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The Cathodes
CHAPTER
The Cathodes
5
Chapter Outline 5.1 5.2 5.3 5.4 5.5 5.6
Lithium iron phosphate .................................................................................... 103 Lithium cobalt oxide ........................................................................................ 105 Lithium manganese oxide ................................................................................ 106 Lithium nickel manganese cobalt/nickel cobalt manganese ............................... 107 Lithium nickel cobalt aluminum oxide .............................................................. 112 Other cathodes ................................................................................................ 113
The cathode is the positively charged electrode of an electrochemical lithium-ion cell where the lithium-ion reduction occurs, or in simpler terms it is the positive side of the battery cell. And as we talked about earlier, that could be either electrode depending on whether it is charged or discharged. But just to remain consistent with the industry understanding and terminology, we will herein consider the cathode to be the electrode that is coated with the NMC, NCA, LFP, LMO, or LCO active materials regardless of its state of charge. The cathode electrode is made up of three separate parts: (1) the current collector; (2) a conductive binder; and (3) the active material, which is the main focus of this chapter. We tend to describe a lithium-ion cell by using the description of the active cathode material in the positive electrode. The main active cathode materials used in lithium-ion cells today were introduced earlier and they include lithium iron phosphate (LFP), lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium nickel cobalt aluminum (NCA), and lithium nickel manganese cobalt (NMC). You will note here that we do not include lithium titanium oxide (LTO) in this list. That is because it is actually an anode material that gets paired with one of these cathode materials, but we will discuss it in the next chapter. For a material to be considered as a viable option for a lithium-ion cathode it must demonstrate a couple of key characteristics. First, the material must have high free energy reaction with lithium which results in a high voltage. It must be able to reversibly incorporate a large number of lithium-ions, which gives it high energy density and rechargeability. It must do this without causing structural changes to the material to deliver long cycle life. Large structural changes will cause the material to degrade, fracture, delaminate from the current collector and literally break apart causing severe reductions in life and performance. A good cathode material should also be able to rapidly intercalate the lithium-ions by offering a high level of lithiumion diffusivity, which also can translate into high power capability. A cathode Lithium-Ion Battery Chemistries. https://doi.org/10.1016/B978-0-12-814778-8.00005-3 # 2019 Elsevier Inc. All rights reserved.
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material should be a good electronic conductor and yet still be insoluble in the electrolyte, meaning that the electrolyte must not react with the material and cause it to break down. Finally, the material must also be low cost and commercially available to ensure the final cell cost is low (Dahn & Ehrlich, 2011; Xu, Qian, Wang, & Meng, 2012). The active materials are made up of compounds of lithiated metal oxide or lithiated metal phosphate molecules as described in the names of the chemistries, such as lithium iron phosphate or lithium manganese oxide. The cathode is effectively a “host” material capable of allowing the lithium-ions to be inserted and reversibly removed during the charge and discharge intercalation process. Most of the current lithium-ion cathode materials are drawn from the transition metal oxides in the periodic table, since these metals tend to be good materials for energy storage since they have incomplete outer energy shells, which allow the creation of cations when electrons are removed. But they are also excellent conductors of heat and electricity and yet are much less reactive than the Group 1 and 2 metals like lithium. These materials can be further divided up based on their crystalline structure as being either a layered, spinel, or olivine structure, each of which we will discuss in this chapter (Challoner, 2014; Nitta, Wu, Lee, & Yushin, 2015). A crystal structure is created when a material undergoes treatment, usually by bringing it to very high temperatures, that brings the atoms into a repeating, structural alignment. In lithium-ion cathodes the term unit-cell is used to describe the smallest unit of the crystal structure that is made up of the active material compounds. The layered structure is the simplest crystal structure as it consists of layers of the cathode host materials. Fig. 55A shows an example of a layered crystal structure which is seen in the LCO- and NMC-type cathodes. In the layered structure, the lithium-ions are inserted (intercalated) in between the layers of cathode material. The spinel structure is a bit more complex in that it is more of a repeating lattice framework as shown in Fig. 55B where the lithium-ions are inserted into the tunnels that are formed as the material crystalizes. This is the type of structure that an LMO material crystalizes in.
FIG. 55 Crystal structures of Li-ion chemistries.
CHAPTER 5 The Cathodes
Finally, the olivine structure consists of hexagonal close packed array of oxygen atoms in half of the octagonal empty spaces with iron (Fe) and phosphorus (P) in a very complex structure as shown in Fig. 55C. While this one looks a bit more random, it is actually a very repeatable structure wherein the lithium-ions are intercalated into the spaces between the crystalized molecules. LFP is the most common cathode that crystalizes in this form (Birle, Gibbs, Moore, & Smith, 1968; Matson & Orbeak, 2013). If we look at the market penetration of different cathodes from 2015, across all applications the market share by chemistry is pretty evenly split among the top three chemistries with NMC, LCO, and LFP making up about 80% of all lithium-ion cells built today (Fig. 56). However, with the rapid acceptance of electric vehicles and grid energy storage systems the market is shifting. If we were to look at just the electric vehicle segment, we would find that LFP is the most commonly used chemistry in 2015 due to its low cost and because of the rapid growth of all electric vehicles in China. These figures also include lithium-ion batteries for both consumer electronics and large format applications such as electric vehicles. LCO is not used in any large format applications today. However, with much higher energy density targets being set by the U.S. Advanced Battery Consortium of 350 Wh/kg (United States Advanced Battery Consortium, 2017) and the Chinese government target of 350 Wh/kg (Hao, Cheng, Liu, & Zhao, 2017) the product mix is beginning to shift away from LFP in the automotive EV segment and into more NMC chemistries as LFP simply cannot meet these much higher energy targets. For these electric vehicles nickel manganese cobalt (NMC) cathodes with both graphite, and more frequently, and with graphite/ silicon blended anodes will continue to be the “go-to” chemistry for high energy density applications. In fact, many of the traditional LFP manufacturers are either
FIG. 56 2015 Cathode market share by chemistry.
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developing or have already launched new NMC-based chemistries to remain competitive in these markets. Another interesting approach to developing new and unique materials is the use of blended cathodes. Several manufacturers have already launched lithium-ion cells that use a cathode with a combination of two different cathode materials such as NMC and LMO or NCA and LMO, or LCO and LMO for instance. The goal of blending materials is to get the best attributes from each one to supplement the poorer characteristics of the other and end up with a more balanced performance than with either individually. For instance, LCO which has good energy density but is poor on safety and is high cost may be blended with LMO to help improve the safety without giving up its energy density and reducing the cost (Chikkannanavar, Bernardi, & Liu, 2014). Jouanneau, Patoux, Reynier, and Martinet (2014) describe some of the benefits of blending cathode chemistries as reducing the costs and toxicity while improving the rate performance, energy density, capacity, and stability. When looking at materials and chemistries one thing to keep in mind is that the way that energy density is compared is different at the material level and at the cell level and there is no clear comparison. Chemists tend to think in terms of milliampere hours per gram (mAh/g) but that doesn’t always translate well to the cell level where we tend to think in watt hour per kilogram (Wh/kg). In Table 7 I have attempted to show a rough comparison of material energy density in milliampere hour per gram (mAh/g) and cell energy density in watt hour per kilogram (Wh/kg). Keep in mind that there are a lot of different items that go into calculating the energy density of a material and a cell including the voltage, anode, mix ratio, morphology, electrolytes, salts, binders, and so on. And most importantly the anode material that is selected will ultimately be the “yang” to the cathodes “yin” and will determine the energy density (Linden & Reddy, 2011, pp. 1.10–1.11). Therefore this table can only ever really be directionally correct and used for reference purposes. But it is a question that I have often been asked especially when talking to materials scientists. How does mAh/g relate to Wh/g? In both cases, materials and cells, I have included a range because of the various factors that influence energy density.
