Improved electrochemical performance of 5 V spinel LiNi0.5Mn1.5O4 by La2O3 surface coating for Li-ion batteries

LiNi0.5Mn1.5O4 coated with 1.5 wt.% La2O3 was prepared and investigated as cathode materials for lithium ion batteries. The samples was characterized by XRD, SEM and EDX. After the 1.5 wt.% La2O3 coating, the lattice structure of LiNi0.5Mn1.5O4 is not destroyed and a La2O3 coating layer has formed on the surface of LiNi0.5Mn1.5O4 particles. The 1.5 wt.% La2O3-coated LiNi0.5Mn1.5O4 exhibits a higher rate capacity, with discharge capacity between 3.5 and 5 V of 131.9, 127.1, 126.5, 120.7, 114.1 and 101.5 mAh g at rates of 0.2, 0.5, 1, 2, 3 and 5 C (1 C = 140 mAh g), respectively. The results indicate that the La2O3 coating could reduce the electrode polarization and enhance the rate capacities.


Introduction
High voltage cathode materials are playing an important role in the development of high energy and power lithium-ion batteries for their potential applications in hybrid electric vehicles and renewable energy storage devices [1][2][3][4]. Spine LiNi0.5Mn1.5O4 (LNMO) is considered as one of the most attractive cathode materials due to its high operating voltage (at about 4.7V vs Li + ) and high specific energy of 658 Wh kg -1 , which is much higher than commercial cathode materials such as LiCoO2 (518 Wh kg -1 ), LiMn2O4 (400 Wh kg -1 ), LiFePO4 (495 Wh kg -1 ), and LiCo1/3Ni1/3Mn1/3O2 (576 Wh kg -1 ). However, the LNMO acts as cathode material may exist two problem. On the one hand, the Mn dissolution into the electrolyte could result in the structure collapse and capacity degradation. On the other hand, the decomposition of the electrolyte would generate HF, which would also corrode the LNMO material, leading to the capacity fading.
In this paper, 1.5 wt.% La2O3 was successfully coated on the surface of LNMO. The structure and electrochemical performance of the La2O3 coated LNMO sample was investigated and reported below.

Synthesis and characterization
Pure LNMO powders were synthesized by a solid-state ball-milling and spray drying method. Specifically, a stoichiometric amount of NiO, Li2CO3, MnO2 and deionized water were mixed by ball milling at 300 rpm for 10 h using zirconia as the milling media, The mixture was subsequently used the spray drying to get the precursor powders. Finally, the precursor powders were calcined at 500°C for 5h and 800°C for 8 h in an air atmosphere to obtain pure LNMO. The molar ratio of Li:Ni:Mn was 1:0.5:1.5 and excessive Li (5%) was added to compensate for the volatilization of Li during the calcined process.
La(NO3)3· 6H2O (Aladdin) and polyvinyl alcohol (PVA, degree of polymerization is 1500) were purchased for this work. The La2O3-coated LNMO sample was achieved by the following steps: LNMO (5 g) was dispersed in deionized water, then the mixture stirred for 2 hours. The calculated 1.5 wt.% of La(NO3)3· 6H2O to form La2O3 and 1.5 wt.% polyvinyl alcohol were dissolved in warm deionized water and added dropwise to the dispersed LNMO. The mixture was continuously stirred for 1 hours at 25 °C and continuously stirred at 100 °C to evaporate the water, then the mixed solid powder was calcined at 500 °C for 5 hours in air to obtain the 1.5 wt.% La2O3-coated LNMO sample. Powder X-ray diffraction (XRD, Ultima IV, Riguku) with Cu Kα radiation was used to characterize the phase composition and crystal structures of all the samples. The diffraction patterns were collected at room temperature by step scanning in the range of 10-90° at a scanning rate of 0.02• per 10 s. The morphology of the materials was characterized by SEM (Hitachi SU8020, Japan). The microstructure and EDX mapping of all samples were also observed by SEM (INCA Penta-FETx3).

Electrochemical measurements
The electrochemical properties of all samples were tested by adopting a CR2032 coin-type half-cell with Li foil as the counter electrode. The cathode slurry was prepared by homogeneously mixing the active materials, Super-P, and polyvinylidene fluoride (PVDF) in a mass ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP) solvent. Then, the slurry was cast onto a Al foil and dried for 12 h in vacuum at 105°C. Finally, the electrode laminate was punched into disks (10 mm in diameter) and dried in a vacuum oven at 105°C for 24 h. The coin cell was assembled entirely in an argon-filled glovebox. The separator was Celgard 2400 (Celgard Inc., USA), and the electrolyte (Capchem Technology (Shenzhen) Co., Ltd.) was a solution of 1 mol L−1 LiPF6 in ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate (1:1:1, volume ratio).
The galvanostatic charge-discharge tests were conducted on an automatic galvanostatic chargedischarge unit (Land 2001A, Wuhan, China) between 3.5 and 5 V and at charge/discharge C-rates between 0.2 and 5 C (1 C = 140 mAh g −1 ) at 25°C.

Material characterization
The XRD patterns of pure LNMO and the La2O3-coated LNMO are presented in Fig. 1. All the XRD patterns are indexed to the standard spinel structure of LNMO (card NO. 80-2162) with Fd-3m space group. The structure of LNMO was not destroyed by La2O3 coating. There is a small peak (marked with '*'), which is corresponding to an impurity phase of LixNi1−xO2, demonstrating an oxygen loss reaction at high temperatures during the calcining process. Furthermore, there were no peaks that indicate the existence of La2O3. This may be attribute to the low La2O3 content or its amorphous state.

Conclusion
In summary, the 1.5 wt.% La2O3 has been successfully coated on the surface of LNMO.The characterization results show that the La2O3 coating does not destroy the formation of LNMO and a La2O3 coating layer distributes uniformly on the surface of LNMO particles. The electrochemical results indicate that the 1.5 wt.% La2O3-coated LNMO exhibits a higher rate capability over pure LNMO. This is attributed to the fact that the La2O3 coating layer could protect the transition metal ions from dissolving into the electrolyte and minimizing harmful side reactions between the LNMO and electrolyte. Therefore, Our studies display that the La2O3 coating offers an important benefit to increase the rate