Magnetic analysis of lamellar oxides for Li-ions batteries

International audienceWe show that the investigation of magnetic properties is the best tool to identify and quantify the impurities and defects that limit the ability of the lamellar intercalation compounds for use as cathode for Li-ion batteries. The results are illustrated for LiNiO2, LiNi1-yCoyO2, LiCoO2, LiNi0.5Mn0.5O2, and LiNi1/3Mn1/3Co1/3O2 (LNMCO). Despite the extensive studies of these ionic compounds in the past, not only for practical use, but also for themselves, the present work reveals that the magnetic properties of these lamellar compounds have been largely misunderstood, and that, at contrast with the common belief, they do not belong to the family of two dimensional frustrated antiferromagnets. The misunderstanding comes from confusion between extrinsic and intrinsic effects. This distinction allows for an overall understanding of the intrinsic properties of these materials, and opens the route to their optimization, in particular for LNMCO that is the most promising element of this family

Study of the Li-insertion / extraction process in LiFePO4/FePO4

International audienceThe structural properties of LiFePO4 prepared by the hydrothermal route and chemically delithiated have been studied using analytical electron microscopy and Raman spectroscopy. High-resolution transmission electron microscopy and selected area electron diffraction measurements indicate that the partially delithiated particles include LiFePO4 regions with cross-sections of finite size along the ac-plane, as a result of tilt grain boundary in the bc-plane, and dislocations in other directions. Only the boundary along the bc-plane is accompanied by a disorder over about 2 nm on each side of the boundary. The Raman spectrum shows the existence of both LiFePO4 and FePO4 phases in the shell of the particles at a delithiation degree of 50%, which invalidates the core–shell model. This result also invalidates the recent model according to which each particle would be single-domain, i.e. either a LiFePO4 particle or a FePO4 particle. On the other hand, our results, like prior ones, can be understood within the framework of a model similar to the spinodal decomposition of a two-phase system, which is discussed within the framework of morphogenesis of patterns in systems at equilibrium. Both end-members, however, are well crystallized, suggesting a recovery similar to that observed in superplastic alloys, with dynamics that are due to the motion of nucleation fronts and dislocations, and not due to a diffusion phenomenon associated with a concentration gradient

Magnetic properties of LiNi0.5Mn1.5O4 spinels prepared by wet chemical methods

International audienceWe present the magnetic properties of LiNi0.5Mn1.5O4 spinels prepared by the sol–gel and pyrolysis techniques. Structural properties show that the material synthesized by pyrolysis method exhibit an ordered spinel structure. Characterization methods include SQUID magnetometry and ESR spectroscopy. Magnetic measurements have evidenced the ferromagnetic ordering below T c 1⁄4 129 K in LiNi0.5Mn1.5O4. Results show that actually no impurity phase is detected in LiNi0.5Mn1.5O4, thus the ferrimagnetic behavior is attributed to an intrinsic property of this material. The magnetic order in this case is trivially a collinear ferrimagnetic ordering in which both the Ni sublattice and the Mn sublattice are ferromagnetic. ESR measurements show a two-component signal with a complex shape. The dominant band is assigned to Mn4+ ions that are the only paramagnetic entities in this compound

Minimization of the cation mixing in Li1+x(NMC)1−xO2 as cathode material

International audienceLi1+x(Ni1/3Mn1/3Co1/3)1−xO2 layered materials were synthesized by the co-precipitation method with different Li/M molar ratios (M=Ni +Mn + Co). Elemental titration evaluated by inductively coupled plasma spectrometry (ICP), structural properties studied by X-ray diffraction (XRD), Rietveld analysis of XRD data, scanning electron microscopy (SEM) and magnetic measurements carried out by superconducting quantum interference devices (SQUID) showed the well-defined -NaFeO2 structure with cationic distribution close to the nominal formula. The Li/Ni cation mixing on the 3b Wyckoff site of the interlayer space was consistent with the structural model [Li1−yNiy]3b[Lix+yNi(1−x)/3−yMn(1−x)/3Co(1−x)/3]3aO2 (x = 0.02, 0.04) and was very small. Both Rietveld refinements and magnetic measurements revealed a concentration of Ni2+-3b ions lower than 2%; moreover, for the optimized sample synthesized at Li/M= 1.10, only 1.43% of nickel ions were located into the Li sublattice. Electrochemical properties were investigated by galvanostatic charge-discharge cycling. Data obtained with Li1+x(Ni1/3Mn1/3Co1/3)1−xO2 reflected the high degree of sample optimization. An initial discharge capacity of 150mAhg−1 was delivered at 1 C-rate in the cut-off voltage of 3.0-4.3 V. More than 95% of its initial capacity was retained after 30 cycles at 1 C-rate. Finally, it is demonstrated that a cation mixing below 2% is considered as the threshold for which the electrochemical performance does not change for Li1+x(Ni1/3Mn1/3Co1/3)1−xO2

