Artificial Solid Electrolyte Interphase To Address the Electrochemical Degradation of Silicon Electrodes

Electrochemical degradation on silicon (Si) anodes prevents them from being successfully used in lithium (Li)-ion battery full cells. Unlike the case of graphite anodes, the natural solid electrolyte interphase (SEI) films generated from carbonate electrolytes do not self-passivate on Si, causing continuous electrolyte decomposition and loss of Li ions. In this work, we aim at solving the issue of electrochemical degradation by fabricating artificial SEI films using a solid electrolyte material, lithium phosphorus oxynitride (Lipon), which conducts Li ions and blocks electrons. For Si anodes coated with Lipon of 50 nm or thicker, a significant effect is observed in suppressing electrolyte decomposition, while Lipon of thinner than 40 nm has a limited effect. Ionic and electronic conductivity measurements reveal that the artificial SEI is effective when it is a pure ionic conductor, but electrolyte decomposition is only partially suppressed when the artificial SEI is a mixed electronic–ionic conductor. The critical thickness for this transition in conducting behavior is found to be 40–50 nm. This work provides guidance for designing artificial SEI films for high-capacity Li-ion battery electrodes using solid electrolyte materials

Influence of Lithium Salts on the Discharge Chemistry of Li–Air Cells

In this work, we show that the use of a high boiling point ether solvent (tetraglyme) promotes the formation of Li₂O₂ in a lithium–air cell. However, another major constituent in the discharge product of a Li–air cell contains halides from the lithium salts and C–O from the tetraglyme used as the solvent. This information is critical to the development of Li–air electrolytes, which are stable and promote the formation of the desired Li₂O₂ products

Self-Assembly of Large Gold Nanoparticles for Surface-Enhanced Raman Spectroscopy

Performance of portable technologies from mobile phones to electric vehicles is currently limited by the energy density and lifetime of lithium batteries. Expanding the limits of battery technology requires <i>in situ</i> detection of trace components at electrode–electrolyte interphases. Surface-enhance Raman spectroscopy could satisfy this need if a robust and reproducible substrate were available. Gold nanoparticles (Au NPs) larger than 20 nm diameter are expected to greatly enhance Raman intensity if they can be assembled into ordered monolayers. A three-phase self-assembly method is presented that successfully results in ordered Au NP monolayers for particle diameters ranging from 13 to 90 nm. The monolayer structure and Raman enhancement factors (EFs) are reported for a model analyte, rhodamine, as well as the best performing polymer electrolyte salt, lithium bis­(trifluoro­methane)­sulfonimide. Experimental EFs for the most part correlate with predictions based on monolayer geometry and with numerical simulations that identify local electromagnetic field enhancements. The EFs for the best performing Au NP monolayer are between 10⁶ and 10⁸ and give quantitative signal response when analyte concentration is changed

Controlled Formation of Mixed Nanoscale Domains of High Capacity Fe₂O₃–FeF₃ Conversion Compounds by Direct Fluorination

We report a direct fluorination method under fluorine gas atmosphere using a fluidized bed reactor for converting nanophase iron oxide (n-Fe₂O₃) to an electrochemically stable and higher energy density iron oxyfluoride/fluoride phase. Interestingly, no noticeable bulk iron oxyfluoride phase (FeOF) phase was observed even at fluorination temperature close to 300 °C. Instead, at fluorination temperatures below 250 °C, scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) and X-ray photoelectron spectroscopy (XPS) analysis showed surface fluorination with nominal composition, Fe₂O_3‑<i>x</i>F_2<i>x</i> (<i>x</i> < 1). At fluorination temperatures of 275 °C, STEM-EELS results showed porous interconnected nanodomains of FeF₃ and Fe₂O₃ coexisting within the same particle, and overall the particles become less dense after fluorination. We performed potentiometric intermittent titration and electrochemical impedance spectroscopy studies to understand the lithium diffusion (or apparent diffusion) in both the oxyfluoride and mixed phase FeF₃ + Fe₂O₃ composition, and correlate the results to their electrochemical performance. Further, we analyze from a thermodynamical perspective, the observed formation of the majority fluoride phase (77% FeF₃) and the absence of the expected oxyfluoride phase based on the relative formation energies of oxide, fluoride, and oxyfluorides

