Quantifying Interfacial Ion Transfer at Operating Potassium-Insertion Battery Electrodes within Highly Concentrated Aqueous Solutions.

Electrode/electrolyte interfacial ion transfer is a fundamental process occurring during insertion-type redox reactions at battery electrodes. The rate at which ions move into and out of the electrode, as well as at interphase structures, directly impacts the power performance of the battery. However, measuring and quantifying these ion transfer phenomena can be difficult, especially at high electrolyte concentrations as found in batteries. Herein, we report a scanning electrochemical microscope method using a common ferri/ferrocyanide (FeCN) redox mediator dissolved in an aqueous electrolyte to track changes in alkali ions at high electrolyte concentrations (up to 3 mol dm-3). Using voltammetry at a platinum microelectrode, we observed a reversible E1/2 shift of ∼60 mV per decade change in K+ concentrations. The response showed high stability in sequential measurements and similar behavior in other aqueous electrolytes. From there, we used the same FeCN mediator to position the microelectrode at the surface of a potassium-insertion electrode. We demonstrate tracking of local changes in the K+ concentration during insertion and deinsertion processes. Using a 2D axisymmetric, finite element model, we further estimate the effective insertion rates. These developments enable characterization of a key parameter for improving batteries, the interfacial ion transfer kinetics, and future work may show mediators appropriate for molar concentrations in nonaqueous electrolytes and beyond.

Electrochemical intercalation of rubidium into graphite, hard carbon, and soft carbon.

The electrochemical insertion of Rb into carbonaceous materials, including graphite, was achieved herein. Rubidium ions were reversibly inserted into and extracted from graphite via electrochemical processes using different non-aqueous electrolytes containing rubidium bis(trifluoromethanesulfonyl)amide (RbTFSA) salts in carbonate esters, glymes, and ionic liquids, similar to the process used for other lighter alkali metal ions such as Li+ and K+. The chemical compositions of the rubidiated graphite were determined to be RbC8, RbC24, and RbC36 at each step of the electrochemical reduction process. Graphite underwent a phase transition to RbC8 exhibiting a stage-1 structure, with stage-3 RbC36 and stage-2 RbC24 as intermediates, as confirmed by ex situ and in situ X-ray diffraction and ex situ Raman spectroscopy, similar to the electrochemical phase evolution of staged potassium graphite intercalation compounds (K-GICs). Furthermore, Rb was reversibly inserted into and extracted from graphitizable and non-graphitizable carbons such as pitch-derived soft carbon and commercial hard carbon, along with other alkali metals such as Li, Na, and K.

Surfactant-Free Formate/O2 Biofuel Cell with Electropolymerized Phenothiazine Derivative-Modified Enzymatic Bioanode.

A formate (HCOO-) bioanode was developed by utilizing a phenothiazine-based electropolymerized layer deposited on sucrose-derived carbon. The electrode modified with NAD-dependent formate dehydrogenase and the electropolymerized layer synergistically catalyzed the oxidation of the coenzyme (NADH) and fuel (HCOO-) to achieve efficient electron transfer. Further, the replacement of carbon nanotubes with water-dispersible sucrose-derived carbon used as the electrode base allowed the fabrication of a surfactant-free bioanode delivering a maximum current density of 1.96 mA cm-2 in the fuel solution. Finally, a separator- and surfactant-free HCOO-/O2 biofuel cell featuring the above bioanode and a gas-diffusion biocathode modified with bilirubin oxidase and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) was fabricated, delivering a maximum power density of 70 µW cm-2 (at 0.24 V) and an open-circuit voltage of 0.59 V. Thus, this study demonstrates the potential of formic acid as a fuel and possibilities for the application of carbon materials in bioanodes.

Impact of electrolyte decomposition products on the electrochemical performance of 4 V class K-ion batteries.

