Hongda Du

Facile synthesis of Li4Ti5O12/C composite with super rate performance

The Li4Ti5O12/C composite with lump morphology and excellent rate performance are synthesized using a facile hydrothermal method followed by a low temperature heat treatment. In the hydrothermal process, the introduction of cetyltrimethylammonium bromide (CTAB) as a surfactant significantly improves the rate performance of Li4Ti5O12/C composite as anode material for lithium ion battery (LIB). The specific capacities of the obtained composite at charge and discharge rates of 0.1, 1, 5, 10 and 20 C are 176, 163, 156, 151 and 136 mA h g−1, respectively, which is apparently larger than those of the Li4Ti5O12/C free from CTAB in the preparation. The Li4Ti5O12/C prepared in presence of CTAB also shows excellent cycling performance at high rate, which is attributed to its larger diffusion coefficient of lithium ion (6.82 × 10−12 cm2 s−1) and smaller charge-transfer resistance (Rct) (19.2 Ω) than those of the composite (1.22 × 10−13 cm2 s−1 and 50.2 Ω) free from CTAB in the preparation. The Li4Ti5O12 particles obtained in presence of CTAB are coated uniformly by a thin carbon layer with a thickness of ∼1 nm, whereas the Li4Ti5O12 particles obtained in absence of CTAB are covered by relatively thick surface layers with a thickness of ∼2.5 nm, which is too thick, blocks the lithium ion diffusion and leads to low ionic conductivity.

Recent progress on manganese dioxide based supercapacitors

The increasing worldwide interest in MnO 2 for supercapacitor applications is based on anticipation that MnO 2 -based high-voltage aqueous supercapacitors will ultimately serve as a safe and low-cost alternative to state-of-the-art commercial organic-based electrochemical double-layer capacitors or RuO 2 -based acid systems. In this paper, the physicochemical features, synthesis methods, and charge storage mechanism of MnO 2 as well as the current status of MnO 2 -based supercapacitors are summarized and discussed in detail. The future opportunities and challenges related to MnO 2 -based supercapacitors have also been proposed.

Gassing in Li 4 Ti 5 O 12 -based batteries and its remedy

Destructive gas generation with associated swelling has been a major challenge to the large-scale application of lithium ion batteries (LIBs) made from Li4Ti5O12 (LTO) anodes. Here we report root causes of the gassing behavior, and suggest remedy to suppress it. The generated gases mainly contain H2, CO2 and CO, which originate from interfacial reactions between LTO and surrounding alkyl carbonate solvents. The reactions occur at the very thin outermost surface of LTO (111) plane, which result in transformation from (111) to (222) plane and formation of (101) plane of anatase TiO2. A nanoscale carbon coating along with a stable solid electrolyte interface (SEI) film around LTO is seen most effective as a barrier layer in suppressing the interfacial reaction and resulting gassing from the LTO surface. Such an ability to tune the interface nanostructure of electrodes has practical implications in the design of next-generation high power LIBs.

Asymmetric Activated Carbon-Manganese Dioxide Capacitors in Mild Aqueous Electrolytes Containing Alkaline-Earth Cations

Manganese dioxide exhibits the ideal capacitive behavior in aqueous electrolytes containing alkaline-earth cations (Mg 2+ , Ca 2+ , or Ba 2+ ). A specific capacitance value as high as 325 F g -1 was obtained in 0.1 mol L -1 Mg(NO 3 ) 2 electrolyte at 2 mV s -1 . The ideal capacitive behavior, high specific capacitance, good coulombic efficiency, and rate ability of manganese dioxides in aqueous electrolytes containing bivalent cations (Mg 2+ , Ca 2+ , or Ba 2+ ) indicate that these alkaline-earth cations might be good alternatives for the state-of-the-art univalent alkaline cations. Moreover, activated carbon/MnO 2 asymmetric capacitors with 2 V operating voltage were built up based on the aqueous electrolytes containing these alkaline-earth cations. The energy density of the AC/MnO 2 asymmetric capacitor with Ca 2+ cation at a current density of 0.3 A g -1 was found to be 21 Wh Kg -1 .

Structure and Electrochemical Properties of Zn-Doped Li4Ti5O12 as Anode Materials in Li-Ion Battery

Zinc-doped lithium titanates with the formula of Li 4-x Zn x Ti 5 O 12 (x = 0, 0.25, 0.5, 1) were synthesized as anode materials via a solid-state reaction. The phase composition does not change by Zn doping when x < 1 but results in the increase in the relative peak intensity ratio of I (311) /I (111) and I (400) /I (111) in X-ray diffraction. The conductivity is improved by Zn doping via the generation of mixing Ti 4+ /Ti 3+ . The specific capacity at 0.5C decreases by Zn doping, which is ascribed to the location of Zn 2+ at the 8a site; however, the capacity retention is improved remarkably compared to that of the undoped one when discharged at a high rate.

