For the extensive applications of lithium-ion batteries (LIBs) to consumer electronics and electric vehicle, high performance electrode materials with high energies, high power densities, and good cyclic stabilities should be developed. In the last two decades, many transition-metal oxide nanostructures based on conversion reactions have been extensively investigated as potential LIB anodes because they have higher theoretical specific capacities (∼1000 mAh g⁻¹) than the currently commercially used graphite (372 mAh g⁻¹) and they show significantly improved reversibility. Furthermore, nanostructured materials are known to provide short diffusion pathways for lithium ions, resulting in high rate capabilities.

Despite these advantages, these nanostructured materials face two major challenges for practical commercial applications. First, they generally suffer from poor capacity retention, which is attributed to their huge volume expansion during lithiation/delithiation processes. Although the mechanical stress associated with volume expansion and contraction during cycling can be reduced with small-sized nanoparticles, such particles are likely to aggregate into larger particles that are pulverized again after long cycles, ultimately leading to rapid capacity fading.

Second, the solid−electrolyte interphase (SEI) layer formed by electrolyte decomposition on the surface of anode materials can degrade battery performance tremendously. Repeated expansion and contraction upon cycling can cause fracture, which can provide new active surfaces for SEI growth...nanoparticles (NPs) provide a huge surface area over which electrolytes can decompose, and a thick SEI layer can be formed easily and continuously with each charge/discharge cycle.

...Herein, we report the successful preparation of mesoporous iron oxide nanoparticle clusters (MIONCs) with carbon coatings through bottom-up self-assembly approach...we used a bottom-up self-assembly strategy because the secondary structure exhibits not only the characteristics of individual constituting NPs, but also new collective properties derived from the self-assembled structure.

—Lee et al.

The researchers said they chose iron oxide NPs as the representative component NPs because of their well-characterized properties and facile production. They compared the performance of MIONCs with that of random aggregates of iron oxide nanoparticles (RAIONs) and commercialized bare iron oxides (CBIOs).

To test durability of the materials, they charged and discharged the MIONCs, RAIONs, and CBIOs galvanostatically in the range of 0.01−3.00 V (vs Li⁺/Li) at a current density of 100 mA g⁻¹. The reversible capacity of the MIONCs was around 867 mAh g^-1 in the first cycle. The MIONCs showed high cyclic stability up to 100 cycles even though capacity slightly decreased in the initial ∼10 cycles. In contrast, the RAIONs showed a high specific capacity of 970 mAh g⁻¹ initially and maintained about 76.6% of the initial capacity after 80 cycles.

CBIOs showed a capacity of 800 mAh g⁻¹ initially but faded very rapidly until 50 cycles. At 50 cycles, they exhibited only 19.6% of capacity retention.

The MIONCs and RAIONs showed initial Coulombic efficiencies (CEs) of 73.4% and 69.0%, respectively. The CEs of the MIONCs increased steadily after several cycles and rapidly reached up to 99.7%. The RAIONs exhibited an oscillating CE of 95−97% after two cycles, indicating their lower reversibility than that of the MIONCs.

Rate properties of MIONCs and RAIONs. Credit: ACS, Lee et al. Click to enlarge.

They found that at low current densities(less than 400 mA g⁻¹), the specific capacities of the MIONCs were lower than those of the RAIONs. At current above 800 mA g⁻¹), however, the MIONCs exhibited higher reversible capacities than the RAIONs.

Even at 1600 mA g⁻¹, the reversible capacity delivered by the MIONCs (473 mAh g⁻¹) was 61% of the value at 100 mA g−1; this is even higher than the theoretical capacity of commercially available graphite (372 mAh g⁻¹). Only 27% of the capacity at 100 mA g^-1 was delivered at 1600 mA g⁻¹ in the RAIONs. The researchers suggested that the change in the geometric configurations allows lithium ions to reach the active materials of the MIONCs rapidly, despite their larger overall size.

Resources

• Soo Hong Lee, Seung-Ho Yu, Ji Eun Lee, Aihua Jin, Dong Jun Lee, Nohyun Lee, Hyungyung Jo, Kwangsoo Shin, Tae-Young Ahn, Young-Woon Kim, Heeman Choe, Yung-Eun Sung, and Taeghwan Hyeon (2013) Self-Assembled Fe₃O₄ Nanoparticle Clusters as High-Performance Anodes for Lithium Ion Batteries via Geometric Confinement. Nano Letters doi: 10.1021/nl401952h