Polymorph with Triphase Heterojunctions Enhancing Interfacial Stability for Long‐Lasting Quasi‐Solid‐State Lithium Metal Batteries

ABSTRACT The ionic conductivity and interfacial stability of composite solid electrolytes (CSEs) are arduous to satisfy the practical application requirements of quasi‐solid‐state lithium metal batteries. Herein, bifunctional Fe‐TiO 2 polymorph (anatase, rutile, and brookite) is incorporated into a poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) based composite solid electrolyte (PLFT), with Li 0.305 La 0.565 TiO 3 (LLTO) as an auxiliary conductive component. The doping of Fe drives the crystalline phase transition of TiO 2 from a single‐phase to a multiphase structure, inducing the formation of oxygen vacancies, while generating phase heterojunctions among different crystalline phases. The oxygen vacancies effectively immobilize lithium salt anions, facilitate salt dissociation, and release free Li + , which migrates preferentially from the polymer matrix toward LLTO. These coupling effects enable PLFT to have a high ionic conductivity (7.3 × 10 −4 S cm −1 ) and Li + migration number (0.83) at 25°C. Meanwhile, the built‐in electric fields also weaken the space charge layer (SCL) between electrolyte and electrode, reducing the interfacial potential and side reactions. Therefore, Li|PLFT|Li symmetrical battery cycles stably for 4800 h at 0.1 mA cm −2 . Li|PLFT|LiFePO 4 (Li|PLFT|LFP) battery retains 83% capacity after 3000 cycles at 1 C. Even high‐voltage Li|PLFT|LiNi 0.5 Co 0.2 Mn 0.3 O 2 (Li|PLFT|NCM523) battery retains a capacity of 147 mAh g −1 after 500 cycles at 1 C.