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

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.

Alumina - Stabilized SEI and CEI in Potassium Metal Batteries.

Aluminum oxide (Al2O3) nanopowder is spin-coated onto both sides of commercial polypropene separator to create artificial solid-electrolyte interphase (SEI) and artificial cathode electrolyte interface (CEI) in potassium metal batteries (KMBs). This significantly enhances the stability, including of KMBs with Prussian Blue (PB) cathodes. For example, symmetric cells are stable after 1,000 cycles at 0.5 mA/cm2-0.5 mAh/cm2 and 3.0 mA/cm2-0.5 mAh/cm2. Alumina modified separators promote electrolyte wetting and increase ionic conductivity (0.59 vs. 0.2 mS/cm) and transference number (0.81 vs. 0.23). Cryo-stage focused ion beam (cryo-FIB) analysis of cycled modified anode demonstrates dense and planar electrodeposits, versus unmodified baseline consisting of metal filaments (dendrites) interspersed with pores and SEI. Alumina-modified CEI also suppresses elemental Fe crossover and reduces cathode cracking. Mesoscale modeling of metal - SEI interactions captures crucial role of intrinsic heterogeneities, illustrating how artificial SEI affects reaction current distribution, conductivity and morphological stability.

Influence of Potassium Metal-Support Interactions on Dendrite Growth.

Combined synchrotron X-ray nanotomography imaging, cryogenic electron microscopy (cryo-EM) and modeling elucidate how potassium (K) metal-support energetics influence electrodeposit microstructure. Three model supports are employed: O-functionalized carbon cloth (potassiophilic, fully-wetted), non-functionalized cloth and Cu foil (potassiophobic, nonwetted). Nanotomography and focused ion beam (cryo-FIB) cross-sections yield complementary three-dimensional (3D) maps of cycled electrodeposits. Electrodeposit on potassiophobic support is a triphasic sponge, with fibrous dendrites covered by solid electrolyte interphase (SEI) and interspersed with nanopores (sub-10 nm to 100 nm scale). Lage cracks and voids are also a key feature. On potassiophilic support, the deposit is dense and pore-free, with uniform surface and SEI morphology. Mesoscale modeling captures the critical role of substrate-metal interaction on K metal film nucleation and growth, as well as the associated stress state.

Emerging trends in anion storage materials for the capacitive and hybrid energy storage and beyond.

Electrochemical capacitors charge and discharge more rapidly than batteries over longer cycles, but their practical applications remain limited due to their significantly lower energy densities. Pseudocapacitors and hybrid capacitors have been developed to extend Ragone plots to higher energy density values, but they are also limited by the insufficient breadth of options for electrode materials, which require materials that store alkali metal cations such as Li+ and Na+. Herein, we report a comprehensive and systematic review of emerging anion storage materials for performance- and functionality-oriented applications in electrochemical and battery-capacitor hybrid devices. The operating principles and types of dual-ion and whole-anion storage in electrochemical and hybrid capacitors are addressed along with the classification, thermodynamic and kinetic aspects, and associated interfaces of anion storage materials in various aqueous and non-aqueous electrolytes. The charge storage mechanism, structure-property correlation, and electrochemical features of anion storage materials are comprehensively discussed. The recent progress in emerging anion storage materials is also discussed, focusing on high-performance applications, such as dual-ion- and whole-anion-storing electrochemical capacitors in a symmetric or hybrid manner, and functional applications including micro- and flexible capacitors, desalination, and salinity cells. Finally, we present our perspective on the current impediments and future directions in this field.

Revealing the Solid-State Electrolyte Interfacial Stability Model with Na-K Liquid Alloy.

