Potassium Secondary Batteries - ACS Applied Materials & Interfaces

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Potassium Secondary Batteries Ali Eftekhari,*,†,‡ Zelang Jian,§ and Xiulei Ji*,§ †

The Engineering Research Institute, Ulster University, Newtownabbey BT37 OQB, United Kingdom School of Chemistry and Chemical Engineering, Queen’s University Belfast, Stranmillis Road, Belfast BT9 5AG, United Kingdom § Department of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003, United States

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ABSTRACT: Potassium may exhibit advantages over lithium or sodium as a charge carrier in rechargeable batteries. Analogues of Prussian blue can provide millions of cyclic voltammetric cycles in aqueous electrolyte. Potassium intercalation chemistry has recently been demonstrated compatible with both graphite and nongraphitic carbons. In addition to potassium−ion batteries, potassium−O2 (or −air) and potassium−sulfur batteries are emerging. Additionally, aqueous potassium−ion batteries also exhibit high reversibility and long cycling life. Because of potentially low cost, availability of basic materials, and intriguing electrochemical behaviors, this new class of secondary batteries is attracting much attention. This mini-review summarizes the current status, opportunities, and future challenges of potassium secondary batteries. KEYWORDS: potassium-ion battery, potassium-air battery, potassium−sulfur battery, graphite intercalation compound, Prussian blue

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Cell Voltage. A key benefit of potassium batteries is the exceptionally negative potential of the K+/K redox couple. The well-known standard potentials for Li+/Li, Na+/Na, K+/K, Rb+/ Rb, and Cs+/Cs are −3.040, −2.714, −2.936, −2.943, and −3.027 vs standard hydrogen electrode (SHE), respectively.48 In fact, Li is somehow an exception as the redox potential generally shifts toward more negative potentials by increasing the atomic number in the alkali metal group, which is due to Liion’s very high desolvation energy in aqueous electrolyte. This exceptional behavior of Li is not strong enough in nonaqueous electrolytes (i.e., the common environment of lithium-ion batteries) where, in fact, the standard potential of K+/K is even more negative than that of Li+/Li. The standard potentials in a well-known battery solvent, propylene carbonate (PC), have been theoretically calculated to be −2.79, − 2.56, − 2.88, − 2.95, and −3.10 for Li+/Li, Na+/Na, K+/K, Rb+/Rb, and Cs+/ Cs, respectively.48 However, experimental measurements showed that the standard potential of K+/K in PC is the most negative one among all alkali metal redox couples.49 Indeed, the standard potential of K+/K has an advantage over Li+/Li in nonaqueous medium. In ethylene carbonate/diethyl carbonate (EC/DEC) electrolyte, i.e., a common electrolyte solvent combination for LIBs, it was determined that K+/K is −0.15 V vs the Li+/Li reference.50 This is somehow symbolically interesting, as it is quite

INTRODUCTION Lithium-ion batteries (LIBs), as a ubiquitous power source, have achieved tremendous successes in portable electronics and electric vehicles. Usage of LIBs is ever increasing; however, economies of scale do not apply to LIBs, where larger production of LIBs may not reduce the price. LIBs may suffer from the technological unsustainability because of rarity and uneven distribution of lithium resources, which limits their applications, especially in the large-scale batteries for stationary storage. In pursuit of alternatives to LIBs, two nearest-neighbor elements (Na and Mg) have been the focus of attention during the past two decades. Although it seems that the closest neighboring elements are the best choices for the replacement because of their property similarity, K is indeed a competitive option, where in selections of anodes and cathodes, there exist highly promising materials for K-ion intercalation. The first prototype of potassium battery was introduced in 2004 by Eftekhari,1 utilizing a Prussian blue (PB) cathode. Since then, PB and its analogues have been widely employed as low-cost cathode materials in lithium,2−16 sodium,4,17−30 calcium,31,32 and aluminum ion batteries.33,34 Electrochemistry of transition metal hexacyanoferrates has been extensively studied since the 1970s as conventional electroactive materials of chemically modified electrodes (CMEs),35,36 which were used in various electrochemical systems, including secondary batteries.37−44 The fascinating capability of PB as a cathode is its excellent cyclability for K-ion insertion/extraction over millions of reversible cycles.45−47 In almost any aspect, potassium batteries can compete with their emerging rivals, sodium (ion) batteries. Even in comparison with the well-established lithium batteries, potassium batteries have some unique advantages. © 2016 American Chemical Society

Special Issue: New Materials and Approaches for Beyond Li-ion Batteries Received: June 30, 2016 Accepted: October 7, 2016 Published: October 7, 2016 4404

DOI: 10.1021/acsami.6b07989 ACS Appl. Mater. Interfaces 2017, 9, 4404−4419

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ACS Applied Materials & Interfaces uncommon to deal with negative potentials when using a Li+/Li reference electrode. This low potential means that in the conventional cell design like Li batteries, K-based batteries may deliver a higher cell voltage by about 0.15 V, when assuming that the cathodes in potassium batteries exhibit the same operating potentials as their analogues in LIBs. This provides a uniquely superior position for K-based batteries among possible alternatives in replacement of Li-based batteries. For other alkali metals, the standard potential of Na+/Na is significantly more positive than Li and K, while heavier alkali metals than K are not suitable for battery systems because of their high cost and low specific capacity. Furthermore, electrochemical behavior at negative potentials in K-based electrolytes is somewhat more straightforward in comparison to that in Libased and Na-based electrolyte, undergoing less complicated interfacial reactions.51 Abundance and Low Cost. Both sodium and potassium are abundant elements with comparable low cost, but lithium is much more expensive because of its rarity as well as uneven distribution in the Earth’s crust, i.e., lithium is about 3 orders of magnitude less abundant than sodium or potassium.52 Lithium is unevenly distributed geographically, where the “lithium triangle” in South America holds ∼70% of the world’s total lithium reserve. This would mean that for most countries lithium would have to be imported, potentially creating a “lithium crisis” because of the lithium commodity price volatility. This causes a concern of sustainability to meet the rapidly growing demands for LIBs. In addition to the challenged future supply of lithium to power millions of electric vehicles, recycling lithium from LIBs cheaply remains an unsolved challenge. It would be impossible to meet the needs for massive stationary energy storage by LIBs as it would starve the supplies of lithium rapidly, causing unaffordable prices. However, there has been a huge demand for stationary batteries, where the energy density or specific energy is not the priority. The two most critical metrics are the cost and cycling stability, which requires sustainable affordability and minimal maintenance. Suitable technologies will facilitate wider installation of solar or wind energy for load leveling, smart grids, and microgrids. For strategic reasons, it is vital to develop battery technologies that are sustainable and based on cheaper and more abundant materials. Electrochemical Behavior with Respect to PB Electrodes. Among alkali metal elements, potassium salts are commonly used as both supporting electrolytes and electroactive species; this is due to their high conductivity, low cost, abundance, and straightforward electrochemical behavior.53−56 Sodium salts are comparable, but the application of lithium salts in aqueous electrochemical systems (except for Lithium Batteries) is negligible. In aqueous electrolytes, large hydrated Li-ions are usually considered unfavorable for solid-state insertion/extraction in PB type electrodes.57−59 In addition to the structural destruction of PB by Li-ion insertion/extraction, which is accompanied by poor cyclability; the electrochemical behavior of PB in Li supporting electrolyte is not as welldefined as that in K supporting electrolyte.60,61 While redox peaks in K supporting electrolyte are sharp, indicating that the redox system occurs at a narrow range of potentials, resulting in a flat plateau in battery performance, they are quite broad in Li supporting electrolyte. Although the rate-determining factor in this class of batteries is solid-state diffusion within the electrode materials, diffusion of the charge carrier within the electrolyte solution is of

