Enhanced Lithium‑Ion Battery Anodes: Polypyrrole‑Coated MnO₂ Core–Shell Micromaterials Deliver Superior Cyclic Stability

Background

Since Tarascon’s seminal proposal of 3d transition‑metal oxides as high‑capacity anodes for Li‑ion batteries, extensive research has focused on tailoring the morphology of metal‑oxide nanostructures to enhance electrochemical performance. For instance, monodisperse Fe₃O₄ and γ‑Fe₂O₃ microspheres have delivered initial capacities of 1,307 and 1,453 mAh g⁻¹, yet their capacities fall to 450 and 697 mAh g⁻¹ after 110 cycles. NiO hollow spheres, while synthetically intriguing, exhibit limited lithium storage. MnO₂ is particularly attractive, boasting a theoretical capacity of ~1,230 mAh g⁻¹; however, bare MnO₂ suffers rapid capacity decay due to low electronic conductivity and severe volume expansion that leads to pulverization and loss of electrical contact.

Coating strategies—carbon, conducting polymers, graphene, and inorganic shells—have proven effective in enhancing cycling stability by providing mechanical buffering and improved electronic pathways. Polypyrrole (PPy), an intrinsically conductive polymer, has already shown promising results when coated onto CuO, achieving 760 mAh g⁻¹ versus the bare CuO’s 360 mAh g⁻¹. These successes motivate the present study, wherein we apply a PPy shell to MnO₂ to preserve its high capacity while eliminating the rapid fade.

Methods

Sample Preparation

Analytical‑grade reagents (Shanghai Chemical Co.) were used without further purification, except for pyrrole, which was distilled under reduced pressure and stored under light‑protected, 0–5 °C conditions. MnO₂ micromaterials were synthesized via a hydrothermal route analogous to Yu et al., producing caddice‑clew‑like and sea‑urchin‑like morphologies. The urchin‑like structure was obtained by dissolving 1.70 g MnSO₄·H₂O in 15 mL water, adding 20 mL K₂S₂O₈, stirring until clear, then autoclaving at 110 °C for 6 h. The resulting black precipitate was filtered, washed, and dried at 80 °C. For the urchin‑like variant, 2 mL H₂SO₄ was added prior to autoclaving.

PPy coating was achieved by dispersing 0.2 g MnO₂ in 50 mL 0.01 M BSNa solution (3:1 pyrrole:BSNa molar ratio) at 0–5 °C. After 30 min, pyrrole was introduced, followed by a dropwise addition of FeCl₃ to initiate polymerization. The mixture was stirred at 0–5 °C for 12 h, yielding MnO₂@PPy core–shell particles. Varying pyrrole volumes (10–50 µL) allowed control of shell thickness. The composites were washed with water and ethanol, then vacuum‑dried at 60 °C for 4 h.

Characterization

SEM (QUANTA‑200) and EDS assessed morphology and elemental composition. XRD (Rigaku D/max‑2200/PC, Cu Kα, 7° min⁻¹, 10–70° 2θ) confirmed crystal phases. FT‑IR (Nicolet IS10) identified functional groups. TGA (MELER/1600H) evaluated PPy content via weight loss from 25–800 °C. XPS (Ulvac‑PHI, PHI5000 Versaprobe‑II, Al Kα) probed surface chemistry. Electrochemical measurements employed CR2025 coin cells: a slurry of 60 wt.% active material, 10 wt.% acetylene black, and 30 wt.% PVDF in NMP was cast on Cu foil and dried at 80 °C. Cells used a Li metal counter electrode and Celgard 2320 separator in 1 M LiPF₆/EC:DMC (1:1). Galvanostatic cycling (0.2 C, 0.01–3.00 V) and EIS (0.1 Hz–100 kHz, 5 mV) were performed with a Land CT2001A system and CHI604D workstation, respectively.

Results and Discussion

Morphology

SEM images (Fig. 1) reveal that pure PPy forms ~800 nm spheres that tend to agglomerate. The sea‑urchin‑like MnO₂ consists of ~3 µm cores with 1 µm radially grown nanorods. As pyrrole volume increases, PPy nucleates within the nanorod gaps, gradually filling the space and transforming the morphology into a compact sphere. At 50 µL pyrrole, the PPy shell is thick and uniform, fully coating the MnO₂ core. Similar behavior is observed for the caddice‑clew‑like MnO₂, which aggregates into 2–4 µm spheres; PPy coverage evolves from isolated particles to a continuous block as pyrrole amount rises.

FT‑IR

Characteristic PPy peaks at 1550, 1448, 1283, and 1130 cm⁻¹ confirm successful coating. The I₁₅₅₀/I₁₄₄₈ ratio increases with pyrrole loading, indicating higher conjugation and doping levels that correlate with improved conductivity. The 30 µL and 50 µL samples exhibit stronger PPy signatures, supporting their superior electrochemical performance.

XPS

Surface analysis shows pronounced O 1s, N 1s, and C 1s signals in the core–shell samples, while Mn signals diminish—consistent with a ~5–10 nm PPy overlayer. Minor Fe and Cl peaks originate from the FeCl₃ oxidant. These results confirm the presence of a PPy shell encapsulating the MnO₂ core.

TGA

TGA curves (Fig. 4) reveal that 30 µL and 50 µL PPy-coated samples contain ~22 % and ~30 % PPy, respectively, matching theoretical expectations. This demonstrates that the shell thickness can be precisely tuned by pyrrole dosage.

XRD

All coated samples retain the α‑MnO₂ fingerprint, with peak intensities decreasing as PPy content rises due to the amorphous polymer overlay. The crystalline structure remains intact, ensuring that the intrinsic electrochemical pathways of MnO₂ are preserved.

Electrochemical Performance

Charge‑discharge profiles (Fig. 6) show that PPy‑coated anodes maintain the same voltage windows as bare MnO₂ but with significantly higher capacity retention. While bare MnO₂ drops below 200 mAh g⁻¹ after 10 cycles, the 30 µL and 50 µL coated samples retain 480 and 620 mAh g⁻¹ after 300 cycles, respectively. The improved stability arises from the PPy shell’s ability to accommodate volume expansion, preserve electronic contact, and suppress SEI growth.

Rate capability tests (Fig. 7) demonstrate that the 50 µL PPy‑coated caddice‑clew‑like MnO₂ delivers 508 mAh g⁻¹ at 0.2 C and 160 mAh g⁻¹ at 5 C, outperforming the bare counterpart (only 170 mAh g⁻¹ at 0.2 C). Similar trends are observed for the urchin‑like morphology. EIS (Fig. 8) reveals that charge‑transfer resistance drops from ~140 Ω (bare) to 77–95 Ω (coated), confirming enhanced electronic conductivity.

Conclusions

Polypyrrole‑coated MnO₂ core–shell micromaterials have been successfully fabricated via a scalable chemical polymerization route. By adjusting pyrrole volume, the PPy shell thickness can be finely tuned, directly influencing electrochemical performance. The 50 µL PPy coating delivers the best results: 620 mAh g⁻¹ after 300 cycles, a dramatic improvement over bare MnO₂. The flexible PPy shell buffers volume changes, preserves electrical connectivity, and mitigates pulverization, thereby extending the cycling life of transition‑metal‑oxide anodes. This strategy offers a viable pathway to high‑performance, durable Li‑ion batteries.