Titanium Activation in Prussian Blue Based Electrodes for Na-ion Batteries: A Synthesis and Electrochemical Study

Abstract

Sodium titanium hexacyanoferrate (TiHCF, Na0.86Ti0.73[Fe(CN)6]·3H2O) is synthesized by a simple co-precipitation method in this study. Its crystal structure, chemical composition, and geometric/electronic structural information are investigated by X-ray powder diffraction (XRPD), microwave plasma-atomic emission spectroscopy (MP-AES), and X-ray absorption spectroscopy (XAS). The electroactivity of TiHCF as a host for Li-ion and Na-ion batteries is studied in organic electrolytes. The results demonstrate that TiHCF is a good positive electrode material for both Li-ion and Na-ion batteries. Surprisingly, however, the material shows better electrochemical performance as a Na-ion host, offering a capacity of 74 mAh g−1 at C/20 and a 94.5% retention after 50 cycles. This is due to the activation of Ti towards the redox reaction, making TiHCF a good candidate electrode material for Na-ion batteries.

Symmetric Aqueous Batteries of Titanium Hexacyanoferrate in Na+, K+, and Mg2+ Media

“…The decrease in R ct value may be due to the total discharge state of the electrode after the CV test [33]. However, the decrease in R s value indicates that the intrinsic resistance was reduced after CV test, and this may be related to the progressive activation of the Ti-site inside the TiHCF structure during cycles, as previously reported in organic media [27]. Based on the Randles-Sevcik equation (Equation ( 1)), the relationship between the peak currents (I p ) and the square root of the scanning rate (v 1/2 ) are linear, and from the slope, the diffusion coefficient (D Na + ) can be estimated,…”

“…When decreasing the scan rate to 1 mV/s, two pairs of wellseparated redox peaks appeared at voltage 0.21/−0.21 V and 1.11/0.65 V (Figure 1b). Based on the reports about TiHCF in organic electrolyte [27][28][29], the peak at around 1.11/0.65 V and 0.21/−0.21 V can be attributed to the redox of Fe 3+ /Fe 2+ and Ti 4+ /Ti 3+ pairs in TiHCF.…”

“…As discussed in the previous work [27], the stoichiometry of as-prepared TiHCF sample can be written as Na 0.86 Ti 0.73 [Fe (CN) 6 ]•3H 2 O. The refinement of XRPD data led to a cubic structure (space group: Pm-3m) with a lattice parameter a = 9.862(5) Å.…”

“…The synthesis of TiHCF was based on a simple and reproducible method by mixing 50 mL 0.1 M tetrabutyl titanate (Sigma Aldrich, St. Louis, MO, USA) ethanol solution with 100 mL 0.1 M sodium ferrocyanide (Na 4 Fe(CN) 6 , Sigma Aldrich) aqueous solution containing 1.5 M HCl under continuous stirring to generate a precipitate, as reported in the article [27]. After mixing, the suspension was maintained at 60 • C for another 4 h under the N 2 atmosphere and then aged for 4 days at room temperature.…”

“…Indeed, compared to most transition metals, titanium is abundant within the earth's crust; meanwhile, Ti-based/doped phosphorates [15][16][17][18][19] and oxidates, such as Na 0.8 Ni 0.4 Ti 0.6 O 2 [25] and P2-Na 0.66 Ni 0.17 Co 0.17 Ti 0.66 O 2 [26], have been widely used in symmetric batteries due to the low potential of Ti 4+ /Ti 3+ and Ti 3+ /Ti 2+ redox couples. We have reported the electrochemical performance of titanium hexacyanoferrate (TiHCF) in organic Li-ion and Na-ion half cells, and all of them show two-well separated redox peaks due to the redox of Fe 3+ /Fe 2+ and Ti 4+ /Ti 3+ pairs [27].…”