In the pursuit of long-life K-ion batteries (KIBs), half-cell measurements using highly reactive K metal counter electrodes are a standard practice. However, there is increasing evidence of electrolyte decomposition by K metal impacting electrode performance. Herein, we systematically explored the K metal-treated electrolytes KPF6, KN(SO2F)2 (KFSA), and their combination in ethylene carbonate/diethyl carbonate (EC/DEC), referred to as K-KPF6, K-KFSA, and K-KPF6:KFSA, respectively, after storage in contact with K metal. Through mass spectrometry analysis, we identified significant formation of carbonate ester-derived decomposition products such as oligocarbonates for K-KPF6, while K-KFSA predominantly generates anions combining FSA- with the solvent structures. Using three-electrode cells, we delineated the positive effects of the K-KFSA and K-KPF6:KFSA electrolytes on graphite negative electrode performance and the negative impact of oligocarbonates in K-KPF6 on K2Mn[Fe(CN)6] positive electrodes. The interactions between the decomposition products and the electrodes were further evaluated using density functional theory calculations. Full cell measurements using K-KPF6:KFSA showed an improved energy density and capacity retention of 78% after 500 cycles compared with an untreated electrolyte (72%). Hard X-ray photoelectron spectroscopy indicated the incorporation of the FSA-derived structures into the solid electrolyte interphase at graphite, which was not observed in K metal-free cells. Overall, this work indicates further complexities to consider in KIB measurements and suggests the potential application of decomposition products as electrolyte additives.

In situ Observation of Evolving H2 and Solid Electrolyte Interphase Development at Potassium Insertion Materials within Highly Concentrated Aqueous Electrolytes.

The solid-electrolyte interphase (SEI) is key to stable, high voltage lithium-ion batteries (LIBs) as a protective barrier that prevents electrolyte decomposition. The SEI is thought to play a similar role in highly concentrated water-in-salt electrolytes (WISEs) for emerging aqueous batteries, but its properties remain unknown. In this work, we utilized advanced scanning electrochemical microscopy (SECM) and operando electrochemical mass spectrometry (OEMS) techniques to gain deeper insight into the SEI that occurs within highly concentrated WISEs. As a model, we focus on a 55 mol/kg K(FSA)0.6 (OTf)0.4 electrolyte and a 3,4,9,10-perylenetetracarboxylic diimide negative electrode. For the first time, our work showed distinctly passivating structures with slow apparent electron transfer rates alike to the SEI found in LIBs. In situ analyses indicated stable passivating structures when PTCDI was stepped to low potentials (≈-1.3 V vs. Ag/AgCl). However, the observed SEI was discontinuous at the surface and H2 evolution occurred as the electrode reached more extreme potentials. OEMS measurements further confirmed a shift in the evolution of detectable H2 from -0.9 V to <-1.4 V vs. Ag/AgCl when changing from dilute to concentrated electrolytes. In all, our work shows a combined approach of traditional battery measurements with in situ analyses for improving characterization of other unknown SEI structures.

Impact of Ti and Zn Dual-Substitution in P2 Type Na2/3 Ni1/3 Mn2/3 O2 on Ni-Mn and Na-Vacancy Ordering and Electrochemical Properties.

High-entropy layered oxide materials containing various metals that exhibit smooth voltage curves and excellent electrochemical performances have attracted attention in the development of positive electrode materials for sodium-ion batteries. However, a smooth voltage curve can be obtained by suppression of the Na+ -vacancy ordering, and therefore, transition metal slabs do not need to be more multi-element than necessary. Here, the Na+ -vacancy ordering is found to be disturbed by dual substitution of TiIV for MnIV and ZnII for NiII in P2-Na2/3 [Ni1/3 Mn2/3 ]O2 . Dual-substituted Na2/3 [Ni1/4 Mn1/2 Ti1/6 Zn1/12 ]O2 demonstrates almost non-step voltage curves with a reversible capacity of 114 mAh g-1 and less structural changes with a high crystalline structure maintained during charging and discharging. Synchrotron X-ray, neutron, and electron diffraction measurements reveal that dual-substitution with TiIV and ZnII uniquely promotes in-plane NiII -MnIV ordering, which is quite different from the disordered mixing in conventional multiple metal substitution.

Sulfated Alginate as an Effective Polymer Binder for High-Voltage LiNi0.5Mn1.5O4 Electrodes in Lithium-Ion Batteries.

