Lithium cobalt oxide (LCO), the first commercialized cathode active material for lithium-ion batteries, is known for high voltage and capacity. However, its application has been limited by relatively low capacity and stability at high C-rates. Reducing particle size is considered one of the most straightforward and effective strategies to enhance ion transfer, thus increasing the rate performance. However, side reactions are simultaneously enhanced as the specific surface area increases. Herein, we investigate the impact of LCO particles with varying size distributions and optimize the particle size. To modulate the side reactions associated with particle size reduction, an ultrathin carbon nanotube film (UCNF) is introduced to coat the cathode surface. With this simple process and optimized particle size, the rate performance improves significantly, normal commercial LCO achieves 118 mA·h·g⁻¹ at 3.0–4.3 V and 20 C (0.72 mA·h·cm⁻²), corresponding to power density of 8732 W·kg⁻¹. This method is applied to high voltage as well, 152 mA·h·g⁻¹ at 3.0–4.6 V and 20 C (0.99 mA·h·cm⁻²) was achieved with high-voltage LCO (HVLCO), corresponding to power density of 11,552 W·kg⁻¹. The cycling stability is also enhanced, with the capacity retention maintaining more than 96% after 100 cycles at 0.1 C. For the first time, UCNF is demonstrated to suppress the excessive decomposition of the electrolytes and solvents by blocking electron injection/extraction between LCO and electrolyte solution. Our findings provide a simple method for improving LCO rate performance, especially at high C-rates.