Inactive materials

CHAPTER

Inactive materials

7

Chapter outline 7.1 Binders ...........................................................................................................140 7.2 Electrolytes ....................................................................................................141 7.2.1 Characteristics of electrolytes and salts ...........................................144 7.2.2 Electrolyte additives .......................................................................146 7.2.3 Nonaqueous electrolytes .................................................................149 7.2.4 Aqueous electrolytes ......................................................................151 7.2.5 Other types of electrolytes ..............................................................152 7.3 Separators ......................................................................................................155 7.3.1 Properties of separators ..................................................................157 7.3.2 Ceramic separators ........................................................................158 7.3.3 Nonwoven separators .....................................................................160 7.3.4 Other separator materials ...............................................................161 7.4 Current collectors and metal foils ....................................................................162 7.4.1 Tabs/terminals ...............................................................................166 7.5 Cases .............................................................................................................168

The anode and cathode are referred to as the active materials in the cell, so the inactive materials account for the rest of the components in the cell including the electrolyte, the separator, the current collecting foils, the packaging, the binders, the solvents, and the safety devices. Each of these components is required for the operation of the cell but, generally, do not contribute to the actual energy storage reactions. And while the majority of the research is happening on the active materials, there are some great opportunities for improvement of the cost and performance of the cells by examining the inactive materials. For instance, binders can make up as much as 10% of the active materials’ content by weight. If we were able to find a way to improve the ability of the active materials to bind to the foils and to itself, we could eliminate or minimize that material. But as a percent of the cell cost it has a pretty small impact. The electrolytes actually receive a large amount of research as it is the material that enables the movement of the lithium-ions and due to the safety concerns over the flammability of the current electrolytes. The separators are an important part of the cell that provide both safety and longevity to the cell, but most manufacturers still use polyethylene- or polypropylene-based materials for this functionality almost exactly the same as have

Lithium-Ion Battery Chemistries. https://doi.org/10.1016/B978-0-12-814778-8.00007-7 # 2019 Elsevier Inc. All rights reserved.

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been used for the past 20 years or so. But there are a couple of companies looking at novel new solutions to making better separators. Similarly, the foils have not historically seen much development other than adding priming coatings and getting thinner. But, again, there are some interesting new technologies emerging that may add some major improvements to safety and cost. Finally, the hardware of the cells including the tabs, the enclosures and cases are necessary parts of the cell but do not add to the energy storage capability so for these types of components reducing cost is critical. Each of these areas will be reviewed in the next sections.

7.1 Binders Binders are the materials that hold the active material molecules together and hold the active material to the current collector. Think of the binder as the glue, literally, that holds the active material together and keeps everything from falling apart. In concert with the conductive carbons it also helps the active material to maintain good electrical connection with the current collector and creates a good path for electrical conduction. Most binders are made from a soft carbon called carbon black, commercially it is often branded as “Super P” carbon black (SPCB). The characteristics of a good binder are that it will be inert and will not influence the electrochemical reactions taking place at the anode and cathode. It must also be flexible enough to allow for both the manufacturing process as well as the expansion experienced during cycling. It must be insoluble in the electrolyte so it does not decompose or begin any side reactions within the cell. Other factors that should be considered in evaluating binder materials are its ionic conductivity, tensile strength, water absorption, adhesion properties, swelling in electrolyte, melting point, crystalline properties, and its purity. Finally, the binder materials should be stable across a wide temperature and voltage range (Targray, 2017). Different types of binders are used for the anode and cathode. On the anode side a styrene butadiene (SBR) copolymer is often used, this copolymer allows the flexibility needed during expansion of anode active materials as lithium-ions are moved in and out. It also allows for the graphite or other anode material to maintain contact with the copper current collector. Other binders that have been used frequently are carboxymethylcellulose (CMC) and ethylene-propylene-diene methylene (EPDM) (Tagawa & Brodd, 2009). On the cathode side the binder must be resistant to oxidation, remember that the reduction part of the RedOx process occurs at the cathode, so the binder material must be highly resistant to oxidation. It must also be electrochemically stable up to at least 5 volts and thermally stable to high temperatures. One binder material that is frequently used on both anode and cathode is a polyvinylidene fluoride (PVDF) (Micrometitics, 2017; Solvay, 2018).

7.2 Electrolytes

7.2 Electrolytes Without some form of ionically conducting medium such as the electrolyte, the lithium-ions could not pass from the cathode to the anode and back so the electrolyte is vital to the function of the electrochemical cell. There would simply be no path that would allow the ions to pass back and forth. That is the purpose of the electrolyte, to create that path for the lithium-ions to traverse back and forth on their journey between the electrodes. Think of the electrolyte as the highway that allows the lithium-ions to drive back and forth. A lithium-ion electrolyte is a solution, most often liquid, of organic solvents mixed with a solute made up of lithium salts and other additives. Perhaps at this point some brief definitions may help us clarify some of these concepts throughout the rest of the chapter. First, a solution is simply a liquid mixture of at least two components. The major component of the solution is the solvent in which the solutes are evenly distributed as shown in Fig. 75. In this simplified image the large, white atom represents the lithium-cation and the combination of the green atoms with six fluorine atoms and the center red atom which represents the phosphate atom which makes up the major component of the salt the lithium hexafluorophosphate (LiPF6) compound in a liquid solvent solution. The LiPF6 molecule is surrounded by four ethyl carbonate (EC) molecules which provide the transport through the electrolyte solution, which is again shown in Fig. 76 more simply. One of the main functions of the solvent in the electrolyte is to reduce the forces that interact between the ions, this means that the solvent must not drive a change in the makeup of the ions, allowing them to remain charged ions without changes in their electron count (Wright, 2007).

FIG. 75 Nonaqueous electrolyte solution.

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FIG. 76 Solvated lithium cation.

The electrolyte consists of a highly concentrated ionically conducting medium which is made up of nonelectroactive ions that play no part in the redox reactions at the interfaces. In other words, the electrolyte is simply a solution that does not contribute to the reduction/oxidation reactions that occur at the electrodes during the charge/discharge of the battery but which has anions that help to conduct the lithium-ions. It contributes to the current flow and to the current capacity at the electrode interfaces but not to the electron flow through the electrolyte. In fact, in order for an electrolyte to conduct current it must contain charged particles in the form of ions but it does not conduct electrons. Both cations and anions can carry current in the electrolyte solution, with the cations carrying current while moving to the negative electrode, and the anions carrying current while moving to the positive electrode. The cations and anions move in opposite directions when a current is being passed through the solution, but it is the movement of these ions that creates the current flow through the electrolyte. The cations move in the same direction as the current flow, while the anions move in the opposite direction of the current flow. These cations and anions are the “charged particles” and are the lithium salts, solutes, previously mentioned (Lefrou, Fabry, & Poignet, 2012; Wright, 2007). Once the lithium salts have dissolved into the solution the negatively charged solvates join up with the positively charge lithium cations. Three to four solvent particles will connect, or coordinate, with the lithium-ions to ferry them through the solution (Fig. 76). These are referred to as solvated particles which is how the lithium-ions are transferred through the solution during charging and discharging. Once the solvated lithium-ions reach the electrodes the strength of the charge of

