Week 1 Understanding Different Battery Chemistry
Week 1
Understanding Different Battery Chemistry
QUESTIONS:
1.Prepare a table which includes materials & chemical reactions occurring at the anode and cathode of LCO, LMO, NCA, NMC, LFP and LTO type of lithium ion cells.Give your detailed explanation on it.
Tables of lithium ion battery materials:
Chemical reaction:
In the figure, the negative and positive electrodes are drawn as crystal structures comprising layers of electrode material. Lithium, drawn as small spheres, can be added to or removed from the spaces between the layers. Within the electrodes, lithium is stored as independent charge neutral atoms. Each lithium atom’s valence electron is very loosely shared with neigh boring atoms in the crystal structure. As such, the lithium is not tightly bonded in one place and is actually quite free to move around. Lithium enters and exits the surface of the electrode,
but diffuses within the layered open crystal structure to equalize the concentration of lithium within the vacant spaces of the electrode. During discharge, lithium atoms at the surface of the negative electrode give up electrons—which travel through the external circuit— and become positive lithium ions, Li+—which exit the crystal structure of the electrode and dissolve into the electrolyte.
We can write,
Li ⇒ Li+ + e−.
Conversely, lithium-ions proximate to the surface of the positive electrode receive electrons from the external circuit, and the resulting charge-neutral lithium atoms enter the crystal structure of the electrode.
We can write,
Li+ + e− ⇒ Li.
The process is completely reversible. Thus the lithium ions pass back and forth between the electrodes during charging and discharging. This has given rise to the terms rocking chair, swing, or shuttlecock cells to describe lithium-ion cells. The intercalation mechanism is much gentler than an electrochemical reaction, so lithium-ion cells have much longer lives than other secondary cells.
Negative electrodes
We now begin to describe the materials that are commonly used as electrodes for lithium-ion cells. Presently, the vast majority of commercial lithium-ion cells use some form of graphite (C6) for the negative-electrode material. Graphite comprises multiple grapheme layers, in which hexagonal C6 structures are tightly bonded together. The graphene layers are stacked loosely on top of each other, held together only by weak van der Waals forces; lithium intercalates between these layers.
This is shown in Fig. 1.11, where carbon atoms are drawn as gray spheres joined by covalent bonds to neigh boring carbon atoms in the graphene layers, and the lithium atoms are drawn as purple spheres. There is sufficient room between the graphene layers for the lithium atoms to be able to move freely.
Several different forms of graphite are used in lithium-ion cells.
Two examples are illustrated in Fig. 1.12, where the lines depict graphene segments, and lithium is able to reside in the voids between graphene layers. The difference is in the degree of uniformity of the graphene layers within the microstructures of the particles. Natural and synthetic graphite tends to be the most uniform; natural “hard” or disordered carbons are less uniform, having many small pockets
of graphene layers, arranged in random configurations. The different types of graphite have different voltage properties, capacities, and aging characteristics, but operate in essentially the same fashion. The maximum amount of lithium that can be stored in graphite is one atom of lithium per six atoms of carbon; the minimum amount is zero. Therefore, when talking about the degree of lithiation of a graphite electrode, we use notation LixC6, where 0 ≤ x ≤ 1. Clearly, when viewed at the atomistic level, there is either a single lithium atom or no lithium atom at all for any given C6 site. But, when the entire electrode is considered, some fraction of the total number of C6 sites is occupied, and that fraction is the value of x. When the cell is charged, the negative electrode is highly lithiated, and x is close to 1. When the cell is discharged, the negative electrode is largely depleted of lithium, and x is close to zero. Alternative materials are being investigated for use as negative electrodes. Lithium titanate (Li4Ti5O12, also known as lithium titanate oxide or LTO) allows much faster charging—without harmful side reactions—than graphite. The suppression of side reactions also
yields cell lifetimes of many tens of thousands of charge/discharge cycles. However, the use of LTO also results in a cell voltages being reduced by about 1.4V with respect to an equivalent cell having a graphite negative electrode and thus yields lower energy density. Silicon structures are also being researched, with the potential of much higher energy densities than graphite since up to four atoms of lithium can be stored per atom of silicon. However, this causes very
large changes in the volume of the silicon structures when lithium intercalates, leading to rapid structural breakdown and poor cell lifetimes. At this point it remains to be seen what technology will replace graphite negative electrodes in the future.
