Interlayer Design of Pillared Graphite by Na-Halide Cluster Intercalation for Anode Materials of Sodium-Ion Batteries

Graphite is currently utilized as anode materials for Li-ion batteries, but it is well-known that graphite does not show good electrochemical performances as the anode material for sodium-ion batteries (SIBs). It was also reported that the low electrochemical performances of graphite originated from the larger ionic radius of the sodium ion due to the required higher strain energy for sodium-ion intercalation into graphite leading to an unstable sodium-ion intercalated graphite intercalation compound (GIC). In this work, using first-principles calculations, we introduce pillaring effects of Na X (n = 3 and 4; X = F, Cl, or Br) halide clusters in GICs, which become electrochemically active for Na redox reactions. Specifically, to enable sodium-ion intercalation into graphite, the interlayer spacing of graphite is required to increase over 3.9 Å, and Na X halide cluster GICs maintain an expanded interlayer spacing of >3.9 Å. This enlarged interlayer spacing of Na X halide cluster GICs facilitates stable intercalation of sodium ions. Na₃F, Na₄Cl, and Na₄Br halide clusters are identified as suitable pillar candidates for anode materials because they not only expand the interlayer spacing but also provide reasonable binding energy for intercalated sodium ions for reversible deintercalation. Based on the model analysis, theoretical capacities of Na₃F, Na₄Cl, and Na₄Br halide cluster GICs are estimated respectively to be 186, 155, and 155 mA h g^-1. These predictions would provide a rational strategy guiding the search for promising anode materials for SIBs.