LiFePO4/C Research Articles

Effects of particle size and carbon coating on electrochemical properties of LiFePO4/C prepared by hydrothermal method

In this study, well-crystallized phase pure LiFePO4/C (LFP/C) powders were synthesized using the hydrothermal reaction method. To improve the electronic conductivity of the LFP/C powder after ball-milling, the LFP/C powders were double-coated with carbon. Scanning electron microscopy and transmission electron microscopy were employed to observe the micromorphology of the samples and the carbon coating, which was analyzed using Raman spectroscopy. Furthermore, the electrochemical properties were evaluated using cyclic voltammetry, electrochemical impedance spectra, and the charge–discharge cycling test. The ball-mill and the process for double-coating carbon decrease the particle size and increase the conductivity of the LFP/C, thereby reducing the Li-ion diffusion length and improving the reversibility of the Li-ion intercalation/de-intercalation in the LFP/C crystallites. The capacity of the small-particle LFP/C with the double-layer carbon coating was 133 mAh/g at 0.1 °C, and remained at 83 mAh/g as the charge–discharge rate increased to 10 °C. In addition, good cycle stability was observed, with a retention rate of 98 % after 50 cycles at 1 °C.

LiFePO4/C composites derived from precipitated FePO4 precursor: effects of mixing processes

Grinding, general incipient wetness and competitive incipient wetness were investigated on the preparation of LiFePO4/C (LFP) composites from precipitated FePO4 precursor. This article focuses on the fundamental details of mixing processes for LFP production, of which the literature is sparse. The structure features of LiFePO4/C composites were analyzed by X-ray diffraction (XRD), while the morphologies were characterized by field emission scanning electron microscopy (FESEM). The electrochemical performances were studied by galvanostatic charge/discharge measurements. Both mixing processes and lithium sources are capable of affecting the electrochemical performances by altering the crystallinity, the quality of carbon coating, or even the impurity formation of LiFePO4/C composites. Liquid alkalis caused the formation of impurities and mild decrease of crystallinity, whereas competitive cations did not produce impurities but significantly decreased the crystallinity of LiFePO4/C composites.

The Study of Theoretic Coated Carbon Amount and Excess Lithium in LiFePO<sub>4</sub>/C Synthesis by Means of Carbon Thermal Reduction Route

LiFePO4/C(LFP/C) composite cathode material was synthesized by carbon thermal reduction route with the sucrose as reducing agent and characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). Some parameters were optimized by means of determining the amount of sucrose and Li2CO3. The electrochemical performances were also tested through assembling batteries. The experimental results demonstrated that the proper theoretic coated carbon amount was 11.4% and 4.21% respectively, and the proper lithium usage is the normal stoichiometric proportion, namely, the lithium amount is not excess.

Structure and electrochemical performances of LiFe1−2xTixPO4/C cathode doped with high valence Ti4+ by carbothermal reduction method

Abstract LiFePO 4 /C (LFPC) and LiFe 1−2 x Ti x PO 4 /C (LFTPC) were prepared by carbothermal reduction method using FePO 4 ·2H 2 O as iron source and phenol–formaldehyde resin as reducing agent and carbon source. Different ratios of TiO 2 (IV) with high valence and small radius were applied to dope LiFePO 4 to enhance its electrochemical performances. Results show that LFPC and LFTPC are synthesized successfully by carbothermal reduction method. The optimal carbon content in LFPC is 5 wt.% and its discharge capacity at 0.1 C is 150.8 mA h g −1 . The crystallite structure of LFTPC becomes stable. They possess the smaller particle size compared with LiFePO 4 . LFTPC-2 possesses the best C-rate and cycle performances among all the samples. Its discharge capacities at 0.1 C, 1 C and 3 C are 132.7 mA h g −1 , 98.7 mA h g −1 and 83.1 mA h g −1 . The discharge curve can maintain its stable and flat platform of 3.3 V at 3 C. The electronic conductivity of LFTPC, which is coated with carbon and doped with Ti, can reach ∼10 −4 S cm −1 . The charge transfer resistance of LFTPC-2 is 33.68 Ω, which is much lower than that of other samples.

