of a significantly low coulombic efficiency during the first cycles. To overcome this obstacle, much effort is needed to clarify the nature and the role of SEI in the overall performance of this family of materials. The promising results reported in this work do not probably yet meet the standards needed to take carbodiimides/cyanamides into the practical applications, but they clearly evidence the rich possibilities offered by this young family of molecular inorganic materials.Les batteries Li-ion constituent actuellement la technologie de choix pour tous les équipements portables, les moyens de transports électriques et le stockage stationnaire des énergies renouvelables. Actuellement, le graphite est incontestablement le matériau d'anode le plus utilisé pour les batteries Li-ion en raison de ses excellentes propriétés telles que sa durabilité, son abondance et son faible coût. Cependant, sa faible densité d'énergie est son talon d'Achille. En plus, le graphite présente certains problèmes de sécurité, en particulier à des puissances élevées. En conséquence, d’autres matériaux sûrs, économiques, à haute densité énergétique et à longue durée de vie, font l’objet d’importants travaux de recherche notamment des candidats comme le silicium et l’étain. Depuis 2015, la possibilité d'utiliser des carbodiimides de métaux de a été rapportée, et certains d'entre eux ont montré des performances électrochimiques prometteuses en tant que matériaux anodiques pour les batteries aux ions Li et Na. Comme tous les matériaux d'électrode à base de métaux divalents, les carbodiimides souffrent d'une capacité irréversible initiale et d'un potentiel de fonctionnement élevés, mais présentent une meilleure tenue en cyclage. L'application de carbodiimides de métaux de transition dans le domaine du stockage (et de la conversion) de l'énergie en est encore à ses débuts malgré les progrès réalisés en terme d'évaluation électrochimique. Il reste encore beaucoup à faire pour établir les mécanismes réactionnels qui régissent les performances prometteuses observées. Outre les carbodiimides de métaux de transition, il reste encore de nombreux carbodiimides inorganiques à explorer. Par conséquent, les principaux objectifs de cette thèse sont (i) d’évaluer la possibilité d’application de nouveaux carbodiimides comme matériaux d’électrode pour les batteries Li-ion et (ii) d’établir les mécanismes réactionnels électrochimique de ces matériaux au moyen de techniques de caractérisation operando avancées couplées à des calculs DFT. En ce qui concerne les performances électrochimiques, Cr2(NCN)3 s'est révélé être le meilleur matériau d'anode, avec une capacité spécifique stable de plus de 600 mAh.g-1 sur plus de 900 cycles à un régime de 2C. CoNCN et FeNCN ont également d’excellentes propriétés électrochimiques, car ils peuvent maintenir une capacité spécifique supérieure à 500 mAh.g-1 pendant plus de 100 cycles à un régime de 2C. Des performances plus modestes ont été observées pour PbNCN, Ag2NCN et ZnNCN car les capacités pratiques sont bien inférieures aux capacités théoriques. Ces phases montrent également une chute de la capacité sur les premiers 20 cycles. Ces trois catégories de performances sont bien corrélées avec les trois mécanismes différents réactionnels établis pour toutes les phases étudiées. Jusqu'à présent, trois types de mécanismes réactionnels ont été identifiés, à savoir (i) un processus combinant une étape d’intercalation suivie d’une étape de conversion dans le cas de Cr2(NCN)3, (ii) une réaction de conversion pure dans le cas de CoNCN et enfin (iii) un mécanisme combiné de conversion et d’alliage dans le cas des composés Pb, Zn et Ag. Il convient de noter que, quelle que soit le mécanisme réactionnel, tous les matériaux d'anode carbodiimide sont confrontés à la limitation d'une faible efficacité coulombique au cours des premiers cycles. Pour surmonter cet obstacle, il faut déployer plus d'efforts pour clarifier la nature et le rôle de la SEI dans la performance globale de cette famille de matériaux. Bien que les résultats prometteurs présentés dans ce travail ne répondent probablement pas aux normes requises pour intégrer les carbodiimides dans des applications commerciales, ils ont au moins le mérite de montrer la richesse de la chimie des carbodiimides et de stimuler davantage de travaux de recherche sur cette famille de matériaux inorganiques moléculaires relativement jeune

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

International audienceWe 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

Reversible High Capacity and Reaction Mechanism of Cr 2 (NCN) 3 Negative Electrodes for Li‐Ion Batteries

International audienceA detailed study of the electrochemical reaction mechanism between lithium and the trivalent transition-metal carbodiimide Cr2(NCN)3, which shows excellent performance as a negative electrode material in Li-ion batteries, is conducted combining complementary operando analyses and state-of-the-art density functional theory (DFT) calculations. As predicted by DFT, and evidenced by operando X-ray diffraction and Cr K-edge absorption spectroscopy, a two-step reaction pathway involving two redox couples (Cr3þ/Cr2þ and Cr2þ/Cr0) and a concomitant formation of Cr metal nanoparticles is apparent, thus indicating that the conversion reaction of this carbodiimide upon lithiation occurs only after a preliminary intercalation step involving two Li per unit formula. This mechanism, evidenced for the first time in transition-metal carbodiimides, is likely behind its outstanding electrochemical performance as Cr2(NCN)3 can maintain more than 600 mAh g1 for 900 cycles at a high rate of 2 C

Cobalt Carbodiimide as Negative Electrode for Li‐Ion Batteries: Electrochemical Mechanism and Performance

International audienceCobalt carbodiimide, CoNCN, shows outstanding performance as negative electrode material for Li‐ion batteries, maintaining a reversible capacity of 530 mAh g−1 over 140 cycles at a current density of 540 mA g−1. The electrochemical lithiation/delithiation mechanism of cobalt carbodiimide was investigated using complementary in situ X‐ray diffraction and X‐ray absorption spectroscopy. Upon lithiation, CoNCN undergoes a reversible conversion reaction, forming Li2NCN and fcc Co metal nanoparticles, which are transformed back into CoNCN upon delithiation. However, the CoNCN obtained electrochemically after delithiation do not recover the local structure of the pristine phase, and might contain the NCN2− ligand in the cyanamide isomer form (N−C≡N2−). It would be the first time that a transition metal cyanamide isomer is obtained at ambient conditions