In Situ Fe-Substituted Hexacyanoferrate for High-Performance Aqueous Potassium Ion Batteries.

The eco-friendliness, safety, and affordability of aqueous potassium batteries (AKIBs) have made them popular for large-scale energy storage devices. However, the cycling and rate performance of research materials, particularly cobalt hexacyanoferrate, have yet to meet satisfactory standards. Herein, a room-temperature drafted K1.66 Fe0.25 Co0.75 [Fe(CN)6 ]·0.83H2 O (KFCHCF) sample is reported using an in situ substitution strategy. A higher concentration of ferrocyanide ions decreases the water content and increases the potassium content, while citric acid works as a chelating agent and is responsible for Fe-substitution in the KFCHCF sample. The resultant KFCHCF sample exhibits good rate performance, and about 97% and 90.6% of discharge capacity are conserved after 400 and 1000 cycles at 100 and 200 mA g-1 , respectively. The full cell using the KFCHCF cathode and 1,4,5,8-naphthalenetetracarboxylic dianhydride-derived polyimide (PNTCDA) anode maintains ≈74.93% and 74.35% of discharge capacity at 200 mA g-1 and 1000 mA g-1 for 1000 and >10,000 cycles, respectively. Furthermore, ex situ characterizations demonstrate the high reversibility of K-ions and structural stability during the charge-discharge process. Such high performance is attributed to the fast K-ion migration and crystal structure stabilization caused by in situ Fe-substitution in the KFCHCF sample. Other hexacyanoferrates can be synthesized using this method and used in grid-scale storage systems.

Relaxation of Stress Propagation in Alloying-Type Sn Anodes for K-Ion Batteries.

Alloying-type metallic tin is perceived as a potential anode material for K-ion batteries owing to its high theoretical capacity and reasonable working potential. However, pure Sn still face intractable issues of inferior K+ storage capability owing to the mechanical degradation of electrode against large volume changes and formation of intermediary insulating phases K4 Sn9 and KSn during alloying reaction. Herein, the TiC/C-carbon nanotubes (CNTs) is prepared as an effective buffer matrix and composited with Sn particles (Sn-TiC/C-CNTs) through the high-energy ball-milling method. Owing to the conductive and rigid properties, the TiC/C-CNTs matrix enhances the electrical conductivity as well as mechanical integrity of Sn in the composite material and thus ultimately contributes to performance supremacy in terms of electrochemical K+ storage properties. During potassiation process, the TiC/C-CNTs matrix not only dissipates the internal stress toward random radial orientations within the Sn particle but also provides electrical pathways for the intermediate insulating phases; this tends to reduce microcracking and prevent considerable electrode degradation.

High Entropy-Induced Kinetics Improvement and Phase Transition Suppression in K-Ion Battery Layered Cathodes.

Layered oxides are widely accepted to be promising cathode candidate materials for K-ion batteries (KIBs) in terms of their rich raw materials and low price, while their further applications are restricted by sluggish kinetics and poor structural stability. Here, the high-entropy design concept is introduced into layered KIB cathodes to address the above issues, and an example of high-entropy layered K0.45Mn0.60Ni0.075Fe0.075Co0.075Ti0.10Cu0.05Mg0.025O2 (HE-KMO) is successfully prepared. Benefiting from the high-entropy oxide with multielement doping, the developed HE-KMO exhibits half-metallic oxide features with a narrow bandgap of 0.19 eV. Increased entropy can also reduce the surface energy of the {010} active facets, resulting in about 2.6 times more exposure of the {010} active facets of HE-KMO than the low-entropy K0.45MnO2 (KMO). Both can effectively improve the kinetics in terms of electron conduction and K+ diffusion. Furthermore, high entropy can inhibit space charge ordering during K+ (de)insertion, and the transition metal-oxygen covalent interaction of HE-KMO is also enhanced, leading to suppressed phase transition of HE-KMO in 1.5-4.2 V and better electrochemical stability of HE-KMO (average capacity drop of 0.20%, 200 cycles) than the low-entropy KMO (average capacity drop of 0.41%, 200 cycles) in the wide voltage window.

