Fredrick Omenya

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.

What can we learn about battery materials from their magnetic properties

Electrode materials for Li-ion batteries should combine electronic and ionic conductivity, structural integrity, and safe operation over thousands of lithium insertion and removal cycles. The quest for higher energy density calls for better understanding of the redox processes, charge and mass transfer occurring upon battery operation. A number of techniques have been used to characterize long-range and local structure, electronic and ionic transport in bulk of active materials and at interfaces, with an ongoing move toward in situ techniques determining the changes as they happen. This paper reviews several representative examples of using magnetic properties toward understanding of Li-ion battery materials with a notion to highlight the intimate connection between the magnetism, electronic and atomic structure of solids, and to demonstrate how this connection has been used to reveal the fine electronic and atomic details related to the electrochemical performance of the battery materials.

Structure Stabilization by Mixed Anions in Oxyfluoride Cathodes for High-Energy Lithium Batteries

Mixed-anion oxyfluorides (i.e., FeOxF2-x) are an appealing alternative to pure fluorides as high-capacity cathodes in lithium batteries, with enhanced cyclability via oxygen substitution. However, it is still unclear how the mixed anions impact the local phase transformation and structural stability of oxyfluorides during cycling due to the complexity of electrochemical reactions, involving both lithium intercalation and conversion. Herein, we investigated the local chemical and structural ordering in FeO0.7F1.3 at length scales spanning from single particles to the bulk electrode, via a combination of electron spectrum-imaging, magnetization, electrochemistry, and synchrotron X-ray measurements. The FeO0.7F1.3 nanoparticles retain a FeF2-like rutile structure but chemically heterogeneous, with an F-rich core covered by thin O-rich shell. Upon lithiation the O-rich rutile phase is transformed into Li-Fe-O(-F) rocksalt that has high lattice coherency with converted metallic Fe, a feature that may facilitate the local electronic and ionic transport. The O-rich rocksalt is highly stable over lithiation/delithiation and thus advantageous to maintain the integrity of the particle, and due to its predominant distribution on the surface, it is expected to prevent the catalytic interaction of Fe with electrolyte. Our findings of the structural origin of cycling stability in oxyfluorides may provide insights into developing viable high-energy electrodes for lithium batteries.

Thermal Stability and Reactivity of Cathode Materials for Li-Ion Batteries

The thermal stability of electrochemically delithiated Li0.1Ni0.8Co0.15Al0.05O2 (NCA), FePO4 (FP), Mn0.8Fe0.2PO4 (MFP), hydrothermally synthesized VOPO4, LiVOPO4, and electrochemically lithiated Li2VOPO4 is investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis, coupled with mass spectrometry (TGA-MS). The thermal stability of the delithiated materials is found to be in the order of NCA < VOPO4 < MFP < FP. Unlike the layered oxides and MFP, VOPO4 does not evolve O2 on heating. Thus, VOPO4 is less likely to cause a thermal run-away phenomenon in batteries at elevated temperature and so is inherently safer. The lithiated materials LiVOPO4, Li2VOPO4, and LiNi0.8Co0.15Al0.05O2 are found to be stable in the presence of electrolyte, but sealed-capsule high-pressure experiments show a phase transformation of VOPO4 → HVOPO4 → H2VOPO4 when VOPO4 reacts with electrolyte (1 M LiPF6 in EC/DMC = 1:1) between 200 and 300 °C. Using first-principles calculations, we confirm that the charged VOPO4 cathode is indeed predicted to be marginally less stable than FP but significantly more stable than NCA in the absence of electrolyte. An analysis of the reaction equilibria between VOPO4 and EC using a multicomponent phase diagram approach yields products and reaction enthalpies that are highly consistent with the experiment results.

Understanding the stability of MnPO4

We have revealed the critical role of carbon coating in the stability and thermal behaviour of olivine MnPO4 obtained by chemical delithiation of LiMnPO4. (Li)MnPO4 samples with various particle sizes and carbon contents were studied. Carbon-free LiMnPO4 obtained by solid state synthesis in O2 becomes amorphous upon delithiation. Small amounts of carbon (0.3 wt%) help to stabilize the olivine structure, so that completely delithiated crystalline olivine MnPO4 can be obtained. Larger amount of carbon (2 wt%) prevents full delithiation. Heating in air, O2, or N2 results in structural disorder (<300 °C), formation of an intermediate sarcopside Mn3(PO4)2 phase (350–450 °C), and complete decomposition to Mn2P2O7 on extended heating at 400 °C. Carbon coating protects MnPO4 from reacting with environmental water, which is detrimental to its structural stability.

