A novel Li₂Mn_2.9Ni_0.9Co_0.2O₈ spinel composite interweaved with carbon nanotube architecture as a lithium battery cathode

Abstract

Li₂Mn_2.9Ni_0.9Co_0.2O₈ spinel nanoflakes, synthesized for the first time using a template assisted co-precipitation method, form an interconnected composite with MWCNTs and demonstrate excellent lithium intercalation/de-intercalation performance upon cycling. As a cathode, the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite exhibits exceptionally interesting properties such as high specific capacity, acceptable rate performance upto 5C rate conditions, appreciable coulombic efficiency and better cycling stability compared with that of pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈, especially due to the effect of the interweaved MWCNTs. A steady state discharge capacity of ∼210 mA h g⁻¹ at a current density of 0.1C has been achieved with the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite cathode compared to an inferior capacity of 135 mA h g⁻¹ exhibited by the pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ cathode. Even at a discharge rate of 1C, the titular cathode, belonging to the category of spinel oxides, delivers a high capacity of 100 mA h g⁻¹ after 100 cycles, which is noteworthy. The encouraging electrochemical properties result from the synergistic effect of the enhanced high lithium-ion diffusion kinetics of the nanosized flakes of Li₂Mn_2.9Ni_0.9Co_0.2O₈ and the high electronic conductivity of the multiwalled carbon nanotube network. The current findings leave ample scope to tailor make and exploit a wide variety of solid solutions based on Li₂MMn₃O₈ (M-transition metal) family spinel cathodes, possibly for high capacity and high rate lithium battery applications, especially when deployed in the form of a composite with MWCNTs.

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1. Introduction

In the modern green energy revolution context, rechargeable lithium batteries are the choicest, and a globally accepted, energy storage medium for application in emission free vehicles and portable electronic devices.^1–5 Even though these lithium batteries are capable of exhibiting better electrochemical performance than other battery systems, their safety, lifetime, energy density and high voltage characteristics are required to be improved to meet with the energy needs of consumer market based applications. Commercialized LiCoO₂ suffers from a lower potential value of 3.7 V and its questionable safety, especially above 4.2 V, has triggered an ever pressing need to explore a few other novel, safe and economically viable cathode materials.^6–9 Particularly, the demand for lithium-ion batteries with alternative cathode materials is augmented, thus provoking the identification and development of cathode materials that are bestowed with high electronic and ionic conductivity, a high working voltage, appreciable stability and cyclability as is the current need.

After the successful exploration of lithium rich layered oxide materials, viz. xLi₂MnO₃·(1 − x)LiMO₂ (M = Mn, Ni, Co), as high capacity cathode materials, researchers, including our group, are extensively trying to develop newer cathode materials, capable of exhibiting capacities as high as 200 mA h g⁻¹.^10–13 However, these Li₂MnO₃ stabilized cathode materials suffer from large irreversible capacity loss behaviour in the first formation cycle that prevents their commercial exploitation. On the other hand, spinel type transition metal oxides of the general formula AB₂O₄ form a large class of materials, wherein the inherent and diverse properties ensure their wide application in areas such as lithium-ion batteries, catalysis and supercapacitors.^14–18 Depending upon the structure and composition, spinel oxides exhibit high electronic and ionic conductivity and demonstrate themselves as potential cathode materials. Among them, LiMn₂O₄ and nickel doped LiMn₂O₄ are widely studied spinel materials, which are capable of delivering even second level charge capability in aqueous electrolytes.^19–23 Herein, facile lithium ion diffusion through the three dimensional ion channels and the high electrochemical potential of these spinel type cathodes make them more suitable for practical applications.

