Liquid Metal-Modified 3D Cu Foam for Dendrite-Free Sodium Plating.

Sodium metal is regarded as one of the most promising anode materials due to its high theoretical capacity (1166 mAh g-1) and low redox potential (-2.714 V vs standard hydrogen electrode). However, the practical application of sodium metal is hindered by the formation of dendrites during Na stripping and plating, which can degrade performance and cause potential safety hazards. To address this issue, previous work focuses on leveraging either 3D current collectors or liquid metal modification on current collectors. In this work, both strategies are simultaneously leveraged to design a 3D Cu foam with liquid metal modification (LM@Cu) for dendrite-free sodium plating. The 3D configuration of Cu effectively reduces local current density and evenly distributes electric fields, while the introduction of liquid metal enhances the sodiophilicity of Cu to lower the nucleation barrier for sodium, thereby promoting its uniform plating. As a result, symmetric cells of Na with LM@Cu maintain stable cycling for over 2800 h. Additionally, full cells comprising Na-LM@Cu and Na3V2(PO4)3 sustain 97.5% of the capacity upon 1000 cycles, underscoring the great potentiality of liquid metal-mediated 3D current collectors in energy storage.

Natural Silkworm Cocoon-Derived Separator with Na-Ion De-Solvated Function for Sodium Metal Batteries.

The commercialization of sodium batteries faces many challenges, one of which is the lack of suitable high-quality separators. Herein, we presented a novel natural silkworm cocoon-derived separator (SCS) obtained from the cocoon inner membrane after a simple degumming process. A Na||Na symmetric cell assembled with this separator can be stably cycled for over 400 h under test conditions of 0.5 mA cm-2-0.5 mAh cm-2. Moreover, the Na||SCS||Na3V2(PO4)3 full cell exhibits an initial capacity of 79.3 mAh g-1 at 10 C and a capacity retention of 93.6% after 1000 cycles, which far exceeded the 57.5 mAh g-1 and 42.1% of the full cell using a commercial glass fiber separator (GFS). The structural origin of this excellent electrochemical performance lies in the fact that cationic functional groups (such as amino groups) on silkworm proteins can de-solvate Na-ions by anchoring the ClO4- solvent sheath, thereby enhancing the transference number, transport kinetics and deposition/dissolution properties of Na-ions. In addition, the SCS has significantly better mechanical properties and thinness indexes than the commercial GFS, and, coupled with the advantages of being natural, cheap, non-polluting and degradable, it is expected to be used as a commercialized sodium battery separator material.

Interdependence of Support Wettability -Electrodeposition Rate - Sodium Metal Anode and SEI Microstructure.

This study examines how current collector support chemistry (sodiophilic intermetallic Na2Te vs. sodiophobic baseline Cu) and electrodeposition rate affect microstructure of sodium metal and its solid electrolyte interphase (SEI). Capacity and current (6 mAh cm-2, 0.5-3 mA cm-2) representative of commercially relevant mass loading in anode-free sodium metal battery (AF-SMBs) are analyzed. Synchrotron X-ray nanotomography and grazing-incidence wide-angle X-ray scattering (GIWAXS) are combined with cryogenic focused ion beam (cryo-FIB) microscopy. Highlighted are major differences in film morphology, internal porosity, and crystallographic preferred orientation e.g. (110) vs. (100) and (211) with support and deposition rate. Within the SEI, sodium fluoride (NaF) is more prevalent with Te-Cu versus sodium hydride (NaH) and sodium hydroxide (NaOH) with baseline Cu. Due to competitive grain growth the preferred orientation of sodium crystallites depends on film thickness. Mesoscale modelling delineates the role of SEI (ionic conductivity, morphology) on electrodeposit growth and onset of electrochemical instability.

Weakly Coordinating Diluent Modulated Solvation Chemistry for High-Performance Sodium Metal Batteries.

Diluents have been extensively employed to overcome the disadvantages of high viscosity and sluggish kinetics of high-concentration electrolytes, but generally do not change the pristine solvation structure. Herein, a weakly coordinating diluent, hexafluoroisopropyl methyl ether (HFME), is applied to regulate the coordination of Na+ with diglyme and anion and form a diluent-participated solvate. This unique solvation structure promotes the accelerated decomposition of anions and diluents, with the construction of robust inorganic-rich electrode-electrolyte interphases. In addition, the introduction of HFME reduces the desolvation energy of Na+, improves ionic conductivity, strengthens the antioxidant, and enhances the safety of the electrolyte. As a result, the assembled Na||Na symmetric cell achieves a stable cycle of over 1800 h. The cell of Na||P'2-Na0.67MnO2 delivers a high capacity retention of 87.3 % with a high average Coulombic efficiency of 99.7 % after 350 cycles. This work provides valuable insights into solvation chemistry for advanced electrolyte engineering.

