Benchmarking the electrochemical parameters of the LiNi0.8Mn0.1Co0.1O2 positive electrode material for Li-ion batteries.

The layered oxide LiNi0.8Mn0.1Co0.1O2 (NMC811, NCM811) is of utmost technological importance as a positive electrode (cathode) material for the forthcoming generation of Li-ion batteries. In this contribution, we have collected 548 research articles comprising >950 records on the electrochemical properties of NMC811 as a cathode material in half-cells with metallic Li counter electrode. The analysis of distribution histograms provided statistically-relevant values of such key characteristics of NMC811 as the first cycle discharge capacity and Coulombic efficiency, discharge capacities at different upper cut-off voltages, capacity fade and capacity retention at the 0.1C-5C current densities. We derived equations describing the relationships between discharge capacity and upper cut-off voltage, Ni content in the LiNixMnyCozO2 compositions in vicinity of NMC811, antisite disorder, and the C-rate. Additionally, the distribution histograms were used for a qualitative comparison between various groups of NMC811 materials, such as benchmarks in various optimizations vs obtained in course of synthesis development, lab-made vs commercial, polycrystalline vs single-crystal. The results of this analysis provide justified values to be used as benchmarks in further works related to optimizing and improving NMC811 and related materials, eliminating random picking up from a huge pool of published data.

NaGaPO4F - a KTiOPO4-structured solid sodium-ion conductor.

Advanced ionic conductors are crucial for a large variety of contemporary technologies spanning solid state ion batteries, fuel cells, gas sensors, water desalination, etc. In this work, we report on a new member of KTiOPO4-structured materials, NaGaPO4F, with sodium-ion conductivity. NaGaPO4F has been obtained for the first time via a facile two-step synthesis consisting of a hydrothermal preparation of an ammonia-based precursor, NH4GaPO4F, followed by an ion exchange reaction with NaNO3. Its crystal structure was precisely refined using a combination of synchrotron X-ray powder diffraction and electron diffraction tomography. The material is thermally stable upon 450 °C showing no significant structural transformations or degradation but only a ∼1% cell volume expansion. Na-ion mobility in NaGaPO4F was investigated by a joint experimental and computational approach comprising solid-state nuclear magnetic resonance (NMR) and density functional theory (DFT). DFT and bond-valence site energy (BVSE) calculations reveal 3D diffusion of sodium in the [GaPO4F] framework with migration barriers amounting to 0.22 and 0.44 eV, respectively, while NMR yields 0.3-0.5 eV that, being coupled with a calculated bandgap of ∼4.25 eV, makes NaGaPO4F a promising fast Na-ion conductor.

Decoupling the roles of Ni and Co in anionic redox activity of Li-rich NMC cathodes.

Li[LixNiyMnzCo1-x-y-z]O2 (lithium-rich NMCs) are benchmark cathode materials receiving considerable attention due to the abnormally high capacities resulting from their anionic redox chemistry. Although their anionic redox mechanisms have been much investigated, the roles of cationic redox processes remain underexplored, hindering further performance improvement. Here we decoupled the effects of nickel and cobalt in lithium-rich NMCs via a comprehensive study of two typical compounds, Li1.2Ni0.2Mn0.6O2 and Li1.2Co0.4Mn0.4O2. We discovered that both Ni3+/4+ and Co4+, generated during cationic redox processes, are actually intermediate species for triggering oxygen redox through a ligand-to-metal charge-transfer process. However, cobalt is better than nickel in mediating the kinetics of ligand-to-metal charge transfer by favouring more transition metal migration, leading to less cationic redox but more oxygen redox, more O2 release, poorer cycling performance and more severe voltage decay. Our work highlights a compositional optimization pathway for lithium-rich NMCs by deviating from using cobalt to using nickel, providing valuable guidelines for future high-capacity cathode design.

Multi-functional Single-Source Molecular Precursors for Carbon-Coated Mixed-Metal Phosphates.

