A Lithium-ion cell is built of various components and sub-components. This article discusses the functionality and importance of selecting the right type of (sub) components.
A Lithium-ion cell has four major components:
- Cathode – Positive electrode
- Anode – Negative electrode
- Electrolyte – Medium for the movement of lithium ions
- Separator – Prevents contact between cathode and anode
When it comes to Lithium-ion batteries, Electrolyte is comparatively a less talked about subject. One reason could be that Lithium-ion batteries are named after the type of cathode (LCO, LFP, NMC or NCA batteries) or anode (LTO batteries) they use.
Electrolyte only conducts Lithium ions and does not participate in the chemical reactions that happen during charging and discharging.
Liquid and solid-state electrolytes are the most commonly spoken about types of electrolytes for Lithium-ion cells. Although liquid electrolytes are being used in Lithium-ion cells today, the future belongs to solid-state electrolytes. We will be discussing in depth both of them.
A liquid electrolyte can come in various forms, but salts dissolved in the solution are popularly used. The conductivity and stability of the solvent to work at a voltage play a vital role. Aqueous-based electrolytes have higher conductivity, but they cannot be stable at the voltage generally required for Lithium-ion batteries. On the other hand, organic solvent-based electrolytes have lower conductivity but offer stability at more than 4V. The latter is expensive but preferred because of its stability in high voltage.
This kind of electrolyte (organic solved based) is sensitive to air and moisture, and hence strict measures are taken during its manufacturing, storage, transportation and at the time of being filled inside the cell (cylindrical, pouch or prismatic).
Hence, when Lithium-ion cell manufacturing takes off in India at a large scale, manufacturing electrolytes locally will be the need of the hour because of the difficulty in transporting them.
The following are the deciding parameters for selecting a liquid electrolyte:
- Ionic Conductivity – It decides how fast the transportation of charge happens
- Viscosity: The lower value of viscosity ensures that ions move easily
- Voltage Stability: Voltage stability ensures that electrolyte does not undergo electrochemical reactions at the working voltage of the cell
- Thermal and Chemical Stability: It ensures that the electrolyte decomposition happens at higher temperatures and it is chemically stable against all the components of the cell
Although various base salt (generally inorganic) can be used for electrolyte making of Lithium-ion batteries, LiPF6 (Lithium hexafluorophosphate) salt is the most popular. Other popular salts are LiBF4 (Lithium tetrafluoroborate) and LiClO4 (Lithium perchlorate).
One of these salt is dissolved in various organic solvents such as EC (Ethylene Carbonate), DEC (Diethyl Carbonate), EMC (Ethyl-methyl Carbonate), DMC (Dimethyl Carbonate) and PC (Propylene Carbonate). Each solvent has its own advantages, such as operation in low temperature, operation in high temperature, how it affects the battery capacity degradation (it happens as a side reaction with its electrodes), etc. Hence, these solvents are mixed in a desired ratio to make use of all the advantages that of each solvent.
Additives such as VC (Vinylene Carbonate) are added for stability in very small quantities. VC increases the stability of the SEI (Solid Electrolyte Interface) layer formed on the graphite anode.
There are many ways an electrolyte for Lithium-ion batteries can be made. The molar ratio of LiPF6 can be 1.0M, 1.2M, etc. The selection of organic solvents can mean that there are different combinations possible, and their usage ratio can again mean multiple possibilities. Moreover, the type of additives and their ratio also increases the possible types of electrolytes that can be made. For example, 1.0M LiPF6 in EC:EMC:DMC=1:1:1 (v/v/v) + 2% wt. VC.
Different chemistries of cells have their own preferred electrolyte formula. Prices vary according to the formulation. For example, the electrolyte used for cells made with NMC 811 cathode are more expensive than the electrolyte used for cells made with LFP cathode.
Solid State Electrolyte
Significant development is happening in the space of solid-state electrolytes globally because of the various advantages they can offer. However, a solid-state electrolyte is not produced by many companies, and only a few companies have started pilot production till now, and that too recently.
Below are the advantages of solid-state electrolyte based Lithium-ion batteries:
- Faster charging and discharging
- Safer because of the absence of a liquid electrolyte, which can catch fire
- Over-charging, over-voltage, over-current and over-temperature do not lead to battery failures
- Higher cycle life, durability and stability
- Higher gravimetric energy density (Wh/Kg)
- Nail penetration and short circuit do not lead to fire
- No structural limitation. It can be used in various shapes, which can be smaller and thinner.
Solid-state electrolytes can be made up of lithium-metal oxides/sulfides/phosphates, and they offer higher energy densities and are non-flammable even in the harshest of usage conditions. The plastic-based separator (which will be discussed in detail in part 4) used along with liquid electrolyte is single-handedly replaced by a solid-state electrolyte.
While liquid electrolytes can cause side reactions with the electrodes resulting in battery capacity degradation, solid-state electrolytes do not directly contribute to battery capacity degradation. Hence, solid-state electrolytes can be operated at higher temperatures under harsh conditions without much battery capacity degradation.
Moreover, the liquid electrolyte has lower stability to temperature, and it can get heated up while fast charging and decompose. But this is not the case with solid-state electrolytes, and it allows for fast charging with ease.
Until solid-state electrolyte is accessible to every Lithium-ion battery manufacturing company, liquid electrolyte-based Lithium-ion batteries will continue to be manufactured. In India, only JLNPhenix Energy has access to solid-state electrolyte-based Lithium-ion batteries. They are testing these batteries with top OEMs.
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Also Read :
- Components of a Lithium-ion Cell – Part 1 | Cathode
- Components of a Lithium-ion Cell – Part 2 | Anode
- Components of a Lithium-ion Cell – Part 4 | Separator
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