Hollow Nanostructured Anode Materials for Li-Ion Batteries
© The Author(s) 2010
Received: 28 July 2010
Accepted: 2 August 2010
Published: 13 August 2010
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© The Author(s) 2010
Received: 28 July 2010
Accepted: 2 August 2010
Published: 13 August 2010
Hollow nanostructured anode materials lie at the heart of research relating to Li-ion batteries, which require high capacity, high rate capability, and high safety. The higher capacity and higher rate capability for hollow nanostructured anode materials than that for the bulk counterparts can be attributed to their higher surface area, shorter path length for Li+ transport, and more freedom for volume change, which can reduce the overpotential and allow better reaction kinetics at the electrode surface. In this article, we review recent research activities on hollow nanostructured anode materials for Li-ion batteries, including carbon materials, metals, metal oxides, and their hybrid materials. The major goal of this review is to highlight some recent progresses in using these hollow nanomaterials as anode materials to develop Li-ion batteries with high capacity, high rate capability, and excellent cycling stability.
Past decades have witnessed tremendous progress in Li-ion batteries [1–5]; however, there are continuous demands for batteries with higher power and energy density and longer cycling life to power newly emerging electronic devices, advanced communication facilities. As the performance of Li-ion batteries strongly depends on the electrode properties, significant improvements in the electrochemical properties of electrode materials are essential to meet the demanding requirements these applications. One example of this improvement is the rapid development of nanostructured electrode materials . The size reduction into nanoscale leads to increased electrode/electrolyte contact areas and shortened Li+ transport distance, permitting batteries to operate at higher power. Among these nanostructured materials, hollow nanomaterials, such as hollow micro/nanospheres and nanotubes, are special and unique. Due to their well-defined interior voids, low density, large surface area, and surface permeability, these nanostructured materials have wide applications in a number of areas including Li-ion batteries, catalysts, optoelectronic sensors, drug-delivery carries, and chemical reactors [6–28]. Besides their large surface area and short effective diffusion distance for Li+, the cavities in hollow structured electrodes for Li-ion batteries may provide extra space for the storage of Li+, beneficial for enhancing specific capacity. Furthermore, the void space in hollow structures buffers against the local volume change during Li insertion/desertion and is able to alleviate pulverization and aggregation of the electrode materials, hence improving cycling performance. The present article provides a simple overview on the hollow nanostructured anode materials for Li-ion batteries, including carbon materials, metals, metal oxides, and their hybrid materials.
Commercial Li-ion batteries usually employ carbonaceous materials as anodes, in which Li+ is inserted during charging. The resulting Li-interacted carbons exhibit a low potential close to that of metal Li electrode. With carbonaceous materials as anodes, the knotty problems of dendrite formation in the initially employed metal Li anode can be avoided, and the safety of Li-ion batteries are improved greatly. Carbon materials are usually specified into three groups, namely, graphite and graphitized materials, ungraphitized soft carbon, and hard carbon . Graphite is most widely used due to its stable specific capacity (a theoretical capacity of 372 mA h g−1, forming LiC6), small irreversible capacity, and good cycling performance. Soft carbon materials exhibit a very high reversible Li-storage capacity but a serious voltage hysteresis during delithiation. Hard carbon shows a high Li-storage capacity of 200–600 mA h g−1 and good power capability, but poor electrical conductivity and a large irreversible capacity.
Although carbon anode materials have received wide-range applications in Li-ion batteries, it is recognized that graphitic carbon anodes suffer from solvent co-intercalation in propylene-carbonate-based electrolytes, which results in large interlayer expansion and subsequent degradation of structure. Furthermore, the gravimetric and volumetric capacity of carbon materials is limited. The rapid development of electronic devices and electric vehicles demands a much higher energy density. Therefore, some other metal or alloy and transition-metal oxides have been explored as anodes for Li-ion batteries. These transition-metal oxide anodes can be specified into insertion-type materials (such as Li4Ti5O12, TiO2) [32–35], alloying-type materials (such as SnO2, SnO) [36–44], and conversion-type materials (such as Co3O4, Fe2O3) .
Among these insertion-type oxide anode materials for Li-ion batteries, Li4Ti15O12 has been considered as one of the most promising alternatives due to its special characters, such as a small volume change during charge/discharge process (zero strain insertion materials), which enables a long and stable cycle life, and a stable insertion potential at 1.55 V versus Li, which avoids the reduction reaction of electrolyte. Additionally, Li4Ti15O12 also has an excellent Li+ mobility, hence promising for high rate battery applications. Zhou et al. reported the fabrication of Li4Ti5O12 hollow microspheres by a sol–gel process using carbon microspheres as templates . The Li4Ti5O12 hollow microspheres show higher Li storage capacity, especially at higher current rates. It is believed that the short Li+ diffusion distance and large contact area between Li4Ti5O12 electrode and electrolyte increased both the efficiency of Li+ and electronic conductivity, hence the rate capability. Jiang et al. prepared hollow spherical Li4Ti5O12 by the emulsion method . This hollow spheres can be charged/discharged at 20C (3.4 A g−1) with the specific capacity of 95 mA h g−1. Over 500 cycles charge and discharge at 2C, the specific capacity stays very stable at 140 mA h g−1 with a loss of 0.01% per cycle. Besides these common hollow spheres of Li4Ti5O12 anode materials, three-dimensional hierarchical hollow microspheres assembled by thin nanosheets were reported . Hollow structured nanomaterials of other insertion-type oxide materials such as TiO2 have also been reported [35, 36].
In addition to these hollow micro/nanospheres, some one-dimensional hollow structured SnO2-based nanomaterials have also been synthesized and employed as anodes for Li-ion batteries [42–45]. Qi et al. reported the synthesis of SnO2 nanotubes with controllable morphologies using a variety of one-dimensional SiO2 mesostructures (chiral nanorods, nonchiral nanofibers, and helical nanotubes) as effective sacrificial templates . The as-obtained SnO2 short nanotubes, which were fabricated with SiO2 chiral nanorods as templates, show a specific discharge capacity of 468 mA h g−1 after 30 cycles. Lee et al. reported the synthesis of C/SnO2 porous nanotubes through template deposition method with carbon nanotubes as templates . These C–SnO2 hollow nanocomposites exhibit a reversible capacity of 600 mA h g−1 and a good cyclability in Li+ storage and retrieval.
On the other hand, lithium reaction of some 3d transition-metal oxides is a conversion reaction, similar to non-alloy metal oxides, such as Co3O4, CuO, and Fe2O3, which can be reversibly reduced and oxidized, coupled with the formation and destruction of lithium oxide, respectively.
The field of nanostructured electrodes for Li-ion batteries is an area of growing interest from both the fundamental and application points of view. In this review, we summarized recent researches in the synthesis and application of hollow nanostructured anode materials used in Li-ion batteries based on carbon materials, metals, metal oxides, and their hybrid materials. Hollow nanomaterials play a great role in improving the performance of Li-ion batteries, because in nanoscale electrodes the distance over which Li+ diffuses is dramatically shortened; the hollow core can buffer against the local volume change during charge/discharge and provide extra space for the storage of Li+; and hollow nanomaterials have large surface area and fast diffusion rates along the many grain boundaries existing in hollow nanomaterials. On the other hand, hollow nanostructured materials also have some disadvantages such as high side reactions, low thermodynamic stability, and low volumetric energy density. To gain commercial success of these nanostructured electrodes, however, requires continued fundamental advances in the science and engineering of materials and in fabrication technologies to enable further improved performance.
The financial support of the National Natural Science Foundation of China (grant nos. 50872016, 20973033) is acknowledged.
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