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LiNixCoyMnzO2 cathode materials are technologically important for high-energy-density Li-ion batteries. However, poor electronic conductivity limits their practical use compared to conventional LiC...

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 Cu1.8S, based on its crystallographic study. It is revealed that copper sulfides (CuxS) 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 Cu1.8S 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–1) 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.

The high energy density graphite anode for the commercial LIBs has critical problems on Li+-ion kinetics due to decreases both in electrode porosity and electrolyte permeability. To overcome issues, interfaces of graphite particles in the anode are polarized using poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP) with the high dielectric constant (e = 8.4), high solubility with lithium salt, and ability to trap a large amount of liquid electrolyte. The PVDF-HFP treatment promoted electrolyte permeability into the graphite electrode with a high mass loading of 13.8 mg cm–2 and a density of 1.7 g cc–1 (a current density over 5 mA cm–2) which particularly leads to an improvement of capacity retention from 77% of a bare electrode to 95% over 40 cycles. These achievements were attributed not only to the enhancement of the lithium-ion kinetics but also to the stable formation of a solid electrolyte interface (SEI) layer on the graphite surface.

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.