Metal Oxide Negative Electrodes for Na-Ion Cells
Authors: Brown Zachary Lee , Smith Stephanie , Obrovac Mark N 
Published: February 2020

Introduction

The desire for sustainable energy sources and energy storage materials has increased, in part, due to the increase in monetary and environmental cost for fossil fuels[1, 2]. Sodium ion batteries have the potential to fulfill these energy storage requirements on a large scale [1, 2]. Finding a suitable negative electrode material for sodium ion batteries is a challenge given that the most widely used negative electrode material for lithium ion batteries, graphite, does not intercalate sodium. Lithium reacts with simple metal oxides undergoing conversion reactions that deliver capacities of up to 1000 mAh/g [3, 4]. Analogous sodium reactions theoretically should react at lower potentials, making them promising candidates for negative electrode materials.
In this work, several metal oxide materials undergoing possible conversion reactions were investigated by ex-situ and in-situ X-ray diffraction measurements of sodium cells. All of the oxides studied undergo conversion reactions in lithium cells. In sodium cells some underwent conversion reactions at low voltage, while some did not. Some oxides underwent conversion reactions at low voltages and have high volumetric capacities, making them interesting for use as negative electrodes in sodium cells.

Experimental

Electrodes comprised the active metal oxide, carbon black and polyimide binder in an 80/12/8 weight ratio. Electrodes were cycled in 2325 coin type cells with sodium or lithium metal counter/reference electrodes and 1M NaPF6 or LiPF6 in EC/DEC/FEC 3/6/1 electrolyte between 4.5 V - 2.5 V to 5 mV at rates C/10 and C/40 with a trickle discharge to C/20 and C/80, respectively.

Results

CuO is an example of an oxide that undergoes a conversion reaction with Na [5]. The voltage curve is reversible and occurs at a low average voltage with a reversible volumetric capacity of 1264 Ah/L.
Figure 1 shows the voltage curve of a CuO/Na cell with ex-situ XRD measurements labelled along the first discharge. As the cell is discharged, the CuO peaks decrease as CuO is consumed and a broad peak appears around the most intense Cu metal peak at 43°. This suggests the following reaction with amorphous products,
2Na + CuO → Na2O + Cu
Figure 2 shows the ex-situ measurements labelled along the first charge of the voltage curve. The Cu and Na2O peak intensity decreases with broad increase in the region containing the main CuO and Cu2O peaks between 35°- 40° as the cell is charged. The CuO influence could be from unreacted starting material. If this is true the following reaction could be occurring,
Na2O + 2Cu → Cu2O + 2Na
Figure 3 shows the ex-situ measurements labelled along the second discharge of the voltage curve. As the cell is discharged the broad Cu2O peak disappears and the broad Cu peak reappears. This suggests the amorphous reaction,
2Na + Cu2O → Na2O + 2Cu
As suggested previously [5], the mechanism for the significant capacity found in the CuO/Na cell is difficult to determine due to formation of Cu2O.
In this work, unexpected behaviour of metal oxides with sodium will be discussed.

Acknowledgements

The authors acknowledge the support of this research by 3M Co. and NSERC.
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