Effects of microwave irradiation on the electrochemical performance of manganese-based cathode materials for lithium-ion batteries

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Effects of microwave irradiation on the electrochemical performance of manganese-based cathode materials for lithium-ion batteries Aderemi B. Haruna and Kenneth I. Ozoemena Abstract

Microwave-assisted synthesis has continued to be adopted for the preparation of high-performance manganese-based cathode materials for lithium-ion batteries. The technique is fast, energy-efficient and has significant positive impacts on the general physico-chemical properties of the cathode materials: LiMn2O4, LiMn1.5Ni0.5O4, and lithium nickel manganese cobalt oxides. Despite the advantages of microwave-assisted synthesis, this review reveals that the application is still limited. In our opinion, increased basic knowledge of the microwave process and availability of safe and reliable instrumentation could be a great opportunity for the commercial realization of low-cost and energy-dense Mn-based cathode materials for the next-generation lithium-ion batteries. Addresses Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Private Bag 3, PO Wits, Johannesburg, 2050, South Africa Corresponding author: Ozoemena, Kenneth I (Kenneth.ozoemena@ wits.ac.za)

Current Opinion in Electrochemistry 2019, 18:16–23 This review comes from a themed issue on Energy Storage

nickel-manganese-cobalt oxides (NMC) [1]. These materials are safer to use, less costly and possess higher energy densities compared with the state-of-the art LiCoO2 (LCO). LMO and LCO are both commercialized, but their available rechargeable capacity is theoretically limiting. LMNO is considered a nextgeneration cathode material because of its high voltage (5 V) and flat plateau at 4.7 V. The NMC with different stoichiometric ratios such as NMC 333 or NMC 111 (LiNi1/3Mn1/3Co1/3O2) and the Ni-rich complexes (NRNMC, e.g., NMC 532 [LiNi0.5Mn0.3Co0.2O2], NMC 622 [LiNi0.6Mn0.2Co0.2O2], and NMC 811 [LiNi0.8Mn0.1Co0.1O2]) deliver high capacity (between 150 and 200 mAh/g). The lithium- and manganese-rich counterparts (LMR-NMC, commonly denoted as xLi2MnO3(1-x) e.g., Li1.2Mn0.54Co0.13Ni0.13O2 and LiMO2, Li1.2Mn0.52Co0.13Ni0.13Al0.02O2 [2]) deliver even much higher capacity (>250 mAh/g), which is almost twice that of the LCO. LMO is already commercialized but still suffers poor cycling performance especially at elevated temperatures due to the Jahn-Teller distortion as well as the disproportionation of Mn3þ (Equation (1)) and dissolution of Mn2þ in the electrolyte.

Edited by Xiaobo Ji For a complete overview see the Issue and the Editorial Available online 5 September 2019

4þ 2þ 2Mn3þ ðsolidÞ / MnðsolidÞ þ MnðsolutionÞ

(1)

https://doi.org/10.1016/j.coelec.2019.08.005 2451-9103/© 2019 Elsevier B.V. All rights reserved.

Keywords Microwave irradiation, Mn-based cathode materials, Lithium-ion batteries.

Introduction Cathode materials constitute approximately 25% of the total cost of a lithium-ion battery pack. Manganesebased cathode materials continue to attract tremendous interest because of their advantageous properties for the development of lithium-ion batteries (LIBs) for use, especially, in electric vehicles and plug-in hybrid electric vehicles. These cathode materials include the two spinel materials (LiMn2O4 [LMO] and LiMn1.5Ni0.5O4 [LMNO]) and the layered materials lithiumCurrent Opinion in Electrochemistry 2019, 18:16–23

The high-voltage LMNO has yet to be commercialized despite being the most promising candidate among the 5 V cathode materials because of poor rate capability at elevated temperatures as well as the instability with the conventional electrolytes. On the other hand, the NRNMC cathode materials are known for their excellent electrochemical properties and stable structure. It is thought that high Ni content confers its high specific capacity; high Mn content leads to increased structural stability and low cost, while an appropriate amount of Co leads to enhancing electrical conductivity by suppression of the Li/Ni cation mixing [3e7]. The full commercialization of the NR-NMC and LMR-NMC materials has been hampered by their low cycling performance and rate capacity associated with structural impurities, phase transitions, and substantial Li/Ni cation mixing [8e10]. The LMR-NMC in particular are www.sciencedirect.com

