**4. Case studies in battery materials**

inducing a transformation of the unstable cycled material [49]. The effects of sampling deviations are also eluded since the sample remains in the same position during the whole measurement series. Finally, the whole study can be performed on a single test cell suppressing the effects of uncontrolled differences in a set of cells which are needed for a stepwise *ex situ* study of the electrochemical mechanism. To perform such an experiment, a special *in situ* electrochemical cell, obeying to the specific requirements of XAS, has to be used. This cell consists of an electrode containing the active material, a lithium foil, a separator, which is typically a polymeric membrane such as Celgard, and an electrolyte, usually based on organic carbonate solvents such as propylene carbonate (PC), dimethyl carbonate (DMC)

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

**Figure 4** displays two different types of *in situ* electrochemical cells. The first one (left) is a typical pouch cell which is characterized by a large dimension of the cathode. In this case, a film containing the active material is previously deposited on a square Al (or Cu) current

material, depending on the energy of the X-ray. Sometimes, a small tube (visible in the right part of the cell) can be used as a sink for the gas, which may be released during the electrochemical processes and which can be analyzed in line, if necessary. The figure on the right displays a typical stainless steel cell [50], which uses self-supported films or pellets of electrode material of smaller dimension (1 cm diameter). The versatility of this second approach is testified by the successfully use of this cell in transmission and fluorescence geometry, as well as in other techniques including *in situ* XRD [51], Mössbauer [52] and Raman spectros-

**Figure 4.** Typical *in situ* electrochemical cells used for *operando* XAS studies of batteries: A pouch cell (left) and a stainless

a separator and the electrolyte. The mass loading varies between 2 and 15 g/cm<sup>2</sup>

and assembled in a glove box together with a Li (Na) counter-electrode,

of active

and ethylene carbonate (EC).

collector of 4 cm<sup>2</sup>

copy [53] measurements.

steel cell (right) mounted on different XAS beamlines.

Given the large amount of physico-chemical information that it usually carries, already mentioned in the previous sections, XAS has been largely applied to the study of battery materials [18, 19]. A few particular case studies, specifying specific features of this technique in particular cases involving nanostructured species, are presented in the following paragraphs. It will be stressed, in particular, the importance of performing *in situ* studies compared to more simple, but also often less reliable, *ex situ* measurements.

### **4.1.** *Ex situ* **studies of lithium-excess manganese layered oxides**

The relative abundance of manganese coupled with their variety of oxides structures, which provides generally a three-dimensional array of edge-shared MnO<sup>6</sup> octahedra for the lithium insertion and release, has aroused the interest of developing positive-electrode materials based on manganese oxide. Due to the well-known poor cycling capability of the spinel structure LiMn<sup>2</sup> O4 , where a cooperative Jahn-Teller distortion of the Mn3+ ion causes a cubic-to-tetragonal phase transition leading to a rapid degradation of the electrode, an intensive research has been focused on alternative materials. Solid solutions of layered cathode materials such as the combination of Li<sup>2</sup> MnO3 and LiMO<sup>2</sup> (M = Mn, Co, Ni, etc.) have been proposed as promising candidates for cheaper, higher capacity and safer positive electrode for lithium batteries. However, the occurrence of an initial activation process during the first delithiation step (first charge) is always accompanied by a large irreversibility in terms of specific capacity. To gain a deeper understanding of the initial activation step and to study the following delithiation-lithiation process, an electronic and local structural characterization of the host material is required and the XAS is the technique of choice. A series of electrodes with different lithium concentration (state of charge, SOC) were studied in a series of lithium-rich, cobalt-poor Li[Li0.2Ni0.16Mn0.56Co0.08]O2 electrode material (NMC), as an examples of *ex situ* XAS investigation [54, 55]. Due to the strong sensitivity of the XAS to the metal site, spectra at the three different metal edges can be measured, allowing the study of the evolution of the physico-chemical properties and of the local structure of each metal site.

**Figure 5** shows the voltage profile of the cell during charge-discharge operation. The numbered points in the curve indicates predetermined states of charge (SOC) at which cells were prepared for the XAS measurements. **Figure 6** summarizes the XAS analysis conducted on the materials, where all the several portions of the X-ray absorption spectrum carry valuable information on the local and electronic structure: pre-edge, XANES and EXAFS. The preedge analysis (the Mn K-edge is displayed in the figure, showing two components) allowed the authors to check the variation of the Mn local site, in terms of symmetry and charge. XANES traces can provide the identification of the electroactive sites at different SOC and the EXAFS analyses the local structural information of the selected metal site. This information is complementary with respect to XRD which probes the long-range order in crystalline materials.

**Figure 5.** Voltage profile of two successive charge and discharge curves of Li-rich NCM at 20 mA/g. Representative points of 1–10 in the process of XAS measurements are indicated. Reference and counter electrode: Li. electrolyte: 1 M LiPF<sup>6</sup> in EC/DMC. Temperature: 20°C ± 2°C. On the upper X axis the capacity detected in each step is reported. Reproduced from Ref. [54].

**Figure 6.** XAS data analysis for the cathode material. The picture displays analysis of the pre-edge data obtained at the Mn K-edge (left panel) including the fitting of the observed peaks (at the bottom). These data provide both charge and symmetry information around the investigated metal. Data at the right panel refer to XANES behavior at the Ni K-edge (up) and the best-fit of the EXAFS data in terms of single contribution to the total EXAFS oscillation (right). At the bottom the fourier transform (FT) behavior of the corresponding EXAFS is displayed. Reproduced from ELETTRA Highlights 2014–15, page 12.

The study here highlighted demonstrates that the manganese is not taking part of the initial electrochemical oxidation process, but a complete Ni2<sup>+</sup> /Ni4+ and a partial Co3+/Co4+ redox processes occur during the first charge of the battery. The electrochemical performance of the material, considering the full and partial redox inactivity of Mn and Co, also reveals the participation of oxygen in the overall electrochemical redox process. Analysis of EXAFS at the three metal edges has revealed that the first charge of the lithium-rich cathode can be described by two separate reactions occurring at the two components, Li<sup>2</sup> MnO3 and LiMO<sup>2</sup> : an activation of the Li<sup>2</sup> MnO3 component with a phase transition to an *hexagonal* layered structure and the oxidation/reduction of both Ni and Co which is not only demonstrated by pre-edge/XANES data but also corroborated by the first shell M-O distances behavior and their corresponding Debye-Waller factors.