7.2 Electrolytes

the active materials allows the weak bonds with the solvates to break and the lithiumions move into the anode or cathode to begin the oxidation or reduction processes. In fact, the electrolyte is often considered the “secret sauce” of the lithium-ion battery. This is because while the major components are generally all the same, each manufacturer uses different ratios of materials and different additives to provide different performance characteristics. For instance, one additive might improve the low temperature performance of the cell, while giving up some of the power capability. Other additives may improve the power capability but at the expense of the temperature extremes. The ultimate mixture of the electrolyte solution of a cell is very specific to each lithium-ion cell manufacturer as they each have different markets, applications, and performance characteristics that they are trying to achieve. Electrolytes fall into two different categories, aqueous and nonaqueous. Early electrolytes were aqueous, meaning that they were generally water based. Currently the predominant electrolytes for lithium-ion batteries are nonaqueous using mainly carbonate-based solvent mixtures with a lithium salt dissolved into it. Lithium-ion cell electrolytes are composed of a combination of organic solvents like ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), or propylene carbonate (PC) and lithium salts like LiPF6 or LiBF4 with a mixture of various additives (An et al., 2016; Ue, Sasaki, Tanaka, & Morita, 2014). The solvents used in lithium-ion batteries are generally made up of aprotic molecules, which means that they are molecules that cannot donate hydrogen atoms, and dipolar molecules, which means that it has two magnetic poles, since they must be able to dissolve the inorganic lithium salts (Chagnes & Swiatokska, 2012). The electrolyte itself has no free electrons, which is what ensures that electrons flow through the metals of the current collectors instead of in the electrolyte mixture. This also makes the electrolyte nonconductive to electron flow, effectively making it an electron insulator (Lefrou et al., 2012). During the cell manufacturing process, the cell is inactive and safe to handle prior to the addition of the electrolyte. This is because we have not yet built the ion highway. It is not until the electrolyte is added that the cells begin to be capable of generating current. Even with the electrolyte the cell must still go through a “formation” process to activate the cell. So a battery without electrolyte will neither store nor discharge energy thus making the electrolyte the enabler. A simple analogy here would be comparing a cell without electrolyte to a car without wheels, neither of them goes very far. In almost all high-volume lithium-ion batteries in production today the electrolytes are in a liquid form; however, some lithium-ion cells have used a polymer-type electrolyte gel. With the continued trend toward higher energy density, higher voltages, wider operating temperature ranges, and safer operation the need for better electrolytes continues to grow. These demands are driving a new generation of electrolytes that will make the next generation of lithium-ion batteries even better than those of today ( Jow, Xu, Borodin, & Ue, 2014). And as we move toward higher energy density batteries new solid-state cells use solid electrolyte materials which are beginning to get greater attention.

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Finally, as we begin to dive into these electrolytes we will see some very long names of materials and compounds that are used. Do not fear, the important aspect here is to recognize that they are there and understand how they interact with each other rather than to be able to name each of them in turn. In fact, I will only describe a small number of these materials since a comprehensive list would be very long and would not serve our purposes here.

7.2.1 Characteristics of electrolytes and salts To be effective an electrolyte must have several characteristics. First, it must be stable with both the negative and positive electrodes and other components in the cell. This means that it must not break down or otherwise generate a chemical reaction when it contacts the active materials of the electrodes, separators, current collectors, or other cell components. It must be stable across the entire voltage window of the cell. At high voltages it must not react with the cathode materials and dissolve them which would generate gases. It must also have low reactivity with lithium and must not form a coating on the lithium surface. Second, a good electrolyte must provide good ionic conductivity throughout a wide range of temperatures and across the entire voltage window. At low temperatures it should remain liquid and maintain good ionic conductivity. Ionic conductivity is, in fact, the inverse of the viscosity, decreasing the viscosity increases the ionic conductivity of the electrolyte. Third, a good electrolyte must have low viscosity. Viscosity is the measure of the liquid’s resistance to deformation, in other words its thickness. The higher the viscosity the thicker the material or liquid is. In terms of electrolytes viscosity is one of the most important characteristics of an electrolyte and is impacted by temperature, concentration of lithium salts, and solvents. Fourth, the electrolyte must keep the electrons from being transported between the electrodes allowing them to move only through the current carriers. In other words, it must be nonconductive to electron flow. Fifth, a good electrolyte must also have no reactive protons or hydrogen atoms. This means that the electrolyte must not consume the lithium-ions as they pass through it. Sixth, it must also remain liquid over a wide temperature range. If the electrolyte freezes at cold temperature it will no longer conduct ions, at least not efficiently. Seventh, it should have a very high level of purity. Eighth, it should have a low melting point, below 20°C, a high boiling point, above 180°C, and a high flash point. And finally, 9, 10, and 11 are that it should have low cost, have a low vapor pressure, and be nontoxic and nonflammable (Aurbach et al., 2004; Blomgren, 2011; Chagnes & Swiatokska, 2012; Henderson, 2014). The typical solvents that are used in lithium-ion batteries fall into a several main categories with the most commonly used solvents falling into the alkyl carbonate category as shown in the partial list of solvents presented in Table 11. Ethylene carbonate (EC), propylene carbonate (PC), dimethylcarbonate (DMC), and diethylcarbonate (DEC), bolded in Table 11, are the most often used solvents and are often used in combination with one another in today’s lithium-ion batteries.

7.2 Electrolytes

Table 11 Partial list of lithium-ion electrolyte solvents Solvent type

Solvent

Alkyl Carbonates

Ethylene carbonate (EC) Propylene carbonate (PC) Dimethylcarbonate (DMC) Diethylcarbonate (DEC) Methylpropylcarbonate (MPC) Ethylpropylcarbonate (EPC) Diethylether 1,2-Dimethoxyethane Tetrahydrofuran Diglyme 2-Methyltetrahydrofuran γ-Butyrolactone (BL) γ-Valerolactone (VL) Sulfonomethane Acetonitrile Adiponitrile

Ethers

Lactones Sulfones Nitriles

When it comes to comparing electrolytes, the SAE International (SAE) has released a recommended practice, “J3042_201502 Measuring Properties of Li-Battery Electrolyte,” for testing and characterization of lithium-ion electrolytes to create a common set of processes to compare different electrolytes from different manufacturers (Society of Automotive Engineers, 2015). In addition, the United States Advanced Battery Consortium LLC (USABC), a coalition of General Motors, Ford, Fiat-Chrysler, and the U.S. Department of Energy (DOE) has created the “USABC Goals for Advanced Electrolytes,” shown in full in Appendix B, which includes the technical performance requirements including stability, conductivity, viscosity, impurities, vapor pressure, flashpoint, lithium solubility, and cost (U.S. Advanced Battery Consortium, 2016a, 2016b). Some of those goals for advanced electrolytes are laid out in Table 12 with an additional set of requirements added for companies basing their work on conventional electrolytes presented in Table 13.

Table 12 USABC LLC Goals for improved electrolytes #

Parameter

Unit

Goal

1 2 3 4 5 6

Cost Low temperature ( 30°C) conductivity High voltage stability Vapor pressure at 30°C Flashpoint Components purity

$/kg mS/cm V vs Li/Li+ mm Hg o C %

<10 >4 5.0 <1 >100 >99.99

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Table 13 USABC LLC requirements for conventional electrolytes #

Parameter

1 2 3

Conductivity at 30°C Li+ transference No. Viscosity 30°C 30°C Water content HF content Components purity

5 6 7

Unit mS/cm – cP

ppm ppm %

Goal >12 >0.35 <5 <20 <20 <50 >99.99

The lithium salts used in the electrolyte mixture must also have a similar set of performance characteristics to the final electrolyte itself. These include having a high level of ionic conductivity and having a high lithium cation transport rate to achieve high power and ionic conductivity. The lithium cation mobility is one of the main sources of internal impedance in the cell, so the cation conductivity is important in the overall operation of the cell. A low ionic conductivity electrolyte means that the lithium-ions will have to work harder to get through the material to overcome this resistance. The salts must have a high level of solubility with the electrolyte solvents to provide enough charge carriers for the ionic conduction. The salts must be stable across the same voltage and temperature range as the electrolyte and not react with the other cell components. The lithium salts should only consume a small amount of materials during the SEI formation process. It should also prevent corrosion of the current collectors. And finally, a lithium salt should be hydrolysis stable, meaning that when exposed to water it should limit the production of hydrofluorocarbon (HF) gases which has a big impact on life, health and safety (Henderson, 2014).

