Ellis B, Subramanya Herle P, Rho Y H, Nazar L F, Dunlap R, Perry Laura K, Ryan D H
Universitv of Waterloo, Department of Chemistry, Waterloo, Ontario, Canada.
Faraday Discuss. 2007;134:119-41; discussion 215-33, 415-9. doi: 10.1039/b602698b.
Lithium metal phosphates are amongst the most promising cathode materials for high capacity lithium-ion batteries. Owing to their inherently low electronic conductivity, it is essential to optimize their properties to minimize defect concentration and crystallite size (down to the submicron level), control morphology, and to decorate the crystallite surfaces with conductive nanostructures that act as conduits to deliver electrons to the bulk lattice. Here, we discuss factors relating to doping and defects in olivine phosphates LiMPO4 (M = Fe, Mn, Co, Ni) and describe methods by which in situ nanophase composites with conductivities ranging from 10(-4)-10(-2) S cm(-1) can be prepared. These utilize surface reactivity to produce intergranular nitrides, phosphides, and/or phosphocarbides at temperatures as low as 600 degrees C that maximize the accessibility of the bulk for Li de/insertion. Surface modification can only address the transport problem in part, however. A key issue in these materials is also to unravel the factors governing ion and electron transport within the lattice. Lithium de/insertion in the phosphates is accompanied by two-phase transitions owing to poor solubility of the single phase compositions, where low mobility of the phase boundary limits the rate characteristics. Here we discuss concerted mobility of the charge carriers. Using Mössbauer spectroscopy to pinpoint the temperature at which the solid solution forms, we directly probe small polaron hopping in the solid solution Li(x)FePO4 phases formed at elevated temperature, and give evidence for a strong correlation between electron and lithium delocalization events that suggests they are coupled.
锂金属磷酸盐是高容量锂离子电池中最具前景的阴极材料之一。由于其固有的低电子导电性，优化其性能以最小化缺陷浓度和微晶尺寸（降至亚微米级别）、控制形态，并在微晶表面修饰导电纳米结构作为将电子输送到体晶格的通道至关重要。在此，我们讨论与橄榄石磷酸盐LiMPO4（M = Fe、Mn、Co、Ni）中的掺杂和缺陷相关的因素，并描述制备电导率范围为10^(-4)-10^(-2) S cm^(-1)的原位纳米相复合材料的方法。这些方法利用表面反应性在低至600摄氏度的温度下生成晶间氮化物、磷化物和/或磷碳化物，从而最大限度地提高锂脱嵌/嵌入体相的可达性。然而，表面改性只能部分解决传输问题。这些材料中的一个关键问题还在于揭示晶格内离子和电子传输的控制因素。由于单相组合物的溶解度较差，磷酸盐中的锂脱嵌/嵌入伴随着两相转变，其中相界的低迁移率限制了速率特性。在此我们讨论电荷载流子的协同迁移。利用穆斯堡尔光谱确定固溶体形成的温度，我们直接探测高温下形成的固溶体Li(x)FePO4相中小极化子的跳跃，并给出电子和锂离域事件之间强相关性的证据，这表明它们是耦合的。
