Jahn-Teller Distortion Induced Mn2+-Rich Cathode Enables Optimal Flexible Aqueous High-Voltage Zn-Mn Batteries.

Although one of the most promising aqueous batteries, all Zn-Mn systems suffer from Zn dendrites and the low-capacity Mn4+/Mn3+ process (readily leading to the occurrence of Jahn-Teller distortion, which in turn causes structural collapse and voltage/capacity fading). Here, the Mn3+ reconstruction and disproportionation are exploited to prepare the stable, Mn2+-rich manganese oxides on carbon-cloth (CMOs) in a discharged state through an inverted design, which promotes reversible Mn2+/Mn4+ kinetics and mitigates oxygen-related redox activity. Such a 1.65 V Mn2+-rich cathode enable constructing a 2.2 V Zn-Mn battery, providing a high area capacity of 4.16 mA h cm-2 (25 mA h cm-2 for 10 mL electrolyte) and superior 4000-cycle stability. Moreover, a flexible hybrid 2.7 V Zn-Mn battery is constructed using 2-pH hydrogel electrolytes to demonstrate excellent practicality and stability. A further insight has been gained to the commercial application of aqueous energy storage devices toward low-cost, high safety, and excellent energy density.

Biphasic P2/O3-Na2/3Li0.18Mn0.8Fe0.2O2: a structural investigation.

The P2/O3 layered oxide system is thought to benefit from a synergistic enhancement, resulting from the presence of both phases, which makes it a promising cathode material for Na-ion battery applications. Here, biphasic P2/O3-Na2/3Li0.18Mn0.8Fe0.2O2 is investigated via a combination of neutron and X-ray scattering techniques. Neutron diffraction data indicates that the O3 alkali metal site is fully occupied by Li. Real time operando X-ray diffraction data shows the structural evolution of the composite electrode - at the charged state there is no evidence of O2, OP4 or Z phases. The results presented herein provide new insight into site preference of Li in biphasic materials and highlights the value of utilizing multiple phases to achieve high performance layered cathode materials for sodium battery applications.

Stabilization of O-O Bonds by d0 Cations in Li4+ xNi1- xWO6 (0 ≤ x ≤ 0.25) Rock Salt Oxides as the Origin of Large Voltage Hysteresis.

Multinary lithium oxides with the rock salt structure are of technological importance as cathode materials in rechargeable lithium ion batteries. Current state-of-the-art cathodes such as LiNi1/3Mn1/3Co1/3O2 rely on redox cycling of earth-abundant transition-metal cations to provide charge capacity. Recently, the possibility of using the oxide anion as a redox center in Li-rich rock salt oxides has been established as a new paradigm in the design of cathode materials with enhanced capacities (>200 mAh/g). To increase the lithium content and access electrons from oxygen-derived states, these materials typically require transition metals in high oxidation states, which can be easily achieved using d0 cations. However, Li-rich rock salt oxides with high valent d0 cations such as Nb5+ and Mo6+ show strikingly high voltage hysteresis between charge and discharge, the origin of which is uninvestigated. In this work, we study a series of Li-rich compounds, Li4+ xNi1- xWO6 (0 ≤ x ≤ 0.25) adopting two new and distinct cation-ordered variants of the rock salt structure. The Li4.15Ni0.85WO6 (x = 0.15) phase has a large reversible capacity of 200 mAh/g, without accessing the Ni3+/Ni4+ redox couple, implying that more than two-thirds of the capacity is due to anionic redox, with good cyclability. The presence of the 5d0 W6+ cation affords extensive (>2 V) voltage hysteresis associated with the anionic redox. We present experimental evidence for the formation of strongly stabilized localized O-O single bonds that explain the energy penalty required to reduce the material upon discharge. The high valent d0 cation associates localized anion-anion bonding with the anion redox capacity.

Integrating Catalysis of Methane Decomposition and Electrocatalytic Hydrogen Evolution with Ni/CeO2 for Improved Hydrogen Production Efficiency.

