Direct synthesis of nanocrystalline Li0.90FePO4: observation of phase segregation of anti-site defects on delithiation
Direct synthesis of nanocrystalline Li0.90FePO4: observation of phase segregation of anti-site defects on delithiation
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DOI：
10.1039/c0jm04378h
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发表时间：
2011-01-01
影响因子：
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通讯作者：
Nazar, Linda F.
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文献类型：
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作者：
Badi, Shri-Prakash;Wagemaker, Marnix;Nazar, Linda F.
LixFePO4固溶体由于离子输运性能的提高而引起人们的极大兴趣，但在高温下形成这些固溶体是困难的，如果不是不可能的话，直接合成也是困难的，而且很少有报道。在这里，我们报道了直接合成纳米晶Li1-yFePO4的改进多元醇合成方法，其中在320℃合成温度下保持在M1位上的最大Li亚化学计量比约为10%。高的靶锂空位浓度促进了Li+和Fe2+反位无序的增加，因为这一过程是由空位稳定驱动的。在部分脱氢的亚化学计量比橄榄石上，中子和X射线的联合衍射显示出分离的无缺陷(Li被提取的地方)和充满缺陷(Li仍然存在的地方)的区域。这证明(1)反位缺陷阻碍了Li+的扩散，解释了有害的电化学；(2)反位缺陷形成了团簇。最后，各向异性应变在BC面上的择优展宽表明贫锂相和富锂相之间存在共格界面。伴随着脱氢后尺寸的扩大，这证明了在纳米LixFePO4中，两个相共存于单个粒子中，这是基于热力学的论点，因为与共格界面相关的能量损失而不是预期的。这些结果为研究纳米LiFePO4的性质提供了重要而独特的见解和理解。
Solid solutions of LixFePO4 are of tremendous interest because of a proposed increase in ion transport properties, but the formation of these solutions at high temperatures is difficult if not impossible and direct synthesis is difficult and rarely reported. Here we report modified polyol syntheses which produce nanocrystalline Li1-yFePO4 directly, where the maximum Li substoichiometry on the M1 site sustained at synthesis temperatures of 320 degrees C is about 10%. High target lithium vacancy concentrations promote the increase in anti-site disorder of Li+ and Fe2+, as this process is driven by vacancy stabilization. Combined neutron and X-ray diffraction on partial delithiated substoichiometric olivines reveals segregated defect-free (where Li is extracted) and defect-ridden (where Li remains) regions. This proves (1) that the anti-site defects obstruct Li+ diffusion explaining the detrimental electrochemistry and (2) that the anti-site defects form clusters. Finally, preferential anisotropic strain broadening in the bc-plane indicates the existence of a coherent interface between the Li-poor and Li-rich phases. Along with the size broadening upon delithiation this proves that in nano-sized LixFePO4 the two phases coexist within a single particle, which is not expected based on thermodynamics arguments due to the energy penalty associated with the coherent interface. Thereby, these results give important and unique insight and understanding in the properties of nano sized LiFePO4.