Effects of microwave irradiation Haruna and Ozoemena

plagued by severe initial irreversible capacity loss due to irreversible loss of oxygen from the lattice, very poor rate capability, and voltage decay [2,11]. The need to curb the various technical challenges of the Mn-based cathode materials and expedite their widespread commercialization has led the exploration of microwave-assisted synthesis (where microwave irradiation steps are included in the conventional solidstate, coprecipitation, Pechini reactions, to name a few). Figure 1 summarizes the percentage publication of each of the three Mn-based cathode materials (LMO, LMNO, and NMC) obtained via microwaveassisted synthesis published from 2010 to date. LMO is the most reported (unsurprisingly, it is the most mature, commercialized Mn-based cathode technologies), followed by the NMC (NMC 111 is commercialized, and other family members of the low cobalt NMC cathode materials have been licensed for commercialization), while LMNO is the least reported material (yet to be commercialized). This short review provides the underlying principles of microwave irradiation and the various opinions on the effects of microwave irradiation on the electrochemical performance of the three Mn-based cathode materials for lithiumion batteries. Figure 2 describes the preparation of a typical LMRNMC (Li1.2Mn0.56Ni0.16Co0.08O2) [12] using both microwave-assisted synthesis and conventional heating process. Unlike the microwave process, which is extremely fast (1 h) and energy-efficient, the

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traditional heating process is extremely slow (16 h) and energy-consuming.

Underlying principles of microwave irradiation: an overview Microwave energy is a form of electromagnetic energy in the frequency range of 300 MHz (i.e., 0.3 GHz) to 300 GHz, with corresponding wavelengths of 1 mm to 1 m. From the basic law of physics (i.e., the longer the frequency and the shorter the wavelength, the larger the energy: E = hc/l = hf), it means that other forms of electromagnetic radiation (such as visible, ultraviolet, and infrared light) with much longer frequency and shorter wavelengths are more energetic than the microwave radiation. Microwave-enhanced chemistry is the efficient heating of a specific material by microwave dielectric heating. In other words, it is the unique ability of a material to absorb electromagnetic energy volumetrically and then transform it into heat. Microwave heating is quite different from the conventional methods of heating that involve heat transfer by conduction, radiation, or convection. This should be expected if one considers that the energy of a microwave photon at a frequency of 2.45 GHz is only 1.0  105 eV (ca. 1 J mol1) [13], which is too weak to break chemical bonds [14,15], and even weaker than the Brownian motion and only affects molecular rotations [16]. The conventional heating method follows the “surfaceto-inside” mechanism (i.e., the surface of the material is first heated up, followed by the heat moving inside the

Figure 1

Percentage publication of each of the three manganese-based cathode materials for lithium-ion batteries obtained via microwave-assisted synthesis (2010 to June 2019).

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materials). On the other hand, microwave heating may be said to follow the “inside-to-surface” mechanism whereby heat is first generated within the target material followed by rapid heating of the entire volume [17]. The microwave dielectric heating follows two main mechanisms, namely (i) dipolar polarization and (ii) ionic conduction [18]. Microwave field is an oscillating field, so when the sample is subjected to microwave irradiation, the dipoles will align themselves in the electric field. This alignment leads to rotation, resulting in friction and ultimately in heat generation. For the ionic conduction, the charged particles or ions oscillate (i.e., move back and forth) in the microwave field, colliding with neighboring atoms and molecules and ultimately generating heat. The ability of a material to be heated in a microwave field is related to the loss tangent, tan d, which is expressed in Equation (2) [18e20]: tan d ¼

ε00 m0  ε0 m00 ; ε0 m0 þ ε00 m00

(2)

where ε’’ is the dielectric loss, which describes the ability or efficiency of a material to convert electromagnetic radiation into heat; ε0 is the dielectric constant, which describes the ability of a material to be polarized in the electric field; m0 is the magnetic permeability; and m” is the magnetic loss. Thus, in microwave processing, both dielectric and magnetic properties are considered. In fact, for magnetic materials, it is thought that the dipoles may couple with the magnetic component of the electromagnetic field and provide an additional heating mechanism.

For materials with negligible magnetic loss (m” = 0), the loss tangent is simplified as in Equation (3): tan d ¼

ε00 ε0

(3)

The loss tangent (also known as the “loss factor”) is a measure of the efficiency with which a particular material is able to convert the electromagnetic energy into heat at a given frequency and temperature. Thus, a material or reaction medium with a high value of loss factor is indicative that such a material or reaction medium is able to undergo efficient microwave

Figure 2

This scheme summarizes a typical microwave-assisted experimental concept discussed in this review. Here, the authors [12] compare the preparation of layered Li- and Mn-rich NMC (i.e., Li1.2Mn0.56Ni0.16Co0.08O2) with conventional heating and full microwave synthesis. Note that the microwave step is extremely fast (1 h) with excellent energy economy compared with the time- and energy-consuming conventional heating process (~16 h). Indeed, it is interesting to observe that microwave-assisted hydrothermal step (30 min) avoids the rigorous conventional coprecipitation step of preparing spherical precursor and product.

