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

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 I 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.