Carbonized Leaf Cathodes with Melt‑Diffusion Selenium for High‑Performance Sodium‑Selenium Batteries

Introduction

The surge of portable electronics and electric vehicles demands batteries that combine high energy density with long‑term stability. Lithium‑ion batteries dominate today, but their cost and resource constraints limit scalability. Sodium‑ion batteries (SIBs) emerge as a compelling alternative, leveraging abundant Na resources and comparable electrochemical potential. Among SIB chemistries, sodium‑selenium (Na‑Se) cells have attracted recent attention due to selenium’s superior electronic conductivity (10⁻³ S cm⁻¹) and high theoretical energy density (~3.3 kWh kg⁻¹). However, the polyselenide shuttle, akin to the polysulfide problem in Li‑S batteries, hampers cycle life. Encapsulating Se within conductive, porous carbons is a proven strategy to suppress this shuttle. Traditional approaches rely on complex, multi‑step syntheses and hazardous additives, whereas renewable biomass—such as leaves—offers heteroatom‑rich, hierarchically porous carbon precursors that can be processed in a single furnace step. Here, we report a free‑standing Se‑impregnated leaf cathode that achieves 84 % of selenium’s theoretical capacity and remarkable rate capability, paving the way for sustainable, high‑performance Na‑Se batteries.

Methods

Preparation of Carbonized Leaf

Dry leaf wafers (17 mm diameter) were sealed between ceramic slides to prevent curling during pyrolysis. The wafers were carbonized at 800 °C for 2 h under N₂ (5 °C min⁻¹ ramp). The resulting R800 film was acid‑washed in 3 M HCl (12 h) to remove inorganic salts, then treated with 1 M KOH (12 h) and activated at 600 °C for 2 h under N₂, yielding the porous R800A. Samples were rinsed with deionized water and dried at 70 °C.

Preparation of Se‑R800A

A layer of Se powder was placed in a porcelain boat; the R800A film was suspended above it in a sealed tube. With a 2:1 Se:C weight ratio, Se was melted at 260 °C under N₂ for 10 h to promote vapor diffusion into the carbon matrix. Final Se loading was quantified by TGA.

Materials Characterization

Morphology was examined by SEM, FE‑SEM, and TEM. Structure and Raman spectra were obtained by XRD (Cu‑Kα) and a Renishaw Raman microscope. N₂ adsorption–desorption isotherms (BELSORP‑max) provided BET surface areas and pore distributions. XPS (Thermo K‑Alpha⁺) assessed chemical states.

Electrochemical Measurements

Coin cells (CR2032) were assembled in an Ar glove box (O₂/H₂O < 1 ppm). Na foil served as counter electrode; Whatman GF separator; electrolyte 1 M NaClO₄ in EC/DEC (1:1). The Se‑R800A film was used directly as the cathode, no binder or conductive additive. CVs were recorded on a CHI660D; galvanostatic charge–discharge tests (0.005–3.0 V) were conducted on a Land CT‑2001A. EIS (CHI 760D) measured impedance (5 mV, 10⁻²–10⁵ Hz). GITT assessed Na⁺ diffusion by 10‑min charge/discharge steps followed by 40‑min rest. All capacities were calculated per Se mass.

Results and Discussion

Fabrication of Se‑R800A. The free‑standing electrode is produced by successive pyrolysis, KOH activation, and Se vapor impregnation (Fig. 1). After carbonization at 800 °C, the wafer shrinks modestly (17 mm → 12 mm) but thins dramatically (800 µm → 240 µm) with 74 % weight loss. KOH activation reduces the mass by ≈9 %, while Se impregnation increases it by 90 % (Fig. 1d,e). The melt‑diffusion strategy ensures uniform Se distribution without isolated particles.

