Gel Polymer Electrolytes with Talc as a Natural Mineral Filler and a Biodegradable Polymer Matrix in Sodium-Ion Batteries

A gel polymer electrolyte based on poly(vinyl alcohol) (PVA) is used in sodium-ion batteries (SIBs). The use of biodegradable and water-soluble polymer potentially reduces the negative environmental impact. The other components include sodium salt (NaPF₆), sulfolane (TMS) as a plasticizer and talc. For the first time, natural and abundant talc has been used as an inert filler in a gel polymer electrolyte. The best results were obtained for moderate amounts of filler (1 and 3 wt%). Then, an increase in the conductivity, transference numbers, and thermal stability of the membranes was observed. Moreover, the presence of talc had a positive effect on the cyclability of the hard carbon electrode. The discharge capacity after 50 cycles of HC|1 % T_TMS|Na and HC|3 % T_TMS|Na was 243 and 225 mAh g⁻¹, respectively. The use of talc in gel polymer electrolytes containing sodium ions improves the safety and efficiency of SIBs.     

Kinetic and galvanostatic studies of a polymer electrolyte for lithium-ion batteries

Quaternary polymer electrolyte (PE) based on poly(acrylonitrile) (PAN), 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid (EMImBF₄), sulfolane (TMS) and lithium hexafluorophosphate salt (LiPF₆) (PAN-EMImBF₄-sulfolane-LIPF₆) was prepared by the casting technique. Obtained PE films of ca. 0.2–0.3 mm in thickness showed good mechanical properties. They were examined using scanning electron microscopy (SEM), thermogravimetry (TGA, DSC), the flammability test, electrochemical impedance spectroscopy (EIS) and galvanostatic charging/discharging. SEM images revealed a structure consisting of a polymer network (PAN) and space probably occupied by the liquid phase (LiPF₆ + EMImBF₄ + sulfolane). The polymer electrolyte in contact with an outer flame source did not ignite; it rather underwent decomposition without the formation of flammable products. Room temperature specific conductivity was ca. 2.5 mS cm^?1. The activation energy of the conding process was ca. 9.0 kJ mol^?1. Compatibility of the polymer electrolyte with metallic lithium and graphite anodes was tested applying the galvanostatic method. Charge transfer resistance for the C₆Li → Li⁺ + e^? anode processes, estimated from EIS curve, was ca. 48 Ω. The graphite anode capacity stabilizes at ca. 350 mAh g^?1 after the 30th cycle (20 mA g^?1).     

Properties of LiNiO2 cathode and graphite anode in N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide

Electrochemical properties of LiNiO₂|Li and LiNiO₂|graphite cells were analysed in ionic liquid electrolyte [Li⁺][MePrPyrr⁺][NTf₂^-] (based on N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulphonyl)imide, [MePrPyrr⁺][NTf₂^-]) using impedance spectroscopy and galvanostatic techniques. The ionic liquid is incapable of protective solid electrolyte interface (SEI) formation on metallic lithium or lithiated graphite. However, after addition of VC, the protective coating is formed, facilitating a proper work of the Li-ion cell. Scanning electron microscopy images of pristine electrodes and those taken after electrochemical cycling showed changes which may be interpreted as a result of SEI formation. The charging/discharging capacity of the LiNiO₂ cathode is between 195 and 170 mAh g⁻¹, depending on the rate. The charging/discharging efficiency of the graphite anode drops after 50 cycles from an initial value of ca. 360 mAh g⁻¹ to stabilise at 340 mAh g⁻¹. The replacement of a classical electrolyte in molecular liquids (cyclic carbonates) with an electrolyte based on the MePrPyrrNTf₂ ionic liquid highly increases in the cathode/electrolyte non-flammability.     

Solid electrolyte interphase formation on metallic lithium

Solutions of three salts (LiBF₄, LiNTf₂, LiPF₆) in N-methyl-2-pyrrolidone (NMP), selected arbitrarily as a reference solvent, were investigated by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy techniques. The lithium surface in contact with LiPF₆ in NMP electrolyte was covered with a protective layer (SEI) which morphology comprise small particles (of ca. 0.2 μm in radius). This salt was selected for further studies. The impedance of the Li|(LiPF₆ in NMP?+?additive)|Li system was measured immediately after cell assembly and after galvanostatic charging/discharging. Fifteen different additives (10 wt.%) were used. The efficiency of individual additives was evaluated in terms of the Li|electrolyte system resistance (ΔR) or total cell impedance reduction, both deduced from EIS. Some of the additives were able to form the SEI layer and to reduce resistance/impedance of the Li|electrolyte interphase. In such cases, the lithium surface was covered with relatively uniform conglomerates, or regions separated by cracks, of ca. 1–2 μm in dimension.     

Kinetics of Li-ion transfer reaction at LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript>, LiCoO<Subscript>2</Subscript>, and LiFePO<Subscript>4</Subscript> cathodes

Kinetics of LiFePO₄, LiMn₂O₄, and LiCoO₂ cathodes operating in 1 M LIPF₆ solution in a mixture of ethylene carbonate and dimethyl carbonate was deduced from impedance spectra taken at different temperatures. The most striking difference of electrochemical impedance spectroscopy (EIS) curves is the impedance magnitude: tens of ohms in the case of LiFePO₄, hundreds of ohms for LiMn₂O₄, and thousands of ohms for LiCoO₂. Charge transfer resistances (R _ct) for lithiation/delitiation processes estimated from the deconvolution procedure were 6.0 Ω (LiFePO₄), 55.4 Ω (LiCoO₂), and 88.5 Ω (LiMn₂O₄), respectively. Exchange current density for all the three tested cathodes was found to be comparable (0.55–1·10^?2 mAcm^?2, T = 298 K). Corresponding activation energies for the charge transfer process, \( {E}_{ct}^{\#} \), differed considerably: 66.3, 48.9, and 17.0 kJmol^?1 for LiMn₂O₄, LiCoO₂, and LiFePO₄, respectively. Consequently, temperature variation may have a substantial influence on exchange current densities (j _o) in the case of LiMn₂O₄ and LiCoO₂ cathodes.     

Capacity of graphene anode in ionic liquid electrolyte

The graphene anode was investigated in an ionic liquid electrolyte (0.7 M lithium bis(trifluoromethanesulfonyl)imide (LiNTf₂)) in room temperature ionic liquid (N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (MPPyrNTf₂)). SEM and TEM images suggested that the electrochemical intercalation/deintercalation process in the ionic liquid electrolyte without vinylene carbonate (VC) leads to small changes on the surface of graphene particles. However, a similar process in the presence of VC results in the formation of a coating (SEI—solid electrolyte interface) on the graphene surface. During charging/discharging tests, the graphene electrode working together with the 0.7 M LiNTf₂ in MPPyrNTf₂ electrolyte lost its capacity, during cycling and stabilizes at ca. 200 mAh g^?1 after 20 cycles. The addition of VC to the electrolyte (0.7 M LiNTf₂ in MPPyrNTf₂?+?10 wt.% VC) considerably increases the anode capacity. Electrodes were tested at different current regimes: ranging between 50 and 1,000 mA g^?1. The capacity of the anode, working at a low current regime of 50 mA g^?1, was ca. 1,250 mAh g^?1, while the current of 500 mA g^?1 resulted in capacity of 350 mAh g^?1. Coulombic efficiency was stable and close to 95 % during ca. 250 cycles. The exchange current density, obtained from impedance spectroscopy, was 1.3?×?10^?7 A cm^?2 (at 298 K). The effect of the anode capacity decrease with increasing current rate was interpreted as the result of kinetic limits of the electrode operation.