Toward High Performance Anodes for Sodium-Ion Batteries: From Hard Carbons to Anode-Free Systems

Biomass-based carbon material for next-generation sodium-ion batteries: insights and SWOT evaluation

In recent years, energy storage has significantly transitioned from lithium to sodium ion due to sodium's abundance and economical and optimal redox potential. Biomass-based carbon anode materials are extensively studied in sodium-ion batteries because of their economic advantages and eco-sustainable approach. Their distinctive microstructural characteristics resulting in higher specific capacitance. The present review explores hard carbon and a few emerging sustainable materials derived from various biomass sources. It also covers their production, focusing on micro-structures, morphological defects, and heteroatom doping-related aspects. However, the sodium storage mechanism within carbon anodes, particularly hard carbon, is a subject of debate due to its diverse microstructural states in contrast to the specific layered structure of graphite. It also integrates strengths, weaknesses, and opportunities with threat evaluation by highlighting detailed insights about recent developments in hard carbon. This review also highlights bibliographic analysis through network visualization map of international research collaboration in the field of biomass based anode material for sodium ion battery. It also offers a cohesive framework for advancing biomass-derived hard carbon and other carbon materials as an independent or complementary anode material for next-generation sodium-ion batteries.

Advanced Interphases Layers for Dendrite-Free Sodium Metal Anodes

Sodium (Na) metal anode is considered the cornerstone of next-generation energy storage technology, owing to its high theoretical capacity and cost-effectiveness. However, the development of Na metal batteries is hindered by the instability and nonuniformity of the solid electrolyte interphase (SEI) and notorious formation of Na dendrites. Recently, various advanced artificial interphase designs have been developed to control notorious dendrite growth and stabilize the SEI layer. In this Review, we provide a comprehensive overview of artificial interphase designs, focusing on inorganic interphase layer, organic interphase layer, and hybrid inorganic/organic interphase layer, all aimed at inhibiting the notorious Na dendrites growth. Finally, future interphase engineering strategies are also envisioned to offer new insights into the optimization of Na anodes.

Anion-mediated approach to overcome oxidation in ether electrolytes for high-voltage sodium-ion batteries

The ether-based electrolytes are acknowledged for their compatibility with a diverse array of sodium-ion battery anodes, as well as their capability to enable efficient and reversible electrochemical reactions. However, they encounter a challenge of oxidation at high voltages. We find that a standard diglyme-based electrolyte starts to oxidize and break down at voltages exceeding 3.9 V (vs. Na+/Na). This deterioration is attributed to the nucleophilic nature of the diglyme solvent and the presence of oxygen atoms that possess two unpaired electrons. To address this issue, we incorporate foreign anions into the electrolyte system to passivate the reactive sites of terminal H on diglyme solvents, inhibiting further dehydrogenation and oxidation during battery operation. The constructed cathode electrolyte interphase, enriched with NaF and NaNxOy, substantially boosts the oxidation resistance of electrolyte to over 4.8 V (vs. Na+/Na), expanding the stability window and rendering it feasible for various high-voltage cathode materials. Our approach also ensures compatibility with either hard carbon or commercial graphite anodes, guaranteeing operation in pouch cells. This study elucidates the relationship between interfacial chemistry and oxidation tolerance at high voltages, offering an approach to the development of practical ether-based electrolytes for high-energy-density battery technologies.

Mechanisms and Mitigation Strategies of Gas Generation in Sodium-Ion Batteries

The transition to renewable energy sources has elevated the importance of SIBs (SIBs) as cost-effective alternatives to lithium-ion batteries (LIBs) for large-scale energy storage. This review examines the mechanisms of gas generation in SIBs, identifying sources from cathode materials, anode materials, and electrolytes, which pose safety risks like swelling, leakage, and explosions. Gases such as CO2, H2, and O2 primarily arise from the instability of cathode materials, side reactions between electrode and electrolyte, and electrolyte decomposition under high temperatures or voltages. Enhanced mitigation strategies, encompassing electrolyte design, buffer layer construction, and electrode material optimization, are deliberated upon. Accordingly, subsequent research endeavors should prioritize long-term high-precision gas detection to bolster the safety and performance of SIBs, thereby fortifying their commercial viability and furnishing dependable solutions for large-scale energy storage and electric vehicles.

