Layered Na₂Ti₃O₇ as an Ultrastable Intercalation-Type Anode for Non-Aqueous Calcium-Ion Batteries

Calcium ion batteries (CIBs) are a promising energy storage device due to the low redox potential of the Ca metal and the abundant reserves of the Ca element. However, the large radius and divalent nature of Ca²⁺ lead to its slow ion diffusion kinetics and the lack of suitable electrode materials for Ca storage. Here, a layered structure of Na₂Ti₃O₇ (NTO) is presented as an anode material for nonaqueous CIBs. This NTO anode demonstrates a high discharge capacity of 165 mA h g^-1 at 100 mA g^-1 and a remarkable capacity retention rate of 80%, even after 2000 cycles at 500 mA g^-1, surpassing the performance of all reported intercalation-type anode materials for CIBs. The NTO transfers to layered Ca^VIINa^IXTi₃O₇ (CNTO) with intercalation of Ca²⁺ and extraction of Na⁺ during the first discharge process. Then, the CNTO undergoes the reversible insertion/extraction of Ca²⁺ during subsequent cycling. Additionally, density functional theory calculations reveal that NTO possesses a rapid two-dimensional diffusion pathway for Ca²⁺. Moreover, the full CIBs based on NTO as the anode further underscore its potential for CIBs. This work presents promising anode materials for CIBs, offering opportunities to promote the development of high-performance CIBs.