Structural Transformations in High-Capacity Li₂Cu_0.5Ni_0.5O₂ Cathodes

Cathode materials that can cycle >1 Li⁺ per transition metal are of substantial interest for increasing the overall energy density of lithium-ion batteries. Li₂Cu_0.5Ni_0.5O₂ has a very high theoretical capacity of ∼500 mAh/g assuming both Li⁺ ions are cycled reversibly. The Cu^2+/3+ and Ni^2+/3+/4+ redox couples are also at high voltage, which could further boost the energy density of this system. Despite such promise, Li₂Cu_0.5Ni_0.5O₂ undergoes irreversible phase changes during charge (delithiation) that result in large first-cycle irreversible loss and poor long-term cycling stability. Oxygen evolves before the Cu^2+/3+ or Ni^3+/4+ transitions are accessed. In this contribution, X-ray diffraction, transmission electron microscopy (TEM), and transmission X-ray microscopy combined with X-ray absorption near edge structure (TXM–XANES) are used to follow the chemical and structural changes that occur in Li₂Cu_0.5Ni_0.5O₂ during electrochemical cycling. Li₂Cu_0.5Ni_0.5O₂ is a solid solution of orthorhombic Li₂CuO₂ and Li₂NiO₂, but the structural changes more closely mimic the changes that the Li₂NiO₂ endmember undergoes. Li₂Cu_0.5Ni_0.5O₂ loses long-range order during charge, but TEM analysis provides clear evidence of particle exfoliation and the transformation from orthorhombic to a partially layered structure. Linear combination fitting and principal component analysis of TXM–XANES are used to map the different phases that emerge during cycling <i>ex situ</i> and <i>in situ</i>. Significant changes in the XANES at the Cu and Ni K-edges correlate with the onset of oxygen evolution

Crystal Chemistry and Electrochemistry of Li_<i>x</i>Mn_1.5Ni_0.5O₄ Solid Solution Cathode Materials

For ordered high-voltage spinel LiMn_1.5Ni_0.5O₄ (LMNO) with the <i>P</i>4₃2₁ symmetry, the two consecutive two-phase transformations at ∼4.7 V (<i>vs</i> Li⁺/Li), involving three cubic phases of LMNO, Li_0.5Mn_1.5Ni_0.5O₄ (L_0.5MNO), and Mn_1.5Ni_0.5O₄ (MNO), have been well-established. Such a mechanism is traditionally associated with poor kinetics due to the slow movement of the phase boundaries and the large mechanical strain resulting from the volume changes among the phases, yet ordered LMNO has been shown to have excellent rate capability. In this study, we show the ability of the phases to dissolve into each other and determine their solubility limit. We characterized the properties of the formed solid solutions and investigated the role of non-equilibrium single-phase redox processes during the charge and discharge of LMNO. By using an array of advanced analytical techniques, such as soft and hard X-ray spectroscopy, transmission X-ray microscopy, and neutron/X-ray diffraction, as well as bond valence sum analysis, the present study examines the metastable nature of solid-solution phases and provides new insights in enabling cathode materials that are thermodynamically unstable

Chemical Evolution in Silicon–Graphite Composite Anodes Investigated by Vibrational Spectroscopy