Although the increasing demand for high-energy-density lithium-ion batteries (LIBs) has inspired extensive research on high-voltage cathode materials, such as LiNi0.5Mn1.5O4 (LNMO), their commercialization is hindered by problems associated with the decomposition of common carbonate solvent-based electrolytes at elevated voltages. To address these problems, we prepared high-voltage LNMO composite electrodes using five polymer binders (two sulfated and two nonsulfated alginate binders and a poly(vinylidene fluoride) conventional binder) and compared their electrochemical performances at ∼5 V vs Li/Li+. The effects of binder type on electrode performance were probed by analyzing cycled electrodes using soft/hard X-ray photoelectron spectroscopy and scanning transmission electron microscopy. The best-performing sulfated binder, sulfated alginate, uniformly covers the surface of LNMO and increased its affinity for the electrolyte. The electrolyte decomposition products generated in the initial charge-discharge cycle on the alginate-covered electrode participated in the formation of a protective passivation layer that suppressed further decomposition during subsequent cycles, resulting in enhanced cycling and rate performances. The results of this study provide a basis for the cost-effective and technically undemanding fabrication of high-energy-density LIBs.

Active material and interphase structures governing performance in sodium and potassium ion batteries.

Development of energy storage systems is a topic of broad societal and economic relevance, and lithium ion batteries (LIBs) are currently the most advanced electrochemical energy storage systems. However, concerns on the scarcity of lithium sources and consequently the expected price increase have driven the development of alternative energy storage systems beyond LIBs. In the search for sustainable and cost-effective technologies, sodium ion batteries (SIBs) and potassium ion batteries (PIBs) have attracted considerable attention. Here, a comprehensive review of ongoing studies on electrode materials for SIBs and PIBs is provided in comparison to those for LIBs, which include layered oxides, polyanion compounds and Prussian blue analogues for positive electrode materials, and carbon-based and alloy materials for negative electrode materials. The importance of the crystal structure for electrode materials is discussed with an emphasis placed on intrinsic and dynamic structural properties and electrochemistry associated with alkali metal ions. The key challenges for electrode materials as well as the interface/interphase between the electrolyte and electrode materials, and the corresponding strategies are also examined. The discussion and insights presented in this review can serve as a guide regarding where future investigations of SIBs and PIBs will be directed.

Superconcentrated NaFSA-KFSA Aqueous Electrolytes for 2 V-Class Dual-Ion Batteries.

Superconcentrated aqueous electrolytes containing NaN(SO2F)2 and KN(SO2F)2 (for which sodium and potassium bis(fluorosulfonyl)amides (FSA), respectively, are abbreviated) have been developed for 2 V-class aqueous batteries. Based on the eutectic composition of the NaFSA-KFSA (56:44 mol/mol) binary system, the superconcentrated solutions of 35 mol kg-1 Na0.55K0.45FSA/H2O and 33 mol kg-1 Na0.45K0.55FSA/H2O are found to form at 25 °C. As both electrolytes demonstrate a wider potential window of ∼3.5 V compared to that of either saturated 20 mol kg-1 NaFSA or 31 mol kg-1 KFSA solution, we applied the 33 mol kg-1 Na0.45K0.55FSA/H2O to two different battery configurations, carbon-coated Na2Ti2(PO4)3∥K2Mn[Fe(CN)6] and carbon-coated Na3V2(PO4)3∥K2Mn[Fe(CN)6]. The former cell shows highly reversible charge/discharge curves with a mean discharge voltage of 1.4 V. Although the latter cell exhibits capacity degradation, it demonstrates 2 V-class operations. Analysis data of the two cells confirmed that Na+ ions were mainly inserted into the negative electrodes passivated by a Na-rich solid electrolyte interphase, and both Na+ and K+ ions were inserted into the positive electrode. Based upon the observation, we propose new sodium-/potassium-ion batteries using the superconcentrated NaFSA-KFSA aqueous electrolytes.

Na Diffusion in Hard Carbon Studied with Positive Muon Spin Rotation and Relaxation.

The diffusive nature of Na+ in Na-inserted hard carbon (C x Na), which is the most common anode material for a Na-ion battery, was studied with a positive muon spin rotation and relaxation (µ+SR) technique in transverse, zero, and longitudinal magnetic fields (TF, ZF, and LF) at temperatures between 50 and 375 K, where TF (LF) denotes the applied magnetic field perpendicular (parallel) to the initial muon spin polarization. At temperatures above 150 K, TF-µ+SR measurements showed a distinct motional narrowing behavior, implying that Na+ begins to diffuse above 150 K. The presence of two different muon sites in C x Na was confirmed with ZF- and LF-µ+SR measurements; one is in the Na-inserted graphene layer, and the other is in the Na-vacant graphene layer adjacent to the Na-inserted graphene layer. A systematic increase in the field fluctuation rate (ν) with increasing temperature also evidenced a thermally activated Na diffusion, particularly above 150 K. Assuming the two-dimensional diffusion of Na+ in the graphene layers, the self-diffusion coefficient of Na+ (D Na J) at 300 K was estimated to be 2.5 × 10-11 cm2/s with a thermal activation energy of 39(7) meV.

