CuGeO3 Nanowires: A High‑Capacity, Stable Anode for Advanced Sodium‑Ion Batteries

Abstract

Germanium offers an attractive theoretical capacity for sodium‑ion batteries, yet its practical use is limited by poor cyclability due to sluggish kinetics and pronounced volume change. We report the first synthesis of high‑purity CuGeO₃ (CGO) nanowires via a simple hydrothermal route and evaluate their sodium‑storage performance. The CGO nanowires deliver an initial discharge capacity of 306.7 mAh g⁻¹ with a high initial coulombic efficiency (61.7 %) and maintain 171 mAh g⁻¹ after 60 cycles, demonstrating excellent capacity retention and rate capability. These results establish CGO nanowires as a promising anode material for sodium‑ion batteries.

Background

Since their emergence, lithium‑ion batteries (LIBs) have dominated portable and automotive energy storage, but the scarcity of lithium threatens their future scalability. Sodium, an earth‑abundant alkali metal with similar electrochemical characteristics, has emerged as a compelling alternative for rechargeable batteries. While significant progress has been made on sodium‑ion cathodes, anode development lags due to the large Na⁺ ion size, which induces sluggish kinetics, substantial volume change, and unstable solid‑electrolyte interphase (SEI) layers.

Germanium (Ge) has been extensively studied as a sodium‑ion anode because of its high theoretical capacity (369 mAh g⁻¹ for NaGe). However, elemental Ge achieves high capacities only in thin films or amorphous forms. Incorporating Ge into carbon composites or binary/ternary compounds can mitigate volume expansion and improve rate performance. Ternary Ge‑based oxides, such as CuGeO₃ (CGO), are of particular interest because their intermediate products can form an inert matrix that buffers volume changes and enhances sodiation kinetics. Although CGO exhibits superior lithium storage, its sodium‑ion performance remains largely unexplored.

Methods

Material Preparation

CGO nanowires were synthesized hydrothermally. A 15‑mL aqueous solution of 0.1 g cetyltrimethylammonium bromide (CTAB) was stirred for 1 h at room temperature. Subsequently, 5 mM Cu(CH₃COO)₂·H₂O and 5 mM GeO₂ were added and stirred for another hour. The mixture was sealed in a Teflon‑lined autoclave (20 mL) and heated at 180 °C for 24 h. After cooling, the product was washed with distilled water and ethanol thrice, then dried at 60 °C for 24 h. For comparison, elemental Ge was prepared by high‑energy ball‑milling of commercial Ge powder (Alfa Aesar).

Material Characterization

X‑ray diffraction (XRD) was performed on a Bruker‑AXS D8 ADVANCE using CuKα radiation (λ = 1.5406 Å). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out on a HITACHI S‑4800 and a JEM 2100HR, respectively. Selected area electron diffraction (SAED) patterns were obtained from the TEM.

Electrochemical Measurements

Electrodes were prepared by mixing 80 wt % CGO, 10 wt % Super P, and 10 wt % poly(acrylic) acid binder with water to form a slurry, then casting onto copper foil and drying at 60 °C under vacuum. The electrolyte was 1 M NaClO₄ in EC/DMC (1:1 v/v) with 5 vol % fluoroethylene carbonate (FEC). Coin cells (CR2032) were assembled in an argon glove box with Na metal as counter electrode. Cyclic voltammetry (CV) was conducted on a CHI 660B, and galvanostatic charge/discharge tests were performed on a LAND 2001A between 0.05 and 2.0 V vs. Na/Na⁺. The active‑material loading was ≈1.0 mg cm⁻².

Results and Discussion

The hydrothermal synthesis yields uniform CuGeO₃ nanowires. In solution, GeO₂ dissolves to H₂GeO₃, which reacts with Cu(CH₃COO)₂·H₂O to form orthorhombic CGO (see schematic in Figure 1a). XRD (Figure 1b) confirms a single‑phase orthorhombic structure (JCPDS No. 32‑0333) with sharp peaks, indicating high crystallinity.

SEM images (Figure 2a) show nanowires longer than 1 µm with an average diameter of ~20 nm (Figure 2b). TEM (Figures 2c,d) corroborates the uniform morphology. The high surface area and short diffusion paths inherent to the nanowire architecture enhance sodium‑ion kinetics and accommodate volume change.

CV curves (Figure 3a) reveal a broad reduction peak at 0.8 V in the first cathodic sweep, corresponding to the multi‑step conversion of CGO to Cu, Ge, Na_xO_y, and Na_kGe_lO_m, plus SEI formation. Subsequent cycles show a split peak (~0.6 V and 0.75 V) indicating reduced irreversible reactions. A reduction peak near 0.01 V corresponds to Na_zGe alloying, while an anodic peak at 0.2 V indicates reversible de‑alloying. An anodic feature at 1.5 V reflects further oxidation of discharge products.

Ex situ XRD of the discharged electrode (0.05 V) (Figure 4a) shows disappearance of CGO peaks and emergence of Cu, Ge₄Na, Na₂O₂, NaO₃, and Na_kGe_lO_m (e.g., Na₄GeO₄, Na₂Ge₂O₅, Na₆Ge₂O₇), confirming conversion reactions. Upon charging to 2.0 V (Figure 4b), only weak CGO peaks remain, suggesting an amorphous or poorly crystalline state that facilitates Na⁺ diffusion. SAED patterns (Figures 4c,d) support these observations.

The sodium‑storage mechanism involves conversion of CGO to Cu, Ge, and Na_xO_y followed by alloying of Ge with Na (see equations below):

CuGeO3 + Na+ → Cu + Ge + NaxOy + NakGelOm
Ge + Na+ → NazGe

Galvanostatic cycling (Figure 3c) demonstrates that CGO delivers an initial discharge capacity of 306.7 mAh g⁻¹ and an initial CE of 61.7 %. After the 10th cycle, capacity stabilizes at 205 mAh g⁻¹ and declines slowly to 171 mAh g⁻¹ by cycle 60 (≈0.68 mAh g⁻¹ per cycle). In contrast, elemental Ge shows an initial capacity of only 27.1 mAh g⁻¹ and rapid fade (15 mAh g⁻¹ after 30 cycles). Rate tests (Figure 3d) reveal capacities of 261, 212, 164, and 130 mAh g⁻¹ at 50, 100, 200, and 500 mA g⁻¹, respectively, and recovery to 175 mAh g⁻¹ when the current is reduced back to 100 mA g⁻¹.

Conclusions

We have successfully synthesized uniform CuGeO₃ nanowires by a scalable hydrothermal method and demonstrated their superior sodium‑ion performance. The nanowire anode delivers an initial capacity of 306.7 mAh g⁻¹, high initial CE (61.7 %), excellent cycling stability (171 mAh g⁻¹ after 60 cycles), and robust rate capability. The ternary composition and nanostructure effectively buffer volume changes and accelerate sodiation kinetics, making CGO a promising candidate for next‑generation sodium‑ion batteries.

Abbreviations

CE

Coulombic efficiency

CGO

CuGeO₃

CTAB

Cetyltrimethylammonium bromide

CV

Cyclic voltammetry

EC/DMC

Ethylene carbonate/dimethyl carbonate

EVs

Electric vehicles

FEC

Fluoroethylene carbonate

Ge

Germanium

LIB

Lithium‑ion battery

SEI

Solid electrolyte interphase

SEM

Scanning electron microscopy

SIBs

Sodium ion batteries

TEM

Transmission electron microscopy

XRD

X‑ray diffraction