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In recent years, emerging fields such as new energy vehicles, energy storage, communications, and data centers have experienced rapid development, significantly driving the advancement of large-capacity lithium-ion batteries. As these industries evolve, there is an increasing demand for higher energy density in lithium-ion batteries. [1].
The active materials in a lithium-ion battery—specifically the cathode and anode—play a critical role in determining its energy density. To enhance this, the cathode should have a higher discharge voltage and capacity, while the anode needs to offer high capacity and a low average delithiation voltage. In third-generation lithium-ion batteries, where energy density is the primary focus, both cathode and anode materials are undergoing continuous improvements [2-3]. Future advancements may lead to the use of metal lithium anodes, which could further boost energy density.
Calculating the energy density of a lithium battery is therefore essential. This involves considering both the active and inactive materials, excluding packaging and tabs, to estimate the energy density of different cell types. For example, the cylindrical 18650 monomer is analyzed, and the expected energy density is calculated. This leads to an estimation of the battery’s cost, as shown in the accompanying figure.
Figure 1: Development chart of energy density of lithium-ion batteries from 1990 to 2030.
I. Energy Density Calculation of Lithium-Ion Battery Cells with Different Anode Materials
The energy density of a battery is primarily determined by its cathode and anode materials. However, many studies calculate energy density based solely on the mass of the cathode material, often neglecting the contribution of the anode or the inactive components. This can lead to overestimations of actual performance. According to existing literature [4], the energy densities of various cathode and anode materials were calculated, with their specific capacities and voltages listed in Tables 1 and 2.
Recent developments have seen an increase in the capacity of cathode materials, though they still fall short of theoretical limits. When selecting materials, feasibility and technical viability are considered rather than just maximum reported values. Challenges such as volume expansion, rate capability, and cycle life remain significant obstacles. Table 3 provides typical parameters for removing packaging and tab materials, offering a more accurate representation of real-world performance.
It's important to note that variations in battery shape, electrode coating thickness, and inactive material characteristics can affect the accuracy of calculations. Therefore, results from tables may differ slightly from actual battery performance, depending on the manufacturing process.
Figures 2(a)–(j) illustrate the energy density calculations of cells formed by combining 10 different anode materials with 16 cathode materials. Figure 2(i) highlights the Li-rich-300 vs. Si-C-2000 system, which achieves the highest mass energy density of 584 Wh/kg and the highest volumetric energy density of 1645 Wh/L (excluding packaging and tabs).
Table 1: Calculation of positive active materials and their specific capacity and voltage.
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Table 2: Calculation of negative active materials and their specific capacity and voltage.
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Figure 2: Cell energy density calculation for different anode and cathode materials.
II. Energy Density Calculation of Metal Lithium Ion Battery Cells
While traditional anode materials like graphite have a theoretical specific capacity of 372 mAh/g, current reversible capacity reaches up to 365 mAh/g. High-capacity anode materials such as silicon-carbon can reach 1000–1500 mAh/g, but practical performance is limited by issues like volume expansion and poor cycle life. In contrast, metallic lithium has a theoretical specific capacity of 3860 mAh/g, making it highly promising if utilization rates can be improved.
This study calculated the energy density of lithium-metal batteries using different lithium utilization rates (100%, 80%, 50%, and 33%). Comparing these results with those of conventional lithium-ion batteries, it was found that lithium-metal systems show significantly higher energy density. For instance, when using Li-rich-300 as the cathode, the energy density reached 649 Wh/kg with full lithium utilization, and even at 33% utilization, it remained at 521 Wh/kg.
Figure 3: Energy density calculation of metal lithium as a negative electrode.
III. Estimation of 18650 Single Cell Energy Density
Considering the impact of tabs and packaging materials, the energy density of individual cells can be estimated. Tables 4 and 5 provide performance parameters for the NCR18650 cylindrical battery and prismatic soft-pack batteries [6]. For example, the NCR18650 typically has a 15–20% mass fraction of tabs and packaging. Table 6 summarizes the highest energy densities for different anode materials, while Table 7 shows the energy density of Si-C-1000 anodes paired with various cathode materials. The LCO-220 cell had an energy density of 492 Wh/kg, and the monomer energy density was 416 Wh/kg. However, the inclusion of packaging materials reduces the overall energy density further.
IV. Relationship Between Battery Energy Density and Driving Range
The driving range is a key performance indicator for electric vehicles. Increasing the battery's energy density allows for longer ranges without increasing the battery pack's size or weight. For example, the Beiqi EV200, with a total mass of 1.29 tons, consumes 14 kWh per 100 km, and the battery box has a volume of 220L. At a mass energy density of 180 Wh/kg, the vehicle can travel about 200 km under normal conditions. If the energy density increases to 400 Wh/kg, the driving range could extend to 521 km, meeting the 200,000 km lifecycle requirement.
V. Cost of High-Energy-Density Lithium Batteries
Based on current industrialized battery core compositions and production processes, the raw material costs for different battery cells can be estimated. Table 9 outlines the cost breakdown for a 100Ah battery. For example, the cost of cathode materials and electrolytes accounts for 37–56% of the total battery cost, while silicon-carbon anodes account for 38–48%. Using metallic lithium as the anode offers a significant cost reduction compared to silicon-carbon anodes.
It should be noted that material costs make up 60–70% of the total battery manufacturing cost. After adjusting for this, the actual single-cell cost becomes even lower. With further technological improvements, the cost of metal lithium-ion batteries could potentially drop below that of lead-acid batteries.
VI. Comprehensive Technical Indicators
Battery applications require not only high energy density but also other performance metrics such as power density, charging speed, cycle life, safety, and cost. Whether a battery can be applied depends on whether it meets the minimum requirements for a given application—a concept known as the "cask effect." Figures 5(a) and 5(b) show spider diagrams comparing the main technical indicators for different applications and the expected versus actual values for pure electric vehicles. There is still a significant gap between current levels and expectations, highlighting the need for new power battery technologies.
From 1990 to now, the energy density of batteries has mainly been increased by optimizing the ratio of active to inactive materials. While reducing the thickness and mass of separators, copper, aluminum foils, and packaging materials is technically possible, it remains a major challenge. Choosing new cathode and anode material systems is a more feasible approach to boosting energy density. With a high-capacity silicon-carbon anode and a lithium-rich manganese-based cathode, the energy density of an 18650 battery can reach 442 Wh/kg, with a cost of 0.4 yuan/W·h, meeting the requirements for pure electric vehicles. A lithium-rich manganese-based metal ion battery can achieve a mass energy density of 521 Wh/kg and a cost of 0.2 yuan/W·h. Although challenges remain in chargeable metallurgical batteries using liquid electrolytes, solid-state batteries are expected to address these issues. Ultimately, achieving higher energy density while maintaining other performance standards requires extensive research and development, presenting both challenges and opportunities.