Towards this direction, quite different from the conventional and popularly known AB₂O₄ type of 4 V spinel cathodes, yet another class of spinel compounds with a general formula of Li₂MMn₃O₈ (M = Cu, Co, Ni and Fe) are capable of performing as a lithium battery cathode over the 4.5 V region, depending upon the transition metal, and thus gain importance as high voltage cathode materials.^24–30 These spinels, although identical to the LiMn₂O₄ spinel, are bestowed with entirely different cation ordering in the octahedral sites, i.e., 1 : 3 instead of 1 : 1 ordering (found with AB₂O₄ type), and hence could not be considered as simple binary spinels.²⁴ H. Kawai et al. reported for the first time that a Li₂CoMn₃O₈ cathode exhibits a very high working voltage, c.a. 5 V, with excellent cycling stability.²⁵ Subsequently, the same authors reported on the application of a Li₂FeMn₃O₈ cathode with a voltage plateau at 4.9 V.²⁶ Even though a few other reports are available on the high voltage characteristics of the Li₂MMn₃O₈ class of compounds, their long term cycleability and their suitability for high rate applications are yet to be explored. In general, electrolyte decomposition, structural distortion and cation ordering based issues upon extended and high rate cycling are certain hampering parameters, due to which the intriguing behavior of such 5 V spinels faces innumerable challenges. Towards this direction, the synthesis–structure–property relationship pertinent to customised Li₂CoMn₃O₈ based spinel cathode formulations needs to be investigated in detail, as it remains a less studied area of research.

Of late, reports on spinel composite cathodes provide a tangible platform and enormous scope to fine tune the properties of native spinel cathodes to best suit the requirements of high capacity cathodes for use in rechargeable lithium batteries. Similarly, the provision of shorter lithium diffusion pathways through reducing the particle size to the nanoscale along with conductive surface modification has been reported to address the rate performance related issues associated with spinel cathodes. Recently, H. K. Noh et al. proposed a simple strategy to harvest a capacity of 220 mA h g⁻¹ from lithium manganese oxide at a cut off voltage of 2.5 V from a composite consisting of a flexible skinny graphitic layer.³¹ Similarly, S. Lee et al. demonstrated the high energy and high power capability of single crystal LiMn₂O₄ clusters through surface modification with carbon coating.³² From these reports, it is clear that surface modification with a suitable carbon might provide a facile electron transport and a stable spinel structure in the 3 V region. However, no report on the possibility of qualifying a Li₂MMn₃O₈ spinel, otherwise known as a 5 V cathode, for high capacity and/or high rate lithium battery applications is available. In this regard, the current piece of work assumes significant importance, as it demonstrates for the first time the suitability of the Li₂MMn₃O₈ type of high voltage cathodes for high capacity and rated capacity applications, especially with the carefully chosen and tailor-made formulation of solid solutions.

Herein, we report a simple solution based synthesis of an interconnected Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite material by interweaving the spinel Li₂Mn_2.9Ni_0.9Co_0.2O₈ compound within a multiwalled carbon nanotube matrix. This newly designed heterostructured material offers a significantly high discharge capacity and better rate performance, facilitated mainly by the interconnecting and the conductive MWCNT network. In other words, MWCNTs provide the desirable carbon wiring and improve the conductivity, thereby increasing the capacity and rate capability. Herein, pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ has been prepared using a template assisted co-precipitation method (see the Experimental section for details). Without requiring any complex synthesis procedure or experimental conditions, the as-prepared Li₂Mn_2.9Ni_0.9Co_0.2O₈, especially when deployed in the form of a Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite cathode, demonstrates excellent electrochemical properties. A high specific capacity of 210 mA h g⁻¹ at a 0.1C current rate and an appreciable capacity of 100 mA h g⁻¹ at the end of the 100^th cycle at a 1C current rate have been observed, which is the highlight of the study.

2. Experimental section

2.1. Synthesis of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite

All reagents used were of analytical grade (Sigma-Aldrich and Alfa Aesar) and were used without further purification. In a typical synthesis, polyethyleneimine (M_W = 1300) (4 mL) was added to deionised water (50 mL) and a stoichiometric ratio of Mn, Co and Ni precursors was mixed with magnetic stirring. After the mixture became transparent, 2 M KOH was added drop wise until the solution became a chocolate brown colour. The solution was left under overnight stirring and the obtained brown colored precipitate was centrifuged, washed three times with deionised water and absolute alcohol (to completely remove uncomplexed polyethyleneimine) and dried in a hot air oven at 120 °C for 12 h. The dried powder was subsequently mixed with LiOH and ethanol with the help of a mortar and pestle and further calcined at 450 °C for 6 h. To the obtained powder, MWCNTs (5 wt%) were added and dispersed using ultrasonication in ethanol for 2 h. The mixture was dried at 80 °C for 10 h to get the final Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite.

