High-Temperature Thermal Reactivity and Interface Evolution of the NMC-LATP-Carbon Composite Cathode.

Temperature-assisted densification methods are typically used in oxide-based solid-state batteries to suppress resistive interfaces. However, chemical reactivity among the different cathode components (which include a catholyte, the conducting additive, and the electroactive material) still represents a major challenge and processing parameters need thus to be carefully selected. In this study, we evaluate the impact of temperature and heating atmosphere in the LiNi0.6Mn0.2Co0.2O2 (NMC), Li1+xAlxTi2-xP3O12 (LATP), and Ketjenblack (KB) system. A rationale of the chemical reactions between components is proposed from the combination of bulk and surface techniques and overall involves a cation redistribution in the NMC cathode material that is accompanied by the loss of lithium and oxygen from the lattice enhanced by LATP and KB, which act as lithium and oxygen sinks. The final result is the formation of several degradation products, starting at the surface, that lead to a rapid capacity decay above 400 °C. Both the reaction mechanism and threshold temperature depend on the heating atmosphere, with the air atmosphere being more favorable compared to oxygen or any other inert gases.

Influence of Transition-Metal Order on the Reaction Mechanism of LNMO Cathode Spinel: An Operando X-ray Absorption Spectroscopy Study.

An operando dual-edge X-ray absorption spectroscopy on both transition-metal ordered and disordered LiNi0.5Mn1.5O4 during electrochemical delithiation and lithiation was carried out. The large data set was analyzed via a chemometric approach to gain reliable insights into the redox activity and the local structural changes of Ni and Mn throughout the electrochemical charge and discharge reaction. Our findings confirm that redox activity relies predominantly on the Ni2+/4+ redox couple involving a transient Ni3+ phase. Interestingly, a reversible minority contribution of Mn3+/4+ is also evinced in both LNMO materials. While the reaction steps and involved reactants of both ordered and disordered LNMO materials generally coincide, we highlight differences in terms of reaction dynamics as well as in local structural evolution induced by the TM ordering.

Role of the voltage window on the capacity retention of P2-Na2/3[Fe1/2Mn1/2]O2 cathode material for rechargeable sodium-ion batteries.

P2-Na2/3[Fe1/2Mn1/2]O2 layered oxide is a promising high energy density cathode material for sodium-ion batteries. However, one of its drawbacks is the poor long-term stability in the operating voltage window of 1.5-4.25 V vs Na+/Na that prevents its commercialization. In this work, additional light is shed on the origin of capacity fading, which has been analyzed using a combination of experimental techniques and theoretical methods. Electrochemical impedance spectroscopy has been performed on P2-Na2/3[Fe1/2Mn1/2]O2 half-cells operating in two different working voltage windows, one allowing and one preventing the high voltage phase transition occurring in P2-Na2/3[Fe1/2Mn1/2]O2 above 4.0 V vs Na+/Na; so as to unveil the transport properties at different states of charge and correlate them with the existing phases in P2-Na2/3[Fe1/2Mn1/2]O2. Supporting X-ray photoelectron spectroscopy experiments to elucidate the surface properties along with theoretical calculations have concluded that the formed electrode-electrolyte interphase is very thin and stable, mainly composed by inorganic species, and reveal that the structural phase transition at high voltage from P2- to "Z"/OP4-oxygen stacking is associated with a drastic increased in the bulk electronic resistance of P2-Na2/3[Fe1/2Mn1/2]O2 electrodes which is one of the causes of the observed capacity fading.

Stacking Versatility in Alkali-Mixed Honeycomb Layered NaKNi2TeO6.

The reaction between P2-type honeycomb layered oxides Na2Ni2TeO6 and K2Ni2TeO6 enables the formation of NaKNi2TeO6. The compound is characterized by X-ray diffraction and 23Na solid-state nuclear magnetic resonance spectroscopy, and the structure is discussed through density functional theory calculations. In addition to the honeycomb Ni/Te cationic ordering, NaKNi2TeO6 exhibits a unique example of alternation of sodium and potassium layers instead of a random alkali-mixed occupancy. Stacking fault simulations underline the impact of the successive position of the Ni/Te honeycomb layers and validate the presence of multiple stacking sequences within the powder material, in proportions that evolve with the synthesis conditions. In a broader context, this work contributes to a better understanding of the alkali-mixed layered compounds.

Impact of Stacking Faults and Li Substitution in LixMnO3 (0 ≤ x ≤ 2) Structural Transformations upon Delithiation.

