Direct Monitoring of Li2 S2 Evolution and Its Influence on the Reversible Capacities of Lithium-Sulfur Batteries.

The polysulfide (PS) dissolution and low conductivity of lithium sulfides (Li2 S) are generally considered the main reasons for limiting the reversible capacity of the lithium-sulfur (Li-S) system. However, as the inevitable intermediate between PSs and Li2 S, lithium disulfide (Li2 S2 ) evolutions are always overlooked. Herein, Li2 S2 evolutions are monitored from the operando measurements on the pouch cell level. Results indicate that Li2 S2 undergoes slow electrochemical reduction and chemical disproportionation simultaneously during the discharging process, leading to further PS dissolution and Li2 S generation without capacity contribution. Compared with the fully oxidized Li2 S, Li2 S2 still residues at the end of the charging state. Therefore, instead of the considered Li2 S and PSs, slow electrochemical conversions and side chemical reactions of Li2 S2 are the determining factors in limiting the sulfur utilization, corresponding to the poor reversible capacity of Li-S batteries.

Lithium Storage Mechanism and Application of Micron-Sized Lattice-Reversible Binary Intermetallic Compounds as High-Performance Flexible Lithium-Ion Battery Anodes.

A strategy of lattice-reversible binary intermetallic compounds of metallic elements is proposed for applications in flexible lithium-ion battery (LIB) anode with high capacity and cycling stability. First, the use of metallic elements can ensure excellent electronic conductivity and high capacity of the active anode substance. Second, binary intermetallic compounds possess a larger initial lattice volume than metallic monomers, so that the problem of volume expansion can be alleviated. Finally, the design of binary intermetallic compounds with lattice reversibility further improves the cycle stability. In this work, the feasibility of this strategy is verified using an indium antimonide (InSb) system. The volumetric expansion and lithium storage mechanism of InSb are investigated by in situ Raman characterization and theoretical calculations. The active material utilization is significantly improved and the growth of In whiskers is inhibited in the micron-sized ball-milled and carbon coated InSb (bInSb@C) anode, which exhibits a reversible capacity of 733.8 mAh g-1 at 0.2 C, and provides a capacity of 411.5 mAh g-1 after 200 cycles at 3 C with an average Coulombic efficiency of 99.95%. This strategy is validated in pouch cells, illustrating the great potential of lattice-reversible binary intermetallic compounds for use as commercial flexible LIB anodes.

Boosting the Oxidative Potential of Polyethylene Glycol-Based Polymer Electrolyte to 4.36 V by Spatially Restricting Hydroxyl Groups for High-Voltage Flexible Lithium-Ion Battery Applications.

Cross-linked polyethylene glycol-based resin (c-PEGR) is constructed by a ring-opening reaction of polyethylene glycol diglycidyl ether (PEGDE) with epoxy groups and polyether amine (PEA) with amino groups. By confining the hydroxyl groups with inferior oxidative stability to the c-PEGR backbone, the oxidation potential of the PEG-based polymer material with reduced reactivity is boosted to 4.36 V. The c-PEGR based gel electrolyte shows excellent flexibility, lithium-ion transport, lithium compatibility, and enhanced oxidation stability, and is successfully applied to a 4.35 V lithium cobaltate (LCO)||lithium (Li) battery system. A quasi-static linear scanning voltammetry (QS-LSV) method is proposed for the first time to accurately measure the oxidation potential and electrochemical stability window of materials with low conductivities such as polymers, which possesses the advantages of high accuracy and short test time. This work provides new insights and research techniques for selecting polymer electrolytes for high-voltage flexible lithium-ion batteries (LIBs).

Efficient polysulfide trapping in lithium-sulfur batteries using ultrathin and flexible BaTiO3/graphene oxide/carbon nanotube layers.

