Voltage Dependent Solid Electrolyte Interphase Formation in Silicon Electrodes: Monitoring the Formation of Organic Decomposition Products

The solid electrolyte interphase (SEI) passivating layer that grows on all battery electrodes during cycling is critical to the long-term capacity retention of lithium-ion batteries. Yet, it is inherently difficult to study because of its nanoscale thickness, amorphous composite structure, and air sensitivity. Here, we employ an experimental strategy using ¹H, ⁷Li, ¹⁹F, and ¹³C solid-state nuclear magnetic resonance (ssNMR) to gain insight into the decomposition products in the SEI formed on silicon electrodes, the uncontrolled growth of the SEI representing a major failure mechanism that prevents the practical use of silicon in lithium-ion batteries. The voltage dependent formation of the SEI is confirmed, with the SEI growth correlating with irreversible capacity. By studying both conductive carbon and mixed Si/C composite electrodes separately, a correlation with increased capacity loss of the composite system and the low-voltage silicon plateau is demonstrated. Using selective ¹³C labeling, we detect decomposition products of the electrolyte solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) independently. EC decomposition products are present in higher concentrations and are dominated by oligomer species. Lithium semicarbonates, lithium fluoride, and lithium carbonate products are also seen. Ab initio calculations have been carried out to aid in the assignment of NMR shifts. ssNMR applied to both rinsed and unrinsed electrodes show that the organics are easily rinsed away, suggesting that they are located on the outer layer of the SEI

Structural Stability from Crystallographic Shear in TiO₂–Nb₂O₅ Phases: Cation Ordering and Lithiation Behavior of TiNb₂₄O₆₂

The host structure and reversible lithium insertion and extraction of an intercalation compound, TiNb₂₄O₆₂, are described. Neutron diffraction, applied for the first time to TiNb₂₄O₆₂, allowed an accurate refinement of the complex block superstructure, particularly with respect to the oxygen sublattice. Analysis of the transition-metal sites revealed significant cation ordering in the mixed-metal oxide. Electrochemical analysis demonstrated highly reversible lithium intercalation with ca. 190 mA·h·g^–1 after 100 cycles (<i>C</i>/10 rate, 3 months). The effect of the potential window on the capacity, polarization, and reversibility was carefully examined; a minimum voltage limit of 1.1–1.2 V is critical for efficient and reversible cycling. The galvanostatic intermittent titration technique revealed three solid–solution regions, with different lithium diffusivities, in addition to the two-phase plateau that was clearly observed in the <i>V</i> versus <i>Q</i> discharge/charge profile. Lithium-ion diffusion decreases by over 3 orders of magnitude from the dilute lithium limit early in the discharge to the lithium-stuffed phase Li_37.5(1.0)TiNb₂₄O₆₂. Nevertheless, prior to lithium stuffing, TiNb₂₄O₆₂ possesses intrinsically rapid lithium-ion kinetics, as demonstrated by the high-rate performance in thick films of ca. 10 μm particles when interfaced with a carbon-coated aluminum foil substrate. The TiO₂·Nb₂O₅ phase diagram is examined and electrochemical results are compared to related superstructures of crystallographically sheared blocks of octahedra in the TiO₂·Nb₂O₅ homologous series including the H–Nb₂O₅ end member

Structural Stability from Crystallographic Shear in TiO₂–Nb₂O₅ Phases: Cation Ordering and Lithiation Behavior of TiNb₂₄O₆₂

The host structure and reversible lithium insertion and extraction of an intercalation compound, TiNb₂₄O₆₂, are described. Neutron diffraction, applied for the first time to TiNb₂₄O₆₂, allowed an accurate refinement of the complex block superstructure, particularly with respect to the oxygen sublattice. Analysis of the transition-metal sites revealed significant cation ordering in the mixed-metal oxide. Electrochemical analysis demonstrated highly reversible lithium intercalation with ca. 190 mA·h·g^–1 after 100 cycles (<i>C</i>/10 rate, 3 months). The effect of the potential window on the capacity, polarization, and reversibility was carefully examined; a minimum voltage limit of 1.1–1.2 V is critical for efficient and reversible cycling. The galvanostatic intermittent titration technique revealed three solid–solution regions, with different lithium diffusivities, in addition to the two-phase plateau that was clearly observed in the <i>V</i> versus <i>Q</i> discharge/charge profile. Lithium-ion diffusion decreases by over 3 orders of magnitude from the dilute lithium limit early in the discharge to the lithium-stuffed phase Li_37.5(1.0)TiNb₂₄O₆₂. Nevertheless, prior to lithium stuffing, TiNb₂₄O₆₂ possesses intrinsically rapid lithium-ion kinetics, as demonstrated by the high-rate performance in thick films of ca. 10 μm particles when interfaced with a carbon-coated aluminum foil substrate. The TiO₂·Nb₂O₅ phase diagram is examined and electrochemical results are compared to related superstructures of crystallographically sheared blocks of octahedra in the TiO₂·Nb₂O₅ homologous series including the H–Nb₂O₅ end member

