INTRODUCTION TO LITHIUM ION BATTERIES
Lithium, it’s the lightest metal on the periodic table, and the one most willing to donate its electrons (the most powerful reducing agent) — you really can’t find an element that’s more suitable for creating light weight, high energy density batteries. From portable electronics like the smartphone in your pocket, to high performance electric cars like the Tesla Model S, lithium ion batteries are currently the most promising chemistry on the market for meeting our renewable energy storage needs. Let’s take a closer look at lithium ion batteries.
How Does a Lithium Ion Battery Work?
Lithium ion batteries capitalize on the strong reducing potential of lithium ions to power the redox reaction central to all battery technologies — reduction at the cathode, oxidation at the anode. Connecting the positive and negative terminals of a battery through a circuit, unites the two halves of the redox reaction, allowing the device attached to the circuit to extract energy from the movement of electrons.
While there are many different types of lithium based chemistries used in the industry today, we’ll use Lithium Cobalt Oxide(LiCoO2) — the chemistry that allowed lithium ion batteries to replace the nickel-cadmium batteries that had been the norm for consumer electronics up until the 90s — to demonstrate the basic chemistry behind this popular technology.
The full reaction for a LiCoO2 cathode and a graphite anode is as follows:
LiCoO2 + C ⇌ Li1-xCoO2 + LixC
Where the forward reaction represents charging, and the reverse reaction represents discharging. This can be broken up into the following half reactions:
At the positive electrode, reduction at the cathode occurs during discharge (see reverse reaction).
LiCo3+O2 ⇌ xLi+ + Li1-xCo4+xCo3+1-xO2 + e-
At the negative electrode, oxidation at the anode occurs during discharge (see reverse reaction).
C + xLi+ + e- ⇌ LixC
During discharge, Lithium ions (Li+) move from the negative electrode (graphite) through the electrolyte (lithium salts suspended in solution) and the separator to the positive electrode (LiCoO2). At the same time electrons move from the anode (graphite) to the cathode (LiCoO2) which are connected via an external circuit. If an external power source is applied, the reaction is reversed along with the roles of the respective electrodes, charging the cell.
What’s in a Lithium Ion Battery
Your typical cylindrical 18650 cell, which is the common form factor used by the industry for commercial applications from laptops to electric vehicles, has an OCV (open circuit voltage) of 3.7 volts. Depending on the manufacturer it can deliver around 20 amps with a capacity of 3000mAh or more. The battery pack will be composed of multiple cells, and generally include a protective microchip to prevent overcharging and discharging below the minimum capacity, which can both lead to overheating, fires, and explosions. Let’s take a closer look at the internals of a cell.
The key to designing a positive electrode is to pick a material that has an electropotential greater than 2.25V when compared with pure lithium metals. Cathode materials in lithium ion vary greatly, but they are generally layered lithium transition metal oxides, like the LiCoO2 cathode design we explored earlier. Other materials include spinels (i.e. LiMn2O4) and olivines (i.e. LiFePO4).
In an ideal lithium battery, you would use pure lithium metal as an anode, because it provides the optimum combination of low molecular weight and high specific capacity possible for a battery. There are two main problems that prevent lithium from being used as an anode in commercial applications: safety and reversibility. Lithium is highly reactive and prone to catastrophic failure modes of the pyrotechnic kind. Also during charge, lithium will not plate back into its original uniform metallic state, instead adopting a needlelike morphology known as a dendrite. Dendrite formation can lead to punctured separators which can lead to shorts.
The solution researchers devised to harness the pros of lithium metal without all the cons was lithium intercalation — the process of layering lithium ions within carbon graphite or some other material, to allow the easy movement of lithium ions from one electrode to another. Other mechanisms involve using anode materials with lithium that make reversible reactions more possible. Typical anode materials include graphite, silicon based alloys, tin, and titanium.
The role of the separator is to provide a layer of electrical insulation between the negative and positive electrodes, while still allowing ions to travel through it during charge and discharge. It must also be chemically resistant to degradation by the electrolyte and other species in the cell and mechanically strong enough to resist wear and tear. Common lithium ion separators are generally highly porous in nature, and consist of polyethylene (PE) or polypropylene (PP) sheets.