High S-doped amorphous carbon/carbon quantum dots coupled micro-frame for highly efficient potassium storage.

Anode materials with excellent rate capability, capacity, and cycle life have been a challenge in obtaining cost-effective K-ion batteries (KIBs). Based on the concept of waste recycling, we prepared the S-doped (21.5%) amorphous carbon/carbon quantum dots coupled micro-frame (SCMF) by combining chemical exfoliation and S/Se-assisted carbonization. SCMF exhibited the advantages of integrating amorphous carbon and carbon quantum dots (CQDs). The CQDs serve as fast electron channels, while amorphous carbon can accommodate more large-size K-ions and mitigate volume expansion. In KIBs, SCMF maintained a high reversible capacity (414.0 mAh g-1, after 100 cycles at 100 mA g-1), a good rate capability (224.0 mAh g-1, 2000 mA g-1), and excellent capacity retention (208.9 mAh g-1, after 2000 cycles at 1000 mA g-1). The molecular dynamic simulation revealed that CQDs provided fast electron transport channels and that C, O and S atoms had suitable interactions with K, facilitating potassium storage. Moreover, the potassium-ion capacitor (PIC) assembled from SCMF and activated carbon exhibited stable electrochemical performance, proving its potential for application. The research provided valuable insights into the reuse of biomass waste in new secondary batteries.

A Conformal Protective Skin Producing Stable Cathode-Electrolyte Interface for Long-Life Potassium-Ion Batteries.

The development of K-based layered oxide cathodes is essential for boosting the competitiveness of potassium-ion batteries (PIBs) in grid-scale energy storage. However, their service life is dramatically limited by interfacial instability issues, which is still poorly understood. In this work, amorphous FePO4 (a-FP) is built on K0.5 Ni0.1 Mn0.9 O2 (KNMO) as the protective skin, whose elasticity for strain relaxation and the K-conducting nature guarantee its integrity during fast and constant K-ion insertion/extraction. The conformal coating leads to a robust interphase on the cathode surface, which qualifies excellent K-transport ability and significantly suppresses the mechanical cracking and transition metal dissolution. Breakthrough in cycle life of the K-based layered cathodes is therefore achieved, which of the amorphous FePO4 coated K0.5 Ni0.1 Mn0.9 O2 (KNMO@a-FP) reaches 2500 cycles. The insights gained from the surface protection layer construction and the in-depth analysis of its working mechanism pave the way for further development of K-based layered cathodes with both bulk structural and interfacial stability.

Small-Molecule Organic Cathodes with Carbon Coating for Highly Efficient Potassium-ion Batteries.

Small-molecule organic cathodes face dissolution in potassium-ion batteries (PIBs). For the first time, an interesting and effective strategy is unveiled to resolve this issue by designing a new soluble small-molecule organic compound namely [N,N'-bis(2-anthraquinone)]-1,4,5,8-naphthalenetetracarboxdiimide (NTCDI-DAQ, 237 mAh g-1 ): Through the precise manipulation of carbonization temperature and time, the molecules on the surface of NTCDI-DAQ particles can be transformed into amorphous carbon with controllable thickness. This strategy called surface self-carbonization can form a carbon protective layer on organic cathodes and significantly increase their insolubility against liquid electrolytes without affecting the electrochemical behavior of bulk particles. As a result, the as-obtained NTCDI-DAQ@C sample displays significantly improved cathode performance in PIBs. In half cells, NTCDI-DAQ@C shows superior capacity stability of 84 % compared to 35 % of NTCDI-DAQ during 30 cycles under the same conditions. In full cells with a KC8 anode, NTCDI-DAQ@C delivers a peak discharge capacity of 236 mAh g-1 cathode and a high energy density of 255 Wh kg-1 cathode in 0.1-2.8 V, with 40 % capacity retention during 3000 cycles at 1 A g-1 . To the best of our knowledge, the integrated performance of NTCDI-DAQ@C is among the best of soluble organic cathodes reported in PIBs.

Single-Layered MoS2 Fabricated by Charge-Driven Interlayer Expansion for Superior Lithium/Sodium/Potassium-Ion-Battery Anodes.