7.2.2 Electrolyte additives Most lithium-ion cell manufacturers use a variety of additives in the solvent and lithium salts to achieve different performance characteristics. While the additives generally make up no more than 5% of the electrolyte, they will offer significant improvements in life, safety, and performance. Most of these additives are very targeted both at specific parts of the cell, such as the anode or the cathode, and then are targeted at achieving a specific set of performance improvements. These are often referred to as role-assigned electrolytes due to this targeting strategy. Additives may be used to improve a variety of different performance characteristics, including improving the SEI formation, protecting the cathode, stabilizing the LiPF6 salts, improving safety, improving lithium deposition, enhancing solvation, inhibiting corrosion, and improving wetting (Zhang, 2006). We will review some of the different additives at a very high level, but certainly not all of them as the most

7.2 Electrolytes

important factor in the spirit of this book is to get an understanding of what is meant by the term additive and what it does. Additives that are targeted at improving the anode performance include compounds containing unsaturated carbon-carbon bonds such as vinylene carbonate (VC), vinyl ethylene carbonate (VEC), allyl ethyl carbonate (AEC), vinyl acetate (VA), and many others that can be used to help suppress the decomposition that occurs at the graphite anode as well as improve the SEI formation and stability. Other anode-targeted additives include the use of carboxylic acid anhydrides; oxalates; sulfur-, halogen-, and phosphorous-containing compounds; and nitrogen-containing compounds (Abe, 2014). The use of acetic anhydride and benzoic anhydride additives has been shown to be effective at reducing the temperature-driven increase in resistance that is experienced in most lithium-ion cells, while the use of 2-cyanofuran has been shown to suppress the exfoliation of graphite (Zhang, 2006). On the cathode sides, additives generally fall into the categories of sulfurcontaining compounds and aromatic compounds. And yes, aromatic compounds may actually have an odor but that is not how they are defined, an aromatic compound is one that is characterized by having a molecular structure that interconnects in a circle. These are used to assist in preventing the performance deterioration of the cathode due to water and acidic impurities and due to the irreversible oxidation of the electrolyte. These compounds include sulfides, such as diphenyl sulfide, dimethoxydiphenyl sulfide, and bis(p-methoxyphenylthio)ethane and have been used to oxidize the surface of the cathode, thereby allowing the organic solvents to avoid decomposition (Abe, 2014; Zhang, 2006). Other additives are used to improve safety by improving the protection during an overcharge event either as a redox shuttle additive or as a shutdown additive. These may include the use of biphenyl, chlorothiophene, and furan which have been shown to create a polymerization layer on the cathode during overcharge events, effectively creating a layer of insulation against the overcharge. One of the areas that is receiving the greatest attention today is in reducing the flammability of electrolytes. This commonly comes by one of two different methodologies, either adding a flame retardant to the electrolyte or moving to solvents that are already nonflammable (Orendorff et al., 2012). Flame-retardant additives that have been used include triethyl phosphate (TEP), tris(2,2,2,-triflouroethyl) phosphate, fluorinated propylene carbonates, and methyl nonafluorobutyl ether (MFE). These types of additives reduce the flammability of the electrolytes either by building up an isolating layer between the condensed and gas phases or by a chemical radical scavenging process that terminates the chain reactions that cause combustion by using up the compounds that would otherwise become additional fuel. Triethyl phosphate (TEP) has been shown to be self-extinguishing and methyl difluoroacetate (MFA) has been shown to offer improvements in thermal stability. Although there is often a trade off between flame retardant features and ionic conductivity, which is why all electrolytes do not have some form of flame retardant today. Finally, some additives such as tris(2-ethylhexyl) phosphate and triphenyl phosphate may be used to improve wettability when the electrolyte wets the separator,

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Table 14 Partial list of electrolyte additives Benefit

LiPF6 electrolyte additives

Flame/Fire retardant

Trimethyl phosphate (TMP), triethyl phosphate (TEP), triphenyl phosphate (TPP), tris(trifluoroethyl) phosphate (TFP) Vinylene carbonate (VC), vinyl ethylene carbonate (VEC), allyl ethyl carbonate, 2-vinyl pyridine, maleic anahydride Acetic anhydride, benzoic anhydride

SEI formation improvement and improved rechargeability at anode by suppressing anodic decomposition Suppress temperature-dependent resistance increase Prevent organic solvent decomposition Cathode protection

Creates polymer insulation layer at cathode during overcharge LiPF6 salt stabilizer

Improve thermal stability Improve wettability Improve corrosion resistance Overcharge protection

Diphenyl sulfide, dimethoxydiphenyl sulfide, bis(p-methoxyphenylthio)ethane Butylamine, N,Ndicyclohexylcarbodiimide, N,N-diethylamino trimethyl-silane, lithium bis(oxalto) borate (LiBOB) Biphenyl, chlorothiophene, furan Tris(2,2,2,-trifluoroethyl) phosphite, amides, carbamates, fluorocarbamates, pyrrolidinone, hexamethyl-phosphoramide Methyl difluoroacetate (MFA) Tris(2-ethylhexyl) phosphate, triphenyl phosphite Adiponitrile Tetracyanoethylene, tetramethylphenylenedi-amine, dihydrophenazine, ferrocene

while adiponitrile, LiBOB, and LiODFB may improve resistance to corrosion of the aluminum in the cell (Abe, 2014; Zhang, 2006). Many of these are presented in Table 14 but again this is not an all-inclusive list of potential additives, only some of the most commonly used and studied ones today (Chagnes & Swiatokska, 2012; Jow et al., 2014; Linden & Reddy, 2011; Orendorff et al., 2012; Wright, 2007). Keep in mind that this list is changing almost daily as new additives and new combinations of additives are developed. It is also important to keep in mind that each time an additive is added, removed, or the amount that is used is changed in an electrolyte solution it will change the performance of the electrolyte. Consequently, solving one problem may create problems in other areas that were not anticipated or that negatively affect the performance. For instance, adding a small amount of a phosphate material to act as a flame retardant may then reduce the ionic conductivity of the electrolyte solution thereby reducing its efficiency and making it no longer to be a viable product.

7.2 Electrolytes

7.2.3 Nonaqueous electrolytes Secondary lithium-ion batteries almost exclusively use nonaqueous electrolytes in either a liquid, gel polymer, or solid polymer form. Liquid electrolytes are the most commonly used form and are based on a solution of lithium salt in one or more types of organic liquid solvents. A gel electrolyte is an ionically conductive material where the lithium salt and solvents are dissolved in a mixture of polymers forming a gelled matrix for the solution. Finally, a solid electrolyte is an electrolyte material that is in the form of solid matter instead of liquid or gel. As noted earlier in this chapter, the electrolyte is a mixture made up of a liquid carbonate solvent that has a lithium salt dissolved in it. Lithium hexafluorophosphate LiPF6 .is the typical lithium salt that is used in nonaqueous electrolytes and is mixed with one or more alkyl carbonates such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), or ethyl methyl carbonate (EMC). The alkyl carbonate family is most often used in modern lithium-ion batteries due to its stability with cathodes allowing for voltages over 4 volts, reasonable temperature range, good conductivity, and generally low toxicity (Aurbach et al., 2004). The salts used in the electrolyte solutions are anions or negatively charged particles which allow them to pair with the lithium cations. Lithium hexafluorophosphate is used due to its high conductivity and relatively good safety properties. However, it is important to note that LiPF6, being a hydrocarbon, is flammable so when a cell fails and goes into a thermal event the electrolyte will burn. As shown in the simplified example in Fig. 77 the LiPF6 molecule is made up of a phosphate atom (red) which is bonded with six fluorine atoms (green) to form an anion molecule that can in turn bonded with a lithium cation (silver). LiPF6 forms a stable interface with the aluminum current collector at high voltage potentials. It also forms a stable SEI interface layer with graphite-based electrodes. One challenge with LiPF6 is that it tends to absorb water, or undergoes hydrolysis, when exposed to the environment and has a relatively low thermal stability window which

FIG. 77 Representative LiPF6 molecule.

