Tin (Sn) shows promise as a robust electrode material for rechargeable sodium-ion (Na-ion) batteries, according to a new study by a team from the University of Pittsburgh and Sandia National Laboratory. A paper on their study of the microstructural changes and phase transformations of tin nanoparticles during electrochemical sodiation is published in the ACS journal Nano Letters.
Rechargeable Na-ion batteries work on the same basic principle as Li-ion batteries—i.e., reversible and rapid ion insertion and extraction, but using sodium ions rather than lithium. The researchers built a nanosized Na-ion battery using tin nanoparticles as a model system to study the fundamental science of the Na insertion and extraction processes.
Motivated by the success in the development of Li-ion batteries, there is growing interest in Na-ion batteries for electrical vehicles and power backup applications. Advantages of Na-ion batteries over Li-ion batteries include the natural abundance and low cost of Na, especially for future large-scale applications like solar or wind farms. Guided by the chemistry of Li-ion batteries, potential candidate materials for Na-ion batteries include hard carbon, other Group-IV elements like Si, Ge, Sn, and various metallic oxides for the negative electrode, and NaCrO2, NaTi2(PO4)3, and Na(Ni0.5Mn0.5)O2 for the positive electrode.
Theoretical investigations have shown that the Na alloys and sodiated hard carbon have significantly lower volumetric energy densities than that of Li alloys and lithiated graphite, due to the increased size of Na atoms as compared to the Li atoms. Therefore, high-energy electrodes are desired in the first place for the development of competitive Na-ion batteries. However, in contrast to the rich literature on Li-ion batteries, the science of Na-ion batteries is much less understood in most material systems.—Wang et al.
They found that the electrochemical sodiation of the crystalline Sn nanoparticles (NPs) occurs in a two-step reaction at room temperature, characterized by transition from a two-phase mechanism to a single-phase mechanism.
- In the first step, an amorphous NaxSn phase is growing by consuming the pristine Sn with a moving phase boundary, and formation of such a Na-poor phase accounts for a modest volumetric expansion around 60%.
In the second step, continued Na insertion leads to formation of several Na-rich amorphous phases and finally the crystalline Na15Sn4 phase. The total volumetric expansion after full sodiation is about 420%.
The separation of the two-phase and single-phase regimes during sodiation of the Sn NPs is quite unique. Compared to the analogous lithiation process of Si, the first step of two-phase sodiation is similar to the structural evolutions in Si and Ge upon Li insertion, where a sharp interface is migrating during the growth of new phases and depletion of the old ones. However, unlike approaching the final compositions during the initial amorphization of Si24 and SnO2, more Na+ ions could be inserted into the Na-poor a-NaxSn particles without an obvious reaction front, and as a result the volume of the a-NaxSn NPs continued to grow. In fact, the second single-phase stage accounts for most Na insertion and volumetric expansion and thus plays an important role in the mechano-electrochemical coupling process. Particularly, despite the large volumetric expansion over 400% at full sodiation, no Sn NPs cracked or fractured, which was quite surprising and unexpected.—Wang et al.
They also found that the tin nanoparticles showed excellent cyclability during the sodiation/desodiation cycles. In the first three cycles, the Sn NPs underwent nearly reversible volumetric expansion and shrinkage. In the following three sodiation/desodiation cycles, the Sn NPs were fully sodiated and a larger volumetric expansion was obtained. The shape of desodiated Sn NPs became more and more irregular with increasing cycles.
The shape change of Sn NPs might be induced by the structure reconstruction as observed in Li ion batteries using Sn NPs as the anode, the researchers suggested, and also observed that, as a low-melting-point metal, and with the reconstruction of tin after Na extraction, “it seems that Sn may be engineered to make a self-healing electrode yet with high energy density”.
Jiang Wei Wang, Xiao Hua Liu, Scott X. Mao, and Jian Yu Huang (2012) Microstructural Evolution of Tin Nanoparticles during In Situ Sodium Insertion and Extraction. Nano Letters doi: 10.1021/nl303305c