Table 7 Comparison of material vs cell energy densities Material energy density (mAh/g)
Chemistry
Cell energy density (Wh/kg)
150–170 140–180 100–120 160–170 175–195 200–220 180–200
LFP LCO LMO NMC 111 NMC 622 NMC 811 NCA
90–120 150–200 100–150 140–180 190–230 220–280 200–300
5.1 Lithium iron phosphate
One last comment on this table, the material energy density that is typically discussed is the theoretically maximum energy density that the material could achieve in a perfect cell with 100% efficiency and zero losses. In reality we know that we cannot achieve these theoretical limits and the cell level energy densities have taken those losses into account. Also note that there is a change in terms from ampere hours per gram to watt hours per gram when we go from materials to cells.
5.1 Lithium iron phosphate Lithium iron phosphate, which is usually abbreviated as either LiFePO4 or simply LFP, is a three-dimensional olivine-type cathode material. It is also called a polyanion material, with the iron (Fe2+) arranged with six atoms symmetrically located around a central atom—this is known as an octahedral. The LFP also has phosphorus (P) arranged with five atoms arranged in a triangular fashion—this is known as a tetrahedral formation as shown in Fig. 57. The phosphate in the LFP also bonds tighter with the oxygen atoms which is what makes the LFP chemistry considered safer than other lithium-ion chemistries. LFP benefits from using low-cost materials, namely, iron, and is thermally stable (American Elements, 2017b) which has given it a leading position in the market as being both low cost and safe. It does not react destructively with the electrolytes until it reaches temperature above 350 °C which is what gives it its excellent thermal stability. The use of iron as the main reactive material also makes it inherently nontoxic and environmentally safe. However, it has a relatively low nominal voltage of about 3.3 volts which gives it lower energy density than other chemistries. LFP energy density typically ranges from 90 Wh/kg to about 120 Wh/kg at the cell level. There have been some claims that have driven the LFP energy density up beyond 120 Wh/kg but
FIG. 57 LiFePO4 crystal structure.
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these claims have not been the rule and may have actually reached or exceeded the maximum limit that can be achieved by this chemistry (Birle et al., 1968; Dahn & Ehrlich, 2011; Gaberscek, Jamnik, Weichert, Samuelis, & Maier, 2014). Many LFP cathodes also use smaller particle sizes than are used by some of the other transition materials. In most cases LFP particles are in the nanometer size range. This offers a couple of interesting benefits to the chemistry. The smaller particle size allows for faster charging and discharging and better lithium-ion transport than with larger particle sizes. This means faster charging and discharging, and more power with higher charge/discharge rates than in other chemistries that are based on larger particles (Dahn & Ehrlich, 2011; Gaberscek et al., 2014; Nitta et al., 2015). Finally, LFP has a very flat voltage curve when compared to other chemistries with a nominal voltage of between 3.2 volts and 3.3 volts. And when compared with lithium metal, which is a common measure in electrochemistry, it has a voltage of 3.45 volts. Remember this is a measure of the voltage potential difference between the lithium metal and the LFP. LFP has an operating voltage range of 2.0 volts to 3.6 volts. From a system-level perspective, the lower voltage and relatively low volumetric energy density means that a system using LFP cells will require more cells in series to achieve higher voltages when compared to a metal oxide chemistry. There are many variations on the traditional LFP chemistry, but a couple of the most common variations use a blending strategy to create lithium iron magnesium phosphate (LiFeMgPO4), lithium iron manganese phosphate (LiFeMnPO4), and lithium iron cobalt phosphate (LiFeCoPO4). The lithium iron magnesium chemistry (LiFeMgPO4) has been in production from Valence/Lithium Werks for many years now as an off-the-shelf solution to replace lead acid and nickel-based batteries. The lithium iron manganese phosphate material was originally developed by the Dow Chemical Company, but they have stopped work on it some time ago. The LiFeMnPO4 offers the lower cost of LFP with a slightly higher energy density, about 10% over LFP. The LiFeMnPO4 chemistry has a theoretical energy density of about 145–150 mAh/g compared to LFP at about 120 mAh/g. The most popular use of the manganese version appears to have been in the earliest BYD vehicles. However, it appears that they have shifted away from this version in favor of a more traditional LFP chemistry or some other variation such as LiFeCoPO4 (DeMorro, 2014; Dow Energy Materials, n.d.; Lithium Werks, 2018). A couple of other interesting variations are the lithium cobalt phosphate (LiCoPO4) which keeps the olivine structure but eliminates the iron and lithium manganese phosphate (LiMnPO4), which is being worked on by several labs including U.S. Army Research Laboratory and materials manufacturers such as SigmaAldrich. Today, these materials still exhibit poor cycle life and with a material capacity of about 100–125 mAh/g still needs some major development before making it viable for electric vehicle applications. The LiCoPO4 is believed to be able to operate at higher voltages, around 4.8 V nominally, and to have the potential to offer high cell energy densities of around 670 Wh/kg with additional work and development. In early 2018 HydroQuebec and the U.S. Army Research Laboratory were able to show some significant improvements in the traditional LiCoPO4 by substituting
5.2 Lithium cobalt oxide
chromium, iron, and silicon in the chemistry and by coating the materials with carbon. This created a material with more than 167 mAh/g theoretical energy and is capable of operating at a much higher 4.8 V with a very high cell theoretical energy of nearly 800 Wh/kg (Green Car Congress, 2018a). The LiMnPO4 shows higher theoretical energy density of the materials at around 150 mAh/g, but suffers from low ionic and electronic conductivity among other challenges that have yet to be resolved. But these chemistries, while promising, are still not close to being ready for production yet (Oh, Myung, & Sun, 2012; Taniguchi, 2018; Xing et al., 2012).
5.2 Lithium cobalt oxide Lithium Cobalt Oxide, also known as LiCO2 or LCO, was the first commercially available lithium-ion chemistry developed in 1980 and commercialized by Sony in 1991 using a lithiated cobalt oxide active material. LCO is a layered crystal structure with six cobalt atoms arranged in octahedral sites around the oxygen atom and that material in alternating layers of cobalt oxide and lithium as shown in Fig. 58 (Nitta et al., 2015). LCO material is a compound containing at least one oxygen anion and one metallic cobalt cation. It is insoluble in aqueous solutions such as water but is stable which is what makes it so useful in electrochemical applications because of its excellent ionic conductivity. Metal oxide compounds are basic anhydrides which just means that they are created through a process that involves elimination of water and can react with acids and with strong reducing agents in redox reactions (American Elements, 2017a).
FIG. 58 Layered crystal structure of LCO.
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LCO continues to be one of the major chemistries in use today, especially in small, portable applications such as smart phones, and laptop and tablet computers due to its high energy density, high voltage, and good cycling performance. On the other end of the spectrum, LCO has seen only limited use in large applications due to several major challenges. First, due to the high cost of cobalt, LCO is an expensive cathode material. Second, from a safety point of view LCO is not very thermally stable at high temperatures and can experience the onset of thermal runaway at temperatures around 150 °C and experience full thermal runaway about 200 °C. Finally, LCO tends to have lower cycle life compared to other lithium-ion chemistries due to the distortions that occur in the layered lattice crystal structure during cycling (Nitta et al., 2015). LCO has a nominal voltage of about 3.8 volts and a voltage of 3.9 volts when compared with lithium and has an operating voltage range from 3.0 volts to 4.2 volts. The higher voltage helps to enable the higher energy density of the chemistry. The voltage discharge curve of LCO has a sloping fit much like the other chemistries with layered crystal structures, NMC and NCA.
5.3 Lithium manganese oxide Lithium Manganese Oxide, known as LiMn2O4 or LMO, differs from LFP and LCO in that its crystal form is that of a lattice-type spinel structure. The manganese atoms are in a three-dimensional octahedral site formation with the manganese taking up one-fourth of the location in the lithium layer leaving one quarter of the manganese sites vacant as shown in Fig. 59. In the active materials of this chemistry the oxidized
FIG. 59 Spinel crystal structure of LMO.
