Lithium Ion Battery

A lithium-ion battery is a family of rechargeable battery types in which lithium ions move from the negative electrode to the positive electrode during discharge and back when charging.

From: Functional Nanofibers and their Applications, 2012

Na-Ion Batteries

Mojtaba Mirzaeian, ... Rizwan Raza, in Reference Module in Materials Science and Materials Engineering, 2021

Introduction

Lithium-ion batteries (LIBs) are considered the pioneering technology that has been successfully adopted as a power source for wide range of applications including portable electronics and electric/hybrid electric vehicles (EVs/HEVs) after their commercialization by Sony Corporation in 1991 (Perveen et al., 2020; Duan et al., 2020; Tian et al., 2020). Despite their commercial success in numerous applications, LIBs have not been deployed in large-scale electrical energy storage (EES) applications due to elevated cost and limited supply of lithium resources over the coming years as shown in Fig. 1, restricting their wider use. Total reserves of lithium are estimated between 15–30 Mt (million tons) with uneven geographical distribution whereas, recycling rate of Li from spent LIBs is only around 1% which can be considered as more or less non-existent (Vikström et al., 2013; Speirs et al., 2014). This scenario requires immediate effort to find substitute material to replace lithium-ion battery systems based on more cost-effective and easily sourced materials.

Fig. 1. Comparison of lithium metal reserves and production from 2013 to 2018. Perveen, T., Siddiq, M., Shahzad, N., et al., 2020. Prospects in anode materials for sodium ion batteries – A review. Renewable and Sustainable Energy Reviews 119, 109549.

Sodium-ion batteries (SIBs) have emerged as suitable alternative owing to their similar physiochemical/electrochemical properties and essentially inexhaustible/low cost elemental sodium (comparison between relative reserve of elemental lithium and sodium in earth crust is shown in Fig. 2) making SIBs a promising candidate to meet future energy storage requirements particularly large scale EES applications. Key benefit of using Na+ is the economy of scale since Na+ is the fifth most abundant element in earth crust as shown in Fig. 2. Furthermore, cast can be reduced even further by using low cost aluminum (Al) as a current collector since it does not form alloy with Na+ based anode and cathode.

Fig. 2. Abundance of elements in the Earth’s crust. The energy carrying elements for comparison are highlighted in red for Na and blue for Li.

Adapted from Chayambuka, K., Mulder, G., Danilov, D.L., Notten, P.H., 2018. Sodium‐ion battery materials and electrochemical properties reviewed. Advanced Energy Materials 8 (16), 1800079.

Despite these advantageous characteristics, SIBs struggle to compete with LIBs due to their inferior storage capacity, sluggish reaction kinetics and poor rate capability especially when used in conjunction with graphitic carbon, which is the most frequently, used anode material. This is due to the larger ionic radius of 1.02 Å of Na+ compared to ionic radius of 0.76 Å for Li+ making Li-ion intercalation more effortlessly attainable. Up until late 80 s, SIBs were successful in attaining similar research attention as LIBs however, the discovery of more cost-effective, high capacity and electrochemically active anode material based on graphite resulted in abandonment of SIBs altogether (Yabuuchi et al., 2014). Despite this neglection, research activity has recently witnessed reacceleration of research on sodium ion batteries after successful insertion of Na+ ion in hard carbon where high storage capacity of around 300 mAh g−1 was achieved which is similar to that of LIBs using graphite as anode (Stevens and Dahn, 2000).

From earlier discussion, it can be appreciated that SIBs are not considered as direct competitor of LIBs especially in area of volumetric or gravimetric energy densities, but they will rather complement Li-ion battery systems in large-scale application due to their competitive pricing, ultimately resulting in the price stabilization of lithium-based batteries. Therefore, success of SIBs will be governed by further research activity and investment in commercialization of the technology in order to close the technological gap between these two state of the art battery systems. In this brief article, comparison between lithium and sodium based rechargeable batteries will be made, followed by detail discussion on various anode, cathode and electrolyte materials and their effect on performance of SIBs.