Ni/CeO2 enables either methane decomposition or water electrolysis for pure hydrogen production. Ni/CeO2 , prepared by a sol-gel method with only one heat treatment step, was used to catalyze methane decomposition for the generation of H2 . The solid byproduct, Ni/CeO2 /carbon nanotube (CNT), was further employed as an electrocatalyst in the hydrogen evolution reaction (HER) for H2 production. The Ni/CeO2 catalyst exhibits excellent activity for methane decomposition because CeO2 prevents carbon encapsulation of Ni nanoparticles during the preparation process and forms a special metal-support interface with Ni. The derived CNTs act as antenna to improve conductivity and promote the dispersion of agglomerated Ni/CeO2 . In addition, they provide H2 diffusion paths and prevent Ni/CeO2 from peeling off the HER electrode. Although long-term methane decomposition reduces the HER activity of Ni/CeO2 /CNTs (owing to degradation of the delicate Ni/CeO2 interface), the tunable nature of the synthesis makes this an attractive sustainable approach to synthesize future high-performance materials.

Exploiting Anti-T-shaped Graphene Architecture to Form Low Tortuosity, Sieve-like Interfaces for High-Performance Anodes for Li-Based Cells.

Graphitic carbon anodes have long been used in Li ion batteries due to their combination of attractive properties, such as low cost, high gravimetric energy density, and good rate capability. However, one significant challenge is controlling, and optimizing, the nature and formation of the solid electrolyte interphase (SEI). Here it is demonstrated that carbon coating via chemical vapor deposition (CVD) facilitates high electrochemical performance of carbon anodes. We examine and characterize the substrate/vertical graphene interface (multilayer graphene nanowalls coated onto carbon paper via plasma enhanced CVD), revealing that these low-tortuosity and high-selection graphene nanowalls act as fast Li ion transport channels. Moreover, we determine that the hitherto neglected parallel layer acts as a protective surface at the interface, enhancing the anode performance. In summary, these findings not only clarify the synergistic role of the parallel functional interface when combined with vertical graphene nanowalls but also have facilitated the development of design principles for future high rate, high performance batteries.

In Situ Study of Li Intercalation into Highly Crystalline Graphitic Flakes of Varying Thicknesses.

An in situ Raman spectroelectrochemical study of Li intercalation into graphite flakes with different thicknesses ranging from 1.7 nm (3 graphene layers) to 61 nm (ca. 178 layers) is presented. The lithiation behavior of these flakes was compared to commercial microcrystalline graphite with a typical flake thickness of ∼100 nm. Li intercalation into the graphitic flakes was observed under potential control via in situ optical microscopy and Raman spectroscopy. As graphite flakes decreased in thickness, a Raman response indicative of increased tensile strain along the graphene sheet was observed during the early stages of intercalation. A progressively negative wavenumber shift of the interior and bounding modes of the split G band (E2g2(i) and E2g2(b)) is interpreted as a weakening of the C-C bonding. Raman spectra of Li intercalation into thin graphitic flakes are presented and discussed in the context of implications for Li ion battery applications, given that intercalation induced strain may accelerate carbon negative electrode aging and reduce long-term cycle life.

In situ Raman study of lithium-ion intercalation into microcrystalline graphite.

The first and second order Raman spectra of graphite during the first lithiation and delithiation have been investigated in a typical lithium-ion battery electrolyte. In situ, real-time Raman measurements under potential control enable the probing of the graphitic negative electrode surface region during ion insertion and extraction. The experimental results reveal the staging formation of a single particle within a free standing graphitic electrode. In particular, the in situ behaviour of the double resonance 2D band during the lithiation and delithiation of graphitic carbon has not been previously reported. The 2D band was observed to shift from 2681 to 2611 cm(-1) and the band shape transformed into a single Lorentzian from 0.24 to 0.15 V vs. Li/Li(+), providing further information on the electronic structure and C-C bonding of stage 3 and 4 graphite intercalation compounds. The behaviour of the 2D band is in keeping with the Daumas-Hérold model of electrochemically derived intercalation, where the graphene layers are flexible and deform around domains of intercalating lithium ions.

Direct detection of discharge products in lithium-oxygen batteries by solid-state NMR spectroscopy.

A closer look: Solid-state (7) Li and (17) O NMR spectroscopy is a valuable tool in the characterization of products formed in the lithium-oxygen battery, a necessary step in the development of a viable cell. Since lithium peroxide, the desired discharge product, has a unique (17) O NMR signature, it can be clearly identified.