in-Operando EC-STM, XRD and Eqcm Studies on the Formation of Ternary Na Graphite-Intercalation-Compounds
Authors: Seidl Lukas , Bucher Nicolas , Hartung Steffen , Martens Sladjana , Schneider Oliver , Stimming Ulrich 
Published: February 2020

The use of graphite as an anode material initialized the commercialization of Li+-ion batteries (LIBs) in the early nineties.1 Since then LIBs experienced a boost in plenty areas of application, like portable electronic devices or electric mobility.2 LIBs, however, suffer from one severe drawback, which is its low abundance in the earth crust, resulting in a high battery price. This issue can be tackled by replacing the Li ions by alternative ions like Na.3
,
4 Due to the higher atomic mass of the Na+-ions and especially the more positive redox potential of the Na/Na+-redox couple compared to the Li-system, the gravimetric and volumetric energy densities of Na+-ion batteries (NIBs) are negatively affected.5
,
6 On the other hand NIBs profit from the fact that Na does not form alloys with Al during battery operation, in contrast to the Li/Al-system. Thus, the costly and heavy Cu-current collectors of the LIB-anodes can be replaced by the cheaper and lighter Al, wherefrom another cost- and a weight-advantage of NIBs arises.6

In order to commercialize the NIB-technology, electrochemical attempts to make use of graphitic anodes were undertaken, however only with moderate success.7 In these studies it was tried to form binary graphite-intercalation-compounds (GICs), which were shown to be energetically unfavorable.8
,
9
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10 However, recently it has been shown that Na+-ions can reversibly intercalate into graphite, if a suitable electrolyte is used from which ternary GICs can be formed.11

In this study a variety of electrochemical techniques has been supplemented by in-operando techniques such as EC-STM, XRD and EQCM in order to deepen the understanding of the intercalation process of Na+-ions into graphite anodes. Different electrolytes permitting the formation of ternary GICs were used for the studies. Consequences for the practical application in batteries are discussed.

References

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