A vanadium-based oxide-phosphate-pyrophosphate framework as a 4 V electrode material for K-ion batteries.

K-ion batteries (KIBs) are promising for large-scale electrical energy storage owing to the abundant resources and the electrochemical specificity of potassium. Among the positive electrode materials for KIBs, vanadium-based polyanionic materials are interesting because of their high working voltage and good structural stability which dictates the cycle life. In this study, a potassium vanadium oxide phosphate, K6(VO)2(V2O3)2(PO4)4(P2O7), has been investigated as a 4 V class positive electrode material for non-aqueous KIBs. The material is synthesized through pyrolysis of a single metal-organic molecular precursor, K2[(VOHPO4)2(C2O4)] at 500 °C in air. The material demonstrates a reversible extraction/insertion of 2.7 mol of potassium from/into the structure at a discharge voltage of ∼4.03 V vs. K. Operando and ex situ powder X-ray diffraction analyses reveal that the material undergoes reversible K extraction/insertion during charge/discharge via a two-phase reaction mechanism. Despite the extraction/insertion of large potassium ions, the material demonstrates an insignificant volume change of ∼1.2% during charge/discharge resulting in excellent cycling stability without capacity degradation over 100 cycles in a highly concentrated electrolyte cell. Robustness of the polyanionic framework is proved from identical XRD patterns of the pristine and cycled electrodes (after 100 cycles).

Development of advanced electrolytes in Na-ion batteries: application of the Red Moon method for molecular structure design of the SEI layer.

This review aims to overview state-of-the-art progress in the collaborative work between theoretical and experimental scientists to develop advanced electrolytes for Na-ion batteries (NIBs). Recent investigations were summarized on NaPF6 salt and fluoroethylene carbonate (FEC) additives in propylene carbonate (PC)-based electrolyte solution, as one of the best electrolytes to effectively passivate the hard-carbon electrode with higher cycling performance for next-generation NIBs. The FEC additive showed high efficiency to significantly enhance the capacity and cyclability of NIBs, with an optimal performance that is sensitive at low concentration. Computationally, both microscopic effects, positive and negative, were revealed at low and high concentrations of FEC, respectively. In addition to the role of FEC decomposition to form a NaF-rich solid electrolyte interphase (SEI) film, intact FECs play a role in suppressing the dissolution to form a compact and stable SEI film. However, the increase in FEC concentration suppressed the organic dimer formation by reducing the collision frequency between the monomer products during the SEI film formation processes. In addition, this review introduces the Red Moon (RM) methodology, recent computational battery technology, which has shown a high efficiency to bridge the gap between the conventional theoretical results and experimental ones through a number of successful applications in NIBs.

MgO-Template Synthesis of Extremely High Capacity Hard Carbon for Na-Ion Battery.

Extremely high capacity hard carbon for Na-ion battery, delivering 478 mAh g-1 , is successfully synthesized by heating a freeze-dried mixture of magnesium gluconate and glucose by a MgO-template technique. Influences of synthetic conditions and nano-structures on electrochemical Na storage properties in the hard carbon are systematically studied to maximize the reversible capacity. Nano-sized MgO particles are formed in a carbon matrix prepared by pre-treatment of the mixture at 600 °C. Through acid leaching of MgO and carbonization at 1500 °C, resultant hard carbon demonstrates an extraordinarily large reversible capacity of 478 mAh g-1 with a high Coulombic efficiency of 88 % at the first cycle.

Effect of Particle Size and Anion Vacancy on Electrochemical Potassium Ion Insertion into Potassium Manganese Hexacyanoferrates.

