The renewable energy sources with intermittent nature call for fast development of electrical energy storage (EES) devices for practical applications. [1] Over the past decades, lithium-ion batteries (LIBs) have pervaded our daily lives, ranging from portable electronics to large-scale EES systems. [2] However, the cost of rare lithium resources involving electrical grid and large-scale storage purposes have raised widespread concerns. In this regard, sodium-ion batteries (SIBs) are highly promising to meet these demands due to that sodium is practically inexhaustible and easily accessible around the globe. [3] However, the higher standard electrochemical potential of Na + /Na (−2.71 V versus SHE) than that of Li + / Li (−3.04 versus SHE) and the larger ion radius of Na + compared with Li + (1.02 Å versus 0.76 Å) mean that SIBs possess a lower energy density, and most conventional electrode materials of LIBs are not suitable for SIBs. Hence, it is of great significance to explore advanced electrode materials that could provide satisfactory specific capacities and rapid ion diffusion kinetics. So far, the development of the cathode materials for SIBs has progressed rapidly, including layered oxides [4] and polyanionic compounds. [5] As for the anodes, although hard carbon as a hotspot has been widely studied due to its high capacity and lower voltage platform, [6] the random adsorption sites and irregular channels for Na + migration lead to a relatively poor sodium-ion diffusion. 2D transition metal chalcogenides (TMCs) have been broadly reported as a kind of promising electrode materials for both LIBs and SIBs due to their open framework and unique electrochemical properties. [7,8] Among them, WS 2 as a typical 2D TMCs has a much larger interlayer spacing of 0.62 nm and weaker van der Waals interaction, which enables fast reversible Na + diffusion and avoids terrible volume expansion during Na + intercalation/deintercalation processes. [9] However, the terrible issue of pure WS 2 anode applied in SIBs is its low intrinsic electronic conductivity, significantly limiting the specific capacity, and rate performance. [10] Generally, the electrochemical properties of materials are strongly dependent on the conductivity of electrode materials as well as the diffusion rate of Na +. Thus, the scrupulous design and rational controllable synthesis of Engineering novel electrode materials with unique architectures has a significant impact on tuning the structural/electrochemical properties for boosting the performance of secondary battery systems. Herein, starting from well-organized WS 2 nanorods, an ingenious design of a one-step method is proposed to prepare a bimetallic sulfide composite with a coaxial carbon coating layer, simply enabled by ZIF-8 introduction. Rich sulfur vacancies and WS 2 /ZnS heterojunctions can be simultaneously developed, that significantly improve ionic and electronic diffusion kinetics. In addition, a homogeneous carbon protective layer around the surface of the composite guarantees an outstandi...

Nevertheless, the research on SIBs was barely conducted after the successful commercialization of LIBs in 1990s, and this situation continued until the end of the 20th century. An obstacle toward the development of SIBs is the lack of suitable anode materials with acceptable performance. The early work conducted by Dahn et al. [5] suggested that hard carbon (HC) has a reversible capacity of 300 mAh g −1 for sodium, approaching the lithium storage capacity in graphite. Extensive attention has been focused on the development of SIBs recently, with a variety of materials being considered as potential anodes for SIBs, which includes alloys, [6-8] organic materials, [9-11] and carbonaceous materials. [12-14] Because of high sodium-ion storage capacity, appropriate working potential, excellent cycling stability, and natural abundance, HC represents the most promising anode for SIBs. Nowadays, increasing interests have been concentrated on revealing sodium intercalation process in HC, [15-19] but the steady state of sodium stored in HC still remains unexplored, which leads us to investigate the steady state of sodium ions in HC from thermodynamic and kinetic aspects. Heretofore, the steady state of sodium in HC has been incidentally proposed, but remains a controversial issue. Specifically, Stevens and Dahn [20] originally revealed the metallic nature of sodium absorbed in nanopores at the voltage plateau as the adsorption potential approaching the deposition potential of sodium metal. Meanwhile, the formation of metallic sodium was also confirmed with operando 23 Na solid-state nuclear magnetic resonance (NMR) and in situ Raman at the plateau region. [21,22] Moreover, Ji and co-workers [23] suggested that sodium adsorbed onto the pore surface is atomic even close to the cutoff potential (0.05-0 V). On the other hand, Liu et al. [19] demonstrated that neither metallic nor quasi-metallic sodium is presented in the whole discharge region over 0 V, based on the results of ex situ 23 Na NMR and electron paramagnetic resonance (EPR). Recently, Guo et al. [13] claimed that sodium stored in the HC is in the ionic state above 0.1 V, whereas metallic sodium clusters form in the nanopores at the plateau voltage, with the methods of EPR, XRD, and Raman. Apparently, scattered efforts have been devoted to uncover the state of sodium stored in hard carbon recently, nevertheless, systematic investigations are Hard carbon (HC) is the most promising anode material for sodium-ion batteries (SIBs), nevertheless, the understanding of sodium storage mechanism in HC is very limited. As an important aspect of storage mechanism, the steady state of sodium stored in HC has not been revealed clearly to date. Herein, the formation mechanism of quasi-metallic sodium and the quasi-ionic bond between sodium and carbon within the electrochemical reaction on the basis of theoretical calculations are disclosed. The presence of quasi-metallic sodium is further confirmed with the assistance of a specific reaction between the sodiated HC electrode and eth...