particular importance. Contrary to common sense, solvated potassium cation is smaller than those of Li-ion and Na-ion in solution. This results in a faster diffusion and higher ionic conductivity, accompanied by a greater transference number.

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CLASSIFICATION OF POTASSIUM SECONDARY BATTERIES Depending on the choice of electrolyte, there are nonaqueous and aqueous potassium secondary batteries. For the former, we will cover (i) potassium-ion batteries (KIBs) that employ the rocking-chair operation principle as LIBs do, in which both the cathode and anode materials employ topotactic intercalation chemistry for charge storage, and (ii) potassium metal batteries, including K−O2 (or K−air) and K−sulfur batteries. Furthermore, we will highlight the recent progress on aqueous K-ion based chemistry.

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NONAQUEOUS POTASSIUM-ION BATTERIES (KIBS) KIB Cathodes. Iron Hexacyanoferrate. In nonaqueous potassium-ion batteries, one main issue is the cathode material, in which K-ions can be reversibly intercalated/deintercalated. Following the first prototype of a potassium-ion battery,1 the top choice is still metal hexacyanoferrates as Cui et al. and Padigi et al. recently demonstrated,62,63 which have excellent cyclability for K-ion insertion/extraction. PB, KFeIIIFeII(CN)6, and its analogues are well-known electroactive materials because of their excellent electrochemical behaviors. PB exhibits a rigid structure, where (CN)− groups bond with both Fe(II) and Fe(III) forming a cubic framework of iron(II) hexacyanide, or known as iron hexacyanoferrate (II), and iron(III) hexaisocyanide (Figure 1a). The “holes” in this framework

Figure 1. Lattice structure of (a) Prussian blue, KFeIIIFeII(CN)6, and (b) layered LiCoO2 and hypothetically layered KCoO2, where in (b) the counterions are shown in purple, oxygen red, and alkali metal blue. The viewing direction for LiCoO2 and KCoO2 is from the crosssection of the layered structure, which is evident in the case of LiCoO2 but almost indistinguishable in the case of KCoO2.

are large enough to host large alkali ions, such as K-ions, 1.52 Å in radius, and water molecules. Note that the oxidation state of Fe-ions in the framework determines whether counterions are allowed in the holes of the framework. When both irons coordinated in hexacyanide and hexaisocyanide are Fe(III), no counterions are allowed in the structure. Comparing the lattice structures of PB analogues to those of common cathodes of LIBs simply reveals why the electrochemical behavior of PB is accompanied by exceptional cyclability. Alkali metals are, indeed, the pillars in the architecture of layered metal oxides, e.g., LiCoO2 and KCoO2 (Figure 1b); upon full extraction, the lattice undergoes severe structural changes. This is the reason that only half of the 4405


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counterions, solid-state diffusion is still the rate-determining step like all cathode materials of LIBs. In general, the chemical synthesis controls the lattice structure and morphology of the electroactive material, and the electrochemical behavior is heavily influenced by the initial structure of the electrodes. The largest advantage of FeHCF as a KIB cathode is its unique crystal structure, which may be translated to long cycling lifekey for stationary energy storage. Figure 1 displays that PB lattice is literally an MOF (metal organic framework) structure with a robust skeleton independent of the presence of the counterions. However, the lattice of a simple layered metal oxide is highly dependent on the presence of the counterions (LiCoO2 and KCoO2 in Figure 1). Thus, it is quite difficult to reversibly extract K+ from KCoO2 and intercalate it again without any structural change. Even in the case of LiCoO2, only half of the theoretical capacity is achievable, as the lattice undergoes several structural changes depending on the Li+ concentration.64 There is a hypothetical layered structure for KCoO2 by exchanging Li+ with K+, but in practice, the lattice structure is substantially changed to P4/nmm symmetry.68 Similar behavior is observed when replacing Li+ with Na+ in a layered structure, and not only the lattice structure is altered but also the morphology undergoes a large change.69 The FeHCF lattice is not as compact as metal oxides as LIB cathodes, such as LiCoO2, where PB exhibits a density of ca. 1.8 g/cm3. Thus, its volumetric capacity would be lower, which may limit the applications of FeHCF-cathode-based KIBs in portable electronics or electric vehicles. Despite excellent cyclability of PB and its analogues in KIBs, KIBs may need new types of cathode materials to satisfy all practical requirements. Other Cathodes. Recham et al. investigated potassium-based cathode materials in LIBs,70 where K-ions are substituted by Liions during the first cycle. This indicates that K-ion deintercalation from most of common LIB cathode materials is feasible, but the question is whether it is possible to reversibly reintercalate K-ions into these cathode materials. Mathew et al. was able to reversibly intercalate/deintercalate K-ions into/ from a nanoporous amorphous FePO4 prepared by a classic sol−gel method. The FePO4 electrode delivered an initial discharge capacity of 156 mAh/g within the range of 3.5−1.5 V vs K+/K in an electrolyte of 1 M KPF6 in EC/DEC, and retained 70% of its theoretical capacity during 50 cycles.71 Most recently, Vaalma et al. reported a layered birnessite of K0.3MnO2 as a cathode, which exhibits a high capacity of 136 mAh/g with an average potential of 2.8 V in a voltage window from 1.5 to 4.0 V vs K+/K.72 However, at this high cutoff potential, the reversibility of K0.3MnO2 cathode is poor, probably due to irreversible phase transition at high voltages. In the voltage window of 1.5 to 3.5 V, this compound exhibits good cycling performance, retaining 68% of its 10th cycle’s capacity after 675 cycles. Because the cathode may not contain removable K-ions, the authors assembled full cells with a prepotassiated carbon anode of hard carbon/carbon black composite. Such a full cell exhibits an initial capacity of 80 mAh/g (based on cathode) at a current rate of 32 mA/g with an average operating potential of ∼2.0 V in an electrolyte of 1.5 M potassium bis(fluorosulfonyl)imide (KFSI) solvated in 1:1 EC/DMC (ethyl carbonate/dimethyl carbonate).72 The cell capacity fades to about 50 mAh/g after 100 cycles. Potassium titanium phosphate has also been employed as a cathode material of KIBs.73 Similar to the performance of this class of materials in LIBs, the battery performance is originally