Ultrathin and flexible layers containing BaTiO3 (BTO) nanoparticles, graphene oxide (GO) sheets, and carbon nanotube (CNT) films (BTO/GO@CNT) are used to trap solvated polysulfides and alleviate the shuttle effect in lithium-sulfur (Li-S) batteries. In the functional layers, the CNT films build a conductive framework, and the GO sheets form a support membrane for the uniform dispersion of BTO nanoparticles. BTO nanoparticles without ferroelectricity (nfBTO) can trap polysulfides more effectively by chemical interaction compared to BTO nanoparticles with ferroelectricity (fBTO). A Li-S cell with the nfBTO/GO@CNT functional layer exhibits a reversible capacity of 824.5 mA h g-1 over 100 cycles at 0.2 C. At a high sulfur loading of 5.49 mg cm-2, an electrode with the functional layer shows an areal capacity of 5.15 mA h cm-2 at 0.1 C, demonstrating the nfBTO/GO@CNT functional layer's potential in developing high-performance Li-S batteries.

Ultra-stretchable supercapacitors based on biaxially pre-strained super-aligned carbon nanotube films.

Super-aligned carbon nanotube (SACNT) films with wrinkled structures are prepared by a biaxial pre-strain method and can withstand repetitive stretching of large strains in multiple directions. Ultra-stretchable supercapacitors were fabricated with the SACNT film and active carbon (AC) powders. The initial specific capacitance without strain and with 150% strains in the X, Y and 45° axes was 91, 88, 89 and 90 F g-1, respectively. Moreover, the capacitance retentions were 97%, 98.5% and 98.6% after 2000 tensile cycles at 0-150% strain in the X, Y and 45° axes, respectively, demonstrating the excellent strain durability of the SACNT/AC supercapacitors. The stretchable circuit with the combination of stretchable SACNT/AC supercapacitors and SACNT conductors demonstrates a promising method in developing self-contained stretchable functional devices for a variety of applications. The low-cost and scalable biaxial pre-strain process presents a potential route for designing high performance stretchable electronic and energy storage devices.

Ultrastretchable carbon nanotube composite electrodes for flexible lithium-ion batteries.

Ultra-stretchable carbon nanotube (CNT) composite electrodes for lithium-ion batteries are fabricated by coating CNT films and active material powders on biaxially pre-strained polydimethylsiloxane (PDMS) substrates. The wrinkled structures that form during the pre-straining and release process extend along the strain axis to protect the CNT composite structures from fracture. The CNT composites demonstrate excellent stability and high durability with resistance increase of less than 12% after 2000 cycles at 150% strain. Both CNT/Li4Ti5O12 (LTO) anodes and CNT/Li(Ni1/3Co1/3Mn1/3)O2 (NCM) cathodes maintain excellent electrochemical properties at cyclic 150% strain in different axes. The full lithium-ion battery consisting of the stretchable CNT/LTO anode and CNT/NCM cathode is able to withstand 150% strain in different axes without large decreases in performance. Stretchable batteries fabricated by the scalable, highly efficient, and low-cost biaxial pre-strain process with excellent durability and electrochemical properties will have potential applications in flexible devices.

TiO2-Nanocoated Black Phosphorus Electrodes with Improved Electrochemical Performance.

Black phosphorus (BP) is a promising electrode material with high energy density for lithium-ion batteries. However, volumetric expansion of BP upon lithiation leads to rapid capacity fading of the electrode. Herein, BP composite electrodes are prepared by mixing microsized BP particles with carbon nanotubes and KetjenBlack as dual conducting agents, which facilitate the construction of stable and conductive networks in the electrodes. An ultrathin TiO2 nanocoating is deposited on the surface of the BP composite electrode by electron-beam evaporation. The TiO2 nanocoating acts as a protective layer to prevent the BP particles from directly contacting the electrolyte by forming a Li xTi yO z passivation coating on the electrode surface. The Li xTi yO z passivation layer suppresses propagation of the formed irreversible solid electrolyte interlayer on the BP particles, resulting in an improved Coulombic efficiency of the BP electrode. Moreover, the Li xTi yO z passivation layer facilitates lithium-ion diffusion and electron transfer and thus superior cycling and rate performance of the BP electrode are achieved.

Multifunctional Interlayer Based on Molybdenum Diphosphide Catalyst and Carbon Nanotube Film for Lithium-Sulfur Batteries.

