1、 Lithium cobalt oxide (LiCoO2)
Its high specific energy makes lithium cobalt oxide a popular choice for mobile phones, laptops, and digital cameras. The drawbacks of lithium cobalt oxide are its relatively short lifespan, low thermal stability, and limited load capacity (specific power). Like other cobalt mixed lithium-ion batteries, lithium cobalt oxide uses a graphite negative electrode, and its cycle life is mainly limited by the solid electrolyte interface (SEI), mainly manifested in the gradual thickening of the SEI film and the problem of lithium plating on the negative electrode during rapid or low-temperature charging processes. Newer material systems have added nickel, manganese, and/or aluminum to improve lifespan, load capacity, and reduce costs.
2、 Lithium manganese oxide (LiMn2O4)
The spinel lithium manganese battery was first published in a material research report in 1983. In 1996, Moli Energy Company commercialized lithium-ion batteries using lithium manganese oxide as the positive electrode material. This architecture forms a three-dimensional spinel structure that 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 are limited.
Low battery internal resistance can achieve fast charging and high current discharge. 18650 type battery cell, lithium manganese oxide battery can discharge at a current of 20-30A and has moderate heat accumulation. The battery temperature cannot exceed 80 ° C. Lithium manganese oxide is used in electric tools, medical devices, as well as hybrid and pure electric vehicles. The capacity of lithium manganese oxide is approximately one-third lower than that of lithium cobalt oxide. Design flexibility allows engineers to choose to maximize battery life or increase maximum load current or capacity.
Most lithium manganate is mixed with lithium nickel manganese cobalt oxide (NMC) to increase specific energy and extend lifespan. This combination brings the best performance for each system, and most electric vehicles such as the Nissan Leaf, Chevrolet Volt, and BMW i3 all use LMO (NMC). The LMO part of the battery can reach around 30%, providing higher current during acceleration; The NMC section provides a long range. Research on lithium-ion batteries tends to combine lithium manganese oxide with cobalt, nickel, manganese, and/or aluminum as active cathode materials. In some architectures, a small amount of silicon is added to the negative electrode. This provides a 25% capacity increase; However, silicon expands and contracts with charging and discharging, causing mechanical stress, and capacity improvement is usually closely related to short cycle life.
3、 Nickel cobalt lithium manganese (NMC)
One of the most successful lithium-ion systems is the positive electrode combination of nickel manganese cobalt (NMC). Similar to lithium manganese oxide, this system can be customized for use as an energy or power battery. For example, the NMC in a 18650 battery under moderate load conditions has a capacity of approximately 2800mAh and can provide discharge currents ranging from 4A to 5A; The same type of NMC has a capacity of only 2000mAh when optimized for specific power, but can provide a continuous discharge current of 20A. The silicon based negative electrode will reach over 4000mAh, but the load capacity will decrease and the cycle life will be shortened. The silicon added to graphite has defects, that is, the negative electrode expands and contracts with charging and discharging, resulting in unstable mechanical stress structure of the battery.
The secret of NMC lies in the combination of nickel and manganese. Similar to this is table salt, where the main components sodium and chloride are themselves toxic, but when mixed together, they serve as seasoning salts and food preservatives. Nickel is famous for its high specific energy, but its stability is poor; The manganese spinel structure can achieve low internal resistance but low specific energy. The advantages of two active metals complement each other. NMC is the preferred battery for electric tools, electric bicycles, and other electric power systems. The combination of positive electrodes is usually one-third nickel, one-third manganese, and one-third cobalt, also known as 1-1. This provides a unique mixture that 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 different amounts of positive electrode materials can also be used. Due to the high cost of cobalt, battery manufacturers have shifted from cobalt based to nickel positive electrodes. 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 to above 4.4V, thereby increasing its battery capacity.
Due to the good economic and comprehensive performance of this system, NMC hybrid lithium-ion batteries are receiving increasing attention. Nickel, manganese, and cobalt are three active materials that can be easily mixed to adapt to the wide range of applications in automobiles and energy storage systems that require frequent cycling. The diversity of the NMC family is increasing.
4、 Lithium iron phosphate (LiFePO4)
In 1996, the University of Texas discovered that phosphate can be used as a positive electrode material for rechargeable lithium batteries. Lithium phosphate has excellent electrochemical performance and low resistance. This is achieved through nanoscale phosphate cathode materials. The main advantages are high rated current and long cycle life; Good thermal stability enhances safety and tolerance for abuse. If maintained at high voltage for a long time, lithium phosphate has stronger resistance to all charging conditions and has lower stress than other lithium-ion systems. The disadvantage is that the lower nominal voltage of 3.2V batteries results in lower specific energy compared to cobalt doped lithium-ion batteries. For most batteries, low temperature will reduce performance, while increasing storage temperature will shorten 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 bring about balance problems. Although it can be compensated by selecting high-quality batteries or using advanced battery management systems, both of these methods increase the cost of the battery pack. The battery life is very sensitive to impurities in the manufacturing process and cannot withstand water doping. Due to the presence of water impurities, some batteries have a minimum lifespan of only 50 cycles. Figure 7 summarizes the properties of lithium phosphate.
Lithium phosphate is commonly used as a substitute for lead-acid starting batteries. Four series connected batteries generate 12.80V, which is similar to the voltage generated by six 2V lead-acid batteries in series. The vehicle charges lead-acid to 14.40V (2.40V/battery) and maintains a floating charge state. The purpose of float charging is to maintain a fully charged 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 point, the charging should be disconnected, but continue to charge while driving. Lithium phosphate tolerates some overcharging; However, due to the fact that most vehicles maintain a voltage of 14.40V for a long time during long-distance travel, it may increase the mechanical stress of lithium phosphate batteries. Time will tell us how long lithium phosphate can withstand overcharging as a substitute for lead-acid batteries. Low temperatures can also reduce the performance of lithium ions, which may affect their starting ability in extreme situations.
5、 Lithium nickel cobalt aluminate (NCA)
Lithium nickel cobalt aluminate batteries or NCA have been used since 1999. It has high specific energy, quite good specific power, and long service life similar to NMC. Less pleasing are safety and cost.
NCA is a further development of lithium nickel oxide; Adding aluminum gives the battery better chemical stability. High energy and power density, as well as good service life, make NCA a candidate for EV power systems. High costs and marginal safety have negative impacts.
6、 Lithium titanate
Since the 1980s, lithium titanate negative electrode batteries have been known. Lithium titanate replaces graphite in the negative electrode of typical lithium-ion batteries, and the material forms a spinel structure. The positive electrode can be lithium manganate or NMC. The nominal battery voltage of lithium titanate is 2.40V, which can quickly charge and provide 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 and has excellent low-temperature discharge characteristics, achieving 80% capacity at -30 ° C (-22 ° F).
LTO (usually Li4Ti5O12) has zero strain, no SEI film formation, and no lithium electroplating phenomenon during fast and low temperature charging, thus it has better charging and discharging performance than traditional cobalt doped Li ions and graphite anodes. The thermal stability at high temperatures is also better than other lithium-ion systems; However, batteries are expensive. Low specific energy, only 65Wh/kg, equivalent to NiCd. Lithium titanate is charged to 2.80V and discharged to 1.80V at the end. Figure 13 shows the characteristics of lithium titanate batteries. Typical applications are electric power transmission systems, UPS, and solar street lights.