2.2. Material characterization

The phase purity, crystallinity and structural stability of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT compound were examined using a Bruker D8 advance powder X-ray diffractometer. Unit cell lattice constants were calculated from the XRD data using “unitcell” software. Field emission scanning electron microscopy (Zeiss Field Emission Scanning Electron Microscope) was used to investigate the morphology of the sample and high resolution transmission electron microscopy (JEOL JEM 2100 Transmission Electron Microscope) was used to determine the particle size and also for the recording of the SAED pattern. The Raman spectrum was recorded on a Renishaw InVia Raman spectrometer with an argon ion laser and charge coupled device detector. FTIR spectra were recorded using a Bruker Tensor 27 FT-IR Spectrometer in the range of 400–4000 cm⁻¹ in the transmittance mode. The conductivity measurements were made using a Wayne Kerr 6500 P high-frequency LCR meter. Powder samples were made into pellets of 2 mm thickness and 10 mm diameter. Conductive silver paste was applied on both sides of the pellets and they were sandwiched between stainless steel blocking electrodes to measure the conductivity.

2.3. Electrochemical studies

CR 2032 coin type cells (Hohsen) were used for evaluating the electrochemical performance of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite cathode. Coin cells were assembled in a glovebox (Mbraun) under an argon atmosphere. The working electrode was fabricated by mixing the active material, acetylene black and a polyvinylidene fluoride binder in the weight ratio of 80 : 10 : 10. Lithium ribbon was used as both the counter and reference electrode and Celgard membrane was used as the separator. A 1 M solution of LiPF₆ in a 1 : 1 (v/v) mixture of ethylene carbonate and diethyl carbonate was used as the electrolyte. The charge–discharge studies were performed using an Arbin battery tester in the voltage range of 2.0–4.9 V at room temperature. Electrochemical impedance spectroscopy (EIS) measurements were carried out using a Biologic VMP3 electrochemical work station in the frequency range from 0.25 MHz to 10 mHz with an amplitude voltage of 10 mV at room temperature.

3. Results and discussion

Scheme 1 illustrates the approach used for the synthesis of Li₂Mn_2.9Ni_0.9Co_0.2O₈ using a simple template assisted co-precipitation method. Polyethyleneimine (PEI) has been used as the template in this synthesis. It is well known that polyethyleneimine has the ability to form complexes with a wide variety of metal ions.^33–35 The metal ions initially react with PEI molecules during stirring and form metal–PEI complexes. On adding potassium hydroxide, these metal ions are precipitated as mixed metal hydroxide. The separated metal hydroxide precursor has a flaky morphology, as evidenced in Fig. 1a and b. The homogenous distribution of metal hydroxide nanoflakes is believed to result from the attachment of the polyelectrolyte (PEI) on the surface of the metal, which in turn reduces the size of the particles by preventing the coalescence of particles due to the repulsive interaction between like charges on the polyethyleneimine. Calcination of this precursor after mixing with LiOH leads to the formation of Li₂Mn_2.9Ni_0.9Co_0.2O₈ nanoflakes as the final product (Fig. 1c and d). The flaky morphology of the precursor is preserved even after calcination at 450 °C. The flakes are uniformly distributed in size, possessing a width of 200 nm and an average thickness of 15–20 nm. The multiwalled carbon nanotube interweaved heterostructure has been prepared by dispersing Li₂Mn_2.9Ni_0.9Co_0.2O₈ nanoflakes and MWCNTs in ethanol with the help of a simple sonication process. Solvent evaporation from this mixture upon drying leads to the formation of the spinel Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite material. The morphology of the spinel Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite material is displayed in Fig. 1e and f, which evidence the presence of heterostructured material, consisting of spinel nanoparticles interconnected with each other through the MWCNT network. The conductive carbon wiring offered by the MWCNTs enhances the electronic conductivity of the pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ material and thereby improves the electrochemical performance of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite cathode ultimately.

