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Electrochemical energy storage was an important enabler of the wireless revolution and it is touted as a key component of a society that shifts away from its dependence on fossil fuels. Li-ion batteries are the primary technology when high energy devices are required. However, despite their improved functionality over older systems (e.g. lead-acid car batteries), they do not quite yet meet the emerging energy demands in transportation and grid markets. This roadblock sparked interest in the development of batteries that utilize Mg2+ as ionic carrier. Theoretical predictions indicate that couples exist between a Mg metal negative electrode and oxide positive electrodes that could surpass the current practical limits of current devices. Among the candidate oxides, those showing a spinel structure have been predicted as the most suitable for the reversible intercalation of ions such as Mg2+ or even Ca2+ [1], the critical reaction in the positive electrode. However, experimental validation, while incipient [2], has not been fully achieved. In this talk, we will present the most up-to-date insight into the ability of spinel oxides to diffuse and reversibly intercalate Mg2+. In this task, the ability to synthesize particles at small dimensions is vital, as is the characterization of chemical and physical phenomena using a combination of tools providing information at different scales. We will rely on data from X-ray diffraction, spectroscopy and scattering, electron microscopy and nuclear magnetic resonance to probe the reactions that occur when spinel oxides are used as working electrodes in cells with electrolytes containing Mg2+. The rationale for the choice of techniques and the key pieces they provided to complete the picture will be discussed. Our ultimate aim in the talk will be to establish relationships between crystal-chemistry, charge carrier and outcomes of the electrochemical reaction. Liu, M., et al., Energy Environ. Sci. 2015, 8, 964. Kim, C. et al., Adv. Mater. 2015, 27, 3377.

An effective way of synthesizing graphene-wrapped Li4Ti5O12 particles was developed by solid-state reaction between graphene oxide-wrapped P25 (TiO2) and Li2CO3. Compared to the previously reported graphene/Li4Ti5O12 composites, prior wrapping of TiO2 with subsequent chemical lithiation led to more effectively confined Li4Ti5O12. The Li4Ti5O12 tightly bound by graphene exhibited a remarkable specific capacity of 147 mA h g−1 at a rate of 10 C (1 C = 175 mA g−1) after 100 cycles. This rate capability is one of the highest values among reported Li4Ti5O12 with 150 ± 50 nm grains. The improved rate capability was attributed to the enhanced electronic conductivity of each Li4Ti5O12 grain via uniform graphene wrapping, with single-grain growth during annealing from the initial ∼25 nm TiO2 nanoparticles enclosed by outer graphene sheets. Graphene-eliminated Li4Ti5O12 by thermal decomposition was also directly compared to the graphene-coated sample, to clarify the role of graphene with nearly equivalent particle size/morphology distributions.

Al-substituted Li7La3Zr2O12 (LLZO) pellets with a grain size of 100–200 μm and a relative density of 94% were prepared by conventional solid-state processing at a sintering temperature of 1100 °C, 130 °C lower than previously reported. Morphological features and the presence of impurities were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Femtosecond Laser Induced Breakdown Spectroscopy (LIBS) was used to visualize the distribution of impurities. The results suggest that chemical composition of the powder cover strongly affects morphology and impurity formation, and that particle size control is critical to densification. These properties, in turn, strongly affect total ionic conductivity and interfacial resistance of the sintered pellets.

Storing electrochemical energy by means of fully reducing transition metal compounds to their metallic state is attractive due to the large amounts of charge that are produced. However, while more reversible than originally envisioned, electrochemical conversion reactions are accompanied by large inefficiencies. These inefficiencies are associated with hysteresis between the potentials of reduction and re-oxidation, as well as the loss of capacity when reversing back to the initial state. This work presents a series of results collected from different kinds of electroanalytical experiments. The use of data from conventional powder electrodes as well as thin films offered the opportunity to compare measurements of common occurrence in the literature with more sophisticated experiments carried out at higher temperature, which provided a complete picture of the kinetic nature of the processes involved. This picture is supportive of the thermodynamic nature of potential hysteresis, and indicates that coulombic inefficiencies stem from the low reversibility of the conversion reaction. Taken together, the results cast a clear light on the significant challenges that lie ahead if this kind of reactivity is to become technologically relevant in devices such as Li-ion batteries.

Members of the spinel solid solution series ZnFe2–yCryO4 (y = 0, 0.5, 1.0, 1.5, and 2) were synthesized using high-energy ball milling followed by annealing at 1000 °C. The structural study of the samples was performed by Fourier transform infrared spectroscopy (FTIR), X-ray absorption spectroscopy (XAS), and powder X-ray diffraction (XRD). While XRD verified the formation of single spinel phases with lattice parameters reduced by increasing Cr substitution, FTIR and XAS provided insight into the subsequently increased covalence of the chemical bonding of the spinels. The mixed transition-metal spinel oxides were employed as working electrodes in Li metal batteries. In agreement with the literature, the spinel oxides experience amorphization during the first discharge, as shown by ex situ XRD and selected area electron diffraction (SAED). The electrochemical activity of the spinel oxides was found to diminish with Cr content so that ZnCr2O4 is completely inactive even when the material is nanosized and in...

Pulsed laser deposition (PLD) was attempted for lithium conducting electrolyte layers of Li7La3Zr2O12 on a porous membrane of anodized aluminum oxide. Al-doping made it possible to obtain dense PLD targets with the cubic garnet phase structure successfully using a solid-state reaction and sintering at 1100°C. Substrate heating during deposition led to homogeneous film morphology, but caused formation of La2ZrO7 impurity phase. Aluminum migration from the substrate to the film surface was observed when the sample was annealed at 700°C. The aluminum migration seemed to be facilitated by the lithium in the environment of the PLD chamber.

For reasons of cost and supply security issues, there is growing interest in the development of rechargeable sodium ion batteries, particularly for large-scale grid storage applications. Like the much better known and technologically important lithium ion analogs, the devices operate by shuttling alkali metal cations between two host materials, which undergo insertion processes at different electrochemical potentials. A particular challenge for the sodium systems is identification of a suitable anode material due to the fact that sodium does not intercalate into graphite. Although several alternatives, including disordered carbons and alloys are being investigated, the most promising options at present lie with titanates, not in the least because of attractive characteristics such as low toxicity, ease of synthesis, wide availability, and low cost. A large variety of sodium titanate compounds can be prepared, many of which have tunnel or layered structures that can readily undergo reversible reductive intercalation reactions. A brief overview of the physical, structural, and electrochemical characteristics of several of the most promising materials for sodium-ion battery applications is given in this paper, and a comparison is made between the sodium and the lithium insertion behaviors. For some of these compounds, insertion of sodium occurs at unusually low potentials, a feature that has important implications for the design of high-energy sodium-ion systems.