Potassium manganese hexacyanoferrate (KMnHCF) can be used as a positive electrode for potassium-ion batteries because of its high energy density. The effect of particle size and [Fe(CN)6 ]n- vacancies on the electrochemical potassium insertion of KMnHCFs was examined through experimental data and theoretical calculations. When nearly stoichiometric KMnHCF was synthesized and tested, smaller particle sizes were found to be important for achieving superior electrochemical performance in terms of capacity and rate capability. However, even in the case of larger particles, introducing a suitable number of anion vacancies enabled KMnHCF to exhibit comparable electrode performance. Electrochemical tests and density functional theory calculations indicated that anion vacancies contribute to the enhancement of K+ ion diffusion, which realizes good electrochemical performance. Structural design, including crystal vacancies and particle size, is the key to their high performance as a positive electrode.

Elucidating Influence of Mg- and Cu-Doping on Electrochemical Properties of O3-Nax [Fe,Mn]O2 for Na-Ion Batteries.

Although O3-NaFe1/2 Mn1/2 O2 delivers a large capacity of over 150 mAh g-1 in an aprotic Na cell, its moist-air stability and cycle stability are unsatisfactory for practical use. Slightly Na-deficient O3-Na5/6 Fe1/2 Mn1/2 O2 (O3-Na5/6 FeMn) and O3-Na5/6 Fe1/3 Mn1/2 Me1/6 O2 (Me = Mg or Cu, O3-FeMnMe) are newly synthesized. The Cu and Mg doping provides higher moist-air stability. O3-Na5/6 FeMn, O3-FeMnCu, and O3-FeMnMg deliver first discharge capacities of 193, 176, and 196 mAh g-1 , respectively. Despite partial replacement of Fe with redox inactive Mg, oxide ions in O3-FeMnMg participate in the redox reaction more apparently than O3-Na5/6 FeMn. X-ray diffraction studies unveil the formation of a P-O intergrowth phase during charging up to >4.0 V.

Development of KPF6/KFSA Binary-Salt Solutions for Long-Life and High-Voltage K-Ion Batteries.

A series of binary-salt electrolytes of KPF6/KN(SO2F)2 (KFSA) in carbonate ester solvents have been developed for high-voltage K-ion batteries by clarifying the effect of salt ratio and different solvents on the physical properties of the electrolyte solutions and electrochemical performance of K-ion batteries. The KPF6/KFSA carbonate ester solutions, such as KPF6/KFSA ethylene carbonate (EC)/diethyl carbonate (DEC), exhibit higher ionic conductivity than single-salt KPF6 one, and higher KFSA content results in higher ionic conductivity. The KPF6-rich binary-salt electrolytes with KPF6/KFSA ratios of ≥3 (mol/mol) provide enough oxidation stability and passivation against Al corrosion at 4.6 V over 100 h, ensuring reversible operation of a 4 V class positive electrode, K2Mn[Fe(CN)6] in half-cell. Graphite negative electrodes exhibit higher Coulombic efficiency and better rate performance in 0.75 mol kg-1 K(PF6)0.9(FSA)0.1/EC/DEC and 1 mol kg-1 K(PF6)0.75(FSA)0.25/EC/DEC electrolytes than those in the KPF6 one. Surface analysis by hard X-ray photoelectron spectroscopy reveals that the decomposition product of N(SO2F)2- anion contributes to stabilizing solid electrolyte interphase on a graphite electrode. From comparing different solvents of EC/DEC, EC/ethyl methyl carbonate, and EC/propylene carbonate (PC), the K2Mn[Fe(CN)6] electrode demonstrates the highest Coulombic efficiency in the EC/PC binary electrolyte, while graphite electrodes exhibit no significant difference. Based on the half-cell tests, we successfully achieve the 3.6 V class full cell of graphite|K(PF6)0.75(FSA)0.25/EC/PC|K2Mn[Fe(CN)6] showing excellent cyclability over 500 cycles, which is far superior to that of the conventional KPF6/EC/DEC electrolyte cell.

Structural Investigation of Quaternary Layered Oxides upon Na-Ion Deinsertion.

Na-ion batteries are emerging alternatives to Li-ion chemistries for large-scale energy storage applications. Quaternary layered oxide Na0.76Mn0.5Ni0.3Fe0.1Mg0.1O2 offers outstanding electrochemical performance in Na-ion batteries compared to pure-phase layered oxides because of the synergistic effect of the P/O-phase mixing. The material is indeed constituted by a mixture of P3, P2, and O3 phases, and a newly identified Na-free phase, i.e., nickel magnesium oxide phase, which improves heat removal and enhances the electrochemical performance. Herein, we structurally investigate, through synchrotron-radiation X-ray diffraction, the modifications occurring after full desodiation, detailing the material structural rearrangement upon Na removal and revealing the effect of two different charging protocols, i.e., constant current (CC) and constant current-constant voltage (CCCV). While the Na-free phase is electrochemically inactive, likely helping in homogenization of the thermal gradient in the electrode during cycling, O-P intergrown phases appear during the extraction of Na ions from interslab layers, and they are dependent on the desodiation level. The application of a constant voltage step at the end of the galvanostatic charge is responsible for a shortening of the interslab distance and a significant volume contraction (-11.9%).

