Metal hydrides as anode materials for Li-ion batteries are receiving increased attention due to the high theoretical capacities and low cost of many of these materials. Among conversion-type electrodes, metal hydrides have relatively high energy density and low polarization (ΔV ≈ 0.2 V) [1]. Similar to other conversion-type materials, the main challenges of metal hydrides are the large volume change and particle growth during operation, which leads to poor capacity retention.

Nanoconfinement, a bottom-up approach for preparing nano-sized materials, effectively limits the size of particles to the pore size of the nano-scaffold. In addition, nano-scale diffusion distances facilitate transport of Li-ions and enhance the kinetics of the conversion reactions.

In this study, we present the improved electrochemical performance of sodium aluminum hydride, NaAlH₄ through nanoconfinement in mesoporous carbon scaffolds with different pore sizes. NaAlH₄ has a theoretical capacity of 1985 mAh g^-1. Although NaAlH₄ is shown to be electrochemically active, large capacity fade is observed already during the first charge with a typical reversible capacity of only 30 % [2]. In contrast, nanoconfined NaAlH₄ shows a 70 % recovery of capacity during the initial discharge-charge cycle.

We have investigated the reaction mechanism of the conversion reaction by operando synchrotron radiation powder X-ray diffraction (SR-PXD), and found important differences between nano-confined and non-confined samples. The mechanism and electrochemical performance are further investigated by detailed electroanalysis and conductivities of NaAlH₄ electrodes are examined by electrochemical impedance spectroscopy (EIS). The electrochemical reactivity of empty carbon scaffold and its contribution to the battery performance will also be discussed.

This work is supported by European Marie Curie Actions under ECOSTORE grant agreement n° 607040.

References

[1] Y. Oumellal, A. Rougier, G. A. Nazri, J-M. Tarascon, L. Aymard, Nat. Mater., 2008, 7, 916.

[2] J. A. Teprovich, J. Zhang, H. Colón-Mercado, F. Cuevas, B. Peters, S. Greenway, R. Zidan, M. Latroche, J. Phys. Chem. C, 2015, 119, 4666.