Na-K Alloy Anode for High-Performance Solid-State Sodium Metal Batteries.

Rechargeable solid-state Na metal batteries (SSNMB) can offer high operational safety and energy density. However, poor solid-solid contact between the electrodes and the electrolyte can dramatically increase interfacial resistance and Na dendrite formation, even at low current rates. Therefore, we developed a carbon-fiber-supported liquid Na-K alloy anode that ensures close anode-electrolyte contact, enabling superior cycle stability and rate capability. We then demonstrated the first cryogenic transmission electron microscopy (cryo-TEM) characterization of an SSNMB, capturing the evolution of solid-electrolyte interphase (SEI) and revealing both crystalline and amorphous phases, which could facilitate ion transport and prevent continuous side reactions. By enhancing contact between the Na-K alloy and solid-state electrolyte, these symmetric cells are capable of cycling for over 800 h without notable increased polarization and enable an unprecedented critical current density (CCD) at 40 mA cm-2. Our liquid Na-K alloy approach offers a promising strategic avenue toward commercial SSNMBs.

Molecular Engineering on Solvation Structure of Carbonate Electrolyte toward Durable Sodium Metal Battery at -40 °C.

Carbonate electrolytes have excellent chemical stability and high salt solubility, which are ideally practical choice for achieving high-energy-density sodium (Na) metal battery at room temperature. However, their application at ultra-low temperature (-40 °C) is adversely affected by the instability of solid electrolyte interphase (SEI) formed by electrolyte decomposition and the difficulty of desolvation. Here, we designed a novel low-temperature carbonate electrolyte by molecular engineering on solvation structure. The calculations and experimental results demonstrate that ethylene sulfate (ES) reduces the sodium ion desolvation energy and promotes the forming of more inorganic substances on the Na surface, which promote ion migration and inhibit dendrite growth. At -40 °C, the Na||Na symmetric battery exhibits a stable cycle of 1500 hours, and the Na||Na3 V2 (PO4 )3 (NVP) battery achieves 88.2 % capacity retention after 200 cycles.

Room-Temperature All-Solid-State Sodium Battery Based on Bulk Interfacial Superionic Conductor.

All-solid-state sodium batteries (ASSSBs) are attractive alternatives to lithium-ion batteries for grid-scale energy storage due to their high safety and ubiquitous distribution of Na sources. A critical component for ASSSB is sodium-ion conducting solid-state electrolyte (SSE). Here, we report a high-performance sodium-ion SSE with the recently developed bulk interfacial superionic conductor (BISC) concept. The ionic conductivity and areal conductance of the Na+ BISC at 25 °C reaches 6.5 × 10-4 S cm-1 and 260 mS cm-2, respectively. Using NaxCo0.7Mn0.3O2 (x ≈ 1.0, NaCMO) as the cathode active material, all-solid-state Na||NaCMO batteries exhibiting small overpotential and ∼180 cycle life are demonstrated under room temperature. This approach may also be used to prepare other metal ion, such as Mg2+, Al3+, and K+, based all-solid-state batteries.

Interfacial Protection Engineering of Sodium Nanoparticles toward Dendrite-Free and Long-Life Sodium Metal Battery.

The instability of interfacial solid-electrolyte interphase (SEI) layer of metallic sodium (Na) anode during cycles results in the rapid capacity decay of sodium metal batteries (SMBs). Herein, the concept of interfacial protection engineering of Na nanoparticles (Na-NPs) is proposed first to achieve stable, dendrite-free, and long-life SMB. Employing an ion-exchange strategy, conformal Sn-Na alloy-SEI on the interface of Na-NPs is constructed, forming Sn@Na-NPs. The stable alloy-based SEI layer possesses the following three advantages: 1) significantly enhancing the transport dynamics of Na+ ions and electrons; 2) enabling the well-distributed deposition of Na+ ions to avoid the growth of dendrites; and 3) protecting the Sn@Na-NPs anode from the attack of electrolyte, thereby reducing the parasitic reaction and boosting the Coulombic efficiency of SMBs. Because of these virtues, the symmetric Sn@Na-NPs cell shows an ultralow voltage hysteresis of 0.54 V at 10 mA cm-2 after 600 h. Paired with the Na3 V2 (PO4 )2 O2 F (NaVPF) cathode, the NaVPF-Sn@Na-NPs full cell exhibits an initial discharge capacity of 89.2 mAh g-1 at 1 C and a high capacity retention of 81.6% after 600 cycles.

