Polymer‐Directed Crystal Growth Regulation: Template‐Free Synthesis of Mesoporous Single‐Crystalline Mn 2 OBO 3

Transition metal borates are attractive anode candidates for lithium‐ion batteries (LIBs) because of their high specific capacity and environmental benignity; however, their structural collapse induced by large volume changes during cycling leads to rapid capacity degradation. In this study, we report a polymer‐directed crystal growth regulation strategy that enables the template‐free synthesis of mesoporous single‐crystalline Mn 2 OBO 3 (MSC Mn 2 OBO 3 ). Polyethyleneimine (PEI) acts as a bifunctional regulator, coordinating Mn 2+ ions and hydrogen bonding with borate ions to modulate hydrolysis and condensation, thereby yielding a hybrid precursor. Subsequent hydrothermal treatment yields single‐crystalline MnBO 2 OH nanobelts, which are converted by calcination into MSC Mn 2 OBO 3 . As an anode active material for lithium‐ion batteries, MSC Mn 2 OBO 3 delivers a reversible capacity of 1440 mAh g −1 at 0.2 A g −1 , more than twice that of bulk Mn 2 OBO 3 . Notably, this polymer‐directed strategy is readily extendable to doped MSC Mn 2 OBO 3 (e.g., Fe, V, and Mg), providing a versatile platform for the design of porous single‐crystalline materials for energy storage.