Yongfei Liu

Electrode activation via vesiculation: improved reversible capacity of γ-Fe2O3@C/MWNT composite anodes for lithium-ion batteries

Capacity fading caused by pulverization is the basic issue for transition-metal-oxide anodes in lithium-ion batteries (LIBs). Here we report a simple and scalable fabrication of core–shell structured γ-Fe2O3@C nanoparticle based composites incorporated with multi-walled carbon nanotubes (MWNTs), through a vacuum-carbonization of the synthesized metal–organic complex and MWNT hybrids. In the constructed γ-Fe2O3@C/MWNT architecture, the carbon shell layers can not only buffer the volume change of γ-Fe2O3 nanoparticles but also improve their conductivity; while the flexible and conductive MWNT networks can maintain the structural and electrical integrity of the electrodes during the charge/discharge cycles. As a result, such γ-Fe2O3@C/MWNT electrodes, tested as anodes for LIBs, exhibit excellent cycling performance with monotonically increased reversible capacities along with cycles. For instance, the specific capacity rises at a rate of ∼6.8 mA h g−1 per cycle to 1139 mA h g−1 after 60 cycles at the current density of 100 mA g−1. Such electrode activation was revealed to be closely related to the increased active surface area of the electrode arising from the gradual vesiculation in Fe2O3@C nanoparticles during lithiation/delithiation in the as-prepared robust γ-Fe2O3@C/MWNT architecture.

Large enhancement of anisotropic magnetoresistance and thermal stability in Ta/NiFe/Ta trilayers with interfacial Pt addition

Ta/NiFe/Ta trilayers, extensively used for anisotropic magnetoresistance (AMR) sensors, exhibit severely reduced MR ratio at small NiFe thickness and appreciable moment loss, especially after annealing. By inserting ultrathin Pt layers at the interfaces of the trilayers, AMR can be significantly enhanced for thin NiFe films due to the strong electron spin-orbit scattering at Pt/NiFe interfaces along with suppression of interfacial magnetic dead layers. Furthermore, the Pt layers also reduce Ta and NiFe interdiffusion and result in negligible moment loss and AMR degradation after annealing at 350 °C.

Effect of niobium doping on the microstructure and electrochemical properties of lithium-rich layered Li[Li0.2Ni0.2Mn0.6]O2 as cathode materials for lithium ion batteries

Niobium-doped lithium-rich layered cathode materials, Li[Li0.2Ni0.2Mn0.6−xNbx]O2 (x = 0, 0.02, 0.04, and 0.06), were prepared and the effects of Nb doping on the microstructure and electrochemical properties were investigated. Upon Nb doping, the layered α-NaFeO2 structure is maintained but with an expanded interlayer spacing and the electrochemical properties are significantly enhanced. In particular, the sample with x = 0.04 delivers a large reversible discharge capacity of 254 mA h g−1 at 0.1 C rate with a high capacity retention rate of 92.3% after 100 cycles. Furthermore, it delivers 198 mA h g−1 at 1 C rate, much larger than that of the undoped sample (125 mA h g−1). Capacity differential results reveal that strong Nb–O bond can stabilize the material structure and thus lead to a stable cycling performance. Electrochemical impedance spectroscopy (EIS) analysis shows that Nb doping can decrease the whole cell impedance and expand the Li+ diffusion path in the lithium-rich layered cathode materials, resulting in the excellent rate capability.

Transport properties and enhanced thermoelectric performance of aluminum doped Cu3SbSe4

The electrical transport and thermoelectric properties of Cu3Sb1−xAlxSe4 (x = 0, 0.01, 0.02 and 0.03) compounds are investigated in the temperature range of 300–600 K. The results indicate that with increasing Al content from x = 0 to x = 0.03, hole concentration increases monotonically from 8.04 × 1017 to 1.19 × 1019 cm−3 due to the substitution of Al3+ for Sb5+, thus leading to a large decrease in the electrical resistivity of Cu3Sb1−xAlxSe4. Meanwhile, the increase in hole concentration leads to a transition from a non-degenerate (x = 0) to a partial degenerate (x = 0.01, 0.02) and then to a degenerate state (x = 0.03). The power factor (PF) of all the Al-doped Cu3Sb1−xAlxSe4 samples is remarkably improved due to the optimization of hole concentration. Lattice thermal conductivity κL of the heavily doped sample (x = 0.03) is reduced. As a result, a large thermoelectric figure of merit ZT = 0.58 is obtained for Cu3Sb0.97Al0.03Se4 at 600 K, which is around 1.9 times as large as that of the un-doped Cu3SbSe4.