We introduce a new synthetic concept that can be broadly adopted for the low-temperature preparation of mixed-metal energy storage materials, such as phosphates, silicates, fluorides, fluorophosphates, and fluorosulfates that exhibit intrinsic low electronic conductivity and thus require a carbon modulation. The development of novel low-temperature approaches for assembling energy-related materials with a complex core-shell microstructure is of great importance for expanding their application scope. The traditional definition of single-source precursors refers to their ability to yield a phase-pure material upon thermal decomposition. We have developed a new way for the utilization of heterometallic molecular precursors in synthesis that goes beyond its common delineation as a single-phase maker. The utility of this approach has been demonstrated upon the low-temperature synthesis of lithium-iron phosphate@C, which represents a celebrated cathode material for Li-ion batteries. The first atomically precise carbonaceous molecular precursors featuring a desired Li:Fe:P ratio of 1:1:1, divalent iron, and sufficient oxygen content for the target LiFeIIPO4 phosphate were shown to enable a spontaneous formation of both the olivine core and conductive carbon shell, yielding a carbon-coated mixed-metal phosphate.

Electrochromic Corundum-like Compound Based on the Reversible (De)insertion of Lithium: Li2Ni2W2O9.

The search for efficient anodic electrochromic materials is essential to the development of electrochromic devices, such as smart windows. Magnetron-sputtered lithium-nickel-tungsten mixed oxides are good candidates to tackle this issue; however, they display a complicated microstructure, making it difficult to pinpoint the origin of their electro-optical properties. Herein, by exploring the Li2O-NiO-WO3 phase diagram, we obtained a new phase, Li2Ni2W2O9, that crystallizes in the orthorhombic Pbcn space group. This material can reversibly uptake/release 0.75 Li+ (31 mA h·g-1) when cycled between 2.5 and 5.0 V versus Li+/Li. Moreover, through operando optical microscopy, we show that this new phase is electrochromic, and crucial information can be accessed about the diffusion-limited insertion of lithium at the single-particle scale. This study sets the ground for future syntheses of electrochemically active materials crystallizing in the ramsayite structure and details how the electrochromic properties of battery materials can be used to shed some light on their electrochemical mechanisms.

Understanding the High Voltage Behavior of LiNiO2 Through the Electrochemical Properties of the Surface Layer.

Nickel-rich layered oxides are adopted as electrode materials for EV's. They suffer from a capacity loss when the cells are charged above 4.15 V versus Li/Li+ . Doping and coating can lead to significant improvement in cycling. However, the mechanisms involved at high voltage are not clear. This work is focused on LiNiO2 to overcome the effect of M cations. Galvanostatic intermittent titration technique (GITT) and in situ X-ray diffraction (XRD) experiments are performed at very low rates in various voltage ranges (3.8-4.3 V,). On the "4.2-4.3 V" plateau the R2 phase is transformed simultaneously in R3, R3 with H4 stacking faults and H4. As the charge proceeds above 4.17 V cell polarization increases, hindering Li deintercalation. In discharge, such polarization decreases immediately. Upon cycling, the polarization increases at each charge above 4.17 V. In discharge, the capacity and dQ/dV features below 4.1 V remain constant and unaffected, suggesting that the bulk of the material do not undergo significant structural defect. This study shows that the change in polarization results from the electrochemical behavior of the grain surface having very low conductivity above 4.17 V and high conductivity below this threshold. This new approach can explain the behavior observed with dopants like tungsten.

Ferromagnetic Layers in a Topological Insulator (Bi,Sb)2Te3 Crystal Doped with Mn.

Magnetic topological insulators (MTIs) have recently become a subject of poignant interest; among them, Z2 topological insulators with magnetic moment ordering caused by embedded magnetic atoms attract special attention. In such systems, the case of magnetic anisotropy perpendicular to the surface that holds a topologically nontrivial surface state is the most intriguing one. Such materials demonstrate the quantum anomalous Hall effect, which manifests itself as chiral edge conduction channels that can be manipulated by switching the polarization of magnetic domains. In the present paper, we uncover the atomic structure of the bulk and the surface of Mn0.06Sb1.22Bi0.78Te3.06 in conjunction with its electronic and magnetic properties; this material is characterized by naturally formed ferromagnetic layers inside the insulating matrix, where the Fermi level is tuned to the bulk band gap. We found that in such mixed crystals septuple layers (SLs) of Mn(Bi,Sb)2Te4 form structures that feature three SLs, each of which is separated by two or three (Bi,Sb)2Te3 quintuple layers (QLs); such a structure possesses ferromagnetic properties. The surface obtained by cleavage includes terraces with different terminations. Manganese atoms preferentially occupy the central positions in the SLs and in a very small proportion can appear in the QLs, as indirectly indicated by a reshaped Dirac cone.

