This paper investigates the role of the stacking fault (5%, 20% and 50%) and the morphology of Li2MnO3 lamellar materials, issued from coprecipitation method with three annealing temperatures, on the surface reactivity. The structure and the morphology have been characterized by XRD, SEM and TEM. We studied the surface reactivity of these materials by combining X-ray photoemission spectroscopy (XPS), gaseous adsorption and first-principle calculations. An evolution of the reactivity toward the SO2 acid gaseous probe has been observed for the three materials, from pure redox mechanism toward mixed acid-base/redox mechanisms, respectively for 5% and 50% of stacking faults. We demonstrated that the electronic structure of Li2MnO3 being not modified by stacking faulted. Thus, the surface reactivity of faulted Li2MnO3 is not linked to the SF rate but only governed by the accessible crystalline surfaces and the manganese environments at the surface atomic layer. The formation of (0 0 1)-Li surface according to the Li-overstoichiometry on the extreme surface and the random particles shape of the more faulted materials are responsible of the reactivity tuning.

Phase Change Materials as those of the Ge-Sb-Te ternary system are of great interest for technological applications. Properties of these compounds are strongly related to presence of vacancies and structural investigations remain challenging. In this paper we evidence that 125Te NMR in natural abundance and using commercial systems at intermediate field (14.1 ​T) together with NMR parameters prediction can contribute to improve understanding of electronic structure of such systems. GeTe is a typical phase change material, whose structure contains germanium vacancies, even in its stoichiometric form, giving it metallic properties. Here, we use nominal Ge50Te50 and Ge48Te52 crystalline samples as an example to optimize the WURST-CPMG technique, a powerful technique to record wide NMR spectra which has not yet been used on 125Te. The goal was to minimize the time devoted to experiments as well as maximize the signal-to-noise ratio in order to detect small intensity signals directly linked to vacancies. Virtual Crystal Approximation (VCA) calculations performed with WIEN2K helped to interpret the NMR spectra. For Te-based crystalline conducting samples the best experimental results were obtained using 3.2 ​mm thin wall rotors with diluted samples 40 ​vol% GeTe-60 ​vol% SiO2. In addition to the WURST-CPMG technique, high resolution spectra using MAS as implemented in the pj-MAT technique allowed us to identify the distributions of chemical shift parameters in the high intensity contribution of the 1D spectra. The NMR spectra recorded on the samples showed that an addition of Tellurium in the stoichiometric Ge50Te50 sample leads to an important broadening of the spectrum together with a shift of the lines. According to VCA calculations it could be attributed to a distribution of concentrations of germanium vacancies in the sample and it would appear that Knight Shift but also Chemical Shift could contribute in similar proportion to the NMR line position when metavalent bonding is invoked.

Li and Mn-rich layered oxides, i.e. Li1+xM1-xO2 (M = Mn, Ni and Co), are attractive positive electrode materials for Li-ion batteries due to their promising high specific capacities. Unfortunately, these materials provide an energy-density fading due to a continuous voltage decay resulting from chemical instability of their surface structure upon cycling. The purpose of this paper is to discuss the main insights got from syntheses of materials targeted to be concentration-gradients of global compositions Li1+x(Ni0.29Mn0.53Co0.18)1-xO2 with: (i) Li and Mn-rich layered oxides in the core to deliver high capacity, and (ii) layered oxides enriched in Ni and in Co moving to the surface of the spherical aggregates to promote improved chemical and thermal stability for the electrode material. Concentration-gradient, core-shell or re-homogenized materials were obtained depending on the temperature and excess of lithium used for the high temperature thermal treatment (i.e. the second step of the synthesis). Despite complex to master, the engineering of layered oxide materials was shown to be a track to follow to optimize the performance of an electrode material.

Among the series of polyanionic positive electrodes for sodium -ion batteries having the general formula Na3V2(PO4)(2)F3-yOy (0 <= y <= 2), the composition Na3V2(PO4)(2)F-3 (y = 0) has the highest theoretical energy that offers competitive electrochemical performances compared to sodium transition metal oxides. Recently, the structural phase diagram from Na3V2(PO4)(2)F-3 to Na1V2(PO4)(2)F-3 has been thoroughly investigated by operando synchrotron X-ray diffraction revealing an unexpected structural feature for the end member composition. In fact, the crystal structure of Na1V2(PO4)(2)F-3 has two very different vanadium environments within each bioctahedron that suggests a charge disproportionation of two V-IV into V-III and V-v. This work shows an operando X-ray absorption spectroscopy at vanadium K edge during the electrochemical extraction of Na+ in order to monitor the redox processes involved in this compound. The large data set provided by this experiment has been processed by the principal component analysis combined with multivariate curve resolution. The results suggest that the bioctahedra have to be considered as the basic structural unit. The peculiar geometry of this material combined with the mixed vanadium valence, directly investigated here along the reaction, seems to allow original electronic configurations. In particular, the two vanadium sites into the basic bioctahedra unit evolve from V-III-V-III to V-III-V-IV and to a final V-III-V-IV configuration. These observations are completed with V-51 NMR sensitive to diamagnetic V-V.