Intercalation-Induced Localized Conversion Reaction in h-CuSe for Ultrafast-Rechargeable and Long-Cycling Sodium Metal Battery.

Cathode materials of sodium-based batteries with high specific capacity and fast charge-discharge mode, as well as ultralong reversible cycles at wide applied temperatures, are essential for future development of advanced energy storage system. Developing transition metal selenides with intercalation features provides a new strategy for realizing the above cathode materials. Herein, this work reports a storage mechanism of sodium ion in hexagonal CuSe (h-CuSe) based on the density functional theory (DFT) guidance. This work reveals that the two-dimensional ion intercalation triggers localized redox reaction in the h-CuSe bulk phase, termed intercalation-induced localized conversion (ILC) mechanism, to stabilize the sodium storage structure by forming localized Cu7Se4 transition phase and adjusting the near-edge coordination state of the Cu sites to achieve high reversible capacity and ultra-long cycling life, while allowing rapid charge-discharge cycling over a wide temperature range.

Regulating Sodium Deposition Behavior by a Triple-Gradient Framework for High-Performance Sodium Metal Batteries.

An efficient method for the synthesis of a self-supporting carbon framework (denoted Gra-GC-MoSe2) is proposed with a triple-gradient structure-in sodiophilic sites, pore volume, and electrical conductivity-which facilitates the highly efficient regulation of Na deposition. In situ and ex situ measurements, together with theoretical calculations, reveal that the gradient distribution of Se heteroatoms in MoSe2, and its derivatives tailor the sodiophilicity, while the gradient distribution of porous nanostructures homogenizes the Na+ diffusion. Therefore, Na deposition occurs from the bottom to the top of the Gra-GC-MoSe2 framework without dendrite formation. In addition, the gradient in electrical conductivity ensures the stripping process does not lead to dead Na. As a result, a Gra-GC-MoSe2 modified Na anode (Na@Gra-GC-MoSe2) shows impressive cycling stability with a high average Coulombic efficiency in an asymmetric cell. In symmetric cells, it also exhibits a long cycling life of 2000 h with a low polarization voltage and works stably even under a large capacity of 10 mAh cm-2. Moreover, a Na@Gra-GC-MoSe2|| Na3V2(PO4)3 full cell delivers a high energy density with an excellent cycling performance.

Aggregate-Dominated Dilute Electrolytes with Low-Temperature-Resistant Ion-Conducting Channels for Highly Reversible Na Plating/Stripping.

Developing rechargeable batteries with high power delivery at low temperatures (LT) below 0 °C is significant for cold-climate applications. Initial anode-free sodium metal batteries (AFSMBs) promise high LT performances because of the low de-solvation energy and smaller Stokes radius of Na+, nondiffusion-limited plating/stripping electrochemistry, and maximized energy density. However, the severe reduction in electrolyte ionic conductivity and formation of unstable solid electrolyte interphase (SEI) hinder their practical applications at LT. In this study, a 2-methyltetrahydrofuran-based dilute electrolyte is designed to concurrently achieve an anion-coordinated solvation structure and impressive ionic conductivity of 3.58 mS cm-1 at -40 °C. The dominant aggregate solvates enable the formation of highly efficient and LT-resistant Na+ hopping channels in the electrolyte. Moreover, the methyl-regulated electronic structure in 2-methyltetrahydrofuran induces gradient decomposition toward an inorganic-organic bilayer SEI with high Na+ mobility, composition homogeneity, and mechanical robustness. As such, a record-high Coulombic efficiency beyond 99.9% is achieved even at -40 °C. The as-constructed AFSMBs sustain 300 cycles with 80% capacity maintained, and a 0.5-Ah level pouch cell delivers 85% capacity over 180 cycles at -25 °C. This study affords new insights into electrolyte formulation for fast ionic conduction and superior Na reversibility at ultralow temperatures.

Multiple Na+ transport pathways and interfacial compatibility enable high-capacity, room-temperature quasi-solid sodium batteries.

