Graphene modulated LiMn1.5Ni0.4Cr0.1O4 spinel cathode for lithium ion battery

We report the development of a rechargeable lithium ion battery with homogeneous mixing of 10% of graphene oxide in active LiMn1.5Ni0.4Cr0.1O4 cathode material for enhanced electrochemical performance. The redox behavior of the cell, which is normally too slow for practical applications, is accelerated with highly conductive graphene. Intimate mixing of the two materials is achieved by a slurry maker using an organic solution for a cathode paste. The fabricated electrode is repeatedly charged/discharge at C/10 and C/2 rates without any significant degradation in the electrochemical capacity. The gravimetric energy density of the composite cathode material exceeds that of the LiMn1.5Ni0.4Cr0.1O4 oxide electrode in lithium-ion batteries, and the addition of graphene in active material is likely to prove advantageous in applications where weight, rather than volume, is a critical factor.


Introduction
The development of low cost rechargeable lithium ions batteries is of considerable technological importance because of their relatively high energy densities. The rechargeable lithium ion batteries exhibit high theoretical energy storage capability with relatively lower weight and higher mechanical strength. Further, lithium-ion batteries (LIBs) with high energy density, design flexibility, and low weight are useful for energy storage, which have been widely used in various applications such as portable mobile electronic devices, hybrid electric vehicles, and military applications [1][2][3][4][5][6]. Among LiMn2O4 spinel group materials, LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 cathode materials has attracted attention as an efficient cathode material because of its high operating voltage, high specific capacity, and structural stability [7][8][9]. However, the poor electronic conductivity of LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 cathode ends up with relatively lower electrochemical performance, thus restricting its practical applications. The conventional reaction synthesis method is not suitable for preparing low conductivity cathode materials in large scales for any practical applications [10,11]. Multilayer graphene powder may be a very good alternative additive to the active cathode material to realize the enhanced electronic conductivity through percolating network and also provide the structural stability simultaneously for high energy density and capacity of oxide materials [12][13][14].
There are some studies on highly conductive nano-structured carbon particles inserted in the active host materials, showing improved electronic conductivity and thereby enhance the discharge capacity as well as cycle stability at high current densities [15,16]. In order to achieve excellent structural stability with large surface-tovolume ratio and high electronic conductivity, highly conductive graphene electrode is added for the development of more efficient LIB [17][18][19][20] Also graphene leads to larger reversible capacity due to efficient Li intercalation between adjacent layers [21]. Graphene is the ideal material for blocking release of oxygen into the Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. electrolyte. Graphene can effectively improve electron and ion transportation of the electrode materials, so the addition of graphene can greatly enhance lithium ion battery's properties and provide better chemical stability, higher electrical conductivity and higher capacity. It is impermeable to oxygen, electrically conductive, flexible, and is strong enough to withstand conditions within the battery. It is only a few nanometers thick so there would be no extra mass added to the battery. Our research shows that its use in the cathode can reliably reduce the release of oxygen and could be one way that the risk for fire in these batteries [22][23][24][25][26]. Improved surface conductivity, capacity, rate capability, and cyclic stability of graphene-containing electrode materials are also confirmed for composite materials [27][28][29]. In the present work, we study the effect of adding graphene into LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 cathode material to understand its impact on the electrochemical performance and charge/discharge cyclability.

