A Review of Cathode and Anode Materials for Lithium-Ion Batteries

Yemeserach Mekonnen Aditya Sundararajan Arif I. Sarwat IEEE Student Member IEEE Student Member IEEE Member Department of Electrical & Department of Electrical & Department of Electrical & Computer Engineering Computer Engineering Computer Engineering Florida International University Florida International University Florida International University Email: [email protected] Email: [email protected] Email: [email protected]

Abstract—Lithium ion batteries are one of the most technologies such as plug-in HEVs. For greater application use, commercially sought after energy storages today. Their batteries are usually expensive and heavy. Li-ion and Li- based application widely spans from Electric Vehicle (EV) to portable batteries show promising advantages in creating smaller, devices. Their lightness and high energy density makes them lighter and cheaper battery storage for such high-end commercially viable. More research is being conducted to better applications [18]. As a result, these batteries are widely used in select the materials for the anode and cathode parts of Lithium (Li) ion cell. This paper presents a comprehensive review of the common consumer electronics and account for higher sale existing and potential developments in the materials used for the worldwide [2]. Lithium, as the most electropositive element making of the best cathodes, anodes and electrolytes for the Li- and the lightest metal, is a unique element for the design of ion batteries such that maximum efficiency can be tapped. higher density energy storage systems. The discovery of Observed challenges in selecting the right set of materials is also different inorganic compounds that react with alkali metals in a described in detail. This paper also provides a brief history of reversible way has opened doors to the design of rechargeable battery technology and their wide applicability in the energy Li-ion batteries [15]. This phenomenon, as defined later, is market today, the chemistry and principle of operation behind called intercalation, which is the reversible inclusion of the batteries, and their potential applications even beyond the molecules between two other molecules [2]. energy sector. Safety concerns related to Li-ion batteries have also been taken into account considering recent events.

Index Terms—Cathode, Anode, Graphite, Lithium ion, Battery, Safety

I. INTRODUCTION Lithium-ion batteries are used in different technologies such as the Hybrid Electric Vehicles (HEV), which use both battery as well as electric motor engines to increase the fuel efficiency [1]. A battery is essentially many electrochemical cells connected in series or parallel to provide voltage and capacity. Each cell contains a positive (cathode) and negative (anode) electrode divided by an electrolytic solution, simply called as an electrolyte, with dissociated salt that allows ion transfer between electrodes [2]. When these electrodes are connected to an external source, electrons are released as a result of chemical reaction and therefore for current to be tapped [25]. The electrical energy that a battery is able to give Figure 1 Energy density of different batteries [1] is a function of both the cell and its capacity which are dependent on the chemistry of the battery. For the purpose of application, Nickel Metal Hydride (Ni-MH) is the common II. CHEMISTRY battery technology currently being used [1]. However, different The materials involved in Li-ion batteries consist of carbon research efforts have proven that Lithium ion (Li-ion) which is porous in nature, usually graphite, as the anode, and chemistry has twice the power efficiency and density of Ni- metal oxide for the cathode [15][24]. Like most battery MH. Out of the common batteries used in various applications, technologies, the working principle of Li-ion batteries involves lead acid, Nickel Cadmium (Ni-Cd), Nickel Metal Hydroxide Lithium stored in the anode terminal that is transported to the (Ni-MH), and Li-ion batteries have higher energy density, as cathode terminal by an electrolyte [2]. Some of the most shown in Fig.1. These advances are reshaping the current common cathode components are Lithium Nickel, Manganese

This material is based on work supported by the National Science Foundation under Grant No. 1541108. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

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Cobalt Oxide, Nickel Oxide, Cobalt Oxide, Manganese spinel, flexibility and shape versatility when compared to the Iron Phosphate, and Titanate. Among them, Lithium Nickel cylindrical, prismatic or coin cell geometries. and Manganese Cobalt Oxide have a higher energy density and cell voltage. The electrolytic solution is lithium salt in organic carbonate solvent containing “lithiated” ions [25]. The operating principle behind Li-ion batteries is a recurring transmission of lithium ions between the anode and the cathode [11][12]. During the discharge process, solid state Li disperses to the surface of the anode material to undergo an electrochemical reaction which enables it to transfer Li+ ion into the electrolytic solution [1]. The equilibrium equation for such a reaction with graphite as the cathode material is as follows:

