Boosting LiMn 2 O 4 Diffusion Coefficients and Stability via Fe/Mg Doping and MWCNT Synergistically Modulating Microstructure

Te dissolution of manganese and its deposition on the anode surface cause poor cycling stability in lithium-ion batteries. To alleviate these issues, this study probes the electrochemical activity of highly crystalline and cation-adjusted lithium manganese oxide (LMO) carbon spinel composite obtained via a modifed sol-gel synthesis procedure. Te pristine LMO cathode was functionalized with a Fe and Mg alloy and fused with purifed multiwalled carbon nanotubes (MWCNTs) to form a catalytically stabilized LiMn 1.98 Fe 0.01 Mg 0.01 O 4 /MWCNT (LMO-FeMg/MWCNT) framework. High-resolution SEM analysis showed well-dispersed particles in the nanometer size range. Te electrochemical characteristics of the novel composite materials yielded favourable electrochemical results with difusion coefcients of 1.91 × 10 − 9 cm 2 · s − 1 and 5.83 × 10 − 10 cm 2 · s − 1 for LMO-FeMg and LMO-FeMg/MWCNT, respectively. Tis improvement was supported by impedance studies which showed a considerable R ct reduction of 0.27 Ω and 0.71 Ω . Te cation stabilized system outperformed the pristine LMO material with specifc capacities around 145mAh · g − 1 , due to an enhancement in electrochemical activity and structural stability.


Introduction
Rechargeable lithium-ion batteries (LIBs) are focal electrical power components for next-generation energy applications.For energy-intensive applications, the electrochemical rechargeable cells should provide high energy density, good structural stability upon cycling, high-rate capability, and sufcient safety.In this context, the positive electrode (cathode) materials have to satisfy and maintain the abovementioned requirements.However, the advancements made to the current cathode materials used in lithium-ion batteries are not progressing, due to the high cost and sustainability issues associated with the use of cobalt or nickel elements.Lithium manganese oxides (LMOs) are one of the most promising alternative cathode materials for Li-ion batteries as they can reversibly (de)intercalate lithium at high potential diferences compared to a carbon anode and are cheap and environmentally benign [1].However, Mn 3+ dissolution occurs at temperatures higher than 55 °C after repeated cycling.Tis results in subsequent capacity decay and structural instability due to a Jahn-Teller (JT) distortion of the high-spin Mn 3+ at a deep state of discharge (SOD) [2].Despite LiMn 2 O 4 being a semiconductor with a mixed ionic (10 −4 S•cm −1 ) and electronic (2 × 10 −6 S•cm −1 ) conductivities, upgrading the electrochemical properties via modifcation is considered as benefcial.
Scientists have thus employed diferent techniques in order to negate these downfalls towards improving the electrochemistry and stability of LMO spinels in LIBs.Although surface coating is a promising strategy for improving the structural stability of LMO, the commonly used coating process is tedious and complicated.Elemental doping into the spinel microstructure has proved to be useful in most cases.Exotic metals such as Na, Mg, In, Sn, Au, and Sb are often used as dopants in lithium-ion batteries as they serve as stabilizing agents within the lattice which not only improve the durability of the battery but also catalyse its electrontransferkinetics [3,4].Billaud et al. employed a coprecipitation doping technique in order to mitigate the irregular conductivity and concluded that substitution at two distinct sites gives rise to impressive ion conductivity as well as improved electrochemical performance [5].In a recent study by Lee et al. [6], Fe allowed for increased Li + ion difusivity as a result of enlarged Li layer spacing and increased oxygen vacancy in a Fe-doped LiMn 2 O 4 truncated octahedral system.Li et al. found that Fe-doped Ni hydroxide powders experienced an enhanced lithium-ion storage capacity with improved cycling stability and rate capability [7].A similar study claimed that the replacement of Mn by Fe can effectively activate the redox reactions and improve the ionicelectronic conductivities through increasing the lower oxidation state transition metal ions [8].Te nontransition metal, manganese has also been shown to provide additional storage capacity and suppress capacity fading by increasing the average valence state of Mn which improves the difusion kinetics during Li insertion/extraction. Additionally, the electron-transfer pathway is ensured by carbonaceous additive, as it facilitates good contact bridges between particles and overcomes interparticle resistance [9].Carbon materials can provide excellent conducting support for the LiMn 2 O 4 electrode, suppress aggregation of nanoparticles, and minimize structural degradation due to volume changes induced by lithium insertion/extraction [4].
