Development and characterization of high power density cathode materials for lithium-ion batteries

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Tezin Türü: Doktora

Tezin Yürütüldüğü Kurum: Orta Doğu Teknik Üniversitesi, Mühendislik Fakültesi, Mühendislik Bilimleri Bölümü, Türkiye

Tezin Onay Tarihi: 2015

Öğrenci: ŞAFAK DOĞU

Eş Danışman: ZAFER EVİS, MEHMET KADRİ AYDINOL

Özet:

In this thesis, facile and cost efficient aqua based synthesis method is developed to synthesize power dense and fast rechargeable LiFePO4 cathode materials. In order to obtain nano sized crystal morphology, nucleation controlled techniques were studied on precursor synthesis. These techniques are freeze (cryogenic) drying with co-precipitation and ultrasound assisted sub-sequential precipitation with vacuum drying at low temperatures (<350 K). In co-precipitation with freeze drying synthesis, star-like platelet LiFePO4 was synthesized while ultrasound assisted sub-sequential precipitation synthesis yielded 2D polycrystalline nano-plate structures around 100 nm thickness which has high tap density. The morphology of particles was diverted by manipulation of nucleation and crystallization processes and then preserved with carbon encapsulation strategy before LiFePO4 formation through calcination. The highest discharge capacity is found as 140.3 mAhg–1 at 0.1C cycling rate for LiFePO4/C synthesized by ultrasound assisted sub-sequential precipitation and carbonized via CVD. The highest and endurable electrochemical performance was achieved with sucrose encapsulated LiFePO4/C where the thinnest plate-like LiFePO4 (40 – 100 nm) was synthesized via ultrasonicated formation of vivianite precursors. The rechargeable capacities are found as 125.1 and 89.2 mAhg–1 at slow (0.1C) and fast (1C) discharge rates, respectively. Regarding to these slow and fast rated discharge capacities, higher capacity retentions (82 – 90%) were observed as 103.8 and 81 mAhg–1 coulombic capacities even after less than 1 hour fast charging. Through this synthesis technique, especially in fast charging, it is able to achieve more discharge capacity than other high temperature hydro/solvothermal synthesis.