Conversion Reaction Mechanisms in Lithium Ion Batteries: Study of the Binary Metal Fluoride Electrodes

Abstract

Materials that undergo a conversion reaction with lithium (e.g., metal fluorides MF(2): M = Fe, Cu, ...) often accommodate more than one Li atom per transition-metal cation, and are promising candidates for high-capacity cathodes for lithium ion batteries. However, little is known about the mechanisms involved in the conversion process, the origins of the large polarization during electrochemical cycling, and why some materials are reversible (e.g., FeF(2)) while others are not (e.g., CuF(2)). In this study, we investigated the conversion reaction of binary metal fluorides, FeF(2) and CuF(2), using a series of local and bulk probes to better understand the mechanisms underlying their contrasting electrochemical behavior. X-ray pair-distribution-function and magnetization measurements were used to determine changes in short-range ordering, particle size and microstructure, while high-resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) were used to measure the atomic-level structure of individual particles and map the phase distribution in the initial and fully lithiated electrodes. Both FeF(2) and CuF(2) react with lithium via a direct conversion process with no intercalation step, but there are differences in the conversion process and final phase distribution. During the reaction of Li(+) with FeF(2), small metallic iron nanoparticles (<5 nm in diameter) nucleate in close proximity to the converted LiF phase, as a result of the low diffusivity of iron. The iron nanoparticles are interconnected and form a bicontinuous network, which provides a pathway for local electron transport through the insulating LiF phase. In addition, the massive interface formed between nanoscale solid phases provides a pathway for ionic transport during the conversion process. These results offer the first experimental evidence explaining the origins of the high lithium reversibility in FeF(2). In contrast to FeF(2), no continuous Cu network was observed in the lithiated CuF(2); rather, the converted Cu segregates to large particles (5-12 nm in diameter) during the first discharge, which may be partially responsible for the lack of reversibility in the CuF(2) electrode.

Phase evolution for conversion reaction electrodes in lithium-ion batteries

“…Secondly, the electrochemically produced nickel nanoparticles are constructed into a bent nanoporous metallic framework that inhibits pulverization and maintains the integrity of NiO nanosheets to allow extended charge-discharge cycles. This result resolves the previous misconception about the morphology of transition metal nanostructures that were exclusively identified as discrete nanoparticles 20,24,49,50 , amorphous domains 51 or localized interconnected nanoparticles 7 . Furthermore, the nanoporous framework creates an electron diffusion network to enable high-rate capability for conversion reactions 55 .…”

“…The response of a material to phase conversions on the nanoscale can directly dictate the performance of various energy materials in electrochemical reactions, such as fuel cell nanocatalysts 4,5 and battery electrodes 2,3,[6][7][8][9][10][11][12][13] . Specifically, phase conversion reactions have provided a rich playground for lithium-ion battery technologies with potential to improve specific/rate capacity and achieve high resistance to lithium metal plating [14][15][16][17][18][19] .…”

Advanced analytical electron microscopy for lithium-ion batteries

“…For example, because the cycling of conversion materials typically leads to the formation of metallic nanoparticles and lithium oxide/fluoride, which differ significantly in their average atomic number, the phase conversion process can be directly visualized using Z-contrast HAADF-STEM imaging and the associated elemental mapping (Figures 3c and d). [40][41][42] In addition, the intermediate and final reaction products and volume change of Si, which is the most important alloying anode material, can also be determined by a combination of STEM/EELS and electron diffraction. The results provide valuable insight into the structural and morphological optimization of Si-based materials.…”

“…With abundant mobile ions and a small electronic conductivity, the solid electrolytes undergo much more severe electron beam irradiation than the cathode materials. 42,46,47 As a result, the atomic-resolution (S)TEM studies on solid electrolyte materials are quite rare. 44,[47][48][49][50][51][52][53][54][55] Regardless, the limited number of papers have provided unique insight into their ionic conduction behaviors and paved the way for the design and discovery of high-performance solid electrolytes.…”

Transport of External Lithium Along Phase Boundary in LiF-Ti Nanocomposite Thin Films

“…One valuable in situ TEM investigations on the reaction of Li with FeF 2 in a solid cell showed that FeF 2 can be fully lithiated and converted into the LiF-Fe nanocomposite. 17 Accordingly, Wang et al suggested that the massive interface formed between nanoscaled solid phases provides pathways for ionic transport during the conversion process. Since either LiF or Li 2 O or M is not lithium ion conductors, it is quite reasonable that lithium should diffuse through the phase boundaries of the LiX-M nanocomposites, although direct evidences are absent.…”