5.4 Lithium nickel manganese cobalt/nickel cobalt manganese
manganese structure looks very much like the lattice that surrounds many gardens. Those lattice-type structures then form layers with the lithium-ions filling the “tunnels” within that structure (Julien, 2000; Xu et al., 2012). Like other metal oxides the LMO is a compound containing at least one oxygen anion and one metallic manganese cation. It is insoluble in aqueous solutions such as water but the combination is stable and has good electrical and ionic conductivity which makes it useful in electrochemical applications such as lithium-ion batteries. Much like the LCO materials, the LMO is a metal oxide compound based on anhydrides and can react with acids and with strong reducing agents in redox reactions which is part of what makes them useful in energy storage applications (American Elements, 2017c). Cells with the LMO chemistry are used in applications where cost and stability are key performance factors. Manganese is a cheaper raw material and is much less toxic than cobalt or nickel. Additionally, LMO operates at a higher nominal voltage of about 3.9 volts and it has a midpoint voltage of 4.05 volts versus lithium metal. The voltage discharge curve of LMO has a sloping fit much like the layered crystal chemistries, with an operating voltage range from 2.5 volts to 4.2 volts. However, these benefits may be offset by several challenges including its lower capacity, higher capacity loss during storage and cycling, and poor performance at high temperatures. Additionally, only about half of the lithium can be removed before the oxide begins to lose oxygen or to oxidize the electrolyte. The poor cycling performance is believed to be because the manganese tends to get dissolved into the electrolyte during cycling causing higher impedance and agglomeration of the manganese on the anode SEI layer which may begin forming dendrites and cause safety issues (Dahn & Ehrlich, 2011; Goodenough, 2007; Jouanneau et al., 2014; Nitta et al., 2015). Because of these factors, most companies have moved away from pure LMO but instead are using it as a blending agent with other chemistries. This helps to mitigate some of those challenges by relying on the other chemistry’s strengths.
5.4 Lithium nickel manganese cobalt/nickel cobalt manganese Lithium Nickel Manganese Cobalt Oxide, known as LiNMnCoO2 or NMC, is quickly becoming one of the most frequently used lithium-ion chemistries due to its energy density, relatively low cost, and high voltage. Some manufacturers call it Nickel Cobalt Manganese (NCM) instead of NMC but there is no major difference other than perhaps the order of the mixture with the elements in order of higher percentages. NMC is a layered crystal structure arranged in octahedral sites in alternating layers of nickel and cobalt atoms, manganese and cobalt atoms, and lithium as shown in Fig. 60 (Dahn & Ehrlich, 2011). The NMC material is a thermally stable compound containing at least one oxygen anion and one metallic cation each of cobalt, manganese, and nickel (American Elements, 2017e). The combination of different transition metals together offers different performance benefits than any used alone. For instance, nickel-rich combinations of
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FIG. 60 Layered atomic structure of NMC.
NMC offer high discharge capacity, while manganese-rich combinations offer better cycle life and thermal stability, and cobalt-rich combinations offer good rate capability. Cells using the NMC chemistry benefit due to its being what I consider a “well-balanced” chemistry, as it is capable of offering very good energy and power density over a wide operating temperature range and can easily be biased toward either energy or power depending on the needs of the application. Additionally, NMC has proven to be able to achieve very high cycle life, in some cases from up to 6000 full depth of discharge cycles have been exhibited which makes it an ideal solution for long-life applications. Of course, with each benefit there is a challenge presented that needs to be overcome. The nickel-rich combinations suffer from structural degradation which reduces the cycle life, manganese-rich combinations suffer from reduced capacity which impacts energy density, and cobalt-rich combinations suffer from both high cost and safety concerns (Sun & Zhao, 2017). Much like other chemistries, NMC chemistries when paired with graphite anodes must also use a reduced depth of discharge to achieve the best performance. In many cases only about 80% of the cell’s capacity is usable in the final application. Traditional NMC chemistries have used nickel, manganese, and cobalt material combinations with a ratio of 1:1:1 (or 3:3:3) which means that the three materials were used in equal amount in the cathode materials 33% nickel, 33% manganese, and 33% cobalt, written as LiNi0.33 Mn0.33Co0.33O2. However, recently much development work has gone into increasing the nickel content as a means of both reducing the amount of cobalt and therefore reducing the cost and increasing the energy density
5.4 Lithium nickel manganese cobalt/nickel cobalt manganese
FIG. 61 Nickel Manganese Cobalt chemistries.
of the chemistry. As shown in the image Fig. 62 below increasing the percentage of nickel in the NMC chemistry can have a significant impact on energy density. Today materials manufacturers have created a wide variety of NMC “flavors” depending on the needs of the cell manufacturer. Many manufacturers have transitioned to mixes of 4:3:3, representing 40% nickel, 30% manganese, and 30% cobalt, written as LiNi0.4 Mn0.3Co0.3O2. and 5:3:2, representing 50% nickel, 30% manganese, and 20% cobalt, written as LiNi0.5 Mn0.3Co0.2O2 which has become the defacto standard mixture. Today, manufacturers are transitioning to 6:2:2 blends, 60% nickel, 20% manganese, and 20% cobalt, written as LiNi0.6 Mn0.2Co0.2O2. Mixes of 6:2:2 are coming into mass production in 2017. Additionally, we are seeing the emergence 7:2:1 and 8:1:1 chemistries with 70% nickel, 20% cobalt, and 10% manganese, written as LiNi0.7 Mn0.2Co0.1O2 and 80% nickel, 10% cobalt, and 10% manganese, written as LiNi0.8 Mn0.1Co0.1O2, respectively. These are expected to move into volume production from multiple manufacturers in 2018. Fig. 61 gives a visual comparison of the materials breakdown in these chemistries showing the change in material quantities with the nickel increasing to become the largest amount of material in the chemistry. The drivers for this change and the evolution of the NMC chemistry mixture is for a couple of reasons. First, NMC has proven to be a very good chemistry for use in lithium-ion batteries, so finding ways to improve something that already works helps to minimize the development risk. Second, the increase in nickel content has a positive effect on energy density. As shown in Fig. 62 increasing the percentage of nickel in the NCM chemistries causes an increase in the energy density of the materials and thereby the cell. The 1:1:1 NCM materials have an energy density of about 158 mAh/g, while the 6:2:2 materials increase energy density to about 180 mAh/g. That is an increase in energy density of about 14%. The 8:1:1 mixture increases the energy density to about 190 mAh/g, an energy density increase of about 20% from the 1:1:1.
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FIG. 62 Nickel content vs. energy density in NCM materials.
