Soft Chemical Approach to Surface Modification of Olivine-Type LiFePO4 nanoplatelets and Their Electrochemical Properties

Abstract: Olivine phosphate (LiFePO₄) is a remarkable cathode material^1-4 for lithium ion (Li-ion) batteries due to its high theoretical capacity (Q= 170 mA h g^-1), exceptional cycling stability, low cost, environmental compatibility and safety.  Conventionally, on this material a carbon coating⁵ has widely been attempted for better Li ions kinetics and cycling (charge/discharge). Here, we report a colloidal synthesis route to prepare platelet-shaped (~10 nm thick) LiFePO₄  nanocrystals (NCs). We then explore soft chemical strategies for the surface preparation of phospho-olivine platelet NCs by a treatment with either lithium fluoride (LiF) or lithium hexafluorophosphate (LiPF₆).  The NCs were given, for instance, a coating of LiF to yield core-shell type structures (LiFePO₄ core and LiF shell) as shown in Figure.1a-b. The core-shell NCs were extensively characterized with the aid of several structural and compositional techniques (via X-ray diffraction, high-resolution electron microscopy, and electron energy loss spectroscopy), highlighting their morphological/structural features relevant for high-rate capable electrodes. We then illustrate the electrochemical properties (via cyclic voltammetry, galvanostatic charge/discharge, and impedance spectroscopy) of core-shell NCs in Li metal half-cells by comparing their carbon coated NC counter parts.

Reference:

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Figure.1 a) Transmission electron microscopy (TEM) image of   LiFePO₄NC samples a) pristine; b) after a treatment with lithium fluoride (LiF)