Boosting the Na-Ion Conductivity in the Cluster-Ion Based Anti-Perovskite Na2BH4NH2

Solid-state sodium-ion/metal batteries (SSSBs) are highly desirable for next-generation energy storage systems, while very limited Na-ion solid-state electrolytes are explored. The borohydride-based solid electrolytes are expected to achieve the high energy density target, due to their low redox potential, low Young's modulus as well as high stability toward alkali metals. However, the biggest challenge of borohydride-based electrolyte is the low ionic conductivity. In this study, an anti-perovskite solid-state electrolyte (SSE) material rich in vacancy defects is explored, Na₂BH₄NH₂, to solve above problems. Benefitting from rich vacancy defects, a high ionic conductivity of 7.56 × 10⁻⁴ S cm⁻¹ with a low activation energy for Na⁺ migration of 0.67 eV at 90 °C are achieved. The NaSn|Na₂BH₄NH₂|NaSn symmetric cell cycles at a current density of 0.1 mA cm⁻² for 500 h. Moreover, the universality of Na₂BH₄NH₂ electrolyte is verified by TiS₂ cathode, indicating that Na₂BH₄NH₂ has good compatibility with electrode material. These outstanding performances suggest that it is a viable strategy to increase the ionic conductivity by forming vacancy defects, leading to the further development of solid electrolytes with superior properties.     

Regulating Na-ion Solvation in Quasi-Solid Electrolyte to Stabilize Na Metal Anode

Sodium metal batteries are promising for cost-effective energy storage, however, the sluggish ion transport in electrolytes and detrimental sodium-dendrite growth stall their practical applications. Herein, a cross-linking quasi-solid electrolyte with a high ionic conductivity of 1.4 mS cm⁻¹ at 25 °C is developed by in-situ polymerizing poly (ethylene glycol) diacrylate-based monomer. Benefiting from the refined solvation structure of Na⁺ with a much lower desolvation barrier, random Na⁺ diffusion on the Na surface is restrained, so that the Na dendrite formation is suppressed. Consequently, symmetrical Na||Na cells employing the electrolyte can be cycled >2000 h at 0.1 mA cm⁻². Na₃V₂(PO₄)₃||Na batteries reveal a high discharge specific capacity of 66.1 mAh g⁻¹ at 15 C and demonstrate stable cycling over 1000 cycles with a capacity retention of 83% at a fast rate of 5 C.     

A Low-Temperature Sodium-Ion Full Battery: Superb Kinetics and Cycling Stability

The increasingly stringent requirement in large-scale energy storage necessitates the development of high-performance sodium-ion batteries (SIBs) that can operate under low-temperature (LT) environment. Although SIBs can achieve good cycling stability and rate performance at room temperature, the sluggish electrochemical reaction kinetics at low temperature remains a great challenge for SIBs. Here, a superior LT SIB composed of 3D porous Na₃V₂(PO₄)₃/C (NVP/C-F) and NaTi₂(PO₄)₃/C foams (NTP/C-F) is developed. First-principles calculations reveal that the intrinsic Na⁺ diffusivity in NASICON-type NVP and NTP is extremely high (maximum 3.84 × 10⁻⁵ for NVP and 2.94 × 10⁻⁹ cm² s⁻¹ for NTP) at –20 °C. In addition, the designed 3D interconnected porous foam structures demonstrate excellent electrolyte absorption ability and Na⁺ transport performance at low temperature. As a result, under −20 °C, the NVP/C F and NTP/C F electrodes (half-cell configuration) can attain reversible capacities close to their theoretical values, and are able to be charged and discharged rapidly (20 C) for 1000 cycles. Based on these features, the designed NTP/C F||NVP/C F full cell also displays superb LT kinetics and cycling stability, making a great stride forward in the development of LT SIBs.     

Dilute Aqueous Hybrid Electrolyte with Regulated Core-Shell-Solvation Structure Endows Safe and Low-Cost Potassium-Ion Energy Storage Devices