Sodium-metal batteries (SMBs) are ideal for large-scale energy storage due to their stable operation and high capacity. However, they have safety issues caused by severe dendrite growth and side reactions, particularly when using liquid electrolytes. Therefore, it is critically important to develop electrolytes with high ionic conductivity and improved safety that are non-flammable and resistant to dendrites. Here, we developed polymerized polyethylene glycol diacrylate (PEGDA)-modified poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) electrolytes (PPEs) with highly conductive sodium bis(trifluoromethanesulfonyl)imide and corrosion-inhibitive sodium bis(oxalato)borate salts for SMBs. Well-complexed PEGDA not only increases the amorphicity of the PVDF matrix, but also offers numerous Lewis basic sites through the polar groups of carbonyl and ether groups (i.e., electron donors). The presence of the Lewis basic sites facilitates the dissociation of sodium salt and transportation of Na+ within the PVDF matrix. This results in the generation of additional Na+ transport pathways, which can enhance the performance of the battery. Among PPEs, the optimized PPE-50 exhibits a high ionic conductivity of 3.42 × 10-4 S cm-1 and a mechanical strength of 14.0 MPa. A Na||Na symmetric cell with PPE-50 displays high stability at 0.2 mA cm-2 for 800 h. PPE-50 further displays high capacity, e.g., a Na3V2(PO4)3|PPE-50|Na battery delivers a decent discharge capacity of 101.5 mAh g-1 at 1.0C after 650 cycles. Our work demonstrates the development of high-performance quasi-solid polymer electrolytes with multiple transport pathways suitable for room-temperature SMBs.

Restructuring Electrolyte Solvation by a Versatile Diluent Toward Beyond 99.9% Coulombic Efficiency of Sodium Plating/Stripping at Ultralow Temperatures.

The reversible and durable operation of sodium metal batteries at low temperatures (LT) is essential for cold-climate applications but is plagued by dendritic Na plating and unstable solid-electrolyte interphase (SEI). Current Coulombic efficiencies of sodium plating/stripping at LT fall far below 99.9%, representing a significant performance gap yet to be filled. Here, the solvation structure of the conventional 1 m NaPF6 in diglyme electrolyte by facile cyclic ether (1,3-dioxolane, DOL) dilution is efficiently reconfigured. DOL diluents help shield the Na+-PF6 - Coulombic interaction and intermolecular forces of diglyme, leading to anomalously high Na+-ion conductivity. Besides, DOL participates in the solvation sheath and weakens the chelation of Na+ by diglyme for facilitated desolvation. More importantly, it promotes concentrated electron cloud distribution around PF6 - in the solvates and promotes their preferential decomposition. A desired inorganic-rich SEI is generated with compositional uniformity, high ionic conductivity, and high Young's modulus. Consequently, a record-high Coulombic efficiency over 99.9% is achieved at an ultralow temperature of -55 °C, and a 1 Ah capacity pouch cell of initial anode-free sodium metal battery retains 95% of the first discharge capacity over 100 cycles at -25 °C. This study thus provides new insights for formulating electrolytes toward increased Na reversibility at LT.

Role of Interface Strain and Chemo-Mechanical Effects during Electroplating in Sodium Metal Battery Anodes.

In this study, we demonstrate that elastic strain applied to a current collector can influence the overall thermodynamic and kinetic picture of sodium metal electrodeposition and hence the performance of a sodium metal battery. To controllably study the role of strain in electrochemical performance, we utilize NiTi foil as a stable current collector, nucleation interface, and superelastic material. Our findings demonstrate that a locked-in elastic tensile strain near 8% results in 40 mV lower onset potential for sodium electrodeposition, 19% decrease in charge transfer resistance, and 20% lower cumulative sodium loss, among other effects. These performance improvements are correlated primarily to the control of the irreversible behavior in the first few minutes of electroplating. Given the prevalence of strain buildup in commercial battery cell configurations, our work highlights that strained current collector interfaces can result in significant long-term chemo-mechanical performance outcomes broadly relevant to sodium and other metal battery design considerations.

Highly Performing Sodium Metal Batteries Reinforced by a Self-Regulated Dual-Layered Solid Electrolyte Interphase via a Metal-Organic Framework.