Current Opinion in Electrochemistry 2019, 18:16–23

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Effects of microwave irradiation Haruna and Ozoemena

absorption and fast heating. However, materials and reaction media with poor loss factor can still undergo microwave reaction in the presence of additives (such as catalysts, ionic liquids) that can improve their microwave-absorbing capacity [15]. There are two other key parameters that determine the uniformity of microwave heating throughout the material: the absorbed power (P) and the depth of microwave penetration (D). The electric field is considered to be uniform throughout the volume; thus, the power absorbed per unit volume, P, in the material (i.e., Watts per volume, W m3) is expressed as in Equation (4) [21,22]: P ¼ sjEj2 ¼ 2pf εo ε}eff jEj2 ¼ 2pf εo ε0r tan djEj2 ;

(4)

where s is the material dielectric conductivity (U1 m1), E is the magnitude of the internal electric field (V m1), f is the microwave frequency (Hz), εo is the permittivity of free space (8,86  1012 F m1), ε”eff is the relative effective dielectric loss factor, and ε0 r is the relative dielectric constant. Equation (3) indicates that the power dissipated in electrode materials (including manganesebased cathode materials for lithium-ion battery) during microwave processing is directly proportional to the applied microwave frequency and the conductivity of the material.

The power absorbed by the material is immediately converted into heat, causing an increase in temperature, expressed as in Equation (5) [21]: 2pf εo ε}eff jEj2 DT ¼ ; Dt rCp

(5)

where T is the temperature (K), t is the time (s), r is the density (kg m3), and CP is the heat capacity (J kg1 K1). The electric field attenuation distance (also known as the penetration depth or skin depth), D (m), describes how deep the microwave irradiation can penetrate into the material. It is the depth at which the intensity of microwave radiation inside the material falls to 1/e (i.e., 36.8%) of its original value at (or just beneath) the surface. Penetration depth is strongly dependent on the electric and magnetic properties of the materials [20,23], Equation (6): D¼

1 4p ¼ ; a lu2 ðε0 m} þ ε}m0 Þ

(6)

where a is the attenuation factor, l is the wavelength of the plane wave inside the material, u is the applied frequency (u = 2pf), and other maintain their usual definitions. The equation suggests that a material with high conductivity (s = uε”) and permeability will present a lower penetration depth. Also, the changes in the conductivity and permeability suggest a certain level of temperature dependence. www.sciencedirect.com

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The penetration depth of metals is relatively quite small (in the order of microns), and direct heating generally remains superficial; however, metal powders with high surface areas (e.g., micron-, sub-micron-, and nano-sized powder materials) possess higher penetration depths and can undergo efficient volumetric heating when subjected to microwave heating [24e27]. The loss factor (tan d) and the penetration depth (D) of ceramic materials (i.e., nonmagnetic dielectric materials, including inorganic, nonmetallic, crystalline oxide, nitride, or carbide material) vary with temperature: generally, loss factor increases with temperature while the penetration depth decreases with temperature.

Effects of microwave on Mn-based cathode materials for Li-ion batteries General effects

The microwave processing of cathode materials provides several advantages over the conventional heating method, ranging from rapid heating energy-efficiency to tuning the general physicochemical properties (morphology, impurity reduction, crystal growth) [16,21,28e39]. Morphology, crystal growth, and crystallinity

Conventional sintering process leads to excessive grain growth of the Mn-based cathode materials, resulting in poor rate capability of the LiB. Microwave irradiation plays a critical role in crystal growth and morphology of the product. Gao et al. [40] showed that microwave irradiation led to rapid and preferential crystal growth of the spinel LMNO crystal. For Li-rich Mn-based cathode materials, microwave offers pure phase and well-formed layered structure, stable surface structure, well-formed channels for lithium-ion transfer, higher hexagonal ordering, and more ordered ion arrangement [12]. In situ transmission electron microscope (TEM) showed that rapid crystal growth tends to yield defect-free microstructures and stable crystal morphology, resulting in enhanced electrochemical performance [41]. Particle size

