Selenium/g-C₃N₄ with a Solid Li₄Ti₅O₁₂ Blocking Layer for Selective Li⁺ Ion Diffusion in Long-Lived Li–Se Batteries

A durable Li–Se battery is fabricated by implementing a rational and facile design strategy for the cathode. The cathode consists of a Se/graphitic carbon nitride (g-C 3 N 4 ) composite combined with a solid lithium titanate (Li 4 Ti 5 O 12, LTO) layer applied as a solid coating over the separator. The solid LTO layer has pores large enough to allow Li + ions to diffuse through to the anode during charging but small enough to effectively block the polyselenides formed during the discharging cycle and confine them to the cathode, thus inhibiting capacity fade and improving the operational lifespan. The micrometer-thick LTO barrier layer affords selective diffusion of Li + ions, with an average Li + diffusion coefficient of 6 × 10 –11 cm 2 s –1, enabling reasonably fast ion transport and kinetics. Simultaneously, g-C 3 N 4, which is rich with active heteroatom (N) sites, also affords a very high Li + ion adsorption, leading to an improved capacity compared to pristine Se. The Se/g-C 3 N 4 /LTO@separator-Li + /Li cell delivers a superior capacity (750 versus 538 mAh g –1 at 0.1 C-rate), rate capability (39% versus 22% capacity retention ongoing from 0.1 to 2 C-rate), and cycle life (498 versus 283 mAh g –1 reversible capacity after 400 cycles) compared to the equivalent cell devoid of the LTO layer. Extended long-term cycling metrics showed a 310 mAh g –1 reversible capacity at 2 C, with 78% retention after 1600 cycles, and reiterated the potential it has for scale-up and even commercial applications.