Positive electrodes
There is much more variability in the choice of materials to be used in positive electrodes for lithium-ion cells. In 1980, John B. Good enough discovered that lithium cobalt oxide (LixCoO2, also known as LCO) was a viable intercalation compound. Fig. 1.13 shows the crystal structure of LCO. The blue spheres represent cobalt atoms, the red spheres represent oxygen atoms, and the purple spheres represent lithium atoms. The amount of lithium in the structure is variable, but the cobalt-oxygen structures are fixed, and these are drawn as linked
blue polyhedra (specifically, octahedra) to highlight the fact that the layers of cobalt oxide do not change—only the degree of lithiation of the structure can change. It is because of these layers, which behave somewhat like graphene in graphite, that this material is often called a layered cathode. A difference is that the lithiumatoms in LCO act like pillars in the structure, keeping the cobalt-oxide layers apart. If too much lithium is removed, then the crystal structure collapses, and lithium can no longer enter between the layers. To avoid this collapse,
only about half the theoretic capacity is usable (“x” in LixCoO2 is permitted to use only about half of its theoretic range of 0 to 1). LCO is commonly used in lithium-ion cells for portable electronics but suffers some problems when trying to scale up to larger cells for grid storage and vehicular applications. The principal problem is that cobalt is rare, toxic, and expensive. Nickel can be substituted for the cobalt sites, resulting in higher energy density (higher voltages at same capacity), but the resulting cell is not very stable thermally (it tends to catch fire). Aluminum, chromium, and manganese can be substituted as well, resulting in somewhat different properties. Often, a combination of transition metals is used. For example, a lithium nickel manganese cobalt oxide (NMC) electrode comprises a blend of nickel, manganese, and cobalt, which retains the layered structure, and has properties from all three constituent metals. A lithium nickel
cobalt aluminum oxide (NCA) electrode blends nickel, cobalt, and aluminum. In 1983, Goodenough and Thackery proposed lithium manganese oxide (LixMn2O4, or LMO) as an alternate intercalation compound.
This structure is drawn in Fig. 1.14. Manganese is drawn as blue spheres, oxygen as red spheres, and lithium as purple spheres. The manganese oxide forms octahedral crystals in a pattern known as a cubic spinel structure, which is quite complicated and difficult to view in a two-dimensional projection. An interesting feature of this
structure is that there are tunnels through it from each side, allowing lithium movement from front to back, from left to right, and from top to bottom.18 These degrees of freedom make it easy for lithium to move within the structure, decreasing the resistance of the cell. The value of “x” in LixMn2O4 typically varies between 0 and 1, but can go as high as 2. Values of x greater than one are usually avoided because the structure of LMO becomes unstable in acidic conditions when highly lithiated—the crystal structure disintegrates as manganese is attacked by the acid and dissolves into the electrolyte. Additives can be introduced to the electrolyte to neutralize acidity to help prevent this, but the “art” of additive design is presently in the realm of black magic and trade secrets. Despite this serious limitation, LMO is common because it is less expensive and safer than LCO and has similar energy-storage densities.
More recently, in 1997, Good enough proposed olivine style phosphates as a third major category of positive-electrode material. Lithium iron phosphate (LixFePO4, or LFP) is the most common in this family. Its crystal structure is drawn in Fig. 1.15,
where iron is represented by brown spheres, phosphorus by gray spheres, oxygen by red spheres, and lithium by purple spheres. Within this overall crystal structure, FeO6 forms the brown octahedra, and PO4 forms the gray tetrahedra. Lithium is free to move only in one-dimensional linear tunnels,
which makes this material quite resistive. In order to compensate for this high resistance, the electrode particle size is usually chosen to be very small, minimizing the diffusion length (and hence resistance). LFP is quite popular due to the material’s low-cost, low-toxicity, and very stable voltage profile. However, it also produces a lower cell voltage—by about 0.5 V—than other common positive-electrode materials and hence also a lower energy density. The amount of lithium by mass in LFP is also low, resulting in lower specific energies.
Alternative materials are being investigated for use as positive electrodes. Many of these are doped with vanadium, which tends to yield higher cell voltages. However, at present, the challenge is to develop an electrolyte that will function properly as a lithium ion conductor, but will not break down at the higher voltages.