Large particle size, nonuniformity, and irregular morphology have negative effect on the performance of electrode materials. Thus, small and uniform particle size is an important parameter for enhancing the electrochemistry of cathode materials for Li-ion batteries. It is generally observed in Mn-based electrodes that microwave process can effectively control the chemical composition, increases crystallinity, and generates small particle size and uniform particle morphology [34,42]. A recent study [43] cautioned on the general perception that smaller particle size will always give better electrochemical performance than the larger particle. The authors studied the electrochemical features of different LMO nanoparticle sizes and found, contrary to the commonly held expectation, that the smallest Current Opinion in Electrochemistry 2019, 18:16–23

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Figure 3

(a) Cyclic voltammograms (0.5 mV/s) of the spinel LMO samples with different average nanoparticle sizes (4, 9, and 14 nm) obtained in 1 M LiClO4 containing ethylene carbonate and dimethyl carbpnate EC/DMC (2:1 v/v). (b) Energy barrier profiles calculated for a Li ion along the different diffusion paths increased energy barrier for the Li-ion diffusion within the LMO crystallite (0.33 eV to at least 0.59 eV); the smallest nanoparticles gave the highest energy barrier. Figure adapted from Ref. [43] with permission.

nanoparticle (4 nm) gave the worse electrochemical activities than the larger counterparts (9 and 14 nm, Figure 3a). Using ab initio density functional theory (DFT), it was found that the redox voltage associated with the Li-ion extraction and insertion processes increased (from 3.93 to 4.64 V) as the particle size decreased (from 14 to 1.3 nm). The study also established that this increase in the redox voltage of the smaller nanoparticles was due to the increased energy barrier for the Li-ion diffusion within the LMO crystallite (0.33 eV to at least 0.59 eV, Figure 3b), arising from the atomic relaxations (mainly of oxygen atoms) that occur on the surface of the smaller nanoparticles. The reason for the poor electrochemical performance of the smaller LMO nanoparticles was attributed to the generation of high voltage that is beyond the stability potential window of the electrolyte solution used. So, it was concluded that if small LMO nanocrystallites are to be used for high-performing LIBs, the average nanoparticle should not be smaller than the critical value of w15 nm. Although this finding is specific for LMO, it may be well be the same for other cathode materials, only further studies will tell. Impurity reduction, controlling the Mn3+ content, creation of oxygen vacancy etc.

Li/Ni cation mixing in NMC (Li þ/Ni 2þ mix due to their close ionic radii, i.e., Ni 2þ [w0.69A] and Li þ [w0.76A]) increases the degree of disorder, negatively affecting electrochemical performance (i.e., low Li þ diffusion coefficient, low capacity, Current Opinion in Electrochemistry 2019, 18:16–23

and poor cycling stability). Hsieh et al. [44] proved that microwave irradiation of NMC-333 induces low cation mixing and highly crystalline layered product, thus enhancing higher Li þ diffusion rate electronic conduction. Conventional sintering method of preparing spinel LMNO usually generates secondary phases or impurities (notably, NiO, LixNi1xO and Li2MnO3) due to the decomposition at high temperatures of Mn4þ to Mn3þ and creation of oxygen vacancy in the crystalline structure. These impurities negatively impact on the electrochemical performance of LMNO. Other researchers [45,46] have also shown that LMNO obtained from microwave-assisted methods were able to diminish the impurity level of NiO, thereby increasing the discharge capacity and the rate capability. Microwave irradiation accelerates the diffusion of Ni and Mn cations to rapidly grow LiMn1.5Ni0.5O4, and the rapid preparation limits the formation of impurity. Gao et al. [40] showed that unlike conventional heating, microwave-assisted synthesis of LMNO led to a nearabsence of the Li2MnO3 impurity. It has long been proposed [47] that long reaction time at high temperature leads to minor decomposition of the spinel LMNO (Equation (7)):

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Effects of microwave irradiation Haruna and Ozoemena

LiNi0:5 Mn1:5 O4 / 2Li2 MnO3 þ 2Mn2 O3 þ 2NiO þ O2 (7)

