The asymmetric behaviour of Li metal electrodes in Li/Li symmetric cells is demonstrated in terms of electrochemical performance and changes in the morphology of Li metal.

Li metal batteries have been considered a promising alternative to Li-ion batteries because of the high theoretical capacity of the Li metal. There have been remarkable improvements in the electrochemical performance of Li metal electrodes, although the current Li metal technology is not sufficiently practical in terms of cycle performance, safety, and volume change during cycling. Herein, the role of pore size distribution in the Li metal plating behavior of porous frameworks is clarified to attain the ideal pore structure of the framework as a Li metal host. The monodisperse pore framework shows the conformal electrodeposition of the Li metal, whereas the pore size gradient framework exhibits the superconformal plating of the Li metal. The conformal and superconformal electrodepositions of the Li metal are elucidated in terms of variations along the pore depth direction in the charge-transfer resistance on the pore walls and the ionic resistance of electrolytes confined in pores. The pore size gradient framework also shows excellent electrochemical performance, such as stable capacity retention over 760 cycles with 0.5 mAh cm–2 at 2 mA cm–2. These findings provide fundamental insights into strategies to improve the electrochemical performance of porous frameworks for Li metal batteries.

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distance of state-of-the-art electric vehicles powered by lithium ion batteries is ≈200 km per charging for 20 min, which is significantly lower than gasoline vehicles. Therefore, it is essential to increase the energy density of lithium ion batteries for extending driving distance. Unfortunately, the energy density of lithium ion batteries has almost reached the theoretical value, when assuming that layered oxides and carbons are used as cathode and anode materials, respectively. [2] This is attributed to the limitation of intercalation chemistry of lithium ion batteries. In this connection, lithium metal has been considered one of promising anode materials because of its high theoretical specific capacity (3860 mA h g −1). Furthermore, since the use of lithium metal anodes is essential for lithium-sulfur and lithium-oxygen batteries containing Li-free cathodes, it is necessary to develop Li metal anodes. [3] Li metal is fascinating in terms of energy density, but some electrochemical properties of current Li metal are unsatisfactory. First of all, most liquid electrolytes are so highly unstable against Li metal that Li metal gives rise to irreversible electrochemical decomposition of electrolytes, eventually leading to poor Coulombic efficiency. [4] In addition, Li metal grows in the form of dendrites during a plating process. Li dendrites are able to penetrate through a porous separator, resulting in the internal short-circuit of cells and cell explosion. [5] Moreover, porous deactivated layers, which form from the accumulation of "dead Li," on the Li metal surface thicken gradually during cycling. As a result, the mass transport of Li + ions is interrupted near the Li metal surface and cell polarization increases substantially during cycling. [6] To overcome these challenging issues, a lot of promising strategies have been introduced, including (i) electrolyte additives, [7] (ii) artificial solid electrolyte interphase (SEI) layers through chemical and physical treatments, [8] and (iii) guiding uniform Li plating through lithiophilic hosts and confining Li metal in three-dimensional structures. [9] In particular, physical protective layers have been intensively explored as one of artificial SEI layers to stabilize Li metal. Various physical protective layers were prepared using organic polymers, [10] inorganic Li metal has been considered a promising anode for high-energy density batteries because of its high specific capacity. However, uncontrolled Li dendrite growth and severe electrolyte decomposition are detrimental obstacles hindering the practical applications of lithium metal batteries. Herein, electrochemically active red P-based protective layers are introduced to suppress Li dendrite growth during cycling. Red P particles confined in the protective layer react with Li dendrites, forming Li 3 P, as Li dendrites are penetrating through the protective layer. As a result, Li metal is no longer plated on the Li 3 P surface because Li 3 P is electrically insulating, eventually leading to suppressing Li de...