Patrick Willmann

Effect of cobalt substitution on cationic distribution in LiNi1 − y CoyO2 electrode materials

A crystal chemistry study of LiNi1 − yCoyO2 phases, used as positive electrode in lithium batteries, is presented. These materials crystallize in the rhombohedral system (space group: R3m) with a layered structure. Rietveld profile refinement of the X-ray data shows that for low substitution amounts (≤ 0.20) extra-nickel ions are always present leading to the Li1 − zNi1 + z − tCotO2 (t = y(1 + z)) formula (z ∗ > 0), while for y ≥ 0.30, a pure 2D structure is obtained (z = 0). The stabilization of the 2D character of the structure by cobalt substitution in lithium nickelate leads to the improvement of the electrochemical properties.

Electroreduction of graphite in LiClO4-ethylene carbonate electrolyte. Characterization of the passivating layer by transmission electron microscopy and Fourier-transform infrared spectroscopy

Abstract Electrochemical intercalation of unsolvated lithium into pitch carbon fibres P100 and natural graphite UF4 has been carried out in LiClO4-ethylene carbonate electrolyte. The reversible electrochemical capacity for a current equal to 7 μA/mg is 260 mAh/g for P100 carbon fibres and about 350 mAh/g for UF4 graphite, respectively. During the first discharge (reduction) an electrochemical capacity greater than the theoretical value (372 mAh/g) corresponding to LiC6 is obtained. This excess of capacity can be related to the formation of a passivating layer on the carbon surface. Analysis of this layer by means of transmission electron microscopy (electron diffraction, electron energy loss spectroscopy, and imaging) and Fourier-transform infrared spectroscopy has shown that this layer is composed of lithium carbonate Li2CO3 and alkylcarbonates of lithium ROCO2Li. Formation of Li2CO3 occurs at potentials in the 1−0.8 V range versus Li + Li , and formation of lithium alkylcarbonates then follows at potentials below 0.8 V. We then attributed the voltage plateau at 0.9 V versus Li + Li observed in the electrochemical waves to the reduction of ethylene carbonate into Li2CO3. Transmission electron spectroscopy revealed the presence of lithium chloride in the electrolyte which appears as small rods.

The relationship between the composition of lithium nickel oxide and the loss of reversibility during the first cycle

Abstract A quantitative relationship between the reversibility of the intercalation/de-intercalation process and the amount z of extra-nickel ions within the lithium layers has been demonstrated by specific electrochemical studies of Li1 − zNi1 + zO2 cells. The oxidation of extra-nickel ions from divalent to trivalent state during the first electrochemical charge, leads to an irreversible shrinkage of the interslab space, and as a consequence, to a loss of reversibility in the first cycle. The re-intercalation composition limit is thus directly related to the concentration of extra-nickel ions. However, the overall reversibility can be recovered by a forced discharge up to the starting composition.

Electrochemical synthesis of binary graphite-lithium intercalation compounds

Abstract Electrochemical intercalation of lithium has been carried out using a cathode of pyrographite in a LiClO 4 -ethylene carbonate (EC) electrolyte. Charge and discharge curves present slopes and plateaus related to the existence of pure phases and biphasic systems respectively. Binary graphite-lithium intercalation compounds of stages I, II, III and IV were isolated and identified based on their (00 l ) X-ray diffraction diagrams. Our experimental conditions allow the obtention of compounds which do not contain coinserted solvent molecules, even for low stage materials.

Revisited structures of dense and dilute stage II lithium-graphite intercalation compounds

Abstract Electrochemical intercalation of unsolvated lithium into pyrographite has been carried out in LiClO4-ethylene carbonate electrolyte. Pure phases have been isolated and characterized by X-ray diffraction studies. The orientated texture of pyrographite has allowed the study of the different families of Bragg reflections (001, hk0, hkl) especially for stage I LiC6, 3D stage II LiC12 and dilute stage II ‘LiC18’ compounds. Experimental data and some theoretical models have been compared.