Tis work reports on the synergistic efect of codoping low valent Fe and Mg ions into the spinel lattice of LMO and its infusion with MWCNT, in order to improve the ion transport kinetics and stability within the cathode system.A lowtemperature and short-time processing at high temperatures is employed to prevent oxygen defciency at the surface of LiMn 2 O 4 .We uncover the dynamics of cation modifcation within the spinel LMO matrix and determine the efects of MWCNT on further boosting the electrochemical performance and durability of the intercalating material., Aldrich), and lithium hydroxide (LiOH) were used as received.Te cyclic voltammetry of the composite material was probed by using a 3-electrode system in 1 M LiPF 6 in 2 :1 (v/v) ethylene carbonate/ethylene carbonate/ diethyl carbonate.NMP served as the binding agent and facilitated the adhesion of the active material onto the glassy carbon electrode (GCE).A silver-silver chloride (Ag/AgCl) electrode was used as the reference electrode and a platinum wire as the counter electrode.

Purifcation of MWCNT.
MWCNTs were baked to 300 °C in a mufe furnace for 1 h before treatment with 1 M HNO 3 solution to remove impurities.Te suspension was placed under ultrasonication for 2 h.After settling at the bottom of the fask, the suspension was centrifuged and washed with distilled water before drying at 60 °C overnight.

(i) Pristine LiMn 2 O 4 (LMO), (ii) LMO/MWCNT, (iii)
LMO-FeMg, and (iv) LMO-FeMg/MWCNT.Te spinel (i) LiMn 2 O 4 (LMO) was prepared by dissolving LiOH and C 4 H 14 MnO 8 in deionized water with a stoichiometry/molar ratio of Li/Mn � 1 : 2. Te solution was mixed well and evaporated at 120 °C for 12 h to obtain the precursor powder.Te precursor was further heated at 400 °C for 1 h, followed by calcination at 800 °C for 20 h in a mufe furnace to form the LMO spinel.(ii) LMO/MWCNT was obtained after calcination of LMO with purifed MWCNT at 600 °C [10].Te molar ratio of LMO was controlled to be 4 :1.Te cation-modifed (iii) LMO-FeMg cathode material was prepared by dissolving stoichiometric amounts of LiOH, C 4 H 14 MnO 8 , C 14 H 27 Fe 3 O 18 , and Mg(CH₃COO)₂ with a molar ratio of Li/Mn/Fe/Mg � 1 : 1.98 : 0. : 0.1 in deionized water, followed by a gentle stirring at 120 °C, over 12 h.It was then heated at 400 °C for 1 h to complete the evaporation and calcined at 800 °C for 20 h in a mufe furnace to form FeMg-doped LMO crystalline powders.(iv) Te LMO-FeMg/MWCNT composite was prepared by simply mixing the AS-prepared LMO-FeMg with purifed MWCNT in a mass ratio of 4 : 1 (LMO-FeMg/ MWCNT) in 100 ml methanol and subjected to sonication for 3 h.Te LMO-FeMg/MWCNT suspension was washed and dried before calcination at 600 °C.
Te cycling stability test was performed by using a coin cell.Te cathode was prepared by mixing LMO, polyvinylidene fuoride (PVDF), and carbon black in a mass ratio of 92 : 6 : 2 in N-methyl-2-pyrrolidone (NMP) at 500 rpm for 24 h.Te ink was then coated onto an aluminum foil substrate by the doctor blade method.Te coating thickness was 60 µm.Te coated foil was then dried at 120 °C overnight before being punched into 16 mm diameter circles.Te anode was composed of a 90 : 6 : 2 : 2 mixture of graphite, water-processable binder materials carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and carbon black in water.Carbon black is a fundamental component as it increases the electrical conductivity of the system, prevents agglomeration of anode materials during cycling, and maintains stability [11].Te separator used was Celgard polypropylene flm.Te punched electrodes were assembled into 2032 coin cells in a dry room with a humidity of 1.0% h.r.Te electrolyte used was 1 M LiPF 6 in 2 :1 (v/v) ethylene carbonate/diethyl carbonate.