In this work, the Na-K liquid alloy with a charge selective interfacial layer is developed to achieve an impressively long cycling life with small overpotential on a sodium super-ionic conductor solid-state electrolyte (NASICON SSE). With this unique multi-cation system as the platform, we further propose a unique model that contains a chemical decomposition domain and a kinetic decomposition domain for the interfacial stability model. Based on this model, two charge selection mechanisms are proposed with dynamic chemical kinetic equilibrium and electrochemical kinetics as the manners of control, respectively, and both are validated by the electrochemical measurements with microscopic and spectroscopic characterizations. This study provides an effective design for high-energy-density solid-state battery with alkali Na-K anode, but also presents a novel approach to understand the interfacial chemical processes that could inspire and guide future designs.

Emerging Potassium Metal Anodes: Perspectives on Control of the Electrochemical Interfaces.

ConspectusPotassium 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 pristine Cu. Moving forward, there are three inter-related thrusts to be pursued: First, K salt-based electrolyte formulations have to mature and become further tailored to handle the increased reactivity of a metal rather than an ion anode. Second, the K-based SEI structure needs to be further understood and ultimately tuned to be less reactive. Third, the energetics of the K metal-current collector interface must be controlled to promote planar wetting/dewetting throughout cycling.

Understanding the Strength of the Selenium-Graphene Interfaces for Energy Storage Systems.

We present comprehensive first-principles density functional theory (DFT) analyses of the interfacial strength and bonding mechanisms between crystalline and amorphous selenium (Se) with graphene (Gr), a promising duo for energy storage applications. Comparative interface analyses are presented on amorphous silicon (Si) with graphene and crystalline Se with a conventional aluminum (Al) current collector. The interface strengths of monoclinic Se (0.43 J m-2) and amorphous Si with graphene (0.41 J m-2) are similar in magnitude. While both materials (c-Se, a-Si) are bonded loosely by van der Waals (vdW) forces over graphene, interfacial electron exchange is higher for a-Si/graphene. This is further elaborated by comparing the potential energy step and charge transfer (Δq) across the graphene interfaces. The interface strength of c-Se on a 3D Al current collector is higher (0.99 J m-2), suggesting a stronger adhesion. Amorphous Se with graphene has comparable interface strength (0.34 J m-2), but electron exchange in this system is slightly distinct from monoclinic Se. The electronic characteristics and bonding mechanisms are different for monoclinic and amorphous Se with graphene as they activate graphene via surface charge doping divergently. The implications of these interfacial physicochemical attributes on electrode performance have been discussed. Our findings highlight the complex electrochemical phenomena in Se interfaced with graphene, which may profoundly differ from their "free" counterparts.

Sodium Metal Anodes: Emerging Solutions to Dendrite Growth.

This comprehensive Review focuses on the key challenges and recent progress regarding sodium-metal anodes employed in sodium-metal batteries (SMBs). The metal anode is the essential component of emerging energy storage systems such as sodium sulfur and sodium selenium, which are discussed as example full-cell applications. We begin with a description of the differences in the chemical and physical properties of Na metal versus the oft-studied Li metal, and a corresponding discussion regarding the number of ways in which Na does not follow Li-inherited paradigms in its electrochemical behavior. We detail the major challenges for Na-metal systems that at this time limit the feasibility of SMBs. The core Na anode problems are the following interrelated degradation mechanisms: An unstable solid electrolyte interphase with most organic electrolytes, "mossy" and "lath-like" metal dendrite growth for liquid systems, poor Coulombic efficiency, and gas evolution. Even solid-state Na batteries are not immune, with metal dendrites being reported. The solutions may be subdivided into the following interrelated taxonomy: Improved electrolytes and electrolyte additives tailored for Na-metal anodes, interfacial engineering between the metal and the liquid or solid electrolyte, electrode architectures that both reduce the current density during plating-stripping and serve as effective hosts that shield the Na metal from excessive reactions, and alloy design to tune the bulk properties of the metal per se. For instance, stable plating-stripping of Na is extremely difficult with conventional carbonate solvents but has been reported with ethers and glymes. Solid-state electrolytes (SSEs) such as beta-alumina solid electrolyte (BASE), sodium superionic conductor (NASICON), and sodium thiophosphate (75Na2S·25P2S5) present highly exciting opportunities for SMBs that avoid the dangers of flammable liquids. Even SSEs are not immune to dendrites, however, which grow through the defects in the bulk pellet, but may be controlled through interfacial energy modification. We conclude with a discussion of the key research areas that we feel are the most fruitful for further pursuit. In our opinion, greatly improved understanding and control of the SEI structure is the key to cycling stability. A holistic approach involving complementary post-mortem, in situ, and operando analyses to elucidate full battery cell level structure-performance relations is advocated.

