Developing a Novel 2-Step Synthesis for High-Voltage Li2CoPO4f for Li-Ion-Batteries
Authors: Schoiber Jürgen , Berger Raphael J. F. , Yada Chihiro , Miki Hidenori , Hüsing Nicola 
Published: February 2020

Materials providing higher energy densities for next generation Li ion batteries (LiBs) are of high interest for the use in electronic vehicles (EV) (1). As promising candidates LiCoPO4, Li3V2(PO4)3, LiNi0.5Mn1.5O4 and Li2CoPO4F are under investigation. Li2CoPO4F was first synthesized and applied in LIBs in 2005 by Okada et. al. and crystallizes in the space group Pnma (2). The framework is built of CoO4F2 octahedra and phosphate tetrahedra. This open network allows for a 3D diffusion of the Li ions (3). Li2CoPO4F has a potential of 4.9 V vs. Li/Li+ and a theoretical capacity of 286 mAh/g, assuming both Li ions can be extracted. With that the highest energy density of all candidates can be expected. However, so far only one Li per formula unit could be extracted. This leads to a capacity of ~145 mAh/g, which was obtained for Li2CoPO4F synthesized via a sol-gel approach (4). In further studies, different synthesis routes based on solid-state reactions of, e.g. LiCoPO4 with LiF or by using stoichiometric amounts of Co2+, Li+, PO4
3- ­and F- precursor salts were developed (5, 6, 7). One problem of Li2CoPO4F is its low cyclability due to the high operating voltage (5, 6). To overcome this drawback coating of the Li2CoPO4F particles with Li3PO4, Al2O3 or ZrO2 is one possibility, while increasing the surface area is another (8, 9, 10).

We herein present a novel 2-step synthesis route towards Li2CoPO4F. In the first step we synthesize a LiCoPO4/LiF powder via solvothermal synthesis. We investigated the influence of different solvents, such as ethylene glycol, diethylene glycol, tetraethylene glycol and glycerol, on particle size and phase purity. The obtained powder is converted into Li2CoPO4F in a second step in a tube furnace under steady Ar-flow at 660 °C for 1 h and subsequent quenching to room temperature. Characterization of the resulting product was performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen sorption, simultaneous thermal analysis (STA), inductively coupled plasma optical emission spectroscopy (ICP-OES) and electrochemical methods.

Our new procedure allows us to directly convert the LiCoPO4/LiF nano-powder obtained from solvothermal synthesis without any additional steps such as ball milling or pelletizing. Furthermore, the rapid conversion time at 660 °C of only 1 h, is highly advantageous compared to the recent literature (2, 4, 6, 8).

We obtained submicron Li2CoPO4F particles after the two step synthesis as can be seen in Figure 1. The different solvents applied in the solvothermal synthesis have a strong influence on phase purity and particle size of Li2CoPO4F.

 Figure 1: SEM image of as synthesized Li2CoPO4F

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Figure 1