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is what limits the temperature range of most lithium-ion cells. LiPF6 may show the presence of impurities such as hydrofluorocarbons (HF) which has a big impact on cell life and performance (Henderson, 2014). Other electrolyte salts in development today are lithium tetrafluoro borate, or LiBF4, lithium bis-triflouro methane LiN(CF3SO2)2, lithium bis-oxalto borate (LiBOB), and lithium diflouro(oxalto)borate (LiDBOB). LiBF6 has experienced a lot of interest over the years because it is more thermally stable and less susceptible to hydrolysis than LiPF6. However, it has failed to gain commercial use due to its much lower conductivity than LiPF6. Yet, it may still offer benefits as an additive salt. Both LiBOB and LiDBOB offer benefits in improving high temperature performance and increasing the upper end voltage range, to greater than4.5 volts for LiBOB and 5.0 volts for LiDBOB. But they also suffer from lower conductivity than LiPF6 and are a much more complex molecule (Dahn & Ehrlich, 2011; Henderson, 2014). In evaluating the potential of new lithium salts, they must not only meet the performance properties described earlier, but they should also be simple to manufacture at low costs and without toxic chemicals. They should have low hydrolysis properties, not reacting with water to form HF either at high temperatures or during the manufacturing process. This reduces the costs throughout the cell manufacturing process. They may have divalent anions, which mean that they would need less salt to retain the same number of cations. They need to continue to act as a redox shuttle and be thermally stable. They should offer improved low temperature performance capabilities and shall form stable SEI layers with the active materials and current collectors. Finally, new salts should also work with new solvents (Henderson, 2014). The solvents used in nonaqueous lithium-ion cells are typically a cyclic carbonate such as ethylene carbonate (EC) due to its high dielectric constant and stable SEI formation or propylene carbonate (PC). A cyclic carbonate is an ester, an organic compound made by replacing the hydrogen of an acid by an alkyl, of weak carbonic acid. But EC suffers from a high viscosity and low melting point (36°C) which means that it generally requires an additive as a thinning agent in the form of a linear carbonate such as dimethyl carbonate (DMC), diethyl carbonate (DEC), or ethyl methyl carbonate (EMC) (An et al., 2016). The addition of DMC offers better electrolytic conductivity improving higher power performance in applications. The lithium salts generally make up about 30% to 50% concentration of the electrolyte on a volume basis, with the preferred concentration at about 30% volume. Within the electrolyte mixture there are often a variety of different additives that are used in order to achieve different performances. These may include vinylene carbonate (VC), propene sultone (PES), methylene methanedislfonate (MMDS), or tris (Trimethylsilyl phosphite (TTSPi) which have all been discussed previously. Today there is also ongoing research into a number of other solvents including fluorine, boron, phosphorous, and sulfur. Fluorinating the anions appears to decrease the anion and lithium cation interactions which may increase the conductivity of the electrolyte. It may also improve the stability at high voltage potentials and may improve the oxidation stability and temperature range that the electrolyte is in liquid

7.2 Electrolytes

form and may even add nonflammability characteristics to the electrolyte but at the expense of the solubility of the lithium salts (Henderson, 2014; Ue et al., 2014). The other area for continued research and development of nonaqueous electrolytes is in finding ways of making the electrolytes nonflammable. Some approaches to reduce the flammability of electrolytes include moving to a solid polymer electrolyte, using room temperature ionic liquids as solvents, using flame-retardant additives and cosolvents, adding alkylphosphates additives, and using inorganic solid electrolytes. Each possible solution offers the chance to improve the safety of the electrolytes, yet most still suffer from decreasing the cell rate performance and overall life (Ue et al., 2014).

7.2.4 Aqueous electrolytes Aqueous electrolytes can be divided on a scale based on their acidity, as either alkaline (low acidity), neutral (mildly acidic), and strong acidic. An example of a strongly acidic electrolyte is the sulfuric acid that is used as the electrolyte in lead acid batteries. While modern lithium-ion batteries do not use aqueous electrolytes, there have been enough other chemistries that have used them in the past and there is continued research into reducing the flammability of the lithium-ion cells that it is worth a short review here. Part of the reason that aqueous electrolytes have not been successfully used in lithium-ion batteries up until today is due to the low voltage window of around 1.2 volts versus lithium. Aqueous electrolytes also suffer from the problem of anode corrosion and cathode gassing. However, due to the increased safety of a nonflammable aqueous electrolyte these are beginning to receive greater interest (Blomgren, 2011). There is much work and a lot of promise looking into aqueous electrolytes for lithium-ion batteries due to the potential for low flammability, low cost, and greater environmental safety. Part of the challenge with adoption of an aqueous electrolyte for lithium-ion batteries is simply due to the low electrochemical window of water which is about 1.2 volts and the low temperature performance limitations (Li, Chen, Fan, Kong, & Lu, 2016).

7.2.4.1 Alkaline electrolytes While alkaline electrolytes are not used in secondary lithium-ion batteries, they are widely used in many primary batteries using zinc-based chemistries, secondary batteries using nickel metal hydride (NiMh) chemistries such as is used in the Toyota Prius, and nickel-cadmium (NiCd) chemistries that are used in a wide variety of consumer electronics and industrial applications. As the name states, alkaline electrolytes use metals from the alkaline elements on the periodic table as their foundation. Alkaline electrolytes tend to be a mild acid electrolyte which gives them better ionic conductivity than neutral electrolytes but cells using them still suffer from gassing, low cycle life, and dendrite growth (Blomgren, 2011).

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7.2.4.2 Neutral electrolytes There are not many batteries that use neutral electrolytes today. The LeClance primary cell which was introduced at the beginning of the 20th century and was used until relatively recently was one of the few cells to use a neutral electrolyte. Additionally, primary cells using carbon-zinc materials also use electrolytes with a neutral acid type (Blomgren, 2011).

7.2.4.3 Acidic electrolytes The main cells using highly acidic aqueous electrolytes are the many variations of lead acid cells that are on the market today. These cells use the highly acidic sulfuric acid as the electrolyte. The lead acid battery has a long history using sulfuric acid since Gaston Plante first introduced the lead acid battery with a diluted sulfuric acid electrolyte in the mid-1800s. But in lithium-ion batteries there is no use of this type of highly acidic electrolyte (Blomgren, 2011).

7.2.5 Other types of electrolytes While the traditional nonaqueous electrolytes still make up the majority of those used in lithium-ion batteries today, and the aqueous types continue to be widely used in other types of batteries, there are also several other types of electrolytes that are worth mentioning here. These may not yet be ready for “prime time” but certainly show some very interesting characteristics and features that may push one or more of them into the forefront of the next generation of lithium-ion batteries or even into the beyond lithium chemistries. New electrolytes are also being developed for the beyond lithium batteries such as lithium-sulfur, lithium-air, aluminum-air, sodium-ion, manganese-ion, and solid-state batteries along with many other new electrochemistries that are still in development. We will not cover these beyond lithium electrolytes here other than solid-state electrolytes as the focus of this book is on current, lithium-ion batteries but each of these chemistries will be reviewed in Chapter 10.

7.2.5.1 Polymer electrolytes Polymer electrolytes are very similar to the nonaqueous electrolytes in general chemical makeup; however, instead of using a liquid the lithium salts are dissolved into a polymer gel matrix. The benefit of using a polymer electrolyte is that it reduces the volatility of the battery when compared to cells using the LiPF6-type nonaqueous electrolytes. A polymer gel electrolyte is also stable at higher temperatures, all of which means it is considered a somewhat safer electrolyte alternative. On the other side of the coin, gel polymer electrolytes have shown poor mechanical properties and tend to have worse “wetting” capabilities than liquid electrolytes due to the viscosity of the gel. Solid polymer electrolytes go back as far as the 1970s with the use of poly (ethylene oxide), or PEO, which has grown to become one of the most commonly used polymer solid electrolytes used in secondary batteries. These PEO-based solid

7.2 Electrolytes

electrolytes have shown good dimensional stability, good safety, and can prevent dendritic growth. However, they still struggle with poor ionic conductivity. The most commonly used polymer electrolyte is one you have likely already heard of, poly(vinylidenefluoride), or PVDF, has been used in lithium-ion batteries successfully for many years. Although PVDF is most commonly used in a liquid electrolyte, it can be used in a gel also. Other gel polymer electrolytes that are being seen include poly(methyl methacrylate), or PMMA, and poly(acrylonitrile), or PAN. Both of these have been used both as the gel polymer electrolyte itself as well as an additive to a PVDF electrolyte (Li et al., 2016). You may also hear the term “li-poly” used to refer to lithium-ion cells that use a polymer gel electrolyte. Many of the early “pouch” type cells used this type of electrolyte almost exclusively and so even today after transitioning to the liquid form of the electrolyte the pouch-type cells may be referred to as li-poly cells even though they may not use a polymer gel electrolyte. But remember that even when a chart shows li-poly and lithium-ion as separate items they are still both lithium-ion cells. I generally struggle to find the differentiation that some people attempt to assign to them.

