Understanding Lithium Batteries


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Ref:© 2015 Isidor Buchmann. All rights reserved. Site by Coalescent Design. Battery University


BU-204: Lithium-based Batteries

Discover why lithium-ion is a superior battery system.

Pioneer work with the lithium battery began in 1912 under G.N. Lewis, but it was not until the early 1970s that the first non-rechargeable lithium batteries became commercially available. Attempts to develop rechargeable lithium batteries followed in the 1980s but the endeavor failed because of instabilities in the metallic lithium used as anode material.

Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest specific energy per weight. Rechargeable batteries with lithium metal on the anode (negative electrodes)* could provide extraordinarily high energy densities; however, it was discovered in the mid 1980s that cycling produced unwanted dendrites on the anode. These growth particles penetrate the separator and cause an electrical short. When this occurs, the cell temperature rises quickly and approaches the melting point of lithium, causing thermal runaway, also known as “venting with flame.” A large number of rechargeable metallic lithium batteries sent to Japan were recalled in 1991 after a battery in a mobile phone released flaming gases and inflicted burns to a man’s face.

The inherent instability of lithium metal, especially during charging, shifted research to a non-metallic solution using lithium ions. Although lower in specific energy than lithium-metal, Li ion is safe, provided cell manufacturers and battery packers follow safety measures in keeping voltage and currents to secure levels. [BU-304, Protection Circuits] In 1991, Sony commercialized the first Li ion battery, and today this chemistry has become the most promising and fastest growing on the market. Meanwhile, research continues to develop a safe metallic lithium battery.

Credit for inventing the lithium-cobalt-oxide should go to John Goodenough (1922). It is said that Goodenough was working with a graduate who was employed by Japanese giant Nippon Telephone & Telegraph (NTT). Shortly after Goodenough had invented Li-ion, the student traveled back to Japan, taking the discovery with him. In 1991, Sony announced having acquired an international patent on a lithium-cobalt-oxide cathode. Years of litigation ensued, but Sony was able to keep the patent and Goodenough received nothing for his efforts.

The specific energy of Li ion is twice that of NiCd, and the high nominal cell voltage of 3.60V as compared to 1.20V for nickel systems contributes to this gain. Improvements in the active materials of the electrode have the potential of further increases in energy density. The load characteristics are good, and the flat discharge curve offers effective utilization of the stored energy in a desirable voltage spectrum of 3.70 to 2.80V/cell. Nickel-based batteries also have a flat discharge curve that ranges from 1.25 to 1.0V/cell.

In 1994, the cost to manufacture Li-ion in the 18650** cylindrical cell with a capacity of 1,100mAh was more than $10. In 2001, the price dropped to $2 and the capacity rose to 1,900mAh. Today, high energy-dense 18650 cells deliver over 3,000mAh and the costs have dropped further. Cost reduction, increase in specific energy and the absence of toxic material paved the road to make Li-ion the universally accepted battery for portable application, first in the consumer industry and now increasingly also in heavy industry, including electric powertrains for vehicles.

In 2009, roughly 38 percent of all batteries by revenue were Li ion. Li-ion is a low-maintenance battery, an advantage many other chemistries cannot claim. The battery has no memory and does not need exercising (deliberate full discharge) to keep in shape. Self-discharge is less than half that of nickel-based systems. This makes Li ion well suited for fuel gauge applications. The nominal cell voltage of 3.60V can directly power cell phones and digital cameras, offering simplifications and cost reductions over multi-cell designs. The drawbacks are the need for protection circuits to prevent abuse, as well as high price.

Types of Lithium-ion Batteries

Similar to the lead- and nickel-based architecture, lithium-ion uses a cathode (positive electrode), an anode (negative electrode) and electrolyte as conductor. The cathode is a metal oxide and the anode consists of porous carbon. During discharge, the ions flow from the anode to the cathode through the electrolyte and separator; charge reverses the direction and the ions flow from the cathode to the anode. Figure 1 illustrates the process.

