Potassium‐based solid electrolyte interphases (SEIs) have a much smaller damage threshold than their lithium counterpart; thus, they are significantly more beam sensitive. Here, an ultralow‐dose cryogenic transmission electron microscopy (cryo‐TEM) technique (≈8 e Å−2 s−1 × 10 s), which enables the atomic‐scale chemical imaging of the electron‐beam‐sensitive potassium metal and SEI in its native state, is adapted. The potassium‐based SEI consists of large brackets of diverse inorganic phases (≈hundreds of nanometers) interspersed with amorphous phases, which are different from the tiny nanocrystalline inorganic phases (≈a few nanometers) formed in a lithium‐based SEI. Organic phosphate‐based electrolyte solvents induce the formation of a thin and stable SEI layer for enhanced cycling performance, while the carbonate ester‐based electrolytes result in large quantities of metastable KHCO3, and K4CO4 products in the SEI, depleting the potassium reserves in the battery. The findings provide deep insights and guidance in the selection of optimum electrolytes that should be used for potassium batteries.

Aprotic lithium–oxygen batteries (LOBs) are promising energy storage systems characterized by ultrahigh theoretical energy density. Extensive research has been devoted to this battery technology, yet the detailed operational mechanisms involved, particularly unambiguous identification of various discharge products and their specific distributions, are still unknown or are subjects of controversy. This is partly because of the intrinsic complexity of the battery chemistry but also because of the lack of atomic-level insight into the oxygen electrodes acquired via reliable techniques. In the current study, it is demonstrated that electron beam irradiation could induce crystallization of amorphous discharge products. Cryogenic conditions and a low beam dosage have to be used for reliable transmission electron microscopy (TEM) characterization. High-resolution cryo-TEM and electron energy loss spectroscopy (EELS) analysis of toroidal discharge particles unambiguously identified the discharge products as a dominating amorphous LiO2 phase with only a small amount of nanocrystalline Li2O2 islands dispersed in it. In addition, uniform mixing of carbon-containing byproducts is identified in the discharge particles with cryo-EELS, which leads to a slightly higher charging potential. The discharge products can be reversibly cycled, with no visible residue after full recharge. We believe that the amorphous superoxide dominating discharge particles can lead researchers to reconsider the chemistry of LOBs and pay special attention to exclude beam-induced artifacts in traditional TEM characterizations.

Rechargeable solid-state Na metal batteries (SSNMB) can offer high operational safety and energy density. However, poor solid−solid contact between the electrodes and the electrolyte can dramatically increase interfacial resistance and Na dendrite formation, even at low current rates. Therefore, we developed a carbon-fiber-supported liquid Na−K alloy anode that ensures close anode−electrolyte contact, enabling superior cycle stability and rate capability. We then demonstrated the first cryogenic transmission electron microscopy (cryo-TEM) characterization of an SSNMB, capturing the evolution of solid− electrolyte interphase (SEI) and revealing both crystalline and amorphous phases, which could facilitate ion transport and prevent continuous side reactions. By enhancing contact between the Na− K alloy and solid-state electrolyte, these symmetric cells are capable of cycling for over 800 h without notable increased polarization and enable an unprecedented critical current density (CCD) at 40 mA cm −2 . Our liquid Na−K alloy approach offers a promising strategic avenue toward commercial SSNMBs.

Solid‐state lithium metal batteries (SSLMBs) are a promising candidate for next‐generation energy storage systems due to their intrinsic safety and high energy density. However, they still suffer from poor interfacial stability, which can incur high interfacial resistance and insufficient cycle lifespan. Herein, a novel poly(vinylidene fluoride‑hexafuoropropylene)‐based polymer electrolyte (PPE) with LiBF4 and propylene carbonate plasticizer is developed, which has a high room‐temperature ionic conductivity up to 1.15 × 10−3 S cm−1 and excellent interfacial stability. Benefitting from the stable interphase, the PPE‐based symmetric cell can operate for over 1000 h. By virtue of cryogenic transmission electron microscopy (Cryo‐TEM) characterization, the high interfacial compatibility between Li metal anode and PPE is revealed. The solid electrolyte interphase is made up of an amorphous outer layer that can keep intimate contact with PPE and an inner Li2O‐dominated layer that can protect Li from continuous side reactions during battery cycling. A LiF‐rich transition layer is also discovered in the region of PPE close to Li metal anode. The feasibility of investigating interphases in polymer‐based solid‐state batteries via Cryo‐TEM techniques is demonstrated, which can be widely employed in future to rationalize the correlation between solid‐state electrolytes and battery performance from ultrafine interfacial structures.

The high specific capacity of alkalic metal (Li, Na, and K) anodes has drawn widespread interest; however, the practical applications of alkalic metal anodes have been hampered by dendrite growth and interfacial instability, resulting in performance deterioration and even safety issues. Here, we describe a simple method for building tunable fluoride-based artificial solid-electrolyte interphase (SEI) from the fluorination reaction of alkali metals with a mild organic fluorinating reagent. Comprehensive characterization by advanced electron microscopes shows that the LiF-based artificial SEI adopts a crystal−glass structure, which enables efficient Li ion transport and improves structural integrity against the volume changes that occur during Li plating/stripping. Compared with bare Li anode, the ones with artificial SEI exhibit decreased voltage hysteresis, enhanced rate capability, and prolonged cycle life. This method is also applied to generate fluoride-based artificial SEI on Na and K metal anodes that brings significant improvement in battery performance.