Amorphous Dual-Layer Coating: Enabling High Li-Ion Conductivity of Non-Sintered Garnet-Type Solid Electrolyte

Garnet-type oxide Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) has attracted considerable attention as a highly promising solid state electrolyte. However, its high ionic conductivity is achievable only after high temperature sintering (≈1200 °C) to form dense pellets but with detrimental brittleness and poor contact with electrodes. Herein, a novel strategy to achieve high Li⁺ ion conductivity of LLZTO without sintering is demonstrated. This is realized by ball milling LLZTO together with LiBH₄, which results in a LLZTO composite with unique amorphous dual coating: LiBO₂ as the inner layer and LiBH₄ as the outer layer. After cold pressing the LLZTO composite powders under 300 MPa to form electrolyte pellets, a high Li⁺ ion conductivity of 8.02 × 10^–5 S cm^–1 is obtained at 30 °C, which is four orders of magnitude higher than that of the non-sintered pristine LLZTO pellets (4.17 × 10^–9 S cm^–1). The composite electrolyte displays an ultrahigh Li⁺ transference number of 0.9999 and enables symmetric Li–Li cells to be cycled for 1000 h at 60 °C and 300 h at 30 °C. The significant improvements are attributed to the continuous ionic conductive network among LLZTO particles facilitated by LiBH₄ that is chemically compatible and electrochemically stable with Li metal electrode.