Replacement of Li-ion liquid-state electrolytes by solid-state counterparts in a Li-ion battery (LIB) is a major research objective as well as an urgent priority for the industry, as it enables the use of a Li metal anode and provides new opportunities to realize safe, non-flammable, and temperature-resilient batteries. Among the plethora of solid-state electrolytes (SSEs) investigated, garnet-type Li-ion electrolytes based on cubic Li7La3Zr2O12 (LLZO) are considered the most appealing candidates for the development of future solid-state batteries because of their low electronic conductivity of ca. 10−8 S cm−1 (RT) and a wide electrochemical operation window of 0–6 V vs. Li+/Li. However, high LLZO density (5.1 g cm−3) and its lower level of Li-ion conductivity (up to 1 mS cm−1 at RT) compared to liquid electrolytes (1.28 g cm−3; ca. 10 mS cm−1 at RT) still raise the question as to the feasibility of using solely LLZO as an electrolyte for achieving competitive energy and power densities. In this work, we analyzed the energy densities of Li-garnet all-solid-state batteries based solely on LLZO SSE by modeling their Ragone plots using LiCoO2 as the model cathode material. This assessment allowed us to identify values of the LLZO thickness, cathode areal capacity, and LLZO content in the solid-state cathode required to match the energy density of conventional lithium-ion batteries (ca. 180 Wh kg−1 and 497 Wh L−1) at the power densities of 200 W kg−1 and 600 W L−1, corresponding to ca. 1 h of battery discharge time (1C). We then discuss key challenges in the practical deployment of LLZO SSE in the fabrication of Li-garnet all-solid-state batteries.

While significant progress has been achieved in the field of Li‐garnet solid‐state batteries, their further development, is hindered by the formation of cavities at the Li7La3Zr2O12 (LLZO)/Li interface at practically relevant current densities and areal capacities exceeding 1 mA cm−2 and 1 mAh cm−2. As a result, the cells exhibit limited cycling stability due to the inhomogeneous distribution of the applied current density, and therefore, the formation of Li dendrites. Another aspect of high importance is associated with the development of the fabrication methodology of thin LLZO electrolytes for achieving the high energy density of Li‐garnet solid‐state batteries. To contribute to these two challenging problems, in this work, a facile intermediate‐stage sintering method of 50‐µm thin and porous LLZO membranes with a mean pore size of 2.5 µm is presented. The employment of such porous LLZO membranes not only provides an effective means of mitigating the formation of voids at the LLZO/Li interface due to the increased LLZO/Li surface area, but also maximizes achievable energy densities. It is demonstrated that fabricated porous LLZO membranes exhibit long cycling stability of over 1480 h at a current density of 0.5 mA cm−2.

Li dendrites form inLi 7 La 3 Zr 2 O 12 (LLZO) solid electrolytes due to intrinsic volume changes of Li and the appearance of voids at the Li metal/LLZO interface. Bilayer dense-porous LLZO membranes make for a compelling solution of this pertinent challenge in the field of Li-garnet solid-state batteries (SSB). Lithium is thus stored in the pores of the LLZO, thereby avoiding i) dynamic changes of the anode volume and ii) the formation of voids during Li stripping due to increased surface area of the Li/LLZO interface. The dense layer then additionally reduces the probability of short circuits during cell charging. In this work, a method for producing such bilayer membranes utilizing sequential tape-casting of porous and dense layers is reported. The minimum attainable thicknesses are 8-10 μm for dense and 32-35 μm for porous layers, enabling gravimetric and volumetric energy densities of Li-garnet SSBs of 279 Wh kg −1 and 1003 Wh L −1 , respectively. Bilayer LLZO membranes in symmetrical cell configuration exhibit high critical current density up to 6 mA cm −2 and cycling stability of over 160 cycles at a current density of 0.5 mA cm −2 at an areal capacity limitation of 0.25 mAh cm −2 .