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Functional nanofibers in lithium-ion batteries

H. Qiao, Q. Wei, in Functional Nanofibers and their Applications, 2012

10.1 Introduction

A lithium-ion battery is a family of rechargeable battery types in which lithium ions move from the negative electrode to the positive electrode during discharge and back when charging. Chemistry, performance, cost and safety characteristics vary across lithium-ion battery types. Unlike lithium primary batteries (which are disposable), lithium-ion batteries use an intercalated lithium compound as the electrode material instead of metallic lithium.

Lithium-ion batteries are common in consumer electronics. They are one of the most popular types of rechargeable battery for portable electronics, with one of the best energy-to-weight ratios, high open circuit voltage, low self-discharge rate, no memory effect and a slow loss of charge when not in use. Beyond consumer electronics, lithium-ion batteries are growing in popularity for military, electric vehicle and aerospace applications due to their high energy density.

Lithium-ion batteries were first proposed by M. S. Whittingham at Binghamton University in the 1970s.1 Whittingham used titanium(II) sul-fide as the cathode and lithium metal as the anode. The electrochemical properties of lithium intercalation in graphite were first discovered in 1980 by Rachid Yazami et al., who showed the reversible intercalation of lithium into graphite in a lithium/polymer electrolyte/graphite half cell.2 In 1981, Bell Labs developed a workable graphite anode to provide an alternative to the lithium metal battery. Following cathode research performed by a team led by John Goodenough, in 1991 Sony released the first commercial lithium-ion battery. Their cells used layered oxide chemistry, specifically lithium cobalt oxide. In 1983, Dr Michael Thackeray, Goodenough and cowork-ers identified manganese spinel as a cathode material.3 Spinel showed great promise, given low cost, good electronic and lithium ion conductivity and three-dimensional structure, which gives it good structural stability. Although a pure manganese spinel fades with cycling, this can be overcome with chemical modification of the material. A manganese spinel is currently used in commercial cells.

In 1989, Goodenough showed that cathodes containing polyanions – for example, sulfates – produce higher voltages than oxides due to the inductive effect of the polyanion.4 In 1996, Goodenough, Akshaya Padhi and coworkers identified lithium iron phosphate (LiFePO4) and other phospho-olivines (lithium metal phosphates with olivine structure) as cathode materials.5 In 2002, Yet-Ming Chiang and his group at MIT showed a substantial improvement in the performance of lithium batteries by boosting the material’s conductivity by doping it with aluminum, niobium and zirconium. The exact mechanism causing the increase became the subject of a debate. In 2004, Chiang again increased performance by utilizing iron phosphate particles of less than 100 nm in diameter. This decreased particle density by almost a hundredfold, increased the cathode’s surface area and improved capacity and performance.

In recent years, with the rapid development of nanotechnology, nano-materials are promising candidates for lithium-ion battery electrodes. As lithium-ion battery electrode materials, nanomaterials have some unique physical and chemical properties, such as the large surface area, shorter transport length, high reversible capacity and long cycle life. These properties can significantly improve specific capacity and high-rate performance of lithium-ion batteries.

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Sustainability Issues in Manufacturing and Operation of Second-Generation Flow Batteries

Concetta Semeraro, ... Michele Dassisti, in Reference Module in Materials Science and Materials Engineering, 2021

Lithium-Ion Batteries (LIBs)

Lithium-ion batteries are rechargeable batteries that employ nonaqueous electrolyte (Yoshizawa, 2009) which were commercialized in the early 1990s. There are four components in a lithium-ion cell: anode, cathode, separator, and the nonaqueous electrolyte. During the charging process, the lithium ions move from the cathode, through the electrolyte, to the anode, and then return during discharge (Zubi et al., 2018). Lithium-ion battery cells are manufactured as stack or cylindrical cells. In the first configuration, the cathode, anode, and separator are encapsulated in a laminate film. In the second one, the layers are rolled and sealed in a metal can.

