| It is of great significance to develop the next-generation electric vehicles (EVs) to protect environment and use sustainable energy. Up to now, graphite and LiCoO2 are still the dominant electrode materials for LIBs, but both of them are not suitable for high power lithium-ion batteries. There was a worldwide search for alternative materials with superior properties. Spinel Li4Ti5O12, with its high Li+ insertion voltage (-1.5 V vs. Li+/Li), nearly zero structural change during Li-ion insertion/extraction and low cost, has been recognized as the most potential candidate for anode materials. Among the cathode candidates,0.5Li2MnO3·0.5 LiNi0.4Co0.2Mn0.4O2 cathodes have been regarded as the most promising cathode materials because of its high reversible capacity, low cost and good thermal stability. LiNi0.8Co0.15Al0.05O2, with the higher specific capacity, lower cost and less toxic than LiCoO2, was seen as the substitution of LiCoO2. These three materials can be applied to the high power lithium-ion batteries. These electrode materials should have a high tap density to provide an improved volumetric energy density, so as to meet the power and energy demands in practical EVs. Therefore, how to prepare these materials with high tap density effectively and economically for mass production should be considered.The nanosized carbon-coated TiO2 was prepared as precursor. Then the TiO2 precursor with a carbon layer was ball milled with Li2CO3. The resulting slurry was spray-dried to generate microspheres, followed by calcining at 80 ℃ for 12 h to obtain i2 microspheres. The Li4Ti5O12 microspheres showed a spherical-like morphology with a particle size in the range of 10-20 μm, which is accumulated by primary nanoparticles. The carbon layer is 5-8 nm thick, and its content is 4 wt%. Nanopores which are mainly concentrated at 3-4 nm existed in the Li4Ti5O12 microspheres, indicating the nanoporous structure of the LTO microspheres. The specific surface area calculated by the BET method is 30.8 m2 g-1. This structure ensure the particles can well be permeated by the electrolyte. The LTO microspheres deliver a tap density of 1.04 g cm-3. The as-prepared LTO microspheres have a reversible capacity of 170 mAh g-1 at 0.1 C and a capacity retention of 97% after 250 cycles at 1 C. The discharge capacity of LTO microspheres was 150,140,118,86 and 73 mAh g-1 at 1,2, 5,10 and 20 C, respectively.The 0.5Li2MnO3·0.5 LiNi0.4Co0.2Mn0.4O2 microspheres were synthesized from Li2CO3 and (Ni0.2Co0.1Mn0.7) (OH)2. The (Ni0.2Co0.1Mn0.7) (OH)2 precursor was prepared by a co-precipitation process using NiSO4·6H2O, CoSO4·7H2O, MnSO4·H2O (Ni:Co:Mn molar ratio was 0.2:0.1:0.7). Then the dried (Ni0.2Co0.1Mn0.7) (OH)2 was ball milled with Li2CO3 to form a slurry. The slurry was pumped to the spray dryer for sphere-making. Finally, the 0.5Li2MnO3·0.5 LiNi0.4Co0.2Mn0.4O2 microspheres were collected by calcining at 750 ℃ for 12 h under air atmosphere. The 0.5Li2MnO3·0.5 LiNi0.4Co0.2Mn0.4O2 microspheres have an approximate spherical morphology with a diameter of 10-20 μm. The specific surface area of the as-prepared 0.5Li2MnO3·0.5 LiNi0.4Co0.2Mn0.4O2 microspheres is 28.7 m2 g-1, and nanopores between 3-4 nm existed in the NCM microspheres. The molar ratio of Ni, Co, and Mn in the 0.5Li2MnO3·0.5 LiNi0.4Co0.2Mn0.4O2 microspheres is 19.8:9.3:70.9, which is close to the feed ratio of 20:10:70. The tap density of the