This article describes the carbon-free titania composite battery electrodes constructed from anisotropic, rod-shaped nanocrystals has very high and stable capacities, as well as outstanding rate capability, when compared to carbon-contg. counterparts. Monodisperse anatase titania nanorods are obtained by low-temp. base-catalyzed hydrolysis of titanium tetraisopropoxide and monodisperse nanospheres are synthesized in solvothermal conditions. Com. available 25 nm and 300 nm titania particles are used for comparison purposes. The voltage-capacity, specific capacity-cycle no. and electrochem. impedance characteristics of titania electrodes are evaluated.

The physical and electrochemical properties of a 60% Li 3 PO 4 –40% Li 4 SiO 4 (60LP) solid solution phase have been investigated with the goal of developing an ion conductive protective layer for metallic lithium in secondary battery configurations. This phase was chosen because it has the highest ionic conductivity of 4 × 10 − 6 S/cm at room temperature in the Li 3 PO 4 –Li 4 SiO 4 solid solution series. Addition of 0.5 wt.% lithium borate or a mixture of its precursors lowered the sintering temperature of 60LP by as much as 150 °C without compromising the ionic conductivity of the dense pellets. Both AC and DC experiments showed that a stable interface is formed between the lithium and the phosphosilicate layer, allowing reversible lithium stripping and plating. The results are consistent with the assumption that the primary mobile species in the ceramic layer are lithium ions.

Battery function is determined by the efficiency and reversibility of the electrochemical phase transformations at solid electrodes. The microscopic tools available to study the chemical states of matter with the required spatial resolution and chemical specificity are intrinsically limited when studying complex architectures by their reliance on two-dimensional projections of thick material. Here, we report the development of soft X-ray ptychographic tomography, which resolves chemical states in three dimensions at 11 nm spatial resolution. We study an ensemble of nano-plates of lithium iron phosphate extracted from a battery electrode at 50% state of charge. Using a set of nanoscale tomograms, we quantify the electrochemical state and resolve phase boundaries throughout the volume of individual nanoparticles. These observations reveal multiple reaction points, intra-particle heterogeneity, and size effects that highlight the importance of multi-dimensional analytical tools in providing novel insight to the design of the next generation of high-performance devices., Here the authors show the development of soft X-ray ptychographic tomography to quantify the electrochemical state and resolve phase boundaries throughout the volume of individual nano-particles from a composite battery electrode.

There is considerable interest in developing solid electrolytes for rechargeable lithium batteries as they have the potential to increase both energy density due to incorporation of a lithium metal anode and safety of batteries due to the fact that they are nonflammable. Block copolymers with a mechanically hard non-conducting block and a soft ion-conducting block provide an avenue for obtaining highly conducting rigid solids. In this study we add surface-modified TiO2 nanoparticles to a mixture of polystyrene-block-poly(ethylene oxide) and bis(trifluoromethane)sulfonimide lithium salt. The presence of BF4 − moieties on the surface of the particles was essential for obtaining macroscopically homogeneous electrolytes; macrophase separation was observed with the same nanoparticles with surfaces covered with oleic acid. The stability of these composite electrolytes against lithium metal electrodes was tested in symmetric lithium-composite electrolyte-lithium cells. The surprising result was that electrolytes with 24 wt% nanoparticles exhibited optimum stability; the amount of charge passed before dendrite formation observed in the optimized composite electrolyte was a factor of 4.7 larger than that of the neat block copolymer electrolyte. Both tensile and shear moduli of the electrolytes were non-monotonic functions of particle concentration with peaks in the vicinity of 17 to 20 wt%.

Graphene CVD-grown on Cu has been studied using Raman spectroscopy, X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES). Raman data indicate the presence of weak compressive strain at the interface of graphene/Cu. Compared with highly ordered pyrolytic graphite (HOPG), new electronic states in the conduction band are observed for graphene/Cu, which are mainly ascribed to the defect states and interfacial interaction between the single graphene layer and Cu surface. Moreover, polarization dependent XAS measurements demonstrate that the graphene/Cu exhibits a high degree of alignment and weak corrugation on the surface. Significant intensity modulation in the resonant XES spectral shape upon different excitation energies near the C K-edge indicates that graphene layer preserves an intrinsic momentum as that of HOPG and the interaction between graphene and Cu shows weak influence on the valence band structure of graphene. However, broad inelastic features and subtle peak shifts are observed in the resonant XES spectra of graphene/Cu in comparison of HOPG, which can be mainly attributed to the electron–phonon scattering and charge transfer from the interfacial interaction of graphene and Cu substrate.

Functional electrodes for batteries share a common design rule by which high electronic and ionic conductivity pathways must exist throughout the electrode in its pristine state. Notable amounts of conductive carbon additive in the composite electrode are usually included to form an electronically conductive matrix. However, excellent high rate cycling performance has been achieved in electrodes composed of the insulating Li4Ti5O12 without any conductive additives. This behavior opens the possibility of a new paradigm for designing functional electrodes by which high electronic conductivity in the pristine electrode is not required. The mechanism of operation that enables such unexpected electrochemical behavior is evaluated and discussed. Electronically conductive pathways due to the reduction of Ti4+ to Ti3+ form and percolate throughout the Li4Ti5O12 electrode in the early stage of Li insertion, eliminating the need for conductive additives. This work highlights the importance of the mass and charge transport properties of the intermediate states during cycling and of good interparticle ohmic contact in the electrode. This physical behavior can lead to novel system designs with improved battery utilization and energy density.

Single-phase LiNi0.45Mn0.45Co0.1–yAlyO2 layered oxide materials with 0 ≤ y ≤ 0.10 were prepared using the glycine-nitrate combustion method. Al-substitution has a minimal effect on the defect concentration and rate capability of the materials, but raises the operating voltage and reduces the capacity fade of the materials during prolonged cycling compared to the unsubstituted system. In situ X-ray diffraction suggests the presence of Al has a significant structural impact during battery operation. It acts to limit the changes in lattice parameters observed during electrochemical charging and cycling of the materials. High-resolution X-ray diffraction reveals structural distortions in the transition metal layers of as-synthesized powders with high Al-contents, as well as a structural evolution seen in all materials after cycling.

A study of the correlations between the stoichiometry, secondary phases and transition metal ordering of LiNi(0.5)Mn(1.5)O(4) was undertaken by characterizing samples synthesized at different temperatures. Insight into the composition of the samples was obtained by electron microscopy, neutron diffraction and X-ray absorption spectroscopy. In turn, analysis of cationic ordering was performed by combining neutron diffraction with Li MAS NMR spectroscopy. Under the conditions chosen for the synthesis, all samples systematically showed an excess of Mn, which was compensated by the formation of a secondary rock salt phase and not via the creation of oxygen vacancies. Local deviations from the ideal 3:1 Mn:Ni ordering were found, even for samples that show the superlattice ordering by diffraction, with different disordered schemes also being possible. The magnetic behavior of the samples was correlated with the deviations from this ideal ordering arrangement. The in-depth crystal-chemical knowledge generated was employed to evaluate the influence of these parameters on the electrochemical behavior of the materials.