Research Development on K-Ion Batteries.

Li-ion batteries (LIBs), commercialized in 1991, have the highest energy density among practical secondary batteries and are widely utilized in electronics, electric vehicles, and even stationary energy storage systems. Along with the expansion of their demand and application, concern about the resources of Li and Co is growing. Therefore, secondary batteries composed of earth-abundant elements are desired to complement LIBs. In recent years, K-ion batteries (KIBs) have attracted significant attention as potential alternatives to LIBs. Previous studies have developed positive and negative electrode materials for KIBs and demonstrated several unique advantages of KIBs over LIBs and Na-ion batteries (NIBs). Thus, besides being free from any scarce/toxic elements, the low standard electrode potentials of K/K+ electrodes lead to high operation voltages competitive to those observed in LIBs. Moreover, K+ ions exhibit faster ionic diffusion in electrolytes due to weaker interaction with solvents and anions than that of Li+ ions; this is essential to realize high-power KIBs. This review comprehensively covers the studies on electrochemical materials for KIBs, including electrode and electrolyte materials and a discussion on recent achievements and remaining/emerging issues. The review also includes insights into electrode reactions and solid-state ionics and nonaqueous solution chemistry as well as perspectives on the research-based development of KIBs compared to those of LIBs and NIBs.

Stable and Unstable Diglyme-Based Electrolytes for Batteries with Sodium or Graphite as Electrode.

We study the stability of several diglyme-based electrolytes in sodium|sodium and sodium|graphite cells. The electrolyte behavior for different conductive salts [sodium trifluoromethanesulfonate (NaOTf), NaPF6, NaClO4, bis(fluorosulfonyl)imide (NaFSI), and sodium bis(trifluoromethanesulfonyl)imide (NaTFSI)] is compared and, in some cases, considerable differences are identified. Side reactions are studied with a variety of methods, including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, online electrochemical mass spectrometry, and in situ electrochemical dilatometry. For Na|Na symmetric cells as well as for Na|graphite cells, we find that NaOTf and NaPF6 are the preferred salts followed by NaClO4 and NaFSI, as the latter two lead to more side reactions and increasing impedance. NaTFSI shows the worst performance leading to poor Coulombic efficiency and cycle life. In this case, excessive side reactions lead also to a strong increase in electrode thickness during cycling. In a qualitative order, the suitability of the conductive salts can be ranked as follows: NaOTf ≥ NaPF6 > NaClO4 > NaFSI ≫ NaTFSI. Our results also explain two recent, seemingly conflicting findings on the degree of solid electrolyte interphase formation on graphite electrodes in sodium-ion batteries [ Maibach , J. ; ACS Appl. Mater. Interfaces 2017 , 9 , 12373 - 12381 ; Goktas , M. ; Adv. Energy Mater. 2018 , 8 , 1702724 ]. The contradictory findings are due to the different conductive salts used in both studies.

Potassium Metal as Reliable Reference Electrodes of Nonaqueous Potassium Cells.

Potassium metal electrochemical cells are widely utilized to examine potassium insertion materials for nonaqueous potassium-ion batteries. However, large polarization during K plating-stripping and unstable rest potential are found at the potassium electrodes, which leads to an underestimation of the electrochemical performance of insertion materials. In this study, the electrochemical behavior of K-metal electrodes is systematically investigated. Electrolyte salts, solvents, and additives influence the polarization of K metals. Although a highly concentrated electrolyte of 3.9 M KN(SO2F)2/1,2-dimethoxyethane realizes the smallest polarization of 25 mV among all the electrolytes investigated in this study, the polarization of K metals is still larger than those of Li and Na metals. The issue of inaccurate rest potential is solved by pretreating the K electrodes with a plating-stripping process, which is essential in evaluating the intrinsic electrode performance of potassium insertion materials.