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Author(s): Zhong, P; Cai, Z; Zhang, Y; Giovine, R; Ouyang, B; Zeng, G; Chen, Y; Clement, R; Lun, Z; Ceder, G | Abstract: Using both computations and experiments, we demonstrate that the performance of Li-excess cation-disordered rocksalt cathodes can be improved by Mg substitution. Mg reduces the amount of Li in the compound that is strongly bound to F and thereby increases the capacity. This enables the use of fluorination as a tool to improve stability of the compounds without significant loss of capacity. Mg emerged as the most optimal substitution element from a systematic computational study aimed at identifying inactive doping elements with a strong enough bonding strength to fluorine to displace Li from the F environments. Our results also show that capacity can be traded for cycle life depending on whether Mg is substituted for Li or for the redox metal. This design strategy should be considered in fluorinated cathodes, which will facilitate the design of optimized disordered rocksalt oxyfluoride cathodes.

The rapid market growth of rechargeable batteries requires electrode materials that combine high power and energy and are made from earth-abundant elements. Here we show that combining a partial spinel-like cation order and substantial lithium excess enables both dense and fast energy storage. Cation overstoichiometry and the resulting partial order is used to eliminate the phase transitions typical of ordered spinels and enable a larger practical capacity, while lithium excess is synergistically used with fluorine substitution to create a high lithium mobility. With this strategy, we achieved specific energies greater than 1,100 Wh kg–1 and discharge rates up to 20 A g–1. Remarkably, the cathode materials thus obtained from inexpensive manganese present a rare case wherein an excellent rate capability coexists with a reversible oxygen redox activity. Our work shows the potential for designing cathode materials in the vast space between fully ordered and disordered compounds. There is an intensive search for high-performance cathode materials for rechargeable batteries. Here the authors report that oxyfluorides with partial spinel-like cation order, made from earth-abundant elements, display both exceptionally high energy and power.

In cation-disordered rocksalt Li-ion cathode materials, an excess of Li with respect to the transition metal content is necessary for the creation of percolating pathways for Li transport. Because of the lower amount of redox-active transition metal, a substantial part of the charge transfer must occur via less reversible oxygen redox. Fluorination can be used to minimize this dependence on oxygen redox by increasing the amount of low-valent transition metal in the compound, but it adds complexity to materials design. Here, we investigate the feasibility of using computationally constructed phase diagrams to facilitate the search for optimal oxyfluorides. We use the phase diagram of LiF–Li3NbO4–NiO to identify Li1.13Ni0.57Nb0.3O1.75F0.25 and Li1.19Ni0.59Nb0.22O1.46F0.54 as two promising compositions and demonstrate that they can be successfully synthesized. These compounds exhibit significantly reduced hysteresis and higher energy density than the previously reported Li1.3Ni0.27Nb0.43O2 compound in this s...

The Fiber Bragg grating sensor (FBG) is a kind of optical fiber sensor, compared with the electric sensor, it has many irreplaceable advantages for aerospace applications. The challenge of FBG for cryogenic application is how to increase the sensitivity of bare FBG and reduce nonlinear error. Two different thermosetting epoxy materials were studied to recoat bare FBG and presented a specific packaging method for thermosetting coatings. The verification of the accuracy of the model that is used to describe the FBG coated with thermosetting polymers by comparing the calculated results with the warming-up experiment conducted in the temperature range of 120K-328K using epoxy coated FBGs and bare FBG has been conducted. The results show a great improvement of sensitivity and good linearity, and the model can also be used to predict the characteristics of FBG coated with epoxy resins at cryogenic temperature and determine the stability of the epoxy.

Metal-containing nanocrystals with well-designed surface structures represent a class of model systems for revealing the fundamental physical and chemical processes involved in heterogeneous catalysis. Herein it is shown how surface modification can be utilized as an efficient strategy for controlling the surface electronic state of catalysts and, thus, for tuning their catalytic activity. As model catalysts, the Pd-tetrahedron-TiO2 nanostructures, modified on the surface with different foreign atoms, showed a varied activity in the catalytic decomposition of formic acid towards H2 production. The catalytic activity increases with a reduction in the work function of modified atoms; this reduction can be well explained by a surface polarization mechanism. In this hybrid system, the difference in the work functions of Pd and modified atoms results in surface polarization on the Pd surface and, thus, in the tuning of its charge state. Together with the Schottky junction between TiO2 and metals, the tuned charge state enables the promotion of catalytic efficiency in the catalytic decomposition of formic acid to H2 and CO2 .