Sodium-metal batteries, heralded for high energy density and cost-effectiveness, are compromised by an unstable solid electrolyte interphase (SEI) and dendrite formation, which hinder practical applications. Herein, a zirconium-based metal-organic framework nanostructure coating (ZMOF-NSC) was constructed in a low-loss, flexible manner. Comprehensive studies show that ZMOF-NSC, with its periodically ordered nanochannels and organized pore structures, enhances ion transport and decreases the Na+ migration energy barrier, thus ensuring uniform ion flux and achieving uniform spherical deposition. Additionally, ZMOF-NSC facilitates partial desolvation, catalyzing the formation of an inorganic-rich, dual-layered SEI that effectively protects the anode and suppresses dendrite formation. Consequently, the ZMOF-NSC@Na symmetric battery exhibits an impressive lifespan of over 2500 h, demonstrating extended operational longevity. The Na3V2(PO4)3∥ZMOF-NSC@Na batteries demonstrate exceptional cycling stability with 81% capacity retention after 2000 cycles at 10 C, maintaining stability over 3000 cycles at 20 C. Moreover, the NVP∥ZMOF-NSC@Na battery achieves an energy density of 370 Wh kg-1 and a power density of 10,484 W kg-1, indicating superior durability and performance. This significant finding highlights the significant potential of structured MOFs to induce a dual-layered SEI, advancing the commercialization of durable, dendrite-free sodium metal batteries. The precise design of self-assembled pore structures and surface active sites in MOFs demonstrates significant potential in advancing the commercialization of durable, dendrite-free electrodes of metal-based rechargeable batteries.

NaSn2F5 nanocluster composed of nanoparticles with matched lattices induced by dislocations: Accelerated sodium-ion transport via in situ oxidation in solid-state sodium metal battery.

Na metal batteries using inorganic solid-state electrolytes (SSEs) have attracted extensive attention due to their superior safety and high energy density. However, their development is plagued by the unclear structural/volumetric evolution of SSEs and the corresponding Na+ migration mechanisms. In this work, NaSn2F5 (NSF) clusters are composed of nanoparticles (NPs) with matched lattices induced by dislocations, which can mitigate the volume swelling/shrinkage of the NPs. NSF behaves like a single ion conductor with a high Na+ transference number (tNa+) of 0.79. Specially, the ionic conductivity (σ) of NSF is increased from 7.64 × 10-6 to 5.42 × 10-5 S cm-1 after partial irreversible oxidation of Sn2+ (0.118 Å) â†’ Sn4+ (0.069 Å) with the shrunk ionic radius during the charge process, giving more spaces for Na+ migration. Furthermore, a poly(acrylonitrile)-NaSn2F5-NaPF6 composite polymer electrolyte (NSF CPE) was fabricated with a σ of 4.13 × 10-4 S cm-1 and a tNa+ of 0.60. The NSF CPE-based symmetric cell can operate over 3000 h due to the couplings between the different components in NSF CPE, which is beneficial for ion transfer and the construction of stable solid electrolyte interface. And the quasi-solid-state Na|NSF CPE|Na3V2(PO4)3 full cell displays excellent electrochemical performance.

Understanding Na+ Diffusion, Physicochemical Behavior, and Electrochemical Performance of a Gel Polymer Electrolyte.

Gel polymer electrolytes (GPEs) represent a credible alternative to organic liquid electrolytes (LEs) for safer sodium metal batteries. As a compromise between solid polymer electrolytes and LEs, GPEs ensure a good ionic conductivity, improve the electrolyte/electrode interface, and prevent solvent leaks. Herein, a GPE based on acrylate-bifunctionalized polyethylene glycol chains mixed with an ether solvent (TEGDME) and a polyethylene glycol diacrylate (PEG600DA) in a 50/50 wt % ratio was prepared by ultraviolet photopolymerization. Sodium bis(fluorosulfonyl)imide salt (NaFSI) was added at different concentrations to study its interactions with the solvent and/or the cross-linked polymer. Infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and swelling ratio characterizations were combined to determine the physicochemical properties of the GPE. Complementary characterizations including electrochemical impedance spectroscopy, chronopotentiometry, and cyclic voltammetry allowed correlating the physicochemical properties of the GPE to its electrochemical performance. Then, improvements were obtained by careful combination of its components. The cross-linking agent allowed us to obtain a polymer matrix that traps the organic solvent and prevents leakage. Such a solvent inclusion reduces the rigidity of the membrane and lowers its viscosity, offering a room temperature ionic conductivity of 4.8 × 10-4 Ω-1 cm-1. The control of polymer's tortuosity leads to a stable cycling vs sodium metal over several hundred hours without increase of the polarization. Finally, optimization of the salt loading plays a major role in electrostatic cross-linking, leading to an improvement of the mechanical properties of the GPE without reducing its conductivity.

