A key issue with Na-ion batteries is the development of active materials with stable electrochemical reversibility through the understanding of their sodium storage mechanisms. We report a sodium storage mechanism and properties of a new anode material, digenite CuS, based on its crystallographic study. It is revealed that copper sulfides (Cu S) can have metal-rich formulas ( x ≥ 1.6), due to the unique oxidation state of +1 found in group 11 elements. These phases enable the unit cell to consist of all strong Cu-S bonds and no direct S-S bonds, which are vulnerable to external stress/strain that could result in bond cleavage as well as decomposition. Because of its structural rigidness, the CuS shows an intercalation/deintercalation reaction mechanism even in a low potential window of 0.1-2.2 V versus Na/Na without irreversible phase transformation, which most of the metal sulfides experience through a conversion reaction mechanism. It uptakes, on average, 1.4 Na ions per unit cell (∼250 mAh g) and exhibits ∼100% retention over 1000 cycles at 2C in a tuned voltage range of 0.5-2.2 V through an overall solid solution reaction with negligible phase separation.
Lithium-sulfur (Li-S) batteries have been intensively investigated as a next-generation rechargeable battery due to their high energy density of 2600 W·h kg(-1) and low cost. However, the systemic issues of Li-S batteries, such as the polysulfide shuttling effect and low Coulombic efficiency, hinder the practical use in commercial rechargeable batteries. The introduction of a conductive interlayer between the sulfur cathode and separator is a promising approach that has shown the dramatic improvements in Li-S batteries. The previous interlayer work mainly focused on the physical confinement of polysulfides within the cathode part, without considering the further entrapment of the dissolved polysulfides. Here, we designed an ultrathin poly(acrylic acid) coated single-walled carbon nanotube (PAA-SWNT) film as a synergic functional interlayer to address the issues mentioned above. The designed interlayer not only lowers the charge transfer resistance by the support of the upper current collector but also localizes the dissolved polysulfides within the cathode part by the aid of a physical blocking and chemical bonding. With the synergic combination of PAA and SWNT, the sulfur cathode with a PAA-SWNT interlayer maintained higher capacity retention over 200 cycles and achieved better rate retention than the sulfur cathode with a SWNT interlayer. The proposed approach of combining a functional polymer and conductive support material can provide an optimiztic strategy to overcome the fundamental challenges underlying in Li-S batteries.
Gluconacetobacter xylinus (G. xylinus) metabolism is activated by oxygen, which makes the formation of an air-medium interface critical. Here we report solid matrix-assisted 3D printing (SMAP) of an incubation medium surface and the 3D fabrication of bacterial cellulose (BC) hydrogels by in situ biosynthesis of G. xylinus. A printing matrix of polytetrafluoroethylene (PTFE) microparticles and a hydrogel ink containing an incubation medium, bacteria, and cellulose nanofibers (CNFs) are used in the SMAP process. The hydrogel ink can be printed in the solid matrix with control over the topology and dimensional stability. Furthermore, bioactive bacteria produce BC hydrogels at the surface of the medium due to the permeability of oxygen through the PTFE microparticle layer. The flexibility of the design is verified by fabricating complex 3D structures that were not reported previously. The resulting tubular BC structures suggest future biomedical applications, such as artificial blood vessels and engineered vascular tissue scaffolding. The fabrication of a versatile free-form structure of BC has been challenged due to restricted oxygen supplies at the medium and the dimensional instability of hydrogel printing. SMAP is a solution to the problem of fabricating free-form biopolymer structures, providing both printability and design diversity.
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