Nanocrystalline LiFePO4 Research Articles

(Invited) Unravelling Structural Implications on Charge Transport Mechanism in Nanostructured Electrode Materials for Energy Storage Devices

The development of new and novel electrode materials for energy storage devices has become the intensive research by the materials science community because of its importance in the portable electronic devices, hybrid electric vehicles and many other applications. Since the discovery by Goodenough and his co-workers on the electrochemical behavior of olivine LiFePO4 (LFP), different theoretical and experimental investigations have been undertaken to optimize it as cathode material in lithium-ion batteries and trying to understand the charge/discharge mechanism. Despite having many fascinating electrochemical properties, the main drawback of LFP lies with its low gravimetric density and poor electrical conductivity (both electronic and ionic) which limits the lithium intercalation/deintercalation rates, and hence the practical specific capacity. Therefore, it becomes necessary to gain the fundamental understanding of electronic structure of the LFP system by adopting appropriate experimental technique. We exploit the combined Mössbauer and X-ray absorption spectroscopy to unravel the electronic structure and local site symmetry of Fe in olivine structured LFP with different crystallite sizes (CS). The lattice parameters are found to contract with decrease in CS monotonously, whereas the electronic structural parameters exhibit two different regions with threshold anomaly around ≈30 nm CS. The 57Fe Mössbauer studies reveal the coexistence of Fe2+ and Fe3+sites and their relative concentration are mainly determined by the CS, which provides the comprehensive insight into the electronic structure of LFP at mesoscopic level. The soft X-ray absorption unravels unequivocally the valance states of Fe 3d electrons in the proximity of Fermi level, which are prone to the local lattice distortion. An obtained spectra fingerprint the effect of CS supplying rich information on valence state of iron, lithium-ion vacancy concentration, covalency and crystal field. The unique structural and electronic properties of the LFP are closely interlinked with changes in the bonding character, which shows the strong dependency on the CS. The evolution of the 3d states is in overall agreement with the local lattice distortion and provides the origin of the size effects on the electronic structure olivine phosphate and other transition metal ion containing materials. We observe polaronic conductivity enhancement of approximately two orders of magnitude at the nanoscale level as compared with its bulk counterpart. The volumetric changes with respect to crystallite size are related to the compressive strain resulting into the improvement in the electronic diffusivity. The nano-crystalline LFP with better kinetics will open the new avenue for its usage as cathode material in rechargeable batteries.

Electrochemical Properties of Pristine and Vanadium Doped LiFePO4 Nanocrystallized Glasses

In our recent papers, it was shown that the thermal nanocrystallization of glassy analogs of selected cathode materials led to a substantial increase in electrical conductivity. The advantage of this technique is the lack of carbon additive during synthesis. In this paper, the electrochemical performance of nanocrystalline LiFePO4 (LFP) and LiFe0.88V0.08PO4 (LFVP) cathode materials was studied and compared with commercially purchased high-performance LiFePO4 (C-LFP). The structure of the nanocrystalline materials was confirmed using X-ray diffractometry. The laboratory cells were tested at a wide variety of loads ranging from 0.1 to 3 C-rate. Their performance is discussed with reference to their microstructure and electrical conductivity. LFP exhibited a modest electrochemical performance, while the gravimetric capacity of LFVP reached ca. 100 mAh/g. This value is lower than the theoretical capacity, probably due to the residual glassy matrix in which the nanocrystallites are embedded, and thus does not play a significant role in the electrochemistry of the material. The relative capacity fade at high loads was, however, comparable to that of the commercially purchased high-performance LFP. Further optimization of the crystallites-to-matrix ratio could possibly result in further improvement of the electrochemical performance of nanocrystallized LFVP glasses.

