Advances in cell manufacturing can play a critical role in helping to reduce costs in lithium ion battery production for the energy storage industry. Robert Galyen, CTO of Contemporary Amperex Technology Limited (CATL), discusses some of the developments in this area lithium ion battery cell technology and manufacturing.
Today’s energy storage battery systems are complex operational devices which must provide power and energy upon command based on the application’s demands. We must understand how the requirements are driven from the application to the fundamental building blocks of the system, all the way to the cell chemistry. We’ll start at the system level and drill down to the cell level.
In most battery systems the cells typically carry about 60-80% of the overall costs. The cells are simply an ‘electron fuel tank’ package to fit within the larger system to meet application requirements. While the cells form the core of any battery pack and system, from modules to cabinets to containers, the manufacturing required to assemble battery packs is becoming more sophisticated by going more towards automated assembly systems. These systems typically include robotics, parts delivery systems, welding, automated torque drivers and highly advanced tracking systems. Key drivers include traceability, quality improvements and
Traceability is important due to the potential of a quality issue and the large number of parts involved. Tracking of these parts allows for isolated identification, minimising large recall potential and enabling field replacement. Tracking is not only limited to the parts but also the processes used. As an example, for the sophisticated battery packs for the automotive industry the torque values and contact resistance measurements are recorded into a master file for every battery pack built. There are many examples of this type of recorded data.
In terms of quality and maintaining consistency in production, machines are far more reliable than human assembly. Typically hand assembly is in the tenths/hundredths of percentage failure rates improving to parts-per-million/billion failure rates for automated assembly. Automation also affords cost reduction in labour costs due to the large number of parts of assembly. Throughput rates can be significantly more than double those of human assembly processes. However, there are some parts of the assembly which require human input for these complicated systems.
When examining the requirement drivers for these battery systems for the stationary energy storage industry this is substantial cross-over with drivers for other markets that use battery technology, such as consumer electronics, transportation and various industrial sectors. A typical requirements hierarchy can be described in the following pictogram:
The most widely accepted guidance in the transportation market segments comes from the organisation SAE International. These same guidance principles can be applied to energy storage which are stated in order of priority:
Regardless of market, the preservation of human life is of paramount importance. Therefore, safety must be number one on the list of requirements. An enormous amount of worldwide resources have been poured into the automotive industry to make battery systems safe. Examples of these are SAE International J2929 and J2964 safety standards for battery systems. Many of the automotive standards can be applied to energy storage systems. The major difference is the large amount of energy in some of the larger energy storage systems, which can be in the range of multiple MWh.
Performance in the form of energy density and power density are important to the particular application and must be defined by the engineers creating the requirements of the use case. The performance metrics must consider the environment topics of temperature, altitude, moisture, infestation, seismic activity, flooding, and so forth. The application engineer may use a chart like the one at the bottom of the page to get the correct power to energy ratio of a use case.
If it does not last long enough the value proposition of the system is inadequate to provide the return on investment (ROI) necessary for the system. The cycle life expectancy of most stationary storage systems exceeds 10 years of service. The real difficulty is running the tests to prove such long life. A surrogate method known as Highly Accelerated Life Testing (HALT), is employed by battery manufacturers to estimate the life expectancy of their products. Accelerated life testing is used in many industries where it is not feasible to monitor a product’s performance in real-time. One good example is the solar photovoltaic (PV) industry, where it is routine to expect panels to last for 25 years.
Clearly cost is important, as it is the basis for financial investment and recovery. But, if the product is not safe and fails to meet performance/life metrics, then cost is meaningless.
Due to concerns over sharing proprietary information in a highly competitive market, some common information from automotive sector cells will be used as examples. These cells are typically the same as, if not interchangeable, with energy storage applications.
Within all leading battery manufacturers, there is ongoing R&D to improve anodes and cathodes. The chart below shows a relative juxtaposition of the cathodes and anodes which provide both voltage and energy capacity of these components of the cell. It is the various combinations of these anodes and cathodes which produce the energy/power ratios for the applications they service. As an example, the combination of the nickel, manganese and cobalt (NMC) cathode to the carbon/graphite anodes are used for high energy density automotive applications (due to volume constraints) even though the cost is higher than lithium iron phosphate. Iron phosphate chemistries are favoured for most stationary storage applications because volume constraints are not as important.
An enormous amount of worldwide resources have been poured into the automotive industry to make battery systems safe. Examples of these are SAE International J2929 and J2964. Many of these standards can be applied to energy storage systems.
There are several formats of cells on the market:
Pouch is a form of a cell container. Typically an aluminium sheet coated with a polymer serves as the electrochemical contents casing which is flexible, sealed by hermetic seals of the polymer around all edges. These containers usually in a rectangular format but can be cylindrical, trapezoidal, or obtuse in shape.
Can is a form of a cell container where an aluminium or steel container houses the electrochemical contents and is welded to complete the enclosure. These containers can be cylindrical or rectangular in shape and varies in size based on capacity and chemistry used.
Wound is a method of assembling the internal cell components by winding the cathode, anode and separator around a mandrill, then assembled into the cell container.
Prismatic is a method of assembling the internal cell components, typically made by stacking the anode-separator-cathode-separator in a repeating manner to create the cell element, which is then inserted into the cell container.
Here are two examples of high volume production automotive cells. One is a prismatic pouch cell from a pure electric vehicle application and the second is a wound can cell used in a hybrid vehicle. These same terms and descriptors serve all market segments, including the energy storage market.
Cell production steps
Value creation for the energy storage industry, using lithium ion batteries, lies within the chemistry and the ability to manufacture cells in a cost-effective, high-quality manner. Lithium ion cells are one of the most efficient ways of storing energy directly with least amount of loss compared to other storage methods.
Contemporary Amperex Technology Limited (CATL) is headquartered in Hong Kong, with three sites in China. Founded in 1999 the company is now in the top five global lithium ion battery suppliers and is the world’s largest supplier of pouch/polymer batteries, supplying to global top tier brands across the transportation, energy storage and industrial markets
At its factory in Ningde, Fujian province, CATL has one of the largest rooftop solar PV installations in China, with nearly 9000 panels generated electricity that is used to power the company’s battery manufacturing operations.
CATL’s energy storage projects and installations include the provision of two sets of 100KW/120KWh systems for a smart grid for China’s State Grid as well as larger battery systems for the State Grid, for integrating renewables, such as wind and solar farms. The company is also working with leading automotive brands that are looking into the concept of developing secondary life batteries, where batteries that come out of warranty for EVs can be used in the stationary storage market.