Sodium

Sodium-ion batteries (SIBs) have attracted much interest as a sustainable alternative to lithium-ion batteries (LIBs) due to their low cost and natural abundance of sodium resources. Among the various post-lithium options, this technology is the most mature one and it also represents one of the strongest research focus within POLiS. Indeed, in POLiS, the topic “sodium” is explored in all different aspects: from the synthesis of new cathode and anode materials to theoretical calculations and elucidation of the underlying storage mechanism with the help of advanced techniques; from the electrolyte formulation and its impact on the electrode surface to the influence on safety/degradation aspects. With strong synergies among chemists, physicists, engineers, and the infrastructures available within POLiS at the different locations (Ulm, Karlsruhe and Gießen), we aim to develop a novel POLiS Na-ion battery cell which is made out of sustainable, abundant materials. The battery cells will be benchmarked against a LFP/ Gr (Lithium iron phosphate/ Graphite) cell system, namely the state of the art LIB cell chemistry that is comparable to new generation SIBs.

It is to be investigated which aspects of SIBs will outperform the LFP/ Gr chemistry and the following will be investigates: capacity, rate capability, safety & sustainability, long-term performance, self-discharge, low- and high temperature performance.

On the way of this ambitious journey, we have already achieved important breakthroughs such as:

Successful scaling up of robust and stable cathode materials such as Na₃V₂(PO₄)₃/C (NVP/C). This composite material has an exceptional cycling stability with more than 900 cycles with a capacity retention of 90% and great rate capability.

For the optimization of the electrochemical performance as well as the synthesis process for the upscalability different synthesis parameter were varied. Beside various carbon contents different calcination temperatures adm atmospheres were used. Hereby the syntheses of 600 g Na₃V₂(PO₄)₃/C-composite material per batch is possible in a reproducible way. This material exhibits capacities up to 113 mAh/g with a high capacity retention, whereby over 80 mAh/g remain at high C-rates of 10 C.

Development of Co-free high-energy layered oxide cathode materials based on abundant and sustainable elements. High-potential spherical P2-Na_0.67Mn_0.75Ni_0.25O₂ (P2-MNO) and doped P2-Na_0.67Mn_0.75Ni_0.20Mg_0.05O₂ (P2-MNMO) cathode materials are synthesized via a scalable co-precipitation route followed by thermal treatment. The Mg-doping improves cycle life as well as coulombic efficiency, see Fig. 3. A combined experimental and theoretical approach demonstrate that the Mg-doping stabilizes the high potential region by suppressing the P2-O2 phase transition and leading to the formation of a more stable Z-phase.
Achievements in understanding the Na insertion into hard carbon as this material is widely used as a negative electrode in NIBs.
Comprehensive characterization of propylene carbonate-based liquid electrolyte mixtures for NIBs with understanding of the impact of electrolytes on the surface of metallic sodium. These two findings in combaniation allowed for stable and high capacity retention measurements of hard carbon vs. sodium.

For the practical investigation of cell performance using novel SIB active materials, at first small single electrode containing pouch cells will be build and tested to account for realistic electrolyte to active material ratios. In the second step large multi-layered electrode pouch cells will be assembled which meet industry cell type standards to demonstrate cell properties under application conditions (cell pressure, thermal behaviour). Such cells can be assembled at the Battery Technology Center at KIT.