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.

A study of the correlations between the stoichiometry, secondary phases and transition metal ordering of LiNi0.5Mn1.5O4 was undertaken by characterizing samples synthesized at different temperatures. Insight into the composition of the samples was obtained by electron microscopy, neutron diffraction and X-ray absorption spectroscopy. In turn, analysis of cationic ordering was performed by combining neutron diffraction with Li MAS NMR spectroscopy. Under the conditions chosen for the synthesis, all samples systematically showed an excess of Mn, which was compensated by the formation of a secondary rock salt phase and not via the creation of oxygen vacancies. Local deviations from the ideal 3:1 Mn:Ni ordering were found, even for samples that show the superlattice ordering by diffraction, with different disordered schemes also being possible. The magnetic behavior of the samples was correlated with the deviations from this ideal ordering arrangement. The in-depth crystal-chemical knowledge generated was employed to evaluate the influence of these parameters on the electrochemical behavior of the materials.

High voltage spinel LiNi(0.5)Mn(1.5)O(4) is a very promising cathode material for lithium ion batteries that can be used to power hybrid electrical vehicles (HEVs). Through careful control of the cooling rate after high temperature calcination, LiNi(0.5)Mn(1.5)O(4) spinels with different disordered phase and/or Mn(3+) contents have been synthesized. It is revealed that during the slow cooling process (<3 °C min(-1)), oxygen deficiency is reduced by the oxygen intake, thus the residual Mn(3+) amount is also decreased in the spinel due to charge neutrality. In situ X-ray diffraction (XRD) demonstrates that the existence of a disordered phase fundamentally changes the spinel phase transition pathways during the electrochemical charge-discharge process. The presence of an appropriate amount of oxygen deficiency and/or Mn(3+) is critical to accelerate the Li(+) ion transport within the crystalline structure, which is beneficial to enhance the electrochemical performance of LiNi(0.5)Mn(1.5)O(4). LiNi(0.5)Mn(1.5)O(4) with an appropriate amount of disordered phase offers high rate capability (96 mAh g(-1) at 10 °C) and excellent cycling performance with 94.8% capacity retention after 300 cycles. The fundamental findings in this work can be widely applied to guide the synthesis of other mixed oxides or spinels as high performance electrode materials for lithium ion batteries.

Vanadium is shown to substitute for iron in the olivine LiFePO4 up to at least 10 mol %, when the synthesis is carried out at 550 °C. In the solid solution LiFe1–3y/2V y PO4, the a and b lattice parameters and cell volume decrease with increasing vanadium content, while the c lattice parameter increases slightly. However, when the synthesis is performed at 650 °C, a NASICON phase, Li3V2(PO4)3, is also formed, showing that solid solution is a function of the synthesis temperature. X-ray absorption near-edge structure indicates vanadium is in the 3+ oxidation state and in an octahedral environment. Magnetic studies reveal a shift of the antiferromagnetic ordering transition toward lower temperatures with increasing vanadium substitution, confirming solid solution formation. The addition of vanadium enhances the electrochemical performance of the materials especially at high current densities.

For the first time, (003) cracking is observed and determined to be the major cracking mechanism for the primary particles of Ni-rich layered dioxides as the positive electrode for Li-ion batteries. Using transmission electron microscopy techniques, here we show that the propagation and fracturing of platelet-like rock-salt phase along the (003) plane of the layered oxide are the leading cause for the cracking of primary particles. The fracturing of the rock-salt platelet is induced by the stress discontinuity between the parent layered oxide and the rock-salt phase. The high nickel content is considered to be the key factor for the formation of the rock-salt platelet and thus the (003) cracking. The (003)-type cracking can be a major factor for the structural degradation and associated capacity fade of the layered positive electrode.