TYPES OF LITHIUM ION BATTERY:
Lithium batteries are more popular today than ever before. You’ll find them in your cell phone, laptop computer, cordless power tools, and even electric vehicles. However, just because all of these electronics use lithium batteries doesn’t mean they use the same type of lithium batteries. We’ll take a closer look at the six main types of lithium batteries reveals pros and cons, as well as the best applications for each.
There are 6 main types of lithium batteries.
What Is A Lithium Battery?
Lithium batteries rely on lithium ions to store energy by creating an electrical potential difference between the negative and positive poles of the battery. An insulating layer called a “separator” divides the two sides of the battery and blocks the electrons while still allowing the lithium ions to pass through.
During the charging phase, lithium ions move from the positive side of the battery to the negative side through the separator. While you discharge the battery, the ions move in the reverse direction.
This movement of lithium ions causes the electrical potential difference mentioned before. This electrical potential difference is called “voltage.” When you connect your electronics to a lithium battery, the electrons which are blocked by the separator are forced to pass through your device and power it.
What Are The 6 Main Types Of Lithium Batteries?
Different types of lithium batteries rely on unique active materials and chemical reactions to store energy. Each type of lithium battery has its benefits and drawbacks, along with its best-suited applications.
The different lithium battery types get their names from their active materials. For example, the first type we will look at is the lithium iron phosphate battery, also known as LiFePO4, based on the chemical symbols for the active materials. However, many people shorten the name further to simply LFP.
#1. Lithium Iron Phosphate
Lithium iron phosphate (LFP) batteries use phosphate as the cathode material and a graphitic carbon electrode as the anode. LFP batteries have a long life cycle with good thermal stability and electrochemical performance.
What Are They Used For:
LFP battery cells have a nominal voltage of 3.2 volts, so connecting four of them in series results in a 12.8-volt battery. This makes LFP batteries the most common type of lithium battery for replacing lead-acid deep-cycle batteries.
Benefits:
There are quite a few benefits to lithium iron phosphate batteries that make them one of the most popular options for applications requiring a large amount of power. The primary benefits, however, are durability, a long life cycle, and safety.
LFP batteries typically have a lifecycle rating of 2,000 cycles or more. Unlike lead-acid batteries, depth of discharge has a minimal impact on the lifespan of LFP batteries. Most LFP manufacturers rate their batteries at 80% depth of discharge, and some even allow 100% discharging without damaging the battery.
Dragonfly Energy lithium iron phosphate batteries can be discharged 100% without damage.
The materials used in lithium iron phosphate batteries offer low resistance, making them inherently safe and highly stable. The thermal runaway threshold is about 518 degrees Fahrenheit, making LFP batteries one of the safest lithium battery options, even when fully charged.
Drawbacks:
There are a few drawbacks to LFP batteries. The first is that compared to other lithium battery types, they have a relatively low specific energy. Their performance can also suffer in low temperatures. Combining the low specific energy and reduced performance in cold temperatures means LFP batteries may not be a great fit in some high cranking applications.
#2. Lithium Cobalt Oxide
Lithium cobalt oxide (LCO) batteries have high specific energy but low specific power. This means that they do not perform well in high-load applications, but they can deliver power over a long period.
What Are They Used For:
LCO batteries were common in small portable electronics such as mobile phones, tablets, laptops, and cameras. However, they are losing popularity to other types of lithium batteries due to the high cost of cobalt and concerns around safety.
Laptops and other portable electronics commonly use LCO batteries.
Benefits:
The key benefit to LCO batteries is their high specific energy. This allows them to deliver power over a relatively long period under low-load applications.
Drawbacks:
LCO batteries have some significant drawbacks resulting in them becoming less popular in recent years. First, LCO batteries suffer from a relatively short lifespan, usually between 500-1,000 cycles. Additionally, cobalt is fairly expensive. Expensive batteries that don’t last a long time are not cost-effective.
LCO batteries also have low thermal stability, which leads to safety concerns. Furthermore, their low specific power limits the ability of LCO batteries to perform in high-load applications.
#3. Lithium Manganese Oxide
Lithium Manganese Oxide (LMO) batteries use lithium manganese oxide as the cathode material. This chemistry creates a three-dimensional structure that improves ion flow, lowers internal resistance, and increases current handling while improving thermal stability and safety.
What Are They Used For:
LMO batteries are commonly found in portable power tools, medical instruments, and some hybrid and electric vehicles.
