Jordi Cabana

Beyond Intercalation‐Based Li‐Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions

Despite the imminent commercial introduction of Li-ion batteries in electric drive vehicles and their proposed use as enablers of smart grids based on renewable energy technologies, an intensive quest for new electrode materials that bring about improvements in energy density, cycle life, cost, and safety is still underway. This Progress Report highlights the recent developments and the future prospects of the use of phases that react through conversion reactions as both positive and negative electrode materials in Li-ion batteries. By moving beyond classical intercalation reactions, a variety of low cost compounds with gravimetric specific capacities that are two-to-five times larger than those attained with currently used materials, such as graphite and LiCoO(2), can be achieved. Nonetheless, several factors currently handicap the applicability of electrode materials entailing conversion reactions. These factors, together with the scientific breakthroughs that are necessary to fully assess the practicality of this concept, are reviewed in this report.

Three-Dimensional Imaging of Chemical Phase Transformations at the Nanoscale with Full-Field Transmission X-Ray Microscopy

The ability to probe morphology and phase distribution in complex systems at multiple length scales unravels the interplay of nano- and micrometer-scale factors at the origin of macroscopic behavior. While different electron- and X-ray-based imaging techniques can be combined with spectroscopy at high resolutions, owing to experimental time limitations the resulting fields of view are too small to be representative of a composite sample. Here a new X-ray imaging set-up is proposed, combining full-field transmission X-ray microscopy (TXM) with X-ray absorption near-edge structure (XANES) spectroscopy to follow two-dimensional and three-dimensional morphological and chemical changes in large volumes at high resolution (tens of nanometers). TXM XANES imaging offers chemical speciation at the nanoscale in thick samples (>20 µm) with minimal preparation requirements. Further, its high throughput allows the analysis of large areas (up to millimeters) in minutes to a few hours. Proof of concept is provided using battery electrodes, although its versatility will lead to impact in a number of diverse research fields.

Monodisperse Sn Nanocrystals as a Platform for the Study of Mechanical Damage during Electrochemical Reactions with Li

Monodisperse Sn spherical nanocrystals of 10.0 ± 0.2 nm were prepared in dispersible colloidal form. They were used as a model platform to study the impact of size on the accommodation of colossal volume changes during electrochemical lithiation using ex situ transmission electron microscopy (TEM). Significant mechanical damage was observed after full lithiation, indicating that even crystals at these very small dimensions are not sufficient to prevent particle pulverization that compromises electrode durability.

X-ray Absorption Spectra of Dissolved Polysulfides in Lithium-Sulfur Batteries from First-Principles.

The X-ray absorption spectra (XAS) of lithium polysulfides (Li2Sx) of various chain lengths (x) dissolved in a model solvent are obtained from first-principles calculations. The spectra exhibit two main absorption features near the sulfur K-edge, which are unambiguously interpreted as a pre-edge near 2471 eV due to the terminal sulfur atoms at either end of the linear polysulfide dianions and a main-edge near 2473 eV due to the (x - 2) internal atoms in the chain, except in the case of Li2S2, which only has a low-energy feature. We find an almost linear dependence between the ratio of the peaks and chain length, although the linear dependence is modified by the delocalized, molecular nature of the core-excited states that can span up to six neighboring sulfur atoms. Thus, our results indicate that the ratio of the peak area, and not the peak intensities, should be used when attempting to differentiate the polysulfides from XAS.

The Formation Mechanism of Fluorescent Metal Complexes at the LixNi0.5Mn1.5O4−δ/Carbonate Ester Electrolyte Interface

Electrochemical oxidation of carbonate esters at the Li(x)Ni(0.5)Mn(1.5)O(4-δ)/electrolyte interface results in Ni/Mn dissolution and surface film formation, which negatively affect the electrochemical performance of Li-ion batteries. Ex situ X-ray absorption (XRF/XANES), Raman, and fluorescence spectroscopy, along with imaging of Li(x)Ni(0.5)Mn(1.5)O(4-δ) positive and graphite negative electrodes from tested Li-ion batteries, reveal the formation of a variety of Mn(II/III) and Ni(II) complexes with β-diketonate ligands. These metal complexes, which are generated upon anodic oxidation of ethyl and diethyl carbonates at Li(x)Ni(0.5)Mn(1.5)O(4-δ), form a surface film that partially dissolves in the electrolyte. The dissolved Mn(III) complexes are reduced to their Mn(II) analogues, which are incorporated into the solid electrolyte interphase surface layer at the graphite negative electrode. This work elucidates possible reaction pathways and evaluates their implications for Li(+) transport kinetics in Li-ion batteries.