Single-layered MoS2 is a promising anode material for lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and potassium-ion batteries (PIBs) due to its high capacity and isotropic ion transport paths. However, the low intrinsic conductivity and easy-agglomerated feature hamper its applications. Here, a charge-driven interlayer expansion strategy that Co2+ replaces Mo4+ in the doping form to endow MoS2 layers with negative charges, thus inducing electrostatic repulsion, together with the insertion of gaseous groups, to drive interlayer expansion which once breaks the confinement of interlayer van der Waals force, single-layered MoS2 is obtained and uniformly dispersed into carbon matrix arising from the transformation of carbonaceous gaseous groups under high vapor pressure, is proposed. Co atom doping helps enhance the intrinsic conductivity of single-layered MoS2 . Carbon matrix effectively prevents agglomeration of single-layered MoS2 . The doped Co atoms can be fully transformed into ultrasmall Co nanoparticles during conversion reaction, which enables strong spin-polarized surface capacitance and thus significantly boosts ion transport and storage. Consequently, the prepared material delivers superb Li/Na/K-ion storage performances, which are best in the reported MoS2 -based anodes. The proposed charge-driven interlayer expansion strategy provides a novel perspective for preparing single-layered MoS2, which shows huge potential for energy storage.

Comprehensively Understanding the Role of Anion Vacancies on K-Ion Storage: A Case Study of Se-Vacancy-Engineered VSe2.

Anion vacancy engineering (AVE) is widely used to improve the Li-ion and Na-ion storage of conversion-type anode materials. However, AVE is still an emerging strategy in K-ion batteries, which are promising for large-scale energy storage. In addition, the role of anion vacancies on ion storage is far from clear, despite several proposed explanations. Herein, by employing VSe2 as a model conversion-type anode material, Se vacancies are intentionally introduced (labeled as P-VSe2-x ) to investigate their effect on K+ storage. The P-VSe2-x shows excellent cyclability in half cells (143 mA h g-1 at 3.0 A g-1 after 1000 cycles) and high energy density in coin-type full cells (206.8 Wh kg-1 ). By applying various electrochemical techniques, the effects of Se vacancies on the redox potentials of K-ion insertion/extraction and the K-ion diffusion in electrodes upon cycling are uncovered. In addition, the structural evolution of Se vacancies during potassiation/de-potassiation using various operando and ex characterizations is revealed. Moreover, it is demonstrated that Se vacancies can facilitate the breaking of VSe bonds upon the P-VSe2-x conversion using theoretical calculations. This work comprehensively explains the role of anion vacancies in ion storage for developing high-performance conversion-type anode materials.

Three-dimensional Ti3C2Tx and MnS composites as anode materials for high performance alkalis (Li, Na, K) ion batteries.

Exploring capable and universal electrode materials could promote the development of alkalis (Li, Na, K) ion batteries. 2D MXene material is an ideal host for the alkalis (Li, Na, K) ion storage, but its electrochemical performance is limited by serious re-stacking and aggregation problems. Herein, we cleverly combined electrostatic self-assembly with gas-phase vulcanization method to successfully combine Ti3C2Tx-MXene with ultra-long recyclability and high conductivity with MnS, which presents high specific capacity but poor conductivity. The as-prepared 3D hierarchical Ti3C2Tx/MnS composites have an unique sandwich-like constituent units. The tiny MnS nanoparticles are restricted between the Ti3C2Tx layers and play a key role in expanding the Ti3C2Tx interlayer spacing. As a result, the 3D Ti3C2Tx/MnS composites as the anode of LIBs exhibits a superior capacities of 826 and 634 mAh/g after 1000 and 3000 cycles at 0.5 and 1.0 A/g, respectively. More importantly, we reveal the reaction mechanism that the specific capacity first increases and then gradually stabilizes with the increase of charge and discharge cycle times when the as-prepared 3D Ti3C2Tx/MnS was used as the anode of LIBs. In addition, we have also used this material in SIBs and PIBs and achieved remarkable electrochemical capability, with a specific capacity of 107 mAh/g after 2500 cycles at 0.5 A/g or 127 mAh/g after the 2000th cycle at 0.2 A/g, respectively.

