Sodium transition metal oxides are one of the most promising cathode materials for future sodium ion batteries. Chemical flexibility of layered Na-oxides including cobalt enables its partial substitution by other...
Layered Na0.8Co0.8Ti0.2O2 oxide crystallizes in the β-RbScO2 structure type (P2 modification) with Co(III) and Ti(IV) cations sharing the same crystallographic site in the metal-oxygen layers. It was synthesized as a single-phase material and characterized as a cathode in Na- and Na-ion batteries. A reversible capacity of about 110 mA h g−1 was obtained during cycling between 4.2 and 1.8 V vs. Na+/Na with a 0.1 C current density. This potential window corresponds to minor structural changes during (de)sodiation, evaluated from operando XRD analysis. This finding is in contrast to Ti-free NaxCoO2 materials showing a multi-step reaction mechanism, thus identifying Ti as a structure stabilizer, similar to other layered O3- and P2-NaxCo1−yTiyO2 oxides. However, charging the battery with the Na0.8Co0.8Ti0.2O2 cathode above 4.2 V results in the reversible formation of a O2-phase, while discharging below 1.5 V leads to the appearance of a second P2-layered phase with a larger unit cell, which disappears completely during subsequent battery charge. Extension of the potential window to higher or lower potentials beyond the 4.2–1.8 V range leads to a faster deterioration of the electrochemical performance. After 100 charging-discharging cycles between 4.2 and 1.8 V, the battery showed a capacity loss of about 20% in a conventional carbonate-based electrolyte. In order to improve the cycling stability, different approaches including protective coatings or layers of the cathodic and anodic surface were applied and compared with each other.
A graphite anode with a polymer gel binder (PGB), consisting of chitosan, the ionic liquid PYR 14 DCA, and the lithium salt LiTFSI, was developed for high-power Li-ion batteries. The electrochemical properties of graphite with a PGB and polyvinylidene difluoride (PVDF) as a reference were investigated at 10 °C, 25 °C, and 60 °C and compared with each other. During cycling, a PGB has an effect on the composition of the solid-electrolyte-interface (SEI) layer by forming a LiF-enriched component. The charge/discharge behavior of graphite is determined to some extent by the stability and resistance of the SEI, especially at elevated and low temperatures. A stable SEI was formed on graphite/PGB electrodes leading to an excellent cycling and rate performance. Graphite/PGB delivered a discharge capacity of 230 mAh•g −1 after 1000 cycles at 1.86 A•g −1 current density (5C) at room temperature. At 60 °C, the replacement of PVDF by a PGB resulted in an enhancement of the electrochemical performance with a high capacity of 345 mAh• g −1 after 200 cycles at 5C, while the conventional graphite/PVDF electrode only displayed 57 mAh•g −1 . The high Li + diffusion provided by a PGB and the LiF-rich SEI on the graphite/PGB surface also contributed to a better electrochemical performance at a low temperature.
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