Birnessite is a widespread, naturally occurring layered manganese oxide that exerts significant influence over the geochemical cycling of environmentally relevant cations due to its high-interlayer adsorption capacity. Triclinic sodium birnessite was used as a synthetic analog to gain a better understanding of the nature of cation adsorption in this important phyllomanganate. Drawing from previous work correlating observed 23Na NMR shifts in manganese oxides with local environment and Mn oxidation state, the 23Na NMR spectra of metastable buserite and two birnessite samples, NaBi-H-I and NaBi-II, were analyzed to determine the nature of the bound interlayer sodium ions in these materials. The small 23Na chemical shift of buserite shows that its interlayer sodium is fully hydrated. X-ray diffraction indicates that NaBi-H-I is a disordered birnessite while NaBi-II is highly crystalline. High-field (14.1 T) fast MAS NMR spectra of NaBi-H-I and NaBi-II supports these observations, resolving multiple sodium environments for NaBi-H-I and only two sodium environments for NaBi-II. The observed hyperfine shifts were less than expected for sodium environments with manganate layers composed of 2/3 Mn4+ and 1/3 Mn3+ ions, and the 23Na line shapes indicated that the Na+ ions are in distorted environments. Both these factors suggest that the sodium ions are offset in the interlayers toward a single oxide layer and located near Mn3+-rich environments within the layer.

The structural and electronic effects of Al-substitution in LiNi0.45Mn0.45Co0.1−yAlyO2 electrode materials were explored on a local scale using X-ray absorption spectroscopy (XAS). Homogeneous substitution of Al is observed between 0 ≤ y ≤ 0.10, additionally causing relaxation within the strained edge-sharing metal octahedra composing the transition metal plane. Ex situ measurements on electrochemically charged electrodes confirm that the main charge compensation mechanism occurs via oxidation of Ni, though a contribution centered at the Co-sites was detected in the early stages of charging. Al-substitution, meanwhile, appears to have a negligible local order effect. Analysis of electrochemically cycled materials suggests that there is a beneficial structural consequence of the Al-substitution, which limits the disorder within the local structure of the substituted samples compared to the fresh layered oxides.

Copper substituted lithium iron phosphate, with the olivine structure, has been synthesized hydrothermally as dark-brown single crystals with 15% of the iron sites occupied by cupric ions. Single crystal X-ray diffraction and magnetic susceptibility,combined with Li solid state NMR studies performed on powder sample, showed that 5% of the lithium ions also reside on the iron site, giving the formula Li0.95[(Fe2+)0.70(Fe3+)0.10(Cu2+)0.15Li0.05)]PO4. The compound exhibits anisotropic magnetic susceptibility and orders antiferromagnetically at 48 K. The magnetic moments are aligned along [010] as in LiFePO4. The optical measurements indicate that the dark brown color originates from interionic d-d transitions. A part of the lithium ions can be cycled in an electrochemical cell, corresponding to the oxidation/reduction of the iron ions. An onset of a second electrochemical process is observed at about 4.1 V, tentatively attributed to the removal of Li ions from the Fe site. The first principles calculations i...

The complexity of layered-spinelyLi2MnO3·(1 –y)Li1+xMn2–xO4(Li:Mn = 1.2:1; 0 ≤x≤ 0.33;y≥ 0.45) composites synthesized at different temperatures has been investigated by a combination of x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), and nuclear magnetic resonance (NMR). While the layered component does not change substantially between samples, an evolution of the spinel component from a high to a low lithium excess phase has been traced with temperature by comparing with data for pure Li1+xMn2–xO4. The changes that occur to the structure of the spinel component and to the average oxidation state of the manganese ions within the composite structure as lithium is electrochemically removed in a battery have been monitored using these techniques, in some cases in situ. Our6Li NMR results constitute the first direct observation of lithium removal from Li2MnO3and the formation of LiMnO2upon lithium reinsertion.

Controlling the reactions occurring at electrode-electrolyte interfaces is key to long-lasting energy storage technologies, such as Li-ion batteries, especially when pushing voltages of operation to increase their energy density. Extreme potentials reached at the electrodes in a fully charged battery render them too reactive toward the electrolyte, triggering deleterious decomposition reactions. While the description of products resulting from electrolyte decomposition has been an object of study for decades, the definition of reactions at the cathode side of the interface is less extensive. The cathodes in a battery are composed of transition metal oxides, which are known to present a rich redox chemistry. During this talk, tools employed to track the structure and chemistry of the surface of cathode particles vis-à-vis their electrochemical properties will be presented. Particular emphasis will be placed on X-ray absorption spectroscopy, a technique with surface specificity that probes redox chemistry. The results revealed that side reactions on the electrolyte are synchronized with the redox processes at the oxide surface, which ultimately lead to the dissolution of transition metal ions into the electrolyte. Classical strategies to prevent these undesired reactions and increase electrode durability generally involved post-synthetic coatings of the cathode powders or the use of electrolyte additives. More recently, new effective options have been added through the demonstration, in our group, of core-(epitaxial) shell architectures at the level of primary particles. Yet the description of the exact modes of degradation and the means by which these strategies are effective remains elusive. The use of objects of study designed at the highest definition and fidelity, in the form of highly dispersed nanocrystals with conformal shells, enhances the understanding of these interfacial phenomena. The high fidelity ensures coherence in the data and the representative character of the characterization methods, especially when highly local probes are employed. The interplay between understanding of electrode-electrolyte processes and the design of novel cathode architectures will be discussed.

A systematic exploratory study of the Li–Nb–N–O system at low oxygen and high lithium contents has been performed. As lithium and oxygen increase, an order–disorder transition has been identified using powder neutron diffraction data between Li 16 Nb 2 N 8 O, which crystallizes in an antifluorite-type superstructure with cationic and anionic ordering, and Li 11 NbN 4 O 2 , a new antifluorite-type oxynitride that shows structural disorder. A description of the synthetic conditions required to prepare these phases and their structural characterization is presented.