Lithium-ion batteries have the characteristics of high-power density, long life, low self-discharge, low maintenance costs and low environmental impact. However, lithium has high reactivity, so there are technical limitations related to the safety of building batteries (Table 2).

Table 2. Lithium-ion battery

Advantages Disadvantages References
High energy efficiency Relatively expensive (Zubi et al., 2018)
Good cycle life Safety issue (Yoshio et al., 2009)
High power density Weak recovery (Yoshizawa, 2009)
High reliability Nominal 3-h charge
Good high-rate capability
Reasonable self-discharge rate

According to Wen et al. (2012) the safety issues of lithium-ion batteries usually are caused by the following chemistry parameters: high voltage (HV) (overcharge), high temperature (HT) (thermal runaway), high pressure (HP) (gas generation) and high current (HC) (dendritic lithium short-circuit). Several techniques and methods have been developed to achieve safety issues of LIBs controlling the internal voltage (V), the temperature (T), the pressure (P) and the current (I). Also, materials can be explored to address safety issues. Recent key progress in materials design to improve LIBs safety have been presented in Liu et al. (2018a). Several optimizations could be achieved regarding the existing materials of anode and cathode due to the limited electrical conductivity, slow Li transport, dissolution or other unfavorable interactions with electrolyte, low thermal stability, high volume expansion, and mechanical brittleness (Nitta et al., 2015). To achieve energy sustainability at a global scale (Zubi et al., 2018) Lithium-ion batteries could be design to: (1) reduce the specific carbon footprint of the power sector; (2) minimize dependence on cobalt; (3) conceive an appropriate recycling processes (Zeng et al., 2014). The research fields on lithium-ion batteries is focused on the development of new electrode materials to improve the performances in terms of manufacturing cost, energy density, power density, cycle life, and safety (Nitta et al., 2015). Commercially available lithium-ion batteries are: lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium iron phosphate (LFP), lithium nickel cobalt aluminum oxide (NCA) and lithium nickel manganese cobalt oxide (NMC) (Blomgren, 2016). All these are manufactured using graphite as anode material, whereas cathode varies according to the battery type: LCO is made from LiCoO2, LMO from LiMn2O4 cathode, LFP from LiFePO4, NCA from LiNiCoAlO2 and NMC from LiNiMnCoO2. LCO and NCA batteries are the most technologically mature at the present. The main disadvantage in operating the LCO battery is its low inherent safety due to the low thermal stability of cobalt-oxide. LMO and LFP are safer than the other batteries. LFP and NMC present a longer life cycle and lower energy density, while NCA batteries have an outstanding specific energy affordability and performance. Lithium cobalt oxide (LCO) finds application in smaller portable electronics while lithium manganese oxide (LMO) in higher power applications such as power tools and electric motive power. Lithium nickel cobalt aluminum oxide (NCA) are widely applied in electronic applications. Lithium nickel manganese cobalt oxide (NMC) are applied in portable and high-power applications including power tools and electric vehicles. Lithium iron phosphate (LFP) are mainly used in high power such as power tools and energy storage applications (Blomgren, 2016). Li-ion batteries are widely employed in portable electronic devices such as mobile phones, tablets, and laptops (Yoshizawa, 2009). Lithium-ion batteries have also found applications in hybrid and electric vehicles (Lu et al., 2013). In the latter type, the key performance is the optimal charge/discharge control of the battery. Accurate information about the state of charge (SOC) and state of health (SOH) of the battery should be provided to ensure the safe and reliable operation of the battery. In addition, the ambient temperature is a key factor affecting the accuracy of SOC estimation (Xing et al., 2014). Machine learning methods, including artificial neural networks, fuzzy logic-based models and support vector machines, are increasingly used in the literature to estimate the state of charge of Li-ion batteries (Charkhgard and Farrokhi, 2010; Chen et al., 2012).

