BASIC KNOWLEDGE - LITHIUM-ION BATTERY Lithium-ion batteries explained
Lithium-ion batteries – also called Li-ion batteries - are used by millions of people every day. This article looks at what lithium-ion batteries are, gives an evaluation of their characteristics, and discusses system criteria such as battery life and battery charging.
Li-ion batteries are almost everywhere. They are used in applications from mobile phones and laptops to hybrid and electric vehicles. Lithium-ion batteries are also increasingly popular in large-scale applications like Uninterruptible Power Supplies (UPSs) and stationary Battery Energy Storage Systems (BESSs).
What are lithium-ion batteries?
A battery is a device consisting of one or more electrochemical cells with external connections for powering electrical devices. When a battery is supplying electric power, its positive terminal is the cathode, and its negative terminal is the anode. The terminal marked negative is the source of electrons that will flow through an external electric circuit to the positive terminal.
When a battery is connected to an external electric load, a redox (reduction-oxidation) reaction converts high-energy reactants to lower-energy products, and the free-energy difference is delivered to the external circuit as electrical energy. Historically the term "battery" specifically referred to a device composed of multiple cells; however, the usage has evolved to include devices composed of a single cell.
Lithium-ion batteries conform to this generic battery definition. Other examples include lead-acid and nickel cadmium (Ni-Cad).
How does a lithium-ion battery work?
Most Li-ion batteries share a similar design consisting of a metal oxide positive electrode (cathode) coated onto an aluminum current collector, a negative electrode (anode) made from carbon/graphite coated on a copper current collector, a separator and electrolyte made of lithium salt in an organic solvent.
While the battery is discharging and providing an electric current, the electrolyte carries positively charged lithium ions from the anode to the cathode and vice versa through the separator. The movement of the lithium ions creates free electrons in the anode which creates a charge at the positive current collector. The electrical current then flows from the current collector through a device being powered (cell phone, computer, etc.) to the negative current collector. The separator blocks the flow of electrons inside the battery.
During charging , an external electrical power source (the charging circuit) applies an over-voltage (a higher voltage than the battery produces, of the same polarity), forcing a charging current to flow within the battery from the positive to the negative electrode, i.e. in the reverse direction of a discharge current under normal conditions. The lithium ions then migrate from the positive to the negative electrode, where they become embedded in the porous electrode material in a process known as intercalation.
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Primary and secondary cells
Primary batteries, or cells, are not rechargeable, and must be discarded once their charge is exhausted. By contrast, secondary types can be recharged using an external electric charger.
Today, most attention is given to secondary types, particularly Li-ion batteries, because of their widespread application in cell phones and electric vehicles. However, primaries still play an important role, especially when charging is impractical or impossible, such as in military combat, rescue missions and forest-fire services . Regulated under IEC 60086, primary batteries also service pacemakers in heart patients, tire pressure gauges in vehicles, smart meters, intelligent drill bits in mining, animal-tracking, remote light beacons, as well as wristwatches, remote controls, electric keys and children’s toys.
Alkaline is the most popular primary battery chemistry, while lithium-metal is used for heavier loads.
Cells, modules, and batteries
The fundamental battery unit, as described in ‘How does a lithium-ion battery work?’ above, is called a battery cell . The three most common form factors are prismatic (rectangular), pouch, and cylindrical.
However, one battery cell is not always enough to power a practical load. Instead, battery cells are connected in series and parallel, into a so-called battery pack, to achieve the desired voltage and energy capacity. An electric car for example requires 400-800 V while one single battery cell typically supplies 3-4 V.
A battery pack is a complete enclosure that delivers power to a final product, such as an electric car. The pack contains battery cells, software (battery management system) and often a cooling and heating system, depending on where and how the battery pack is to be used. In large battery packs, the battery cells are arranged in modules to achieve serviceable units.
Lithium-ion battery applications
Lithium-ion batteries are popular because of their high energy density and other properties – and as the technology improves and prices reduce, they are proliferating in many applications. Here are some examples for Li-ion battery applications:
- Portable power packs: Li-ion batteries are lightweight and more compact than other battery types, which makes them convenient to carry around within cell phones, laptops and other portable personal electronic devices.