Results and Discussion
3.1.Structural Characterization.Te X-ray difraction overlay patterns in Figure 1 represent the pristine LMO, LMO-FeMg, and LMO-FeMg/MWCNT materials.Te diffraction patterns were taken at room temperature in the range of 5 < 2 theta <75 °using step size and measurement time of 0.027 °and 1 sec/step, respectively.Te patterns have been indexed to the Fd-3m phase (JCPDS no.01-083-0358; space group Fd-3m, a = 8.19190 Å, and a unit cell volume of 543.66 Å3 ) suggestive of high purity and crystalline samples.
where D is the crystallite size (nm), λ is the difraction wavelength ( Å), β is the full width at half maximum (FWHM) value, and θ is the difraction peak position.Te values are given in Table 1.Te particle size contributes to enhancing the electrochemical performance (especially improving rate capability).To evaluate the efect of the doping ions on the crystalline lattice of the obtained spinel, the lattice parameter a was calculated from the XRD data by the least-squares method.
Te a value of 8.246 Å for the pure spinel was obtained which is in close agreement with that of the stoichiometric LiMn 2 O 4 spinel.Te values for the doped samples were 8.244 and 8.243 Å, respectively, which are indicative of an efective doping since this process is associated with the substitution of Mn 3+ ions in the octahedral 16d sites by cations of smaller ionic radius.Tis efect is further demonstrated through the charge and discharge tests.
Figure 2 shows the surface morphology of the doped LMO-FeMg spinel, where the material consists of closepacked octahedrally shaped particles.It is evident that the FeMg cations are well incorporated within the LMO particles, with the brighter contrast being the Fe and Mg atoms.analysis (see following).Te inclusion of the MWCNT gives the material a fufer appearance with an increased surface area which serves as an ideal medium for Li-ion extraction and intercalation.Te composite facilitates electronic contact between the LiMn 1.98 Fe 0.01 Mg 0.01 O 4 electrode and the current collector through an overlap of the electrochemically active energies of the conductive MWCNT.Tis multidimensional aspect is viable as Li-ion difusion through the electrolyte is enhanced [12].
Te energy dispersive spectrum shows the elemental composition obtained from HRSEM analysis for LMO-FeMg and LMO-FeMg/MWCNT.While EDS does not detect elemental Li and Mg in the composite, it was evident from XRD analysis which deduced the spinel crystal structure.Additionally, increased carbon levels were detected which refect the successful infusion with MWCNT.Te average Fe content in the sample was calculated to be 1.08% and a small amount of 0.25% for Mg was detected, respectively.In the LMO-FeMg/ MWCNT sample, the average Fe content was found to be 0.73% and the Mg amounted to be 0.85%.Te dopant concentrations are close to 1% as expected.
From Figure 3(a), well-defned lattice fringes are observable and are roughly spaced at 0.42 nm apart, which are indicative of the crystallinity of the LMO-FeMg material.Te inset shows the particles at higher magnifcation, where the particle shapes appear to be octahedrally shaped, which is well in agreement with the HRSEM.In Figure 3(b), the particles grow well within the nanotube network with the SAED image confrming the deduced crystal orientation in Figure 3(c) and average particle size in Figure 3(d) ranging between 100 and 140 nm which is slightly smaller than conventionally doped Li-Mn-O particles.No defect was observed at the interface between the host spinel and layered surface phase, which provides an efcient path for the ionic and electronic mobility [13].
Figure 4 shows the FTIR spectra of LMO, LMO-FeMg, and LMO-FeMg/MWCNT in the wavenumber range of 0-4000 cm −1 at room temperature.Te distinct absorption bands observed at 507.1 and 612.3 cm −1 are characteristic of LMO spinels and are assigned to the stretching vibration of the MO 6 octahedra.Tese distinctive symmetrical bands are a direct result of the Mn ion being split into two octahedrons, namely, [Mn 3+ O 6 ] and [Mn 4+ O 6 ].Tere is a small shift of two peaks towards the higher wavenumbers at 509.1 and 614.3 cm −1 .Tis slight shift could be attributed to the presence of Fe and Mg dopants [14].Te octahedral site preferences of the codoped cations are clearly demonstrated by these fndings.Specifc interactions such as Mg-O/Mn-O interactions cause peak shifts in the FTIR spectra, with higher wavenumbers and higher Mn-O bond strengths [15].Te results show that doping enhances the Mn-O bond, further enhancing the stability of the spinel structure.Te occurrence and intensity of the absorption band at Particle size reduction has a limitation as extremely small particles show negative efect in performance.More critically, independent of the particle size distribution, the existence of coarse particles are found to promote lithium plating, which lowers cell performance and threatens the safety of battery operation.Tis is the average crystallite size of 44 nm.Te particle size contributes to enhancing the electrochemical performance (especially improving rate capability).Te normalized Raman spectrographs of LMO, LMO-FeMg, and LMO-FeMg/MWCNT in the spectral regions of 400-800 and 1400-1700 cm −1 are shown in Figure 5. Te strong band in (a) is representative of the A 1g Raman vibrational mode of LMO which is indicative of the MnO 6 octahedrons and the LiO 4 tetrahedrons combined [16].A slight blue shift of about 10 cm −1 is observed between the cation-doped and pristine materials.Tis is indicative of the Fe and Mg dopants which substitute with Mn atoms in the spinel.Te shoulder at 580 cm −1 can be ascribed to the F 2g mode which becomes a noticeably broader A 1g band as a result of bond length variations which were introduced by the dopants.Tis could be due to the increase of Mn 4+ in the LMO crystallite [17].An intense D band and less intense G band shown in (d) can be ascribed to the MWCNT-coated spinel [18], which indicates that the MWCNT in LMO-FeMg composites has fewer surface defects.Terefore, spinel rigidity and improved Li difusion pathways are maintained.