Silicon–graphite composites are under development for the next generation of high-capacity lithium-ion anodes, and vibrational spectroscopy is a powerful tool to identify the different mechanisms that contribute to performance loss. With alloy anodes, the underlying causes of cell failure are significantly different in half-cells with lithium metal counter electrodes compared to full cells with standard cathodes. However, most studies which take advantage of vibrational spectroscopy have only examined half-cells. In this work, a combination of FTIR and Raman spectroscopy describes several factors that lead to degradation in full pouch cells with LiNi_0.5Mn_0.3Co_0.2O₂ (NMC532) cathodes. The spectroscopic signatures evolve after longer term cycling compared to the initial formation cycles. Several side-reactions that consume lithium ions have clear FTIR signatures, and comparison to a library of reference compounds facilitates identification. Raman microspectroscopy combined with mapping shows that the composite anodes are not homogeneous but segregate into graphite-rich and silicon-rich phases. Lithiation does not proceed uniformly either. A basis analysis of Raman maps identifies electrochemically inactive regions of the anodes. The spectroscopic results presented here emphasize the importance of improving electrode processing and SEI stability to enable practical composite anodes with high silicon loadings

Correlating Local Structure with Electrochemical Activity in Li₂MnO₃

Li₂MnO₃ is believed to be a critical component of the high capacity Li-rich–manganese-rich oxide materials; however, the mechanism of its electrochemical activity remains controversial. Here, Raman spectroscopy and mapping are used to follow the chemical and structural changes that occur in Li₂MnO₃ during electrochemical cycling. Conventional composite electrodes cast from a slurry and thin films are studied as a function of the state of charge (voltage) and cycle number. Thin films have similar electrochemical properties as electrodes prepared from slurries but enable spectroscopy of uniform samples without carbon additives and binder. First-principles density functional theory is used to calculate the phonon spectra and identify the Raman-active modes. On the basis of the calculations of phonon spectra for pristine Li₂MnO₃ and structures with Li vacancies, we discuss the origin of Raman-active peaks observed during the electrochemical cycling. The spectral changes correlate well with the electrochemical behavior and support a mechanism whereby capacity is lost upon extended cycling due to the formation of new manganese oxide phases

Nanoscale Morphological and Chemical Changes of High Voltage Lithium–Manganese Rich NMC Composite Cathodes with Cycling

Understanding the evolution of chemical composition and morphology of battery materials during electrochemical cycling is fundamental to extending battery cycle life and ensuring safety. This is particularly true for the much debated high energy density (high voltage) lithium–manganese rich cathode material of composition Li_{1 + <i>x</i>}M_{1 – <i>x</i>}O₂ (M = Mn, Co, Ni). In this study we combine full-field transmission X-ray microscopy (TXM) with X-ray absorption near edge structure (XANES) to spatially resolve changes in chemical phase, oxidation state, and morphology within a high voltage cathode having nominal composition Li_1.2Mn_0.525Ni_0.175Co_0.1O₂. Nanoscale microscopy with chemical/elemental sensitivity provides direct quantitative visualization of the cathode, and insights into failure. Single-pixel (∼30 nm) TXM XANES revealed changes in Mn chemistry with cycling, possibly to a spinel conformation and likely including some Mn­(II), starting at the particle surface and proceeding inward. Morphological analysis of the particles revealed, with high resolution and statistical sampling, that the majority of particles adopted nonspherical shapes after 200 cycles. Multiple-energy tomography showed a more homogeneous association of transition metals in the pristine particle, which segregate significantly with cycling. Depletion of transition metals at the cathode surface occurs after just one cycle, likely driven by electrochemical reactions at the surface

Anomalous Discharge Product Distribution in Lithium-Air Cathodes

Using neutron tomographic imaging, we report for the first time the three-dimensional spatial distribution of lithium products in electrochemically discharged lithium-air cathodes. Neutron imaging finds a nonuniform lithium product distribution across the electrode thickness, with the lithium species concentration being higher near the edges of the Li-air electrode and relatively uniform in the center of the electrode. The experimental neutron images were analyzed in context of results obtained from 3D modeling that maps the spatiotemporal variation of the lithium product distribution using a kinetically coupled diffusion based transport model. The origin of such anomalous behavior is due to the competition between the transport of lithium and oxygen and the accompanying electrochemical kinetics. Quantitative understanding of these effects is a critical step toward rechargeability of Li-air electrochemical systems