Lithium metal batteries (LMBs), sodium metal batteries (SMBs), and potassium metal batteries (KMBs) are receiving extensive attention in scientific literature. [1,2] The specific capacity of lithium, sodium, and potassium metal anodes is 3861 − , 1165 − , and 678 mAh g −−1 , which is substantially higher than that of graphite or hard carbons employed for ion battery anodes. For all three metal battery systems, achieving long-term stable and safe metal anode performance would be potentially transformative. However, growth of dendrites is a ubiquitous problem for each system, to date inhibiting wide scale commercial application of LMBs, SMBs, and KMBs in rechargeable cells. [3-6] The well-known risk is that dendrites will penetrate the separator and reach the cathode, resulting in a short circuit, potentially causing thermal runaway, burning, and even explosions. This is largely why Li metal anodes were originally abandoned in favor of graphite. [2,7] Less dramatically, dendrites result in a severe cell impedance increase, pouch swelling, as well as electrolyte drying. [3,8] The science around Li, Na, and K metal anodes is rapidly advancing. It is recognized that the structure of the solid electrolyte interface (SEI) plays a crucial role in determining the cyclability of metal anodes. [4,5] The SEI serves multiple roles, including limiting further side-reactions between the metal and the electrolyte, as well as promoting uniform metal deposition by regulating the solid-state ion flux. An ideal SEI layer has properties along the following: High cation conductance but also high electrical resistance, stable thickness close to a few nanometers, high mechanical toughness (combination of strength and ductility) leading to a tolerance to charginginduced volumetric changes, insolubility in the electrolyte, and stability over a wide range of operating temperatures and voltages. A stable SEI is an accepted prerequisite for safe battery performance, be it with a metal or an ion insertion anode. [6] The characteristics of SEI growth, gradual and uniform versus rapid and heterogeneous, is a key indicator whether or not dendrites form. [9-11] The dominant focus of many reports is on the spatially averaged SEI chemistry. [12,13] However recent progress brings fourth site-specific concepts in the SEI analysis, Anodes for lithium metal batteries, sodium metal batteries, and potassium metal batteries are susceptible to failure due to dendrite growth. This review details the structure-chemistry-performance relations in membranes that stabilize the anodes' solid electrolyte interphase (SEI), allowing for stable electrochemical plating/stripping. Case studies involving Li, Na, and K are presented to illustrate key concepts. "Classical" versus "modern" understandings of the SEI are described, with an emphasis on the new structural insights obtained through novel analytical techniques, including in situ liquid-secondary ion mass spectroscopy, titration gas chromatography, and tip-enhanced Raman spectroscopy. This Review highlights diver...

Potassium (K) metal anodes suffer from a challenging problem of dendrite growth. Here, it is demonstrated that a tailored current collector will stabilize the metal plating–stripping behavior even with a conventional KPF6‐carbonate electrolyte. A 3D copper current collector is functionalized with partially reduced graphene oxide to create a potassiophilic surface, the electrode being denoted as rGO@3D‐Cu. Potassiophilic versus potassiophobic experiments demonstrate that molten K fully wets rGO@3D‐Cu after 6 s, but does not wet unfunctionalized 3D‐Cu. Electrochemically, a unique synergy is achieved that is driven by interfacial tension and geometry: the adherent rGO underlayer promotes 2D layer‐by‐layer (Frank–van der Merwe) metal film growth at early stages of plating, while the tortuous 3D‐Cu electrode reduces the current density and geometrically frustrates dendrites. The rGO@3D‐Cu symmetric cells and half‐cells achieve state‐of‐the‐art plating and stripping performance. The symmetric rGO@3D‐Cu cells exhibit stable cycling at 0.1–2 mA cm−2, while baseline Cu prematurely fails when the current reaches 0.5 mA cm−2. The half‐cells cells of rGO@3D‐Cu (no K reservoir) are stable at 0.5 mA cm−2 for 10 000 min (100 cycles), and at 1 mA cm−2 for 5000 min. The baseline 3D‐Cu, planar rGO@Cu, and planar Cu foil fails after 5110, 3012, and 1410 min, respectively.

Two friends are skipping stones across a still ocean inlet. The scene represents the joy of collaborating, each rock being one advance, in aggregate making a mountain of new science. The smooth water is the dendrite‐free metal anode, the topic of article number 2002297 by Wei Liu, David Mitlin and co‐worker. The yellow mountains on the other side are composed of sulfur, the cathode in a metal battery.

Conspectus Potassium metal serves as the anode in emerging potassium metal batteries (KMBs). It also serves as the counter electrode for potassium ion battery (KIB) half-cells, with its reliable performance being critical for assessing the working electrode material. This first-of-its-kind critical Account focuses on the dual challenge of controlling the potassium metal-substrate and the potassium metal–electrolyte interface so as to prevent dendrites. The discussion begins with a comparison of the physical and chemical properties of K metal anodes versus the much oft studied Li and Na metal anodes. Based on established descriptions for root causes of dendrites, the problem should be less severe for K than for Li or Na, while in fact the opposite is observed. The key reason that the K metal surface rapidly becomes dendritic in common electrolytes is its unstable solid electrolyte interphase (SEI). An unstable SEI layer is defined as being non-self-passivating. No SEI is perfectly stable during cycling, and all SEI structures are heterogeneous both vertically and horizontally relative to the electrolyte interface. The difference between a “stable” and an “unstable” SEI may be viewed as the relative degree to which during cycling it thickens and becomes further heterogeneous. The unstable SEI on K metal leads to a number of interrelated problems, such as low cycling Coulombic efficiency (CE), a severe impedance rise, large overpotentials, and possibly electrical shorting, all of which have been reported to occur as early as in the first 10 plating/stripping cycles. Many of the traditional “interface fixes” employed for Li and Na metal anodes, such as various artificial SEIs, surface membranes, barrier layers, secondary separators, etc., have not been attempted or optimized for the case of K. This is an important area for further exploration, with an understanding that success may come harder with K than with Li due to K-based SEI reactivity with both ether and ester solvents. The second critical problem with K metal anodes is that they do not thermally or electrochemically wet a standard (untreated) Cu foil current collector. Published experimental and modeling research directly highlights the weak bonding between the K atoms and a Cu surface. Existing surface treatment approaches that achieve improved K wetting are discussed, along with the general design rules for future studies. Also discussed are geometry-based methods to tune nucleation as well dual approaches where nucleation and SEI structure are tuned through complementary schemes to achieve extended half-cell and full battery stability. We hypothesize that K metal never achieves a planar wetting morphology even at cycle one, making the dendrites “baked-in”. We propose that classical thin film growth models, Frank van der Merwe (F-M), Volmer–Weber (V-W), and Stranski-Krastanov (S-K), can be employed to describe early stage plating behavior. It is demonstrated that islandlike V-W growth is the applicable description for the natural plating behavior of K on p...

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal−sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li−S and the emerging Na−S and K−S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li−S are relatively mature, less progress has been made with Na−S and K−S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal−selenium and the metal−selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal−sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li-and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li−S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure−battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.