Rapid and Low‐Carbon Emission Synthesis of Stable LiNi0.5Mn1.5O4 Cathode for Li‐Ion Batteries

The surging adoption of lithium‐ion batteries, driven by diverse applications including electric vehicles, renewable energy storage, and portable electronics, has intensified the demand for high‐energy cathode materials. The existing methods to produce cathode materials typically include high‐temperature (>800 °C) sintering for a long period of time (>15 h), which are not only energy‐intensive but also significantly contribute to CO2 emissions. This investigation presents the ultrafast microwave solid‐state process to produce battery cathode materials by synthesizing high‐voltage LiNi0.5Mn1.5O4 (LNMO). Benefiting from rapid direct heating with an ultrahigh heating rate (>50 °C), high‐purity and well‐defined LNMO crystals are produced in a much shorter time (2 h) as compared to the conventional heating methods (>15 h). It is revealed that microwave irradiations substantially accelerate the reaction kinetics and enhance the homogeneity of LNMO. The as‐synthesized LNMO cathode delivers an excellent discharge capacity of 137.96 mAh g−1 (at 0.05 C), and outstanding cycling stability (≈63% retention over 280 cycles). The in situ characterizations elucidate the charge kinetics, suggesting high structural stability of the LNMO cathode during the reversible intercalation of Li‐ions. These encouraging results pave the way toward the manufacturing of other oxide materials for energy storage and other applications using the rapid microwave method.


Introduction
Lithium-ion batteries (LIBs) are dominating the energy storage market due to their exceptional electrochemical features including high-energy storage capacity and long cycle life.The recent adoption of electric vehicles and large-scale stationary energy storage systems has boosted the demands of LIBs and consequently the electrode materials required for their manufacturing.][7][8][9][10][11]16,20] Both approaches involve high-temperature (>800 °C) heating for long period of time.The existing electricity-powdered furnaces used to produce battery materials are energy intensive and require long reaction times (>15 h).As the demand for battery materials is expected to increase 40-fold by 2050, truly sustainable products to meet net zero emissions targets necessitate energy-efficient production processes.Moreover, advanced processes are desirable for efficient production of battery cathode materials.
Unlike conventional heating, microwave (MW) processing is faster, more energy efficient, and can be switched between on and off states more rapidly.MW synthesis is now commonplace in solution-phase chemistry, but is relatively underdeveloped in solid-state heating for synthesis and calcination to produce battery materials due to poor understanding of solid-MW interactions. [21,22]MW heating (MWH) operates on a fundamentally different mechanism compared to conventional heating.While conventional heating relies on heat transfer through conduction, convection, or radiation, MWH employs electromagnetic radiation in the MW frequency range to transfer energy directly to the material's molecules.This electromagnetic radiation consists DOI: 10.1002/aesr.202300199 The surging adoption of lithium-ion batteries, driven by diverse applications including electric vehicles, renewable energy storage, and portable electronics, has intensified the demand for high-energy cathode materials.The existing methods to produce cathode materials typically include high-temperature (>800 °C) sintering for a long period of time (>15 h), which are not only energyintensive but also significantly contribute to CO 2 emissions.This investigation presents the ultrafast microwave solid-state process to produce battery cathode materials by synthesizing high-voltage LiNi 0.5 Mn 1.5 O 4 (LNMO).Benefiting from rapid direct heating with an ultrahigh heating rate (>50 °C), high-purity and welldefined LNMO crystals are produced in a much shorter time (2 h) as compared to the conventional heating methods (>15 h).It is revealed that microwave irradiations substantially accelerate the reaction kinetics and enhance the homogeneity of LNMO.The as-synthesized LNMO cathode delivers an excellent discharge capacity of 137.96 mAh g À1 (at 0.05 C), and outstanding cycling stability (%63% retention over 280 cycles).The in situ characterizations elucidate the charge kinetics, suggesting high structural stability of the LNMO cathode during the reversible intercalation of Li-ions.These encouraging results pave the way toward the manufacturing of other oxide materials for energy storage and other applications using the rapid microwave method.
of oscillating electric and magnetic fields.One of the key distinctions is that MWH follows an "inside-to-surface" heating mechanism (Figure 1a). [23]This means that MW radiation is absorbed by the material's molecules, leading to rapid and uniform heating throughout the entire volume of the material.As a result, temperature gradients and hotspots within the material are minimized, promoting consistent and efficient heating.In contrast, conventional heating methods, typically employ a "surface-toinside" heating mechanism.Heat is transferred from the outer surface of the material to its interior through conduction, which can be relatively slow and less uniform.This often necessitates long reaction times, especially when high temperatures are involved, to ensure that the entire material reaches the desired temperature.Especially for solid metallic materials, the dominant heating mechanism is resistive heating caused by a current which is in phase with the field generated by the alternating MW field; this mechanism is commonly referred to as conduction heating. [21,24]As a result, MW irradiation is highly beneficial for solid-state reactions because substances are heated uniformly at the molecular level followed by rapid heating of the entire volume. [8,25]MWH can ensure not only rapid, simple, and costeffective process but also high-yield, high-purity products. [22,26,27]owever, despite its advantages, MW synthesis has some challenges, and more research is needed to optimize the process parameters and conditions. [21,22][30][31][32][33][34] Herein, we present an ultrafast and efficient MW solid-state process for the synthesis of high-voltage LiNi 0.5 Mn 1.5 O 4 (LNMO) cathode material.The use of a MW furnace allows for rapid heating and efficient energy transfer, resulting in a significant reduction in the synthesis time (>2 h) compared to traditional methods (>20 h).The aim of this study is to demonstrate the feasibility of implementing MW synthesis to produce highpurity cathode materials, as it offers a more environmentally friendly and energy-efficient alternative to traditional heating methods.The as-synthesized LNMO cathode delivered a capacity close to its theoretical specific capacity (147 mAh g À1 ) and good cycling stability.The lower energy consumption and CO 2 emissions of MW synthesis can also help reduce the carbon footprint of relevant industries and contribute to global efforts in mitigating climate change.The process can be further employed to synthesize other inorganic battery materials within short time and less energy consumption.

