Insights into the Phase Purity and Storage Mechanism of Nonstoichiometric Na<sub>3.4</sub>Fe<sub>2.4</sub>(PO<sub>4</sub>)<sub>1.4</sub>P<sub>2</sub>O<sub>7</sub> Cathode for High‐Mass‐Loading and High‐Power‐Density Sodium‐Ion Batteries
Ziwei Fan, Wande Song, Yang Nian, Chenjie Lou, Ruiyuan Tian, Weibo Hua, Mingxue Tang, Fei Du
Abstract
Abstract Mixed‐anion‐group Fe‐based phosphate materials, such as Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 , have emerged as promising cathode materials for sodium‐ion batteries (SIBs). However, the synthesis of pure‐phase material has remained a challenge, and the phase evolution during sodium (de)intercalation is debating as well. Herein, a solid‐solution strategy is proposed to partition Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 into 2NaFePO 4 ⋅ Na 2 FeP 2 O 7 from the angle of molecular composition. Via regulating the starting ratio of NaFePO 4 and Na 2 FeP 2 O 7 during the synthesis process, the nonstoichiometric pure‐phase material could be successfully synthesized within a narrow NaFePO 4 content between 1.6 and 1.2. Furthermore, the proposed synthesis strategy demonstrates strong applicability that helps to address the impurity issue of Na 4 Co 3 (PO 4 ) 2 P 2 O 7 and nonstoichiometric Na 3.4 Co 2.4 (PO 4 ) 1.4 P 2 O 7 are evidenced to be the pure phase. The model Na 3.4 Fe 2.4 (PO 4 ) 1.4 P 2 O 7 cathode (the content of NaFePO 4 equals 1.4) demonstrates exceptional sodium storage performances, including ultrahigh rate capability under 100 C and ultralong cycle life over 14000 cycles. Furthermore, combined measurements of ex situ nuclear magnetic resonance, in situ synchrotron radiation diffraction and X‐ray absorption spectroscopy clearly reveal a two‐phase transition during Na + extraction/insertion, which provides a new insight into the ionic storage process for such kind of mixed‐anion‐group Fe‐based phosphate materials and pave the way for the development of high‐power sodium‐ion batteries.