The design of a LiFePO4/carbon nanocomposite with a core-shell structure and its synthesis by an in situ polymerization restriction method
The design of a LiFePO4/carbon nanocomposite with a core-shell structure and its synthesis by an in situ polymerization restriction method
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DOI：
10.1002/anie.200802539
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发表时间：
2008-01-01
影响因子：
16.6
通讯作者：
Zhou, Haoshen
中科院分区：
文献类型：
--
作者：
Wang, Yonggang;Wang, Yarong;Zhou, Haoshen
用于锂离子电池的纳米尺寸电极材料最近引起了广泛关注[1-4]，因为它们减小的尺寸能够实现更高的功率。然而，由它们的尺寸引起的大的电解质/电极界面导致更多不期望的反应，这导致差的循环性能。[1，2]此外，通过低温方法合成的一些纳米尺寸的阴极材料结晶性差，这也降低了它们的电化学稳定性。合成完全包覆有导电碳（或碳壳）的高度结晶的纳米材料将是消除这些问题的有效手段。然而，这种合成是一个重大的挑战，因为高度结晶的结构及其随后的导电碳涂层必须在高温下实现，其中较大的微晶尺寸几乎是不可避免的。橄榄石（LiFePO 4）由于其低毒性、低成本和高安全性而被认为是下一代锂电池最有前途的正极材料之一。[5，6]然而，其功率性能受到锂离子穿过两相边界的缓慢扩散和/或低电导率的极大限制。[7，8]在过去的几年中，人们已经做出了许多努力，通过使用低温路线来获得定制的颗粒或碳涂层来改善固相的导电性，从而改善LiFePO 4的功率性能。然而，这些以往的研究一直集中在“纳米尺寸”或“涂层与导电碳”分开，而不是考虑两者在一起。已经开发了各种低温方法（合成温度低于600 ℃），例如低温陶瓷路线[9-13]或水热合成[14，15]，以降低LiFePO 4的粒度，尽管它们都不能确保碳涂层的导电性。此外，一些低温路线不能产生所需的高度结晶的橄榄石结构，从而降低了LiFePO 4的电化学稳定性。由产品的纳米尺寸产生的高表面积也大大增加了不期望的电极/电解质反应，这导致了差的循环性能[1，2]从对纳米尺寸的LiFePO 4（小于100 nm）的先前研究的回顾中，我们可以看到在高充电/放电深度（90%）下的“完美”循环寿命（> 200次循环）几乎是闻所未闻的。基于含碳前体的热分解的方法也被广泛研究用于制备碳包覆的LiFePO 4颗粒。[16-23]然而，这些方法通常涉及高温处理，在此期间微晶尺寸的增加是不可避免的，以确保所得碳材料的导电性。因此，基于含碳前体的热分解的那些方法只能产生具有部分碳涂层的LiFePO 4颗粒（图1a）。[17]如图1a所示，在嵌入过程中，电子不能到达Li+离子嵌入的所有位置，从而导致电极极化。鉴于框架中的一维Li+离子迁移率，[24]用碳完全包覆，确保LiFePO 4颗粒从各个方向获得电子，可以进一步缓解这种极化现象。根据我们对以前研究的分析，高性能LiFePO 4的理想结构应该包含完全包覆有导电碳的纳米颗粒（图1b）。应该指出的是，许多以前的研究涉及纳米级LiFePO 4的合成采用Fe 2+盐作为前体。
Nano-sized electrode materials for lithium-ion batteries have attracted much attention recently [1–4] because their reduced dimensions enable much higher power. However, the large electrolyte/electrode interface arising from their size leads to more undesired reactions, which result in poor cycling performance.[1, 2] Moreover, some nano-sized cathode materials synthesized by low-temperature methods are poorly crystalline, which also reduces their electrochemical stability. The synthesis of highly crystalline nanomaterials completely coated with conductive carbon (or a carbon shell) would be an effective means of eliminating these problems. Such a synthesis is a significant challenge, however, as the highly crystalline structure and its subsequent coating with conductive carbon have to be achieved at high temperature, where larger crystallite sizes are almost inevitable. Olivine (LiFePO4) is considered to be one of the most promising cathode materials for the next generation of lithium batteries due to its low toxicity, low cost, and high safety.[5, 6] However, its power performance is greatly limited by slow diffusion of lithium ions across the two-phase boundary and/or low conductivity.[7, 8] Many efforts have been made over the past few years to improve the power performance of LiFePO4 by using low-temperature routes to obtain tailored particles or carbon painting to improve the conductivity of the solid phase. However, these previous studies have always focused on the “nano-size” or the “coating with conductive carbon” separately, rather than considering both of them together. Various low-temperature methods (synthesis temperature below 6008C), such as lowtemperature ceramic routes [9–13] or hydrothermal syntheses,[14, 15] have been developed to lower the particle size of LiFePO4, although none of them have been able to ensure the conductivity of the carbon coating. Furthermore, some lowtemperature routes are not able to produce the required highly crystalline olivine structure, thus reducing the electrochemical stability of LiFePO4. The high surface area arising from the nano-size of the products also greatly increases the undesirable electrode/electrolyte reactions, which leads to a poor cycling performance [1, 2] From a review of previous studies of nano-sized LiFePO4 (less than 100 nm), we can see that a “perfect” cycle-life (> 200 cycles) at high charge/discharge depth (90%) is almost unheard of. Approaches based on the thermal decomposition of carbon-containing precursors have also been widely studied for the preparation of carbon-coated LiFePO4 particles.[16–23] However, these methods generally involve a high-temperature treatment, during which an increase in crystallite size is inevitable, to ensure the conductivity of the resulting carbon materials. Accordingly, those approaches based on the thermal decomposition of carbon-containing precursors can only produce LiFePO4 particles with a partial coating of carbon (Figure 1 a).[17] As shown in Figure1a, during the intercalation process, the electrons cannot reach all the positions where Li+ ion intercalation takes place, thus resulting in polarization of the electrode. In view of the one-dimensional Li+ ion mobility in the framework,[24] full coating with carbon, which ensures LiFePO4 particles get electrons from all directions, could further alleviate this polarization phenomenon. According to our analysis of previous studies, the ideal structure for high-performance LiFePO4 should contain nano-size particles completely coated with conductive carbon (Figure 1b). It should be noted that many previous studies involving the synthesis of nano-sized LiFePO4 employ Fe2+ salts as precursors …