7.2.5.2 Solid electrolytes As solid-state and thin-film batteries have grown in interest and are becoming commercialized today, at least for smaller applications, the use of solid electrolytes based on polymer, glass, or ceramic has been given great attention. The earliest works in this area used glassy phosphorus oxysulfide materials made using a magnetron sputtering process to coat them onto the cathodes. However, this process effectively limited the cells to very small sizes and capacities. Later work involved the introduction of a material with LIthium, Phosphorus, Oxygen, and Nitrogen called “LIPON.” However, it still relied on the same manufacturing sputtering process thus limiting its use to very small cells with very thin solid electrolyte layers. In sum, even with the potential safety and other improvements they offer, the processes to make these types of electrolytes are prohibitive due to the slowness and cost of the manufacturing processes (Blomgren, 2011). More recently there has been a great deal of excitement with the latest announcement from Dr. John Goodenough and the research team at the University of Texas due to their recently published discovery of a new, glass-based electrolyte solid-state battery that is noncombustible, has long cycle life, high volumetric energy density, and fast charge and discharge capabilities. In this instance the team at the University of Texas used a lithium metal anode with a sulfur-based cathode and the solid glass electrolyte that has lithium or sodium conductivity. Due to its low cost, sulfur may be used in place of lithium metal anode. The potential with this type of development may be great, including the elimination of the solid electrolyte interphase (SEI) layer, the elimination of the potential for dendrites to form, the elimination of the expansion experienced in current lithium-ion cells that eliminates the main sources of capacity fade during cycling. This cell design also benefits from using the more traditional cathode electrode manufacturing process and the current cell assembly processes,

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with only minor changes due to the lithium-metal (Braga, Grundish, Murchison, & Goodenough, 2017; University of Texas at Austin, 2017). Other types of solid electrolytes include garnet-type Lithium Lanthanum Zirconate (LLZO, or Li7La3Zr2O12); Lithium Super Ionic Conductor (LISICON) such as Lithium Tin Phosphorous Sulfide (LSPS, or Li10SnP2S12); sulfur based glassy and glass-ceramic solid electrolytes such as Li2S–SiS2 and Li2S–P2S5; and Solid Polymer Electrolytes (SPE) using low lattice energy lithium salts like LiClO4, LiN(CF3SO2)2, LiCF3SO3, or LiBC4O8.

7.2.5.3 Ionic liquids Ionic liquids have gained significant interest as a potential electrolyte for secondary lithium-ion batteries in recent years. But what exactly is an ionic liquid and why should we care? The short answer is that an ionic liquid is, simply, a salt that is in liquid form without any of the organic molecules and solvents such as the LiPF6 used in standard electrolytes. These liquids are made up mainly of ions and pairs of ions, which have very good conductivity. Some cations used in ionic liquids include imidazolium, quaternary ammonium, pyrrolidinium, and piperidinium and are paired with anions of PF6, BF4 or bis(triflouromethanesulfonyl)imide (TFSI) (Li et al., 2016). One of the reasons that ionic liquids have gained interest recently is due to some of the properties that may offer improved safety in lithium-ion batteries, including a reduction in flammability and volatility. But they are also being evaluated due to their high ion conductivity, stability, and solubility of both organic and nonorganic compounds. Yet many of these liquids have failed to gain use as the rate performance that they offer is typically below that of standard nonaqueous liquid electrolytes. And while they have proven more difficult to ignite, when they are ignited they will burn at very high temperatures. Because of this their nonflammability has been questioned especially during fully charged conditions. Ionic liquids also have higher viscosities, making them more difficult to fill a cell quickly and taking longer to wet the electrodes. Because of these challenges ionic liquids have not yet gained purchase but continue to be examined as potential future electrolytes as the industry continues to search for greater safety and nonflammability (Blomgren, 2011; Johnson, 2007; Li et al., 2016; Matsumoto, 2014).

7.2.5.4 Water-in-salt electrolyte Water-in-Salt Electrolytes (WISE) are a form of aqueous electrolytes that is also receiving much attention. This aqueous, water-based, electrolyte uses a high concentration of salts in the mixture to achieve good performance while being nonflammable. An example was shown during a presentation at the NAATBatt annual meeting in Phoenix, Arizona in March 2017 by the U.S. Army Research Laboratory who reported having achieved energy densities of more than200 Wh/kg and 300 Wh/kg at the cell level (NAATBatt presentation 2017 annual meeting). The researchers at the University of Maryland and the U.S. Army Research Laboratory reported developing a water-based salt electrolyte solution that is capable of reaching 4.0 volts,

7.3 Separators

while the water base makes it nonexplosive and cannot catch fire. This new WISE electrolyte is a gel polymer electrolyte coating that is applied directly to the graphite anode. This is followed by coating the gel electrolyte with a hydrophobic film that allows the lithium-ions to pass to the graphite anode without allowing the water to contact the graphite anode (Cell Press, 2017; Chang et al., 2016; Yang et al., 2017). Some WISE electrolytes are made through dissolving lithium bis(triflouromethanesulfonyl) imide, or LiTFSI, in very high concentrations in water. This has been shown to offer almost 100% columbic efficiency at more than 1000 cycles which means that it has the potential to become a competitor to the current nonaqueous electrolytes in terms of power and energy density (Li et al., 2016; Suo et al., 2015). The energy density of the WISE batteries, around 200 Wh/kg, will need continued development if they are to compete with nonaqueous lithium-ion batteries which are coming in sight of 300 Wh/kg. The use of the solid-state hydrophobic coating on the graphite anode also needs continued work as this is currently an expensive process to undertake, even while the actual materials are inexpensive (Chang et al., 2016). But the potential to eliminate the risk of fire and thermal events makes WISE and other aqueous electrolytes worth continuing to develop. Today, however WISE electrolytes still require a trade-off between safety and energy density.

7.3 Separators The separator, which is occasionally also referred to as an ion-conducting membrane (ICM), is perhaps one of the most important components of the lithium-ion battery that is also perhaps one of the least discussed. The separator is a type of ultrathin, porous membrane that allows for the physical separation of the positive and negative electrodes. It is responsible for keeping the anode and cathode electrodes electrically separated and thereby preventing a short circuit. Subsequently, it is a primary safety component within a cell. But it is also a critical component in the operation of the cell as it must have enough porosity to allow the lithium-ions in the electrolyte to pass back and forth between anode and cathode, but not allow the electrons to pass, which means that it must be ionically conductive but electrically isolating. Brian Morin, CEO and founder of Dreamweaver International, has described three generations of lithium-ion battery separators. The first generation are the traditional PE and PP polyolefin separators. The second generation are the ceramic-coated PE and PP separators, while the third generation are the newest types of nonwoven separators. Separators are made from extremely thin polymer films such as polyolefins, ceramics, or polymer/ceramic blends. Typical commercial separators are between 15 and 40 μm thick but are trending to lower thicknesses and are created through either a dry extrusion process or a wet solvent-based process. The most commonly used separator is made from polyethylene (PE) or polypropylene (PP). In Fig. 78 we see scanning electron microscope images of three different types of polyolefin separators. Fig. 78A shows an example of a single component extruded polymer film

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FIG. 78 Polyolefin separator types.

using a dry process with no solvents. With this type of material and process the pores are generally small and there is limited ability to control the pores size and structure. Fig. 78B shows an example of a two-component separator made using a wet process. In this instance the polymer is mixed with a plasticizer before it is extruded into a film. It is considered a “wet” process because a solvent is used to remove the plasticizers which allows for better control of the pore size and structure. Finally, Fig. 78C is very similar to the previous one in that it is a wet process but in addition to the plasticizer there is an inorganic filler added. Both the inorganic filler and the plasticizer are removed using the solvent. With these three processes we can see that we can get different sizes of pores and therefore different performances (Dahn & Ehrlich, 2011; Yoshino, 2014). In addition to these polyolefin materials we also see a combination of the two materials polyethylene and polypropylene which some manufacturers refer to as a “tri-layer” separator. This separator includes a layer of polyethylene sandwiched between two layers of polypropylene (Fig. 80). This takes advantage of the characteristics of both materials to create a safety shutdown feature with the middle polyethylene layer melting first and closing the pores to prevent any additional lithium-ions from passing through. This is possible because the melting point of polyethylene is about 135°C, while the melting point of polypropylene is about 165°C. In the event of a battery that is in a failure mode and begins heating up the polyethylene layer will melt first, stopping the flow of ions (Fig. 79). If the temperature of the cell continues to rise the polypropylene layers will also melt at which point the anode and cathode electrodes will contact each other and a short circuit will follow as the cell moves into an uncontrollable thermal runaway event. However, it is important to note that these polyolefin-based separators may begin shrinking at about 110°C. This means that in large format cells in high voltage applications the cells are likely to incur an internal short circuit and be driven to a thermal event, before the safety separators reach their melting points. Therefore a safety separator will only keep the battery safe up to a certain temperature, usually up to the 110°C when the separators begin shrinking.