Tutorial review on structure - dendrite growth relations in metal battery anode supports.

This tutorial review explains the emerging understanding of the surface and bulk chemistry - electrochemical performance relations in anode supports (aka secondary current collectors, substrates, templates, hosts) for lithium, sodium and potassium metal batteries (LMBs, SMBs or NMBs, and KMBs or PMBs). In relation to each section, the possible future research directions that may yield both new insight and improved cycling behavior are explored. Representative case studies from Li, Na and K metal anode literature are discussed. The tutorial starts with an overview of the solid electrolyte interphase (SEI), covering both the "classic" understanding of the SEI structure and the "modern" insights obtained by site-specific cryogenic stage TEM analysis. Next, the multiple roles of supports in promoting cycling stability are detailed. Without an optimized support architecture, the metal-electrolyte interface becomes geometrically unstable at a lower current density and cycle number. Taking into consideration the available literature on LMBs, SMBs and KMBs, it is concluded that effective architectures are geometrically complex and electrochemically lithiophilic, sodiophilic or potassiophilic, so as to promote conformal electrochemical wetting of the metal during plating/stripping. One way that philicity is achieved is through support oxygen surface chemistry, which yields a reversibly reactive metal-support interface. Examples of this include the well-known oxygen-carbon moieties in reduced graphene oxide (rGO), as well as classic ion battery reversible conversion reaction oxides such as SnO2. Unreactive surfaces lead to dewetted island growth of the metal, which is a precursor to dendrites, and possibly to non-uniform dissolution. Surveying the literature on various Li, Na and K metal supports, it is concluded that the key bulk thermodynamic property that will predict electrochemical wetting behavior is the enthalpy of infinite solution (ΔsolH∞) of the metal (solute) into the support (solvent). Large and negative ΔsolH∞ promotes uniform metal wetting on the support surface, corresponding to relatively low plating overpotential. Positive ΔsolH∞ promotes dewetted islands and a relatively high overpotential. This simple rule explains a broad range of studies on Li, Na and K metal - support interactions, including the previously reported correlation between mutual solubility and wetting.

Site-Specific Sodiation Mechanisms of Selenium in Microporous Carbon Host.

We combined advanced TEM (HRTEM, HAADF, EELS) with solid-state (SS)MAS NMR and electroanalytical techniques (GITT, etc.) to understand the site-specific sodiation of selenium (Se) encapsulated in a nanoporous carbon host. The architecture employed is representative of a wide number of electrochemically stable and rate-capable Se-based sodium metal battery (SMB) cathodes. SSNMR demonstrates that during the first sodiation, the Se chains are progressively cut to form an amorphous mixture of polyselenides of varying lengths, with no evidence for discrete phase transitions during sodiation. It also shows that Se nearest the carbon pore surface is sodiated first, leading to the formation of a core-shell compositional profile. HRTEM indicates that the vast majority of the pore-confined Se is amorphous, with the only localized presence of nanocrystalline equilibrium Na2Se2 (hcp) and Na2Se (fcc). A nanoscale fracture of terminally sodiated Na-Se is observed by HAADF, with SSNMR, indicating a physical separation of some Se from the carbon host after the first cycle. GITT reveals a 3-fold increase in Na+ diffusivity at cycle 2, which may be explained by the creation of extra interfaces. These combined findings highlight the complex phenomenology of electrochemical phase transformations in nanoconfined materials, which may profoundly differ from their "free" counterparts.