SYNTHESIS OF LiFePO4 NANOCRYSTALS IN THE ION-LIQUID MEDIUM USING MICROWAVE HEATING

The liquid-phase method of synthesis of lithium iron(II) phosphate (LiFePO4) in the medium of choline chloride and diethylene glycol under the action of microwave heating is proposed. With a power of microwave radiation of 920 and 1150 W, a nanocrystalline LiFePO4 without impurities was obtained. Obtained samples of microwave processes contain amorphous phase and require long annealing resulting in nanocrystalline LiFePO4/C composites with small impurities Li3PO4, Li3Fe2(PO4)3, Fe2O3. For samples obtained in the choline chloride with diethylene glycol microwave heating characteristic is lamellar morphology – the same as for LiFePO4 obtained by thermal heating, but in the case of using microwave irradiation plates are smaller. This indicates that the reaction mechanism of LiFePO4 synthesis does not change in the microwave synthesis, but the reaction rate is significantly increased (up to 6 times faster than thermal synthesis). Using the Raman spectroscopy, the nature of the carbon coating on the crystal of LiFePO4 was studied. The Raman spectra of the LiFePO4/C composites obtained from an annealed powder with glucose and malic acid have pronounced D (~ 1340 cm-1) and G (~ 1600 cm-1) peaks, as well as two additional bands at ~ 1200 and ~ 1520 cm-1 obtained after the expansion of main peaks. The ratio of peak intensities of lines D and G (ID/IG) has a value of 1.06 for the material obtained after glucose carbonation and 1.01 for LiFePO4/C composites annealed with malic acid, which correlates with the results of other investigations of the carbon coating LiFePO4 (ID/IG ~ 1-3) That means the choice of an organic precursor does not affect the nature of the carbon coating (ID/IG ~ 1). Capacity of cathode material based on LiFePO4/C composites is ~ 130 mAh/g for a current of 0.1C.

Effect of carbon coating on the crystal orientation and electrochemical performance of nanocrystalline LiFePO4

Extensive efforts have been devoted to carbon coated LiFePO4 to optimize its performance by increase conductivity, however, the coating effect on crystalline size and shape have never been touched. Herein, we reveal the influence of carbon content on the crystal orientation, particle size, and electrochemical performance of the nanocrystalline LiFePO4, which is synthesized by a hydrothermal method with predominantly (010) faces exposure. We found that an increase of the carbon content from 1.65 to 6 wt% modified the preferential orientation of the LiFePO4 crystal from the (010) plane to the (100) plane. The solid electrolyte interface resistance Rsf also increased, causing a decrease in the electrochemical performance of LiFePO4/C cathode materials. Among all the carbon coated samples, LiFePO4/C composites coated with 1.65 wt% carbon exhibited the best initial discharge capacity and efficiency of 162 mAh g−1 and 95.4% at 0.1C, respectively. This performance can be attributed to the low Rsf value and preferential orientation of the (010) plane.

Effect of Carbon Sources and Synthesis Conditions on the LiFePo4/C Cathode Properties

Abstract The sol-gel synthesis and precipitation from dimethyl sulfoxide (DMSO)-water mixture were used to obtain LiFePO4. Sucrose (sucr), citric acid (CA), phthalic acid (phth), polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyvinylidene fluoride (PVDF), polyethylene glycol (PEG), polyacrylic acid (PAA) were investigated as carbon precursors. Increasing of the annealing temperature from 600 °» to 800 °» leads to a certain perfection of graphite structure with simultaneous particle size growth. Higher capacities were observed for the materials synthesized at 600 °». The best results for the sol-gel synthesis were obtained when PVDF was used as a source of carbon coating due to the partial fluorination of LiFePO4 (discharge capacity of LiFePO4/ C-PVDF was ~160 and 70 mAh g-1 at 20 mA g-1 and 800 mA g-1 currents, respectively). For nanocrystalline LiFePO4 obtained by precipitation from DMSO-water mixture the best results were obtained when PEG was used as carbon source (discharge capacity LiFePO4/C-PEG was 158 and 77 mAh g-1 at 20 mA g-1 and 800 mA g-1, respectively).

Effect of lithium phosphate on the structural and electrochemical performance of nanocrystalline LiFePO4 cathode material with iron defects

The influence of excessive lithium on the crystal structure, morphology, and electrochemical properties of Fe-deficient LixFePO4 (x = 1.00, 1.02, 1.04, 1.05) cathode material was investigated using co-precipitation and the carbon thermal method. The X-ray diffraction pattern and Rietveld refinement results show that excessive Li+ combines with the phosphate group to form Li3PO4 rather than occupying the Fe site to form antisite pairs. Benefitting from the intrinsic ion conductivity of lithium phosphate, the lithium diffusion coefficient of LixFePO4 increases. However, the charge transfer impedance (Rct) also increases with increasing Li content due to the presence of Li3PO4, a nonconductive phase that can block the electric transfer channel. The biphasic LixFePO4 (x = 1.02) compound shows the best performance. This work suggests an effective and easy way to improve the electrochemical performance by adding excessive Li, which leads to the formation of Li3PO4.

