Materials for Electrodes of Li-Ion Batteries: Issues

Critical Reviews in Solid State and Materials Sciences

ISSN: 1040-8436 (Print) 1547-6561 (Online) Journal homepage: http://www.tandfonline.com/loi/bsms20

Materials for Electrodes of Li-Ion Batteries: Issues Related to Stress Development Joyita Banerjee & Kingshuk Dutta To cite this article: Joyita Banerjee & Kingshuk Dutta (2017) Materials for Electrodes of Li-Ion Batteries: Issues Related to Stress Development, Critical Reviews in Solid State and Materials Sciences, 42:3, 218-238, DOI: 10.1080/10408436.2016.1173011 To link to this article: http://dx.doi.org/10.1080/10408436.2016.1173011

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CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 2016, VOL. 42, NO. 3, 218–238 http://dx.doi.org/10.1080/10408436.2016.1173011

Materials for Electrodes of Li-Ion Batteries: Issues Related to Stress Development Joyita Banerjee and Kingshuk Dutta Department of Polymer Science and Technology, University of Calcutta, Kolkata, India

ABSTRACT

KEYWORDS

Presently, rechargeable Li-ion batteries, possessing highest energy densities among all batteries, are used in a major fraction of all portable electronic devices. However, for bestowing the Li-ion batteries suitable for such advanced applications, further improvements in the energy densities (Li-capacities) and in the cycle life are essential. In a broader sense, this can be achieved by replacing the presently used electrode materials by materials possessing higher Li-capacities and minimization of the degradation of such materials with electrochemical cycling. It has been realized that the major reason for degradation in battery performance in terms of capacity with cycling is the disintegration/fragmentation of the active electrode materials due to stresses generated during Li-intercalation/de-intercalation in every cycle. Such stresses arise from the reversible volume changes of the active electrode materials during Liinsertion and removal. In quest of higher energy densities, replacement of the presently used graphitic carbon by potentially higher capacity metallic anode materials (like Si, Sn, and Al) is likely to further accrue this stress related disintegration due to »30 times higher volume changes experienced by such materials. It has also been recently realized that passivating layer formed on the surface of the electrodes also contributes toward the stress development. After briefly introducing the mechanistic aspects of Li-ion batteries, this article focuses on the reasons and consequences associated with stress developments in different electrode materials, highlighting the various strategies, in terms of designing new electrode compositions or reducing the microstructural scale, that are being presently adopted to address the stress-related issues. Considering that experimental determination of such stresses is essential toward further progress in Li-ion battery research, this article introduces a recently reported technique developed for real-time measurement of such stresses. It finally concludes by raising some critical issues that need to be resolved through further research in this area.

Li-ion battery; electrode; storage device; SEI; MOSS

Table of Contents 1. Introduction ........................................................................................................................................................................................... 218 2. Basics of lithium-ion batteries........................................................................................................................................................... 219 2.1. Various aspects of cathode materials in li-ion batteries......................................................................................................... 220 2.2. Various aspects of anodes materials in li-ion batteries .......................................................................................................... 223 2.3. Electrolytes used in li-ion batteries ............................................................................................................................................ 225 3. Stress development in electrode materials – issues and perspectives...................................................................................... 227 4. Possibilty of measurement of stress development by in situ measurement during cycling .............................................. 229 5. Conclusion and future perspectives ................................................................................................................................................. 232 Funding.................................................................................................................................................................................................... 233 References................................................................................................................................................................................................ 233

1. Introduction Most of the countries of the world are now interested in renewable sources of energy because of scarcity in fossil

fuel and other non-renewable energy sources.1–4 In recent years it has been realized that energy storage is an important essentiality for the future, and thus, various

CONTACT Kingshuk Dutta [email protected] Color versions of one or more of the figures in this article can be found online at www.tandfonline.com/bsms. © 2016 Taylor & Francis Group, LLC

CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES

energy storage devices are getting demonstrated.5,6 In the present era it is much more important to store energy than it was in the early days. Among such demonstrated devices for energy storage, the battery is one of the most important. The Nickel Cadmium (NiCd) battery has for long served as the only suitable battery for portable applications, including mobile computing and wireless communications. However, this particular category of batteries possesses an undesirable “memory” effect, which is caused by partial discharges. The possible wayout from this effect is to perform a complete discharging of the battery before charging it again. Next to appear on the scene were the Nickel Metal Hydride (NiMH) and the Lithium-ion (Li-ion) batteries. Emerging in 1990– 1991,7 these batteries offered higher capacities compared to the NiCd batteries. This commercial appearance of the Li-ion batteries was preceded by several separate works, including those of Goodenough, Yazami, and others.8,9 However, it has been realized that the battery technology has failed to cope up with the rate at which the advancement of computer and electronics has taken place (Moore’s law). Demand for portable electronic devices directly influences the commercial market of Li-ion batteries. This market, in recent years, has witnessed explosive growth. However, Li metal suffers from poor rechargeability and associated problems, caused mostly owing to the formed dendrites and mossy Li metal deposits. These problems were eliminated by utilizing intercalated carbon/graphite anode, with only a minor voltage penalty of 0.05 mV. Moreover, the use of this particular anode resulted in greatly enhancing the safety aspects of the high-energy battery system. Intense effort has been going on toward development of new materials which will render both safety and enhancement of capacity of the batteries. Increase in the capacity of the battery can only be realized through a cumulative outcome of (a) engineering improvements in the manufacturing processes and (b) introduction of new materials toward the fabrication of the separator, the cathode and the anode. For example, in order to stabilize the crystal structure and to enhance the capacity, additives have been added to the original lithium cobalt oxide. Similarly, researchers have been active in identifying new forms of carbon-based materials to replace the originally used hard carbon anode. As a result, graphites have been identified as a possible alternative, possessing a practical capacity of up to 350 mAhg¡1. Again, alternative forms based on surfacetreated mesophase pitch precursor carbon fibers exhibited capacities higher than 350 mAhg¡1. Similarly, alternate anode materials that have been developed include lithium titanium oxide and nanostructured stabilized tin alloys. On the other hand, new cathode materials include

