**1. Introduction**

With lithium ion batteries becoming a mature technology, it is now clear that applications such as electric vehicles require chemistries with higher energy density to reach acceptable driving ranges without compromising performance and comfort already common with conventional vehicles. This pushed high research efforts in the area of lithium‐air (or more precisely lithium‐oxygen) batteries, presenting the highest energy density among known scalable battery devices, and of metal‐air batteries in general. However, the difficulty to obtain a

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

high reversibility and long cycle life still implies a significant barrier to become a technology. In many applications the precise knowledge of composition and morphology of materials at the nanoscale is a key to control performance and reliability. Metal‐oxygen batteries are one of these cases; in fact, they involve complex reaction and precipitation processes that need to be understood in detail to obtain true reversible operation. The determination of composition as a function of position in the nano‐sized precipitate at different charging states is the most valuable information for this understanding. A few physical techniques are routinely used to reveal the processes underlying battery behavior, e.g., XRD, TEM, SEM, XPS, FTIR, Raman, and mass spectroscopy. In this chapter we present the application to this problem of energy‐ dependent full‐field transmission soft X‐ray microscopy. This spectromicroscopical technique based on synchrotron radiation is able to give a full picture at the nanometer scale of the oxidation state and spatial distribution in the cathode of oxygen, the most relevant element in any metal‐air battery. Although a host of physical techniques are routinely used to reveal the processes underlying battery behavior (e.g., XRD, TEM, SEM, XPS, FTIR, Raman, and mass spectroscopy), this technique is unique in providing high‐resolution imaging with chemical information.

We will first introduce the basic concepts of metal‐air batteries and transmission X‐ray microscopy, then detail how the technique can be applied to battery electrodes, and finally provide some examples of studies that we performed on lithium‐oxygen cathodes, which can be easily extended to other metal‐air systems and all those materials where oxygen redox reactivity is involved.