Lithium-rich layered oxides appear in most roadmaps as next generation Li-ion cathode materials owing to their superior capacity. Within this family, Li2MnO3 represents the archetype material and is often taken as model compound to better understand the complex structural modifications occurring in the first charging cycle. In this work, density functional theory (DFT) calculations have been used to understand the impact of stacking faults in the structural transformations occurring in Li2MnO3 upon delithiation, which are found to hinder the phase transformations leading to structural degradation. The formation energies of both ideal and defective LixMnO3 compositions and the analysis of the encountered ground states have been used to rationalize the predicted differences in terms of structural evolution. From the understanding of the origin in the O1 phase transformation, Mg substitution is proposed as alternative strategy to improve the structural stability in this family of materials.

Exploring new hydrated delta type vanadium oxides for lithium intercalation.

Three hydrated double layered vanadium oxides, namely Na0.35V2O5·0.8(H2O), K0.36(H3O)0.15V2O5 and (NH4)0.37V2O5·0.15(H2O), were obtained by using mild hydrothermal conditions. Their delta type structural frameworks were solved by high-resolution synchrotron X-ray powder diffraction and the interlayer spacings were interpreted from difference Fourier maps. The inter-slab distances are modulated by the water content and the special arrangements of the alkali and ammonium cations. The XPS measurements denote mixed valence systems with high contents of V4+ ions up to 40%. The monitoring of the V4+ EPR signal over time suggests a reduction of the electronic delocalization on account of the partial oxidation to V5+. The electrochemical performance of the active phases is strongly conditioned by the vacuum-drying process of the electrodes, showing better capacity retention when vacuum is not applied. In situ X-ray diffraction shows a structural mechanism of contraction/expansion of the bilayers upon lithium insertion/extraction where the alkali ions behave as structural stabilizers. Galvanostatic cycling at very low current density implies migration of the alkali "pillars" triggering the collapse of the structure.

DFT-Assisted Solid-State NMR Characterization of Defects in Li2MnO3.

The complete description of defective structures and their impact on materials behavior is a great challenge due to difficulties associated with their reliable characterization in the nanoscale. In this paper, density functional theory (DFT) calculations are used to elucidate the solid-state nuclear magnetic resonance (NMR) spectra of Li2MnO3 which, combined with X-ray diffraction (XRD), provide a full description of disorder in this compound. While XRD allows accurate quantification of planar defects, the use of solid-state NMR reveals limited vacancy concentrations that were undetected by XRD as NMR is highly sensitive to the atomic local environments. The combination of these methods is here proved highly effective in overcoming the challenges of describing in great detail limited concentrations of disorder in transition metal oxides, providing information about structural variables that are essential to their application.

Coulombic self-ordering upon charging a large-capacity layered cathode material for rechargeable batteries.

Lithium- and sodium-rich layered transition-metal oxides have recently been attracting significant interest because of their large capacity achieved by additional oxygen-redox reactions. However, layered transition-metal oxides exhibit structural degradation such as cation migration, layer exfoliation or cracks upon deep charge, which is a major obstacle to achieve higher energy-density batteries. Here we demonstrate a self-repairing phenomenon of stacking faults upon desodiation from an oxygen-redox layered oxide Na2RuO3, realizing much better reversibility of the electrode reaction. The phase transformations upon charging A2MO3 (A: alkali metal) can be dominated by three-dimensional Coulombic attractive interactions driven by the existence of ordered alkali-metal vacancies, leading to counterintuitive self-repairing of stacking faults and progressive ordering upon charging. The cooperatively ordered vacancy in lithium-/sodium-rich layered transition-metal oxides is shown to play an essential role, not only in generating the electro-active nonbonding 2p orbital of neighbouring oxygen but also in stabilizing the phase transformation for highly reversible oxygen-redox reactions.

Enhanced electrochemical performance of Li-rich cathode materials through microstructural control.

The microstructural complexity of Li-rich cathode materials has so far hampered understanding the critical link between size, morphology and structural defects with both capacity and voltage fadings that this family of materials exhibits. Li2MnO3 is used here as a model material to extract reliable structure-property relationships that can be further exploited for the development of high-performing and long-lasting Li-rich oxides. A series of samples with microstructural variability have been prepared and thoroughly characterized using the FAULTS software, which allows quantification of planar defects and extraction of average crystallite sizes. Together with transmission electron microscopy (TEM) and density functional theory (DFT) results, the successful application of FAULTS analysis to Li2MnO3 has allowed rationalizing the synthesis conditions and identifying the individual impact of concurrent microstructural features on both voltage and capacity fadings, a necessary step for the development of high-capacity Li-ion cathode materials with enhanced cycle life.

Composition and evolution of the solid-electrolyte interphase in Na2Ti3O7 electrodes for Na-ion batteries: XPS and Auger parameter analysis.

