Transition metal dissolution and degradation in nmc811-graphite electrochemical cells

Nickel-rich lithium nickel-manganese-cobalt oxide cathodes, in particular Li(Ni0.8Mn0.1Co0.1)O2 (NMC811), are currently being commercialized as next generation cathode materials, due to their increased capacities compared to current materials. Unfortunately, the higher nickel content has been shown to accelerate cell degradation and a better understanding is needed to maximize cell lifetimes. NMC811/graphite cells were tested under stressed conditions (elevated temperature and cell voltages) to accelerate degradation focusing on transition metal (TM) dissolution from the cathode. Increasing the cell temperature, upper cut-off voltage (UCV) and number of cycles all accelerated capacity fade and diffraction studies showed that under stressed conditions, additional degradation mechanisms beyond lithium loss to the SEI are present. Significant TM dissolution and subsequent deposition on the graphite anode is seen, particularly at stressed conditions. The concentration of TMs in the electrolyte remained invariant with cycling conditions, presumably reflecting the limited solubility of these ions and emphasizing the role that TM deposition on the anode plays in continuing to drive dissolution. Significant deposits of metals from the cell casings and current collectors were also detected at all cycling conditions, indicating that corrosion and metal leaching can be as important as TM dissolution from the active material in some cell formats.We thank Ms. Jennifer Allen, Prof. Mary Ryan and Dr Daniel Abraham for helpful discussions. We thank Stephen Young and Nigel Howard for assistance with the ICP-OES measurements. This work is supported by the Faraday Institution under grant no. FIRG00

Phase Behavior during Electrochemical Cycling of Ni-Rich Cathode Materials for Li-Ion Batteries

Although layered lithium nickel-rich oxides have become the state-of-the-art cathode materials for lithium-ion batteries in EV applications, they can suffer from rapid performance failure – particularly when operated under conditions of stress (temperature, high voltage), the underlying mechanisms of which are not fully understood. In this essay, we aim to connect the electrochemical performance with changes in structure during cycling. First, we compare the structural properties of LiNiO2, to the substituted Ni-rich compounds NMCs (LiNixMnyCo1-x-yO2) and NCAs (LiNixCoyAl1-x-yO2). Particular emphasis is placed on decoupling intrinsic behaviour and extrinsic “two-phases” reactions observed during initial cycles, as well as after extensive cycling for NMC and NCA cathodes. We highlight the need to revisit the various high-voltage structural change processes that occur in LiNiO2 with modern characterization tools to aid the understanding of the accelerated degradation for Ni- rich cathodes at high voltages

Infrared-active optical phonons in LiFePO4 single crystals

Infrared-active optical phonons were studied in olivine LiFePO4 oriented single crystals by means of both rotating analyzer and rotating compensator spectroscopic ellipsometry in the spectral range between 50 and 1400 cm-1. The eigenfrequencies, oscillator strengths, and broadenings of the phonon modes were determined from fits of the anisotropic harmonic oscillator model to the data. Optical phonons in a heterosite FePO4 crystal were measured from the delithiated ab-surface of the LiFePO4 crystal and compared with the phonon modes of the latter. Good agreement was found between experimental data and the results of solid-state hybrid density functional theory calculations for the phonon modes in both LiFePO4 and FePO4

Mechanistic insights into sodium storage in hard carbon anodes using local structure probes

Operando $^{23}$Na solid-state NMR and pair distribution function analysis experiments provide insights into the structure of hard carbon anodes in sodium-ion batteries. Capacity results from "diamagnetic" sodium ions first adsorbing onto pore surfaces, defects and between expanded layers, before pooling into larger quasi-metallic clusters/expanded carbon sheets at lower voltages.J. M. S. acknowledges funding from EPSRC and the European Commission under grant agreement no. 696656 (Graphene Flagship). P. K. A. acknowledges the School of the Physical Sciences of the University of Cambridge for funding through an Oppenheimer Research Fellowship and a Junior Research Fellowship from Gonville and Caius College, Cambridge. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 655444 (O. P.)

Screening and Characterization of Ternary Oxides for High-Temperature Carbon Capture

Carbon capture and storage (CCS) is increasingly being accepted as a necessary component of any effort to mitigate the impact of anthropogenic climate change, as it is both a relatively mature and easily implemented technology. High-temperature CO2 absorption looping is a promising process that offers a much lower energy penalty than the current state of the art amine scrubbing techniques, but more effective materials are required for widespread implementation. This work describes the experimental characterisation and CO2 absorption properties of several new ternary transition metal oxides predicted by high-throughput DFT screening. One material reported here, Li5SbO5, displays reversible CO2 sorption, and maintains 72 % of its theoretical capacity out to 25 cycles. The results in this work are used to discuss major influences on CO2 absorption capacity and rate, including the role of the crystal structure, the transition metal, the alkali or alkaline earth metal, and the competing roles of thermodynamics and kinetics. Notably, this work shows the extent and rate to which ternary metal oxides carbonate is driven primarily by the identity of the alkali or alkaline earth ion and the nature of the crystal structure, whereas the identity of the transition ion carries little influence in the systems studied here

Ring current effects: Factors affecting the NMR chemical shift of molecules adsorbed on porous carbons

