Enhancing Lithium‑Ion Battery Cathodes: LiNi0.8Co0.15Al0.05O2/Carbon Nanotube Composite with Superior Electrochemical Performance

Abstract

We report a straightforward mechanical grinding approach that incorporates carbon nanotubes (CNTs) onto the surface of LiNi_0.8Co_0.15Al_0.05O₂ (NCA) cathodes without compromising crystal structure or morphology. The resulting NCA/CNT composite delivers a reversible capacity of 181 mAh g^-1 after 60 cycles at 0.25 C, a 18 % improvement over pristine NCA (153 mAh g^-1). At a high rate of 5 C, the composite maintains 160 mAh g^-1 versus 140 mAh g^-1 for unmodified NCA. Homogeneous CNT dispersion enhances electronic conductivity and mitigates side reactions with electrolyte, underscoring the viability of conductive nanomaterials as surface modifiers for high‑energy cathodes.

Background

High‑performance lithium‑ion batteries (LIBs) rely on cathode materials that combine high capacity, stability, and safety. While LiCoO₂ offers excellent reversible capacity (~150 mAh g^-1), its cobalt content drives up cost and toxicity. Nickel‑based layered oxides, such as LiNiO₂, promise higher capacities (10–30 % above LiCoO₂ in practice) but suffer from unstable Ni⁴⁺ ions and poor cycling stability. Substituting a fraction of Ni³⁺ with Co³⁺ and Al³⁺—forming LiNi_1−x−yCo_xAl_yO₂—improves both capacity and thermal stability. In particular, LiNi_0.8Co_0.15Al_0.05O₂ (NCA) balances high energy density with structural robustness, yet residual Ni²⁺ migration, side reactions with electrolyte, and limited electronic conductivity still hinder performance. Surface modification is a proven strategy to protect cathode particles. Conventional wet‑coating methods deposit insulating layers (TiO₂, MnO₂, AlF₃, etc.) that require high‑temperature post‑treatments and add processing steps. Mechanical ball‑milling with inorganic nanoparticles offers a cleaner alternative but typically employs inert additives that can increase electrode polarization. Here, we introduce conductive CNTs as a surface modifier via a gentle mechanical grinding process, aiming to preserve crystal integrity while improving electronic pathways and suppressing parasitic reactions.

Methods

Commercial NCA powder and CNTs were ground together at room temperature using a pestle and agate mortar for 1 h, producing composites with 5 %, 10 %, and 20 % CNT by weight. Structural characterization employed field‑emission scanning electron microscopy (FESEM, Quanta FEI), powder X‑ray diffraction (XRD, Rigaku Smart Lab III, Cu Kα), Raman spectroscopy (LabRAM HR, 532 nm laser), and energy‑dispersive X‑ray spectroscopy (EDS) for elemental mapping.

Electrodes were fabricated by mixing 80 % active material, 10 % acetylene black, and 10 % PVDF in NMP, cast onto aluminum foil, and dried at 100 °C under vacuum. CR2032 coin cells with lithium metal counter electrodes were assembled in an argon glove box. Galvanostatic charge–discharge tests (2.8–4.3 V vs. Li/Li⁺) were performed at 0.25 C, while rate capability was examined up to 5 C. Cyclic voltammetry (CV) was conducted from 2.8 to 4.5 V at 0.1 mV s^-1, and electrochemical impedance spectroscopy (EIS) measured 5 mV perturbation over 100 kHz–0.01 Hz using a Biologic VMP3 workstation.

Results and Discussion

SEM images show that pristine NCA consists of 5–8 µm secondary microspheres made of 100–500 nm primary particles. Gentle grinding preserves these structures, whereas high‑energy ball‑milling causes fragmentation and agglomeration (Fig. 1e‑f). Increasing CNT loading enhances surface coverage; 10 % CNT yields a uniform, tight coating without excessive aggregation (Fig. 1b‑d).

EDS dot‑mapping confirms homogeneous distribution of Ni, Co, Al, and C across composite microspheres, indicating uniform CNT adhesion (Fig. 2). XRD patterns reveal that the α‑NaFeO₂ layered structure (R3m) remains intact after CNT incorporation; no CNT‑specific peaks appear, confirming that the grinding process does not introduce impurities or phase changes (Fig. 3).

Raman spectra display characteristic NCA vibrations (~500 cm^-1) and distinct CNT bands: G‑band at 1588 cm^-1 and D‑band at 1337 cm^-1, confirming the presence of well‑graphitized CNTs (Fig. 4).

CV curves for pristine NCA exhibit two oxidation peaks at 3.9 and 4.2 V in the first cycle, shifting to 3.75 V in subsequent cycles, reflecting phase transitions (H1→M, M→H2, H2→H3). The NCA/CNT composite shows similar profiles with slightly delayed irreversible changes, indicating that CNTs modestly stabilize the structure (Fig. 5a‑b). Initial charge–discharge tests at 0.25 C reveal lower polarization for the composite (lower charge plateau, higher discharge plateau) and a higher initial discharge capacity (187 mAh g^-1 vs. 170 mAh g^-1 for pristine NCA). EIS analysis shows a reduced total resistance (110.83 Ω vs. 145.13 Ω) for the composite, underscoring enhanced charge‑transfer kinetics (Fig. 5c‑d).

Cycling performance at 0.25 C demonstrates that after 60 cycles, the composite retains 181 mAh g^-1 (96 % capacity retention) versus 153 mAh g^-1 for pristine NCA (90 % retention). Coulombic efficiency exceeds 99 % from the second cycle onward. Rate capability tests confirm that the composite delivers 160 mAh g^-1 at 5 C, while pristine NCA drops to 140 mAh g^-1. Restoring to 0.25 C fully recovers capacity, indicating excellent reversibility (Fig. 6).

Conclusions

We have demonstrated that a simple mechanical grinding strategy can effectively integrate highly conductive CNTs onto NCA cathodes without compromising their crystal structure. The CNT coating enhances electronic conductivity, suppresses deleterious side reactions, and markedly improves both cycling stability and high‑rate performance. After 60 cycles at 0.25 C, the NCA/CNT composite delivers 181 mAh g^-1, an 18 % increase over pristine NCA. At 5 C, it maintains 160 mAh g^-1, surpassing unmodified NCA. This approach offers a scalable route to high‑energy, high‑rate LIB cathodes with improved safety.