Preventing Structural Rearrangements on Battery Cycling: A First-Principles Investigation of the Effect of Dopants on the Migration Barriers in Layered Li_0.5MnO₂

Layered LiMnO₂ is a potential Li ion cathode material that is known to undergo a layered to spinel transformation upon delithiation, as a result of Mn migration. A common strategy to improve the structural stability of LiMnO₂ has been to replace Mn with a range of metal dopants, although the mechanism by which each dopant stabilizes the structure is not well understood. In this work we characterize ion-migration barriers using hybrid eigenvector-following (EF) and density functional theory to study how trivalent dopants (Al³⁺, Cr³⁺, Fe³⁺, Ga³⁺, Sc³⁺, and In³⁺) affect Mn migration during the initial stage of the layered to spinel transformation in Li_0.5MnO₂. We demonstrate that dopants with small ionic radii, such as Al³⁺ and Cr³⁺, can increase the barrier for migration, but only when they are located in the first cation coordination sphere of Mn. We also demonstrate how the hybrid EF approach can be used to study the migration barriers of dopant species within the structure of Li_0.5MnO₂ efficiently. The transition state searching methodology described in this work will be useful for studying the effects of dopants on structural transformation mechanisms in a wide range of technologically interesting energy materials

Insights into Electrochemical Sodium Metal Deposition as Probed with <i>in Situ</i> ²³Na NMR

Sodium batteries have seen a resurgence of interest from researchers in recent years, owing to numerous favorable properties including cost and abundance. Here we examine the feasibility of studying this battery chemistry with <i>in situ</i> NMR, focusing on Na metal anodes. Quantification of the NMR signal indicates that Na metal deposits with a morphology associated with an extremely high surface area, the deposits continually accumulating, even in the case of galvanostatic cycling. Two regimes for the electrochemical cycling of Na metal are apparent that have implications for the use of Na anodes: at low currents, the Na deposits are partially removed on reversing the current, while at high currents, there is essentially no removal of the deposits in the initial stages. At longer times, high currents show a significantly greater accumulation of deposits during cycling, again indicating a much lower efficiency of removal of these structures when the current is reversed

Probing Cation and Vacancy Ordering in the Dry and Hydrated Yttrium-Substituted BaSnO₃ Perovskite by NMR Spectroscopy and First Principles Calculations: Implications for Proton Mobility

Hydrated BaSn_1–<i>x</i>Y_<i>x</i>O_{3–<i>x</i>/2} is a protonic conductor that, unlike many other related perovskites, shows high conductivity even at high substitution levels. A joint multinuclear NMR spectroscopy and density functional theory (total energy and GIPAW NMR calculations) investigation of BaSn_1–<i>x</i>Y_<i>x</i>O_{3–<i>x</i>/2} (0.10 ≤ <i>x</i> ≤ 0.50) was performed to investigate cation ordering and the location of the oxygen vacancies in the dry material. The DFT energetics show that Y doping on the Sn site is favored over doping on the Ba site. The ¹¹⁹Sn chemical shifts are sensitive to the number of neighboring Sn and Y cations, an experimental observation that is supported by the GIPAW calculations and that allows clustering to be monitored: Y substitution on the Sn sublattice is close to random up to <i>x</i> = 0.20, while at higher substitution levels, Y–O–Y linkages are avoided, leading, at <i>x</i> = 0.50, to strict Y–O–Sn alternation of B-site cations. These results are confirmed by the absence of a “Y–O–Y” ¹⁷O resonance and supported by the ¹⁷O NMR shift calculations. Although resonances due to six-coordinate Y cations were observed by ⁸⁹Y NMR, the agreement between the experimental and calculated shifts was poor. Five-coordinate Sn and Y sites (i.e., sites next to the vacancy) were observed by ¹¹⁹Sn and ⁸⁹Y NMR, respectively, these sites disappearing on hydration. More five-coordinated Sn than five-coordinated Y sites are seen, even at <i>x</i> = 0.50, which is ascribed to the presence of residual Sn–O–Sn defects in the cation-ordered material and their ability to accommodate O vacancies. High-temperature ¹¹⁹Sn NMR reveals that the O ions are mobile above 400 °C, oxygen mobility being required to hydrate these materials. The high protonic mobility, even in the high Y-content materials, is ascribed to the Y–O–Sn cation ordering, which prevents proton trapping on the more basic Y–O–Y sites

Identification of Cation Clustering in Mg–Al Layered Double Hydroxides Using Multinuclear Solid State Nuclear Magnetic Resonance Spectroscopy

