Remarkably improved ionic conductivity and lithium metal compatibility of LLZTO-Li4(BH4)3I composite solid-state electrolyte

With superior ionic conductivity and (electro)chemical stability, garnet-type oxide Li _6.4La ₃Zr _1.4Ta _0.6O ₁₂ (LLZTO) has been extensively studied and explored as advanced solid-state electrolyte (SSEs) for lithium ion batteries (LIBs). However, high-temperature sintering treatment at ∼1200 °C to fabricate dense sheets is necessary to attain high ionic conductivity, whereas at the expense of degraded mechanical properties and poor contact with electrodes. Herein, we demonstrate a novel LLZTO-Li ₄(BH ₄) ₃ I composite solid electrolyte prepared by mechanical milling, which features a bilayer coating structure where LiBO ₂ in the inner layer and Li ₄(BH ₄) ₃I in the outer layer. The electrolyte pellets fabricated by a simple cold pressing treatment deliver an ionic conductivity of 3.69 × 10 ⁻⁵ S cm ⁻¹ at 30 °C and a Li ⁺ transference number of 0.9999, which successfully prevents the formation of lithium dendrites. Using this electrolyte, Li|Li symmetric batteries can work continuously for more than 2200 h at 35 and 60 °C. Such performance improvement is ascribed to the sharp reduction in porosity, the enhanced mechanical properties and the improved interface contact after cold pressing the Li ₄(BH ₄) ₃I-modified LLZTO composite to form an electrolyte pellet.