Lithium-Ion Batteries

Lithium metal electrode batteries first attracted interest decades ago due to their potential for very high energy density. However, safety concerns, particularly the risk of explosions, hindered their commercial development. This changed with the advent of lithium-ion batteries, which have since transformed the portable electronics industry.

Lithium-ion batteries are composed of a layered structure composed of electrodes and separators. Active electrode materials are typically lithium cobalt oxide (LiCoO₂) and graphite (Li_xC₆). The electrolyte is typically a mixture of lithium hexafluorophosphate dissolved in ethylene carbonate and dimethyl carbonate. The layered structures allows for the efficient transport of ions and electrons during battery operation, resulting in a high cell potential of around 3.6 to 7.2 V.

To address safety concerns, lithium-ion batteries are equipped with built-in safety features that prevent overcharging and handle emergency situations by automatically venting. Their high energy density, whether by weight or volume, combined with the ability to recharge without loss of performance, has made lithium-ion batteries the go-to choice for powering high-end portable devices.

These advantages have driven the rapid adoption of lithium-ion batteries, making them essential in powering devices ranging from smartphones and laptops to electric vehicles.

 

Where are lithium-ion batteries used?

The battery market is categorically divided into consumer, automotive, industrial, and special applications, which include aerospace and military sectors. 

In consumer electronics, lithium-ion batteries have become the major rechargeable power sources due to their high energy density, lightweight nature, and long cycle life. This chemistry is favored for its ability to deliver sustained and reliable performance in devices such as smartphones, laptops, cameras, and portable electronic gadgets.

Smart phones, laptops, and personal digital assistants
Rechargeable lithium-ion batteries have become the top choice for smartphones, laptops, and other portable devices due to their durability, high energy density, and lightweight design. They offer the convenience of easy recharging without needing to fully discharge and require minimal maintenance. Additionally, their environmentally friendly disposal makes them a preferred option for manufacturers and consumers alike.

Personal mobility
A rechargeable lithium-ion-powered personal mobility scooter can cover impressive distances, with a range of up to twelve miles on a single charge. One of the standout benefits of lithium-ion batteries is their lightweight design, making it easy for users to bring the battery indoors for recharging. Additionally, lithium-ion batteries are an environmentally friendly alternative to lead-acid batteries, offering a longer lifespan and quicker charging times. The reduced weight also contributes to an extended travel range and less mechanical wear, making them the perfect choice for personal mobility.

Solar energy storage
Lithium-ion batteries are the go-to solution for solar energy storage as the world increasingly relies on renewable energy. In the U.S., solar power is expected to meet 20% of energy needs by 2050, making efficient storage crucial. Lithium-ion batteries are perfect for storing excess solar energy, ensuring power availability even when the sun isn’t shining or during maintenance. Their low-resistance charging matches the output of solar panels, allowing for rapid charging and maximizing energy capture during daylight hours. This efficiency makes lithium-ion batteries a key player in securing a sustainable energy future.

Backup power systems and uninterrupted power supply
Power instability and outages remain a challenge, making reliable backup power essential. Lithium-ion batteries are increasingly used in backup systems and uninterruptible power supplies (UPS) due to their ability to provide almost instant power. Unlike generators, lithium-ion batteries offer immediate energy, crucial for keeping critical equipment like medical devices, communication systems, and computers running smoothly. Their rapid response, compact size, and low maintenance make them an ideal choice for ensuring uninterrupted power during emergencies.

e-Mobility
Lithium-ion batteries are revolutionizing the electric vehicle industry with their superior energy density, efficiency, and performance. These batteries are favored for their ability to provide long driving ranges, quick charging times, and a lightweight design, making them ideal for modern electric vehicles. Compared to traditional battery technologies, lithium-ion batteries offer extended lifespan and reduced maintenance, enhancing overall vehicle reliability. As the demand for sustainable transportation grows, lithium-ion batteries play a crucial role in powering electric vehicles, contributing to a cleaner and more efficient future for mobility.

 

How do lithium-ion batteries work?

Secondary battery chemistries, distinct from primary batteries, are rechargeable systems where the electrochemical reactions are reversible. Unlike primary batteries that are typically single-use, secondary batteries, such as lithium-ion and nickel-metal hydride, allow for repeated charging and discharging cycles. The key differentiator lies in the ability of secondary batteries to be replenished with electrical energy, making them suitable for applications requiring long-term use and energy storage. The reversible nature of the electrochemical processes in secondary batteries involves the movement of ions between the positive and negative electrodes during both charging and discharging, enabling a sustainable and rechargeable power source for various devices and systems.

