Two-Dimensional Mixed Lead/Bismuth Oxychloride Based Materials As Anodes for Sodium-Ion Batteries

The continuous pursuit on building low cost and sustainable batteries to efficiently harvest renewable energy from intermittent sources is ever expanding. The research on Na-ion technology has gained significant interest due to inexpensive and broad availability of precursor materials. Until now, various Na-ion anodes including carbonaceous materials such hard carbon, alloy-based materials such as Bi, Pb, Sb and Sn based systems and conversion materials have been explored¹. Along this line, the recent chase on two dimensional (2D) materials such as chalcogenides, MXenes and van der Waals types oxyhalide heterostructures, have become more intense due to their excellent electrical and structural properties ². Among them, 2D Sillén-type metal oxyhalide heterostructures (MOX), due to their unique layered structure consisting of anionic sheets interleaved with cationic layers have gain particular interest as electrode materials. BiOCl is one such candidate with fluorite-like layers, [Bi₂O₂]²⁺ intergrown with halide Cl^– anions, have been studied as anode in rechargeable batteries. However, its application is mainly challenged by rapid capacity decay originating from particle pulverization and unstable solid electrolyte interphase (SEI).

In this work, 2D layered mixed lead bismuth oxychloride nanosheets (BiOCl⁴ and PbBiO₂Cl ³) were synthesized by new rapid room temperature chemical precipitation route. In former anode, the electrolyte engineering approach has been used to enhance the cycling stability of BiOCl and at the same time, its electrochemical (de)sodiation mechanism is also clarified through in-operando XRD studies, which reveals the reversible formation of Bi ↔ NaBi ↔ cubic-Na₃Bi phases. On the other hand, the later anode, i.e., 2D-PbBiO₂Cl was further investigated with the combined in-operando XRD and ex-situ TEM, XPS studies, which reveal a distinct (de)sodiation process of the 2D-PbBiO₂Cl in comparison with reported Na-(Pb/Bi) phase diagram. This 2D heterostructure demonstrated stable sodium storage performance (~250 mAh g^–1 for 250 cycles at 100 mA g^–1). Henceforth, the detailed understanding of formation process of these two structures (BiOCl and PbBiO₂Cl) at room temperature using material characterizations and electrochemical studies combined with operando findings will be discussed to comprehend the reversible storage mechanism of 2D-MOX type heterostructures as anodes in SIBs.

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