Investigations of LiCoPO$_{4}$ as a cathode material for high-voltage lithium ion batteries

Aachen (2019)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

Due to the ever-growing demand for higher energy density of lithium ion batteries, intensive research is underway to identify and develop the new cathode materials with high intercalation voltage over 4.5 V (vs. Li/Li+). LiCoPO4 is a promising candidate for cathode materials of high-voltage lithium ion batteries to obtain high energy, due to its high theoretical capacity (167 mAh g-1), high operating voltage (4.7 V vs. Li/Li+) and the thermal stability thanks to P-O covalent bonding. LiCoPO4 materials have not been widely used in practical applications, since there are several problems hindering its utilization, including low electronic conductivity, poor Li+ diffusion and limited stability of electrolytes at high voltage. To solve these critical problems and improve the electrochemical performance of LiCoPO4 cathode in high-voltage lithium ion batteries, this thesis uses solvothermal methods to synthesize various LiCoPO4-based materials, mainly focuses on three parts: carbon coating strategies, Fe substitution and particle morphologies control. The details are described as followed: Carbon free, ex-situ carbon coated and in-situ carbon coated olivine polymorph LiCoPO4 materials were prepared by solvothermal and a subsequent annealing process. With the addition of citric acid in the solvothermal reaction, a carbon layer was coated via an in-situ approach. To systematically compare the different carbon coating routes, the structure and morphology of the LiCoPO4 materials were investigated by XRD, Raman, and SEM. HAADF-STEM combined with EDX was applied to analyze the homogeneity of the carbon layer and corresponding antisite defects. Electrochemical properties were analyzed by half-cells measuring cyclic-voltammograms, charge/discharge cycling behavior stability and rate-capability. It was found that the in-situ carbon coated LiCoPO4/C exhibited a superior electrochemical performance due to the relatively uniform and complete surface-layer formation. As a result, an appropriate carbon layer improves the electronic and ionic transport properties, ensures fast electron-transfer kinetics at the electrode particle surfaces and suppresses unwanted side reactions with the electrolyte. Carbon coated olivine Pnma LiCoPO4 (LCP/C) and Fe-substituted LiCo0.8Fe0.2PO4 (LCFP/C) were synthesized by a solvothermal method and their structural features and electrochemical properties were investigated. The electrochemical performance of LCFP/C was better than that of LCP/C, owing to the partial substitution of Co by Fe which efficiently suppresses the increment of antisite exchange between Li+ and Co2+ ions within the structure during cycling, although the Li-Co antisite exchange amount in pristine LCP/C and LCFP/C was similar. Furthermore, direct visualization of Co in Li sites in the pristine samples and after 50 cycles was achieved through IIhigh-resolution scanning transmission electron microscopy for both LCP/C and LCFP/C. It was found that LCP/C locally formed a new cation-ordered structure after cycling due to the Li-Co antisite exchange, while the structure of LCFP/C remains almost the same. This study provides direct evidence that Fe substitution reduced the Li-Co exchange and improved the electrochemical cycling life of the LiCoPO4 cathode for high-voltage lithium ion batteries. Various LiCoPO4 materials have been synthesized by solvothermal methods and a subsequent annealing process. By adding citric acid, PVP or CTAB, different morphologies of LiCoPO4 materials were achieved, including unstructured nanoparticle, nanosheet, nanorod and microrod shape. Electrochemical analysis showed that the controllable morphology has an influence in electronic and ionic pathways, thus affects the electrochemical performance. The nanosheet shape LiCoPO4 shows the largest discharge capacity and the best rate capability, while the nanorod shape LiCoPO4 displays the relatively better cycling stability. Furthermore, the apparent Li+ diffusion coefficients of LiCoPO4 samples were determined to investigate the influence of particle size and shape on the Li+ migration.