Fabrication of a Cation Exchange Membrane with Largely Reduced Anion Permeability for Advanced Aqueous K-ion Battery in an Alkaline-Neutral Electrolyte Decoupling System.

Herein, an efficient method to prepare sulfonated polyether ether ketone (SPEEK) based cation exchange membranes (CEMs) is developed, where polyethersulfone (PES) is used as an additive. The optimized membrane of 30 wt.%PES/SPEEK-M exhibits a rather low anion permeability and a high ionic conductivity of 9.52 mS cm-1 together with low volume swelling in water. Meanwhile, tensile strength of the membrane is as high as 31.4 MPa with a tensile strain of 162%. As separators for aqueous K-ion batteries (AKIBs) with decoupled gel electrolytes (Zn anode in alkaline and Prussian blue (FeHCF) cathode in neutral). Discharge voltage of the AKIB can reach 2.3 V. Meanwhile, Zn dendrites can be effectively suppressed in the gel anolyte. Specific capacities of the FeHCF cathode are 116.7 mAh g-1 at 0.3 A g-1 (close to its theoretical value), and 95.0 mAh g-1 at 1.0 A g-1 , indicating good rate performance. Capacity retention of the cathode is as high as 91.2% after 1000 cycles' cycling owing to the well remained neutral environment of the catholyte. There is almost no pH change for the catholyte after cycling, indicating good anion-blocking or cation-selecting ability of the 30 wt.%PES/SPEEK-M, much better than other membranes.

Advanced flame-retardant electrolyte for highly stabilized K-ion storage in graphite anode.

Although graphite anodes operated with representative de/intercalation patterns at low potentials are considered highly desirable for K-ion batteries, the severe capacity fading caused by consecutive reduction reactions on the aggressively reactive surface is inevitable given the scarcity of effective protecting layers. Herein, by introducing a flame-retardant localized high-concentration electrolyte with retentive solvation configuration and relatively weakened anion-coordination and non-solvating fluorinated ether, the rational solid electrolyte interphase characterized by well-balanced inorganic/organic components is tailored in situ. This effectively prevented solvents from excessively decomposing and simultaneously improved the resistance against K-ion transport. Consequently, the graphite anode retained a prolonged cycling capability of up to 1400cycles (245 mA h g-1, remaining above 12mon) with an excellent capacity retention of as high as 92.4%. This is superior to those of conventional and high-concentration electrolytes. Thus, the optimized electrolyte with moderate salt concentration is perfectly compatible with graphite, providing a potential application prospect for K-storage evolution.

A Novel Two-Dimensional ZnSiP2 Monolayer as an Anode Material for K-Ion Batteries and NO2 Gas Sensing.

Using the crystal-structure search technique and first-principles calculation, we report a new two-dimensional semiconductor, ZnSiP2, which was found to be stable by phonon, molecular-dynamic, and elastic-moduli simulations. ZnSiP2 has an indirect band gap of 1.79 eV and exhibits an anisotropic character mechanically. Here, we investigated the ZnSiP2 monolayer as an anode material for K-ion batteries and gas sensing for the adsorption of CO, CO2, SO2, NO, NO2, and NH3 gas molecules. Our calculations show that the ZnSiP2 monolayer possesses a theoretical capacity of 517 mAh/g for K ions and an ultralow diffusion barrier of 0.12 eV. Importantly, the ZnSiP2 monolayer exhibits metallic behavior after the adsorption of the K-atom layer, which provides better conductivity in a period of the battery cycle. In addition, the results show that the ZnSiP2 monolayer is highly sensitive and selective to NO2 gas molecules.

High-performance K-ion half/full batteries with superb rate capability and cycle stability.

SignificanceThe present work might be significant for exploring advanced K-ion batteries with superb rate capability and cycle stability toward practical applications. The as-assembled K-ion half cell exhibits an excellent rate capability of 428 mA h g-1 at 100 mA g-1 and a high reversible specific capacity of 330 mA h g-1 with 120% specific capacity retention after 2,000 cycles at 2,000 mA g-1, which is the best among those based on carbon materials. The as-constructed full cell delivers 98% specific capacity retention over 750 cycles at 500 mA g-1, superior to most of those based on carbon materials that have been reported thus far.