specific capacity of LiCoO2, which remains as a common cathode material of LIBs, can be achieved in practice: because extracting further Li-ions will result in lattice breakdown.64,65 However, the rigid cubic structure of PB is independent of the existence of counterions, which are accommodated within the cube. This allows fast and reversible K-ion insertion/extraction. Like any other intercalation materials, the reversibility of the redox systems depends on the size of the counterion, which should match well with the diffusion channels. In the case of the PB family, the lattice structure perfectly matches with K+. Although PB can be successfully cycled for K+ insertion/ extraction, the lattice structure is destroyed after hundreds of cycles in Li electrolyte. This makes this class of electroactive materials promising candidates particularly for KIBs from the cyclability standpoint. In addition to being an alternative to LIBs, the prototypes of KIBs exhibited the potential for fabricating ever-lasting batteries, because millions of cycles were realized in cyclic voltammetry tests in aqueous conditions.45−47 Recent computational studies by Ling et al. showed how the cation size can affect the lattice energies during the intercalation process.66 While the preferred site for the intercalation of Li+ and Na+ is the face-centered 24d sites, the preferable interstitial site for larger K+, Rb+, and Cs+ intercalation is the bodycentered 8c sites. They also reported a remarkable correlation between the insertion voltage in iron hexacyanoferrate (FeHCF) and the ionic radii of different alkali metals, where intercalation of larger ions leads to a higher voltage. It is 3.08, 3.23, and 3.70 V for Li+, Na+, and K+, respectively, where the much higher potential provides an edge for KIBs regarding energy density over sodium-ion batteries (NIBs) if PB is used for both devices. Experimental investigation of the battery performance of PB-based Na- and K-ion batteries also revealed that K-ion battery delivers a higher cell voltage, and the capacity retention is significantly better.67 The rigid structure of PB is quite stable during cycling, and the corresponding battery does not suffer from the common capacity fading as a result of structural changes or distortions. On the other hand, the conductivity is higher, and diffusion is much faster in comparison with conventional cathode materials of lithium-ion batteries. The open framework structure of the lattice allows fast solid-state diffusion, which is a critical parameter in rate capability of batteries. For the case of Li-ion intercalation, thin film PB cathode with a thickness of 120 nm has been successfully charged/discharged with an extremely high rate of 3000 C, delivering a considerable specific capacity of 85 mAh/g.13 Electrochemical behavior of iron hexacyanoferrate is based on the following two sequential redox reactions transforming Berlin green (BG, aka Prussian green) to Prussian blue (PB) and subsequently Prussian white (PW).46 Fe IIIFe III(CN)6 (BG) + K+ + e− → KFe IIIFe II(CN)6 (PB) (1) KFe IIIFe II(CN)6 (PB) + K+ + e− → K 2Fe IIFe II(CN)6 (PW) (2)

PB can be oxidized to BG (1) or reduced to PW (2), which is, indeed, a general mechanism for the insertion of all monovalent cations into PB and its analogues. Padigi et al. prepared BG particles sized 50−75 nm and PB sized 2−10 μm, delivering the specific capacities of 121.4 mAh/g and 53.8 mAh/g, respectively, in an aqueous electrolyte of KNO3.63 Although the PB lattice is spacious enough for the diffusion of 4406


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Figure 2. Enthalpies of the formation of various alkali metal intercalated graphite compounds. (a) Li, (b) K, and (c) Na. Reprinted with permission from ref 78. Copyright 2014 Royal Society of Chemistry.

poor because of low electrical conductivity and the addition of conductive agents, such as carbon black, is mandatory. As a matter of fact, most of the available cathode materials of LIBs with spacious and preferably cubic channels for diffusion might also be employed in KIBs. Organic molecular solids can be promising for accommodating large ions due to their better structural flexibility with strains. Perylene tetracarboxylic dianhydride (PTCDA) has

been recently introduced as a cathode material for potassiumion battery with a specific capacity of about 130 mAh/g by Chen et al. and Xing et al.74,75 The performance is comparable when Na-ions are stored in this molecular solid.76 Following successful applications of Poly(anthraquinonyl sulfide) (PAQS) as a cathode material in LIBs, NIBs, and Mg-ion batteries (MIBs), Jian et al. employed PAQS as a promising cathode material in KIBs.77 Although the cell voltage is not as high as 4407


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Figure 3. DFT calculations for different theories based on various pathways of K intercalation into graphite. (a) Calculated potential profile for Kions intercalation into graphite for different staging scenarios. Blue line corresponds to an intercalation staging: KC24 (Stage III) → KC16 (Stage II) → KC8 (Stage I). The green dotted line corresponds to calculated values for the previously reported staging: KC24 (Stage II) → KC8 (Stage I). The red dotted line corresponds to the averaged experimental data shifted by 26 mAh/g to correct the capacity contribution from SEI formation. (b) Scheme of the different stages of K-intercalated graphite, K is shown in blue and C in yellow. Reprinted with permission from ref 96. Copyright 2015American Chemical Society.

In the case of NIBs, as there is yet a mass-scale commercialization, where one of its biggest challenges is the anode. Although there are hundreds of species that can be successfully intercalated into graphite, intercalation of desolvated sodium into the gallery of graphite is thermodynamically unfavorable.78,79 One promising approach is to expand the graphite layers by simultaneous intercalation of solvent molecules,78 which is, of course, accompanied by volume expansion. For NIBs, nongraphitic carbon, including hard carbon and soft carbon, are promising candidates.80,81 Interestingly, the most stable form of alkali metal graphite intercalation compound (GIC) is that of potassium. This stability is so much dominant that K-intercalated graphite is even employed as an anode material of LIBs.82 Potassium-GIC is of a stage-one structure, where one layer of potassium atoms are inserted between every graphene layer (Figure 2). Obviously, the expansion of graphite interlayers as a result of the interaction of potassium is inevitable. The energies of intercalation of alkali metals into graphite have been calculated using van der Waals density functionals and density functional theory (DFT) calculations.78,83 Figure 2 graphically summarizes the thermodynamics of possible GICs accommodating Li, Na, and K. NaCX is unstable, and its formation is thermodynamically unfavorable, whereas the formation enthalpies of LiCX and KCX are sufficiently negative to form stable GICs, but KCX is even more favorable thermodynamically. Figure 3 shows computational investigations of different stages of potassium intercalation in graphite, which is in agreement with the experimental results reported in the literature. 84,85 K-GICs have been well known since 1932,8,86,87 where in most prior art, K-GICs were prepared by thermally annealing the mixture of potassium metal and graphite under vacuum or an inert atmosphere.88−90 However, electrochemical intercalation of potassium into graphite had not been attempted until very recently. Tossici et al. investigated KC8 as a precharged anode for LIBs, in which K-ions are extracted during the first full-cell discharge, and during charge, incoming Li-ions are inserted to form a Li-GIC.91,92 KC8 anode that can be formed more easily than LiC6 by thermal treatment may allow usage of a Li-free cathode in LIBs. From the perspective of using graphite containers, e.g., crucibles, for electrolysis of molten salts, Liu et al. investigated the electrochemical intercalation of potassium into graphite in molten KF, but the GIC formed is quite