Mechanism of the Fe Reduction at Low Temperature for LiFePO Synthesis from a Polymeric Additive

International audienceComparison is made between the use of either a carbon powder or a polymer additive to the precursors in the synthesis of LiFePO4 from the Fe(III) compound FePO4(H2O)2 and Li2CO3. The evolution of the structural properties and phase purity with temperature and time have been monitored at all length scales by X-ray diffraction, Fourier transformed infrared spectroscopy, and magnetic susceptibility. The reactor temperature was decreased to 300 °C to investigate the early stages of the reaction. Formation of crystalline LiFePO4 begins in the range 300-400 °C only if the polymer is used as the carbonaceous additive. This LiFePO4 formation is made possible by the reduction of Fe(III) species by gases such as H2 or gaseous hydrocarbons evolved during the calcination of the polymer. Moreover, decomposition of the polymer results in a carbonaceous deposit on the surface of the LiFePO4 particles. An Li3Fe2(PO4)3 impurity found after sintering at 400 °C for 4 h was greatly reduced after sintering at 400 °C for 24 h, and phase- pure LiFePO4 was attained at 700 °C. Where the solid carbon powder was used as the reducing agent, no Fe(II) species could be detected after sintering at 400 °C. Carbothermal reduction of Fe(III) is ruled out in this temperature range

Synthesis and characterization of LiNi1/3Mn1/3Co1/3O2 by wet-chemical method

International audienceA series of LiNi1/3Mn1/3Co1/3O2 samples with ˛-NaFeO2 structure belonging to the R¯3m D3d 5 space group were synthesized using tartaric acid as a chelating agent by wet-chemical method. Different acid to metal-ion ratios R have been used to investigate the effect of this parameter on the physical and electrochemical properties. We have characterized the reaction mechanism, the structure, and morphology of the powders by TGA, XRD, SEM and TEM imaging, completed by magnetic measurements, Raman scattering spectroscopy, and complex impedance experiments. We find that the LiNi1/3Mn1/3Co1/3O2 sintered at 900 ◦C for 15 h with an acid to metal-ion ratio R = 2 was the optimum condition for this synthesis. For this optimized sample, only 1.3% of nickel-ions occupied the 3b Wyckoff site of the lithium-ions sublattice. The electrochemical performance has been investigated using a coin-type cell containing Li metal as the anode. The electronic performance is correlated to the concentration of the Ni(3b) defects that increase the charge transfer resistance and reduce the lithium diffusion coefficient. The optimized cell delivered an initial discharge capacity of 172mAhg−1 in the cut-off voltage of 2.8-4.4 V, with a coulombic efficiency of 93.4%

Structural and magnetic properties of Lix(MnyFe1-y)PO4 electrode materials for Li-ion batteries