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Metal-organic framework for batteries and supercapacitors

M. Ramesh, ... S. Praveen, in Metal-Organic Frameworks for Chemical Reactions, 2021

2.3.1 Lithium-ion batteries

LIBs have an anode and electrode, as well as an electrolyte in three main components. When LIBs is charged, lithium ions are removed from the cathode electrode. The decomposition of lithium ions then travels through the electrolyte and transfers into the anode electrode, and the energy is stored in LIBs during this cycle. When the LIBs stop storing, the lithium ions move back to the cathode electrode; and the stored energy has been released. The selection of cathode and anode materials is very important, and this is the main focus of various researchers [12,13]. The conventional electrode material is subject to complicated synthesis, low energy/power density, and limited life cycle. MOFs are promising electro-materials for LIBs due to their unique character, nonspecific surfaces, well-developed porosity, and high storage ability. Metal captions serve as the active redox MOF site and enable efficient and reversible ion insert/extraction in open crystal frameworks [14]. Combarieu et al. [15] have been identified as a cathode material Li+ to reversibly insert FeIII(OH)0.8F0.2[O2C–C6H4–CO2] [MIL-53(Fe)]. Experimental findings showed that MOFs are excellent cathode materials for LIBs, with no structural modification, up to 0.6 Li+ per Fe3+ that could be split into MIL-53(Fe) at C/40. Except for cathode materials, MOFs may be used as anode materials for LIBs. Maiti et al. [16] synthesized a single solvothermal strategy for Mn-1,3,5-benzenetricarboxylate MOF. COO groups play a significant role in the insertion/extraction of Li+ in Mn-BTC MOFs.

Lin et al. [17], with hydrophobic and polar-functionalized MOFs (bimetallic metal organic frameworks (BMOFs)), showed excellent thermal and chemical stability. They have demonstrated that lithium ions are deposited primarily by pores in BMOF. It also contributed to the highly required capacities of nitrogen atoms in BMOF amine groups. The maximum ability of the BMOF could be further increased by the maximum surface area, the pore volume, and the current nitrogen-rich functionality content. The prevailing concentrations were the small particle size and the rapid movement of lithium-ion through extensive open skeleton passages. Most of the MOFs in air and moisture are unstable [18]. In the complicated electrical climate of LIBs, chemical stability in MOFs is suffering from tougher challenges. Thus MOFs are in high demand for thermal, chemical, and structural stability that is critical in practical applications for LIBs cycling performance. Besides, for practical capabilities and rate efficiency, the electrical conductivity of MOFs is critical. For LIBs, MOFs are thus desirable electrode materials with high electrical conductivity and stability.

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Applications

Gary M. Gladysz, Krishan K. Chawla, in Voids in Materials, 2015

7.4.1 Lithium Ion Battery

Lithium ion batteries are rechargeable batteries that are characterized by very high power densities. Such batteries have become very commonplace: from everyday electronic products such as cell phones to electric vehicles. What is not commonly appreciated is that voids play a very important role in such batteries. As this example will illustrate the void structure in a material, it does not always need to be spherical. Let us first briefly describe the main features of a lithium ion battery and then point out the important role of voids in it.

There are four components in a lithium ion cell: anode, cathode, separator, and the nonaqueous electrolyte. Different chemistries are used; the anode is graphite, the cathode is an oxide (LiCoO2), and the alternating layers of anode and cathode are separated by a porous polymer separator, which is generally made of polypropylene (PP), polyethylene (PE), or a laminate of PP and PE. In all cases a critical feature of the separator is a controlled amount and uniform size of porosity in the separator.

The electrolyte consists of an organic solvent and dissolved lithium salt, it provides the media for Li ion transport. Lithium ions move from the anode to the cathode during discharge and are intercalated into, i.e., are inserted into, open spaces in the voids in the cathode. The Li ions make the reverse journey during charging. A lithium ion battery (or battery pack) is made from one or more individual cells packaged together with their associated protection electronics.

