Engineering Solution-Processed Non-Crystalline Solid Electrolytes for Li Metal Batteries.

Non-crystalline Li-ion solid electrolytes (SEs), such as lithium phosphorus oxynitride, can uniquely enable high-rate solid-state battery operation over thousands of cycles in thin film form. However, they are typically produced by expensive and low throughput vacuum deposition, limiting their wide application and study. Here, we report non-crystalline SEs of composition Li-Al-P-O (LAPO) with ionic conductivities > 10-7 S cm-1 at room temperature made by spin coating from aqueous solutions and subsequent annealing in air. Homogenous, dense, flat layers can be synthesized with submicrometer thickness at temperatures as low as 230 °C. Control of the composition is shown to significantly affect the ionic conductivity, with increased Li and decreased P content being optimal, while higher annealing temperatures result in decreased ionic conductivity. Activation energy analysis reveals a Li-ion hopping barrier of ≈0.4 eV. Additionally, these SEs exhibit low room temperature electronic conductivity (< 10-11 S cm-1) and a moderate Young's modulus of ≈54 GPa, which may be beneficial in preventing Li dendrite formation. In contact with Li metal, LAPO is found to form a stable but high impedance passivation layer comprised of Al metal, Li-P, and Li-O species. These findings should be of value when engineering non-crystalline SEs for Li-metal batteries with high energy and power densities.

Thin Solid Electrolyte Separators for Solid-State Lithium-Sulfur Batteries.

The lithium-sulfur battery is one of the most promising "beyond Li-ion" battery chemistries owing to its superior gravimetric energy density and low cost. Nonetheless, its commercialization has been hindered by its low cycle life due to the polysulfide shuttle and nonuniform Li-metal plating and stripping. Thin and dense solid electrolyte separators could address these issues without compromising on energy density. Here, we introduce a novel argyrodite (Li6PS5Cl)-carboxylated nitrile butadiene rubber (XNBR) composite thin solid electrolyte separator (TSE) (<50 µm) processed by a scalable calendering technique and compatible with Li-metal. When integrated in a full cell with a commercial tape-cast sulfur cathode (3.54 mgS cm-2) in the presence of an in situ polymerized lithium bis(fluorosulfonyl)imide-polydioxolane catholyte and a 100 µm Li-metal foil anode, we demonstrate stable cycling for 50 cycles under realistic operating conditions (stack pressure of <1 MPa and 30 °C).

Effect of current density on the solid electrolyte interphase formation at the lithium∣Li6PS5Cl interface.

Understanding the chemical composition and morphological evolution of the solid electrolyte interphase (SEI) formed at the interface between the lithium metal electrode and an inorganic solid-state electrolyte is crucial for developing reliable all-solid-state lithium batteries. To better understand the interaction between these cell components, we carry out X-ray photoemission spectroscopy (XPS) measurements during lithium plating on the surface of a Li6PS5Cl solid-state electrolyte pellet using an electron beam. The analyses of the XPS data highlight the role of Li plating current density on the evolution of a uniform and ionically conductive (i.e., Li3P-rich) SEI capable of decreasing the electrode∣solid electrolyte interfacial resistance. The XPS findings are validated via electrochemical impedance spectrsocopy measurements of all-solid-state lithium-based cells.

LiNi0.5Mn1.5O4 Cathode Microstructure for All-Solid-State Batteries.

Solid-state batteries (SSBs) have received attention as a next-generation energy storage technology due to their potential to superior deliver energy density and safety compared to commercial Li-ion batteries. One of the main challenges limiting their practical implementation is the rapid capacity decay caused by the loss of contact between the cathode active material and the solid electrolyte upon cycling. Here, we use the promising high-voltage, low-cost LiNi0.5Mn1.5O4 (LNMO) as a model system to demonstrate the importance of the cathode microstructure in SSBs. We design Al2O3-coated LNMO particles with a hollow microstructure aimed at suppressing electrolyte decomposition, minimizing volume change during cycling, and shortening the Li diffusion pathway to achieve maximum cathode utilization. When cycled with a Li6PS5Cl solid electrolyte, we demonstrate a capacity retention above 70% after 100 cycles, with an active material loading of 27 mg cm-2 (2.2 mAh cm-2) at a current density of 0.8 mA cm-2.

Elucidating the Formation and Structural Evolution of Platinum Single-Site Catalysts for the Hydrogen Evolution Reaction.

