From Back to Stage: Superior Cycling Stability of 2LiBH 4 −MgH 2 Enabled by Anion Tuning

2LiBH 4 −MgH 2 (Li‐RHC) is a potential hydrogen storage candidate with a capacity of 11.5 wt.%. However, its further application is severely limited by the long nucleation induction period and poor reversibility of end‐product. Unlike the traditional focus on the catalytical role of cation, herein, a new perspective of previously ignored anion tuning has been proposed, in which S 2− bridges the reversible conversion between Li 2 S and MgS during de‐/hydrogenation cycling of Li‐RHC. Such conversion not only enhances the migration of Li + , leading to the rapid destabilization of B─H bonds without obvious nucleation period of MgB 2 , but also improves the cycling stability of Li‐RHC with capacity retention over 90% even after 50 cycles. The universal of this anion modulation has been verified, along with the matching criteria of the corresponding transition metal cation. The results highlight the contribution of anion, push anion modulation from back to stage and eventually broaden the selection of catalysts for Li‐RHC.