Revisiting metal fluorides as lithium-ion battery cathodes.

Metal fluorides, promising lithium-ion battery cathode materials, have been classified as conversion materials due to the reconstructive phase transitions widely presumed to occur upon lithiation. We challenge this view by studying FeF3 using X-ray total scattering and electron diffraction techniques that measure structure over multiple length scales coupled with density functional theory calculations, and by revisiting prior experimental studies of FeF2 and CuF2. Metal fluoride lithiation is instead dominated by diffusion-controlled displacement mechanisms, and a clear topological relationship between the metal fluoride F- sublattices and that of LiF is established. Initial lithiation of FeF3 forms FeF2 on the particle's surface, along with a cation-ordered and stacking-disordered phase, A-LixFeyF3, which is structurally related to α-/ß-LiMn2+Fe3+F6 and which topotactically transforms to B- and then C-LixFeyF3, before forming LiF and Fe. Lithiation of FeF2 and CuF2 results in a buffer phase between FeF2/CuF2 and LiF. The resulting principles will aid future developments of a wider range of isomorphic metal fluorides.

Electrolyte-Induced Surface Transformation and Transition-Metal Dissolution of Fully Delithiated LiNi0.8Co0.15Al0.05O2.

Enabling practical utilization of layered R3̅m positive electrodes near full delithiation requires an enhanced understanding of the complex electrode-electrolyte interactions that often induce failure. Using Li[Ni0.8Co0.15Al0.05]O2 (NCA) as a model layered compound, the chemical and structural stability in a strenuous thermal and electrochemical environment was explored. Operando microcalorimetry and electrochemical impedance spectroscopy identified a fingerprint for a structural decomposition and transition-metal dissolution reaction that occurs on the positive electrode at full delithiation. Surface-sensitive characterization techniques, including X-ray absorption spectroscopy and high-resolution transmission electron microscopy, measured a structural and morphological transformation of the surface and subsurface regions of NCA. Despite the bulk structural integrity being maintained, NCA surface degradation at a high state of charge induces excessive transition-metal dissolution and significant positive electrode impedance development, resulting in a rapid decrease in electrochemical performance. Additionally, the impact of electrolyte salt, positive electrode surface area, and surface Li2CO3 content on the magnitude and character of the dissolution reaction was studied.

Mesoscale effects in electrochemical conversion: coupling of chemistry to atomic- and nanoscale structure in iron-based electrodes.

The complex coupling of atomic, chemical, and electronic transformations across multiple length scales underlies the performance of electrochemical energy storage devices. Here, the coupling of chemistry with atomic- and nanoscale structure in iron conversion electrodes is resolved by combining pair distribution function (PDF) and small-angle X-ray scattering (SAXS) analysis for a series of Fe fluorides, oxyfluorides, and oxides. The data show that the anion chemistry of the initial electrode influences the abundance of atomic defects in the Fe atomic lattice. This, in turn, is linked to different atom mobilities and propensity for particle growth. Competitive nanoparticle growth in mixed anion systems contributes to a distinct nanostructure, without the interconnected metallic nanoparticles formed for single anion systems.

Comprehensive insights into the structural and chemical changes in mixed-anion FeOF electrodes by using operando PDF and NMR spectroscopy.

In-depth analysis of operando X-ray pair distribution function (PDF) data is combined with Li NMR spectroscopy to gain comprehensive insights into the electrochemical reaction mechanism of high-performance iron oxyfluoride electrodes. While the full discharge capacity could be recovered upon charge, implying reversibility of the electrochemical reaction, the atomic structure of the electrode formed after cycling (discharge-charge) differs from the pristine uncycled electrode material. Instead, the "active" electrode that forms upon cycling is a nanocomposite of an amorphous rutile phase and a nanoscale rock salt phase. Bond valence sum analysis, based on the precise structural parameters (bond lengths and coordination number) extracted from the in situ PDF data, suggests that anion partitioning occurs during the electrochemical reaction, with the rutile phase being F-rich and the rock salt phase being O-rich. The F- and O-rich phases react sequentially; Fe in a F-rich environment reacts preferentially during both discharge and charge.

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.

How Bulk Sensitive is Hard X-ray Photoelectron Spectroscopy: Accounting for the Cathode-Electrolyte Interface when Addressing Oxygen Redox.

Sensitivity to the "bulk" oxygen core orbital makes hard X-ray photoelectron spectroscopy (HAXPES) an appealing technique for studying oxygen redox candidates. Various studies have reported an additional O 1s peak (530-531 eV) at high voltages, which has been considered a direct signature of the bulk oxygen redox process. Here, we find the emergence of a 530.4 eV O 1s HAXPES peak for three model cathodes-Li2MnO3, Li-rich NMC, and NMC 442-that shows no clear link to oxygen redox. Instead, the 530.4 eV peak for these three systems is attributed to transition metal reduction and electrolyte decomposition in the near-surface region. Claims of oxygen redox relying on photoelectron spectroscopy must explicitly account for the surface sensitivity of this technique and the extent of the cathode degradation layer.

Surface Chemistry Dependence on Aluminum Doping in Ni-rich LiNi0.8Co0.2-yAlyO2 Cathodes.

