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From basic research to industry process, battery energy storage systems have played a great role in the informatization, mobility, and intellectualization of modern human society. Some potential systems such as Li, Na, K, Mg, Zn, and Al secondary batteries have attracted much attention to maintain social progress and sustainable development. As one of the components in batteries, electrolytes play an important role in the upgrade and breakthrough of battery technology. Since room‐temperature ionic liquids (ILs) feature high conductivity, nonflammability, nonvolatility, high thermal stability, and wide electrochemical window, they have been widely applied in various battery systems and show great potential in improving battery stability, kinetics performance, energy density, service life, and safety. Thus, it is a right time to summarize these progresses. In this review, the composition and classification of various ILs and their recent applications as electrolytes in diverse metal‐ion batteries (Li, Na, K, Mg, Zn, Al) are outlined to enhance the battery performances. This manuscript reviews the classification of ionic liquids, and their potential application as electrolytes in metal‐ion batteries (Li, Na, K, Mg, Zn, Al). Their merits of nonflammable property, thermal stability, and high safety suggest that they could be a promising solution to realize high safety and high energy density for next generational battery systems.

The incorporation of heteroatoms into carbon aerogels (CAs) can lead to structural distortions and changes in active sites due to their smaller size and electronegativity compared to pure carbon. However, the evolution of the electronic structure from single‐atom doping to heteroatom codoping in CAs has not yet been thoroughly investigated, and the impact of codoping on potassium ion (K+) storage and diffusion pathways as electrode material remains unclear. In this study, experimental and theoretical simulations were conducted to demonstrate that heteroatom codoping, composed of multiple heteroatoms (O/N/B) with different properties, has the potential to improve the electrical properties and stability of CAs compared to single‐atom doping. Electronic states near the Fermi level have revealed that doping with O/N/B generates a greater number of active centers on adjacent carbon atoms than doping with O and O/N atoms. As a result of synergy with enhanced wetting ability (contact angle of 9.26°) derived from amino groups and hierarchical porous structure, ON‐CA has the most optimized adsorption capacity (−1.62 eV) and diffusion barrier (0.12 eV) of K+. The optimal pathway of K+ in ON‐CA is along the carbon ring with N or O doping. As K+ storage material for supercapacitors and ion batteries, it shows an outstanding specific capacity and capacitance, electrochemical stability, and rate performance. Especially, the assembled symmetrical K+ supercapacitor demonstrates an energy density of 51.8 Wh kg−1, an ultrahigh power density of 443 W kg−1, and outstanding cycling stability (maintaining 83.3% after 10,000 cycles in 1 M KPF6 organic electrolyte). This research provides valuable insights into the design of high‐performance potassium ion storage materials. O/N/B functionalized carbon aerogel (CA) derived from agarose was synthesized. Electronic states near the Fermi level have revealed that O/N/B codoping generates more active centers on adjacent carbon atoms than doping with O and O/N atoms. Synergy with enhanced wetting ability (contact angle of 9.26°) derived from amino groups and hierarchical porous structure, ON‐CA has the most optimized adsorption capacity (−1.62 eV), diffusion barrier (0.12 eV) of potassium ion, and outstanding K+ storage performance for supercapacitor and ion battery.

Electrochemical potassium ion intercalation into two-dimensional layered MoS2 was studied for the first time for potential applications in the anode in potassium-based batteries. X-ray diffraction analysis indicated that an intercalated potassium compound, hexagonal K0.4MoS2, formed during the intercalation process. Despite the size of K^＋, MoS2 was a long-life host for repetitive potassium ion intercalation and de-intercalation with a capacity retention of 97.5% after 200 cycles. The diffusion coefficient of the K^＋ ions in KxMoS2 was calculated based on the Randles-Sevcik equation. A higher K^＋ intercalation ratio not only encountered a much slower K^＋ diffusion rate in MoS2, but also induced MoS2 reduction. This study shows that metal dichalcogenides are promising potassium anode materials for emerging K-ion, K-O2, and K-S batteries.

Potassium‐ion batteries (PIBs) are considered promising alternatives to lithium‐ion batteries owing to cost‐effective potassium resources and a suitable redox potential of −2.93 V (vs. −3.04 V for Li+/Li). However, the exploration of appropriate electrode materials with the correct size for reversibly accommodating large K+ ions presents a significant challenge. In addition, the reaction mechanisms and origins of enhanced performance remain elusive. Here, tetragonal FeSe nanoflakes of different sizes are designed to serve as an anode for PIBs, and their live and atomic‐scale potassiation/depotassiation mechanisms are revealed for the first time through in situ high‐resolution transmission electron microscopy. We found that FeSe undergoes two distinct structural evolutions, sequentially characterized by intercalation and conversion reactions, and the initial intercalation behavior is size‐dependent. Apparent expansion induced by the intercalation of K+ ions is observed in small‐sized FeSe nanoflakes, whereas unexpected cracks are formed along the direction of ionic diffusion in large‐sized nanoflakes. The significant stress generation and crack extension originating from the combined effect of mechanical and electrochemical interactions are elucidated by geometric phase analysis and finite‐element analysis. Despite the different intercalation behaviors, the formed products of Fe and K2Se after full potassiation can be converted back into the original FeSe phase upon depotassiation. In particular, small‐sized nanoflakes exhibit better cycling performance with well‐maintained structural integrity. This article presents the first successful demonstration of atomic‐scale visualization that can reveal size‐dependent potassiation dynamics. Moreover, it provides valuable guidelines for optimizing the dimensions of electrode materials for advanced PIBs. The tetragonal FeSe nanoflakes with different sizes are designed and their atomic‐scale potassiation/depotassiation mechanisms are revealed for the first time through in situ high‐resolution transmission electron microscopy. FeSe undergoes two distinct structural evolutions, sequentially characterized by intercalation and conversion reactions, and the initial intercalation behavior is size‐dependent originating from the combined effect of the mechanical and electrochemical interaction.