Scheme 1 Synthesis of Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT heterostructures using a co-precipitation method.

Fig. 1 FESEM images of (a and b) the mixed metal hydroxide precursor, (c and d) the pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ synthesized at 450 °C and (e and f) the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite at different magnifications, showing the presence of a uniform network, formed out of multiwalled carbon nanotubes.

The morphology and particle size of pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ and the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite are evident from the recorded HRTEM images. Fig. 2a and b evidence the presence of flakes possessing a thickness of 15–20 nm and a diameter of 150–200 nm. High resolution images show the presence of lattice fringes (Fig. 2c and S1†), pertinent to the spinel Li₂Mn_2.9Ni_0.9Co_0.2O₈ framework, and the SAED pattern confirms the polycrystalline nature of the spinel Li₂Mn_2.9Ni_0.9Co_0.2O₈ nanoflakes (Fig. 2d). This observation has been corroborated using the recorded XRD pattern and Raman spectra (displayed in Fig. 3 and 4 respectively), which are characteristic of spinel materials. Additionally, Fig. 2c and d confirm the crystalline nature of low temperature synthesized spinel nanoflakes, which is quite interesting. The representative HRTEM images of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite are displayed in Fig. 2e and f, and indicate that the multiwalled carbon nanotube networks interconnect the spinel nanoflakes, leading to possibly improved electronic conductivity and electrochemical properties.

Fig. 2 (a and b) Low resolution HRTEM images of Li₂Mn_2.9Ni_0.9Co_0.2O₈ showing flaky morphology, (c) high resolution image showing the lattice fringes of spinel Li₂Mn_2.9Ni_0.9Co_0.2O₈, (d) SAED pattern showing the polycrystalline nature of the Li₂Mn_2.9Ni_0.9Co_0.2O₈ compound and (e and f) carbon wiring found in Li₂Mn_2.9Ni_0.9Co_0.2O₈ with MWCNTs.

Fig. 3 XRD pattern of pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ and the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite.

Fig. 4 Raman spectra of pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ and the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite (the D and G bands of the multiwalled carbon nanotubes are also indicated).

Fig. 3 shows the powder XRD pattern of the pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ compound prepared at 450 °C and the corresponding Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite. The XRD patterns match well with the Fd3m space group of the spinel compound, as per the standard JCPDS card number: 802162.^36–38 Intriguingly, no impurity peaks are found in the diffractogram and the lattice parameter value ‘a’ has been calculated as 8.1869, which shows striking similarity with the reported results. Hence, it is understood that the reaction of LiOH with the metal hydroxide precursor at 450 °C produces phase pure Li₂Mn_2.9Ni_0.9Co_0.2O₈, wherein the polyethyleneimine template has been removed completely during calcination and the metal hydroxide precursor has been ultimately transformed into the desired final product.

It is well known that spinel compounds may possess two different types of arrangements of atoms with respect to space groups, viz., Fd3m and P4₃32. Discrimination of such crystallographic arrangements of space groups can be done through the careful analysis of the local cation environment. It is in this context that a thorough analysis of the FTIR spectra and the complementary Raman spectra assumes paramount importance, especially when key information on the local cation environment and crystallographic arrangement of space groups of spinel compounds is required. The FTIR spectra of pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ and the corresponding composite (ESI, Fig. S2†) contain typical peaks corresponding to those of Mn–O (626 cm⁻¹) and Ni–O (500 and 577 cm⁻¹) stretching vibrations.³⁹ Bands due to multiwalled carbon nanotubes are clearly visible around 1668 (C=O), 2925 (CH₃), 1582 (C–C), 1018 (C–O) and 3293 cm⁻¹ (O–H). At this juncture, it is very difficult to differentiate the crystallographic arrangement of the spinel structure with two different space groups using FTIR spectra, whereas Raman spectroscopy could be used to distinguish such space groups, apart from gaining information on the cation ordering in spinel compounds. Evidently, no splitting of the signal has been found in the 550–600 cm⁻¹ region in the Raman spectrum of the bare Li₂Mn_2.9Ni_0.9Co_0.2O₈ and the composite Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT material, which in turn confirms the presence of the Fd3m space group, as inferred from the XRD pattern. The band at 635 cm⁻¹ is due to the Mn–O stretching vibration (A1g) (oxidation state) and the Ni–O stretching vibration is visible at 592 (F2g⁽¹⁾), 499 (F2g⁽²⁾) and 400 cm⁻¹ (Eg).³⁹ Furthermore, the Raman spectrum of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite (Fig. 4) clearly shows the presence of the characteristic D and G bands of multiwalled carbon nanotubes.

