Ali Darwiche

Better Cycling Performances of Bulk Sb in Na-Ion Batteries Compared to Li-Ion Systems: An Unexpected Electrochemical Mechanism

Pure micrometric antimony can be successfully used as negative electrode material in Na-ion batteries, sustaining a capacity close to 600 mAh g(-1) at a high rate with a Coulombic efficiency of 99 over 160 cycles, an extremely high capacity compared to any other compound tested against both Li and Na. The reaction mechanism with Na does not simply go through the alloying mechanism observed for Li where the intermediate species are those expected from the phase diagram. In the case of Na, the intermediate phases are mostly amorphous and could not be precisely identified. Surprisingly, we evidenced that a competition takes place at the end of the discharge of the Sb/Na cell between the formation of the hexagonal and the cubic polymorphs of Na(3)Sb, the last being described in the literature as unstable at atmospheric pressure and only synthesized under high pressure (1-9 GPa). In addition, fluoroethylene carbonate added to the electrolyte combined with an appropriate electrode formulation based on carboxymethyl cellulose, carbon black, and vapor ground carbon fibers seems to be determinant in the excellent performances of this material.

NiP3: a promising negative electrode for Li- and Na-ion batteries

Due to the abundance and low cost of sodium-containing precursors ambient temperature sodium ion batteries are promising for large scale grid storage. The low melting point of Na (97.7 °C) compared to 180.6 °C for Li represents a significant safety hazard for the use of Na metal anodes at ambient temperatures, which emphasizes the need for scientists and engineers to identify, design and develop new negative electrodes for Na-ion batteries. The identification of a suitable negative electrode is a crucial challenge for any further successful development of new cells, and to date efficient and competitive negative electrodes for NaB are still very rare. In this work we demonstrate that NiP3 could be a good challenger for this purpose. NiP3 based electrodes are evaluated as negative electrode materials for Li-ion batteries (LiB) and Na-ion batteries (NaB). The study of the reaction mechanism reveals the formation of a phase of composition close to Li3P and Na3P embedding Ni nanoparticles as the final reaction product after a full discharge. While the direct conversion of NiP3 into Na3P is identified for the reaction versus Na, it is still unclear whether an amorphous phase exists during the first discharge for the reaction versus Li before the conversion. Furthermore, thanks to the carboxymethyl cellulose/carbon black (CMC/CB) electrode formulation, the NiP3 electrode possesses a very promising capacity with a reversible storage capacity higher than 1000 mA h g−1 after 50 cycles for LiB and 900 mA h g−1 after 15 cycles for NaB, which represents one of the highest capacities ever sustained in Na-ion batteries.

Tracking Sodium-Antimonide Phase Transformations in Sodium-Ion Anodes: Insights from Operando Pair Distribution Function Analysis and Solid-State NMR Spectroscopy

Operando pair distribution function (PDF) analysis and ex situ 23Na magic-angle spinning solid-state nuclear magnetic resonance (MAS ssNMR) spectroscopy are used to gain insight into the alloying mechanism of high-capacity antimony anodes for sodium-ion batteries. Subtraction of the PDF of crystalline NaxSb phases from the total PDF, an approach constrained by chemical phase information gained from 23Na ssNMR in reference to relevant model compounds, identifies two previously uncharacterized intermediate species formed electrochemically; a-Na3–xSb (x ≈ 0.4–0.5), a structure locally similar to crystalline Na3Sb (c-Na3Sb) but with significant numbers of sodium vacancies and a limited correlation length, and a-Na1.7Sb, a highly amorphous structure featuring some Sb–Sb bonding. The first sodiation breaks down the crystalline antimony to form first a-Na3–xSb and, finally, crystalline Na3Sb. Desodiation results in the formation of an electrode formed of a composite of crystalline and amorphous antimony networks. We link the different reactivity of these networks to a series of sequential sodiation reactions manifesting as a cascade of processes observed in the electrochemical profile of subsequent cycles. The amorphous network reacts at higher voltages reforming a-Na1.7Sb, then a-Na3–xSb, whereas lower potentials are required for the sodiation of crystalline antimony, which reacts to form a-Na3–xSb without the formation of a-Na1.7Sb. a-Na3–xSb is converted to crystalline Na3Sb at the end of the second discharge. We find no evidence of formation of NaSb. Variable temperature 23Na NMR experiments reveal significant sodium mobility within c-Na3Sb; this is a possible contributing factor to the excellent rate performance of Sb anodes.

Exceptionally highly performing Na-ion battery anode using crystalline SnO2 nanoparticles confined in mesoporous carbon

Confined and unconfined SnO2 nanoparticles in the pores of mesoporous carbon were prepared and tested as anode materials vs. Na. Both composites present small crystalline SnO2 particles (∼3 nm) but different location and dispersion in the carbon matrix. When the particles are homogeneously distributed and confined in the carbon pores, an initial reversible capacity of 780 mA h g−1 is achieved with unprecedented capacity retention of 80 and 54% after 100 and 4000 cycles, respectively, at a high current rate (50 C, 1800 mA g−1). Unexpectedly, over two current rate variation cycles from 1 C to 500 C, the composite recovers 81% and 97%, respectively after returning from the 500 C to the 1 C rate. To our knowledge, no other material with such a long cycling life and superior performance in terms of capacity and rate capability has been reported so far for sodium ion batteries. HRTEM, XRD, N2 adsorption, XPS and galvanostatic cycling results suggest that confined SnO2 particles undergo an enhanced sodium alloying/dealloying process due to their special confinement inside the pores, which increases their conductivity, facilitates the diffusion of Na+ ions and buffers the large volumetric changes during charge/discharge. These high performances cannot be delivered when SnO2 is not confined and not well dispersed in the carbon pores. This work demonstrates that nano-confinement of anode species in carbon is a valuable concept affording the modification of the fundamental properties of guest species along with their electrochemical performances leading to highly stable and performing materials with a long life for Na-ion batteries.

Transition-Metal Carbodiimides as Molecular Negative Electrode Materials for Lithium- and Sodium-Ion Batteries with Excellent Cycling Properties

We report evidence for the electrochemical activity of transition-metal carbodiimides versus lithium and sodium. In particular, iron carbodiimide, FeNCN, can be efficiently used as negative electrode material for alkali-metal-ion batteries, similar to its oxide analogue FeO. Based on (57)Fe Mössbauer and infrared spectroscopy (IR) data, the electrochemical reaction mechanism can be explained by the reversible transformation of the Fe-NCN into Li/Na-NCN bonds during discharge and charge. These new electrode materials exhibit higher capacity compared to well-established negative electrode references such as graphite or hard carbon. Contrary to its oxide analogue, iron carbodiimide does not require heavy treatments (such as nanoscale tailoring, sophisticated textures, or coating) to obtain long cycle life with current density as high as 9 A g(-1) for hundreds of charge-discharge cycles. Similar to the iron compound, several other transition-metal carbodiimides M(x)(NCN)y with M=Mn, Cr, Zn can cycle successfully versus lithium and sodium. Their electrochemical activity and performance open the way to the design of a novel family of anode materials.