Garnet‐type solid‐state electrolytes (SSEs) are promising for the realization of next‐generation high‐energy‐density Li metal batteries. However, a critical issue associated with the garnet electrolytes is the poor physical contact between the Li anode and the garnet SSE and the resultant high interfacial resistance. Here, it is reported that the Li|garnet interface challenge can be addressed by using Li metal doped with 0.5 wt% Na (denoted as Li*) and melt‐casting the Li* onto the garnet SSE surface. A mechanistic study, using Li6.4La3Zr1.4Ta0.6O12 (LLZTO) as a model SSE, reveals that Li2CO3 resides within the grain boundaries of newly polished LLZTO pellet, which is difficult to remove and hinders the wetting process. The Li* melt can phase‐transfer the Li2CO3 from the LLZTO grain boundary to the Li*’s top surface, and therefore facilitates the wetting process. The obtained Li*|LLZTO demonstrates a low interfacial resistance, high rate capability, and long cycle life, and can find applications in future all‐solid‐state batteries (e.g., Li*|LLZTO|LiFePO4).

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High interfacial impedance is a major obstacle in the application of solidstate Li metal batteries (SSLMBs). Understanding the atomic-scale structure of the interfaces in SSLMBs is thus critical to their practical implementations. However, due to the beam sensitivity of battery materials, such information is not accessible by conventional electron microscopy (EM). Herein, by using cryogenic-EM (cryo-EM), the atomic-scale structures of interfaces in garnet electrolyte based SSLMBs are revealed. A LiF-rich interlayer exhibiting intimate contacts with both Li and LLZTO is shown, thus rendering uniform Li + transport across the interface in turn inhibiting Li dendrite growth. Consequently, the Li symmetric cell based on the LiF-rich interlayer exhibits a high critical current density of 3.2 mA cm −2 and a long lifespan over 1800 cycles at 1 mA cm −2 . Moreover, a full cell with a LiNi 0.88 Co 0.1 Al 0.02 O 2 cathode at a high mass loading ≈12 mg cm −2 reached over 400 cycles at 1.2 mA cm −2 , which represents a major progress in the performance of the garnet-type SSLMBs. This study provides atomic-scale understanding of interfaces in SSLMBs and an effective strategy to design dendrite-free SSLMBs for practical applications.

Understanding polysulfide electrochemistry is critical for mitigation of the polysulfide shuttle effect in Li-S batteries. However, in situ imaging polysulfides evolution in Li-S batteries has not been possible. Herein, we constructed a hollow carbon nanotubule (CNT) wet electrochemical cell that permits real-time imaging of polysulfide evolutions in Li-S batteries in a Cscorrected environmental transmission electron microscope. Upon discharge, sulfur was electrochemically reduced to long-chain polysulfides, which dissolved into the electrolyte instantly and were stabilized by Py 14 + cations solvation. Metastable polysulfides prove to be problematic for Li-S batteries, therefore, destabilizing the Py 14 + -solvated polysulfides by adding low polarized solvents into the electrolyte to weaken the interaction between Py 14 + cation and long-chain polysulfides renders a rapid polysulfides-to-Li 2 S transition, thus efficiently mitigating polysulfide formation and improving the performance of Li-S batteries dramatically. Moreover, the CNT wet electrochemical cell proves to be a universal platform for in situ probing electrochemistry of various batteries.