Optimized atomic layer deposition of homogeneous, conductive Al2O3 coatings for high-nickel NCM containing ready-to-use electrodes.

Atomic layer deposition (ALD) derived ultrathin conformal Al2O3 coating has been identified as an effective strategy for enhancing the electrochemical performance of Ni-rich LiNixCoyMnzO2 (NCM; 0 ≤x, y, z < 1) based cathode active materials (CAM) in Li-ion batteries. However, there is still a need to better understand the beneficial effect of ALD derived surface coatings on the performance of NCM based composite cathodes. In this work, we applied and optimized a low-temperature ALD derived Al2O3 coating on a series of Ni-rich NCM-based (NCM622, NCM71.51.5 and NCM811) ready-to-use composite cathodes and investigated the effect of coating on the surface conductivity of the electrode as well as its electrochemical performance. A highly uniform and conformal coating was successfully achieved on all three different cathode compositions under the same ALD deposition conditions. All the coated cathodes were found to exhibit an improved electrochemical performance during long-term cycling under moderate cycling conditions. The improvement in the electrochemical performance after Al2O3 coating is attributed to the suppression of parasitic side reactions between the electrode and the electrolyte during cycling. Furthermore, conductive atomic force microscopy (C-AFM) was performed on the electrode surface as a non-destructive technique to determine the difference in surface morphology and conductivity between uncoated and coated electrodes before and after cycling. C-AFM measurements on pristine cathodes before cycling allow clear separation between the conductive carbon additives and the embedded NCM secondary particles, which show an electrically insulating behavior. More importantly, the measurements reveal that the ALD-derived Al2O3 coating with an optimized thickness is thin enough to retain the original conduction properties of the coated electrodes, while thicker coating layers are insulating resulting in a worse cycling performance. After cycling, the surface conductivity of the coated electrodes is maintained, while in the case of uncoated electrodes the surface conductivity is completely suppressed confirming the formation of an insulating cathode electrolyte interface due to the parasitic side reactions. The results not only show the possibilities of C-AFM as a non-destructive evaluation of the surface properties, but also reveal that an optimized coating, which preserves the conductive properties of the electrode surface, is a crucial factor for stabilising the long-term battery performance.

A Rapid and Facile Approach for the Recycling of High-Performance LiNi1-x-y Cox Mny O2 Active Materials.

The demand for lithium-ion batteries has risen dramatically over the years. Unfortunately, many of the essential component materials, such as cobalt and lithium, are both costly and of limited abundance. For this reason, the recycling of lithium-ion battery electrodes is crucial to ensuring the availability of such resources and protecting the environment. Herein, a simple and scalable recycling process was developed for the prototypical cathode active material Li1.02 (Ni0.8 Co0.1 Mn0.1 )0.98 O2 (NCM-811). By a combination of thermal decomposition and dissolution steps, spent NCM could be converted into Li2 CO3 and a transition metal oxalate blend, which served as precursors for new NCM. Importantly, it was also possible to individually separate each transition metal during the recycling process, thereby extending the utility of this method to a wide variety of NCM compositions. Each intermediate in the process was investigated by scanning electron microscopy and X-ray diffraction. Additionally, the elemental composition of the recycled NCM-811 was confirmed using inductively coupled plasma optical emission spectroscopy and energy-dispersive X-ray spectroscopy. The electrochemical performance of the recycled NCM-811 exhibited up to 80 % of the initial capacity of pristine NCM-811. The method presented herein serves as an efficient and environmentally benign alternative to existing recycling methods for lithium-ion battery electrode materials.

Evidence for a Solid-Electrolyte Inductive Effect in the Superionic Conductor Li10Ge1-xSnxP2S12.

Strategies to enhance ionic conductivities in solid electrolytes typically focus on the effects of modifying their crystal structures or of tuning mobile-ion stoichiometries. A less-explored approach is to modulate the chemical bonding interactions within a material to promote fast lithium-ion diffusion. Recently, the idea of a solid-electrolyte inductive effect has been proposed, whereby changes in bonding within the solid-electrolyte host framework modify the potential energy landscape for the mobile ions, resulting in an enhanced ionic conductivity. Direct evidence for a solid-electrolyte inductive effect, however, is lacking-in part because of the challenge of quantifying changes in local bonding interactions within a solid-electrolyte host framework. Here, we consider the evidence for a solid-electrolyte inductive effect in the archetypal superionic lithium-ion conductor Li10Ge1-xSnxP2S12. Substituting Ge for Sn weakens the {Ge,Sn}-S bonding interactions and increases the charge density associated with the S2- ions. This charge redistribution modifies the Li+ substructure causing Li+ ions to bind more strongly to the host framework S2- anions, which in turn modulates the Li+ ion potential energy surface, increasing local barriers for Li+ ion diffusion. Each of these effects is consistent with the predictions of the solid-electrolyte inductive effect model. Density functional theory calculations predict that this inductive effect occurs even in the absence of changes to the host framework geometry due to Ge → Sn substitution. These results provide direct evidence in support of a measurable solid-electrolyte inductive effect and demonstrate its application as a practical strategy for tuning ionic conductivities in superionic lithium-ion conductors.