as-prepared 0.5Li2MnO3·0.5 LiNi0.4Co0.2Mn0.4O2 microspheres is 2.07 g cm-3. The discharge specific capacity of 0.5Li2MnO3·0.5 LiNi0.4Co0.2Mn0.4O2 microspheres is 270 mAh g-1 and coulombic efficiency is 88% at 0.03 C (1C=300 mA g-1). It delivered an initial discharge capacity of 270,243 and 225 mAh g-1 at 0.03,0.1 and 0.2 C, after 50 cycles, the capacity retention is 90%,85% and 85%, respectively. The 0.5Li2MnO3·0.5 LiNi0.4Co0.2Mn0.4O2 microspheres delivered a discharge capacity of 246,228,203,182 and 165 mAh g-1 with capacity retention of 91, 84,75,67 and 61% (vs.0.03 C) at 0.1,0.2,0.5,1 and 2 C, respectively.NiSO4·6H2O, CoSO4·7H2O and A12(SO4)3·12H2O were used to prepare the (Ni0.8Co0.15Al0.05) (OH)2 precursor. The collected hydroxide was filtered with deionized water and alcohol, then vacuum-dried overnight. The as-prepared (Ni0.8Co0.15Al0.05) (OH)2 was ball milled with Li2CO3 to form a slurry. Then the slurry were pumped into the spray dryer for sphere-making. Finally, the collected powders were annealed at 750℃ for 12 h in O2 atmospheres to obtain the LiNi0.8Co0.15Al0.05O2 microspheres. (Ni0.8Co0.15Al0.05) (OH)2/Li2CO3 precursor delivered a spherical-like morphology with a diameter of 10-20 μm. The final LiNi0.8Co0.15Al0.05O2 microspheres maintain the origin spherical morphology but deliver a smaller size. From the SEM, LiNi0.8Co0.15Al0.05O2 secondary particles was assembled by nanoparticles of about 100-200 nm. The tap density of the LiNi0.8Co0.15Al0.05O2 microspheres is 2.38 g cm-3. The specific surface area is 1.91 m2 g-1, and there are nanaopores mainly between 4-5 nm were existed in the secondary microspheres. The LiNi0.8Co0.15Al0.05O2 microspheres delivered a discharge capacity of 192 mAh g-1 with a coulombic efficiency of 93% at 0.1 C (1C=200 mA g-1). The capacity retention of LiNi0.8Co0.15Al0.05O2 microspheres is 93% after 100 cycles at 1 C with the initial discharge capacity of 146 mAh g-1. The LiNi0.5Co0.15Al0.05O2 microspheres delivered a discharge capacity of 170,147,133,124,100 and 68 mAh g-1 at 0.5,1,2,5,10 and 20 C, respectively. From the differential capacity curves, the first charge of LiNi0.8Co0.15Al0.05O2 microspheres is an irreversible process with capacity loss. The oxidation-reduction processes in the subsequent cycles show good reversibility.The Li4Ti5O12 microspheres provides a flat voltage and excellent cycling performance. The cathode-limited nature of the cells can ensure the voltage is controllable and cycling performance is stable. So the cathode-limited Li4Ti5O12/0.5Li2MnO3·0.5LiNi0.4Co0.2Mn0.4O2 was assembled. Li4Ti50i2/0.5Li2Mn03·0.5LiNi0.4Co0.2Mn0.4O2 full cell delivered a specific discharge capacity (based on NCM) of 273 mAh g-1 and initial Coulombic efficiency of 88%. The shape of the curves of the cell is similar to that of the Li/0.5Li2MnO3·0.5 LiNi0.4Co0.2Mn0.4O2 half cell, but shows a lower voltage range (0.5-3.3 V) because the LTO anode has a flat voltage plateau at about 1.55 V. The specific energy delivered by this full cell was 205Wh kg-1, and the volumetric energy density of this full cell is approximately 82 Wh L-1 (volume is calculated on the proportion of mass/tap density). The cells delivered a cathode capacity of 243 mAh g-1 at 0.1 C and 224 mAh g-1 at 0.2 C. After 50 cycles, the capacity retention was nearly 80%. Some improvements are still needed to be made. |
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