However, with microwave irradiation, the diffusion of Ni and Mn cations are accelerated to rapidly grow the LMNO, thereby eliminating the formation of impurities. The observed oxygen deficiency in the microwave-based LMNO, according to the authors [40], may be related to the incomplete oxidation of the Mn to Mn4þ. LMO contains higher concentration of Mn3þ than the LMNO, which explains why the advantageous disproportionation reaction (Equation (1)) is worse with LMO than LMNO. Ordered LMNO (LiMn1.5Ni0.5O4) contains only the redox-inactive Mn4þ while the disordered/oxygen-deficient phase (LiMn1.5Ni0.5O4-d) contains both Mn4þ and redox-active Mn3þ. While the electrochemical performance of the disordered spinel is better than the ordered one (especially in terms of enhanced Li-ion diffusion), there is a need to control the concentration of the Mn3þ and restrain the formation of the impurity LixNi1yO phase that lowers the obtainable capacity. Ozoemena and co-workers [48,49] proved that microwave-assisted methods not only impact on the physicochemistry of the disordered spinel phases but also can control the Mn4þ/Mn3þ ratio, eliminate the impurity, and significantly improve the electrochemical performance. In other reports, the same group showed that the use of a combined process of “doping-with-microwave irradiation” [35,36] was able to decrease the concentration of Mn3þ in the pristine LMO, thereby significantly enhancing its electrochemical performance (including increased discharge capacity, Li-ion diffusion, and cycling stability at room and elevated temperatures). Like the LMNO, the Mn valency of the NMC or LMRNMC is also affected by microwave irradiation due to the creation of the oxygen vacancies, which cause some Mn4þ ions to be converted to Mn3þ due to charge compensation [2].

Concluding remarks Microwave-assisted synthesis has emerged as extremely fast, low-energy, and potentially low-cost technique to prepare pure phase manganese-based cathode materials for lithium-ion batteries. Importantly, several researchers across the globe continue to prove that microwave irradiation provides tremendous improvement on the physicochemical properties of the Mn-based cathode materials, notably those of the spinel LiMn2O4 (LMO), high-energy spinel LiMn1.5Ni0.5O4 (LMNO), and high-capacity layered NMC. The reasons for the microwave effects can be found, as we clearly explained, on the unique or special heating process (i.e., the low energy of the microwave, the inside-to-surface mechanism of heating, high reaction temperature within short reaction time, etc). Despite the advantages of microwave-assisted synthesis, this review shows that the research in this field remains hugely limited. As attempted in this short review, increased fundamental understanding of the microwave science and application could unravel the opportunity for the fast realization of low-cost and energy-dense Mn-based cathode materials for the next-generation lithium-ion batteries.

Conflict of interest statement Nothing declared

Acknowledgments This study was supported by the University of the Witwatersrand (Wits) and the National Research Foundation (NRF: Grant No: 113638) of South Africa. AB Haruna is grateful to Wits University for PhD bursary.

References Papers of particular interest, published within the period of review, have been highlighted as:  of special interest  of outstanding interest 1.

Ozoemena KI, Kebede M: Next-generation nanostructured lithium-ion cathode materials: critical challenges for new directions in R&D. In Nanomaterials in advanced batteries and supercapacitors. Edited by Ozoemena KI, Chen SW, New York, USA: Springer Publishing; 2016:1–24. Chapter 1.

2.

Jafta CJ, Raju K, Mathe MK, Manyala N, Ozoemena KI: Microwave irradiation controls the manganese oxidation states of nanostructured (Li[Li0.2Mn0.52Ni0.13Co0.13Al0.02]O2) layered cathode materials for high-performance lithium ion batteries. J Electrochem Soc 2015, 162:A768–A773.

3.

Sun HH, Choi W, Lee JK, Oh IH, Jung HG: Control of electrochemical properties of nickel-rich layered cathode materials for lithium ion batteries by variation of the manganese to cobalt ratio. J Power Sources 2015, 275:877–883.

4.

Noh HJ, Youn S, Yoon CS, Sun YK: Comparison of the structural and electrochemical properties of layered Li[NixCoyMnz] O2 (X [ 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) cathode material for lithium-ion batteries. J Power Sources 2013, 233:121–130.

5.

Yoon CS, Choi MH, Lim BB, Lee EJ, Sun YK: Review-high-capacity Li[Ni1-XCoX/2MnX/2]O2 (X [ 0.1, 0.05, 0) cathodes for next-generation Li-ion battery. J Electrochem Soc 2015, 162: A2483–A2489.

6.

Li LJ, Wang ZX, Liu QC, Ye C, Chen ZY, Gong L: Effects of chromium on the structural, surface chemistry and

Lithiation

Recently, Manthiram’s group [50] reported chemical lithiation for spinel LMO and LMNO materials using microwave-assisted synthesis which can insert one extra lithium per formula unit. They proved that the microwave-assisted lithiation technique can provide precise control over the amount of lithium added to the spinel cathode materials without change in the overall structure and morphology because of the low operating temperature.