Enhancing the Electrochemical Performance of LiNi0.70Co0.15Mn0.15O2 Cathodes Using a Practical Solution-Based Al2O3 Coating.

Ni-rich Li[NixCoyMn1-x-y]O2 (NCM) cathode materials have attracted great research interest owing to their high energy density and relatively low cost. However, capacity fading because of parasitic side reactions, mainly occurring at the interface with the electrolyte, still hinders widespread application in advanced Li-ion batteries (LIBs). Surface modification via coating is a feasible approach to tackle this issue. Nevertheless, achieving uniform coatings is challenging, especially when using wet chemistry methods. In this work, a protective alumina shell on NCM701515 (70% Ni) was prepared through the reaction of surface-active -OH groups with trimethylaluminum as the precursor. The coated NCM701515 shows significantly improved capacity retention over uncoated (pristine) NCM701515. Part of the reason is the lower impedance buildup during cycling due to the effective suppression of adverse side reactions and secondary particle fracture. Taken together, the solution-based coating strategy described herein offers an easy way to apply surface treatment to stabilize Ni-rich NCM cathode materials in next-generation LIBs.

Local Structure and Influence of Sb Substitution on the Structure-Transport Properties in AgBiSe2.

Owing to their intrinsically low thermal conductivity and chemical diversity, materials within the I-V-VI2 family, and especially AgBiSe2, have recently attracted interest as promising thermoelectric materials. However, further investigations are needed in order to develop a more fundamental understanding of the origin of the low thermal conductivity in AgBiSe2, to evaluate possible stereochemical activity of the 6s2 lone pair of Bi3+, and to further elaborate on chemical design approaches for influencing the occurring phase transitions. In this work, a combination of temperature-dependent X-ray diffraction, Rietveld refinements of laboratory X-ray diffraction data, and pair distribution function analyses of synchrotron X-ray diffraction data is used to tackle the influence of Sb substitution within AgBi1-xSbxSe2 (0 ⩽ x ⩽ 0.15) on the phase transitions, local distortions, and off-centering of the structure. This work shows that, similar to other lone-pair-containing materials, local off-centering and distortions can be found in AgBiSe2. Furthermore, electronic and thermal transport measurements, in combination with the modeling of point-defect scattering, highlight the importance of structural characterizations toward understanding changes induced by elemental substitutions. This work provides new insights into the structure-transport correlations of the thermoelectric AgBiSe2.

Ionic Conductivity of the NASICON-Related Thiophosphate Na1+x Ti2-x Gax (PS4 )3.

Inspired by the recent interest in fast ionic conducting solids for electrolytes, the ionic conductivity of a novel ionic conductor Na1+x Ti2-x Gax (PS4 )3 has been investigated. Using X-ray diffraction and impedance spectroscopy the sodium ionic conductivity in this compound was demonstrated, in which bond valence sum analysis suggests a tunnel diffusion for Na+ . Substitution with Ga3+ leads to an increasing Na+ content, an expansion of the lattice and an increasing conductivity with increasing x in Na1+x Ti2-x Gax (PS4 )3 . Given the relation to the NASICON family, upon replacement of the phosphate by a thiophosphate group, a rich structural chemistry can be expected in this class of materials. This work demonstrates the potential for making NaTi2 (PS4 )3 an ideal system to study structure-property relationships in ionic conductors.

Investigation of Fluorine and Nitrogen as Anionic Dopants in Nickel-Rich Cathode Materials for Lithium-Ion Batteries.

