High‑Performance Na4Mn9O18/Carbon Nanotube Cathodes for Aqueous Sodium‑Ion Batteries

Background

Li‑ion batteries (LIBs) dominate portable electronics due to their high energy density, yet the scarcity of lithium and the cost of extraction pose long‑term challenges. Aqueous LIB systems, first reported by the Dahn group in 1994, have attracted renewed interest because of their low cost, intrinsic safety, and high‑rate capability.1–3 However, most early research focused on lithium‑based electrodes, overlooking the potential of sodium, which is far more abundant and cheaper.4–8

Replacing lithium with sodium gives rise to aqueous sodium‑ion batteries (ASIBs), which combine the advantages of aqueous chemistry with the low cost of sodium.9–12 Promising cathode chemistries for ASIBs include Na₃V₂(PO₄)₃, Na₂FeP₂O₇, Na₂CuFe(CN)₆, and Na₄Mn₉O₁₈ (NMO).13,14 Na₄Mn₉O₁₈ is particularly attractive because its orthorhombic lattice contains two distinct tunnel types (S‑type and O‑type) that facilitate Na⁺ diffusion.15,16 Nonetheless, NMO suffers from low electronic conductivity and pronounced volume expansion during Na⁺ insertion/extraction, leading to pulverization and rapid capacity fade.17–19

One effective strategy to overcome these limitations is to embed NMO within a conductive carbon scaffold that buffers volume changes and enhances charge transport. Carbon nanotubes (CNTs) are ideal for this role, forming a percolating network that boosts electron pathways and mechanical resilience.20–23 Additionally, the morphology of the cathode—particle size, shape, and surface area—critically influences electrochemical performance. Micron‑sized, uniform spheres typically deliver higher tap density and faster ion transport than irregular powders.24–26 Spray‑drying is a scalable technique for producing such microspheres, yet it has been underexplored in sodium‑ion research.

In this work, we report the first spray‑drying synthesis of NMO/CNT microspheres and assemble them into an ASIB with a Zn anode and a Na⁺/Zn²⁺ mixed‑ion electrolyte. The resulting cathode exhibits remarkable rate capability and long‑term cyclability, underscoring the synergy between the CNT network and the microspherical architecture.

Methods

Material Preparation

First, 4.0 mg of a 9 wt % CNT aqueous dispersion was combined with 30 mL of 0.1 M KMnO₄ and 3.0 M NaOH solution under vigorous stirring. After adding 30 mL of 0.28 M MnSO₄, a brown precipitate formed immediately. This precipitate was centrifuged, aged for 24 h, and then dispersed in 100 mL of 15 M NaOH, stirred for 25 min, and hydrothermally treated at 180 °C for 24 h in a stainless‑steel autoclave with a Teflon liner. The resulting precursor was washed with deionized water and dried at 80 °C.

To fabricate the final NMO/CNT composite, 0.6 g of the precursor was ultrasonicated in 150 mL of water for 15 min to form a brown suspension. The suspension was fed into a Holve v spray‑dryer (peristaltic pump, 6 mL min⁻¹) and atomized at 205 °C with a two‑fluid nozzle (0.8 MPa) and outlet temperature of 110 °C. The resulting powder consisted of uniform microspheres (~5–7 µm) containing CNTs intertwined with NMO rods. A reference NMO sample without CNT was prepared under identical conditions.

Material Characterization

Powder X‑ray diffraction (XRD, Bruker D8 Discover, Cu Kα) confirmed the orthorhombic NMO phase (JCPDS #27‑0750). Thermogravimetric analysis (TGA, TA SDT Q‑600) measured the CNT content (≈13 wt %) and verified the thermal stability of the composite. Raman spectroscopy (Jobin‑Yvon T6400, 532‑nm laser) revealed characteristic D and G bands of CNTs and Mn–O stretching vibrations of NMO. Scanning electron microscopy (SEM, Hitachi S‑4800) and high‑resolution TEM (HR‑TEM, JEOL JEM‑2800) illustrated the microspherical morphology, rod‑shaped NMO nanostructure (~30–50 nm), and the interpenetrating CNT network. Selected‑area electron diffraction (SAED) patterns confirmed the crystallinity of NMO and the presence of CNTs. Inductively coupled plasma optical emission spectroscopy (ICP‑OES, PRODIGY XP) quantified Mn dissolution in the electrolyte.

Electrochemical Measurements

The NMO/CNT electrode was fabricated by mixing 80 wt % composite, 10 wt % acetylene black, and 10 wt % PVDF in N‑methyl‑2‑pyrrolidone (NMP). The slurry was coated on a carbon‑foil current collector and dried at 75 °C for 12 h. Coin cells (2025) were assembled with Zn foil anode, a 1 M Na₂SO₄/0.5 M ZnSO₄ electrolyte (pH = 4), and a glass‑mat separator. Electrochemical testing (Neware) was conducted in the 1–1.85 V (vs. Zn²⁺/Zn) window. Cyclic voltammetry (CV) was performed at 0.1 mV s⁻¹, and electrochemical impedance spectroscopy (EIS) was measured over 0.01–100 kHz.

