Operando X-Ray Diffraction As a Tool to Monitor Compositional Gradients in Battery Electrodes

Batteries are a crucial technology for a society to move away from fossil fuels. The amount of energy a battery can store is proportional to the amount of charge transferred by the cathode and anode [1]. To maximize this energy, electrode thickness is maximized. These electrodes are made of a porous active material which allows for lithium transport in and out of the materials. Inhomogeneities will be observed due to the non-uniformity of the pores, as well as the conductive additive within the cell. Therefore, the rate of lithium insertion/removal is dependent on the location within the electrode. As the electrode thickens, the inhomogeneities will increase as a function of the distance from the anode, resulting in gradients normal to the electrode. The development of these gradients is also associated with transport within the active material.

X-ray diffraction (XRD) is a powerful technique that can provide structural information on the unit cell [2]. Performing operando experiments can reveal information on the dynamic composition of the electrode. The materials of interest are various compositions of LiMO₂ (M=Ni, Mn, Co, Al), crystallizing in a layered α-NaFeO₂-type structure. These materials have high theoretical charge capacities, yet can only stably cycle at a fraction of them during operation [3]. During each charge cycle, the material transforms following a second-order phase transition involving continuous miscibility between end states, which is easily followed by diffraction. In the presence of chemical gradients that take place during the transformation, the XRD peaks would progressively broaden. These gradients will vanish when the circuit is open. We validated this hypothesis with LiNi_0.33Co_0.33Mn_0.33O₂ (NCM111) electrodes. Peaks broadened during the reaction and faded during relaxation, especially at high rates. Figure 1 demonstrates this phenomenon when charging to 4.3V at a 1C rate (60 minutes for full charge). We leveraged the high penetration enabled by the 115 keV beam in beamline 11-ID-C at Advanced Photon Source to do the measurements in coin cells without thin windows, to ensure constant isostatic pressure and avoid undesired sources of chemical gradients. In this presentation, we will summarize the results obtained in a variety of experimental conditions, such as rate, voltage window and electrode porosity. The observations help explain how gradients are formed within positive electrodes based on layered oxides and can provide information on why the theoretical capacity has yet to be achieved in application.

References

1. A. Van der Ven, J. Bhattacharya, A. A. Belak, Accounts of Chemical Research 46, 1216-1225 (2012)

2. He, B. Two-dimensional X-ray Diffraction. 2009.

3. Goodenough, J. B.; Kim, Y. Chem. Mater. 2010, 22, 587-603.