Cathode Material (+) [or LIC Chemistries]
The Li-ion is a very generic term and as these ions can be generated from Cathode using a variety of chemistries, bringing a range of performance characteristics. The most common chemistry combinations used in LICs are: LFP, LMO, LTO, LCO, NCA, and NMC (see below), and cell manufacturers also use their combination to get an improved performance. Some of the LIC chemistries are indicated below:
- Anode (-): The negative side of the electrode is known as the anode (-). Today, most anodes (-) are made of a mixture of one of two materials either graphite, or soft or hard carbons. The quality of cell majorly depends upon the quality and this material & it plays a critical role in the performance of the LICs.
- Separators: The separator is the single most important part in any LIC design and separates anode (-) & cathode (+) by creating a physical partition. It is often made of plastic or ceramic. While the separator must maintain the isolation of two electrodes, it must also be able to withstand the corrosive hydrocarbon (HC) based electrolytes used in LICs. However, if this separator fails due to any reason, the two electrodes will come into direct contact, leading to an internal short circuiting, causing cell failure & in some cases, a fire. While some manufactures also use multilayer PP/ PE allowing it to sustain a higher temperatures (~135 °C), new technologies are also allowing to integrate ceramics in these separators as they allow higher temperatures at separator surface, increasing the safety of LIC.
- Electrolytes: This is usually a liquid or gel based solution in which anode (-) and cathode (+) are submerged & acts as a conductor for the Li-ions to move between the anode (-) and the cathode (+). This electrolyte is typically a HC based mixture that includes multiple additives to provide different functionality within the LIC. These additives are the “secret sauce” of any cell manufacturer and are one of the key research areas of the cell makers. Typical electrolytes may include a mixture of alkyl carbonates such as ethylene carbonate, dimethyl, diethyl, and ethyl methyl carbonates and lithium salts (LiPF6). The main function of the additives are to facilitate the formation of the solid electrolyte interphase (SEI) layer, to reduce the irreversible capacity and gas generation to enhance the thermal stability of the cell.
LIC Shape, Types and Sizes
There are essentially three main types of LIC form factors: small cylindrical, large prismatic, and pouch (or polymer) cells.
- Cylindrical Cells: They are made by rolling long strips of cathode foil, separator, and anode foil together and inserting into a rigid tubular stainless steel or aluminum cell housing or “can”. The “can” is filled with liquid electrolyte, safety disks are inserted into the top, and the electrodes are welded to the outer battery terminals (in this case, the top and bottom of the cell). The cell is hermetically sealed by crimping the top disk assembly closed. These cells are identified by their nomenclature such as 18650 (18 cm is its diameter while 65 cm is its length) & come in different nomenclatures such as 10440 (AAA), 14500 (AA), 16340 (CR123A), 18650, 21700, 26650 & 32650 & 32330 depending upon their application.
- Prismatic Cells: These are similar in construction to cylindrical cells but use a flat rectangular housing to lower the overall thickness of the cell. The electrode/separator assembly can be rolled, as with cylindrical cells, or it can be a rectangular stack of individual electrodes (similar to a deck of cards). The battery terminals can be placed as contact pads on the top or side of the housing. The prismatic cell thin form factor is well suited to use in consumer electronics, particularly when ease of battery replacement is desirable.
- Pouch Cell: They also have a thin rectangular form factor & are composed of rectangular stacks of individual electrode/separator layers, but instead of a rigid metal case they use a laminated flexible polymer/aluminium “bag”. The electrodes have tabs along one side; these are welded together with battery terminal tabs that stick out of the top of the bag. The assembly is saturated with a liquid electrolyte and the bag is heat-sealed. By eliminating the rigid housing, pouch cells save on cost, weight, and thickness. The flexible pouch is, however, prone to swelling and this can pose problems with lifetime, capacity loss, and safety.
