Meinan Liu

High-Rate, Ultralong Cycle-Life Lithium/Sulfur Batteries Enabled by Nitrogen-Doped Graphene

Nitrogen-doped graphene (NG) is a promising conductive matrix material for fabricating high-performance Li/S batteries. Here we report a simple, low-cost, and scalable method to prepare an additive-free nanocomposite cathode in which sulfur nanoparticles are wrapped inside the NG sheets (S@NG). We show that the Li/S@NG can deliver high specific discharge capacities at high rates, that is, ∼ 1167 mAh g(-1) at 0.2 C, ∼ 1058 mAh g(-1) at 0.5 C, ∼ 971 mAh g(-1) at 1 C, ∼ 802 mAh g(-1) at 2 C, and ∼ 606 mAh g(-1) at 5 C. The cells also demonstrate an ultralong cycle life exceeding 2000 cycles and an extremely low capacity-decay rate (0.028% per cycle), which is among the best performance demonstrated so far for Li/S cells. Furthermore, the S@NG cathode can be cycled with an excellent Coulombic efficiency of above 97% after 2000 cycles. With a high active S content (60%) in the total electrode weight, the S@NG cathode could provide a specific energy that is competitive to the state-of-the-art Li-ion cells even after 2000 cycles. The X-ray spectroscopic analysis and ab initio calculation results indicate that the excellent performance can be attributed to the well-restored C-C lattice and the unique lithium polysulfide binding capability of the N functional groups in the NG sheets. The results indicate that the S@NG nanocomposite based Li/S cells have a great potential to replace the current Li-ion batteries.

Chemical routes toward long-lasting lithium/sulfur cells

Lithium/sulfur (Li/S) cells have great potential to become mainstream secondary batteries due to their ultra-high theoretical specific energy. The major challenge for Li/S cells is the unstable cycling performance caused by the sulfur’s insulating nature and the high-solubility of the intermediate polysulfide products. Several years of efforts to develop various fancy carbon nanostructures, trying to physically encapsulate the polysulfides, did not yet push the cell’s cycle life long enough to compete with current Li ion cells. The focus of this review is on the recent progress in chemical bonding strategy for trapping polysulfides through employing functional groups and additives in carbon matrix. Research results on understanding the working mechanism of chemical interaction between polysulfides and functional groups (e.g. O–, B–, N–and S–) in carbon matrix, metal-based additives, or polymer additives during charge/discharge are discussed.

Polyaniline-modified cetyltrimethylammonium bromide-graphene oxide-sulfur nanocomposites with enhanced performance for lithium-sulfur batteries

Conductive polymer coatings can boost the power storage capacity of lithium-sulfur batteries. We report here on the design and preparation-by combining a facile and green chemical deposition method with an oxidative polymerization approach-of polyaniline (PANI)-modified cetyltrimethylammonium bromide (CTAB)-graphene oxide (GO)-sulfur (S) nanocomposites with significantly enhanced performance in lithium-sulfur batteries. Such conductive polymer modified CTAB-GO-S nanocomposites as sulfur cathode materials can deliver high specific discharge capacities and long-term cycling performance, i.e., ∼970 mAh·g−1 at 0.2 C and ∼715 mAh·g−1 after 300 cycles, ∼820 mAh·g−1 at 0.5 C and ∼670 mAh·g−1 after 500 cycles, ∼770 mAh·g−1 at 1 C and ∼570 mAh·g−1 after 500 cycles. The capacity decay was as low as 0.036% per cycle at 0.5 C, and 0.051% per cycle at 1 C. Under the same condition, batteries using PANI-modified CTAB-GO-S as cathodes exhibited higher specific capacity and higher average coulombic efficiency compared with CTAB-decorated GO-S and GO-S nanocomposites. The improved performance can be attributed to the lower charge transfer resistance and the alleviated dissolution of polysulfides in the PANImodified CTAB-GO-S cathodes.

A high energy density Li2S@C nanocomposite cathode with a nitrogen-doped carbon nanotube top current collector

Lithium sulfide (Li2S), with a high theoretical capacity of 1166 mA h g−1, is considered as one of the most promising cathode materials for the next-generation lithium-ion batteries. In this work, a novel cell configuration with a top current collector was designed for Li2S based batteries. The nitrogen-doped carbon nanotube (N-CNT) film was applied on top of the cathode, which serves not only as a top current collector but also as a barrier layer to effectively impede the polysulfide diffusion and enhance the utilization of active materials. A sheet-like Li2S@C nanocomposite was synthesized from low-cost and environmentally friendly raw materials of lithium sulfate (Li2SO4) and activated graphite. The as-prepared Li2S@C composites were directly used as cathode materials without adding any binder or carbon additive, which enabled a high Li2S loading up to 68% in the total cathode weight. The cells exhibited superior electrochemical performance. The specific energy at 0.5C was 804 W h kg−1 based on the total electrode weight including the N-CNT top current collector, which is among the highest values demonstrated so far for sulfur and Li2S cathodes.

