Biphenylene Concentric Nanorings as High-Performance Anode Materials for Lithium-Ion Batteries: A DFT-Based Study on Lithium Intercalation and Capacity Enhancement

Biphenylene network (BPN), a newly discovered two-dimensional sp2-hybridized carbon allotrope composed of 4-6-8 carbon rings, shows great potential for energy storage applications. In this study, biphenylene concentric nanorings (BPNCRs), derived from hydrogen-terminated finite-sized BPN units, are explored as anode materials for lithium-ion batteries (LIBs) using density functional theory (DFT) based simulations. The lithium intercalation and adsorption on BPNCRs of varying sizes are investigated. BPNCR with an inner-outer ring diameter of 5-17 Å is found to exhibit an impressive specific capacity of 1509 mA h g-1 and an energy density of ∼4500 mW h g-1, with a low open-circuit voltage of 0.01 V (average voltage: 0.102 V). An increase in inter-ring spacing offers more lithium intercalation, which leads to further capacity enhancement and open-circuit voltage reduction. For example, BPCNR with an inner-outer ring diameter of 5-19 Å delivers a capacity of 1973 mA h g-1 with an OCV of 0.001 V. Notably, for every 1 Å increase in inter-ring spacing, the capacity increases by ∼500 mA h g-1. Finally, a three-dimensional assembly of lithiated BPNCR is modelled to evaluate its stability in the bulk form. Bulk-BPCNR is not only found to be stable but also provides experimental viability and promises the best features of both nano-particles and micro-particles at the same time. It is also noted that all intercalated lithium atoms are charged, thereby, ruling out lithium plating. These promising results suggest BPNCRs as high-performance anode materials for next-generation LIBs.