Atomically tailoring vacancy defects in FeF2.2(OH)0.8 toward ultra-high rate and long-life Li/Na-ion batteries

Abstract

As for FeF_2.2(OH)_0.8, introducing appropriate vacancy defects has been recently discovered to be a new experimental method for the improvement of the lithium storage performance. However, owing to the limitation of experimental methods, for Li/Na rechargeable batteries, the vacancy formation mechanism of FeF_2.2(OH)_0.8 and its regulation mechanism on the electrochemical properties associated with rate-performance and cycling stability are still poorly understood. Therefore, we carried out first principles calculations to investigate the defect chemistry in FeF_2.2(OH)_0.8. Eleven representative vacancy defects, such as neutral iron vacancy (V⁰_Fe), charged iron vacancy (V_Fe²⁻ and V_Fe³⁻), neutral oxygen vacancy (V⁰_O), charged oxygen vacancy (V_O⁺ and V_O²⁺), neutral hydrogen vacancy (V⁰_H), charged hydrogen vacancy (V_H⁻), neutral hydroxide vacancy (V⁰_OH), charged fluorine vacancy (V_F⁺) and neutral fluorine vacancy (V⁰_F) are included. The vacancy formation energies clearly reveal that FeF_2.2(OH)_0.8 with neutral hydroxide vacancy (V⁰_OH) (FeF_2.2(OH)_0.64O_0.08□_0.08) is most likely to form under hydrogen-poor (H-poor) conditions. With the introduction of V⁰_OH, the band gap of FeF_2.2(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 the ion diffusion channel of FeF_2.2(OH)_0.8 is broadened. Besides, the balance between Li/Na ions and surrounding anions also occurs at the saddle point, which induces higher ionic conductivity to appear in FeF_2.2(OH)_0.64O_0.08□_0.08 relative to FeF_2.2(OH)_0.8 (5.16 × 10⁻⁴ S cm⁻¹vs. 1 × 10⁻⁶ S cm⁻¹ for Li; 6.88 × 10⁻² S cm⁻¹vs. 2.03 × 10⁻² S cm⁻¹ for Na). Accordingly, FeF_2.2(OH)_0.64O_0.08□_0.08 possesses higher rate performance and more stable discharge voltage than FeF_2.2(OH)_0.8. As a whole, our theoretical results successfully clarify the origin of the favorable electrochemical properties of FeF_2.2(OH)_0.64O_0.08□_0.08 during the Li intercalation/deintercalation process in the experiment. Furthermore, they also clearly simulate the process of Na diffusion kinetics and electrochemistry in FeF_2.2(OH)_0.8 and FeF_2.2(OH)_0.64O_0.08□_0.08. Furthermore, it is verified that introducing V⁰_OH is an effective strategy to design FeF_2.2(OH)_0.8-based cathode materials for ultra-high rate and long-life Li/Na-ion batteries.