We should also note that these are not the only combinations that NMC can be made in. Materials manufacturers have been working on a wide variety of different ratios as a means of improving chemistry behavior and cost. In addition to the chemistries mentioned before, there are also 4:2:4 and even 9:1:0 versions that have or are being developed by various materials companies. Therefore with the increase in energy and the decrease in cost, cells using the higher nickel content will see a significant improvement in cost per Wh. That of course does not yet take into account the material manufacturers capital expense as they retool for the new materials, so the actual reductions that a cell manufacturer will see are likely to be somewhat smaller than those shown here and these are only directional materials reductions. The third driver for this change to higher nickel content is that there is a significant decrease in the cost of the material due to the change in material mix. As an example of the difference in the material costs let’s take a comparison based on raw material costs. Today, in October 2018, the London Metal Exchange (LME) lists nickel at $12,620 USD per metric ton, cobalt at $62,000 USD per metric ton, and manganese is listed on the Shanghai Metals Exchange at $2562 USD per metric ton. In the next example we will compare everything to manganese as it is the least expensive metal in the group. Nickel’s cost is about five (5) times that of manganese and cobalt is twenty-four (24) times the price of manganese. If we use those numbers as a base and allocate across the different ratios of NMC chemistries as presented in Table 8, by increasing from 33% nickel to 60% nickel, and decreasing the other two accordingly, there is about a 19% reduction in material cost going from the 1:1:1 (or 3:3:3) to a 6:2:2. When you continue to reduce the cobalt percentage going to the 8:1:1 NMC chemistry the material cost savings nearly doubles to 34% less than
5.4 Lithium nickel manganese cobalt/nickel cobalt manganese
Table 8 NCM material cost comparison. x Mn Nickel Manganese Cobalt
5 1 24
Relative material cost
1:1:1 33 33 33
5:3:2 $165 $33 $792 $990
% Cost reduction from 1:1:1
50 30 20
6:2:2 $250 $30 $480
60 20 20
7:2:1 $300 $20 $480
70 20 10
8:1:1 $350 $20 240
80 10 10
$400 $10 $240
$760
$800
$610
$650
(23%)
(19%)
(38%)
(34%)
the 1:1:1. It is also interesting to note that the 7:2:1 combination actually offers the best material cost reduction from the base 1:1:1 by almost 40%, which is even better than the 8:1:1 blend. You can see from Table 8 that the trend has been to develop combinations that reduce the amount of cobalt in the NMC chemistry. There is no question that it is the most expensive metal used in the chemistry and while we have seen cobalt prices rising steadily since later 2016 it appears that they may have peaked in March 2018 and have now begun to come down again. And it may not make sense to eliminate it completely from the chemistry as it does offer great benefits in the area of electrical and ionic conductivity. So even with the higher costs we may want to leave some amount of cobalt in the chemistry. One reason that NMC chemistries have been so successful is that they operate at relatively higher voltages generally with nominal voltages ranging from 3.60 volts to 3.75 volts depending on the respective chemistry mixture and a voltage of 3.8 volts when compared to lithium metal. The operating range voltages are from a minimum of 2.5 volts up to a maximum of 4.2 volts with a sloping voltage discharge curve which makes it easy to identify state of charge based on the cell voltage.
FIG. 63 NMC Core shell gradient examples.
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Another area of research for NMC chemistries is in the development of what are called core-shell gradient materials. This is effectively using the same materials but in such a way that it varies the elements from the center of the molecule to the outside edge to achieve different performance characteristics. Fig. 63 shows two different examples of this type of compound structure. The image on the left shows a gradual transition from a more nickel-rich center to a more manganese-rich outside, while the image on the right shows a material with a sharper transition from nickel core to manganese shell. This second image may be more akin to what we may find in a surface coating process such as atomic layer deposition, which will be discussed in more detail in Chapter 8.
5.5 Lithium nickel cobalt aluminum oxide Lithium Nickel Cobalt Aluminum Oxide, known as LiNiCoAlO2 or simply NCA, is also seeing increased use in no small part due to the fact the Tesla uses an NCA chemistry in their electric vehicles. Much like LCO and NMC, NCA is a layered crystal structure arranged in octahedral sites in alternating layers of nickel and cobalt atoms, aluminum and cobalt atoms, and lithium as shown in Fig. 64 (Dahn & Ehrlich, 2011). Much like how with NMC chemistries we saw many different ratios of NMC materials, NCA is typically made of 80% nickel, 15% cobalt, and 5% aluminum and may be written as LiNi0.8Co0.15Al0.05O2 but we may also find different configurations in the future. The reason to add the aluminum is that it was found that by doping the lithium nickel cobalt oxide with aluminum it stabilizes the thermal and charge
FIG. 64 Layered atomic structure of NCA.
5.6 Other cathodes
transfer resistance, thereby making it a highly thermally stable cathode material with high discharge capacity and a long storage life (American Elements, 2017d). NCA offers some advantages compared to some of the other cathode chemistries. First, is the very high energy density that can be achieved especially when combined with a blended graphite and silicon anode. An early concern with cells using the NCA chemistry was cost due to the high cost of cobalt used in the cell. If we compare to the NMC chemistry costs in Table 8 and use the same methodology we would find that the cost of NCA materials would be $960, far more expensive than the high nickel content NMC chemistries. And while that is still a concern, long-term raw material supply agreements and high volume manufacturing of cylindrical cells by Panasonic and Tesla appears to have mitigated the cost risk and in fact has developed a solution that leads the market as one of the lowest price and highest energy density lithiumion batteries available today. However, NCA does suffer from several major challenges. First, at high temperatures it tends to exhibit very high capacity fade because of the rapid SEI growth on the anode at high temperatures. Second, while it is safer than LCO the inclusion of aluminum has been thought to make the NCA chemistry suffer from somewhat lower safety at least in part due to its lower thermal runaway onset temperature of 150 °C. Cycle life is also a concern with NCA as it generally achieves only about 500 full depth of discharge cycles. While it has an excellent energy density it is generally necessary to derate cells in order to achieve adequate usable energy and usable life (Kam & Doeff, 2012). Much like the other layered crystal chemistries NCA has a nominal voltage of about 3.65 volts and about 3.7 volts when compared with lithium metal. The higher voltage helps to enable the higher energy density of the chemistry. NCA has an operating voltage range of 3.0 volts to 4.2 volts, with a voltage curve with a sloping fit much like LCO and NMC the other chemistries with layered crystal structures.
5.6 Other cathodes The chemistries discussed so far make up the most popular commercialized lithiumion cathode chemistries that are on the market today. However, they are certainly not the only ones that have been used or are being investigated. Early work on lithiumion cathode materials also evaluated other transition metals such as nickel and chromium. The work that was done on lithium nickel oxide, known as LiNiO2 or LNO, has formed the basis for many of the current chemistries that use nickel. Lithium chromium manganese oxide, LiCrMnO4, showed some early potential due to its high voltage versus lithium, 4.0 volts versus lithium with a maximum voltage of 5.4 volts. However, due to the side reactions that occurred with the electrolyte at the high charging voltages and the general toxicity of chromium this work was eventually abandoned (Islam, Ammundsen, Jones, & Roziere, 2000; Legallasalle, Vverbaere, Piffard, & Guyomard, 2000).
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The Chinese company BYD has used a blended chemistry based on LiFeMnPO4 chemistry that adds manganese into the traditional lithium iron phosphate chemistry. In addition, there are some very interesting new cathode chemistries that could see use in the future, including lithium nickel manganese oxide Li[Li0.11Ni0.33Mn0.56]O4 which adds nickel and lithium to the traditional LMO chemistry and may offer energy densities up to nearly 300 mAh/g. Another variation on this LMO is the LiNi0.5Mn0.5O2 variation which attempts to maintain the same energy density as LCO while also achieving lower costs. In the same family is the LiNi0.5Mn1.5O4 which is also based on the LMO chemistry but blends in about 25% nickel. This chemistry looks very interesting as it has a voltage of 4.6 volts versus lithium, meaning that the top end of its operating range will be at or above 5.0 volts. However, for high voltage chemistries such as these new electrolytes will be required in order to operate safely and to achieve high cycle life (Dahn & Ehrlich, 2011; Nitta et al., 2015). There has also been some very interesting work being done by Argonne National Laboratory, the Department of Energy, UC Berkley and others in the area of developing manganese-rich chemistries instead of nickel-rich chemistries. Manganese offers some very interesting potential benefits, not the least of which is that manganese can exchange two electrons instead of just one like some of the other transition metals which means that it should be able to hold more charge and reach higher energy densities. Recent work done at UC Berkley has demonstrated more than 300 mAh/g material capacity which equates to about 1000 Wh/kg at the cell level. And because manganese is both a more abundant material and has low toxicity there is less concern about demand driving up cost or safety (Green Car Congress, 2018b; Lee et al., 2018). Many other chemistries are under development including those from the olivine phosphate family: Lithium cobalt phosphate (LiCoPO4), lithium manganese phosphate (LiMnPO4), lithium nickel cobalt phosphate (LiNiCoPO4), and lithium manganese iron cobalt phosphate (LiMnFeCoPO4). In the layered crystal family: lithium nickel oxide (LiNiO2) and lithium-rich lithium manganese oxide (Li2MnO3). And finally, in a crystal structure family that has not yet been discussed, the tavorite crystal structure work is ongoing on chemistries such as the lithium iron fluorosulfate (LiFeSO4F) (Nitta et al., 2015).