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electrochemical of layered LiNi0.8-XCo0.1Mn0.1+XO2. Electrochim Acta 2012, 77:89–96. 7.

8.

9.

Zheng JM, Kan WH, Manthiram A: Role of Mn Content on the electrochemical properties of nickel-rich layered LiNi0.8XCo0.1Mn0.1+XO2 (0.0 £ X £ 0.08) cathodes for lithium-ion batteries. ACS Appl Mater Interfaces 2015, 7:6926–6934. Liao JY, ManthiramA: Surface-modified concentration-gradient Ni-rich layered oxide cathodes for high-energy lithium-ion batteries. J Power Sources 2015, 282:429–436. Sun HH, Manthiram A: Impact of microcrack generation and surface degradation on a nickel-rich layered Li [Ni0.9Co0.05Mn0.05]O2 cathode for lithium-ion batteries. Chem Mater 2017, 29:8486–8493.

10. Lee JH, Yoon CS, Hwang JY, Kim SJ, Maglia F, Lamp P, Myung ST, Sun YK: High-energy-density lithium-ion battery using a carbon-nanotube-Si composite anode and a compositionally graded Li[Ni0.85Co0.05Mn0.10]O2 cathode. Energy Environ Sci 2016, 9:2152–2158. 11. Johnson CS, Li N, Lefief C, Thackeray MM: Anomalous capacity and cycling stability of xLi2MnO3·(1 −x)LiMO 2 electrodes (M [ Mn, Ni, Co) in lithium batteries at 50  C. Electrochem Commun 2007, 9:787–795. 12. Shi Shaojun, Zhang Saisai, Wu Zhijun, Wang Ting, Zong Jianbo,  Zhao Mengxi, Yang Gang: Full microwave synthesis of advanced Li-rich manganese based cathode material for lithium ion batteries. J Power Sources 2017, 337:82–91. This work describes the ability of microwave-assisted hydrothermal step to avoid the rigorous conventional industry-preferred co-precipitation step of preparing spherical precursor and product. 13. Nuchter M, Ondruschka MB, Bonrath W, Gum A: Microwave assisted synthesis – a critical technology overview. Green Chem 2004, 6:128–141. 14. Stuerga DAC, Gaillard P: Microwave athermal effects in chemistry: a myth’s autopsy: part I: historical background and fundamentals of wave-matter interaction. J Microw Power Electromagn Energy 1996, 31:87–100. 15. Stuerga DAC, Gaillard P: Microwave athermal effects in chemistry: a myth’s autopsy: part II: orienting effects and thermodynamic consequences of electric field. J Microw Power Electromagn Energy 1996, 31:101–113. 16. Kappe CO: Controlled microwave heating in modern organic synthesis. Angew Chem, Int Ed 2004, 43:6250–6284. 17. Yadoji P, Peelamedu R, Agrawal D, Roy R: Microwave sintering of Ni–Zn ferrites: comparison with conventional sintering. Mater Sci Eng B 2003, 98:269–278. 18. Jennifer M Kremsner, Stadler Alexander: A chemist’s guide to microwave synthesis: basics, equipment & application examples. Austria: Anton Paar GmbH; 2013. 19. Menezes Romualdo R, Souto Pollyane M, Kiminami Ruth HGA: Microwave fast sintering of ceramic materials: sintering of ceramics – new emerging techniques. In Chapter 1. Edited by Lakshmanan Arunachalam, Rijeka, Croatia: InTech; 2012. 20. Peng Z, Hwang J-Y, Mouris J, Hutcheon R, Huang X: Microwave penetration depth in materials with non-zero magnetic susceptibility. ISIJ Int 2010, 50:1590–1596. 21. Clark DE, Folz DC, West KK: Processing materials with microwave energy. Mater Sci Eng A 2000, 287:153–158. 22. Sutton WH: Microwave processing of ceramic materials. Am Ceram Soc Bull 1989, 68:376–386. 23. Peng Z, Hwang J–Y, Park CL, Kim BG, Onyedika G: Numerical analysis of heat transfer characteristics in microwave heating of magnetic dielectrics. Metall Mater Trans A 2012, 43: 1070–1078. 24. Roy R, Agrawal D, Cheng J, Gedevanishvili S: Full sintering of powdered-metal bodies in a microwave field. Nature 1999, 399:668–670.

