Hydrothermal Sintering Yields Porous, Hollow LiNb3O8 Anodes with Record Discharge Capacity

Background

Hollow and porous nanostructures have attracted intense interest across catalysis, energy storage, environmental remediation, drug delivery, and sensing [1–4]. In lithium‑ion batteries (LIBs), electrodes with open frameworks provide abundant active sites and continuous channels for rapid Li⁺ intercalation, leading to enhanced rate capability and cyclability [5–6]. Nevertheless, fabricating such nanostructures remains challenging.

Beyond the conventional graphite anode, transition‑metal oxides (TMOs), MoS₂, and graphene hybrids have been explored for high‑capacity anodes [13–14]. Niobium‑based oxides, with a theoretical capacity of 389 mAh g⁻¹ and dual Nb redox couples (Nb⁵⁺/Nb⁴⁺, Nb⁴⁺/Nb³⁺), offer both high capacity and intrinsic SEI suppression [15–19]. LiNb₃O₈ is a well‑known impurity phase in LiNbO₃ synthesis due to Li loss, yet its potential as an anode has been largely overlooked [20]. Prior work using solid‑state synthesis reported initial capacities up to 351 mAh g⁻¹ and limited cycling stability [18–19].

In this study, we present a hydrothermal‑assisted sintering method that yields porous, hollow LiNb₃O₈ with superior electrochemical performance, and we detail the phase evolution and structural origins.

Methods

Preparation of Samples

LiNb₃O₈ powders were synthesized via hydrothermal treatment of LiOH·H₂O and Nb₂O₅ (Li:Nb = 8:1). 3.5 mmol Nb₂O₅ was dispersed in 35 mL LiOH·H₂O with stirring for 1 h, then sealed in a 50 mL Teflon autoclave, heated at 260 °C for 24 h, and cooled naturally. The precipitate was washed, dried at 60 °C for 12 h, and calcined at 500–800 °C for 2 h (5 °C min⁻¹ ramp).

Characterization

TG/DSC (Netzsch STA 409 PC/PG) measured 25–1200 °C under N₂ (10 °C min⁻¹). XRD (Bruker D8 Discover, Cu Kα) identified crystal phases. SEM (JSM‑6700F) examined morphologies. XPS (Thermo‑Fisher Escalab 250Xi) analyzed elemental states, calibrated to C 1s = 284.6 eV.

Electrochemical Measurements

Electrodes were cast from a slurry of LiNb₃O₈, carbon black, and PVDF (8:1:1) onto Al foil, dried at 120 °C under vacuum, and punched into 16 mm disks. Coin cells (CR2025) used Li foil, Celgard 2320 separator, and 1.0 M LiPF₆ in EC/DMF (1:1). Galvanostatic cycling (Land system) ran 0–3 V (vs Li/Li⁺) at 0.1–1 C (1 C = 389 mAh g⁻¹). CV (CHI604E) scanned 1–3 V at 0.1 mV s⁻¹.

Results and Discussion

Figure 1 shows TG/DSC of the hydrothermally treated powder. Weight loss is modest (~5 %) up to 1100 °C, reflecting Li volatilization. Endothermic peaks at 330 °C (LiNbO₃ formation) and 580 °C (LiNbO₃ + Nb₂O₅ → LiNb₃O₈) confirm the reaction pathway, while a pronounced exotherm above 1100 °C indicates LiNb₃O₈ decomposition.

TG/DSC curves of the Li‑Nb‑O powder from room temperature to 1200 °C at 10 °C min⁻¹ in N₂

Calcined XRD patterns (Fig. 2) reveal LiNbO₃ and Nb₂O₅ peaks at 500 °C. At 700 °C, monoclinic LiNb₃O₈ dominates, and a single‑phase material is obtained at 800 °C (JCPDS 36‑0307, space group P2₁/a). The hydrothermal route thus facilitates pure LiNb₃O₈ formation more readily than conventional solid‑state synthesis.

XRD patterns of the Li‑Nb‑O powder calcined at different temperatures for 2 h

SEM images (Fig. 3) show a honeycomb‑like porous and hollow framework composed of ~200 nm LiNb₃O₈ nanoparticles. The irregular, warped morphology contrasts sharply with the aggregated particles from solid‑state routes. The high porosity and internal voids arise from Li loss; excess Li migrates to particle surfaces, forming a transient liquid phase that nucleates new LiNb₃O₈ and interconnects particles into a network.

a–c SEM images of LiNb₃O₈ powder with different magnifications

XPS confirms the oxidation states: Nb 3d peaks at 207.1 and 209.8 eV (Nb⁵⁺), and O 1s deconvolution shows lattice (530.3 eV) and non‑lattice (532.0 eV) oxygen [22–23].

XPS spectra of (a) Nb 3d and (b) O 1s for the porous, hollow LiNb₃O₈

Electrochemical behavior (Fig. 5) shows two reduction peaks at 1.13 and 1.30 V during the first cycle (Nb⁴⁺→Nb³⁺ and Nb⁵⁺→Nb⁴⁺), which vanish in subsequent cycles, indicating irreversible phase changes. Oxidation peaks at 1.71 and 1.96 V remain stable, reflecting reversible Li extraction.

The initial three CV curves of the LiNb₃O₈ powder at a scan rate of 0.05 mV s⁻¹ between 3–1 V

Galvanostatic cycling (Fig. 6) yields an initial discharge capacity of 285.1 mAh g⁻¹ at 0.1 C. The first charge capacity (106.4 mAh g⁻¹) indicates that ~1.6 Li⁺ are reversibly extracted, while the remaining capacity loss (~2.8 Li⁺) remains to be elucidated. After 50 cycles, the reversible capacity retains 77.6 mAh g⁻¹, a 2.5‑fold improvement over solid‑state LiNb₃O₈ (≈30 mAh g⁻¹) [18].

Galvanostatic charge‑discharge profiles of the LiNb₃O₈ powder at 0.1 C between 3 and 1 V

Rate performance (Fig. 7) shows 250 mAh g⁻¹ at 0.5 C and 228 mAh g⁻¹ at 1 C, with capacities after 50 cycles of 39.7 and 29.4 mAh g⁻¹, respectively. The porous, hollow structure provides abundant active sites and short Li⁺ pathways, accounting for the high capacity and rate capability. Further surface modification could enhance long‑term stability.

Cycling performance of the LiNb₃O₈ powder at different current rates of 0.1 C, 0.5 C and 1 C

Conclusions

Hydrothermal‑assisted sintering successfully produces porous, hollow LiNb₃O₈ anodes with record electrochemical performance. The unique structure originates from localized liquid‑phase formation during Li volatilization. The material delivers an initial discharge capacity of 285.1 mAh g⁻¹ at 0.1 C and retains 77.6 mAh g⁻¹ after 50 cycles, exceeding conventional solid‑state counterparts by more than two‑fold. The high capacity and rate capability are attributed to the abundant active sites and short Li⁺ diffusion channels inherent to the porous, hollow architecture.