The impetus to study and develop polymer electrolytes for metal-ion batteries is due to their enhanced safety compared to flammable organic liquid electrolytes, promising ionic conductivity, and broad electrochemical stability window, making them to viable candidates for battery application. In the current work, we present a simple fabrication procedure and a comprehensive physico–chemical study of various PVDF-HFP-based electrolyte formulations with a sufficient addition of PEO polymer, LiTFSI conducting salt, and EMIMTFSI ionic liquid. The ionic conductivity, activation energy for ionic movement and thickness of the resulting polymer electrolyte show a non-linear dependency on the PVDF-HFP/PEO ratio. The electrolyte composition with a 0.35PEO-0.65PVDF-HFP/1LiTFSI/1EMIMTFSI mass fraction exhibits the highest ionic conductivity among the compositions, revealing $7.7\times {10}^{-5} \mathrm{S} {\mathrm{c}\mathrm{m}}^{-1}$ at 30 °C. Electrochemical tests in half full and full Li-ion batteries with a LiFePO₄ cathode and Li₄Ti₅O₁₂ anode also emphasized this composition as the most promising one, providing an initial capacity in full cells of 120 mAh g⁻¹ and a capacity retention of about 75% after 50 charge/discharge cycles at a 0.1 C current rate. In the PEO/PVDF-HFP polymer blend with EMIMTFSI as a plasticizer, the amount of crystalline parts, which are detrimental to a fast ionic diffusion, is significantly reduced.

Layered Na_0.8Co_0.8Ti_0.2O₂ 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 Na_xCoO₂ materials showing a multi-step reaction mechanism, thus identifying Ti as a structure stabilizer, similar to other layered O3- and P2-Na_xCo_1−yTi_yO₂ oxides. However, charging the battery with the Na_0.8Co_0.8Ti_0.2O₂ 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.

The P2- and O3-Na_0.67Co_0.33Ti_0.67O₂ cathodes undergo a very slow transformation during charge and discharge in sodium batteries due to a valence and spin-state transition of cobalt cations. As a result, a big potential hysteresis of 2.5–3 V arises.