Cell chemistry & parts
The battery cells are the most important part of the battery system and the most important factor both for cost and performance on EV's.
The anode is one of the two electrodes in an electric vehicle (EV) battery, the other being the cathode. The anode is the electrode where oxidation occurs during the discharge cycle of the battery, releasing electrons into the external circuit to power the device or vehicle. During the charging cycle, the anode is where reduction occurs, absorbing electrons from the external circuit and storing energy in the battery.
In a lithium-ion battery, the anode is typically made of graphite, which has a layered structure that allows lithium ions to intercalate, or insert, between the layers. When the battery is being discharged, lithium ions move from the anode to the cathode through the electrolyte, while electrons flow through the external circuit to power the device. During the charging cycle, the reverse process occurs, with lithium ions moving from the cathode back to the anode, where they are stored.
The performance of the anode is a critical factor in the overall performance and safety of an EV battery. A high-quality anode material should have high lithium-ion storage capacity, good conductivity, good structural stability, and be resistant to degradation over multiple charge and discharge cycles. The graphite anode used in most lithium-ion batteries meets these requirements, but researchers are also exploring other anode materials such as silicon, which has a much higher storage capacity but is more prone to degradation.
In summary, the anode is the electrode where oxidation occurs during the discharge cycle of an EV battery, releasing electrons into the external circuit to power the device. It is typically made of graphite and plays a critical role in the performance and safety of the battery.
The cathode is the electrode where reduction occurs during the discharge cycle of the battery, accepting electrons from the external circuit to power the device or vehicle. During the charging cycle, the cathode is where oxidation occurs, releasing electrons into the external circuit and storing energy in the battery.
In a lithium-ion battery, the cathode is typically made of a metal oxide, such as lithium cobalt oxide (LCO), lithium nickel cobalt manganese oxide (NMC), or lithium iron phosphate (LFP). The choice of cathode material can have a significant impact on the performance, safety, and cost of the battery.
Lithium nickel manganese cobalt oxides (NMC)
NMC cathodes are a popular choice for EV batteries due to their high energy density and good thermal stability. They offer a balance between energy density and power density, making them suitable for a wide range of EV applications.
There are different versions of NMC (Lithium Nickel Manganese Cobalt Oxide) cathodes used in electric vehicle (EV) batteries. The composition and ratio of the metals used in the cathode can vary, resulting in different properties and performance characteristics.
The most common NMC cathodes used in EV batteries are NMC111, NMC532, and NMC622, which refer to the ratios of nickel, manganese, and cobalt in the cathode. For example, NMC111 contains one part nickel, one part manganese, and one part cobalt, while NMC523 contains five parts nickel, three parts manganese, and two parts cobalt. The higher the nickel content, the higher the energy density of the cathode, but also the higher the cost.
More recently, there have been developments in NMC cathode technology that use even higher nickel content, such as NMC811 (eight parts nickel, one part manganese, and one part cobalt) and NMC622. These cathodes offer even higher energy density, but may also have some challenges related to thermal stability and cycle life. NCM9 is the most recent evolution where Nickel is 90% of the content.
In addition to the metal ratios, the particle size and morphology of the cathode material can also impact performance. For example, smaller particle sizes can improve the rate capability and power density of the battery, while larger particles may improve the energy density.
Overall, the choice of NMC cathode composition and morphology depends on the specific requirements of the EV battery, including energy density, power density, thermal stability, durability, and cost. Battery manufacturers are constantly researching and developing new NMC cathode materials to improve the performance and efficiency of EV batteries.
Lithium nickel cobalt aluminum oxides (NCA)
NCA cathodes have a high energy density and are used in EV batteries, particularly in Tesla’s vehicles. NCA cathodes offer high energy density and good performance, making them a popular choice for many EV manufacturers.
The composition of NCA cathodes typically includes nickel, cobalt, aluminum, and oxygen. The exact ratio of these elements can vary depending on the specific application and requirements of the battery. NCA cathodes offer a high energy density, which means they can store more energy per unit of weight or volume compared to some other cathode materials. This results in a longer driving range for EVs equipped with NCA batteries.
NCA cathodes also have good power density, which means they can deliver high power output when needed, such as during acceleration or regenerative braking. NCA batteries also have a long cycle life, which means they can withstand many charge-discharge cycles without significant degradation, making them a reliable and durable option for EV applications.
However, NCA cathodes can be sensitive to high temperatures, which can cause thermal degradation and reduce their lifespan. To address this, EV manufacturers often use sophisticated battery thermal management systems to maintain the temperature of the battery within a safe operating range.
Overall, NCA cathodes offer a good balance between energy density and power density, making them a popular choice for EVs that require high performance and long driving range. However, their sensitivity to high temperatures means they require careful thermal management to maintain their performance and lifespan.
Lithium iron phosphate battery (LFP)
Lithium Iron (Ferro) Phosphate (LFP) is a cathode material commonly used in electric vehicle (EV) batteries due to its high thermal stability and long cycle life. LFP cathodes have a lower energy density compared to some other cathode types, but they offer good safety, durability, and cost-effectiveness.
The composition of LFP cathodes consists of lithium iron phosphate (LiFePO4), which is a relatively stable and non-toxic material compared to other lithium-ion battery chemistries. This makes LFP cathodes a popular choice for EV batteries in China, where safety regulations are strict.