Stabilization of high energy cathode-electrolyte interface is required to meet the criteria for the power source and stable cycling performance in Li-ion batteries. [1] The undesired reactions at the interface could be minimized by introducing redox-inactive ions on the surface of active materials without affecting original properties. [2] To demonstrate the roles of passivating layers more effectively, high energy nanocrystals were introduced due to its high surface area that can induce access of charges with electrolytes, accelerating chemical degradation at the interface. [3] A synthetic method based on colloidal chemistry was employed to grow a thin shell enriched in Al3+ in the same reaction environment where the particles were grown in dispersible form, ensuring effective coverage of all surfaces. The resultant core-shell nanocrystal consists of active lithium transition metal oxides as a core and enriched Al3+ shells as a passivating layers to suppress the unfavorable reaction on the surface. Careful post-synthetic annealing was used to produce the final heterostructures and tailor the specific chemistry of the shells. The modified nanocrystals improved capacity retentions in various conditions, such as elevated temperature and high applying potentials compared to the bare counterpart where a shell is not presented, strongly suggesting a role of protective layers. The chemical identity and specific structural information of the bulk and passivation layers were mainly characterized by a combination of solid state nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and X-ray absorption spectroscopy (XAS). Reference Lee, K. T.; Jeong, S.; Cho, J. Acc. Chem. Res, 2013, 46, 1161-1170. Kim, C.; Phillips, P. J.; Xu, L. P.; Dong, A. G.; Buonsanti, R.; Klie, R. F.; Cabana, J. Chem. Mater, 2015, 27, 394-399. Bruce, P. G.; Scrosati, B.; Tarascon, J. M. Angew. Chem. Int. Ed, 2008, 47, 2930-2946.

The performance of Li7.9MnN3.2O1.6 and Li7MnN4 as electrode materials in lithium batteries was analyzed. At 1C rate, capacities of 180 and 230 mAh/g, respectively, were obtained after 50 cycles. If the first charge is done at 0.1C, outstanding capacities of 120–135 mAh/g are observed after 100 cycles at 5C. More lithium can be removed during the charge at 0.1C, leading to a large amount of lithium vacancies that enhance mobility and rate capability. It is proposed that incomplete filling of the vacancies occurs upon cycling, so that the mobility remains high. This performance compares well to that of Li4Ti5O12. Keywords: Lithium batteries, Lithium manganese nitride and oxynitride, Rate capability, Negative electrodes

6,7Li and 31P NMR experiments were conducted on a series of single- or two-phase samples in the LiFePO4−FePO4 system with different overall lithium contents, and containing the two end-members and/or two metastable solid solution phases, Li0.6FePO4 or Li0.34FePO4. These experiments were carried out at different temperatures in order to search for vacancy/charge ordering and ion/electron mobility in the metastable phases. Evidence for Li+−Fe2+ interactions was observed for both Li0.6FePO4 and Li0.34FePO4. The strength of this interaction leads to the formation of LiFePO4-like clusters in the latter, as shown by the room temperature data. Different motional processes are proposed to exist as the temperature is increased and various scenarios are discussed. While concerted lithium-electron hopping and/or correlations explains the data below 125 °C, evidence for some uncorrelated motion is found at higher temperatures, together with the onset of phase mixing.

A systematic study has been performed to investigate the structural changes of Li[Ni{sub 0.05}Mn{sub 0.05}Co{sub 0.90}]O{sub 2}, one member of the Li[Ni{sub y}Mn{sub y}Co{sub (1-2y)}]O{sub 2} series with low Ni/Mn content, upon electrochemical lithium deintercalation. X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), and nuclear magnetic resonance (NMR) measurements were performed, and the results from these experiments provided a detailed picture of the whole delithiation process. Oxidation of not only Ni{sup 2+}, but also some Co{sup 3+}, is seen in the beginning of Li extraction (less than 0.15 mol removed), the ions located closest to Mn{sup 4+} being extracted first. Further deintercalation (additional 0.2 mol of Li removal) induces an insulator to metal transition that is similar to that reported for LiCoO{sub 2}. However, this reaction follows a solid solution mechanism even for this low level of substitution, rather than the two-phase reaction reported for the Ni, Mn-free oxide. When half of the Li ions are extracted, the electrochemical signature for lithium vacancy ordering in the host framework is observed. The NMR results for deintercalation of more than 50% Li were compared to those for Li{sub x}CoO{sub 2} at similar stages of charge, which are reported here for the firstmore » time; they indicate that the behavior of these two phases at these potentials is very similar. When the batteries are charged to voltages higher than 4.6 V, very few lithium ions remain in the structure and the O3 to O1 phase transition occurs.« less

Among the metal-organic frameworks (MOFs), MIL-53(Fe) or FeIII(OH)0.8F0.2[O2C−C6H4−CO2] was the first ever reported member to reversibly insert Li+ electrochemically. A variety of electroactive sorbents has been investigated in an attempt to increase its electrochemical capacity vs Li+/Li0. Here, we describe the synthesis and characterization of a new composite hybrid material involving MIL-53(Fe) as the host for the guest electroactive 1,4-benzoquinone molecule in a 1:1 molar ratio, using complementary high-resolution X-ray diffraction (XRD), differential scanning calorimetry (DSC), and magic angle spinning nuclear magnetic resonance (MAS NMR) measurements. Its room-temperature structure has been solved and shows that the quinone molecules are located within the channels nearly parallel to each other, and to the benzene rings of the skeleton, in order to maximize π−π interactions. When heated in a sealed container, a flip-flop reorganization of the quinone molecules occurred above 140 °C, whereas in an o...

Lithium-sulfur (Li-S) batteries are expected to overcome the limit of current energy storage devices by delivering high specific energy with low material cost. However, the potential of Li-S batteries has not yet been realized because of several technical barriers. Poor electrochemical performance is mainly attributed to the low electrical conductivity of the fully charged and discharged species, the irreversible loss of polysulfide anions and the decrease in the number of electrochemically active reaction sites during battery operation. Here, we report that the introduction of graphene quantum dots (GQDs) into the sulfur cathode dramatically enhanced sulfur/sulfide utilization, yielding high performance. In addition, the GQDs induced structural integrity of the sulfur-carbon electrode composite by oxygen-rich functional groups. This hierarchical architecture enabled fast charge transfer while minimizing the loss of lithium polysulfides, which is attributed to the physicochemical properties of GQDs. The mechanisms through which excellent cycling and rate performance are achieved were thoroughly studied by analyzing capacity versus voltage profiles. Furthermore, experimental observations and theoretical calculations further clarified the role played by GQDs by proving that C-S bonding occurs. Thus, the introduction of GQDs into Li-S batteries will provide an important breakthrough allowing their use as high-performance and low-cost batteries for next-generation energy storage systems.

