Silver‑Coated Spherical Li₄Ti₅O₁₂ Anodes: A Sol‑Gel‑Assisted Hydrothermal Route for High‑Performance Lithium‑Ion Batteries

Abstract

We report the successful synthesis of Ag‑coated spherical Li₄Ti₅O₁₂ (LTO) particles by a sol‑gel‑assisted hydrothermal method. Using ethylene glycol and silver nitrate as the precursor, we varied the silver content to investigate its influence on electrochemical performance. X‑ray diffraction confirmed that the LTO spinel structure remains intact, while silver appears as a distinct metallic phase. Electrochemical impedance spectroscopy (EIS) revealed that the highly conductive Ag layer dramatically reduces charge‑transfer resistance, yielding superior rate capability. At an optimal 5 wt.% Ag loading, the composite delivers 186.34 mAh g⁻¹ at 0.5 C and retains 92.69 % of its capacity after 100 cycles at 5 C; even at 10 C, 89.17 % capacity retention is achieved after 100 cycles. These results demonstrate that a thin, uniform Ag coating enhances electronic conductivity and cycling stability, positioning Ag‑coated spherical LTO as a promising anode for high‑power lithium‑ion batteries.

Highlights

• Spherical LTO/Ag composites were fabricated via a sol‑gel‑assisted hydrothermal process using ethylene glycol and silver nitrate, yielding a uniform Ag coating that improves electronic conductivity and electrochemical performance.
• The spherical morphology provides a high tap density, enhancing volumetric energy density.

Background

Rechargeable lithium‑ion batteries (LIBs) are prized for their lightweight, high voltage, and energy density, yet challenges such as cost, safety, and power density limit large‑scale deployment. Lithium titanate (Li₄Ti₅O₁₂, LTO) has emerged as a leading anode material due to its zero‑strain intercalation, operating at ~1.55 V vs. Li⁺/Li, which mitigates solid electrolyte interface formation and enhances safety. However, LTO suffers from low electronic conductivity and sluggish Li⁺ diffusion, restricting rate capability. Strategies to overcome these limitations include size reduction, high‑valence doping, and conductive surface coatings. Among them, metallic surface modification offers a straightforward way to form a conductive network that facilitates electron and ion transport. Previous studies have demonstrated Ag coating via electroless deposition, but a sol‑gel‑assisted hydrothermal synthesis of micron‑sized Ag‑coated LTO spheres has not been reported.

Experimental

Synthesis of Pristine LTO and Ag Surface Modification

Preparation of Spherical Titanium Glycolate (TG) Precursor

Tetrabutyl titanate was slowly added to a solution of AgNO₃ (concentration adjusted for solubility in 50 mL ethylene glycol) under vigorous stirring to form the precursor solution. The mixture was then transferred to a 150 mL acetone bath containing 0.1 mL Tween 80 and stirred for 1 h at room temperature to precipitate TG. The precipitate was aged 8 h, filtered, washed twice with anhydrous alcohol, and dried at 80 °C for 6 h before grinding.

Hydrothermal Synthesis of LTO/Ag

A stoichiometric mixture of LiOH·H₂O and TG (3.9:1 molar ratio) was dissolved in 40 mL ethanol and stirred for 1 h. The solution was sealed in Teflon‑lined autoclaves and heated at 180 °C for 12 h. The resulting precipitate was collected by centrifugation, washed with ethanol, dried at 80 °C for 2 h, then calcined at 700 °C (5 °C min⁻¹) for 2 h in air to yield spherical LTO/Ag powders.

Characterization

Phase identification employed X‑ray diffraction (Rigaku D/max‑PC2200, Cu Kα, 4 ° min⁻¹). Morphology and particle size were examined by SEM (Zeiss Supra 55) and TEM (JEOL‑2100). Electrochemical performance was evaluated in CR2025 coin cells using 80 wt.% active material, 10 wt.% Super‑P, and 10 wt.% PVDF in NMP. Electrolyte comprised 1 M LiPF₆ in EC/DMC/EMC (1:1:1). Tests were conducted in an Ar‑filled glove box (<1 ppm H₂O/O₂) using a LAND CT2001A system. Cyclic voltammetry (0.1 mV s⁻¹, 1.0–2.5 V) and EIS (100 kHz–10 mHz, 5 mV) were recorded.

Results and Discussion

Structural and Morphological Analysis

XRD patterns confirm the spinel LTO structure with additional peaks at 38.1°, 44.3°, and 64.4° corresponding to metallic Ag. Increasing Ag loading enhances these peaks, indicating a uniform surface coating without lattice penetration. SEM images reveal uniformly spherical particles (5–10 µm) for all samples; the Ag‑coated particles display a slightly rougher surface but maintain good dispersion. TEM of the 5 wt.% Ag sample shows a 3–4 nm Ag shell, forming a continuous conductive network.

Electrochemical Performance

Charge–discharge profiles exhibit a flat plateau at 1.55 V, characteristic of the Li₄Ti₅O₁₂ ↔ Li₇Ti₅O₁₂ two‑phase transition. Ag coating extends the plateau and reduces polarization. The 5 wt.% Ag sample achieves the highest capacity: 186.34 mAh g⁻¹ at 0.5 C, 172.47 mAh g⁻¹ at 1 C, 154.12 mAh g⁻¹ at 5 C, and 136.06 mAh g⁻¹ at 10 C. Excessive Ag (7 wt.%) slightly reduces capacity due to diminished active surface area. Rate capability tests show the 5 wt.% Ag composite retains >97 % of its 5 C capacity after 30 cycles and >89 % at 10 C after 100 cycles, outperforming uncoated LTO.

CV curves at 0.1 mV s⁻¹ display symmetric redox peaks with minimal potential gap (0.244 V for 5 wt.% Ag vs. 0.240 V for uncoated), indicating low polarization and high coulombic efficiency. EIS analysis reveals a reduced charge‑transfer resistance for the 5 wt.% Ag sample, and the calculated Li⁺ diffusion coefficient (6.73 × 10⁻¹¹ cm² s⁻¹) is an order of magnitude higher than that of pristine LTO (8.69 × 10⁻¹² cm² s⁻¹).

Conclusions

We have demonstrated a scalable sol‑gel‑assisted hydrothermal route to produce Ag‑coated spherical Li₄Ti₅O₁₂ anodes with high tap density. A 5 wt.% Ag coating optimally balances electronic conductivity and active surface area, delivering high capacity, excellent rate performance, and robust cycling stability. These findings underscore the potential of Ag‑coated spherical LTO as a high‑power, safe anode for next‑generation lithium‑ion batteries.