Pristine Amorphous Vanadium Oxide Thin Films Deliver High‑Performance Cathodes for Li‑ and Na‑Ion Batteries

Abstract

We report pristine, additive‑free amorphous vanadium oxide (a‑VOx) thin films, deposited by pulsed laser deposition (PLD), that serve as high‑capacity cathodes for both lithium‑ion (LIB) and sodium‑ion (SIB) batteries. The 650‑nm‑thick films, grown on stainless steel substrates under controlled oxygen partial pressures (pO₂ = 0, 6, 13, 30 Pa), exhibit O/V ratios ranging from 0.76 to 2.25. Films prepared at moderate pO₂ (6–13 Pa) display a dominant V⁵⁺ oxidation state and superior electrochemical performance, achieving reversible capacities of 300 mAh g⁻¹ (Li) and 164 mAh g⁻¹ (Na) at 0.1 C. After 100 cycles, both chemistries retain ~90 % of their initial capacity, with Coulombic efficiencies near 100 %. These results demonstrate that the stoichiometric balance of V⁵⁺ in a‑VOx is key to stable, high‑performance battery cathodes.

Introduction

Amorphous vanadium oxides (a‑VOx) have emerged as promising cathode materials for secondary Li‑ and Na‑ion batteries due to their short diffusion paths, high theoretical capacities, and resistance to irreversible phase transformations that plague crystalline V₂O₅. While crystalline V₂O₅ can store up to 441 mAh g⁻¹ (1 C), practical cycling is limited to ~292 mAh g⁻¹ because of irreversible ω‑LiₓV₂O₅ formation below 1.9 V. In contrast, a‑VOx circumvents these constraints, achieving reversible capacities above 400 mAh g⁻¹ in sol‑gel and electrodeposited systems, though cycling stability has often been poor. Prior PLD studies reported high capacities (~346 mAh g⁻¹) but involved electrochemically active SnO₂ substrates, obscuring the intrinsic performance of a‑VOx. A systematic, pristine PLD investigation—free of additives, binders, and residual water—has been lacking. This work addresses that gap by correlating oxygen stoichiometry, oxidation state distribution, and electrochemical behavior in a‑VOx thin films.

Methods

Thin Film Deposition

Amorphous VOₓ films were deposited from a crystalline V₂O₅ target using a KrF excimer laser (λ = 248 nm, 200 mJ, 5 Hz). Stainless steel (SS 304) substrates were pre‑heated to 100 °C, and films were grown under a high‑vacuum base pressure of 6 × 10⁻⁶ mbar. Oxygen was introduced to achieve pO₂ of 0, 6, 13, and 30 Pa, yielding film thicknesses of ~650 nm after 44 min of deposition. The four film types were denoted a‑VOx‑0 Pa, a‑VOx‑6 Pa, a‑VOx‑13 Pa, and a‑VOx‑30 Pa.

Characterization

Surface morphology was examined by JEOL 7600F FESEM (5 kV). Structural analysis employed Bruker D8 Advance XRD (Cu‑Kα). Atomic force microscopy (AFM) provided topography. X‑ray photoelectron spectroscopy (XPS) with monochromatic Mg Kα quantified V oxidation states. Elemental ratios were obtained via EDAX. All measurements were conducted under standard laboratory conditions.

Electrochemical Characterization

As‑deposited films served directly as cathodes in CR2016 coin cells, with Li or Na metal counter electrodes. Whatman glass microfiber separators were used. Electrolytes were 1 M LiPF₆ in EC/DEC (1:1) for LIBs and 1 M NaClO₄ in PC + 5 % FEC for SIBs. Cells were assembled in an Ar‑filled glove box (< 0.1 ppm H₂O/O₂). Cyclic voltammetry (CV) was performed from 1.5 to 4.0 V at 0.1 mV s⁻¹. Galvanostatic charge‑discharge (GCD) tests spanned 0.1–10 C (1 C = 294 mA g⁻¹ for LIB, 236 mA g⁻¹ for SIB). Electrochemical impedance spectroscopy (EIS) covered 100 kHz–0.01 Hz with a 10 mV perturbation.

