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(a) Schematic showing evenly spread out Sn nanoclusters on a SnO2 nanowire surface. (b) SEM image showing a network of Sn-nanocluster-covered SnO2 nanowires. (c) HRTEM image showing well-spaced crystalline Sn nanoclusters on the nanowire periphery. Credit: ACS. Click to enlarge.

Researchers at the University of Louisville (Kentucky) are proposing a simple, generic design for stable, high-capacity lithium-ion anode materials consisting of hybrid structures involving metal nanoclusters supported on metal oxide nanowires.

In a paper published online 21 January in the ACS journal Nano Letters, the team presented a study demonstrating the design concept using a tin and tin oxide (Sn and SnO₂) material system. The design, they noted, can be extended to a wide range of metal oxides, nitrides, and other semiconductors such as Si (silicon) and Ge (germanium).

Nanowire-based systems show promise as Li-ion electrode material due to quicker charge transport, better conducting pathways and excellent strain relaxation. But, noted the Louisville team, the stability of nanowire based materials over cycling is either unknown or has been observed to fade rapidly.

After demonstrating high-capacity Si nanowire materials with excellent capacity retention over 10 charge-discharge cycles last year (earlier post), for example, Yi Cui and Stanford and his colleagues recently demonstrated a new core-shell design of crystalline-amorphous (c-a) silicon nanowires (NW) to enable higher power and longer-life lithium-ion battery electrodes (earlier post).

A schematic illustrating the reversible Li alloying and dealloying steps in the Sn-nanocluster-covered-SnO2 nanowires. Credit: ACS. Click to enlarge.

The basic design principle of Associate Professor Mahendra Sunkara and his colleagues is that the metal oxide nanowires are covered with metal nanoclusters with regular spacing larger than the diameter of each cluster. The spacing accomodates the volume expansion during alloying, thereby preventing agglomeration. The quicker electron transport through the underlying nanowires is expected to allow for efficient lithium alloying and dealloying, while the exposed metal nanoclusters and metal oxide nanowire surfaces serve as lithium alloying sites.

Tin and tin oxide offer excellent semiconducting properties combined with high capacity (Sn, 994 mAhg^-1 and SnO₂, 781 mAhg^-1) compared to that of graphite (372 mAhg^-1). But, a number of recent studies have shown significant capacity fading with these types of tin systems.

Sunkara and his colleagues synthesized SnO₂ nanowires and reduced them using H₂ plasma exposure producing nano-meter sized Sn clusters on the nanowire surfaces. The Sn nanoclusters are spaced at ~1.4 times the
diameter of each cluster.

All systems in the study were tested using anodic measurements over a potential window of 0 to 2.2 V (versus Li/Li⁺).

The data using pure SnO₂ nanowires showed a high initial capacity of 2400 mAhg^-1 but severe capacity degradation occurred within the next 15 cycles leading to a reversible capacity of 166 mAhg^-1 after 40 cycles…In comparison, Sn-nanocluster-covered SnO₂ nanowires exhibited a reversible capacity of 845 mAhg^-1 after 40 cycles…Other types of Sn/SnO₂ composite nanowire systems (metal Sn nanoclusters distributed in between the SnO₂ nanowire networks) showed an initial capacity of 2800 mAhg-1 with a final reversible capacity of 490 mAhg^-1 after 40 cycles. This result is similar to that obtained in prior studies using Sn/SnO₂ composites.

…The hybrid structures show an initial irreversible capacity of 413 mAhg^-1, which accounts to a columbic efficiency of 74%, notably the highest reported until now in SnO₂ systems. The columbic efficiency in
the subsequent cycles is shown to be over 98%. The capacity fading at a rate of ~1.3% for the initial 15 cycles and ~0.8% after the 15^th cycle is considerably lower than that reported for other nanoscale SnO₂ material systems.

—Meduri at al. (2009)

Capacity of the hybrid material over 100 cycles. Credit: ACS. Click to enlarge.

Overall, the SnO₂ nanowires covered with Sn nanoclusters exhibited a capacity of >800 mAhg^-1 over 100 cycles with a low capacity fading of less than 1% per cycle. Post lithiation analyses after 100 cycles showed small structural degradation of the hybrid nanowires.

Resources

• Praveen Meduri, Chandrashekhar Pendyala, Vivekanand Kumar, Gamini U. Sumanasekera and Mahendra K. Sunkara (2009) Hybrid Tin Oxide Nanowires as Stable and High Capacity Anodes for Li-Ion Batteries. Nano Lett., Article ASAP doi: 10.1021/nl802864a

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