Microstructure. Optical and SEM images reveal that the leaf retains its natural hierarchical porosity: a flat upper surface, a stomata‑rich backside, and an internal sponge of palisade and spongy cells (Fig. 2). Cross‑sectional TEM shows overlapping carbon sheets (< 100 nm) that shorten Na⁺ diffusion paths. The Se‑R800A electrode maintains this architecture; HRTEM confirms lattice fringes (0.2 nm) corresponding to Se (111) planes, indicating successful infiltration.

Elemental Mapping. EDX mapping (Fig. 3) shows a homogeneous Se distribution throughout the film, with N and O from the biochar further stabilizing polyselenides through heteroatom doping.

Structural Analysis. XRD patterns (Fig. 4a) show the disappearance of crystalline Se peaks, confirming amorphous Se dispersion. Raman spectra (Fig. 4b) exhibit broad C–Se stretching bands (250–300 cm⁻¹) and an I_D/I_G ratio of 0.92, indicating moderate graphitization and high conductivity. BET surface areas rise from 270 m² g⁻¹ (R800) to 934 m² g⁻¹ (R800A), then drop to 434 m² g⁻¹ after Se loading, reflecting pore filling. TGA reveals 47 % Se content in Se‑R800A. XPS confirms Se–O–C bonding, which anchors Se and suppresses the shuttle.

Electrochemical Performance. CVs (Fig. 5a) display a single cathodic peak at ~1.2 V (Se → Na₂Se) and a single anodic peak at 1.7 V (Na₂Se → Se), indicative of a reversible, one‑step conversion and effective shuttle suppression. Galvanostatic curves (Fig. 5b) show an initial discharge capacity of 1,100 mAh g⁻¹, settling to 700 mAh g⁻¹ after a few cycles. Over 120 cycles at 100 mA g⁻¹, the electrode retains 520 mAh g⁻¹ (84 % of theoretical). Rate tests (Fig. 5d) deliver 745, 674, 655, 610, 573, and 486 mAh g⁻¹ at 20–600 mA g⁻¹, and remarkably 300 mAh g⁻¹ after 500 cycles at 2 A g⁻¹ (Fig. 5e), outperforming most carbon‑Se cathodes.

Impedance and Diffusion. EIS (Fig. 6a) shows lower charge‑transfer (R_ct) and ion‑diffusion (R_id) resistances for Se‑R800A, attributed to the microporous framework and Se‑induced conductivity. GITT (Fig. 6d) yields Na⁺ diffusion coefficients of ~10⁻¹⁶ cm² s⁻¹, higher than R800 or R800A alone.

Cycling Stability. Post‑cycling SEM (Fig. 7) confirms the structural integrity of Se‑R800A after 500 cycles; the leaf’s hierarchical pores and heteroatom doping preserve morphology and suppress polyselenide migration.

Conclusions

The melt‑diffusion Se‑impregnated leaf cathode demonstrates that biomass‑derived, binder‑free electrodes can achieve high capacities, excellent rate capability, and durable cycling in Na‑Se batteries. With a reversible capacity of 520 mAh g⁻¹ at 100 mA g⁻¹ and 300 mAh g⁻¹ at 2 A g⁻¹ after 500 cycles, this free‑standing architecture offers a sustainable, high‑performance alternative to conventional electrode materials.

Abbreviations

BET:

Brunauer-Emmett-Teller

CV:

Cyclic voltammogram

DEC:

Diethyl carbonate

EC:

Ethylene carbonate

EIS:

Electrochemical impedance spectroscopy

FESEM:

Field emission scanning electron microscopy

GITT:

Galvanostatic intermittent titration technique

LIBs:

Lithium ion batteries

Li‑S:

Lithium‑sulfur

Na‑Se:

Sodium‑selenium

SEI:

Solid electrolyte interface

SEM:

Scanning electron microscopy

SIBs:

Sodium ion batteries

TEM:

Transmission electron microscopy

TGA:

Thermogravimetric analysis

XPS:

X‑ray photoelectron spectroscopy

XRD:

X‑ray diffraction