Ellis B L, Makahnouk W R M, Makimura Y, Toghill K, Nazar L F
Department of Chemistry, University of Waterloo, 200 University Ave. W., Waterloo, Ontario, N2L 3G1, Canada.
Nat Mater. 2007 Oct;6(10):749-53. doi: 10.1038/nmat2007. Epub 2007 Sep 9.
In the search for new positive-electrode materials for lithium-ion batteries, recent research has focused on nanostructured lithium transition-metal phosphates that exhibit desirable properties such as high energy storage capacity combined with electrochemical stability. Only one member of this class--the olivine LiFePO(4) (ref. 3)--has risen to prominence so far, owing to its other characteristics, which include low cost, low environmental impact and safety. These are critical for large-capacity systems such as plug-in hybrid electric vehicles. Nonetheless, olivine has some inherent shortcomings, including one-dimensional lithium-ion transport and a two-phase redox reaction that together limit the mobility of the phase boundary. Thus, nanocrystallites are key to enable fast rate behaviour. It has also been suggested that the long-term economic viability of large-scale Li-ion energy storage systems could be ultimately limited by global lithium reserves, although this remains speculative at present. (Current proven world reserves should be sufficient for the hybrid electric vehicle market, although plug-in hybrid electric vehicle and electric vehicle expansion would put considerable strain on resources and hence cost effectiveness.) Here, we report on a sodium/lithium iron phosphate, A(2)FePO(4)F (A=Na, Li), that could serve as a cathode in either Li-ion or Na-ion cells. Furthermore, it possesses facile two-dimensional pathways for Li+ transport, and the structural changes on reduction-oxidation are minimal. This results in a volume change of only 3.7% that--unlike the olivine--contributes to the absence of distinct two-phase behaviour during redox, and a reversible capacity that is 85% of theoretical.
在寻找锂离子电池新型正极材料的过程中，近期的研究聚焦于纳米结构的锂过渡金属磷酸盐，这类材料展现出诸如高储能容量与电化学稳定性相结合等理想特性。到目前为止，这一类材料中只有一个成员——橄榄石型LiFePO₄（参考文献3）——因其包括低成本、低环境影响和安全性等其他特性而崭露头角。这些特性对于诸如插电式混合动力汽车等大容量系统至关重要。尽管如此，橄榄石型材料存在一些固有缺点，包括一维锂离子传输以及两相氧化还原反应，这两者共同限制了相界的迁移率。因此，纳米微晶是实现快速充放电行为的关键。也有人提出，大规模锂离子储能系统的长期经济可行性最终可能会受到全球锂储量的限制，尽管目前这仍具有推测性。（目前已探明的全球储量对于混合动力汽车市场应该是足够的，不过插电式混合动力汽车和电动汽车的扩张将给资源带来相当大的压力，进而影响成本效益。）在此，我们报道一种钠/锂铁磷酸盐A₂FePO₄F（A = Na、Li），它可作为锂离子电池或钠离子电池的正极。此外，它具有便捷的二维Li⁺传输通道，并且氧化还原过程中的结构变化极小。这导致体积变化仅为3.7%，与橄榄石型材料不同的是，这使得在氧化还原过程中不存在明显的两相行为，并且可逆容量为理论值的85%。
