Oxide cathodes Olivine

This section aims at demonstrating the importance and relevance of the surface chemistry developed on cathodes for Li-ion battery systems to their performance. The topics selected for discussion are the effect of nano-size, surface chemical aspects of lithiated transition metal oxide cathodes and a comparison with the surface chemistry of L1MPO4 olivine-type cathodes. 

Based on their structure, cathodes can be separated into three basic families layered oxides, spinels, and olivine phosphates. The three families differ greatly in their cell performance as well as in thermal properties. Layered oxide cathodes typically have higher specific capacities than spinels or phosphates. As a consequence of their structure, layered oxide materials are more energetic upon thermal runaway events due to the ease with which oxygen is liberated from their framework upon decomposition. 

Reversible electrochemical lithium deintercalation from 2D and 3D materials is important for applications in lithium-ion batteries. New developments have been realized in two classes of materials that show exceptionally promising properties as cathode materials. The first includes mixed layered oxides exemplified by LijMn Nij, Co ]02, where the Mn remains inert to oxidation/reduction and acts as a framework stabilizer while the other elements carry the redox load. Another class that shows much potential is metal phosphates, which includes olivine-type LiFeP04, and the NASICON-related frameworks Li3M2(P04)3. 

The case of air cathodes is complicated, as is presented later, and definitely deserves a separate discussion in this chapter. Therefore, the most interesting and important discussion on the surface chemistry of the positive electrodes in Li batteries relates to lithiated transition metal oxides and Li olivine compounds. 

It should be noted that the type of cathode reaction has no direct effect on its surface chemistry. The most important aspects are the redox potentials, the particle size, and the level of reactivity of the surface oxygen atoms. Another important aspect relates to the ease of transition metal ion dissolution from the cathode material to the solution phase. In general, as the redox potential is lower, the cathode material is less reactive with the solution species. However, the redox potential is not the main important factor. The nucleophilicity and basicity of the oxygen atoms of the cathode compounds are also highly important. Li MOy compounds are much more basic and nucleophilic than LiMP04 compounds [13]. The phosphorous atoms at the 5+ oxidation state in the olivine compounds moderate the basic nature of the oxygen atoms. Thereby, olivine compounds are much less reactive to solution species than LLMOy compounds [ 14]. Consequently, they can be used as nanoparticles, which help to overcome their poor transport properties. The fluorine atoms in FeFs and its reduction product, LiF, are neither basic nor nucleophilic, and thereby this cathode material does not develop surface chemisfiy in conventional electrol)de solutions. Finally, as the particle size of cathode materials is smaller, they are supposed to be more surface reactive. 

Transition Metal Oxide Electrodes, and LiMPOa Olivine Cathodes Some Examples and Introductory Remarks... 

I = 7/2-1—> 5/2-F). The materials that can be studied, thanks to the Mossbauer effect of the above-mentioned nuclei, are also varied. Both cathode and anode materials can be examined. Moreover, the electrochemical reactions in which they are involved may vary from intercalation to conversion and/or alloying. Table 28.1 shows some examples. Fe MS provides useful information in the study of insertion cathodes, such as olivine LiFeP04, as well as layered solids structurally related to LiCo02. Fe MS is also useful to analyze anodes consisting of binary or ternary oxides for conversion reactions, or tin intermetallics that react with lithium by alloying processes. In the latter case, a multiisotope approach can be developed, due to the Mossbauer effect of both Fe and Sn nuclei. 

This book offers various comprehensive ideas on anode and cathode nanomaterials for new-generation lithium-ion batteries. Silicon and tin nano-alloy systems as the anode, discussed in the first several chapters, and olivine and spinel oxides as the cathode, in the latter chapters, are presented as nano-electrode materials with high power and high energy capacity. Readers will get some important electrode concepts from this book. ... 

---