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lithium cobalt-nickel oxide, lithium manganese oxides, and olivine-structured lithium iron phosphates. Moreover, fabrication of batteries based on polymer electrolytes has resulted in generation of greater Whkg¡1. Again, in order to reduce the first cycle loss associated with the solid electrolyte interphase (SEI) layer that gets formed on the electrode surfaces during cell operation, development of additives has taken place. Additives have also been designed to enhance adhesion to the current collectors and to increase the longevity of cycle and storage lives. As has been mentioned above, constraints on availability of fossil fuel have induced government and people to demanding for alternative sources of power. Over the last decade, Li-ion batteries have been successfully used in portable devices like laptop computers, mobile phones, etc. In the present scenario, with respect to depletion of fossil fuels, it is imperative to utilize the high energy densities of Li-ion batteries for more advanced and heavy duty applications, like in automobiles. Other advantages possessed by Li-ion batteries over the previously developed battery types are: (a) they are lighter, (b) they hold charge longer, and (c) they are easy to design and fabricate. Nevertheless, due to intercalation and de-intercalation Li-ion batteries exhibit capacity fade—a prime drawback for these types of batteries. Therefore, an extensive research is going on to improve upon the capacity fade, which is resulting in increasing the cycle life of the Li-ion battery. In this article, a clear description of the development of Li-ion battery has been depicted. The pros and cons of several methods which are being used to improve the performance of the battery have been discussed. A detail review has been done regarding the stress development issues within the electrode. So far, researchers have provided glimpses of the work they have done specifically either in the electrode or in the electrolyte development. But in this article an exhaustive literature survey has been done and presented in a compact model.

2. Basics of lithium-ion batteries An illustration of the operation of a typical Li-ion cell has been presented in Figure 1.10 Depending in whether the cell is in a charged or discharged state, its operation takes place via intercalation and de-intercalation of Liions into the anode or the cathode, respectively. The net effect of charging and discharging is dependent upon the transport of Li-ions through the electrolyte, exchanging between the two electrodes, and getting manifested by the flow of electrons in the external circuits. This phenomenon of cell operation has been variously termed, viz. a “swing”, a “rocking chair”, or “Li-ion” concept. As

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J. BANERJEE AND K. DUTTA

Figure 2. Schematic representation of the working principle of rechargeable Li-ion battery. (© Nature Publishing Group. Reprinted with permission from Tarascon and Armand.12 Permission to reuse must be obtained from the rightsholder.) Figure 1. Depiction of charge-discharge operation of Li-ion cells. (© The Authors. Reprinted with permission from Goriparti et al.10 Permission to reuse must be obtained from the rightsholder.)

can be seen from the figure, there exist no lithium metal within the cell but only LiC ions. The electrolyte present is a mixture of alkyl carbonate solvents, while lithium hexafluoro phosphate salt provides for the conductivity. The fact that Li metal exhibit the most electropositive character (i.e., ¡3.04 V vs. standard hydrogen electrode) coupled with its lightest attribute (i.e., an equivalent weight (M) of 46.94 gmol¡1 and a specific gravity (r) of 40.53 gcm¡3) has rendered the initial motivation toward development of a battery technology based on Li metal as the anode. This, in turn, has facilitated designing of high energy density storage systems. These advantage associated with the use of Li metal was first demonstrated as early as in the 1970 s with the fabrication and operation of primary Li cells.11 Subsequently, the demand of Li-ion batteries increased owing to their high power density, long cycle life, volumetric energy and self-discharge properties. 12–16 In 1972, Exxon commercialized Li-ion battery utilizing positive electrode made of TiS2,8,17 negative electrode made of Li metal and electrolyte composed of lithium perchlorate in dioxolane. In those times, TiS2 served as the best intercalation compound, since it possessed a very favorable layered-type structure. The obtained results got published in huge range of publications and have attracted a wide range of audience. However, in spite of the high standard of performance of the positive electrode, the overall system was not found to be viable.18,19 This is because of the uneven growth of Li (dendritic growth) that took place in each subsequent discharge-recharge cycle. Figure 2 shows the dendritic growth of Li on the surface of the electrode. It was concluded that for such advanced applications, the energy densities of the batteries need to be further enhanced, which can be achieved with using electrodes possessing higher Licapacities. It is to be mentioned here that the limitation toward usage of the abovementioned electrode is not only