The effects of moderate thermal treatments under air on LiFePO 4-based nano powders

The thermal behavior under air of LiFePO4-based powders was investigated through the combination of several techniques such as temperature-controlled X-ray diffraction, thermogravimetric analysis and Mossbauer and NMR spectroscopies. The reactivity with air at moderate temperatures depends on the particle size and leads to progressive displacement of Fe from the core structure yielding nano-size Fe2O3 and highly defective, oxidized LixFeyPO4 compositions whose unit-cell volume decreases dramatically when the temperature is raised between 400 and 600 K. The novel LiFePO4-like compositions display new electrochemical reactivity when used as positive electrodes in Li batteries. Several redox phenomena between 3.4 V and 2.7 V vs.Li were discovered and followed by in-situX-ray diffraction, which revealed two distinct solid solution domains associated with highly anisotropic variations of the unit-cell constants.

Effective wrapping of graphene on individual Li4Ti5O12 grains for high-rate Li-ion batteries

An effective way of synthesizing graphene-wrapped Li4Ti5O12 particles was developed by solid-state reaction between graphene oxide-wrapped P25 (TiO2) and Li2CO3. Compared to the previously reported graphene/Li4Ti5O12 composites, prior wrapping of TiO2 with subsequent chemical lithiation led to more effectively confined Li4Ti5O12. The Li4Ti5O12 tightly bound by graphene exhibited a remarkable specific capacity of 147 mA h g−1 at a rate of 10 C (1 C = 175 mA g−1) after 100 cycles. This rate capability is one of the highest values among reported Li4Ti5O12 with 150 ± 50 nm grains. The improved rate capability was attributed to the enhanced electronic conductivity of each Li4Ti5O12 grain via uniform graphene wrapping, with single-grain growth during annealing from the initial ∼25 nm TiO2 nanoparticles enclosed by outer graphene sheets. Graphene-eliminated Li4Ti5O12 by thermal decomposition was also directly compared to the graphene-coated sample, to clarify the role of graphene with nearly equivalent particle size/morphology distributions.

Titanate Anodes for Sodium Ion Batteries

For reasons of cost and supply security issues, there is growing interest in the development of rechargeable sodium ion batteries, particularly for large-scale grid storage applications. Like the much better known and technologically important lithium ion analogs, the devices operate by shuttling alkali metal cations between two host materials, which undergo insertion processes at different electrochemical potentials. A particular challenge for the sodium systems is identification of a suitable anode material due to the fact that sodium does not intercalate into graphite. Although several alternatives, including disordered carbons and alloys are being investigated, the most promising options at present lie with titanates, not in the least because of attractive characteristics such as low toxicity, ease of synthesis, wide availability, and low cost. A large variety of sodium titanate compounds can be prepared, many of which have tunnel or layered structures that can readily undergo reversible reductive intercalation reactions. A brief overview of the physical, structural, and electrochemical characteristics of several of the most promising materials for sodium-ion battery applications is given in this paper, and a comparison is made between the sodium and the lithium insertion behaviors. For some of these compounds, insertion of sodium occurs at unusually low potentials, a feature that has important implications for the design of high-energy sodium-ion systems.

Study of the transition metal ordering in layered Na(x)Ni(x/2)Mn(1-x/2)O2 (2/3 ≤ x ≤ 1) and consequences of Na/Li exchange.

A series of layered oxides within the Na(x)Ni(ix/2)Mn(1-x/2)O2 (2/3 ≤ x ≤ 1) system were synthesized by classical solid-state methodologies. A study of their long and short-range structure was undertaken by combining X-ray diffraction and NMR spectroscopy. A transition from P2 to O3 stacking was observed at x > 0.8 when samples were made at 900 °C, which was accompanied by disordering of ions in the transition metal layer. The magnetic properties of the materials were consistent with this picture of ordering, with all samples showing antiferromagnetic character. At x = 2/3, competition between a P2 and a P3 structure, with different degrees of transition metal ordering, was found depending on the synthesis temperature. Na/Li exchange led to structures with octahedral or tetrahedral coordination of the alkali metal, and Li/Ni crystallographic exchange in the resulting O3 phases. The transition from alkali metal prismatic coordination to octahedral/tetrahedral coordination involves [TMO6]∞ layer shearing that induces some structural disorder through the formation of stacking faults.

Visualization of electrochemically driven solid-state phase transformations using operando hard X-ray spectro-imaging

In situ techniques with high temporal, spatial and chemical resolution are key to understand ubiquitous solid-state phase transformations, which are crucial to many technological applications. Hard X-ray spectro-imaging can visualize electrochemically driven phase transformations but demands considerably large samples with strong absorption signal so far. Here we show a conceptually new data analysis method to enable operando visualization of mechanistically relevant weakly absorbing samples at the nanoscale and study electrochemical reaction dynamics of iron fluoride, a promising high-capacity conversion cathode material. In two specially designed samples with distinctive microstructure and porosity, we observe homogeneous phase transformations during both discharge and charge, faster and more complete Li-storage occurring in porous polycrystalline iron fluoride, and further, incomplete charge reaction following a pathway different from conventional belief. These mechanistic insights provide guidelines for designing better conversion cathode materials to realize the promise of high-capacity lithium-ion batteries.