Several members of the compositional series Li[NixMnxCo(1–2x)]O2 (0.01 ≤ x ≤ 1/3) were synthesized and characterized. X-ray diffraction results confirm the presence of the layered α-NaFeO2-type structure, while X-ray absorption near-edge spectroscopy experiments verify the presence of Ni2+, Mn4+, and Co3+. Their local environment and short-range ordering were investigated by using a combination of 6Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and neutron pair distribution function (PDF) analysis, associated with reverse Monte Carlo (RMC) calculations. The 6Li MAS NMR spectra of compounds with low Ni/Mn contents (x ≤ 0.10) show several well-resolved resonances, which start to merge when the amount of Ni and Mn increases, finally forming a broad resonance at high Ni/Mn contents. Analysis of the 6Li MAS NMR 6Li[Ni0.02Mn0.02Co0.96]O2 spectrum, is consistent with the formation of Ni2+ and Mn4+ clusters within the transition-metal layers, even at these low-doping levels. The oxida...

The mechanism of the chemical and electrochemical alkali metal intercalation reactions in β-HfNCl has been investigated through electrochemical potential spectroscopy (EPS), in-situ powder X-ray diffraction during electrochemical intercalation and room temperature chemical intercalation experiments. EPS experiments in lithium cells reveal the presence of a plateau, at 1.8 V vs. Li + /Li 0 accounting for ca. 0.14 mol Li, that indicates the formation of a new intermediate phase, and then a gradual decrease of potential with composition that extends up to very high lithium contents (ca. 1.1 per formula), consistent with the formation of a solid solution. Sodium electrochemical intercalation experiments showed a relatively similar behaviour with a plateau at 1.4 V vs. Na + /Na 0 , corresponding to ca. 1.7 V vs. Li + /Li 0 . In-situ monitored powder X-ray diffraction electrochemical intercalation experiments showed that the electrolyte solvent (ethylene carbonate/dimethyl carbonate, EC/DMC or propylene carbonate, PC) co-intercalated with the alkaline atom. This leads to a large expansion of the interlayer spacing that reaches a value of 21.06 A in the lithium co-intercalated phase with EC/DMC, Li x (EC/DMC) y HfNCl, and 22.01 A in the sodium co-intercalated phase with PC, Na x (PC) y HfNCl. Chemical intercalation using naphthyl-sodium solutions in tetrahydrofuran (THF) leads to solvent-free, multiple-phase samples showing in different proportions the pristine and the superconducting stage 2 and stage 1 phases. The composition of the intercalated samples depends on the pristine sample, the concentration of the naphthyl-sodium solution, the ratio Na:HfNCl and the reaction time. Pristine samples exhibiting low lithium intercalation degree upon electrochemical reduction gave the second stage as the major phase when treated with short reaction times or using low Na:HfNCl ratios, coexisting either with the host or with the first stage phase, whereas stage 1 is obtained as the major phase from pristine samples showing high electrochemical capacities. The staging behaviour and the multiphase nature of these samples account for the wide superconducting transitions and the different critical temperatures observed in these superconductors.

We report an approach to control the reversible electrochemical activity (i.e., extraction/insertion) of Mg(2+) in a cathode host through the use of phase-pure epitaxially stabilized thin film structures. The epitaxially stabilized MgMn2O4 (MMO) thin films in the distinct tetragonal and cubic phases are shown to exhibit dramatically different properties (in a nonaqueous electrolyte, Mg(TFSI)2 in propylene carbonate): tetragonal MMO shows negligible activity while the cubic MMO (normally found as polymorph at high temperature or high pressure) exhibits reversible Mg(2+) activity with associated changes in film structure and Mn oxidation state. These results demonstrate a novel strategy for identifying the factors that control multivalent cation mobility in next-generation battery materials.

Extensive and reversible electrochemical Mg intercalation into spinel-type Mn2O4 is demonstrated by electrochemical measurements, XRD, STEM, XAS, and solid state NMR.

The performance of battery electrode materials is strongly affected by inefficiencies in utilization kinetics and cycle life as well as size effects. Observations of phase transformations in these materials with high chemical and spatial resolution can elucidate the relationship between chemical processes and mechanical degradation. Soft X-ray ptychographic microscopy combined with X-ray absorption spectroscopy and electron microscopy creates a powerful suite of tools that we use to assess the chemical and morphological changes in lithium iron phosphate (LiFePO4) micro- and nanocrystals that occur upon delithiation. All sizes of partly delithiated crystals were found to contain two phases with a complex correlation between crystallographic orientation and phase distribution. However, the lattice mismatch between LiFePO4 and FePO4 led to severe fracturing on microcrystals, whereas no mechanical damage was observed in nanoplates, indicating that mechanics are a principal driver in the outstanding electrode performance of LiFePO4 nanoparticles. These results demonstrate the importance of engineering the active electrode material in next generation electrical energy storage systems, which will achieve theoretical limits of energy density and extended stability. This work establishes soft X-ray ptychographic chemical imaging as an essential tool to build comprehensive relationships between mechanics and chemistry that guide this engineering design.

LiMn0.8Fe0.1M0.1PO4/C (M: Fe, Co, Ni, Cu) nanocomposite cathode materials are prepared by freeze-drying of aqueous solutions of citric acid, Mn(OAc)3, LiOH, NH4H2PO4, and M(OAc)2 in stoichiometric quantities (-80 °C, 48 h, 3 x 10-1 mbar).

In situ techniques with high temporal, spatial and chemical resolution are key to understand ubiquitous solid-state phase transformations, which are crucial to many technological applications. Hard X-ray spectro-imaging can visualize electrochemically driven phase transformations but demands considerably large samples with strong absorption signal so far. Here we show a conceptually new data analysis method to enable operando visualization of mechanistically relevant weakly absorbing samples at the nanoscale and study electrochemical reaction dynamics of iron fluoride, a promising high-capacity conversion cathode material. In two specially designed samples with distinctive microstructure and porosity, we observe homogeneous phase transformations during both discharge and charge, faster and more complete Li-storage occurring in porous polycrystalline iron fluoride, and further, incomplete charge reaction following a pathway different from conventional belief. These mechanistic insights provide guidelines for designing better conversion cathode materials to realize the promise of high-capacity lithium-ion batteries., Hard X-ray spectro-imaging using synchrotron radiation can be used to monitor electrochemical reactions. Here, the authors present X-ray absorption data and resolve phase evolution for the conversion of iron fluoride, a high-capacity Li-ion battery conversion cathode, with nanoscale resolution.