Aluminum is a common dopant across oxide cathodes for improving the bulk and cathode-electrolyte interface (CEI) stability. Aluminum in the bulk is known to enhance structural and thermal stability, yet the exact influence of aluminum at the CEI remains unclear. To address this, we utilized a combination of X-ray photoelectron and absorption spectroscopy to identify aluminum surface environments and extent of transition metal reduction for Ni-rich LiNi0.8Co0.2-yAlyO2 (0%, 5%, or 20% Al) layered oxide cathodes tested at 4.75 V under thermal stress (60 °C). For these tests, we compared the conventional LiPF6 salt with the more thermally stable LiBF4 salt. The CEI layers are inherently different between these two electrolyte salts, particularly for the highest level of Al-doping (20%) where a thicker (thinner) CEI layer is found for LiPF6 (LiBF4). Focusing on the aluminum environment, we reveal the type of surface aluminum species are dependent on the electrolyte salt, as Al-O-F- and Al-F-like species form when using LiPF6 and LiBF4, respectively. In both cases, we find cathode-electrolyte reactions drive the formation of a protective Al-F-like barrier at the CEI in Al-doped oxide cathodes.

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.

Sodiation via heterogeneous disproportionation in FeF2 electrodes for sodium-ion batteries.

Sodium-ion batteries utilize various electrode materials derived from lithium batteries. However, the different characteristics inherent in sodium may cause unexpected cell reactions and battery performance. Thus, identifying the reactive discrepancy between sodiation and lithiation is essential for fundamental understanding and practical engineering of battery materials. Here we reveal a heterogeneous sodiation mechanism of iron fluoride (FeF2) nanoparticle electrodes by combining in situ/ex situ microscopy and spectroscopy techniques. In contrast to direct one-step conversion reaction with lithium, the sodiation of FeF2 proceeds via a regular conversion on the surface and a disproportionation reaction in the core, generating a composite structure of 1-4 nm ultrafine Fe nanocrystallites (further fused into conductive frameworks) mixed with an unexpected Na3FeF6 phase and a NaF phase in the shell. These findings demonstrate a core-shell reaction mode of the sodiation process and shed light on the mechanistic understanding extended to generic electrode materials for both Li- and Na-ion batteries.

Transport, phase reactions, and hysteresis of iron fluoride and oxyfluoride conversion electrode materials for lithium batteries.

Potentiostatic intermittent titration technique (PITT) was applied to FeF2, FeF3, and FeO0.67F1.33 to gain insight into the transport-related aspects of the conversion reaction by quantitative analysis of Li(+) diffusion and hysteresis. PITT derived diffusion coefficient measurements were benchmarked relative to values extracted by electrochemical impedance spectroscopy (EIS). A reverse-step PITT methodology was used to evaluate true hysteresis by eliminating nucleation induced overpotentials. This method evaluates the minimum potential hysteresis and allowed an accurate representation of the potential required to move conversion reactions forward at C/1000 rates in both lithiation and delithiation. The high resolution PITT data were also used to gain further insight into reaction mechanisms involved in the reversible conversion reactions. Physical evidence, based on pair distribution function (PDF) structural analysis, and electrochemical evidence are presented regarding a new step in the reaction during the rutile FeF2 reconversion reaction.

Tracking lithium transport and electrochemical reactions in nanoparticles.

Expectations for the next generation of lithium batteries include greater energy and power densities along with a substantial increase in both calendar and cycle life. Developing new materials to meet these goals requires a better understanding of how electrodes function by tracking physical and chemical changes of active components in a working electrode. Here we develop a new, simple in-situ electrochemical cell for the transmission electron microscope and use it to track lithium transport and conversion in FeF(2) nanoparticles by nanoscale imaging, diffraction and spectroscopy. In this system, lithium conversion is initiated at the surface, sweeping rapidly across the FeF(2) particles, followed by a gradual phase transformation in the bulk, resulting in 1-3 nm iron crystallites mixed with amorphous LiF. The real-time imaging reveals a surprisingly fast conversion process in individual particles (complete in a few minutes), with a morphological evolution resembling spinodal decomposition. This work provides new insights into the inter- and intra-particle lithium transport and kinetics of lithium conversion reactions, and may help to pave the way to develop high-energy conversion electrodes for lithium-ion batteries.

EELS spectroscopy of iron fluorides and FeFx/C nanocomposite electrodes used in Li-ion batteries.

A new type of positive electrode for Li-ion batteries has been developed recently based on FeF3/C and FeF2/C nanocomposites. The microstructural and redox evolution during discharge and recharge processes was followed by electron energy loss spectroscopy (EELS) to determine the valence state of Fe by measuring the Fe L3 line energy shift and from Fe L3/L2 line intensity ratios. In addition, transition metal fluorides were found to be electron beam sensitive, and the effect of beam exposure on EELS spectra was also investigated. The EELS results indicate that for both FeF3/C and FeF2/C nanocomposite systems, a complete reduction of iron to FeO is observed upon discharge to 1.5 V with the formation of a finer FeO/LiF subnanocomposite ( approximately 7 nm). Upon complete recharging to 4.5 V, EELS data reveal a reoxidation process to a Fe2+ state with the formation of a carbon metal fluoride nanocomposite related to the FeF2 structure.