Inside the battery, chemical reactions, namely oxidation and reduction, occur at the electrodes. During discharging, oxidation occurs at the anode, releasing electrons that travel through the external circuitry to the cathode. At the cathode, these electrons are used up during the reduction reaction. At the same time, ions move through the electrolyte. This movement maintains charge neutrality within the battery. The flow of electron through the external circuit creates an electric current, therefore discharging the battery. 

To charge secondary batteries, an external voltage is applied. This applied voltage reverses the electrochemical processes. The electrons are forced to move from the cathode to the anode, essentially restoring the reactants and allowing the battery to be reused. As long as there are reactants in the anode and cathode, and the circuit is closed, secondary batteries continue to produce electrical energy through the electrochemical reactions. Over time, the chemical reactions deplete the reactants in the anode and cathode, which reduces the battery's ability to generate electrical energy.

For example, the following processes occur when discharging a lithium-ion battery.

1. Oxidation occurs at the anode. The graphite intercalation compound LiC₆ is broken down into graphite (C₆) and lithium ions, according to the reaction:
   LiC₆ → C₆ + Li⁺ + e^-
2. The lithium ions move through the electrolyte to the separator until they reach the positive electrode.
3. The electrons produced in (1) move from the anode to the cathode through the external circuitry (wiring).
4. Reduction occurs at the cathode using the electrons and lithium ions produced in (1). Cobalt oxide (CoO₂) reacts with the lithium ions, producing LiCoO₂ according to the reaction
   CoO₂ + Li⁺ + e^- → LiCoO₂

To charge a lithium ion battery, external voltage is applied, which forces reverses the electrochemical processes.

1. The applied voltage forces the electrons to move from the cathode to the anode. This reverses the reduction reaction during the discharge process, causing the deintercalation of lithium ions from the cathode material:
   LiCoO₂ → CoO₂ + Li⁺ + e^-
2. The electrons move from the cathode to the anode through the external circuitry (wiring).
3. The lithium ions move through the electrolyte to the separator until they reach the negative electrode.
4. Intercalation happens at the anode, where lithium ions are inserted back into the anode material (graphite C₆), according to the reaction:
   C₆ + Li⁺ + e^- → LiC₆

Lithium Ion Battery Components

A lithium-ion cell consists of four key components: the anode, cathode, nonaqueous electrolyte, and separator.

 

Cathode

 

Lithium-ion batteries use different cathode chemistries, each offering unique performance traits.  In many designs, the cathode is an aluminum foil coated with the active cathode material. The most common cathode materials include LFP (Lithium Iron Phosphate), NMC (Nickel Manganese Cobalt), LCO (Lithium Cobalt Oxide), NCA (Nickel Cobalt Aluminum), and LMO (Lithium Manganese Oxide). Manufacturers often tailor or combine these chemistries within cells to enhance specific performance characteristics, such as energy density, cycle life, and safety, depending on the application.

Lithium iron phosphate (LFP)
LFP is widely used in automotive applications due to its high power capability, making it ideal for handling rapid charging, such as regenerative braking, and delivering quick bursts of acceleration. It’s also one of the more cost-effective chemistries, as iron phosphate is abundant and inexpensive. LFP is considered safer than some other chemistries because it is more tolerant of overcharging and high temperatures. However, it has a lower energy density, which means it stores less energy per unit weight, resulting in less severe consequences in the event of a failure.

Lithium cobalt oxide (LCO)
LCO is commonly used in consumer electronics like smartphones, cameras, and laptops, where high energy density and long cycle life are critical. While LCO excels in these areas, it has drawbacks, including instability at higher temperatures and a tendency to become reactive above 130 °C, which can lead to thermal runaway. Due to these safety concerns and its higher cost (owing to the significant cobalt content), LCO is less favored for large-scale applications, such as automotive use, though it remains a mainstay in small electronics.

Lithium mangenese oxide (LMO)
LMO is known for its high energy and power capabilities, making it suitable for applications where long run time is needed, such as in portable power tools. However, LMO has a shorter cycle life, meaning it doesn’t last as long under repeated charging and discharging. This limitation makes it less suitable for automotive applications where longevity is crucial, but it remains valuable in scenarios where immediate power and energy are prioritized over long-term durability.

Anode

In lithium-ion batteries, the anode plays a critical role in determining the performance and efficiency of the cell. Currently, most anodes are made from graphite or variations of carbon, such as soft or hard carbons. Graphite is particularly favored because of its stable structure and ability to intercalate lithium ions effectively. The quality of the graphite used can significantly influence the battery's behavior, with high-grade graphite offering better conductivity and durability. However, research is constantly pushing the boundaries to explore new materials that could enhance battery performance.