Ion transport theory of nonaqueous electrolytes. LiClO4 in γ-butyrolactone: the quasi lattice approach

As a part of a study on the optimisation of the electrolyte for high-density energy lithium batteries, transport properties of concentrated LiClO4 solutions in γ-butyrolactone (BL) have been investigated. The effect of the salt concentration (C) on the viscosity (η) of BL solutions has been discussed in term of the Jones–Dole equation. At concentrations higher than 0.2 M, the molar conductivity (Λ) of LiClO4 solutions follow a C1/3 cube root law which is predicted by the quasi lattice model first introduced by Gosh. In this model, the ions of the strong binary electrolyte are distributed in a lattice-like arrangement (fcc). The experimental value found for the slope of Λ vs. C1/3 relation is in fair agreement with the calculated one. The effect of the temperature on the viscosity and the conductivity of electrolyte solutions have been examined. These two transport processes are well described by Arrhenius type laws from which the activation energies for the viscosity Eaη and conductivity EaΛ are deduced. The variations of Eaη and EaΛ with salt concentration are respectively dependent on C and C4/3 as predicted by the quasi lattice model.

Electrochemical behavior of α-cobalted nickel hydroxide electrodes

Cobalt substituted α-nickel hydroxides obtained by precipitation techniques have been used as positive electrodes of NiCd batteries. As these materials are indefinitely stable in KOH solution, the electrochemical cycling between α and γ phases allows up to 1.3 e− per (Ni+Co) atom to be reversibly exchanged at the C/5 rate. Nevertheless, during long-range cycling, a slow evolution from the γ/α couple to the β(III)/β(II) couple occurs. It has been assigned to a partial reduction of Co3+ ions to Co2+ ions at the end of the discharge process. The evolution of the number of exchanged electrons (NEE) versus the cobalt amount is discussed.

Preparation and characterization of cobalt-substituted α-nickel hydroxides stable in KOH medium Part I. α′-hydroxide with an ordered packing

Abstract Ab α′-cobalt-substituted nickel hydroxide has been prepared from a β(II)-substituted nickel hydroxide by an oxidation to a γ-type phase followed by a reduction. This α′ Co 3+ -hydroxide crystallizes in the hexagonal system with a P 3-type hydroxyl packing ( a = 3.1 A, c = 23.5 A). β(II)- and α′-cobalted phases have been characterized by chemical analysis, TGA, u.v.-visible and i.r. spectroscopies. In both materials cobalt ions remain in the trivalent state. In α′ Co 3+ -hydroxide, the excess of positive charge is compensated by carbonate anions which are either inserted between the M(OH) 2 slabs, or adsorbed on the particles. The infrared study shows that inserted anions are totally hydrogen bonded with intercalated water molecules ( D 3 h symmetry), while the adsorbed ions are linked to the nickel or cobalt ions in substitution of hydroxyl groups. In β Co 3+ -hydroxide, the charge compensation due to the presence of trivalent cobalt ions is provided by chemisorbed carbonate anions and by a proton deficiency in the sheets. The bonding between the inserted carbonate anions and the Co 3+ cations increases the slab cohesion; as a result the α′ Co 3+ -phase ( x Co ⩾ 0.2) is stable indefinitely in KOH medium.

Preparation and characterization of cobalt-substituted α-nickel hydroxide stable in KOH medium Part II. α-Hydroxide with a turbostratic structure

Abstract α-Turbostratic cobalted nickel hydroxides have been obtained by precipitation, with an NaOH solution, from nickel and cobalt salts. Several preparation methods have been developed in order to obtain a material containing Ni 2+ and Co 3+ ions. Depending on the experimental procedure, carbonate and sulfate anions are inserted between the (Ni, Co)(OH) 2 slabs, in order to compensate for the excess of positive charge due to the presence of trivalent cobalt ions in the hydroxide. These materials have been characterized by chemical analysis, X-ray diffraction, TGA, and u.v.-visible spectroscopy. The resulting α Co 3+ phase is stable in KOH medium if the cobalt amount is at least equal to 20%. By contrast, the α Co 2+ hydroxide is spontaneously transformed to a β(II) hydroxide, whatever the cobalt amount.

Lithium insertion into new graphite–antimony composites

Metal-based composites are under investigation as possible negative-electrode materials in lithium-ion batteries. In this paper, we present a new composite material constituted of antimony particles dispersed on graphite. The antimony–graphite compound is prepared by antimony pentachloride reduction with KC8 in tetrahydrofuran. The high reversible capacity of 420 mAh g−1 and the good stability suggest that the association of antimony with graphite allows not only to improve reversible capacity but also to prevent the metal from particle pulverisation generally occurring during lithium alloying.