The Cathodes
5
Chapter Outline 5.1 5.2 5.3 5.4 5.5 5.6
Lithium iron phosphate .................................................................................... 103 Lithium cobalt oxide ........................................................................................ 105 Lithium manganese oxide ................................................................................ 106 Lithium nickel manganese cobalt/nickel cobalt manganese ............................... 107 Lithium nickel cobalt aluminum oxide .............................................................. 112 Other cathodes ................................................................................................ 113
The cathode is the positively charged electrode of an electrochemical lithium-ion cell where the lithium-ion reduction occurs, or in simpler terms it is the positive side of the battery cell. And as we talked about earlier, that could be either electrode depending on whether it is charged or discharged. But just to remain consistent with the industry understanding and terminology, we will herein consider the cathode to be the electrode that is coated with the NMC, NCA, LFP, LMO, or LCO active materials regardless of its state of charge. The cathode electrode is made up of three separate parts: (1) the current collector; (2) a conductive binder; and (3) the active material, which is the main focus of this chapter. We tend to describe a lithium-ion cell by using the description of the active cathode material in the positive electrode. The main active cathode materials used in lithium-ion cells today were introduced earlier and they include lithium iron phosphate (LFP), lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium nickel cobalt aluminum (NCA), and lithium nickel manganese cobalt (NMC). You will note here that we do not include lithium titanium oxide (LTO) in this list. That is because it is actually an anode material that gets paired with one of these cathode materials, but we will discuss it in the next chapter. For a material to be considered as a viable option for a lithium-ion cathode it must demonstrate a couple of key characteristics. First, the material must have high free energy reaction with lithium which results in a high voltage. It must be able to reversibly incorporate a large number of lithium-ions, which gives it high energy density and rechargeability. It must do this without causing structural changes to the material to deliver long cycle life. Large structural changes will cause the material to degrade, fracture, delaminate from the current collector and literally break apart causing severe reductions in life and performance. A good cathode material should also be able to rapidly intercalate the lithium-ions by offering a high level of lithiumion diffusivity, which also can translate into high power capability. A cathode Lithium-Ion Battery Chemistries. https://doi.org/10.1016/B978-0-12-814778-8.00005-3 # 2019 Elsevier Inc. All rights reserved.
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material should be a good electronic conductor and yet still be insoluble in the electrolyte, meaning that the electrolyte must not react with the material and cause it to break down. Finally, the material must also be low cost and commercially available to ensure the final cell cost is low (Dahn & Ehrlich, 2011; Xu, Qian, Wang, & Meng, 2012). The active materials are made up of compounds of lithiated metal oxide or lithiated metal phosphate molecules as described in the names of the chemistries, such as lithium iron phosphate or lithium manganese oxide. The cathode is effectively a “host” material capable of allowing the lithium-ions to be inserted and reversibly removed during the charge and discharge intercalation process. Most of the current lithium-ion cathode materials are drawn from the transition metal oxides in the periodic table, since these metals tend to be good materials for energy storage since they have incomplete outer energy shells, which allow the creation of cations when electrons are removed. But they are also excellent conductors of heat and electricity and yet are much less reactive than the Group 1 and 2 metals like lithium. These materials can be further divided up based on their crystalline structure as being either a layered, spinel, or olivine structure, each of which we will discuss in this chapter (Challoner, 2014; Nitta, Wu, Lee, & Yushin, 2015). A crystal structure is created when a material undergoes treatment, usually by bringing it to very high temperatures, that brings the atoms into a repeating, structural alignment. In lithium-ion cathodes the term unit-cell is used to describe the smallest unit of the crystal structure that is made up of the active material compounds. The layered structure is the simplest crystal structure as it consists of layers of the cathode host materials. Fig. 55A shows an example of a layered crystal structure which is seen in the LCO- and NMC-type cathodes. In the layered structure, the lithium-ions are inserted (intercalated) in between the layers of cathode material. The spinel structure is a bit more complex in that it is more of a repeating lattice framework as shown in Fig. 55B where the lithium-ions are inserted into the tunnels that are formed as the material crystalizes. This is the type of structure that an LMO material crystalizes in.
FIG. 55 Crystal structures of Li-ion chemistries.
CHAPTER 5 The Cathodes
Finally, the olivine structure consists of hexagonal close packed array of oxygen atoms in half of the octagonal empty spaces with iron (Fe) and phosphorus (P) in a very complex structure as shown in Fig. 55C. While this one looks a bit more random, it is actually a very repeatable structure wherein the lithium-ions are intercalated into the spaces between the crystalized molecules. LFP is the most common cathode that crystalizes in this form (Birle, Gibbs, Moore, & Smith, 1968; Matson & Orbeak, 2013). If we look at the market penetration of different cathodes from 2015, across all applications the market share by chemistry is pretty evenly split among the top three chemistries with NMC, LCO, and LFP making up about 80% of all lithium-ion cells built today (Fig. 56). However, with the rapid acceptance of electric vehicles and grid energy storage systems the market is shifting. If we were to look at just the electric vehicle segment, we would find that LFP is the most commonly used chemistry in 2015 due to its low cost and because of the rapid growth of all electric vehicles in China. These figures also include lithium-ion batteries for both consumer electronics and large format applications such as electric vehicles. LCO is not used in any large format applications today. However, with much higher energy density targets being set by the U.S. Advanced Battery Consortium of 350 Wh/kg (United States Advanced Battery Consortium, 2017) and the Chinese government target of 350 Wh/kg (Hao, Cheng, Liu, & Zhao, 2017) the product mix is beginning to shift away from LFP in the automotive EV segment and into more NMC chemistries as LFP simply cannot meet these much higher energy targets. For these electric vehicles nickel manganese cobalt (NMC) cathodes with both graphite, and more frequently, and with graphite/ silicon blended anodes will continue to be the “go-to” chemistry for high energy density applications. In fact, many of the traditional LFP manufacturers are either
FIG. 56 2015 Cathode market share by chemistry.
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developing or have already launched new NMC-based chemistries to remain competitive in these markets. Another interesting approach to developing new and unique materials is the use of blended cathodes. Several manufacturers have already launched lithium-ion cells that use a cathode with a combination of two different cathode materials such as NMC and LMO or NCA and LMO, or LCO and LMO for instance. The goal of blending materials is to get the best attributes from each one to supplement the poorer characteristics of the other and end up with a more balanced performance than with either individually. For instance, LCO which has good energy density but is poor on safety and is high cost may be blended with LMO to help improve the safety without giving up its energy density and reducing the cost (Chikkannanavar, Bernardi, & Liu, 2014). Jouanneau, Patoux, Reynier, and Martinet (2014) describe some of the benefits of blending cathode chemistries as reducing the costs and toxicity while improving the rate performance, energy density, capacity, and stability. When looking at materials and chemistries one thing to keep in mind is that the way that energy density is compared is different at the material level and at the cell level and there is no clear comparison. Chemists tend to think in terms of milliampere hours per gram (mAh/g) but that doesn’t always translate well to the cell level where we tend to think in watt hour per kilogram (Wh/kg). In Table 7 I have attempted to show a rough comparison of material energy density in milliampere hour per gram (mAh/g) and cell energy density in watt hour per kilogram (Wh/kg). Keep in mind that there are a lot of different items that go into calculating the energy density of a material and a cell including the voltage, anode, mix ratio, morphology, electrolytes, salts, binders, and so on. And most importantly the anode material that is selected will ultimately be the “yang” to the cathodes “yin” and will determine the energy density (Linden & Reddy, 2011, pp. 1.10–1.11). Therefore this table can only ever really be directionally correct and used for reference purposes. But it is a question that I have often been asked especially when talking to materials scientists. How does mAh/g relate to Wh/g? In both cases, materials and cells, I have included a range because of the various factors that influence energy density.
