there have been tremendous efforts to improve their performance, safety, costeffectiveness, and sustainability. [1] In the last four decades, dedicated research in material science toward developing electrodes, separators, electrolytes, current collectors, packaging materials, etc., have significantly contributed to the advancement of LIB technology. [2] Indeed, the progress in cell engineering and processing conditions has aided the evolution of high energy density LIBs (energy density >250 Wh kg −1 ), leading to their widespread application spanning from electronic devices to electric vehicles and stationary/ grid energy storage. [1] Considering the contributions that led to the invention of LIB technology, John B. Goodenough, Stanley Whittingham, and Akira Yoshino received the Chemistry Nobel Prize in 2019. [3] Despite their huge success, research and development on LIBs and allied battery technologies are still a hot topic in both academia and industry offering opportunities for breakthrough innovations and discoveries. Figure 1a presents a schematic illustration of the state-of-theart LIB and its main components. Liquid electrolytes based on organic solvents and lithium salts are the primary choice as electrolytes in LIBs due to their high ionic conductivity and good Polymer composite electrolytes (PCEs), i.e., materials combining the disciplines of polymer chemistry, inorganic chemistry, and electrochemistry, have received tremendous attention within academia and industry for lithiumbased battery applications. While PCEs often comprise 3D micro-or nanoparticles, this review thoroughly summarizes the prospects of 2D layered inorganic, organic, and hybrid nanomaterials as active (ion conductive) or passive (nonion conductive) fillers in PCEs. The synthetic inorganic nanofillers covered here include graphene oxide, boron nitride, transition metal chalcogenides, phosphorene, and MXenes. Furthermore, the use of naturally occurring 2D layered clay minerals, such as layered double hydroxides and silicates, in PCEs is also thoroughly detailed considering their impact on battery cell performance. Despite the dominance of 2D layered inorganic materials, their organic and hybrid counterparts, such as 2D covalent organic frameworks and 2D metal-organic frameworks are also identified as tuneable nanofillers for use in PCE. Hence, this review gives an overview of the plethora of options available for the selective development of both the 2D layered nanofillers and resulting PCEs, which can revolutionize the field of polymer-based solid-state electrolytes and their implementation in lithium and post-lithium batteries.