Experiment procedures
The highly oriented pyrolytic graphite powder from (Alfa aesar) was added to a desired quantity of concentrated H 2 SO 4 at room temperature and the mixture was continuously stirred for one hour. The container was transferred into an ice bath to lower the reaction temperature. This process was followed by the addition of potassium permanganate (KMnO4) slowly and the solution is allowed to mix thoroughly for few hours. A large quantity of distilled water was then added to the solution followed by the slow addition of hydrogen peroxide until the gas evolution has stopped. Further, excess water was added and the solution was filtered several times to get graphene oxide. A film of graphene oxide powder was deposited on the filter paper after filtration, which was isolated after drying properly. It was then placed on a manual motorized stage for laser heating using a 532 nm line of a diode laser (incident power was ∼500 mW). This process resulted in reduced graphene oxide. It was further confirmed with Raman scattering, where noticed increase in the intensity of D peak as compared to that of G peak substantiated the successful reduction of graphene oxide. The LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 cathode material was synthesized by a sol-gel method which is reported elsewhere [30]. The graphene and pure phase cathode powder were mixed in 10:90 weight ratio to get a graphene modulated LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 active cathode materials. The phase purity and the crystallinity of the LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 cathode material were investigated using a Siemens D5000 x-ray powder diffractometer [CuKα radiation, 1.5405 Å]. X-ray photoelectron spectroscopy (XPS) measurements of LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 cathodes were performed before and after chargedischarge cycle, using PHI ESCA system (Physical Electronics) using Al Kα radiation. Curve fitting of the slowscanned XPS spectra was carried out using a non-linear least-squares fitting program with a Gaussian-Lorentz sum function. The cathode slurry was prepared by mixing of graphene modulated LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 active cathode material with polyvinylidene fluoride (weight ratio 95:5) in N-methyl pyrolidone (NMP). The resulting paste was casted uniformly onto aluminum foil followed by drying at about 60-70°C in vacuum furnace overnight. The coin cells were fabricated in an argon atmosphere, inside a Glove Box (MBraun Inc., USA), using LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 electrode as cathode, Li foil as anode, and 1 M lithium hexafluoride (LiPF6), dissolved in ethyl carbonate (EC) and dimethyl carbonate (DMC) [1:2, v/v ratio] as electrolyte. The electrochemical behavior of the cells was studied at room temperature by cyclic voltammetry and charge-discharge characteristics, using solartron battery tester, Model 1470E. The impedance measurements of the cells were carried out using Gamry Instruments potentiostat and EIS 300 electrochemical software.

Results and discussion
The XRD pattern of graphene modulated LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 is shown in figure 1. All diffraction peaks are indexed to cubic crystal structure with Fd3m space group and thus confirming the spinel structure. The sharpness of peaks is suggesting that the prepared material is well-crystallized [31]. Additionally, a carbon peak is also observed in XRD pattern of graphene modulated LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 composite, substantiating the presence of graphene in cathode matrix [32][33][34]. However the superstructure peaks which are characteristics of Ni and Mn ordering cannot be resolved from the XRD patterns of the spinels because of their low intensities [35]. Therefore, further structural investigation by Raman is required to confirm the P4332 symmetry of the synthesized spinel. The recorded Raman spectra as shown in figures 2(a) and (b) represents spectrum of LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 and graphene modulated LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 respectively. The Raman modes at 492 and 640 cm −1 are assigned to Eg and A1g active Raman-modes for the transition metal oxygen arrangements in the spinel structure of lithium metal oxide with Fd3m symmetry [36,37]. The characteristic of cubic spinel Raman active modes at 600 cm-1 indicates that the material is a single phase spinel structure [38].    figure 4. The voltammograms showed well-defined redox peaks along with an anodic peak centered at 3.7 V. This peak explains the oxidation of Mn3+/Mn4+and another peaks correspond to nickel and chromium with respective 2+and 3+oxidation states indicating their perfect stoichiometry. These peaks are known to be electrochemically active and undergo redox transitions after few charge discharge cycles [30,31]. An indication of anodic peaks at 4.65 V in the first charge-discharge cycle corresponds to the oxidation of Ni2+to Ni4+via Ni3+ [10,11] is probably due to redox couples of Ni2+/Ni3+and Ni3+/Ni4+and or Li vacancy ordering [41]. A continuous cycling at C/10 showed the disappearance of intense anodic peak at 4.65 V and appearance of distinguishable redox peaks in between 3.0-4.9 V in both charging as well as discharging cycles. The redox peaks were observed when the cells are charged at C/10, indicating the activation of graphene in the composite system. The smaller current density  will increase the respective activation of graphene element in active cathode materials and thus, it will exhibits high capacity.
