How Lithium-Ion Batteries Work
Lithium-ion batteries use the reversible lithium intercalation reaction. The battery has several important components to enable this intercalation. A lithium-rich cathode battery material supplies the lithium ions, and an electrically conductive anode allows a current to power the circuit. A non-electrically conductive electrolyte and separator material prevent the battery from short circuiting. These materials also allow for lithium-ion transfer while keeping the electrons isolated at either the cathode or anode.
History of Rechargeable Batteries
In 1976, Stanley Whittingham demonstrated the first rechargeable lithium-ion battery cell. It featured a titanium disulfide (TiS2) cathode, a lithium-metal anode, and a lithium salt (LiCIO4) dissolved in an organic solvent. The battery cell discharged with a voltage below 2.5V and could be recharged. However, lithium-metal anode dendrite growths occurred during the charge cycle, leading to issues such as internal shorting and thermal runaway. Despite these challenges, the rechargeability of the battery triggered extensive research into the use of metal dichalcogenides. The underlying process is known as ion intercalation and, in the case of lithium batteries, lithium intercalation.
In the 1980s, John Goodenough discovered three classes of oxide cathodes:
- Polyanion
- Layered
- Spinel
Each class was worked on by a separate scientist, with no overlap; Koichi Mizushima from Japan worked on layered oxide cathodes, Michael Thackeray from South Africa worked on spinel oxide cathodes, and Arumugam Manthiram from India worked on polyanion cathodes. The classes vary by how many dimensions their ions can diffuse through. Polyanions allow ions to travel in only one dimension, layered oxides allow two dimensions, and spinel oxides allow three dimensions.
Following research into the three cathode types and their material designs, the switch from a lithium anode to an intercalated anode was tested by Akira Yoshino. The change was prompted by safety concerns relating to lithium metal being unstable and prone to dendrite formation, increasing the short circuit potential. The first commercial lithium-ion battery was patented by Yoshino. It utilised a soft carbon anode in addition to Goodenough’s lithium cobalt oxide cathode. Sony would later begin producing and selling the world’s first rechargeable lithium-ion battery.
Battery Components and Their Functions
Lithium intercalation is the process that underlies all lithium-ion batteries. A battery cell consists of four components:
- Cathode
- Anode
- Electrolyte
- Separator
By applying a voltage to a battery, the lithium ions are carried through an electrolyte medium to intercalate with the anode material. A separator moderates the ion flow and separates the anode and cathode to prevent instantaneous discharging.
The charged battery is used to power a circuit which results in the battery discharging. When discharging, electrons flow from the anode through a circuit and return to the cathode. At the same time, the lithium ions held at the anode flow back to the cathode material, through the electrolyte. Once all the lithium ions are intercalated with the cathode, the battery requires charging to be used again.
The materials of lithium-ion batteries are important because they can alter the specific energy, specific power, cycle life, and the safety of the battery.
Materials and Properties
Cathode Materials
Battery research has focused largely on the cathode material as it is the most limiting of the four main components. In typical lithium-ion batteries, the cathode plays a vital role in supplying the ions. Therefore, research is essential to determine the best morphology and chemistry for each specific battery application.
The choice of materials is crucial as the properties depend on the elements and morphology of the cathode. The morphology of the cathode affects how fast the ions can diffuse through its own structure. If the integrity of the morphology is not stable enough, there will be a limit on the number of cycles the electrode can take before degrading to an unusable state. The chemistry of the cathode determines how many lithium ions are available for intercalation.
The most common cathode materials include:
- Lithium cobalt oxide (LCO)
- Lithium Manganese Oxide (LMO)
- Lithium Nickel Manganese Oxide (LNMO)
- Lithium Nickel Cobalt Aluminum Oxide (NCA)
- Lithium Nickel manganese cobalt oxide (NMC)
- Lithium Nickel Manganese Cobalt Oxide (NCM)
- Lithium Iron Phosphate (LFP)
Anode Materials
The anode must supply a large discharge voltage while remaining isometric. Graphite is commonly used. It has a large discharge profile and porous nature which allows lithium ions to interlap with the structure during charging and vice versa.
The most common anode materials currently feature graphite or hard carbon, but research is testing new anode materials. Other anode materials include metal oxides. They offer a higher charge capacity but tend to expand or contract during charging.
Common anode materials include:
- Graphite
- Lithium titanate (LTO)
- Silicon or carbon
- Tin and cobalt alloys
- Bismuth oxide
Electrolytes
The electrolyte carries lithium ions from the cathode to the anode, and vice versa, without allowing a flow of electrons. It is important to consider the chemical and physical reactions that could occur between the electrolyte and other battery components. The electrolyte must be chosen carefully to prevent voltage induced electrochemical decomposition or rapid oxidation.
Generally, electrolytes are a mix of electrolyte salts in either an organic or inorganic solvent. Electrolytes are also available in either an aqueous or solid solution, although solid electrolytes are a fairly new research area. Solid state electrolytes are emerging as a new substitute for aqueous solutions. They have a higher energy density and improved safety, through a larger range of operating temperatures and dendrite prevention.
Separator
The separator material provides structural integrity and acts as an inbuilt safety feature to moderate ion travel. If ions move through the electrolyte too fast there is potential for thermal runaway, so the separator is designed to close or open their pores depending on temperature.
An interesting area of research is the design of separators to help prevent dendrite and solid electrolyte interphase formation. This prolongs the battery lifetime.An uneven distribution of lithium ions moving through the separators can cause a spike like formation, creating a risk of penetration and short circuiting. By maintaining a uniform ion distribution, the dendrite growth can remain evenly distributed to prolong the life of the battery.