A novel sintering additive based on LiNO₃ was used to overcome the drawbacks of poor sinterability and low grain boundary conductivity in BaZr_0.8Y_0.2O_3−δ (BZY20) protonic conductors. The Li-additive totally evaporated during the sintering process at 1600 °C for 6 h, which led to highly dense BZY20 pellets (96.5% of the theoretical value). The proton conductivity values of BZY20 with Li sintering-aid were significantly larger than the values reported for BZY sintered with other metal oxides, due to the fast proton transport in the “clean” grain boundaries and grain interior. The total conductivity of BZY20–Li in wet Ar was 4.45 × 10⁻³ S cm⁻¹ at 600 °C. Based on the improved sinterability, anode-supported fuel cells with 25 μm-thick BZY20–Li electrolyte membranes were fabricated by a co-firing technique. The peak power density obtained at 700 °C for a BZY–Ni/BZY20–Li/La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)–BZY cell was 53 mW cm⁻², which is significantly larger than the values reported for fuel cells using electrolytes made of BZY sintered with the addition of ZnO and CuO, confirming the advantage of using Li as a sintering aid.