Atomic Tailoring Vacancy Defects in FeF2.2(OH)0.8 toward Ultra-high Rate and Long-life Li/Na-ion Batteries
As for FeF2.2(OH)0.8, introducing appropriate vacancy defects is recently discovered to be a new experimental method for the improvement of the lithium storage performance. However, owing to the limitation of experiment technology, for Li/Na rechargeable batteries, the vacancy formation mechanism of FeF2.2(OH)0.8 and its regulation mechanism on the electrochemical properties associated with the rate-performance and cycling stability are still poorly understood. Therefore, we carried out first principles calculations to investigate the defect chemistry in FeF2.2(OH)0.8. Eleven representative vacancy defects, such as neutral iron vacancy (VFe0), charged iron vacancy (VFe2-, VFe3-), neutral oxygen vacancy (VO0), charged oxygen vacancy (VO-, VO2-), neutral hydrogen vacancy (VH0), charged hydrogen vacancy (VH-) neutral hydroxide vacancy (VOH0), charged fluorine vacancy (VF+) and neutral fluorine vacancy (VF0) are included. The vacancy formation energies clearly reveal that FeF2.2(OH)0.8 with neutral hydroxide vacancy (VOH0) (FeF2.2(OH)0.64O0.08□0.08) is most likely to form under Hydrogen-poor (H-poor) condition. With introduced, the band gap of FeF2.2(OH)0.8 reduces from 1.47 eV to 0.99 eV, which contributes to the improvement of electronic conductivity. Moreover, by analysis of the defect structure and Li/Na diffusion process, it was proposed that ion diffusion channel of FeF2.2(OH)0.8 is broadened and the balance between Li/Na ion and surrounding anions also takes place at saddle point, which induces the higher ionic conductivity to appear in FeF2.2(OH)0.64O0.08□0.08 relative to FeF2.2(OH)0.8 (5.16×10-4 S cm-1 vs. 1×10-6 S cm-1 for Li; 6.88×10-2 S cm-1 vs. 2.03×10-2 S cm-1 for Na). Accordingly, FeF2.2(OH)0.64O0.08□0.08 possesses higher rate performance and more stable discharge voltage than FeF2.2(OH)0.8. As a whole, our theoretical results successfully clarify the origin of the favorable electrochemical properties of FeF2.2(OH)0.64O0.08□0.08 during Li intercalation/deintercalation behavior in the experiment. Furthermore, it also clearly simulates the process of Na diffusion kinetics and electrochemistry in FeF2.2(OH)0.8 and FeF2.2(OH)0.64O0.08□0.08. Furthermore, it is verified that introducing VOH0 is effective strategy to design FeF2.2(OH)0.8-based cathode material for ultra-high rate and long-life Li/Na-ion batteries.