| dc.description.abstract | The spinels investigated in the present work
were obtained by solid state reaction at 750 ºC for 72 h among the precursors
ε-MnO2, LiOH and the respective oxides/salt of the doping ions. In order to control
the particles size, all the spinels were milled in a ball milling for 30 min. The
mechanical milling, associated or not to further calcination, was also used for
obtaining the pure spinel LiMn2O4. The different obtained spinels were
characterized by the average manganese valence (n), X ray diffractometry,
electronic scanning microscopy, particles size distribution and specific superficial
area. From the X ray diffractograms, spinels of single cubic phase belong to the
Fd3m space group were identified. The calculated values for the unity cell
parameter (a) were lower for the doped spinels (8,221 Å at 8,229 Å) than for the
pure one (8,234 Å), decreasing in the following order: a(Li1,05Ga0,02Mn1,98O4) >
a(Li1,05Co0,02Mn1,98O4) > a(Li1,05Al0,02Mn1,98O4) > a(Li1,05M0,02Mn1,98O3,98S0,02) >
a(Li1,05M0,02Mn1,98O3,98F0,02). The values of n were higher for the doped spinels
(3,56 3,50) than for the pure one (3,53 ± 0,01), decreasing in the following order:
n(Li1,05Ga0,02Mn1,98O4) > n(Li1,05Co0,02Mn1,98O4) > n(Li1,05Al0,02Mn1,98O4) >
n(Li1,05M0,02Mn1,98O3,98F0,02) > n(Li1,05M0,02Mn1,98O3,98S0,02). The mechanical milling
after calcination changed the spinel particles in three aspects: formation of
nanometer-scale particles, generation of lattice strain and partial oxidation of
manganese ions. Before calcination, it allowed obtaining of pure spinel LiMn2O4
with highly disordered lattice structure, nanometer particles and high number of
structural defects. A gel polymer electrolyte of the polyacrylonitrile
(PAN)/polyvinylacrylate (PVA) was developed with different mixtures of organic
solvents and lithium salts. From impedance measurements, the highest value of
ionic conductivity (1,47 x 10-3 S cm-1) was obtained, at 25 oC, for the electrolyte
prepared from the mixture (1:1) of ethyl (EC) and dimethyl (DMC) carbonates
containing 1 mol L-1 of lithium tetrafluoroborate (LiBF4). Cyclic voltammetric
measurements showed that this gel electrolyte only oxidizes at potentials close to
5 V vs. Li/Li+. In the charge and discharge tests, the cathodes of the doped spinels
presented values of specific discharge capacity decreasing in the following order:
C(Li1,05Al0,02Mn1,98S3,02O3,98) > C(Li1,05Al0,02Mn1,98F3,02O3,98) >
C(Li1,05Ga0,02Mn1,98S3,02O3,98) > C(Li1,05Ga0,02Mn1,98F3,02O3,98) >
C(Li1,05Co0,02Mn1,98S3,02O3,98) > C(Li1,05Co0,02Mn1,98F3,02O3,98). Although the
Li1,05Al0,02Mn1,98S3,02O3,98 cathode presented the highest initial capacity value (126
mA h g-1), the Li1,05Ga0,02Mn1,98S3,02O3,98 cathode presented the lowest capacity
fading (from 120 mA h g-1 to 115 mA h g-1, that is, only 4%) after three hundred
cycles of charge and discharge. | |
