Lithium Batteries and Separators
Lithium batteries power everything from laptops to electric cars. But they’re also a fire hazard and cost recycling, landfill and solid waste operations billions of dollars in lost revenue each year.
The key to this battery chemistry is the movement of lithium ions between electrodes, creating an electrical potential difference that we use to power devices and tools. Let’s take a closer look.
The Anode
It was 2007. Apple launched its first iPhone, J.K. Rowling wrapped up the seventh and final Harry Potter book, and the worst financial crisis since the Great Depression was on its way. It was also the year that Gene Berdichevsky, an engineer and employee number 7 at electric car pioneer Tesla, began to question why gains in lithium battery energy density were tapering off.
Graphite electrodes are the heart of lithium batteries, providing most of the cell’s energy storage capacity. During discharge, the anode absorbs lithium ions from the electrolyte through a process called intercalation. The ions cause the material to expand, which drives electrons from the anode through the external circuit to wherever they dissipate energy—typically, the device being powered or a charging circuit.
The expansion of the graphite anode, however, eats away at the cell’s energy storage capacity. To counter this, manufacturers add silicon to the anode—adding 3-5 percent of its mass to achieve good cycle life. This enables the anode to bind twice as many lithium ions as graphite, boosting energy density by 20-40%.
The problem with adding silicon is that it can lead to an increase in battery size and cost. Moreover, it’s difficult to make a high-performance anode from pure silicon because it takes six carbon (graphite) atoms to bind just one lithium ion. Thin lithium foil solves this issue, but it introduces a tradeoff between volumetric energy density and cost. In addition, it is impractical to produce a thin lithium foil that is sufficiently consistent to prevent the formation of dendrites—metal Lithium atoms that form on the anode’s surface and degrade its capacity.
The Cathode
The cathode of a lithium battery is the positive electrode and is where conventional current flows out of the device during operation. When the battery is discharging, lithium battery lithium ions move to the anode from the cathode through the electrolyte and separator. During charging, the process reverses and the ions move back to the cathode. This movement of ions creates the electric current, which powers devices.
Lithium ions move easily between the anode and cathode of a lithium battery. Because of this, the battery has a very high voltage. Each cell produces 3.7 volts, which is much higher than the 1.5 volts produced by an alkaline battery like those found in clocks and TV remote controls. The high voltage makes lithium batteries more compact and allows them to be used in a wide variety of devices.
Most lithium-ion batteries have a metal oxide (such as Lithium cobalt oxide, or LiCoO2) cathode and a porous carbon anode. The carbon is typically graphite, although Sony’s early lithium-ion cells used coke. Most modern lithium-ion batteries use a coated carbon to attain a flatter discharge curve.
Button-cell, coin and single-use lithium batteries have a special metal case that holds the negative and positive electrodes apart. If this case is punctured or crushed, the battery will heat up very quickly and explode. For this reason, you should never attempt to remove a lithium battery from a product.
The Electrolyte
Lithium batteries are used in everything from laptops and iPods to electric cars and hybrid vehicles. They are popular because, pound for pound, they have the lithium battery highest energy density of any rechargeable battery. But they have also made the news recently because, from time to time, they can burst into flames. This isn’t common — two or three batteries in a million do this — but it can be frightening.
A separator — a thin sheet of micro-perforated plastic — sits between the positive and negative electrodes in the lithium battery. The separator lets lithium ions pass through while blocking electrons. When the battery charges, ions of lithium move from the positive electrode (Lithium Cobalt Oxide, or LiCoO2) through the electrolyte solution to the negative electrode, where they attach themselves to carbon. During discharge, the lithium ions travel back through the electrolyte to the positive electrode. This releases the electrons that were tying them to the anode, and they flow through an external wire.
Most lithium batteries have a liquid electrolyte, although it may be a solid material such as ceramic. Some are a combination of both liquid and solid electrolyte; others have a gel-like electrolyte. In all, electrolyte solutions of various concentrations are available to provide the optimum balance of safety, performance and cost.
The Separator
A separator is a microporous film that separates the positive and negative electrodes while allowing the electrolyte to pass through. It is a key component of the lithium battery and directly influences its performance. Its performance is important for the battery cycle life, power density and safety. Early separators were made of rubber, glass fiber mat, cellulose and porous polyolefin films (polyethylene or polypropylene) or combinations of these materials. Commercially available Li-ion batteries have a separator made of porous ethylene or polypropylene.
When a lithium battery is discharged, the lithium atoms of the anode are forced to jump from their original positions due to the electrical charge and move through the separator. They re-unify with their long lost electrons in the cathode and form lithium ions. The ions are carried by the electrolyte and then through the separator to re-unite with the anode on the opposite side of the cell. The cathode then reverses the oxidation reaction, and the cycle begins again.
Numerical modeling allows the morphological characteristics of separators to be better understood and optimized for battery applications. For example, a FEA approach can be used to simulate the stress-strain behavior of battery separators and determine the effect of their microstructure on their mechanical properties. In addition, simulation studies can help predict separator degradation processes and their consequences on battery safety.
