Synthesis and Characterization of Carbon Coated LiCoPO4 as Cathode Materials for Lithium Ion Batteries
Authors: Li Ye , Taniguchi Izumi 
Published: February 2020

Introduction:

During the past two decades, tremendous amount of researches with regard to lithium ion battery has been studied because of its great energy density and environmental friendliness. The cathode gradually came to be the increasingly critical component of lithium ion batteries. However, although given to application for a long time, traditional cathode material LiCoO2 could not satisfy the increasing demand of higher energy density and superior safety.
Accordingly, this work focused attention on cathode material LiCoPO4 (theoretical capacity: 167mAh g-1) that possesses the advantages of both very high energy density resulted from high working voltage (about 4.8V) and promising thermal stability [1]. Nevertheless, poor electronic and ion conductivities blocked the way of this material for further application.
In order to solve those problems, carbon coated LiCoPO4 was synthesized by spray pyrolysis followed by wet ball milling with carbon source as well as heat treatment in this work. Moreover, physical and electrochemical properties of the material were studied.

Experimental

Firstly, the precursor solution was prepared by dissolving lithium orthophosphate monometallic (LiH2PO4) and cobaltous nitrate hexahydrate (Co(NO3)2‧6H2O) into distilled water as a molar ratio of 1:1. The molar concentration of the solution was 0.2 mol L-1. Afterwards, it was atomized to form 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 mixed with carbon source such as PVP by wet ball milling with different rotation speed ranging from 200 to 600 rpm for 6 h. Then, the mixtures were annealed at 700℃ for 4 h under a steady flow of Ar gas after completing free drying and ultimately carbon coated LiCoPO4 was obtained.
The characterization of physical properties of the material, such as crystal structure and morphology, was achieved 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 carbon coated LiCoPO4 to investigate the electrochemical performance of this material. First, calculated based on LiCoPO4, 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 the coin-type cell (CR2032) in a glove box under a high purity Ar atmosphere. 1 mol L-1 LiPF6 dissolved in EC/DMC was used as the electrolyte. Li foil was used as an opposite electrode. The cells were galvanostatically cycled between 3.5 and 5.1 V at 0.1C rate (1C= 167 mA g-1) on multichannel battery testers (Hokuto Denko, HJ1010mSM8A).

Results and discussion

Fig. 1 displays the XRD patterns of carbon coated LiCoPO4 synthesized by spray pyrolysis at 400℃followed by wet ball milling with 0-20 wt.% of PVP and heat treatment at 700℃in pure Ar atmosphere. According to these XRD patterns, all peaks of the samples are mainly indexed to olivine structure in a Pnma space group. Nevertheless, when 10 wt.% and 20 wt.% of PVP are added into the sample, weak impurity peaks of Co2P appear in the XRD pattern of sample. The electrochemical performance of these samples are also investigated by testing coin cells assembled with using these samples. The sample added with 10 wt.% of PVP showed the highest initial discharge capacity reaching to approximately 100 mAh g-1 for the carbon coated LiCoPO4. The further data and figures in detail will be displayed during the presentation of symposium.

Reference

[1] J. Ludwig, C. Marino, D. Haering, C. Stinner, H.A. Gasteiger, T. Nilges, J. Power Sources. 342 (2017) 214–223.

Figure 1