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.

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.

Rechargeable sodium‐ion batteries are promising sustainable energy storage systems, owing to their low cost and high energy density. However, the existing cathode chemistries, particularly layered sodium transition metal oxides, exhibit restricted electrochemical performance, which hinders their extensive applications. To overcome challenges, originating from kinetically limited structural transformations, various cation substitutions are reported in the literature. Especially the partial Na–K replacement is discussed recently, which can increase bulk Na‐diffusivity and suppress undesirable phase transitions. Here, a critical study of potassium doping in different layered materials P3‐K_0.5Co_0.33Mn_0.67O₂ and P3‐K_0.5Co_0.33Mn_0.67O₂*0.5H₂O with redox‐active Co (and Mn), and P2‐Na_0.7Co_0.8Ti_0.2O₂ and P2‐K_0.1Na_0.7Co_0.8Ti_0.2O₂ with Co activity is presented. It is found that the presence of K significantly enhances the moisture sensitivity: P3‐K_0.5Co_0.33Mn_0.67O₂*0.5H₂O is quickly formed in the air from K_0.5Co_0.33Mn_0.67O₂, while Na_0.7Co_0.8Ti_0.2O₂ remains stable under the same conditions. In electrochemical Na‐cells, a partial K–Na cation exchange in P3‐K_0.5Co_0.33Mn_0.67O₂ and P3‐K_0.5Co_0.33Mn_0.67O₂*0.5H₂O occurs without applying current. The electrochemical performance of P3‐K_0.5Co_0.33Mn_0.67O₂ is slightly increased in comparison to K‐free P2‐Na_0.67Co_0.33Mn_O.67O₂ from the literature, staying, however, behind the performance of Na_0.7Co_0.8Ti_0.2O_2. K_0.1Na_0.7Co_0.8Ti_0.2O₂ shows weaker rate capability than isostructural Na_0.7Co_0.8Ti_0.2O₂, reflecting a negative impact of K on Na‐diffusivity. Therefore, effect of K‐incorporation in layered structures on the battery performance strongly depends on the cathode composition.

Increased attention to sodium-containing materials during the last several years is caused by the rapid development of sodium-ion batteries (NIBs), considered as a potential successor for lithium-ion batteries (LIBs) [1]. Especially layered sodium oxides have gained large interest due to their potential applicability as electrode materials (both, as cathodes and anodes). Similar to LiCoO₂ in LIBs, NaCoO₂ shows an immense potential as a cathode in NIBs. A partial replacement of Co by Ti in a Na(Co,Ti)O₂ cathode can stabilize the crystal structure during (de)sodiation and reduce the number of phase transformations. Although the phase diagram of the Na-Co-Ti-O system has not been studied yet, existence of single-phase and multi-phase regions in dependence on the Na-content and temperature is reported in the literature [2]. Typically, layered Na,Co,Ti-oxides adopt three very close structure types P2, O3 and P3, which differ in the oxygen surrounding for Na-cations (octahedral or prismatic) and in the number of the formula units in the crystal unit cell (2 or 3). For application in Na-batteries, however, a trial-and-error approach is mostly needed to evaluate a composition with outstanding electrochemical performance. The redox activity of Co and Ti cations is usually characterized by two well-separated cell potentials. The Co-oxidation (Co²⁺ → Co³⁺ and Co³⁺→ Co⁴⁺) occurs mostly at potentials between 3 and 4 V vs. Na⁺/Na, qualifying the material as cathode while Ti⁴⁺/Ti³⁺ redox process takes place below 1.5 V, as common for anode materials. Based on our experimental studies of layered Na,Co,Ti-oxides with different compositions as NaCo_0.5Ti_0.5O₂ (O3) [3], Na_0.65Co_0.5Ti_0.5O₂ (P3) [3], Na_0.67Co_0.33Ti_0.67O₂ (P2- and O3-structures) [4] and Na_0.8Co_0.8Ti_0.2O₂ (P2) [5] as well as on literature report [6], we can conclude that the most stable cycling behavior can be realized, if the composition in the initial (discharged) state contains mainly Co³⁺ and not Co²⁺. The reason for this is a first-order spin-state transition of high-spin HS-Co²⁺ to low-spin LS-Co³⁺ and back upon (de)sodiation, which is accompanied by a phase transformation, leading to a huge potential hysteresis of about 2 V. This hysteresis only slightly depends on the temperature between 0 °C and 40 °C. Among the compositions mentioned above, Na_0.8Co_0.8Ti_0.2O₂ with the P2-structure is the best candidate for application as a cathode in Na-batteries. However, a deep discharge of the material below 1.6 V to reach the Na_1.0Co_0.8Ti_0.2O₂ composition is detrimental as well, since some amount of Co²⁺ appears, negatively influencing the cycling behavior. References [1] J.-Y. Hwang, S.-T. Myung, Y.-K. Sun, Chem. Soc. Rev. 2017, 46, 3529. [2] Y.-J. Shin, M.-H. Park, J.-H. Kwak, H. Namgoong, O. H. Han, Solid State Ionics 2002, 150, 363. [3] S. Maletti, L. Giebeler, S. Oswald, A. A. Tsirlin, A. Senyshyn, A. Michaelis, D. Mikhailova, ACS Appl. Mater. Interfaces 2018, 10, 36108. [4] D. Mikhailova, M. V. Gorbunov, H. B. A. Nguyen, B. Pohle, S. Maletti, C. Heubner, Mat. Chem. A 2023, 11, 187. [5] B. Pohle, M V. Gorbunov, Q Lu, A. Bahrami, K. Nielsch, D. Mikhailova, Energies 2022, 15, 3371 [6] E. Watanabe, W. Zhao, A. Sugahara, B. Mortemard de Boisse, L. Lander, D. Asakura, Y. Okamoto, T. Mizokawa, M. Okubo, A. Yamada, Chem. Mat. 2019, 31, 23582365