Structural Engineering in Graphite‐Based Metal‐Ion Batteries

With the increasing demand for sustainable energy and concerns about the scarcity of lithium resources, sodium and potassium ion batteries have emerged as promising alternative energy storage technologies. MXene, as a novel two-dimensional material, possesses exceptional electrical conductivity, high surface area, and tunable structural features that make it an ideal candidate for high-performance electrode materials. However, its limited theoretical capacity hinders its widespread application. To overcome this limitation, MXene has been combined with other materials through synergistic effects between different components to enhance the overall electrochemical performance and expand its application in sodium/potassium ion batteries. Recently, substantial advancements have been realized in the exploration of MXene-based composites as energy storage materials, encompassing their synthesis, design, and the comprehension of charge storage mechanisms. This paper aims to propose a comprehensive summary of the latest developments in MXene-based composites as electrode materials for sodium ion batteries and potassium ion batteries, with a particular emphasis on the enhanced physicochemical properties resulting from composite formation. Moreover, the challenges faced by MXene materials in sodium ion batteries and potassium ion batteries are thoroughly discussed, and future research directions to further advance this field are proposed.

Sodium-based dual-ion batteries have shown great promise for large-scale energy storage applications due to their wide operating voltages, environmental friendliness, abundant sodium resources, and low cost, which are widely investigated by researchers. However, the development of high-performance anode materials is a key requirement for the realization of such electrochemical energy storage systems at the practical application level. Carbonaceous anode materials based on intercalation/deintercalation mechanisms typically exhibit low discharge capacities, while metal-based materials based on conversion or alloying reactions show unsatisfactory stability in performance. On the contrary, organic materials display high theoretical capacities due to their flexible molecular structure designability and stable cyclic performance with fast reaction kinetics based on the unique enolization reaction. Herein, we report an organic polymer anode material of polyimide (PNTO), combined with a high-concentration electrolyte; the sodium-based dual-ion battery system constructed exhibits outstanding electrochemical performance. The full battery shows an ultra-high specific discharge capacity of 293.2 mAh g-1 and can be cycled stably for 3200/5600/4100 cycles at ultra-high rates of 60/120/150 C without degradation. Furthermore, the dual-ion battery system demonstrates an extremely low self-discharge rate of 0.03% h-1 and superior fast-charging-slow-discharging performance. It is one of the best performances reported up to now for a dual-ion full battery based on an organic polymer anode. This novel battery system design strategy will facilitate the advancement of high-performance organic-based dual-ion batteries and is expected to be a promising candidate for large-scale energy storage applications.

Recently, sodium ion batteries (NIBs) and potassium-ion battery (KIBs) have been attracted much attention since sodium and potassium resource is natural abundant, environmental friendly and showing similar chemical properties with lithium ion batteries. The red P anode for Na-ion and K-ion batteries has attracted great attention recently due to the high theoretical capacity, but the poor intrinsic electronic conductivity and large volume expansion restrains its widespread applications. The intrinsic low electronic conductivity of red P leads to the poor electronic kinetics, large polarization and further low active material utilization. In addition, the large volume expansion (~400%) during Na insertion/extraction in red P leads to pulverization of red P and loose contact between the red P and the current collector, resulting in the severe capacity fading. Herein, we successfully encapsulate the red P into the carbon-based material (such as MOF-derived carbon, N-doped carbon, carbon-nanofibers) via the vaporization-condensation method.1, 2 Superior cycling stability (high capacity retention of about 93% at 2 A g-1 after 100 cycles) and excellent rate performance (502 mAh g-1 at 10 A g-1) could be obtained for the P/C electrode. The superior electrochemical performance could be ascribed to the successful incorporation of red P into the unique carbon matrix with large surface area and pore volume, interconnected porous structure, excellent electronic conductivity and superior structural stability.The porous carbon matrix with abundant micro/meso pores, large pore volume could effectively facilitate the infiltration of the electrolyte, and accommodate large volume expansion of red P during electrochemical process. The excellent conductivity of the carbon could ensure the essential electrical contact to the insulating red P, leading to excellent sodium storage performances. The rational design of the unique red P/ carbon structure could be served as a model for other anode material with poor intrinsic electronic conductivity and large volume expansion.

