HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. On the mechanism of the P2–Na0.70CoO2→O2–LiCoO2 exchange reaction-Part I: proposition of a model to describe the P2–O2 transition Frédéric Tournadre, Laurence Croguennec, Ismaël Saadoune, Dany Carlier-Larregaray, Yang Shao-Horn, P. Willmann, Claude Delmas

A significant cationic disorder is evidenced on Li3FeN2 prepared through solid-state reaction under controlled atmosphere. This derivative anti fluorite type structure (orthorhombic, space group Ibam, a=4.870(1)Å, b=9.652(1)Å and c=4.789(1)Å), 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 [FeN4/2]3− edge-sharing tetrahedral chains. From 7Li/6Li Nuclear Magnetic Resonance spectroscopy, 57Fe Mössbauer 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.

Li3FeN2 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 Mössbauer spectroscopy and X-ray diffraction, we evidence 3 biphasic reactions involving four orthorhombic phases. Despite a derived anti-fluorine type structure, Li3FeN2 oxidation induces a unidirectional contraction along b axis. Mössbauer 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 Fe3+ during the oxidation of Li3FeN2. Based on the cationic mixing recently demonstrated and the anisotropic structural response, two possible explanations are discussed; (i) a significant deformation of 8g lithium sites, which contain ∼10% of iron cations or (ii) migration of these cations into the neighboring octahedron 8j. This High Spin Fe+3 contribution remains almost constant until the end of the oxidation.

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, E-a = 0.28 eV against E-a = 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. (C) 2015 Elsevier B.V. All rights reserved.

An optimization of the specific capacity exhibited by the best layered lithiated cobalt nitride Li2.20Co0.40N is proposed by using a conditioning electrochemical oxidation up to 1.1V before cycling in the 1.1V–0.02V potential range. This initial charge process allows the Co3+/Co2+ redox couple to be involved in the cycling process in addition to the Co2+/Co+ couple as in the 1V–0.02V voltage range. A new electrochemical fingerprint is obtained with a single step at 0.4/0.8V for the discharge-charge process and a specific capacity of 300mAhg−1 at C/5 which constitutes a huge improvement compared to 130mAhg−1 recovered in the conventional 1V–0.02V potential window. This high capacity value and the excellent capacity retention of 100% over at least 75 cycles make Li2.20Co0.40N a promising anode material for Li-ion batteries.

The reactivity of pure molecular fluorine F2 allows the creation of new materials with unique electrochemical properties.

The high rate performance of ball-milled Li7MnN4 as negative electrode material in lithium-ion batteries has been investigated at C and 5C 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 240mAhg−1 at 1C rate in the potential range 1.6V/1V. Even at 5C the promoting effect of ball-milling results in a remarkable high and stable capacity of 120mAhg−1 upon cycling, which compares very well with the behavior achieved for Li4Ti5O12 while the pristine material is practically inactive.

Thermal evolution of the layered oxide Li2/3Co2/3Mn1/3O2, showing a T(#)2 stacking and prepared by a Na/Li ion exchange in P2-Na2/3Co2/3Mn1/3O2, was investigated by thermal analyses and X-ray diffraction. A thermal expansion of the T(#)2 orthorhombic unit cell is observed from 25 to 350 degrees C; from 350 degrees C the T(#)2 stacking is destabilized to the benefit of an O6-type stacicing obtained from the former through slab gliding. The T(#)2 to O6 phase transformation is allowed to occur from a stacicing with larger interlayer distances and the lithium ions in tetrahedral sites to a stacking with smaller interlayer distances and the lithium ions in octahedral sites. This phase transition from T(#)2, to O6 is reversible, even though its kinetic can be very slow: the thermal treatment of the T(#)2-type Li2/3Co2/3Mn1/3O2 phase at 450 degrees C with a quenching in air has shown to stabilize the O6(HT)-Li2/3Co2/3Mn1/3O2 phase. At temperatures higher than 450 degrees C, the layered oxide Li2/3Co2/3Mn1/3O2 is gradually decomposed into Li2MnO3 and Co3O4. First electrochemical tests performed in lithium batteries have revealed that O6(HT)-Li2/3Co2/3Mn1/3O2 delivers as positive electrode material a high reversible capacity of similar to 230 mAh.g(-1) over two voltage domains around 3 and 4 V vs Li+/Li

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.