Cells are constructed by stacking alternating layers of electrodes such as in prismatic cells or by winding long strips of electrodes into a “jelly roll” configuration typical for cylindrical cells, see Fig. 7.14. Generally, cell form factors are classified as prismatic, cylindrical, and pouch cells (also known as polymer, soft-pack polymer, or lithium polymer).

Figure 7.14. Polypropylene separators of various widths.

(Courtesy of Celgard.)

A separator is nothing but a porous membrane that separates the anode and the cathode in a lithium ion battery. It allows flow of ionic charge carriers but prevents electrical contact between the electrodes (Arora and Zhang, 2004). All separators contain pores or voids. They can be made of nonwoven fibers (e.g., cotton, polyester, nylon, or glass); films of PE, PP; or laminates of PP and PE. Lithium based batteries use nonaqueous electrolytes because of the reactivity of lithium with water. Most of these batteries use porous membranes made of polyolefins. There are two processes of making separators: dry and wet. In the dry process, a polyolefin resin is melted, extruded into a film and annealed, and subjected to a controlled tensile stretching to form pores (∼40% by volume). Examples of the microstructure of such separators made by Celgard are shown in Fig. 7.15. Note the slitlike form of voids. The mechanical properties of the separator are obviously anisotropic. The wet process is an example of a χ-induced syneresis technique of introducing voids discussed in Chapter 5. In the wet process, a hydrocarbon liquid is mixed with a polyolefin resin, followed by heating, melting, and extrusion into a sheet. The sheet is oriented and the hydrocarbon liquid is extracted with a volatile solvent. Typically, a controlled amount of porosity (∼40% by volume) and submicrometer pore size are specified.

Figure 7.15. A characteristic of the Celgard material is that as stress on the material is increased there is a decrease in pore volume fraction.

This is illustrated in the micrographs where the stress state is (a) unstressed, (b) 5 MPa, (c) 10 MPa, and (d) 30 MPa (Peabody and Arnold, 2011).

(Courtesy of Celgard.)

It is worth mentioning that pore closure can occur with increasing applied compressive stress. Figure 7.15 shows the situation under different states of stress (a) unstressed, (b) 5 MPa, (c) 10 MPa, and (d) 30 MPa. With increasing applied stress, there is a decrease in the pore volume fraction (Peabody and Arnold, 2011).

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Battery Carbons

S. Flandrois, S. Revathi, in Reference Module in Materials Science and Materials Engineering, 2016

Abstract

The demand for rechargeable lithium ion battery has remarkable growth in portable devices and automotive applications. In such scenario, the development of high-performance lithium ion battery with high capacity retention, high coulombic efficiency, high energy density, and low cost becomes inevitable. To optimize the performance and to increase the electrical conductivity of batteries, graphite, and conductive carbon black are used as active material in positive or negative electrode of rechargeable lithium ion batteries. The structure of the carbon material has high influence on the reversibility of the lithium intercalation carbon. This article outlines the main achievements toward the comprehension and the improvement of carbon performances for lithium ion reversible absorption.

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Solid Waste Materials for Energy Storage Applications

Ali C. Zaman, ... Cem B. Üstündağ, in Reference Module in Materials Science and Materials Engineering, 2020