Nucleation and Growth Controlled Polyol Synthesis of Size-Focused Nanocrystalline LiFePO4 Cathode for High Performance Li-Ion Batteries

Mono-dispersed and densely packed, LiFePO4 cathodes with high rate performance versus lithium were synthesized using a two-step polyol process. The particle-size distribution and packing density of LiFePO4 nanoparticles were fine-tuned via separation of the nucleation and particle growth processes in the polyol medium by varying reaction time and temperature. Precisely, the LiFePO4 samples were synthesized by heating the reaction mixtures at 200 °C for various reaction times (0, 1, 3 and 6 h) to facilitate nucleation and thereafter the solution temperature was raised and maintained at 320 °C to facilitate the crystal growth process. XRD studies confirmed the formation of olivine phase in all the samples. The FE-SEM, FE-TEM and particle-size distribution studies illustrate that the particle-size of olivine nanocrystals may be tuned to approximately 200 nm by controlling the aging time at 200 °C or in other words separating the nucleation and crystal growth process. The olivine cathodes synthesized by maintaining 3-6 h aging time demonstrate improved discharge capacities and cycle performances in comparison to those of the remaining samples. In particular, the 6 h olivine cathode registered average discharge capacities of 130, 87 and 64 mAh/g at 5 C, 10 C and 15.7 C current rates respectively.

Synthesis and characterization of LiFePO4 cathode preparation by low temperature method

We review in detail the physics and technology of the novel material LiFePO4, a potential cathode material for Li-ion batteries. In the present work, nano crystalline LiFePO4 film has been synthesized in both powder and thin film forms from a non-aqueous sol–gel synthesis route based on oxalates of Li and Fe (II). Ferrous oxalate has been synthesized indigenously using a ferrous sulphate based chemical reaction and characterized. Nano powders and thin films of LiFePO4 have been fabricated and coated on stainless steel substrates with the aim of device development in future. The material has been characterized extensively by XRD for crystal structure, FESEM for microstructure, EDS for elemental analysis and FTIR for the internal modes of phosphate ion. Fe3+ impurity characterization has been done by using ESR.

Lithium potential variations for metastable materials: case study of nanocrystalline and amorphous LiFePO4.

Much attention has been paid to metastable materials in the lithium battery field, especially to nanocrystalline and amorphous materials. Nonetheless, fundamental issues such as lithium potential variations have not been pertinently addressed. Using LiFePO4 as a model system, we inspect such lithium potential variations for various lithium storage modes and evaluate them thermodynamically. The conclusions of this work are essential for an adequate understanding of the behavior of electrode materials and even helpful in the search for new energy materials.

The chemical, physical and electrochemical effects of carbon sources on the nano-scale LiFePO4 cathode surface

LiFePO4–C composite cathode active materials were synthesized by a sol–gel-assisted carbothermal reduction method using tannic (TA), tartaric (TRA) and stearic (SA) acids as both the carbon coating and reducing agent. The effect of different carbon sources on the structural, morphological and electrochemical properties of LiFePO4 are studied in this paper. The samples were characterized by optical-analytical methods and galvanostatic charge–discharge tests. The scanning electron microscopy (SEM) images revealed that the majority of the particles lay between 300 and 500nm for pure LiFePO4, while the carbon-coated LiFePO4 particles were from 50 to 300nm. Furthermore, we found that TA is suitable for producing a LiFePO4–C composite with a fine particle size and a uniformly-coated carbon conductive layer (3.6nm thickness) by high resolution transmission electron microscopy (HR-TEM). The cycling studies indicated a high and stable discharge capacity of 151mAhg−1 for the carbon-coated nanocrystalline LiFePO4 with TA at room temperature. The current rate capability studies between 0.2–10 C (1 C=170mAhg−1) demonstrated an excellent capacity retention efficiency of over 96.9% after 300 cycles. A schematic illustration was proposed based on the physical and chemical adhesion behaviors on the LiFePO4 surface of the three organic structures used as the carbon source.

Microscopic and spectroscopic properties of hydrothermally synthesized nano-crystalline LiFePO4 cathode material

LiFePO4 is one of the most promising cathode materials as it offers low cost, high abundance, appropriate theoretical capacity and environmental friendly. In the present investigation, nano-crystalline LiFePO4 has been synthesized by hydrothermal method and studied spectroscopic and microscopic properties. The XRD and TEM studies exhibited the formation of orthorhombic structure with Pnma space group. The X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy studies confirm the phase purity of the LiFePO4 powder. The electrochemical characteristics of LiFePO4 are measured in non-aqueous region. It exhibited a good reversible cyclic voltammogram on sweeping the potential upward and downward. The chronopotentiometric measurements performed at various C-rates and the cell exhibited a discharge capacity of 143mAh/g at 0.1C with good cycling stability.

