Insight into the Structures and Properties of Morphology-Controlled Legs of Tetrapod-Like ZnO Nanostructures

The fine structure characterization of individual legs is essential for understanding the detailed formation mechanism and the origin of the unique properties of tetrapod-like ZnO nanostructures. We have synthesized tetrapod-like ZnO nanostructures with thin needle (TN-ZnO), uniform hexagonal prism (TU-ZnO), and hierarchical hexagonal prism (TH-ZnO) legs through oxidization of zinc vapor followed by ZnO condensation at relatively lower temperatures. The individual legs of as-synthesized ZnO nanotetrapods were characterized complementarily by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron dispersive spectrum (EDS), and cathodoluminescence (CL). We demonstrated that the legs have wurtzite structure and prefer to grow along the [0001] direction. We found that all legs grew from similar ZnOx nuclei, where x is about 0.3, and all of them showed a strong visible luminescent property. EDS and CL spectra obtained from different regions in an individual leg illustrated that the strong visible luminescence resulted from their surface states rather than the heavy oxygen vacancy. The possible nucleation and growth mechanisms of the legs with different morphologies are discussed

Heat Generation Power of the LiNi_0.8Co_0.1Mn_0.1O₂ Cathode at Higher Charging Cutoff Voltages

Lithium-ion batteries using the LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode usually exhibit a high energy density, but structure fading and heat accumulation occurring during charge/discharge are their drawbacks. Herein, the heat generation power of the NCM811 cathode at relatively higher charging cutoff voltages and elevated temperatures is studied systematically. Specifically, the heat generation powers of NCM811 after various charge/discharge cycles under charging cutoff voltages of 4.3, 4.5, and 4.8 V at 50 °C are measured. First-principles calculation results combined with experimental measurements show that more structural variation can be generated at higher charging cutoff voltages, such as vacancies of oxygen, lithium, and transition metal ions, as well as cation mixing of Li+/Ni2+, all of which can increase the energy barrier for Li+ diffusion and instantly enhance the heat generation power of the NCM811 cathode

Electrospraying Graphene Nanosheets on Polyvinyl Alcohol Nanofibers for Efficient Thermal Management Materials

With the rapid development of electronic devices, the demand for heat-dissipating materials is increasing. Graphene is a two-dimensional (2D) material with high thermal conductivity. In this study, reduced graphene oxide (rGO) nanosheets are electrostatically sprayed on the surface of electrospun polyvinyl alcohol (PVA) nanofibers. A multilayer rGO/PVA film is fabricated by performing multi-step electrospinning and electrospraying. Finally, the multilayer rGO/PVA films are cut into small pieces and stacked for hot pressing to obtain a flexible H-rGO/PVA-t film (where t denotes the rGO-layer electrospraying time). Scanning electron microscopy images show that an rGO-layer electrospraying time of 4 h resulted in a uniform distribution of rGO sheets on the PVA nanofiber surface to form continuous heat transfer channels. Hence, the H-rGO/PVA-4 composite exhibits high in-plane and cross-plane thermal conductivities of 6.15 and 0.89 W m–1 K–1, respectively, and can be used as a thermal-interface material (TIM) to facilitate effective thermal management. Compared to a hot-pressed pure PVA film, the H-rGO/PVA-4 TIM facilitates a lower surface temperature for light-emitting diode (LED) lamps. This study provides a universal approach for fabricating heat-dissipating composites of 2D materials and polymer films

Boosting Sodium Storage of Hierarchical Nanofibers with Porous Carbon-Supported Anatase TiO₂/TiO₂(B) Nanowires

Structural and phase regulation of TiO2 anode materials has been confirmed to significantly promote sodium storage for sodium-ion batteries (SIBs). Herein, TiO2/C hierarchical nanofibers with anatase/TiO2(B) mixed phases are synthesized by a dual-template method, which employs an amphiphilic triblock copolymer (F127) and SiO2 as templates. The hierarchical structure of TiO2/C nanofibers provides effective contact for both the anode and electrolyte, and the anatase/TiO2(B) mixed phase causes rapid sodium-ion transmission. Moreover, the TiO2/C anode delivers a high reversible discharge capacity (262 mAh g–1, 0.1 A g–1), remarkable rate capacity (97 mAh g–1, 2.0 A g–1), and stable sodium storage performance (∼109 mAh g–1 over 1000 cycles, 1.0 A g–1). This work provides a dependable approach to build hierarchical TiO2 anode materials for SIBs with superior performance

Sn₄P₃ Encapsulated in Carbon Nanotubes/Poly(3,4-ethylenedioxythiophene) as the Anode for Pseudocapacitive Lithium-Ion Storage

