Enhancement of Reversible Nonaqueous Fe(III/VI) Cathodic Charge Transfer

Abstract

The electrochemical preparation of Fe(III/VI) super-iron thin films cathodes on an extended conductive matrix significantly facilitates such film’s nonaqueous, reversible charge transfer. Fe(VI) salts can exhibit higher cathodic capacity and environmental advantages, and the films are of relevance toward the next generation charge storage chemistry for reversible cathodes. These films were electrochemically deposited on either smooth or platinized platinum by electrochemical reduction of Na2FeO4, which retains an intrinsic 3 e− cathodic charge storage capacity of 485 mAh g−1. The influence on cathode reversibility, capacity, and charge retention was probed as function of film deposition conditions (including the deposition potential and stirring rate, and the concentration of NaOH and K2FeO4, in the deposition electrolyte). The highest storage capacity films were formed from an 8.0 M NaOH electrolyte containing 50.0 mM of K2FeO4 and examined by FTIR. Those “super-iron” cathode films, which are up to 191 nm thick and electrodeposited on smooth platinum, sustained up to 300 reversible three-electron galvanostatic charge/discharge cycles to a high depth of discharge in 1 M LiPF6 in PC:DME electrolyte lithium cells. Thicker (greater than 500 nm thick) films deposited on smooth platinum evidence increasing passivity to Fe(VI) charge transfer and charge storage. Films deposited on platinized platinum overcome this passivity, permitting sustained nonaqueous charge/discharge cycling with a thicker (571 nm) Fe(VI) film cathode, and provide a first demonstration of significant Fe(VI)/lithium thin-film secondary cell behavior.

Synthesis and Reversible Li-intercalation Behavior of BaFeO4 films

“…The Electrochemical Society of Japan (111) and (211) Figure 3 shows an XPS depth profile for BaFeO 4 deposited on the Fe current collector by anodic oxidation. The XPS spectra indicate the existence of Ba, Fe and O, which is in good agreement with the results obtained by the TEM observation.…”

“…The film directly synthesized on the Fe substrate could have advantages due to the highly reversible characteristics of the charge-discharge reaction. 4 films obtained by XPS analysis. F, O, C, N, Ba, and S species were detected on the surface of the electrode that was cathodically polarized to 0.5 V. No Fe was observed on the electrode surface, which indicates that lithium carbonates and fluorides were formed on the surface and covered the entire electrode surface.…”

“…Figure 6 shows XRD patterns of the film after discharge to 0.5 V, after charge to 4.5 V, and the pristine film. Although signal intensities for (a) after discharge to 0.5 V and (b) after charge to 4.5 V were small because of large X-ray incident angle, the peaks corresponding to (002) and (102) of BaFeO 4 were observed, which indicates that the BaFeO 4 structure was maintained during the electrode reactions, and thus exhibits highly reversible characteristics.…”

“…Composite cathodes of Fe(VI) compounds such as K 2 FeO 4 and BaFeO 4 have been previously examined for use as a reversible electrode in Li-ion batteries. 13 An electrode containing BaFeO 4 was synthesized through a two-step process, oxidation using hypochlorites followed by a double decomposition reaction, and the resulting electrode containing BaFeO 4 exhibited a reversible electrode capacity of approximately 200 mAh g ¹1 during the first lithiation process.…”

“…Iron oxide films with high oxidation state have been previously synthesized on platinum substrates by the electrochemical deposition method using the electrochemical reduction of Na 2 FeO 4 solution. 4 The 191 nm thick deposited film contained the Fe(III/VI) mixed valence state and showed 300 reversible lithium (de)intercalation cycles in an electrolyte of 1 M LiPF 6 in propylene carbonate/1,2-dimethoxyethane (PC/DME). However, such films contain a large amount of Fe(III) ions due to the presence of nanocrystalline C-Fe 2 O 3 as a principal component of the reductive synthesis.…”