available LIBs, the battery performance is comparable. Since there is a wider range of organic materials, this is just the beginning in the quest for organic cathode materials. Quest for Ideal Cathodes. The cathode materials tested so far are promising but still far from being ideal candidates for the practical development of KIBs. The cyclability of PB and its analogues is a noticeable advantage, but the volumetric capacity is much lower than those of LIBs due to the low density of hexacyanoferrates. Another issue is the intrinsic role of water molecules within the hexacyanoferrate lattice, where there might be of safety risk. In general, a rigid but more compact lattice is required to accommodate a considerable amount of Kions. The key point is that Li-ion is fairly small and a simple metal oxide (e.g., LiCoO2) lattice can retain its original architecture with no structural change upon extraction of at least a large portion of Li-ions. This is not the case of K-based materials because of the large size of K-ions, which are critical in the spatial architecture of the lattice. According to the available reports,1,72,86 the capacity of Prussian blue cathode can reach 80 mAh/g, and its average operating redox potential is 3.8 V vs K+/K. Considering the carbon anode capacity of 262 mAh/g at C/10 and its average depotassiation potential of 0.3 V vs K+/K, a hypothetical full cell would be able to deliver an energy density of 201 Wh/kg based on the mass of the both electrodes. On the basis of the state-of-the-art battery manufacturing technology, the real battery may deliver an energy density of ∼110 Wh/kg in pouch cells. Although this is lower than that of LIBs or even NIBs, if extremely long cycling life of KIBs can be demonstrated in nonaqueous electrolyte, KIBs may become a serious competitor in the market. KIB Anodes. Carbon Anodes. Another critical component of alkali metal ion batteries is the anode material. It is generally believed that metallic anodes may not be practical because of dendrite-formation related safety issues. However, potassium is very soft as the low potassium melting point of 64 °C compared to 98 and 179 °C of Na and Li, respectively, renders potassium dendrites mechanically much weaker, which might avoid a dendrite-caused thermal runaway. Nevertheless, to be on the safer side, it is desirable that the anode is an intercalating material, and consequently, its operating potential and cyclability directly affect the battery performance. For LIBs, carbon-based anodes have been the most common ones not only in research but also in commercial products. 4408


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Figure 4. (A) First-cycle galvanostatic potassiation/depotassiation potential profiles at C/10. (B) XRD patterns of electrodes corresponding to marked SOCs in A. (C) Structure diagrams of different K-GICs, side view (up) and top view (down). Reprinted with permission from ref 82. Copyright 2015 American Chemical Society.

unstable, where exfoliated graphene layers are formed.93 Liu and Wang et al. studied hollow nongraphitic carbon nanofibers as the anode for KIBs, which showed a high initial depotassiation capacity of about 300 mAh/g. However, the cell showed relatively poor cycling performance as the specific capacity fades to 200 mA/g on the second cycle and about 80 mA/g after 20 cycles.94 Jian et al., for the first time, reported room-temperature electrochemical potassium insertion/extraction in graphite in a nonaqueous electrolyte of 0.8 M KPF6 solvated in EC/DEC. The Graphite/K half-cell exhibited a high reversible depotassiation capacity of 273 mAh/g at C/40 (1C = 279 mA/g in forming KC8), close to its theoretical capacity of 279 mAh/g, making it a promising anode material for KIBs. Ex situ XRD studies confirm that stage-III KC36, stage-II KC24, and stage-I KC8 form sequentially upon potassiation while depotassiation recovers graphite through phase transformations in an opposite sequence (Figure 4). Moderate rate capability and cycling performance were observed, where at C/2 in 50 cycles, the capacity retention is 50%.82 The theoretical volume expansion for graphite is 61% upon full potassiation, which causes a detrimental impact on cycling stability. The cycling performance of graphitic materials for potassiation/depotassiation can be improved by introducing enclosed nanoporosity. Xing et al. reported the synthesis of a low-density (0.92 g/cm3) polynanocrystalline graphite by infiltrating a nanoporous graphene with chemical vapor deposition of an ethanol precursor.95 Note that the density of bulk graphite is around 2.2 g/cm3. The hollowness of this polynanocrystalline graphite is most likely enclosed without allowing the imbibing of the electrolyte as its N2 Brunauer− Emmett−Teller (BET) surface area is only 91 m2/g. This new

carbon material exhibit much better cycling stability as it better accommodates the volumetric expansion. Right after Jian et al.’s report on graphite KIB anode, Komaba et al. and Luo et al. also reported highly reversible electrochemical intercalation of potassium into graphite.50,96 The former group reported a capacity of 244 mAh/g for their Graphite/K half-cells, where they also investigated different binders, including polyvinylidene fluoride (PVdF), sodium carboxymethylcellulose (CMC-Na), and sodium polyacrylate (PANa). It was shown that the latter two binders help increase the first-cycle Coulombic efficiency although the reversible capacity is slightly compromised. In the study by Luo et al., a reversible capacity of 208 mAh/g was reported for the graphite anode. They also investigated the K-ion storage properties of reduced graphene oxide (RGO), where a reversible capacity of 222 mAh/g was observed.96 Remarkably, K-ion intercalation into RGO significantly increases the optical transparency of RGO from 29.0 to 84.3%. Not only does this indicate the uniform stacking of graphene layers but it also provides an opportunity for potentially utilizing this electroactive material in photoelectrochemical cells. To address the volumetric change of the graphite electrode during (de)potassiation and improve its cycling stability, Jian et al. synthesized bulk-sized nongraphitic carbons that have somewhat low density, i.e., ca. 1.6 g/cm3 vs ca. 2.2 g/cm3 for graphite.82,97 These are very typical nongraphitic carbons (often called amorphous carbons): graphitizable soft carbon by pyrolysis of PTCDA and nongraphitizable hard carbon spheres (HCS) by hydrothermal carbonization of sucrose. Soft carbon displayed a high specific capacity of 270 mAh/g, comparable with that of graphite, where a pure sloping behavior is observed in its potential profiles, similar to its performance as an anode for Na-ion batteries.81 The cyclability of soft carbon/K half-cells 4409