To further probe the modifed materials and interfaces, X-ray photoelectron spectroscopy (XPS) was used to provide quantitative chemical insights.XPS spectra were measured at room temperature in an ultra-high vacuum (UHV) chamber at a base pressure of 2 × 10 −10 mbar.Te measurement chamber was equipped with a SPECS XR 50M monochromatised X-ray source with an Al anode (Al Kα excitation line, or h] � 1486.71eV) and a SPECS PHOIBOS 150 hemispherical electron energy analyser.A food gun set was used to operate at the following parameters: electron energy of 2.0 eV and electron fux of 25 µA.Te overall energy resolution of the combined analyser + photon source system was set to 1.0 eV for the survey scans and to 0.8 eV for all the other high-resolution core-level spectra.
Te survey scan for the pristine and modifed cathode materials presented in Figure 6 shows minute Na contamination in the sample, deriving from the synthesis process.Te amount of Fe and Mg ions present in LMO-FeMg and LMO-FeMg/MWCNT was below the detection limit of the XPS system (approximately 1% to 5%), making these two elements not detectable.Te O1 core level spectra for the samples are displayed in Figure 7. Tese spectra have been ftted by adding the three Voigt line-shaped singlets labelled O1, O2, and O3 in the fgure, to a Shirley background.Tis is consistent with the literature reported for similar compounds [19].Te ftted components and the background are appended to the spectra, and the overall ft shows very good agreement with the experimental data.O1, the component at the lower binding energy (BE) side, is assigned to stoichiometric oxygen in the main matrix.Component O2 can be ascribed to oxygen vacancies and/or defects within the crystalline structure [20].Finally, O3 is attributed to surface contaminants, i.e., oxygen that is chemisorbed on the surface of the sample.Te BEs and the relative percentage areas of the three ftted components for O1s are reported in Table 2. Te values in the table show that about 60% of the spectral weight of the core level in LMNO and LMO-FeMg is derived from stoichiometric oxygen, while this percentage decreases to 45% in LMO-FeMg/MWCNT because the percentage ratio of component O3 increases tremendously with respect to the other two samples.
Figure 8 displays the ftted Mn 3s core level spectra for the three samples.Te line shape of the Mn 3s core level is a good indicator of the oxidation state of Mn in a compound.In fact, it consists of two separated components resulting from the exchange interaction between the 3s core hole and the 3d unflled shell [21].Te main peak at lower BE can be assigned to the high-spin (HS) state, or Mn (1), and its satellite at higher BE to the low-spin (LS) state, or Mn (2), due to the 3s core electron spin being coupled, respectively, parallel or antiparallel to the 3d electrons' majority spin.Each spectrum has been ftted with two singlets having a Voigt line shape and a Shirley-type background.Te ftted components, the background, and the overall ft to the data have been appended to the fgure.Te magnitude of the exchange splitting ΔE 3s is proportional to 2S + 1, where S is the total spin moment of the unpaired 3d electrons in the ground state.Tis means that an exchange splitting, ΔE 3s , around 5 eV is a signature of a majority Mn 3+ , or 3 d 4 , for Mn ions.Deviations from this value hint at changes in the 3+/4+ oxidation state ratio [22,23].In this study, the addition of Fe and Mg to LMO in LMO-FeMg causes ΔE 3s to decrease by about 0.56 eV (from 5.49 eV in LMO to 4.93 eV in LMO-FeMg).Tis hints at an increase in the percentage of the Mn 4+ contribution in LMO-FeMg, which is supported by the change in the Mn 2p core level line shape as is shown below.Te addition of MWCNT in LMO-FeMg/MWCNT causes the Mn 3s exchange splitting to increase again to 5.18 eV, showing that the amount of Mn 3+ is increasing again with respect to that of LMO-FeMg (a fact that is again supported by the change in the Mn 2p line shape).It can therefore be inferred that the addition of FeMg and MWCNT causes changes in the oxidation state of Mn ions which can then be related to the chemical performance of the composites.