Deciphering the Double-Layer Structure and Dynamics on a Model LixMoO3 Interface by Advanced Electrogravimetric Analysis.

A major feature of the electrolyte/electrode interface (EEI) that affects charge storage in lithium-ion batteries is the electrical double layer (EDL), but most of the available experimental approaches for probing its structuration have limitations due to electrical field and redox reaction disturbances, hence explaining why it is frequently overlooked. Herein we show that this is no longer true by using an advanced electrochemical quartz crystal microbalance (EQCM)-based method in the form of ac-electrogravimetry. For proof of concept, we studied the effect of various solvent/salt combinations, differing in their dipole moment and size/weight, respectively, on the structure of the EDL forming at the EEI of LixMoO3. We show that a significant amount of solvated lithium ions and anions contribute to charge compensation at the interface, and by varying the nature of the solvents (cyclic vs noncyclic), we provide a solid experimental proof of the direct relationship between the ions' solvation and solvent polarity. Moreover, we demonstrated a disappearance of the anionic motion in the less polar solvent (DMC) most likely due to plausible formation of contact ion pairs and agglomerates at the EDL level. Altogether, ac-electrogravimetry, when combined with classical EQCM, stands as an elegant and powerful method to experimentally assess the chemical structure and dynamics of the electrical double layer. We hope that the community will start to adopt it to better engineer interfaces of electrochemical energy storage devices.

Thermodynamics as a Driving Factor of LiCoO2 Grain Growth on Nanocrystalline Ta-LLZO Thin Films for All-Solid-State Batteries.

All-solid-state batteries primarily focus on macrocrystalline solid electrolyte/cathode interfaces, and little is explored on the growth and stability of nanograined Li-garnet and cathode ones. In this work, a thin (∼500 nm) film of LiCoO2 (LCO) has been grown on top of the polycrystalline layer of Ta-doped Li7La3Zr2O12 (Ta-LLZO) solid electrolyte using the pulsed laser deposition (PLD) technique. Scanning transmission electron microscopy, electron diffraction, and electron tomography demonstrated that the LCO film is formed by columnar elements with the shape of inverted cones. The film appears to be highly textured, with the (003) LCO crystal planes parallel to the LCO/Ta-LLZO interface and with internal pores shaped by the {104} and {102} planes. According to density functional theory (DFT) calculations, this specific microstructure is governed by a competition between free energies of the corresponding crystal planes, which in turn depends on the oxygen and lithium chemical potentials during the deposition, indicating that thermodynamics plays an important role in the resulting LCO microstructure even under nonequilibrium PLD conditions. Based on the thermodynamic estimates, the experimental conditions within the LCO stability domain are proposed for the preferential {104} LCO orientation, which is considered favorable for enhanced Li diffusion in the positive electrode layers of all-solid-state batteries.

Topochemical Synthesis of Ca3 CrN3 H Involving a Rotational Structural Transformation for Catalytic Ammonia Synthesis.

Topochemical reactions have led to great progress in the discovery of new metastable compounds with novel chemical and physical properties. With these reactions, the overall crystal structure of the host material is generally maintained. Here we report a topochemical synthesis of a hexagonal nitride hydride, h-Ca3 CrN3 H, by heating an orthorhombic nitride, o-Ca3 CrN3 , under hydrogen at 673 K, accompanied by a rotational structural transformation. The hydrogen intercalation modifies the Ca-N rock-salt-like atomic packing in o-Ca3 CrN3 to a face-sharing octahedral chain in h-Ca3 CrN3 H, mimicking a "hinged tessellation" movement. In addition, the h-Ca3 CrN3 H exhibited stable ammonia synthesis activity when used as a catalyst.

The Influence of Synthesis Method on the Local Structure and Electrochemical Properties of Li-Rich/Mn-Rich NMC Cathode Materials for Li-Ion Batteries.