7.3 Separators

FIG. 79 Tri-layer separator.

7.3.1 Properties of separators The United States Advanced Battery Consortium LLC (USABC), which is a subgroup of United States Center for Automotive Research LLC (USCAR), has developed a set of requirements for lithium-ion battery separators presented in Table 15 and in complete in Appendix B. A lithium-ion battery separator typically has a Table 15 General requirements for lithium-ion battery separators Note

Parameter

Goal

1 2 3 4

Sales price, $/m Thickness, μm Permeability (MacMullin #, dimensionless) Wettability

5 6 7 8 9 10 11 12

Chemical stability Pore size, μm Puncture strength Thermal stability Purity Tensile strength Skew Pin removal

2

1.00 25 11 Complete wet out in typical battery electrolytes Stable in battery for 10 years <1 >300 g/25.4 μm <5% shrinkage after 60 min at 90°C <50 ppm H2O <2% offset at 1000 psi <2 mm/m Easy removal from all major brands of winding machines

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thickness of between 20 and 25 nm, but development is occurring that may reduce the thickness of the separator down to 10–15 nm with pores of about 1 nm. It should also be able to withstand temperatures of up to 90°C for up to 60 minutes without shrinking by >5% (United States Advanced Battery Consortium, 2010). It should have high strength to permit high-speed automated winding processes and it should not yield or shrink in the width direction. It should be resistant to puncturing or penetration by the electrode materials to prevent dendrite penetration. It should have a pore size of less than1 μm and be easily wetted by the electrolyte but be stable with the electrolyte and active materials (Dahn & Ehrlich, 2011; Lundgren, Xu, Jow, Allen, & Zhang, 2017). Both the USABC and the Society for Automotive Engineers (SAE) have developed recommended practices for testing separator characteristics to ensure consistent comparisons across different organizations and companies. The SAE standard number J2983 “Recommended Practice for Determining Material Properties of Li-Battery Separator” defines a recommended process for testing the properties of separators while the USABC “Procedure for Determining Shutdown Temperature of Battery Separators” defines a procedure to determine the temperature at which the separator shutdown function is reached (Society of Automotive Engineers, 2012; United States Advanced Battery Consortium, 2013). It is desirable to have high porosity in a separator as it improves the ionic conductivity, but it also acts like an electrolyte holding tank. This function of holding excess electrolyte acts to increase the energy density and reduce the heat generation. The size of the pores in the separators is a key dimension of lithium-ion cells. Separators manufactured using the dry process tend to have a very consistently oriented and large pore structure which is better for fast ion passage through the separator. While separators manufactured using the wet process tend to have “tortuous” pore structures making ion passage somewhat slower, but the tortuous pores make it more difficult for dendrites to penetrate the separator. In Table 15 we see that the USABC preferred pore size is less than1 μm. But the size needs to be determined based on the size of the particles in the electrodes; they must be small enough so that the active materials cannot pass but that ions can. The pores cannot be too large, however, as that would eliminate the safety shutdown functionality as the pores would not close during a thermal event. Fig. 80 offers two higher magnification SEM views of the pores in dry processed separators. These holes or gaps are what we are referring to as pores and it is through these that the lithium-ions will pass. Most commercial separators have a porosity of 40% to 50%, with some separators being up to 70% porous (AZO Materials, 2017; Lundgren et al., 2017).

7.3.2 Ceramic separators One evolution on the traditional separator technologies has come through the process of coating the separator with an inorganic coating such as alumina, silica, titanium, magnesia, or other ceramic particles combined with a polymeric material (Yoshino, 2014). Today many separator manufacturers offer their standard PE, PP, or tri-layer

7.3 Separators

(A)

1µ 2400

20000×

2400 FSM

5 kV

(B)

1µ 2500

20000×

5 kV

2500 FSM

FIG. 80 Dry processed separators.

separators with what is referred to as a ceramic coating, primarily using alumina or one of the other materials noted before. The benefit of ceramic coating a PE or PP separator is that it improves the safety due to the much higher melting point of ceramic compared to the polyolefin materials. This process creates a protective coating on the separator but leaves the pores open.

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One of the main challenges with the ceramic-coated separators is the manufacturing process. It is important to ensure the right combination of polymers and ceramics such that the ceramic coating does not become brittle forcing it to fracture, delaminate, and break apart during manufacturing process. This brittleness is also known to cause cracking of the coating during electrode punching operations and cell assembly. It is also important to note that the ceramic coating process is not limited to the separators. Many manufacturers have begun using the ceramic coating process on both the cathode and anode electrodes to improve the high temperature performance and life.

7.3.3 Nonwoven separators Another type of separator that has been growing in usage and interest is the nonwoven separator formed from materials such as liquid crystalline polyester, aromatic polyamide, and cellulose. One of the benefits of the nonwoven separator is that they tend to have high temperature stability which means that this class of separator will not shrink thereby preventing short circuiting due to separator shrinkage. Nonwoven separators also tend to be nonflammable, especially the cellulosic types which is a major benefit. The other benefit that has been seen is that the ionic conductivity is higher than that of the traditional separator. One of the challenges with nonwoven separators is that they have tended to be thicker than the traditional separators ranging between 25 and 50 nm but with some manufacturers working on 10 to 15 nm thicknesses (Yoshino, 2014; Zhang, Ramadass, & Fang, 2014; Zhang et al., 2014). Dreamweaver International is one company that has been developing cellulosic nonwoven separators for the lithium-ion batteries and ultracapacitors. With a parent company founded based on paper manufacturing, Dreamweaver was able to apply some of the same processes toward creating a low-cost, safe, nonflammable separator that enables high performance. Dreamweaver’s separators use a combination of nanofibers and microfibers that are blended using a high shear mixing and then are cast together with the microfibers forming a scaffolding over which the nanofiber drape, giving strength and dimensional stability as well as small pore size. In Fig. 81 you can clearly see the microfibers, the big ones, and the nanofibers, the smaller ones draping over the larger microfibers. Other manufacturers of this third generation of nonwoven and fibrous separators include Freudenberg, Electrovaya, Optodot, and Mitsubishi Paper. Freudenberg uses a wet process with a polyester nonwoven base that it impregnated with inorganic particles to create a nonwoven, ceramic separator. Electrovaya uses a similar process with a PET-based nonwoven material with embedded ceramic materials in their Separion™ branded nonwoven separators. In addition to their more traditional ceramic coated separators, Optodot has developed an all-ceramic separator, branded NPORE® that offers high dimensional stability, high thermal conductivity, and flame resistance. Mitsubishi Paper Mills have developed a nonwoven separator based on a cellulosic nanofiber combined with synthetic fibers in their NanoBase2 and

7.3 Separators

FIG. 81 Dreamweaver separator courtesy of Dreamweaver International.

NanoBaseX brands. This list is certainly not all inclusive, but these few examples are intended to show that the market for separators is continuing to experience new developments as the demands on the lithium-ion batteries get more challenging and as the limits of safety continue to push manufacturers to find better solutions to make their lithium-ion batteries safer.

7.3.4 Other separator materials While virtually all separators today are made from the polyolefin plastics films, there is development work that has been looking into the use of different materials including silicone rubber, fluororubber, aromatic polyamide resins, liquid crystalline, and even Kevlar®. Separators made from Kevlar® nanofibers are very interesting as it may be possible to create separators that are as small as 10 nm thick, compared to 20 to 25 nm for current separators (Fig. 82). With a reduced thickness the cell will end up with a greater energy density. Additionally, the same properties of Kevlar® that make it good for creating bulletproof vests make it good at preventing dendrites from penetrating it. The Kevlar® material is also very stable at high temperatures and in fact may be able to operate up to temperatures exceeding 400°C. This means that it will not experience the shrinkage that accompanies the traditional polyolefin separators. Finally, it may be possible to achieve better power rate capabilities as the distance between the anode and cathode electrodes is reduced from that of traditional separators (Moore, 2015; Tung, Ho, Yang, Zhang, & Kotov, 2015; Yoshino, 2014). One other emerging type of promising nonwoven separator is using polyester materials. Poly(butylene) terephthalate (PBT) may be electrospun into sheet of 30

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FIG. 82 Kevlar separator clockwise from top left, TEM image of ANF Kevlar separator; SEM side view; optical photograph and bottom left, SEM top view. Reprinted by permission from Springer Nature: Tung, S., Ho, S., Yang, M., Zhang, R., & Kotov, N. A. (2015), A dendrite-suppressing composite ion conductor from aramid nanofibers. Nature Communications.

to 55 μm which offers about 75% porosity while ionic conductivity is just less than that of traditional PE or PP separators. From the thermal perspective the PBT separator has a melting point of 210°C, which is much higher than the PE or PP commercial separators. A couple of areas for continued development may be the electrochemical stability of the PBT separators with the electrolytes and in reducing the thickness of the material. Early development appears that it is a viable candidate for use in lithium-ion cells (Orendorff et al., 2012).