Graphite has been widely used as a negative electrode in LIBs, where the reversible intercalation of Li + into the graphite layers forms binary compounds (b-GICs) with the stoichiometric composition of LiC 6. [9-11] However, an early attempt to use graphite as an anode for SIBs was unsuccessful because capacities less than 35 mAh g −1 were achieved. [12-14] This may be mistakenly attributed to the higher radius of Na + (0.102 nm) as compared with that of Li + (0.076 nm). [15-19] However, the instability of the Na-GICs resulted in poor sodium storage in graphite. [20-22] Owing to the diglyme-graphene vdW interaction, [23] the co-intercalation of the solvated Na ions effectively formed stable ternary graphite intercalation compounds (t-GICs) with the common formula Na(solv) y C 20 (y = 1 or more likely 2). [24,25] The formation of t-GICs at approximately 0.5-0.6 V (vs Na + /Na) is accompanied by a pronounced volume expansion in the ether-based electrolyte. The average electrode thickness reportedly increases by approximately 100 µm (50 µm in the pristine electrode) after sodiation. Furthermore, the original value is 3.4 Å which increase to the value within the range of 11.3 to 11.9 Å that indicates a 300-340% volume expansion. [26,27] The huge volume changes resulted in the formation of an unstable interphase that causes a rapid decay in the capacity of the anode material. This is typified by the silicon-based materials in LIBs. [28,29] The solid electrolyte interphase (SEI) that is formed during the initial cycle hinders Considerable efforts have been exerted to understand the formation and properties of the solid electrolyte interphase (SEI) in sodium ion batteries. However, the puzzling existence and role of SEI behind the huge volume changes of the graphite electrodes need to be answered. Herein, the reason of how ether-derived SEI maintains excellent reversibility despite the huge volume changes during cycling is unraveled. Theoretical simulations and Fourier-transform infrared spectroscopy demonstrate the formation mechanism of an SEI between the graphite anode and electrolyte. Furthermore, the high mechanical tolerance of the ether-derived SEI is confirmed in atomic force microscopy. A depth profile of X-ray photoelectron spectroscopy points to a multilayer structure of the ether-derived SEI. The outer layer comprises organics (sodium alkoxide), while the inorganics (Na 2 CO 3 , NaF) in interior region are mixed with some organics. Notably, the presence of organics ensures the adaptability of the SEI to the volume expansion of graphite during cycling, and the concentrated distribution of inorganics improves the Young's modulus (resistance to deformation). Therefore, the graphite anode exhibits high cycle stability (96.6% capacity retention ratio at 1 A g-1 over 860 cycles) and efficiency (≈99.5%).

An aluminum‐ion battery was assembled with potassium nickel hexacyanoferrate (KNHCF) as a cathode and Al foil as an anode in aqueous electrolyte for the first time, based on Al3+ intercalation and deintercalation. A combination of ex situ XRD, X‐ray photoelectron spectroscopy (XPS), galvanostatic intermittent titration technique (GITT), and differential capacity analysis was used to unveil the crystal structure changes and the insertion/extraction mechanism of Al3+. Al3+ could reversibly insert/extract into/from KNHCF nanoparticles through a single‐phase reaction with reduction/oxidation of Fe and Ni. Over long‐term cycling, it was Fe rather than Ni that contributed to more capacity owing to the dissolution of Ni from the KNHCF structure, which could be expressed as a compensation effect of mixed redox centers in KNHCF. KNHCF delivered an initial discharge capacity of 46.5 mAh g−1. The capacity decay could be attributed to the unstable interface between Al foil and the aqueous electrolyte owing to the catalytic activity of the Ni transferring from Ni dissolution of KNHCF to the Al foil anode, rather than KNHCF structure collapse; KNHCF maintained its 3 D framework structure for 500 cycles. This work is expected to inspire more exhaustive investigations of the mechanisms that occur in aluminum‐ion batteries.

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