- Uninterruptible Power Supplies (UPSs): Li-ion batteries provide emergency back-up power during power loss or fluctuation events. Office equipment like computers, as well as IT servers and complete data centers, must be protected from power interruptions to prevent data loss. Back-up power is also needed in the medical and health care industries to guarantee consistent power supply to life-saving medical equipment.
- Electric vehicles: The automotive industry is creating a demand for Li-ion battery packs to provide power sources for electric, hybrid or plug-in hybrid electric vehicles. As Li-ion batteries can store large amounts of energy and can be recharged many times, they offer good charging capacity and long lifespans.
- Marine vehicles: Li-ion batteries are emerging as an alternative to gasoline and lead-acid batteries in powering work or tug boats and leisure craft like speed boats and yachts. Li-ion batteries provide quiet and efficient power sources and can also be used to provide electricity to appliances inside the boat while it is in dock.
- Personal mobility: Lithium-ion batteries are used in wheelchairs, bikes, scooters and other mobility aids for individuals with disability or mobility restrictions. Unlike cadmium and lead batteries, lithium-ion batteries contain no chemicals that may further harm a person’s health.
- Renewable energy storage: Li-ion batteries are also used for storing energy from solar panels and wind turbines as they can be charged quickly. They are lighter, more compact and can hold higher amounts of energy than lead-acid batteries. Within battery energy storage systems, they fulfil a valuable role in balancing supply and demand, and in grid stabilization.
System evaluation of lithium-ion batteries
Lithium-ion battery characteristics
Li-ion technology is competing with lead-acid, Valve-Regulated Lead-Acid (VRLA) and nickel-cadmium products across its various markets due primarily to its high energy density. Nevertheless, it has traditionally faced two barriers to acceptance: initial purchase price, and fears about safety. More recently, however, both these barriers have been eroding.
According to BloombergNEF’s Battery Price Survey, published in December 2019, pricing has declined significantly over the last decade. Pricing that was above USD1,100 per kilowatt-hour in 2010 has fallen by over 87 % in real terms to USD156 per kWh in 2019. By 2023, average prices will be close to USD100/kWh. This price is seen as the point around which EVs will start to reach price parity with internal combustion engine vehicles. However, this varies depending on the region of sale and vehicle segment.
Cost reductions in 2019 are thanks to increasing order size, growth in battery electric vehicle sales and the continued penetration of high energy density cathodes. The introduction of new pack designs and falling manufacturing costs will drive prices further down in the near term.
Balancing safety and performance: The inherent instability of lithium metal led to the development of a non-metallic lithium battery using lithium ions. Although slightly lower in energy density, the lithium-ion system is safe when it is properly designed, and used with a built-in associated Battery Management System (BMS) which ensures certain precautions are met when charging and discharging.
While establishing an optimal design, manufacturers must take care not to improve performance at the cost of safety. With consumers always wanting longer battery runtimes, battery manufacturers have responded by packing more active material into a cell and making the electrodes and separator thinner. This enabled a doubling of energy density since lithium-ion was introduced in 1991.
The high energy density comes at a price. Manufacturing methods become more critical the denser the cells become. With a separator thickness of only 20-25 µm, any small intrusion of metallic dust particles can have devastating consequences. Appropriate measures will be needed to achieve the mandated safety standard required by UL 1642. Lithium-ion batteries are nearing their theoretical energy density limit and battery manufacturers are beginning to focus on improving manufacturing methods and increasing safety.
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Although battery manufacturers strive to minimize the presence of metallic particles, complex assembly techniques make the elimination of all metallic dust nearly impossible. Accordingly, on rare occasions microscopic metal particles may come into contact with other parts of the battery cell, leading to a short circuit within the cell.
A mild short will only cause an elevated self-discharge. Little heat is generated because the discharging energy is extremely low. If, however, enough microscopic metal particles converge on one spot, a major electrical short can develop and a sizable current will flow between the positive and negative plates. This causes the temperature to rise, leading to a thermal runaway.