due to the dendritic growth of Li in liquid electrolytes, but also owing to the possibility of potential explosion. These drawbacks have motivated the replacement of Li metal with other prospective materials containing Li-ions in their unit cells, such as LiCoO2, LiMn2O4, and LiFePO4. It was later found that TiS2 holds potential compatibility in a thin film solid state construction. Trevey et al. investigated the effect of the reduced size of TiS2, fabricated by using high energy ball milling, on the performance of rechargeable batteries.20 For this purpose, high-power all-solid-state batteries, composed of nanosized TiS2, were developed. These batteries exhibited high power density of >1000 Wkg¡1 maintained for over 50 cycles, with a maximum power density value of 1400 Wkg¡1. 2.1. Various aspects of cathode materials in li-ion batteries It has been demonstrated that potential anode materials, such as Si, Al, and Sn, possess higher capacity with respect to presently used graphite by up to a factor of 10. Furthermore, the cathode materials like LiCoO2, which are presently used to half its theoretical capacities, need to be utilized to full capacity. Not only the capacities, but also the cycle lives of the batteries need to be improved, which can be achieved by minimization of the degradation of the electrode materials (and electrolyte) with electrochemical cycling. Another issue is rate capability, which can be improved by improving the Li-insertion/ removal kinetics to and from the electrode materials. It must be mentioned here that the atomic radius of Liatom is 0.205 nm, which is considered to be higher than ordinary metals like Sn, Sb, and Ni. So incorporation of Li-ion leads to change in the density, as well as, the size of the atom that engulfs it. This generates stress inside the electrode, which leads to subsequent capacity fading and low cycle life. The charging-discharging process in the battery generates intercalation induced to be developed over the oxide, which in turn affects the lithium


transport through LiCoO2. The electrochemical quartz microbalance (EQCM) technique has been effectively utilized to study the electrochemical processes in the intercalation electrode. This stress developed within the electrode can be estimated quantitatively by beam optical stress sensor (MOSS). Formation of passivating layer of the electrode surface is the central issue in rechargeable Li-ion batteries. This layer gets formed due to reduction/ decomposition of the electrolyte. This SEI layer is considered to be one of the reasons for stress generation within the electrode. In order to enhance the performance of the Li-ion battery, further work on the development and modification of the electrodes is essential. For example, LiCoO2 possesses a rock-salt structure composed of closelypacked network of oxygen atoms, where LiC and Co3C are arranged in alternative plane of (111) in the structure. Now, removal of lithium from LiCoO2 results in the oxidation of Co3C to Co4C (an unstable oxidation state of Co). Large concentration of Co3C destroys the electrode. It undergoes a transition to hexagonal phase to monoclinic phase,21 accompanying with a large decrease in c-axis dimension up to 1.7%. This non-uniform dimensional change leads to differential stress development within the system, which results in destruction of the electrode.22 The cathode reaction can be presented as: LiCoO2 ! Li1 ¡ x CoO2 C xLi C C xe ¡

(1)

Hence, different types of spinel, viz. LiFePO4, LiMn2O4, and LiNO2, have been analyzed as the cathode

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material (Figure 3). However, they have not been commercialized either due to thermal instability or due to lower efficiency compared to LiCoO2. Dahn et al. suggested the replacement of LiCoO2 by LiNiO2, but the possibility was dismissed due to safety reasons.23 The de-lithiated structure of LiNiO2 was not thermally more stable compared to LiNiO2 due to exothermic oxidation of the de-lithiated structure. Rougier et al. suggested that the main problem of LiNiO2 lies in the existence of over stoichiometric phase due to excess of Ni present.24 The presence of these extra Ni-ions results in a low electrochemical performance. Another approach has been adopted to order to improve the cathode performance, i.e., utilization of the soft chemistry of LiFeO2 and LiMnO2 via redox reaction. The advantages of using the newly developed cathode over LiCoO2 is that it has a lower cost, greener in nature and is abundantly found in nature. Armstrong et al. suggested that the structural instability associated with reversing of the layered phase to the spinel LixMn2O4 upon cycling can be minimized through substitution of the cation by chromium (Li1CxMn0.5Cr0.5O2).25 The 3 D spinel material LiMn2O4 is a challenging material, whose performance was found to get improved by several ways. Thackeray et al. proposed the use of LiMn2O4 due to the following four advantages: (a) a potential lower cost of Mn over that of Ni or Co, (b) a larger thermal stability domain especially at overcharged conditions, (c) a higher discharge voltage useful for telecom appliance, and (d) more environmental friendly nature. Nevertheless, it is still inferior to other cathode systems owing to lower capacity, lower power and lower density. In the early

Figure 3. Crystal structure of three spinel Li-insertion compounds used as cathode. (© The Authors. Reprinted with permission from Julien et al.29 Permission to reuse must be obtained from the rightsholder.)

