(Battery Division Student Research Award) Real-time Diffraction Studies of Electrode Materials for Li-ion and Na-ion Batteries

Performing in-situ and operando measurements on electrode materials for Li-ion and Na-ion batteries is of extreme importance for their improvement. These materials need to be studied in their environment (in-situ), without exposure to reactive agents (O₂, H₂O, etc). They also need to be studied in real time while they function (operando), since they normally operate in non-equilibrium conditions and the reactions involved are thus extremely dynamical. Real-time experiments upon charge/discharge of the electrodes (i.e. upon lithium or sodium extraction/insertion from/into the electrodes) unveil dynamics that are not accessible by other means and allow a more complete understanding of the electrodes’ functioning. In particular our work deals with the use of operando diffraction techniques, to study the structural modifications occurring in several different electrochemical systems. The use of different probes is another important requirement for the study of such reactions. Combined use of X-Ray Powder Diffraction (XRPD), Synchrotron radiation XRPD and Neutrons Powder Diffraction (NPD) allows observing any atomic element in any crystalline electrode. However, custom setups are required to carry out operandodiffraction experiments on batteries. Those for XRPD are common nowadays and can be purchased or, in the case of synchrotron XRPD, they were developed by several groups (ours included).  The most complicated setups are those for NPD, making it a less mature technique. This is because of the many difficulties it holds when applied to a real battery. Several scattering contributions are present, coming from different components and generating unwanted Bragg reflections and very high background.

I will address the development of a sample environment suitable for both electrochemistry and NPD. The task was accomplished designing an electrochemical cell featuring a (Ti,Zr) alloy and a deuterated version of the electrolyte. The resulting cell is able to function using massive electrodes (massive on the battery scale, small samples for neutrons) with good performances and able to give NPD patterns of high quality for data analysis. Our studies are conducted on the D20 high-flux diffractometer at Institut Laue-Langevin (ILL) in Grenoble(France), where the feasibility of real operando experiments using commercial LiFePO₄ was demonstrated. Importantly, we showed the possibility to succeed in reliable structural refinements (by the Rietveld method) and thus to observe structural modifications in details, from unit cell parameters to atomic coordinates and even site occupancy factors. I will discuss a few studies done with this setup, namely the observation of lithium extraction from different samples in the family of spinels Li_1+xMn_2-xO₄. We performed NPD in real time on three samples (LiMn₂O₄, Li_1.05Mn_1.95O₄ and Li_1.10Mn_1.90O₄) and showed how the Li/Mn ratio influences the phase diagram of the material (Figure 1). I will also discuss more recent experiments about high-voltage spinels such as LiNi_0.4Mn_1.6O₄.

As neutrons alone are not sufficient, we often combine them with other probes. A significant case is the study of lithium (de)intercalation in the oxyphosphate LiVPO₄O, an interesting 2-electron positive electrode material for Li-ion batteries (working on the V⁴⁺/V⁵⁺ and V⁴⁺/V³⁺ couple). For both domains (lithium extraction at 4 V vs. Li and lithium insertion around 2.2 V) we combined in-situ XRPD and ex-situXRPD and NPD to observe extremely rich sequences of biphasic reactions, often involving strain in the unit cell of the material (Figure 2). I will review the most significant findings on the phase diagram of this material and on the previously unknown crystal structures involved.

Another important example is related to the field of Na-ion batteries. We focused our efforts on Na₃V₂(PO₄)₂F₃ that demonstrates extremely good capacity and rate capabilities. In recent synchrotron radiation XRPD experiments we showed that the tetragonal space group used for the last 15 years to describe the structure was not the correct one, because of a subtle orthorhombic distortion observed thanks to high angular resolution measurements. This led to a new structural determination in the space group Amam, preserving the tridimensional framework of the material but inducing a different arrangement of sodium ions, which is the key property to understand sodium dynamics and therefore to improve the materials’ performances. Thanks to our new structure and to subsequent operando Synchrotron XRPD experiments the phase diagram of Na₃V₂(PO₄)₂F₃ upon Na⁺extraction was determined, showing its amazing richness in terms of intermediate phases involved and ordering/disordering phenomena.

Bibliography:

[1]M.Bianchini et al. Journal of The Electrochemical Society(2013), 160(11), A2176.

[2]M.Bianchini et al., Journal of Physical Chemistry C (2014), 118(45), 25947.

[3]M.Bianchini et al., Journal of Materials Chemistry A (2014), 2, 10182.

[4]M.Bianchini et al., Chemistry of Materials (2014), 26(14), 4238.

[5]M.Bianchini et al., Chemistry of Materials (2015), 27(8), 3009.