Ordered Mesoporous Titanium Nitride as a Promising Carbon-Free Cathode for Aprotic Lithium-Oxygen Batteries

Despite the extraordinary gravimetric energy densities, lithium-oxygen (Li-O₂) batteries are still facing a technological challenge; limited round trip efficiency leading to insufficient cycle life. Recently, carbonaceous electrode materials were found to be one of the primary origins of the limited cycle life, as they produce irreversible side products during discharge. A few investigations based on noncarbonaceous materials have demonstrated largely suppressed accumulation of irreversible side products, but such studies have focused mainly on the materials themselves rather than delicate morphology control. As such, here, we report the synthesis of mesoporous titanium nitride (m-TiN) with a 2D hexagonal structure and large pores (>30 nm), which was templated by a block copolymer with tunable chain lengths, and introduce it as a stable air-cathode backbone. Due to the well-aligned pore structure and decent electric conductivity of TiN, the battery reaction was quite reversible, resulting in robust cycling performance for over 100 cycles under a potential cutoff condition. Furthermore, by protecting the Li metal with a poreless polyurethane separator and engaging a lithium iodide redox mediator, the original capacity was retained for 280 cycles under a consistent capacity condition (430 mAh g^–1). This study reveals that when the appropriate structure and material choice of the air-cathode are coupled with an advanced separator and an effective solution-phase redox mediator, the cycle lives of Li-O₂ batteries can be enhanced dramatically

Block Copolymer Directed Ordered Mesostructured TiNb₂O₇ Multimetallic Oxide Constructed of Nanocrystals as High Power Li-Ion Battery Anodes

In order to achieve high-power and -energy anodes operating above 1.0 V (vs Li/Li⁺), titanium-based materials have been investigated for a long time. However, theoretically low lithium charge capacities of titanium-anodes have required new types of high-capacity anode materials. As a candidate, TiNb₂O₇ has attracted much attention due to the high theoretical capacity of 387.6 mA h g^–1. However, the high formation temperature of the TiNb₂O₇ phase resulted in large-sized TiNb₂O₇ crystals, thus resulting in poor rate capability. Herein, ordered mesoporous TiNb₂O₇ (denoted as m-TNO) was synthesized by block copolymer assisted self-assembly, and the resulting binary metal oxide was applied as an anode in a lithium ion battery. The nanocrystals (∼15 nm) developed inside the confined pore walls and large pores (∼40 nm) of m-TNO resulted in a short diffusion length for lithium ions/electrons and fast penetration of electrolyte. As a stable anode, the m-TNO electrode exhibited a high capacity of 289 mA h g^–1 (at 0.1 C) and an excellent rate performance of 162 mA h g^–1 at 20 C and 116 mA h g^–1 at 50 C (= 19.35 A g^–1) within a potential range of 1.0–3.0 V (vs Li/Li⁺), which clearly surpasses other Ti-and Nb-based anode materials (TiO₂, Li₄Ti₅O₁₂, Nb₂O₅, etc.) and previously reported TiNb₂O₇ materials. The m-TNO and carbon coated m-TNO electrodes also demonstrated stable cycle performances of 48 and 81% retention during 2,000 cycles at 10 C rate, respectively

Direct Access to Hierarchically Porous Inorganic Oxide Materials with Three-Dimensionally Interconnected Networks

Hierarchically porous oxide materials have immense potential for applications in catalysis, separation, and energy devices, but the synthesis of these materials is hampered by the need to use multiple templates and the associated complicated steps and uncontrollable mixing behavior. Here we report a simple one-pot strategy for the synthesis of inorganic oxide materials with multiscale porosity. The inorganic precursor and block copolymer are coassembled into an ordered mesostructure (microphase separation), while the in situ-polymerized organic precursor forms organic-rich macrodomains (macrophase separation) around which the mesostructure grows. Calcination generates hierarchical meso/macroporous SiO₂ and TiO₂ with three-dimensionally interconnected pore networks. The continuous 3D macrostructures were clearly visualized by nanoscale X-ray computed tomography. The resulting TiO₂ was used as the anode in a lithium ion battery and showed excellent rate capability compared with mesoporous TiO₂. This work is of particular importance because it (i) expands the base of BCP self-assembly from mesostructures to complex porous structures, (ii) shows that the interplay of micro- and macrophase separation can be fully exploited for the design of hierarchically porous inorganic materials, and therefore (iii) provides strategies for researchers in materials science and polymer science