Table 7 Comparison of material vs cell energy densities Material energy density (mAh/g)
Chemistry
Cell energy density (Wh/kg)
150–170 140–180 100–120 160–170 175–195 200–220 180–200
LFP LCO LMO NMC 111 NMC 622 NMC 811 NCA
90–120 150–200 100–150 140–180 190–230 220–280 200–300
5.1 Lithium iron phosphate
One last comment on this table, the material energy density that is typically discussed is the theoretically maximum energy density that the material could achieve in a perfect cell with 100% efficiency and zero losses. In reality we know that we cannot achieve these theoretical limits and the cell level energy densities have taken those losses into account. Also note that there is a change in terms from ampere hours per gram to watt hours per gram when we go from materials to cells.
5.1 Lithium iron phosphate Lithium iron phosphate, which is usually abbreviated as either LiFePO4 or simply LFP, is a three-dimensional olivine-type cathode material. It is also called a polyanion material, with the iron (Fe2+) arranged with six atoms symmetrically located around a central atom—this is known as an octahedral. The LFP also has phosphorus (P) arranged with five atoms arranged in a triangular fashion—this is known as a tetrahedral formation as shown in Fig. 57. The phosphate in the LFP also bonds tighter with the oxygen atoms which is what makes the LFP chemistry considered safer than other lithium-ion chemistries. LFP benefits from using low-cost materials, namely, iron, and is thermally stable (American Elements, 2017b) which has given it a leading position in the market as being both low cost and safe. It does not react destructively with the electrolytes until it reaches temperature above 350 °C which is what gives it its excellent thermal stability. The use of iron as the main reactive material also makes it inherently nontoxic and environmentally safe. However, it has a relatively low nominal voltage of about 3.3 volts which gives it lower energy density than other chemistries. LFP energy density typically ranges from 90 Wh/kg to about 120 Wh/kg at the cell level. There have been some claims that have driven the LFP energy density up beyond 120 Wh/kg but
FIG. 57 LiFePO4 crystal structure.
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these claims have not been the rule and may have actually reached or exceeded the maximum limit that can be achieved by this chemistry (Birle et al., 1968; Dahn & Ehrlich, 2011; Gaberscek, Jamnik, Weichert, Samuelis, & Maier, 2014). Many LFP cathodes also use smaller particle sizes than are used by some of the other transition materials. In most cases LFP particles are in the nanometer size range. This offers a couple of interesting benefits to the chemistry. The smaller particle size allows for faster charging and discharging and better lithium-ion transport than with larger particle sizes. This means faster charging and discharging, and more power with higher charge/discharge rates than in other chemistries that are based on larger particles (Dahn & Ehrlich, 2011; Gaberscek et al., 2014; Nitta et al., 2015). Finally, LFP has a very flat voltage curve when compared to other chemistries with a nominal voltage of between 3.2 volts and 3.3 volts. And when compared with lithium metal, which is a common measure in electrochemistry, it has a voltage of 3.45 volts. Remember this is a measure of the voltage potential difference between the lithium metal and the LFP. LFP has an operating voltage range of 2.0 volts to 3.6 volts. From a system-level perspective, the lower voltage and relatively low volumetric energy density means that a system using LFP cells will require more cells in series to achieve higher voltages when compared to a metal oxide chemistry. There are many variations on the traditional LFP chemistry, but a couple of the most common variations use a blending strategy to create lithium iron magnesium phosphate (LiFeMgPO4), lithium iron manganese phosphate (LiFeMnPO4), and lithium iron cobalt phosphate (LiFeCoPO4). The lithium iron magnesium chemistry (LiFeMgPO4) has been in production from Valence/Lithium Werks for many years now as an off-the-shelf solution to replace lead acid and nickel-based batteries. The lithium iron manganese phosphate material was originally developed by the Dow Chemical Company, but they have stopped work on it some time ago. The LiFeMnPO4 offers the lower cost of LFP with a slightly higher energy density, about 10% over LFP. The LiFeMnPO4 chemistry has a theoretical energy density of about 145–150 mAh/g compared to LFP at about 120 mAh/g. The most popular use of the manganese version appears to have been in the earliest BYD vehicles. However, it appears that they have shifted away from this version in favor of a more traditional LFP chemistry or some other variation such as LiFeCoPO4 (DeMorro, 2014; Dow Energy Materials, n.d.; Lithium Werks, 2018). A couple of other interesting variations are the lithium cobalt phosphate (LiCoPO4) which keeps the olivine structure but eliminates the iron and lithium manganese phosphate (LiMnPO4), which is being worked on by several labs including U.S. Army Research Laboratory and materials manufacturers such as SigmaAldrich. Today, these materials still exhibit poor cycle life and with a material capacity of about 100–125 mAh/g still needs some major development before making it viable for electric vehicle applications. The LiCoPO4 is believed to be able to operate at higher voltages, around 4.8 V nominally, and to have the potential to offer high cell energy densities of around 670 Wh/kg with additional work and development. In early 2018 HydroQuebec and the U.S. Army Research Laboratory were able to show some significant improvements in the traditional LiCoPO4 by substituting
5.2 Lithium cobalt oxide
chromium, iron, and silicon in the chemistry and by coating the materials with carbon. This created a material with more than 167 mAh/g theoretical energy and is capable of operating at a much higher 4.8 V with a very high cell theoretical energy of nearly 800 Wh/kg (Green Car Congress, 2018a). The LiMnPO4 shows higher theoretical energy density of the materials at around 150 mAh/g, but suffers from low ionic and electronic conductivity among other challenges that have yet to be resolved. But these chemistries, while promising, are still not close to being ready for production yet (Oh, Myung, & Sun, 2012; Taniguchi, 2018; Xing et al., 2012).
5.2 Lithium cobalt oxide Lithium Cobalt Oxide, also known as LiCO2 or LCO, was the first commercially available lithium-ion chemistry developed in 1980 and commercialized by Sony in 1991 using a lithiated cobalt oxide active material. LCO is a layered crystal structure with six cobalt atoms arranged in octahedral sites around the oxygen atom and that material in alternating layers of cobalt oxide and lithium as shown in Fig. 58 (Nitta et al., 2015). LCO material is a compound containing at least one oxygen anion and one metallic cobalt cation. It is insoluble in aqueous solutions such as water but is stable which is what makes it so useful in electrochemical applications because of its excellent ionic conductivity. Metal oxide compounds are basic anhydrides which just means that they are created through a process that involves elimination of water and can react with acids and with strong reducing agents in redox reactions (American Elements, 2017a).
FIG. 58 Layered crystal structure of LCO.
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LCO continues to be one of the major chemistries in use today, especially in small, portable applications such as smart phones, and laptop and tablet computers due to its high energy density, high voltage, and good cycling performance. On the other end of the spectrum, LCO has seen only limited use in large applications due to several major challenges. First, due to the high cost of cobalt, LCO is an expensive cathode material. Second, from a safety point of view LCO is not very thermally stable at high temperatures and can experience the onset of thermal runaway at temperatures around 150 °C and experience full thermal runaway about 200 °C. Finally, LCO tends to have lower cycle life compared to other lithium-ion chemistries due to the distortions that occur in the layered lattice crystal structure during cycling (Nitta et al., 2015). LCO has a nominal voltage of about 3.8 volts and a voltage of 3.9 volts when compared with lithium and has an operating voltage range from 3.0 volts to 4.2 volts. The higher voltage helps to enable the higher energy density of the chemistry. The voltage discharge curve of LCO has a sloping fit much like the other chemistries with layered crystal structures, NMC and NCA.
5.3 Lithium manganese oxide Lithium Manganese Oxide, known as LiMn2O4 or LMO, differs from LFP and LCO in that its crystal form is that of a lattice-type spinel structure. The manganese atoms are in a three-dimensional octahedral site formation with the manganese taking up one-fourth of the location in the lithium layer leaving one quarter of the manganese sites vacant as shown in Fig. 59. In the active materials of this chemistry the oxidized
FIG. 59 Spinel crystal structure of LMO.