Four LiMn0.8Fe0.1M0.1PO4/C (M = Fe, Co, Ni, Cu) cathode materials have been synthesized via a freeze-drying method. The samples have been characterized by powder X-ray diffraction, transmission electron microscopy, magnetic susceptibility, and electrochemical measurements. The composition and effective insertion of the transition-metal substituents in LiMnPO4 have been corroborated by elemental analysis, the evolution of the crystallographic parameters, and the magnetic properties. The morphological characterization of the composites has demonstrated that the phosphate nanoparticles are enclosed in a matrix of amorphous carbon. Among them, LiMn0.8Fe0.1Ni0.1PO4/C is the most promising cathode material, providing a good electrochemical performance in all aspects: high voltage and specific capacity values, excellent cyclability, and good rate capability. This result has been attributed to several factors, such as the suitable morphology of the sample, the good connection afforded by the in situ generated carbon, and the amelioration of the structural stress provided by the presence of Ni(2+) and Fe(2+) in the olivine structure.

Electrochemical oxidation of carbonate esters at the Li(x)Ni(0.5)Mn(1.5)O(4-δ)/electrolyte interface results in Ni/Mn dissolution and surface film formation, which negatively affect the electrochemical performance of Li-ion batteries. Ex situ X-ray absorption (XRF/XANES), Raman, and fluorescence spectroscopy, along with imaging of Li(x)Ni(0.5)Mn(1.5)O(4-δ) positive and graphite negative electrodes from tested Li-ion batteries, reveal the formation of a variety of Mn(II/III) and Ni(II) complexes with β-diketonate ligands. These metal complexes, which are generated upon anodic oxidation of ethyl and diethyl carbonates at Li(x)Ni(0.5)Mn(1.5)O(4-δ), form a surface film that partially dissolves in the electrolyte. The dissolved Mn(III) complexes are reduced to their Mn(II) analogues, which are incorporated into the solid electrolyte interphase surface layer at the graphite negative electrode. This work elucidates possible reaction pathways and evaluates their implications for Li(+) transport kinetics in Li-ion batteries.

Li 7 MnN 4 was prepared and characterised by both long and short range probes of structure. The results of the Rietveld refinement of its crystal structure from Powder Neutron Diffraction data are in agreement with previous determination from single crystal X-ray diffraction data. The 6 Li MAS-NMR spectrum of this compound contained five main resonances, which could be assigned to the five different crystallographic sites based on the multiplicities of the different sites and an analysis of the mechanisms that contribute to the observed NMR shifts. Variable temperature and two-dimensional NMR spectra showed that the Li ions in three out of the five sites are extremely mobile in this material, and a model to explain this mobility was developed involving jumps via nearby empty octahedral sites. In order to get a deeper insight in the structural changes taking place during the reversible lithium extraction/insertion redox process in this compound, partially oxidised samples were electrochemically prepared and studied both by 6 Li MAS-NMR and powder neutron diffraction. On extracting Li, distinct 6 Li NMR resonances due to the five sites were no longer resolved, exchange occurring rapidly between the different crystallographic positions. Comparison of the experimental neutron diffraction patterns of partially oxidised samples with calculated neutron diffractograms assuming lithium vacancies in the different sites, coupled with an analysis of the Li–Mn distances, indicated that lithium extraction takes place from the Li1 and Li5 sites.

Antifluorite-type phases have been prepared in the Li-M-N-O (M = Ti, V) systems by solid state reaction under nitrogen starting from mixtures of lithium nitride, different amounts of lithium oxide and the transition metal nitride. The two series of samples obtained can be formulated as Li5TiN3 · xLi2O and Li7VN4 · xLi2O solid solutions. X-ray diffraction data indicate that, as the amount of oxygen increases, the superstructure peaks decrease in intensity without changing their position, whereas the peaks corresponding to the simple anti-fluorite structure increase. Thus, samples with low oxygen content are a mixture of two phases: the corresponding ternary nitride (either Li5TiN3 or Li7VN4) and the quaternary oxinitride with a disordered antifluorite structure and hence structural disorder does not seem to be induced progressively, as previously believed. In agreement with previous studies, a decrease of the cell parameter with oxygen content is observed in both solid solutions. Even though no deintercalation/intercalation reactions were expected (no cationic vacancies exist in the sample and the transition metals are present in their highest oxidation states) an evaluation of the electrochemical performances of the samples vs. metallic lithium was done to test the possibility of conversion reactions taking place. However, none of them showed any electroactivity.

Direct evidence of Mg2+ intercalation into a spinel-type Mn2O4 is provided. By com­bining tools with different sensitivities, from atomic-resolution X-ray spectro­scopy to bulk X-ray diffraction, it is demonstrated that Mg2+ reversibly occupies the tetrahedral sites of the spinel structure through the reduction of Mn when the electrochemical reaction is performed.

Recently, new energy storage chemistries based on nonaqueous electrolytes and multivalent metals (e.g., Mg, Zn, Ca and Al) have drawn the attention of the researchers as a promising advanced energy storage technology due to their higher theoretical volumetric capacity, limited dendrite formation and low cost.1 A major developmental need for these systems is the identification of electrolytes compatible with both electrodes while showing reversible deposition/dissolution on an anode and multivalent intercalation into a cathode.1,2 In the case of nonaqueous Mg or Ca ion-based systems, electrolyte compatibility issues (e.g., low Coulombic efficiency, a high overpotential and corrosion) have held back the development of Mg or Ca metal batteries.3 However, the nonaqueous Zn2+ ion chemistry utilized in a Zn metal cells with a reversible intercalation cathode is an exception with a number of promising features including highly-efficient reversible Zn deposition/dissolution on a Zn metal anode with a wide electrochemical window,3 similar ionic radius compared with Li+ and Mg2+ ions,4 relatively lower activation barrier energy for diffusion in cathode materials (e.g., FePO4, CoO2 and V2O5)5 and high volumetric capacity.1 Considering these advantages, a nonaqueous Zn system provides an opportunity to delve into the mechanisms in multivalent-ion cell chemistry and solve the present issues in multivalent cell design and prototyping.3 In this study, the intercalation chemistry on a variety of cathodes materials (e.g., V2O5, Mn2O4 and FePO4) have been investigated in various nonaqueous Zn electrolytes. The electrochemical and transport properties of the electrolytes (e.g., reversible Zn deposition, anodic/cathodic stability, ionic conductivity and diffusion coefficient) were characterized utilizing the experimental and computational analysis.3 Among various Zn metal cells, a Zn/nanostructured bilayered V2O5 cell with a selected acetonitrile(AN)-Zn(TFSI)2 electrolyte demonstrates good reversibility and stability for 120+ cycles with nearly 100% Coulombic efficiency and ~170 mAhg-1 of gravimetric capacity, albeit operating at a cell voltage of 0.7 V vs. Zn/Zn2+.6 A Zn/nanostructured layered δ-MnO2 cell with an AN-Zn(TFSI)2 electrolyte also shows good reversibility (~100% Coulombic efficiency) and stability for 50+ cycles with ~100 mAhg-1 capacity with an operating voltage of 1.2 V vs. Zn/Zn2+.7 By utilizing a combination of analytical tools, we address numerous factors affecting capacity fade, and issues associated with the second phase formation including Mn dissolution in Zn/δ-MnO2 cells that have been extensively cycled.7 References 1. J. Muldoon, C. B. Bucur and T. Gregory, Chem. Rev. 2014, 114, 11683-11720. 2. H. D. Yoo, I. Shterenberg, Y. Gofer, G. Gershinsky, N. Pour and D. Aurbach, Energy Environ. Sci. 2013, 6, 2265-2279. 3. S.-D. Han, N. N. Rajput, X. Qu, B. Pan, M. He, M. S. Ferrandon, C. Liao, K. A. Persson and A. K. Burrell, ACS Appl. Mater. Inter. 2016, 8, 3021-3031. 4. R. D. Shannon, Acta Cryst. 1976, A32, 751-767. 5. Z. Rong, R. Malik, P. Canepa, G. Gautam, M. Liu, A. Jain, K. Persson and G. Ceder, Chem. Mater. 2015, 27, 6016-6021. 6. P. Senguttuvan, S.-D. Han, S. Kim, A. L. Lipson, S. Tepavcevic, T. T. Fister, I. D. Bloom, A. K. Burrell and C. S. Johnson, Adv. Energy Mater. 2016, 6, 1600826. 7. S.-D. Han, S. Kim, D. Li, V. Petkov, H. D. Yoo, P. J. Phillips, H. Wang, J. J. Kim, K. L. More, B. Key, R. F. Klie, J. Cabana, V. Stamenkovic, T. T. Fister, N. M. Markovic, A. K. Burrell, S. Tepavcevic, J. T. Vaughey, 2017, in revision.