Solid Wastes for Lithium-ion Batteries

Lithium-ion batteries are the dominant energy storage device for portable electronics since their commercialization in 1991 by SONY due to the high energy density they provide. One of the waste types that can be utilized for Li-ion batteries is biomass waste (Long et al., 2017), which can be used as carbon material source but there are several other options. Meng et al. (2018) used spent Li-ion batteries and Vanadium bearing slag to produce LiNi1/3Co1/3Mn1/3O2 (NCM) -V2O5 cathodes. NH4VO3 was recovered from Vanadium bearing slag and mixed with NCM powder. After spray drying and sintering (350°C for 4 h) cathodes were obtained. They found a simple solution for the preparation of cathode materials: rather than employing complicated consecutive methods like washing with bases and acids; simple sintering, spray drying enabled production of recycled NCM cathodes. There is a tremendous supply and demand for Liquid-crystal displays (LCDs) and it generates a considerable amount of waste. Some parts such as indium tin oxide films or printed circuit boards are recycled. However, glass substrates are not completely recycled. Glass substrates are composed of SiO2 (~60 wt%), Al2O3,B2O3, CaO, and SrO (Xie et al., 2018). Kang et al. have chosen magnesiothermic reduction method (MR) to produce Si metal from LCD glass substrates (Kang et al., 2019). Consequently, the recycled Si anode in Li-ion batteries show 4083 and 3438 mA h g−1 initial capacity for the first and second cycles. Fang et al. used an industrial waste of AlSiFe powders for the production of silicon composite anode (Fang et al., 2019). They attribute the success of the anode to the surrounding electrochemically active Si nanoparticles with SiO2 network to compensate for the volume changes and enabling charge transfer and partly to nitrogen doped carbon formation on Si nanoparticle surface, enabling improved electronic conductivity. Increasing consumption of aquatic animals especially fish generates huge amounts of solid waste, which are discarded into the environment (Amaral Corrêa and França Holanda, 2019). Shan et al. utilized fish bones (tuna bones) as a source of fibrous carbon foam production to utilize the material as a capacitor-type Li-ion cathode material (Shan et al., 2018). The key aspect of the production method they employed is the exclusion of tedious templating, template removal and activation steps. When the materials are employed both as anode and cathode to form lithium-ion hybrid supercapacitors (LIHC), it delivered an energy density of 131 Wh kg−1 at a power density of 131 W kg−1. Eggshells were used as a source of coating material for Li-ion battery cathodes (Senthil et al., 2019). 92.6% capacity was acquired compared to 82.9% for bare cathode after 50 cycles. The success is because of the suppression of the corrosion of Ni-rich material by electrolyte. Yu et al. used Si waste of organosilane industry to produce Li-ion battery anodes (Yu et al., 2013). They found that more Si accounts for higher specific capacity, but more carbon provides better stability. The drawback of the material studied is the presented cycle stability is only for 20 cycles, which is much lower than required for commercial batteries.

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All-Solid-State Li Battery for Future Energy Technology

Masashi Kotobuki, ... Kiyoshi Kanamura, in Handbook of Advanced Ceramics (Second Edition), 2013

1 Introduction

Rechargeable lithium ion batteries have been widely used as a power source for portable electronic devices such as mobile phones, laptop computers, and digital cameras because of their high operation voltage, high energy density, and no memory effect [1]. Recently, their application to electric vehicles has been accelerated; however, the safety reason caused by flammable organic liquid electrolyte is preventing them from the application. All-solid-state battery composed of nonflammable solid electrodes and solid electrolyte has been recognized as a battery with ultimate safety [2]. Hence, both polymer and ceramic solid electrolytes have been studied [3–5]. Particularly, ceramic electrolytes have been given much attention because of their high durability against high temperature. Among them, Li0.35La0.55TiO3 (LLT) with perovskite structure [6,7] and Li1+xAlxTi2−x (PO4)3 (LATP) with NASICON-type structure [8–12] have been given much attention owing to their high Li ion conductivity of approximately 10−3–10−4 S cm−1, which is acceptable for practical all-solid-state battery.

One of the critical issues of the all-solid-state battery is poor contact between solid electrode and solid electrolyte compared with the present liquid electrolyte (Figure 1). This poor contact leads to high internal resistance, resulting in low performance [13]. Hence, 3-dimensional (3D) battery has been considered to improve the performance [14–16]. Several prospective configurations of 3D batteries have been suggested (Figure 2) [17]. All of these configurations can provide not only large contact between solid electrode and solid electrolyte, but also high loading of active materials and short Li ion transport distance, which improves energy density and leads to high power density, respectively.

FIGURE 1. Illustration of electrode–electrolyte contact of (a) liquid electrolyte and (b) solid electrolyte.