Platinum single-site catalysts (SSCs) are a promising technology for the production of hydrogen from clean energy sources. They have high activity and maximal platinum-atom utilization. However, the bonding environment of platinum during operation is poorly understood. In this work, we present a mechanistic study of platinum SSCs using operando, synchrotron-X-ray absorption spectroscopy. We synthesize an atomically dispersed platinum complex with aniline and chloride ligands onto graphene and characterize it with ex-situ electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectroscopy (XANES), and extended X-ray absorption fine structure spectroscopy (EXAFS). Then, by operando EXAFS and XANES, we show that as a negatively biased potential is applied, the Pt-N bonds break first followed by the Pt-Cl bonds. The platinum is reduced from platinum(II) to metallic platinum(0) by the onset of the hydrogen-evolution reaction at 0 V. Furthermore, we observe an increase in Pt-Pt bonding, indicating the formation of platinum agglomerates. Together, these results indicate that while aniline is used to prepare platinum SSCs, the single-site complexes are decomposed and platinum agglomerates at operating potentials. This work is an important contribution to the understanding of the evolution of bonding environment in SSCs and provides some molecular insights into how platinum agglomeration causes the deactivation of SSCs over time.

Gently does it!: in situ preparation of alkali metal-solid electrolyte interfaces for photoelectron spectroscopy.

The key charge transfer processes in electrochemical energy storage devices occur at electrode-electrolyte interfaces, which are typically buried, making it challenging to access their interfacial chemistry. In the case of Li-ion batteries, metallic Li electrodes hold promise for increasing energy and power densities and, when used in conjunction with solid electrolytes, the adverse safety implications associated with dendrite formation in organic liquid electrolytes can potentially be overcome. To better understand the stability of solid electrolytes when in contact with alkali metals and the reactions that occur, here we consider the deposition of thin (∼10 nm) alkali metal films onto solid electrolyte surfaces, where the metal is thin enough that X-ray photoelectron spectroscopy can probe the buried electrode-electrolyte interface. We highlight the importance of in situ alkali metal deposition by assessing the contaminant species that are present after glovebox handling and the use of 'inert' transfer devices. Consequently, we compare and contrast three available methods for in situ alkali-metal deposition; Li sputter deposition, Li evaporation, and Li plating induced by e- flood-gun irradiation. Studies on both a sulphide solid electrolyte (Li6PS5Cl), and a single-layer graphene probe surface reveal that the more energetic Li deposition methods, such as sputtering, can induce surface damage and interfacial mixing that are not seen with thermal evaporation. This indicates that the appropriate selection of the Li deposition method for in situ studies is required to observe representative behaviour, and the results of previous studies involving energetic deposition may warrant further evaluation.

High Energy Density Single-Crystal NMC/Li6PS5Cl Cathodes for All-Solid-State Lithium-Metal Batteries.

To match the high capacity of metallic anodes, all-solid-state batteries require high energy density, long-lasting composite cathodes such as Ni-Mn-Co (NMC)-based lithium oxides mixed with a solid-state electrolyte (SSE). However in practice, cathode capacity typically fades due to NMC cracking and increasing NMC/SSE interface debonding because of NMC pulverization, which is only partially mitigated by the application of a high cell pressure during cycling. Using smart processing protocols, we report a single-crystal particulate LiNi0.83Mn0.06Co0.11O2 and Li6PS5Cl SSE composite cathode with outstanding discharge capacity of 210 mA h g-1 at 30 °C. A first cycle coulombic efficiency of >85, and >99% thereafter, was achieved despite a 5.5% volume change during cycling. A near-practical discharge capacity at a high areal capacity of 8.7 mA h cm-2 was obtained using an asymmetric anode/cathode cycling pressure of only 2.5 MPa/0.2 MPa.

Pseudopotential-based studies of electron transport in graphene and graphene nanoribbons.

The theoretical understanding of electron transport in graphene and graphene nanoribbons is reviewed, emphasizing the help provided by atomic pseudopotentials (self-consistent and empirical) in determining not only the band structure but also other fundamental transport parameters such as electron-phonon matrix elements and line-edge roughness scattering. Electron-phonon scattering in suspended graphene sheets, impurity and remote-phonon scattering in supported and gated graphene, electron-phonon and line-edge roughness scattering in armchair-edge nanoribbons are reviewed, keeping in mind the potential use of graphene in devices of the future very large scale integration technology.

Flexible transparent metal/polymer composite materials based on optical resonant laminate structures.

Suitable design of periodic metal/polymer composite materials is shown to facilitate resonant tunneling of light at absorbing wavelengths and to provide a means to significantly reduce optical absorption losses in polymer-based metallodielectric composite structures. The conditions for resonant tunneling are established based on the concept of "photonic band edge alignment" in 1D-periodic systems. For the particular case of a four-layer gold/polystyrene laminate structure, it is shown that the matching of the lower band edge of the 1D-periodic structure with the plasma frequency of the metal component facilitates the increase of optical transmission by about 500% as compared to monolithic film structures of equal total thickness. The effect of sheet thickness on the optical properties of thin metal films is determined and shown to be an important prerequisite for the reliable prediction of resonant metallodielectric structures. The resonant 1D-periodic metal/polymer heterostructures are shown to retain the flexural stability of the polymer matrix and thus could find application as flexible transparent conductors in areas such as "plastic electronics".