The materials currently attracting most interest as positive electrodes for Lithium-ion batteries are Li and Mn-rich layered oxides that exhibit outstanding energy densities at an affordable cost.1 A common feature for all these layered oxides is a high capacity “plateau”, observed only at the end of the first charge, once all the transition metal ions are already at the tetravalent state. That behavior has been explained by the reversible participation of oxygen anions in the redox processes, thanks to hybridization between their p levels and the d levels of the transition metals. This reaction is reversible within the bulk, occurring without any major structural modification, while oxidized oxygen ions are lost at the surface causing irreversible structural reorganizations at the outer part of the particles, those being at the origin of a continuous voltage decay upon cycling.2,3 We will show how we tried to stabilize concentration gradients and core-shell composites with Li and Mn-rich layered oxides in the bulk and stoichiometric layered oxides at the outer part of the spherical aggregates,4 with the goal to combine high energy density and chemical stability respectively. We will also highlight that Tavorite-type compositions offer a very rich crystal chemistry, among which LiVPO4F has the highest theoretical energy density (i.e. 655 Wh/kg).5 New Tavorite-type compositions were recently obtained: LiVPO4OH and LiVPO4F1-yOy, for these latter by direct syntheses or by aging of LiVPO4F upon oxidation in air.6-8 We will show how we can tailor the structure, the potential and the reaction mechanism involved, playing with the composition of the Tavorite-type phases. We will discuss how detrimental/positive the defects can be on the electrochemical properties of the mixed oxy-fluorophosphates LiVPO4F1-yOy. Acknowledgements: These researches are funded by Région Nouvelle Aquitaine for layered oxides and by the French National Research Agency ANR (Labex STORE EX and project HIPOLITE) for polyanionic materials. The authors thank also the French network RS2E (http://www.energie-rs2e.com), the European network ALISTORE-ERI (http://www.alistore.eu), FEDER and Région Haut-de-France. Reference s : [1] Croguennec, L.; Palacin, M. R., Journal of the American Chemical Society 2015, 137, 3140-3156 [2] Koga, H.; Croguennec, L.; Ménétrier, M.; Douhil, K.; Belin, S.; Bourgeois, L.; Suard, E.; Weill, F.; Delmas, C.,, Journal of the Electrochemical Society 2013, 160, A786-A792 [3] Genevois, C. ; Koga, H. ; Croguennec, L. ; Ménétrier, M. ; Delmas, C. ; Weill, F., Journal of Physical Chemistry C 2015, 119, 75-83 [4] Pajot et al., in preparation [5] C. Masquelier and L. Croguennec, Chemical Reviews 2013, 113, 6552−6591 [6] Boivin, E.; Chotard, J.-N.; Ménétrier, M.; Bourgeois, L.; Bamine, T.; Carlier, D.; Fauth, F.; Suard, E.; Masquelier, C.; Croguennec, L., J ournal of Mater ial Chem istry A 2016, 4, 11030–11045. [7] Boivin, E.; Chotard, J.-N., Ménétrier, M.; Bourgeois, L.; Bamine, T.; Carlier, D.; Fauth, F.; Masquelier, C.; Croguennec, L., Journal of Physical Chemistry C 2016, 120(46), 26187-26198 [8] Boivin et al., in preparation

In a recent study, we showed by solid-state NMR that LiVPO4F, which is a promising material as positive electrode for Li-ion batteries, often exhibits some defects that may affect its electrochemical behavior. In this paper, we use DFT calculations based on the projector augmented-wave (PAW) method in order to model possible defects in this (paramagnetic) material and to compute, the Fermi contact shifts expected for Li nuclei located in their proximity. The advantage of the PAW approach versus FP-LAPW we have beenpreviously using is that it allows considering large supercells suitable to model a diluted defect. In the first part of this paper, we aim to validate the Fermi contact shifts calculation using the PAW approach within the VASP code. Then we apply this strategy for modeling possible defects in LiVP0(4)F. By analogy with the already existing homeotypic LiVOPO4 phase, we first replace one fluoride ion, along the VO2F4 chains, :by an oxygen one and consider, in a second step, an association with a lithium vacancy. As a result, the agreement between the calculated NMR spectra and the experimental one is satisfying. In both cases, the local electronic structure and the spin transfer mechanisms from V3+ or V4+ ions to the Li nuclei are analyzed.