Concentrated Nonaqueous Polyelectrolyte Solutions: High Na-Ion Transference Number and Surface-Tethered Polyanion Layer for Sodium-Metal Batteries.

Na metal is a promising anode material for the preparation of next-generation high-energy-density sodium-ion batteries; however, the high reactivity of Na metal severely limits the choice of electrolyte. In addition, rapid charge-discharge battery systems require electrolytes with high Na-ion transport properties. Herein, we demonstrate a stable and high-rate sodium-metal battery enabled by a nonaqueous polyelectrolyte solution composed of a weakly coordinating polyanion-type Na salt, poly[(4-styrenesulfonyl)-(trifluoromethanesulfonyl)imide] (poly(NaSTFSI)) copolymerized with butyl acrylate, in a propylene carbonate solution. It was found that this concentrated polyelectrolyte solution exhibited a remarkably high Na-ion transference number (tNaPP = 0.9) and a high ionic conductivity (σ = 1.1 mS cm-1) at 60 °C. Furthermore, the surface of the Na electrode was modified with polyanion chains anchored via the partial decomposition of the electrolyte. The surface-tethered polyanion layer effectively suppressed the subsequent decomposition of the electrolyte, thereby enabling stable Na deposition/dissolution cycling. Finally, an assembled sodium-metal battery with a Na0.44MnO2 cathode demonstrated an outstanding charge/discharge reversibility (Coulombic efficiency >99.8%) over 200 cycles while also exhibiting a high discharge rate (i.e., 45% capacity retention at 10 mA cm-2).

NaBr-Assisted Sintering of Na3Zr2Si2PO12 Ceramic Electrolyte Stabilizes a Rechargeable Solid-state Sodium Metal Battery.

Solid-state metal batteries with nonflammable solid-state electrolytes are regarded as the next generation of energy storage technology on account of their high safety and energy density. However, as for most solid electrolytes, low room temperature ionic conductivity and interfacial issues hinder their practical application. In this work, Na super ionic conductor (NASICON)-type Na3Zr2Si2PO12 (NZSP) electrolytes with improved ionic conductivity are synthesized by the NaBr-assisted sintering method. The effects of the NaBr sintering aid on the crystalline phase, microstructure, densification degree, and electrical performance as well as the electrochemical performances of the NZSP ceramic electrolyte are investigated in detail. Specifically, the NZSP-7%NaBr-1150 ceramic electrolyte has an ionic conductivity of 1.2 × 10-3 S cm-1 (at 25 °C) together with an activation energy of 0.28 eV. A low interfacial resistance of 35 Ω cm2 is achieved with the Na/NZSP-7%NaBr-1150 interface. Furthermore, the Na/NZSP-7%NaBr-1150/Na3V2(PO4)3 battery manifests excellent cycling stability with a capacity retention of 98% after 400 cycles at 1 C and 25 °C.

A PVDF/g-C3N4-Based Composite Polymer Electrolytes for Sodium-Ion Battery.

As one of the most promising candidates for all-solid-state sodium-ion batteries and sodium-metal batteries, polyvinylidene difluoride (PVDF) and amorphous hexafluoropropylene (HFP) copolymerized polymer solid electrolytes still suffer from a relatively low room temperature ionic conductivity. To modify the properties of PVDF-HEP copolymer electrolytes, we introduce the graphitic C3N4 (g-C3N4) nanosheets as a novel nanofiller to form g-C3N4 composite solid polymer electrolytes (CSPEs). The analysis shows that the g-C3N4 filler can not only modify the structure in g-C3N4CSPEs by reducing the crystallinity, compared to the PVDF-HFP solid polymer electrolytes (SPEs), but also promote a further dissociation with the sodium salt through interaction between the surface atoms of the g-C3N4 and the sodium salt. As a result, enhanced electrical properties such as ionic conductivity, Na+ transference number, mechanical properties and thermal stability of the composite electrolyte can be observed. In particular, a low Na deposition/dissolution overpotential of about 100 mV at a current density of 1 mA cm-2 was found after 160 cycles with the incorporation of g-C3N4. By applying the g-C3N4 CSPEs in the sodium-metal battery with Na3V2(PO4)3 cathode, the coin cell battery exhibits a lower polarization voltage at 90 mV, and a stable reversible capacity of 93 mAh g-1 after 200 cycles at 1 C.