In the rapidly evolving landscape of energy storage, the demand for short-duration, high-frequency, and high-power output systems has become paramount across various sectors, including renewable energy integration, power grid stabilization, aerospace applications, and electromagnetic or directed-energy equipment. Among the available technologies, the lithium-ion battery stands out due to its relatively high energy density, but traditional high-power lithium-ion batteries often struggle to achieve power densities exceeding 10 kW/kg, which limits their applicability in scenarios requiring ultrahigh-power bursts. The core challenge lies in balancing the inherent trade-off between energy density and power density within a lithium-ion battery system. To address this, significant research efforts have focused on optimizing electrode materials, interfaces, and cell architectures. In this work, we explore a novel approach by designing a cathode material through particle size gradation, combining nano-scale and micro-scale LiCoO2 particles, to achieve an exceptional synergy that enhances both rate capability and cycling stability, thereby pushing the boundaries of what is possible for ultrahigh-power lithium-ion batteries.
The fundamental limitations of electrode materials in lithium-ion batteries often stem from ionic and electronic transport kinetics. Micro-sized particles, while offering high volumetric energy density and good cycling stability, suffer from long solid-state lithium-ion diffusion paths, leading to significant polarization and capacity fade under high current rates. Conversely, nano-sized materials boast shortened diffusion lengths and enlarged surface areas, which drastically improve reaction kinetics and rate performance. However, they introduce drawbacks such as low initial Coulombic efficiency, reduced volumetric energy density, difficulties in achieving high mass loadings, poor cycling stability due to exacerbated side reactions, and complex, costly synthesis routes. Therefore, a hybrid strategy that intelligently combines both nano and micro particles presents a compelling solution to overcome these individual shortcomings and unlock the full potential of lithium-ion batteries for ultrahigh-power applications.
Our strategy centers on the cathode, as it is frequently the rate-limiting component in high-power lithium-ion batteries. We selected lithium cobalt oxide (LiCoO2), a well-established cathode material known for its high volumetric energy density and good rate capability, as our base material. The objective was to engineer a composite cathode where micro-sized LiCoO2 (M-LCO) particles provide the structural backbone for high packing density and cycling robustness, while nano-sized LiCoO2 (N-LCO) particles fill the interstitial voids, creating efficient ion/electron transport networks and offering abundant active sites for rapid lithium (de)intercalation. This nano/micro-structured graded-particle design aims to minimize electrode tortuosity, enhance electrolyte wettability, and establish a percolating conductive matrix, all critical for sustaining ultrahigh current densities.
The synthesis of the nano/micro composite materials was achieved through a facile and scalable wet ball-milling process. Commercial micro-sized LiCoO2 (M-LCO) with a median particle diameter (D50) of approximately 5 μm was used as the starting material. A portion of this M-LCO was subjected to high-energy wet ball-milling using stainless steel balls in an anhydrous ethanol medium. The ball-to-powder mass ratio was maintained at 3:1, with a strategic distribution of ball sizes (10 mm, 20 mm, and 40 mm diameter balls in a 5:3:2 mass ratio) to ensure efficient size reduction. Milling at 600 rpm for 8 hours effectively broke down the polycrystalline micro-aggregates into primary nanoparticles, resulting in nano-scale LiCoO2 (N-LCO) with particle dimensions below 500 nm. The obtained N-LCO slurry was then vacuum-dried at 80°C for 12 hours and gently ground into a fine powder.
To create the graded-particle composites, the synthesized N-LCO was blended with the pristine M-LCO in different mass ratios. Specifically, composites with N-LCO mass fractions of 5 wt%, 10 wt%, and 15 wt% were prepared. These mixtures were again subjected to low-energy wet ball-milling (200 rpm for 4 hours) in ethanol to ensure homogeneous dispersion of the nano particles within the micro particle matrix, without causing further significant size reduction of the M-LCO. The final products, denoted as x%N-LCO (where x = 5, 10, 15), were dried and ground similarly. This method emphasizes process simplicity and scalability, which are vital for the practical deployment of advanced lithium-ion battery technologies.