Sustainable Solid-State Sodium-Ion Batteries Featuring Ferroelectric Electrolytes

Solid-state batteries offer significant advantages but present several challenges. Given the complexity of these systems, it is good practice to begin the study with simpler models and progressively advance to more complex configurations, all while maintaining an understanding of the physical principles governing solid-state battery operation. The results presented in this work pertain to cells without traditional electrodes, thus providing a foundation for guiding the development of fully functional solid-state cells. The open circuit voltage (OCV) of the Cu/Na2.99Ba0.005ClO composite in a cellulose/Zn pouch cell achieves 1.10 V, reflecting the difference in the chemical potentials of the current collectors (CCs), Zn and Cu, serving as electrodes. After 120 days, while set to discharge, conversely to what was expected, a higher potential difference of 1.13 V was attained (capacity of 5.9 mAh·g-1electrolyte). By incorporating a layer of carbon felt, the OCV became 0.85 V; however, after 95 days, the potential difference increased to 1.20 V. Ab initio simulations were additionally performed on a Cu/Na3ClO/Zn heterojunction showing the formation of dipoles and the Na deposition on Zn which is demonstrated experimentally. The sodium plating on the negative CC (Zn) takes place as the cell is set to discharge at room temperature but is not observed at 40 °C.

Boosting sodium storage performance of Na0.44MnO2 through surface modification with conductive polymer PPy utilizing sonication-assisted dispersion

The search for suitable electrode materials for sodium storage in sodium-ion batteries (SIBs) poses significant challenges. Na0.44MnO2 (NMO) has emerged as a promising candidate among various cathode materials due to its distinct three-dimensional tunnel structure, which facilitates Na+ diffusion and governs structural stress fluctuations during Na+ intercalation/deintercalation. However, NMO faces obstacles such as limited electronic conductivity, lattice distortion induced by the Jahn-Teller effect of Mn3+ during cycling, and Mn3+ disproportionation leading to material dissolution, which affects cycling durability. To overcome these problems, Na0.44MnO2/polypyrrole (NMO/PPy) composites were fabricated through surface modification of the conductive PPy using an ultrasonically assisted dispersion method. Experimental results show that NMO/PPy with a 7 wt% PPy content exhibits superior sodium storage capabilities. Specifically, at a current density of 0.5C, the initial specific discharge capacity reaches 135.2 mA h g-1, a 12.1% increase over pristine NMO, with a capacity retention of 94.5% after 100 cycles. Of particular note is a capacity retention of 82% after 500 cycles at 1C, attributed to the PPy coating, which suppresses Mn3+ side reactions, enhances the structural stability and electronic conductivity of NMO, and accelerates Na+ diffusion. These results suggest that the use of conductive polymer coatings represents a simple and effective strategy to improve the sodium storage capacity of NMO, paving the way for the further development of high-performance SIB cathodes.

Reconstructing Helmholtz Plane Enables Robust F-Rich Interface for Long-Life and High-Safe Sodium-Ion Batteries

Hard carbon (HC) is the most commonly used anode material in sodium-ion batteries. However, the solid-electrolyte-interface (SEI) layer formed in carbonate ester-based electrolytes has an imperceptible dissolution tendency and a sluggish Na+ diffusion kinetics, resulting in an unsatisfactory performance of HC anode. Given that electrode/electrolyte interface property is highly dependent on the configuration of Helmholtz plane, we filtrated proper solvents by PFBE (PF6 - anion binding energy) and CAE (carbon absorption energy) and disclosed the function of chosen TFEP to reconstruct the Helmholtz plane and regulate the SEI film on HC anode. Benefiting from the preferential adsorption tendency on HC surface and strong anion-dragging interaction of TFEP, a robust and thin anion-derived F-rich SEI film is established, which greatly enhances the mechanical stability and the Na+ ion diffusion kinetics of the electrode/electrolyte interface. The rationally designed TFEP-based electrolyte endows Na||HC half-cell and 2.8 Ah HC||Na4Fe3(PO4)2P2O7 pouch cell with excellent rate capability, long cycle life, high safety and low-temperature adaptability. It is believed that this insightful recognition of tuning interface properties will pave a new avenue in the design of compatible electrolyte for low-cost, long-life, and high-safe sodium-ion batteries.

Unlocking the Na-Storage Behavior in Hard Carbon Anode by Mass Spectrometry

Hard carbon (HC) is a promising anode candidate for Na-ion batteries (NIBs) because of its excellent Na-storage performance, abundance, and low cost. However, a precise understanding of its Na-storage behavior remains elusive. Herein, based on the D2O/H2SO4-based TMS results collected on charged/discharged state HC electrodes, detailed Na-storage mechanisms (the Na-storage states and active sites in different voltage regions), specific SEI dynamic evolution process (formation, rupture, regeneration and loss), and irreversible capacity contribution (dead Na0, NaH, etc.) were elucidated. Moreover, by employing the online electrochemical mass spectrometry (OEMS) to monitor the gassing behavior of HC-Na half-cell during the overdischarging process, a surprising rehydrogen evolution reaction (re-HER) process at around 0.02 V vs Na+/Na was identified, indicating the occurrence of Na-plating above 0 V vs Na+/Na. Additionally, the typical fluorine ethylene carbonate (FEC) additive was demonstrated to reduce the accumulation of dead Na0 and inhibit the re-HER process triggered by plated Na.