Spinodal Decomposition Method for Structuring Germanium–Carbon Li-Ion Battery Anodes

To increase the energy density of lithium-ion batteries (LIBs), high-capacity anodes which alloy with Li ions at a low voltage against Li/Li+ have been actively pursued. So far, Si has been studied the most extensively because of its high specific capacity and cost efficiency; however, Ge is an interesting alternative. While the theoretical specific capacity of Ge (1600 mAh g–1) is only half that of Si, its density is more than twice as high (Ge, 5.3 g cm–3; Si, 2.33 g cm–3), and therefore the charge stored per volume is better than that of Si. In addition, Ge has a 400 times higher ionic diffusivity and 4 orders of magnitude higher electronic conductivity compared to Si. However, similarly to Si, Ge needs to be structured in order to manage stresses induced during lithiation and many reports have achieved sufficient areal loadings to be commercially viable. In this work, spinodal decomposition is used to make secondary particles of about 2 μm in diameter that consist of a mixture of ∼30 nm Ge nanoparticles embedded in a carbon matrix. The secondary structure of these germanium–carbon particles allows for specific capacities of over 1100 mAh g−1 and a capacity retention of 91.8% after 100 cycles. Finally, high packing densities of ∼1.67 g cm–3 are achieved in blended electrodes by creating a bimodal size distribution with natural graphite

One-Pot Synthesis of Tin-Embedded Carbon/Silica Nanocomposites for Anode Materials in Lithium-Ion Batteries

We report a facile “one-pot” method for the synthesis of Sn-embedded carbon–silica (CS) mesostructured (nanostructured) composites through the selective interaction of resol (carbon precursor), tetraethylorthosilicate (TEOS), and tributylphenyltin (Sn precursor) with an amphiphilic diblock copolymer, poly(ethylene oxide-<i>b</i>-styrene), PEO-<i>b</i>-PS. A unique morphology transition from Sn nanowires to spherical Sn nanoparticles embedded in CS framework has been obtained. Metallic Sn species are homogeneously embedded in a rigid CS framework and are effectively confined within the nanostructures. The resulting composites are used as anode materials for lithium-ion batteries and exhibit high specific capacities (600 mA h g ^–1 at a current density of 45 mA g^–1, and 440 mA h g^–1 at a current density of 300 mA g^–1) and an excellent cyclability of over 100 cycles with high Coulombic efficiency. Most of all, the novel method developed in this work for synthesizing functional hybrid materials can be extended to the preparation of various functional nanocomposites owing to its versatility and facileness

Soft-Template Simple Synthesis of Ordered Mesoporous Titanium Nitride-Carbon Nanocomposite for High Performance Dye-Sensitized Solar Cell Counter Electrodes

Ordered mesoporous titanium nitride-carbon (denoted as OM TiN-C) nanocomposite with high surface area (389 m² g^–1) and uniform hexagonal mesopores (ca. 5.5 nm) was facilely synthesized via the soft-template method. As a structure-directing agent, Pluronic F127 triblock copolymer formed an ordered structure with inorganic precursors, resol polymer, and prehydrolyzed TiCl₄, followed by a successive heating at 700 °C under nitrogen and ammonia flow. In this study, the amorphous carbon within the parent OM TiO₂-C acted as a rigid support, preventing structural collapse during the conversion process of TiO₂ nanocrystals to TiN nanocrystals. The OM TiN-C was then successfully applied as counter electrode material in dye-sensitized solar cells (DSCs). The organic electrolyte disulfide/thiolate (T₂/T^–) was introduced to study the electrocatalytic property of the OM TiN-C nanocomposite. Because of the existence of TiN nanocrystals and the defect sites of the amorphous carbon, the DSCs using OM TiN-C as a counter electrode showed 6.71% energy conversion efficiency (platinum counter electrode DSCs: 3.32%) in the organic electrolyte system (T₂/T^–). Furthermore, the OM TiN-C counter electrode based DSCs showed an energy conversion efficiency of 8.41%, whereas the DSCs using platinum as a counter electrode showed a conversion efficiency of only 8.0% in an iodide electrolyte system. The superior performance of OM TiN-C counter electrode resulted from the low charge transfer resistance, enhanced electrical conductivity, and abundance of active sites of the OM TiN-C nanocomposite. Moreover, OM TiN-C counter electrode showed better chemical stability in organic electrolyte compared with the platinum counter electrode

