Carbon coated spinel LiNi0.5Mn1.5O4 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 130 mAh·g−1, and the corresponding capacity retention was 98.8%. The 100th cycle specific discharge capacity at 10 C rate of carbon coated LiNi0.5Mn1.5O4 was 105.4 mAh·g−1, and even the corresponding capacity retention was 95.2%.
Spinel LiNi0.5Mn1.5O4 is a promising and attractive cathode material because of its good electrochemical properties and high working potential (~5.0 V) [
Carbon coating is easy and popular method at present [
The synthesis route was as follows: LiOH, Ni(NO3)2·6H2O and MnSO4·H2O were used as starting precursors with molar ratio of Li : Ni : Mn = 1.0 : 0.5 : 1.5. Ni(NO3)2·6H2O and MnSO4·H2O were dispersed in deionized water under continuous stirring and then slowly dripped the (NH4)2C2O4 aqueous solution. After reaction at 50°C for 12 h, then centrifugal washing to obtain the Ni0.5Mn1.5(C2O4)2 precipitates. Ni0.5Mn1.5(C2O4)2, LiOH, and PMF (10 wt.%) were dispersed in deionized water by ultrasonic dispersion 30 min into suspension. The suspension was dried by spray-drying method (inlet temperature 220°C and feed rate 6 mL·min). The dried products were presintered at 500°C in N2 atmosphere for 5 h and then sintered at 800°C in N2 atmosphere for 20 h to obtain the carbon coated LiNi0.5Mn1.5O4. For comparison, LiNi0.5Mn1.5O4 was also synthesized without PMF.
The structure of the LiNi0.5Mn1.5O4 materials was characterized using powder X-ray diffraction (XRD, XD-3, Beijing Purkinje General, China) with Cu K
For electrochemical characterizations, 2032 type coin cells were used. The coin-type cells (2032) were assembled with Li plate as anode and LiNi0.5Mn1.5O4 electrode as cathode. The LiNi0.5Mn1.5O4 electrodes were prepared by coating the slurry of a mixture composed of LiNi0.5Mn1.5O4 (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 LiNi0.5Mn1.5O4 electrode was 3.0 mg·cm−2. The electrolyte was 1 mol·L−1 LiPF6 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 LiNi0.5Mn1.5O4 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 3.5–5.0 V 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.
Figure
XRD patterns of prepared LiNi0.5Mn1.5O4.
FE-SEM images of prepared LiNi0.5Mn1.5O4.
Figure
The charge and discharge curves of prepared LiNi0.5Mn1.5O4 at 0.5 C.
No-PMF
PMF-10%
Figure
The charge and discharge curves of prepared LiNi0.5Mn1.5O4 at 10 C.
No-PMF
PMF-10%
Figure
Cyclic voltammogram curves of prepared LiNi0.5Mn1.5O4.
Figure
EIS spectra of prepared LiNi0.5Mn1.5O4.
PMF carbon coated LiNi0.5Mn1.5O4 was prepared as the 5 V cathode materials of lithium ion batteries for the first time. PMF carbon coating increases the crystallinity of LiNi0.5Mn1.5O4. The as-prepared LiNi0.5Mn1.5O4 has the cubic face-centered spinel structure with a space group of Fd3m. The PMF carbon coating greatly promotes electrochemical performance and reduces the LiNi0.5Mn1.5O4 electrode polarization in the process of charging and discharging. The cycle life of LiNi0.5Mn1.5O4 was significantly improved by PMF carbon coating, especially at high rate discharging and charging.
The authors declare that they have no competing interests.
This work was supported by the National Natural Science Foundation of China (no. 21376056 and no. 21463030), China Postdoctoral Science Foundation (2016M590781), and Science and Technology Project of Guangzhou (no. 201509030005 and no. 201510010131).