Na Vs. Li - Thermodynamic Surprises for Graphite and CuO As Anode Materials for Sodium-Ion Batteries
Authors: Adelhelm Philipp , Klein Franziska , Jache Birte , Pinedo Ricardo , Hering Philipp , Polity Angelika , Janek Jürgen 
Published: February 2020

Research on sodium-ion batteries (NIBs) is well motivated by the large abundance of sodium and strategies for commercializing such systems are mainly based on principles known from lithium-ion technology. From a scientific perspective it is an intriguing questions of how lithium and sodium compare in their redox chemistry with identical electrodes. Graphite, for example, was long thought to be inactive as electrode in NIBs but has been shown to be active under the right conditions1 . In metal-batteries, Li2O2 forms in the case of lithium whereas NaO2 can form in the case of sodium2, 3. The differences in physic-chemical properties might also provide an opportunity for improved performance in next-generation electrodes that are so far not performing well enough for LIBs. For example, conversion reactions are being studied for many years with the hope to achieve higher capacity electrodes but many challenges remain.
In this presentation, will focus on lithium and sodium ion storage in graphite and copper oxide (CuO). We will first discuss the peculiar properties of sodium-ion intercalation into graphite. Inactive when carbonates are used as electrolyte solvents, a highly reversible storage mechanisms (> 1000 cycles without significant capacity decay) with low irreversible capacity and fast kinetics is found when ether solvent molecules are used. This is because ether molecules can co-intercalate into the graphite structure without leading to exfoliation. This redox process is more reversible as compared to lithium. In the second example, we will discuss CuO as model system for conversion reactions. We prepared CuO thin films by sputtering and studied the surface film formation by XPS (surface analysis and depth profiling) and SEM. As important finding, the reaction mechanism changes when replacing lithium by sodium. In the latter case, CuO conversion is only partially reversible but oxygen becomes redox active as well and Na2O2 forms as additional intermediate. On the contrary, oxygen is inactive in Li-CuO cells. Further, significant differences occur in the surface film formation on CuO electrodes in Li and Na cells.4 The results clearly demonstrate that significant differences in the redox chemistry in LIBs and NIBs exist for identical electrode materials and these differences need to be understood and used accordingly to achieve high performance electrodes.
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2016. - advance online - DOI: 10.1021/acs.jpcc.5b10642