These temperature-related safety considerations impact on the choice of chemistry for Li-ion battery cathodes. To achieve maximum runtime, cell phones, digital cameras and laptops use cobalt. However, lithium-ion cells with cobalt cathodes should never rise above 130 °C (265 °F). At 150 °C (302 °F) the cell becomes thermally unstable, a condition that can lead to a thermal runaway in which flaming gases are vented.
Manganese, which is a newer chemistry, offers superior thermal stability. Manganese with a spinel structure (LiMn2O4) can sustain temperatures of up to 250 °C (482 °F) before becoming unstable. Additionally, manganese has a very low internal resistance and can deliver high current on demand. Increasingly, these batteries are used for power tools and medical devices as well as hybrid and electric vehicles.
This section has shown how lithium-ion’s most important characteristics relate to energy density, thermal stability and safety. Further discussion on these topics appears below. However lithium-ion batteries additionally have many other characteristics, which also influence how well they fit into various applications. These have been added to the discussion.
Energy density of lithium-ion batteries
The 18650 form factor provides a useful reference point, as it is very common in applications from laptop battery packs and flashlights to cordless tools and electric vehicles. The first four digits indicate the physical dimensions while the fifth digit shows that it is a cylinder cell. The standard 18650 battery is 18 mm around by 65 mm long.
A paper titled ‘A Brief Review of Current Lithium Ion Battery Technology and Potential Solid State Battery Technologies’ , written by Andrew Ulvestad, provides some energy density calculations for these form factor lithium-ion battery cells as used within an electric vehicle. He says:
“Assuming Tesla is using state of the art Panasonic batteries in their vehicles, the 100 kWh battery pack in the Model S P100D uses 8,256 18650 form factor cells, which is a total cell volume of 136.5 L leading to a volumetric energy density of 732 Wh/L. If we assume each 18650 cell weighs about 45 g then the gravimetric energy density is ~ 270 Wh/kg. The performance limit of this chemistry with the industry standard cathode thickness was recently estimated to be 1200 Wh/L (400 Wh/kg).
“Note that if zero inactive components are used, the fundamental chemistry limit is ~ 470 Wh/kg. If the same cathode is used but the electrode thickness is increased by 60 % and Li metal replaces carbon as the anode, the performance is estimated at 1300 Wh/L (475 Wh/kg). Note that increasing the electrode thickness increases energy density and decreases cost by decreasing the relative weight and volume contribution of the inactive materials.”
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High current capability of Li-ion batteries
The performance characteristics of a Li-ion battery can be modified by changing the choice of materials used for the electrolyte, cathode, and anode. The cathode is a metal oxide while the anode is made of a porous carbon material.
Manufacturers substitute materials and/or use additives to affect how a particular cell performs. Some batteries, for example, are engineered to maximize energy capacity and allow long runtime measured in hours . These are often referred to as “energy cells”. “Power cells” on the other hand, have moderate capacity but can deliver a high current.
For example, an 18650 cell rated at 2,000 mAh can provide a continuous load current of 20 A (30 A with Li-phosphate). The superior performance is achieved in part by lowering the internal resistance and by optimizing the surface area of active cell materials. Low resistance enables high current flow with minimal temperature rise. Running at the maximum permissible discharge current, the Li-ion Power Cell heats to about 50 °C (122 °F); the temperature is limited to 60 °C (140 °F).
Cell voltage of a Li-ion battery
The voltage produced by each lithium-ion cell is about 3.6 V, which is higher than that of standard nickel cadmium, nickel metal hydride and even standard alkaline cells at around 1.5 V and lead-acid at around 2 V per cell. Li-ion with cathode additive materials of cobalt, nickel, manganese and aluminum typically charge to 4.20V/cell. The tolerance is +/–50mV/cell. Some nickel-based varieties charge to 4.10V/cell; high capacity Li-ion may go to 4.30V/cell and higher. Higher voltage means that fewer cells are needed in many applications. Smartphones, for example, need only a single cell; this simplifies power management.
System criteria of lithium-ion batteries
Lithium-ion battery life
Life of a lithium-ion battery is typically defined as the number of full charge-discharge cycles to reach a failure threshold in terms of capacity loss or impedance rise. Manufacturers' datasheet typically uses the word "cycle life" to specify lifespan in terms of the number of cycles to reach 80 % of the rated battery capacity. Inactive storage of these batteries also reduces their capacity, so calendar life is used to represent the whole battery life cycle involving both the cycle and inactive storage operations.
