Attila Gören

Improved performance of rare earth doped LiMn2O4 cathodes for lithium-ion battery applications

Different rare-earth element (Dy, Gd, Tb and Yb) doped LiMn2O4 spinel active materials were prepared by a sol–gel method. The rare earth doping elements decrease the particle size but do not affect the formation of a cubic spinel structure. Furthermore, these dopants have a strong influence on the overall electrical properties of the cathodes prepared from them. In cyclic voltammetry measurements, two pairs of separated redox peaks are observed, regardless of the dopant. LiMn2O4 materials doped with gadolinium (Gd) and dysprosium (Dy) exhibit comparable room temperature rate capabilities to pristine LiMn2O4, with a discharge capacity of 94.5 mA h g−1 and 82.3 mA h g−1, respectively, versus 73.4 mA h g−1 for pristine LiMn2O4 at a rate of C5. Terbium (Tb) and ytterbium (Yb) doping shows, on the other hand, lower performance. After 50 cycles at C2, the capacity fade is 7% for LiMn1.95Dy0.05O4 and 14% for LiMn1.95Gd0.05O4, whereas for LiMn2O4 it is 32%. The improved cycling performance of LiMn2O4 doped with Gd and Dy is attributed to the powder size and atomic radii of the elements. The differences of the capacity retention on cycling are attributed to superior structural stability due to the rare earth doping. These results indicate that improved cathode materials doped with rare earth elements are suitable for lithium-ion battery applications.

Poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) lithium-ion battery separator membranes prepared by phase inversion

Separator membranes based on poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) were prepared by a solvent casting technique based on its phase diagram in N,N-dimethylformamide (DMF) solvent. The microstructure of the PVDF-CTFE separator membranes depends on the initial position (temperature and concentration) of the solution in the phase diagram of the PVDF-CTFE/DMF system. A porous microstructure is achieved for PVDF-CTFE membranes with solvent evaporation temperatures up to 50 °C for a polymer/solvent relative concentration of 20 wt%. The ionic conductivity of the separator depends on the degree of porosity and electrolyte uptake, the highest room temperature value being 1.5 mS cm−1 for the sample with 20 wt% of polymer concentration and solvent evaporation temperature at 25 °C saturated with 1 mol L−1 lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in propylene carbonate (PC). This PVDF-CTFE separator membrane in Li/C–LiFePO4 half-cell shows good cyclability and rate capability, showing a discharge value after 50 cycles of 92 mA h g−1 at 2C, which is still 55% of the theoretical value. PVDF-CTFE separators are thus excellent candidates for high-power and safe lithium-ion battery applications.