Potassium‐ion batteries (PIBs) are considered as promising candidates for lithium‐ion batteries due to the abundant reserve and lower cost of K resources. However, K+ exhibits a larger radius than that of Li+, which may impede the intercalation of K+ into the electrode, thus resulting in poor cycling stability of PIBs. Here, an N/O dual‐doped hard carbon (NOHC) is constructed by carbonizing the renewable piths of sorghum stalks. As a PIB anode, NOHC presents a high reversible capacity (304.6 mAh g−1 at 0.1 A g−1 after 100 cycles) and superior cycling stability (189.5 mAh g−1 at 1 A g−1 after 5000 cycles). The impressive electrochemical performances can be ascribed to the super‐stable porous structure, expanded interlayer space, and N/O dual‐doping. More importantly, the NOHC can be prepared in large scale in a concise way, showing great potential for commercialization applications. This work may impel the development of low‐cost and sustainable carbon‐based materials for PIBs and other advanced energy storage devices. A low‐cost and sustainable N/O dual‐doped hard carbon (NOHC) is constructed by carbonizing the renewable piths of sorghum stalks. As an anode material in potassium‐ion batteries, the NOHC exhibits a high reversible capacity and superior cycling stability, outperforming most carbonaceous materials.

A hybrid ion capacitor (HIC) based on potassium ions (K+) is a new high‐power intermediate energy device that may occupy a unique position on the Ragone chart space. Here, a direct performance comparison of a potassium ion capacitor (KIC) versus the better‐known sodium ion capacitor is provided. Tests are performed with an asymmetric architecture based on bulk ion insertion, partially ordered, dense carbon anode (hard carbon, HC) opposing N‐ and O‐rich ion adsorption, high surface area, cathode (activated carbon, AC). A classical symmetric “supercapacitor‐like” configuration AC–AC is analyzed in parallel. For asymmetric K‐based HC–AC devices, there are significant high‐rate limitations associated with ion insertion into the anode, making it much inferior to Na‐based HC–AC devices. A much larger charge–discharge hysteresis (overpotential), more than an order of magnitude higher impedance RSEI, and much worse cyclability are observed. However, K‐based AC–AC devices obtained on‐par energy, power, and cyclability with their Na counterpart. Therefore, while KICs are extremely scientifically interesting, more work is needed to tailor the structure of  “Na‐inherited” dense carbon anodes and electrolytes for satisfactory K ion insertion. Conversely, it should be possible to utilize many existing high surface area adsorption carbons for fast rate K application. A hard carbon ion insertion anode gives much higher overpotential with K+ versus Na+. This significantly lowers the energy and power of symmetric hybrid potassium ion capacitor (KIC) versus sodium ion capacitor. However, high surface area ion adsorption carbon works well with both K and Na, opening the possibility for high‐performance symmetric KICs.

Heteroatom doping effectively tunes the electronic conductivity of transition metal selenides (TMSs) with rapid K+ accessibility in potassium ion batteries (PIBs). Although considerable efforts are dedicated to investigating the relationship between the doping strategy and the resulting electrochemistry, the doping mechanisms, especially in view of the ion and electronic diffusion kinetics upon cycling, are seldom elucidated systematically. Herein, the crystal structure stability, charge/ion state, and bandgap of the active materials are found to be precisely modulated by favorable heteroatom doping, resulting in intrinsically fast kinetics of the electrode materials. Based on the combined mechanisms of intercalation and conversion reactions, electron and K+ ion transfer in Ni‐doped CoSe2 embedded in carbon nanocomposites (Ni‐CoSe2@NC) can be significantly enhanced via electronic engineering. Benefiting from the synthetic controlled Ni grains, the heterointerface formed by the intermediate products of electrochemical reactions in Ni‐CoSe2@NC strengthens the conversion kinetics and interdiffusion process, developing a low‐barrier mesophase with optimized potassium storage. Overall, an electronic tuning strategy can offer deeper atomic insights into the conversion reaction of TMSs in PIBs. Heteroatom doping has a significant impact on boosting the performance of secondary battery systems. By engineering the electrodes with controllable composites, ionic and electronic diffusion kinetics are simultaneously obtained. The underlying electrochemical K storage mechanisms based on the intercalation/deintercalation and conversion reactions are illustrated in detail by electrochemical kinetic analysis, theoretical calculations, and X‐ray absorption spectroscopy.

