In Situ X‑ray Absorption Spectroscopy Study of the Capacity Fading Mechanism in Hybrid Sn₃O₂(OH)₂/Graphite Battery Anode Nanomaterials

In situ X-ray absorption spectroscopy (XAS) of an electrode material under electrochemical control has enabled a detailed examination of the capacity fading mechanism during charge–discharge cycling in a hybrid nanomaterial, Sn₃O₂(OH)₂/graphite, that is considered for use as a high-capacity lithium-ion battery anode. By the use of an original one-pot solvothermal synthesis technique, Sn₃O₂(OH)₂ nanoparticles were directly deposited on the surface of nanothin graphite and were charged/discharged in situ for several cycles while XAS spectra at the Sn K-edge were taken. Modeling of the collected extended X-ray absorption fine structure (EXAFS) spectra provides detailed information on the Sn–O, Sn–Sn, and Sn–Li coordination numbers and atomic distances for each charged and discharged electrode state. On the basis of electrochemical data and the changes in atomic arrangement deduced from the EXAFS fitting results, including the first unambiguous observation of Sn–Li near neighbors, a capacity fading mechanism is proposed that is different from widely accepted volume expansion for tin metal and tin oxides. Our experimental results suggest that atomic clusters of metallic tin surrounded by highly disordered Li₂O shells are formed on first charge. The metallic tin clusters participate in lithiation and delithiation on the following charge/discharge cycles; however, because of continued segregation of tin and Li₂O phases, the tin clusters eventually lose electrical contact with the rest of the electrode and become excluded from further participation in electrochemical reactions, resulting in reduced capacity of this anode material

Potential-Resolved In Situ X‑ray Absorption Spectroscopy Study of Sn and SnO₂ Nanomaterial Anodes for Lithium-Ion Batteries

This work provides detailed analysis of processes occurring in metallic Sn and SnO₂ anode materials for lithium ion batteries during first lithiation, studied in situ with rapid continuous X-ray absorption spectroscopy (XAS). The X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra provide information on dynamic changes in the Sn atomic environment, including type and number of neighboring atoms and interatomic distances. A unique methodology was used to model insertion of Li atoms into the electrode material structure and to analyze the formation of SnLi phases within the electrodes. Additionally, analysis of fully lithiated and delithiated states of Sn and SnO₂ electrodes in the first two cycles provides insight into the reasons for poor electrochemical performance and rapid capacity decline. Results indicate that use of SnO₂ is more promising than metallic Sn as an anode material, but more effort in nanoscale and atomic engineering of anodes is required for commercially feasible use of Sn-based materials

Surface Modification Approach to TiO₂ Nanofluids with High Particle Concentration, Low Viscosity, and Electrochemical Activity

This study presents a new approach to the formulation of functional nanofluids with high solid loading and low viscosity while retaining the surface activity of nanoparticles, in particular, their electrochemical response. The proposed methodology can be applied to a variety of functional nanomaterials and enables exploration of nanofluids as a medium for industrial applications beyond heat transfer fluids, taking advantage of both liquid behavior and functionality of dispersed nanoparticles. The highest particle concentration achievable with pristine 25 nm titania (TiO₂) nanoparticles in aqueous electrolytes (pH 11) is 20 wt %, which is limited by particle aggregation and high viscosity. We have developed a scalable one-step surface modification procedure for functionalizing those TiO₂ nanoparticles with a monolayer coverage of propyl sulfonate groups, which provides steric and charge-based separation of particles in suspension. Stable nanofluids with TiO₂ loadings up to 50 wt % and low viscosity are successfully prepared from surface-modified TiO₂ nanoparticles in the same electrolytes. Viscosity and thermal conductivity of the resulting nanofluids are evaluated and compared to nanofluids prepared from pristine nanoparticles. Furthermore, it is demonstrated that the surface-modified titania nanoparticles retain more than 78% of their electrochemical response as compared to that of the pristine material. Potential applications of the proposed nanofluids include, but are not limited to, electrochemical energy storage and catalysis, including photo- and electrocatalysis

Dispersion of Nanocrystalline Fe₃O₄ within Composite Electrodes: Insights on Battery-Related Electrochemistry

Aggregation of nanosized materials in composite lithium-ion-battery electrodes can be a significant factor influencing electrochemical behavior. In this study, aggregation was controlled in magnetite, Fe₃O₄, composite electrodes via oleic acid capping and subsequent dispersion in a carbon black matrix. A heat treatment process was effective in the removal of the oleic acid capping agent while preserving a high degree of Fe₃O₄ dispersion. Electrochemical testing showed that Fe₃O₄ dispersion is initially beneficial in delivering a higher functional capacity, in agreement with continuum model simulations. However, increased capacity fade upon extended cycling was observed for the dispersed Fe₃O₄ composites relative to the aggregated Fe₃O₄ composites. X-ray absorption spectroscopy measurements of electrodes post cycling indicated that the dispersed Fe₃O₄ electrodes are more oxidized in the discharged state, consistent with reduced reversibility compared with the aggregated sample. Higher charge-transfer resistance for the dispersed sample after cycling suggests increased surface-film formation on the dispersed, high-surface-area nanocrystalline Fe₃O₄ compared to the aggregated materials. This study provides insight into the specific effects of aggregation on electrochemistry through a multiscale view of mechanisms for magnetite composite electrodes

Multi-Stage Structural Transformations in Zero-Strain Lithium Titanate Unveiled by <i>in Situ</i> X‑ray Absorption Fingerprints

Zero-strain electrodes, such as spinel lithium titanate (Li_4/3Ti_5/3O₄), are appealing for application in batteries due to their negligible volume change and extraordinary stability upon repeated charge/discharge cycles. On the other hand, this same property makes it challenging to probe their structural changes during the electrochemical reaction. Herein, we report <i>in situ</i> studies of lithiation-driven structural transformations in Li_4/3Ti_5/3O₄ via a combination of X-ray absorption spectroscopy and <i>ab initio</i> calculations. Based on excellent agreement between computational and experimental spectra of Ti K-edge, we identified key spectral features as fingerprints for quantitative assessment of structural evolution at different length scales. Results from this study indicate that, despite the small variation in the crystal lattice during lithiation, pronounced structural transformations occur in Li_4/3Ti_5/3O₄, both locally and globally, giving rise to a multi-stage kinetic process involving mixed quasi-solid solution/macroscopic two-phase transformations over a wide range of Li concentrations. This work highlights the unique capability of combining <i>in situ</i> core-level spectroscopy and <i>first-principles</i> calculations for probing Li-ion intercalation in zero-strain electrodes, which is crucial to designing high-performance electrode materials for long-life batteries