Synthesis, Crystal Structure, and Electrochemical Properties of a Simple Magnesium Electrolyte for Magnesium/Sulfur Batteries

Most simple magnesium salts tend to passivate the Mg metal surface too quickly to function as electrolytes for Mg batteries. In the present work, an electroactive salt [Mg(THF)6 ][AlCl4 ]2 was synthesized and structurally characterized. The Mg electrolyte based on this simple mononuclear salt showed a high Mg cycling efficiency, good anodic stability (2.5 V vs. Mg), and high ionic conductivity (8.5 mS cm(-1) ). Magnesium/sulfur cells employing the as-prepared electrolyte exhibited good cycling performance over 20 cycles in the range of 0.3-2.6 V, thus indicating an electrochemically reversible conversion of S to MgS without severe passivation of the Mg metal electrode surface.

Liquid‐Phase Electrochemical Scanning Electron Microscopy for In Situ Investigation of Lithium Dendrite Growth and Dissolution

An in situ electrochemical scanning electronic microscopy method is developed to systematically study the lithium plating/stripping processes in liquid electrolytes. The results demonstrate that the lithium dendrite growth speed and mechanism is greatly affected by the additives in the ether-based electrolyte.

All-Solid-State High-Energy Asymmetric Supercapacitors Enabled by Three-Dimensional Mixed-Valent MnOx Nanospike and Graphene Electrodes

Three-dimensional (3D) nanostructures enable high-energy storage devices. Here we report a 3D manganese oxide nanospike (NSP) array electrode fabricated by anodization and subsequent electrodeposition. All-solid-state asymmetric supercapacitors were assembled with the 3D Al@Ni@MnOx NSP as the positive electrode, chemically converted graphene (CCG) as the negative electrode, and Na2SO4/poly(vinyl alcohol) (PVA) as the polymer gel electrolyte. Taking advantage of the different potential windows of Al@Ni@MnOx NSP and CCG electrodes, the asymmetric supercapacitor showed an ideal capacitive behavior with a cell voltage up to 1.8 V, capable of lighting up a red LED indicator (nominal voltage of 1.8 V). The device could deliver an energy density of 23.02 W h kg(-1) at a current density of 1 A g(-1). It could also preserve 96.3% of its initial capacitance at a current density of 2 A g(-1) after 10000 charging/discharging cycles. The remarkable performance is attributed to the unique 3D NSP array structure that could play an important role in increasing the effective electrode surface area, facilitating electrolyte permeation, and shortening the electron pathway in the active materials.

Synthesis of three-dimensional hyperbranched TiO2 nanowire arrays with significantly enhanced photoelectrochemical hydrogen production

The three-dimensional (3D) hierarchical nanostructure is one of the promising candidates for high performance photoelectrochemical (PEC) water splitting electrodes due to the reduced carrier diffusion distance, improved light absorption efficiency and charge collection efficiency. Here, by growing omnidirectional, densely packed branches on TiO2 nanowires, we demonstrated a 3D hyperbranched hierarchical TiO2 nanowire (HHNW) architecture that could significantly enhance the performance of PEC water splitting. Under a solar simulator with chopped AM 1.5G light of 100 mW cm−2 intensity, the HHNW electrode yielded a photocurrent density of 1.21 mA cm−2 at 1.23 V with respect to the reversible hydrogen electrode (RHE), which was about four times higher than that of TiO2 nanowires (NWs) (0.34 mA cm−2). The highest incident photon-to-current conversion efficiency (IPCE) obtained from our HHNWs was 77% at 365–425 nm. This greatly improved PEC performance can be attributed to the improved light absorption efficiency and the increased contact surface areas at the TiO2/electrolyte interface.

Interfacial Energy-Level Alignment for High-Performance All-Inorganic Perovskite CsPbBr3 Quantum Dot-Based Inverted Light-Emitting Diodes

All-inorganic perovskite light-emitting diode (PeLED) has a high stability in ambient atmosphere, but it is a big challenge to achieve high performance of the device. Basically, device design, control of energy-level alignment, and reducing the energy barrier between adjacent layers in the architecture of PeLED are important factors to achieve high efficiency. In this study, we report a CsPbBr3-based PeLED with an inverted architecture using lithium-doped TiO2 nanoparticles as the electron transport layer (ETL). The optimal lithium doping balances the charge carrier injection between the hole transport layer and ETL, leading to superior device performance. The device exhibits a current efficiency of 3 cd A-1, a luminance efficiency of 2210 cd m-2, and a low turn-on voltage of 2.3 V. The turn-on voltage is one of the lowest values among reported CsPbBr3-based PeLEDs. A 7-fold increase in device efficiencies has been obtained for lithium-doped TiO2 compared to that for undoped TiO2-based devices.