Deciphering the Failure Mechanism and Improvement Strategy of High‐Sulfur‐Loading Na‐S Batteries and Promoting Polysulfide Conversion by Electrochemically Reconstructed Cu 1.84 Mo 6 S 8 /Na 2 Cu 4 S 3 Heterostructure

ABSTRACT High‐sulfur‐loading RT Na‐S batteries usually encounter the “under‐voltage failure” problem, which will lead to battery damage and long‐term cycling termination. Herein, we investigate the failure causes and mitigate the failure issue at both the battery level (polyethylene@ketjen black‐polyacrylonitrile‐glass fiber@ketjen black (PE@KB‐PAN‐GF@KB) composite separator construction) and material level (Te addition) through alleviating chemical/physical micro‐short circuits. To catalyze the conversion of sodium polysulfides (NaPSs), by referring to the PE@Te/KB‐PAN‐GF@KB separator model, we reassemble a high‐sulfur‐loading RT Na‐S battery with a PE@Te/resin carbon‐PAN‐GF@MoO 3 /resin carbon composite separator, and innovatively propose an electrochemical reconstruction strategy for generating a Cu 1.84 Mo 6 S 8 /Na 2 Cu 4 S 3 heterojunction catalyst encapsulated in resin carbon. It is uncovered that at the interface between the conductive Cu 1.84 Mo 6 S 8 and the adsorptive Na 2 Cu 4 S 3 , a built‐in electric field spontaneously emerges to achieve the redistribution of interface charges and expand the active area for capture‐migration‐transformation of NaPSs. Moreover, the lower conversion barrier heightens the catalytic activity of Cu 1.84 Mo 6 S 8 /Na 2 Cu 4 S 3 during the bidirectional sulfur conversion process. Consequently, the high‐sulfur‐loading RT Na‐S battery featuring the reconstructed Cu 1.84 Mo 6 S 8 /Na 2 Cu 4 S 3 heterojunction delivers outstanding electrochemical performance and excellent cycle life. This comprehensive research deepens the understanding of the failure behaviors and performance improvement mechanisms of RT Na‐S batteries, and guides the synthesis of heterostructures for advanced batteries.