Further, the study of chemical state of graphene modulated LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 were investigated by XPS. The collected elemental XPS binding energy curves for each element (Li1s, Mn3d, Mn3s, C1s, Cr2p, Mn2p, O1s, Ni2p) are shown in figures 5(a)-(g) for as prepared i.e. before and after charging as well. These data are calibrated with respect to C1s (284.6 eV) carbon elemental peak, as shown in figure 5(c). In addition to the C1s peak at 284.6 eV, another peak at around 288.75 eV [42] is observed, which has relatively lower intensity in pristine sample and became very intense in case after electrochemical cycling. This is associated with the bonding of graphitic carbon to the oxygen atoms of cathode material. The de-intercalation of Li during charging is observed in Li1s region ( figure 5(a)), suggesting the presence of Li and the peak around 54.5 eV attributed to Li 1s [38] in transition metal oxide, consistent with our findings. The lithium 1s peak becomes more clear in after electrochemical cycle XPS spectra figure 5(a). The binding energy of We also noticed characteristic Mn3d ∼48 eV, and Mn 3s between 85-90 eV XPS peaks figure 5(b). The O1s spectra are shown in figure 5(d) and two peaks∼531 eV and 528 eV are observed, which correspond to spinel O and interface or/and defect O. These are dominating features in the spectra before charging than after charging. These peaks are also present with reduced intensity in the XPS spectrum after electrochemical cycling XPS spectra, however a new peak has evolved at 535.5 eV. This high binding energy peak may correspond to the residual oxygen containing functional groups (such as -OH and -COOH) bonded with C atoms in graphene [43]. The Cr XPS spectrum is shown in figure 5(e), showing the signature of Cr3+peak, whereas no significant Cr6+peak is observed ( figure 5(e)). The intensity of Cr2P peak is also very poor and is because of the very small atomic fraction of Cr in LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 cathode material and relatively similar results are observed for after cycle XPS spectrum. Further, Mn2p XPS spectra is shown in figure 5(f), showing dominant Mn4+XPS peak with respect to Mn3+in before charging case. The Mn3+intense peak is observed after cycling and Mn4+is relatively not seen (figure 5(f)). A weak Ni2p (figure 5(g)) XPS peaks were observed in both the samples with Ni 2p3/2 located at 853.3 eV and 2p1/2 at 871.7 eV with an energy splitting (Δ)=18.4 eV, and results are consistent with the nickel positions in oxide materials. Interestingly, we noticed that XPS binding energy peaks for most of the elements are shifted to higher binding energy values after charging. The multiplet splitting observed in these elements is the consequence of the interaction between the spins of unpaired electrons in the valence levels i.e. in 3d-transition metals such as Cr and Mn, giving rise to the complex peak structure [33].
