Multifunctional Separator Allows Stable Cycling of Potassium Metal Anodes and of Potassium Metal Batteries

Abstract

This is the first report of a multifunctional separator for potassium‐metal batteries (KMBs). Double‐coated tape‐cast microscale AlF3 on polypropylene (AlF3@PP) yields state‐of‐the‐art electrochemical performance: symmetric cells are stable after 1000 cycles (2000 h) at 0.5 mA cm−2 and 0.5 mAh cm−2, with 0.042 V overpotential. Stability is maintained at 5.0 mA cm−2 for 600 cycles (240 h), with 0.138 V overpotential. Postcycled plated surface is dendrite‐free, while stripped surface contains smooth solid electrolyte interphase (SEI). Conventional PP cells fail rapidly, with dendrites at plating, and “dead metal” and SEI clumps at stripping. Potassium hexacyanoferrate(III) cathode KMBs with AlF3@PP display enhanced capacity retention (91% at 100 cycles vs 58%). AlF3 partially reacts with K to form an artificial SEI containing KF, AlF3, and Al2O3 phases. The AlF3@PP promotes complete electrolyte wetting and enhances uptake, improves ion conductivity, and increases ion transference number. The higher of K+ transference number is ascribed to the strong interaction between AlF3 and FSI− anions, as revealed through 19F NMR. The enhancement in wetting and performance is general, being demonstrated with ester‐ and ether‐based solvents, with K‐, Na‐, or Li‐ salts, and with different commercial separators. In full batteries, AlF3 prevents Fe crossover and cycling‐induced cathode pulverization.

Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal–Sulfur and Selenium Batteries

“…Such a transfer method avoids having to pretreat the metal anodes before assembling the cell, making functional separators a potentially scalable technology. For example, our recent work on functional separators with K metal anodes focused on double-coated tape-cast microscale AlF 3 on polypropylene (AlF 3 @PP) . This layer resulted in a distinctly stable artificial SEI film containing KF, AlF 3 , and Al 2 O 3 phases on a K metal surface, which for a range of electrolytes promoted electrolyte wetting on the current collector, stabilized the SEI, and prevented dendrites.…”

“…(f) Top-down SEM images and EDXS maps of the post-100-cycle metal anodes extracted from full batteries based on the potassium hexacyanoferrate­(III) cathode, tested at 100 mA g –2 . Reproduced with permission from ref . Copyright 2021 John Wiley & Sons, Inc.…”

“…Employing site-specific techniques such as cryo-stage FIB and cryo-stage TEM in combination with global electroanalytical (EIS, overpotential analysis) and analytical methods (synchrotron and X-ray, XPS, and TOF-SIMS) is likely the best path forward. For example, such a combined approach provided unique microstructural information regarding the role of AlF 3 multifunctional coatings in extending the cycling stability of potassium metal batteries, as well as specific early failure modes in baseline cells employing a conventional separator …”

Highly Potassiophilic Graphdiyne Skeletons Decorated with Cu Quantum Dots Enable Dendrite‐Free Potassium‐Metal Anodes

“…Despite the appealing features of PMBs, their development has been impeded by a multi tude of obstacles at the anode side thus far, including unstable solid electrolyte interphase (SEI), large volume expansion and uncontrollable growth of K dendrites. [15][16][17][18] To address these challenges, strategic solutions have recently been proposed: (i) modifying the electrolyte to enable a durable SEI layer on the surface of K metal, which is expected to initiate with reversible plating/stripping process; [19][20][21] (ii) engineering the interface between K anode and electrolyte via the creation of artificial coating to inhibit the generation of dendrites; [22][23][24][25][26] (iii) strengthening the interaction between current collector and K anode to homogenize the local current and mitigate the dendritic growth. [27][28][29] To date, the current collectors employed for the anode of alkali metal batteries mainly encompass two types: 3D free-standing supports and commercial Al/Cu foils.…”

Precise Control of Li+ Directed Transport via Electronegative Polymer Brushes on Polyolefin Separators for Dendrite‐Free Lithium Deposition

“…Further increasing the current density to 1 mA cm -2 , the Li nuclei starts to grow into dense and smooth Li deposits with the PPFPA-g-Celgard separator, while apparent Li dendrites with loosely packed porous morphology are observed for the cell with the Celgard separator. It has been reported that a carbonate-based electrolyte (i.e., 1 m LiFP 6 in EC/DEC) generates a more weak and thinner SEI layer that is easily fractured during unstable Li plating/stripping, [47] but evenly distributed Li nuclei is still observed with the PPFPAg-Celgard separator in the carbonate-based electrolyte. However, irregular island-like patterns occur in the plated fibrous Li metal with the Celgard separator.…”