Delicately Tailored Ternary Phosphate Electrolyte Promotes Ultrastable Cycling of Na3V2(PO4)2F3-Based Sodium Metal Batteries

Na3V2(PO4)3 cathode materials for advanced sodium-ion batteries: Modification strategies and density functional theory calculations

With the rapid development of electric vehicles and smart grids, the demands for energy supply systems such as secondary batteries are increasing exponentially. Despite the world-renowned achievements in portable devices, lithium-ion batteries (LIBs) have struggled to meet the demands due to the constraints of total lithium resources. As the most promising alternative to LIBs, sodium-ion batteries (SIBs) are generating widespread research enthusiasm around the world. Among all components, the cathode material remains the primary obstacle to the practical application of SIBs due to its inability to match the performance of other components. Na3V2(PO4)3 (NVP) stands out as a promising cathode material for SIBs, given its suitable theoretical specific capacity, appropriate operating voltage, robust structural stability, and excellent ionic conductivity. In this article, we first review recent modification strategies for NVP, including conductive substance coating, ion doping (single-, dual- and multi-site doping) and morphology modulation (from zero-dimensional (0D) to three-dimensional (3D)). Subsequently, we summarize five ways in which density functional theory (DFT) calculations can be applied in guiding NVP modification studies. Furthermore, a series of emerging studies combining DFT calculations are introduced. Finally, the remaining challenges and the prospects for optimization of NVP in SIBs are presented.

Synergetic Dual-Additive Electrolyte Enables Highly Stable Performance in Sodium Metal Batteries

Sodium (Na)-metal batteries (SMBs) are considered one of the most promising candidates for the large-scale energy storage market owing to their high theoretical capacity (1,166 mAh g-1) and the abundance of Na raw material. However, the limited stability of electrolytes still hindered the application of SMBs. Herein, sulfolane (Sul) and vinylene carbonate (VC) are identified as effective dual additives that can largely stabilize propylene carbonate (PC)-based electrolytes, prevent dendrite growth, and extend the cycle life of SMBs. The cycling stability of the Na/NaNi0.68Mn0.22Co0.1O2 (NaNMC) cell with this dual-additive electrolyte is remarkably enhanced, with a capacity retention of 94% and a Coulombic efficiency (CE) of 99.9% over 600 cycles at a 5 C (750 mA g-1) rate. The superior cycling performance of the cells can be attributed to the homogenous, dense, and thin hybrid solid electrolyte interphase consisting of F- and S-containing species on the surface of both the Na metal anode and the NaNMC cathode by adding dual additives. Such unique interphases can effectively facilitate Na-ion transport kinetics and avoid electrolyte depletion during repeated cycling at a very high rate of 5 C. This electrolyte design is believed to result in further improvements in the performance of SMBs.

Solvation Effect: The Cornerstone of High-Performance Battery Design for Commercialization-Driven Sodium Batteries

Sodium batteries (SBs) emerge as a potential candidate for large-scale energy storage and have become a hot topic in the past few decades. In the previous researches on electrolyte, designing electrolytes with the solvation theory has been the most promising direction is to improve the electrochemical performance of batteries through solvation theory. In general, the four essential factors for the commercial application of SBs, which are cost, low temperature performance, fast charge performance and safety. The solvent structure has significant impact on commercial applications. But so far, the solvation design of electrolyte and the practical application of sodium batteries have not been comprehensively summarized. This review first clarifies the process of Na+ solvation and the strategies for adjusting Na+ solvation. It is worth noting that the relationship between solvation theory and interface theory is pointed out. The cost, low temperature, fast charging, and safety issues of solvation are systematically summarized. The importance of the de-solvation step in low temperature and fast charging application is emphasized to help select better electrolytes for specific applications. Finally, new insights and potential solutions for electrolytes solvation related to SBs are proposed to stimulate revolutionary electrolyte chemistry for next generation SBs.

In Situ Construction of Inorganics-Rich Cathode-Electrolyte Interface toward Long-Life Prussian White Cathode

Prussian white (PW) is one of the most promising candidates as a cathode for sodium-ion batteries (SIBs) because of its high theoretical capacity, excellent rate performance, and low production cost. However, PW materials suffer severe capacity decay during long-term cycling. In this work, a robust cathode electrolyte interface (CEI) is designed on the PW cathode by employing cresyl diphenyl phosphate (CDP) and adiponitrile (ADN) as electrolyte additives. CDP and ADN possess higher highest occupied molecular orbital energy levels (HOMO) than other solvents, leading to the preferential decomposition of CDP and ADN to construct an inorganics-rich CEI layer in situ on the PW cathode. Benefiting from this CEI layer, the degradation of PW is effectively inhibited during the long cycling. The Na||PW cell achieves an excellent cycling performance with a capacity retention of 85.62% after 1400 cycles. This work presented here provides a feasible strategy for improving the cycling performance of PW by electrolyte modification.