Electrospun core–shell silicon/carbon fibers with an internal honeycomb-like conductive carbon framework as an anode for lithium ion batteries

Core–shell silicon/carbon (Si/C) fibers with an internal honeycomb-like carbon framework are prepared based on the coaxial electrospinning technique. For this hierarchical structure, the fibers core is composed of a porous carbon framework and embedded Si nanoparticles, which is further wrapped by a compact carbon shell. The well-defined Si/C composite anode shows high specific capacities, good capacity retention, and high accessibility of Si in lithium-ion batteries. An initial reversible capacity of 997 mA h g−1 and a capacity retention of 71% after 150 cycles are demonstrated with a current density of 0.2 A g−1. At a higher current density of 0.5 A g−1, a reversible capacity of 603 mA h g−1 can be maintained after 300 cycles. The accessibility of Si in the Si/C anode is up to 3612 mA h g−1 in the 1st cycle. The excellent electrochemical properties are attributed to the hierarchical structure of Si/C fibers. The porous carbon framework in the core region could not only accommodate the volume expansion of Si, but also enhance the conductivity inside these fibers. The compact carbon shell is able to prevent the electrolyte from permeating into the core section, therefore a stable solid-electrolyte interphase can be formed on the fiber surface.

Conductive graphene-based macroscopic membrane self-assembled at a liquid–air interface

Free-standing graphene-based macroscopic membranes, which are characterized by a layered structure and tunable conductivity, are prepared by a self-assembly process at a liquid–air interface. Since the preliminary results indicate that it is hard to construct macroscopic graphene membranes solely by low-temperature exfoliated graphene nanosheets (LGNs) at the liquid–air interface, graphene oxide nanosheets (GONs), as the stacking template and sticking component, are introduced into the assembly process of graphene layers to promote the formation of layer-by-layer stacking structure and help form a conductive macroscopic membrane. The conductivity of such a graphene-based membrane can be tuned by changing the GON fraction in the LGN/GON hybrid membrane.

Highly Crystalline Lithium Titanium Oxide Sheets Coated with Nitrogen‐Doped Carbon enable High‐Rate Lithium‐Ion Batteries

Sheets of Li4Ti5O12 with high crystallinity are coated with nitrogen-doped carbon (NC-LTO) using a controlled process, comprising hydrothermal reaction followed by chemical vapor deposition (CVD). Acetonitrile (CH3 CN) vapor is used as carbon and nitrogen source to obtain a thin coating layer of nitrogen-doped carbon. The layer enables the NC-LTO material to maintain its sheet structure during the high-temperature CVD process and to achieve high crystallinity. Doping with nitrogen introduces defects into the carbon coating layer, and this increased degree of disorder allows fast transportation of lithium ions in the layer. An electrode of NC-LTO synthesized at 700 °C exhibits greatly improved rate and cycling performance due to a markedly decreased total cell resistance and enhanced Li-ion diffusion coefficient (D(Li)). Specific capacities of 159.2 and 145.8 mA h g(-1) are obtained using the NC-LTO sheets, at charge/discharge rates of 1 and 10 C, respectively. These values are much higher than values for LTO particles did not undergo the acetonitrile CVD treatment. A capacity retention value as high as 94.7% is achieved for the NC-LTO sheets after 400 cycles in a half-cell at 5 C discharge rate.

Reversible Insertion Properties of Zinc Ion into Manganese Dioxide and Its Application for Energy Storage

The reversible intercalation of Zn 2+ ions into manganese dioxide was first reported in an aqueous system and a large capacity (210 mAh g -1 ) was measured. A cycle life test was performed at 1 A g -1 , and after 50 cycles, no capacity fading was found, which indicates the good cycling properties of manganese dioxide toward the insertion of zinc ions. X-ray photoelectron spectrum measurements indicated that Zn 2+ ions in the electrolyte are involved in the charge storage process of manganese dioxide. X-ray diffraction results exhibited the kinetic stability of the host structure in allowing high-capacity, single-phase, and reversible zinc-ion intercalation.

Investigation of cyano resin-based gel polymer electrolyte: in situ gelation mechanism and electrode–electrolyte interfacial fabrication in lithium-ion battery

Cyanoethyl polyvinyl alcohol (PVA-CN)-based gel polymer electrolyte (GPE) is a high-performance electrolyte for lithium-ion batteries (LIBs), which is in situ synthesized from a stable monomer without using additional initiators. Unfortunately, the gelation mechanism of PVA-CN is still unclear. Furthermore, for general GPEs prepared by in situ polymerization, the electrode–GPE interface in batteries remains to be further optimized. Here we present the gelation mechanism of the PVA-CN-based GPE and fabricate an electrode–GPE interface with less resistance during battery formation. The cross-linkable PVA-CN-based organogel is formed via in situ cationic polymerization of the cyano resin initiated by PF5, a strong Lewis acid produced by the thermo-decomposition of LiPF6. It is interesting to find that the battery formation process completed in the precursor solution instead of gel can greatly reduce the interfacial resistance of graphite–GPE and benefit the formation of a more stable solid electrolyte interface (SEI) on the anode, which contributes to a dramatic improvement in battery performance. This work gives useful guidance towards designing new GPE materials and promoting their practical application in LIBs.