The electrochemical performance of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode has been evaluated by fabricating coin-type half cells with lithium metal as the counter and reference electrodes. Fig. 5a shows a cyclic voltammogram of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode, recorded at a scan rate of 0.1 mV s⁻¹. Generally, for nickel doped spinels, the presence of Mn³⁺ related oxidation peak at 3.9–4.2 V is barely detected.^40,41 By contrast, a broad peak due to the Mn³⁺/Mn⁴⁺ couple is observed around 4.0 V (Fig. 5a), indicating the presence of Mn³⁺ ions in the spinel framework. The peak above 4.6 V is due to the oxidation of nickel ions, as in the case of similar types of nickel doped spinel oxides, viz. LiNi_0.5Mn_1.5O₄. In other words, the oxidation peak at 4.7 V could be corroborated with the presence of the Ni^2+/4+ redox couple. Basically, the presence of a broad and unresolved peak shows the presence of the less crystalline framework of nickel doped spinels. But in the case of Li₂Mn_2.9Ni_0.9Co_0.2O₈, prepared using the low temperature synthesis protocol, clear splitting of the CV peak is observed, which is an indication of the crystallinity of the titular cathode.⁴² Similarly, upon discharge, peaks corresponding to the reduction of Ni⁴⁺ and Mn⁴⁺ are visible at 4.6 and 3.8 V respectively. The transition from a cubic to a tetragonal phase occurs at a potential below 3.0 V during reduction, which corresponds to the peak observed at 2.7 V. Subsequently, the cubic phase is found to be resumed during oxidation, taking place around 3.1 V (Fig. 5a). The galvanostatic charge/discharge behavior of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode, recorded at a constant rate of 0.1C in the voltage range of 2.0 to 4.9 V, is depicted in Fig. 5b. The initial formation cycle of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode is depicted in the ESI Fig. S3.† The larger capacity values of the titular cathode in the first few cycles are believed to be due to the occurrence of electrolyte oxidation at high voltage and the structural re-organisation of the spinel framework above 4.5 V.⁴³ Further, the charge/discharge capacity is found to stabilise after ten cycles and a steady state reversible capacity of ∼210 mA h g⁻¹ has been exhibited by the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode, which is noteworthy. However, the origin of the observed capacity obtained from a combination of intercalation/de-intercalation of lithium ions and the corresponding redox couples related to the presence of manganese and nickel ions could be better understood from the dQ/dV behavior of the titular cathode. Fig. 5c depicts the dQ/dV versus voltage curve of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode, obtained after the formation cycle. The dQ/dV behavior clearly demonstrates the involvement of a series of electrochemical processes occurring at different voltages. There are three peaks, observed at 3.1, 4.0 and 4.7 V, which are ascribed to the de-intercalation of lithium ions from the cubic spinel structure. The peak at 4.0 V corresponds to the oxidation of Mn³⁺ ions and the strong redox peak located at 4.7 V is caused by the oxidation of Ni²⁺ to Ni⁴⁺. In the reversible cycling process, peaks at 4.6 and 3.8 V, which are associated with the discharge process, correspond to the reduction of the respective redox couples aided by the reversible insertion of extracted lithium ions, which is consistent with the cyclic voltammetry results. Here again, the presence of a peak at 3.1 V upon oxidation and one at 2.7 V during reduction substantiates the cubic-tetragonal conversion of the Li₂Mn_2.9Ni_0.9Co_0.2O₈ spinel cathode.