Current Opinion in Electrochemistry 2019, 18:16–23

25. Mishra P, Upadhyaya A, Sethi G: Modeling of microwave heating of particulate metals. Metall MaterTrans B 2006, 37: 839–845. 26. Ripley EB, Oberhaus JA: Melting and heat-treating metals using microwave heating. Ind Heat 2005, 72:65–69. 27. Clark DE, Sutton WH: Microwave processing of materials. Annu Rev Mater Sci 1996, 26:299–331. 28. Agrawal D: Microwave sintering of ceramics, composites and metallic materials, and melting of glasses. J Mater Education 1999, 19:49–58. 29. Leonelli C, Veronesi P, Denti L, Gatto A, Iuliano L: Microwave assisted sintering of green metal parts. J Mater Process Technol 2008, 205:489–496. 30. Menezes RR, Souto PM, Kiminami RHGA: Microwave hybrid fast sintering of porcelain bodies. J Mater Process Technol 2007, 190:223–229. 31. Morteza O, Omid M: Microwave versus conventional sintering: a review of fundamentals, advantages and applications. J Alloy Comp 2010, 494:175–189. 32. Chen K, Donahoe A, Noh Y, Li K, Komarneni S, Xue D: Conventional- and microwave-hydrothermal synthesis of LiMn2O4: effect of synthesis on electrochemical energy storage performances. Ceram Int 2014, 40:3155–3163. 33. Rao B, Muralidharan P, Kumar P, Venkateswarl M, Satyanarayana N: Fast and facile synthesis of LiMn2O4 nanorods for Li ion battery by microwave assisted hydrothermal and solid state reaction methods. Int. J. Electrochem. Sci. 2014, 9:1207–1220. 34. Miao X, Yan Y, Wang C, Cui L, Fang J, Yang G: Optimal microwave-assisted hydrothermal synthesis of nanosized xLi2MnO3.(1-x)LiNi1/3Co1/3Mn1/3O2 cathode materials for lithium ion battery. J Power Sources 2014, 247:219–227. 35. Nkosi F, Jafta C, Kebede M, Roux L, Mathe M, Ozoemena K: Microwave-assisted optimization of the manganese redox states for enhanced capacity and capacity retention of LiAlxMn2-xO4 (x[ 0 and 0.3) spinel materials. RSC Adv 2015, 5:32256–32262. 36. Raju K, Nkosi F, Viswanathan E, Mathe M, Damodaran K,  Ozoemena K: Microwave-enhanced electrochemical cycling performance of the LiNi0.2Mn1.8O4 spinel cathode material at elevated temperature. Phys Chem Chem Phys 2016, 18: 13074–13083. The report showed the uncommon use of microwave irradiation to tune the average manganese redox state of the spinel material for enhanced the cycling performance of the LiNi0.2Mn1.8O4 at elevated temperature. They used microwave irradiation to increase the Mn4+ concentration around Li-ion, optimized the Mn3+ content and expose the {111} facets of the spinel. 37. Zhang H, Xu Y, Liu D, Zhang X, Zhao C: Structure and performance of dual-doped LiMn2O4 cathode materials prepared via microwave synthesis method. Electrochim Acta 2014, 125: 225–231. 38. Zhang H, Liu D, Zhang X, Zhao C, Xu Y: Microwave synthesis of LiMg0.05Mn1.95O4 and electrochemical performance at elevated temperature for lithium-ion batteries. J Solid State Electrochem 2014, 18:569–575. 39. Silva J, Biaggio S, Bocchi N, Rocha-Filho R: Practical microwave-assisted solid-state synthesis of the spinel LiMn2O4. Solid State Ion 2014:268 42–47. 40. Gao P, Wang L, Chen L, Jiang X, Pinto J, Yang G: Microwave  rapid preparation of LiNi0.5Mn1.5O4 and the improved high rate performance for lithium-ion batteries. Electrochim Acta 2013, 100:125–132. The report described the use of microwave irradiation to limit the formation of impurity and minor Li loss in LMNO. The observed oxygen deficiency observed in the microwave-based LMNO, according to the authors, may be related to the incomplete oxidation of the Mn to Mn4+. 41. Wu J, Liu X, Han Bi H, Song Y, Wang C, Cao Q, Liu Z, Wang M,  Renchao Che R: Microwave sintering and in-situ transmission electron microscopy heating study of Li1.2(Mn0.53Co0.27)O2

