Evidence of Zintl Intermediate Phase and Its Impacts on Li and Na Storage Performance of Pb-Based Alloying Anodes

Anodematerials based on conversion and alloying reactions arepromising to achieve high energy density of advanced sodium-ion batteries(SIBs). While the chemical similarities between sodium and lithiumas alkali elements make the benchmarking strategy practical in developingnew high-performance anodes, simply borrowing the anode material fromone system to the other does not always guarantee success unless itis based on sound understanding of both Li- and Na-reaction mechanisms.In this work, we report the Na storage performance of a Pb-based anodeand its fundamental reaction dynamics. In contrast to its excellentelectrochemical performances in Li cells (reversible similar to 600 mAh/g),the newly developed Pb@PbO-C nanocomposite anode has limitedelectrochemical Na reaction properties showing moderate capacity andrate performances (similar to 300 mAh/g at 20 mA/g). Synchrotron-basedX-ray diffraction and absorption spectroscopy studies reveal the fundamentaldifferences in the Na and Li reaction mechanism of the Pb-based anode.Unlike Li reaction, the unique Na reaction mechanism involves theformation of a highly ionic NaPb Zintl phase, which comprises tetrahedralPb(4) clusters, as an intermediate phase. The strong covalentcharacter of the Pb-4 Zintl clusters adversely affects theelectronic conductivity and thus limits the electrochemical performanceof the Pb-based anode in Na cells. These findings provide new insightsapplicable to developing high-performance alloying anode materials.