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19900 s, extensive research was performed in order to improve the performance of the spinel. The primary reason behind the phenomenon of capacity fading is mainly attributed to the Jahn-Teller effect26 and manganese dissolution.27 Transmission electron microscope (TEM) image of nanostructured LiMn2O4 spinel obtained on cycling layered LiMnO2 and a schematic representation of the nanodomain structure of LiMn2O4 spinel are presented in Figure 4. Padhi et al. focused on the development in phosphate materials. For instance, LiFePO4 was found to be used at 90% of its theoretical capacity (i.e., 165 mAhg¡1) with acceptable rate capabilities.21,28,29 However, LiFePO4 delithiated into FePO4 as the Fe2Cgets oxidized to Fe3C. Usually, the electrical conductivity of LiFePO4 is quite low (i.e., in the order of 10¡9 Scm¡1), and it was observed that the charge transport is dominated by the polaron hopping mechanism.30 Other phosphates which have been used as anode materials are LiCoPO4, LiMnPO4, and LiNiPO4. Both LiCoPO4 (4.8V)31,32 and LiMnPO4 (4.1V)33,34 have higher open circuit voltages as compared to LiFePO4; however, they have lower capacity as compared to LiFePO4. It was reported that a mixture of LiMnPO435 or LiCoPO4 with LiFePO436 can also be used as the cathode of Li batteries. It has been observed that the capacity of Li batteries increases with the Fecontent,37 whereas the operating voltage increases with the Mn-content.38 Researchers also reported the use of composite cathode materials for Li batteries.39–41 It was found that addition of LiCoO242,43 or other electrodes to LiFePO4 may lead to formation of a new electrode with

improved capacity retention after several cycles. Sahana et al. reported the use of V2O5 as a cathode material for Li batteries.44 Vanadium may exist in several forms; however, the orthorhombic V2O545–48 and the monoclinic LiV3O849,50 have been used as cathode materials with high capacities. V2O5 possesses high potential as a Li-intercalation material for fabrication of cathodes; however, it suffers from limited use in commercial Liion batteries. Performance of cathodes composed of V2O5 depends on its morphology and crystallinity. For example, V2O5 in its crystalline form enjoys high specific capacity. However, the main disadvantage of using crystalline V2O5 is that it suffers from low average potentials, which results in a decrease in the output energy density value compared to other prospective cathode materials. This drawback arises due to the following two reasons: (a) successive deep intercalaction/de-intercalation charge-discharge cycles reduce the Li-intercalation capacity of crystalline V2O5 and (b) low LiC diffusion coefficient and electrical conductivity of crystalline V2O5.44 Increased resistance to movement of electrical charge, caused by mechanical stress-induced structural distortions upon trapping of ions, is seen as the reason behind the observed decrease in Li-intercalation capacity of V2O5 over successive charge-discharge cycles.44,45 These structural distortions are caused by change in volume due to electrochemical Li-intercalaction/de-intercalaction, transport of solvent in and out of the material during redox cycling and redox transition induced change of the coordination geometry at the metal center.45 This, in turn, leads to loss of long-term

Figure 4. (a) TEM image of nanostructured LiMn2O4 spinel obtained on cycling layered LiMnO2. (b) A schematic representation of the nanodomain structure of LiMn2O4 spinel derived from layered LiMnO2. (© Wiley-VCH Verlag GmbH & Co. KGaA. Reprinted with permission from Bruce et al.27 Permission to reuse must be obtained from the rightsholder.)


dimensional stability, available conductive pathways, charge storage capacity and charge extraction rate.45 As a result, both the specific capacity and the specific energy get decreased. However, use of amorphous V2O5 can solve these drawbacks as it allows faster diffusion rate, higher ionic conductivity and significant cyclability owing to easier absorption of Li-intercalation/de-intercalation induced mechanical stress by small crystallites having a high surface area.47 LiMn2O4 was found to exhibit a Li-ion diffusivity of 10¡8 cm2s¡1 to 10¡11 cm2s¡1 in its solid state. This low diffusion value was attributed to the availability of only fraction of the bulk Li-ion for insertion/extraction at high charge-discharge rates, as well as, to the instability of the de-lithiated phase of LiMn2O4 in organic electrolytes.51 Several researchers investigated the effect of deposition of nanosized Si on LiFePO4 electrodes, deposited by employing ultrasonic spray technique, on the capacity of the electrodes.52 It was found that the deposition of nanosized Si leads to protection of the active material from further side reaction(s), and thus, results in increasing the stability of the battery. Recently, a wide number of studies have been conducted using LiAlO2coated Li1.2Ni0.2Mn0.6O2 electrode as a cathode for Li-ion batteries. This electrode exhibited excellent improvement in the electrochemical performance and the cycling capabilities at room temperature, as well as, at lower temperatures. It was found that the first discharge capacity of the coated material was lower than that of the uncoated Li1.2Ni0.2Mn0.6O2 (i.e., 240.4 mAhg¡1). Coating of LiAlO2 on Li1.2Ni0.2Mn0.6O2 was done using a two-step method, which involved pre-coating of Al2O3 on Ni0.25Mn0.75CO3 precursors followed by sintering with Li2CO3 at high temperature.53 Other complex materials, such as layered LiNi0.5Co0.45Fe0.05O2 compounds, have also produced extraordinary improvements in the performance of Li-ion batteries.54 LiFePO4 has also been studied as a cathode material. Recent studies have focused on Sm-doped LiFe1-xSmxPO4/C (x D 0, 0.02, 0.04, 0.06, and 0.08) composites, synthesized by sol-gel method.55 It was observed that among all the different types of composites studied, LiFe0.94Sm0.06PO4/C composite exhibited higher capacity and cycling performance compared to undoped LiFePO4/C composites. Attempts have also been made to synthesize micro nanostructured LiFePO4/C electrode as a cathode, prepared by solvothermal route using triethylamine.56 It was found that this as-synthesized LiFePO4/C composites exhibited open mesoporous star-like morphology, with a particle size of 10 mm and an average pore size of 3 nm. A specific capacity value of 157.5 mAhg¡1 at 1 C was produced by this novel cathode. Upon increasing the discharge rate to 20 C, this cathode still exhibited a high initial discharge capacity value of 86.7 mAhg¡1. Recently, it has been reported that addition of