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Effects of microwave irradiation Haruna and Ozoemena

with improved electrochemical performance. J Power Sources 2016, 326:104–111. The group strategically showed the significant role of microwave sintering temperature on the crystal growth, morphology, particle agglomeration and eventually the electrochemical performance of the cathode materials. They produced defect-free microstructure and a stable crystal morphology of Li1.2(Mn0.53Co0.27)O2 cathode material. 42. Rao N, Padmaraj O, Narsimulu D, Venkateswarlu M, Satyanarayana N: A.C conductivity and dielectric properties of spinel LiMn2O4 nanorods. Ceram Int 2015, 41: 14070 – 14077. 43. Velasquez E, Silva D, Falqueto J, Mejõa-Lopez J, Bocchi N,  Rio R, Mazo-Zuluaga J, Rocha-Filho R, Biaggio S: Understanding the loss of electrochemical activity of nanosized LiMn2O4 particles: a combined experimental and ab initio DFT study. J Mater Chem A 2018, 6:14967–14974. The report cautioned on the general perception that smaller particles size produce better electrochemical performances by showing that it is not true for of LiMn2O4 nanoparticles below certain critical average size. They used an ab initio DFT methodology to show that, this occurrence could be clarified by theoretical explanation for the decreased electrochemical activity of the smallest LiMn2O4 nanoparticles. The group concluded that when LiMn2O4 nanoparticles are to be used in lithium-ion batteries, the average nanoparticle size should not be less than 15 nm; otherwise Li-ion extraction–insertion becomes significantly mired or impossible. 44. Hsieh C, Mo C, Chen Y, Chung Y: Chemical-wet synthesis and  electrochemistry of LiNi1/3Co1/3Mn1/3O2 cathode materials for Li-ion batteries. Electrochim Acta 2013, 106:525–533. The group addressed the drawbacks (low electronic conductivity, high reactivity between delithiated cathode and electrolyte, and serious dissolution of transition metal ions and electrolyte) of NCM by reducing the cation mixing of LI and Ni ions. They concluded that an optimum microwave heating of NCM precursors induces low cation mixing and highly crystalline degree of the layered lattices, enhancing higher Li+ diffusion rate in the solid solution and easier electronic conduction across the electrode for improved performance

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46. Feng X, Shen C, Xiang H, Liu H, Wu Y, Chen C: High rate capability of 5 V LiNi0.5Mn1.5O4 cathode material synthesized via a microwave assist method. J Alloy Comp 2017, 695: 227–232. 47. Yang T, Sun K, Lei Z, Zhang N, Lang Y: The influence of holding time on the performance of LiNi0.5Mn1.5O4 cathode for lithium ion battery. J Alloy Comp 2010, 502:215–219. 48. Jafta CJ, Mkhulu K, Mathe MK, Manyala N, Ross WD,  Ozoemena KI: Microwave-assisted synthesis of high-voltage nanostructured LiMn1.5Ni0.5O4 spinel: tuning the Mn3+ content and electrochemical performance. ACS Appl Mater Interfaces 2013, 5:7592–7598. Ozoemena group used microwave-assisted synthesis as an elegant strategy to fine tune the Mn3+ content of spinel LiMn1.5Ni0.5O4. The microwave-assisted synthesis eliminated the drawbacks of the state-of the-art procedures such as (i) the use of energy-intensive and timeconsuming, careful controlling of the cooling rates after hightemperature calcination or the postsynthesis annealing; (ii) partial substitution of Ni or Mn with expensive metals (e.g., Ru), which may be toxic and environmentally unfriendly (e.g., Cr); and (iii) long-hour acid treatment 49. Kebede M, Yannopoulo SN, Sygellou L, Ozoemena KI: High voltage LiNi 0.5 Mn 1.5 O4- d spinel material synthesized by microwave-assisted thermo-polymerization: some insights into the microwave-enhancing physico-chemistry. J Electrochem Soc 2017, 164:A3259–A3265. The group showed that the microwave treatment not only led to nanosizing of the oxygen-deficient LMNO spinel, but also enhance the electrochemical property by adjusting the lattice parameter, nickel content, and Mn3+ content. 50. Moorhead-Rosenberg Zachary, Allcorn Eric, Manthiram Arumugam: In situ mitigation of first-cycle anode irreversibility in a new spinel/FeSb lithium-ion cell enabled via a microwave-assisted chemical lithiation process in situ mitigation of first-cycle anode irreversibility in a new spinel/ FeSb lithium-ion cell enabled via a microwave-assisted chemical lithiation process. Chem Mater 2014, 26:5905–5913.

45. Zhang M, Wang J, Xia Y, Liu Z: Microwave synthesis of spherical spinel LiNi0.5Mn1.5O4 as cathode material for lithium-ion batteries. J Alloy Comp 2012, 518:68–73.

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Current Opinion in Electrochemistry 2019, 18:16–23