Developing lithium-ion batteries (LIBs) with higher capacity is crucial for renewable energy utilization, such as large-scale energy storage systems, as well as portable and flexible electronics. As a conversion-reaction type LIB anode material, Sn4P3 could deliver a theoretical gravimetric capacity of 1132 mA h g–1. However, the usage of Sn4P3 in real LIB applications has been impeded by large volume expansion, low electronic conductivity, and limited Li+ charging speed upon cycling. Therefore, Sn4P3 is usually combined with carbon materials to improve its electrochemical performance. Herein, Sn4P3 nanoparticles were encapsulated inside the inner cavities of carbon nanotubes (CNTs) using a low-pressure vapor approach. This stemlike CNT network was further coated using poly­(3,4-ethylenedioxythiophene) (PEDOT) as the electron-boosting buffer layer. In this special design, CNT/PEDOT bilayers could relieve the volume expansion of Sn4P3 during charge–discharge, as well as provide robust electron and ion transportation. As anode materials for LIBs, Sn4P3@CNT/PEDOT exhibits superior rate performances (reversible capability of 499 mA h g–1 at 2000 mA g–1) and superior long-term cycling stability (701 mA h g–1 after 500 cycles at 500 mA g–1 and 1208 mA h g–1 after 230 cycles at 100 mA g–1). In addition, a high pseudocapacitive contribution of 80% was delivered by Sn4P3@CNT/PEDOT, satisfying potential fast-charging demands. The present study provides a novel train of thought for improving the electrochemical performance of other conversion-reaction-type anode materials with large volume expansion

Fluorine-Terminated Self-Assembled Monolayers Grafted Graphite Anode Inducing a LiF-Dominated SEI Inorganic Layer for Fast-Charging Lithium-Ion Batteries

The electrochemical kinetic processes of Li+ ions, including the desolvation of the Li+ ions from the electrolyte to the solid electrolyte interphase (SEI), the transportation of desolvated Li+ ions across the SEI, and the charge transfer at the interface between the SEI and graphite, determine the rate performance and cycling stability of the graphitic anode in lithium-ion batteries (LIBs). In this work, fluorine-terminated self-assembled monolayers were grafted on the surface of spherical graphite particles to regulate the chemical composition and structure of SEI formed on the graphite surface in the presence of conventional ester electrolytes. The comprehensive characterization and first-principles calculation results illustrate that a uniform LiF-dominated SEI film can be generated on the as-functionalized graphite anode due to the carbon–fluorine bonds’ cleavage of fluorine-terminated self-assembled monolayers. The LiF-dominated SEI film is particularly beneficial for desolvated lithium-ion transport across the SEI, affording LiCoO2//graphite full cells with substantially enhanced fast-charging capability and cycle stability. This strategy should be potentially useful for modifying other anode materials to regulate the interfacial chemistry between the anode and electrolyte in lithium-ion batteries

Flexible Fluorinated Graphene/Poly(vinyl Alcohol) Films toward High Thermal Management Capability

Graphene is widely used in heat dissipation, owing to its inherently high in-plane thermal conductivity and excellent mechanical properties. However, its poor cross-plane thermal conductivity limits its use in some electronic applications. The electron distribution of graphene and the interaction with the base material can be greatly altered by introducing F, the most electronegative element, giving fluorinated graphene oxide (FG) with a high thermal conductivity. Herein, FG is prepared by grafting F atoms onto the surface of graphene oxide in a low-temperature solid-phase reaction with poly­(vinylidene fluoride) as a fluorine source. This method can effectively avoid the use of dangerous substances such as HF and F2. The FG dispersion and aqueous poly­(vinyl alcohol) (PVA) solution are sequentially vacuum-filtered to obtain the FG/PVA composite film. After natural drying and hot-pressing, the thermal conductivity of the N-FG/PVA film is enhanced by the hydrogen bond between F of FG and the hydroxyl group of PVA. The in-plane and cross-plane thermal conductivity of an N-FG/PVA film containing 10.4 wt % FG are 7.13 and 1.42 W m–1 k–1, respectively. The film has a tensile strength of 60 MPa and an elongation at a break of 28%, which is promising for the thermal management of flexible electronic devices

Regulating Lithium-Ion Transference Number of a Poly(vinyl alcohol)-Based Gel Electrolyte by the Incorporation of H₃BO₃ as an Anion Trapper

Gel electrolytes hold great promise for improving the safety of lithium-ion batteries (LIBs); however, their low lithium-ion transference number (tLi+ < 0.4) remains to be improved. Herein, boron-containing poly­(vinyl alcohol) (B-PVA) solutions are prepared based on the simple reaction between PVA and H3BO3 and then electrospun to obtain the B-PVA nanofiber matrices. The gel electrolytes are formed by loading the liquid electrolytes into the as-obtained porous B-PVA matrices. We demonstrate that the tLi+ of B-PVA8 (with 8 wt % H3BO3 added) gel electrolyte can reach 0.81, much higher than that of the conventional PVA gel electrolyte (0.32). The B-PVA8 gel electrolyte also shows the highest Li+ conductivity of 1.81 × 10–3 S cm–1 compared to PVA. The B-PVA8 gel electrolyte enables a small Li nucleation overpotential of 129.8 mV when Li is plated and reduces the concentration polarization of Li||Li batteries. The mesocarbon microbead (MCMB) half-coin cells with the B-PVA8 gel electrolyte exhibit excellent rate capability and exceptional charge/discharge cycling stability