Results and Discussion

The XRD patterns of NMO/CNT and pristine NMO (Fig. 2a) match the standard orthorhombic phase, with additional broad peaks at ~26° and 44° confirming the presence of graphitic CNT layers. Raman spectra (Fig. 2b) display the Mn–O stretch (600–650 cm⁻¹) and the CNT D (≈1347 cm⁻¹) and G (≈1575 cm⁻¹) bands, confirming the composite’s dual‑phase nature. TGA (Fig. 2c) indicates ~87 wt % NMO and ~13 wt % CNT.

SEM images (Fig. 3a) reveal densely packed microspheres (~5–7 µm) with intertwined CNTs. TEM (Fig. 3b) shows rod‑shaped NMO (~30–50 nm) ensnared by CNTs, while HRTEM (Fig. 3c) resolves lattice fringes corresponding to NMO (0.45 nm) and CNT (0.33 nm). SAED (Fig. 3d) confirms the single‑crystal nature of NMO and the homogeneous CNT distribution.

CV curves (Fig. 4a) display two reduction peaks at ~1.20 and 1.37 V and a single oxidation peak at ~1.53 V during the first scan, indicative of Na⁺ de‑insertion/ insertion in the orthorhombic lattice. Subsequent cycles exhibit symmetrical redox couples (1.50/1.20 V and 1.62/1.37 V), underscoring reversible Na⁺ dynamics. The minor oxidation peak near 2 V arises from water decomposition.

Galvanostatic cycling (Fig. 4b) at 4 C shows an initial charge capacity of 35.8 mAh g⁻¹ and discharge capacity of 85.6 mAh g⁻¹, yielding a high initial coulombic efficiency (~239 %). After the first cycle, the capacity stabilizes, and the coulombic efficiency approaches 100 %. At 4 C, the NMO/CNT cathode delivers 53.2 mAh g⁻¹ after 150 cycles, outperforming pristine NMO (40 mAh g⁻¹). The enhanced performance is attributed to the CNT‑mediated electron transport and the sphere‑shaped morphology that facilitates Na⁺ diffusion.

Replacing the mixed‑ion electrolyte with 1 M Na₂SO₄ alone reduces the 150‑cycle capacity to ~24 mAh g⁻¹, highlighting the importance of the Zn²⁺ component in stabilizing the electrode reaction.

To isolate the effect of morphology, a non‑spherical NMO/CNT composite was prepared via ball‑milling. Its cycling performance (Fig. 5c) is markedly inferior to the spray‑drying product, confirming that microspherical architecture is essential for high capacity.

Rate capability tests (Fig. 6) demonstrate reversible capacities of 96, 77, 66, and 58 mAh g⁻¹ at 1, 2, 3, and 4 C, respectively. The capacity recovers to ~50 mAh g⁻¹ when the rate returns to 1 C, evidencing excellent structural robustness.

EIS (Fig. 7) reveals a charge‑transfer resistance (R_CT) of 133 Ω for NMO/CNT, significantly lower than 207 Ω for NMO, confirming the conductivity benefit of CNTs. R_CT remains stable over 100 cycles, indicating minimal impedance growth.

Compared with other Na_1−xMnO₂‑based cathodes (Table 2), the spray‑drying NMO/CNT delivers superior capacity and rate performance, particularly at high current densities.

Conclusions

The spray‑drying route successfully produced NMO/CNT microspheres that, when paired with a Zn anode and a Na⁺/Zn²⁺ electrolyte, form an ASIB with high reversible capacity (96 mAh g⁻¹ at 1 C, 53.2 mAh g⁻¹ at 4 C) and excellent cyclability (150 cycles). The combination of a CNT conductive network and a uniform spherical morphology underpins the observed performance gains. These results position NMO/CNT as a promising cathode for safe, high‑rate aqueous sodium‑ion batteries.

Abbreviations

ASIBs:

Aqueous sodium‑ion batteries

CV:

Cyclic voltammetry

EIS:

Electrochemical impedance spectroscopy

HR‑TEM:

High‑resolution transmission electron microscopy

LIBs:

Lithium‑ion batteries

NMO:

Na₄Mn₉O₁₈

NMO/CNT:

Na₄Mn₉O₁₈/carbon nanotube

NMP:

N‑methyl‑2‑pyrrolidone

PVDF:

Polyvinylidene fluoride

SAED:

Selected area electron diffraction

SEM:

Scanning electron microscopy

TG:

Thermo‑gravimetric analysis

XRD:

X‑ray powder diffraction