Cordless power tools often use the lithium manganese oxide battery type.
Benefits:
LMO batteries charge quickly and offer high specific power. This means they can deliver higher current than LCO batteries, for example. They also offer better thermal stability than LCO batteries, meaning they can operate safely at higher temperatures.
One other benefit to LMO batteries is their flexibility. Tuning the internal chemistry allows LMO batteries to be optimized to handle high-load applications or long-life applications.
Drawbacks:
The main downside to LMO batteries is their short lifespan. Typically, LMO batteries will last 300-700 charge cycles, significantly fewer than other lithium battery types.
#4. Lithium Nickel Manganese Cobalt Oxide
Lithium nickel manganese cobalt oxide (NMC) batteries combine the benefits of the three main elements used in the cathode: nickel, manganese, and cobalt. Nickel on its own has high specific energy but is not stable. Manganese is exceptionally stable but has a low specific energy. Combining them yields a stable chemistry with a high specific energy.
What They Are Used For:
Similar to LMO batteries, NMC batteries are popular in power tools as well as electronic powertrains for e-bike, scooters, and some electric vehicles.
Benefits:
The benefits of NMC batteries include high energy density and a longer lifecycle at a lower cost than cobalt-based batteries. They also have higher thermal stability than LCO batteries, making them safer overall.
Drawbacks:
The major drawback to NMC batteries is that they have a slightly lower voltage than cobalt-based batteries.
Electric cars, like Teslas, often use NMC and NCA lithium batteries.
#5. Lithium Nickel Cobalt Aluminium Oxide
Lithium nickel cobalt aluminum oxide (NCA) batteries offer high specific energy with decent specific power and a long lifecycle. This means they can deliver a relatively high amount of current for extended periods.
What They Are Used For:
The ability to perform in high-load applications with a long battery life makes NCA batteries popular in the electric vehicle market. Specifically, NCA is the battery of choice for Tesla.
Benefits:
The biggest benefits of NCA batteries are high energy and a decent lifespan.
Drawbacks:
With NCA technology, the batteries aren’t as safe as most other lithium technologies and are expensive in comparison.
#6. Lithium Titanate
All of the previous lithium battery types we have discussed are unique in the chemical makeup of the cathode material. Lithium titanate (LTO) batteries replace the graphite in the anode with lithium titanate and use LMO or NMC as the cathode chemistry.
The result is an extremely safe battery with a long lifespan that charges faster than any other lithium battery type.
What Are They Used For:
Many applications use LTO batteries. Electric vehicles and charging stations, uninterrupted power supplies, wind and solar energy storage, solar street lights, telecommunications systems, and aerospace and military equipment are just some of the use cases.
Lithium titanate batteries offer extremely wide operating temperatures making them useful in aerospace applications.
Benefits:
LTO batteries offer many benefits, including fast charging, an extremely wide operating temperature, a long lifespan, and superb safety because of their stability.
Drawbacks:
There are a couple of significant hurdles for LTO batteries to overcome. They offer low energy density, which means it stores a lower amount of energy relative to its weight when compared to some other lithium technologies. Additionally, they are very expensive.
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2.Compare the differences between each type of Li+ion batteries based on their characteristics.
Lithium Cobalt Oxide(LiCoO2) — LCO
Its high specific energy makes Li-cobalt the popular choice for mobile phones, laptops and digital cameras. The battery consists of a cobalt oxide cathode and a graphite carbon anode. The cathode has a layered structure and during discharge, lithium ions move from the anode to the cathode. The flow reverses on charge. The drawback of Li-cobalt is a relatively short life span, low thermal stability and limited load capabilities (specific power). Figure 1 illustrates the structure.
Figure 1: Li-cobalt structure.
The cathode has a layered structure. During discharge the lithium ions move from the anode to the cathode; on charge the flow is from cathode to anode.
The drawback of Li-cobalt is a relatively short life span, low thermal stability and limited load capabilities (specific power). Like other cobalt-blended Li-ion, Li-cobalt has a graphite anode that limits the cycle life by a changing solid electrolyte interface (SEI), thickening on the anode and lithium plating while fast charging and charging at low temperature. Newer systems include nickel, manganese and/or aluminum to improve longevity, loading capabilities and cost.