This article deals with the surface reactivity of (001)-oriented Li2MnO3 crystals investigated from a multitechnique approach combining material synthesis, X-ray photoemission spectroscopy (XPS), scanning electron microscopy, Auger electron spectroscopy, and first-principles calculations. Li2MnO3 is considered as a model compound suitable to go further in the understanding of the role of tetravalent manganese atoms in the surface reactivity of layered lithium oxides. The knowledge of the surface properties of such materials is essential to understand the mechanisms involved in parasitic phenomena responsible for early aging or poor storage performances of lithium-ion batteries. The surface reactivity was probed through the adsorption of SO2 gas molecules on large Li2MnO3 crystals to be able to focus the XPS beam on the top of the (001) surface. A chemical mapping and XPS characterization of the material before and after SO2 adsorption show in particular that the adsorption is homogeneous at the micro- and nanoscale and involves Mn reduction, whereas first-principles calculations on a slab model of the surface allow us to conclude that the most energetically favorable species formed is a sulfate with charge transfer implying reduction of Mn.

This article focuses on the surface reactivity of two spinel samples with different stoichiometries and crystal morphologies, namely Li1+xMn2-xO4 with x = 0.05 and 0.10. LiMn2O4 compounds are good candidates as positive electrode of high-power lithium-ion batteries for portable devices. The samples were investigated using both experimental and theoretical approaches. On the experimental point of view, they were characterized in depth from X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS) analyses. Then, the reactivity was investigated through the adsorption of (SO2) gaseous probes, in controlled conditions, followed by XPS characterization. First-principle calculations were conducted simultaneously to investigate the electronic properties and the reactivity of relevant surfaces of an ideal LiMn2O4 material. The results allow us to conclude that the reactivity of the samples is dominated by an acido-basic reactivity and the formation of sulfite species. Nonetheless, on the x = 0.05 sample, both sulfite and sulfate species are obtained, the later, in lesser extent, corresponding to a redox reactivity. Combining experimental and theoretical results, this redox reactivity could be associated with the presence of a larger quantity of Mn4+ cations on the last surface layers of the material linked to a specific surface orientation.

Tavorite-type compositions offer a very rich crystal chemistry, among which (LiVPO4F)-P-III has the highest theoretical energy density (i.e., 655 Wh/kg). In this article, an in-depth study of vanadyl-type defects generated by temperature-controlled oxidation of (LiVPO4F)-P-III under air is proposed, and the influence of the defects on the electrochemical properties is demonstrated. A combination of high resolution synchrotron diffraction, infrared spectroscopy, and magic angle spinning nuclear magnetic resonance was used to fully characterize the materials thus generated, from their average long-range structure to their local structure with the presence of defects. The increase of the annealing temperature tends to substitute oxygen for fluorine with the formation of a series of LiVPO4F1-xOx compositions. The miscibility domains appear to be narrow at the two ends of the solid solution (i.e., in the composition ranges LiVPO4F[1.0.9]O[0.0.1] and LiVPO4F[0-0.1]O[1-0.9]). The presence of vanadyl-type defects obtained as localized or more extended ones, depending on the annealing conditions, affects drastically the electrochemical properties of these Tavorite LiVPO4F-type materials.

Polyanionic materials such as Na3V2(PO4)(2)F3-yOy (0 <= y <= 2) are of high interest as positive electrode for Na-ion batteries since they offer competitive electrochemical performances compared to sodiated transition metal oxides. The composition Na3V2(PO4)(2)F-3 (y = 0) has the highest theoretical energy density among the series, but surprisingly a lot of discrepancies are reported throughout the literature considering its structure and its electrochemical properties. We will show that most of the compounds reported as being Na3V2III(PO4)(2)F-3 are in fact slightly oxidized due to synthesis conditions resulting in a partial oxygen substitution for fluorine. To get an in-depth understanding of this system, a series of compositions Na3V2(PO4)(2)F3-yOy (0 <= y <= 0.5, i.e., near the fluorine-rich composition) was synthesized and characterized combining synchrotron X-ray diffraction, X-ray absorption spectroscopy, solid state nuclear magnetic resonance spectroscopy, and galvanostatic electrochemical tests. The structural features and electrochemical signatures of these oxidized compounds will be carefully compared to those recently obtained for Na3V2III(PO4)(2)F-3 by Bianchini et al.