Supports are commonly implemented in the industrial application of heterogeneous catalysts to improve the stability and recyclability of catalysts. The supported catalysts often show the enhanced activity and selectivity in various catalytic reactions. However, the specific contributions of electronic and steric effects to a catalytic system often remain elusive due to the lack of well-defined model systems. In this work, two types of uniform Pd nanocrystals covered by {111} facets in tetrahedral and octahedral shapes, respectively, are synthesized with identical chemical environment and loaded on TiO2 supports to form hybrid structures (Pd{111}-TiO2) towards the application of formic acid decomposition. Our observation suggests that the polarization effect at the interface of Pd-TiO2 enhances its activity in formic acid decomposition. Moreover, the Pd tetrahedrons-TiO2 hybrid structure whose Pd{111}-TiO2 interface possesses a larger angle shows higher catalytic activity, owing to the reduced steric effect as compared to Pd octahedrons-TiO2. This study reveals the nature of interface effects in formic acid decomposition, and provides a guidance for the related catalyst design.

Pd is a versatile catalyst in various hydrogen-related catalytic applications; however, it typically exhibits low activity in electrocatalytic hydrogen evolution reaction (HER) as too strong Pd-H bonding makes the electronic desorption of H adatoms (H ad ) hardly occur. We herein report a selective etching-deposition approach to implant trace Pd atoms in the near-surface region of Ag nanocrystals, forming a heteratomic-rich Pd-Ag structure on Ag surface. This near-surface dilution of Pd atoms can dramatically facilitate the electronic desorption of H ad . As a result, this approach enhances the electrocatalytic HER activity of Pd catalysts about 14 times with excellent performance durability, approaching the high level of Pt catalysts. While enhancing the catalytic performance, this atomic implantation strategy allows the substantial reduction of material costs. This work thus represents a step toward the high-performance, low-cost catalyst design through near-surface lattice engineering.

Conventional Li-ion cathodes are stoichiometric intercalation materials (e.g., LiCoO2, NCM, LiMn2O4). We will present here our efforts to make high capacity, high rate materials based on the earth abundant Mn redox center by partially disordering a cation-excess spinel. In an ordered spinel, the two-phase reaction that occurs at low voltage (~2.7 V) is not only intrinsically slow, but also detrimental to the structural stability because of the large difference in lattice constant between the two end points (cubic and tetragonal phases)1. Recently, Li-excess cation disordered rock salt materials have shown remarkable cycling performance-large capacities and high energy densities2,3. We applied the Li-excess strategy to the spinel structure to synthesize metal-excess, Li-excess partially-disordered spinels. Disorder is induced to remove the two-phase region, and metal excess and Li-excess is used to create high mobility Li-ions and enablement of O-redox. Two partially disordered spinels with compositions between spinel and rocksalt stoichiometry are presented4: Li1.68Mn1.6O3.7F0.3 (LMOF03) and Li1.68Mn1.6O3.7F0.3 (LMOF06). A remarkably high specific energy density greater than 1,100 Wh kg–1 (and capacity >360 mA h g–1) can be achieved in LMOF03, with nearly half of the capacity coming from oxygen redox. On top of that, facilitated by the beneficial spinel-like cation order and the elimination of two-phase reactions, both LMOF03 and LMOF06 show a very high rate capability with a capacity larger than 100 mA h g-1 at 20 A g-1. Our strategy of combining Li-excess and cation over-stoichiometry in a spinel structure to enable high energy and power at the same time can also be applied to other materials. 1 Thackeray, M. M., David, W. I. F., Bruce, P. G. & Goodenough, J. B. Lithium insertion into manganese spinels. Materials Research Bulletin 18, 461-472 (1983). 2 Lee, J. et al. Reversible Mn 2+/Mn 4+ double redox in lithium-excess cathode materials. Nature 556, 185 (2018). 3 Lee, J. et al. Unlocking the potential of cation-disordered oxides for rechargeable lithium batteries. Science 343, 519-522 (2014). 4 Ji, H. et al. Ultrahigh power and energy density in partially ordered lithium-ion cathode materials. Nature Energy, doi:10.1038/s41560-020-0573-1 (2020).

Structure plays a vital role in determining materials properties. In lithium ion cathode materials, the crystal structure defines the dimensionality and connectivity of interstitial sites, thus determining lithium ion diffusion kinetics. In most conventional cathode materials that are well-ordered, the average structure as seen in diffraction dictates the lithium ion diffusion pathways. Here, we show that this is not the case in a class of recently discovered high-capacity lithium-excess rocksalts. An average structure picture is no longer satisfactory to understand the performance of such disordered materials. Cation short-range order, hidden in diffraction, is not only ubiquitous in these long-range disordered materials, but fully controls the local and macroscopic environments for lithium ion transport. Our discovery identifies a crucial property that has previously been overlooked and provides guidelines for designing and engineering cation-disordered cathode materials., The average crystal structure largely governs the Li diffusion kinetics in well-ordered cathode materials. Here the authors show this rule does not hold true for cation-disordered analogues. Cation short-range order is not only ubiquitous but also controls the Li transport behavior.