Mesoporous Ge/GeO₂/Carbon Lithium-Ion Battery Anodes with High Capacity and High Reversibility

We report mesoporous composite materials (m-GeO₂, m-GeO₂/C, and m-Ge-GeO₂/C) with large pore size which are synthesized by a simple block copolymer directed self-assembly. m-Ge/GeO₂/C shows greatly enhanced Coulombic efficiency, high reversible capacity (1631 mA h g^–1), and stable cycle life compared with the other mesoporous and bulk GeO₂ electrodes. m-Ge/GeO₂/C exhibits one of the highest areal capacities (1.65 mA h cm^–2) among previously reported Ge- and GeO₂-based anodes. The superior electrochemical performance in m-Ge/GeO₂/C arises from the highly improved kinetics of conversion reaction due to the synergistic effects of the mesoporous structures and the conductive carbon and metallic Ge

Advanced Hybrid Supercapacitor Based on a Mesoporous Niobium Pentoxide/Carbon as High-Performance Anode

Recently, hybrid supercapacitors (HSCs), which combine the use of battery and supercapacitor, have been extensively studied in order to satisfy increasing demands for large energy density and high power capability in energy-storage devices. For this purpose, the requirement for anode materials that provide enhanced charge storage sites (high capacity) and accommodate fast charge transport (high rate capability) has increased. Herein, therefore, a preparation of nanocomposite as anode material is presented and an advanced HSC using it is thoroughly analyzed. The HSC comprises a mesoporous Nb₂O₅/carbon (m-Nb₂O₅–C) nanocomposite anode synthesized by a simple one-pot method using a block copolymer assisted self-assembly and commercial activated carbon (MSP-20) cathode under organic electrolyte. The m-Nb₂O₅–C anode provides high specific capacity with outstanding rate performance and cyclability, mainly stemming from its enhanced pseudocapacitive behavior through introduction of a carbon-coated mesostructure within a voltage range from 3.0 to 1.1 V (<i>vs</i> Li/Li⁺). The HSC using the m-Nb₂O₅–C anode and MSP-20 cathode exhibits excellent energy and power densities (74 W h kg^–1 and 18 510 W kg^–1), with advanced cycle life (capacity retention: ∼90% at 1000 mA g^–1 after 1000 cycles) within potential range from 1.0 to 3.5 V. In particular, we note that the highest power density (18 510 W kg^–1) of HSC is achieved at 15 W h kg^–1, which is the highest level among similar HSC systems previously reported. With further study, the HSCs developed in this work could be a next-generation energy-storage device, bridging the performance gap between conventional batteries and supercapacitors

Facile Synthesis of Nb₂O₅@Carbon Core–Shell Nanocrystals with Controlled Crystalline Structure for High-Power Anodes in Hybrid Supercapacitors

Hybrid supercapacitors (battery-supercapacitor hybrid devices, HSCs) deliver high energy within seconds (excellent rate capability) with stable cyclability. One of the key limitations in developing high-performance HSCs is imbalance in power capability between the sluggish Faradaic lithium-intercalation anode and rapid non-Faradaic capacitive cathode. To solve this problem, we synthesize Nb₂O₅@carbon core–shell nanocyrstals (Nb₂O₅@C NCs) as high-power anode materials with controlled crystalline phases (orthorhombic (<i>T</i>) and pseudohexagonal (<i>TT</i>)) <i>via</i> a facile one-pot synthesis method based on a water-in-oil microemulsion system. The synthesis of ideal <i>T</i>-Nb₂O₅ for fast Li⁺ diffusion is simply achieved by controlling the microemulsion parameter (<i>e.g.,</i> pH control). The <i>T</i>-Nb₂O₅@C NCs shows a reversible specific capacity of ∼180 mA h g^–1 at 0.05 A g^–1 (1.1–3.0 V <i>vs</i> Li/Li⁺) with rapid rate capability compared to that of <i>TT</i>-Nb₂O₅@C and carbon shell-free Nb₂O₅ NCs, mainly due to synergistic effects of (i) the structural merit of <i>T</i>-Nb₂O₅ and (ii) the conductive carbon shell for high electron mobility. The highest energy (∼63 W h kg^–1) and power (16 528 W kg^–1 achieved at ∼5 W h kg^–1) densities within the voltage range of 1.0–3.5 V of the HSC using <i>T</i>-Nb₂O₅@C anode and MSP-20 cathode are remarkable