Na2Ti3O7 is considered a promising negative electrode for Na-ion batteries; however, poor capacity retention has been reported and the stability of the solid-electrolyte interphase (SEI) could be one of the main actors of this underperformance. The composition and evolution of the SEI in Na2Ti3O7 electrodes is hereby studied by means of X-ray photoelectron spectroscopy (XPS). To overcome typical XPS limitations in the photoelectron energy assignments, the analysis of the Auger parameter is here proposed for the first time in battery materials characterization. We have found that the electrode/electrolyte interface formed upon discharge, mostly composed by carbonates and semicarbonates (Na2CO3, NaCO3R), fluorides (NaF), chlorides (NaCl) and poly(ethylene oxide)s, is unstable upon electrochemical cycling. Additionally, solid state nuclear magnetic resonance (NMR) studies prove the reaction of the polyvinylidene difluoride (PVdF) binder with sodium. The powerful approach used in this work, namely Auger parameter study, enables us to correctly determine the composition of the electrode surface layer without any interference from surface charging or absolute binding energy calibration effects. As a result, the suitability for Na-ion batteries of binders and electrolytes widely used for Li-ion batteries is questioned here.

Structure of H2Ti3O7 and its evolution during sodium insertion as anode for Na ion batteries.

H2Ti3O7 was prepared as a single phase material by ionic exchange from Na2Ti3O7. The complete ionic exchange was confirmed by (1)H and (23)Na solid state Nuclear Magnetic Resonance (NMR). The atomic positions of H2Ti3O7 were obtained from the Rietveld refinement of powder X-ray diffraction (PXRD) and neutron diffraction experimental data, the latter collected at two different wavelengths to precisely determine the hydrogen atomic positions in the structure. All H(+) cations are hydrogen bonded to two adjacent [Ti3O7](2-) layers leading to the gliding of the layers and lattice centring with respect to the parent Na2Ti3O7. In contrast with a previous report where protons were located in two different positions of H2Ti3O7, 3 types of proton positions were found. Two of the three types of proton are bonded to the only oxygen linked to a single titanium atom forming an H-O-H angle close to that of the water molecule. H2Ti3O7 is able to electrochemically insert Na(+). The electrochemical insertion of sodium into H2Ti3O7 starts with a solid solution regime of the C-centred phase. Then, between 0.6 and 1.2 inserted Na(+) the reaction proceeds through a two phase reaction and a plateau at 1.3 V vs. Na(+)/Na is observed in the voltage-composition curve. The second phase resembles the primitive Na2Ti3O7 cell as detected by in situ PXRD. Upon oxidation, from 0.9 to 2.2 V, the PXRD pattern remains mostly unchanged probably due to H(+) removal instead of Na(+), with the capacity quickly fading upon cycling. Conditioning H2Ti3O7 for two cycles at 0.9-2.2 V before cycling in the 0.05-1.6 V range yields similar specific capacity and better retention than the original Na2Ti3O7 in the same voltage range.

The mechanism of NaFePO4 (de)sodiation determined by in situ X-ray diffraction.

The reaction mechanism occurring during the (de)intercalation of sodium into the host olivine FePO4 structure is thoroughly analysed through a combination of structural and electrochemical methods. In situ XRD experiments have confirmed that the charge and discharge reaction mechanisms are different and have revealed the existence of a solid solution domain from 1 < x < 2/3 in Na(x)FePO4 upon charge. The second part of the charge proceeds through a 2-phase reaction between Na(2/3)FePO4 and FePO4 with strongly varying solubility limits. The strong cell mismatch between Na(2/3)FePO4 and FePO4 enhances the effects of the diffuse interface and therefore varying solubility limits are first observed here in micrometric materials.

Room-temperature single-phase Li insertion/extraction in nanoscale Li(x)FePO4.

Classical electrodes for Li-ion technology operate by either single-phase or two-phase Li insertion/de-insertion processes, with single-phase mechanisms presenting some intrinsic advantages with respect to various storage applications. We report the feasibility to drive the well-established two-phase room-temperature insertion process in LiFePO4 electrodes into a single-phase one by modifying the material's particle size and ion ordering. Electrodes made of LiFePO4 nanoparticles (40 nm) formed by a low-temperature precipitation process exhibit sloping voltage charge/discharge curves, characteristic of a single-phase behaviour. The presence of defects and cation vacancies, as deduced by chemical/physical analytical techniques, is crucial in accounting for our results. Whereas the interdependency of particle size, composition and structure complicate the theorists' attempts to model phase stability in nanoscale materials, it provides new opportunities for chemists and electrochemists because numerous electrode materials could exhibit a similar behaviour at the nanoscale once their syntheses have been correctly worked out.