Nucleation and Growth Controlled Polyol Synthesis of Size-Focused Nanocrystalline LiFePO4 Cathode for High Performance Li-Ion Batteries

Mono-dispersed and densely packed, LiFePO4 cathodes with high rate performance versus lithium were synthesized using a two-step polyol process. The particle-size distribution and packing density of LiFePO4 nanoparticles were fine-tuned via separation of the nucleation and particle growth processes in the polyol medium by varying reaction time and temperature. Precisely, the LiFePO4 samples were synthesized by heating the reaction mixtures at 200°C for various reaction times (0, 1, 3 and 6 h) to facilitate nucleation and thereafter the solution temperature was raised and maintained at 320°C to facilitate the crystal growth process. XRD studies confirmed the formation of olivine phase in all the samples. The FE-SEM, FE-TEM and particle-size distribution studies illustrate that the particle-size of olivine nanocrystals may be tuned to approximately 200 nm by controlling the aging time at 200°C or in other words separating the nucleation and crystal growth process. The olivine cathodes synthesized by maintaining 3–6 h aging time demonstrate improved discharge capacities and cycle performances in comparison to those of the remaining samples. In particular, the 6 h olivine cathode registered average discharge capacities of 130, 87 and 64 mAh/g at 5 C, 10 C and 15.7 C current rates respectively.

Defect chemistry of phospho-olivine nanoparticles synthesized by a microwave-assisted solvothermal process

Nanocrystalline LiFePO 4 powders synthesized by a microwave-assisted solvothermal (MW-ST) process have been structurally characterized with a combination of high resolution powder neutron diffraction, synchrotron X-ray diffraction, and aberration-corrected HAADF STEM imaging. A significant level of defects has been found in the samples prepared at 255 and 275 °C. These temperatures are significantly higher than what has previously been suggested to be the maximum temperature for defect formation in LiFePO 4 , so the presence of defects is likely related to the rapid MW-ST synthesis involving a short reaction time (∼5 min). A defect model has been tentatively proposed, though it has been shown that powder diffraction data alone cannot conclusively determine the precise defect distribution in LiFePO 4 samples. The model is consistent with other literature reports on nanopowders synthesized at low temperatures, in which the unit cell volume is significantly reduced relative to defect-free, micron-sized LiFePO 4 powders. Temperature-dependent antisite defect formation has been observed in nanocrystalline LiFePO 4 powders synthesized by a microwave solvothermal process, using high resolution diffraction and STEM imaging. • LiFePO 4 nanopowders synthesized by a microwave-assisted solvothermal process. • Defects directly observed by aberration-corrected HAADF STEM imaging. • Antisite defects present from synthesis at 255 and 275 °C. • Defects present from higher temperature synthesis than previously reported. • Powder diffraction data have been analyzed in detail for various defect models.

Synthesis and modification of nanocrystalline LiFePO4 as a cathode material for lithium-ion batteries

Poly-aniline (PANI) coated nano-powder FePO4/PANI was synthesized. LiOH·H2O, FePO4/PANI and polyvinyl alcohol (PVA) were used as the raw materials to synthesize the nanocrystalline LiFePO4 and then it was coated by carbon and doped by titanium. The properties of the samples are investigated using XRD, SEM, TEM and electrochemical methods. Electrochemical measurements were conducted in half cells with nanocrystalline LiFePO4, LiFePO4/C and LiFe0.96Ti0.02PO4/C as cathode materials and lithium metal as the anode. Results have shown that nano-structure helps the doping ion to enter into crystal lattice of LiFePO4 easily and distribute uniformly. LiFe0.96Ti0.02PO4/C has well coating structure with higher crystalline, smaller grain size, higher capacity and better rate performance than other samples.

Electrical and electrochemical properties of nanocrystalline LiFePO4 cathode

Lithium iron phosphate (LiFePO4) cathode material has been prepared by hydrothermal synthesis. The XRD spectrum exhibited different characteristic peaks along with (311) predominant orientation corresponding to orthorhombic crystal structure with Pnma space group. Electric and dielectric properties were studied over a frequency range of 1 Hz–1 MHz at different temperatures. The conductivity was found to be increased with increasing temperature following Arrhenius relation with an estimated activation energy of 0.44 eV. The dielectric properties were analyzed in the framework of complex dielectric permittivity and complex electric modulus formalisms. The complex permittivity as a function of frequency and temperature was investigated. Several important parameters, such as activation energy, ionic hopping frequency, carrier concentration, ionic mobility, and diffusion coefficient, etc., were determined. The electrochemical characteristics of LiFePO4 are examined in aqueous region. It exhibited a good reversible cyclic voltammogram on sweeping the potential upward and downward with discharge capacity of 140 mAh/g.

