Degradation Mechanisms of Positive Electrode Materials for Sodium-Ion Batteries

Abstract

Sodium-ion batteries (SIB) require less critical materials and lead to a significant cost reduction compared to state-of-the-art lithium-ion batteries [1]. Although a large-scale commercialization of SIBs is within reach [2], the understanding of the degradation mechanisms of these batteries is still incomplete. The larger ionic radius of sodium in comparison to that of lithium can induce larger stress changes in the host material causing poor long-term stability. Minimizing degradation of positive electrode materials is of high importance for the advancement of SIB technology. Here, we report on degradation of the important structure types: layered oxides, polyanionic compounds, and Prussian blue analogues. Our results show that the polyanionic Na3V2(PO4)3 is a highly reliable cathode material, which can be used as a benchmark or reference material.During battery operation, mechanical stresses are caused by volume changes of the active materials, when sodium ions are inserted or extracted into/from them. We use operando methods such as the substrate curvature technique to measure stresses in the electrode and draw conclusions about electrochemical and physical processes in the materials during cycling [3]. To investigate phenomena like volume changes, crack formation and overall morphology, ex situ scanning electron microscopy (SEM) is used intermittently. Here, we compare the same site on the electrode in different states of charge and for different cycle numbers. Additionally, X-ray diffraction (XRD) measurements are conducted to study phase transitions.Na2/3Fe1/3Mn2/3O2 is a layered P2-type oxide that offers a high specific capacity and is composed of abundant elements. Due to its intrinsic sodium deficiency, diverse presodiation methods become important [4]. We investigate Na2C4O4 as sacrificial salt and its effects on electrode morphology during decomposition. Our goal is to understand the resulting reaction pathways and optimize the full cell performance of layered oxides.Sodium hexacyanoferrate NaxFe[Fe(CN)6] is a Prussian blue analogue and one of the most prominent candidates for commercial SIBs [2]. It also offers high specific capacities and a long cycle life, but its high moisture sensitivity and interstitial water demand appropriate electrode drying procedures to avoid side reactions during cell operation. Crack formation and growth were studied with SEM after and during cycling. Phase transitions are observed by operando XRD and operando substrate curvature measurements.Sodium vanadium phosphate Na3V2(PO4)3 (NVP) with its highly stable three-dimensional network of a NASICON-type structure and its large diffusion channels for sodium ions is another promising electrode material that deserves attention. NVP can be sodiated and desodiated at two different voltage plateaus and therefore can serve either as cathode or anode material. SEM observations at different states of charge show how preexisting cracks open/close during desodiation/sodiation due to volume shrinkage/expansion. We compare symmetric NVP−NVP cells to full cells with NVP cathodes and anodes of Na metal, hard carbon, Sb/C, and SnSb/C. Our data suggests that problems of Na and Na-ion cells frequently result from the anode. NVP itself is extremely reliable and balanced symmetric NVP−NVP cells exhibit very high cycling stability. More than one thousand cycles can be easily achieved. Due to its distinct plateaus and high stability, we suggest NVP as a reference material for cathode and anode materials. For example, NVP can serve as a reliable anode material for testing the stability of different cathode materials. NVP as a cathode may be used to test anode materials or as a benchmark for other cathode materials [5].Acknowledgements:This work contributes to the research performed at CELEST (Center for Electrochemical Energy Storage Ulm-Karlsruhe) and was funded by the German Research Foundation (DFG) under Project ID 390874152 (POLiS Cluster of Excellence, EXC 2154).Literature:[1] C. Vaalma et al., Nature Reviews Materials 2018, 3, 4, 1-11.[2] M. A. Sawhney et al., ChemPhysChem 2022, 23, 5, 1-19.[3] Z. Choi et al., Journal of Power Sources 2013, 240, 245-251.[4] J. M. De Ilarduya et al. Electrochimica Acta 2019, 321, 134693.[5] T. Akçay et al., ACS Applied Energy Materials 2021, 4, 11, 12688-12695.