Battery cycle life is affected by many different stress factors including temperature, discharge current, charge current, and state of charge ranges (depth of discharge). Batteries are not fully charged and discharged in real applications such as smartphones, laptops and electric cars, so defining battery life via full discharge cycles can be misleading. To avoid this confusion, researchers sometimes use cumulative discharge defined as the total amount of charge (Ah) delivered by the battery during its entire life or equivalent full cycles, which represents the summation of the partial cycles as fractions of a full charge-discharge cycle.
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Battery degradation during storage is affected by temperature and battery State Of Charge (SOC) and a combination of full charge (100 % SOC) and high temperature (usually > 50 °C) can result in sharp capacity drop and gas generation.
Multiplying the battery cumulative discharge (in Ah) by the rated nominal voltage gives the total energy delivered over the life of the battery. From this one can calculate the cost per kWh of the energy (including the cost of charging).
Li-ion batteries are comparatively low maintenance, and do not require scheduled cycling to maintain their battery life. They have no memory effect, a detrimental process where repeated partial discharge/charge cycles can cause a battery to ‘remember’ a lower capacity.
Some indication of manufacturers’ expectations for their Li-ion battery products can be revealed by looking at the warranties they offer. Nissan, for example, offers a warranty of up to five years or 60,000 miles to cover their Leaf’s battery and electric motor, while Tesla offers an eight year warranty on their Model S. For power tools, Bosch, DeWalt, Metabo HPT (Hitachi), Makita, Milwaukee, and Ridgid all warranty their lithium-ion batteries for 2–3 years.
Lithium-ion battery charging
The charging procedures for single Li-ion cells, and complete Li-ion batteries vary slightly. A single Li-ion cell is charged in two stages: Constant Current (CC) and Constant Voltage (CV). A Li-ion battery (a set of Li-ion cells in series) is charged in three stages: Constant Current, Balance (not required once a battery is balanced) and Constant Voltage.
During the constant current phase, the charger applies a constant current to the battery at a steadily increasing voltage, until the voltage limit per cell is reached.
During the balance phase, the charger reduces the charging current (or cycles the charging on and off to reduce the average current) while the state of charge of individual cells is brought to the same level by a balancing circuit, until the battery is balanced. Some fast chargers skip this stage, and some accomplish the balance by charging each cell independently.
During the constant voltage phase, the charger applies a voltage to the battery equal to the maximum cell voltage times the number of cells in series, as the current gradually declines towards zero, until the current is below a set threshold of about 3 % of initial constant charge current.
A typical electric car (60 kWh battery) takes just under eight hours to charge from discharged to fully charged with a 7 kW charging point. For many electric cars, up to 100 miles of range can be added in about 35 minutes with a 50 kW rapid charger.
Lithium-ion battery safety
A lithium-ion battery comprises not only cells, but also a Battery Management System (BMS) that manages its operation and ensures that it does not depart from its safe operating area. This is vital for Li-ion batteries as they are sensitive to overcharging, shorts and excessively deep discharge, and can be permanently damaged. Battery management systems can contain a switch that can disconnect the battery once charged to capacity, to prevent continued charging and consequent damage.
Battery management systems can monitor almost everything that the battery does, ensuring that temperature is regulated, that power output is kept even and that battery pressure does not exceed advisable limits. These systems can even detect if the battery has shifted and may be discharging its current incorrectly, for example into the conductive body of a vehicle.
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Accordingly, lithium-ion batteries are generally safe and unlikely to fail, but only so long as there are no defects and the batteries are not damaged. When lithium-ion batteries fail to operate safely or are damaged, they may present a fire and/or explosion hazard. Damage from improper use, storage, or charging may also cause lithium-ion batteries to fail. Testing batteries, chargers, and associated equipment in accordance with an appropriate test standard (e.g., UL 2054), NRTL certification (where applicable), and product recalls, help identify defects in design, manufacturing, and material quality.
