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 ultralow 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 future research directions on the Na₂Ti₃O₇ anode are highlighted. This review offers insights into the design of high-energy and high-safety NIBs.