Dual-ion batteries (DIBs) have been attracting great attention recently for the storage of stationary energy due to their low cost, environmental friendliness, and high working voltage. However, most reports on DIBs involve low mass-loading electrodes (<2.5mg), while the use of high mass-loading electrodes (>10mg cm-2 ), which are critical for practical application, is often overlooked. In the present study, an integrated free-standing functional carbon positive electrode MSCG with a "point-line-plane" hierarchical architecture at the practical level of ultrahigh mass-loading (>50mg cm-2 ) is developed for high-energy-density DIBs. The rationally designed microstructure and the advanced assembly method that was adopted produce a well-interconnected ion/electron transport channel in the MSCG positive electrode, which confers rapid ion/electron kinetic properties to the positive electrode while maintaining good mechanical properties. Consequently, the DIBs with ultrahigh mass-loading MSCG positive electrodes exhibits a high discharge capacity of 100.5 mAh g-1 at 0.5 C (loading mass of 50mg cm-2 ), and a long-term cycling performance is also achieved with a capacity retention of 87.7% at 1 C after 500 cycles (loading mass of 23mg cm-2 ). Moreover, the DIB with the ultrahigh mass-loading positive electrode achieves a high energy density of 379Wh kg-1 based on the mass of electrode materials, which is the highest value recorded to date for any DIBs. This article is protected by copyright. All rights reserved.

Progress and perspectives of 2D materials as anodes for potassium-ion batteries

Surface-dominated ultra-stable sodium and potassium storage enabled by N/P/O tri-doped porous carbon

SnO2 has been extensively investigated as an anode material for sodium‐ion batteries (SIBs) and potassium‐ion batteries (PIBs) due to its high Na/K storage capacity, high abundance, and low toxicity. However, the sluggish reaction kinetics, low electronic conductivity, and large volume changes during charge and discharge hinder the practical applications of SnO2‐based electrodes for SIBs and PIBs. Engineering rational structures with fast charge/ion transfer and robust stability is important to overcoming these challenges. Herein, S‐doped SnO2 (S–SnO2) quantum dots (QDs) (≈3 nm) encapsulated in an N, S codoped carbon fiber networks (S–SnO2–CFN) are rationally fabricated using a sequential freeze‐drying, calcination, and S‐doping strategy. Experimental analysis and density functional theory calculations reveal that the integration of S–SnO2 QDs with N, S codoped carbon fiber network remarkably decreases the adsorption energies of Na/K atoms in the interlayer of SnO2–CFN, and the S doping can increase the conductivity of SnO2, thereby enhancing the ion transfer kinetics. The synergistic interaction between S–SnO2 QDs and N, S codoped carbon fiber network results in a composite with fast Na+/K+ storage and extraordinary long‐term cyclability. Specifically, the S–SnO2–CFN delivers high rate capacities of 141.0 mAh g−1 at 20 A g−1 in SIBs and 102.8 mAh g−1 at 10 A g−1 in PIBs. Impressively, it delivers ultra‐stable sodium storage up to 10,000 cycles at 5 A g−1 and potassium storage up to 5000 cycles at 2 A g−1. This study provides insights into constructing metal oxide‐based carbon fiber network structures for high‐performance electrochemical energy storage and conversion devices.

