What is energy density?

Energy density refers to the amount of energy stored in a unit of space or mass of matter. The energy density of a battery is the average amount of electrical energy released per unit volume or mass of the battery. The energy density of a battery is generally divided into two dimensions: weight energy density and volume energy density.


Battery weight and energy density=battery capacity x discharge platform/weight, with the basic unit being Wh/kg (watt hours/kilogram)


Battery volumetric energy density=battery capacity x discharge platform/volume, with the basic unit being Wh/L (watt hours/liter)


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


What is individual energy density?


The energy density of a battery often refers to two different concepts, one is the energy density of individual cells, and the other is the energy density of the battery system.


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


Single cell energy density, as the name suggests, refers to the energy density at the level of a single cell.


According to "Made in China 2025", the development plan for power batteries has been clarified: by 2020, the energy density of batteries will reach 300Wh/kg; By 2025, the energy density of batteries will reach 400Wh/kg; By 2030, the energy density of batteries will reach 500Wh/kg. This refers to the energy density at the level of a single battery cell.


What is system energy density?


System energy density refers to the ratio of the total amount of electricity in the entire battery system after the completion of individual unit combinations to the weight or volume of the entire battery system. Because the battery system contains internal components such as battery management system, thermal management system, high and low voltage circuits, which occupy a portion of the weight and internal space of the battery system, the energy density of the battery system is 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 chemical system behind the battery is the main reason that cannot escape blame.


Generally speaking, the four parts of a lithium battery are crucial: positive electrode, negative electrode, electrolyte, and diaphragm. The positive and negative poles are the places where chemical reactions occur, equivalent to the Ren and Du meridians, and their important position is evident. We all know that battery pack systems with ternary lithium as the positive electrode have higher energy density than those with lithium iron phosphate as the positive electrode. Why is that?


The existing negative electrode materials for lithium-ion batteries are mainly graphite, with a theoretical capacity of 372mAh/g. The theoretical gram capacity of the positive electrode material lithium iron phosphate is only 160mAh/g, while the ternary material nickel cobalt manganese (NCM) is about 200mAh/g.


According to the barrel theory, the level of water is determined by the weakest point of the barrel, and the lower limit of energy density for lithium-ion batteries depends on the positive electrode material.


The voltage plateau of lithium iron phosphate is 3.2V, while the ternary index is 3.7V. Compared with the two phases, the difference in energy density is 16%.


Of course, in addition to the chemical system, the level of production technology such as compaction density and foil thickness can also affect energy density. Generally speaking, the higher the compaction density, the higher the capacity of the battery in a limited space, so the compaction density of the main material is also considered as one of the reference indicators for battery energy density.


In the fourth episode of "Great Power Heavy Industries II", Ningde Times used 6-micron copper foil and advanced technology to improve energy density.


If you can persist in reading each line until here. Congratulations, your understanding of batteries has reached a new level.


How to increase energy density?


The adoption of new material systems, precise adjustment of lithium battery structures, and improvement of manufacturing capabilities are the three stages for R&D engineers to excel. Below, we will explain from two dimensions: monomer and system.


——Individual energy density mainly relies on breakthroughs in chemical systems


1. Increase battery size


Battery manufacturers can achieve the effect of expanding battery capacity by increasing the size of the original battery. The most familiar example is undoubtedly Tesla, a well-known electric vehicle company that was the first to use Panasonic 18650 batteries, which will be replaced with a new 21700 battery.


But the "weight gain" or "growth" of battery cells is only a temporary solution, not the root cause. The ultimate solution is to find the key technology to improve energy density from the positive and negative electrode materials and electrolyte components that make up the battery cell.


2. Chemical System Transformation


As mentioned earlier, the energy density of a battery is limited by its positive and negative electrodes. Due to the fact that the energy density of negative electrode materials is much higher than that of positive electrodes, increasing the energy density requires continuous upgrading of positive electrode materials.


High nickel positive electrode


Ternary materials generally refer to the family of nickel cobalt manganese oxide lithium oxides. We can change the performance of batteries by changing the ratio of nickel, cobalt, and manganese.


Silicon carbon negative electrode in the diagram


The specific capacity of silicon-based negative electrode materials can reach 4200mAh/g, far higher than the theoretical specific capacity of 372mAh/g for graphite negative electrodes, making it a powerful alternative to graphite negative electrodes.


At present, using silicon carbon composite materials to enhance battery energy density is widely recognized as one of the development directions for lithium-ion battery negative electrode materials in the industry. Tesla's Model 3 uses a silicon carbon anode.


In the future, if we want to make further progress and break through the barrier of 350Wh/kg for single cell batteries, industry peers may need to focus on lithium metal negative electrode battery systems. However, this also means a change and improvement in the entire battery manufacturing process. In several typical ternary materials, it can be seen that the proportion of nickel is increasing while the proportion of cobalt is decreasing. The higher the nickel content, the higher the specific capacity of the battery cell. In addition, due to the scarcity of cobalt resources, increasing the proportion of nickel will reduce the usage of cobalt.


3. System energy density: improving the grouping efficiency of battery packs


The grouping of battery packs tests the ability of battery "siege lions" to deploy individual cells and modules, with safety as the premise and maximizing the use of every inch of space.


There are several ways to slim down battery packs.


Optimize the layout structure


In terms of external dimensions, the internal layout of the system can be optimized to make the internal components of the battery pack more compact and efficient.


topological optimization


We achieve weight reduction design through simulation calculations while ensuring rigidity, strength, and structural reliability. Through this technology, topology optimization and morphology optimization can be achieved, ultimately helping to achieve lightweight battery enclosures.


Material selection


We can choose low-density materials, such as battery pack covers that have gradually shifted from traditional sheet metal covers to composite material covers, which can reduce weight by about 35%. For the lower body of the battery pack, the traditional sheet metal solution has gradually shifted to an aluminum profile solution, reducing weight by about 40% and achieving significant lightweighting effects.


Integrated vehicle design


The integrated design and structural design of the entire vehicle are considered comprehensively, and structural components are shared as much as possible, such as collision prevention design, to achieve ultimate lightweight


Batteries are a comprehensive product, and if you want to improve one aspect of performance, you may sacrifice other aspects of performance. This is the understanding foundation of battery design and research and development. Power batteries are specifically designed for vehicles, so energy density is not the only measure of battery quality.