Lepidocrocite-type Layered Titanate Structures: New Lithium and Sodium Ion Intercalation Anode Materials

The electrochemical characteristics of lepidocrocite-type titanates derived from K_0.8Ti_1.73Li_0.27O₄ are presented for the first time. By exchanging sodium ions for potassium, the practical specific capacity of the titanate in both sodium and lithium half cells is considerably enhanced. Although the gross structural features of the titanate framework are maintained during the ion exchange process, the symmetry changes because sodium occupies different sites from potassium. The smaller size of the sodium ion as compared to potassium and the change in site symmetry allow more alkali metal cations to be inserted reversibly into the structure during discharge in sodium and lithium cells than in the parent compound. Insertion of lithium cations takes place at an average of about 0.8 V vs Li⁺/Li while sodium intercalation occurs at 0.5 V vs Na⁺/Na, with sloping voltage profiles exhibited for both cell configurations, implying single-phase processes. Ex situ synchrotron X-ray diffraction measurements show that a lithiated lepidocrocite is formed during discharge in lithium cells, which undergoes further lithium insertion with almost no volume change. In sodium cells, insertion of sodium initially causes an overall expansion of about 12% in the <i>b</i> lattice parameter, but reversible uptake of solvent minimizes changes upon further cycling. In the case of the sodium cells, both the practical capacity and the cyclability are improved when a more compliant binder (polyacrylic acid) that can accommodate volume changes associated with insertion processes is used in place of the more common polyvinylidene fluoride. The ability to tune the electrochemical properties of lepidocrocite titanate structures by varying compositions and utilizing ion exchange processes make them especially versatile anode materials for both lithium and sodium ion battery configurations

Control of Size and Composition of Colloidal Nanocrystals of Manganese Oxide

A comprehensive study on the effects of experimental parameters on the composition and size of manganese oxide nanocrystals was completed using colloidal chemistry. The reactions studied involved the thermolysis of Mn2+ acetate and Mn3+ acetylacetonate in oleylamine. Temperature was found to be the dominant factor affecting the composition and size of the products. Reactions completed below 200 ᵒC favored the formation of nanocrystals smaller than 20 nm, with the presence of even impurity amounts of oxidizing agents leading to the formation of Mn3O4. Nanocrystals of MnO could only be synthesized below 200 ᵒC if Mn2+ acetate was used, and the reaction was carefully controlled to have no O2 and H2O contamination. In turn, particle growth was rapid above this temperature. In this case, regardless of the oxidizing agents used or oxidation state of the Mn precursor, nanocrystals of MnO formed after annealing for at least one hour at temperatures higher than 200 ᵒC. This finding suggests the role of oleylamine as solvent, surfactant and reducing agent at sufficiently high annealing temperatures. These results increase the understanding of redox stability of manganese during the colloidal synthesis of semiconductor metal oxide nanocrystals

Control of Chemical Structure in Core–Shell Nanocrystals for the Stabilization of Battery Electrode/Electrolyte Interfaces

Undesired reactions at electrode/electrolyte interfaces impose challenges in the durability of Li-ion battery. Traditional strategies of interfacial stabilization involve coating with inactive oxide films on aggregated powders of active cathode oxides. Despite generating gains in electrode performance, the lack of control of film growth of existing methods limits the ability to design its chemical structure and enhance functionality. The complexity of these coated materials also complicates efforts to define the specific chemical and structural features that determine function. Core–shell heterostructures at the nanocrystal level offer opportunities for precise control of chemistry and homogeneity. This ability is demonstrated with the compositional and structural tailoring of passivating layers based on Al³⁺, grown conformally onto LiCoO₂ nanoplates, using thermal treatments. They result in heterostructures from core–shell (LiCoO₂ nanoplates@2 nm aluminum oxide) to LiCo_1–<i>x</i>Al_<i>x</i>O₂ gradient structures composed by an Al-rich outer layer on a Co-rich core. While all samples presented improvements in electrochemical performance compared to the bare material, the LiCo_1–<i>x</i>Al_<i>x</i>O₂ gradient heterostructure presented the greatest advantage compared to pure aluminum oxide shells. The presence of a high Al/Co ratio at the surface, combined with the structural epitaxy and presence of Li throughout the particle, was considered to be critical to the best electrode properties and electrode/electrolyte interface stabilization. This work advances our ability to build complex heterostructures that both offer engineering solutions and create novel fundamental insight into the origins of battery durability

Visualization of the Phase Propagation within Carbon-Free Li₄Ti₅O₁₂ Battery Electrodes

The electrochemical reactions occurring in batteries involve the transport of ions and electrons among the electrodes, the electrolyte, and the current collector. In Li-ion battery electrodes, this dual functionality is attained with porous composite electrode structures that contain electronically conductive additives. Recently, the ability to extensively cycle composite electrodes of Li₄Ti₅O₁₂ without any conductive additives generated questions about how these structures operate, the answers to which could be used to design architectures with other materials that reduce the amount of additives that do not directly store energy. Here, the changes occurring in carbon-free Li₄Ti₅O₁₂ electrodes during lithiation were studied by a combination of ex situ and operando optical microscopy and microbeam X-ray absorption spectroscopy (μ-XAS). The measurements provide visualizations of the percolation of lithiated domains through the thick (∼40-μm) structure after a depth of discharge of only 1%, followed by a second wave of propagation starting with regions in closest contact with the current collector and progressing toward regions in contact with the bulk electrode. These results emphasize the interplay between the electronic and ionic conductivities of the phases involved in a battery reaction and the formation of the phases in localized areas in the electrode architecture. They provide new insights that could be used to refine the design of these architectures to minimize transport limitations while maximizing energy density