Materials Synthesis
LNMO was synthesized by a modified solid-state method with a MW tube furnace utilizing stoichiometric amounts of NiO (Sigma), MnO 2 (Sigma), and LiOH•H 2 O (Sigma) as raw materials.First, NiO and MnO 2 were thoroughly mixed by ball milling (MM400) for 3 min followed by annealing at 900 °C for 10 min in a MW tube furnace to obtain spinel-structured manganesenickel binary oxide.Then, the mixture of the as-prepared precursor and 3% excess LiOH.H 2 O were mixed by ball milling for 3 min and irradiated by MW at 800 °C for different holding times of 15, 30, 45, and 60 min in air.The samples were collected and labelled as MW15, MW30, MW45, and MW60.For comparison, LNMO was also prepared using a conventional tube furnace, that is, the mixture of NiO and MnO 2 was annealed at 900 °C for 5 h, then as-prepared precursor was mixed with 3% excess LiOH.H 2 O, followed by calcination at 800 °C for 12 h in air with a heating rate of 5 °C min À1 .The sample was labelled as CONV.

Materials Characterizations
The crystal structure of samples was analyzed through X-Ray powder diffraction (XRD) using a Rigaku SmartLab Diffractometer (Cu Kα radiation, λ = 0.15409 nm) in the range of 10°-90°(2θ).The surface composition was determined by X-Ray photoelectron spectroscopy (XPS) measurements using a Kratos AXIS Supra spectrometer.Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses were conducted on a IONTOF M6 to further characterize the composition and chemical structure of materials at the nanoscale.A 30 kV Biþ source was employed for analysis.The morphology and microstructure of the samples were explored by using field emission scanning electron microscopy (SEM) (Tescan-MIRA3), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (JEOL 2100-200 kV) imaging.A Renishaw Raman microscope was used to obtain the Raman spectra using a selective laser source at 532 nm.

Electrochemical Tests
The electrochemical performance of samples as cathodes in LIB was conducted with CR2032 coin cell (half-cells) assembled in an argon-filled glove box.The electrodes were created by combining the active materials (MW15, MW30, MW45, MW60, and CONV), along with super P and poly(vinylidene fluoride) binders.These components were mixed in a proportion of 80:10:10 by weight, and a slurry was produced using a solution of N-methyl-2-pyrrolidone. Subsequently, the slurry was casted on an aluminum foil and subjected to drying in a vacuum oven at 80 °C overnight.The cell configuration encompassed a lithium metal counter electrode, a glass microfiber separator, and the previously synthesized LNMO serving as the active material.The mass loading was in the range of 0.8-1.2mg over 12 mm diameter electrode.An electrolyte solution containing 1.0 M LiPF6 in a mixture of ethylene carbonate, dimethyl carbonate, and diethyl carbonate at a volume ratio of 1:1:1 (Sigma) was employed.These cells were cycled within the voltage range of 3.5 and 4.9 V (vs Li/Li þ ) at room temperature (RT).The galvanostatic charge/discharge testing was carried out using Neware battery testers.