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reduced graphene oxide (rGO) to metal-organic phosphate open framework (MOPOF) helped in improving the Li diffusivity and electrical conductivity of MOPOF at room temperature.57 2.2. Various aspects of anodes materials in li-ion batteries In order to improve the capacity of Li batteries, similar improvements have to be observed in case of the anode as well. In this respect, starting from alloy anodes and anode coated with tin or other metals are also documented. Apart from this, serious attention has also been given toward development of nanoparticulate materials for electrode development. It has been observed that metals and semiconductors, like Sn and Si, may react electrochemically with Li to form alloys by a partially reversible reaction. However, these anodes are usually brittle and undergo pulverization due to volume change during charging or discharging, thereby, reducing the number of cycles.58 Si is one of the most prospective candidate to be used as the anode (exhibiting low discharge potential and highest theoretical charge capacity of 4,200 mAhg¡1), with high charge capacity as compared to the existing graphite, nitride, and oxide materials. However, it has a large volume expansion of »400%, which results in severe pulverization; thus, limiting its application.59,60 Several methodologies were adopted by researchers to overcome this constrain by using Si-Carbon,61 Si-C composites were prepared by mechanical mixing,62,63 or by carrying out pyrolysis of carbon on Si surface.64–66 Si/C/CNTs composites have also been used as an anode material.67–69 It was reported that incorporation of CNTs in Si/graphite/CNTs improves the electrochemical properties of the anodes due to uniform dispersion of CNTs within the interspaces of graphite and silicon and alleviation of the SEI formation.70 Scientists have also postulated that deposition of amorphous Si films on metals like Cu increases the cycle efficiency of the anode for constant electrical contact.71 It was also reported that the cyclic reversibility and capacities of the anode increases by using post annealed Co/Si films with alternating layers72 and Fe/Si multilayer films.73 Another highly promising anode material is the spinel lithium titanium oxide Li4Ti5O12 (LTO). It is believed that this structure can accommodate LiC with a theoretical capacity of 175 mAhg¡1, along with an operational voltage plateau at 1.55 V vs. LiC/Li. As a result, the electrochemical reduction of some specific electrolyte can be avoided.74–76 However, such spinel suffers from low electrical conductivity. In order to overcome it, researchers have used composites of LTO/TiO2/C/CNTs,77 which resulted in an improved electrochemical performance (with voltage

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window of 1.0–2.5 V) and electrical conductivity of LTO. Apart from LTO, TiO2 is another promising material which has been used as an anode due to its abundance, safety, structural diversity, and cost effectiveness.78–83 It was observed that the capacity of the Li-ion batteries can be improved upon decreasing the particle size of TiO2, independent of its form. The improvement in the lithiation and de-lithiation is due to the resulting ease in Li-ion diffusion and shortening of charge transport length.84,85 Although these titanium oxide materials, such as TiO2 and Li4Ti5O12, are considered to be high safety materials. However, the main obstacles of these materials are low capacities, even below »200 mAhg¡1. Besides Ti-based electrodes, Nbbased electrodes have also been received tremendous attention for research. Several studies were done in to investigate the properties of Nb-based electrodes. Recently, Cheng et al. developed a new anode material, TiNb2O7 with a practical capacity of »280 mAhg¡1 with good rate performance and a long cycle life of more than 1000 cycles with capacity loss of 0.0033% per cycle.86 Composites of ZnO/C prepared by solid state solution has been observed to provide twice the value of the capacity offered by ZnO NPs/C composites at 4000 mAg¡1. It was further studied that even when the current density increased to 40 times, the discharge capacity still maintained 53.5% of the original value.87 Li et al. studied the improved properties of Na2Ti6O13 nanorods prepared at 900 C by traditional solid state reaction which reveals that it has shown a higher Li diffusion rate of 6.98£ 10¡15 cm2s¡1.88 Studies have reported that Na2Li2Ti6O14 electrode have shown a particle size of 200–400 nm with a charge capacity of 69.5–200 mAg¡1, 64.7 mAhg¡1 at 300 mAg¡1, 60.7 mAhg¡1 at 400 mAg¡1, and 57.9 mAhg¡1 at 500 mAg¡1.89 It is studied that nanocage Co3O4 displays good rate capability, with a reversible capacity of 1069 mAhg¡1, 1063 mAhg¡1, 850 mAhg¡1, and 720 mAhg¡1 at specific current of 100 mAg¡1, 200 mAg¡1, 800 mAg¡1, and 1000 mAg¡1, respectively.90 3 D cobalt orthophosphate synthesized by wet chemical process at low temperature followed by dehydration was for the first time used as anode for Li-ion battery. It is reported that this anode material exhibits a specific capacity of 260 mAhg¡1 and good cycle stability even after 100 cycles.91 Another type of materials which have been used as anode materials are the oxides, nitrides, phosphides, and sulphides of transition metals. The equation for the electrochemical reaction associated with the conversation reaction has been presented below: Mx Ny C zLi C C Ze ¡ ! Liz Ny C xM; where M D Fe, Co, Ni, Cu, and Mn.