Brands like Tesla has started using LFP in their lower range models.
Advantages with LFP
One of the key advantages of LFP cathodes is their high thermal stability, which makes them less susceptible to thermal runaway and overheating compared to other cathode types. This is because the chemical bond between iron, oxygen, and phosphorous in the cathode structure is stronger than other cathode materials. As a result, LFP batteries can be operated at high temperatures without the need for active cooling systems, which reduces the complexity and cost of the battery pack.
Another advantage of LFP cathodes is their long cycle life, which means they can withstand many charge-discharge cycles without significant degradation. This makes LFP batteries a good choice for applications that require long life and high reliability, such as EVs and energy storage systems.
Cost is lower than nickel based chemistries. Typical about 20% less for each kWh.
LFP cathodes have a lower energy density compared to some other cathode types, which means they have a lower capacity to store energy per unit of weight or volume. This makes them less suitable for EVs that require high driving range and energy density.
LFP battery performance in cold weather is worse than other lithium-ion batteries, such as NCA/NMC batteries. Cold temperatures slow down the chemical reactions inside batteries, reducing their discharge capacity and charging speed.
LFP batteries charge more slowly in cold weather than NCA/NMC batteries and their range decreases somewhat more than NCA batteries in cold weather. However, LFP batteries can still operate safely and reliably in temperatures ranging from -20°C to 60°C (-4°F to 140°F).
Lithium manganese iron phosphate (LMFP)
LMFP battery chemistry is a type of lithium-ion battery chemistry that uses lithium manganese iron phosphate (LiMn x Fe 1−x PO 4 ) as the cathode material and a graphitic carbon electrode with a metallic backing as the anode. LMFP battery chemistry combines the high safety of lithium iron phosphate (LFP) and the high energy density of lithium manganese phosphate (LMP). LMFP battery chemistry is a promising cathode material for high performance lithium ion batteries, especially for electric vehicles.
Some of the advantages of LMFP battery chemistry are:
- High thermal stability and low risk of thermal runaway
- High power density and fast charging capability
- High cycle life and good rate performance
- Low cost and environmental friendliness
- High voltage platform and improved specific capacity
Some of the challenges of LMFP battery chemistry are:
- Low electronic conductivity and lithium ion diffusion coefficient
- Phase transition and lattice distortion during cycling
- Manganese dissolution at high temperatures
- Electrolyte compatibility and interface stability
Summary Cathode materials
The below table summaries the differences
|Gravimetric density||240-272 Wh/kg||200-272 Wh/kg||90-120Wh/kg|
|Volummetric density||580-720 Wh/l||500 - 620 Wh/l||220-300 Wh/l|
|Cycles||500-1000||1000-2000||2000 - 5000|
A separator is an essential component of an electric vehicle (EV) battery that is placed between the cathode and anode to prevent them from touching and causing a short circuit. The separator is typically a thin, porous membrane made of a polymer material that allows the flow of lithium ions between the cathode and anode, while preventing the flow of electrons.
The separator serves several important functions in an EV battery. Firstly, it provides a physical barrier between the cathode and anode, preventing them from coming into direct contact and causing a short circuit. Secondly, it allows the flow of lithium ions, which are necessary for the battery to function. Thirdly, it helps maintain the uniformity of the battery’s internal structure by preventing the formation of dendrites, which are tiny, needle-like structures that can grow from the anode and puncture the separator, leading to a short circuit.
The choice of separator material can have a significant impact on the performance and safety of the battery. A good separator material should have high ion conductivity, low electrical conductivity, and good thermal stability. It should also be able to withstand the stresses of repeated charge and discharge cycles without degrading, and be resistant to thermal and chemical degradation.
There are several types of separator materials used in EV batteries, including polyethylene (PE), polypropylene (PP), and ceramic materials such as lithium aluminum titanium phosphate (LATP). Each type of separator has its own unique advantages and disadvantages, and the choice of separator material depends on the specific requirements of the battery.
In summary, the separator is a critical component of an EV battery that helps prevent short circuits and maintains the uniformity of the battery’s internal structure, while allowing the flow of lithium ions necessary for the battery to function.
Liquid electrolytes in lithium-ion batteries consist of lithium salts in an organic solvent, such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate.
A liquid electrolyte acts as a conductive pathway for the movement of cations passing from the negative to the positive electrodes during discharge and the opposite direction when charging.
Electrolytes based on solid materials are areas that many works on. These are called solid-state batteries. Currently, there are no concrete plans for any mass production EV with Solid-state batteries.
The current collector is a component of a battery cell that facilitates the flow of electrical current between the electrode and the external circuit. In most battery cells, the current collector is a thin metal foil that is placed on either side of the electrode material.
In a lithium-ion battery cell, for example, the current collector is typically made of copper or aluminum and is coated with a thin layer of carbon to improve conductivity and prevent corrosion. The current collector is typically placed on the surface of the electrode material, and when the cell is charged or discharged, the current flows from the current collector through the electrode material and back out to the external circuit.
The design and materials used for the current collector can have a significant impact on the performance and durability of the battery cell. For example, a current collector with a high conductivity and low resistance can help to improve the efficiency of the battery cell, while a current collector that is prone to corrosion can reduce the lifespan of the cell.