Advanced lithium-ion batteries are of great interest for consumer electronics and electric vehicle applications; however, they still suffer from drawbacks stemming from cathode active material limitations (e.g., insufficient capacities and capacity fading). One approach for alleviating such limitations and stabilizing the active material structure may be anion doping. In this work, fluorine and nitrogen are investigated as potential dopants in Li1.02(Ni0.8Co0.1Mn0.1)0.98O2 (NCM) as a prototypical nickel-rich cathode active material. Nitrogen doping is achieved by ammonia treatment of NCM in the presence of oxygen, which serves as an unconventional and new approach. The crystal structure was investigated by means of Rietveld and pair distribution function analysis of X-ray diffraction data, which provide very precise information regarding both the average and local structure, respectively. Meanwhile, time-of-flight secondary-ion mass spectroscopy was used to assess the efficacy of dopant incorporation within the NCM structure. Moreover, scanning electron microscopy and scanning transmission electron microscopy were conducted to thoroughly investigate the dopant influences on the NCM morphology. Finally, the electrochemical performance was tested via galvanostatic cycling of half- and full-cells between 0.1 and 2 C. Ultimately, a dopant-dependent modulation of the NCM structure was found to enable the enhancement of the electrochemical performance, thereby opening a route to cathode active material optimization.

Inducing High Ionic Conductivity in the Lithium Superionic Argyrodites Li6+ xP1- xGe xS5I for All-Solid-State Batteries.

Solid-state batteries with inorganic solid electrolytes are currently being discussed as a more reliable and safer future alternative to the current lithium-ion battery technology. To compete with state-of-the-art lithium-ion batteries, solid electrolytes with higher ionic conductivities are needed, especially if thick electrode configurations are to be used. In the search for optimized ionic conductors, the lithium argyrodites have attracted a lot of interest. Here, we systematically explore the influence of aliovalent substitution in Li6+ xP1- xGe xS5I using a combination of X-ray and neutron diffraction, as well as impedance spectroscopy and nuclear magnetic resonance. With increasing Ge content, an anion site disorder is induced and the activation barrier for ionic motion drops significantly, leading to the fastest lithium argyrodite so far with 5.4 ± 0.8 mS cm-1 in a cold-pressed state and 18.4 ± 2.7 mS cm-1 upon sintering. These high ionic conductivities allow for successful implementation within a thick-electrode solid-state battery that shows negligible capacity fade over 150 cycles. The observed changes in the activation barrier and changing site disorder provide an additional approach toward designing better performing solid electrolytes.

Comparing the Descriptors for Investigating the Influence of Lattice Dynamics on Ionic Transport Using the Superionic Conductor Na3PS4- xSe x.

Recent work on superionic conductors has demonstrated the influence of lattice dynamics and the softness of the lattice on ionic transport. When examining either the changes in the acoustic phonon spectrum or the whole phonon density of states, both a decreasing activation barrier of migration and a decreasing entropy of migration have been observed, highlighting that the paradigm of "the softer the lattice, the better" does not always hold true. However, both approaches to monitor the changing lattice dynamics probe different frequency ranges of the phonon spectrum, and thus, it is unclear if they are complementary. In this work, we investigate the lattice dynamics of the superionic conductor Na3PS4- xSe x by probing the optical phonon modes and the acoustic phonon modes, as well as the phonon density of states via inelastic neutron scattering. Notably, Raman spectroscopy shows the evolution of multiple local symmetry reduced polyhedral species, which likely affect the local diffusion pathways. Meanwhile, density functional theory and the ionic transport data are used to compare the different approaches for assessing the lattice dynamics. This work shows that, while acoustic and inelastic methods may be used to experimentally assess the overall changing lattice stiffness, calculations of the average vibrational energies between the mobile ions and the anion framework are important to assess and computationally screen for ionic conductors.

Competing Structural Influences in the Li Superionic Conducting Argyrodites Li6PS5- xSe xBr (0 ≤ x ≤ 1) upon Se Substitution.

Lithium-ion conducting argyrodites have recently attracted significant interest as solid electrolytes for solid-state battery applications. In order to enhance the utility of materials in this class, a deeper understanding of the fundamental structure-property relationships is still required. Using Rietveld refinements of X-ray diffraction data and pair distribution function analysis of neutron diffraction data, coupled with electrochemical impedance spectroscopy and speed of sound measurements, the structure and transport properties within Li6PS5- xSe xBr (0 ≤ x ≤ 1) have been monitored with increasing Se content. While it has been previously suggested that the incorporation of larger, more polarizable anions within the argyrodite lattice should lead to enhancements in the ionic conductivity, the Li6PS5- xSe xBr transport behavior was found to be largely unaffected by the incorporation of Se2- due to significant structural modifications to the anion sublattice. This work affirms the notion that, when optimizing the ionic conductivity of solid ion conductors, local structural influences cannot be ignored and the idea of "the softer the lattice, the better" does not always hold true.

Refinement of the crystal structure of Li4P2S6 using NMR crystallography.