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potential quasi-plateau, whereas sodiation plateau takes place around 0.05 V vs Na+/Na. Note that the low-potential plateau of lithiation into graphite could occur around 0.1 vs Li+/Li. This is why upon high sodiation current rates, even a low extent of overpotential could shift the HCS/Na cells to the lower cutoff potential. This higher relative operating potential of carbon with respect to K+/K not only gives an edge for carbon anodes in KIBs in high-rate performance but also mitigates the possibility of metal plating for KIBs in full-cell practical applications. It should be emphasized that there remain knowledge gaps in terms of K-ion storage in carbon-based materials toward a practically acceptable battery performance. Intercalation of desolvated Na-ions in graphite is thermodynamically unfavorable; however, recent studies on cointercalation with suitable electrolyte solvents provide a new opportunity for design of graphite anodes for NIBs.98−100 It would be interesting to learn about coinsertion of K-ions with solvent molecules in carbon materials. Other Anodes. At the infancy of the field of KIBs, computational studies may play a large role in predicting potential electrodes. Er et al. showed that Ti3C2 MXene is a potential electrode material for KIBs with a theoretical capacity of 192 mAh/g.101 Quite recently, Sn−C has also been investigated as a potential anode material for KIBs, displaying a reversible charge/discharge performance.102 Similar to the case of sodium,80,103 insertion of potassium into Sn−C is not straightforward and is accompanied by some slow reactions, resulting in the formation of a composite rather than a crystalline structure.102

is more stable than that of graphite/K cells with capacity retention of 81.4% after 50 cycles at 2C. Soft carbon exhibits good rate capability as well, where the specific capacities of 210, 185, and 140 mAh/g were obtained at discharge rates of 1C, 2C, and 5C, respectively. The rate performance of HCS was comparable to soft carbon with specific capacities of 190 and 136 mAh/g achieved at rates of 2C and 5C, respectively. HCS exhibited, to date, the best cycling stability among graphite, soft carbon, and hard carbon, where the electrode retained 83% of its initial capacity after 100 cycles at C/10. All the above results were obtained by using PVdF as the binder for the carbon electrodes. Ongoing research from the Ji group reveals even better cycling stability when NaCMC is employed as the binder for these nongraphitic carbon anodes. It is very likely that PVdF reacts more with potassium or even sodium but less with lithium in the electrolyte/electrode interface, where, indeed, similar issues have been observed for NIBs as well. Electrochemical studies of Na and K intercalation in HCS showed different rate behaviors (Figure 5).97 While the specific

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Figure 5. Typical discharge/charge potential profiles of HCS/Na cells and HCS/K cells. Reprinted with permission from ref 97. Copyright 2016 Wiley.

AQUEOUS POTASSIUM-ION BATTERIES Among various applications of PB in electrochemical systems, PB analogues were also investigated as electrode materials for aqueous batteries. The focus has been symmetrical PB batteries where PB analogues serve as both anode and cathode. In this cell design, two redox couples of PB provide the required anodic and cathodic processes, resulting in a cell voltage of 0.68 V.104 The principle is via “disproportionation” redox reactions, as shown in the eqs 1 and 2, corresponding to 0.82 and 0.18 V vs SHE, respectively. During the charging process, the PB cathode is oxidized to BG, and the PB anode is reduced to PW. To increase the operational potential, researchers developed asymmetrical hexacyanometallate cells, in which a more negative anode and a more positive cathode were selected from a wide range of transition metal hexacyanometallates.37,44 An example is an aqueous KIB based on a KCrIIICrII(CN) 6

capacity is larger for HCS/Na at low current rates, surprisingly, compared to HCS/Na cells, HCS/K half-cells exhibit a much better rate performance. The authors measured the diffusivity values of HCS/K half-cells and HCS/Na half-cells as a function of the state of charge by galvanostatic intermittent titration technique (GITT) measurements, where the diffusivity of the former is slightly higher. The interesting comparative kinetic performance between HCS/K and HCS/Na calls for further studies, particularly by calculation/simulation. Another large factor contributing to the better rate behavior of HCS/K than HCS/Na is the higher operating potential of the former relative to its metal reference/counter electrode. On average, potassiation occurs at above 0.2 V vs K+/K for the lower-

Figure 6. (a) Galvanostatic cycling of KCuFe(CN)6 at various current densities between 0.6 and 1.4 V vs SHE. (b) Long-term cycling of CuHCF at a rate of 17 C between 0.8 and 1.2 V vs SHE shows 83% capacity retention after 40 000 cycles (right). Reprinted with permission from ref 107. Copyright 2011 Nature Publishing Group. 4410


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ions to form Li2O2.122 Since then, considerable attention has been paid to Li−O2 batteries.117,123−127 In a sense, the cathode of Li−O2 batteries is similar to fuel cells, as a part of the reactive species comes from outside the cell. In theory a Li−O2 battery can exhibit an energy density higher than any other known battery systems. The low atomic weight of lithium guarantees the highest possible specific capacity of a battery. However, its theoretical capacity is not practically achievable because of severe limitations. However, achieving a fraction would make Li−O2 batteries impressive. It should be emphasized the terminology of this topic is somewhat misleading, as the term Li−air and Li−O2 are used interchangeably. The target battery is obviously Li−air because it should work by consuming O2 from the ambient air rather than the constant supply of pure O2. However, most research works are conducted under pure saturated O2. Inappropriate comparison of different cases without considering the O2 concentration has caused some concerns. Because of the challenges of Li−O2 batteries, Na−O2 batteries have recently become possible candidates, despite sacrificing some specific capacity. Although Na−O2 batteries have some advantages over Li−O2, K−O2 batteries can be even more advantageous. For Li−O2 batteries, a critical issue is the electrochemical pathway for the formation of lithium peroxide (Li2O2), the final discharge product, where the process involves the thermodynamically unstable intermediate of lithium superoxide (LiO2).128−130 The unstable nature of LiO2 and its reactions with organic electrolyte directly contributes to the irreversible cell processes, e.g., drying up the cells. In contrast, as reported by Hartmann et al., the final discharge product of the Na−O2 batteries is crystalline sodium superoxide (NaO2), which is at least kinetically stable and is also formed with a similar mechanism.131 In this family of alkali metal superoxides, KO2 as the final discharge product of K−O2 batteries, first reported by Ren and Wu,132 is exceptionally stable both kinetically and thermodynamically, as it is even available as a chemical in the market. For Li−O2 systems, some primary interest is to stabilize LiO2 and to increase reaction reversibility and efficiency associated with O2−. Beside catalysts, one approach is to employ ionic liquids as an electrolyte due to their wide stable electrochemical window, wh ere 1-et hyl-3-methy limidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI) is, indeed, a common ionic liquid in electrochemical experiments. The impact of cations in the ionic liquid electrolyte is evident by simply comparing the CV curves of MO2/M redox systems in an electrolyte of EMI-TFSI (Figure 7). It was shown that large (soft) cations greatly enhance the electrochemical behavior of O2/O2− redox reactions. For detailed information about Li−O2 batteries, readers may read the recent review articles.133−135 Another interesting feature of a K−O2 battery is the reversibility of O2/KO2 in comparison with O2/NaO2, as shown by the cyclic voltammetry (CV) results in Figure 8 (compare the left and right columns side by side). While the KO2 peak is slightly weakened after 20 cycles, NaO2 peak is almost disappeared. This is due to the mechanism of the KO2 formation, which is through one straightforward process (as judged by the well-defined voltammetric peak). Moreover, the stability of KO2 guarantees the system reversibility for the reverse reaction. Changing the potential window in the cathodic region only affects the peak intensity with a little