O-Doping Configurations Reduce the Adsorption Energy Barrier of K-Ions to Improve the Electrochemical Performance of Biomass-Derived Carbon.

In recent years, atomic-doping has been proven to significantly improve the electrochemical performance of biomass-derived carbon materials, which is a promising modification strategy. Among them, there are relatively few reports about O-doping. Here, porous carbon derived from orange peel was prepared by simple carbonization and airflow-annealing processes. Under the coordination of microstructure and surface groups, the derived carbon had excellent electrochemical performance for the K-ion batteries' anode, including a high reversible specific capacity of 320.8 mAh/g, high rate performance of 134.6 mAh/g at a current density of 2000 mA/g, and a retention rate of 79.5% even after 2000 long-term cycles, which shows great application potential. The K-ion storage mechanisms in different voltage ranges were discussed by using various characterization techniques, that is, the surface adsorbed of K-ionswas in the high-potential slope area, and the intercalation behavior corresponded to the low-potential quasi-plateau area. In addition, the density functional theory calculations further confirmed that O-doping can reduce the adsorption energy barrier of K-ions, change the charge density distribution, and promote the K-ion storage. In particular, the surface Faraday reaction between the C=O group and K-ions plays an important role in improving the electrochemical properties.

Hollow CoS/C Structures for High-Performance Li, Na, K Ion Batteries.

Alkali ion (Li, Na, and K) batteries as a new generation of energy storage devices are widely applied in portable electronic devices and large-scale energy storage equipment. The recent focus has been devoted to develop universal anodes for these alkali ion batteries with superior performance. Transition metal sulfides can accommodate alkaline ions with large radius to travel freely between layers due to its large interlayer spacing. Moreover, the composite with carbon material can further improve electrical conductivity of transition metal sulfides and reduce the electron transfer resistance, which is beneficial for the transport of alkali ions. Herein, we designed zeolitic imidazolate framework (ZIF)-derived hollow structures CoS/C for excellent alkali ion (Li, Na, and K) battery anodes. The porous carbon framework can improve the conductivity and effectively buffer the stress-induced structural damage. The ZIF-derived CoS/C anodes maintain a reversible capacity of 648.9, and 373.2, 224.8 mAh g-1 for Li, Na, and K ion batteries after 100 cycles, respectively. Its outstanding electrochemical performance is considered as a universal anode material for Li, Na, and K ion batteries.

A Stabilized Polyacrylonitrile-Encapsulated Matrix on a Nanolayered Vanadium-Based Cathode Material Facilitating the K-Storage Performance.

Layered vanadium-based metal oxides were regarded as promising cathode materials accounting for suitable K+ transport channels as well as high work potential in K-ion batteries. Nevertheless, because of the large radius of K+ and the rigid structure of inorganic materials, the typical K0.486V2O5 suffers from volume expansion seriously in the repeated charging and discharging processes along with poor ionic and electronic conductivity, consequently determining inevitably poor electrochemical properties. Herein, we proposed a stabilized polymer (PAN) matrix on K0.486V2O5 nanobelts by a liquid-assisted methodology and further electrospinning technology. As a result, a nanocomposite containing a 3D conductive and interconnected mesh structure was thus constructed. By avoiding the full carbonization of polyacrylonitrile (PAN) with appropriate thermal treatment, the elastic properties of the PAN precursor can be retained, effectively inhibiting the volume effect, and the stabilized PAN-encapsulated matrix can also greatly accelerate transport rates of K+ and electrons at a high rate as well as restrict the decomposition of organic electrolytes and side reactions. This work can supply significant basic scientific value of the polymer surface coating methodology for the far-reaching development of inorganic cathode materials in K-ion batteries.

Pomegranate-like structured Nb2O5/Carbon@N-doped carbon composites as ultrastable anode for advanced sodium/potassium-ion batteries.

