Effect of Fluoride Doping on Lithium Diffusivity in Layered Molybdenum Oxide

Abstract

The effects of cation doping on cathode performance has been extensively studied; however, the field of anion doping has historically received much less attention. Fluoride doping can greatly increase the initial diffusivity of the layered MoO3 system. The first discharge cycle of the layered α-MoO3 and MoO2.8F0.2 phases were investigated and compared using the galvanostatic intermittent titration technique (GITT) in a lithium ion cell. The analysis revealed that a slight reduction of the oxide by fluoride doping to form a fluorobronze (MoO2.8F0.2) eliminated a slow electrochemical process observed in α-MoO3. Galvanostatic cycling studies show that while α-MoO3 has a higher initial capacity, it exhibits a first cycle Coulombic efficiency of only 86% with rapid capacity fade which has been associated with lithium trapping within the MoO3 layer. In contrast, the Li+ intercalation process in the fluorobronze was found to have a 94% Coulombic efficiency on the first cycle. By the third cycle Coulombic efficiencies greater than 99% were observed for five cycles. A thorough investigation of the synthesis of MoO2.8F0.2 is also presented. Under mild hydrothermal conditions, the fluorination of α-MoO3 to form MoO2.8F0.2 is topotactic, while a competing reaction in solution forms MoO2.4F0.6 (ReO3 structure). Methods to prevent the solution phase reaction from occurring are discussed.

Toward Deterministic 3D Energy Storage Electrode Architectures via Electrodeposition of Molybdenum Oxide onto CNT Foams

“…As was observed from cyclic voltammetry, the intrinsic instability of MoO 3 − led to a poor capacity retention of ∼44% (67.2 mA h g –1 ) after 1000 cycles. For long-term cycling, further work is necessary to either stabilize the MoO 3 crystal structure, which has been the topic of recent investigation, ,,− or determine an aqueous electrodeposition route for an oxide material that does not undergo such significant structural change.…”

Synthesis of NaMoO3F and Na5W3O9F5 with Morphological Controllability in Non-Aqueous Solvents

“…These facts seem to show NaMoO 3 F was obtained in topotactic manner, in which MoO 3 layers were cut by insertion of NaF with substitution of oxygen to fluorine. Such topotactic reaction may be possible, because it was reported that one of molybdenum oxyfluorides, MoO 2.8 F 0.2 , was obtained by topotactic reaction in the hydrothermal reaction …”

“…The difference in the rod length between the pristine MoO 3 and MoO 3 _NaF_CH 3 CN_200 may be derived from the fracturing by the dissolution or the mechanical force during the reaction. Therefore, two mechanisms are possible in the case of CH 3 CN: (i) the complete dissolution–reprecipitation mechanism or (ii) simultaneous occurrence of the dissolution and the topotactic reaction and the subsequent growth on the topotactically formed NaMoO 3 F. Although we cannot understand which manner is real yet, the topotactic fluorination under solvothermal condition offers a new design concept for synthesizing mixed anion materials . However, the SEM image of the sample after the reaction in EtOH for 1 h (Figure S7E) shows the mixture of the rod shape and polyhedral morphology, while that of the samples after the reaction in EtOH for 5 h (Figure S7F) shows only the polyhedral morphology.…”

“…Such topotactic reaction may be possible, because it was reported that one of molybdenum oxyfluorides, MoO 2.8 F 0.2 , was obtained by topotactic reaction in the hydrothermal reaction. 43 For evaluation of the mechanism, the time-elapsed reactions were investigated. The reaction time was shortened to 1 and 5 h in the case of the reaction of MoO 3 with NaF at 150 °C in CH 3 CN and EtOH.…”

Investigation of Ca Insertion into α-MoO3 Nanoparticles for High Capacity Ca-Ion Cathodes

“…Figure a shows the α-MoO 3 electrode delivered 236 and 165 mA h g –1 at a current density of 20 mA g –1 for the first discharge and charge capacity, respectively, with 83 mA h g –1 irreversible capacity in the first cycle, which will be discussed later. This irreversible capacity is no longer observed after the first cycle . The α-MoO 3 electrode reversibly delivered a specific capacity of 165 mA h g –1 (discharge capacity basis), corresponding to 0.44 mol of Ca 2+ /MoO 3 .…”