How to Improve Lithium Battery Energy Density?

Energy density refers to the amount of energy stored per unit volume or mass of a substance. For batteries, energy density represents the average electrical energy released per unit volume or mass. Battery energy density is typically categorized into two dimensions: weight energy density and volume energy density.

What is Energy Density?

Energy density refers to the amount of energy stored per unit volume or mass of a substance. For batteries, energy density represents the average electrical energy released per unit volume or mass. Battery energy density is generally categorized into two dimensions: weight energy density and volume energy density.

Battery weight energy density = Battery capacity × Discharge platform / Weight, with the basic unit being Wh/kg (watt-hours per kilogram)
Battery volume energy density = Battery capacity × Discharge platform / Volume, with the basic unit being Wh/L (watt-hours per liter)
The higher the energy density of a battery, the greater the amount of electrical energy stored per unit volume or weight.

What is cell energy density?

Battery energy density often refers to two distinct concepts: the energy density of individual cells and the energy density of the battery system. A cell is the smallest unit within a battery system. M cells form a module, and N modules form a battery pack—the fundamental structure of automotive power batteries. Cell energy density, as the name implies, refers to the energy density at the individual cell level.

China’s “Made in China 2025” initiative outlines the development roadmap for power batteries:
– By 2020, battery energy density should reach 300 Wh/kg;
– By 2025, battery energy density should reach 400 Wh/kg;
– By 2030, battery energy density should reach 500 Wh/kg.
These targets specifically refer to the energy density at the individual cell level.

What is system energy density?

System energy density refers to the ratio of the total electrical capacity of a battery system to its total weight or volume after individual cells are assembled. Since the battery system includes components like the battery management system, thermal management system, and high/low voltage circuits that occupy part of the system’s weight and internal space, the energy density of the battery system is always lower than that of individual cells.

System energy density = Battery system capacity / Battery system weight OR Battery system volume

What exactly limits the energy density of lithium batteries? The underlying chemical systems of batteries are primarily responsible for this limitation.

Generally, four components are critical in lithium batteries: the cathode, anode, electrolyte, and separator. The cathode and anode are where chemical reactions occur, akin to the body’s vital meridians, highlighting their crucial role. We know that battery packs using ternary lithium cathodes achieve higher system energy density than those with lithium iron phosphate cathodes. Why is this?

Current lithium-ion battery anodes primarily use graphite, which has a theoretical specific capacity of 372 mAh/g. The cathode material, lithium iron phosphate, has a theoretical specific capacity of only 160 mAh/g, while ternary materials like nickel-cobalt-manganese (NCM) offer approximately 200 mAh/g.

According to the bucket theory, the height of the water level is determined by the shortest plank. Similarly, the lower limit of a lithium-ion battery’s energy density is dictated by the cathode material.

LFP has a voltage plateau of 3.2V, while NCM reaches 3.7V. Comparing these values reveals a clear difference in energy density: a 16% gap.

Of course, beyond the chemical system, manufacturing processes—such as packing density and foil thickness—also impact energy density. Generally, higher packing density allows greater capacity within a limited space, making the packing density of the main material another key indicator for battery energy density.

If you’ve made it this far, reading line by line, congratulations—your understanding of batteries has reached a new level.

How can energy density be improved?

The adoption of new material systems, fine-tuning of lithium battery structures, and enhancement of manufacturing capabilities are key focuses for R&D engineers. Below, we will explain this from both the cell and system dimensions.

Cell Energy Density: Primarily Depends on Breakthroughs in Chemical Systems

1. Increasing Battery Size

Battery manufacturers can achieve capacity expansion by increasing the size of existing cells. The most familiar example is Tesla, the renowned electric vehicle manufacturer that pioneered the use of Panasonic’s 18650 batteries, now transitioning to the newer 21700 battery format.

However, making cells “fatter” or “taller” is merely a temporary fix, not a fundamental solution. The root cause requires identifying key technologies to enhance energy density within the positive and negative electrode materials and electrolyte composition that constitute the battery cell.

2. Chemical System Transformation

As mentioned earlier, battery energy density is constrained by its positive and negative electrodes. Since current anode materials possess significantly higher energy density than cathodes, advancing cathode materials is essential for boosting overall energy density.

3. High-Nickel Cathodes

Ternary materials refer to the family of lithium nickel cobalt manganese oxide compounds. By adjusting the ratio of nickel, cobalt, and manganese, we can modify battery performance.

4. Silicon-Carbon Anodes

Silicon-based anode materials can achieve specific capacities of up to 4200 mAh/g, far exceeding the theoretical capacity of graphite anodes at 372 mAh/g. This makes them a strong contender to replace graphite anodes.

Currently, utilizing silicon-carbon composite materials to enhance battery energy density is widely recognized as one of the key development directions for lithium-ion battery anodes. Tesla’s Model 3 incorporates silicon-carbon anodes.

Looking ahead, to achieve the next breakthrough—surpassing the 350Wh/kg threshold for individual cells—industry peers may need to focus on lithium metal anode battery systems. This, however, implies a fundamental overhaul and refinement of the entire battery manufacturing process. Among several typical ternary materials, nickel’s proportion is steadily increasing while cobalt’s is decreasing. Higher nickel content translates to greater specific capacity for the cell. Additionally, given cobalt’s scarcity, increasing nickel’s proportion reduces the amount of cobalt required.

System Energy Density: Enhancing Battery Pack Assembly Efficiency

Battery pack assembly tests engineers’ ability to strategically arrange individual cells and modules, prioritizing safety while maximizing every inch of space. Key approaches to “slimming down” battery packs include:

Optimizing Layout Structure: From a dimensional perspective, internal system arrangements can be refined to achieve more compact and efficient component placement within the pack.

Topology Optimization: Through simulation calculations, we achieve weight reduction while ensuring structural rigidity and reliability. This technology enables topology and morphology optimization, ultimately contributing to battery box lightweighting.

Material Selection: We can opt for low-density materials. For instance, battery pack lids have transitioned from traditional sheet metal to composite materials, achieving approximately 35% weight reduction. For the battery pack lower case, we have transitioned from traditional sheet metal solutions to aluminum extrusions, achieving approximately 40% weight reduction with significant lightweighting effects.

Batteries are highly multifaceted products. Improving performance in one aspect may compromise others—this fundamental understanding underpins battery design and development. As power batteries are vehicle-specific, energy density alone does not serve as the sole measure of battery quality.