**2. Metal‐air batteries and the nature of discharge products**

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 high reversibility and long cycle life still implies a significant barrier to become a technology. Compared to conventional lithium‐ion batteries the reversibility and cycle life of metal‐air batteries is generally frustrating [1]. While in a lithium‐ion battery the main process is essentially ion transport through different phases that essentially leave electrode interfaces unaffected, in metal‐air batteries reversible electrodeposition processes have to take place at the electrodes. For instance, with a lithium anode and aprotic electrolytes molecular oxygen is reduced and precipitates as insulating lithium peroxide inside a porous carbonaceous cathode [2]:

Anode:2 Li → 2 Li<sup>+</sup> + 2 e<sup>−</sup> Cathode:O2 <sup>+</sup> <sup>2</sup> Li<sup>+</sup> <sup>+</sup> <sup>2</sup> <sup>e</sup><sup>−</sup> <sup>→</sup> Li2 O2<sup>↓</sup> The latter reaction may proceed through two subsequent one‐electron oxygen reduction steps or through chemical disproportionation of electrochemically generated superoxide:

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

96 X-ray Characterization of Nanostructured Energy Materials by Synchrotron Radiation

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

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 high reversibility and long cycle life still implies a significant barrier to become a technology. Compared to conventional lithium‐ion batteries the reversibility and cycle life of metal‐air batteries is generally frustrating [1]. While in a lithium‐ion battery the main process is essentially ion transport through different phases that essentially leave electrode interfaces unaffected, in metal‐air batteries reversible electrodeposition processes have to take place at the electrodes. For instance, with a lithium anode and aprotic electrolytes molecular oxygen is reduced and precipitates as insulating lithium peroxide inside a porous

Anode:2 Li → 2 Li<sup>+</sup> + 2 e<sup>−</sup>

Cathode:O2 <sup>+</sup> <sup>2</sup> Li<sup>+</sup> <sup>+</sup> <sup>2</sup> <sup>e</sup><sup>−</sup> <sup>→</sup> Li2 O2<sup>↓</sup>

**2. Metal‐air batteries and the nature of discharge products**

information.

involved.

carbonaceous cathode [2]:

$$\rm Li^{\cdot} + O\_{2} + e^{-} \to LiO\_{2}$$

$$2\,\mathrm{LiO\_2} \rightarrow \mathrm{Li\_2O\_2} \downarrow \mathrm{O\_2}$$

This implies problems of controlling nucleation and growth processes, but more importantly the oxygen chemistry hugely increases complexity and triggers parasitic reactions with the organic electrolyte and with both electrodes [3, 4].

However, the control of texture, composition, and crystallinity of the discharge products can also have important consequences on the capacity, rate capability, and reversibility. In fact, capacity directly depends on the discharged peroxide volume, which depending on its morphology can be more or less well distributed in a given porous network before it becomes clogged or passivated [5]. The size of the precipitate particles also affects rechargeability: conditions that favor deposition of smaller particles obtain better reversibility [6, 7], likely for the more favorable surface‐to‐volume ratio. Evidence of the reaction intermediate superoxide in the precipitate has been reported [8–11] and it has been demonstrated that this is easier to oxidize than peroxide [12]. In effect Na/O2 and K/O2 cells, where superoxide prevails, have remarkably higher reversibility than Li/O2 [13, 14].

A precise characterization of the nature and the evolution of the discharge products, and in particular the oxidation state of oxygen, is therefore essential to understand the processes underlying the electrochemical behavior of the cell, and can lead to improvements in materials and operating conditions. Given the light elements involved and the poor stability of incompletely reduced Li‐O compounds many imaging, spectroscopic, or diffraction techniques are not suitable for their compositional analysis. In addition, amorphous phases are occasionally possible with Li2 O2 (which may benefit rechargeability) [15] and usual with Li superoxide [10]. It is then evident that a technique such as full‐field transmission spectromicroscopy has high value, being able to accurately spatially resolve distributions of superoxide, peroxide, and other oxygen compounds even within a single particle, and regardless of crystallinity.