Mn 2p core level spectra are displayed in Figure 9(a).Te line shape of this core level is composed of two broad peaks whose centroids are located at ∼642 eV and ∼654 eV BE, corresponding to Mn 2p 3/2 and Mn 2p 1/2 spin-orbit components, respectively.Te oxidation state of Mn ions can be inferred by comparing the line shape of this core level with the relevant literature.Colour-coded arrows in the fgure indicate the centroid of the main Mn 3+ /Mn 4+ features.Te line shape of the Mn 2p core level of LMO is in very good agreement with that of Mn 2 O 3 [24], showing a majority 3+ oxidation state of Mn ions in this composite.Te addition of FeMg causes a change in the line shape: the spectral weight of the low BE shoulder (which is the defning feature of a 3+ oxidation state) evidently decreases, confrming the shift towards a majority 4+ oxidation state in LMO-FeMg, as already discussed for the Mn 3s core level.Te further addition of MWCNT sees an increase in the intensity of the 3+ shoulder, confrming that the oxidation state changes to be more similar to that of the LMO composite.
Finally, Figure 9(b) shows the C 1s core level spectrum for LMO-FeMg/MWCNT.Te spectrum has been ftted with 4 components, representing the carbon bonds with its ligands, consistent with respect to the relevant literature [25,26].C1 and C2 are ascribed to double and single carbon-to-carbon chemical bonds and are the most intense features in the spectrum.

6
Journal of Nanotechnology Te inset shows the plot of anodic and cathodic peak currents (i pa and i pc ) against the square root of the scan rate (] 1/2 ).Te plot shows a linear dependence between i p and ] 1/2 at almost all scan rates, and the peak currents (i p ) are proportional to the square root of the scan rate (] 1/2 ), i.e., i p α] 1/2 at diferent scan rates.Tis indicates that the electrode reaction is a difusion-controlled process.Te system displays better performance at a slower scan rate; however, as the rate increased the reversibility was maintained in accordance with equation ( 1) and illustrated good comprehensive stability with a stable increasing current density. ( After the addition of MWCNT, the electrochemical behaviour of LMO-FeMg/MWCNT was probed by using similar parameters as for LMO-FeMg, for comparison.Te cyclic voltammogram of LMO-FeMg/MWCNT shown in Figure 11(a) is obtained at a scan rate of 1 mV•s −1 .Four distinct oxidation peaks were observed, with the peaks at −0.44 and −0.19 V being attributed to the Mn 3 /Mn 4+ redox couple and the peaks at 0.22 and 0.36 V attributed to the Fe 3+ /Fe 4+ transitions [27].Te current density of the LMO-FeMg/MWCNT sample was signifcantly enhanced compared to LMO-FeMg, which suggests that the nanotubes facilitate the ease of movement of electrons within the 3D framework and the reduction of the polarization loss.
Te scan rate-dependent study of the LMO-FeMg/ MWCNT composite is shown in Figure 12(b), where two broad oxidation peaks were observed that shift to higher voltage regions as the scan rate increases.Te Ragone plot of peak current vs scan rate (inset) shows that the redox reaction taking place on the electrode surface is indeed a difusion-controlled process in which Li-ions migrate between the electrode interface and electrolyte.MWCNTs facilitate the interaction with LMO-FeMg particles to form a 3D network that promotes lithium-ion transport.Te charge/discharge redox reactions during the charge and discharge processes are represented in the following equations:   10 Journal of Nanotechnology Additionally, it is widely known that the difusion of Liions is the rate-determining for Li-ion batteries [28].Due to the combined synergistic efect of Fe and Mg, the composite ofers increased Li + ion difusivity resulting from the increased oxygen vacancy as the path of Li + ion and additional storage capacity, mainly due to the high theoretical volumetric capacity of metallic magnesium.A difusion coefcient of 5.83 × 10 − 10 cm 2 •s − 1 was obtained for LMO-FeMg and 1.91 × 10 − 9 cm 2 •s − 1 for LMO-FeMg/MWCNT, by using the following equation, where I p , n, A, D + Li , and v 1/2 are the peak current, number of electrons, area of the electrode, difusion coefcient, and scan rate, respectively.