Heavily Tungsten-Doped Sodium Thioantimonate Solid-State Electrolytes with Exceptionally Low Activation Energy for Ionic Diffusion.

A strategy for modifying the structure of solid-state electrolytes (SSEs) to reduce the cation diffusion activation energy is presented. Two heavily W-doped sodium thioantimonate SSEs, Na2.895 W0.3 Sb0.7 S4 and Na2.7 W0.3 Sb0.7 S4 are designed, both exhibiting exceptionally low activation energy and enhanced room temperature (RT) ionic conductivity; 0.09 eV, 24.2 mS/cm and 0.12 eV, 14.5 mS/cm. At -15 °C the Na2.895 W0.3 Sb0.7 S4 displays a total ionic conductivity of 5.5 mS/cm. The 30 % W content goes far beyond the 10-12 % reported in the prior studies, and results in novel pseudo-cubic or orthorhombic structures. Calculations reveal that these properties result from a combination of multiple diffusion mechanisms, including vacancy defects, strongly correlated modes and excessive Na-ions. An all-solid-state battery (ASSB) using Na2.895 W0.3 Sb0.7 S4 as the primary SSE and a sodium sulfide (Na2 S) cathode achieves a reversible capacity of 400 mAh g-1 .

Review of Hybrid Ion Capacitors: From Aqueous to Lithium to Sodium.

In this critical Review we focus on the evolution of the hybrid ion capacitor (HIC) from its early embodiments to its modern form, focusing on the key outstanding scientific and technological questions that necessitate further in-depth study. It may be argued that HICs began as aqueous systems, based on a Faradaic oxide positive electrode (e.g., Co3O4, RuO x) and an activated carbon ion-adsorption negative electrode. In these early embodiments HICs were meant to compete directly with electrical double layer capacitors (EDLCs), rather than with the much higher energy secondary batteries. The HIC design then evolved to be based on a wide voltage (∼4.2 V) carbonate-based battery electrolyte, using an insertion titanium oxide compound anode (Li4Ti5O12, Li xTi5O12) versus a Li ion adsorption porous carbon cathode. The modern Na and Li architectures contain a diverse range of nanostructured materials in both electrodes, including TiO2, Li7Ti5O12, Li4Ti5O12, Na6LiTi5O12, Na2Ti3O7, graphene, hard carbon, soft carbon, graphite, carbon nanosheets, pseudocapacitor T-Nb2O5, V2O5, MXene, conversion compounds MoS2, VN, MnO, and Fe2O3/Fe3O4, cathodes based on Na3V2(PO4)3, NaTi2(PO4)3, sodium super ionic conductor (NASICON), etc. The Ragone chart characteristics of HIC devices critically depend on their anode-cathode architectures. Combining electrodes with the flattest capacity versus voltage characteristics, and the largest total voltage window, yields superior energy. Unfortunately "flat voltage" materials undergo significant volume expansion/contraction during cycling and are frequently lifetime limited. Overall more research on HIC cathodes is needed; apart from high surface area carbon, very few positive electrodes demonstrate the necessary 10 000 or 100 000 plus cycle life. It remains to be determined whether its lithium ion capacitors (LICs) or sodium ion capacitors (NICs) are superior in terms of energy-power and cyclability. We discuss unresolved issues, including poorly understood fast-charge storage mechanisms, prelithiation and presodiation, solid electrolyte interface (SEI) formation, and high-rate metal plating.

Synergy of Epoxy Chemical Tethers and Defect-Free Graphene in Enabling Stable Lithium Cycling of Silicon Nanoparticles.