Electrochemical energy storage plays a vital role in combating global climate change. Nowadays lithium-ion battery technology remains the most prominent technology for rechargeable batteries. A key performance-limiting factor of lithium-ion batteries is the active material of the positive electrode (cathode). Lithium- and manganese-rich nickel manganese cobalt oxide (LMR-NMC) cathode materials for Li-ion batteries are extensively investigated due to their high specific discharge capacities (>280 mAh/g). However, these materials are prone to severe capacity and voltage fade, which deteriorates the electrochemical performance. Capacity and voltage fade are strongly correlated with the particle morphology and nano- and microstructure of LMR-NMCs. By selecting an adequate synthesis strategy, the particle morphology and structure can be controlled, as such steering the electrochemical properties. In this manuscript we comparatively assessed the morphology and nanostructure of LMR-NMC (Li1.2Ni0.13Mn0.54Co0.13O2) prepared via an environmentally friendly aqueous solution-gel and co-precipitation route, respectively. The solution-gel (SG) synthesized material shows a Ni-enriched spinel-type surface layer at the {200} facets, which, based on our post-mortem high-angle annual dark-field scanning transmission electron microscopy and selected-area electron diffraction analysis, could partly explain the retarded voltage fade compared to the co-precipitation (CP) synthesized material. In addition, deviations in voltage fade and capacity fade (the latter being larger for the SG material) could also be correlated with the different particle morphology obtained for both materials.

Development of vanadium-based polyanion positive electrode active materials for high-voltage sodium-based batteries.

Polyanion compounds offer a playground for designing prospective electrode active materials for sodium-ion storage due to their structural diversity and chemical variety. Here, by combining a NaVPO4F composition and KTiOPO4-type framework via a low-temperature (e.g., 190 °C) ion-exchange synthesis approach, we develop a high-capacity and high-voltage positive electrode active material. When tested in a coin cell configuration in combination with a Na metal negative electrode and a NaPF6-based non-aqueous electrolyte solution, this cathode active material enables a discharge capacity of 136 mAh g-1 at 14.3 mA g-1 with an average cell discharge voltage of about 4.0 V. Furthermore, a specific discharge capacity of 123 mAh g-1 at 5.7 A g-1 is also reported for the same cell configuration. Through ex situ and operando structural characterizations, we also demonstrate that the reversible Na-ion storage at the positive electrode occurs mostly via a solid-solution de/insertion mechanism.

Polyoxocationic antimony oxide cluster with acidic protons.

The success and continued expansion of research on metal-oxo clusters owe largely to their structural richness and wide range of functions. However, while most of them known to date are negatively charged polyoxometalates, there is only a handful of cationic ones, much less functional ones. Here, we show an all-inorganic hydroxyiodide [H10.7Sb32.1O44][H2.1Sb2.1I8O6][Sb0.76I6]2·25H2O (HSbOI), forming a face-centered cubic structure with cationic Sb32O44 clusters and two types of anionic clusters in its interstitial spaces. Although it is submicrometer in size, electron diffraction tomography of HSbOI allowed the construction of the initial structural model, followed by powder Rietveld refinement to reach the final structure. The cationic cluster is characterized by the presence of acidic protons on its surface due to substantial Sb3+ deficiencies, which enables HSbOI to serve as an excellent solid acid catalyst. These results open up a frontier for the exploration and functionalization of cationic metal-oxo clusters containing heavy main group elements.

Capturing dynamic ligand-to-metal charge transfer with a long-lived cationic intermediate for anionic redox.

Reversible anionic redox reactions represent a transformational change for creating advanced high-energy-density positive-electrode materials for lithium-ion batteries. The activation mechanism of these reactions is frequently linked to ligand-to-metal charge transfer (LMCT) processes, which have not been fully validated experimentally due to the lack of suitable model materials. Here we show that the activation of anionic redox in cation-disordered rock-salt Li1.17Ti0.58Ni0.25O2 involves a long-lived intermediate Ni3+/4+ species, which can fully evolve to Ni2+ during relaxation. Combining electrochemical analysis and spectroscopic techniques, we quantitatively identified that the reduction of this Ni3+/4+ species goes through a dynamic LMCT process (Ni3+/4+-O2- → Ni2+-On-). Our findings provide experimental validation of previous theoretical hypotheses and help to rationalize several peculiarities associated with anionic redox, such as cationic-anionic redox inversion and voltage hysteresis. This work also provides additional guidance for designing high-capacity electrodes by screening appropriate cationic species for mediating LMCT.

