Exploring Redox Activity and Interfacial Stability of All-Solid-State Sodium Batteries with High-Voltage (> 4 V vs. Na+/Na) Phosphate Cathode Material and Polymer Electrolyte (ERA SSB).

• Applicants: Dr. Gennady Cherkashinin; Maider Zarrabeitia, Ph.D.
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 552238462

The key challenge of sodium-ion batteries (SIBs) is to increase the energy density, which is still lower than in lithium-ion batteries. The energy density is mainly defined by the capacity and operation potential of the cathode materials, while the long-term stability is correlated with the structural stability of the materials, as well as the properties of the interfaces. Among the various cathodes, phosphate-based materials exhibit the highest intrinsic stability vs. high voltage provided by the robust polyanion structure. While the specific capacity of NaFePO4 cathode approaches its theoretical limit, the energy density of NaMPO4 (M=Co, Mn, Ni) cathodes can be increased to 690 Wh/kg via replacement of Fe by Ni, which exhibits a higher redox couple (~4.45 V vs. Na+/Na). The challenge is to overcome a steady and strong drop in the discharge capacity in the order of Co>Mn>Fe and a complete collapse of the redox activity for NaNiPO4. The aim of our project is the development of All-Solid-State >4 V phosphate-based batteries via designing high-voltage cathode material(s), high-voltage stable solid polymer electrolytes, artificial electrode/electrolyte interfaces, and a Na-free anode. For this purpose, the fundamental atomic-level understanding of the functionality of the battery components, the redox activity of high-voltage cathode(s), the interfacial properties, and the stability of the polymer electrolyte(s) against oxidation are needed for developing high-performing SIBs. The following concrete scientific challenges will be addressed: 1) increase of the intrinsic electronic conductivity and the opening/widening of Na+ diffusion channels through doping of NaNiPO4, 2) exploring the intrinsic voltage limit of the Na[NixM1-x]PO4 by tuning the material with a suitable dopant, 3) design of a polymer electrolyte with high oxidation stability, 4) interface stabilization at a high operation voltage, in particularly via interface engineering able to facilitate Na+ transport across the electrode-electrolyte interface. The NaNiMPO4 thin films (as reference materials) and composite cathodes will be implemented in different types of all-solid-state batteries. The correlation of electrochemical performance with the evolution of the electronic and structural properties and the interfacial chemical composition at different states of (dis-)charge will be studied by our unique in-situ and operando electron spectroscopy and structure-sensitive techniques (including synchrotron measurements). As a result, this project will provide a strong scientific foundation towards sustainable and green electrochemical energy storage devices.

• DFG Programme: Research Grants