The article is mainly focused on the impact, causes and reduction of particle generation during the manufacturing of Li-ion cells. Therefore, it is sub-divided into three sections as each aspect is equally important to study the topic in detail. Sections are listed below.
1. Effect on the cell
2. Sources of generation
3. Mitigation techniques
We are going to start with the first section i.e.; effects of particle generation on cell, in order to understand the need to study this topic. This section will establish the strong WHY to delve well into the topic.
Effect on the cell
Let us consider the typical 21700 Li-ion cell with NMC as a cathode material and graphite as an anode material with Aluminum and Copper as a current collector respectively. Any impurities (foreign particles) present inside the cell affects the cell capacity and performance drastically. To understand the concept well let’s see what causes cells to fail to meet their functional requirement.
- Internal Short circuit (ISC)
We know that the cell is layered structure of active material with some binders, conductive agents coated over a current collector. Both the electrodes are separated by the separator made up of non-conducting material in order to avoid the direct contacts. Direct contact between the electrode results in the internal short circuit (ISC) of the cell.
If we consider the structure of lithium ion cells, there are four different mode of contact responsible for ISC. They are mentioned below.
a. Cathode – Anode
b. Cathode – Copper foil
c. Anode – Aluminum foil
d. Copper foil – Aluminum foil
In a good quality cell, all the surfaces are properly separated and positioned so that the probability of ISC is almost negligible. But when the impurities are present on the cell layers, it acts as medium that causes the short circuit in the cell.
This raises a question that how can a particle cause the surfaces to come in direct contact? So, when the particles are trapped inside the cell during the manufacturing phase, it can cause damage to the separator by penetrating into it and making a path for the electrodes and current collector to come in contact with each other. This defects reflect during the inline short circuit test of the cell.
Sometimes, these cells with particles inside comes to charging process where the cells are charged to their full capacity. During charging, high current flows through the cell and causes a sudden short circuit inside the cell resulting into a heat generation which can lead to a fire and explosion in the charging chamber.
- Voltage drop
In another way, these particles are also responsible for voltage drop during discharge of cells by causing an increase in the internal resistance of the cell. Internal resistance is the resistance that the cell offers to flow of current. This resistance is very small. But, when there are particles present on the layer of electrodes they create a microstructure that can increase the resistance which leads to a voltage drop of the cell.
Both the phenomenon explained above results in not good (NG) cells during the production. This directly impacts the overall yield of the factory. It also increases the scrap hence; additional attention needs to be given to the treatment of this scrap.
- Dendrite Formation
If the size of these particles are very small than it is undetected during all the inline inspections and cells are considered good cells and used in the battery pack. Then over the period of time while its working, one more phenomenon takes place called a lithium dendrite formation. The reason for the formation of these dendrite is different and not related to the presence of particles. It takes place due to the deposition of lithium ion on the anode particle surface as a result of the difference in rate of transfer of lithium ion from cathode to anode and the diffusion rate of lithium ion towards the center of anode particles. So, metallic lithium is formed and extends as the time passes.
By offering a physical base for dendrite growth, these particles contribute to the aforementioned occurrence. Particles give dendrites the nucleation site they need to develop on once they start to form. It speeds up dendritic development and causes an early loss of battery capacity.
Source of particle generation
When it comes to generation of particles in the manufacturing plant, there are hundreds of things causing it. It does not matter how small or big the reason is because even particles of a few micron size can cause deteriorating effect on the cell.
The major factors that are responsible for the generation of particles includes personnel, process equipment, leakage in the dry room and malfunctioning of AHUs.
Personnel : Personnel presence inside the dry room is considered as the main reason for generation of particles. Human body is considered to generate millions of dead skin cells every day which can contribute to the particle generation. During breathing, particles get airborne due to exhalation. Hair dandruffs can also be the reason for the generation of particles. While performing the routine tasks inside the dry room particles can get airborne due to man movement and movement of equipment by the operators.
Process Equipment : During production, the cell is processed through various steps such as electrode preparation which comprises of mixing electrode powder, coating and calendaring, slitting and drying. Second step is winding and assembly of the cell and last step is cell finishing. All the process equipment has one or other moving parts that can be responsible for generating the particles due to abrasion and wearing of moving parts. Other operations like cutting processes, welding processes contribute in the particle generation.
Leakage in the dry room : Leakages can be other factor contributing to particle generation. During the construction of dry room, as a result of worker negligence or improper commissioning there can be some leakages present in the walls through which particles can sneak in and contaminate the environment. Over the period of time degradation of wall material can also lead to formation of leakages.
Malfunctioning of AHUs : All the dry rooms are required to be maintained at some specific environmental conditions. These conditions are achieved by supplying the air with desired parameters and for that this incoming air needs to pass through series of devices such as dehumidifier, cooling coils, dampers and air filters. Damage to any of these components results in a variation of air quality and unwanted particles can enter the dry room causing the contamination in the environment.
So far we have gone through effects and sources of particle generation and understood how important it is to keep track of them. In order to do that, we will see methods to mitigate the cause of particle generation.
In this section, we will go through standard protocols followed for particle monitoring in manufacturing factories. ISO 14644 is the international standard specifically made for dry rooms.
In order to have the least probability for generation of particles inside the dry rooms, there should be strict protocols followed by all the operators working on the equipment. We will cover one by one various methods implemented throughout the industry for tracking and managing the generation of particles.
Personal protective equipment (PPE) : All the operators working inside the dry room should wear the proper PPE kit as per the defined protocols before entering the room. Everyone must pass through the air showers installed near the entrance of room. Operators must make sure that all the body parts are covered except nose. While working inside the dry room, operator should not move rapidly as particles can get airborne. Operators should be properly trained for working inside the dry room so that they follow the discipline during the work.
Particle count monitoring : According to ISO 14644, cleanrooms are defined on the basis of ISO classes ranging from 1-9. ISO class 1 is the most stringent class whereas class 9 is the least stringent. These classes have a predefined limit for the number of particles of particular size to be allowed inside the room.
Track must be kept for the number of particles of particular size reported inside the room. Room is divided into number of zones where the particles are counted with the help of a optical particle counter and reported in the spreadsheet. So, any malfunction happened inside the room is reflected into the count of particles reported and necessary actions can be taken to address the issue. Particles can be monitored in real time as well as periodically depending upon the company.
Dust collectors : This equipment is used where the processes are responsible for generating the particles in the form of chips, fibers etc.; Processes such as cutting, slitting and welding generate particles and these are detrimental to the product. So, a dust collector creates a vacuum suction and sucks the particles keeping the product safe.
Cleaning and mopping : Cleaning products such as mops, wipes should be cleanroom compatible and properly sterilized before the use. Personnel responsible for the cleaning must be trained with the protocols and should adhere to them every time the cleanroom is cleaned. Floors are mopped with a cleanroom specific detergent and HEPA filter vacuum. Walls are wiped with a damp sponge and then vacuum dry. The frequency of cleaning depends on the ISO class maintained inside the room and protocols defined by the company.
Monitoring of AHU system : Regular checks must be conducted for testing the functionality of AHU as it supplies the air to the cleanroom as per the ISO specification. As and when there is increase in the particle generation, periodic air flushing must be performed in order to supply the fresh air and take out the contaminated air.
Apart from above techniques, a dedicated team should be assigned to conduct the testing of samples and report the cause of the particle generation. This way the exact issue can be focused and addressed.
All the three sections are covered and effort has been taken to cover as much information possible. I hope it will be useful to the people working as well as people willing to start their career in Li-ion battery manufacturing industry.
Author: Ankit Fule