High-concentration “water-in-salt” (WIS) electrolytes with the wider electrochemical stability window (ESW) can give rise to safe, non-flammable, and high-energy aqueous potassium-ion energy storage devices, thus highlighting the prospect for applications in grid-scale energy storage. However, WIS electrolytes usually depend on highly concentrated salts, leading to serious concerns about cost and sustainability. Here, an aqueous low-concentration-based potassium-ion hybrid electrolyte is demonstrated with the regulated core-shell-solvation structure by using an aprotic solvent, i.e., trimethyl phosphate, to limit the water activity. This aqueous hybrid electrolyte has a low salt concentration (1.6 mol L⁻¹) of potassium trifluoromethanesulfonate but with an expanded ESW up to 3.4 V and the nonflammable property. Based on this dilute aqueous hybrid electrolyte, electrochemical double-layer capacitors are capable of working within a large voltage range (0–2.4 V) at a wide range of temperatures from −20 to 60 °C. An aqueous potassium-ion battery consisting of an organic 3,4,9,10-perylenetetracarboxylic diimide anode, Prussian blue K_1.5Mn_0.61Fe_0.39[Fe(CN)₆]_0.77·H₂O cathode and this dilute aqueous hybrid electrolyte can operate well at rates between 0.2 and 4.0 C and deliver a high energy density of 66.5 Wh kg⁻¹ as well as a durable cycling stability with a capacity retention of 84.5% after 600 cycles at 0.8 C.     

Brine Refrigerants for Low-cost,Safe Aqueous Supercapacitors with Ultra-long Stable Operation at Low Temperatures

Traditional aqueous energy storage devices are difficult to operate at low temperatures owing to the poor ionic conductivity and sluggish interfacial dynamics in frozen electrolytes. Herein, the low-cost brine refrigerants for food freezing and preservation as electrolytes, and unexpectedly realize high ionic conductivity and stable operation of an aqueous storage device at low temperatures are demonstrated. A CaCl₂ brine refrigerant electrolyte (BRE) with a low freezing point −55 °C and high ionic conductivity (10.1 mS cm⁻¹ at −50 °C) is developed for supercapacitors (SCs), which retains 80% of the room temperature capacity at −50 °C and exhibits ultra-long cycle life with excellent capacity retention of 92% over 98,500 cycles, outperforming the other SCs which can be operated below −40 °C in literature. Moreover, the SCs with MgCl₂ and NaCl BREs can also be operated successfully with excellent cycle stability and high-capacity retention at low temperatures of −30 and −20 °C, respectively. Fundamental correlation between various cations and their effect on the freezing point reduction of aqueous electrolytes is revealed via Raman investigation and molecular dynamics simulations. This study provides a rational design strategy for green, inexpensive, and safe low-temperature aqueous electrolytes for energy storage devices.     

Alkali Metal Cations as Charge-Transfer Bridge for Polarization Promoted Solar-to-H2 Conversion

Utilization of spontaneous polarization electric field of ferroelectric materials to realize the spatial separation and fast transfer of photogenerated charges has been regarded as a promising strategy to fabricate highly efficient photocatalysts. Herein, a novel heterostructure is constructed by coupling potassium poly(heptazine imide) (K-PHI) with ferroelectric Ba_xSr_1-xTiO₃ (B_xST) through a facile electrostatic self-assembly strategy. The ionic species of K-PHI can neutralize the polarized charges in B_xST to form intimate interfacial contact, substantially boosting the internal electric field. Notably, K⁺ cations intercalated into K-PHI act as charge-transfer bridge to promote migration and separation of photogenerated charge carriers. As a result, a significantly improved H₂-evolution rate of 1087.4 µmol h⁻¹ g⁻¹ with an apparent quantum yield (AQY) of 8.05% at 420 nm is achieved over 5% K-PHI/B_0.8ST, standing among the best polymeric carbon nitride-based photocatalysts reported up to date. Moreover, the extreme stability of the catalysts is evidenced by remaining outstanding catalytic performance even after storage for half a year. This strategy can be extended to other alkali metal (Na⁺ and Cs⁺) modified polymeric materials, highlighting the key role of the bridging ions in constructing polarized heterostructure, which sheds light on the design of ferroelectric-assisted photocatalysts.     

Salty Ice Electrolyte with Superior Ionic Conductivity Towards Low-Temperature Aqueous Zinc Ion Hybrid Capacitors

Aqueous electrochemical energy storage (EES) devices have attracted considerable attention due to their advantages of low cost and high safety. However, the freeze of aqueous electrolytes usually causes the dramatic loss of ionic conduction capacity, thereby seriously restricting the low-temperature application of such EES devices. Herein, different from traditional frozen electrolytes, a Zn(ClO₄)₂ salty ice with superior ionic conductivity (1.3 × 10⁻³ S cm⁻¹ even at −60 °C) is discovered. It is attributed to the unique 3D ionic transport channels inside such ice, which enables the fast transport of both Zn²⁺ ions and ClO₄⁻ ions inside the ice at low temperatures. Using this Zn(ClO₄)₂ salty ice as an electrolyte, as-built zinc ion hybrid capacitor is able to work even at −60 °C (with 74.2% of the room temperature capacity), and exhibits an ultra-long cycle life of 70 000 cycles at low temperature. This discovery provides a new insight for constructing low-temperature EES devices using salty ices as electrolytes.     