In situ XRD
The procedure for preparing the LNMO cathode for in situ XRD tests was consistent with the method outlined in the preceding section.In situ XRD data were acquired using a Rigaku SmartLab diffractometer in Bragg-Brentano geometry, utilizing Cu Kα radiation (40 kV, 40 mA).The cathode prepared as previously described was subjected to cycling at an approximate rate of 0.05 C within an in situ beryllium window battery cell holder (Beijing SciStar Co. Ltd.).A total of 250 in situ XRD patterns were gathered over a span of 48 h, effectively covering two complete cycles of charge and discharge for the studied LNMO cathode.

Determination of CO 2 Emissions
CO 2 emissions during the manufacturing process of cathode materials for LIBs have become a significant challenge for the battery industry.The production of cathode materials typically involves energy-intensive processes that consume substantial quantities of electricity and produce considerable volumes of greenhouse gas emissions.The determination of CO 2 emissions during cathode manufacturing is critical for assessing the environmental impact of the LIB industry and developing effective strategies to reduce its carbon footprint.The MWH provides more efficient and uniform heating, which is demonstrated by the smaller fluctuations in the temperature and the faster temperature increase compared to the conventional heating method.The effect of LNMO cathodes production using conventional and MW methods on residence time and CO 2 emission was studied and shown in Figure 1b.The amounts of CO 2 emission are calculated from electricity consumption and emission factor shown in the Supporting Information.The synthesis of the precursor and cathode derivates using MW furnace has significantly lowered CO 2 emission in comparison with that of conventional process.The main reason is the faster heating rate and short retention time of MW method.In detail, it took less than 90 min to heat up samples from RT to 800 °C for the MW furnace, which is approximately 1.7-fold faster than that of the conventional procedure.The difference in the heating efficiency is attributed to the unique mechanism of MWH, which directly heats the material through molecular dipole rotation and friction.This results in a more efficient and uniform heating of the material, leading to improved heating speed and reduced thermal gradients.Moreover, the steep thermal gradient observed in the MWH curve may also result in a significant reduction in CO 2 emission compared to the conventional heating method.This effect is due to the faster heating speed and improved heating efficiency of MWH and could significantly reduce energy consumption and greenhouse gas emissions during manufacturing and processing of materials.