ZnSe is regarded as a promising anode material for energy storage due to its high theoretical capacity and environment friendliness. Nevertheless, it is still a significant challenge to obtain superior electrode materials with stable performance owing to the serious volume change and aggregation upon cycling. Herein, a willow-leaf-like nitrogen-doped carbon-coated ZnSe (ZnSe@NC) composite synthesized through facile solvothermal and subsequent selenization process is beneficial to expose more active sites and facilitate the fast electron/ion transmission. These merits significantly enhance the electrochemical performances of ZnSe@NC for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). The obtained ZnSe@NC exhibits outstanding rate performance (440.3 mAh g-1 at 0.1 A g-1 and 144.4 mAh g-1 at 10 A g-1 ) and ultralong cycle stability (242.2 mAh g-1 at 8.0 A g-1 even after 3200 cycles) for SIBs. It is noted that 106.5 mAh g-1 can be retained after 550 cycles and 71.4 mAh g-1 is still remained after 1500 cycles at 200mA g-1 when applied as anode for PIBs, indicating good cycle stability of the electrode. The possible electrochemical mechanism and the ionic diffusion kinetics of the ZnSe@NC are investigated using ex situ X-ray diffraction, high-resolution transmission electron microscopy, and a series of electrochemical analyses.

Development of sodium and potassium ion batteries with greater energy density is gaining great attention. Although recently proposed alloying anodes (e.g., Sn and Bi) demonstrate much higher capacity than classic carbon anodes, their severe capacity fading hinders their practical applications. The failure mechanism has traditionally been attributed to the large volumetric change and/or their fragile solid electrolyte interphase (SEI). However, herein we present a completely new concept and approach based on electrolyte engineering to stabilize alloying anodes. This approach results in unprecedented high capacity (>650 mAh g–1) and stability (>500 cycles) of alloying anodes by simply tuning the electrolyte compositions, without the need for nanostructural control and/or carbon modification. We confirm that the cation solvation structure, particularly the type and location of the anions in the electrolyte, plays a critical role in alloying anode stabilization. We further present a new anionic and alloying anode reaction model showing that the root cause of the capacity fading in these alloys is dictated by the properties of the anions and not only the volume change or fragile SEI effect. Our model elucidates the failure mechanism in alloying anodes and provides a new guideline for electrolyte design that stabilizes alloying anodes in emerging mobile ion batteries.

Recently demand for Li-ion batteries is growing as power sources for various applications such as electric vehicles and electric energy storage systems, because their high energy density and light weight properties are suitable for these applications. However, lithium resources may be insufficient to meet the increasing demand for these applications. Na-ion batteries could be a promising alternative to Li-ion batteries thanks to the natural abundance and lower cost of sodium. Non-graphitized carbon materials such as hard carbon are well-studied as high capacity anode materials for Na-ion batteries. The discharge capacity of 200 - 300 mAh/g, which is lower than conventional graphite anode materials for Li-ion batteries, has been reported [1,2]. In addition, the insertion mechanism of Na-ion into non-graphitized carbon materials is proposed that Na-ions are first inserted between stacked graphene layers in the voltage range from 2.0 V to 0.2 V (vs. Na metal), and then inserted into nano-pores in the voltage range from 0.2 V to 0.0 V (vs. Na metal) [2]. In this study, we focused on the pore structure of carbon materials and investigated the relationship between the carbon structures and electrochemical properties to clarify the optimum pore structure and propose a new carbon material as anode for high energy Na-ion batteries. Non-graphitized carbon samples were synthesized by heating various raw materials at between 1000 and 2600 °C under flowing argon. The pore structure of the synthesized carbon samples was characterized by using argon and nitrogen adsorption measurement and helium gas densitometry. Electrochemical properties were evaluated by using laminate-type cell with Na metals as the counter electrode and the reference electrode. Open and closed pore volumes of the synthesized carbons were calculated and the relationship between the reversible capacity and pore volume was investigated. As seen in Figure 1, an almost linear relationship between the reversible capacity in the voltage range from 0.2 to 0.0 V (vs. Na metal) and the closed pore volume was confirmed. Some carbon materials with large closed pore volume on the basis of this relationship were designed and successfully synthesized. Figure 2 shows charge/discharge curves of one of synthesized carbon samples. This carbon sample had the closed pore volume of 0.37 cc/g and its discharge capacity reached 413 mAh/g. This result suggests that Na-ion could be inserted reversibly in the closed pore and non-graphitized carbons with large closed pore volume may be promising for high capacity anode materials for Na-ion batteries. [1] D. A. Stevens et al, J. Electrochem. Soc., 148 (2001) A803 [2] S.Komaba et al, ACS Appl. Mater. Interfaces, 3 (2011) 4165 Figure 1