International audienceA series of LiMnyFe1-yPO4 samples have been prepared in the whole range 0≤y≤1. Chemical delithiation could be achieved to obtain MnyFe1-yPO4 in the range 0≤y≤0.8, keeping the same crystal phase (olivine structure, space group Pnma). The composition y=0.8 is the limit where the delithiated phase is still crystallized, but abruptly suffers strains at the molecular scale evidenced by both optical spectroscopy and X-ray diffraction. The analysis of the magnetic properties shows that all the samples are the concentration of impurities is negligible. The concentration of polarons, either holes associated to Li vacancies in LiMnyFe1-yPO4 or electrons associated to the existence of Li left in the matrix of MnyFe1-yPO4, is found to be small (≤ 1%) in all the samples. For y ≤0.6, all the Mn3+ ions MnyFe1-yPO4 are in the high spin state (S=2). At larger manganese concentration, however, the Mn3+ ions in excess of the critical concentration yc=0.6 undergo a transition to the low-spin state (S=1). As a consequence, and in contrast with prior works, we find that Mn0.8Fe0.2PO4 has magnetic interactions that are much smaller, and no antiferromagnetic ordering in this compound is detected, at least above 20 K. Antiferromagnetic ordering that had been reported so far for MnyFe1-yPO4 at large y-composition might come from incomplete delithiation. The spin transition of Mn3+ in concentration (y-yc) to the low-spin state is at the origin of the strain fields at the molecular scale that increase with y for y>0.6, and ultimately prevents the full delithiation for y>0.8. This result sheds light on the reason for the degradation of cathode properties in Mn-rich compounds of the heterosite-purpurite series, while the electrochemical properties are good in the range y ≤0.6 but only at slow rates, due to the very small hopping mobility of the small polaron

Aging of LiFePO4 upon exposure to H2O

International audienceThe effect of H2O on carbon-coated LiFePO4 particles was investigated by chemical analysis, structural analysis (X-ray diffraction, SEM, TEM), optical spectroscopy (FTIR, Raman) and magnetic measurements. Upon immersion in water, part of the product floats while the main part sinks. Both the floating and the sinking part have been analyzed. We find that the floating and sinking part only differ by the amount of carbon that partly detaches from the particles upon immersion in water. Exposure to H2O results in rapid attack, within minutes, of the surface layer of the particles, because the particles are no longer protected by carbon. The deterioration of the carbon coat is dependent on the synthesis process, either hydrothermal or solid-state reaction. In both cases, however, the carbon coat is permeable to water and fails to protect the surface of the LiFePO4 particles. The consequence is that this immersion results in the chemical attack of LiFePO4, but is restricted to the surface layer of the particles (few nanometers-thick). In case the particles are simply exposed to humid air, the carbon coat protects the particles more efficiently. In this case, the exposure to H2O mainly results in the delithiation of the surface layer, due to the hydrophilic nature of Li, and only the surface layer is affected, at least for a reasonable time of exposure to humid air (weeks). In addition, within this timescale, the surface layer can be chemically lithiated again, and the samples can be dried to remove the moisture, restoring the reversible electrochemical propertie

Wave-front error breakdown in laser guide star multi-object adaptive optics validated on-sky by CANARY

Context. Canary is the multi-object adaptive optics (MOAO) on-sky pathfinder developed in the perspective of multi-object spectrograph on extremely large telescopes (ELTs). In 2013, Canary was operated on-sky at the William Herschel telescope (WHT), using three off-axis natural guide stars (NGS) and four off-axis Rayleigh laser guide stars (LGS), in open-loop, with the on-axis compensated turbulence observed with a H-band imaging camera and a Truth wave-front sensor (TS) for diagnostic purposes. Aims. Our purpose is to establish a reliable and accurate wave-front error breakdown for LGS MOAO. This will enable a comprehensive analysis of Canary on-sky results and provide tools for validating simulations of MOAO systems for ELTs. Methods. To evaluate the MOAO performance, we compared the Canary on-sky results running in MOAO, in single conjugated adaptive optics (SCAO) and in ground layer adaptive optics (GLAO) modes, over a large set of data acquired in 2013. We provide a statistical study of the seeing. We also evaluated the wave-front error breakdown from both analytic computations, one based on a MOAO system modelling and the other on the measurements from the Canary TS. We have focussed especially on the tomographic error and we detail its vertical error decomposition. Results. We show that Canary obtained 30.1%, 21.4% and 17.1% H-band Strehl ratios in SCAO, MOAO and GLAO respectively, for median seeing conditions with 0.66′′ of total seeing including 0.59′′ at the ground. Moreover, we get 99% of correlation over 4500 samples, for any AO modes, between two analytic computations of residual phase variance. Based on these variances, we obtain a reasonable Strehl-ratio (SR) estimation when compared to the measured IR image SR. We evaluate the gain in compensation for the altitude turbulence brought by MOAO when compared to GLAO