In Situ Carbon Coated LiNi 0 . 5 Mn 1 . 5 O 4 Cathode Material Prepared by Prepolymer of Melamine Formaldehyde Resin Assisted Method

Carbon coated spinel LiNi 0.5 Mn 1.5 O 4 were prepared by spray-drying using prepolymer of melamine formaldehyde resin (PMF) as carbon source of carbon coating layer. The PMF carbon coated LiNi0.5Mn1.5O4 was characterized by XRD, SEM, and other electrochemical measurements. The as-prepared lithium nickel manganese oxide has the cubic face-centered spinel structure with a space group of Fd3m. It showed good electrochemical performance as a cathode material for lithium ion battery. After 100 discharge and charge cycles at 0.5 C rate, the specific discharge capacity of carbon coated LiNi0.5Mn1.5O4 was 130mAh⋅g , and the corresponding capacity retentionwas 98.8%.The 100th cycle specific discharge capacity at 10 C rate of carbon coated LiNi0.5Mn1.5O4 was 105.4mAh⋅g, and even the corresponding capacity retention was 95.2%.

Carbon coating is easy and popular method at present [17].In our prophase research, we found that the prepolymer of melamine formaldehyde resin would evenly coat on the surface of metal oxide and obtained the carbon coated metal catalysts with abundant pore structure after high temperature sintering [18].In this study, in order to prepare the LiNi 0.5 Mn 1.5 O 4 as well as to improve the high rate discharge performance, polymer-complex-assisted method was applied.Prepolymer of melamine formaldehyde resin (PMF) is used as carbon source to accomplish the carbon coated LiNi 0.5 Mn 1.5 O 4 .at 500 ∘ C in N 2 atmosphere for 5 h and then sintered at 800 ∘ C in N 2 atmosphere for 20 h to obtain the carbon coated LiNi 0.5 Mn 1.5 O 4 .For comparison, LiNi 0.5 Mn 1.5 O 4 was also synthesized without PMF.

Characterization.
The structure of the LiNi 0.5 Mn 1.5 O 4 materials was characterized using powder X-ray diffraction (XRD, XD-3, Beijing Purkinje General, China) with Cu K radiation at 36 kV and 20 mA.The morphology of LiNi 0.5 Mn 1.5 O 4 was identified by field emission scanning electron microscopy (FE-SEM, JSM-5600, JEOL, Japan).
For electrochemical characterizations, 2032 type coin cells were used.The coin-type cells (2032) were assembled with Li plate as anode and LiNi 0.5 Mn 1.5 O 4 electrode as cathode.The LiNi 0.5 Mn 1.5 O 4 electrodes were prepared by coating the slurry of a mixture composed of LiNi 0.5 Mn 1.5 O 4 (80 wt.%), conducting agent (Super-p, 10 wt.%), and binder (polyvinylidene difluoride, 10 wt.%) onto an aluminum foil, and then dried at 120 ∘ C for 24 h in a vacuum drier.The weight of active material in the LiNi 0.5 Mn 1.5 O 4 electrode was 3.0 mg⋅cm −2 .The electrolyte was 1 mol⋅L −1 LiPF 6 in the mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1 : 1.A polypropylene (PP) film (Celgard 2300) was used as the separator.Afterwards, the coin-type cells were assembled in argon-filled glove box.NEWARE multichannel battery-testing unit (CT-3008W, China) was employed to test the cycling and rate performances of LiNi 0.5 Mn 1.5 O 4 over a voltage range between 3.5 and 5.0 V versus Li/Li + electrode at room temperature.Firstly, the cell should discharge and charge two cycles at 0.5 C and then discharge and charge 100 cycles at different rates.The cyclic voltammetric (CV) tests were carried out on an electrochemical workstation (CHI660A) at a scan rate of 0.1 mV⋅s −1 in the range of 3.5-5.0V versus Li/Li + .The electrochemical impedance spectroscopy (EIS) data of the electrodes were acquired at room temperature by a electrochemical workstation (CHI660A) before cycling in the frequency range 10 mHz-100 kHz by imposing an alternate current with an amplitude of 10 mV on the electrode.