High Energy Storage Performance of Opposite Double‐Heterojunction Ferroelectricity–Insulators

In this study, the excellent energy storage performance is achieved by constructing opposite double‐heterojunction ferroelectricity–insulator–ferroelectricity configuration. The PbZr_0.52Ti_0.48O₃ films and Al₂O₃ films are chosen as the ferroelectricity and insulator, respectively. The microstructures, polarization behaviors, breakdown strength, leakage current density, and energy storage performance are investigated systematically of the constructed PbZr_0.52Ti_0.48O₃/Al₂O₃/PbZr_0.52Ti_0.48O₃ opposite double‐heterojunction. The ultrahigh electric field breakdown strength (≈5711 kV cm^?1) is obtained, which is beneficial to achieve high energy storage density. Meanwhile, the high linearity of hysteresis loops with low energy dissipation is obtained at a proper annealing temperature, which is induced by partially crystallized and is in favor of achieving high energy storage efficiency η. The PbZr_0.52Ti_0.48O₃/Al₂O₃/PbZr_0.52Ti_0.48O₃ annealed at 550 °C exhibits excellent energy storage performance with a storage density of 63.7 J cm^?3 and efficiency of 81.3%, which is ascribed to the synergetic effect of electric breakdown strength (E_BDS = 5711 kV cm^?1) and the polarization (P_m–P_r = 23.74 µC cm^?2). The proposed method in this study opens a new door to improve the energy storage performance of inorganic ferroelectric capacitors.     

Antifreezing Zwitterionic Hydrogel Electrolyte with High Conductivity of 12.6 mS cm−1 at −40 °C through Hydrated Lithium Ion Hopping Migration

Hydrogel electrolytes have high room-temperature conductivity and can be widely used in energy storage device. However, hydrogels suffer from the inevitable freezing of water at subzero temperatures, resulting in the diminishment of their conductivity and mechanical properties. How to achieve high conductivity without sacrificing hydrogels’ flexibility at subzero temperature is an important challenge. To address this challenge, a new type of zwitterionic polymer hydrogel (polySH) electrolytes is fabricated. The anionic and cationic counterions on the polymer chains facilitate the dissociation of LiCl. The antifreezing electrolyte can be stretched to a strain of 325% and compressed to 75% at −40 °C and possesses an outstanding conductivity of 12.6 mS cm⁻¹ at −40 °C. A direct hopping migration mechanism of hydrated lithium-ion through the channel of zwitterion groups is proposed. The polySH electrolyte-based-supercapacitor (SC) exhibits a high specific capacitance of 178 mF cm⁻² at 60 °C and 134 mF cm⁻² at −30 °C with a retention of 81% and 71% of the initial capacitance after 10 000 cycles, respectively. The overall merits of the electrolyte will open up a new avenue for advanced ionic conductors and energy storage device in practical applications.     

Ultrahigh Energy Storage Capacitors Based on Freestanding Single-Crystalline Antiferroelectric Membrane/PVDF Composites

Inorganic/organic dielectric composites are very attractive for high energy density electrostatic capacitors. Usually, linear dielectric and ferroelectric materials are chosen as inorganic fillers to improve energy storage performance. Antiferroelectric (AFE) materials, especially single-crystalline AFE oxides, have relatively high efficiency and higher density than linear dielectrics or ferroelectrics. However, adding single-crystalline AFE oxides into polymers to construct composite with improved energy storage performance remains elusive. In this study, high-quality freestanding single-crystalline PbZrO₃ membranes are obtained by a water-soluble sacrificial layer method. They exhibit classic AFE behavior and then 2D–2D type PbZrO₃/PVDF composites with the different film thicknesses of PbZrO₃ (0.1-0.4 µm) is constructed. Their dielectric properties and polarization response improve significantly as compared to pure PVDF and are optimized in the PbZrO₃(0.3 µm)/PVDF composite. Consequently, a record-high energy density of 43.3 J cm⁻³ is achieved at a large breakdown strength of 750 MV m⁻¹. Phase-field simulation indicates that inserting PbZrO₃ membranes effectively reduces the breakdown path. Single-crystalline AFE oxide membranes will be useful fillers for composite-based high-power capacitors.     

