No abstract

Water contamination is generally considered to be detrimental to the performance of aprotic lithium-air batteries, whereas this view is challenged by recent contrasting observations. This has provoked a range of discussions on the role of water and its impact on batteries. In this work, a distinct battery chemistry that prevails in water-contaminated aprotic lithium-oxygen batteries is revealed. Both lithium ions and protons are found to be involved in the oxygen reduction and evolution reactions, and lithium hydroperoxide and lithium hydroxide are identified as predominant discharge products. The crystallographic and spectroscopic characteristics of lithium hydroperoxide monohydrate are scrutinized both experimentally and theoretically. Intriguingly, the reaction of lithium hydroperoxide with triiodide exhibits a faster kinetics, which enables a considerably lower overpotential during the charging process. The battery chemistry unveiled in this mechanistic study could provide important insights into the understanding of nominally aprotic lithium-oxygen batteries and help to tackle the critical issues confronted.

The rare physical property of zero thermal expansion (ZTE) is intriguing because neither expansion nor contraction occurs with temperature fluctuations. Most ZTE, however, occurs below room temperature. It is a great challenge to achieve isotropic ZTE at high temperatures. Here we report the unconventional isotropic ZTE in the cubic (Sc1-xMx)F3 (M = Ga, Fe) over a wide temperature range (linear coefficient of thermal expansion (CTE), αl = 2.34 × 10(-7) K(-1), 300-900 K). Such a broad temperature range with a considerably negligible CTE has rarely been documented. The present ZTE property has been designed using the introduction of local distortions in the macroscopic cubic lattice by heterogeneous cation substitution for the Sc site. Even though the macroscopic crystallographic structure of (Sc0.85Ga0.05Fe0.1)F3 adheres to the cubic system (Pm3̅m) according to the results of X-ray diffraction, the local structure exhibits a slight rhombohedral distortion. This is confirmed by pair distribution function analysis of synchrotron radiation X-ray total scattering. This local distortion may weaken the contribution from the transverse thermal vibration of fluorine atoms to negative thermal expansion, and thus may presumably be responsible for the ZTE. In addition, the present ZTE compounds of (Sc1-xMx)F3 can be functionalized to exhibit high-Tc ferromagnetism and a narrow-gap semiconductor feature. The present study shows the possibility of obtaining ZTE materials with multifunctionality in future work.

Sodium-ion batteries are the primary candidate for a low-cost and resourceabundant alternative to lithium-ion batteries for large-scale electric storage applications. However, the development of sodium-ion batteries is hindered by the lack of suitable cathode materials that have sufficient specific energy density and cycle life. Here, we report layered NaVOPO 4 as a cathode material that exhibits high voltage ($3.5 V versus Na/Na +), high discharge capacity (144 mAh g À1 at 0.05 C), and remarkable cyclability with 67% capacity retention over 1,000 cycles. The excellent performances result from the high Na + ion diffusion rate in the two-dimensional path and the reversible transformation behavior of (de)sodiation. Particularly, this layered structure and its synthetic procedure can be extended to other alkali-metal intercalation materials, leading to other metal-ion battery systems, which opens a new avenue for large-scale energy storage systems with the development of high-energydensity and long-life cathodes for electric storage applications.

The controllable isotropic thermal expansion with a broad coefficient of thermal expansion (CTE) window is intriguing but remains challenge. Herein we report a cubic MZrF series (M = Ca, Mn, Fe, Co, Ni and Zn), which exhibit controllable thermal expansion over a wide temperature range and with a broader CTE window (-6.69 to +18.23 × 10/K). In particular, an isotropic zero thermal expansion (ZTE) is achieved in ZnZrF, which is one of the rarely documented high-temperature isotropic ZTE compounds. By utilizing temperature-dependent high-energy synchrotron X-ray total scattering diffraction, it is found that the flexibility of metal···F atomic linkages in MZrF plays a critical role in distinct thermal expansions. The flexible metal···F atomic linkages induce negative thermal expansion (NTE) for CaZrF, whereas the stiff ones bring positive thermal expansion (PTE) for NiZrF. Thermal expansion could be transformed from striking negative, to zero, and finally to considerable positive though tuning the flexibility of metal···F atomic linkages by substitution with a series of cations on M sites of MZrF. The present study not only extends the scope of NTE families and rare high-temperature isotropic ZTE compounds but also proposes a new method to design systematically controllable isotropic thermal expansion frameworks from the perspective of atomic linkage flexibility.