The chemical stability in air of a layered lithium nitridocobaltate Li2.13Co0.43N has been investigated using in situ XRD diffraction as a function of time. A high reactivity of the nitride with air moisture is found with the rapid emergence of lithium hydroxide. The ageing process finally leads to the decomposition of the nitride into lithium carbonate (Li2CO3), cobalt hydroxide (Co(OH)(2)) and the release of gaseous NH3. The effect of the ageing process on the electrochemical properties of this promising anode material for Li-ion batteries is reported. The discharge-charge properties of the compound slightly deteriorate after few hours while they are dramatically affected after 15 h. The present results suggest handling of these anodic materials in dry air would be possible with satisfactory electrochemical properties. (c) 2012 Elsevier Ltd. All rights reserved.

The kinetics of the electrochemical lithium insertion reaction in nano-sized rutile β-MnO2 has been investigated using ac impedance spectroscopy. The experimental kinetic data are obtained for a rutile compound synthesized by ball-milling the powder produced from the heat treatment of manganese nitrate salts. The results are discussed as a function of the Li content for 0<x<0.6 and the number of cycles in the 4.1–2V window. From a comparison with data obtained on the micro-sized oxide, an improved kinetics is found with DLi values for the apparent chemical diffusion coefficient of lithium much higher by one order of magnitude than in microsized oxide. Impedance behaviour of the ball-milled rutile β-MnO2 vs cycles demonstrates a new system takes place from the second cycle, characterized by a significant improvement of Li diffusion by a factor 5 and a cathode impedance which decreases by a factor 2, remaining thereafter unchanged during cycling.

In this paper, we report an investigation of three tin-based composite materials as negative electrodes for lithium-ion batteries. Theses composites were synthesized by solid state reaction from dispersion of micrometric tin into BPO4, Li-doped BPO4 (LiBPO) and Na-substituted BPO4 (NaBPO) matrix, respectively. We have investigated more particularly the influence of the two alkaline ions (Li+, Na+) introduced into the matrix on electrochemical performances. The morphology of powders was observed by SEM and the composition studied by EDX analysis. The conductivity measurements showed that the modified BPO4 matrixes (Li or Na) exhibit improved conductivity (σRT=2×10−11Scm−1 for NaBPO). A focus of our interest was to relate the nature and structural composition of the composite interface between active tin and inactive matrix to the irreversible capacity in this type of composite materials. The electrochemical analysis shows a decrease of the irreversible capacity for the composite based on modified matrixes (around 150 and 190mAhg−1 for SnNaBPO and SnLiBPO, respectively) with respect to the reference composite SnBPO (245mAhg−1).

We report a systematic study of the layered lithium nitridocuprates Li3−xCuxN with 0.1≤x≤0.39. The structural data obtained from experimental XRD patterns, Rietveld refinements and unit cell parameters calculation vs x, indicate that copper (I) substitute interlayer lithium ions in the parent nitride Li3N to form the Li3−xCuxN compound without any Li vacancy in the Li2N− layer. Electrochemical results report Li insertion into the corresponding layered structures cannot take place in the 1.2/0.02 V voltage range as in the case of lithium into nitridonickelates and nitridocobaltates. However, in the initial charge process of Li3−xCuxN at 1.4 V leading to a specific capacity higher than 1000 mA h/g, the oxidation of copper and nitride ions is probably involved inducing a strong structural disordering process. As a consequence a new rechargeable electrochemical system characterized by discharge–charge potential of ≈0.3 V/1.2 V appears from the second cycle. Cycling experiments 0.02 V voltage/0.02 V range induce a complete destruction of the layered host lattice and the presence of Cu3N in the charge state suggests a conversion reaction. The capacity recovered in the 1.4/0.02 V range practically stabilizes around 500 mA h/g after 20 cycles.

