We often talk about ternary lithium batteries or iron-lithium batteries, which are named after the positive electrode active material. This article summarizes six common lithium battery types and their main performance parameters. Everyone knows that the specific parameters of the batteries of the same technical route are not exactly the same. What this article shows is the general level of the current parameters. The six lithium batteries specifically include: lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium nickel cobalt manganese oxide (LiNiMnCoO2 or NMC), lithium nickel cobalt aluminate (LiNiCoAlO2 or NCA), lithium iron phosphate (LiFePO4) , Lithium titanate (Li4Ti5O12).
Lithium Cobalt Oxide (LiCoO 2)
Its high specific energy makes lithium cobalt oxide a popular choice for mobile phones, notebook computers and digital cameras. The battery consists of a cobalt oxide cathode and a graphite carbon anode. The cathode has a layered structure. During discharging, lithium ions move from the anode to the cathode, while the direction of flow is reversed during the charging process. The structure is shown in Figure 1.
Figure 1: Structure of lithium cobalt oxide
The cathode has a layered structure. During discharge, lithium ions move from the anode to the cathode; when charging, the flow flows from the cathode to the anode.
The disadvantages of lithium cobalt oxide are relatively short life, low thermal stability and limited load capacity (specific power). Like other cobalt hybrid lithium-ion batteries, lithium cobalt oxide uses graphite anodes, and its cycle life is mainly limited by the solid electrolyte interface (SEI), which is mainly manifested in the gradual thickening of the SEI film and the anode plating during fast charging or low temperature charging Lithium problem. Newer material systems have added nickel, manganese and/or aluminum to increase life, load capacity and reduce costs.
Lithium cobalt oxide should not be charged and discharged with a current higher than the capacity. This means that the 18650 battery with 2,400mAh can only be charged and discharged at 2,400mA or less. Forcing fast charging or applying a load higher than 2400mA can cause overheating and overload stress. In order to obtain the best fast charging, the manufacturer recommends a charging rate of 0.8C or about 2,000mA. The battery protection circuit limits the charging and discharging rate of the energy unit to a safe level of about 1C.
The hexagonal spider diagram (Figure 2) summarizes the performance of lithium cobalt oxide in specific energy or capacity related to operation; specific power or ability to provide large current; safety; performance in high and low temperature environments; life includes calendar life and cycle Life; cost characteristics. Other important characteristics not shown in the spider diagram include toxicity, fast charging capability, self-discharge and shelf life.
Due to the high cost of cobalt and the obvious performance improvements brought about by mixing materials with other active cathode materials, lithium cobalt oxide is gradually being replaced by lithium manganate, especially NMC and NCA. (See the description of NMC and NCA below.)
Figure 2: Spider diagram of average lithium cobalt oxide battery
Lithium cobalt oxide is excellent in terms of high specific energy, but it can only provide general performance in terms of power characteristics, safety and cycle life.
Table 3: Characteristics of lithium cobalt oxide
Lithium Manganese Oxide (LiMn2O4)
Spinel lithium manganese oxide batteries were first published in a material research report in 1983. In 1996, Moli Energy Corporation commercialized a lithium-ion battery using lithium manganate as the cathode material. The structure forms a three-dimensional spinel structure, which can improve ion flow on the electrode, thereby reducing internal resistance and improving current carrying capacity. Another advantage of spinel is its high thermal stability and improved safety, but its cycle and calendar life is limited.
Low battery internal resistance can achieve fast charging and large current discharge. 18650 battery, lithium manganate battery can be discharged at a current of 20-30A, and has a moderate heat accumulation. It is also possible to apply up to 50A1 second load pulse. Continuous high load at this current will cause heat to accumulate, and the battery temperature cannot exceed 80°C (176°F). Lithium manganate is used in power tools, medical equipment, and hybrid and pure electric vehicles.
Figure 4 illustrates the formation of a three-dimensional crystal framework on the cathode of a lithium manganate battery. The spinel structure is usually composed of rhombus shapes connected into a lattice, and generally appears after the battery is formed.
Figure 4: Structure of lithium manganate
The lithium manganate cathode crystallizes to form a three-dimensional framework structure formed after formation. Spinel provides low electrical resistance, but has a lower specific energy than lithium cobalt oxide.
The capacity of lithium manganese oxide is about one third lower than that of lithium cobalt oxide. Design flexibility allows engineers to choose to maximize battery life, or increase the maximum load current (specific power) or capacity (specific energy). For example, the long-life version of the 18650 battery has only a moderate capacity of 1,100mAh; the high-capacity version reaches 1,500mAh.