7.4 Current collectors and metal foils One area that does not seem to change much are the current collectors used in lithium-ion batteries; however, today there is quite a bit of work happening in this area. For the active materials to conduct the electrons and current to the electric motor or power source, there needs to be a medium to enable that conduction. The current collectors are that medium. The active anode and cathode materials

7.4 Current collectors and metal foils

are coated upon a metallic foil and this foil allows the flow of electrons and current but not ions. Typically, the two types of metals that are used in the foils are aluminum for the positive cathode electrode and copper for the negative anode electrode. Copper is favored due to its high conductivity of about 5.98  107 Seimens per mole (S/m), which makes it the second most conductive metal know to man. Aluminum is the fourth most conductive metal at 3.5  107 S/m conductivity. Silver is actually more conductive than copper at 6.30  107 S/m and gold is the third most conductive metal at 4.52  107 S/m, but of course due to the cost of these metals they are not used in high volume lithium-ion batteries. In some lithium titanate oxide cells aluminum foils are used on both the positive and negative electrodes. But in either case, the vast majority of commercial cells today use an aluminum and a copper metal foil as the current collectors. Why can’t we use the same current collector for both anode and cathode? The short answer is due to the electrochemical stability windows of the metals. Copper has a wide stability window from 0 to 3.0 volts. It reacts with metallic lithium at about 3.2 volts and begins oxidization above this voltage level. Depending on the electrolyte additives used it can continue to be stable above 3 .0 volts but will ultimately begin oxidizing in these conditions. Aluminum, however, is stable at a high voltage potential but reacts with lithium at a much lower voltage than copper, aluminum reacts with lithium at about 0.6 volts. Both the aluminum and copper foils will form a passivation layer much like the SEI layer discussed in relation to the anode materials. This is a result of the oxidation of the aluminum and copper. However, both copper and aluminum experience corrosion over time that is a result of the generation of hydrofluorocarbons (HF) forming in the electrolyte due to impurities. This will show up as a pitting of the aluminum or copper foil. Dissolution of the copper may be more problematic than the aluminum, as the dissolved copper may plate on the negative electrode surface creating safety concerns due to the potential for short circuiting due to copper dendritic growths (Myung, Hitoshi, & Sun, 2011). There are many reasons why these materials are so widely chosen, including their high conductivity, low cost, stability, low reactivity, flexibility in manufacturing, and abundance. A good current collector must have high strength to be used in the current manufacturing processes. In current manufacturing processes, the metal foils must carry the weight of the electrodes through the coating, drying, pressing, and cutting processes without tearing or breaking. For example, I have seen drying ovens as long as 25 meters, or about 82 feet, long with the coated foil suspended for the entire length of the oven. The foils need to have excellent strength to be able to do this without tearing. As the industry pushes forward with higher energy densities, one somewhat obvious area to look at is simply coating more active materials onto the metal foils and using thinner foils. However, this is not as simple as it sounds. The thicker the active material is, the greater chance that it will pull away from the metal foil, this is called delamination. Consequently, there is a limit to how much active material can be coated onto a traditional metal foil. There are a variety of coatings and surface

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treatments that are frequently applied to the standard aluminum and copper current collectors to help alleviate this problem. The main types of surface treatments use carbon black coatings applied either by vapor deposition or by tape casting of a slurry. The primary purpose of these coatings is to help enhance the adhesion of the active materials to the metal foils, in other words to reduce the potential for delamination. A secondary purpose is to reduce the potential for corrosion of the metal foils. Another issue in energy density is the foil thickness, the thinner the foil is the greater the energy density will be. Most foils today fall between 16 and 20 μm, but there is a strong trend to move to foil thicknesses below 10 μm. However, this leads to some challenges in a couple of areas. First, with foils this thin it becomes more difficult to handle them in the manufacturing process without damaging them. But it has also proven difficult to manufacture films this thin using traditional rolling or electrodeposition methods. They simply cannot get consistent thicknesses and surfaces below about 6–10 μm. Some researchers have recently approached this problem by moving to a copper nanowire foil that can be made as thin as 1.5 μm. If this new form of foil can be processed on the existing cell manufacturing lines it could offer significant energy density benefits (Chu & Tuan, 2017). There has also been some interesting new work happening to develop entirely new types of foils to improve the safety, energy density, and cost of the lithiumion cells. One of these types is the three-dimensional (3D) current collector structures which are currently experiencing a lot of interest. A 3D current collector may come in the form of nanotubes being grown on metal foils, by using porous conductive textiles, through carbon and metal foams, or in the form of a metallic mesh. Each of these approaches offers the possibility for lower weights, and therefore higher energy densities, lower costs, and lower volumes. One method, using a standard nonwoven polyethylene terephthalate (PET) that has been plated with aluminum using physical vapor deposition (PVD) shows some very interesting properties (Poetz, Fuchsbichler, Schmuck, & Koller, 2014). The Soteria Battery Innovation Group is a start-up company that spun off from the Dreamweaver International separator company with a very interesting new concept that has the potential to make a lower cost, lighter, safer type of current collector. The Soteria design is based on this process of metalizing a plastic film. This offers a couple of major benefits. First, it reduces the weight of the current collector by more than 20%. Second, the weight reduction drives an improvement of the gravimetric energy density with a potential to achieve cell energy densities of 400 Wh/kg using current state-of-the-art technologies and chemistries. Third, the structure acts as a fuse at the electrode level. In a short circuit or nail penetration event the metal shorts immediately surrounding the failure spot and does not allow the failure to propagate beyond a single electrode as shown in Fig. 83. When used in combination with their third-generation, nonwoven separator the potential for a “short-free” and safe lithium-ion cell may be at our doorstep. Of course, like with any new technology there are a few challenges that they are working through, but there do not appear to be any “showstoppers.” One challenge that the Soteria Group is working through is the electrode-to-electrode connections.

7.4 Current collectors and metal foils

No separator shrinkage

Conductor retreats from short AI film conductor Cathode Nonwoven separator Anode Cu film conductor Defect

High temperature zone

Conductor retreats from short

FIG. 83 Soteria Innovation Group current collector design.

In a typical cell the electrodes are welded together; however, in the Soteria design it is more challenging to weld these electrodes, so a mechanical connection may need to be developed. This may drive a change to the cell assembly process for cell manufacturers who use the design, but the benefits may outweigh the new challenges. But members of the Soteria consortium are working on solutions that would make welding the electrodes possible and therefore allowing them to be a drop-in design to current processes. Similar to the Soteria solution, 3-D current collectors using an expanded metallic matrix or metalized polymer attempt to gain the advantages of reduced weight and therefore improved energy density and lowered cost, while still maintaining power, performance, and manufacturability. Dexmet has been developing a 3-D current collector based on an expanded metal. Their MicroGrid® branded expanded foils claim up to 20% capacity increase up to 100 cycles while reducing the potential for delamination of the active materials from the foils (Dexmet, 2018). The general concept with a 3-D current collector is to use not only the X- and Y-dimensions of the foil, but also to be able to use the Z-dimension. In the simplified example in Fig. 84 the 2-D current collector is a standard foil and is coated with active material only on the surface in the X- and Y-axis directions. However, in the 3-D example the active material is coated not only in the X- and Y-dimensions, but due to the porous grid it is coated down into the Z-dimension. Some researchers are even attempting to use graphite sheets and carbon nanotubes (CNT) as current collectors. One solution presented by Qu, Hou, Tang, Semenikhin, and Skorobogatiy (2016) proposed the use of a graphite sheet with a submicron level metallic layer that is added via physical vapor deposition (PVD) as a current collector for a more flexible type of lithium-ion battery. The use of vertically aligned carbon nanotubes (VACNT) shows some very interesting properties as current collectors as well. These VACNT’s are grown directly on the current collector without the need for a binder. This vertical alignment offers the