The structure of Li4P2S6 was solved, based on a combination of X-ray powder diffraction data, quantum chemical calculations and solid state nuclear magnetic resonance (NMR). Two-dimensional 31P single quantum/double quantum correlation spectra yielded important constraints regarding the space group symmetry allowing the crystal structure to be solved by the Rietveld method. Li4P2S6 crystallizes in a trigonal space group with a = 10.51452(6) Å; c = 6.59149(8) Å. The structure contains two distinct P2S64- ions in a 2 : 1 ratio: in the first one the two P atoms of the hexahypothiophosphate unit are crystallographically distinct, whereas in the second one they are crystallographically identical.

Degradation Mechanisms at the Li10GeP2S12/LiCoO2 Cathode Interface in an All-Solid-State Lithium-Ion Battery.

All-solid-state batteries (ASSBs) show great potential for providing high power and energy densities with enhanced battery safety. While new solid electrolytes (SEs) have been developed with high enough ionic conductivities, SSBs with long operational life are still rarely reported. Therefore, on the way to high-performance and long-life ASSBs, a better understanding of the complex degradation mechanisms, occurring at the electrode/electrolyte interfaces is pivotal. While the lithium metal/solid electrolyte interface is receiving considerable attention due to the quest for high energy density, the interface between the active material and solid electrolyte particles within the composite cathode is arguably the most difficult to solve and study. In this work, multiple characterization methods are combined to better understand the processes that occur at the LiCoO2 cathode and the Li10GeP2S12 solid electrolyte interface. Indium and Li4Ti5O12 are used as anode materials to avoid the instability problems associated with Li-metal anodes. Capacity fading and increased impedances are observed during long-term cycling. Postmortem analysis with scanning transmission electron microscopy, electron energy loss spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy show that electrochemically driven mechanical failure and degradation at the cathode/solid electrolyte interface contribute to the increase in internal resistance and the resulting capacity fading. These results suggest that the development of electrochemically more stable SEs and the engineering of cathode/SE interfaces are crucial for achieving reliable SSB performance.

Local Tetragonal Structure of the Cubic Superionic Conductor Na3PS4.

The sodium superionic conductor Na3PS4 is known to crystallize in one of two different structural polymorphs at room temperature (i.e., cubic or tetragonal, depending on the synthetic conditions). Experimentally, the cubic structure is known to exhibit a higher ionic conductivity than the tetragonal structure, despite theoretical investigations suggesting that there should be no difference at all. Employing a combination of Rietveld and pair distribution function (PDF) analyses, as well as electrochemical impedance spectroscopy, we investigate the open question of how the crystal structure influences the ionic transport in Na3PS4. Despite the average structures of Na3PS4 prepared via ball-milling and high-temperature routes being cubic and tetragonal, respectively, the structural analysis by PDF indicates that both compounds are best described by the structural motifs of the tetragonal polymorph on the local scale. Ultimately, the high ionic conductivity of Na3PS4 prepared by the ball-milling approach is confirmed to be independent of the crystal structure. This work demonstrates that even in ionic conductors differences can be observed between the average and local crystal structures, and it reasserts that the high ionic conductivity in Na3PS4 is not related to the crystal structure but rather differences in the defect concentration.

The Detrimental Effects of Carbon Additives in Li10GeP2S12-Based Solid-State Batteries.

All-solid-state batteries (SSBs) have recently attracted much attention due to their potential application in electric vehicles. One key issue that is central to improve the function of SSBs is to gain a better understanding of the interfaces between the material components toward enhancing the electrochemical performance. In this work, the interfacial properties of a carbon-containing cathode composite, employing Li10GeP2S12 as the solid electrolyte, are investigated. A large interfacial charge-transfer resistance builds up upon the inclusion of carbon in the composite, which is detrimental to the resulting cycle life. Analysis by X-ray photoelectron spectroscopy reveals that carbon facilitates faster electrochemical decomposition of the thiophosphate solid electrolyte at the cathode/solid electrolyte interface-by transferring the low chemical potential of lithium in the charged state deeper into the solid electrolyte and extending the decomposition region. The occurring accumulation of highly oxidized sulfur species at the interface is likely responsible for the large interfacial resistances and aggravated capacity fading observed.

Influence of Lattice Polarizability on the Ionic Conductivity in the Lithium Superionic Argyrodites Li6PS5X (X = Cl, Br, I).

In the search for novel solid electrolytes for solid-state batteries, thiophosphate ionic conductors have been in recent focus owing to their high ionic conductivities, which are believed to stem from a softer, more polarizable anion framework. Inspired by the oft-cited connection between a soft anion lattice and ionic transport, this work aims to provide evidence on how changing the polarizability of the anion sublattice in one structure affects ionic transport. Here, we systematically alter the anion framework polarizability of the superionic argyrodites Li6PS5X by controlling the fractional occupancy of the halide anions (X = Cl, Br, I). Ultrasonic speed of sound measurements are used to quantify the variation in the lattice stiffness and Debye frequencies. In combination with electrochemical impedance spectroscopy and neutron diffraction, these results show that the lattice softness has a striking influence on the ionic transport: the softer bonds lower the activation barrier and simultaneously decrease the prefactor of the moving ion. Due to the contradicting influence of these parameters on ionic conductivity, we find that it is necessary to tailor the lattice stiffness of materials in order to obtain an optimum ionic conductivity.