(2)

The oxides of these metals involve a completely different mechanism of storage and cycling of Li-ions as compared to graphite anodes. Iron oxides are used for their low cost and high abundance; however, they lack in electrical conductivity, aggregation of iron on the surface of the anode and volume expansion during charging and discharging, along with poor cyclic performance.92–94 In view of this, recent development has been made to produce nanomaterials of iron oxides or by coating carbon onto it or to prepare composite of iron oxide and carbon.95–98 Materials of cobalt oxides, like Co3O4 and CoO, have also been reported as anode materials in Li batteries, with theoretical capacities of 890 mAhg¡1 and 715 mAhg¡1, respectively.99,100 Researchers have reported various forms of cobalt oxide like nanocages,101 nanotubes,92 nanowires, nanocubes, and nanosheets102 prepared by various techniques viz., solid state and hydrothermal, to be used as anodes in Li batteries.94,103 Metal phosphides have also been observed to act as efficient anode materials in Li batteries.104–107 They are usually classified under two groups depending upon the stability of the metal and the phosphide, and the type of transition metal: (a) phosphides undergoing intercalation/de-intercalation mechanism with formation of nanometals and phosphides without rupture of the metal-phosphide bond,108,109 and (b) phosphides undergoing insertion/extraction mechanism involving insertion and extraction of Li-ion without the breakage of the metal-phosphide bond. Apart from these, metal sulphides and nitrides have also been studied widely for their potential use as anode materials,110–112 having advantages of restriction to volume change and insertion of LiC ions within it. It has been shown recently that nanomaterials possessing porous structure hold immense application potential in the field of Li-ion batteries as electrodes. Marzuki et al. reported that carbon-coated Co3O4/C composites (prepared by molten salt method) exhibited improved properties as compared to bare Co3O4, owing to the higher pore volume possessed by the former.113 Again, the use of uncoated Co3O4 resulted in a decrease in discharge capacity to 556 mAhg¡1, whereas with Co3O4/C composites, much higher discharge capacity values after 100 cycles were realized. Copper oxide (CuO)/rGO composites, synthesized by a facile hydrothermal procedure using urea as a foaming agent, have produced improved Li-ion mobility by shortening the distance of Li-ion diffusion by virtue of the high surface area of CuO.114 The applicability of LTO, as an anode material in Li-ion batteries, has been reported recently. This mesoporous structured LTO possesses several advantages, such as: (a) large contact surface area, (b) high safety, (c) high cycling stability, and (d) high accessibility for the electrochemical active sites. In this context, it is important to mention the application of porous TiNb2O7 nanospheres,115 having an average diameter of 500 nm, a BET surface area of


23.4 m2g¡1 and a pore volume of 0.155 cm3g¡1, as an anode material for Li-ion batteries.86 It was realized that such material can resist any deformation to their porous structure even after 10,000 cycles.86 The porous nanostructure of the electrode material of Li-ion batteries may affect cycle life and high rate performance.115 It has been observed that porous-structured tungsten nitride with a two-dimensional nanostructure, prepared using tungsten oxide nanoplates, as an electrode material for Li-ion batteries provided high specific surface area and well-defined pore structure. As a result, a high specific capacity for both insertion/extraction and conversion reaction was achieved.116 Researchers have been made attempts to synthesize ferroso ferric oxide (Fe3O4)/carbon nanotubes (CNTs) composites by hydrothermal method, involving in situ filling of multi-walled nanotubes (MWNTs) with ultrafine Fe3O4 nanoparticles.117 It has been reported that this Fe3O4/CNTs composites, as anode material, have exhibited mesoporous structure with higher surface area as compared to commercial Fe3O4. This, in turn, led to exhibition of higher capacity, excellent cyclic stability and higher performance by the Fe3O4/CNTs composites. The properties of Zn2So4/CNTs composite, as anode

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material, have also been investigated.118 Again, it has also been reported that MgFe2O4 anode, synthesized by the two-step solvothermal method, produced remarkable improvement in electrochemical performance. This improvement has been attributed to their hollow spherical structure.119 Nowadays, attempts regarding the use of bio-based hierarchically porous carbon (HPC)/Co3O4 nanocomposites as anode in Li-ion battery are in progress.120 This HPC nanocomposite showed excellent electrochemical performance, producing an initial discharge value of 1566 mAhg¡1 and a charge capacity value of 1314 mAhg¡1 with a coulombic efficiency of 83.9%. In addition, suitability of materials like hollow nitrogen (N)-doped Fe3O4/C nanocages and Zn5(CO3)2(OH)6 as potential candidates to be used as anode have been tested.121,122 Table 1 depicts various cathodes and anodes used in recent studies. 2.3. Electrolytes used in li-ion batteries The total power output for Li batteries is determined by the ease of the mobility of the ions in the electrodes. It is specifically an electrolyte property and is mainly determined by

Table 1. Various cathode and anode materials used in recent studies. Type of electrode Anode

Material used Co3O4/C composite ZnO/C composite Hollow nitrogen-doped Fe3O4/C nanocage Na2Ti6O13 nanorod