Fig. 5 (a) Cyclic voltammogram of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode recorded at a scan rate of 0.1 mV s⁻¹, (b) galvanostatic charge/discharge curves recorded at a 0.1C rate for the first ten cycles, (c) dQ/dV profile of the second cycle, (d) comparison of the discharge capacity of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT and pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ cathodes, (e) specific capacity and coulombic efficiency of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode upon progressive cycling at a 1C rate and (f) rate capability behavior of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode.

The specific discharge capacity of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode as a function of cycle number has been compared with that of pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ (Fig. 5d). We have observed that the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite exhibits a much higher discharge capacity than pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈, which is quite interesting. Further, the discharge capacity of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode is found to be higher than the reported values of similar category cathodes.^44,45 Based on this encouraging result, we have extended our study to a high current rate of 1C and found that the cathode delivers an exceptional discharge capacity of 240 mA h g⁻¹ in the first cycle. Fig. 5e shows the galvanostatic charge/discharge capacity – cycle number curve for the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite cathode at a 1C current rate. At the end of the 100^th cycle, the cathode delivers a capacity of 100 mA h g⁻¹, which is noteworthy. The coulombic efficiency of the titular cathode is found to be 97% at a 1C rate, which is the first observation of its kind. Fig. 5f shows the rate capability behaviour of the titular cathode at different current values ranging from C/10 to 5C. It is interesting to note that the cathode delivers an excellent capacity of 71 mA h g⁻¹ even at a 5C current rate. Appreciable tolerance against various rates such as 1C, 2C and 5C is evident from the figure. Further, when the cell is switched back to the C/5 rate condition after being subjected to high current rate conditions of 5C, it is capable of resuming a capacity value closer to 145 mA h g⁻¹, which is an indication of the capacity retention behaviour. This in turn validates the suitability of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode for high rate applications, thus leaving ample scope to exploit a wide variety of custom engineered spinel architectures for high rate lithium battery applications. Moreover, the capacity fading identified during the 1C rate may be associated with the wide voltage range selected for the study and the structural reorganisation that occurs below a 3 V discharge. Capacity fading can be minimised by reducing the discharge cut-off voltage to 3.5 V and/or by increasing the amount of Ni²⁺ (which will be active above 4.5 V) and decreasing the concentration of Mn³⁺.

Electrochemical impedance spectroscopy (EIS) measurements further confirm the advantageous effect of the interconnected MWCNTs in enhancing the electrochemical performance of the titular cathode compared with that of the bare Li₂Mn_2.9Ni_0.9Co_0.2O₈ cathode. The Nyquist plots of the pristine and composite cathodes of Li₂Mn_2.9Ni_0.9Co_0.2O₈ are shown in Fig. 6 wherein an AC voltage of 10 mV amplitude in the frequency range of 0.25 MHz to 10 mHz has been applied. The high-frequency semicircle indicates favourable lithium transport kinetics. The diameter of the semicircle in the high frequency region of the as-fabricated pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ cathode and the corresponding Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite electrode implies that the R_ct value of the composite (1700 Ω) is much lower than that of the pristine electrode (2700 Ω), which is in line with the conductivity values obtained with the pristine samples, prior to the electrochemical studies (ESI, Fig. S4†). This is an indication of enhanced charge transfer, which is attributed to the conductive MWCNT network decorating the surface of the Li₂Mn_2.9Ni_0.9Co_0.2O₈ cathode material. The enhanced charge transfer kinetics of the composite cathode further confirms the increased contact area at the electrode/electrolyte interface and enhanced electrical conductivity resulting from the conductive MWCNT network. After five charge/discharge cycles, the cells have been subjected to EIS measurements and the Nyquist plots show decreased charge transfer resistance for both the electrodes, thus favouring high lithium diffusion kinetics. Hence, electrochemical impedance spectroscopy provides more insight into the stability of the chosen Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode, especially upon lithiation and de-lithiation. Further, ex situ XRD studies were performed on the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode with a view to confirming the structural stability of the same upon cycling. Interestingly, a better correlation of structural integrity and electrochemical behavior has been obtained from ex situ XRD studies performed at different voltages. Four different cells have been fabricated and subjected to ex situ XRD with different cycling conditions. The as fabricated electrode shows the presence of all the possible diffraction peaks, corresponding to the spinel framework (Fig. 7a). One cell has been stopped after the initial de-lithiation or charge reaction and it is observed that the XRD pattern of the cathode material recovered from that cell shows a similar diffraction pattern to that of the freshly fabricated Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT electrode, but with decreased intensity of the (111) line, which is reduced especially upon charging (Fig. 7b). Another cell was analysed after one complete cycle (one charge and one discharge), wherein the (111) line is found to resume the same intensity (Fig. 7c).