High performance LiFePO4/CN cathode material promoted by polyaniline as carbon–nitrogen precursor

A facile and efficient solid state synthesis of carbon and nitrogen coated lithium iron phosphate (LiFePO4/CN) cathode material is achieved via polymer-pyrolysis method using polyaniline-chloride (PANI-Cl). The current investigation is comparatively analyzed with the results of the composite of LiFePO4/C (LFP/C) synthesized using sucrose as carbon precursor. The optimized LiFePO4/CN (LFP/CN) composite is synthesized at 700 °C using 10 wt.% PANI-Cl. The composite exhibits remarkable improvement in capacity, cyclability and rate capability compared to those of LFP/C. The specific discharge capacities as high as 164 mAh g−1 (theoretical capacity: 170 mAh g−1) at 0.1 C and 100 mAh g−1 at 10 C rates were achieved with LFP/CN. In addition, the composite exhibits a long-term cycling stability with the capacity loss of only 10% after 1000 cycles. PANI-Cl shifts the size distribution of the composite to nanometer scale (approximately 150 nm), however the addition of sucrose does not have such an effect. LFP/CN contains 1.6 wt.% nitrogen and 15.8 wt.% carbon. LFP particles are mostly coated with a few nanometers thick C–N layer forming a core–shell structure. The possible crosslinking mechanism of PANI-Cl upon pyrolysis on size reduction and formation of uniform carbon/nitrogen coating on LFP are also discussed.

An economic and scalable approach to synthesize high power LiFePO4/C nanocomposites from nano-FePO4 precipitated from an impinging jet reactor

A novel, economic and scalable method is proposed in this context using an impinging jet reactor to continuously precipitate FePO4 (FP) nanoparticles with high purity. The LiFePO4/C (LFP/C) nano-composites synthesized from the FP, lithium and carbon sources show fast charge capability and excellent power performance. The generated FP and LFP/C are characterized with X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and inductively coupled plasma (ICP) spectrometry. The LFP/C nano-composites are cycled at various C-rates to explore the electrochemical performance. When the pH of the working solution is 1.3, the molar ratio of Fe to P equals 1 in the obtained amorphous FP. In addition, the precipitation mechanism is discussed. Compared to the amorphous FP, the hexagonal FP obtained after calcining the former one is preferred to synthesize high power LFP/C. It is proved that 2 mol% excess LiOH is needed to compensate for the possible loss of Li during calcination. With 4.4 wt% of in situ carbon coating, the LFP/C nano-composites exhibit 145 mA h g−1 at 10 C (1.7 A g−1, 6 min) charge (without floating charge) and discharge, without obvious capacity loss up to 100 cycles, which is among the best high power performances reported so far. This strategy offers an inexpensive and easy control method to mass-produce high quality LFP/C nano-composites which might be suitable for powering hybrid electric vehicles and plug-in hybrid electric vehicles.

A comparative study on the low-temperature performance of LiFePO 4/C and Li 3V 2(PO 4) 3/C cathodes for lithium-ion batteries

The electrochemical properties of the LiFePO 4/C (LFP/C) and Li 3V 2(PO 4) 3/C (LVP/C) samples are investigated using Li-ion half cells at various low temperatures of 23, 0, -10 and -20 °C in the electrolyte 1.0 M LiPF 6/EC + DMC (1:1 w/w). In the voltage range of 2.5-4.3 V at 0.3C rate, the LFP/C electrode shows a stable reversible discharge capacity of 45.4 mAh g −1 at -20 °C, which is 31.5% of the capacity obtained at 23 °C. In the voltage range of 3.0-4.3 V at 0.3C rate, the LVP/C electrode delivers a high stable reversible discharge capacity of 108.1 mAh g −1 at -20 °C, being 86.7% of the capacity at 23 °C. The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements illustrate that the resistance and cell polarization of LFP/C at various low temperatures are larger than those of LVP/C. In addition, using CV method, the apparent chemical diffusion coefficient of lithium ions ( D L i + app ) in LFP and LVP at various low temperatures are in the magnitude of 10 −11 and 10 −10 cm 2 s −1, respectively, and the D L i + app decreasing rate of LFP as the temperature is much larger than LVP, meaning that the kinetics of the LFP electrode is relatively slow. The activation energies of LFP and LVP are calculated to be 47.48 and 6.57 kJ mol −1, respectively, which further indicates that the extraction/intercalation of Li + in LVP is much easier than in LFP. The high reversible capacity of LVP/C at -20 °C makes it an attractive cathode for low-temperature lithium ion batteries.