Results and Discussion

Physical and Chemical Characterization

FESEM images (Figure 1) reveal that a‑VOx‑6 Pa and a‑VOx‑13 Pa are smooth, continuous films with minimal particulates, whereas a‑VOx‑30 Pa is rough and non‑continuous, indicating that higher pO₂ increases grain size and surface roughness. XRD patterns show only SS peaks, confirming the amorphous nature of all VOₓ films. AFM analysis (Figure 2) quantifies roughness: 8.6 nm (6 Pa), 9.2 nm (13 Pa), and 24 nm (30 Pa). EDAX data (Table 1) demonstrate O/V ratios of 0.76 (0 Pa), 2.13 (6 Pa), 2.25 (13 Pa), and 2.00 (30 Pa). XPS deconvolution of V 2p₃/₂ (Figure 3) shows V⁵⁺ predominance (~68 %) in 6 Pa and 13 Pa films, with V⁴⁺ increasing as pO₂ rises, corroborating the O/V trends. These findings indicate that moderate oxygen environments promote a V⁵⁺‑rich, highly amorphous matrix optimal for electrochemical activity.

Electrochemical Characterization

Li‑Ion Battery Performance

GCD curves (Figure 4b) for a‑VOx‑6 Pa exhibit pseudocapacitive behavior with a small plateau near 4 V that vanishes in the 2.0–4.0 V window. The first‑cycle reversible capacities are 239 mAh g⁻¹ (2.0–4.0 V) and 298 mAh g⁻¹ (1.5–4.0 V). After 100 cycles, capacity retention reaches 90 % with near‑unity Coulombic efficiency (Figure 5). Rate capability tests (Figure 6) show 300 mAh g⁻¹ at 0.1 C and 50 mAh g⁻¹ at 10 C, outperforming bulk crystalline V₂O₅ powders (which retain only 38–57 %) and most reported PLD a‑VOx films. The superior performance is attributed to low charge‑transfer resistance and fast surface‑controlled Li⁺ insertion, as evidenced by EIS (Figure 9).

Na‑Ion Battery Performance

Na‑ion cycling (Figure 7) demonstrates pronounced pseudocapacitance with nearly rectangular CV curves, especially for a‑VOx‑13 Pa. The initial capacities rise with pO₂, peaking at 162 mAh g⁻¹ for 13 Pa and retaining 84 % after 100 cycles (Figure 8b). The 30 Pa film shows continuous capacity fade due to excessive oxygen deficiency and SEI formation. In contrast, 6 Pa and 13 Pa films maintain 90 % and 84 % retention, respectively, thanks to balanced V⁵⁺/V⁴⁺ ratios that minimize irreversible phase changes. EIS analysis (Figure 10) confirms lower charge‑transfer resistance for the 13 Pa film, aligning with its enhanced cycling stability.

Electrochemical Impedance Spectroscopy

Nyquist plots for LIBs (Figure 9) and SIBs (Figure 10) reveal that a‑VOx‑6 Pa has the lowest surface‑film resistance (R_sf) and charge‑transfer resistance (R_ct), while the 30 Pa film suffers from high R_sf and low ionic capacitance (C_i). In SIBs, the 13 Pa film shows consistent R_ct and bulk resistance (R_b) across charge‑discharge states, supporting its stable performance. These impedance signatures corroborate the observed capacity retention trends.

Conclusions

Pristine, additive‑free a‑VOx thin films grown by PLD exhibit exceptional cathodic performance in LIBs and SIBs. Films deposited at 6 Pa (Li) and 13 Pa (Na) achieve reversible capacities of 300 mAh g⁻¹ and 164 mAh g⁻¹, respectively, with ~90 % capacity retention after 100 cycles. The dominance of V⁵⁺ oxidation state and moderate oxygen stoichiometry underpin the rapid, faradaic surface reactions and low impedance that drive this performance. These results position a‑VOx thin films as attractive, binder‑free cathodes for next‑generation Li‑ and Na‑ion batteries.

Abbreviations

AFM

Atomic force microscope

a‑VOx

Amorphous vanadium oxide

CV

Cyclic voltammetry

DEC

Diethyl carbonate

EC

Ethylene carbonate

FESEM

Field emission scanning electron microscopy

LIB

Lithium‑ion battery

PC

Propylene carbonate

PLD

Pulsed laser deposition

SEI

Solid electrolyte interface

SIB

Sodium‑ion battery

XPS

X‑ray photoelectron spectroscopy

XRD

X‑ray diffraction