Li-cobalt should not be charged and discharged at a current higher than its C-rating. This means that an 18650 cell with 2,400mAh can only be charged and discharged at 2,400mA. Forcing a fast charge or applying a load higher than 2,400mA causes overheating and undue stress. For optimal fast charge, the manufacturer recommends a C-rate of 0.8C or about 2,000mA. The mandatory battery protection circuit limits the charge and discharge rate to a safe level of about 1C for the Energy Cell.
The hexagonal spider graphic (Figure 2) summarizes the performance of Li-cobalt in terms of specific energy or capacity that relates to runtime; specific power or the ability to deliver high current; safety; performance at hot and cold temperatures; life span reflecting cycle life and longevity; and cost. Other characteristics of interest not shown in the spider webs are toxicity, fast-charge capabilities, self-discharge and shelf life.
The Li-cobalt is losing favor to Li-manganese, but especially NMC and NCA because of the high cost of cobalt and improved performance by blending with other active cathode materials.
Figure 2: Snapshot of an average Li-cobalt battery.
Li-cobalt excels on high specific energy but offers only moderate performance specific power, safety and life span.
Summary Table
|
Lithium Cobalt Oxide: LiCoO2 cathode (~60% Co), graphite anode |
|
|
Voltages |
3.60V nominal; typical operating range 3.0–4.2V/cell |
|
Specific energy (capacity) |
150–200Wh/kg. Specialty cells provide up to 240Wh/kg. |
|
Charge (C-rate) |
0.7–1C, charges to 4.20V (most cells); 3h charge typical. Charge current above 1C shortens battery life. |
|
Discharge (C-rate) |
1C; 2.50V cut off. Discharge current above 1C shortens battery life. |
|
Cycle life |
500–1000, related to depth of discharge, load, temperature |
|
Thermal runaway |
150°C (302°F). Full charge promotes thermal runaway |
|
Applications |
Mobile phones, tablets, laptops, cameras |
|
Comments |
Very high specific energy, limited specific power. Cobalt is expensive. Serves as Energy Cell. Market share has stabilized. |
Table 1: Characteristics of Lithium Cobalt Oxide.
Lithium Manganese Oxide (LiMn2O4) — LMO
Li-ion with manganese spinel was first published in the Materials Research Bulletin in 1983. In 1996, Moli Energy commercialized a Li-ion cell with lithium manganese oxide as cathode material. The architecture forms a three-dimensional spinel structure that improves ion flow on the electrode, which results in lower internal resistance and improved current handling. A further advantage of spinel is high thermal stability and enhanced safety, but the cycle and calendar life are limited.
Low internal cell resistance enables fast charging and high-current discharging. In an 18650 package, Li-manganese can be discharged at currents of 20–30A with moderate heat buildup. It is also possible to apply one-second load pulses of up to 50A. A continuous high load at this current would cause heat buildup and the cell temperature cannot exceed 80°C (176°F). Li-manganese is used for power tools, medical instruments, as well as hybrid and electric vehicles.
Figure 3 illustrates the formation of a three-dimensional crystalline framework on the cathode of a Li-manganese battery. This spinel structure, which is usually composed of diamond shapes connected into a lattice, appears after initial formation.
Figure 3: Li-manganese structure.
The cathode crystalline formation of lithium manganese oxide has a three-dimensional framework structure that appears after initial formation. Spinel provides low resistance but has a more moderate specific energy than cobalt.
Li-manganese has a capacity that is roughly one-third lower than Li-cobalt. Design flexibility allows engineers to maximize the battery for either optimal longevity (life span), maximum load current (specific power) or high capacity (specific energy). For example, the long-life version in the 18650 cell has a moderate capacity of only 1,100mAh; the high-capacity version is 1,500mAh.
Figure 4 shows the spider web of a typical Li-manganese battery. The characteristics appear marginal but newer designs have improved in terms of specific power, safety and life span. Pure Li-manganese batteries are no longer common today; they may only be used for special applications.
Figure 4: Snapshot of a pure Li-manganese battery.
Although moderate in overall performance, newer designs of Li-manganese offer improvements in specific power, safety and life span.
Most Li-manganese batteries blend with lithium nickel manganese cobalt oxide (NMC) to improve the specific energy and prolong the life span. This combination brings out the best in each system, and the LMO (NMC) is chosen for most electric vehicles, such as the Nissan Leaf, Chevy Volt and BMW i3. The LMO part of the battery, which can be about 30 percent, provides high current boost on acceleration; the NMC part gives the long driving range.
