Jaephil Cho

Mesoporous and nanowire SnO₂ anode materials for lithium batteries were prepared using KIT-6 and SBA-15 SiO₂ templates, and their electrochemical properties were compared at different current rates. The as-prepared SnO₂nanowires had a diameter of 6 nm and a length of >3 μm and Brunauer–Emmett–Teller (BET) surface area of 80 m² g⁻¹ while mesoporous SnO₂ showed a pore size of 3.8 nm and a BET surface area of 160 m² g⁻¹. The charge capacities of these two anodes were similar to each other at 800 mAh g⁻¹, but mesoporous SnO₂ showed much improved cycle life performance and rate capabilities because of its higher surface area than nanowire SnO₂. Especially, the capacity retention of the mesoporous SnO₂ was 98%, compared with 31% for the SnO₂nanowires at a 10 C rate (= 4000 mA g⁻¹). The improved electrochemical performance of the mesoporous SnO₂ was related to the regular porosity which permitted thorough flooding of the electrolyte between the particles, and the mesopores which acted as a buffer zone during the volume contraction and expansion of Sn.

LiNi0.8Co0.1+xMn0.1−xO2LiNi0.8Co0.1+xMn0.1−xO2 cathodes with x=0x=0 , 0.03, and 0.06 were prepared by firing a mixture of stoichiometric amounts of LiOH⋅H2OLiOH⋅H2O and coprecipitated Ni0.8Co0.1+xMn0.1−x(OH)2Ni0.8Co0.1+xMn0.1−x(OH)2 powders at 800°C800°C for 15 h. Using these powders, their storage characteristics upon exposure to air and electrolytes at 90°C90°C were compared before charging and after charging to 4.3 V with a variation of the storage time. The discharge capacities of the LiNi0.8Co0.1+xMn0.1−xO2LiNi0.8Co0.1+xMn0.1−xO2 cathodes with x=0x=0 , 0.03, and 0.06 were 192, 186, and 184 mAh/g, respectively, while their irreversible capacity ratios were 13, 12, and 8%8% , respectively. As the Co content (x)(x) increased in the cathode, both the Ni2+Ni2+ content in the lithium 3a sites, and the contents of the LiOH and Li2CO3Li2CO3 impurity phases decreased. In particular, changes in the oxidation state of the Ni and Mn ions after 4.3 V charging upon storage at 90°C90°C were monitored using X-ray absorption near-edge spectra, and Ni4+Ni4+ was found to reduce to Ni3+Ni3+ while the oxidation state of the predominant Mn4+Mn4+ did not change. However, residual Mn3+Mn3+ ions in the cathodes dissolved into the electrolytes. Moreover, the cathodes stored at 90°C90°C for 7 days were transformed into a spinel phase (Fd3m)(Fd3m) , regardless of the Co content. In an effort to resolve this dissolution problem, Al2O3Al2O3 and Co3(PO4)2Co3(PO4)2 nanoparticles were coated onto the cathode (LiNi0.8Co0.1Mn0.1O2)(LiNi0.8Co0.1Mn0.1O2) with the highest amounts of metal dissolution at 90°C90°C . The results showed that the Co3(PO4)2Co3(PO4)2 -coated cathode exhibited a greatly decreased metal dissolution and decreased its irreversible capacity by 5%5% , compared with a bare and Al2O3Al2O3 -coated cathode.

LixCoPO4LixCoPO4 phase-grown LiNi0.86Co0.1Al0.04O2LiNi0.86Co0.1Al0.04O2 cathode materials were prepared by varying the coating concentration of precipitated Co3(PO4)2Co3(PO4)2 nanoparticles and the annealing temperature. These materials were characterized by analyzing their electrochemical properties, moisture uptake, and thermal stability. Among the various coating concentrations, 3wt%3wt% coating showed no decrease in the first discharge capacity (193mAh∕g)(193mAh∕g) and a much improved capacity retention (90%) at a 1C1C cycling rate after 40 cycles, compared to the bare cathode ( 193mAh∕g193mAh∕g and 44% for first discharge capacity and capacity retention, respectively). Using this concentration, the annealing time was varied from 2 to 7h7h at 700, 800, and 900°C. The coated cathode annealed at 700°C for 7h7h showed comparable electrochemical properties and moisture uptake to that annealed at 700°C for 5h5h . Even after annealing at 700°C for 7h7h , P ions remained on the particle surface with a similar coating thickness to that at 700°C for 5h5h . However, increasing the annealing temperature led to a further decrease in electrochemical properties and was related to deeper diffusion of P ions into the bulk particle, which likely formed a solid solution consisting of bulk elements and P elements. The amounts of Li2CO3Li2CO3 and LiOHLiOH formation of the coated samples upon exposure to air with a relative humidity of 40% showed a similar result when annealing at 700°C, but increasing the annealing temperature to 800 and 900°C resulted in a significant increase of the amount of Li2CO3Li2CO3 and LiOHLiOH . Thermal stability was also greatly improved by the coating, and the sample annealed at 700°C for 7h7h exhibited a decreased total heat capacity (240J∕g)(240J∕g) compared to the 5h5h annealed samples (380J∕g)(380J∕g) even though those values were much smaller than the bare sample (980J∕g)(980J∕g) .

The electrochemical cycling and storage characteristics of LiCo0.96M0.04O2LiCo0.96M0.04O2 and M3(PO4)2M3(PO4)2 nanoparticle-coated LiCoO2LiCoO2 cathode materials (M=ZnandMg)(M=ZnandMg) were compared at room temperature and 90°C90°C between 3 and 4.5V4.5V . The doped cathodes showed degraded electrochemical performance at room temperature compared to the uncoated cathode. The first discharge capacities of the uncoated and the doped cathodes were 186 and 175mAh∕g175mAh∕g , respectively. The doped cathodes showed 30mAh∕g30mAh∕g after 30 cycles, while the uncoated cathode showed 100mAh∕g100mAh∕g after 50 cycles at a 1C1C rate. Mg3(PO4)2Mg3(PO4)2 and Zn3(PO4)2Zn3(PO4)2 -coated LiCoO2LiCoO2 showed discharge capacities of 179 and 187mAh∕g187mAh∕g , respectively, and hadsignificantly improved capacity retention, showing 133 and 153mAh∕g153mAh∕g , respectively, after 50 cycles. After storage at 90°C90°C , in the electrolytes using 4.5V4.5V charged electrodes, the doped cathodes showed both greatly decreased side reactions with the electrolytes and formation of Co3O4Co3O4 and CoOCoO phases from Li and Co dissolution. However, the coated cathodes did not show either structural transformation into the Co3O4Co3O4 and CoOCoO phases or side reactions with the electrolytes.