A Novel Two-Step Synthesis for Li2CoPO4F as High-Voltage Cathode Material

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, LiNi_0.5Mn_1.5O₄ 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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