anode and a KCrIIIFeII(CN) 6 cathode (at a fully discharged state), which delivers a cell voltage of about 1.5 V.37 Wessells and Cui et al. systematically studied the analogues of the PB family, such as KCuIIFeIII(CN)6, KNiIIFeIII(CN)6 and structures with Ni and Cu coreplacement of Fe in the hexaisocyanide sites.62,105−109 The operation voltage can be adjusted by the ratio of Cu and Ni, from 0.7 to 0.95 V vs Ag/ AgCl. For the KCuIIFeIII(CN)6 electrode, the operation reaction is KCu IIFe III(CN)6 + K+ + e− → K 2Cu IIFe II(CN)6

(3)

which provides a theoretical capacity of 85 mAh/g. The electrode material was prepared by a precipitation method from an aqueous solution, where the obtained material is hydrated as K0.71Cu[Fe(CN)6]0.72·3.7 H2O, and the theoretical capacity of this formula is 62 mAh/g.107 These electrodes show extremely high rate performance and excellent cycling performance. For example, KCuFe(CN)6 exhibits a capacity of 40 mAh/g at 83C, which is about 2/3 of the capacity at 0.83C, and the capacity retention is above 80% after 40 000 cycles at 8.3C (Figure 6). At 0.83C or even 8.3C, the polarization is extremely small, thus demonstrating a high round-trip efficiency, which is critical for the applications of stationary energy storage. The same group also demonstrated that PB analogues exhibit high-rate and long-cycling performance when alkaline earth metal ions, i.e., Mg2+, Ca2+, Sr2+, and Ba2+, are the counterions.106 The large interstitial sites as well as the interaction between (CN)− and divalent ions being weaker compared to the case with O2− in metal oxides may facilitate the great performance, where the activation energy for the transfer of these ions may be low. Potassium titanium oxide has been utilized as a potential anode material.110 The capacity fading of this material is severe during the initial cycles. This is due to the fact that K-ions are too large for insertion/extraction into metal oxides, and the lattice structure will be subjected to severe changes during initial cycling. This limits the application of available metal oxides, which have been successfully employed as anode or cathode in LIBs and NIBs. Suitable lattice structures with good diffusion channels for K-ions have yet to be found. KIB Electrolytes. Unfortunately, no systematic research has been conducted to study the electrolyte and its role on the performance of KIBs yet. In all works devoted to KIBs, electrolytes have been typically chosen based on conventional electrolytes of LIBs. The most common electrolyte is a mixture of alkyl carbonates1 and potassium salts, such as KBF4,1 KPF6,67,74,82,96,97,102 and KFSI (potassium bis(fluorosulfonyl)imide).72 Since PB and its analogues are still the dominant cathodes of KIBs, and their preliminary studies have been conducted in aqueous media, aqueous electrolytes of common potassium salts, such as KNO3, are also employed for the studies of KIBs.62,63,107,109 This is accompanied by rapidly growing interest in aqueous LIBs. Despite being limited by a low cell voltage, aqueous KIBs with low cost, simplicity in design, and good safety are promising candidates for large-scale energy storage. Potassium Metal Batteries. Potassium−Oxygen Battery. The concept of metal−air batteries has been well-known, e.g., the primary zinc-air battery,111−115 but a secondary alkali metal battery utilizing a nonaqueous electrolyte is a relatively new idea.116−121 Abraham and Jiang proposed the possibility of replacing intercalation cathode materials with a reactive cathode, in which O2 (from air) is reduced and reacts with Li 4411


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dependency of these peaks reveals that the second reaction is the rate-determining step. First, a sharper CV peak indicates a flat plateau in charge/discharge profiles, where the battery can deliver a constant potential during its performance. Second, a high peak-to-peak separation in CV means that there is a gap between the anode and cathode processes, i.e., cell polarization due to overpotentials (Figure 9). In other words, a higher cell voltage is needed to charge the battery than that provided during the discharge. In any battery systems, it is vital to reduce this irreversible energy consumption to maximize the roundtrip efficiency. A key problem of Li−O2 batteries is a huge potential difference between charge and discharge potential profiles owing to high overpotentials of the reactions involved. There are two overpotentials in the superoxide reaction for (i) oxygen adsorption, and (ii) LiO2 formation. The former can be reduced by employing catalysts to dissociate O2 to form O adatoms. However, it is difficult to reduce the latter overpotential, because it is thermodynamically related to LiO2. This overpotential is much less in Na−O2 batteries in the range of ∼200 mV.131,136−138 According to the electrochemical behavior described above, the overpotentials are trivial for a K−O2 battery (Figure 9). Ren and Wu have shown that

Figure 7. Cyclic voltammogram of O2/O2− in neat EMITFSI along with various salts at 0.025 M concentration on a glassy carbon (GC) electrode at 100 mV/s, showing OER overpotentials of cation-O2− as a function of cation hardness Reprinted with permission from from ref 129. Copyright 2012 American Chemical Society.

influence on the peak shape. This means that the progress of the cathode process does not change the reaction pathway. It should be pointed out that the redox system of KO2 is composed of two peaks, but they are hardly separable. Scan rate

Figure 8. Cyclic voltammetry studies of 0.025 M (left column) NaPF6, and (right column) 0.025 M KPF6 in EMITFSI. (a) Selected cycles: first (black), second (blue), and 20th (red) scan. (b) Scans extending to various cathodic limits as listed in the inset. (c) Cyclic voltammograms of O2saturated 0.025 M KPF6 in EMITFSI at 100 mV/s on a GC electrode: 1st cycle (black) and 20th cycle (red). (d) Incremental cathodic limits ranging from 2.38 to 1.19 V. (e) Various scan rates (10−300 mV/s). Inset shows expansion on anodic peaks. Reprinted with permission from ref 129. Copyright 2012 American Chemical Society. 4412