Results and Discussion
Figure 1 shows the XRD pattern of the as-prepared LiNi 0.5 Mn 1.5 O 4 .Compared with the PDF powder diffraction data file (JCPDS Card No. 80-2162), the diffraction peaks of as-prepared LiNi 0.5 Mn 1.5 O 4 are in agreement with the standard diffraction peaks.This illustrates that the asprepared LiNi 0.5 Mn 1.5 O 4 has the cubic face-centered spinel structure with a space group of Fd3m [19].There is no change of the station of diffraction peak after carbon coating and no coating.The diffraction peak intensity would be enhanced after PMF carbon coating.The carbon coating would be helpful to improve LiNi 0.5 Mn 1.5 O 4 crystallinity.Figure 2 shows FE-SEM images of LiNi 0.5 Mn 1.5 O 4 with no coating and after 10 wt.% PMF carbon coating.The LiNi 0.5 Mn 1.5 O 4 maintains the micronanostructure after carbon coating, but the surface of samples becomes roughed, and the roughness of samples increases after the PMF carbon coating.The degree of sphericity drops gradually.
Figure 3 shows the 2nd, 50th, and 100th charge and discharge curves of carbon coated LiNi 0.5 Mn 1.5 O 4 at the rate of 0.5 C. The specific discharge capacity of LiNi 0.5 Mn 1.5 O 4 increased after PMF carbon coating at around 4.7 V voltage plateau.The 2nd cycle specific discharge capacity raised from 125.1 mAh⋅g −1 (no coating) to 131.7 mAh⋅g −1 (after coating).After 100 discharge and charge cycles, the specific discharge capacity of carbon coated LiNi 0.5 Mn 1.5 O 4 was 130 mAh⋅g −1 , and even the corresponding capacity retention was 98.8%.And yet the specific discharge capacity of no coated LiNi 0.5 Mn 1.5 O 4 was 88.9 mAh⋅g −1 , and the corresponding capacity retention was 71.1%.The discharge and charge curves indicate that the PMF carbon coating reduces the electrode polarization after repeatedly charging and discharging cycle which suggests that PMF coating was in favour of improving the discharge and charge performance of lithium nickel manganese oxide materials.Another reason might be that the PMF carbon coating prevents the Mn 3+ dissolving in the process of charging and discharging, thereby reducing the fading rate of discharge capacity [20,21].
Figure 4 shows the 2nd, 50th, and 100th charge and discharge curves of carbon coated LiNi 0.5 Mn 1.5 O 4 at the rate of 10 C. The specific discharge capacity of LiNi 0.5 Mn 1.5 O 4 declines sharply after PMF carbon coating at high rate discharge.The 2nd cycle specific discharge capacity decreased from 117.9 mAh⋅g −1 (before coating) to 110.6 mAh⋅g −1 (after coating).The PMF carbon coating hinders the Li + ionic conduction at high rate discharge and charge.But the corresponding capacity retention was greatly improved after PMF carbon coating.The 100th cycle specific discharge capacity of carbon coated LiNi 0.5 Mn 1.5 O 4 was 105.4 mAh⋅g −1 , and even the corresponding capacity retention was 95.2%.And yet the specific discharge capacity of no coated LiNi 0.5 Mn 1.5 O 4 was 85.4 mAh⋅g −1 , the corresponding capacity retention was 69.1%.
Figure 5 shows the CV curves of the PMF carbon coated LiNi 0.5 Mn 1.5 O 4 electrode.In the full range view of CV curves, one pair of redox peaks around 4 V (Mn 3+ /Mn 4+ ) and two pairs of well separated strong redox peaks at 4.6-4.8V can be observed in CV curve of no carbon coated LiNi 0.5 Mn 1.5 O 4 electrode.The two strong redox peaks indicate a two-stage (Ni 2+ /Ni 3+ and Ni 3+ /Ni 4+ ) Li + extraction from or insertion into the spinel framework [22].For PMF carbon coated LiNi 0.5 Mn 1.5 O 4 electrode, the peaks around 4 V should weaken obviously.This indicates that the Mn 3+ ions dissolution amount of the PMF carbon coated LiNi 0.5 Mn 1.5 O 4 is smaller than no carbon coating [23].The presence of Mn 3+ in the LMNO material plays an important role in the capacity retention [9].And the two redox peaks of LiNi 0.5 Mn 1.5 O 4 with 10 wt.% PMF carbon coating are overlapped to a broad peak.The cyclic voltammetry curves of the PMF carbon coated LiNi 0.5 Mn 1.5 O 4 material exhibit smaller potential intervals that indicated higher electrode reaction reversibility and lower polarization.The PMF carbon coating would effectively restrain the Mn 3+ stripping and the side reaction between positive electrode active material and electrolyte at high voltage.Figure 6 depicts the electrochemical impedance spectroscopy of the electrode obtained after the first cycle of charge.The semicircle at high frequency (>100 Hz) in EIS spectra reflects the contact resistances between the active materials and electrolyte or current collector [24].The significant improvements in electric conductivity for LiNi 0.5 Mn 1.5 O 4 electrode could therefore be attributed to the PMF carbon coating [25].

Figure 4 :Figure 5 :
Figure 4: The charge and discharge curves of prepared LiNi 0.5 Mn 1.5 O 4 at 10 C.