Structural and morphological characterization was performed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD patterns for all materials confirmed the preservation of the layered α-NaFeO2 structure (space group R-3m) characteristic of LiCoO2, as all major diffraction peaks corresponded well with the standard pattern (JCPDS 16-0427). However, the diffraction peak intensities for the N-LCO and the composites showed a gradual decrease with increasing N-LCO content, which is typical for nano-crystalline materials due to factors like increased surface disorder, lattice strain, and reduced crystallite size. The SEM images vividly illustrated the morphological transformation. The pristine M-LCO appeared as dense, polyhedral secondary particles composed of tightly packed primary grains. After high-energy milling, the N-LCO consisted of much finer, sub-500 nm particles with reduced thickness. In the composite powders, the SEM analysis revealed a successful integration strategy: for the 5%N-LCO composite, the N-LCO particles were primarily dispersed in the spaces between the larger M-LCO particles. As the N-LCO fraction increased to 10% and 15%, a more uniform distribution was achieved, with nano particles adhering to the surfaces of micro particles, forming a conductive “satellite” structure that is anticipated to facilitate charge transfer.
The electrochemical performance of the cathode materials was first evaluated in half-cell configurations (vs. Li/Li+) within a voltage window of 3.0–4.4 V. The key metrics—initial Coulombic efficiency (ICE), rate capability, and cycling stability—were systematically compared. The data is comprehensively summarized in Table 1.
| Material | Initial Discharge Capacity (mAh/g) | Initial Coulombic Efficiency (%) | Capacity Retention at 10C (vs. 0.1C) (%) | Capacity Retention after 100 cycles at 1C (%) |
|---|---|---|---|---|
| M-LCO (pristine) | 159.5 | 96.80 | 64.93 | 81.26 |
| N-LCO (nano) | 170.1 | 90.83 | 81.11 | — |
| 5%N-LCO | 169.8 | 95.24 | 67.09 | 81.49 |
| 10%N-LCO | 170.1 | 93.83 | 79.30 | 96.33 |
| 15%N-LCO | 170.3 | 91.69 | 83.48 | — |
The initial discharge capacity showed a clear trend: while M-LCO delivered 159.5 mAh/g, the incorporation of N-LCO boosted the reversible capacity to around 170 mAh/g for all composites. This enhancement is attributed to the nano particles providing a much higher specific surface area, exposing more active sites for lithium-ion interaction, and drastically shortening the solid-state diffusion path length ($$L$$), thereby improving the utilization of the material’s bulk. The diffusion time ($$\tau$$) is proportional to the square of the diffusion length ($$\tau \propto L^2$$), so reducing $$L$$ from micrometers to nanometers can decrease $$\tau$$ by several orders of magnitude, which is crucial for high-rate performance in a lithium-ion battery.
However, the increased surface area of nano materials also leads to more extensive side reactions with the electrolyte during the initial cycle, forming a thicker cathode-electrolyte interphase (CEI) and consuming active lithium ions. This explains the observed decrease in ICE with higher N-LCO content: from 96.8% for M-LCO to 90.83% for pure N-LCO. The composites exhibited intermediate ICE values, offering a practical compromise. The rate capability test results were particularly illuminating. The capacity retention at a high discharge rate of 10C (relative to the 0.1C capacity) was only 64.93% for M-LCO, reflecting its kinetic limitations. The pure N-LCO showed superior kinetics with 81.11% retention. Remarkably, the 10%N-LCO composite achieved 79.30% retention, significantly outperforming M-LCO and approaching the performance of pure N-LCO. The 15%N-LCO composite even surpassed pure N-LCO with 83.48% retention. This non-linear improvement suggests a synergistic effect in the graded-particle design. The nano particles facilitate rapid ion transport at the interface and within the electrode pores, while the micro particles maintain structural integrity and reduce overall electrode porosity, mitigating excessive polarization under high currents. The effective electronic conductivity ($$\sigma_{eff}$$) of the composite electrode can be qualitatively described by a percolation model where the nano particles enhance the connectivity between micro particles: $$\sigma_{eff} \propto \phi_{nano}^{t}$$, where $$\phi_{nano}$$ is the volume fraction of nano particles and $$t$$ is a critical exponent. An optimal fraction exists where ionic and electronic transport are simultaneously maximized without compromising other properties.
Cycling stability at 1C revealed another critical advantage of the hybrid design. While M-LCO and 5%N-LCO retained about 81% capacity after 100 cycles, the 10%N-LCO composite demonstrated outstanding stability with 96.33% capacity retention. This indicates that the micro particle framework effectively buffers the mechanical stress and volume changes associated with lithium-ion insertion/extraction, preventing the nano particles from aggregating or detaching, which commonly plagues pure nano-electrodes. The hybrid structure thus alleviates capacity fade, a common failure mode in high-power lithium-ion batteries subjected to frequent deep cycling.