5.4 Lithium nickel manganese cobalt/nickel cobalt manganese
manganese structure looks very much like the lattice that surrounds many gardens. Those lattice-type structures then form layers with the lithium-ions filling the “tunnels” within that structure (Julien, 2000; Xu et al., 2012). Like other metal oxides the LMO is a compound containing at least one oxygen anion and one metallic manganese cation. It is insoluble in aqueous solutions such as water but the combination is stable and has good electrical and ionic conductivity which makes it useful in electrochemical applications such as lithium-ion batteries. Much like the LCO materials, the LMO is a metal oxide compound based on anhydrides and can react with acids and with strong reducing agents in redox reactions which is part of what makes them useful in energy storage applications (American Elements, 2017c). Cells with the LMO chemistry are used in applications where cost and stability are key performance factors. Manganese is a cheaper raw material and is much less toxic than cobalt or nickel. Additionally, LMO operates at a higher nominal voltage of about 3.9 volts and it has a midpoint voltage of 4.05 volts versus lithium metal. The voltage discharge curve of LMO has a sloping fit much like the layered crystal chemistries, with an operating voltage range from 2.5 volts to 4.2 volts. However, these benefits may be offset by several challenges including its lower capacity, higher capacity loss during storage and cycling, and poor performance at high temperatures. Additionally, only about half of the lithium can be removed before the oxide begins to lose oxygen or to oxidize the electrolyte. The poor cycling performance is believed to be because the manganese tends to get dissolved into the electrolyte during cycling causing higher impedance and agglomeration of the manganese on the anode SEI layer which may begin forming dendrites and cause safety issues (Dahn & Ehrlich, 2011; Goodenough, 2007; Jouanneau et al., 2014; Nitta et al., 2015). Because of these factors, most companies have moved away from pure LMO but instead are using it as a blending agent with other chemistries. This helps to mitigate some of those challenges by relying on the other chemistry’s strengths.
5.4 Lithium nickel manganese cobalt/nickel cobalt manganese Lithium Nickel Manganese Cobalt Oxide, known as LiNMnCoO2 or NMC, is quickly becoming one of the most frequently used lithium-ion chemistries due to its energy density, relatively low cost, and high voltage. Some manufacturers call it Nickel Cobalt Manganese (NCM) instead of NMC but there is no major difference other than perhaps the order of the mixture with the elements in order of higher percentages. NMC is a layered crystal structure arranged in octahedral sites in alternating layers of nickel and cobalt atoms, manganese and cobalt atoms, and lithium as shown in Fig. 60 (Dahn & Ehrlich, 2011). The NMC material is a thermally stable compound containing at least one oxygen anion and one metallic cation each of cobalt, manganese, and nickel (American Elements, 2017e). The combination of different transition metals together offers different performance benefits than any used alone. For instance, nickel-rich combinations of
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FIG. 60 Layered atomic structure of NMC.
NMC offer high discharge capacity, while manganese-rich combinations offer better cycle life and thermal stability, and cobalt-rich combinations offer good rate capability. Cells using the NMC chemistry benefit due to its being what I consider a “well-balanced” chemistry, as it is capable of offering very good energy and power density over a wide operating temperature range and can easily be biased toward either energy or power depending on the needs of the application. Additionally, NMC has proven to be able to achieve very high cycle life, in some cases from up to 6000 full depth of discharge cycles have been exhibited which makes it an ideal solution for long-life applications. Of course, with each benefit there is a challenge presented that needs to be overcome. The nickel-rich combinations suffer from structural degradation which reduces the cycle life, manganese-rich combinations suffer from reduced capacity which impacts energy density, and cobalt-rich combinations suffer from both high cost and safety concerns (Sun & Zhao, 2017). Much like other chemistries, NMC chemistries when paired with graphite anodes must also use a reduced depth of discharge to achieve the best performance. In many cases only about 80% of the cell’s capacity is usable in the final application. Traditional NMC chemistries have used nickel, manganese, and cobalt material combinations with a ratio of 1:1:1 (or 3:3:3) which means that the three materials were used in equal amount in the cathode materials 33% nickel, 33% manganese, and 33% cobalt, written as LiNi0.33 Mn0.33Co0.33O2. However, recently much development work has gone into increasing the nickel content as a means of both reducing the amount of cobalt and therefore reducing the cost and increasing the energy density
5.4 Lithium nickel manganese cobalt/nickel cobalt manganese
FIG. 61 Nickel Manganese Cobalt chemistries.
of the chemistry. As shown in the image Fig. 62 below increasing the percentage of nickel in the NMC chemistry can have a significant impact on energy density. Today materials manufacturers have created a wide variety of NMC “flavors” depending on the needs of the cell manufacturer. Many manufacturers have transitioned to mixes of 4:3:3, representing 40% nickel, 30% manganese, and 30% cobalt, written as LiNi0.4 Mn0.3Co0.3O2. and 5:3:2, representing 50% nickel, 30% manganese, and 20% cobalt, written as LiNi0.5 Mn0.3Co0.2O2 which has become the defacto standard mixture. Today, manufacturers are transitioning to 6:2:2 blends, 60% nickel, 20% manganese, and 20% cobalt, written as LiNi0.6 Mn0.2Co0.2O2. Mixes of 6:2:2 are coming into mass production in 2017. Additionally, we are seeing the emergence 7:2:1 and 8:1:1 chemistries with 70% nickel, 20% cobalt, and 10% manganese, written as LiNi0.7 Mn0.2Co0.1O2 and 80% nickel, 10% cobalt, and 10% manganese, written as LiNi0.8 Mn0.1Co0.1O2, respectively. These are expected to move into volume production from multiple manufacturers in 2018. Fig. 61 gives a visual comparison of the materials breakdown in these chemistries showing the change in material quantities with the nickel increasing to become the largest amount of material in the chemistry. The drivers for this change and the evolution of the NMC chemistry mixture is for a couple of reasons. First, NMC has proven to be a very good chemistry for use in lithium-ion batteries, so finding ways to improve something that already works helps to minimize the development risk. Second, the increase in nickel content has a positive effect on energy density. As shown in Fig. 62 increasing the percentage of nickel in the NCM chemistries causes an increase in the energy density of the materials and thereby the cell. The 1:1:1 NCM materials have an energy density of about 158 mAh/g, while the 6:2:2 materials increase energy density to about 180 mAh/g. That is an increase in energy density of about 14%. The 8:1:1 mixture increases the energy density to about 190 mAh/g, an energy density increase of about 20% from the 1:1:1.
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FIG. 62 Nickel content vs. energy density in NCM materials.