Materials Analysis
The XRD patterns of the Mn-Ni binary oxide precursors are shown in Figure S1a (Supporting Information).Both patterns are well indexed by the spinel structure of Mn 2 NiO 4 .Minor peaks of Mn 2 O 3 are shown in P-MW sample, as indicated by the characteristic peak (222).On the other hand, the XRD patterns of the prepared LNMO are presented in Figure 2a.All the patterns exhibit sharp and intense peaks, indicating a high degree of crystallinity in the samples.Moreover, all the XRD patterns are well indexed to the spinel structure of LNMO with the space group of Fd-3m.There are no obvious impurities peaks in all the samples.The lattice parameters of the as-prepared samples are calculated by DIFFRAC.TOPAS v6, as provided in Table S1 (Supporting Information).The lattice parameters and crystal volume values of the samples exhibit negligible variation, suggesting that alterations in heating methods or holding times have minimal impact on the structural integrity of the LNMO material in this study.
The SEM images of Mn-Ni binary oxide from both MW irradiation and conventional sintering (Figure S1a, Supporting Information) show a crystallized spinel structure with a uniform particle size (%500 nm) distribution.The magnified images reveal that the particles first form an octahedral shape, and then grow to form a chamfered polyhedral morphology due to preferential adherence to surfaces with higher surface tension and more catalytically active edges and vertices.Figure 2d-g shows the SEM images of LNMO cathode material MW45 and CONV.The materials produced by the two heating methods have similar particle size distribution, with most particles being micron-sized secondary aggregates, despite the calcination time differing by 10-50 times.The primary particles are chamfered polyhedrons with smooth surfaces, around 60-70 nm in size, consistent with the crystallinity from XRD.The crystal morphology of the Mn-Ni binary oxide particles is well maintained during high-temperature calcination, as shown in the comparison of Figure S1a (Supporting Information), and further grown into LNMO materials with larger particle sizes.This finding indicates that MW irradiation heating, as well as conventional heating, does not affect the formed crystal.The smaller particle size and uniform particle size distribution of the MW45, as well as its chamfered polyhedral morphology with more crystal faces, contribute to improved electrochemical rate performance and cycle stability of the material, which is confirmed by electrochemical tests in the following part.Other MW samples also show perfect crystal structure (Figure S2a-e, Supporting Information).The TEM image in Figure 2b provides valuable insights into the microstructure and composition of the MW45 LNMO sample.A clear spinel structure is shown in the HRTEM image in Figure 2c.TEM and HRTEM images of other MW samples show similar microstructure and composition in Figure S3 (Supporting Information).
The Raman spectra of the MW LNMO in Figure 3a provide insights into the structural properties of the material.The peaks at %245 and %600 cm À1 are characteristic of the P4 3 32 crystal structure, indicating that the material has a well-ordered crystal structure with a high degree of long-range order.On the other hand, the presence of peaks at %175, and %410 cm À1 is characteristic of the Fd3 ¯m crystal structure, indicating that the less ordered crystal structure with a lower degree of long-range order also exists in the material.The observation of both P4 3 32 and Fd3 ¯m crystal structures in the MW LNMO is important, as it suggests that the material is a mixture of different phases.This observation is consistent with the electrochemical performance, as different phases are likely to have different electrochemical properties.
The XPS survey scans of MW samples are shown in Figure S1b (Supporting Information); apparently, the presence of O, Li, Ni, and Mn atoms is further confirmed and illustrated in Figure 3b-e.Figure 3b shows the deconvoluted O 1s core-level peaks.For all the MW samples, the peaks at %531.4 eV (assigned to Ni─O bonds) and at %529.6 eV (assigned to Mn─O bonds) are quite intense; however, the ones at %533.8 eV (assigned to C=O(H)) are insignificant in MW30 and MW60.Moreover, the ones at %534.6 eV (assigned to adsorbed H 2 O) are only present in MW15 and MW45 samples.In Figure 3c, the core-level peaks of Li 1s and Mn 3p are depicted.The composite window is subjected to deconvolution, resulting in distinct Mn 3p and Li 1s core-level peaks.The Li 1s binding energy is %53.6 eV, corresponding to Li─Mn─O bonds.Figure 3d illustrates the Ni 2p core-level peaks.It is worth noting that the binding energy of the MnLVV Auger transition with Al Kα excitation closely aligns with the Ni 2p 3/2 peak.This peak is deconvoluted into the Ni 2p 3/2 peak along with its satellite and MnL3VV components.The binding energy for Ni 2p 3/2 is situated around 854.5 eV, while the satellite peak emerges at around 861 eV, both characteristic of Ni 4þ , NiOOH, or Ni(OH) 2 species.In Figure 3e, the Mn 2p core-level peaks are depicted, with the Mn 2p 3/2 peak residing at %642.6 eV, attributed to MnO 2 .
The ToF-SIMS spectrum and images (Figure 4 and S4, Supporting Information) provide information on elemental composition of the LNMO materials.The presence of Li, Ni, Mn, and O is confirmed in all MW and conventional samples, aligning well with the findings from the XPS analysis.On the other hand, minor surface contaminants and impurities, such as carbon and hydrogen, have also been detected.The MW45 shows relatively high intensity of Li þ as well as CONV.

