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International audience; Nitridocobaltates Li3−2xCoxN, with Li3N-type layered structure, are promising compounds as negative electrode materials for Li-ion batteries. In the present paper, we report the first detailed broadband dielectric spectroscopy (BDS) study on lithiated transition metal nitrides. The ionic and electronic conductivities of Li3−2xCox□xN compounds (0 ≤ x ≤ 0.44) are investigated as a function of the concentration x of cobalt ions, cationic vacancies (□) and lithium ions. Dielectric and conductivity spectra were recorded within the frequency range of 60–1010 Hz from 200 to 300 K. Experimental results exhibit two types of electric conduction: the first one is due to lithium ion diffusion (for 0 ≤ x ≤ 0.25) and the second one due to electronic transfers (for x ≥ 0.3). Furthermore, two percolation transitions are evidenced and associated with 3D ionic transfers (threshold at x ≈ 0.11) on the one hand and 2D electronic transfers (threshold at x ≈ 0.30) on the other hand. Upon increasing the frequency, dielectric relaxations appear from larger to smaller sample scales. These successive polarizations appear with increasing frequency in the following order: (a) sample/silver paint interface; (b) particles (aggregates of grains); (c) grains (crystallites); (d) local ionic and electronic motions within the grains. Evolutions of dielectric relaxation parameters (dielectric strength and relaxation frequency) with Co content confirm the two percolation transitions. Surprisingly, the grain conductivity has a large discontinuity immediately below the electronic percolation threshold where any local- and long-range ionic movement disappears without electronic transfer. This discontinuity would be due to a narrow transition from ionic to electronic conduction when x increases.

Li 3 FeN 2 displays rich and complex structural response upon electrochemical oxidation/reduction. During the first lithium deintercalation, 4 voltage plateaus corresponding to a total charge transfer of 1.14 e − per iron cation take place. Combining operando Mossbauer spectroscopy and X-ray diffraction, we evidence 3 biphasic reactions involving four orthorhombic phases. Despite a derived anti-fluorine type structure, Li 3 FeN 2 oxidation induces a unidirectional contraction along b axis. Mossbauer spectroscopy established a partial iron oxidation (∼90%). Therefore the participation of the nitrogen network as additional redox center is suggested to explain the observed extra capacity. Moreover, an unexpected low spin to high spin crossover took place for ∼10% of Fe 3+ during the oxidation of Li 3 FeN 2 . Based on the cationic mixing recently demonstrated and the anisotropic structural response, two possible explanations are discussed; (i) a significant deformation of 8 g lithium sites, which contain ∼10% of iron cations or (ii) migration of these cations into the neighboring octahedron 8 j . This High Spin Fe +3 contribution remains almost constant until the end of the oxidation.

A large data set of XAS (X-ray Absorption Spectroscopy) Manganese K-edge spectra has been collected operando and studied upon the electrochemical oxidation of the promising Li-ion battery anode material Li7MnN4. Using chemometric tools such as PCA (Principal Component Analysis) and MCR-ALS (Multivariate Curve resolution – Alternating Least Squares), three independent environment spectra were insulated. Based on the faradaic yield and well-chosen comparison of absorption spectrum energies within the frame of the coordination charge model, these environments were ascribed to unusual oxidation states allowed by nitride chemistry at a low potential (∼1.2 V vs. Li+/Li), i.e. Mn5+ (3d2), Mn6+ (3d1) and Mn7+ (3d0). Also, their relative amounts are discussed with regard to the long-range structural variation which can be simply described by two successive biphasic domains followed by a solid-solution behaviour. Gathering this long-range and local structure information provides a complete picture of the redox mechanisms occurring in Li7MnN4.

The kinetics of the electrochemical lithium insertion reaction in the as-prepared Li7MnN4 has been investigated using ac impedance spectroscopy as a function of cycles and temperature. The results are compared and discussed with that obtained for an optimized Li7MnN4 sample obtained by ball-milling. From this comparison, a promoting effect of a lower crystallite size on the electrochemical kinetics is evidenced. For the first time an experimental evaluation of the activation energy for Li diffusion in the as-prepared and the ball-milled Li7MnN4 is obtained. The lower activation energy obtained for the ball-milled nitride, Ea = 0.28 eV against Ea = 0.42 eV for the pristine material is responsible of the better rate capability and significant improvement of the electrochemical behaviour especially in terms of cycling properties: at high rate of 5C, a remarkable and stable capacity of 120 mAh g−1 is obtained over 50 cycles which competes very well with the LTO anode material.

