Improved Performance in FeF₂ Conversion Cathodes through Use of a Conductive 3D Scaffold and Al₂O₃ ALD Coating

Abstract

This article is protected by copyright. All rights reserved. 3 FeF 2 is considered a promising conversion compound for the positive electrode in lithium ion batteries due to its high thermodynamic reduction potential (2.66 V vs. Li/Li +) and high theoretical specific capacity (571 mA h g-1). However, the sluggish reaction kinetics and rapid capacity decay caused by side reactions during cycling limit its practical application. Here, the fabrication of Ni supported 3D Al 2 O 3 coated FeF 2 electrodes is presented and it is shown that these structured electrodes significantly overcome these limitations. The electrodes are prepared by iron electrodeposition on a Ni support followed by a facile fluorination process and Al 2 O 3 coating by atomic layer deposition. The 3D FeF 2 electrode delivers an initial discharge capacity of 380 mA h g − 1 at a current density of 200 mA g-1 at room temperature. The 3D scaffold improves the reaction kinetics and enables a high specific capacity by providing an efficient electron pathway to the insulating FeF 2 and short Li diffusion lengths. The Al 2 O 3 coating significantly improves the cycle life, probably by preventing side reaction through limiting direct electrode-electrolyte contact. The fabrication method presented here can also be applied for synthesis of other metal fluoride materials on different 3D conductive templates.

Advanced Thin Film Cathodes for Lithium Ion Batteries

“…The author observed the increased structural stability of Li 3 PO 4coated cathodes as it acts as an HF scavenger for suppressing the SEI formation. In addition, Al 2 O 3 was coated on FeF 2 with inverse opal Ni 3D scaffold substrates using ALD, which further confirms that combinations between 3D nanostructures and surface coating can maintain good kinetics and suppress side reactions caused by large surface areas [110].…”

Highly Reversible Iron Fluoride Conversion Cathodes Enabled by Deep‐Eutectic Solvent Method and Heterostructure Design

“…However, the reaction voltage decreases alongside the discharge process and another obvious peak appears at 1.35 V. The phenomenon could be caused by the relatively poor Li + diffusion through the bulk FeF 2 domains. [ 28 ] During the recharging to 4 V, the curves in the second and following cycles show a similar tendency with all the oxidation (delithiation) peaks moving to the upper voltages. During the following discharge, the lithiation peaks shift to the positions of the higher voltages of ≈3 and 2 V, and the lower‐voltage peak at 1.51 V is remarkably weakened.…”

Enabling Conversion‐Type Iron Fluoride Cathode by Halide‐Based Solid Electrolyte

“…The reaction mechanisms of FeF 2 with different SEs can be rationalized by the difference in the electrode potential of iron compounds. Based on a thermodynamic analysis, [52] the theoretical electrode potentials of iron fluorides (FeF 2 : 2.63 V and FeF 3 : 2.73 V) are higher than those of iron sulfides (FeS: 1.76 V and FeS 2 : 1.87 V), but are comparable with or slightly lower than [2,5,7,8,49,64] F) Comparison of the cycling performance of amorphous FeF 2 -LYC cathode with the data reported in the literature. [2,5,7,8,23,49,50,57,64,65] All the cells were tested at 60 °C unless specified.…”

“…The excellent rate performance of amorphous FeF 2 -LYC cathode is also demonstrated by comparing with the previously reported iron fluoride cathodes tested at both room temperature and elevated temperatures (Figure 5E). [2,5,7,8,49,64] It should be noted that even a thick SE (≈1 mm) with a low ionic conductivity (≈10 −4 S cm −1 ) was used in this work, the rate performance of amorphous FeF 2 -LYC at room temperature is still comparable with FeF 2 cathode tested in liquid electrolyte cells where a thin separator (tens of microns) is typically used with highly conductive liquid electrolyte (≈10 −2 S cm −1 ). The results indicates that amorphization of the FeF 2 -LYC cathode composite can be used as an effective way to improve its kinetics.…”