The endless pursuit on building low cost and sustainable batteries to effectively harvest renewable energy from intermittent sources is ever increasing. 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 have been explored including carbonaceous materials such hard carbon, alloy-based materials such as Bi, Pb, Sb and Sn based systems and conversion materials [1]. 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. For instance, the 2D-(Bi1-xFex)OCl have increased insertion kinetics in Li-ion storage performances [3] in comparison with unsubstituted BiOCl, whereas 2D-(Bi1-xSbx)OCl heterostructures have shown synergetic effect of (Bi/Sb) alloys on with improved K-ion storage capacity [4]. However, in continuation, the further assessment of other mixed cationic Sillén heterostructures with the crystal structures of [BixM1-xO2]n+ (M = Na+/Ca2+/Pb2+/Fe3+/Sb3+; n = +1, +2 and +3) layers intergrown with single/double/triple halide anionic layers, remain unexplored. This is mainly challenged by their current synthesis strategies which demand high temperature, longer reaction durations, toxic chemicals, and complex reaction setup. In this work, 2D layered lead bismuth oxychloride (PbBiO2Cl) heterostructure nanosheets were synthesized by new rapid room temperature chemical precipitation route within ~45 mins. The stepwise reaction intermediates were harvested and investigated further with XRD and SEM mechanistic studies. The formation proceeds through topotactic ion exchange route from analogous PbBiO2NO3 intermediate. When employed as an anode material in Na half-cell configurations (as presented in Figure 1), this 2D heterostructure demonstrated stable sodium storage performance (~250 mAh g–1 for 250 cycles at 100 mA g–1). Surprisingly, the electrochemical profile of the 2D-PbBiO2Cl significantly differs from those of its individual counterparts Pb, Bi anodes. This 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. Henceforth, the detailed understanding of formation process of 2D-PbBiO2Cl at room temperature using non-operando material characterizations and electrochemical studies combined with operando findings will be discussed to comprehend the reversible storage mechanism of 2D-PbBiO2Cl type heterostructures as anodes in SIBs. References: Y. Hwang, S.T. Myung, Y.K. Sun, Chem. Soc. Rev. 2017, 46, 3529–3614. Zeng, Y. Xiao, J. Liu, K. Yang, L. Fu, Chem. Rev. 2018, 118, 6236–6296. Myung, J. Choi, F. Wu, S. Banerjee, E. H. Majzoub, J. Jin, S. U. Son, P. V. Braun, P. Banerjee, ACS Appl. Mater. Interfaces 2017, 9, 14187–14196. Wang, B. Wang, B. Lu, Adv. Energy Mater. 2020, 10, 2000884. Figure 1