Electrochemical Analysis
The electrochemical characteristic of the MW-delivered LNMO cathode was investigated and is illustrated in Figure 5, S5, S6 (Supporting Information).The differential capacity diagram of MW45 versus Li/Liþ over initial three cycles is shown in Figure 5a, while diagrams of other samples are shown in Figure S5 (Supporting Information).The curves exhibited the typical reversible peaks of LNMO during delithiation and lithiation, as reported previously, [4,7] confirming the material's structure.In particular, the major redox peaks observed in between 4.5 and 4.9 (V) are ascribed to Ni 2þ /Ni 3þ versus Li/Li þ at 4.68 V and Ni 3þ /Ni 4þ versus Li/Li þ at 4.74 V.These observations underscore the electrochemical and chemical insertion/ deinsertion of Li þ within the LNMO framework.Simultaneously, the Mn 3þ /Mn 4þ redox process is depicted by subtle redox peaks positioned within the 3.9 to 4.1 V range, corroborating earlier findings. [4]The galvanostatic charge-discharge curves of fabricated cathodes derived from MW and conventional samples are depicted in Figure 5b and Figure S6 (Supporting Information).Figure 5b shows the first five cycles of chargedischarge performance at a current density of 0.05 C (where 1 C equals to 147 mA g À1 ) in the voltage range from 3.5 to 4.9 V.After the full delithiation process, MW45 delivered a significant initial reversible discharge capacity of 137.96 mAh g À1 and a high initial Coulombic efficiency (CE) of 92.65%.To demonstrate the long-term cycle life of the as-prepared LNMO material, the cycling stability of MW45 sample is further studied at 0.05 C (see Figure 5d).The electrode exhibited excellent longterm cycling stability over 280 cycles, retaining %63% initial capacity of the initial reversible capacities along with %95% CE during the entire cycle measurements.For comparison, the capacity changes at different current rates of CONV half-cells are demonstrated along with that of MW45 in Figure 5c.When a higher current rate was applied, the MW45 delivered a higher specific capacity than CONV, which can be attributed to improved structural stability, optimized morphology, and reduced impurities.Nevertheless, upon reverting from diverse C rates, both samples recovered to %97% initial capacity, showing their ability to recover specific capacity when recurring to a small charge-discharge rate.
To reveal the potential structural evolution of MW LNMO cathode throughout the processes of lithiation and delithiation, in situ XRD data were acquired from the MW45 half-cell, as shown in Figure 6.A geometric correction model (Figure S8, Supporting Information) of the layered structure in the in situ battery cell was built in DIFFRAC.TOPAS v6 software to simultaneously refine the LNMO scale factor from 250 in situ XRD patterns (Figure S7, Supporting Information).The diffraction peak intensity, peak position, and the peak profile were corrected from the 2θ dependent thin layer absorption, thin layer diffraction, and the known dimensions of layer displacements, taking the thickness of Be window, Al-foil, and the coated LNMO cathode into consideration.The variation of normalized scale factors of the MW45 phase is summarized in Figure 6.The Miller indexes of LNMO series diffractions and the diffraction peaks of the window materials and the current collector were labelled.7]

Conclusions
The growing use of LIBs in various sectors has heightened the demand for high-energy cathode materials.The use of the MW solid-state method for the synthesis of LNMO holds great promise in mitigating CO 2 emissions during the production of LIBs.The method offers several advantages over conventional thermal methods, including heightened reaction velocity and efficiency, decreased energy usage, and improved product quality.Furthermore, the MW solid-state method is environmentally friendly, generating no toxic waste, thereby aligning with principles of sustainable and green chemistry.The results of this study have validated the viability of the MW solid-state method for the synthesis of cathode materials for LIBs.We expect its eventual industrial adoption as an effective means of reducing CO 2 emissions and advancing sustainable energy production.

Figure 1 .
Figure 1.a) Schematic of heating mechanism for MW and conventional heating methods; b) residence time and CO 2 equivalent (kg) of MW and conventionally heated samples.

Figure 2 .
Figure 2. a) XRD patterns of MW and conventionally heated samples; b) TEM image of MW45; c) HRTEM image of MW45; d,e) SEM image of MW45; and f,g) SEM images of CONV.

Figure 4 .
Figure 4. a,b) ToF-SIMS spectra with chemical imaging for MW45 and CONV under positive and negative polarities, respectively.

Figure 5 .
Figure 5. Electrochemical assessment of MW45 LNMO cathode in Li-ion half-cell at RT: a) Differential capacity diagram of MW45 versus Li/Liþ of initial three cycles; b) initial galvanostatic charge/discharge curves recorded at 0.05 C (1 C = 147 mA g À1 ); c) comparison of MW45 and CONV on variation of specific capacity with cycling at different C-rates; d) specific capacity with cycling at 0.05 C along with the CE.

Figure 6 .
Figure 6.2D plot of the 250 in situ XRD data aligned with the lattice parameter changes and the relative weight percentages refined from the simultaneous refinement.The Miller indexes of LNMO series diffractions (top) and the diffraction peaks of the window materials (Be, BeO) and the current collector (Al) were labelled.Phase 1 (blue): LiNiMnO 4 , phase 2 (orange): Li 0.5 Ni 0.5 Mn 1.5 O 4 , and phase 3 (gray): Ni 0.5 Mn 1.5 O 4 .