Energy density analysis of lithium batteries

What is energy density?

Energy density refers to the amount of energy stored in a given unit of space or mass of a substance. For batteries, energy density is the electrical energy released per unit volume or mass on average. It is generally measured in two dimensions: gravimetric energy density and volumetric energy density.

Gravimetric energy density of a battery = battery capacity × discharge plateau / weight, with the basic unit being Wh/kg (watt-hours per kilogram).

Volumetric energy density of a battery = battery capacity × discharge plateau / volume, with the basic unit being Wh/L (watt-hours per liter).

The higher the energy density of a battery, the more electricity it can store per unit volume or mass.

What is cell energy density?

Energy density in batteries often refers to two distinct concepts: the energy density of a single cell and the energy density of the battery system.

A cell is the smallest unit of a battery system. M cells form a module, and N modules form a battery pack, which is the basic structure of automotive power batteries.

Cell energy density, as the name suggests, refers to the energy density at the individual cell level.

According to "Made in China 2025," the development plan for power batteries is clear: by 2020, battery energy density should reach 300 Wh/kg; by 2025, it should reach 400 Wh/kg; and by 2030, it should reach 500 Wh/kg. This refers specifically to the energy density at the individual cell level.

What is system energy density?

System energy density refers to the ratio of the total energy of the entire battery system to its total weight or volume after the cells are assembled. Since the battery system includes components such as the battery management system, thermal management system, and high- and low-voltage circuits, which occupy part of the system's weight and internal space, the energy density of the battery system is generally lower than that of the individual cells.

System energy density = total energy of the battery system / total weight OR total volume of the battery system.

What exactly limits the energy density of lithium-ion batteries?

The underlying chemical system of the battery is the primary reason.

Generally, four parts of a lithium-ion battery are critical: the cathode, anode, electrolyte, and separator. The cathode and anode are where chemical reactions occur, akin to vital pathways, highlighting their importance. It is widely known that battery systems with ternary lithium as the cathode have higher energy density than those with lithium iron phosphate as the cathode. Why is that?

Most existing lithium-ion battery anode materials are graphite-based, with 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) have around 200 mAh/g.

According to the "barrel theory," the water level is determined by the shortest stave. Similarly, the lower limit of energy density in lithium-ion batteries depends on the cathode material.

The voltage plateau of lithium iron phosphate is 3.2V, while that of ternary materials is 3.7V. Comparing the two, the difference in energy density is clear: a 16% gap.

Of course, besides the chemical system, production processes such as compaction density and foil thickness also affect energy density. Generally, the higher the compaction density, the greater the battery capacity in a limited space, making the compaction density of the main materials a key reference indicator for energy density.

In the fourth episode of "China's Mega Projects II," CATL used 6-micron copper foil and advanced manufacturing techniques to enhance energy density.

If you’ve read every line up to this point, 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-ion battery structures, and advancements in manufacturing capabilities are the three stages where R&D engineers can showcase their expertise. Below, we will explain from both the cell and system perspectives.

— Cell energy density relies primarily on breakthroughs in the chemical system.

1. Increasing battery size
   Battery manufacturers can expand capacity by increasing the size of existing batteries. A familiar example is Tesla, the pioneering electric vehicle manufacturer that initially used Panasonic’s 18650 batteries and later switched to the new 21700 batteries.

   However, merely making cells "fatter" or "taller" is a temporary solution, not a fundamental one. The key to improving energy density lies in the cathode and anode materials and electrolyte composition that make up the battery unit.

2. Chemical system transformation
   As mentioned earlier, energy density is constrained by the cathode and anode of the battery. Since the energy density of anode materials currently far exceeds that of cathode materials, improving energy density requires continuous upgrades to cathode materials.

   High-nickel cathode
   Ternary materials generally refer to the broad family of lithium nickel-cobalt-manganese oxides. By adjusting the ratios of nickel, cobalt, and manganese, battery performance can be altered.

   In the figure, several typical ternary materials show that the proportion of nickel is increasing while that of cobalt is decreasing. Higher nickel content means higher specific capacity of the cell. Additionally, due to the scarcity of cobalt resources, increasing the proportion of nickel reduces cobalt usage.

   Silicon-carbon anode
   Silicon-based anode materials can achieve a specific capacity of 4,200 mAh/g, far exceeding the theoretical specific capacity of graphite anodes at 372 mAh/g, making them strong candidates to replace graphite anodes.

   Currently, using silicon-carbon composite materials to enhance battery energy density is widely recognized as a key direction for lithium-ion battery anode materials. Tesla’s Model 3, for example, employs a silicon-carbon anode.

   In the future, to break through the 350 Wh/kg barrier for individual cells, the industry may need to focus on lithium metal anode battery systems, though this would require significant changes and refinements in the entire battery manufacturing process.

3. System energy density: Improving battery pack integration efficiency
   Battery pack integration tests the ability of battery engineers to arrange cells and modules strategically, maximizing the use of space while ensuring safety.

   "Slimming down" the battery pack mainly involves the following approaches:

   Optimizing layout structure
   From a dimensional perspective, the internal arrangement of the system can be optimized to make component placement within the battery pack more compact and efficient.

   Topology optimization
   Through simulation calculations, weight reduction designs can be achieved while ensuring rigidity, strength, and structural reliability. This technology enables topology and morphology optimization, ultimately contributing to the lightweight design of the battery casing.

   Material selection
   Low-density materials can be chosen. For example, the battery pack cover has gradually shifted from traditional sheet metal to composite materials, reducing weight by approximately 35%. For the battery pack lower casing, traditional sheet metal solutions are increasingly being replaced by aluminum profiles, achieving about 40% weight reduction and significant lightweight effects.

   Integrated vehicle design
   Integrated vehicle design considers the overall vehicle structure, sharing and reusing structural components as much as possible, such as in crash protection design, to achieve lightweighting.