Enhanced thermoelectric performance through carrier scattering at heterojunction potentials in BiSbTe based composites with Cu3SbSe4 nanoinclusions

Thermoelectric materials with the thermoelectric figure of merit, ZT, being much larger than unit at near room temperature are vital for power generation by using low-grade waste heat. Here we show that by incorporating very small proportion (1 vol%) of Cu3SbSe4 nanoparticles into the BiSbTe matrix to form nanocomposites, besides large (∼50%) reduction of lattice thermal conductivity, both enhanced thermopower through energy-dependent scattering and alleviated reduction of carrier mobility via carrier scattering at heterojunction potentials occur at elevated temperatures, which allow the thermoelectric power factor of the composite material to reach ∼37 μW cm−1 K−2 at 467 K. Consequently, a largest value of ZT = 1.6 is achieved at 476 K. Moreover, it has excellent performance in a broad temperature range (say, ZT = 1.0 at 300 K and ZT = 1.5 at 500 K), which makes this material attractive for cooling and power generation.

Enhanced thermoelectric figure of merit in p-type β-Zn4Sb3/Bi0.4Sb1.6Te3 nanocomposites

The thermoelectric properties of Bi0.4Sb1.6Te3-based composites incorporated with β-Zn4Sb3 nanoparticles are investigated in the temperature range from 300 K to 500 K. The results show that ∼5% increase in Seebeck coefficient and ∼32% reduction of lattice thermal conductivity at 443 K are concurrently realized in the nanocomposite system with 1.3 vol% of β-Zn4Sb3, which originates from energy filtering effect as well as enhanced phonon scattering at dispersed nanoparticles and phase boundaries, respectively. As a result, the largest figure of merit ZT = 1.43 is achieved at 443 K for the sample with 1.3 vol% of β-Zn4Sb3 nanoinclusions, which is ∼18% larger than that (=1.21) of the Bi0.4Sb1.6Te3 matrix.

Graphene modified Li-rich cathode material Li[Li0.26Ni0.07Co0.07Mn0.56]O2 for lithium ion battery

Lithium and Mn rich solid solution materials Li[Li0.26Ni0.07Co0.07Mn0.56]O2 were synthesized by a carbonate co-precipitation method and modified with a layer of graphene. The graphene-modified cathodes exhibit improved rate capability and cycling performance as compared to the bare cathodes. Electrochemical impedance spectroscopy (EIS) analyses reveal that the improved electrochemical performances are due to acceleration kinetics of lithium-ion diffusion and the charge transfer reaction of the graphene-modified cathodes.

Enhanced thermoelectric performance of CuGaTe2 based composites incorporated with graphite nanosheets

CuGaTe2 based composites incorporated with graphite nanosheets (GNs) CuGaTe2/x G (G = GNs, 0 ≤ x ≤ 3.04 vol. %) were prepared, and the thermoelectric properties of the composites were studied from 300 to 875 K. The results show that the incorporation of GNs into the CuGaTe2 matrix can enhance the Seebeck coefficient and power factor over the whole temperature range investigated due to energy filtering effects, and the reduction of thermal conductivity below 750 K owing to interface scattering. Although the resistivity increases, energy filtering significantly raises the Seebeck component, and the overall effect on power factor is positive. The sample with 2.28 vol. % GNs had the largest ZT value, reaching 0.93 at 873 K, which is a ∼21% improvement on pure CuGaTe2.

Optimized thermoelectric properties of AgSbTe2 through adjustment of fabrication parameters

AgSbTe2 bulk sample is obtained by hot-pressing under different fabrication parameters, and their thermoelectric properties are investigated in the temperature range of 300 - 550 K. The highest ZT = 0.86 is achieved at 475 K for the sample hot-pressed at 423 K and 500MPa due to the lower thermal conductivity and higher power factor. The results indicate that the optimized thermoelectric properties can be obtained for AgSbTe2 compound at the sintering temperature of 423 K under the pressure of 500 MPa.

Fabrication and thermoelectric properties of n-type (Sr0.9Gd0.1)TiO3 oxides

The n-type oxides (Sr0.9Gd0.1)TiO3 (SGTO) have been successfully prepared via a sol–gel process followed by solid-state sintering. The effects of sintering temperature on the thermoelectric (TE) properties of the SGTO samples have been investigated. The Seebeck coefficient showed no obvious difference, while the electrical conductivity increased with increasing sintering temperature, benefiting from an enhancement of densification. The maximum power factor (PF) value, ~ 20.5μW/K2cm at 370 K in the metallic region, was observed for the sample sintered at 1748 K. As a result, the peak figure of merit (ZT) values for the samples sintered at higher than 1673 K were in the range of 0.28–0.30. All the results indicate that such synthetic method provides a simple and effective way to prepare TE oxides.