+ The Li ion in turn passes through the electrolytic state through dispersion and ionic conduction to react with the anode Fig. 3 Different types of cell geometry a) cylindrical b) coin c) prismatic d) and change back to its solid state. The equilibrium reaction at pouch [4] cathode in this case with lithium metal oxide is presented as III. APPLICATIONS AND MARKET follows: Having a higher energy density when compared to other battery technologies, rechargeable Li-ion batteries are and will Lithium will be stored inside the cell until the battery is continue to control the market. By 2011, the rechargeable Li- later recharged. At times of high current discharge, there is a ion battery market reached an approximate $11 billion and has possibility that the cell can suddenly lose power depending on continued to grow [5]. These rechargeable batteries are utilized the Li concentration, if saturated or depleted at the electrolyte in market segments where high energy and power density surface. applications are favored. The future of smart grid will heavily consist of the Plug-in Electric Vehicles (PIEVs) as part of the smart home power systems [20] [28] [29]. The EV and the Plug-in Hybrid EVs (PHEVs) are perfect examples of such applications [20] [21]. After many significant research efforts, it is now plausible for consumers to use EVs such as Tesla model S, or PHEVs like Chevrolet Volt, all powered by Li-ion batteries [22]. The challenges for these market segments are the manufacturing cost, higher price of Li-ion batteries and safety concerns. A few of the other Li-ion applications are commercial portable technologies such as cellphones, laptops and tablets, aeronautics, and industrial energy power stations [23]. There are numerous advantages to Li-ion batteries. They are light weight which makes them the perfect candidates for the recently sought-after portable technologies. They have a high open circuit voltage and high energy density. They are characterized by lack of memory, small self-discharge rate, and less environmental impact when disposed. They, however, Fig.2 Schematics showing the working principles of a) Rechargeable Li-metal have their own challenges where recent cases of unprovoked battery, b) rechargeable Li-ion battery [1] inflammation have raised a constant safety concern [26]. The use of Li-ion batteries for the aforementioned There are different types Li-ion cell geometries according applications faces challenges. For one, the battery performance to the current manufacturing practices, namely the prismatic, has to be in tune with the applications it is used for. Safety, as cylindrical, coin, and the pouch cell geometry which is the mentioned, remains a concern. The battery performance, most recent method. Both the cylindrical and prismatic cells usually measured in capacity, energy density, and cell are commonly made of “laser-welded” aluminum can and potential, is directly related to the properties of the materials consist of liquid electrolyte. The pouch cell with aluminized which form the positive and negative electrodes [13]. Safety plastic bag contains Li-ion polymer electrolyte or gel. Bellcore concerns can be addressed through extensive studies of battery researchers were the first to advance their research on the chemistry, and cell engineering [17]. Present research is being polymeric electrolyte called “plastic Li-ion (PLiON)” [2]. This conducted in finding new materials which can act as anode and thin film battery technology gives the advantage of lightness, cathode, to offer better performance arrangements of electrode- electrolyte-electrode [2]. In addition, finding the right

electrolyte combination to avoid damaging reactions associated acceptance of Li, flexibility to temperature control as result of with “electrode-electrolyte interface” is another challenge that its organic structure, and optimal cycling ability [4]. Through is currently being researched about [19]. structural and surface modifications, carbonaceous anodes have shown consistent improvements in their charge-discharge IV. ANODE MATERIALS efficiency and discharge capacity. There have been new Currently, the two most commonly used anode materials developments where artificial graphite has been designed by are those based on carbon (graphite) and lithium alloyed Hitachi by altering pore and particle structures [4]. metals. One of the commercialized lithium alloyed metal is the oxide spinel Li4Ti5O12 the structure of which is shown in Fig.4.

Fig.5. Structures of common electrode material [4] B. Novel graphite and non-graphitic anodes