Damage to all types of lithium batteries can occur when temperatures are too high (e.g., above 130 ° F). Damage can also occur when the batteries or their environment are below freezing (32 °F) during charging.
Charging lithium-ion batteries without following their manufacturer’s instructions may cause damage. For example, some manufacturer-authorized chargers will cycle the power to the battery on and off before it is fully charged to avoid overcharging. Since ultra-fast chargers may not cycle power, do not use them unless the manufacturer’s instructions include them as an option.
Heat released during cell failure can damage nearby cells, releasing more heat in a chain reaction known as a thermal runaway. The high energy density in lithium batteries makes them more susceptible to these reactions. Depending on the battery chemistry, size, design, component types, and amount of energy stored in the lithium cell, lithium cell failures can result in chemical and/or combustion reactions, which can also result in heat releases and/or over-pressurization.
Nevertheless, lithium-ion is one of the most successful and safe battery chemistries available today. Two billion cells are produced every year.
Li-ion battery system load
The load characteristics of a lithium-ion cell are reasonably good. They maintain their nominal voltage of 3.6 V or more before falling off as the last of their charge is used.
The cell’s effective capacity is reduced by very high discharge rates, or conversely increased by low discharge rates.
Design/packaging of a lithium-ion battery
Li-ion cells (as distinct from entire batteries) are available in various shapes, which can] generally be divided into four groups:
- Small cylindrical (solid body without terminals, such as those used in older laptop batteries)
- Large cylindrical (solid body with large threaded terminals)
- Flat or pouch (soft, flat body, such as those used in cell phones and newer laptops; these are lithium-ion polymer batteries.
- Rigid plastic case with large threaded terminals (such as electric vehicle traction packs)
Cells with a cylindrical shape are made in a characteristic "swiss roll" manner (known as a "jelly roll" in the US), which means it is a single long 'sandwich' of the positive electrode, separator, negative electrode, and separator rolled into a single spool. One advantage of cylindrical cells compared to cells with stacked electrodes is faster production speed. A disadvantage can be a large radial temperature gradient inside the cells developing at high discharge currents.
The absence of a case gives pouch cells the highest gravimetric energy density; however, for many practical applications they still require an external means of containment to prevent expansion when their SOC level is high, and for general structural stability of the battery pack of which they are part. Both rigid plastic and pouch-style cells are sometimes referred to as prismatic cells due to their rectangular shapes.
Battery technology analyst Mark Ellis of Munro & Associates sees three basic Li-ion battery types used in modern (~2020) electric vehicle batteries at scale: cylindrical cells (e.g., Tesla), prismatic pouch (e.g., from LG), and prismatic can cells (e.g., from LG, Samsung, Panasonic, and others). Each form factor has characteristic advantages and disadvantages for EV use.
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Lithium-ion battery recycling
As electric vehicles become more popular, the demand for Li-ion battery recycling will grow significantly over the coming decades.
There is some lag to this, as EV batteries have to work through their life of, say, eight years before they become candidates for recycling. Additionally, many of these batteries will find further years’ work in a ‘second life’; although no longer suitable for their vehicle application, they can successfully be used in stationary energy storage systems for grid balancing.
Nevertheless, these factors simply delay the inevitable. From 2025 onwards, retired EV batteries will exceed consumer electronics batteries and dominate the recycling market, while concerns over raw material supplies, especially rare metals such as cobalt, will intensify. Recycling provides a crucial solution to raw material supply insecurity and price fluctuations. Through recovering critical raw materials from Li-ion batteries, manufacturers can shield themselves from supply disruptions and also generate additional revenue streams.
However, recycling presents challenges. An efficient battery collection network is essential, to ensure the availability of volumes sufficient for economic recycling. Lack of battery recycling designs makes battery disassembly and sorting costly and time-consuming; this is exacerbated by the numerous designs and high voltage of EV battery packs. Recyclers will also have to extract more material at higher purities and efficiencies than for consumer electronics batteries to break even.
Most recycling capacity is currently in China, although there is increasing interest from other countries. According to an IDTechEx forecast ‘Li-ion Battery Recycling: 2020-2040’, by 2040 the global Li-ion battery recycling market will be worth USD31 billion annually.