2D Antiferroelectric Hybrid Perovskite with a Large Breakdown Electric Field And Energy Storage Density

Energy conversion and storage devices are highly desirable for the sustainable development of human society. Hybrid organic–inorganic perovskites have shown great potential in energy conversion devices including solar cells and photodetectors. However, its potential in energy storage has seldom been explored. Here the crystal structure and electrical properties of the 2D hybrid perovskite (benzylammonium)₂PbBr₄ (PVK-Br) are investigated, and the consecutive ferroelectric-I (FE1) to ferroelectric-II (FE2) then to antiferroelectric (AFE) transitions that are driven by the orderly alignment of benzylamine and the distortion of [PbBr₆] octahedra are found. Furthermore, accompanied by field-induced AFE to FE transition near room temperature, a large energy storage density of ≈1.7 J cm⁻³ and a wide working temperature span of ≈70 K are obtained; both of which are among the best in hybrid AFEs. This good energy storage performance is attributed to the large polarization of ≈7.6 µC cm⁻² and the high maximum electric field of over 1000 kV cm⁻¹, which, as revealed by theoretical calculations, originate from the cooperative coupling between the [PbBr₆] octahedral framework and the benzylamine molecules. The research clarifies the discrepancy in the phase transition character of PVK-Br and shed light on developing high-performance energy storage devices based on 2D hybrid perovskite.     

Regulating the Water Molecular in the Solvation Structure for Stable Zinc Metal Batteries

Rechargeable aqueous zinc batteries are promising energy storage devices because of their low cost, high safety, and high energy density. However, their performance is plagued by the unsatisfied cyclability due to the dendrite growth and hydrogen evolution reaction (HER) at the Zn anode. Herein, it is demonstrated that the concentrated hybrid aqueous/non-aqueous ZnCl₂ electrolytes constitute a peculiar chemical environment for not only the Zn-ions but also water molecules. The high concentration of chloride ions substitutes the H₂O molecular in the solvation structure of Zn²⁺, while the acetonitrile further interacts with H₂O to decrease its activity. The hybrid electrolytes both inhibit the dendrite formation and HER, enabling an ultrahigh average Coulombic efficiency of 99.9% in the Zn||Cu half-cell and a highly reversible Zn plating/stripping with a low overpotential of 21 mV. Using this hybrid electrolyte, the Zn||polytriphenylamine (PTPAn) full cell deliveres a high discharge capacity of 110 mAh g⁻¹, a high power density of 9200 W kg⁻¹ at 100 °C and maintains 85% of the capacity for over 6000 cycles at 10 °C. This study provides a deep understanding between the solvation structure and columbic efficiency of Zn anode, thus inspiring the development for stable Zn batteries.     

Effects of Annealing Temperature on the Electric Properties of 0.94(Na0.5Bi0.5)TiO3–0.06BaTiO3 Ferroelectric Thin Film

0.94(Na_0.5Bi_0.5)TiO₃–0.06BaTiO₃ (NBT–BT6) ferroelectric thin films have been fabricated on Pt–Ti–SiO₂–Si(100) substrate by metal–organic decomposition. The effects of annealing temperature (650–800°C) on the microstructure, and the piezoelectric, ferroelectric, and dielectric properties of the thin films were studied in detail. The residual stress was evaluated by the orientation average method to clarify its dependence on annealing temperature and grain size, and it was correlated with the electric properties to understand the mechanism of piezoelectric enhancement. Among the thin films, NBT–BT6 thin film annealed at 750°C has the largest effective piezoelectric coefficient, 95.1 pm/V, remnant polarization, 49.7 μC/cm², spontaneous polarization, 105.2 μC/cm², and dielectric constant, 504, and the lowest dielectric loss, 0.05, and tensile residual stress, 24.5 MPa. For the NBT–BT6 thin film annealed at 750°C, a wide temperature range, 183–210°C, around the phase transition temperature (T _m) was observed in the dielectric temperature plots, and the diffusion coefficients (γ) were quantitatively assessed as 1.6, 1.78, and 1.6. Piezoelectric performance is discussed on the basis of the dispersion phase transition and residual stress.     