A lot of advances are being made using altered natural

graphite and other graphitic carbons such as “kish” graphite

[4]. Recent studies have shown that the electrochemical characteristics are better improved on modified graphite as a result of the oxidation of natural graphite in air. Aritifcal Fig.4. The basic chemical structure of Li-ion batteries [17] development to graphitic anodes requires heat treatment at temperatures starting 3000 C, which requires higher energy To avoid issues in the cycling and safety which are and might lead to the production of gaseous materials. Kish associated with dendrite formation on lithium anodes, it is graphite shows Li intercalation capacities well above the set advisable to use the minimal potential intercalation electrode theoretical value equal to 372mAh/g [7]. In addition, the [5]. The element found in the graphite intercalation alloy production of Kish graphite is cheap and can be done at a protects the inserted lithium, making it less reactive towards lower temperature of 1500 C. electrolytes. This will make the amount of Lithium in the Non-graphitic carbons are those having graphene domains but lithiated material less, which comes with both advantage and do not possess the structural order exhibited by graphene. disadvantage. The advantage is that it accounts for any safety They are also known as disordered carbons. Although their concerns regarding the flammability of the electrochemical irreversible capacity does not compare with natural grapheme, reactions. The disadvantage is manifested in the form of loss in these materials are less vulnerable in solid electrolyte interface performance owing to a reduction in the cell voltage, [4] which disruption [5]. This makes them the perfect materials to be further reduces the energy density and power. paired with Li-Manganese Oxide where the dissolution of A. Carbonaceous (carbon-based) anodes metal is challenging. As discussed earlier in this paper, one of the primary C. Lithium alloy anodes carbon materials used as anode is graphite. They consist of Among the top studied Li-alloy anodes, Li-Al (lithium sheets packed in hexagonal (AB) or rhombohedral (ABC) [5] aluminum) is the first to be developed as the anode for Li-ion arrangements as referenced in Fig.4. When lithium ion is batteries. Challenges related to cycling can be improved by inserted, these graphene sheets rearrange themselves on top of introducing substances such as Di-Lithium Phthalocyanine each other in AA arrangement and “staging” occurs. Its which changes the anode surface film. One other challenge for minimal cost, accessibility and favorable electrochemical this material is the volumetric change during lithiation and properties form the pros; carbon is the key anode material in dilithiation processes. However, this problem can be solved by Li-ion batteries. However, compared to Li-ion alloys, graphite the use of “dimensionally stable” anodes. This can be done by carbon has poor lithium intercalation capacity. Graphitic the utilizing a submicron particle alloy which is surrounded by carbons are used an anode material in the frontier of a stabilizing matrix and “intermetallic” host where one metal commercial Li-ion cells, mainly portable devices. Crystalline alloys with Li but the others do not [4]. Group of metals that carbon has also been claiming prominence due to its higher alloy with Li are Al, Bi, Cd, Mg, Sn, and Sb, whereas those

that do not are Co, Cu, Fe, and Ni. Some of these intermetallic elements that have shown promising result as anode materials are Al3Ni, Fe-Sn, Sn-Sb and Sn-Cu [8]. Lithium titanium oxide is another material that has been branded as an alternative to carbon anodes. This material cycle well since it does not exhibit any volumetric changes during Li insertion and extraction process unlike most other intercalation electrodes [6]. Its usage is limited to applications that do not require a high energy density as a result of its high operating voltage. Due to its low conductivity, this material is recommended to be nanostructured.

V. CATHODE MATERIALS Fig. 6 Cubic spinel LiMn2O4 structures [9]

The cathode material in Li-ion battery chemistry is the major and active source of all the Li-ions [17]. The preference B. Lithium metal oxides of positive electrode materials depends on rechargeable Li- Lithiated nickel and cobalt oxides are the most in-depth metal or Li-ion batteries. The Li-metal, when used in studied cathode material for Li-ion batteries. Both are rechargeable Li batteries, the metal acts as a negative electrode, characterized by high structural stability. Limited resources can therefore the positive electrode does not need to be lithiated. In be a challenge for manufacturing making them costly and hard the case of Li-ion, because carbon electrode which acting as to synthesis. A resolution for this has been in the development the negative doesn’t have Li, the positive terminal must act as of solid solutions of these layered compounds. Li Ni0.5Mn0.5O2 source of Li; therefore intercalation compound is required for and Li1.2Cr0.4Mn0.4O2 are the most common solid solution the cell assembly [6]. The most common cathode materials are compounds [6]. A research performed shows that a combination LiCoO2, Li-Mn-O, LiFePO4 and lithium layered metal oxides of low-valent transition metal ions and low strain in the [5][17]. activated state is key to high rate capability cathodes [8]. A. Lithium Manganese spinels (Li-Mn-O) Layered metal oxides are perfect for applications requiring fast charging and discharging. These materials appear to be doing Li-Mn-O is one of the oldest compounds researched that well on capacity when subjected to temperatures above 300oC. dates back to centuries ago; it is still widely used. Its first use was depolarizer. It is easily accessible, has low cost and possesses desirable electrochemical properties. When compared to the high cost and toxic lithium cobalt based (Li- Co-O), and difficult to produce lithium-nickel based (Li-Ni-O), lithium manganese (Li-Mn) is the most widely used battery material. Its different forms make it ideal for the intercalation of small helium and lithium ions. The lambda form with its spinel (Mn2O4) allows for the intercalation of Li-ion [6]. Some of the advantages of Li-Mn spinels are high thermal threshold, great rate capability, and minimal health and environmental + -6 impacts. The diffusion rate for Li ion in this compound is 10 -10 -10 sq. cm/s [5]. Challenges arise in reduced capacity upon Fig.7 Layered lithium metal oxides structure [16] frequent cycling. This is due to the instability of the electrochemically active Mn3+ ion above 55o C temperatures. C. Olivines For such cases, improvements can be made by doping selected They are known by their compound name LiFePO4. They metal ions (Al, Co, Cr, Fe, Mg, Ni, Mg, etc.) and coating acid possess flat discharge plateau and moderate capacity ranging resistant materials on LiMn2O4 to obtain different structural from 150-160 mAh/g [6]. They are non-toxic and show little stability. capacity decline through the life of a battery. These compounds are characterized by smaller volume charges and charge and discharge heat flow when compared with other cathode materials. They offer significant safety advantage over Li- cobalt based cathode, which makes them favorable for higher level applications [23]. The transition metal iron (Fe) is cheap, readily available, and environmentally gentle. However, conductivity for these materials are poor and charging voltage drops below 4V [4]. Two methods have been proposed to improve this challenge. One involves the reduction of cathode particle and the second is to use nanocomposite of LiFePO4 with conductive carbon matrix [4][30]. This has opened doors