We should also note that these are not the only combinations that NMC can be made in. Materials manufacturers have been working on a wide variety of different ratios as a means of improving chemistry behavior and cost. In addition to the chemistries mentioned before, there are also 4:2:4 and even 9:1:0 versions that have or are being developed by various materials companies. Therefore with the increase in energy and the decrease in cost, cells using the higher nickel content will see a significant improvement in cost per Wh. That of course does not yet take into account the material manufacturers capital expense as they retool for the new materials, so the actual reductions that a cell manufacturer will see are likely to be somewhat smaller than those shown here and these are only directional materials reductions. The third driver for this change to higher nickel content is that there is a significant decrease in the cost of the material due to the change in material mix. As an example of the difference in the material costs let’s take a comparison based on raw material costs. Today, in October 2018, the London Metal Exchange (LME) lists nickel at $12,620 USD per metric ton, cobalt at $62,000 USD per metric ton, and manganese is listed on the Shanghai Metals Exchange at $2562 USD per metric ton. In the next example we will compare everything to manganese as it is the least expensive metal in the group. Nickel’s cost is about five (5) times that of manganese and cobalt is twenty-four (24) times the price of manganese. If we use those numbers as a base and allocate across the different ratios of NMC chemistries as presented in Table 8, by increasing from 33% nickel to 60% nickel, and decreasing the other two accordingly, there is about a 19% reduction in material cost going from the 1:1:1 (or 3:3:3) to a 6:2:2. When you continue to reduce the cobalt percentage going to the 8:1:1 NMC chemistry the material cost savings nearly doubles to 34% less than
5.4 Lithium nickel manganese cobalt/nickel cobalt manganese
Table 8 NCM material cost comparison. x Mn Nickel Manganese Cobalt
5 1 24
Relative material cost
1:1:1 33 33 33
5:3:2 $165 $33 $792 $990
% Cost reduction from 1:1:1
50 30 20
6:2:2 $250 $30 $480
60 20 20
7:2:1 $300 $20 $480
70 20 10
8:1:1 $350 $20 240
80 10 10
$400 $10 $240
$760
$800
$610
$650
(23%)
(19%)
(38%)
(34%)
the 1:1:1. It is also interesting to note that the 7:2:1 combination actually offers the best material cost reduction from the base 1:1:1 by almost 40%, which is even better than the 8:1:1 blend. You can see from Table 8 that the trend has been to develop combinations that reduce the amount of cobalt in the NMC chemistry. There is no question that it is the most expensive metal used in the chemistry and while we have seen cobalt prices rising steadily since later 2016 it appears that they may have peaked in March 2018 and have now begun to come down again. And it may not make sense to eliminate it completely from the chemistry as it does offer great benefits in the area of electrical and ionic conductivity. So even with the higher costs we may want to leave some amount of cobalt in the chemistry. One reason that NMC chemistries have been so successful is that they operate at relatively higher voltages generally with nominal voltages ranging from 3.60 volts to 3.75 volts depending on the respective chemistry mixture and a voltage of 3.8 volts when compared to lithium metal. The operating range voltages are from a minimum of 2.5 volts up to a maximum of 4.2 volts with a sloping voltage discharge curve which makes it easy to identify state of charge based on the cell voltage.
FIG. 63 NMC Core shell gradient examples.
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Another area of research for NMC chemistries is in the development of what are called core-shell gradient materials. This is effectively using the same materials but in such a way that it varies the elements from the center of the molecule to the outside edge to achieve different performance characteristics. Fig. 63 shows two different examples of this type of compound structure. The image on the left shows a gradual transition from a more nickel-rich center to a more manganese-rich outside, while the image on the right shows a material with a sharper transition from nickel core to manganese shell. This second image may be more akin to what we may find in a surface coating process such as atomic layer deposition, which will be discussed in more detail in Chapter 8.
5.5 Lithium nickel cobalt aluminum oxide Lithium Nickel Cobalt Aluminum Oxide, known as LiNiCoAlO2 or simply NCA, is also seeing increased use in no small part due to the fact the Tesla uses an NCA chemistry in their electric vehicles. Much like LCO and NMC, NCA is a layered crystal structure arranged in octahedral sites in alternating layers of nickel and cobalt atoms, aluminum and cobalt atoms, and lithium as shown in Fig. 64 (Dahn & Ehrlich, 2011). Much like how with NMC chemistries we saw many different ratios of NMC materials, NCA is typically made of 80% nickel, 15% cobalt, and 5% aluminum and may be written as LiNi0.8Co0.15Al0.05O2 but we may also find different configurations in the future. The reason to add the aluminum is that it was found that by doping the lithium nickel cobalt oxide with aluminum it stabilizes the thermal and charge
FIG. 64 Layered atomic structure of NCA.
5.6 Other cathodes
transfer resistance, thereby making it a highly thermally stable cathode material with high discharge capacity and a long storage life (American Elements, 2017d). NCA offers some advantages compared to some of the other cathode chemistries. First, is the very high energy density that can be achieved especially when combined with a blended graphite and silicon anode. An early concern with cells using the NCA chemistry was cost due to the high cost of cobalt used in the cell. If we compare to the NMC chemistry costs in Table 8 and use the same methodology we would find that the cost of NCA materials would be $960, far more expensive than the high nickel content NMC chemistries. And while that is still a concern, long-term raw material supply agreements and high volume manufacturing of cylindrical cells by Panasonic and Tesla appears to have mitigated the cost risk and in fact has developed a solution that leads the market as one of the lowest price and highest energy density lithiumion batteries available today. However, NCA does suffer from several major challenges. First, at high temperatures it tends to exhibit very high capacity fade because of the rapid SEI growth on the anode at high temperatures. Second, while it is safer than LCO the inclusion of aluminum has been thought to make the NCA chemistry suffer from somewhat lower safety at least in part due to its lower thermal runaway onset temperature of 150 °C. Cycle life is also a concern with NCA as it generally achieves only about 500 full depth of discharge cycles. While it has an excellent energy density it is generally necessary to derate cells in order to achieve adequate usable energy and usable life (Kam & Doeff, 2012). Much like the other layered crystal chemistries NCA has a nominal voltage of about 3.65 volts and about 3.7 volts when compared with lithium metal. The higher voltage helps to enable the higher energy density of the chemistry. NCA has an operating voltage range of 3.0 volts to 4.2 volts, with a voltage curve with a sloping fit much like LCO and NMC the other chemistries with layered crystal structures.
5.6 Other cathodes The chemistries discussed so far make up the most popular commercialized lithiumion cathode chemistries that are on the market today. However, they are certainly not the only ones that have been used or are being investigated. Early work on lithiumion cathode materials also evaluated other transition metals such as nickel and chromium. The work that was done on lithium nickel oxide, known as LiNiO2 or LNO, has formed the basis for many of the current chemistries that use nickel. Lithium chromium manganese oxide, LiCrMnO4, showed some early potential due to its high voltage versus lithium, 4.0 volts versus lithium with a maximum voltage of 5.4 volts. However, due to the side reactions that occurred with the electrolyte at the high charging voltages and the general toxicity of chromium this work was eventually abandoned (Islam, Ammundsen, Jones, & Roziere, 2000; Legallasalle, Vverbaere, Piffard, & Guyomard, 2000).
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The Chinese company BYD has used a blended chemistry based on LiFeMnPO4 chemistry that adds manganese into the traditional lithium iron phosphate chemistry. In addition, there are some very interesting new cathode chemistries that could see use in the future, including lithium nickel manganese oxide Li[Li0.11Ni0.33Mn0.56]O4 which adds nickel and lithium to the traditional LMO chemistry and may offer energy densities up to nearly 300 mAh/g. Another variation on this LMO is the LiNi0.5Mn0.5O2 variation which attempts to maintain the same energy density as LCO while also achieving lower costs. In the same family is the LiNi0.5Mn1.5O4 which is also based on the LMO chemistry but blends in about 25% nickel. This chemistry looks very interesting as it has a voltage of 4.6 volts versus lithium, meaning that the top end of its operating range will be at or above 5.0 volts. However, for high voltage chemistries such as these new electrolytes will be required in order to operate safely and to achieve high cycle life (Dahn & Ehrlich, 2011; Nitta et al., 2015). There has also been some very interesting work being done by Argonne National Laboratory, the Department of Energy, UC Berkley and others in the area of developing manganese-rich chemistries instead of nickel-rich chemistries. Manganese offers some very interesting potential benefits, not the least of which is that manganese can exchange two electrons instead of just one like some of the other transition metals which means that it should be able to hold more charge and reach higher energy densities. Recent work done at UC Berkley has demonstrated more than 300 mAh/g material capacity which equates to about 1000 Wh/kg at the cell level. And because manganese is both a more abundant material and has low toxicity there is less concern about demand driving up cost or safety (Green Car Congress, 2018b; Lee et al., 2018). Many other chemistries are under development including those from the olivine phosphate family: Lithium cobalt phosphate (LiCoPO4), lithium manganese phosphate (LiMnPO4), lithium nickel cobalt phosphate (LiNiCoPO4), and lithium manganese iron cobalt phosphate (LiMnFeCoPO4). In the layered crystal family: lithium nickel oxide (LiNiO2) and lithium-rich lithium manganese oxide (Li2MnO3). And finally, in a crystal structure family that has not yet been discussed, the tavorite crystal structure work is ongoing on chemistries such as the lithium iron fluorosulfate (LiFeSO4F) (Nitta et al., 2015).