One such material is lithium titanate (LTO), which has gained interest due to its high power density and ability to operate at lower temperatures. While LTO anodes offer some advantages, including faster charge times and better thermal stability, they come with the drawbacks of a lower nominal voltage and higher cost compared to traditional options like nickel manganese cobalt (NMC) or lithium iron phosphate (LFP) cells. Despite these challenges, LTO is being actively developed for use in specific applications, such as micro-hybrid electric vehicles, where its benefits can outweigh its limitations. Meanwhile, other promising materials like silicon and tin are under investigation, though they face significant hurdles, such as volumetric expansion during cycling, which impacts their commercial viability. As research continues, these new materials could eventually lead to significant advancements in lithium-ion battery technology.

Separator

In lithium-ion batteries, the separator plays a key role in ensuring the cell's safety and efficiency. It is usually a thin layer made from plastic or ceramic materials that physically keeps the anode and cathode apart. This separation is critical because if these two electrodes were to touch, it could cause an internal short circuit, leading to potential cell failure.

The separator must be robust and chemically resistant, as it needs to endure the harsh, often corrosive, hydrocarbon-based electrolytes found within the battery. Although it may seem delicate, the separator is essential for maintaining the battery's structural integrity and safety. It prevents the electrodes from coming into direct contact while still allowing ions to flow freely during the battery's operation. Therefore, the design and choice of materials for the separator are crucial elements that influence the overall performance and reliability of lithium-ion batteries.

Most Common Separator Materials for Lithium Ion Batteries

The most common materials used for separators in lithium-ion batteries are polypropylene (PP) and polyethylene (PE) plastics. These materials are chosen for their unique properties that balance functionality and safety.

Polypropylene (PP) is a popular choice due to its excellent chemical stability and mechanical strength. It is resistant to the electrolyte solutions used in lithium-ion batteries, which helps prevent degradation over time. PP also has a high melting point, which makes it suitable for applications where thermal stability is important.

Polyethylene (PE) is valued for its flexibility and low cost. PE separators often have a slightly lower melting point compared to PP, which can be advantageous in some designs where a lower temperature threshold is desired.

Together, these plastics are designed to be porous, allowing lithium ions to pass freely between the anode and cathode during the charge and discharge cycles. The porous structure ensures that the flow of lithium ions is not obstructed, which is crucial for maintaining the battery’s efficiency and capacity.

Challenges in Separator Materials for Lithium Ion Batteries

The thermal stability of these separators can be a concern. At elevated temperatures, typically above 90 to 110 °C, the plastic material can start to shrink or melt. This can lead to a situation where the separator loses its effectiveness, potentially causing internal short circuits within the battery. To address this issue, some manufacturers use multilayer separators, combining PP and PE in a trilayer configuration. In such designs, the middle PE layer is engineered to melt at a lower temperature, which helps in managing high-temperature scenarios by closing the pores and preventing short circuits, while the outer PP layers remain intact until higher temperatures are reached. This design approach enhances safety and helps ensure the longevity and reliability of the battery under various operating conditions.

Ceramic (High Purity Alumina) Coatings to Improve Thermal Stability of Separator Materials

Some lithium-ion cell manufacturers are increasingly incorporating ceramic-layered separators into their designs to enhance both safety and performance. Ceramic separators offer significant advantages over traditional plastic separators, particularly in terms of thermal stability and safety.

Ceramic materials can withstand much higher temperatures than polypropylene (PP) or polyethylene (PE), making them more effective in preventing thermal runaway and ensuring battery safety during extreme conditions, such as nail penetration tests. In these tests, the battery is subjected to physical damage that could potentially cause a short circuit or thermal event. Ceramic separators are more robust under such stress, reducing the risk of catastrophic failures.

Additionally, ceramic-layered separators contribute to improved power rate capability by lowering the internal resistance of the cell. This reduction in resistance allows for more efficient ion transport between the anode and cathode, which enhances the battery's performance and efficiency. As a result, cells with ceramic separators can deliver higher power output and maintain better performance under high-current conditions, making them suitable for applications requiring rapid discharge and charge cycles.

High-purity Alumina (HPA) Grades of Lithium Ion Battery Separator Coatings

Currently, 4N grade HPA is commonly used as a ceramic coating lithium ion battery separators. Higher 4N+ grade contains >99.99% alumina. High purity is needed to minimize metallic cation impurities and metal impurities, which should be less than a few ppm. Impurities can leach into the electrolyte, form dendrites, or serve as nuclei that accelerate dendrite formation. Metals in the ceramic layer, introduced either through raw materials or the manufacturing process, are a source of short-circuits due to their proximity to the polymer membrane. Higher grades, 5N or 6N, are offered and carry premium pricing over 4N. Various process techniques allow producers to consistently meet 4N+ grade spec, but at relatively high overall product cost. At 4N grade, HPA has under 100 ppm of total impurities.