A Complexing Agent to Enable a Wide-Temperature Range Bromine-Based Flow Battery for Stationary Energy Storage

Bromine-based flow batteries (Br-FBs) are considered one of the most promising energy storage systems due to their features of high energy density and low cost. However, they generally suffer from uncontrolled diffusion of corrosive bromine particularly at high temperatures. That is because the interaction between polybromide anions and the commonly used complexing agent (N–methyl–N–ethyl–pyrrolidinium bromide [MEP]) decreases with increasing temperatures, which causes serious self-discharge and capacity fade. Herein, a novel bromine complexing agent, 1–ethyl–2–methyl–pyridinium bromide (BCA), is introduced in Br-FBs to solve the above problems. It is proven that BCA can combine with polybromide anions very well even at a high temperature of 60 °C. Moreover, the BCA contributes to decreasing the electrochemical polarization of Br⁻/Br₂ couple, which in turn improves their power density. As a result, a zinc–bromine flow battery with BCA as the complexing agent can achieve a high energy efficiency of 84% at 40 mA cm⁻², even at high temperature of 60 °C and it can stably run for more than 400 cycles without obvious performance decay. This paper provides an effective complexing agent to enable a wide temperature range Br-FB.     

Influence of the annealing temperature on the ferroelectric properties of niobium-doped strontium–bismuth tantalate

Characteristics of ferroelectric thin films of niobium-doped strontium–bismuth tantalate (SBTN), which were deposited by magnetron sputtering on Pt/TiO₂/SiO₂/Si substrates, are investigated. To form the ferroelectric structure, deposited films were subjected to subsequent annealing at 700–800°C in an O₂ atmosphere. The results of X-ray diffraction showed that the films immediately after the deposition have an amorphous structure. Annealing at 700–800°C results in the formation of the Aurivillius structure. The dependences of permittivity, residual polarization, and the coercitivity of SBTN films on the modes of subsequent annealing are established. Films with residual polarization 2P_r = 9.2 µC/cm², coercitivity 2E_c = 157 kV/cm, and leakage current 10^–6 A/cm² are obtained at the annealing temperature of 800°C. The dielectric constant and loss tangent at frequency of 1.0 MHz were ε = 152 and tan δ = 0.06. The ferroelectric characteristics allow us to use the SBTN films in the capacitor cell of high density ferroelectric random-access non-volatile memory (FeRAM).     

A Manganese Phosphate Cathode for Long-Life Aqueous Energy Storage

Mn-based materials for aqueous energy storage are reaching the capacity ceiling due to the limited Mn⁴⁺/Mn³⁺ redox. The disproportionation of Mn³⁺ often occurs, forming soluble Mn²⁺ and thus leading to severer capacity decays. Here, an amorphous manganese phosphate material [AMP, Na_1.8Mn₄O_1.4(PO₄)₃] is fabricated using an electrochemical method for the first time. Benefitting from the open framework and the insoluble nature of Mn²⁺ in AMP, the Mn³⁺/Mn²⁺ and Mn⁴⁺/Mn³⁺ redox couples can participate in the charge storage processes. The AMP electrode shows a high capacity of 253.4 mAh g⁻¹ (912.4 F g⁻¹ or 912.4 C g⁻¹) at the current density 1 A g⁻¹ and good rate capability. Experimental results indicate AMP experiences a mixed charge storage mechanism (i.e., cation intercalation and conversion reactions) in Na₂SO₄ electrolyte. Besides, electrolyte engineering can effectively prevent the decomposition of AMP during cycling test, achieving capacity retention of 97% in 5000 cycles. Importantly, AMP can accommodate different cations (e.g., Mg²⁺, Ca²⁺, etc.), exhibiting great potential for aqueous energy storage.     

Weak Ionization Induced Interfacial Deposition and Transformation towards Fast-Charging NaTi2(PO4)3 Nanowire Bundles for Advanced Aqueous Sodium-Ion Capacitors

Aqueous sodium-ion capacitors (ASICs) offer great promise for inexpensive and safe energy storage. However, their development is plagued by a kinetics imbalance at high rates between battery and capacitive electrodes and a narrow voltage window due to water electrolysis. Here a unique nanowire bundles anode is designed that simultaneously affords ultrahigh rate capability and manifests robust Na⁺ insertion to suppress hydrogen evolution, enabling an advanced ASIC. The NaTi₂(PO₄)₃ (NTP) is grown on thin titanium foil by elaborately utilizing the weak ionization chemistry of NaH₂PO₄ (NHP), where single-agent NHP not only partially etches titanium to release TiO²⁺, but also induces the interfacial phase-transformation of pre-deposited orthomorphic Na₄Ti(PO₄)₂(OH)₂ cubes to hexagonal NTP nanowires. This anode has hierarchical architectures to facilitate charge and mass transport, thus working stably at considerably high rates of 15–150 C with high capacities. The first 2.4 V flexible solid-state NTP-based ASIC is designed with high energy densities (5.8–12.8 mWh cm⁻³; 57.9–62.1 Wh kg⁻¹; total mass loading up to 8.1 mg cm⁻²) comparable to NASICON-based devices using organic electrolytes, demonstrating outstanding stability of 10 000 cycles and no performance decay even after continuous bending at 180^o. This work presents a versatile strategy to construct NASICON phosphate electrodes for advanced energy storage.     