Na2Li2Ti6O14 Porous CuO/rGO composite Co3O4 nanocage TiNb2O7 nanosphere Zn5(CO3)2(OH)6 Zn2SO4/CNTs composite Cathode

Nanoporous LiMn2O4 Layered LiNi0.5Co0.45Fe0.05O2 LiAlO2-coated Li1.2Ni0.2Mn0.6O2 Sm-doped LiFePO4/C composite rGO/MOPOF nanocomposite

Characteristic features

Reference number

It exhibited a discharge capacity of 1164 mAhg¡1 at 0.1C The reversible capacity of SSL ZnO/C nanofibers electrode, maintained at 813.3 mAhg¡1, exhibited a decreased rate of 0.4% per cycle after 100 cycles It exhibited a specific capacity of 878.7 mAhg¡1 after 200 cycles at a specific current of 200 mAg¡1 In a potential range of 0 V to 3 V, it exhibited high charge capacities of 198.6 mAhg¡1, 178.4 mAhg¡1, 165.8 mAhg¡1, 151.8 mAhg¡1 and 149.3 mAhg¡1 at 100 mAg¡1, 150 mAg¡1, 200 mAg¡1, 250 mAg¡1 and 300 mAg¡1, respectively It exhibited a reversible capacity of 74 mAhg¡1 with capacity retention of 98.01% upon 50 cycles It exhibited an initial reversible capacity of 819 mAhg¡1 and a specific capacity of 480 mAhg¡1 after 50 cycles at a current density of 70 mAg¡1 It exhibited a reversible capacity of 810 mAhg¡1 after 100 cycles at a high specific current of 500 mAg¡1 It exhibited a reversible capacity of 160 mAhg¡1 after 10,000 cycles and a good performance of 167 mAhg¡1 with a capacity loss of only 0.0033% per cycle It exhibited an initial discharge capacity of 1248 mAhg¡1 and a charge capacity of 428 mAhg¡1 It exhibited a high initial discharge/charge capacity of 1925.4/ 1064.9 mAhg¡1 at a current density of 100 mAg¡1 It exhibited a discharge capacity of 92.6 mAhg¡1 at a rate of 20 C and a retention of almost 95.9% of the initial capacity (i.e. 94.5 mAhg¡1) after 100 cycles It exhibited an initial discharge capacity of 160.4 mAhg¡1 at 0.2 C, and the capacity retention was 78.5% after 50 cycles It exhibited rate capabilities of 162.6 mAhg¡1 and 112.3 mAhg¡1 at 2 C and 5C, respectively It exhibited a specific discharge capacity of 112.6 mAhg¡1 and a capacity retention of 96% after 20 cycles at a high rate of 10 C It exhibited a lower capacity fading compared to pristine MOPOF; and exhibited a capacity of 100 mAhg¡1 and 57§ 3 mAhg¡1 at a current rate of 0.2 C and 4 C, respectively

113 87 121 88

89 114 90 86 122 118 51 54 53 55 57

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the ongoing electrochemical reaction within the cell. The electrolyte should cater with both the electrodes, and thus, with improvements in the electrodes, invent of much more efficient electrolyte is also required. Many advances have been made in the researches involving the existence of LiC in the electrolyte and their interactions, as well as the interphase formed between the electrolyte and the electrodes.123 The two main important properties that an electrolyte should possess are: (a) high dielectric permittivity to dissolve the salt and (b) low viscosity to help in easy transportation of Li ions. Not a single solvent can serve the purposes efficiently. Thus, usually a mixture of more than one solvent is used. It has been reported that ethylene carbonate (EC) has high dielectric permittivity and a series of acrylic carbonate or carboxylic ester help in reducing the viscosity.124 The dissolution of salt is basically governed by the ion-solvent interactions, and small sized Li-ion is expected to be more strongly binded to the nucleophilic sites of the electrolyte (Figure 5). The interaction between LiC and the solvent is determined by several factors, like the number of solvent molecules forming the solvation sheath of LiC, the type of nucleophilic part of the solvent molecule interacting with LiC and on the preference of LiC toward a specific solvent in

case of solvent mixture. Preferentially, an electrolyte must be polar enough to facilitate the dissociation of the salt and must be electrochemically inert within the potential range of 0–0.5 V vs. Li. Esters are considered as an efficient electrolyte due to their resistance to anodic decomposition, whereas the high cathodic decomposition makes it unsuitable for the anode electrode to act at that potential. However, the use of carbonate as the second solvent may lead to formation of SEI layer which vent the latter issue. Cyclic structured organic carbonate solvents and acyclic structures, like ethylmethyl carbonate (EMC), diethyl carbonate (DEC), and dimethyl carbonate (DMC), form stable interphase with oxides of transition metals.125 The effect of the degree of branching of acrylic carbonates used as a solvent on the performance of Li batteries is that although it increases the viscosity of the medium decreasing the ion mobility, the reversible cycling performance and capacity improves in case of graphitic anode systems.126 Apart from this, onium salts has been used either as an additive or co-solvent along with ether or even as sole ionic electrolyte.127–130 In addition, fluorinations of cyclic and acyclic carbonates or esters are also well known for their use as electrolytes.131–133 These electrolytes has