We report a new approach for nanosilicon-graphene hybrids with uniquely stable solid electrolyte interphase. Expanded graphite is gently exfoliated creating "defect-free" graphene that is non-catalytic towards electrolyte decomposition, simultaneously introducing high mass loading (48 wt. %) Si nanoparticles. Silane surface treatment creates epoxy chemical tethers, mechanically binding nano-Si to CMC binder through epoxy ring-opening reaction while stabilizing the Si surface chemistry. Epoxy-tethered silicon pristine-graphene hybrid "E-Si-pG" exhibits state-of-the-art performance in full battery opposing commercial mass loading (12 mg cm-2 ) LiCoO2 (LCO) cathode. At 0.4 C, with areal capacity of 1.62 mAh cm-2 and energy of 437 Wh kg-1 , achieving 1.32 mAh cm-2 , 340.4 Wh kg-1 at 1 C. After 150 cycles, it retains 1.25 mAh cm-2 , 306.5 Wh kg-1 . Sputter-down XPS demonstrates survival of surface C-Si-O-Si groups in E-Si-pG after repeated cycling. The discovered synergy between support defects, chemical-mechanical stabilization of Si surfaces, and SEI-related failure may become key LIB anode design rule.

Li-Rich Li[Li1/6 Fe1/6 Ni1/6 Mn1/2 ]O2 (LFNMO) Cathodes: Atomic Scale Insight on the Mechanisms of Cycling Decay and of the Improvement due to Cobalt Phosphate Surface Modification.

Lithium-rich Li[Li1/6 Fe1/6 Ni1/6 Mn1/2 ]O2 (0.4Li2 MnO3 -0.6LiFe1/3 Ni1/3 Mn1/3 O2 , LFNMO) is a new member of the xLi2 MnO3 ·(1 - x)LiMO2 family of high capacity-high voltage lithium-ion battery (LIB) cathodes. Unfortunately, it suffers from the severe degradation during cycling both in terms of reversible capacity and operating voltage. Here, the corresponding degradation occurring in LFNMO at an atomic scale has been documented for the first time, using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), as well as tracing the elemental crossover to the Li metal anode using X-ray photoelectron spectroscopy (XPS). It is also demonstrated that a cobalt phosphate surface treatment significantly boosts LFNMO cycling stability and rate capability. Due to cycling, the unmodified LFNMO undergoes extensive elemental dissolution (especially Mn) and O loss, forming Kirkendall-type voids. The associated structural degradation is from the as-synthesized R-3m layered structure to a disordered rock-salt phase. Prior to cycling, the cobalt phosphate coating is epitaxial, sharing the crystallography of the parent material. During cycling, a 2-3 nm thick disordered Co-rich rock-salt structure is formed as the outer shell, while the bulk material retains R-3m crystallography. These combined cathode-anode findings significantly advance the microstructural design principles for next-generation Li-rich cathode materials and coatings.

Tin and Tin Compounds for Sodium Ion Battery Anodes: Phase Transformations and Performance.

Sodium ion batteries (NIB, NAB, SIB) are attracting interest as a potentially lower cost alternative to lithium ion batteries (LIB), with readily available and geographically democratic reserves of the metal. Tin is one of most promising SIB anode materials, which alloys with up to 3.75 Na, leading to a charge storage capacity of 847 mAh g(-1). In this Account, we outline the state-of-the-art understanding regarding the sodiation-induced phase transformations and the associated performance in a range of Sn-based systems, treating metallic Sn and its alloys, tin oxide (SnO2), tin sulfide (SnS2/SnS), and tin phosphide (Sn4P3). We first detail what is known about the sodiation sequence in metallic Sn, highlighting the most recent insight into the reactions prior to the terminal equilibrium Na15Sn4 intermetallic. We explain why researchers argue that the equilibrium (phase diagram) series of phase transitions does not occur in this system, and rather why sodiation/desodiation proceeds through a series of metastable crystalline and amorphous structures. We also outline the recent modeling-based insight regarding how this phase transition profoundly influences the mechanical properties of the alloy, progressively changing the bonding and the near neighbor arrangement from "Sn-like" to "Na-like" in the process. We then go on to discuss the sodiation reactions in SnO2. We argue that while a substantial amount of experimental work already exists where the focus is on synthesis and testing of tin oxide-based nanocomposites, the exact sodiation sequence is just beginning to be understood. Unlike in Sn and Sn alloys, where capacities near the theoretical are reached at least early during cycling, SnO2 never quite achieves anything close to the 1398 mAh g(-1) that would be possible with a combination of fully reversible conversion and alloying reactions. We highlight recent work demonstrating that contrary to general expectations, it is the Sn to Na15Sn4 alloying reaction that is incomplete and hence limits the capacity of the electrode. We also describe how the oxide conversion reaction goes through an intermediate SnO phase, and how its reversibility in a half-cell is highly dependent on the terminal anodic voltage. We then present what is known about sodiation of tin sulfide and of tin phosphide phases, including emerging microstructural evidence that may explain why both the sulfides and the phosphides are unable to achieve their highly promising theoretical capacities under conventional electrode testing conditions. Finally, we provide a broad comparison of the capacity (cycling and rate) performance for a range of Sn based anode materials, and show that there may be indeed an optimum microstructural architecture.

