As a positive material with a low cost, a high energy density and a good power performance, spinel lithium manganese nickel oxide (LiNi_0.5Mn_1.5O₄) has attracted much attention in the industry of lithium ion battery. Reducing the surface oxygen release behavior and the resolution of Ni or Mn during charging and discharging is a main method to improve the electrochemical performance. The conventional coating technique is to construct a coating layer on the surface of the material particles, which can isolate the electrolyte and reduce side reactions at the interface. Since the solid-phase coating method cannot form the uniform and compact coating layer, the improvement of the electrochemical performance is limited. In this paper, a precursor with Li₃PO₄ layer that was produced by a wet coating method was sintered at 850 ℃. In addition, the impact of coating method was also analyzed by the first-principles calculation and characterizations.

According to a molar ratio of LiNi_0.5Mn_1.5O₄, a certain amount of Ni_0.25Mn_0.75(OH)₂ precursor and Li₂CO₃ were weighed and mixed. The mixed material was ground in a mortar evenly. Afterwards, the ground material was sintered in a muffle furnace at 950 ℃ for 10 h, finally obtaining a LNMO positive electrode material (i.e., sample P0) after cooling in the furnace.

According to the designed coating amount of 3% Li₃PO₄, NH₄H₂PO₄ and LiOH·H₂O were weighed and sequentially added to alcohol solution with LNMO particles. After stirring and evaporation, the collected products were sintered in a muffle furnace to obtain conventional coating LNMO@Li₃PO₄ material (i.e., sample P1).

Also, Ni_0.25Mn_0.75(OH)₂ precursor was mixed with deionized water in a mass ratio of 1∶5. LiOH·H₂O was added to the coating at 3% under stirring until fully dissolved. Also, NH₄H₂PO₄ was added in another beaker and mixed with deionized water in a mass ratio of 1∶20 until fully dissolved. The above two solutions were mixed thoroughly and filtered. The obtained filtered product was dried in a blast drying oven at 90 ℃ for 24 h to obtain Ni_0.25Mn_0.75(OH)₂ precursor pre-coated with Li₃PO₄. Li₂CO₃ in a ratio of Li:(Ni+Mn) of 1∶2 was mixed evenly with the pre-coated precursor, and heated in a furnace at 950 ℃ for 10 h to obtain a wet coated LNMO cathode material with Li₃PO₄ after cooling (i.e., sample P2).

The morphologies and structures of the cathode materials were examined by a model S-4800 scanning electron microscope (SEM, Hitachi Co., Japan) a model F20 aberration-corrected transmission electron microscope (TEM) with a cold field emission gun at 200 kV (TECNAI), and a model Panalytical X'Pert X-ray diffractometer (XRD) in the 2θ range of 10°-75° with Cu K_α radiation (λ=1.540 5 Å). The compositions of TMs in materials were measured by a model IRIS IntrepidⅡinductively coupled plasma atomic emission spectroscope (ICP-AES, XSP).

Density functional theory (DFT) calculations were carried out through a named Vienna ab initio package (VASP), in which the cut-off energy was set to 520 eV for an accurate test, and the projector augmented wave (PAW) was used to describe the interaction between ions and electrons. For the optimization of crystal structures, the exchange correlation function was used as the Perdew-BurkesEmzerhof (PBE) form of generalized gradient approximation (GGA). The lattice vector and the atomic position were sufficiently optimized until the resultant force was less than 0.01 eV/(Å·atom^-1). The Brillouin zone was adopted with a 4×4×4 k-mesh.

Each material was mixed with carbon black and polyvinylidene fluoride binder in N-methyl-2-pyrrolidone with a mass ratio of 90∶5∶5 to prepare the electrode slurry. Subsequently, the slurry was casted on the Al foil and dried in vacuum at 100 ℃. Pouch cells were assembled with graphite as an anode, and 1 mol/L LiPF6 solution in EC/EMC/DEC (3∶5∶2 by volume) as an electrolyte. Pouch cells were tested at 3.50-4.95 V and 0.1 C (1 C=150 mA·h·g^-1) for the first cycle on a model Maccor S4000 battery testing system. The cycling performance, EIS, and rate capability at 25 ℃ were tested at 3.5-4.9 V.

Based on the powder XRD patterns of samples P0, P1, and P2, all the samples have a typical spinel structure, since the Li₃PO₄ coating layer has no impact on the LNMO's crystal structure. The SEM images of samples reveal that the coating process has a slight effect on the surface appearance and particle size distribution. Based on the TEM and SEM-EDS analysis, the coating layer of sample P2 has higher homogeneity than that of sample P1, which can provide a better protective effect and reduce the corrosion of the electrolyte. According to the cycling data, the capacity retention is 90.47%, which is obvious improvement from sample P0 after running 300 cycles, and the superior cycling performance indicates an outstanding structural stability of sample P2, which can be proved by the Mn dissolution data. The capacity retention of 92.45% at 2C discharging can verify that sample P2 has a uniform coating layer. As a result, the precursor pre-coating Li₃PO₄ technique affects the phase structure and morphology of spinel LNMO slightly, and improves the electrical properties effectively.

The main phase structure and micromorphology of spinel LNMO remained unchanged after Li₃PO₄ coating, and the characteristic peak of Li₃PO₄ appeared. The rate discharging and cycling performance was significantly improved, and the charge transfer impedance was reduced by Li₃PO₄ layer. This study indicated that precursor pre-coating Li₃PO₄ technique could be an effective approach to reduce the dissolution of transition metal and could be thus a potential method to improve the electrochemical performance of lithium manganese nickel oxide.