Li-ion batteries are invaluable for portable electronics and vehicle electrification. A better knowledge of compositional variations within the electrodes during battery operation is, however, still needed to keep improving their performance. Although essential in the medical field, magnetic resonance imaging of solid paramagnetic battery materials is challenging due to the short lifetime of their signals. Here we develop the scanning image-selected in situ spectroscopy approach, using the strongest commercially available magnetic field gradient. We demonstrate the 7 Li magnetic resonance spectroscopic image of a 5 mm-diameter operating battery with a resolution of 100 μm. The time-resolved image-spectra enable the visualization in situ of the displacement of lithiation fronts inside thick paramagnetic electrodes during battery operation. Such observations are critical to identify the key limiting parameters for high-capacity and fast-cycling batteries. This non-invasive technique also offers opportunities to study devices containing paramagnetic materials while operating.

Li10SnP2S12, the tin analogue of the Li10GeP2S12 superionic conductor is characterized. Rietveld refinement of capillary powder XRD confirms that the material provided by NEI Corp. is mainly composed of Li10SnP2S12 with LGPS structure, with some Li2SnS3 impurity. Very strong reactivity at low voltage and vs. Li metal is shown by impedance measurements using Au and Li electrodes, as well as 3-electrode cyclic voltammetry. Galvanostatic measurements in half-cells with liquid and solid electrolytes confirm that Li10SnP2S12 reacts with respectively 16 and 8 lithium per mole of tin below 0.5 V vs. Li+/Li, most probably following a conversion/alloying reaction. Making use of this reactivity, we tested the concept of a LiCoO2/Li10SnP2S12 cell, were the electrolyte also acts as negative electrode at the contact with the negative current collector. 0.5 Li can be deintercalated from such cells, with very poor reversibility as probably hampered by the conversion/alloying reaction in the solid state, at least with a non-optimized electrode formulation. Although the voltage slowly but strongly relaxes when opening the circuit suggesting propagation of the reaction from the negative electrode to the electrolyte, it does not seem to reach the positive since no short-circuit was observed.

a ICMCB-CNRS, Université de Bordeaux, Bordeaux INP, F-33608 Pessac cedex, France b LRCS, Université de Picardie Jules Verne, F-80039 Amiens Cedex 1, France c ILL, Institut Laue Langevin, F-38000 Grenoble, France d CEMHTI, Université d’Orléans, F-45071 Orléans, France e CELLS - ALBA Synchrotron, Cerdanyola del Vallès, E-08290 Barcelona, Spain Contact: Laurence.Croguennec@icmcb.cnrs.fr Vanadium-rich fluorinated-phosphates are attractive positive electrode materials for Li-ion and Na-ion batteries due to their high capacity, rate capability and long-term cycling stability.1,2 We already reported on a complex phase diagram as a function of the charge state for Na3V2(PO4)2F3.3 From structural determination based on high resolution X-ray powder synchrotron data and bond valence sum analysis we proposed two vanadium environments in NaV2(PO4)2F3, V3+ and V5+, instead of a single one (i.e. V4+). We will report on the operando investigation of the redox processes involved during sodium deintercalation and on the charge compensation mechanism on the V site, from X-ray absorption near edge structure measurements collected at the V k-edge. We will compare that mechanism to those observed for other vanadium-rich phosphates such as LiVPO4(F, O, OH). We will also show how challenging is the control of oxygen over fluorine stoichiometry in these fluorinated phosphates. Existence of characteristic lithium defect environments has been for instance recently revealed using solid-state 7Li nuclear magnetic resonance in well-crystallized Tavorite LiVPO4F,4 despite they were not detected by high resolution X-ray and neutron diffraction as well as scanning transmission electron microscopy. Several studies performed on Na3V2(PO4)2F3 have revealed significant discrepancies in its structural description as well as in the electrochemical properties.5 Unit cell volumes ranging between 871 and 878 Å3 were for instance reported, whereas the phase diagram observed upon cycling was described either as complex with a series of two phase reactions or as a solid solution. Subtle differences in compositions exist. We will especially discuss in details the effect of a partial substitution of oxygen for fluorine and thus of a mixed valence state for vanadium (V3+,4+) on the structure and electrochemical properties of these materials. Acknowledgements CEA-Liten (Grenoble, France) and especially Loïc Simonin are acknowledged for their collaboration. This research is performed in the frame of the French network RS2E (http://www.energie-rs2e.com) and of the European network ALISTORE-ERI (http://www.alistore.eu). This project is partly funded by the French National Research Agency ANR (Descartes project SODIUM and Progelec project HIPOLITE) and by the H2020 European Program (Project NAIADES). References [1] Huang et al. J. Power Sources189, 748-751 (2009) [2] Ponrouch et al. Energy & Environmental Science6(8), 2361-2369 (2013) [3] Bianchini et al. Chem. Mater. 27(8), 3009 (2015) [4] Messinger et al. Chem. Mater.27(15) 5212 (2015) [5] Bianchini et al. Chem. Mater. 26(14), 4238 (2014)