Electrochemical impedance spectroscopy (EIS) further corroborated these findings. The Nyquist plots for all materials featured a semicircle in the high-to-medium frequency region, associated with the charge transfer resistance ($$R_{ct}$$) at the electrode/electrolyte interface, and a sloping line in the low-frequency region, related to lithium-ion diffusion (Warburg impedance). The diameter of the semicircle, representing $$R_{ct}$$, was largest for M-LCO, confirming its sluggish charge transfer kinetics. The composites, especially 10%N-LCO and 15%N-LCO, showed substantially reduced semicircle diameters, indicating a lower $$R_{ct}$$. This reduction is directly linked to the enlarged electrochemically active surface area and improved interfacial contact provided by the nano particles, which accelerates the charge transfer reaction: $$Li^+ + e^- + \text{host} \rightleftharpoons Li-\text{host}$$. The relationship between the exchange current density ($$i_0$$) and $$R_{ct}$$ is given by $$i_0 = \frac{RT}{nFR_{ct}}$$, where $$R$$ is the gas constant, $$T$$ is temperature, $$n$$ is the number of electrons transferred, and $$F$$ is Faraday’s constant. A lower $$R_{ct}$$ implies a higher $$i_0$$, signifying faster electrode kinetics, which is essential for the high-power operation of a lithium-ion battery.
To translate the material-level success into a practical device, we assembled a prototype pouch-type lithium-ion battery. The cathode was fabricated using the optimized 10%N-LCO composite material, and commercial hard carbon (HC) was chosen as the anode due to its relatively good rate capability and safety profile compared to graphite. The full cell was designed with a nominal capacity of 1.4 Ah and an operating voltage window of 2.5–4.3 V to balance performance and longevity. The mass of the complete pouch cell was 44.1 g.
The electrochemical evaluation of this HC//10%N-LCO full cell yielded remarkable results, pushing the envelope for high-energy, ultrahigh-power lithium-ion batteries. The formation cycle at 0.2C provided an initial discharge capacity of 1.48 Ah and an ICE of 72.9%. The lower ICE compared to half-cells is typical for full cells, primarily influenced by irreversible lithium consumption during solid electrolyte interphase (SEI) formation on the hard carbon anode and initial side reactions at both electrodes. The specific energy of the cell was calculated from the 1C discharge data (capacity: 1.40 Ah, discharge energy: 5.15 Wh):
$$E_{specific} = \frac{\text{Discharge Energy}}{\text{Cell Mass}} = \frac{5.15 \text{ Wh}}{0.0441 \text{ kg}} \approx 116.78 \text{ Wh/kg}$$
This specific energy is competitive for a power-optimized lithium-ion battery, demonstrating that the graded-particle cathode design does not come at a severe cost to energy density.
The true breakthrough was observed in rate and power performance. The cell was subjected to extremely high constant-current discharge rates. As summarized in Table 2, the capacity retention remained high even at 200C, a discharge rate where most conventional lithium-ion batteries would fail catastrophically.
| Discharge Rate | Discharge Capacity (Ah) | Capacity Retention vs. 1C (%) | Approximate Discharge Time |
|---|---|---|---|
| 1C (Baseline) | 1.40 | 100.00 | 1 hour |
| 100C | 1.25 | 89.28 | 36 seconds |
| 150C | 1.23 | 87.86 | 24 seconds |
| 200C | 1.10 | 78.57 | 18 seconds |
The ability to deliver over 78% of its nominal capacity in just 18 seconds underscores the exceptional power capability of this lithium-ion battery. During these tests, the surface temperature of the pouch cell was monitored under natural convection cooling. Even at a 200C discharge, the temperature rise was less than 40°C, reaching a maximum of around 63°C from an ambient 25°C. This moderate thermal response is highly advantageous, as excessive heat generation is a primary safety concern and a source of degradation in high-power lithium-ion batteries. The efficient ion/electron transport in the graded-particle cathode minimizes ohmic heating ($$P_{heat} = I^2 R_{internal}$$), contributing to this improved thermal management.