A multifunctional interlayer, composed of molybdenum diphosphide (MoP2 ) nanoparticles and a carbon nanotube (CNT) film, is introduced into a lithium-sulfur (Li-S) battery system to suppress polysulfide migration. Molybdenum diphosphide acts as the catalyst and can capture polysulfides and improve the polysulfide conversion activity during the discharge/charge processes. The CNT film acts as a conductive skeleton to support the MoP2 nanoparticles and to ensure their uniform distribution. The CNT film physically hinders polysulfide migration, acts as a current collector, and provides abundant electron pathways. The Li-S battery containing the multifunctional MoP2 /CNT interlayer exhibits excellent electrochemical performance. It delivers a reversible specific capacity of 905 mA h g-1 over 100 cycles at 0.2 C, with a capacity decay of 0.152% per cycle. These results suggest the introduction of the multifunctional CNT/MoP2 interlayer as an effective and practical method for producing high-performance Li-S batteries.

Self-Expansion Construction of Ultralight Carbon Nanotube Aerogels with a 3D and Hierarchical Cellular Structure.

A novel and simple strategy is developed to construct ultralight and 3D pure carbon nanotube (CNT) aerogels by the spontaneous expansion of superaligned CNT films soaked in a piranha (mixed H2 SO4 and H2 O2 ) solution, followed by cryodesiccation. The macroscopic CNT aerogels have an extremely low apparent density (0.12 mg cm-3 ), ultrahigh porosity (99.95%), high specific surface area (298 m2 g-1 ), and a hierarchical cellular structure with giant and ultrathin CNT sheets as cell walls. The pure CNT aerogels show high adsorption abilities for various kinds of solvents, and have great potential in widespread applications such as energy storage, catalysis, and bioengineering.

Self-assembly of 3D Carbon Nanotube Sponges: A Simple and Controllable Way to Build Macroscopic and Ultralight Porous Architectures.

Macroscopic and 3D superaligned CNT (SACNT) sponges are fabricated through a simple, low-cost, controllable, and scalable self-assembly method without using organic binder. Sponges with specific shapes and densities can be achieved. SACNT sponges are ultralight (1-50 mg cm-3 ), highly porous (97.5%-99.9%) with honeycomb-like hierarchical structure, and highly conductive. Using SACNT sponges as templates, various materials with honeycomb-like structure can be obtained for wide applications.

Cross-stacked carbon nanotube film as an additional built-in current collector and adsorption layer for high-performance lithium sulfur batteries.

Cross-stacked carbon nanotube (CNT) film is proposed as an additional built-in current collector and adsorption layer in sulfur cathodes for advanced lithium sulfur (Li-S) batteries. On one hand, the CNT film with high conductivity, microstructural rough surface, high flexibility and mechanical durability retains stable and direct electronic contact with the sulfur cathode materials, therefore decreasing internal resistivity and suppressing polarization of the cathode. On the other hand, the highly porous structure and the high surface area of the CNT film provide abundant adsorption points to support and confine sulfur cathode materials, alleviate their aggregation and promote high sulfur utilization. Moreover, the lightweight and compact structure of the CNT film adds no extra weight or volume to the sulfur cathode, benefitting the improvement of energy densities. Based on these characteristics, the sulfur cathode with a 100-layer cross-stacked CNT film presents excellent rate performances with capacities of 986, 922 and 874 mAh g(-1) at cycling rates of 0.2C, 0.5C and 1C for sulfur loading of 60 wt%, corresponding to an improvement of 52%, 109% and 146% compared to that without a CNT film. Promising cycling performances are also demonstrated, offering great potential for scaled-up production of sulfur cathodes for Li-S batteries.

Mesoporous Li4Ti5O12 nanoclusters anchored on super-aligned carbon nanotubes as high performance electrodes for lithium ion batteries.

Mesoporous lithium titanate (LTO) nanoclusters are in situ synthesized in a network of super aligned carbon nanotubes (SACNTs) via a solution-based method followed by heat treatment in air. In the LTO-CNT composite, SACNTs not only serve as the skeleton to support a binder-free electrode, but also render the composite with high conductivity, flexibility, and mechanical strength. The homogeneously dispersed LTO nanoclusters among the SACNTs allow each LTO grain to effectively access the electrolyte and the conductive network, benefiting both ion and electron transport. By the incorporation of LTO into the CNT network, mechanical reinforcement is also achieved. When serving as a negative electrode for lithium ion batteries, such a robust composite-network architecture provides the electrodes with effective charge transport and structural integrity, leading to high-performance flexible electrodes with high capacity, high rate capability, and excellent cycling stability.