A combined X-ray diffraction and magic angle spinning nuclear magnetic resonance (MAS NMR) study of a series of layered double hydroxides (LDHs) has been utilized to identify cation clustering in the metal hydroxide layers. High resolution (multiple quantum, MQ) ²⁵Mg NMR spectroscopy was successfully used to resolve different Mg local environments in nitrate and carbonate-containing layered double hydroxides with various Al for Mg substitution levels, and it provides strong evidence for cation ordering schemes based around Al–Al avoidance (in agreement with ²⁷Al NMR), the ordering increasing with an increase in Al content. ¹H MAS double quantum NMR spectroscopy verified the existence of small Mg₃OH and Mg₂AlOH clusters within the same metal hydroxide sheet and confirmed that the cations gradually order as the Al concentration is increased to form a honeycomb-like Al distribution throughout the metal hydroxide layer. The combined use of these multinuclear NMR techniques provides a structural foundation with which to rationalize the effects of different cation distributions on properties such as anion binding and retention in this class of materials

Materials’ Methods: NMR in Battery Research

Improving electrochemical energy storage is one of the major issues of our time. The search for new battery materials together with the drive to improve performance and lower cost of existing and new batteries is not without its challenges. Success in these matters is undoubtedly based on first understanding the underlying chemistries of the materials and the relations between the components involved. A combined application of experimental and theoretical techniques has proven to be a powerful strategy to gain insights into many of the questions that arise from the “how do batteries work and why do they fail” challenge. In this Review, we highlight the application of solid-state nuclear magnetic resonance (NMR) spectroscopy in battery research: a technique that can be extremely powerful in characterizing local structures in battery materials, even in highly disordered systems. An introduction on electrochemical energy storage illustrates the research aims and prospective approaches to reach these. We particularly address “NMR in battery research” by giving a brief introduction to electrochemical techniques and applications as well as background information on both <i>in</i> and <i>ex situ</i> solid-state NMR spectroscopy. We will try to answer the question “Is NMR suitable and how can it help me to solve my problem?” by shortly reviewing some of our recent research on electrodes, microstructure formation, electrolytes and interfaces, in which the application of NMR was helpful. Finally, we share hands-on experience directly from the lab bench to answer the fundamental question “Where and how should I start?” to help guide a researcher’s way through the manifold possible approaches

The Morphology of TiO₂ (B) Nanoparticles

The morphology of a nanomaterial (geometric shape and dimension) has a significant impact on its physical and chemical properties. It is, therefore, essential to determine the morphology of nanomaterials so as to link shape with performance in specific applications. In practice, structural features with different length scales are encoded in a specific angular range of the X-ray or neutron total scattering pattern of the material. By combining small- and wide-angle scattering (typically X-ray) experiments, the full angular range can be covered, allowing structure to be determined accurately at both the meso- and the nanoscale. In this Article, a comprehensive morphology analysis of lithium-ion battery anode material, TiO₂ (B) nanoparticles (described in Ren, Y.; Liu, Z.; Pourpoint, F.; Armstrong, A. R.; Grey, C. P.; Bruce, P. G. <i>Angew. Chem. Int. Ed.</i> <b>2012</b>, <i>51</i>, 2164), incorporating structure modeling with small-angle X-ray scattering (SAXS), pair distribution function (PDF), and X-ray powder diffraction (XRPD) techniques, is presented. The particles are oblate-shaped, contracted along the [010] direction, this particular morphology providing a plausible rationale for the excellent electrochemical behavior of these TiO₂(B) nanoparticles, while also provides a structural foundation to model the strain-driven distortion induced by lithiation. The work demonstrates the importance of analyzing various structure features at multiple length scales to determine the morphologies of nanomaterials

Understanding the Conduction Mechanism of the Protonic Conductor CsH₂PO₄ by Solid-State NMR Spectroscopy

Local dynamics and hydrogen bonding in CsH₂PO₄ have been investigated by ¹H, ²H, and ³¹P solid-state NMR spectroscopy to help provide a detailed understanding of proton conduction in the paraelectric phase. Two distinct environments are observed by ¹H and ²H NMR, and their chemical shifts (¹H) and quadrupolar coupling constants (²H) are consistent with one strong and one slightly weaker H-bonding environment. Two different protonic motions are detected by variable-temperature ¹H MAS NMR and <i>T</i>₁ spin–lattice relaxation time measurements in the paraelectric phase, which we assign to librational and long-range translational motions. An activation energy of 0.70 ± 0.07 eV is extracted for the latter motion; that of the librational motion is much lower. ³¹P NMR line shapes are measured under MAS and static conditions, and spin–lattice relaxation time measurements have been performed as a function of temperature. Although the ³¹P line shape is sensitive to the protonic motion, the reorientation of the phosphate ions does not lead to a significant change in the ³¹P CSA tensor. Rapid protonic motion and rotation of the phosphate ions is seen in the superprotonic phase, as probed by the <i>T</i>₁ measurements along with considerable line narrowing of both the ¹H and the ³¹P NMR signals