Molybdenum Oxide Nitrides of the Mo2(O,N,□)5 Type: On the Way to Mo2O5.

Blue-colored molybdenum oxide nitrides of the Mo2(O,N,□)5 type were synthesized by direct nitridation of commercially available molybdenum trioxide with a mixture of gaseous ammonia and oxygen. Chemical composition, crystal structure, and stability of the obtained and hitherto unknown compounds are studied extensively. The average oxidation state of +5 for molybdenum is proven by Mo K near-edge X-ray absorption spectroscopy; the magnetic behavior is in agreement with compounds exhibiting MoVO6 units. The new materials are stable up to ∼773 K in an inert gas atmosphere. At higher temperatures, decomposition is observed. X-ray and neutron powder diffraction, electron diffraction, and high-resolution transmission electron microscopy reveal the structure to be related to VNb9O24.9-type phases, however, with severe disorder hampering full structure determination. Still, the results demonstrate the possibility of a future synthesis of the potential binary oxide Mo2O5. On the basis of these findings, a tentative suggestion on the crystal structure of the potential compound Mo2O5, backed by electronic-structure and phonon calculations from first principles, is given.

Vacancy and anti-site disorder scattering in AgBiSe2 thermoelectrics.

AgBiSe2 has recently been shown to exhibit promising thermoelectric properties due to the low intrinsic thermal conductivity, stemming from a large degree of lattice anharmonicity. While samples synthesized via solid-state routes usually exhibit n-type behavior, p-type transport is seen in samples based on solution synthetic routes possibly due to Ag vacancies. Using a combined approach of synchrotron diffraction, thermoelectric transport measurements and thermal transport modeling, we show the influence of synthetically induced Ag vacancies on the structure of AgBiSe2 and the thermoelectric transport. We identify the degree of anti-site disorder of Ag and Bi due to the occurring phase transformation and the influence of the vacancy content on metal ordering. Additionally, we show that anti-site disorder and vacancies act as scattering centers for phonons, leading to enhanced point defect scattering in this interesting thermoelectric material.

Composition-dependent surface chemistry of colloidal BaxSr1-xTiO3 perovskite nanocrystals.

BaxSr1-xTiO3 perovskite nanocrystals, prepared by the vapor diffusion sol-gel method and characterized by state of the art surface techniques, display significantly different O-H stretching frequencies and adsorption properties towards CO2 as a function of the alkaline earth composition (Ba vs. Sr). The difference of properties can be associated with the more basic nature of BaO-rich than SrO-rich surfaces.

Lanthanide-activated scheelite nanocrystal phosphors prepared by the low-temperature vapor diffusion sol-gel method.

A series of Eu3+-, Tb3+-, and Tm3+-doped CaWO4 phosphor nanocrystals have been synthesized under benign conditions using the vapor diffusion sol-gel method. The high degree of synthetic flexibility inherent to this approach has enabled the synthesis of a CaWO4:(Eu,Tb) dual-sensitized white light emitting nanocrystal phosphor upon commercial UV excitation at 366 nm with a long lifetime exceeding 1 ms.

Low-temperature synthesis of homogeneous solid solutions of scheelite-structured Ca1-xSrxWO4 and Sr1-xBaxWO4 nanocrystals.

A series of compositionally complex scheelite-structured nanocrystals of the formula A1-xAWO4 (A = Ca, Sr, Ba) have been prepared under benign synthesis conditions using the vapor diffusion sol-gel method. Discrete nanocrystals with sub-20 nm mean diameters were obtained after kinetically controlled hydrolysis and polycondensation at room temperature, followed by composition-dependent thermal aging at or below 60 °C. Rietveld analysis of X-ray diffraction data and Raman spectroscopy verified the synthesis of continuous and phase-pure nanocrystal solid solutions across the entire composition space for A1-xAWO4, where 0 ≤ x ≤ 1. Elemental analysis by X-ray photoelectron and inductively coupled plasma-atomic emission spectroscopies demonstrated excellent agreement between the nominal and experimentally determined elemental stoichiometries, while energy dispersive X-ray spectroscopy illustrated good spatial elemental homogeneity within these nanocrystals synthesized under benign conditions.