Ion flow in lithium-ion battery

Figure 1: Ion flow
in lithium-ion battery.

When the cell charges and discharges, ions shuttle between cathode (positive electrode) and anode (negative electrode). On discharge, the anode undergoes oxidation, or loss of electrons, and the cathode sees a reduction, or a gain
of electrons. Charge reverses the movement.

Li ion batteries come in many varieties but all have one thing in common — the catchword “lithium-ion.” Although strikingly similar at first glance, these batteries vary in performance, and the choice of cathode materials gives them their unique personality.

Common cathode materials are Lithium Cobalt Oxide (or Lithium Cobaltate), Lithium Manganese Oxide (also known as spinel or Lithium Manganate), Lithium Iron Phosphate, as well as Lithium Nickel Manganese Cobalt (or NMC)*** and Lithium Nickel Cobalt Aluminum Oxide (or NCA). All these materials possess a theoretical specific energy with given limits. (Lithium-ion has a theoretically capacity of about 2,000kWh, more than 10 times the specific energy of a commercial Li-ion battery.)

Sony’s original lithium-ion battery used coke as the anode (coal product). Since 1997, most Li ion manufacturers, including Sony, have shifted to graphite to attain a flatter discharge curve. Graphite is a form of carbon that is also used in the lead pencil. It stores lithium-ion well when the battery is charged and has long-term cycle stability. Among the carbon materials, graphite is the most commonly used, followed by hard and soft carbons. Other carbons, such as carbon nanotubes, have not yet found commercial use. Figure 2 illustrates the voltage discharge curve of a modern Li-ion with graphite anode and the early coke version.

Voltage discharge curve of lithium-ion

 

Figure 2: Voltage discharge curve of lithium-ion

A battery should have a flat voltage curve in the usable discharge range. The modern graphite anode does this better than the early coke version.

Courtesy of Cadex

 

The anode is also developing and several additives are being tried, including silicon-based alloys. It takes six carbon (graphite) atoms to bind to a single lithium ion, a single silicon atom can bind to four lithium ions. This means that the silicon anode can theoretically store more than 10 times the energy of graphite, but the problem is an expansion of the anode during charge. Absorbing the ions could expand the anode to four times the original size. Today’s silicon achieves a 20 to 30 percent increase in specific energy at the cost of lower load currents and reduced cycle life.

Nano-structured lithium-titanate as an anode additive shows promising cycle life, good load capabilities, excellent low-temperature performance and superior safety, but the specific energy is low and the cost is high (see following pages).

Experimenting with cathode and anode material allows manufacturers to strengthen intrinsic qualities but one enhancement may compromise another. In the so-called “Energy Cell,” battery makers optimize the specific energy (capacity) to achieve long runtimes but this may lower the specific power and reduce cycle life. On the “Power Cell,” on the other hand, all efforts are made for high specific power to the sacrifice of capacity. There are also the “Hybrid Cell” that has a little of both, and the “Long-life Cell” that is strictly made for longevity. These specialty cells are larger in size and cost more.

Manufacturers can attain a high specific energy and low cost relatively easily by adding nickel in lieu of cobalt, but this makes the cell less stable. While a start-up company may focus on high specific energy to gain quick market acceptance, safety and durability cannot be compromised. Reputable manufacturers place high integrity on safety and longevity.

Never was the competition to improve the battery and find new chemistries more intense than today. Even an incremental improvement will give the battery a decisive advantage over fossil fuel, an energy source that is almost free. Although the media is not shy in publishing battery breakthroughs, there may be no homeruns at time of writing. Once identified, the new- comer will take four years to enter the market.

The battery industry gained its experience mainly in portable applications and the long-term suitability for the electric powertrain is still unknown. Cycle life, lasting performance and operating cost will only be known after having gone through a few generations of battery-powered vehicles and customer’s acceptance has been solidified. Table 3 summarizes the advantages and limitations of Li-ion.