The role of an electrolyte in a lithium ion cell is to provide a medium through which lithium ions can freely flow between the cathode and anode during charge and discharge cycles. The idea is to pick a medium that is both a good Li+ conductor and electronic insulator. The electrolyte should be thermally stable, and chemically compatible with the other components in the cell. Generally, lithium salts like LiClO4, LiBF4, or LiPF6 suspended in an organic solvent like diethyl carbonate, ethylene carbonate, or dimethyl carbonate serve as the electrolyte for conventional lithium ion designs.
Solid Electrolyte Interphase (SEI)
An important design concept to understand about lithium ion cells is the solid electrolyte interphase (SEI) — a passivation film that builds up at the interface between the electrode and the electrolyte as Li+ ions react with degradation products of the electrolyte. The film forms on the negative electrode during the initial charge of the cell. The SEI protects the electrolyte from further decomposition during subsequent charges of the cell. Loss of this passivating layer can adversely impact cycle life, electrical performance, capacity, and the overall life of a cell. On the flipside, manufacturers have found that they can improve battery performance by fine tuning the SEI.
Meet the Lithium Ion Battery Family
The allure of lithium as an ideal electrode material for battery applications has led to many kinds of lithium ion batteries. Here are five of the most common commercially available batteries on the market.
Lithium Cobalt Oxide
We’ve already covered LiCoO2 batteries in depth in this article, because it represents the most popular chemistry for portable electronics like cellphones, laptops, and electronic cameras. LiCoO2 owes its success to its high specific energy. A short lifespan, poor thermal stability, and the price of cobalt has manufacturers to switch to blended cathode designs.
Lithium Manganese Oxide
Lithium manganese oxide batteries (LiMn2O4) use MnO2 based cathodes. Compared with standard LiCoO2 batteries, LiMn2O4 batteries are less toxic, cost less, and are safer to use, but with reduced capacity. While rechargeable designs have been explored in the past, today’s industry typically uses this chemistry for primary (single cycle) cells which are non-rechargeable and meant to be disposed of after use. Durable, high thermal stability, and a long shelf life make them great for power tools or medical devices.
Lithium Nickel Manganese Cobalt Oxide
Sometimes the whole is greater than the sum of its parts, and lithium nickel manganese cobalt oxide batteries (also known as NCM batteries) boast greater electrical performance than LiCoO2. NCM gains its strength in balancing the pros and cons of its individual cathode materials. One of the most successful lithium ion systems on the market, NCM is widely used in powertrains like power tools and e-bikes.
Lithium Iron Phosphate
Lithium Iron Phosphate (LiFePO4) batteries achieve a long cycle life and high current rating with good thermal stability with the help of nanostructured phosphate cathode material. Despite these improvements, it is not as energy dense as cobalt blended technologies and it has the highest self-discharge rate of the other batteries in this list. LiFePO4 batteries are popular as an alternative to lead acid as a car starter battery.
Replacing the graphite anode with lithium titanate nanocrystals greatly increase the surface area of the anode to about 100 m2 per gram. The nanostructured anode increases the number of electrons that can flow through the circuit, giving lithium titanate cells the ability to safely charged and discharged at rates greater than 10C (ten times its rated capacity). The tradeoff for having the fastest charge and discharge cycle of the lithium ion batteries is a relatively lower voltage 2.4V per cell, lithium titanate cells on the lower end of the energy density spectrum of lithium batteries but still higher than alternative chemistries like nickel cadmium. Despite this disadvantage, overall electrical performance, high reliability, thermal stability, and an extra-long cycle life mean that the battery still sees use in electric vehicles.
The Future of Lithium Ion Batteries
There is a big push from companies and governments across the globe to pursue further research and development on lithium ion and other battery technologies to meet the growing demand for clean energy and reduced carbon emissions. Inherently intermittent energy sources like solar and wind could benefit greatly from lithium ion’s high energy density and long cycle life which has already helped the technology corner the electric vehicle market.
To meet this growing demand, researchers have already begun pushing the boundaries of existing lithium ion in new and exciting ways. Lithium polymer (Li-Po) cells replace the hazardous liquid lithium salt based electrolytes with safer polymer gels and semi-wet cell designs, for comparable electrical performance with improved safety and lighter weight. Solid-state lithium is the newest technology on the block, promising improvements in energy density, safety, cycle life, and overall longevity with the stability of a solid electrolyte. It’s difficult to predict which technology will win the race for the ultimate energy storage solution, but lithium ion is certain to continue to play a major role in the energy economy in the years to come.