Li-ion research gravitates heavily towards combining Li-manganese with cobalt, nickel, manganese and/or aluminum as active cathode material. In some architecture, a small amount of silicon is added to the anode. This provides a 25 percent capacity boost; however, the gain is commonly connected with a shorter cycle life as silicon grows and shrinks with charge and discharge, causing mechanical stress.
These three active metals, as well as the silicon enhancement can conveniently be chosen to enhance the specific energy (capacity), specific power (load capability) or longevity. While consumer batteries go for high capacity, industrial applications require battery systems that have good loading capabilities, deliver a long life and provide safe and dependable service.
Summary Table
|
Lithium Manganese Oxide: LiMn2O4 cathode. graphite anode |
|
|
Voltages |
3.70V (3.80V) nominal; typical operating range 3.0–4.2V/cell |
|
Specific energy (capacity) |
100–150Wh/kg |
|
Charge (C-rate) |
0.7–1C typical, 3C maximum, charges to 4.20V (most cells) |
|
Discharge (C-rate) |
1C; 10C possible with some cells, 30C pulse (5s), 2.50V cut-off |
|
Cycle life |
300–700 (related to depth of discharge, temperature) |
|
Thermal runaway |
250°C (482°F) typical. High charge promotes thermal runaway |
|
Applications |
Power tools, medical devices, electric powertrains |
|
Comments |
High power but less capacity; safer than Li-cobalt; commonly mixed with NMC to improve performance. |
Table 2: Characteristics of Lithium Manganese Oxide
Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2) — NMC
One of the most successful Li-ion systems is a cathode combination of nickel-manganese-cobalt (NMC). Similar to Li-manganese, these systems can be tailored to serve as Energy Cells or Power Cells. For example, NMC in an 18650 cell for moderate load condition has a capacity of about 2,800mAh and can deliver 4A to 5A; NMC in the same cell optimized for specific power has a capacity of only about 2,000mAh but delivers a continuous discharge current of 20A. A silicon-based anode will go to 4,000mAh and higher but at reduced loading capability and shorter cycle life. Silicon added to graphite has the drawback that the anode grows and shrinks with charge and discharge, making the cell mechanically unstable.
The secret of NMC lies in combining nickel and manganese. An analogy of this is table salt in which the main ingredients, sodium and chloride, are toxic on their own but mixing them serves as seasoning salt and food preserver. Nickel is known for its high specific energy but poor stability; manganese has the benefit of forming a spinel structure to achieve low internal resistance but offers a low specific energy. Combining the metals enhances each other strengths.
NMC is the battery of choice for power tools, e-bikes and other electric powertrains. The cathode combination is typically one-third nickel, one-third manganese and one-third cobalt, also known as 1-1-1. This offers a unique blend that also lowers the raw material cost due to reduced cobalt content. Another successful combination is NCM with 5 parts nickel, 3 parts cobalt and 2 parts manganese (5-3-2). Other combinations using various amounts of cathode materials are possible.
Battery manufacturers move away from cobalt systems toward nickel cathodes because of the high cost of cobalt. Nickel-based systems have higher energy density, lower cost, and longer cycle life than the cobalt-based cells but they have a slightly lower voltage.
New electrolytes and additives enable charging to 4.4V/cell and higher to boost capacity. Figure 5 demonstrates the characteristics of the NMC.
Figure 5: Snapshot of NMC.
NMC has good overall performance and excels on specific energy.
This battery is the preferred candidate for the electric vehicle and has the lowest self-heating rate.
There is a move towards NMC-blended Li-ion as the system can be built economically and it achieves a good performance. The three active materials of nickel, manganese and cobalt can easily be blended to suit a wide range of applications for automotive and energy storage systems (EES) that need frequent cycling. The NMC family is growing in its diversity.