 

Advantages

High specific energy and commendable energy density

Available in Energy Cells and Power Cells

Rapid charge and high load capabilities

Sealed cells; format choices provide good flexibility

Long cycle and extend shelf-life; no maintenance

High coulombic efficiency; good energy efficiency

Low self-discharge (less than half that of NiCd and NiMH)

Limitations

Requires protection circuit to limit voltage and current

Possibility of venting and thermal runaway if stressed

Degrades at high temperature and when stored at high voltage

No rapid charge possible at freezing temperatures (<0°C, <32°F)

Transportation regulations required when shipping in larger quantities

Higher cost than most other nickel and lead-based systems

Table 3: Advantages and limitations of Li‑ion batteries

 

*          When consuming power, as in a diode, vacuum tube or a battery on charge, the anode is positive; when withdrawing power, as in a battery on discharge, the anode becomes negative.

**       Standard of a cylindrical Li-ion cell developed in the mid 1990s; measures 18mm in diameter and 65mm in length; commonly used for laptops. [BU-301, Battery Formats]

***     Some Lithium Nickel Manganese Cobalt Oxide systems go by designation of NCM, CMN, CNM, MNC and MCN. The systems are basically the same


The casual battery user may think there is only one lithium-ion battery. As there are many species of apple trees, so do also lithium-ion batteries vary and the difference lies mainly in the cathode materials. Innovative materials are also appearing in the anode to modify or replace graphite.

Scientists name batteries by their chemical breakdown. Unless you are also a scientist, this can get a bit complicated and the terms may not mean much to you. In our descriptions, each Li-ion system is listed by its full name, chemical definition, abbreviations and short form. (When appropriate, the writing will use the short form.) All readings are average estimates at time of writing.
 

Lithium Cobalt Oxide(LiCoO2)

Its high specific energy makes Li-cobalt the popular choice for cell 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.

Li-cobalt structure

 

Figure 1Li-cobalt structure

The cathode has a layered structure. Duringdischarge the lithium ions move from the anode to the cathode; on charge the flow is from anode to cathode.

Courtesy of Cadex

Li-cobalt cannot be charged and discharged at a current higher than its 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 1920mA. See BU-402: What is C-rate). The mandatory battery protection circuit limits the charge and discharge rate to a safe level of about 1C.

The hexagonal spider graphic (Figure 2) summarizes the performance of Li-cobalt in terms of specific energy or capacity; specific power or the ability to deliver high current; safety or the chances of venting with flame if abused; performance at hot and cold temperatures; life span reflecting cycle life and longevity; and cost. These spider webs provide do not include all attributes and others of interest are levels of toxicity, fast-charge capabilities, self-discharge and shelf life.

Snapshot of an average Li-cobalt battery

 

Figure 2Snapshot of an average Li-cobalt battery

Li-cobalt excels on high specific energy but offers only moderate performance specific power, safety and life span.

Courtesy of Cadex

 

 


Summary Table

Lithium Cobalt Oxide: LiCoO2 (~60% Co). Graphite anode                                                      Since 1991
Short form: LCO or Li-cobalt.
Voltage, nominal 3.60V
Specific energy (capacity) 150–250Wh/kg
Charge (C-rate) 0.8C, 1C maximum, 4.20V peak (most cells); 3h charge typical
Discharge (C-rate) 1C; 2.50V cut off
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.

Table 3: Characteristics of Lithium Cobalt Oxide
 

Lithium Manganese Oxide (LiMn2O4)

Lithium insertion in manganese spinels was first published in the Materials Research Bulletin in 1983. In 1996, Moli Energy commercialized a Li-ion cell with lithium manganese oxide as a cathode material. The architecture forms a three-dimensional spinel structure that improves ion flow on the electrode, which results in lower internal resistance and improves current handling. A further advantage of spinel is high thermal stability and enhanced safety, but the cycle and calendar life is limited. 

Low internal cell resistance promotes 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 4 shows the crystalline formation of the cathode in a three-dimensional framework. This spinel structure, which is usually composed of diamond shapes connected into a lattice, appears after initial formation.

