Na metal batteries are considered to be one of the most promising large-scale energy storage batteries due to their high theoretical specific capacity and low cost. However, the high reactivity of sodium metal can easily lead to problems such as instability of the solid electrolyte interface (SEI) film, uneven deposition of sodium, and dendrite growth. Here, an Al2O3 in-situ modified Al foil current collector (Al@Al2O3) was fabricated by a facile one-step calcination method to promote uniform Na deposition/stripping. During the discharge process, Al2O3 is sodiumified to form a Na-Al-O film with high ion conductivity, which not only stabilizes the electrode/electrolyte interface, but also regulates the nucleation behavior on the current collector surface, reducing the formation of nuclear energy barrier, improving ion mass transfer kinetics, and achieving uniform deposition of dendrite-free sodium and long cycle life. The results show that Al@Al2O3 can stably deposit/strip sodium for 50 times with an average Coulombic efficiency of 99.6% under 3 mA·cm−2/3 mAh·cm−2; and that the Na-Al@Al2O3‖Na-Al@Al2O3 symmetric battery can be cycled stably for 1000 h at 1 mA·cm−2 and 1 mAh·cm−2. Even at a high current density of 10 C, the NVP‖Na-Al@Al2O3 full cell can be cycled stably for 250 cycles with a high capacity retention of 94%.
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Lithium-ion batteries are extensively utilized in portable electronic devices, electric vehicles, and large-scale energy storage due to their high energy density, long cycle life, and various other advantages. However, the limited natural abundance of lithium resources and their uneven geographical distribution imped the further development of the lithium-ion battery industry. Sodium-ion batteries have an application potential in large-scale energy storage due to their advantages such as abundant sodium resources, low cost, and compatibility with existing lithium-ion battery production lines. Replacing the conventional electrolyte with a solid electrolyte possessing flame retardant properties can effectively address the issues of thermal runaway and explosion associated with sodium-ion batteries. All solid-state sodium batteries (ASSB) offer benefits, including high energy density, enhanced safety, and low cost, aligning with the development goals of energy storage. As a critical component of ASSBs, the electrochemical properties of solid electrolytes play a pivotal role in determining their performance. Composite solid electrolytes (CSE), characterized by a good flexibility, high interfacial compatibility, and ease of processing, are considered as the most promising solid electrolytes for future large-scale commercial applications.
This review summarizes recent research progress on organic-inorganic composite solid electrolytes (CSE) for ASSB and further analyzes the ion transport mechanisms within CSE. CSE are primarily composed of an organic polymer matrix, inorganic fillers, and sodium salts in specific ratios. The polar groups (i.e., −S−, C≡N, −O−, C=O) present in the polymer form group-ion complexes with the dissolved sodium salt. Within the amorphous regions, individual segments of the polymer chains exhibit relative freedom to rotate and bend, facilitating the movement of group-ion complexes within a limited spatial domain. As the chain segments reposition themselves appropriately, the group-ion complexes on these segments begin to segregate, allowing ions to interact with the functional groups of adjacent chain segments, thereby forming new group-ion complexes. This process is reiterated to facilitate ion transport. It is widely accepted that Na+ conduction predominantly occurs in the amorphous phase regions of the polymers above the glass transition temperature. In addition to these amorphous regions, certain crystalline domains exist within the polymers. In inorganic solid electrolytes, ionic hopping migration serves as the primary ion transport mechanism, which are largely affected by defects within the crystal lattice. The complexity of the interfacial region is further compounded in the presence of inorganic fillers that possess highly reactive surface defects, which readily interact with the polymer matrix. Two main explanations for ion transport in the interfacial region are proposed, i.e., 1) the interaction between the functional groups on the surfaces of the inorganic fillers and the polymer matrix, as well as the sodium salt, which weakens the interaction between the polymer matrix and Na+, resulting in a higher concentration of free Na+ on the filler surfaces and the formation of ion-transport channels, and 2) the space charge layer effect arises from the disparity in Na+ concentration between the inorganic filler and the polymer matrix, as well as the electrochemical potential difference. This results in the spontaneous formation of a Na+-rich space charge region, which serves as an efficient channel for Na+ transport. Solid electrolytes, including oxides and sulfides, are commonly utilized as active fillers, wherein the Na+ transport mechanisms encompass both vacancy and interstitial mechanisms.
Based on their ionic conductivity, fillers are classified into inert fillers and active fillers. Inert fillers include Al2O3, ZnO, TiO2, SiO2, Y2O3, ZrO2, MgO, and BaTiO3, while active fillers encompass NASICON-type, calcite-type, and sulfide solid electrolytes. Fillers are further categorized by their shape and dimension into zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) structures. Modifying the chemical properties of the filler surface enhances compatibility and interfacial bonding between the filler and the polymer matrix and allows for the modulation of ionic transport pathways within the electrolyte, significantly impacting overall electrochemical performance. Graft copolymers serve as interfacial modifiers, interacting with both the polymer and the filler to markedly improve interfacial bonding and enhance ionic conductivity. This strategy also mitigates the aggregation of fillers, which arises from the disparity in surface energy between the inorganic filler and the polymer matrix. Furthermore, the quantity of inorganic filler added plays a crucial role in ionic transport within CSE, making it essential to clarify the relationship between the amount of inorganic filler and ionic conductivity to effectively regulate ionic transport.
