On the path toward the design of Li-ion batteries with increased energy densities, efforts are focused on the development of positive electrodes that can maximize the voltage of the full cell. However, the development of novel materials that operate at high voltage, while also showing high efficiency and meeting strict safety standards, is an ongoing challenge. LiCoPO4 is being explored as a possible candidate, as the Co2+/3+ redox couple operates at 4.8 V versus Li+/Li0. The presence of phosphate groups is typically expected to stabilize the compound against oxygen loss, yet the changes in Co–O bonding upon Li extraction have not been ascertained. In addition, LiCoPO4 is riddled with problems relating to poor transport and strain in the crystal structure of the delithiated phase, which handicap its use as a high-voltage electrode. In this work, substituting ions to generate Li1.025Co0.84Fe0.10Cr0.05Si0.01(PO4)1.025 is found to stabilize both the electronic structure and crystal structure and, therefore, substantially improve the ability to fully utilize the redox capacity of the material. A thorough study by spectroscopic tools, combined with computations of the electronic structure, was used to probe changes in chemical bonding. The measurements revealed the existence of redox gradients between surface and bulk that are common in other materials that react at high potential. The study offers a comprehensive understanding of the fundamental reactions in LiCoPO4-type frameworks, while further demonstrating that ion substitution is an effective tool for improving their performance.

One route to accessing site-specific dynamical information available with ultrafast multidimensional infrared spectroscopy is the development of robust and versatile vibrational probes. Here we synthesize and characterize a vibrationally labeled cholesterol derivative, (cholesteryl benzoate) chromium tricarbonyl, to probe model lipid membranes, focusing specifically on the membrane-water interface. Utilizing FTIR and polarized-ATR spectroscopies, we determine the location of the chromium tricarbonyl motif to be situated at the water-membrane interface with an orientation of 46 ± 2° relative to the vector normal to the membrane surface. We test the dynamical sensitivity of the (cholesteryl benzoate) chromium tricarbonyl label with two different nonlinear infrared spectroscopy methods, both of which show that the probe is well-suited to the study of membrane dynamics as well as the dynamics of water at the membrane interface. The metal carbonyl vibrational probe located at the surface of a bicelle exhibits spectral diffusion dynamics induced by membrane hydration water that is roughly three times slower than observed using a nearly identical vibrational probe in bulk water.

In an effort to improve the cycle life and rate capability of olivine LiCoPO4, Cr, Fe, and Si were added to produce nominal Li1.025Co0.84Fe0.10Cr0.05Si0.01(PO4)1.025. This cathode material has an energy density comparable to LiCoPO4, with markedly improved electrochemical performance. Here, we apply operando X-ray diffraction to gain an understanding of the crystallographic delithiation mechanism of this new substituted electrode material, compared to both LiCo0.75Fe0.25PO4 and LiCo0.75Fe0.25PO4. Throughout charging, the extent of solid-solution domains was significantly increased in Li1.025Co0.84Fe0.10Cr0.05Si0.01(PO4)1.025 and LiCo0.75Fe0.25PO4 compared to LiCoPO4. These domains reduce the mechanical strain during electrode function, providing a clear explanation for the high durability with Co substitution. Li1.025Co0.84Fe0.10Cr0.05Si0.01(PO4)1.025 operated at notably higher average potential than LiCo0.75Fe0.25PO4, which would increase the energy density of the cell. Ex situ measurements reveal the persistence of structural irreversibilities in the substituted phase after the first cycle, identifying avenues for further improvement in durability. This finding sheds light on the strategies for judicious cation substitution in LiCoPO4 electrodes to maximize the cycle life while preserving high energy density, especially compared to LiFePO4.

Graphite intercalation compounds continue to be central to technologies for electrochemical energy storage from anodes in established Li-ion batteries to cathodes in beyond Li-ion concepts paired with multivalent anodes. When used as a cathode, graphite intercalates a variety of anions with PF6 – being among the most common. Paired with Li intercalation at the anode, the corresponding dual carbon battery yields high energy and power densities. Given the available choice of anions as intercalants, it is important to elucidate how the graphite structure accommodates them in order to tailor the molecular species to maximize charge and reversibility. However, the changes in electronic structure of the host graphite lattice upon anion intercalation are poorly understood compared to cations, which represent a fundamentally different reaction. In this work, PF6-intercalated graphite has been studied using techniques sensitive to electronic structure, namely, X-ray Raman spectroscopy (XRS), X-ray absorption near-edge spectroscopy (XANES), and X-ray emission spectroscopy (XES). Complementary full-potential, all-electron density functional theory calculations yielded excellent agreement with the spectra, thus providing insight into charge compensation in the graphite lattice. In particular, a pre-π* feature emerged in XRS/XANES, which is direct evidence of the removal of charge from the host lattice to compensate the intercalated anions, leading to an overall lowering of the Fermi energy level. This is expected to be characteristic of many intercalants in anion-intercalated graphite. The unambiguous identification of the origin of the pre-π* spectral feature, which is frequently seen in graphitic systems, is of broad interest to the spectroscopy of graphitic systems beyond the practical implications of anion-induced changes in the electronic properties for real devices.

The energy storage capability of a battery scales with the potential difference between its electrodes. Yet operation of positive electrode materials at high potentials introduces challenges of stabilization of charged states. As of today, no positive electrode material has been demonstrated to durably and safely operate above 4.5 V vs. Li+/Li0. LiCoPO4-based electrodes theoretically offer high specific capacity and high potentials of operation, around 4.8V vs. Li+/Li0, but these electrodes are prone to failure during cycling. Failure occurs through chemical and structural degradation in the bulk of the active material or at its interfaces with cell components, especially the electrolyte. The development of Li-ion battery electrodes operating at high potential is indispensable to meet the specific energy target of 250 kWh/kg at the packaged cell level. Changes in the electronic structure and chemical stability of olivine-type LiCoPO4 and Fe-substituted LiCoPO4 were explored as both a function of ion substitution and oxidation state. Soft Ex situX-Ray absorption spectroscopy (XAS) made it possible to compare the changes in chemical bonding between electrode bulk and surface as a function of lithium content. This technique can probe the density of states at the transition metal and O levels. The evolution of these levels revealed changes in the metal-oxide covalence when lithium was deintercalated from the structure, and, thus, the material was oxidized. An increase in covalence can lead to the destabilization of the anions. If this process takes place in the bulk of the material, this destabilization can lead to thermal degradation via oxygen loss. At the surface, even small degrees of destabilization are sufficient to produce oxidizing species that attack the electron-rich solvent molecules in the electrolyte, leading to irreversible capacity loss. Increased metal-oxygen covalence was universally observed in the spectroscopy in the form of a rising pre-O K-edge peak at ~530 eV as a function of lithium deintercalation in both bulk and surface. However, accompanying changes in the Co spectroscopy were only observed in the Fe-substituted sample. These changes are also indicative of increased hybridization between Co 3d and O 2p orbitals. Co K-edge XANES and EXAFS experiments further corroborated these findings, in which virtually no changes in the Co K-edge were observed upon oxidation of unsubstituted LiCoPO4. Fe doping appears to play a substantial role in getting Co to participate in redox chemistry, and the mechanism by which it occurs is currently being explored using Density Functional Theory. Fe-substituted LiCoPO4 is an exciting new positive electrode material that may prove useful in advancing Li-ion battery technology.