Coupling In Situ TEM and Ex Situ Analysis to Understand Heterogeneous Sodiation of Antimony.

We employed an in situ electrochemical cell in the transmission electron microscope (TEM) together with ex situ time-of-flight, secondary-ion mass spectrometry (TOF-SIMS) depth profiling, and FIB-helium ion scanning microscope (HIM) imaging to detail the structural and compositional changes associated with Na/Na(+) charging/discharging of 50 and 100 nm thin films of Sb. TOF-SIMS on a partially sodiated 100 nm Sb film gives a Na signal that progressively decreases toward the current collector, indicating that sodiation does not proceed uniformly. This heterogeneity will lead to local volumetric expansion gradients that would in turn serve as a major source of intrinsic stress in the microstructure. In situ TEM shows time-dependent buckling and localized separation of the sodiated films from their TiN-Ge nanowire support, which is a mechanism of stress-relaxation. Localized horizontal fracture does not occur directly at the interface, but rather at a short distance away within the bulk of the Sb. HIM images of FIB cross sections taken from sodiated half-cells, electrically disconnected, and aged at room temperature, demonstrate nonuniform film swelling and the onset of analogous through-bulk separation. TOF-SIMS highlights time-dependent segregation of Na within the structure, both to the film-current collector interface and to the film surface where a solid electrolyte interphase (SEI) exists, agreeing with the electrochemical impedance results that show time-dependent increase of the films' charge transfer resistance. We propose that Na segregation serves as a secondary source of stress relief, which occurs over somewhat longer time scales.

Activation with Li enables facile sodium storage in germanium.

Germanium is a promising sodium ion battery (NIB, NAB, SIB) anode material that is held back by its extremely sluggish kinetics and poor cyclability. We are the first to demonstrate that activation by a single lithiation-delithiation cycle leads to a dramatic improvement in the practically achievable capacity, in rate capability, and in cycling stability of Ge nanowires (GeNWs) and Ge thin film (GeTF). TEM and TOF-SIMS analysis shows that without activation, the initially single crystal GeNWs are effectively Na inactive, while the 100 nm amorphous GeTF sodiates only partially and inhomogeneously. Activation with Li induces amorphization in GeNWs reducing the barrier for nucleation of the NaxGe phase(s) and accelerates solid-state diffusion that aids the performance of both GeNWs and GeTF. Low rate (0.1C) Li activation also introduces a dense distribution of nanopores that lead to further improvements in the rate capability, which is ascribed to the lowered solid-state diffusion distances caused by the effective thinning of the Ge walls and by an additional Na diffusion path via the pore surfaces. The resultant kinetics are promising. Tested at 0.15C (1C = 369 mA/g, i.e. Na/Ge 1:1) for 50 cycles the GeNWs and GeTF maintain a reversible (desodiation) capacity of 346 and 418 mAh/g, respectively. They also demonstrate a capacity of 355 and 360 mAh/g at 1C and 284 and 310 mAh/g at 4C. Even at a very high rate of 10C the GeTF delivers 169 mAh/g. Preliminary results demonstrate that Li activation is also effective in promoting cycling stability of Sb blanket films.