Chemistry, Local Molybdenum Clustering, and Electrochemistry in the Li2+xMo1-xO3 Solid Solutions.

A broad range of cationic nonstoichiometry has been demonstrated for the Li-rich layered rock-salt-type oxide Li2MoO3, which has generally been considered as a phase with a well-defined chemical composition. Li2+xMo1-xO3 (-0.037 ≤ x ≤ 0.124) solid solutions were synthesized via hydrogen reduction of Li2MoO4 in the temperature range of 650-1100 °C, with x decreasing with the increase of the reduction temperature. The solid solutions adopt a monoclinically distorted O3-type layered average structure and demonstrate a robust local ordering of the Li cations and Mo3 triangular clusters within the mixed Li/Mo cationic layers. The local structure was scrutinized in detail by electron diffraction and aberration-corrected scanning transmission electron microcopy (STEM), resulting in an ordering model comprising a uniform distribution of the Mo3 clusters compatible with local electroneutrality and chemical composition. The geometry of the triangular clusters with their oxygen environment (Mo3O13 groups) has been directly visualized using differential phase contrast STEM imaging. The established local structure was used as input for density functional theory (DFT)-based calculations; they support the proposed atomic arrangement and provide a plausible explanation for the staircase galvanostatic charge profiles upon electrochemical Li+ extraction from Li2+xMo1-xO3 in Li cells. According to DFT, all electrochemical capacity in Li2+xMo1-xO3 solely originates from the cationic Mo redox process, which proceeds via oxidation of the Mo3 triangular clusters into bent Mo3 chains where the electronic capacity of the clusters depends on the initial chemical composition and Mo oxidation state defining the width of the first charge low-voltage plateau. Further oxidation at the high-voltage plateau proceeds through decomposition of the Mo3 chains into Mo2 dimers and further into individual Mo6+ cations.

Revisited Ti2Nb2O9 as an Anode Material for Advanced Li-Ion Batteries.

Ti2Nb2O9 with a tunnel-type structure is considered as a perspective negative electrode material for Li-ion batteries (LIBs) with theoretical capacity of 252 mAh g-1 corresponding to one-electron reduction/oxidation of Ti and Nb, but only ≈160 mAh g-1 has been observed practically. In this work, highly reversible capacity of 200 mAh g-1 with the average (de)lithiation potential of 1.5 V vs Li/Li+ is achieved for Ti2Nb2O9 with pseudo-2D layered morphology obtained via thermal decomposition of the NH4TiNbO5 intermediate prepared by K+→ H+→ NH4+ cation exchange from KTiNbO5. Using operando synchrotron powder X-ray diffraction (SXPD), single-phase (de)lithiation mechanism with 4.8% unit cell volume change is observed. Operando X-ray absorption near-edge structure (XANES) experiment revealed simultaneous Ti4+/Ti3+ and Nb5+/Nb4+ reduction/oxidation within the whole voltage range. Li+ migration barriers for Ti2Nb2O9 along [010] direction derived from density functional theory (DFT) calculations are within the 0.15-0.4 eV range depending on the Li content that is reflected in excellent C-rate capacity retention. Ti2Nb2O9 synthesized via the ion-exchange route appears as a strong contender to widely commercialized Ti-based negative electrode material Li4Ti5O12 in the next generation of high-performance LIBs.

Enhancement of Catalytic Activity and Stability of La0.6Ca0.4Fe0.7Ni0.3O2.9 Perovskite with ppm Concentration of Fe in the Electrolyte for the Oxygen Evolution Reaction.

The catalytic activity and stability of an iron-nickel based oxygen-deficient perovskite for the oxygen evolution reaction (OER) are drastically improved with the ppm additive of Fe ions to the alkaline electrolyte. The enhancement is attributed to a 1-2 nm restructured Ni0.5Fe0.5Ox(OH)2-x (oxy)hydroxide layer, as demonstrated with scanning transmission electron microscopy. La0.6Ca0.4Fe0.7Ni0.3O2.9 shows almost a four-fold increase in OER activity after Fe addition relative to the as-prepared pristine electrolyte, which demonstrates the low Tafel slope of 44 ± 2.4 mV dec-1 and the superior intrinsic activity of 706 ± 71 A g-1oxide at 1.61 V vs. RHE.

