Review and prospects on the low-voltage Na2Ti3O7 anode materials for sodium-ion batteries

Due to its low cost and natural abundance of sodium, Na-ion batteries (NIBs) are promising candidates for large-scale energy storage systems. The development of ultra-low voltage anode materials is of great significance in improving the energy density of NIBs. Low-voltage anode materials, however, are severely lacking in NIBs. Of all the reported insertion oxides anodes, the Na2Ti3O7 has the lowest operating voltage (an average potential of 0.3 V vs. Na+/Na) and is less likely to deposit sodium, which has excellent potential for achieving NIBs with high energy densities and high safety. Although significant progress has been made, achieving Na2Ti3O7 electrodes with excellent performance remains a severe challenge. This paper systematically summarizes and discusses the physicochemical properties and synthesis methods of Na2Ti3O7. Then, the sodium storage mechanisms, key issues and challenges, and the optimization strategies for the electrochemical performance of Na2Ti3O7 are classified and further elaborated. Finally, remaining challenges and promising future research directions on the Na2Ti3O7 anode are highlighted. This review offers insights into the design of high-energy and high-safety NIBs.