This decade witnesses a mounting number of researches towards lithium ion battery due to its great energy density and environmental friendliness. However, as the considerably significant part of lithium battery, conventional cathode material such as LiCoO₂ could not fulfill the growing requirement for higher energy density and superior safety.

Therefore, with theoretical capacity of about 167mAh g^-1, cathode materials LiMPO₄ (M=Co, Ni) have been attracted more and more attention, owing to their high energy densities derived from high working voltages (4.7-5 V) and outstanding thermal stability ^[1]. Nevertheless, poor electronic and ion conductivities, and especially low electrochemical activity of LiNiPO₄ hindered these kind of material from practical application ^[2].

For the purpose of endeavoring to settle those issues, the LiNi_1-xCo_xPO₄/C (x=0, 0.2, 0.5, 0.8, 1) solid solution composites, in this work, were synthesized by spray pyrolysis followed by wet ball milling as well as heat treatment. Moreover, physical and electrochemical properties of the material were studied.

Experimental

Firstly, the precursor solution was prepared by dissolving lithium orthophosphate monometallic (LiH₂PO₄), nickel nitrate hexahydrate (Ni(NO₃)₂‧6H₂O) and cobaltous nitrate hexahydrate (Co(NO₃)₂‧6H₂O) into distilled water as molar ratios of 1: 1-x: x (x=0, 0.2, 0.5, 0.8, 1). The molar concentration of the solution was 0.2 mol L^-1. They were atomized to generate small droplets that subsequently were carried by air gas flowing into the reactor, in which the temperature was 400℃. The collected spherical-like precursor particles prepared by spray pyrolysis were milled with acetylene black by wet ball milling as a rotation speed of 600 to 800rpm for 6h. Then, the mixture was annealed at 550℃ for 4 h under Ar atmosphere after drying in the oven and finally LiNi_1-xCo_xPO₄/C nanocomposites were obtained.

The characterization of physical properties of the material, such as structure and morphology, was accomplished by X ray diffraction analysis (XRD), scanning electron microscope (SEM), specific surface area measurement, inductively-coupled plasma spectrometer (ICP) and CHNS elemental analysis.

The cathode was made by LiNi_1-xCo_xPO₄/C nanocomposites to investigate the electrochemical performance of this material. First, calculated based on LiNi_1-xCo_xPO₄, the active material was mixed with acetylene black (AB) and polyvinylidene fluoride (PVDF) as a weight ratio of 7:2:1 in appropriate amount of 1-methyl-2-pyrrolidinone (NMP). The formed slurry was coated on an Al foil and was dried in a vacuum oven. The cut cathode was used to assemble coin-type cells (CR2032) in the glove box under a high purity Ar atmosphere. 1 mol L^-1 LiPF₆ dissolved in EC/DMC was used as the liquid electrolyte. Li foil was used as an opposite electrode. The cells were galvanostatically cycled between 3.5 and 5.3 V at 0.1C rate (1C= 167 mA g^-1) on multichannel battery testers (Hokuto Denko, HJ1010mSM8A).

Results and discussion

Fig. 1 shows the XRD patterns of LiNi_1-xCo_xPO₄/C (x=0, 0.2, 0.5, 0.8, 1) synthesized by spray pyrolysis at 400℃followed by wet ball milling and heat treatment at 550℃ in pure Ar atmosphere. It can be confirmed that all peaks of the samples are mainly indexed to olivine structure with a Pnma space group, except that weak impurity peak of Ni shows in the sample of LiNi_0.8Co_0.2PO₄/C. Besides, peak shift towards left can be seen clearly at about 2θ=36.5º because of larger radius of Co ion (0.74Å) than Ni ion (0.69Å). After analyzing these samples by Rietveld refinement, the obtained lattice parameters of samples elevate linearly with the increasing amount of Co doped into LiNiPO₄, also because the radius of Co ion is larger than that of Ni ion. The further data and figures in detail will be displayed during the presentation of symposium.

Reference

[1] J. Wolfenstine, Ni³⁺ /Ni²⁺ redox potential in LiNiPO₄, J. Power Sources, 142 (2005) 389–390.

[2] D. Shanmukaraj, R. Murugan, Synthesis and Characterization of LiNi_yCo_1-yPO₄ ( y = 0-1 ) Cathode Materials for Lithium Secondary Batteries, Ionics, 10 (2004) 88–92.