Li Ion Migration at the Interfaces
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Janko Jamnik and Miran Gaberscek Abstract During the past decade, the electrochemical properties (energy density, power capability, and cycling stability) of practical lithium (Li) batteries have been enormously improved. Surprisingly, although the knowledge exists of how to prepare excellent batteries, a detailed understanding of how they actually work is still lacking. In particular, the impact of interfaces in electrode composites is poorly understood. Here, we collect the most advanced mechanistic studies performed in our laboratory or published in recent literature and try to embed this knowledge into the well-established concepts used in solid-state ionics for many decades. In particular, we focus on the so-called perpendicular and parallel interfacial effects. We show that much of the old wisdom can be applied to batteries, although there are several important differences. We discuss, in some detail, the effects of charge incorporation, electronic interphase contacting, electrode porosity, and heterogeneous doping in selected advanced electrode materials and emphasize the future perspectives.
Introduction Electrodes for lithium batteries are multiphase systems involving chemically and physically very different phases. As can be deduced from Figure 1, some of these phases appear in the form of nanoparticles. Specifically, the active particles (in which Li is stored) are inorganic crystallites of between 10 and 200 nm in size. Between these crystallites is a conductive carbon phase, typically in the form of carbon black particles of the order of 100 nm in size. Both types of nanoparticles are held together mechanically by a third phase—a polymeric binder (not shown in Figure 1). Furthermore, the whole composite is soaked with a liquid electrolyte containing Li ions—the fourth phase. Finally, during the charge/discharge processes, additional phases are usually formed (for example, a solid electrolyte interphase), while some phases may simultaneously be consumed. It is no surprise that such a complex and dynamic multiphase system is still poorly understood. Besides, most Li battery research has been focused on understanding the structural features of the active crystallites,1–5 while much less attention has been devoted to the other phases or the interfaces between the various phases. However, the systematic electrode trans942
port studies6–10 that have been carried out indicate that the role of certain interfaces and non-active phases may be much more important than generally believed in the Li battery community. In this article, we not only present the major results of such interfacial studies in batteries but also try to embed them into the well-established concepts known from polycrystals and ceramic composites as treated by classic work in the field of solid-state ionics (SSI). The impact of interfaces on the conduction and chemical diffusion in polycrystals and in composites is usually discussed separately in terms of the socalled perpendicular and parallel effects (that is, the modifications that occur
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