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

Garnet‐type oxide Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (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 LiBH 4 , which results in a LLZTO composite with unique amorphous dual coating: LiBO 2 as the inner layer and LiBH 4 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 LiBH 4 that is chemically compatible and electrochemically stable with Li metal electrode.