Hybrid device employing three-dimensional arrays of MnO in carbon nanosheets bridges battery-supercapacitor divide.

It is a challenge to meld the energy of secondary batteries with the power of supercapacitors. Herein, we created electrodes finely tuned for this purpose, consisting of a monolayer of MnO nanocrystallites mechanically anchored by pore-surface terminations of 3D arrays of graphene-like carbon nanosheets ("3D-MnO/CNS"). The biomass-derived carbon nanosheets should offer a synthesis cost advantage over comparably performing designer nanocarbons, such as graphene or carbon nanotubes. High Li storage capacity is achieved by bulk conversion and intercalation reactions, while high rates are maintained through stable ∼20 nm scale diffusion distances. For example, 1332 mAh g(-1) is reached at 0.1 A g(-1), 567 mAh g(-1) at 5 A g(-1), and 285 mAh g(-1) at 20 A g(-1) with negligible degradation at 500 cycles. We employed 3D-MnO/CNS (anode) and carbon nanosheets (cathode) to create a hybrid capacitor displaying among the most promising performances reported: based on the active materials, it delivers 184 Wh kg(-1) at 83 W kg(-1) and 90 Wh kg(-1) at 15 000 W kg(-1) with 76% capacity retention after 5000 cycles.

Review of Carbon Support Coordination Environments for Single Metal Atom Electrocatalysts (SACS).

This topical review focuses on the distinct role of carbon support coordination environment of single-atom catalysts (SACs) for electrocatalysis. The article begins with an overview of atomic coordination configurations in SACs, including a discussion of the advanced characterization techniques and simulation used for understanding the active sites. A summary of key electrocatalysis applications is then provided. These processes are oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), and carbon dioxide reduction reaction (CO2 RR). The review then shifts to modulation of the metal atom-carbon coordination environments, focusing on nitrogen and other non-metal coordination through modulation at the first coordination shell and modulation in the second and higher coordination shells. Representative case studies are provided, starting with the classic four-nitrogen-coordinated single metal atom (MN4 ) based SACs. Bimetallic coordination models including homo-paired and hetero-paired active sites are also discussed, being categorized as emerging approaches. The theme of the discussions is the correlation between synthesis methods for selective doping, the carbon structure-electron configuration changes associated with the doping, the analytical techniques used to ascertain these changes, and the resultant electrocatalysis performance. Critical unanswered questions as well as promising underexplored research directions are identified.

Dual-Function Alloying Nitrate Additives Stabilize Fast-Charging Lithium Metal Batteries.

Lithium metal is regarded as the "holy grail" of lithium-ion battery anodes due to its exceptionally high theoretical capacity (3800 mAh g-1) and lowest possible electrochemical potential (-3.04 V vs Li/Li+); however, lithium suffers from the dendritic formation that leads to parasitic reactions and cell failure. In this work, we stabilize fast-charging lithium metal plating/stripping with dual-function alloying M-nitrate additives (M: Ag, Bi, Ga, In, and Zn). First, lithium metal reduces M, forming lithiophilic alloys for dense Li nucleation. Additionally, nitrates form ionically conductive and mechanically stable Li3N and LiNxOy, enhancing Li-ion diffusion through the passivation layer. Notably, Zn-protected cells demonstrate electrochemically stable Li||Li cycling for 750+ cycles (2.0 mA cm-2) and 140 cycles (10.0 mA cm-2). Moreover, Zn-protected Li||Lithium Iron Phosphate full-cells achieve 134 mAh g-1 (89.2% capacity retention) after 400 cycles (C/2). This work investigates a promising solution to stabilize lithium metal plating/stripping for fast-charging lithium metal batteries.