Structure, defects and thermal stability of delithiated olivine phosphates

Studies of thermal decomposition mechanism of olivine Fe1−yMnyPO4 are reported here for inert (He), oxidizing (O2) and oxidizing and moist (air) atmospheres using in situ X-ray diffraction and thermal gravimetric analysis with mass spectroscopy. The results indicate that the olivine structure is inherently stable up to at least 400 °C and y = 0.9 for particle size down to 50 nm. However, structural disorder and oxygen loss in the presence of reductive impurities, e.g. carbon and hydrogen, can occur as low as 250 °C for particles larger than 100 nm and at 150 °C for 50 nm particles. Fe1−yMnyPO4 is hygroscopic at high Mn contents, y ≥ 0.6, and moisture exposure is more detrimental to its thermal stability than carbon or small particle size. Nano-Fe1−yMnyPO4 (y > 0.7) with particle size about 50 nm, when exposed to moisture, disorders at 150 °C and transforms to sarcopside phase by 300 °C, no matter whether the delithiation was done electrochemically or chemically. Contrary, under inert atmosphere the sample produced by chemical delithiation is stable up to 400 °C.

What Happens to LiMnPO4 upon Chemical Delithiation

Olivine MnPO4 is the delithiated phase of the lithium-ion-battery cathode (positive electrode) material LiMnPO4, which is formed at the end of charge. This phase is metastable under ambient conditions and can only be produced by delithiation of LiMnPO4. We have revealed the manganese dissolution phenomenon during chemical delithiation of LiMnPO4, which causes amorphization of olivine MnPO4. The properties of crystalline MnPO4 obtained from carbon-coated LiMnPO4 and of the amorphous product resulting from delithiation of pure LiMnPO4 were studied and compared. The phosphorus-rich amorphous phases in the latter are considered to be MnHP2O7 and MnH2P2O7 from NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The thermal stability of MnPO4 is significantly higher under high vacuum than at ambient condition, which is shown to be related to surface water removal.

Correlating Lithium Hydroxyl Accumulation with Capacity Retention in V2O5 Aerogel Cathodes

V2O5 aerogels are capable of reversibly intercalating more than 5 Li(+)/V2O5 but suffer from lifetime issues due to their poor capacity retention upon cycling. We employed a range of material characterization and electrochemical techniques along with atomic pair distribution function, X-ray photoelectron spectroscopy, and density functional theory to determine the origin of the capacity fading in V2O5 aerogel cathodes. In addition to the expected vanadium redox due to intercalation, we observed LiOH species that formed upon discharge and were only partially removed after charging, resulting in an accumulation of electrochemically inactive LiOH over each cycle. Our results indicate that the tightly bound water that is necessary for maintaining the aerogel structure is also inherently responsible for the capacity fade.

Electrochemical Performance of Nanosized Disordered LiVOPO4

ε-LiVOPO4 is a promising multielectron cathode material for Li-ion batteries that can accommodate two electrons per vanadium, leading to higher energy densities. However, poor electronic conductivity and low lithium ion diffusivity currently result in low rate capability and poor cycle life. To enhance the electrochemical performance of ε-LiVOPO4, in this work, we optimized its solid-state synthesis route using in situ synchrotron X-ray diffraction and applied a combination of high-energy ball-milling with electronically and ionically conductive coatings aiming to improve bulk and surface Li diffusion. We show that high-energy ball-milling, while reducing the particle size also introduces structural disorder, as evidenced by 7Li and 31P NMR and X-ray absorption spectroscopy. We also show that a combination of electronically and ionically conductive coatings helps to utilize close to theoretical capacity for ε-LiVOPO4 at C/50 (1 C = 153 mA h g–1) and to enhance rate performance and capacity retention. The optimized ε-LiVOPO4/Li3VO4/acetylene black composite yields the high cycling capacity of 250 mA h g–1 at C/5 for over 70 cycles.

Role of disorder in limiting the true multi-electron redox in ε-LiVOPO4

Recent advances in materials syntheses have enabled e-LiVOPO4 to deliver capacities approaching, and in some cases exceeding the theoretical value of 305 mA h g−1 for 2Li intercalation, despite its poor electronic and ionic conductivity. However, not all of the capacity corresponds to the true electrochemical intercalation/deintercalation reactions as evidenced upon systematic tracking of V valence through combined operando and rate-dependent ex situ X-ray absorption study presented herein. Structural disorder and defects introduced in the material by high-energy ball milling impede kinetics of the high-voltage V5+/V4+ redox more severely than the low-voltage V4+/V3+ redox, promoting significant side reaction contributions in the high-voltage region, irrespective of cycling conditions. The present work emphasizes the need for nanoengineering of active materials without compromising their bulk structural integrity in order to fully utilize high-energy density of multi-electron cathode materials.