Microwave-Assisted Hydrothermal Synthesis of Na3V2(PO4)2F3 Nanocuboid@Reduced Graphene Oxide as an Ultrahigh-Rate and Superlong-Lifespan Cathode for Fast-Charging Sodium-Ion Batteries

Na3V2(PO4)2F3 (NVPF) has been regarded as a favorable cathode for sodium-ion batteries (SIBs) due to its high voltage and stable structure. However, the limited electronic conductivity restricts its rate performance. NVPF@reduced graphene oxide (rGO) was synthesized by a facile microwave-assisted hydrothermal approach with subsequent calcination to shorten the hydrothermal time. NVPF nanocuboids with sizes of 50-150 nm distributed on rGO can be obtained, delivering excellent electrochemical performance such as a longevity life (a high capacity retention of 85.6% after 7000 cycles at 10 C) and distinguished rate capability (116 mAh g-1 at 50 C with a short discharging/charging time of 1.2 min). The full battery with a Cu2Se anode represents a capacity of 116 mAh g-1 at 0.2 A g-1. The introduction of rGO can augment the electronic conductivity and advance the Na+ diffusion speed, boosting the cycling and rate capability. Besides, the small lattice change (3.3%) and high structural reversibility during the phase transition process between Na3V2(PO4)2F3 and NaV2(PO4)2F3 testified by in situ X-ray diffraction are also advantageous for Na storage behavior. This work furnishes a simple method to synthesize polyanionic cathodes with ultrahigh rate and ultralong lifespan for fast-charging SIBs.

Tailoring Na+ Solvation Environment and Electrode-Electrolyte Interphases with Sn(OTf)2 Additive in Non-flammable Phosphate Electrolytes towards Safe and Efficient Na-S Batteries

Room-temperature sodium-sulfur (RT Na-S) batteries are promising for low-cost and large-scale energy storage applications. However, these batteries are plagued by safety concerns due to the highly flammable nature of conventional electrolytes. Although non-flammable electrolytes eliminate the risk of fire, they often result in compromised battery performance due to poor compatibility with sodium metal anode and sulfur cathode. Herein, we develop an additive of tin trifluoromethanesulfonate (Sn(OTf)2 ) in non-flammable phosphate electrolytes to improve the cycling stability of RT Na-S batteries via modulating the Na+ solvation environment and interface chemistry. The additive reduces the Na+ desolvation energy and enhances the electrolyte stability. Moreover, it facilitates the construction of Na-Sn alloy-based anode solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI). These interphases help to suppress the growth of Na dendrites and the dissolution/shuttling of sodium polysulfides (NaPSs), resulting in improved reversible capacity. Specifically, the Na-S battery with the designed electrolyte boosts the capacity from 322 to 906 mAh g-1 at 0.5 A g-1 . This study provides valuable insights for the development of safe and high-performance electrolytes in RT Na-S batteries.

Non-aqueous Liquid Electrolyte Additives for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have been recognized as one of the most promising new energy storage devices for their rich sodium resources, low cost and high safety. The electrolyte, as a bridge connecting the cathode and anode electrodes, plays a vital role in determining the performance of SIBs, such as coulombic efficiency, energy density and cycle life. Therefore, the overall performance of SIBs could be significantly improved by adjusting the electrolyte composition or adding a small number of functional additives. In this review, the fundamentals of SIB electrolytes including electrode-electrolyte interface and solvation structure are introduced. Then, the mechanisms of electrolyte additive action on SIBs are discussed, with a focus on film-forming additives, flame-retardant additives and overcharge protection additives. Finally, the future research of electrolytes is prospected from the perspective of scientific concepts and practical applications.

Highly Oxidation-Resistant Electrolyte for 4.7 V Sodium Metal Batteries Enabled by Anion/Cation Solvation Engineering

Sodium metal batteries (SMBs) are considered as promising battery system due to abundant Na sources. However, poor compatibility between electrolyte and cathode severely impedes its development. Herein, we proposed an anion/cation solvation strategy for realizing 4.7 V resistant SMBs electrolyte with NaClO₄ and trimethoxy(pentafluorophenyl)silane (TPFS) as dual additives (DA). The ClO₄ ^- can rapidly transfer to the cathode surface and strongly coordinate with Na⁺ to form stable polymer-like chains with solvents. Meanwhile, TPFS can preferentially enter into the PF₆ ^- anion solvation sheath for reducing PF₆ -solvent interaction and effectively scavenge adverse electrolyte species for protecting electrode electrolyte interphases. Thus, such electrolyte elevates the oxidative stability of carbonate electrolytes from 3.77 to 4.75 V, and enables Na||Na₃ V₂ (PO₄ )₂ O₂ F (NVPF) battery with a capacity retention of 93 % and an average Coulombic efficiency (CE) of 99.6 % after 500 cycles at 4.7 V.

Formation of NaF-rich Solid Electrolyte Interphase on Na Anode through Additive Induced Anion-enriched Structure of Na+ Solvation

High-capacity sodium (Na) anode suffer from the trouble of dendrite growth due to high reactivity, which can be overcome through inducing stable NaF-rich solid electrolyte interphase (SEI). Herein, we propose an additive strategy for realizing the anion-enriched structure of Na+ solvation to obtain NaF-rich SEI. The electron withdrawing acetyl group in 4-acetylpyridine (4-APD) increases the coordination number of PF6- in Na+ solvation sheath to facilitate PF6- decompose to NaF. Thus, the NaF-rich SEI with high mechanical stability and interfacial energy is formed to repress the growth of Na dendrites. With the 4-APD-contained electrolyte, the symmetric Na||Na cells show excellent cycling performance over 360 h at 1.0 mA cm-2. Meanwhile, enhanced stability is also achieved of Na||Na3V2(PO4)2O2F full cells with high Coulombic efficiency (97%) and capacity retention (91%) after 200 cycles.