Summary Table
|
Lithium Nickel Manganese Cobalt Oxide: LiNiMnCoO2. cathode, graphite anode |
|
|
Voltages |
3.60V, 3.70V nominal; typical operating range 3.0–4.2V/cell, or higher |
|
Specific energy (capacity) |
150–220Wh/kg |
|
Charge (C-rate) |
0.7–1C, charges to 4.20V, some go to 4.30V; 3h charge typical. Charge current above 1C shortens battery life. |
|
Discharge (C-rate) |
1C; 2C possible on some cells; 2.50V cut-off |
|
Cycle life |
1000–2000 (related to depth of discharge, temperature) |
|
Thermal runaway |
210°C (410°F) typical. High charge promotes thermal runaway |
|
Cost |
~$420 per kWh[1] |
|
Applications |
E-bikes, medical devices, EVs, industrial |
|
Comments 2019 Update: |
Provides high capacity and high power. Serves as Hybrid Cell. Favorite chemistry for many uses; market share is increasing. |
Table 3: Characteristics of Lithium Nickel Manganese Cobalt Oxide (NMC)
Lithium Iron Phosphate(LiFePO4) — LFP
In 1996, the University of Texas (and other contributors) discovered phosphate as cathode material for rechargeable lithium batteries. Li-phosphate offers good electrochemical performance with low resistance. This is made possible with nano-scale phosphate cathode material. The key benefits are high current rating and long cycle life, besides good thermal stability, enhanced safety and tolerance if abused.
Li-phosphate is more tolerant to full charge conditions and is less stressed than other lithium-ion systems if kept at high voltage for a prolonged time. As a trade-off, its lower nominal voltage of 3.2V/cell reduces the specific energy below that of cobalt-blended lithium-ion. With most batteries, cold temperature reduces performance and elevated storage temperature shortens the service life, and Li-phosphate is no exception. Li-phosphate has a higher self-discharge than other Li-ion batteries, which can cause balancing issues with aging. This can be mitigated by buying high quality cells and/or using sophisticated control electronics, both of which increase the cost of the pack. Cleanliness in manufacturing is of importance for longevity. There is no tolerance for moisture, lest the battery will only deliver 50 cycles. Figure 9 summarizes the attributes of Li-phosphate.
Li-phosphate is often used to replace the lead acid starter battery. Four cells in series produce 12.80V, a similar voltage to six 2V lead acid cells in series. Vehicles charge lead acid to 14.40V (2.40V/cell) and maintain a topping charge. Topping charge is applied to maintain full charge level and prevent sulfation on lead acid batteries.
With four Li-phosphate cells in series, each cell tops at 3.60V, which is the correct full-charge voltage. At this point, the charge should be disconnected but the topping charge continues while driving. Li-phosphate is tolerant to some overcharge; however, keeping the voltage at 14.40V for a prolonged time, as most vehicles do on a long road trip, could stress Li-phosphate. Time will tell how durable Li-Phosphate will be as a lead acid replacement with a regular vehicle charging system. Cold temperature also reduces performance of Li-ion and this could affect the cranking ability in extreme cases.
Figure 6: Snapshot of a typical Li-phosphate battery.
Li-phosphate has excellent safety and long life span but moderate specific energy and elevated self-discharge.
Summary Table
|
Lithium Iron Phosphate: LiFePO4 cathode, graphite anode |
|
|
Voltages |
3.20, 3.30V nominal; typical operating range 2.5–3.65V/cell |
|
Specific energy (capacity) |
90–120Wh/kg |
|
Charge (C-rate) |
1C typical, charges to 3.65V; 3h charge time typical |
|
Discharge (C-rate) |
1C, 25C on some cells; 40A pulse (2s); 2.50V cut-off (lower that 2V causes damage) |
|
Cycle life |
2000 and higher (related to depth of discharge, temperature) |
|
Thermal runaway |
270°C (518°F) Very safe battery even if fully charged |
|
Cost |
~$580 per kWh[1] |
|
Applications |
Portable and stationary needing high load currents and endurance |
|
Comments |
Very flat voltage discharge curve but low capacity. One of safest Li-ions. |
Table 4: Characteristics of Lithium Iron Phosphate
See Lithium Manganese Iron Phosphate (LMFP) for manganese enhanced L-phosphate.
Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2) — NCA
Lithium nickel cobalt aluminum oxide battery, or NCA, has been around since 1999 for special applications. It shares similarities with NMC by offering high specific energy, reasonably good specific power and a long life span. Less flattering are safety and cost. Figure 7 summarizes the six key characteristics. NCA is a further development of lithium nickel oxide; adding aluminum gives the chemistry greater stability.
Figure 7: Snapshot of NCA.