Biomimetic-Mineralization-Assisted Self-Activation Creates a Delicate Porous Structure in Carbon Material for High-Rate Sodium Storage

Porous carbons have shown their potential in sodium-ion batteries (SIBs), but the undesirable initial Coulombic efficiency (ICE) and rate capability hinder their practical application. Herein, learning from nature, we report an efficient method for fabricating a carbon framework (CK) with delicate porous structural regulation by biomimetic mineralization-assisted self-activation. The abundant pores and defects of the CK anode can improve the ICE and rate performance of SIBs in ether-based electrolytes, whereas they are confined in carbonate ester-based electrolytes. Notably, ether-based electrolytes enable CK anode to possess excellent ICE (82.9%) and high-rate capability (111.2 mAh g-1 at 50 A g-1). Even after 5500 cycles at a large current density of 10 A g-1, the capacity retention can still be maintained at 73.1%. More importantly, the full cell consisting of the CK anode and Na3V2(PO4)3 cathode delivers a high energy density of 204.4 Wh kg-1, with a power density of 2828.2 W kg-1. Such outstanding performance of the CK anode is attributed to (1) hierarchical pores, oxygen doping, and defects that pave the way for the transportation and storage of Na+, further enhancing ICE; (2) a high-proportion NaF-based solid-electrolyte-interphase (SEI) layer that facilitates Na+ storage kinetics in ether-based electrolytes; and (3) ether-based electrolytes that determine Na+ storage kinetics further to dominate the performance of SIBs. These results provide compelling evidence for the promising potential of our synthetic strategy in the development of carbon-based materials and ether-based electrolytes for electrochemical energy storage.

Competitive Coordination of Sodium Ions for High-Voltage Sodium Metal Batteries with Fast Reaction Speed

The unstable electrode-electrolyte interface and the narrow electrochemical window of normal electrolytes hinder the potential application of high-voltage sodium metal batteries. These problems are actually related to the solvation structure of the electrolyte, which is determined by the competition between cations coordinated with anions or solvent molecules. Herein, we design an electrolyte incorporating ethyl (2,2,2-trifluoroethyl) carbonate and fluoroethylene carbonate, which facilitates a pronounced level of cation-anion coordination within the solvation sheath by enthalpy changes to reduce the overall coordination of cation-solvents and increase sensitivity to salt concentration. Such an electrolyte regulated by competitive coordination leads to highly reversible sodium plating/stripping with extended cycle life and a high Coulombic efficiency of 98.0%, which is the highest reported so far in Na||Cu cells with ester-based electrolytes. Moreover, 4.5 V high-voltage Na||Na3V2(PO4)2F3 cells exhibit a high rate capability up to 20 C and an impressive cycling stability with an 87.1% capacity retention after 250 cycles with limited Na. The proposed strategy of solvation structure modification by regulating the competitive coordination of the cation provides a new direction to achieve stable sodium metal batteries with high energy density and can be further extended to other battery systems by controlling enthalpy changes of the solvation structure.

Improved Interface Construction on Anode and Cathode for Na-Ion Batteries Using Ultralow-Concentration Electrolyte Containing Dual-Additives

Compared with Li+ , Na+ with a smaller stokes radius has faster de-solvation kinetics. An electrolyte with ultralow sodium salt (0.3 M NaPF6 ) is used to reduce the cell cost. However, the organic-dominated interface, mainly derived from decomposed solvents (SSIP solvation structure), is defective for the long cycling performance of sodium ion batteries. In this work, the simple application of dual additives, including sodium difluoro(oxalato)borate (NaDFOB) and tris(trimethylsilyl)borate (TMSB), is demonstrated to improve the cycling performance of the hard carbon/NaNi1/3 Fe1/3 Mn1/3 O2 cell by constructing interface films on the anode and cathode. A significant improvement on cycling stability has been achieved by incorporating dual additives of NaDFOB and TMSB. Particularly, the capacity retention increased from 17 % (baseline) to 79 % (w/w, 2.0 wt % NaDFOB) and 83 % (w/w, 2.0 wt % NaDFOB and 1.0 wt % TMSB) after 200 cycles at room temperature. Insight into the mechanism of improved interfacial properties between electrodes and electrolyte in ultralow concentration electrolyte has been investigated through a combination of theoretical computation and experimental techniques.