Everything about rechargeable Lithium-ion cell
Present days and the pace of technology brings new challenges to the engineers and scientists when it comes to vehicles electrification.
The battery for an electric car, besides the electric motor, plays a key role. And the most common technology used today is rechargeable lithium-ion cells. A battery in general has multiple modules, made up of individual battery cells. And the main components of a battery cell are the anode and the cathode, the separator, the electrolyte and the cell case.
The electrodes consist of a thin metal foil which is a current collector coated with an electrode film. For lithium-ion cells, the aluminum foil of the cathode is frequently coated with an electrode film based on transition metal oxides such as lithium nickel manganese cobalt oxides.
The anode consists of a copper foil coated with a graphite-containing electrode film. The porous electrode films consists of the active material and to a lesser extent of conductive carbon additives and polymeric binders. The two binders electrodes are electrically isolated from each other by the separator in order to prevent a short circuit.
Separators are made of microporous plastics like polyethylene. They can also be stabilized by ceramic particles. The pores in the electrodes and the separators are saturated with an electrolyte that serves as a lithium conductor.
Straight after assembly, a lithium-ion cell is in the uncharged state, i.e. all available lattice sites in the cathode active material are completely occupied by lithium ions. If the cell is charged, lithium-ions move from the cathode through the electrolyte to the anode where it goes into the anode structure. This process is also known as intercalation. In order to balance the charge, electrons flow from the cathode to the anode by means of the connected energy source. When discharging, the exact opposite process takes place, and the electrons and lithium-ions move back in the direction of the cathode. The process of moving back and forwards is also known as rocking chair principle.
Development of the materials for cells
When a lithium-ion cell is charged for the first time, a surface layer forms on the anode. This formation is called SEI(solid electrolyte interface) layer is an inevitable and irreversible process, which results in the loss of lithium ions. But this layer has a good side, by protecting the anode structure against destruction. In order to develop lithium-ion cells you have to take in consideration five criteria: safety, lifetime, power, cost and energy.
What we want is energy, to fulfill a huge demand from electric cars, at the lowest possible mass volume of the battery. So the energy is the product of the average cell voltage U and the cell capacity Q (E=UxQ). In relation to the cell mass/volume, one speaks of the specific energy (Wh/kg) or the energy density (Wh/I). To increase these 2 values, core materials has to be developed.
Cathode active materials
The list of cathode active materials is huge. One important substance class is the transition metal layer oxides with lithium nickel manganese cobalt oxides (NMC, NMC-111 being effective in automotive sector). However, building electric cars with high driving ranges requires cathode active materials that enable higher specific cell energies. NMC-622 has recently become available for automotive lithium-ion cells.
Structure of transition metal oxides (gray:lithium ions, blue: MO6 octahedron)
What's in development process, is cathode active material NMC-811. Usually, the higher the nickel content, the higher the cell energy, but cyclic stability declines. Lithium nickel cobalt aluminium oxide (NCA) has been available for quite some time. However, at over 40 degrees Celsius, the NCS demonstrates a shorter cycle life and a lower current rate capability compared with nickel-rich NMC materials.
What is even fresher in market, sulfur is being investigated as cathode active material, due to its low costs and high specific capacity. But overall, sulfur cells have a lower energy density compared with conventional lithium-ion cells.
Anode active materials
At the moment, graphites are used as anode active materials. And there are 3 types: MCMBs (mesocarbon microbeads), synthetic or natural graphites. MCMBs are spherical particles and provide a very good cycling characteristics but are expensive.
For further improving the specific energy of cells, silicon is moving in the focus of research, having a nine time higher specific capacity than the graphite.
Thinner separators for a higher energy density
The separators are currently made of polyolefins, stabilized by ceramic particles. The trend is to develop separators thinner than 20 microns. The classic separator should be replaced by a very thin layer of solid electrolytes. Nowadays, lithium-ion cells for automotive use solvents such as organic carbonates in which a conductive salt is dissolved.
Lithium-ion cells are very complex systems providing a massive amount of advantages and disadvantages. A precise understanding of lithium-ion cell including the chemical and physical processes is extremely important when it comes to chose the most suitable cell for the respective vehicle.
Photo:Porsche
The battery for an electric car, besides the electric motor, plays a key role. And the most common technology used today is rechargeable lithium-ion cells. A battery in general has multiple modules, made up of individual battery cells. And the main components of a battery cell are the anode and the cathode, the separator, the electrolyte and the cell case.
The electrodes consist of a thin metal foil which is a current collector coated with an electrode film. For lithium-ion cells, the aluminum foil of the cathode is frequently coated with an electrode film based on transition metal oxides such as lithium nickel manganese cobalt oxides.
The anode consists of a copper foil coated with a graphite-containing electrode film. The porous electrode films consists of the active material and to a lesser extent of conductive carbon additives and polymeric binders. The two binders electrodes are electrically isolated from each other by the separator in order to prevent a short circuit.
Separators are made of microporous plastics like polyethylene. They can also be stabilized by ceramic particles. The pores in the electrodes and the separators are saturated with an electrolyte that serves as a lithium conductor.
Straight after assembly, a lithium-ion cell is in the uncharged state, i.e. all available lattice sites in the cathode active material are completely occupied by lithium ions. If the cell is charged, lithium-ions move from the cathode through the electrolyte to the anode where it goes into the anode structure. This process is also known as intercalation. In order to balance the charge, electrons flow from the cathode to the anode by means of the connected energy source. When discharging, the exact opposite process takes place, and the electrons and lithium-ions move back in the direction of the cathode. The process of moving back and forwards is also known as rocking chair principle.
Development of the materials for cells
When a lithium-ion cell is charged for the first time, a surface layer forms on the anode. This formation is called SEI(solid electrolyte interface) layer is an inevitable and irreversible process, which results in the loss of lithium ions. But this layer has a good side, by protecting the anode structure against destruction. In order to develop lithium-ion cells you have to take in consideration five criteria: safety, lifetime, power, cost and energy.
What we want is energy, to fulfill a huge demand from electric cars, at the lowest possible mass volume of the battery. So the energy is the product of the average cell voltage U and the cell capacity Q (E=UxQ). In relation to the cell mass/volume, one speaks of the specific energy (Wh/kg) or the energy density (Wh/I). To increase these 2 values, core materials has to be developed.
Cathode active materials
The list of cathode active materials is huge. One important substance class is the transition metal layer oxides with lithium nickel manganese cobalt oxides (NMC, NMC-111 being effective in automotive sector). However, building electric cars with high driving ranges requires cathode active materials that enable higher specific cell energies. NMC-622 has recently become available for automotive lithium-ion cells.
Structure of transition metal oxides (gray:lithium ions, blue: MO6 octahedron)
What is even fresher in market, sulfur is being investigated as cathode active material, due to its low costs and high specific capacity. But overall, sulfur cells have a lower energy density compared with conventional lithium-ion cells.
Anode active materials
At the moment, graphites are used as anode active materials. And there are 3 types: MCMBs (mesocarbon microbeads), synthetic or natural graphites. MCMBs are spherical particles and provide a very good cycling characteristics but are expensive.
For further improving the specific energy of cells, silicon is moving in the focus of research, having a nine time higher specific capacity than the graphite.
Thinner separators for a higher energy density
The separators are currently made of polyolefins, stabilized by ceramic particles. The trend is to develop separators thinner than 20 microns. The classic separator should be replaced by a very thin layer of solid electrolytes. Nowadays, lithium-ion cells for automotive use solvents such as organic carbonates in which a conductive salt is dissolved.
Photo:Porsche
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