Te difusion coefcient of Li + in the LMO-FeMg/ MWCNT electrode is an order of magnitude larger than that in the LMO-FeMg electrode.Tis confrms the notion that carbon nanotubes not only reinforce the structural stability of the LMO cathode material to mitigate Mn dissolution but also improve its electrochemical properties [29].
Electrochemical impedance spectroscopy (EIS) was performed to investigate charge-transfer activity as well as interfacial properties of the electrodes.Te EIS measurements were recorded at a potential of 3 V at a frequency range of 10 5 -10 −2 Hz [30].Te Nyquist plots of (a) LMO, (b) LMO-FeMg, and (c) LMO-FeMg/MWCNT are shown in Figure 12.Te kinetic diference of the electrodes was extracted by modeling AC impedance spectra based on the Randles equivalent circuit shown in the inset of the spectrograph.Equivalent circuits consist of the Randles circuit, solution resistance, R sol ; charge-transfer resistance, R ct ; double-layer capacity, C dl ; and refective fnite Warburg impedance, W. Each plot presents a single semicircle at high frequency that is well defned and an inclined line at low-frequency attributed to Warburg impedance related to Li+ difusion in the bulk of electrode.Te decreased impedance in the Fe-and Mg-doped materials as compared to pristine LMO correlates well with the difusion coefcient obtained from cyclic voltammetry.Te LMO-FeMg/MWCNT displayed the lowest chargetransfer resistance resulting from catalysed redox kinetics within the composite framework.Te data points of the semicircle at low-frequency regions give the intercept correlating with R s + R ct from the R ct values, which were extrapolated by subtracting the value of R s [31].
Te cycling performance of (i) LMO, (ii) LMO-FeMg, and (iii) LMO-FeMg/MWCNT after 100 cycles is shown in Figure 13(a).Te pristine LMO displays a typical cyclic channel after 100 cycles with a signifcant capacity drop and capability retention of only 69%.
Te specifc capacitance (C s ) of cathode materials electrodes was calculated by using the following equation: where C s (F/g) is the specifc capacitance, I (A/g) stands for current density, m (g) is the coated mass of the active material, V is the total volume (cm 3 ), and s stands for discharging time, t, respectively.Te LMO-FeMg material delivered an initial specifc capacity of about 145 mAh•g −1 with a moderate capacity fade after consecutive cycling.Te improved cycling capacities of LMO-FeMg can be a result of dopant substitution of Mn within the spinel which signifcantly reduces Mn 3+ disproportionation reactions as well as the J-T efects [32].Tis is an interesting advancement, as it signifes the role of cations in improving the chemical interactions and charge difusion.With the functionalization of the LMO-FeMg with MWCNT, the carbon composite cathode delivered an initial specifc capacity of about 138 mAh•g −1 .However, the structure appears more stable displaying improved capacity retention over time.
Te charge-discharge capabilities of LMO-FeMg/ MWCNT are shown in Figure 13   Li-ion transitions taking place between λ-MnO 2 and LMO.Te lithium ions were inserted into the tetrahedral sites at the 4 V plateau with 0 < x ≤ 1. Te remaining lithium ions were inserted into the octahedral sites at the 3 V plateau with 0 < x ≤ 2. It is to be noted that cycles 50-100 have a specifc discharge capacity of 100 mAh•g −1 .Te 1 st , 50 th , and 100 th cycles of LMO-FeMg/MWCNT show an initial dischargespecifc capacity of 142 mAh•g −1 with the 50 th and 100 th cycles in the vicinity of 120 mAh•g −1 , which can be attributed to the synergy between the composite materials.
Te rate performance of the cathode materials was further analysed with the discharge capacity of LMO-FeMg and LMO-FeMg/MWCNT at c-rates ranging from 0.5 to 7 C after 30 cycles with 1C being taken as 100% discharge capacity as shown in Figure 14.
