ABSTRACT
The influence of electrolyte additives on the electrochemical and interfacial properties of SiOx-based anodes for lithium-sulfur batteries (Li-S) was systematically investigated. Four different electrolyte additives, namely, lithium nitrate, vinylene carbonate (VC), vinyl ethylene carbonate, and fluoroethylene carbonate (FEC), were added to the bare electrolyte comprising 1 M LiTFSI in tetraethylene glycol dimethyl ether/1,3 dioxolane in a ratio of 1:1 (v/v). The self-extinguishing time (SET) of the liquid electrolytes was measured. The 2032-type half-cells composed of Li/SiOx/Si/C were assembled, and their charge -discharge studies were analyzed at the 0.1 C-rate. Upon cycling, the electrode materials were subjected to surface morphology and differential scanning calorimetry analyses. The interfacial properties of SiOx-based electrodes were investigated by electrochemical impedance spectroscopy, Fourier transform infrared, and X-ray photoelectron spectroscopy studies. Among the electrolytes examined, FEC-added electrolytes offered the lowest SET and interfacial resistance values. The superior charge-discharge properties of FEC-added electrolytes were attributed to the formation of a stable solid electrolyte interface layer on the electrode surface. The surface chemistry studies revealed the formation of Li2CO3 and ROCO2Li peaks on the electrode surface.
ABSTRACT
Poly(ethylene oxide) (PEO)-based composite polymer electrolytes (CPEs) containing the amine-functionalized, zirconium-based metal-organic framework @silica (UiO-66-NH2@SiO2) and lithium, LiN(CF3SO2)2 salt (LiTFSI) are prepared using a simple hot press method. The electrochemical properties such as compatibility of the electrolyte with the Li metal anode, Li transference number, and ionic conductivity are investigated for the different systems containing different relative concentrations of the additives. The incorporation of UiO-66-NH2@SiO2 in the PEO-LiTFSI matrix not only enhanced ionic conductivity by one order of magnitude but also offered better compatibility and suppressed the formation of lithium dendrites appreciably. X-ray photoelectron spectroscopy studies on post-cycled materials revealed the formation of lithium alkoxide (RO-Li) on the cathode and Li2O on the anode. The coin cell (2032-type) consisting of LiFePO4/CPE/Li with UiO-66-NH2@SiO2 as filler provided a discharge capacity of 151 mA h g-1 at 0.1 C-rate at 60 °C, measurably higher than control experiments utilizing SiO2 and UiO-66-NH2. The notable enhancement of electrochemical properties when incorporating the UiO-66-NH2@SiO2 at the CPE was attributed to formation of more uniform ion conduction pockets and channels within the PEO matrix, facilitated by the presence of the microporous UiO-66-NH2@SiO2. The enhanced distribution of microporous channels, where Li ions are assumed to percolate through within the matrix, is assumed to desirably reduce formation of Li dendrites by increasing diffusion channels and therefore reducing crystallization and growth of dendrites at the electrode surface.
ABSTRACT
Even though lithium-sulfur batteries possess higher theoretical capacity and energy density than conventional lithium-ion batteries, the challenging issues such as poor electronic conductivity of sulfur, dendrite formation and subsequent polysulfide shuttling, and the undesirable interfacial properties of the lithium metal anode with an electrolyte impede this system from commercialization. To circumvent the dissolution of lithium polysulfides and to improve the interfacial properties of the electrolyte with the lithium metal anode, numerous tactics have been employed. Therefore, in this work, hybrid electrolytes composed of room-temperature ionic liquids of different cations with the bis(trifluoromethanesulfonyl)imide (TFSI) anion and a nonaqueous liquid electrolyte [1 M LiTFSI in tetraethylene glycol dimethyl ether/1,3-dioxolane 1:1 (v/v)] have been prepared, and their physicoelectrochemical properties were thoroughly investigated. The lithium surface upon cycling was characterized by Raman, Fourier transform infrared, and X-ray photoelectron spectroscopy analyses. The dendrite and shuttle current measurements also indicated the formation of a stable solid electrolyte interphase and lower polysulfide shuttling between the electrodes. Among the systems examined, the hybrid electrolyte composed of 1-methyl-1-propylpyrrolidinium TFSI exhibited appreciable charge-discharge characteristics, better interfacial properties with the lithium metal anode, and increased ionic conductivity which were attributed to the enhanced ion-pair interaction that is present between the 1-methyl-1-propylpyrrolidinium cation and the TFSI anion in the electrolyte which was substantiated by Raman analysis.
ABSTRACT
Nanocomposite polymer electrolytes (NCPE), with different proportions of poly(ethylene oxide)/LiClO(4)/chitin were prepared by a hot press method. Nanochitin, a biopolymer, poly(beta-(1-->4)-N acetyl-d-glucosamine) was incorporated as a filler in poly(ethylene oxide) (PEO). The ionic conductivity of the composite polymer electrolytes was enhanced by one order upon addition of nanochitin. The lithium transference number, t(Li)(+), was increased from 0.24 to 0.51 upon chitin addition. The membranes were subjected to scanning electron microscopy, thermogravimetric-differential thermal analysis, differential scanning calorimetry, ionic conductivity, and Fourier transform infrared (FTIR) spectroscopy analysis. The free volume V(f) was probed by positron annihilation lifetime spectroscopy studies at 30 degrees C. Li/NCPE/Li symmetric cells were assembled, and the thickness of the solid electrolyte interface as a function of time was analyzed. This paper also describes FTIR spectroscopic studies of the interface between lithium metal and NCPE, which suggests that the surface chemistry of lithium electrodes in contact with NCPE is dominated by compounds with C-N-Li and C-O-Li bonding.