Rechargeable Mg batteries are considered as promising candidates for energy storage. The Mg metal anode with high volumetric capacity (3833 mAh mL-1) and low redox potential (-2.37 V vs. S.H.E.) provides high energy density for the system, while a low price, good safety and long term utilization is maintained by the inexpensiveness, air stability and limited-dendritic growth of Mg. However, the sluggish solid diffusion of the divalent Mg ion presents limited candidates for cathode materials. One strategy to facilitate Mg2+ mobility is to utilize polarizable anions in the lattice so as to weaken the ionic interaction, as noted with the first Mg cathode (the Chevrel phase Mo6S8) as well as with more recent spinel and layered phases of titanium sulfide.1-3 Although the sulfide materials exhibit reasonable Mg2+ mobility, they limit the voltage and have relatively high mass, thus lowering the energy density of the cathode. Switching to oxides is the solution towards the search for high energy density cathodes; however, a structure that allows facile Mg2+de/intercalation is obviously necessary. In this presentation, we will discuss the prospect of oxide materials for Mg2+ de/intercalation. One such cathode candidate is the “calcium ferrite” (CF) –type structured oxide in which a low Mg2+ migration barrier has been predicted by first principles calculations.4 A sodium vanadium-titanium oxide compound with the CF structure was obtained by solid state synthesis. Na+ ions reside in the spacious tunnels of the CF structure in this material, and need to be either removed or replaced by Mg2+ in order to allow Mg2+ de/intercalation. Desodiation (Fig. 1a) was carried out with both electrochemical and chemical methods, resulting in ~ 55% and 85% Na+ extraction, respectively. The chemically desodiated material exhibits ~ 85 mAh g-1 discharge capacity in a Mg full cell (Fig. 1b), showing a promising Mg2+ intercalation capability in the structure, although only half of this is reversible in these preliminary studies. XAS studies suggest that vanadium is the redox centre. We also examined Mg2+ ion exchange for the CF material, finding that ~ half of the Na+ ions were replaced with Mg2+ (Fig. 1a). Interestingly, we observe a phase transformation reaction accompanying major desodiation or ion exchange. This demonstrates a trend towards the rearrangement of the polyhedral framework in the structure when the large ion tunnel is emptied or occupied by the smaller Mg2+ ion, the origins of which will be discussed in this talk. Nevertheless, the promising electrochemistry of the desodiated material upon Mg2+ de/intercalation, as well as the ability to partially exchange Na+ with Mg2+, suggests moderately good Mg2+ diffusion in the CF structure, and sheds light on the possibility of iso-structural compounds as cathode materials for Mg batteries that are stable or metastable upon Mg2+de/intercalation. Other promising cathode materials for Mg batteries, not strictly limited to oxides, will also be the topic of the presentation. References: 1. D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich and E. Levi, Nature 407, 724 (2000). 2. X. Sun, P. Bonnick, V. Duffort, M. Liu, Z. Rong, K. A. Persson, G. Ceder and L. F. Nazar, Energy Environ. Sci. 9, 2273 (2016). 3. X. Sun, P. Bonnick and L. F. Nazar, ACS Energy Lett. 1, 297 (2016). 4. C. Ling and F. Mizuno, Chem. Mater. 25, 3062 (2013). Figure 1

Stable cycling performance with high power density in lithium ion battery (LIBs) is required to meet the criteria for the power source such as electric vehicles. But generally, high power density is hindered by slow diffusion of lithium ions in micrometric electrode materials. [1] From this perspective, nanoparticle electrode materials can enhance the rate of lithium ions because of shortened diffusion pathway of lithium ion. Besides, high surface area of electrode induces facile access of charge via large contact area with electrolyte and conducting additives like carbon. In contrast, chemical degradation at electrode-electrolyte interface is also facilitated by large contact area with nanoparticle electrode. Unfavorable interfacial reactions such as dissolution of cathode species and decomposition of electrolyte mainly occur through energetically unstable surfaces of the active material. [2] In order to minimize side reactions that can negatively affect electrochemical performance, introducing electrochemically inactive ions on the surface of active materials can improve the interfacial stability. However, this substitution should take place as thin passivating layers on individual particles to preserve storage capacity. [3] A strategy toward the stabilization of electrode-electrolyte interfaces has been devised by introducing core-shell nanocrystals, consisting of electroactive lithium transition metal oxide cores and ultra-thin inactive epitaxial oxide shells on the surface. Spinel Li1+xMn2-xO4 nanocrystals were used as a core component, with an Al-rich shell as passivation layers to minimize unfavorable reaction on the surface of cathode particles. The electrochemical performance shows that Spinel Li1+xMn2-xO4 nanoparticle with conformal shell reveals improved capacity retention and rate capability even at elevated temperature, compared to a bare counterpart. Reference 1. Isaac D. Scott, Yoon Seok Jung, Andrew S. Cavanagh, Yanfa Yan, Anne C. Dillon, Steven M. George, and Se-Hee Lee, Nano Letters, 414–418, 11 (2011). 2. Peter G. Bruce, Bruno Scrosati, and Jean-Marie Tarascon, Angew. Chem. Int. Ed, 2930-2946, 47 (2008). 3. Chunjoong Kim, Patrick J. Phillips, Linping Xu, Angang Dong, Raffaella Buonsanti, Robert F. Klie, and Jordi Cabana, Chem. Matter, 394-399, 27 (2014).

