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Abstract: Birnessite-type MnO2 (Bir-MnO2) nanospheres have been successfully synthesized by the microwave–hydrothermal (M–H) method at 75°C for 30min under low pressure. The properties and electrochemical performance of the as-prepared MnO2 are analyzed and evaluated by the field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) measurements and electrochemical tests. The Bir-MnO2 appears mesoporous nanosphere structure with 70–90nm in diameter, and it exhibits a large specific surface (SS) of 213.6m2 g−1 by the results of FE-SEM, XRD and BET. The electrochemical test results show that the specific capacitance (SC) is 210Fg−1 at 200mAg−1 in 1.0M Na2SO4 electrolyte, and the SC retention and coulombic efficiency are over 96% and 98% respectively after 300 cycles at 1.6Ag−1. Compared with the conventional syntheses of MnO2, the performance of the Bir-MnO2 nanospheres synthesized by M–H method is significantly improved. [Copyright &y& Elsevier]

Abstract: Solid solution cathode material 0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2 has been synthesized by a co-precipitation method and a mild acid was adopted to give rise to the H+/Li+ exchange reaction. The inductively coupled plasma-atomic emission spectrometry (ICP-AES) and atomic absorption spectroscopy (AAS) data show that the H+/Li+ exchange reaction actually occurs and the chemical composition is H0.06Li1.15Ni0.13Co0.14Mn0.55O2.03 after the material was treated. The X-ray powder diffraction patterns indicates that the structure doesn't change through the H+/Li+ exchange reaction and remains the hexagonal α-NaFeO2 layered structure with space group of R-3m. The field-emission scanning electron microscope (SEM) and transmission electron microscope (TEM) images show that there are traces of erosion on the surface of the H+/Li+ exchanged sample. The initial charge–discharge curve measured at 0.05C (12.5 mA g−1) demonstrates that the H+/Li+ exchanged electrode delivers a capacity of up to 314.0 mAh g−1 and coulombic increased initial efficiency. Cycle voltammetry (CV) measurement confirms this is attributed to the improvement of the reduction catalytic activity of oxygen released during the initial charging. The processed electrode also displays improved rate performance. [Copyright &y& Elsevier]

Abstract: Understanding the formation of SEI films on Li4Ti5O12 (LTO) anodes offers a major benefit to large-scale applications of lithium ion batteries made therefrom. This paper reveals that an SEI film is formed above 1 V due to the interfacial reaction between the electrode and electrolyte: LTO anodes are previously considered free from SEI films when cycled between 1 and 3 V. The reactivity and the formation of SEI films are much affected by the morphology and surface area of the electrode. To study the above, LTO powders with different morphologies are synthesized using lithium acetate (LA) and lithium hydroxide (LH) as the lithium sources. LTO–LH consisting of agglomerates of primary small particles with a large surface area has higher reactivity than LTO–LA with a cubic structure and small surface area. As a result, the LTO–LH anode with a smooth SEI film offers better cyclic performance than the LTO–LA anode with a porous SEI film. The addition of vinylene carbonate to the electrolyte facilitates rapid formation of a protective SEI film on LTO–LA, greatly improving the rate and cyclic performance: stable specific capacity of 155.6 mAh g−1 and remarkable 135.2 mAh g−1 after 500 cycles at 10 C are recorded. [Copyright &y& Elsevier]

Abstract: This paper demonstrates an asymmetric micro supercapacitor with superior overall performance that combines the advantages of both MnO2 positive electrode and carbon negative electrode for energy storage on a chip. Nano-structured MnO2 with rough surface is synthesized as active material for micro electrode. A self-supporting composite containing MnO2 and another containing nanoporous activated carbon (AC) are separated in an interdigital structure by using micro-electro-mechanical systems (MEMS) fabrication technologies. Measurements of the prototype show that the asymmetric micro supercapacitor has well-performed capacitive behavior, and its working voltage range is extended from 1 V to 1.5 V in aqueous electrolyte due to the MnO2–AC configuration. Calculations prove that it stores much higher energy densities than symmetric ones using either MnO2 or AC. Moreover, this work provides an attractive approach to achieve various asymmetric micro energy storage systems on chips. [Copyright &y& Elsevier]

Abstract: Electrospun carbon nanotubes-embedded carbon nanofibers (CNF) fabric is employed as freestanding substrates for in-situ coating MnO2 nanostructures. Birnessite-type MnO2 nanoflakes are observed to grow vertically on individual CNF, thus building hierarchical coaxial architecture. This work presents an extensive study of the effect of temperature on the pseudo-capacitive behavior of the as-prepared freestanding MnO2@CNF composites electrodes in mild Na2SO4 electrolyte. The results show that the MnO2@CNF composites exhibit excellent pseudo-capacitive behaviors at different temperatures between 0 °C and 75 °C. The specific capacitance at 1 A g−1 increases from 365 F g−1 at 0 °C to 546 F g−1 at 75 °C, whereas the coulombic efficiency decreases with the increasing temperature, especially at lower charging rate. The cycling stability of the composites strongly depends on the temperature, i.e., 95.3% at 25 °C and 82.4% at 75 °C. This study provides a fundamental understanding of the temperature-dependent capacitive properties of MnO2 in mild electrolyte, which gives an insight into the supercapacitor design for industrial applications. [Copyright &y& Elsevier]