Activation of anionic redox in d0 transition metal chalcogenides by anion doping.

Expanding the chemical space for designing novel anionic redox materials from oxides to sulfides has enabled to better apprehend fundamental aspects dealing with cationic-anionic relative band positioning. Pursuing with chalcogenides, but deviating from cationic substitution, we here present another twist to our band positioning strategy that relies on mixed ligands with the synthesis of the Li2TiS3-xSex solid solution series. Through the series the electrochemical activity displays a bell shape variation that peaks at 260 mAh/g for the composition x = 0.6 with barely no capacity for the x = 0 and x = 3 end members. We show that this capacity results from cumulated anionic (Se2-/Sen-) and (S2-/Sn-) and cationic Ti3+/Ti4+ redox processes and provide evidence for a metal-ligand charge transfer by temperature-driven electron localization. Moreover, DFT calculations reveal that an anionic redox process cannot take place without the dynamic involvement of the transition metal electronic states. These insights can guide the rational synthesis of other Li-rich chalcogenides that are of interest for the development of solid-state batteries.

Correlating ligand-to-metal charge transfer with voltage hysteresis in a Li-rich rock-salt compound exhibiting anionic redox.

Anionic redox is a double-edged sword for Li-ion cathodes because it offers a transformational increase in energy density that is also negated by several detrimental drawbacks to its practical implementation. Among them, voltage hysteresis is the most troublesome because its origin is still unclear and under debate. Herein, we tackle this issue by designing a prototypical Li-rich cation-disordered rock-salt compound Li1.17Ti0.33Fe0.5O2 that shows anionic redox activity and exceptionally large voltage hysteresis while exhibiting a partially reversible Fe migration between octahedral and tetrahedral sites. Through combined in situ and ex situ spectroscopic techniques, we demonstrate the existence of a non-equilibrium (adiabatic) redox pathway enlisting Fe3+/Fe4+ and O redox as opposed to the equilibrium (non-adiabatic) redox pathway involving sole O redox. We further show that the charge transfer from O(2p) lone pair states to Fe(3d) states involving sluggish structural distortion is responsible for voltage hysteresis. This study provides a general understanding of various voltage hysteresis signatures in the large family of Li-rich rock-salt compounds.

α-TiPO4 as a Negative Electrode Material for Lithium-Ion Batteries.

To realize high-power performance, lithium-ion batteries require stable, environmentally benign, and economically viable noncarbonaceous anode materials capable of operating at high rates with low strain during charge-discharge. In this paper, we report the synthesis, crystal structure, and electrochemical properties of a new titanium-based member of the MPO4 phosphate series adopting the α-CrPO4 structure type. α-TiPO4 has been obtained by thermal decomposition of a novel hydrothermally prepared fluoride phosphate, NH4TiPO4F, at 600 °C under a hydrogen atmosphere. The crystal structure of α-TiPO4 is refined from powder X-ray diffraction data using a Rietveld method and verified by electron diffraction and high-resolution scanning transmission electron microscopy, whereas the chemical composition is confirmed by IR, energy-dispersive X-ray, electron paramagnetic resonance, and electron energy loss spectroscopies. Carbon-coated α-TiPO4/C demonstrates reversible electrochemical activity ascribed to the Ti3+/Ti2+ redox transition delivering 125 mAh g-1 specific capacity at C/10 in the 1.0-3.1 V versus Li+/Li potential range with an average potential of ∼1.5 V, exhibiting good rate capability and stable cycling with volume variation not exceeding 0.5%. Below 0.8 V, the material undergoes a conversion reaction, further revealing capacitive reversible electrochemical behavior with an average specific capacity of 270 mAh g-1 at 1C in the 0.7-2.9 V versus Li+/Li potential range. This work suggests a new synthesis route to metastable titanium-containing phosphates holding prospective to be used as negative electrode materials for metal-ion batteries.