Figure 5 shows a spider diagram of a typical lithium manganate battery. These characteristic parameters seem to be less than ideal, but the new design has improved in terms of power, safety and life. Pure lithium manganate batteries are no longer common today; they are only used in special circumstances.
Figure 5: Spider diagram of pure lithium manganate battery
Although the overall performance is mediocre, the new lithium manganese oxide design can improve power, safety and life.
Most lithium manganese oxide is mixed with lithium nickel manganese cobalt oxide (NMC) to increase specific energy and extend life. This combination brings the best performance of each system, and most electric vehicles such as Nissan Leaf, Chevrolet Volt and BMW i3 use LMO (NMC). The LMO part of the battery can reach about 30%, which can provide a higher current when accelerating; the NMC part provides a long cruising range.
Li-ion battery research tends to combine lithium manganate with cobalt, nickel, manganese and/or aluminum as the active cathode material. In some architectures, a small amount of silicon is added to the anode. This provides a 25% increase in capacity; however, silicon expands and contracts with charging and discharging, causing mechanical stress. Capacity increase is usually closely related to short cycle life.
These three active metals and silicon enhancements can be easily selected to improve specific energy (capacity), specific power (load capacity) or lifetime. Consumer batteries need large capacity, while industrial applications need battery systems that have good load capacity, long life, and provide safe and reliable services.
Table 6: Characteristics of lithium manganate oxide
Lithium nickel cobalt manganese oxide (LiNiMnCoO 2 or NMC)
One of the most successful lithium ion systems is the cathode combination of nickel manganese cobalt (NMC). Similar to lithium manganate, this system can be customized to be used as an energy battery or power battery. For example, the NMC in the 18650 battery under medium load conditions has a capacity of about 2,800mAh and can provide 4A to 5A discharge current; when optimized for a specific power, the NMC of the same type has a capacity of only 2,000mAh, but it can provide 20A Continuous discharge current. The silicon-based anode will reach more than 4000mAh, but the load capacity will be reduced and the cycle life will be shortened. Silicon added to graphite has a defect, that is, the anode expands and contracts with charging and discharging, which makes the structure of the battery unstable due to mechanical stress.
The secret of NMC lies in the combination of nickel and manganese. Similar to this is table salt, in which the main ingredients sodium and chloride are toxic in themselves, but they are mixed together as seasoning salt and food preservation agent. Nickel is known for its high specific energy, but its stability is poor; the manganese spinel structure can achieve low internal resistance but low specific energy. The two active metals have complementary advantages.
NMC is the battery of choice for electric tools, electric bicycles and other electric power systems. The cathode combination is usually one-third nickel, one-third manganese and one-third cobalt, also known as 1-1-1. This provides a unique blend that also reduces raw material costs due to the reduced cobalt content. Another successful combination is NCM, which contains 5 parts nickel, 3 parts cobalt and 2 parts manganese (5-3-2). Other combinations of cathode materials in different amounts can also be used.
Due to the high cost of cobalt, battery manufacturers switched from cobalt to nickel cathodes. Nickel-based systems have higher energy density, lower cost and longer cycle life than cobalt-based batteries, but their voltage is slightly lower.
New electrolytes and additives can charge a single battery above 4.4V, thereby increasing the power. Figure 7 shows the characteristics of NMC.
Figure 7: NMC's spider map
NMC has good overall performance and outstanding performance in terms of specific energy. This battery is the first choice for electric vehicles and has the lowest self-heating rate.
Due to the relatively good performance of the system's economy and overall performance, NMC hybrid lithium-ion batteries have attracted more and more attention. The three active materials of nickel, manganese and cobalt can be easily mixed to adapt to a wide range of applications in automobiles and energy storage systems (EES) that require frequent cycles. The diversity of the NMC family is growing.
Table 8: Characteristics of lithium nickel manganese cobalt oxide (NMC)
Lithium Iron Phosphate (LiFePO 4)
In 1996, the University of Texas discovered that phosphate can be used as a cathode material for rechargeable lithium batteries. Lithium phosphate has good electrochemical properties and low resistance. This is achieved through nano-scale phosphate cathode materials. The main advantages are high rated current and long cycle life; good thermal stability, enhanced safety and tolerance to abuse.