The distinctive pomegranate-like Nb2O5/Carbon@N-doped carbon (Nb2O5/C@NC) composites are fabricated using hydrothermal method integrated with nitrogen doped carbon coating procedure. For the SIBs anode, the Nb2O5/C@NC composites present superior rate character and sustainable capacity (117 mAh g-1 upon 1000 cycles at 5 A g-1). The in-situ X-ray diffraction (XRD) is utilized to research its sodium storage mechanism. Furthermore, for PIBs, the Nb2O5/C@NC composites present sustainable capacity (81 mAh g-1 upon 1000 cycles at 1 A g-1). The outstanding performance of Nb2O5/C@NC composites is ascribed to its unique architecture, in which Nb2O5 nanocrystals embedded in porous carbon can restrain agglomeration of Nb2O5 nanocrystals, enhance electron/ion diffusion kinetics, and ensure electrolyte accessibility, and moreover, NC shell layer can provide effective active sites and further increase ions/electrons transfer.

Impact of the Salt Anion on K Metal Reactivity in EC/DEC Studied Using GC and XPS Analysis.

To develop K-ion batteries, the potassium metal reactivity in a half-cells must be understood. Here, it is shown first that the K metal leads to the migration of the electrode degradation species to the working electrode surface so that half-cells' solid electrolyte interphase (SEI) studies cannot be trusted. Then, the K metal reactivity was studied by combining gas chromatography (GC)-mass spectrometry, GC/Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis after storage in ethylene carbonate/diethylene carbonate (EC/DEC) wo/w 0.8 M KPF6 or KFSI. A comparison with Li stored in EC/DEC wo/w 0.8 M LiPF6 was also performed. Overall, full electrolyte degradation pathways were obtained. The results showed a similar alkali reactivity when stored in EC/DEC with the formation of a CH3CH2OCO2M-rich SEI. For a MPF6-based electrolyte, the reactivity was driven by the PF6- anion (i) forming mostly LiF (Li metal) or (ii) catalyzing the solvent degradation into (CH2CH2OCOOK)2 and CH3CH2OCOOK as main SEI products with additional C2H6 release (K metal). This highlights the higher reactivity of the K system. With KFSI, the reactivity was driven by the FSI- anion degradation, leading to an inorganic-rich SEI. These results thus explain the better electrochemical performance often reported in half-cells with KFSI compared to that with KPF6. Finally, the understanding of these chemically driven electrolyte degradation mechanisms should help researchers to design robust carbonate-based electrolyte formulations for KIBs.

Electrolyte Effect on a Polyanionic Organic Anode for Pure Organic K-Ion Batteries.

Potassium naphthalene-1,4,5,8-tetracarboxylate (K4NTC, 117 mAh g-1) is a new organic anode for K-ion batteries, which possesses four strong K-O ionic bonds within a -4-valent naphthalene-1,4,5,8-tetracarboxylate skeleton (NTC4-). And thus, K4NTC is a polyanionic organic salt. Simultaneously, new insights are provided by comparing two typical electrolyte systems (carbonate and ether electrolytes) with KPF6 as the same solute. Finally, the pure organic K-ion batteries (OKIBs) are fabricated by using perylene-3,4,9,10-tetracarboxydianhydride (PTCDA) as the organic cathode and the reduced state (K6NTC) of K4NTC as the anode. And this OKIB can deliver a peak discharge capacity of 121 mAh g-1anode and run over 1500 cycles in 0.5-3 V using ether electrolytes.

K-Ion Battery Cathode Design Utilizing Trigonal Prismatic Ligand Field.

The intrinsic physical and chemical properties of materials are largely governed by the bonding and electronic structures of their fundamental building units. The majority of cathode materials contain octahedral TMO6 (TM = transition metal), which dominates the redox chemistry during electrochemical operation. As a less symmetric form of TMO6 , the trigonal prismatic geometry is not a traditionally favored coordination configuration as it tends to lose the crystal-field stabilization energy and thus generate large ligand repulsion. Herein, a K-ion battery cathode design, K2 Fe(C2 O4 )2 , is shown​, where the TMO6 trigonal prism (TP) is not only electrochemically active but stable enough to allow for excellent cycling stability. Detailed synchrotron X-ray absorption spectroscopy measurements reveal the evolution of localized fine structure, evidencing the electrochemical activity, reversibility, and stability of the TP motif. The findings are expected to expand the toolbox for the rational design of electrode materials by taking advantage of TP as a structural gene.