Li-manganese structure

Figure 4: 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. 

Courtesy of Cadex

Li-manganese has a capacity that is roughly one-third lower compared to Li-cobalt but the battery still holds about 50 percent more energy than nickel-based chemistries. 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 1,100mAh; the high-capacity version is 1,500mAh but has a reduced service life.

Figure 5 shows the spider web of a typical Li-manganese battery. In this chart, all characteristics are marginal; however, newer designs have improved in terms of specific power, safety and life span.

Snapshot of a typical Li-manganese battery

Figure 5: Snapshot of a pure Li-manganese battery

Most modern manganese-based Li-ion systems include a blend of nickel and cobalt. Typical designations are LMO/NMC for lithium manages oxide/nickel-manganese-cobalt.

Courtesy of BCG research

Most Li-manganese batteries “partner” with Lithium Nickel Manganese Cobalt Oxide (NMC) to improve the specific energy and prolong the life span. This combination brings out some of the best in each system and the so-called 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 is about 30% on the Chevy Volt, provides high current boost on acceleration, the NMC part gives the long driving range.


Summary Table

Lithium Manganese Oxide: LiMn2O4. Graphite anode                                                              Since 1996
Short form: LMO or Li-manganese (spinel structure) 
Voltage, nominal 3.70V (some may be rated 3.80V)
Specific energy (capacity) 100–150Wh/kg
Charge (C-rate) 0.7–1C recommended, 3C maximum;  4.20V peak (most cells)
Discharge (C-rate) 10C continuous, 30C for 5s pulse, 2.50V cut-off
Cycle life 500–1000 (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 6: Characteristics of Lithium Manganese Oxide

 

Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2 or NMC)

Leading battery manufacturers focus on a cathode combination of nickel-manganese-cobalt (NMC). Similar to Li-manganese, these systems can be tailored for high specific energy or high specific power, but not both. For example, NMC in an 18650 cell for moderate load condition has a capacity of about 2,800mAh and can deliver 4–5A; NMC in the same cell optimized for specific power has a capacity of only about 2,000mWh but delivers a continuous discharge current of 20A. A silicon-based anode will go to 4,000mAh but at reduced loading and shorter cycle life.

The secret of NMC lies in combining nickel and manganese. An analogy of this is table salt, in which the main ingredients of 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 of typically one-third nickel, one-third manganese and one-third cobalt offers a unique blend that also lowers raw material cost due to reduced cobalt content. Other combinations, such as NCM, CMN, CNM, MNC and MCN are also being offered in which the metal content of the cathode deviates from the 1/3-1/3-1/3 formula. Manufacturers keep the exact ratio a well-guarded secret. Figure 7 demonstrates the characteristics of the NMC.

Snapshot of NMC

 

 

Figure 7: 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.

Courtesy of BCG research


Summary Table

Lithium Nickel Manganese Cobalt Oxide: LiNiMnCoO2. Graphite anode                             Since 2008
Short form: NMC (NCM, CMN, CNM, MNC, MCN are similar with different medal combination)
Voltage, nominal 3.60V, 3.70V
Specific energy (capacity) 150–220Wh/kg
Charge (C-rate) 0.7C, 4.20V peak; 3h charge time
Discharge (C-rate) 2C continuous; 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
Applications E-bikes, medical devices, EVs, industrial
Comments Provides high capacity and high power. Serves as Hybrid Cell. This chemistry is often used to enhance Li-manganese.

Table 8: Characteristics of Lithium Nickel Manganese Cobalt Oxide (NMC)

 

Lithium Iron Phosphate(LiFePO4)

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 a high voltage for a pronged time. (See How to Prolong Lithium-Based Batteries in Chapter 8 on page xxx.) As trade-off, the lower voltage of 3.2V/cell reduces the specific energy to less than Li-manganese. In addition, cold temperature reduces performance, and elevated storage temperature shortens the service life but is still better than lead acid, NiCd or NiMH. Li-phosphate has a higher self-discharge than other Li-ion batteries, which can cause balancing issues with aging. Figure 9 summarizes the attributes of Li-phosphate.