Figure 5. LiC transport through interphase: (top) schematic illustration of solvated LiC and bare LiC intercalation at graphite edges and the corresponding activation energy barriers; (bottom) differentiation of the contributions from LiC desolvation and LiC migration across the interphase. (© American Chemical Society. Reprinted with permission from Xu et al.156 Permission to reuse must be obtained from the rightsholder.)


been observed to possess lots of positive characteristics like ease of LiC transport, improved low-temperature performance, high-temperature resistance, and safety due to formation of better SEI on the anode surfaces. For the last few decades and even for the future, LiPF6 has been observed and predicted to be the dominant electrolyte in Li batteries.134 Although LiPF6 has well-balanced properties, however, it has low thermal and chemical stability. Keeping in mind the extreme success of LiPF6, several attempts has been made by researchers to replace the weak P-F bond with a much stronger bond. The second most useful electrolyte is lithium bis(oxalate) borate (LiBOB),135,136 owing to its extraordinary properties like acceptable conductivity at room temperature, ability to form SEI layer on anodes and wide electrochemical window. Unlike LiPF6, LiBOB exhibits stability at high temperatures;137 and even it does not dissolve Mn from LiMnPO4 spinel cathode.138 For the last few years, extensive researches have been carried out on solvent free electrolyte known as “solid polymer electrolyte” (SPE), which is basically extended to “gel polymer electrolyte.” Wright et al. studied that the oligoether functional group of SPE dissolves lithium salt.139 However, the factors that impose challenges to the utilization of this type of electrolytes are as follows: (a) low ion conductivity and (b) low interfacial interaction between the electrolyte and the electrodes. It has also been studied that lithium salt complexes with poly(ethylene oxide) (PEO) showed conductivity lower than 10¡4 Scm¡1 at room temperature, owing to the existence of partial crystallinity present arising out of oligoether section in SPE.140 It has been reported that the use of LiBOB along with PEO resulted in an improvement of the ion conductivity upto 10¡4 Scm¡1 at 40 C.141 Winter et al. proposed the 3 D model for the formation of SEI mechanism for a polycarbonate (PC)-based electrolyte.142 The foremost signature for the formation of SEI is the co-intercalation of solvated LiC ions within graphite interiors and even formation of graphitic intercalation compounds. This phenomenon is reflected by the linear expansion of graphitic composite electrodes after first lithiation cycle above 0.7 V. Moreover, LiC ion remain unstable within the graphite interior due to the reaction between solvating molecules and EC, which has been represented by signals obtained from electrochemical mass spectra.143–146 Recently, a novel electrolyte membrane comprising of a viscous copolymer of polyethylene glycol and styrene maleic anhydride (PEGMEM-co-SMA), Li-salt, and cellulose matrix has been reported. It exhibited a wide window of electrochemical stability, high ionic conductivity, and high thermal stability up to 3158C.147 This material above its melting point infiltrate the electrodes, and thus, maintains contact with the electrodes during charge/discharge process. Porous structure, offered by copolymers like PVDF-co-HFP membrane,148,149 helps in achieving

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high ionic conductivity at ambient temperature,150,151 excellent fire proof capability, high mechanical properties, 152 and outstanding stability.153,154 It has been observed that this electrolyte offer a reversible capacity of 145 mAhg¡1 at 0.2 C, which is higher than that exhibited by Celgard 2400 separator. The usage of an innovative LiC/NaC mixed electrolyte to construct rechargeable batteries, involving the immigration of LiC between electrolytes on one side and the other one refers to the exchange of NaC between electrode and electrolytes, has also been reported.155

3. Stress development in electrode materials – issues and perspectives In order to render ability to electric vehicles to compete with the traditional vehicles running on internal combustion engine, it is imperative to develop Li-ion batteries with higher energy density. Fabrication of next generation of Li-ion batteries demands designing of new electrodes and electrolytes that will hold potential to survive thousands of charge-discharge cycles while exhibiting minimum capacity fading.156 The mechanical integrity, cycle life, and electrochemical performance of electrode materials depend on the stresses developed within them during operation. Therefore, it is a prerequisite to realize the underlying mechanism of evolution of damage within the electrode materials. This realization will help battery designers to develop predictive models and optimal designs of the materials. Electrolyte is another essential member of a battery assembly. Different electrolytes have different polarity and dielectric constant; therefore, depending on the condition required proper electrolyte is chosen. It should be remembered that there exist only a few Li-based salts or polymers that can be employed, with those based on PEO as the most commonly used ones. Different groups of scientist started working on the development of new types of electrodes to satisfy the need of the modern age. The development of insertion compounds paved the way for the solution to this problem. 157 These materials have the ability to perform two functions, i.e., (a) hosting Li-ions within their crystalline structure and (b) reducing transition metals from their higher oxidation state(s). Called “topotactical” electrochemical reaction, it can occur reversibly without major phase change and can be facilitated by a significant conductivity/semi-conductivity. Similar to the problem arising at the positive electrode, lithium corrosion, and dendrite formation on the negative side results in poor cycling efficiency and cell shorting. However, with the advent of powerful anode system, the demand for improved cathode became mandatory.

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Extensive research on the improvement of cathode led to the discovery of low voltage Li-intercalation-de-intercalation of carbonaceous compound like LiCoO2. At a range of 0