If kept at a high voltage for a long time, lithium phosphate is more resistant to all charging conditions and has less stress than other lithium-ion systems. The disadvantage is that the lower nominal voltage of the 3.2V battery makes the specific energy lower than the cobalt-doped lithium-ion battery. For most batteries, low temperatures will reduce performance, and elevated storage temperatures will shorten their service life, and lithium phosphate is no exception. Lithium phosphate has a higher self-discharge than other lithium-ion batteries, which may cause aging and then bring balance problems. Although it can be compensated by selecting high-quality batteries or using advanced battery management systems, both methods increase The cost of the battery pack. The battery life is very sensitive to impurities in the manufacturing process and cannot withstand the doping of moisture. Due to the presence of moisture impurities, the shortest life of some batteries is only 50 cycles. Figure 9 summarizes the properties of lithium phosphate.
Lithium phosphate is commonly used instead of lead-acid starter batteries. Four batteries in series produce 12.80V, which is similar to the voltage of six 2V lead-acid batteries in series. The vehicle charges the lead-acid to 14.40V (2.40V/battery) and maintains a floating charge state. The purpose of floating charge is to maintain a full charge level and prevent sulfation of lead-acid batteries.
By connecting four lithium phosphate batteries in series, the voltage of each battery is 3.60V, which is the correct full charge voltage. At this time, the charging should be disconnected, but continue charging while driving. Lithium phosphate tolerates some overcharging; however, since most vehicles maintain the voltage at 14.40V for long periods of time during long-distance travel, it may increase the mechanical stress of the lithium phosphate battery. Time will tell us how long lithium phosphate can withstand overcharge as an alternative to lead-acid batteries. Low temperature will also reduce the performance of Li-ion, which may affect the starting ability under extreme conditions.
Figure 9: Spider diagram of a typical lithium phosphate battery
Lithium phosphate has good safety and long life, moderate specific energy, and enhanced self-discharge ability.
Table 10: Characteristics of lithium iron phosphate
Lithium nickel cobalt aluminate (LiNiCoAlO2 or NCA)
Nickel-cobalt lithium-aluminate batteries or NCA have been used since 1999. It has a high specific energy, a fairly good specific power and a long service life, which are similar to NMC. The less likable is safety and cost. Figure 11 summarizes the six key features. NCA is a further development of lithium nickel oxide; the addition of aluminum gives the battery better chemical stability.
Figure 11: NCA's spider diagram
High energy and power density and good service life make NCA a candidate for EV power system. High cost and marginal safety have a negative impact.
Table 12: Characteristics of Lithium Nickel Cobalt Aluminum Oxide
Lithium titanate (Li4Ti5O12)
Since the 1980s, batteries with lithium titanate anodes have been known. Lithium titanate replaces graphite in the anode of a typical lithium-ion battery, and the material forms a spinel structure. The cathode can be lithium manganate or NMC. The nominal battery voltage of lithium titanate is 2.40V, which can be quickly charged and provides a high discharge current of 10C. It is said that the number of cycles is higher than that of conventional lithium-ion batteries. Lithium titanate is safe, has excellent low-temperature discharge characteristics, and can obtain 80% of the capacity at -30°C (-22°F).
LTO (usually Li4Ti5 O12) has zero strain, no SEI film formation and no lithium electroplating phenomenon during fast charging and low temperature charging, so it has better charge and discharge performance than traditional cobalt-blended Li-ion and graphite anodes. The thermal stability at high temperatures is also better than other lithium-ion systems; however, batteries are expensive. The specific energy is only 65Wh/kg, which is equivalent to NiCd. The lithium titanate is charged to 2.80V, and it is 1.80V at the end of the discharge. Figure 13 shows the characteristics of lithium titanate batteries. Typical applications are electric power transmission systems, UPS and solar street lights.
Figure 13: Lithium titanate spider diagram
Lithium titanate is excellent in safety, low temperature performance and life. Efforts are being made to increase specific energy and reduce costs.
Table 14: Characteristics of lithium titanate
Figure 15 compares the specific energies based on lead, nickel and lithium systems. Although lithium aluminum (NCA) is a clear winner by storing more capacity than other systems, it is only suitable for power usage in specific scenarios. In terms of specific power and thermal stability, lithium manganate (LMO) and lithium phosphate (LFP) are excellent. Lithium titanate (LTO) may have a lower capacity, but its life span exceeds most other batteries and has the best low temperature performance.
Figure 15: Typical specific energy of lead, nickel and lithium-based batteries
NCA enjoys the highest specific energy; however, lithium manganate and lithium iron phosphate are superior in terms of specific power and thermal stability. Lithium titanate has the best service life.