Mechanistic Insight into the High Voltage Stability of Fe-Rich Transition Metal Layered Oxides As the Positive Electrode of Na-Ion Batteries
Authors: Saurel Damien , Silván Begoña , Gonzalo Elena Candida , Lian Jian Xiang , Carrasco Javier 
Published: December 2020

Transition metal layered oxides (TMLOs) of general formula LiTMO2 are the cornerstone of the present Li-ion technology as these cathode active materials allow the highest energy density thanks to the large capacity they offer. This is due to the high concentration of alkali metal their structure includes in the form of Li layers between TMO2 metal oxide layers, with Van der Walls bounds between the two type of layers. However, this structural particularity also consists in their main drawback: a full charge would require removal of 1/4 of the atoms of the structure, which consists in ½ of its structural layers. The layered structure cannot sustain such heavy change and tends to irreversibly transform into a more 3D structure, such as spinel or rock-salt. As a consequence, only 50 to 70% of the theoretical capacity is effectively used to ensure reversible electrochemical response with good cycle life.
Na-TMLOs share the same advantages that Li-TMLOs, as the best compounds present the highest capacities of all known Na-ion cathodes, but also the same drawbacks as the structural instability under deep deintercalation is even more pronounced than in the case of Li. However, Na-TMLOs have an additional cost-related advantage. Stable compounds can be rich in abundant elements such as Fe, Mn and Ni, without the requirement of incorporating critical metals such as Co. Fe is especially interesting because it allows a fairly high working potential while being from far the most abundant. However, Fe-rich Na-TMLOs show the least structural stability under Na extraction due to the occurrence of TM migration to the Na-layers which is believed to be irreversible and thus responsible for the fast degradation of the electrochemical response of these compounds.[1] The mechanism of Fe migration is thus the focus of the intense research activity so as to understand its features as well as its correlation with the degradation of the electrochemical performance, see e.g. refs. [1-4].
New mechanistic insights on the high voltage structural evolution of Fe-rich Na-TMLOs will be presented based on the combination of various operando techniques, coupled PITT and impedance spectroscopy, and with the support of DFT calculations. The mechanism of Fe migration will be described in detail as well as its correlation with the electrochemical response. It will be showed how surprisingly reversible is the Fe migration phenomenon in certain compounds, as shown in Figure 1,[4] and that the degradation of the electrochemical performance actually occurs when oxygen redox is involved,[5] the two phenomena being closely intercorrelated. How the dilution of Fe and the nature of the initial structure, P2 or O3, influences the dynamics and reversibility of Fe migration will also be shown. Finally, it will be highlighted how the high voltage evolution of Fe-rich TMLOs can easily go unnoticed using operando synchrotron X-ray diffraction depending on the design of the electrochemical cell.

References:

[1] X. Li et al, Chem. Mater., 28, 6575 (2016).
[2] D. Susanto et al., Chem. Mater. 31, 3644 (2019).
[3] Y. Li et al., Nano Energy 47, 519 (2018).
[4] B. Silván et al., J. Mater. Chem.
A 6, 15132 (2018).
[5] B. Silván et al, in preparation.

Figure 1