A vanadium based hydroxy-phosphate of the Tavorite-type structure LiVPO4OH was obtained for the first time, by a hydrothermal route. Li+ and H+ are extracted from the structure at the same equilibrium potential.

Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) and electron nanodiffraction investigations have been carried out to follow changes in the local atomic structure of three Li-rich layered oxides recovered after a single chargedischarge cycle, performed in different conditions. The structure of the pristine material Li1.20Mn0.54Co0.13Ni0.13O2 was fully characterized. It was then compared to those of the materials recovered after one electrochemical cycle in a lithium battery, with the upper voltage limit being either just below the voltage of the irreversible plateau typical of Li-rich layered oxides or just above. An in-depth study of the material obtained chemically, after oxidation to deintercalate Li and then reduction to reintercalate Li, was also performed for comparison. The main message of this paper is that the plateau is not associated with an extended structural reorganization of the material. The irreversible processes associated with cation migration are restricted to the external part of the particles. The results reported here support the oxidation of oxygen ions we earlier proposed to occur reversibly in the core of the particles and to be the actual origin for the exceptional capacity of Li- and Mn-rich layered oxides.

We investigate the origins of phase transformation hysteresis in electrodes of Li-ion batteries, focusing on the alloying reaction of Li with Sb. Electrochemical measurements confirm that the reaction path followed during Li insertion into Sb electrodes differs from that followed upon subsequent Li extraction. Results from first-principles calculations and NMR measurements indicate that Li3Sb is capable of tolerating high Li-vacancy concentrations. An unusually high Li mobility in Li3Sb facilitates over potentials during charging, which leads to a substantially larger driving force for the nucleation of Sb compared to that of Li2Sb. The differences in nucleation driving forces arise from a lever effect that favors phases with large changes in Li concentration over phases that are closer in composition along the equilibrium path. These properties provide an explanation for the observed path hysteresis between charge and discharge in the Li-Sb system and likely also play a role in intercalation compounds and other alloying reactions exhibiting similar phase transformation hysteresis.

Identifying and characterizing defects in crystalline solids is a challenging problem, particularly for lithium-ion intercalation materials, which often exhibit multiple stable oxidation and spin states as well as local ordering of lithium and charges. Here, we reveal the existence of characteristic lithium defect environments in the crystalline lithium-ion battery electrode LiVPO4F and establish the relative subnanometer-scale proximities between them. Well-crystallized LiVPO4F samples were synthesized with the expected tavorite-like structure, as established by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) measurements. Solid-state Li-7 nuclear magnetic resonance (NMR) spectra reveal unexpected paramagnetic Li-7 environments that can account for up to 20% of the total lithium content. Multidimensional and site-selective solid-state Li-7 NMR experiments using finite-pulse radio frequency-driven recoupling (fp-RFDR) establish unambiguously that the unexpected lithium environments are associated with defects within the LiVPO4F crystal structure, revealing the existence of dipole dipole-coupled defect pairs. The lithium defects exhibit local electronic environments that are distinct from lithium ions in the crystallographic LiVPO4F site, which result from altered oxidation and/or spin states of nearby paramagnetic vanadium atoms. The results provide a general strategy for identifying and characterizing lithium defect environments in crystalline solids, including paramagnetic materials with short Li-7 NMR relaxation times on the order of milliseconds.

Operando X-ray absorption spectroscopy investigations have been carried out to follow changes in the atomic and electronic local structures of all three transition metals for the Li1.20Mn0.54Co0.13Ni0.13O2 layered oxide during the first and second charges and discharges of lithium batteries. The experiments were performed using a Quick-XAS monochromator on the SAMBA beamline at Synchrotron SOLEIL to record the three K-edges by edge-jumping between two energy ranges ([Mn, Co] and [Co, Ni]) every 3 min during the cycling of the battery. The results obtained especially at the Mn K-edge fully support the participation of oxygen in the reversible charge-discharge reaction of this Li- and Mn-rich layered material as a redox center and not only with oxygen loss, as was proposed previously.

