Lithium-ion batteries (LIBs) provide high-energy-density electrochemical energy storage, which plays a central role in advancing technologies ranging from portable electronics to electric vehicles (EVs). However, a demand for lighter, more compact devices and for extended range EVs continues to fuel the need for higher energy density storage systems. Li 2 VO 2 F, which is synthesized in its lithiated state, allows two-electron transfer per formula during the electrochemical reaction providing a high theoretical capacity of 462 mAh/g. Herein, the synthesis and electrochemical performance of Li 2 VO 2 F are optimized. The thermal stability of Li 2 VO 2 F, which is related to the safety of a battery is studied by thermal gravimetric analysis. The structure and vanadium oxidation state evolution along Li cycling are studied by ex-situ X-ray diffraction and absorption techniques. It is shown that the rock-salt structure of pristine Li 2 VO 2 F is stable up to at least 250 • C, and is preserved upon Li cycling, which proceeds by the solid-solution mechanism. However, not all Li can be removed from the structure upon charge to 4.5 V, limiting the experimentally obtained capacity.
Carbon‐based anodes are the key limiting factor in increasing the volumetric capacity of lithium‐ion batteries. Tin‐based composites are one alternative approach. Nanosized Sn–Fe–C anode materials are mechanochemically synthesized by reducing SnO with Ti in the presence of carbon. The optimum synthesis conditions are found to be 1:0.25:10 for initial ratio of SnO, Ti, and graphite with a total grinding time of 8 h. This optimized composite shows excellent extended cycling at the C/10 rate, delivering a first charge capacity as high as 740 mAh g−1 and 60% of which still remained after 170 cycles. The calculated volumetric capacity significantly exceeds that of carbon. It also exhibits excellent rate capability, delivering volumetric capacity higher than 1.6 Ah cc−1 over 140 cycles at the 1 C rate.
Vapor-phase polymerized poly(3,4-ethylenedioxythiophene) (PEDOT)/TiO 2 composite fibers were fabricated and applied as the supercapacitor electrode materials. TiO 2 fibers were prepared as substrates for the vapor-phase polymerization process, by electrospinning and calcination in air. The symmetric supercapacitor cells assembled with the resulting composites were studied by a series of electrical measurements including cyclic voltammetry, charge-discharge characterization and electrochemical impedance spectroscopy. To further understand the capacitive behavior, the band gap energy of the composite fibers and the specific surface area of TiO 2 fibers calcined at varied temperatures were measured. The highest specific capacitance of PEDOT on TiO 2 fibers to date, 87.9 F g-1 , was achieved with the composite fibers prepared by vapor-phase polymerization at 50 °C on the TiO 2 fibers calcined at 550 °C. The pseudocapacitance and the reversibility of PEDOT were improved in comparison to other PEDOT/TiO 2 binary composites.
Sn2Fe anode materials were synthesized by
a solvothermal
route, and their electrochemical performance and reaction mechanism
were evaluated. The structural evolution in the first two lithium
cycles was investigated by X-ray absorption spectroscopy (XAS), synchrotron
X-ray diffraction (XRD), and magnetic studies. In the first cycle,
progressive alloying of Sn with Li accompanied by metallic iron displacement
occurs upon lithiation, and the delithiation proceeds by LixSn dealloying and recovery of the Sn2Fe phase. In the second cycle, both XRD and XAS identify Li–Sn
alloying at earlier lithiation stages than in the first cycle, with
low-Li-content alloys evident in the beginning of the lithiation process.
In the fully lithiated state, XAS analysis reveals higher coordination
numbers in both the LixSn and Fe phases,
which points toward more complete reaction and higher crystallinity
of the products. Upon second delithiation, the Sn2Fe phase
is generally reformed as evidenced by XRD. However, XAS indicates
somewhat reduced Sn–Fe coordination and shorter Fe–Fe
distance, which indicates incomplete reconversion and metallic Fe
retention, which is also evident in the magnetic studies. Thus, a
combination of long-range (XRD, magnetic) and local (XAS) techniques
has revealed differences between the first and the second Li cycles
relevant to the understanding of the capacity fading mechanisms.
Needle-like CoO nanowires grown on carbon cloth have been successfully fabricated by a controllable hydrothermal method followed by an annealing process. The as-fabricated nanostructure showed enhanced specific capacitances and excellent cycling stability in supercapacitors.
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