A relatively high discharge capacity between 140 and 145 mAh•g −1 was obtained at 0.5C for LMO-FeMg/ MWCNT.It can be seen that the discharge capacity of LMO-FeMg lessens slightly to 137 mAh•g −1 after 30 cycles.Both materials obtain a capacity retention of 97% at room temperature which indicates that the composite material is both durable and operable at high currents.Te LMO-FeMg/MWCNT demonstrates good reversibility at varying c-rates as well as a reduction in capacity loss in comparison with the pristine LMO as the discharge capacity remains above 105 mAh•g −1 even at 7C [33].Tis capacity retention after deep current cycling is higher than that of LMO-FeMg which was about 140 mAh•g −1 at 0.5C (inset).Tis implies that the network of carbon nanotubes and Fe and Mg cations enhances the capacity retention of the LMO spinel [34][35][36].

Conclusion
Te novel LMO-FeMg and LMO-FeMg/MWCNT composite cathode materials were obtained via the sol-gel method and compared with one another as well as with the pristine LMO cathode material.Te formation of the crystalline LMO-FeMg/ MWCNT composite and its physical and electrochemical properties were probed using microscopic and spectroscopic analyses.HRSEM analysis showed octahedrally shaped 12 Journal of Nanotechnology particles with an average diameter of 140 nm being well dispersed and reinforced within the LMO-FeMg/MWCNT framework.Te modifed cathode yielded favourable electrochemical results with difusion coefcients of 1.91 × 10 − 9 cm 2 • s − 1 and 5.83 × 10 − 10 cm 2 • s − 1 .Tis improvement was also observed in the impedance studies where considerable R ct reductions of 0.27 Ω and 0.71 Ω for LMO-FeMg and LMO-FeMg/MWCNT were obtained.Te FeMg-doped LMO and FeMg/MWCNT within a coin cell setup outperformed that containing the pristine LMO material with specifc capacities around 145 mAh•g −1 compared to about 107 mAh•g −1 for LMO.Te composite cathode delivered enhanced electrontransfer kinetics due to the higher surface area and stability provided by the MWCNTs which also facilitates good dispersion of LMO-FeMg as observed by HRSEM.Te good cycling performance of the LMO-FeMg/MWCNT cathode is attributed to the reduction of the polarization loss for this doped-cation-to-Mn ratio.

Figure 1 (
d) inset represents the crystal structure of the cationdoped LMO through the polyhedral model.Te polyhedral LMO structure shows the Fd-3m geometry of the spinel with 2 Journal of Nanotechnology purple polyhedra representing the substitution and partial reduction of manganese.Te average crystallite size of 44 nm was determined from the Scherrer equation as follows:

3. 2 .
Electrochemical Measurements.Te electrochemical performance of the LMO-FeMg and LMO-FeMg/MWCNT composite materials was probed using a 3-electrode system in 1 M LiPF 6 in 2 : 1 (v/v) ethylene carbonate/ethylene carbonate/diethyl carbonate.Galvanostatic charge/discharge and cyclability analysis was conducted on a BTS battery analyser workstation.Electrochemical measurements (CV and EIS) were conducted to probe the redox activity of LMO-FeMg/MWCNT resulting from the synergy of a chemical interaction.Te cyclic voltammogram of LMO-FeMg shown in Figure10(a) displays multiple redox peaks in the voltage range of −0.5-0.5 at a scan rate of 1 mV•s −1 .Te redox couples at −0.25/−0.19V and 0.28/ 0.19 V can be ascribed to Mn 3+/4+ transitions upon Li-ion insertion and extraction reactions taking place across the electrode surface.Te broader peak pair in the −0.46/0.5 V region is obtained due to the partial substitution of Fe and Mg atoms and can be ascribed as Fe 2+/3+ transitions which contribute towards the improved current densities.Te voltage gap between the reduction and oxidation peaks is attributed to the electrode polarization, closely correlating with the active material's conductivity.Changing the scan rate afects the CV features including the current as is observed for LMO-FeMg at varying scan rates (5-60 mV•s −1 ) in Figure 10(b).
(b).Two distinct voltage plateaus are observed at 4.0 and 3.2 V, which indicates the

Table 1 :
Average crystallite size determined from 2θ positions.

Table 2 :
BEs (in eV), FWHM (in eV), and relative percentage areas (in %) of the three components of the O 1s core level.