Long-term cycle stability at a high current for nanocrystalline LiFePO4 coated with a conductive polymer

Highly uniform hierarchical-microstructured LiFePO4 particles with dumbbell- and donut-shape and individual LiFePO4 nanocrystals were prepared by a hydrothermal method utilizing citric acid or a triblock copolymer (Pluronic P123) as a surfactant. The cathode composed of the individual nanocrystalline LiFePO4 particles exhibited higher specific capacity than the cathodes composed of the hierarchically assembled microparticles. Coating a conductive polymer, poly-3,4-ethylenedioxythiophene (PEDOT), on the surface of LiFePO4 particles improved the battery performances such as large specific capacities, high rate capability and an improved cycle stability. The nanocrystalline LiFePO4 particles coated with PEDOT (20 wt%) exhibited the highest discharge capacities of 175 and 136 mAh g−1 for the first battery cycle and 163 and 128 mAh g−1 after 500 battery cycles, with a degradation rate of 6–7%, at the rates of 1 and 10 C, respectively.

Facile preparation of nanoporous and nanocrystalline LiFePO4 with excellent electrochemical properties

A modified sol–gel process for the preparation of a nanoporous LiFePO4 (LFP) cathode with a well-ordered carbon coating has been developed. The interconnected nanocrystalline LFP was fabricated by addition of sucrose at certain time intervals, which wraps the pore wall. The atomic structure of the LFP was investigated by Rietveld refinement. The surface structure and pore properties have been investigated by SEM, TEM and BET. The structure of the coated residual carbon on the LFP was analyzed by Raman spectroscopy. Lithium ion diffusion and interface properties of the LFP electrodes were investigated by CV and impedance measurements. The LFP displays a good crystal structure, high reversible capacity, good cycle stability, and high energy efficiency. It also shows excellent high rate capability with a 10 C rate capacity of up to 118.5 mA h g−1 at the 100th cycle and a high tap density of 1.8 g cm−3 as a cathode material.

Delineating local electromigration for nanoscale probing of lithium ion intercalation and extraction by electrochemical strain microscopy

Lithium (Li) ion intercalation and extraction are critically important for high performance Li-ion batteries, and they are highly sensitive to local crystalline morphologies and defects that remain poorly understood. Using electrochemical strain microscopy (ESM) in combination with local transport analysis, we demonstrate that we cannot only probe Li-ion concentration and diffusivity with nanometer resolution but also map local energy dissipation associated with electromigration of Li-ions. Using these techniques, we uncover drastic differences in ESM response and energy dissipation between micro- and nano-crystalline lithium iron phosphate (LiFePO4) under different charging states, which explains the superior capacity observed in Li-ion batteries with nanocrystalline LiFePO4 electrode.

ChemInform Abstract: Facile Synthesis of the High‐Pressure Polymorph of Nanocrystalline LiFePO4 at Ambient Pressure and Low Temperature.

AbstractNanocrystalline high‐pressure LiFePO4 having the CrVO4 type structure (space group Cmcm) is synthesized at ambient pressure from a solution of FeCl2, LiCl, diphenylether, and tri‐n‐octylphosphine oxide in MeOH (100 °C, 1 h) mixed with a solution containing H3PO4, tri‐n‐hexylamine, and dihexylether (N2, 150 °C, 1 h).

Low-temperature Electrochemical Performance of LiFePO4/C Cathode with 3D Conducting Networks

The low-temperature electrochemical performance of LiFePO4/C cathode with 3D conducting networks is reported. XRD results show the sample is olivine-type structure. SEM and TEM images indicate that the particles consist of nanocrystalline LiFePO4 and are coated by nanocarbon webs and form conducting 3D networks. The 3D networks contained two effectively mixed conducting networks, one on the nanoscale and the other on the microscale. It is useful to shorten the distance of Li+ diffusion and e− transport and to improve the electrochemical performance. The LiFePO4/C cathodes show excellent capacity retention and cycling performance at −20 °C for lithium-ion batteries.