We reported a rhombohedral Na-rich nickel hexacyanoferrate (r-NiHCF) with high discharge voltage, which also possesses long cycle stability and excellent rate capability when serving as the cathode material of Na-ion batteries. First-principles calculations suggest that the high working voltage of r-NiHCF is correlated to the asymmetric residence of Na ions in the rhombohedral framework in parallel with the low charge density at the Fe ions. In both aqueous and ether-based electrolytes, r-NiHCF exhibits higher voltage than that of cubic NiHCF. Rate and cycle experiments indicate that r-NiHCF delivers a specific capacity of 66.8 mAh g at the current density of 80 mA g, which is approximate to the theoretical capacity of r-NiHCF. A capacity retention of 96% can be achieved after 200 cycles. The excellent stability of r-NiHCF can be assigned to the absence of rhombohedral-cubic phase transition and negligible volume variation during electrochemical redox, as proven by the ex situ XRD patterns at different depths of charge/discharge and the DFT calculations, respectively.

Understanding the thermal and mechanical properties of CH4 and CO2 hydrates is essential for the replacement of CH4 with CO2 in natural hydrate deposits as well as for CO2 sequestration and storage. In this work, we present isothermal compressibility, isobaric thermal expansion coefficient and specific heat capacity of fully occupied single-crystal sI-CH4 hydrates, CO2 hydrates and hydrates of their mixture using molecular dynamics simulations. Eight rigid/nonpolarisable water interaction models and three CH4 and CO2 interaction potentials were selected to examine the atomic interactions in the sI hydrate structure. The TIP4P/2005 water model combined with the DACNIS united-atom CH4 potential and TraPPE CO2 rigid potential were found to be suitable molecular interaction models. Using these molecular models, the results indicate that both the lattice parameters and the compressibility of the sI hydrates agree with those from experimental measurements. The calculated bulk modulus for any mixture ratio of CH4 and CO2 hydrates varies between 8.5 GPa and 10.4 GPa at 271.15 K between 10 and 100 MPa. The calculated thermal expansion and specific heat capacities of CH4 hydrates are also comparable with experimental values above approximately 260 K. The compressibility and expansion coefficient of guest gas mixture hydrates increase with an increasing ratio of CO2-to-CH4, while the bulk modulus and specific heat capacity exhibit the opposite trend. The presented results for the specific heat capacities of 2220-2699.0 J kg(-1) K(-1) for any mixture ratio of CH4 and CO2 hydrates are the first reported so far. These computational results provide a useful database for practical natural gas recovery from CH4 hydrates in deep oceans where CO2 is considered to replace CH4, as well as for phase equilibrium and mechanical stability of gas hydrate-bearing sediments. The computational schemes also provide an appropriate balance between computational accuracy and cost for predicting mechanical and thermal properties of gas hydrates in the high temperature range (≥260 K), and the schemes may be useful for the study of other complex hydrate systems.

Stabilizing lithium polysulfides in cathodes via interactions between polysulfides and affinitive functional groups could prevent polysulfide dissolution, leading to suppressed "shuttle effect" of lithium/sulfur (Li/S) batteries. Herein, four deoxynucleotides (DNs), including A (adenine-DN), T (thymine-DN), G (guanine-DN), and C (cytosine-DN), which own rich polysulfide affinitive groups, are selected to model the anchoring environments of polysulfides. Using the most soluble Li 2 S 8 as probe, our first-principles simulations suggest that the interactions between polysulfides and substrates are highly correlated to the charges of affinitive sites, H-bonding environments and structural tension. The contributions from each type of interactions are quasiquantitatively assessed. The electrostatic attractions between Li + and the strong electron lone-pairs dominate the adsorption energetics, while the H-bonds formed between S 8 2− and substrate give rise to excessive stabilization. In contrast, structural distortion or rearrangement of the substrates is detrimental to the anchoring strengths. The quasi-quantitative resolution on the different interaction modes provides a facile and rational scheme for screening more efficient polysufide affinitive additives to sustain the cathode cyclicity of Li/S batteries.

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The structural stability of electrode materials and their compatibility with electrolytes are the important properties for ion‐intercalative electrochemical energy‐storage devices. In the present work, we employed zinc hexacyanoferrates (ZnHCFs), which occurs as cubic or rhombic phases, as the probe to tailor the mechanism of capacity decay upon electrochemical cycling and the corresponding mitigating strategy. Capacity fading results from the loss of active materials, which is highly correlated to the phase states; this has been identified for both phases, where the cubic phase is demonstrated to be the dominant source of ZnHCF dissolution. In 1 m KNO3 electrolyte, rhombic ZnHCF behaves evidently more stable than the cubic phase for long‐term galvanostatic charge/discharge cycling. Even when simply immersed in an aqueous environment, the rhombic–cubic phase transition can spontaneously occur, which, in particular, can be accelerated considerably by electrochemical redox processes in the potential window of 0.8–1.1 V. Utilizing the common‐ion effect, specifically by incorporating ZnII into aqueous electrolytes, could considerably enhance the capacity retention of ZnHCF. Our results suggest that, if electrode materials are soluble at certain electrochemical stages, introducing electrochemically inert common ions into the electrolyte should be an efficient approach to improve the electrode–electrolyte compatibility for pursuing enhanced cycling performances.