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FIG. 84 2-D versus 3-D current collector.

benefit of creating a short electron and lithium-ion transport pathway. It is also expected to reduce the charge transfer resistance because of these short pathways. The vertical alignment of the carbon nanotubes creates direct pathways for electron transport and offers large surface area with good rate capability. However, CNTs still remain an expensive solution to manufacture and are mainly done in small volumes at lab scale (Pawllitzek, Althues, Schumm, & Kaskel, 2017). Finally, other metals have been evaluated as potential for use as current collectors including iron (Fe), chromium (Cr), nickel (Ni), and stainless steel. Both iron and stainless steel have proven to be best used with low voltage electrode materials such as the metal sulfides like nickel metal hydride, but have not proven effective for use with the higher voltage lithium-ion chemistries due to the dissolution of the iron which is believed to “poison” lithium-ion batteries and speed the end of life when used in these cells. Chromium was found to be stable in the potential range from 2.5 to 4.0 volts making it a potential for use in lithium-ion batteries; however, it does begin to dissolve at potentials above 4.5 volts. Stainless steel, in addition to its use of iron in the metallic makeup, includes chromium and nickel both of which are susceptible to dissolution above 4.5 volts. So none of these materials are mainstream foil candidates today (Myung et al., 2011).

7.4.1 Tabs/terminals In discussing current collectors that are used within the cell it is also worth briefly mentioning that those current collector foils must be interconnected and then attached to the terminals that will extend outside of the cell casing. These electrodes

7.4 Current collectors and metal foils

that extend beyond the cell are referred to as tabs, generally on pouch cells, or terminals on can- and prismatic-type cells. The tabs or terminals may terminate in a weld pad that is used to make the cell-to-cell connection. They may also terminate in a mechanical connection, either a male or female threaded terminal. Regardless of the design, the tab or terminals must be designed, and the material selected, that will be compatible with the environment that the cell will ultimately be used in. And while the tabs must not be the sole mechanical connections, they should be rigid enough to ensure that the contact is maintained through the life of the cell and they must resist permanent deformation (Friel, 2011). When dissimilar metals are used in a joint it is likely for galvanic corrosion and oxidation to take place more quickly. Galvanic corrosion occurs when two dissimilar metals are put in contact and are electrically connected especially when they are in the presence of an electrolyte (Budinski, 1989). In this instance the two metals form a kind of galvanic cell with one metal acting as the anode and the other acting as the cathode. This creates a form of galvanic corrosion at the joint increasing the resistivity of the connection. The oxidation or corrosion of the terminal will reduce the contact area and increase the impedance and resistance until the terminal is isolated from the cell and can no longer transmit current. Therefore these tabs or terminals are most often nickel plated to improve the weldability to other metals and reduce the potential for oxidation and corrosion over the life of the product. Nickel is a good candidate because it has higher thermal and electrical resistivity, in other words it is less thermally and electrically conductive than copper or aluminum which makes it easier to weld to. It is frequently used as a coating to the tab materials and connections. It also allows the connections to be made using similar metals which reduces the potential for galvanic corrosion. Referring to our introduction to chemistry in Chapter 2 we saw that the electrical conductivity of a material is a function of its charge carrier number per unit of volume. In plain English this means the conductivity of a material is a function of how many charge carriers, such as free electrons, the material has per the amount of space it takes up (Lefrou et al., 2012). This is important since the size and thickness of the tab or terminal will be determined based on the cell’s currentcarrying capability. If the cell is intended to be a power cell, it will have greater current capability than would a cell that is intended for energy applications, hence terminals must be sized accordingly. In general, the larger a piece of metal the lower is its resistance to the flow of electrons. Because of this, high-power cells, and systems, will most often use very large copper bus bars as the large size of the bus bars allows the current to flow more freely. The current-carrying ability of a metal depends on several factors: (1) the distance between the points, (2) the size of the current collector, (3) the material that is used, and (4) the temperature. Temperature may seem odd, but it is a factor that must be considered when evaluating the materials. Most metals will see an increase in resistance as the temperature rises.

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FIG. 85 Cell form factors.

7.5 Cases Once the electrodes are coated, rolled, or stacked and are ready to be assembled into a cell they must be contained in some manner. There are currently three different types of cases that are used for lithium-ion cells (Fig. 85). The first type is a deep-drawn cylindrical can. This type of case is most often also aluminum but may also be nickelplated steel. The second type of case is the pouch, or laminate, packaging that is made from an aluminum and polymer lamination process. The pouch is electrically neutral due to the polymers and allows for a wide variety of form factors to be used. Finally, the third case is referred to as the prismatic can. This is typically a rectangular aluminum cube that is deep-drawn aluminum and is sealed with a lid. However, some manufacturers use an injection-molded plastic cube in place of the aluminum can. The most widely used cell format for many years has been the cylindrical “18650” cell, which is a cylindrical can that is 18 mm diameter and 65 mm high. This has been the basis for most laptop computers and portable electronics for many years. For large applications the small cylindrical cells are not as frequently used except for Panasonic/Tesla who have converted to a 2 mm diameter by 70 mm height cylindrical cell (a 21700 or 2170). In place of these small format cells many of the larger manufacturers are using fewer, but larger format prismatic- or pouch-type cells. This strategy reduces the number of cell-to-cell connections but depends on the larger cells which may be more difficult to manage in a thermal event. There are benefits and challenges to each type of cell design and all have been successfully used in many applications. There is no single solution that we can say is the “best” cell type. Due to the rigid mechanical structure both the can and the prismatic cells are able to integrate venting safety devices whereas the pouch cell is not possible to integrate a pressure vent. The vent makes it easier to direct the flow of the exhausting gases during a failure. However, it may also be possible to direct the venting gases in a pouch-type cell at the module and system level. Small, cylindrical cells

7.5 Cases

are often presented as being safer because they contain less energy and should, therefore, be easier to manage in a failure and thermal event. However, there have been numerous and very public cases in the news showing vehicles with both small cells and large cells that have went into cascading thermal failure after catastrophic events. If this small cell argument were true then we would not see catastrophic failures in vehicles since the thermal event would be managed at the single cell level. So I believe this to be a bit of a red herring argument supporting the smaller cells. Pouch cells have a very thin cross section ranging between 5 and 15 mm in total thickness for some of the large form factor pouch cells. These thin cross sections allow for easier thermal management as there is less distance for the heat to travel from the center of the cell to remove it when compared with prismatic can cells which can range from 12 up to over 90 mm in thickness. The pouch, also called a laminate, cell material typically consists of a thin aluminum foil that is laminated between a nylon layer and a polypropylene layer as shown in the example in Fig. 86. This use of laminated aluminum also means that the cell itself is electrically neutral with no current running through the pouch material. Prismatic can cells tend to be easier to handle due to their rigid structure which also allows for very large cells to be manufactured. Prismatic cells may be either aluminum or a plastic, while can cells are either aluminum or nickel-plated steel. The larger cell sizes mean that fewer cells are needed to create large voltage/capacity systems. But it is also more difficult to thermally manage a large format prismatic cell since they can be very thick so moving the heat away from the center of the cell is more difficult. In the case of metal cylindrical or prismatic cans they are most likely to be “can positive” which means that the positive (most frequently) terminal runs

FIG. 86 Aluminum laminate pouch material.

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through the case. These types of cases must therefore be insulated with a dielectric film or similar type of insulator. The key characteristics for the cell casing selection are that it should provide a good mechanical structure for the cell, even for pouch cells. It should not be reactive with the electrolyte, in other words it should not have a chemical reaction with the electrolyte that causes the materials dissolve or cause “side reactions.” It should provide good electrical insulation but should also offer good thermal conductivity with the purpose of removing heat from the cell. It should be robust as it must last the life of the cell in the application. It may include some safety features such as vents. It should also be flame retardant, in the event of a fire the case should not add fuel to the fire. The case design should also take into consideration the expansion of the cell during cycling and be able to accommodate it both during operation and over life (Friel, 2011). For more information on the variety of cells sizes and formats, I would refer the reader to the SAE International technical information report SAE J3125 “Industry Review of xEV Battery Size Standards” that describes the state of the industry in terms of the different types and sizes of cells that were in use at the time the document was published.