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materials of LIBs, it is also possible to examine new candidates for potassium anodes too. K3Sb alloy showed a high capacity of 650 mAh/g in a K−O2 battery.140 Potassium−Sulfur Battery. The mechanism of Li−S battery is somehow between LIBs and Li−O2 battery. Unlike LIBs in which the cathode operates on a topotactic solid-state intercalation reaction, Li−S cathode is based on a conversion or integration reaction of S8 and Li+ at the cathode interface. This is similar to Li−O2 batteries, in which the cathode is involved in a catalytic reaction of oxygen combining with Liions. However, contrary to the Li−O2 batteries, Li−S batteries have a sealed system similar to LIBs. This matters when calculating the specific capacity and energy density, as the reactive species are already within the cell. This also dictates the cathodic reaction pathway. While in a Li−O2 battery, the ratio of oxygen in the resulting cathodic material, LixOy decreaces, the discharge pathway of an S-cathode is from S8 to Li2Sx (3 ≤ x ≤ 8), Li2S2, and finally Li2S. Li−S batteries face challenges associated with the cathode product intermediates of lithium polysulfides, which are soluble in the electrolyte solution. Thus, not only the cathode has a mass loss, but also the dissolved polysulfides interfere with the anode process too. The most common approach is to trap sulfur within a shell of carbon or polymers to avoid direct interaction with the electrolyte.141−150 Beyond Li−S systems, the first immediate alternative analogue is Na−S battery. Similar to the cases described above, potassium has the advantages of sodium over lithium, plus more. Hence, there are emerging interests in K−S batteries.151,152 Similar to Na-ion, K-ion is too big (soft) to complete the reaction to reach K2S, and the final product has been reported by Chen et al. to be K3S2.152 Further effort is needed to capture the full capacity of an S-cathode in potassium−sulfur battery because there is no thermodynamical limit for the formation of K2S in the cathode; Li2S and K2S are similar. Because it is very difficult to overcome solubility of alkali metal polysulfides in a liquid electrolyte, high-temperature alkali metal−sulfur battery was introduced utilizing a molten metal anode and a solid electrolyte.153,154 The solid electrolyte prevents the liquid metal anode from contacting with the sulfur cathode. This design is not suitable for Li−S battery because of the high melting point of lithium (180 °C) but utilized for Na− S.153 Although Na is melted at 98 °C, a temperature of ca. 350 °C is needed to reach an acceptable wettability of the solid electrolyte in contact with the molten anode. However, this type of alkali metal−sulfur battery exhibited a good cyclability.153 Another approach is to make an alloy of Na with K or Cs, which can be a liquid at room temperature.155 However, there exists another problem associated with destructive ion exchange of Na and K in the solid electrolyte.155,156 As depicted in Figure 10a, Al2O3 layer is cracked in contact with melted sodium, but no destructive effect for the case of potassium. Owing to its low melting point (63 °C) and excellent wettability performance, molten K anode can have an acceptable performance at a temperature below 240 °C, as can be witnessed in Figure 10b. A high-temperature K−S battery at operating temperature of 150 °C can deliver a specific capacity of ca. 300 mAh/g and retain it over 1000 cycles, as reported by Liu et al.151

Figure 9. Charge/discharge potential profiles for (a) K−O2 battery (0.5 M KPF6 in DME) and (b) Li−O2 battery (1 M LiCF3SO3 in tetraglyme). Both at a current density of 0.16 mA/cm2. Electrode geometric area is 0.64 cm2. The dashed lines indicate the calculated thermodynamic potentials for the batteries. Reprinted with permission from ref 132. Copyright 2013 American Chemical Society.

the potential difference between charge and discharge of a Kbattery is as low as 50 mV without the use of catalysts.132 High polarization causes low energy (round-trip) efficiencythe ratio between energy output and energy input, where conventional powers, i.e., internal combustion engine, are criticized for, as it is only around 20%. A key challenge for developing alkali metal−oxygen batteries is to find appropriate anode materials, as it may not be practical to directly use alkali metal anodes due to safety risks, where Li is perhaps an exception due to its relatively high melting temperature, 180 °C, if solid-state electrolytes are employed. Formation of an insoluble surface layer on metallic anodes may hinder the parasitic reactions on surface.139 Furthermore, the excellent performance of carbon anodes for K-ion intercalation might satisfy the need of an anode for a K-ion−O2 battery. In other words, not much effort is required to find a practical anode for K−O2 batteries, as they are already identified.96 In contrast, the anode issue makes a Na−O2 battery less practical, because almost all studies have been conducted using a Na metal anode. Of course, the specific capacity of graphite anode is much below the theoretical expectation of alkali metal air batteries, and the quest for finding conversion anodes with high specific capacity is an active area of research for all alkali metal-air batteries. By learning from profound experience on anode 4413


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electrodes and the whole battery with inert mass of electrolyte, current collectors and packaging materials. For example, the K2Fe2(CN)6/C8 system (442.10) represents only 7.85% heavier than the competitive NIB system of Na2Fe2(CN)6/hard carbon (C8 with a capacity of 279 mAh/g), 409.88. Additionally, the Kion systems may have a wider voltage window than NIB systems due to the low operating potential of potassium metal, K-GICs or K-Carbon as an anode. Indeed, the argument about the NIBs’ intrinsic advantage over KIBs may not hold true. Furthermore, similar to NIBs, potassium does not form alloys with aluminum; thus, for both NIBs and KIBs, aluminum foil can be used as a current collector, which will further lower than cost. It is safe to say that neither NIBs nor KIBs would be able to compete against LIBs regarding energy density, where in portable electronics and electric vehicles it is the most important metric unless lithium price turns completely unaffordable. The advantage of NIBs and KIBs is still the low cost and their long-term technological sustainability. Thus, the competition will be most likely between NIBs and KIBs for the market of stationary energy storage. This is not saying that NIBs and KIBs have no future in electric vehicles; there are great potentials for both to improve. Note when LIBs was first commercialized, the energy density of those LIBs was only around 100 Wh/kg, where such values have already been surpassed by some current prototypes of NIBs. When looking back the history of research and development of LIBs, which was relatively rapid due to the tremendous existing demand for portable electronics in the 1990s, considerable attention from both academia and industry paid to LIBs paved the path for establishing an extensive knowledge pool about various features and components of LIBs. Although potassium batteries have attracted much attention during the past few years, our knowledge is still very limited, particularly about the mechanisms at an atomic scale as well as along interfaces, i.e., solid electrolyte interphase (SEI). For SEI of KIBs, it is not surprising that little has been done because even for NIBs, not much is known about the SEI on the hard carbon anode as well as various cathodes. Hence, it is vital to profoundly understand the different basic facets of potassium systems before any practical batteries can be realized. It is our opinion that the following issues are worthwhile being properly investigated/addressed to make potassium batteries practically comparable with their rivals. (i) Identify suitable cathode materials and reveal their K-ion storage mechanisms: The size and charge density of Kions in an electrolyte and solid materials are significantly different from those of Li and Na counterparts. Thus, conventional materials of LIBs and even newer cathode materials for NIBs cannot be simply employed in KIBs. This is the main reason that the PB family is still the most popular cathode material of KIBs, as there is comprehensive knowledge about K-ion insertion/extraction into/from PB structures. Therefore, intercalation of K-ions into possible metal oxides (which may have high specific capacity values) should be computationally and experimentally studied to fundamentally understand possible opportunities and drawbacks. For designing new cathode materials, the largest challenge is to accommodate the large strain caused by accommodation of large K-ions. To this end, conventional crystalline inorganic materials may be limited because of their rigid

Figure 10. (a) Interaction of Na, Na−K alloys, and K with Na−Al2O3 or K−Al2O3 solid electrolytes at 150 °C. (b) Wetting behavior of liquid K on the K−Al2O3 solid electrolyte surface at various temperatures. Reprinted with permission from ref 151. Copyright 2015 Wiley.