Li-ion batteries are the top choice in electrochemical energy storage for portable devices and mobile vehicles. However, the limited storage capacities do not quite yet meet the emerging energy demands in transportation and grid markets. Multivalent batteries based on divalent cations have attracted wide attention with high energy density/capacity in different framework of oxide cathodes. For instance, spinel structure M2O4, has been theoretically predicted as one of the most suitable oxide cathode for the reversible intercalation with Mg2+ [1]. Further, reversible, yet sluggish Mg intercalation into Mn2O4 host, prepared from de-lithiated LiMn2O4 electrode, was reported in aqueous electrolyte environments [2]. In order to enhance kinetics, reduction of particles to the smallest possible sizes is desired. Further, spinel-type Mn­2O4 can only be prepared by delithiation of LiMn2O4. Therefore, the direct synthesis of the parent spinel oxide, MgMn2O4, in the form nanocrystals would be desirable to investigate the fundamental limits of Mg intercalation within the spinel oxide framework, preferably in non-aqueous electrolytes. Here, we report a series of Mg-based spinel nanocrystals with different phase (MgxMn3-xO4 and MgxCr3-xO4), size (5nm~50nm), and shape (cube and sheet) via a low temperature hydrothermal process, followed by calcination (Figure 1). All samples were characterized with X-ray diffraction and spectroscopy, as well as electron microscopy in order to define the products of electrochemical reactions in non-aqueous cells. Our goal was to explore spinel oxide nanocrystals with optimal size, phase, and shape and establish size-structure-property relationship for the experimental realization of Mg reversible intercalation in spinel oxide hosts. 1. Liu, M., et al., Energy Environ. Sci. 2015, 8, 964. 2. Kim, C. et al., Adv. Mater. 2015, 27, 3377. Figure 1 (a) Representative STEM image of 50nm MgMn­2O4 nanocubes (b) Representative STEM image of 2~5nm thickness MgMn­2O4 nanosheets Figure 1

The Li-V2O5 system has been well studied electrochemically, but there is a lack of systematic in-situ studies involving direct investigations of the structural changes that accompany the lithiation process. The open-cell battery setup inside a transmission electron microscope is ideal for studying the reaction pathway of intercalation of Li+ into nanowire cathodes. In this work, we utilize in-situ transmission electron microscopy to study the Li-V2O5 system. More specifically, we employ electron beam diffraction and electron energy-loss spectroscopy (EELS) in an open-cell battery setup to examine the phase changes within α-V2O5 nanowire cathodes upon in-situ lithiation. Our results suggest that the pristine α-V2O5 nanowire forms a Li oxide shell which then acts as a solid state electrolyte to conduct Li+ ions, and the bulk of the V2O5 nanowire undergoes transformation to the γ−Li2V2O5 phase.

X-ray microscopy is powerful in that it can probe large volumes of material at high spatial resolution with exquisite chemical, electronic and bond orientation contrast. The development of diffraction-based methods such as ptychography has, in principle, removed the resolution limit imposed by the characteristics of the X-ray optics. Here, using soft X-ray ptychography, we demonstrate the highest-resolution X-ray microscopy ever achieved by imaging 5â €..nm structures. We quantify the performance of our microscope and apply the method to the study of delithiation in a nanoplate of LiFePO 4, a material of broad interest in electrochemical energy storage. We calculate chemical component distributions using the full complex refractive index and demonstrate enhanced contrast, which elucidates a strong correlation between structural defects and chemical phase propagation. The ability to visualize the coupling of the kinetics of a phase transformation with the mechanical consequences is critical to designing materials with ultimate durability.

Electrochemical conversion reactions of transition metal compounds create opportunities for large energy storage capabilities exceeding modern Li-ion batteries. However, for practical electrodes to be envisaged, a detailed understanding of their mechanisms is needed, especially vis-à-vis the voltage hysteresis observed between reduction and oxidation. Here, we present such insight at scales from local atomic arrangements to whole electrodes. NiO was chosen as a simple model system. The most important finding is that the voltage hysteresis has its origin in the differing chemical pathways during reduction and oxidation. This asymmetry is enabled by the presence of small metallic clusters and, thus, is likely to apply to other transition metal oxide systems. The presence of nanoparticles also influences the electrochemical activity of the electrolyte and its degradation products and can create differences in transport properties within an electrode, resulting in localized reactions around converted domains that lead to compositional inhomogeneities at the microscale. Published version

Dense LLZO (Al-substituted Li7La3Zr2O12) pellets were processed in controlled atmospheres to investigate the relationships between the surface chemistry and interfacial behavior in lithium cells. Laser induced breakdown spectroscopy (LIBS), scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, synchrotron X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (XAS) studies revealed that Li2CO3 was formed on the surface when LLZO pellets were exposed to air. The distribution and thickness of the Li2CO3 layer were estimated by a combination of bulk and surface sensitive techniques with various probing depths. First-principles thermodynamic calculations confirmed that LLZO has an energetic preference to form Li2CO3 in air. Exposure to air and the subsequent formation of Li2CO3 at the LLZO surface is the source of the high interfacial impedances observed in cells with lithium electrodes. Surface polishing can effectively remove Li2CO3 and dramatically improve the interfacial properties. Polished samples in lithium cells had an area specific resistance (ASR) of only 109 Ω cm(2) for the LLZO/Li interface, the lowest reported value for Al-substituted LLZO. Galvanostatic cycling results obtained from lithium symmetrical cells also suggest that the quality of the LLZO/lithium interface has a significant impact on the device lifetime.

The X-ray absorption spectra (XAS) of lithium polysulfides (Li2Sx) of various chain lengths (x) dissolved in a model solvent are obtained from first-principles calculations. The spectra exhibit two main absorption features near the sulfur K-edge, which are unambiguously interpreted as a pre-edge near 2471 eV due to the terminal sulfur atoms at either end of the linear polysulfide dianions and a main-edge near 2473 eV due to the (x -2) internal atoms in the chain, except in the case of Li2S2, which only has a low-energy feature. We find an almost linear dependence between the ratio of the peaks and chain length, although the linear dependence is modified by the delocalized, molecular nature of the core-excited states that can span up to six neighboring sulfur atoms. Thus, our results indicate that the ratio of the peak area, and not the peak intensities, should be used when attempting to differentiate the polysulfides from XAS. © 2014 American Chemical Society.