NaV_1.25Ti_0.75O₄: A Potential Post-Spinel Cathode Material for Mg Batteries

Rechargeable Mg batteries are promising candidates for high energy density storage in theory, when a Mg metal anode is combined with an oxide cathode material. Despite the widely observed sluggish Mg²⁺ diffusion in most oxide lattices, recent first-principles calculations predicted low diffusion barriers in the calcium ferrite (CF)-type post-spinel structures. In the present work, we experimentally examine the prospect of CF-type NaV_1.25Ti_0.75O₄ as a Mg cathode. The Na⁺ ions, which lie in the ion migration pathway, need to be removed or exchanged with Mg²⁺ to allow Mg²⁺ de/intercalation. Partial desodiation was achieved through chemical and electrochemical methods, as proven by X-ray diffraction and X-ray absorption spectroscopy, but deep desodiation was accompanied by partial amorphization of the material. Mg²⁺ ion exchange at moderate temperature (80 °C) resulted in the formation of Na_0.19Mg_0.41V_1.25Ti_0.75O₄; however, phase transformation was observed when higher temperatures were applied to attempt complete ion exchange. Such phenomena point to the instability of the CF lattice when the tunnel is empty or occupied by a small ion (Mg²⁺). Thus, while the low migration barrier predicted by computation is partly based on the relative metastability of the theoretical CF-Mg_<i>x</i>V_1.25Ti_0.75O₄ lattice, the difficulty in stabilizing it also renders the material synthetically nonaccessible, hindering this post-spinel’s application as an electrode material

Ultrathin Lithium-Ion Conducting Coatings for Increased Interfacial Stability in High Voltage Lithium-Ion Batteries

Ultrathin conformal coatings of the lithium ion conductor, lithium aluminum oxide (LiAlO₂), were evaluated for their ability to improve the electrochemical stability of LiNi_0.5Mn_1.5O₄/graphite Li-ion batteries. Electrochemical impedance spectroscopy confirmed the ion conducting character of the LiAlO₂ films. Complementary simulations of the activation barriers in these layers match experimental results very well. LiAlO₂ films were subsequently separately deposited onto LiNi_0.5Mn_1.5O₄ and graphite electrodes. Increased electrochemical stability was observed, especially in the full cells, which was attributed to the role of the coatings as physical barriers against side reactions at the electrode–electrolyte interface. By comparing data from full cells where the coatings were applied to either electrode, the dominating failure mechanism was found to be the diffusion of transition metal ions from the cathode to the anode. The LiNi_0.5Mn_1.5O₄/graphite full cell with less than 1 nm LiAlO₂ on the positive electrode exhibited a discharge capacity of 92 mAh/g at C/3 rate. The chemical underpinnings of stable performance were revealed by soft X-ray absorption spectroscopy. First, both manganese and nickel were detected on the graphite electrode surfaces, and their oxidation states were determined as +2. Second, the ultrathin coatings on the anode alone were found to be sufficient to significantly reduce this deleterious process

Facet-Dependent Rock-Salt Reconstruction on the Surface of Layered Oxide Cathodes

The surface configuration of pristine layered oxide cathode particles for Li-ion batteries significantly affects the electrochemical behavior, which is generally considered to be a thin rock-salt layer in the surface. Unfortunately, aside from its thin nature and spatial location on the surface, the true structural nature of this surface rock-salt layer remains largely unknown, creating the need to understand its configuration and the underlying mechanisms of formation. Using scanning transmission electron microscopy, we have found a correlation between the surface rock-salt formation and the crystal facets on pristine LiNi_0.80Co_0.15Al_0.05O₂ primary particles. It is found that the originally (014̅) and (003) surfaces of the layered phase result in two kinds of rock-salt reconstructions: the (002) and (111) rock-salt surfaces, respectively. Stepped surface configurations are generated for both reconstructions. The (002) configuration is relatively flat with monatomic steps while the (111) configuration shows significant surface roughening. Both reconstructions reduce the ionic and electronic conductivity of the cathode, leading to a reduced electrochemical performance

Facet-Dependent Rock-Salt Reconstruction on the Surface of Layered Oxide Cathodes

The surface configuration of pristine layered oxide cathode particles for Li-ion batteries significantly affects the electrochemical behavior, which is generally considered to be a thin rock-salt layer in the surface. Unfortunately, aside from its thin nature and spatial location on the surface, the true structural nature of this surface rock-salt layer remains largely unknown, creating the need to understand its configuration and the underlying mechanisms of formation. Using scanning transmission electron microscopy, we have found a correlation between the surface rock-salt formation and the crystal facets on pristine LiNi_0.80Co_0.15Al_0.05O₂ primary particles. It is found that the originally (014̅) and (003) surfaces of the layered phase result in two kinds of rock-salt reconstructions: the (002) and (111) rock-salt surfaces, respectively. Stepped surface configurations are generated for both reconstructions. The (002) configuration is relatively flat with monatomic steps while the (111) configuration shows significant surface roughening. Both reconstructions reduce the ionic and electronic conductivity of the cathode, leading to a reduced electrochemical performance

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

The Formation Mechanism of Fluorescent Metal Complexes at the Li_<i>x</i>Ni_0.5Mn_1.5O_4−δ/Carbonate Ester Electrolyte Interface

Electrochemical oxidation of carbonate esters at the Li_<i>x</i>Ni_0.5Mn_1.5O_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_<i>x</i>Ni_0.5Mn_1.5O_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_<i>x</i>Ni_0.5Mn_1.5O_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