To probe the limits of instantaneous power delivery, pulse discharge tests were conducted. The cell successfully endured 7 consecutive 1-second pulses at a staggering 350C rate (approximately 500 A current) without the voltage collapsing below the 2.5 V cutoff. More impressively, it sustained 20-millisecond pulses at a 1000C rate. During this 1000C pulse, the instantaneous power output was nearly constant at around 3900 W. Given the cell mass of 0.0441 kg, the corresponding pulse power density is:
$$P_{density} = \frac{\text{Power}}{\text{Cell Mass}} = \frac{3900 \text{ W}}{0.0441 \text{ kg}} \approx 88.44 \text{ kW/kg}$$
Achieving a power density of 88.44 kW/kg in a lithium-ion battery is a significant milestone, bridging the performance gap towards supercapacitors while retaining much higher energy density. This capability is directly enabled by the nano/micro cathode architecture, which provides ultra-low impedance pathways for both ions and electrons, allowing the cell to source enormous current with minimal polarization.
Furthermore, constant-power discharge tests were performed to simulate real-world dynamic loads. The cell delivered 440 W (10 kW/kg) for 34.1 seconds, 880 W (20 kW/kg) for 11.9 seconds, and 1320 W (30 kW/kg) for 3.1 seconds. The associated temperature rises were again manageable, with less than 30°C increase in all cases. Finally, to assess durability under realistic high-power cycling conditions, the cell was subjected to repeated charge/discharge cycles at a constant power of 30 W. After 1500 such cycles, the capacity retention remained at 81.5%, with Coulombic efficiency consistently near 100%, demonstrating robust long-term performance for a high-power lithium-ion battery.
The underlying mechanism for the success of this graded-particle design can be conceptualized through a model of hierarchical transport. The micro particles (diameter $$D_m$$) create a macro-porous network that ensures good electrolyte penetration and acts as a current collector for electronic transport. The nano particles (diameter $$D_n$$, where $$D_n << D_m$$) populate the meso- and micro-pores between the micro particles. This arrangement drastically reduces the effective tortuosity ($$\tau_{eff}$$) of the electrode for ion transport, which is related to the porosity ($$\epsilon$$) and pore structure. A simplified relation is $$\tau_{eff} \propto \epsilon^{-0.5}$$ for a bimodal pore distribution, but the actual benefit is greater due to the percolating nano-network. Simultaneously, the electronic conductivity is enhanced as the nano particles form bridges between micro particles, increasing the contact area and reducing the contact resistance. The total electrode polarization ($$\eta_{total}$$) under high current ($$I$$) can be expressed as a sum of ohmic, charge-transfer, and diffusion overpotentials:
$$\eta_{total} = I \cdot R_{\Omega} + \frac{RT}{\alpha nF} \ln\left(\frac{I}{I_0}\right) + \frac{RT}{nF} \cdot \frac{I \cdot L}{A \cdot D_{Li} \cdot C_{Li}}$$
where $$R_{\Omega}$$ is the ohmic resistance, $$I_0$$ is the exchange current density, $$\alpha$$ is the charge transfer coefficient, $$A$$ is the electroactive area, $$D_{Li}$$ is the chemical diffusion coefficient of lithium, $$C_{Li}$$ is the concentration of lithium in the solid, and $$L$$ is the diffusion length. Our design effectively minimizes $$R_{\Omega}$$ by improving electronic wiring, maximizes $$I_0$$ by enlarging $$A$$, and crucially, minimizes $$L$$ by incorporating nano particles. Therefore, $$\eta_{total}$$ is kept low even at ultrahigh currents, enabling the unprecedented power performance.
In conclusion, this work demonstrates a highly effective and industrially viable strategy for developing ultrahigh-power lithium-ion batteries. By engineering a cathode through the particle size gradation of nano-scale and micro-scale LiCoO2, we have created a material system that synergistically combines the high-rate capability of nanomaterials with the structural stability and packing efficiency of micromaterials. The resulting lithium-ion battery achieves an exceptional balance, offering a specific energy of over 116 Wh/kg alongside a pulse power density approaching 90 kW/kg. It sustains extreme discharge rates up to 200C with high capacity retention and delivers millisecond-level pulses at 1000C. This performance leap addresses a critical need in applications requiring short, intense bursts of power, such as grid frequency regulation, electromagnetic launchers, pulsed power systems, and advanced aerospace platforms. The wet ball-milling synthesis route is straightforward and scalable, facilitating potential commercial translation. Future work could focus on extending this graded-particle design principle to other cathode chemistries (e.g., NCM, NCA) and optimizing the anode/electrolyte system to further push the energy-power frontier. Ultimately, this research provides a clear pathway for designing the next generation of lithium-ion batteries that do not force a choice between energy and power but deliver both at an unprecedented level.