Uniform Surface Modification of Li2ZnTi3O8 by Liquated Na2MoO4 To Boost Electrochemical Performance

Polycrystal Li2ZnTi3O8/C anode with lotus seedpod structure for high-performance lithium storage

Lotus-seedpod structured Li₂ZnTi₃O₈/C (P-LZTO) microspheres obtained by the molten salt method are reported for the first time. The received phase-pure Li₂ZnTi₃O₈ nanoparticles are inserted into the carbon matrix homogeneously to form a Lotus-seedpod structure, as confirmed by the morphological and structural measurements. As the anode for lithium-ion batteries, the P-LZTO material demonstrates excellent electrochemical performance with a high rate capacity of 193.2 mAh g^-1 at 5 A g^-1 and long-term cyclic stability up to 300 cycles at 1 A g^-1. After even 300 cyclings, the P-LZTO particles can maintain their morphological and structural integrity. The superior electrochemical performances have arisen from the unique structure where the polycrystalline structure is beneficial for shorting the lithium-ion diffusion path, while the well-encapsulated carbon matrix can not only enhance the electronic conductivity of the composite but also alleviate the stress anisotropy during lithiation/delithiation process, leading to well-preserved particles.

Visualizing crystal structure evolution of electrode materials upon doping and during charge/discharge cycles in lithium-ion batteries

Here we propose a systematic approach to reliably visualize the crystal structure evolution of electrode materials of lithium-ion batteries (LIBs) during cyclic charge/discharge process. Using anodic Ta⁵⁺-doped Li₂ZnTi₃O₈ (LZTO) spheres as an example, this protocol describes the doping state modeling by density functional theory (DFT) calculation, their crystal structure parameter determination by X-ray diffraction (XRD) refinement, and formation energy by electron density calculation. This protocol also details the in-situ XRD technique and date processing to visualize the cycling reversibility of Ta⁵⁺-doped LZTO. For complete details on the use and execution of this profile, please refer to Ma et al. (2021).

Solid-state self-template synthesis of Ta-doped Li2ZnTi3O8 spheres for efficient and durable lithium storage

Ta-doped Li2ZnTi3O8 (LZTO) spheres (Li2ZnTi3-x Ta x O8; where x is the synthetic chemical input, x = 0, 0.03, 0.05, 0.07) are synthesized via solid-state reaction using mesoporous TiO2 spheres as the self-template. The majority of Ta5+ ions are uniformly doped into crystal lattices of LZTO through the Ti↔Ta substitution, and the rest forms the piezoelectric LiTaO3 secondary phase on the surface, as confirmed by X-ray diffraction refinement, Raman spectroscopy, density functional theory, and electron microscopy. Electrochemical impedance spectroscopy demonstrates that the Ta5+ doping creates rapid electronic transportation channels for high Li+ ion diffusion kinetics; however, the LiTaO3 surface coating is beneficial to improve the electronic conductivity. At the optimal x = 0.05, Li2ZnTi3-x Ta x O8 spheres exhibit a reversible capacity of 90.2 mAh/g after 2000 cycles with a high coulombic efficiency of ≈100% at 5.0 A/g, thus enabling a promising anode material for lithium-ion batteries with high power and energy densities.

Immobilized Precursor Particle Driven Growth of Centimeter-Sized MoTe2 Monolayer

Molybdenum ditelluride (MoTe₂) has attracted ever-growing attention in recent years due to its novel characteristics in spintronics and phase-engineering, and an efficient and convenient method to achieve large-area high-quality film is an essential step toward electronic applications. However, the growth of large-area monolayer MoTe₂ is challenging. Here, for the first time, we achieve the growth of a centimeter-sized monoclinic MoTe₂ monolayer and manifest the mechanism of immobilized precursor particle driven growth. Microscopic characterizations reveal an obvious trend of immobilized precursor particles being consumed by the monolayer and continuing to provide a source for the growth of the monolayer. Time-of-flight secondary ion mass spectrometry verifies the attachment of hydroxide ions on the surface of the MoTe₂ monolayer, thereby realizing the inhibition of crystal growth along the [001] zone axis and the continuous growth of the MoTe₂ monolayer. The first-principles DFT calculations prove the mechanism of immobilized precursor particles and the absorption of hydroxide ions on the MoTe₂ monolayer. The as-grown MoTe₂ monolayer exhibits a surface roughness of 0.19 nm and average conductivity of 1.5 × 10^-5 S/m, which prove the smoothness and uniformity of the MoTe₂ monolayer. Temperature-dependent electrical measurements together with the transfer characteristic curves further demonstrate the typical semimetallic properties of monoclinic MoTe₂. Our research elaborates the microscopic process of immobilized precursor particles to grow large-area MoTe₂ monolayer and provides a new thinking about the growth of many other two-dimensional materials.

Reversible Movement of Zn2+ Ions with Zero-Strain Characteristic: Clarifying the Reaction Mechanism of Li2ZnTi3O8

Lithium zinc titanate spinel, Li₂ZnTi₃O₈, has received significant attention as a negative electrode material for lithium-ion batteries (LIBs). However, its reaction mechanism has not been fully clarified yet, particularly for the large voltage hysteresis between discharge and charge curves. We hence closely examined (Li_1-xZn_x)[Li_1/3+x/3Ti_5/3-x/3]O₄ (LZTO) with 0 < x ≤ 0.5 by measuring its open-circuit voltage (OCV) and recording synchrotron radiation X-ray diffraction (XRD) patterns. Here, LZTO is a solid solution of Li[Li_1/3Ti_5/3]O₄ (x = 0) and Li₂ZnTi₃O₈ (x = 0.5), both of which have a spinel-framework structure. For the x = 0.5 sample, the OCV of the discharge reaction differed from that of the charge reaction, particularly at a capacity above 50 mAh·g^-1. This difference was due to the migration of Zn²⁺ ions from tetrahedral sites to octahedral sites, and the Zn²⁺ ions moved back to tetrahedral sites during the charge reaction. Despite these drastic movements of Zn²⁺ ions, the cubic lattice parameter of the spinel was maintained during the whole reaction, i.e., zero strain. Perfect zero strain, which has never been reported for any LIB materials, was achieved with the x = 0.25 sample. The reaction mechanism with x = 0.5 and the contributions of the amount of Zn ions are discussed in detail.

3D-Structured Polyoxometalate Microcrystals with Enhanced Rate Capability and Cycle Stability for Lithium-Ion Storage

The unsatisfactory rate capability and poor cycle stability are two major obstacles for polyoxometalates (POMs) in lithium-ion storage. On the other hand, how to endow POMs with 3D macrostructures for further practice is a challenge. To this end, a facile hydrothermal strategy was practiced to fabricate Co₈W₁₂O₄₂(OH)₄(H₂O)₈ microcrystals or CoWO₄ aggregates onto the foamed substrate (denoted as CoW-POM and CoW-Salt, respectively). Integrating the extraordinary redox stability and lattice deformability of POMs with the excellent volume accommodation, the as-prepared CoW-POM presents an extraordinary better electrochemical performance (specific capacity, rate capability, and cycle life) than that of CoW-Salt. In detail, the CoW-POM can deliver a reversible capacity of 737.8 mA h g^-1 at the current density of 0.1 A g^-1 and provide a capacity retention of 90.1% even after 100 cycles. This work not only promotes the application of POMs in energy storage and conversion but also guides an effect methodology to endow POMs with 3D structures.