Fig. 6 Electrochemical impedance spectroscopy of pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ and Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite electrodes, recorded before and after cycling.

Fig. 7 Ex situ XRD pattern of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode recorded under different experimental conditions: (a) as fabricated electrode, (b) electrode after one charge, (c) electrode after one cycle and (d) electrode after completing 10 cycles.

This observation supports the complete reversibility of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode and the facile diffusion of lithium ions upon lithiation and de-lithiation. More interestingly, the ex situ XRD pattern recorded for the cell after completing ten charge/discharge cycles shows striking similarity with that of the as fabricated electrode, thus substantiating the appreciable structural integrity and excellent cycling reversibility of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode upon progressive cycling (Fig. 7d). It is worth mentioning here that the XRD pattern derived lattice parameter value of a = 8.18 corresponding to the as-fabricated electrode has been found to show a minor deviation, such as a = 8.176, upon de-lithiation or charge, driven by the slight change in the atomic radius following the oxidation process. However, after the completion of one cycle, the restored lattice parameter value of a = 8.18 has been observed, which is a strong evidence for the desired structural stability of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT cathode.

4. Conclusion

In conclusion, we have synthesized a novel 5 V category spinel cathode in the form of a composite, viz., Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT, wherein the interweaving of individual nanoparticles of electro-active material with multiwalled carbon nanotubes has been obtained by adopting a simple sonication approach. Herein, a pristine Li₂Mn_2.9Ni_0.9Co_0.2O₈ spinel compound in the form of nanoflakes with an Fd3m space group has been prepared through using a template assisted co-precipitation method, followed by a solid-state reaction at 450 °C. Interestingly, the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite cathode exhibits good electrical conductivity, low charge transfer resistance and good structural stability upon cycling, as evidenced by the conductivity, electrochemical impedance spectroscopy and ex situ X-ray diffraction studies. The unique hetero-structure of the Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite cathode provides a shorter lithium ion diffusion path and good electronic conductivity, resulting in the demonstration of excellent electrochemical properties. The titular composite delivers a discharge capacity of ∼210 mA h g⁻¹ at a 0.1C rate and an excellent rate capability at a 1C rate with a specific discharge capacity of 100 mA h g⁻¹, facilitated by the exceptional structural stability. To our knowledge, this is the first ever report on the high capacity and high rate applicability of a low temperature synthesized spinel category Li₂Mn_2.9Ni_0.9Co_0.2O₈/MWCNT composite, which in turn has been recommended as a potential lithium intercalating cathode material through the present study. The promising results of this study open up a newer gateway to explore series of Li₂MMn₃O₈ spinel cathodes and the corresponding composites for exploitation in high capacity lithium batteries with rate capability.

Acknowledgements

P. Remith is grateful for the Inspire Fellowship support from the Department of Science and Technology, India and N. Kalaiselvi acknowledges the support from MULTIFUN and TAPSUN projects funded by the Council of Scientific and Industrial Research, India.

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Footnote

† Electronic supplementary information (ESI) available: Additional information includes FTIR spectra of pristine and composite materials, the first cycle charge/discharge voltage profile of the composite and room temperature conductivity curves. See DOI: 10.1039/c6ra04344e

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