Ultrasound Asissted LiFePO4 Nano Plate Synthesis Via Subsequential Aqueous Precipitation Method

After the discovery of the olivine type LiFePO₄ as a promising cathode candidate for lithium-ion batteries¹, it performed outstanding cycling stability and safety compared to alternative oxide cathode materials. Pioneer research studies in the field of LiFePO₄ have focused on the improvement of low electronic conductivity (∼10⁻⁹ Scm⁻¹) and slow lithium ion diffusion (∼1.8×10⁻¹⁴ cm² s⁻¹) which causes poor rate capability at quick charge². To increase conductivity by increasing Li⁺ ion diffusion rate is aimed to be achieved by both carbon coating and size refinement^3,4.

As the sequentail aqueous vivianite (Fe₃(PO₄)₂·8H₂O) synthesis followed by ball milling is the early known technique as inexpensive and resulted in high energy cathode⁵, it was not promising due to bad reproducibility, micron size particles and oxidation sensitivity of vivianite⁶. Other than sequential method, coprecipitation technique based on aqueous chemistry have been used by researchers to meet the demand on high rate capable LiFePO₄ synthesis. However, besides its rapid growth effect, co-precipitation technique needs Li excess stoichiometry in solution system which equimolar existence of Li₃PO₄ and Fe₃(PO₄)₂·8H₂O phases is not possible in order to form pure LiFePO₄ as reported in many computational and experimental studies^7,8.

Our study focus on synthesis of fast rechargeable LiFePO₄ cathode material by sequential aqueous precipitation via ultrasound method. This new technique provides high reproducibility with more control on size distrubution and morphology of particles as well as reaction kinetics. Thanks to the slow reactive crystallization rate due to the weak solubility balance – between vivianite and Li₃PO₄– ultrasound is used to gain control on thickness of nano plates and conversion rate. The subsequential iron(II) phosphate formation reactions and vivianite crystallization are driven by ultrasound stimulation which enhances the reaction and nucleation rates within a very small volume and time in atmospheric and inorganic suspension conditions.

Insonation reduces both induction time and metastable zone width of growth region so that crystallization process becomes uniform and controllable⁹. Improved mass transportation and sonic fragmentation also homogenize the mean crytallite size by rapid shock waves while reducing the agglomeration. No other reducing organic substances are added into aqueous media during precursor synthesis, because the reducing effect of ultrasound waves prevents vivianite and Fe²⁺from further oxidation by formation of H· and OH· radicals in aqueous suspension.

For conductivity increment and preservation of the synthesized nano plate (40 – 80 nm) morphology, carbon encapsulation is conducted under reducing atmosphere on vivianite precursors before the calcination step which is a crucial step in most methods for ordered and well crystalline cathode. Following the meta-morphologic phase transformation besides carbon coating, calcination yields to the polycrystalline 50 – 100 nm thick individual LiFePO₄/C nano plates surviving from high temperature treatment at 700 ^oC.

Polycrystalline nano plate morphology positively contributes to the lithium diffusion mechanism by creating ten orders of magnitude higher ionic conduction¹⁰ paths within high number of grain boundaries as thin as the nano plates. XRD and SAED patterns support that vivanite nano plates are synthesized with strong preferred orientation at (010) as single crystalline. After sub-grain formation in LiFePO₄/C nano plates, weak preferred orientation distribution is observed around [010].

The highest discharge capacity is 125.1 mAh g^–1 at 0.1C cycling rate. Into the fast charge at 1C, discharge capacities are obtained as 103.8 and 80.3 mAh g^–1 at slow (0.1C) and fast (1C) discharges, respectively. Especially, in fast charge (1C) rate, it is able to achieve higher discharge capacities by comparing other recent high temperature solvothermal synthesis^11,12.

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