How Do They Work? Battery Chemistry
Lithium-ion batteries work in a similar manner to all rechargeable batteries, in which reversible chemical reactions inside each cell cause ions to move between the cathode (positive electrode) and the anode (negative electrode). When the battery is being charged, an external electric current is applied, and lithium ions move from the cathode to the anode. During discharge, the lithium ions migrate back to the cathode and release energy, which powers the device. The cell is filled with an electrolyte through which the ions travel.
Figures 1a and b: Diagram of the process; Lithium-ion rechargeable battery charge and discharge mechanism.
(Source: Image courtesy of Marshall Brain, originally appearing in “How Lithium-ion Batteries Work,” November 14, 2006. HowStuffWorks.com, http://electronics.howstuffworks.com/everyday-tech/lithium-ion-battery.htm)
Each battery cell contains a separator, usually a polymer membrane, to electrically isolate the anode from the cathode. The integrity of the separator is critical to battery safety.
Lithium is a very reactive metal; this is a valuable characteristic for energy storage, but it also makes lithium-ion batteries potentially risky. If the internal battery temperature becomes too high, reactions will speed up to the point of being unstable. Battery design can incorporate features that minimize the chance of this thermal runaway, such as adding safety vents to release internal pressure and using microporous separators that fuse above a certain temperature, blocking excess ion transport.
As batteries age, the chemicals inside the cells become depleted with each charging cycle, resulting in reduced capacity over time. Solid-electrolyte interphase (SEI) layers build up on the electrodes, limiting further ion transport.
Figure 2: Voltammogram of two batteries – one new and one cycled 100 times.
(Source: Image courtesy of N. Willard, B. Sood, M. Osterman, M. Pecht, originally appearing in “Disassembly Methodology for Conducting Failure Analysis on Lithium-ion Batteries.” J. Mater Sci: Mater Electron 22, 1616 (2011)
Trends in Battery Materials
Lithium-ion batteries have seen improvements in materials and assembly processes since Sony commercialized the technology in 1991.7 U.S. patents issued in the 1990s describe advances in foil morphology and electrolyte materials that are now common in lithium-ion batteries.8,9
Cathode Materials. The original cathode material is lithium cobalt oxide (LiCoO2). Some commercial lithium-ion batteries contain lithium iron phosphate (LiFePO4) or lithium manganese oxide (LiMn2O4) cathodes.
Combining lithium manganese and lithium nickel manganese cobalt oxide (NMC) cathode materials produces a battery with an optimum combination of acceleration and driving range for EVs. Vehicles such as the Nissan Leaf, Chevy Volt, and BMW i3 run on NMC batteries. Tesla vehicles use a lithium nickel cobalt aluminum oxide (NCA) battery.
Anode Materials. Most lithium-ion batteries contain a carbon anode, in the form of graphite. Lithium metal anodes are attractive because of their high energy density, but they are prone to forming dendrites that can cause shorts, increasing the risk of battery fires. Silicon-based anodes are another option to increase energy density, but formation of SEI layers can decrease reliability.
Figure 3: Evolution of anode materials for lithium-ion batteries.
(Source: Image courtesy of N. Willard, B. Sood, M. Osterman, M. Pecht, originally appearing in “Disassembly Methodology for Conducting Failure Analysis on Lithium-ion Batteries.” J. Mater Sci: Mater Electron 22, 1616 (2011)
MIT spinoff SolidEnergy Systems is commercializing a battery with a thin lithium foil anode, which is said to double the energy capacity compared to a standard lithium-ion battery. The company has created a new electrolyte designed to minimize the risk of shorts.
Electrolytes. The electrolyte can be liquid or solid (gel) and is usually polymer-based, consisting of organic solvents and lithium salts. Liquid electrolytes more efficiently transfer lithium ions, but they are highly flammable; solid electrolytes are less conductive, but they are safer because they are not likely to catch fire. Materials selection can improve the safety of liquid electrolytes. One example is lithium hexafluorophosphate (LiPF6) dissolved in a mixture of ethylene carbonate and diethyl carbonate. The mixture of solvents is more stable than either solvent used by itself.
Battery Form Factors for Various Applications
Each battery cell contains layered sheets of anode and cathode materials enclosed in a metal case. A copper foil is coated with anode material (typically graphite powder), and an aluminum foil is coated with cathode material; these are sandwiched with a polymer separator and either stacked vertically or, more commonly, wound into a coil.
Once the electrodes are enclosed in an aluminum case or foil pouch, the case is filled with electrolyte before sealing. Most batteries include a release valve to exhaust gaseous byproducts that form as the electrolyte breaks down during use. The cell shape may be cylindrical, similar to a standard alkaline AA battery, or prismatic (square or rectangular).
A battery pack consists of multiple interconnected cells. Linking cells in series increases the voltage at which the battery operates, and combining multiple cells or rows of cells in parallel increases the current that the batteries can withstand.
Common Battery Failures
Lithium-ion batteries may fail catastrophically when shorts occur between the anode and the cathode. A number of manufacturing defects inside the cells may increase the likelihood of such failure. These include:
- Burrs on the electrode foils
- Voids in the electrode materials
- Chemical contamination
- Inconsistent particle morphology of electrodes
Applications engineers sourcing lithium-ion batteries have no way to easily identify these defects. Accelerated life testing during qualification helps ensure batteries will perform safely and according to expectations.
Batteries can also be considered failed when capacity has decreased beyond a certain point. Overheating and operating at high voltage can accelerate the mechanisms that reduce battery life. Several different problems may occur during use:
- Loss of electroactive ions, leading to decreased power density
- Excessive growth of passivation layers on the electrodes
- Breakdown of the solvent or lithium salts in the electrolyte
- Swelling of electrodes, which irreversibly increases the discharge rate
- Pressure buildup, which can rupture sealed cells
- Delamination of electrodes from metal foils
- Cracking of electrodes due to mechanical stresses
The chart below illustrates the various mechanisms that can cause degradation of lithium-ion battery cells.
Figure 4: Progression of mechanisms that lead to degradation of battery cells.
(Source: Image courtesy of N. Willard, B. Sood, M. Osterman, M. Pecht, originally appearing in “Disassembly
Methodology for Conducting Failure Analysis on Lithium-ion Batteries.” J. Mater Sci: Mater Electron 22, 1616 (2011)