Nuclear magnetic resonance (NMR) spectroscopy is increasingly being used to study the adsorption of molecules in porous carbons, a process which underpins applications ranging from electrochemical energy storage to water purification. Here we present density functional theory (DFT) calculations of the nucleus-independent chemical shift (NICS) near various sp2-hybridized carbon fragments to explore the structural factors that may affect the resonance frequencies observed for adsorbed species. The domain size of the delocalized electron system affects the calculated NICSs, with larger domains giving rise to larger chemical shieldings. In slit pores, overlap of the ring current effects from the pore walls is shown to increase the chemical shielding. Finally, curvature in the carbon sheets is shown to have a significant effect on the NICS. The trends observed are consistent with existing NMR results as well as new spectra presented for an electrolyte adsorbed on carbide-derived carbons prepared at different temperatures.A.C.F., J.M.G., and C.P.G. acknowledge the Sims Scholarship (A.C.F.), EPSRC (via the Supergen consortium; J.M.G.), and the EU ERC (via an Advanced Fellowship to C.P.G.) for funding. CDC synthesis at Drexel University was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award #ER46473. V.P. acknowledges funding from the German Federal Ministry for Research and Education (BMBF) in support of the nanoEES3D project (Award 03EK3013) as part of the strategic funding initiative energy storage framework and thanks Prof. Eduard Arzt (INM) for his continuing support. Mohamed Shamma and Boris Dyatkin (Drexel University) are thanked for their support in the synthesis of CDC material. DFT calculations were performed using the Darwin Supercomputer of the University of Cambridge High Performance Computing Service, provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council.This is the author accepted manuscript. The final version is available from the American Chemical Society via http://dx.doi.org/10.1021/jp502387

The use of strontium ferrite in chemical looping systems

This work reports a detailed chemical looping investigation of strontium ferrite (SrFeO3-δ), a material with the perovskite structure type able to donate oxygen and stay in a nonstoichiometric form over a broad range of oxygen partial pressures, starting at temperatures as low as 250°C (reduction in CO, measured in TGA). SrFeO3-δ is an economically attractive, simple, but remarkably stable material that can withstand repeated phase transitions during redox cycling. Mechanical mixing and calcination of iron oxide and strontium carbonate was evaluated as an effective way to obtain pure SrFeO3-δ. In situ XRD was performed to analyse structure transformations during reduction and reoxidation. Our work reports that much deeper reduction, from SrFeO3-δ to SrO and Fe, is reversible and results in oxygen release at a chemical potential suitable for hydrogen production. Thermogravimetric experiments with different gas compositions were applied to characterize the material and evaluate its available oxygen capacity. In both TGA and in-situ XRD experiments the material was reduced below δ=0.5 followed by reoxidation either with CO2 or air, to study phase segregation and reversibility of crystal structure transitions. As revealed by in-situ XRD, even deeply reduced material regenerates at 900°C to SrFeO3 δ with a cubic structure. To investigate the catalytic behaviour of SrFeO3-δ in methane combustion, experiments were performed in a fluidized bed rig. These showed SrFeO3-δ donates O2 into the gas phase but also assists with CH4 combustion by supplying lattice oxygen. To test the material for combustion and hydrogen production, long cycling experiments in a fluidized bed rig were also performed. SrFeO3-δ showed stability over 30 redox cycles, both in experiments with a 2-step oxidation performed in CO2 followed by air, as well as a single step oxidation in CO2 alone. Finally, the influence of CO/CO2 mixtures on material performance was tested; a fast and deep reduction in elevated pCO2 makes the material susceptible to carbonation, but the process can be reversed by increasing the temperature or lowering pCO2.EPSRC grant no. EP/K030132/1. European Union's Horizon 2020 Marie Skłodowska–Curie grant agreement No. 65976

Revealing the Structure and Oxygen Transport at Interfaces in Complex Oxide Heterostructures via ¹⁷O NMR Spectroscopy

Vertically aligned nanocomposite (VAN) films, comprising nanopillars of one phase embedded in a matrix of another, have shown great promise for a range of applications due to their high interfacial areas oriented perpendicular to the substrate. In particular, oxide VANs show enhanced oxide-ion conductivity in directions that are orthogonal to those found in more conventional thin-film heterostructures; however, the structure of the interfaces and its influence on conductivity remain unclear. In this work, 17O NMR spectroscopy is used to study CeO2–SrTiO3 VAN thin films: selective isotopic enrichment is combined with a lift-off technique to remove the substrate, facilitating detection of the 17O NMR signal from single atomic layer interfaces. By performing the isotopic enrichment at variable temperatures, the superior oxide-ion conductivity of the VAN films compared to the bulk materials is shown to arise from enhanced oxygen mobility at this interface; oxygen motion at the interface is further identified from 17O relaxometry experiments. The structure of this interface is solved by calculating the NMR parameters using density functional theory combined with random structure searching, allowing the chemistry underpinning the enhanced oxide-ion transport to be proposed. Finally, a comparison is made with 1% Gd-doped CeO2–SrTiO3 VAN films, for which greater NMR signal can be obtained due to paramagnetic relaxation enhancement, while the relative oxide-ion conductivities of the phases remain similar. These results highlight the information that can be obtained on interfacial structure and dynamics with solid-state NMR spectroscopy, in this and other nanostructured systems, our methodology being generally applicable to overcome sensitivity limitations in thin-film studies