 

Snapshot of a typical Li-phosphate battery

Figure 9: Snapshot of a typical Li-phosphate battery

Li-phosphate has excellent safety and long life span but moderate specific energy and a lower voltage than other lithium-based batteries. LFP also has higher self-discharge compared to other lithium-ion systems.

Courtesy of BCG research


Summary Table

Lithium Iron Phosphate: LiFePO4, Graphite anode                                                                    Since 1996
Short form: LFP or Li-phosphate
Voltage, nominal 3.20V, 3.20V
Specific energy (capacity) 90–120Wh/kg
Charge (C-rate) 1C typical; 3.65V peak; 3h charge time
Discharge (C-rate) 25-30C continuous, 2V cut-off (lower that 2V causes damage)
Cycle life 1000–2000 (related to depth of discharge, temperature)
Thermal runaway 270°C (518°F) Very safe battery even if fully charged
Applications Portable and stationary needing high load currents and endurance
Comments Very flat voltage discharge curve but low capacity. One of safest
Li-Ions. Elevated self-discharge

Table 10: Characteristics of Lithium Iron Phosphate
 

Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2)

Lithium Nickel Cobalt Aluminum Oxide battery, or NCA, has been around since 1999 for special application and shares similarity with NMC by offering high specific energy and reasonably good specific power and a long life span. These attribute made Elon Musk choose NMC for the Tesla EV’s. Less flattering are safety and cost. Figure 11 demonstrates the strong points against areas for further development.

Snapshot of NCA

Figure 11: Snapshot of NCA

High energy and power densities, as well as good life span, make the NCA
a candidate for EV powertrains. High cost and marginal safety are negatives.

Courtesy of BCG research


Summary Table

Lithium Nickel Cobalt Aluminum Oxide: LiNiCoAlO2. (~9% Co) Graphite anode               Since 1999
Short form: NCA or Li-aluminum.
Voltage, nominal 3.60V
Specific energy (capacity) 200-250Wh/kg
Charge (C-rate) 0.5C standard; 4.20V peak (most cells), 3h charge typical
Discharge (C-rate) 1C continuous; 3.00V cut-off
Cycle life 500 (related to depth of discharge, temperature)
Thermal runaway 150°C (302°F) typical, High charge promotes thermal runaway
Applications Medical devices, industrial, electric powertrain (Tesla)
Comments Shares similarities with Li-cobalt. Serves as Energy Cell.

Table 12: Characteristics of Lithium Nickel Cobalt Aluminum Oxide


Lithium Titanate (Li4Ti5O12)

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. 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). However, the battery is expensive and at 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 13 illustrates the characteristics of the Li-titanate battery. Typical uses are electric powertrains and UPS.
 

Snapshot of Li-titanate

Figure 13: 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.

Courtesy of BCG research

 

Summary Table

Lithium Titanate: Li4Ti5O12. Titanate anode                                                                               Since 2008
Short form: LTO or Li-titanate
Voltage, nominal 2.40V
Specific energy (capacity) 70–80Wh/kg
Charge (C-rate) 1C standard; 5C maximum; 2.85V peak
Discharge (C-rate) 10C continuous, 30C 5s pulse; 1.80V cut-off  on LCO/LTO
Cycle life 3,000–7,000
Thermal runaway One of safest Li-ion batteries
Applications UPS, electric powertrain (Mitsubishi i-MiEV, Honda Fit EV)
Comments Long life, fast charge, wide temperature range but low specific energy and expensive. Among safest Li-ion.

Table 14: Characteristics of Lithium Titanate

Figure 15 compares the specific energy of lead, nickel- and lithium-based systems. While Li-cobalt is the clear winner by being able to store more capacity than other systems, this only applies to specific energy. In terms of specific power (load characteristics) and thermal stability, Li-manganese and Li-phosphate are superior. As we move towards electric powertrains, safety and cycle life will become more important than capacity alone.