Composite solid electrolytes (CSE), which consist of a polymer matrix combined with inorganic fillers, leverage the advantages of both solid polymer electrolytes (SPE) and inorganic solid electrolytes, offering promising prospects for practical applications. However, the development and implementation of CSE are still in their early stages, revealing a significant gap between their existing performance and application requirements, necessitating further enhancements. To address this, CSE with specific morphologies and microstructures must be designed to improve the connectivity of ion migration channels, facilitating efficient ion transport. An effective interfacial conductive network can be established via carefully controlling the ratio, size, dispersion, and other characteristics of the inorganic fillers, significantly enhancing ionic conductivity. The strategic application of simulation and characterization techniques to elucidate the ion transport mechanisms within CSE is crucial for advancing the performance of these materials. Lastly, to lower the costs associated with ASSB, the fabrication process of CSE should be streamlined to be simple, time-efficient, and material-saving, while also being compatible with the existing battery production lines to enhance the practicality.
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Because of the superiority of low cost and high theoretical capacity, sodium metal batteries are considered an attractive option for high energy storage. However, the uncontrollable and random deposition of Na tends to expedite the formation of Na dendrites and increases the risk of thermal runaway. The method of preplant sodiophilic sites can induce the lateral deposition of Na instead of sharp dendrite emergence. Here, we introduce the sodiophilic V2O3 particles to form a protective layer on Na surface (Na/V2O3). The high Na ion adsorption energy and low nucleation overpotential of Na/V2O3 facilitate the diffusion of Na ions and homogeneous Na deposition, which can work well in cubing dendrite development. Thus, the symmetrical cell (Na/V2O3||Na/V2O3) can stably operate for 670 h at 0.5 mA·cm−2/1 mAh·cm−2 with a smaller voltage hysteresis (less than 100 mV). Moreover, full cell constructed by coupling Na/V2O3 anode with Na3V2(PO4)3 cathode displays an outstanding rate performance, maintaining a high capacity of 70 mAh·g−1 at 30 C. On the basis of the design of sodiophilic protection layer, a dendrite-free, outstanding rate performance, and long lifespan sodium metal battery is realized.
VS4 has an one-dimensional chain structure with the chain spacing of 0.583 nm, which is greater than Na+ ion radius of 0.102 nm. Moreover, its theoretical sodium storage capacity is 1196 mA·h/g, making it one of the most promising anode materials for sodium storage. However, the volume of the electrode material expands and contracts dramatically during repeated cycling, resulting in collapse of the structure, poor cycling stability and capacity decay, and severely affecting the practical application of VS4. In this paper, VS4 microspheres (approximately 1 μm in diameter) with a high stability were constructed for the unique three-dimensional microsphere structure to alleviate the volume expansion, improve the cycling stability and the electrochemical reaction kinetics. This microsphere structured material as an anode material for sodium ion batteries exhibits a superior rate performance (i.e., 372 mA·h/g@2.0 A/g and 297 mA·h/g@5.0 A/g) and a long-cycle life (i.e., stable cycling for 100 times at 5.0 A/g).
Sodium-ion batteries (SIBs) have become an auspicious candidate for large-scale energy storage by cause of low cost, natural abundance, and similar working principle with lithium-ion batteries (LIBs). At present, there is an urgent need to explore superior anode materials with rapid and stable sodiation/desodiation. Herein, 3D self-assembled VS4 curly nanosheets hierarchitectures (VS4-CN-Hs) are developed for SIB anodes, where VS4 possesses a large theoretical sodium storage capacity, and the building block of nanosheets has large exposed surface area to the electrolyte as well as the constructed hierarchitectures can provide abundant buffer space to alleviate the volume expansion. As a result, VS4-CN-Hs anode possesses excellent electrochemical performance under a wide voltage window of 0.01–3.0 V, such as high reversible capacity of 863 mA h g−1 at 0.1 A g−1, marvelous rate feature (444 mA h g−1 at 10 A g−1), and extralong cycle stability (386 mA h g−1 after 1000 times at 5 A g−1).
Lithium ion batteries (LIBs) that can be operated under extended temperature range hold significant application potentials. Here in this work, we successfully synthesized Co2V2O7 electrode with rich porosity from a facile hydrothermal and combustion process. When applied as anode for LIBs, the electrode displayed excellent stability and rate performance in a wide range of temperatures. Remarkably, a stable capacity of 206 mAh·g-1 was retained after cycling at a high current density of 10 A·g-1 for 6,000 cycles at room temperature (25 °C). And even when tested under extreme conditions, i.e., -20 and 60 °C, the battery still maintained its remarkable stability and rate capability. For example, at -20 °C, a capacity of 633 mAh·g-1 was retained after 50 cycles at 0.1 A·g-1; and even after cycling at 60 °C at 10 A·g-1 for 1,000 cycles, a reversible capacity of 885 mAh·g-1 can be achieved. We believe the development of such electrode material will facilitate progress of the next-generation LIBs with wide operating windows.
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