However, by far the highest volume LIC format in production today is the 18650 cylindrical cell with nearly 660 million cells produced annually. When it comes to EV applications, nearly all of the major auto manufacturers have identified NiMH for HEVs, while BEVs manufacture have preferred to use 18650 cells or pouch type LICs e.g., the Tesla uses about 7000 no. of 18650 cells in their Model 3 (85 kWh battery pack) whereas the General Motors uses 288 pouch type cells in their Chevrolet Volt (16.5 kWh battery pack) and Nissan uses 192 pouch-type cells in their LEAF (with 24 kWh battery pack).Yet again, in order to improve its EV performance, Tesla, recently, introduced a 4680 form factor for LIC that is expected to have six times power and about five times the energy capacity of 21700.
Safeties in LICs
Like all other energy storage devices, LICs also have some inherent safety challenges. Additionally, there is always the potential for contamination creeping in or errors occurring during the cell manufacturing process, causing cell failure or sudden energy release. In order to help mitigate these risks, few safety features are in-built in these cells during manufacturing. The LICs have a hard can type container, in which the first and most common in-built safety is a cell vent, which is basically designed as an engineered failure point within the cell in the event of a buildup of pressure inside the cell. However, in the pouch cell there is a notch or other weak point to fail in case of pressure build up in the cell.
Secondly, a type of non-resettable fuse (generally it is pressure based) or the “Current Interrupting Device (CID)” is often integrated with in the LIC. In essence, the CID is a two-part mechanism that is designed to separate and break the flow of current to the terminals if the pressure builds up beyond a certain point.
Third, type of safety that is included in some cell designs is a “Positive Temperature Coefficient (PTC)” device, which is essentially a resettable thermal fuse, which is designed to break the current flow if the temperature of the cell rises above a predetermined point and when the temperature of the cell falls back to normal operating temperatures, the PTC will reset and the cell will again be usable.
The biggest benefit of the cylindrical cell is that it uses either a nickel-coated steel or aluminium “can” offering a high-strength packaging requiring a lot of energy to damage it and it provides “stack” pressure on the jellyroll inside the can. However, one of the challenges involved in using such tubular format cells, apart from their sizes, is in their lid assembly. Manufacturers are slowly transitioning to laser welding of the lids onto the cells as crimping process to attach the lid have been found to fail under certain operating conditions. [In 2006, Sony recalled its $250 million dollar worth of cells which had started to fail as some metallic particles began to make their way inside the cells (Mook, 2006)].
Electrical Safety
Since EVs’ eco-system needs electricity, electrical safety become of the most critical factor to avoid any mishap. While the EV connectors must be polarizedand configured so that it is non-interchangeable with other electrical devices such as electric dryers, the method by which the charging equipment couples to the EV (which can be either conductive or inductive), must also be designed so as to prevent against unintentional disconnection. Additionally, the new electrical codes require that EV charging loads be considered continuous; therefore, the premises wiring for the EV charging equipment must be rated at 125% of the charging equipment’s maximum load. All EV charging equipment must have ground-fault circuit interrupter devices for personnel protection. To avoid any physical damage and water ingress in the EV mounted battery packs, it is generally expected that these battery packs must be rated form IP67 ratings. Also an interlock to de-energize the equipment in the event of connector or cable damage must be incorporated. Furthermore, a connection interlock is required to ensure that there is a non-energized interface between the EV charging equipment and the EV until the connector has been fastened to the vehicle. A ventilation interlock is also required in the EV charging equipment; this interlock enables the EV charging equipment to determine whether a vehicle requires ventilation and whether ventilation is available.
If ventilation is included in the system, the ventilation interlock will allow any vehicle to charge. However, if ventilation is not included in the system, the mechanical ventilation interlock will allow the vehicles equipped with non-gassing batteries to charge, but not vehicles equipped with gassing batteries.
…To be continued
Prabhat Khare holds BE (Electrical) & a Gold Medalist from IIT, Roorkee. He is an Automotive (EV) & Engineering Consultant, as well as a Technology Article Writer. He is a Certified Energy Manager (BEE) & Lead Assessor for ISO 9K, 14K, 45K & 50K. He can be reached at LinkedIn: https://www.linkedin.com/in/prabhatkhare2/.