The electrochemical performance of graphene modulated LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 composite electrode was evaluated at different discharge rate in 3.0 and 4.9 V. The electrochemical performance of the composite electrode without graphene has been tested and published by Katiyar et al [29]. The galvanostatic charge/ discharge profiles at C/10 and C/2 current rates are shown in figure 6, and 7, respectively. As shown in figure initially the voltage increases monotonically until 3.73 V and reaches in a plateau region between 3.73 and 4.56 V. We observed low capacity in the initial few cycles for graphene modulated LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 whereas almost negligible irreversible loss is noticed after a few cycles. The maximum discharge capacities observed was ∼154 mAh g −1 at C/10 current rate at 3.73 V. The above characteristic revealed an enhancement of electrochemical performance in the active cathode material by the addition of graphene [13][14][15][16][17][18][19][20][21][22]. Similar discharge capacities observed in the ˃50 cycles indicate reversible nature with well ionic pathways deintercalation of lithium ions in the cathode materials. Although there is a decrease in the discharge capacity of the samples upon increasing current rates, the LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 electrode clearly shows low discharge capacity but enhanced stability ˃50 cycles. The extended cyclic stability measurements of the LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 combined with the graphene electrodes conducted at different current rates are shown in figure 7. Initially, low discharge capacity increased with increasing cycles due to more stable electronics and ionic path by graphene into structure and reached values very close to the theoretical capacity of 148 mAh g −1 . The cell showed improved discharge capacity at C/10 current rates after 50 charge cycles, while a negligible capacity fading was observed for LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 combined with graphene at C/2 current rates that was ascribed to polarization effects. The LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 combined with graphene at C/10 is retained >98% due to the more stable structure obtained by the addition of graphene. The high capacity of the cathode is due to increase in the electronic conductivity, which reduces the cell polarization and prevents the evolution of oxygen from the cathodes. Figure 8 represents a comparative result of cycling performance for pristine to modulated system. Electrochemical Impedance Spectroscopy (EIS) is a non-destructive technique and widely used to understand the electrode and electrolyte interfaces. To understand and the beneficial effect of graphene on the electrochemical performance of LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 cathode, alternating-current (AC) impedance measurements are carried out after five charge discharge cycles at C/10 rate over a wide range of frequencies (MHz to mHz) at constant DC voltage equal to the open circuit voltage. These measurements were carried out for the electrochemical cells before and after cycling and results are summarized in figure 9, with inset showing the equivalent circuit, used for fitting the experimental data using Zsimpwin program. The equivalent circuit consists of Re and Zw in series with two R-C circuits as the cathode-electrolyte and electrolyte-anode interface together with an addition capacitive Q element. The different resistances are extracted from these Nyquist plots and the data is summarized as shown in figure 9 .
The resistance values for the cells are considerably low in comparison with coin cells which were tested before cycling. This observation confirms that the conductivity has been improved after cycling and helps for easier Li ions intercalation and de-intercalation which in turn improves the electrochemical performances like specific capacity and cyclic stability [1,2]. This also explains the observed initial lower capacity in few cycles, as the resistance of cathode and interfaces were higher and thus, poor electronic/ionic transition resulted in relatively lower capacity in initial few cycles. The enhanced electronic/ionic conductivity observed for the combined with graphene cathode resulted in the increased discharge capacity and a high rate capability, facilitating the transfer of lithium ions across the active material/electrolyte interface, as well as the transfer of electrons from the current collector to the active material. In addition, the factors that may have contributed to the superior electrochemical performance of the graphene-containing cathode are improvement in the structural stability, a decrease in the disorder of metal ions in the lattice, and suppression of the dissolution of

Conclusions
Graphene modulated LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 spinel structured composites were synthesized and tested for electrochemical performance. The structural stability of the LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 was confirmed by XRD and ordered spinel structure with p4 3 32 symmetry confirmed by Raman spectroscopy. The FE-SEM images confirm homogenous orientation of graphene throughout the spinel structure. XPS investigation confirms that chromium and manganese ions changed their valence states from Cr3 + to Cr6 + , whereas manganese changes from Mn4 + to Mn3 + state, during charging/discharging process. The discharge capacity of the graphene modulated LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 at C/10 and C/2 discharge rates in the voltage range between 3.0 and 4.9 V obtained 150 mAh g −1 and 45 mAh g −1 respectively. In addition to the fast charge and discharge capability graphene modulated LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 composite revealed promising cyclic stability, with 95% capacity retained after 50 cycles. In comparison with LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 , graphene modulated LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 showed greatly improved electrochemical performance. The improved rate capability and cycling performance of graphene modulated LiMn 1.5 Ni 0.4 Cr 0 . 1 O 4 cathode material attributed to an increase in the grain connectivity and high electronic conductivity for the smart hybrid electrode design.