Yi-Chen Wang

Electrochemical performance of Na/NaFePO4 sodium-ion batteries with ionic liquid electrolytes

Rechargeable Na/NaFePO4 cells with a sodium bis(trifluoromethanesulfonyl)imide (NaTFSI)-incorporated butylmethylpyrrolidinium (BMP)–TFSI ionic liquid (IL) electrolyte are demonstrated with an operation voltage of ∼3 V. High-performance NaFePO4 cathode powder with an olivine crystal structure is prepared by chemical delithiation of LiFePO4 powder followed by electrochemical sodiation of FePO4. This IL electrolyte shows high thermal stability (>400 °C) and non-flammability, and is thus ideal for high-safety applications. The effects of NaTFSI concentration (0.1–1.0 M) on cell performance at 25 °C and 50 °C are studied. At 50 °C, an optimal capacity of 125 mA h g−1 (at 0.05 C) is found for NaFePO4 in a 0.5 M NaTFSI-incorporated IL electrolyte; moreover, 65% of this capacity can be retained when the charge–discharge rate increases to 1 C. This ratio (reflecting the rate capability) is higher than that found in a traditional organic electrolyte. With a 1 M NaTFSI-incorporated IL electrolyte, a 13% cell capacity loss after 100 charge–discharge cycles is measured at 50 °C, compared to the 38% observed in an organic electrolyte under the same conditions.

Ionic liquid electrolytes with various sodium solutes for rechargeable Na/NaFePO4 batteries operated at elevated temperatures.

NaFePO4 with an olivine structure is synthesized via chemical delithiation of LiFePO4 followed by electrochemical sodiation of FePO4. Butylmethylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (BMP-TFSI) ionic liquid (IL) with various sodium solutes, namely NaBF4, NaClO4, NaPF6, and NaN(CN)2, is used as an electrolyte for rechargeable Na/NaFePO4 cells. The IL electrolytes show high thermal stability (>350 °C) and nonflammability, and are thus ideal for high-safety applications. The highest conductivity and the lowest viscosity of the electrolyte are obtained with NaBF4. At an elevated temperature (above 50 °C), the IL electrolyte is more suitable than a conventional organic electrolyte for the sodium cell. At 75 °C, the measured capacity of NaFePO4 in a NaBF4-incorporated IL electrolyte is as high as 152 mAh g(-1) (at 0.05 C), which is near the theoretical value (154 mAh g(-1)). Moreover, 60% of this capacity can be retained when the charge-discharge rate is increased to 1 C.

Ionic liquid electrolytes for high-voltage rechargeable Li/LiNi0.5Mn1.5O4 cells

A high-voltage LiNi0.5Mn1.5O4 cathode material with a cubic spinel structure is synthesized using a citric-acid-assisted sol–gel process. Butylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI)-based ionic liquids (ILs) with various kinds of Li salts, namely LiTFSI, LiPF6, and their mixtures, are used as electrolytes for Li/LiNi0.5Mn1.5O4 cells. The IL electrolytes show high thermal stability (>400 °C) and non-flammability, and are thus ideal for high-safety applications. At 25 °C, LiTFSI is more suitable than LiPF6 as an IL electrolyte in terms of cell capacity, rate capability, and cyclic stability. The IL electrolytes clearly outperform the conventional organic electrolytes at 50 °C, since the latter decomposes at high voltage and corrodes both the Al current collector and LiNi0.5Mn1.5O4, degrading the electrode performance. At such an elevated temperature, using LiPF6 to partially substitute LiTFSI in the IL electrolyte can effectively suppress Al pitting corrosion and thus improves the cell performance. In the 0.4 M LiTFSI/0.6 M LiPF6 mixed-salt IL electrolyte, an LiNi0.5Mn1.5O4 discharge capacity of 115 mA h g−1 (at 0.1 C) is obtained at 50 °C with a high cell voltage of ∼4.7 V.

Electrochemical properties of an AgInS2 photoanode prepared using ultrasonic-assisted chemical bath deposition

This study focuses on preparing a AgInS2 film electrode and studying its electrochemical properties. The AgInS2 film after 400 °C thermal treatment had the orthorhombic structure and a direct energy band gap of 1.98 eV. The thickness of AgInS2 film used in this study was 758.9 ± 40.9 nm. In order to understand the photoelectrochemical properties, electrochemical impedances of the AgInS2 photoanode in response to a light intensity of 75 mW cm−2 were scrutinized. It was found that homogeneous AgInS2 films were obtained with increasing coatings. In addition, these dense films can effectively suppress the dark current. Charge transfer resistance and space charge capacitance can be retrieved from impedance spectra by fitting the experimental data to the models. In fact, Randles model fitted the data better than other complicated models. Under illumination, the space charge capacitance and charge transfer resistance are strongly correlated to the onset of the photo-enhanced current density, suggesting a direct carrier transfer to the electrolyte from the valence band of the semiconductor photoanode, rather than from the surface states.

Nanostructured tin electrodeposited in ionic liquid for use as an anode for Li-ion batteries

Nanostructured tin (Sn) is fabricated via electrodeposition in an ionic liquid (IL). The advantages of electrodeposition in IL (compared to that in conventional aqueous solution) include increased deposition current efficiency and suppressed corrosion of the Cu current collector. The former is associated with the elimination of hydrogen evolution during the cathodic deposition and the latter is attributed to the chemical benignity of ILs. It is found that the Sn morphologies can be effectively manipulated by adjusting the deposition potential in the IL plating solution. The higher the deposition overpotential, the better is the electrochemical performance of the Sn electrode for Li-ion batteries. The electrode deposited at −2.4 V, consisting of Sn nanospheres, shows a reversible discharge (delithiation) capacity of 980 mA h g−1 (at 0.1 A g−1). When the charge–discharge rate is increased to 15 A g−1, the measured capacity is as high as 588 mA h g−1. These properties are clearly superior to those of the Sn electrode deposited in aqueous solution.

Formation of metal coatings on magnesium using a galvanic replacement reaction in ionic liquid

Surface coatings of Cu, Ni, Zn, and Ti on magnesium are deposited using a galvanic replacement reaction in butylmethylpyrrolidinium–dicyanamide ionic liquid. The coated samples show improved corrosion resistance compared to bare Mg.