Bi2O3 particles embedded in carbon matrix as high-performance anode materials for potassium ion batteries

3D printing of porous hollow nanosphere MoS2@NiS/rGO scaffolds empowering long-cycle sodium-ion batteries

Manganese-based chalcogenides have significant potential as anodes for sodium-ion batteries (SIBs) due to their high theoretical specific capacity, abundant natural reserves, and environmental friendliness. However, their application is hindered by poor cycling stability, resulting from severe volume changes during cycling and slow reaction kinetics due to their complex crystal structure. Here, an efficient and straightforward strategy was employed to in-situ encapsulate single-phase porous nanocubic MnS0.5Se0.5 into carbon nanofibers using electrospinning and the hard template method, thus forming a necklace-like porous MnS0.5Se0.5-carbon nanofiber composite (MnS0.5Se0.5@N-CNF). The introduction of Se significantly impacts both the composition and microstructure of MnS0.5Se0.5, including lattice distortion that generates additional defects, optimization of chemical bonds, and a nano-spatially confined design. In situ/ex-situ characterization and density functional theory calculations verified that this MnS0.5Se0.5@N-CNF alleviates the volume expansion and facilitates the transfer of Na+/electron. As expected, MnS0.5Se0.5@N-CNF anode demonstrates excellent sodium storage performance, characterized by high initial Coulombic efficiency (90.8%), high-rate capability (370.5 mAh g-1 at 10 A g-1) and long durability (over 5000 cycles at 5 A g-1). The MnS0.5Se0.5@N-CNF //NVP@C full cell, assembled with MnS0.5Se0.5@N-CNF as anode and Na3V2(PO4)3@C as cathode, exhibits a high energy density of 254 Wh kg-1 can be provided. This work presents a novel strategy to optimize the design of anode materials through structural engineering and Se substitution, while also elucidating the underlying reaction mechanisms.

Recently the demand for rechargeable batteries with high capacity, high power, long life, high reliability, and low cost has been increasing. While Li-ion batteries offer the highest energy density among present battery technologies, the higher cost and insufficient resources of lithium is a bottleneck to enabling their wider use. Na-ion batteries, where sodium acts as the current carrier, are attractive alternatives to Li-ion batteries because of the lower cost and abundance of sodium. However, the energy density of Na-ion batteries is still lower than that of Li-ion batteries, therefore the development of the active materials achieving a higher energy density is eagerly anticipated. For anode materials in Na-ion batteries, a non-graphitized carbon, such as hard carbon, is well-studied in contrast to Li-ion batteries, where graphite is usually used as the anode material. The capacity of non-graphitized carbon in Na-ion batteries is reported to be 200 - 300 mAh/g [1,2]. In Na-ion insertion process, it is proposed that Na-ions are inserted between stacked graphenes in the higher potential above ca. 0.2 V vs. Na+/Na, and then nano-pores are filled during electrochemical reaction in the lower potential region [1,2]. In our previous research, we focused on the effect of the pore structure of carbons, and found a linear relationship between the reversible capacity in the range of 0.2 - 0 V vs. Na+/Na and the closed pore volume of non-graphitized carbons. This result suggests that Na-ion could be inserted reversibly into the closed pore of non-graphitized carbons, and thus the non-graphitized carbon with large closed pore volume is a promising anode material for higher capacity Na-ion batteries [3]. In this study, we synthesized a new carbon material with a large closed pore volume using the carbon sources including a pore-forming agent as the starting material. It has the closed pore volume of 0.49 cc/g and its discharge capacity reached 438mAh/g. [1] D. A. Stevens et al, J. Electrochem. Soc., 148 (2001) A803 [2] S.Komaba et al, ACS Appl. Mater. Interfaces, 3 (2011) 4165 [3] A. Kano et al, 228th ECS meetings, (2015) 221

Functionalized carbon dots for advanced batteries