3D HfO2 Thin Film MEMS Capacitor with Superior Energy Storage Properties

Capacitors are ubiquitous and crucial components in modern technologies. Future microelectronic devices require novel dielectric capacitors with higher energy storage density, higher efficiency, better frequency and temperature stabilities, and compatibility with integrated circuit (IC) processes. Here, in order to overcome these challenges, a novel 3D HfO₂ thin film capacitor is designed and fabricated by an integrated microelectromechanical system (MEMS) process. The energy storage density (ESD) of the capacitor reaches 28.94 J cm⁻³, and the energy storage efficiency of the capacitor is up to 91.3% under an applied electric field of 3.5 MV cm⁻¹. The ESD can be further improved by reducing the minimum period structure size of the 3D capacitor. Moreover, the 3D capacitor exhibits excellent temperature stability (up to 150 °C) and charge-discharge endurance (10⁷ cycles). The results indicate that the 3D HfO₂ thin film MEMS capacitor has enormous potential in energy storage applications in harsh environments, such as pulsed discharge and power conditioning electronics.     

Ultrathin Layered Double Hydroxide Nanosheets Enabling Composite Polymer Electrolyte for All-Solid-State Lithium Batteries at Room Temperature

Solid electrolytes are the most promising substitutes for liquid electrolytes to construct high-safety and high-energy-density energy storage devices. Nevertheless, the poor lithium ion mobility and ionic conductivity at room temperature (RT) have seriously hindered their practical usage. Herein, single-layer layered-double-hydroxide nanosheets (SLN) reinforced poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) composite polymer electrolyte is designed, which delivers an exceptionally high ionic conductivity of 2.2 × 10⁻⁴ S cm⁻¹ (25  ° C), superior Li⁺ transfer number ( ≈ 0.78) and wide electrochemical window ( ≈ 4.9 V) with a low SLN loading ( ≈ 1 wt%). The Li symmetric cells demonstrate ultra-long lifespan stable cycling over ≈ 900 h at 0.1 mA cm⁻², RT. Moreover, the all-solid-state Li|LiFePO₄ cells can run stably with a high capacity retention of 98.6% over 190 cycles at 0.1 C, RT. Moreover, using LiCoO₂/LiNi_0.8Co_0.1Mn_0.1O₂, the all-solid-state lithium metal batteries also demonstrate excellent cycling at RT. Density functional theory calculations are performed to elucidate the working mechanism of SLN in the polymer matrix. This is the first report of all-solid-state lithium batteries working at RT with PVDF-HFP based solid electrolyte, providing a novel strategy and significant step toward cost-effective and scalable solid electrolytes for practical usage at RT.     

Engineering Solid Electrolyte Interface at Nano-Scale for High-Performance Hard Carbon in Sodium-Ion Batteries

Engineering the structure and chemistry of solid electrolyte interface (SEI) on electrode materials is crucial for rechargeable batteries. Using hard carbon (HC) as a platform material, a correlation between Na⁺ storage performance, and the properties of SEI is comprehensively explored. It is found that a “good” SEI layer on HC may not be directly associated with certain kinds of SEI components, such as NaF and Na₂O. Whereas, arranging nano SEI components with refined structures constructs the foundation of “good” SEI that enables fast Na⁺ storage and interface stability of HC in Na-ion batteries. A layer-by-layer SEI on HC with inorganic-rich inner layer and tolerant organic-rich outer flexible layer can facilitate excellent rate and cycling life. Besides, SEI layer as the gate for Na⁺ from electrolyte to HC electrode can modulate interfacial crystallographic structures of HC with pillar-solvent that function as “pseudo-SEI” for fast and stable Na⁺ storage in optimal 1 m NaPF₆-TEGDME electrolytes. Such a layer-by-layer SEI combined with a “pseudo-SEI” layer for HC enables an outstanding rate of 192 mAh g⁻¹ at 2 C and stable cycling over 1100 cycles at 0.5 C. This study provides valuable guidance to improve the electrochemical performance of electrode materials through regulation of SEI in optimal electrolytes.