An EC/DEC [40:60% (v/v)] solvent mixture has been added in various amounts to the ionic liquid (IL) hexyltrimethylammonium bis(trifluoromethylsulfonyl)imide (N1116-NTf2) in the presence of LiNTf2 (lithium bis(trifluoromethylsulfonyl)imide) as lithium salt for possible use as electrolytes in lithium-ion batteries. These electrolytes exhibit a larger thermal stability than the reference electrolyte EC/DEC [40:60]+LiNTf2 1M when the percentage of the IL exceeds 30% (v/v). All studied electrolytes are glass forming ones with an ideal glass transition temperature of ca. −85°C(±5°C), which has been determined by application of the VTF theory to conductivity and viscosity measurements and confirmed by DSC (Tg=−90±5°C). An electrochemical window of about 5V versus Li/Li+ was measured at a glassy carbon electrode. The cycling ability of the optimized electrolyte N1116-NTf2/EC:DEC (40/60% (v/v))+1M LiNTf2 has been investigated at a titanate oxide (Li4Ti5O12) and a cobalt oxide (LixCoO2) electrodes. Cycling the positive and the negative electrodes was conducted successfully with a high capacity and without any significant fading.

Based on the voltammetric behaviour of a series of halogen-substituted dimethoxybenzene in 1M LiPF6/EC:DEC electrolyte, 2,5-difluoro-1,4-dimethoxybenzene (F2DMB) was selected and tested as an electrolyte additive for overcharge protection of Li/Li4Ti5O12 cell. From the galvanostatic study of the cells at different overcharge current (C/20, C/50, C/100 and C/200) in the presence of F2DMB, it was found that the shuttle additive can be adsorbed at the cathode surface and form a dense layer which prevents the intercalation of Li+ ion in the positive electrode. At low overcharge current (C/200 rate) the voltage of the cell levelled off at the oxidation potential of the shuttle molecule for more than 50 cycles, but at higher charge rates (C/50 and C/100), the voltage of the cell was levelled off for only 16 cycles. The reason is that the F2DMB molecules remaining in solution after the formation of the layer at the cathode cannot carry the current even at charge rates as low as C/100.

New lithium nickel nitrides Li3−2xNixN (0.20≤x≤ 0.60) have been prepared and investigated as negative electrode in the 0.85/0.02V potential window. These materials are prepared from a Ni/Li3N mixture at 700°C under a nitrogen flow. Their structural characteristics as well as their electrochemical behaviour are investigated as a function of the nickel content. For the first time are reported here the electrochemical properties of a lithium intercalation compound based on a layered nitride structure. The Li3−2xNixN compounds can be reversibly reduced and oxidized around 0.5V versus Li/Li+ leading to specific capacities in the range 120–160mAh/g depending on the nickel content and the C rate. Due to a large number of lithium vacancies, the structural stability provides an excellent capacity retention of the specific capacity upon cycling.

Redox shuttle electrolyte additives have been suggested as a possible mean of internal overcharge protection of secondary lithium-ion batteries. TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) is one of these redox shuttles for overcharge protection of 3V class Li-ion cells. The electrochemical reversibility and the diffusion coefficient of this molecule has been evaluated by mean of cyclic voltammetry. The redox shuttle voltage was found to be 3.5V versus Li/Li+ and D=cm2s−1. The electrochemical stability of TEMPO in different overcharging conditions has been evaluated by long-term cycling using Li/Li4Ti5O12 cells. Results show that the TEMPO redox system does not act as an ideal shuttle. When dissolved in the electrolyte at 0.5M, this additive is able to level off the cell potential at 3.5V for a long period at low overcharging current (C/200 to C/50). Nevertheless, it appears that the cell capacity fades drastically at the first cycles and with time. This phenomenon is probably related to the stability of the oxidized and reduced form of the TEMPO molecule.