Abstract: Mesoporous MnO2/polyaniline composite (MP-I) has been synthesized using an interfacial synthesis technique. The results of X-ray diffraction and Fourier transform infrared spectroscopy indicate the amorphous structure of MP-I. The morphology of MP-I observed by scanning electron microscopy and transmission electron microscopy is composed of hierarchical hollow submicron spheres. The N2 adsorption/desorption test reveals MP-I has higher surface area (124m2 g−1) and more uniform pore-size distribution than the composite (MP-C) prepared by chemical co-precipitation. The electrochemical measurements show that as electrode materials the mesoporous MP-I composite electrode yields larger specific capacitance (262Fg−1) with better rate capability and higher capacitance retention (93% of its initial capacitance after 800 cycles). The unique microstructure of MP-I and the coexistence of conducting polyaniline with MnO2 are found to be responsible for the superior electrochemical properties. [Copyright &y& Elsevier]

Abstract: The lithium ion batteries using Li4Ti5O12 as the anode material are easily being inflated during charge and discharge, which, however, does not occur in the batteries using graphite as the anode material. The high reduction reactivity of electrolyte on the Li4Ti5O12 material may be the main reason. In this work, the reduction reactivities of electrolyte on the uncoated and carbon-coated Li4Ti5O12 electrodes are compared for the first time. The results show that the reduction decomposition of electrolyte does occur on the uncoated Li4Ti5O12 electrode at around 0.7V, while it only takes place at the first cycle on the carbon-coated Li4Ti5O12 electrode. The carbon coating layers cover the catalytic active sites of Li4Ti5O12 particles and separate the Li4Ti5O12 particles from the electrolyte. A successive solid electrolyte interface (SEI) film is formed on the carbon layer during the formation process, which can prevent the further reduction decomposition of electrolyte at around 0.7V. The impurity phases of rutile and anatase TiO2 do not influence the reduction reactivity of electrolyte. This work is not only important to understand the reduction decomposition mechanism of electrolyte on the Li4Ti5O12 electrode, but also provides an effective solution to suppress the reduction decomposition of electrolyte in the batteries. [Copyright &y& Elsevier]

Abstract: The structural and thermal stabilities of the layered Li(Ni1/3Co1/3Mn1/3)O2 cathode materials under high rate cycling and abusive conditions are investigated using the commercial 18650 Li(Ni1/3Co1/3Mn1/3)O2/graphite high power batteries. The Li(Ni1/3Co1/3Mn1/3)O2 materials maintain their layered structure even when the power batteries are subjected to 200 cycles with 10C discharge rate at temperatures of 25 and 50°C, whereas their microstructure undergoes obvious distortion, which leads to the relatively poor cycling performance of power batteries at high charge/discharge rates and working temperature. Under abusive conditions, the increase in the battery temperature during overcharge is attributed to both the reactions of electrolyte solvents with overcharged graphite anode and Li(Ni1/3Co1/3Mn1/3)O2 cathode and the Joule heat that results from the great increase in the total resistance (R cell) of batteries. The reactions of fully charged Li(Ni1/3Co1/3Mn1/3)O2 cathodes and graphite anodes with electrolyte cannot be activated during short current test in the fully charged batteries. However, these reactions occur at around 140°C in the fully charged batteries during oven test, which is much lower than the temperature of about 240°C required for the reactions outside batteries. [Copyright &y& Elsevier]

Abstract: The co-existence of two mechanisms, the surface chemisorption mechanism (SCM) and the tunnel storage mechanism (TSM), for the charge storage of MnO2 is investigated. Sample α-MnO2 whose tunnels (2×2) are almost vacant is synthesized by chemical co-precipitation technique. To verify whether the charge storage of as-prepared α-MnO2 involves the charges based on the SCM besides the TSM, Ba2+ and K+ are intercalated into the tunnels of α-MnO2 by refluxing and samples Ba-MnO2 and K-MnO2 are obtained, respectively. By powder X-ray diffraction, ICP and the extraction test, it is confirmed that Ba2+ and K+ occupy the tunnels and are difficult to remove. By electrochemical analyses, we have found that the charge storage of Ba-MnO2 and K-MnO2 basically depends on the SCM, as Ba2+ and K+ stabilized in the tunnels in advance block the intercalation/deintercalation of cations from the electrolyte. Thus, it can be inferred that for α-MnO2 two mechanisms contribute to the charge storage. The contributions of α-MnO2''s mechanisms are estimated. The charges based on the TSM predominantly account for 81.2–89.9% of the total charges. The charges on the SCM may contribute to 7.9–16.8% and are not negligible. The charges from double-layer capacitance may be negligible, since it is only 1.1–4.1%. [Copyright &y& Elsevier]