FIGURE 2. Prospective configurations of 3D batteries. (a) Array of interdigitated cylindrical cathodes and anode, (b) Interdigitated plate array of cathodes and anode, (c) rod array of cylindrical anodes coated with a thin layer of electrolyte with remaining free volume filled with the cathode material, and (d) sponge-type battery composed of sponge cathode which is coated with thin electrolyte and the remaining free space is filled with anode.

In this section, the 3D all-solid-state Li batteries with honeycomb-type and 3-dimensionally ordered macroporous (3DOM) structures are described.

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Ionic Liquids Batteries

Qaisar Abbas, Michael R.C. Hunt, in Reference Module in Materials Science and Materials Engineering, 2020

Lithium Based Batteries

Lithium/lithium-ion batteries have become devices of choice for electrical energy storage (EES) over past 29 years after its commercialization by Sony corporation in 1991 (Nazri and Pistoia, 2008), due to their outstanding performance features, well developed/mature technology, long cycle life, high efficiencies and wide spread commercialization. Most important of these properties is their high energy densities (EDs) where metallic lithium can reach exceptionally high energy densities of up to 3860 mAh g−1 (Wakhihara and Yamamoto, 1998; Vincent and Scrosati, 1997) however, due to instability of electrode/electrolyte interface for metallic lithium battery systems, resulting in the formation of dendrites which can lead to short cuts. This has encouraged researchers to investigate and adopt carbon based cathodes which can intercalate lithium with the formation of the C6Li compound with energy desensitize reaching more moderate value of 372 mAh g−1, still superior than many other battery systems (Luan et al., 2019). Whereas SnOx or SiX based compounds are commonly used as anodes (Liu et al., 2019; Kim et al., 2019). Third and most important element of lithium-ion (Li-ion) battery like any battery system is the electrolyte that works not only as bridge between cathode and anode but can also influence the device’s performance. Although, electrodes are used for energy storage however they work in combination with Li conducting electrolytes where organic solvents such as cyclic carbonates with lithium solids (LiPF6 or LiN(CF3SO2)2) dissolved inside are most frequently used as electrolytes. However, these organic solutions contain VOCs due to the flammable nature of these compounds these can result in thermal volatility (Mishra et al., 2020). ILs particularly RTILs have seen surge in research activity for their use as an electrolyte in electrochemical batteries in last decade due to the absence of VOCs and reasonably high ionic conductivities.

Due to exceptionally high operating potentials, ionic liquid based Li-ion batteries can outperform any other battery systems. A recent study by T. Evans et al., has shown that remarkably high energy density of 542 Wh kg−1 normalized to cathode composite mass was achieved where bis(trifluoromethanesulfonyl)imide (TFSI−) anion‐based room temperature ionic liquid (RTIL) was used as an electrolyte (Evans et al., 2014). Energy densities for conventional battery cell operating using organic electrolyte can suffer from inferior energy densities. Energy density of 78 Wh kg−1 was attained at relatively high discharge voltage of 1.7 V when water-in-salt” (21 m LiTFSI in H2O) electrolyte was used in Li-ion battery system which is nearly six fold lower than previous study where RTIL was used as an electrolyte (Sun et al., 2017). Electrochemical batteries suffer from lower power densities (PDs) even though; higher energy densities are achievable using ILs. Recently, research in improving ionic conductivity of ionic liquids has seen a substantial increase, which is producing in positive results for high ionic conductivities. Ionic conductivity of 10−3 S cm−1 was attained at operating temperature and potential of – 20°C and 5V respectively where 0.1LiTFSI-0.3PYR13TFSI-0.6PYR13FSI ionic liquid electrolyte was used (Moreno et al., 2016) which is still inferior than organic solutions and require further investigation.

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Inactive materials

John T. Warner, in Lithium-Ion Battery Chemistries, 2019

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 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).

Fig. 77. Representative LiPF6 molecule.

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 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).

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URL: https://www.sciencedirect.com/science/article/pii/B9780128147788000077