to the advances in synthesizing olivines with other transition considerable occupational health damage. As a result, all Li-ion metals. Although, it can be difficult to synthesize olivines with batteries used for high voltage application (over 60 V) must transition metals and can experience limited capacity, they follow the recommended protection standards such as terminals exhibit high discharge potential and high energy out [6]. and insulation fault controls, to avoid hazardous exposure to the battery [3]. There exist many safeguard tests as tabulated in [26], which are predominantly applied for EVs and are designed to mitigate or even prevent failures. However, thermal stability of active materials within the battery at high temperatures has been a constant concern. Thermal runaways triggered by internal short-circuiting of the batteries are a huge threat. Runaway temperature for Li-ion batteries is typically between 130C and well over 200C. Cathode materials which release oxygen at

high temperatures have known to possess high reaction rates as Fig.8 Olivines structure [14] well as enthalpies, which favors inflammation, short-circuiting and unprovoked combustion of the battery. A comprehensive VI. ELECTROLYTES risk assessment to appraise and evaluate the different failure For a rechargeable Li-ion battery, there are two types of modes through fault tree analysis must be conducted during the electrolyte technologies: polymer base and liquid electrolyte manufacturing process itself. [18]. A sustainable battery technology relies on good Another ensemble of safety evaluation techniques are electrolyte comprising the salt and solvent combination. provided in [27]. Cyclic Voltammetry is used to evaluate the Polymer-based electrolytes add further selection criteria linked electrochemical stability of the battery constituents. to the electrochemical stability of polymer. These become a Differential Scanning Calorimetry (DSC) is an analytic challenge since there are only a few Li-based salts or polymers technique that assesses the implications of thermal abuse on to achieve high ion conduction, Polyethylene Oxide (PEO) batteries by measuring the thermal response of selective being the common one. For liquid electrolytes, there are combinations of cell components over a wide range of different solvents with specific dielectric and viscosity temperatures, maintaining the scanning at a fixed temperature constants that can be selected to achieve higher ionic rate. The thermal stability of cells can be further quantified and conduction. However, there are challenges in both analyzed using Thermal Ramp Testing, where the battery is technologies. In liquid electrolyte, the ion conduction of the heated in a linearly programmed fashion until it fails. These electrolyte is “field-trial” process, guided by the concepts of and many other tests help us understanding the significance of dielectric and viscosity constants [3]. In case of the polymer adherence to safety regulations and standards prescribed for Li- electrolyte, achieving high ionic conduction in Li-based ion batteries. polymer entails an in-depth understanding of ionic dissociation VIII. CONCLUSION and transport. This paper conducted a comprehensive review on the VII. SAFETY evolution of battery technology, the various cathode and anode Lithium-ion battery hazards, as any other battery materials widely employed, and their pros and cons associated technology, are associated with electrical and chemical risks. with the corresponding applications they are deployed in. The The different risks associated with Li-ion batteries are chemical electrochemical reactions behind the battery technologies were and electrical hazards, cumulative effects (both chemical and also elaborated, backed by safety concerns with regard to the electrical) and high voltage hazard, and hazards due to the loss batteries today, were also described and elaborated. There is a of a function of the battery. Chemical hazards stem from any huge demand for lighter, space efficient, and high capacity chemicals used in the battery [8]. The hazard could be as a batteries. This demand will continue to steadily increase with result of spillage or flammable tendencies of substances. technology maturation. Li-ion batteries are most highly Electrical hazard of Li-ion batteries is associated with electrical researched and the future energy storage for higher application energy content based on the state of charge. During high especially in EVs and PHEVs. Through extensive material discharge and charge processes, the heat dissipated by electric research and design, there should be an improvement in the current should be properly thermally managed. Unwanted energy density for Li-ion batteries. Future developments in the exothermic reactions are prone to occur as a result of the Nano approach such as the carbon nanotube anodes, silicon overcharge and discharge of the battery [10]. When batteries anodes and nanoparticles that can be used as cathode are are subjected cyclical discharge and charge, the increase in promising advancements to the future of Li-ion based batteries. battery temperature is accelerated, creating a chemical instability of the battery materials. For such a hazard, it is generally recommended to have electronic protection based on the voltage thresholds [2]. In the past, Li-ion batteries used for large industrial application for over 60V have presented

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