Size engineering is deemed to be an adoptable method to boost the electrochemical properties of potassium‐ion storage; however, it remains a critical challenge to significantly reduce the nanoparticle size without compromising the uniformity. In this work, a series of MoP nanoparticle splotched nitrogen‐doped carbon nanosheets (MoP@NC) is synthesized. Due to the coordinate and hydrogen bonds in the water‐soluble polyacrylamide hydrogel, MoP is uniformly confined in a 3D porous NC to form ultrafine nanoparticles which facilitate the extreme exposure of abundant three‐phase boundaries (MoP, NC, and electrolyte) for ionic binding and storage. Consequently, MoP@NC‐1 delivers an excellent capacity performance (256.1 mAh g−1 at 0.1 A g−1) and long‐term cycling durability (89.9% capacitance retention after 800 cycles). It is further confirmed via density functional theory calculations that the smaller the MoP nanoparticle, the larger the three‐phase boundary achieved for favoring competitive binding energy toward potassium ions. Finally, MoP@NC‐1 is applied as highly electroactive additive for 3D printing ink to fabricate 3D‐printed potassium‐ion hybrid capacitors, which delivers high gravimetric energy/power density of 69.7 Wh kg−1/2041.6 W kg−1, as well as favorable areal energy/power density of 0.34 mWh cm−2/9.97 mW cm−2. Due to the usage of water‐soluble polyacrylamide as molecular skeleton and the strong chemical bond connection inside the hydrogel network, ultrafine MoP nanoparticles can be formed and evenly confined in 3D porous nitrogen‐doped carbon (NC) framework. This can create abundant three‐phase boundaries for efficient response between MoP, NC, and electrolyte, endowing high energy/power 3D‐printed potassium‐ion hybrid capacitors.

Carbon materials, including graphite, hard carbon, soft carbon, graphene, and carbon nanotubes, are widely used as high‐performance negative electrodes for sodium‐ion and potassium‐ion batteries (SIBs and PIBs). Compared with other materials, carbon materials are abundant, low‐cost, and environmentally friendly, and have excellent electrochemical properties, which make them especially suitable for negative electrode materials of SIBs and PIBs. Compared with traditional carbon materials, modifications of the morphology and size of nanomaterials represent effective strategies to improve the quality of electrode materials. Different nanostructures make different contributions toward improving the electrochemical performance of electrode materials, so the synthesis of nanomaterials is promising for controlling the morphology and size of electrode materials. This paper reviews the progress made and challenges in the use of carbon materials as negative electrode materials for SIBs and PIBs in recent years. The differences in Na+ and K+ storage mechanisms among different types of carbon materials are emphasized. Carbon materials represent one of the most promising candidates for negative electrode materials of sodium‐ion and potassium‐ion batteries (SIBs and PIBs). This review focuses on the research progress of carbon materials such as graphite, hard carbon, soft carbon, graphene and carbon nanotubes, and other carbon nanomaterials as negative electrode materials for SIBs and PIBs.

Hard carbon materials are characterized by having rich resources, simple processing technology, and low cost, and they are promising as one of the anode electrodes for commercial applications of sodium‐/potassium‐ion batteries. Simultaneously, exploring the alkali metal ion storage mechanism is particularly important for designing high‐performance electrode materials. However, the structure of hard carbon is more complex, and the description of energy storage behavior is quite controversial. In this study, the Magnolia grandiflora Lima leaf is used as a precursor, combined with simple pyrolysis and impurity removal processes, to obtain biomass‐derived hard carbon material (carbonized Magnolia grandiflora Lima leaf [CMGL]). When it is used as an anode for sodium‐ion batteries, it exhibits a high specific capacity of 315 mAh/g, and the capacity retention rate is 90.0% after 100 cycles. For potassium‐ion batteries, the charge specific capacity is 263.5 mAh/g, with a capacity retention rate of 85.5% at the same cycling. Furthermore, different electrochemical analysis methods and microstructure characterization techniques were used to further elucidate the sodium/potassium storage mechanism of the material. All the results indicate that the high potential slope region represents the adsorption/desorption characteristics on the surface active sites, whereas the low‐potential quasiplateau region belongs to the ion insertion/extraction in the graphitic microcrystallites interlayer. It is noteworthy that potassium ion is randomly intercalated between the graphitic microcrystallite layer without forming a segmented intercalation compound structure. To make full use of waste biomass resources, the Magnolia grandiflora Lima leaf is used as a precursor to obtain derived hard carbon material, which showed an excellent electrochemical performance as the sodium‐/potassium‐ion batteries anode, and the different electrochemical analysis methods and microstructure characterization techniques also further explain the corresponding energy storage mechanisms.