A New Superstructure in Beam Sensitive Cathode Material Revealed by Multimodal STEM Combining ADF, iDPC and EDX Mapping Techniques

Microscopy and Microanalysis, vol. 29, pp. 1764–1765

Abstract

LiCoPO4 is a promising cathode material for the high voltage lithium-ion batteries. However, the moderate cycling stability of the material heavily limits its practical applications. One of the major reasons of the capacity fade of LiCoPO4 and the end of the lifetime of the material is related to its structural degradation: Li and Co site exchange upon charge-discharge cycling leads to the blocking of the Li+ transport channels [1]. The opportunity to stabilize the atomic structure of the material and improve the battery lifetime directly depends on the capability to gain knowledge on the degradation mechanisms though advanced characterization. Cathode materials are obstinate candidates for TEM investigations due to their poor stability under the electron beam. Low electron dose should be applied in order to minimize radiolysis while imaging LiCoPO4 . Here, we exploit simultaneous ADF-STEM and iDPC-STEM [2,3] imaging to investigate the LiCoPO4 cathode material down to atomic scale, as iDPC-STEM allows for extremely low dose imaging of beam sensitive materials [4,5]. By combining the experimental and simulation results a superstructure at the surface of the LiCoPO4 nanoparticle is revealed: every second Li site is partially occupied by higher Z-number atoms as periodic bright contrast is observed. Fig. 1 contains experimental ADF-STEM and iDPC-STEM images as well as corresponding simulations in the inserts and the ball structure of the superlattice. A good match between the experimental data and the proposed structure is shown. To confirm the proposed superstructure, EDX spectroscopy at atomic resolution is performed. Fig. 2 contains high resolution ADF-STEM image of the superstructure formed at the surface of the LiCoPO4 nanoparticle viewed along the [001] zone axis and corresponding atomic resolution EDX elemental map showing Co in purple and P in green. It is clearly visible that indeed every other Li site (also highlighted by the arrows in Fig. 2) is partially occupied by Co atoms. To sum up, in this work we exploited atomic resolution low dose techniques to investigate beam-sensitive cathode battery material. A combination of both HR imaging and spectroscopy opens the doors to precise and trustful investigation of the cathode materials degradation at atomic scale. An example of LiCoPO4 cathode material with the so-called antisite defects formation as one of the major degradation reasons helps to highlight the power of atomic resolution imaging and spectroscopy. Simultaneously recorded, using HAADF and Panther detectors, high resolution ADF- and iDPC-STEM images (from left to right) of the superstructure at the surface of the LiCoPO4 nanoparticle, viewed along the [001] zone axis, together with the corresponding simulations and proposed surface structure. The probe corrected beam conditions: HT 300kV, Convergence semi-angle 30 mrad, beam current 15 pA. HR ADF-STEM image of the superstructure at the surface of the LiCoPO4 nanoparticle, viewed along the [001] zone axis together with the corresponding overlayed EDX elemental map for Co (shown in purple) and P (shown in green). The black and the purple arrows point to the regions where Co occupies Li positions while the blue arrows highlight the regions where no Co occupation in Li sites is monitored.

Authors 10

1. Maria Meledina corresponding

   Thermo Fisher Scientific (Netherlands)

   Affiliation as printed

   Thermo Fisher Scientific, Materials & Structural Analysis , Eindhoven , the Netherlands

2. Alexander Meledin

   Thermo Fisher Scientific (Netherlands)

   Affiliation as printed

   Thermo Fisher Scientific, Materials & Structural Analysis , Eindhoven , the Netherlands

3. Eric Bosch

   Thermo Fisher Scientific (Netherlands)

   Affiliation as printed

   Thermo Fisher Scientific, Materials & Structural Analysis , Eindhoven , the Netherlands

4. Ivan Lazić

   Thermo Fisher Scientific (Netherlands)

   Affiliation as printed

   Thermo Fisher Scientific, Materials & Structural Analysis , Eindhoven , the Netherlands

5. Xiaochao Wu Aachen Institute of Inorganic Chemistry

   RWTH Aachen University

   Affiliation as printed

   Institute of Inorganic Chemistry, RWTH Aachen University , Aachen , Germany

6. Ulrich Simon Aachen Institute of Inorganic Chemistry

   RWTH Aachen University

   Affiliation as printed

   Institute of Inorganic Chemistry, RWTH Aachen University , Aachen , Germany

7. Boy Markus

   Thermo Fisher Scientific (Netherlands)

   Affiliation as printed

   Thermo Fisher Scientific, Materials & Structural Analysis , Eindhoven , the Netherlands

8. Bert H. Freitag

   Thermo Fisher Scientific (Netherlands)

   Affiliation as printed

   Thermo Fisher Scientific, Materials & Structural Analysis , Eindhoven , the Netherlands

9. Sorin Lazar

   Thermo Fisher Scientific (Netherlands)

   Affiliation as printed

   Thermo Fisher Scientific, Materials & Structural Analysis , Eindhoven , the Netherlands

10. Paolo Longo

    Thermo Fisher Scientific (Netherlands)

    Affiliation as printed

    Thermo Fisher Scientific, Materials & Structural Analysis , Eindhoven , the Netherlands