Abstract
Na2Ti3O7 is a promising negative electrode for rechargeable Na-ion batteries; however, its good properties in terms of insertion voltage and specific capacity are hampered by the poor capacity retention reported in the past. The interfacial and ionic/electronic properties are key factors to understanding the electrochemical performance of Na2Ti3O7. Therefore, its study is of utmost importance. In addition, although rather unexplored, the use of metallic Na in half-cell studies is another important issue due to the fact that side-reactions will be induced when metallic Na is in contact with the electrolyte. Hence, in this work the interfacial and transport properties of full Na-ion cells have been investigated and compared with half-cells upon electrochemical cycling by means of X-ray photoelectron spectroscopy (conventional XPS and Auger parameter analysis) and electrochemical impedance spectroscopy. The half-cell has been assembled with C-coated Na2Ti3O7 against metallic Na whilst the full-cell uses C-coated Na2Ti3O7 as negative electrode and NaFePO4 as positive electrode, delivering 112 Wh/kganode+cathode in the 2nd cycle. When comparing both types of cells, it has been found that the interfacial properties, the OCV (open circuit voltage) and the electrode–-electrolyte interphase behavior are more stable in the full-cell than in the half-cell. The electronic transition from insulator to conductor previously observed in a half-cell for Na2Ti3O7 has also been detected in the full-cell impedance analysis.
Highlights
Rechargeable Na-ion batteries (NIBs) are becoming one of the most promising technologies for stationary applications, while Li-ion batteries (LIBs) are more focused on the consumer electronics market and electric vehicle industry [1,2]
Na2 Ti3 O7 electrodes in half-cells and full-cells is gathered
The irreversibility of the first cycle is larger in a full-cell (233.7 mAh/ganode ) than in a half-cell (100.1 mAh/g), which might be because a thicker
Summary
Rechargeable Na-ion batteries (NIBs) are becoming one of the most promising technologies for stationary applications, while Li-ion batteries (LIBs) are more focused on the consumer electronics market and electric vehicle industry [1,2]. Among negative electrode materials for rechargeable NIBs, the most studied are hard carbons (HC), phosphorus/carbon composites and Na alloys with Si, Ge, Sn and Sb elements [3,4,5,6] They exhibit safety hazards and/or high cost which reduces the interest to use them as negative electrode in NIBs. they exhibit safety hazards and/or high cost which reduces the interest to use them as negative electrode in NIBs Metal oxides are another possible negative electrode and, amongst them, the Na2 Ti3 O7 is one of the most promising ones because of its good specific capacity close to 200 mAh/g, non-toxicity, abundant resources, fast processing and low cost.
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