One of the viable options to significantly augment the energy density of lithium-ion batteries (LIBs) is to adopt lithium metal anode in the cell, which offers the highest theoretical capacity (3,860 mAh g-1) among anode materials.¹ However, the practical obstacles such as dendrite growth, low Coulombic efficiency, and safety issues still remain unresolved, despite the extensive efforts to employ the lithium metal anode in LIBs.² Recent progress in solid-state electrolyte development has granted a new promising opportunity for the utilization of lithium metal anodes, whose mechanical rigidity and non-flammable nature are supposed to effectively suppress lithium dendrite short-circuiting, thereby securing battery safety. Nevertheless, there has been no report thus far that demonstrates acceptable levels of electrochemical performance of solid-state lithium metal batteries for real-world applications.^3,4

In our research, a lithium-metal-battery employing tailored garnet-type Li_7-xLa_3-aZr_2-bO₁₂ (LLZO) solid-electrolytes that can meet the lifespan requirements of both electric vehicles and stationary applications has been achieved, affording remarkable stability and energy density over 2,000 cycles. It is demonstrated that the compatibility between LLZO and lithium metal is crucial for the long-term stability, which can be accomplished by regulating bulk dopants and the corresponding dopant-specific interfacial treatment using protonation/etching. The appropriate selection of dopant and dopant-specific protonation/etching agent for LLZO leads to (i) the formation of a stable passivation layer at the interface with lithium metal, (ii) effective release of residual stress in LLZO, and (iii) intact contact at the interface.

The lithium-metal cell with 2 mAh cm^-2 cathode (12 mg cm^-2) delivers a cumulative capacity of over 4,000 mAh cm^-2 at 3 mA cm^-2, which to the best of our knowledge, is the highest long-term cycle value reported for lithium metal batteries with LLZO electrolytes. Moreover, even with the thin 110-µm-LLZO electrolyte and 20-µm lithium metal, a high-loading-capacity cell (3.2 mAh cm^-2) exhibits a superior cycle life (>600 cycles) at 2 mA cm^-2 without short-circuiting, affording a basis for the high-energy-cell design with ultra-thin electrolyte. In addition, an all-solid-state-battery, excluding the ionic liquid electrolyte, was successfully demonstrated using the composite cathode, which could cycle over 1,000 times at a high current density of 3 mA cm^-2 without short-circuiting. To the best of our knowledge, this is the first all-solid-state battery that can operate over 1,000 cycles, enabled by the garnet-type electrolytes and cathode with a commercially acceptable capacity.

These findings are expected to open a new avenue for developing long-lasting solid-state lithium metal batteries by highlighting the efficacy of the coupled bulk and interface doping of solid electrolytes.