A Fiber‐Based 3D Lithium Host for Lean Electrolyte Lithium Metal Batteries

3D hosts are promising to extend the cycle life of lithium metal anodes but have rarely been implemented with lean electrolytes thus impacting the practical cell energy density. To overcome this challenge, a 3D host that is lightweight and easy to fabricate with optimum pore size that enables full utilization of its pore volume, essential for lean electrolyte operations, is reported. The host is fabricated by casting a VGCF (vapor‐grown carbon fiber)‐based slurry loaded with a sparingly soluble rubidium nitrate salt as an additive. The network of fibers generates uniform pores of ≈3 µm in diameter with a porosity of 80%, while the nitrate additive enhances lithiophilicity. This 3D host delivers an average coulombic efficiency of 99.36% at 1 mA cm(−2) and 1 mAh cm(−2) for over 860 cycles in half‐cell tests. Full cells containing an anode with 1.35‐fold excess lithium paired with LiNi(0.8)Mn(0.1)Co(0.1)O(2) (NMC811) cathodes exhibit capacity retention of 80% over 176 cycles at C/2 under a lean electrolyte condition of 3 g Ah(−1). This work provides a facile and scalable method to advance 3D lithium hosts closer to practical lithium‐metal batteries.