Development of new materials for high energy density batteries is essential as they power a wide range of devices ranging from portable electronics to transport and storage applications. Since they are still facing limitations (energy and power density, safety …), intense academic and industrial research activities are devoted to the development of better performing materials with in particular high voltage and safety. Beside the extensively studied layered and spinel oxides, polyanionic materials have aroused huge interest in the past ten years [1]. Among them, the so-called fluorophosphates, actually fluoride phosphates combining phosphate and fluoride anions in the same framework, are very attractive positive electrode materials with increased voltage versus Li and Na as compared to regular (oxide) phosphates for a given M (n+1)+ /M n+ redox couple. We will discuss results obtained recently in our groups for Tavorite-like phosphate materials LiMPO 4 X (M = V, Fe, Ti; X = F, OH, O) used as electrodes in Li-ion batteries, but also for Na 3 V 2 (PO 4 ) 2 F 3 in Li and Na-ion batteries. The vanadium-rich compounds LiVPO 4 F and LiVPO 4 O have been shown to be of particular interest as V is stable at different oxidation states in similar environments (V II , V III , V IV and V V ) in these Tavorite-type structures, leading to a possible exchange of two electrons per vanadium [2-5]. Very distinct values are interestingly observed for the “same” V 3+ /V 4+ redox couple in LiVPO 4 F and LiVPO 4 O: ~ 4.25 V vs. Li upon Li + extraction from LiVPO 4 F versus ~ 2.3 V vs. Li upon Li + insertion into LiVPO 4 O, showing that the potential can be tuned over a wide range moving from a V 3+ -rich material to a vanadyl (VO) 2+ -rich material. Na 3 V 2 (PO 4 ) 2 F 3 is also undoubtedly of high interest for the development of Na-ion batteries: it was shown recently to allow fast sodium diffusion, even at high rates (up to 20C) and despite the formation of successive intermediate phases with Na + /vacancy ordering [6]. The average potential observed for the V 3+ /V 4+ redox couple in Na 3 V 2 (PO 4 ) 2 F 3 is around 3.9 V vs. Na + /Na, i.e. (as expected) significantly higher than that of the Nasicon phase Na 3 V 2 (PO 4 ) 3 . References [1] C. Masquelier and L. Croguennec, Chem. Rev. , 113, 6552 (2013) [2] J. Barker, R.K.B. Gover, P. Burns, A. Bryan, M.Y. Saidi and J.L. Swoyer, J. Power Sources , 146, 516 (2005) [3] J.M. Ateba Mba, C. Masquelier, E. Suard and L. Croguennec, Chem. Mater. , 24, 1223 (2012) [4] J.M. Ateba Mba, L. Croguennec, N.I. Basir, J. Barker and C. Masquelier, J. Electrochem. Soc. , 159, A1171 (2012) [5] M. Bianchini, J.M. Ateba Mba, P. Dagault, E. Bogdan, D. Carlier, E. Suard, C. Masquelier and L. Croguennec, submitted [6] A. Ponrouch, R. Dedryvère, D. Monti, A.E. Demet, J.M. Ateba Mba, L. Croguennec, C. Masquelier, P. Johansson and M.R. Palacin, Energy and Environmental Science , 6, 2361 (2013)

M2+-doped aluminate spinels (M=Co or Ni) were prepared by a polymeric route leading to pure phases for synthesis temperatures equal to 800 or 1200°C and characterized by UV–vis–NIR spectroscopy, 27Al NMR and XRD refinements. Coloration of the synthesized pigments is clearly sensitive to the distribution of doping ions in the aluminate spinel lattice. As the synthesis temperature increased, a color shift from green to blue has been observed for Zn1−xCoxAl2O4 compound while coloration of Zn1−xNixAl2O4 compound remains greenish-gray. Hence, to improve pigment coloration and/or synthesis cost, two different strategies have been proposed: (i) the synthesis of aluminum over-stoichiometric spinel with Zn0.9Co0.1Al2.2O4+δ formal composition in order to force Co2+ to be located in tetrahedral sites and (ii) changing from ZnAl2O4 to MgAl2O4 as host lattices for Ni2+ doping ions in order to force Ni2+ to be located in octahedral sites.