Typical energy densities of lead, nickel- and lithium-based batteries

Figure 15: Typical specific energy of lead, nickel- and lithium-based batteries
Lithium-cobalt enjoys the highest specific energy; however, manganese and phosphate are superior in terms of specific power and thermal stability


Find out why lithium-polymer is so popular.

The polymer hype of the early 2000s is still going strong, but most users cannot distinguish between a regular Li-ion and one with polymer architecture. While many people identify the term “polymer” as a “plastic,” polymers range from synthetic plastics to natural biopolymers and proteins that are from a fundamental biological structure.

Lithium-polymer differs from other battery systems in the type of electrolyte used. The original polymer design dating back to the 1970s used a solid (dry) polymer electrolyte that resembles a plastic-like film. This insulator allows the exchange of ions (electrically charged atoms) and replaces the traditional porous separator that is soaked with electrolyte. A solid polymer has a poor conductivity at room temperature and the battery must be heated to 60C (140F) and higher to enable current flow. The much anticipated “true plastic battery” promised in the early 2000s did not materialize as the conductivity could not be attained at ambient temperature.

To make the modern Li-polymer battery conductive at room temperature, gelled electrolyte has been added. All Li-ion polymer cells today incorporate a micro porous separator with some moisture. Li-polymer can be built on many systems, such as Li-cobalt, NMC, Li-phosphate and Li-manganese, and is not considered unique battery chemistry. Most Li-polymer packs are for the consumer market and are based on Li-cobalt.

With gelled electrolyte added, what is the difference between a normal Li ion and Li ion polymer? As far as the user is concerned, lithium polymer is essentially the same as lithium-ion. Both systems use identical cathode and anode material and contain a similar amount of electrolyte. Li-polymer is unique in that a micro porous electrolyte replaces the traditional porous separator. Li-polymer offers slightly higher specific energy and can be made thinner than conventional Li-ion, but the manufacturing cost is higher by 10–30 percent.

Li-polymer cells also come in a flexible foil-type case (polymer laminate or pouch cell) that resembles a food package. While a standard Li-ion needs a rigid case to press the electrodes together, Li-polymer uses laminated sheets that do not need compression. A foil-type enclosure reduces the weight by more than 20 percent over the classic hard shell. Thin film technology liberates design as the battery can be made into any shape, fitting neatly into stylish mobile phones and laptops. Li-polymer can also be made very slim to resemble a credit card. [BU-301a: Types of Battery Cells]

Charge and discharge characteristics of Li-polymer are identical to other Li-ion systems and do not require a special charger. Safety issues are also similar in that protection circuits are needed. Gas buildup during charge can cause some prismatic and pouch cells to swell and equipment manufacturers must make allowances for expansion. Li-polymer in a foil package may be less durable than Li-ion in the cylindrical package.


Solid Electrolyte with Lithium-metal Batteries

Research with the solid electrolyte (SE) continues and attempts are made by using metallic lithium as anode material. Solid lithium has a higher energy density than in modified lithium-ion form, but lithium anodes have been tried before and battery manufacturers were forced to discontinue production because of safety issues. Lithium tends to form metal filaments, or dendrites, that cause short circuits. Scientists are trying to overcome this invasion by using specially designed separators and other remedies.

The key objectives for the so-called “solid state lithium ion battery” are achieving sufficient conductivity at room temperature and below and delivering a high enough cycle count, a weak point with most new battery designs. Prototypes of the solid state lithium ion only reach 100 cycles. Targeted applications are load leveling for renewable energy source and fulfilling the need for personal transportation in cars that are non-polluting, charge in minutes and do not prompt range anxiety. Commercialization can take 10 years or longer

Optimizing the selection of a Li-ion system that includes specific energy, specific power and runtime.

Batteries can be made to perform as an Energy Cell that stores a large amount of energy, or a Power Cell that is capable to deliver high load currents. An analogy is a water flask that is designed to hold a large volume of liquid while offering a wide opening to permit quick pouring.