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SUMMARY AND PERSPECTIVE: OPPORTUNITIES AND CHALLENGES The current status of LIBs, as the most common energy storage systems of portable devices and electric vehicles, is the result of extensive research conducted during the past four decades. In any case, the need for large energy storage systems of the future cannot be satisfied by LIBs due to the limited availability of natural resources of lithium and expensive cost. Thus, there is no other choice but to find potential alternatives. Without any exaggeration, potassium batteries have certainly invaluable advantages over their sodium (and even lithium) counterparts. Among alkali and alkaline earth metals, potassium represents a unique and long-ignored opportunity for energy storage. Na and Mg batteries have been extensively studied over the past two decades, but still, the performance is not satisfactory enough for practical applications. Potassium batteries are just at the infancy stage, and practical ideas and solutions will evolve in time. In this direction, comprehensive knowledge of LIBs can be the guiding star, but the intrinsic differences of lithium and potassium systems should be carefully taken into account. Another important perspective is that KIBs are not necessarily a replacement to current LIBs for power devices, as can be electrochemical energy sources with their own applicability. If considering the prototype of KIBs with a PB cathode, a conventional electrolyte, and a carbon anode; the resulting battery can be inexpensive and may exhibit a good long-term performance. The only issue is the lower energy density in comparison with available LIBs. The critical requirement for maximizing the energy density is for portable devices and electric vehicles, where the battery should be in its minimum size and weight. If the goal is to build large batteries for household energy storage or load-leveling, the price is much more important than the weight. Alkali metal oxygen and sulfur batteries are, indeed, new types, at least for practical applications. Although the primary focus has been on lithium-based batteries because of resemblance of LIBs, their commercialization may take time. Because of the fact that the development of Li−O2 and Li−S batteries will be limited by the scarcity of natural resources sooner or later, it may be necessary to shift this focus toward K−O2 and K−S batteries at this preliminary stage. Indeed, potassium (39.10) is much heavier than lithium (6.94) and sodium (22.99). However, from a full-cell point of view, in the K-ion systems, the more mass from potassium ions does not occupy a large percentage of the total mass of both 4414


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ACS Applied Materials & Interfaces structures, whereas amorphous or short-range ordered materials may find much better performance. Among these materials, organic solids assembled by van der Waals force with or without crystalline structures can be promising. Of course, polymeric materials can be uniquely advantageous as well because of their structure flexibility and tunability of properties, such as conductivity and redox functionalities. As a unique advantage, KIBs may be more amenable to chargecarrier-free cathode because K-GICs and K-Carbon anode can be easily prepared by various methods, such as annealing or solution intercalation. Indeed, many impractical cathode materials explored in the last 20 years of LIB research may find their more suitable applications in KIBs. (ii) Explore carbonaceous anodes: Compared to LIBs and NIBs, K−C type anodes can be more advantageous due to the higher operation potentials relative to the metal reference electrode, where the metal dendrite concern is less than Li−C and Na−C counterparts. Of course, it is common sense that potassium metal is extremely reactive and can be very dangerous, compared to Li and Na metals. One should not confuse about K-ion batteries with such an issue of the potassium metal. Electrodes containing K-ions can be safe in sealed batteries as long as dendrites and plating of potassium metal do not occur on the anode side. Please note that as most fundamental research is typically conducted with half-cells containing metallic potassium as the counter/reference electrode first, we strongly encourage researchers in the KIB field to take extra caution in handling potassium metal and tested K-containing half-cells. For K-ion full-cell batteries, metallic potassium does not have to be employed; therefore, the safety of K-ion batteries may not necessarily be more concerned about than its Li-ion or Na-ion counterparts. As graphite may work well for KIBs, the existing infrastructure of LIBs will most likely serve the futuristic KIB industry, which presents a large advantage over NIBs. However, the volume expansion of graphite anode upon potassiation is of a large scale, ca. 61%, which may be a stopper of the graphite anode in practical KIBs. Along this line, nongraphitic anodes with less density values, including hard carbon and soft carbon, may still put this technology back on the track of eventual commercialization. The more space in these nongraphitic carbons will be able to accommodate the “lodging” of large K-ions more easily. Thus, on the anode side, to understand the correlation between the local structures of nongraphitic carbons and the electrochemical performance of K-ion storage can be a fruitful pathway forward. We expect that capacity values higher than 279 mAh/g corresponding to C8 can be realized if the structures of nongraphitic carbons are well-designed. (iii) Investigate SEI: It has taken quite a long time to understand the SEI in LIBs. At the current stage with strong interests in high-voltage cathodes and solid-state electrolytes, there remain knowledge gaps regarding fundamentals of SEI of LIBs, particularly on the cathode side and Li metal anode. For NIBs, understanding of SEI is very much limited. For KIBs, the SEI chemistry may be very much different from that of LIBs as K is a very soft cation and it prefers to precipitate with large anions. For SEI, on the surface of carbon anodes in LIBs, LiF, and

Li2CO3 of strong ionic bonding are among the primary components. As for KIBs, SEI can be designed by purposely using large and soft anions. By comparing to the Li and K cases, Na-SEI might be challenging, as Naions are neither “hard” nor “soft.” Like other batteries, SEI formation is a key step in long-term cycling performance of KIBs, as a protective SEI can avoid side reactions of the electrode surface in direct contact with the electrolyte, but a thick SEI can block further diffusion of electroactive species. It has been reported that potassium may be capable of forming a more stable SEI in comparison with Li and Na,51 but profound studies are needed to fully understand the nature and permeability of K to practically plan the optimum cell architecture. (iv) Understand electrode performance in full cells: In lithium batteries, the Li anode, which serves as the reference electrode too, is commonly used to eliminate the influence of the other half-cell while studying the anode or cathode half-cell. In half-cells, the same cell setup has been used for sodium batteries and potassium batteries as for LIBs. However, Na metal and K metal may not be as good reference electrodes as Li metal because they can be more reactive with an organic electrolyte, resulting in large Ohmic resistance and fast electrolyte degradation. In particular, potassium cannot be used at elevated temperatures due to its low melting point. This may cause an inconsistency in the fundamental studies of K cells, which should be carefully taken into account. On the other hand, chemical reactivity of potassium is generally a serious safety issue in designing K batteries.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.J. is thankful for the financial support from the National Science Foundation of the United States, Award 1551693. REFERENCES

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