Origin of self-discharge mechanism in LiMn2O4-based Li-ion cells: A chemical and electrochemical approach

LiMn₂O₄-based Li-ion cells suffer from a limited cycle-life and a poor storage performance at 55 °C, both in their charged and discharged states. To get some insight on the origin of the poor 55 °C storage performance, the voltage distribution through plastic Li-ion cells during electrochemical testing was monitored by means of 3-electrode type measurements. From these measurements, coupled with chemical analysis, X-ray diffraction and microscopy studies, one unambiguously concludes that the poor performance of LiMn₂O₄/C-cells at 55 °C in their discharged state is due to enhanced Mn dissolution that increases with increasing both the temperature and the electrolyte HF content. These results were confirmed by a chemical approach which consists in placing a fresh LiMn₂O₄ electrode into a 55 °C electrolyte solution. A mechanism, based on an ion-exchange reaction leading to the Mn dissolution is proposed to account for the poor storage performance of LiMn₂O₄/C Li-ion cells in their discharged state. In order to minimize the Mn dissolution, two surface treatments were performed. The first one consists in applying an inorganic borate glass composition to the LiMn₂O₄ surface, the second one in using an acetylacetone complexing agent. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Formation of lithium fluoride/metal nanocomposites for energy storage through solid state reduction of metal fluorides

In order to utilize high energy metal fluoride electrode materials as direct replacement electrode materials for lithium ion batteries in the future, a methodology to prelithiate the cathode or anode must be developed. Herein, we introduce the use of a solid state Li₃N route to achieve the lithiation and mechanoreduction of metal fluoride based nanocomposites. The resulting prelithiation was found to be effective with the formation of xLiF:Me structures of very fine nanodimensions analogous to what is found by electrochemical lithiation. Physical and electrochemical properties of these nanocomposites for the bismuth and iron lithium fluoride systems are reported.     

Conversion reaction mechanisms in lithium ion batteries: study of the binary metal fluoride electrodes

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.