The physical dimensions of a battery are specified by volume in liter (l) and kilogram (kg). Adding dimension and weight provides specific energy in Wh/kg, power density in Wh/l and specific power in W/kg. Most batteries are rated in Wh/kg, revealing how much energy a given weight can generate. Wh/l denotes watt/hours per liter. See Battery Definition and what they mean.)

The relationship between energy and power of a battery can best be represented in a Ragone plot. This plot places energy in Wh on the horizontal x-axis and power in W on the vertical y-axis. The diagonal lines across the field disclose the time the battery cells can deliver energy at various loading conditions. The derived power curve provides a clear demarcation line of what level of power a battery can deliver. The Ragone plot is logarithmic to display performance profiles of very high and low values.

Figure 1 illustrates the Ragone plot reflecting the discharge energy and power of four classic lithium-ion systems packaged in 18650 cells. The battery chemistries featured are the most common power-based lithium-ion systems, which include lithium-iron phosphate (LFP), lithium-manganese oxide (LMO), and nickel manganese cobalt (NMC).

18650 Ragone Plot
Figure 1: Ragone plot reflects Li-ion 18650 cells. 
Four Li-ion systems are compared for discharge power and energy as a function of time.
Courtesy of Exponent

Legend: The A123 APR18650M1 is a lithium iron phosphate (LiFePO4) with 1,100mAh and a continuous discharge current of 30A. The Sony US18650VT and Sanyo UR18650W are manganese–based Li-ion cells of 1500mAh each with a continuous discharge current of 20A. The Sanyo UR18650F is a 2,600mAh cell for a moderate 5A.discharge. This cell provides the highest discharge energy but has the lowest discharge power.

The Sanyo UR18650F [4] has the highest specific energy and can power a laptop or e-bike for many hours at a moderate load. The Sanyo UR18650W [3], in comparison, has a lower specific energy but can supply a current of 20A. The A123 [1] has the lowest specific energy but offers the highest power capability by delivering 30A of continuous current.

The Ragone plot helps choosing the best Li-ion system to satisfy optimal discharge power and energy as a function of discharge time. If an application calls for very high discharge current, the 3.3 minute diagonal line on the chart points to the A123 (Battery 1) as a good pick; it can deliver up to 40 Watts of power for 3.3 minutes. The Sanyo F (Battery 4) is slightly lower and delivers about 36 Watts. Focusing on discharge time and following the 33 minute discharge line further down, Battery 1 (A123) only delivers 5.8 Watts for 33 minutes before the energy is depleted whereas the higher capacity Battery 4 (Sanyo F) can provide roughly 17 Watts for the same time; its limitation is lower power.

For best results, battery manufacturers take the Ragone snapshot on new cells, a condition that is only valid for a short time. When calculating power and energy thresholds, design engineers must include battery fade that will develop as part of cycling and aging. A battery operated systems should still provide full function with a battery that has faded to 70 or 80 percent. A further consideration is temperature as a battery loses power when cold. The Ragone plot does not include these discrepancies.  

It should be noted that loading a battery to its full power capability increases stress and shortens life. When a high current draw is needed continuously, the battery pack should be made larger. Tesla does this with their Model S cars by doubling and tripling the battery. An analogy is a heavy truck fitted with a large diesel engine that provides long and durable service as opposed to installing a souped-up engine of sports car with similar horsepower.

The Ragone plot also calculates power requirements of other energy sources and storage devices, such as capacitors, flywheels, flow batteries and fuel cells. As fuel cells and internal combustion engines draw fuel from a tank, a conflict develops because energy-delivery can be made continuous. The Ragone plot may also be deployed to establish the optimal energy/power ratio and loading condition of a renewable power source, such as solar cells and wind turbines


Info Link:

Lithium vs. LeadBruce SchwabBruce Schwab Marine SystemsRepresentative for Genasun Lithium Batterieshttp://www.bruceschwab.com/uploads/li-vs-la.pdf

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