Nitridocobaltates Li3-2xCoxN, with Li3N-type layered structure, are promising compounds as negative electrode materials for Li-ion batteries. In the present paper, we report the first detailed broadband dielectric spectroscopy (BDS) study on lithiated transition metal nitrides. The ionic and electronic conductivities of Li3-2xCox□xN compounds (0 ≤ x ≤ 0.44) are investigated as a function of the concentration x of cobalt ions, cationic vacancies (□) and lithium ions. Dielectric and conductivity spectra were recorded within the frequency range of 60-1010 Hz from 200 to 300 K. Experimental results exhibit two types of electric conduction: the first one is due to lithium ion diffusion (for 0 ≤ x ≤ 0.25) and the second one due to electronic transfers (for x ≥ 0.3). Furthermore, two percolation transitions are evidenced and associated with 3D ionic transfers (threshold at x ≈ 0.11) on the one hand and 2D electronic transfers (threshold at x ≈ 0.30) on the other hand. Upon increasing the frequency, dielectric relaxations appear from larger to smaller sample scales. These successive polarizations appear with increasing frequency in the following order: (a) sample/silver paint interface; (b) particles (aggregates of grains); (c) grains (crystallites); (d) local ionic and electronic motions within the grains. Evolutions of dielectric relaxation parameters (dielectric strength and relaxation frequency) with Co content confirm the two percolation transitions. Surprisingly, the grain conductivity has a large discontinuity immediately below the electronic percolation threshold where any local- and long-range ionic movement disappears without electronic transfer. This discontinuity would be due to a narrow transition from ionic to electronic conduction when x increases.

A large data set of XAS (X-ray Absorption Spectroscopy) Manganese K-edge spectra has been collected operando and studied upon the electrochemical oxidation of the promising Li-ion battery anode material Li

The chemical reactivity in air of the promising metallic nitride Li7MnN4 as negative electrode for Li-ion batteries has been studied. The evolution of the ageing process with exposure time has been examined by XRD and the degradation kinetics has been determined. This cubic metallic nitride is found to be much less sensitive to the air atmosphere than that exhibited by the lamellar nitridocobaltates Li3−2xCoxN (0.1 ⩽ x ⩽ 0.44). A fast oxidation step in air leads to the delithiated Li6.2MnN4 phase after a few hours and is thereafter progressively destroyed by reaction with moisture leading to the formation of LiOH⋅H2O and Li2CO3. Electrochemical experiments on various aged samples support these results. The demonstration of Li7MnN4 storage in dry air is showed to constitute a relevant solution to preserve the integrity of the nitride host lattice and then its excellent electrochemical properties.

The high rate performance of ball-milled Li 7 MnN 4 as negative electrode material in lithium-ion batteries has been investigated at C and 5 C rates. An optimization of ball-milling experimental conditions allows to synthesize this metallic nitride with attractive and improved specific capacity and cycle life compared to the pristine compound. The outstanding finding is its excellent cycle life over 50 cycles with a capacity of 240 mAh g −1 at 1 C rate in the potential range 1.6 V/1 V. Even at 5 C the promoting effect of ball-milling results in a remarkable high and stable capacity of 120 mAh g −1 upon cycling, which compares very well with the behavior achieved for Li 4 Ti 5 O 12 while the pristine material is practically inactive.

A significant cationic disorder is evidenced on Li 3 FeN 2 prepared through solid-state reaction under controlled atmosphere. This derivative anti fluorite type structure (orthorhombic, space group Ibam , a =4.870(1) A, b =9.652(1) A and c =4.789(1) A), solved first through single crystal X-ray diffraction [7] , is usually described by Li + and Fe +3 ordered distribution in tetrahedral sites formed by the nitrogen network, leading to [FeN 4/2 ] 3− edge-sharing tetrahedral chains. From 7 Li/ 6 Li Nuclear Magnetic Resonance spectroscopy, 57 Fe Mossbauer spectroscopy and powder X-ray and neutron diffraction, we demonstrate that about 4% of lithium sites are filled by iron and about 11% of iron sites are occupied by Li, which can explain the discrepancy within the Gudat's model observed on larger scale solid-state synthesis samples.

International audience; The Li7MnN4 structural response upon the first Li extraction insertion cycle is highlighted using in operando XRD experiments. A 3-phases mechanism involving two biphasic regions for 0.1 less than or similar to x less than or similar to 0.8 and 0.8 less than or similar to x less than or similar to 1.2 in Li7-xMnN4 and a solid solution behaviour (1.2 less than or similar to x less than or similar to 1.5) explains its electrochemical fingerprint. These successive structural transitions do not change the cubic symmetry of the cell and induce a limited cell contraction (similar to 7%) associated to a reversible mechanical strain. This finding partly explains the excellent cycle life of this promising negative electrode for Li-ion batteries. (C) 2013 Elsevier B.V. All rights reserved.