High energy and power densities, as well as good life span, make NCA a candidate for EV powertrains. High cost and marginal safety are negatives. Source: Cadex
Summary Table
|
Lithium Nickel Cobalt Aluminum Oxide: LiNiCoAlO2 cathode (~9% Co), graphite anode |
|
|
Voltages |
3.60V nominal; typical operating range 3.0–4.2V/cell |
|
Specific energy (capacity) |
200-260Wh/kg; 300Wh/kg predictable |
|
Charge (C-rate) |
0.7C, charges to 4.20V (most cells), 3h charge typical, fast charge possible with some cells |
|
Discharge (C-rate) |
1C typical; 3.00V cut-off; high discharge rate shortens battery life |
|
Cycle life |
500 (related to depth of discharge, temperature) |
|
Thermal runaway |
150°C (302°F) typical, High charge promotes thermal runaway |
|
Cost |
~$350 per kWh[1] |
|
Applications |
Medical devices, industrial, electric powertrain (Tesla) |
|
Comments |
Shares similarities with Li-cobalt. Serves as Energy Cell. |
Table 5: Characteristics of Lithium Nickel Cobalt Aluminum Oxide
Lithium Titanate (Li2TiO3) — LTO
Batteries with lithium titanate anodes have been known since the 1980s. Li-titanate replaces the graphite in the anode of a typical lithium-ion battery and the material forms into a spinel structure. The cathode can be lithium manganese oxide or NMC. Li-titanate has a nominal cell voltage of 2.40V, can be fast charged and delivers a high discharge current of 10C, or 10 times the rated capacity. The cycle count is said to be higher than that of a regular Li-ion. Li-titanate is safe, has excellent low-temperature discharge characteristics and obtains a capacity of 80 percent at –30°C (–22°F).
LTO (commonly Li4Ti5O12) has advantages over the conventional cobalt-blended Li-ion with graphite anode by attaining zero-strain property, no SEI film formation and no lithium plating when fast charging and charging at low temperature. Thermal stability under high temperature is also better than other Li-ion systems; however, the battery is expensive. At only 65Wh/kg, the specific energy is low, rivalling that of NiCd. Li-titanate charges to 2.80V/cell, and the end of discharge is 1.80V/cell. Figure 8 illustrates the characteristics of the Li-titanate battery. Typical uses are electric powertrains, UPS and solar-powered street lighting.
Figure 8: Snapshot of Li-titanate.
Li-titanate excels in safety, low-temperature performance and life span. Efforts are being made to improve the specific energy and lower cost. Source: Boston Consulting Group
Summary Table
|
Lithium Titanate: Cathode can be lithium manganese oxide or NMC; Li2TiO3 (titanate) anode |
|
|
Voltages |
2.40V nominal; typical operating range 1.8–2.85V/cell |
|
Specific energy (capacity) |
50–80Wh/kg |
|
Charge (C-rate) |
1C typical; 5C maximum, charges to 2.85V |
|
Discharge (C-rate) |
10C possible, 30C 5s pulse; 1.80V cut-off on LCO/LTO |
|
Cycle life |
3,000–7,000 |
|
Thermal runaway |
One of safest Li-ion batteries |
|
Cost |
~$1,005 per kWh[1] |
|
Applications |
UPS, electric powertrain (Mitsubishi i-MiEV, Honda Fit EV), solar-powered street lighting |
|
Comments |
Long life, fast charge, wide temperature range but low specific energy and expensive. |
Table 6: Characteristics of Lithium Nickel Cobalt Aluminum Oxide
Future Batteries
- Solid-state Li-ion: High specific energy but poor loading and safety.
- Lithium-sulfur: High specific energy but poor cycle life and poor loading
- Lithium-air: High specific energy but poor loading, needs clean air to breath and has short life.
Figure 9 compares the specific energy of lead-, nickel- and lithium-based systems. While Li-aluminum (NCA) is the clear winner by storing more capacity than other systems, this only applies to specific energy. In terms of specific power and thermal stability, Li-manganese (LMO) and Li-phosphate (LFP) are superior. Li-titanate (LTO) may have low capacity but this chemistry outlives most other batteries in terms of life span and also has the best cold temperature performance. Moving towards the electric powertrain, safety and cycle life will gain dominance over capacity. (LCO stands for Li-cobalt, the original Li-ion.)
Figure 9: Typical specific energy of lead-, nickel- and lithium-based batteries.
NCA enjoys the highest specific energy; however, manganese and phosphate are superior in terms of specific power and thermal stability. Li-titanate has the best life span.