LiFePO4 Grains Research Articles

Probing the Interface Evolution in Co‐sintered All‐Phosphate Cathode‐Solid Electrolyte Composites

AbstractTo achieve cohesive interfaces of solid components in next‐generation Li‐ion batteries (LIBs) composed of oxide cathodes and solid electrolytes (SEs) high‐temperature sintering is necessary which leads to material degradation and phase decompositions. Thermochemically compatible components can address these issues. In this work, advanced electron microscopy techniques are employed to investigate the structural and chemical evolution taking place at the interfaces between a LiFePO4 (LFP) cathode and an in‐house optimized Li1.3Al0.3Ti1.7(PO4)3 (LATP) SE co‐sintered between 650 °C – 850 °C. While the LFP particles exhibit excellent structural stability, the LATP particles phase transforms at elevated temperatures. At 650 °C, FeLi anti‐site defect is observed at the LFP grain boundaries. Between 750 and 850 °C, the LATP (R‐3c) transforms into two orthorhombic phases (Pbca and Pbcn) of the type Li1.3+xAl0.3FexTi1.7‐x(PO4)3 depending on Fe substitution while forming the cohesive interfaces. The discharge capacity of the composite decreases from 148 mAh g−1 at 650 °C to 130 mAh g‐1 at 850 °C confirming that the new phases are still electrochemically active. This study provides a spatially resolved insight into the formation of cohesive interfaces highlighting the advantage of employing components with similar anionic structures for further improvements in LIBs.

Electrochemical Properties of LiFePO4 Cathodes: The Effect of Carbon Additives

The influence of different conductive additives (carbon nanofibers (CNFs), carbon nanoplatelets, and pyrolytic carbon from sucrose (Sucr) or polyvinylidene fluoride) on the morphology, electron conductivity, and electrochemical performance of LiFePO4-based cathodes was investigated to develop the most efficient strategy for the fabrication of high-rate cathodes. Pyrolytic carbon effectively prevents the growth of LiFePO4 grains and provides contact between them, CNFs provide fast long-range conductive pathways, while carbon nanoplatelets can be embedded in carbon coatings as high-conductive “points” which enhance the rate capability and decrease the capacity fading of LFP. The LiFePO4/CSucr/CNF showed better performance than the other cathodes due to the synergy of the high-conductive CNF network (the electronic conductivity was 1.3 × 10−2 S/cm) and the shorter Li+ ion path (the lithium-ion diffusion coefficient was 2.1 × 10−11 cm2/s). It is shown that the formation of composites based on LFP and carbon nanomaterials via mortar grinding is a more promising strategy for electrode material manufacturing than ball milling.

Porous microspheres consisting of carbon-modified LiFePO4 grains prepared by a spray-drying assisted approach using cellulose as carbon source

Porous microspheres composing of carbon-modified LiFePO4 (C/LFP) nanoparticles have been prepared successfully by a spray-drying process combined with a calcination step using cellulose as carbon source. LiOH plays duple roles as agency for dissolving cellulose with aqueous solution and source of lithium for LiFePO4. The synthesized C/LFP composite microspheres are rich in porosity, and all the C/LFP microspheres consist of numerous LiFePO4 grains with a diameter of about 200 nm. In addition, the individual LiFePO4 grains are encapsulated by the carbon matrix derived from cellulose. The carbon cladding enhances the electronic conductivity of LiFePO4, while the porous structure shortens the Li+ diffusion distance and facilitates the penetration of electrolyte when the C/LFP composites are used as cathode material for the battery. Hence, the unique composites delivered an initial specific discharge capacity of 165 mAh g−1 and showed good rate performance.

Active Layer Thickness Effect on the Behavior of Electrodes Based on Lithium Iron Phosphate

The effect of the active layer thickness (the amount of active material per unit area of the electrode) on the behavior of electrodes based on lithium iron phosphate was first studied by methods of galvanostatic cycling and cyclic voltammetry. When considering the electrode as a system with doubly distributed parameters (distribution of material composition along the individual LiFePO4 grain radius and distribution of the process along the depth of the active layer), it was concluded that the distribution of the process over the depth of the active layer is much more pronounced than in the bulk of individual grains of lithium iron phosphate. It is assumed that such conclusion will be valid for electrodes from other materials.

Rational design of mesoporous LiFePO4@C nanofibers as cathode materials for energy storage

Abstract In this work, the mesoporous LiFePO4@C nanofibers have been successfully fabricated through a facile electrospinning method. The structure, morphology, chemical composition and lithium storage performance have been systematically investigated. The results reveal that the LiFePO4 grains with particle size of ～15 nm are uniformly dispersed in the mesoporous carbon nanofibers. The LiFePO4@C electrode presents a high reversible capacity and excellent rate performance. It delivers a discharge capacity of 107 mAh g−1 and retains 105 mAh g−1 over 200 cycles at 10C. The excellent electrochemical performances are attributed to the novel nanostructure where LiFePO4 nanoparticles are embedded in the carbon fibers. This designed structure can significantly enhance the conductivity of LiFePO4@C, accelerate the diffusion of electrolyte, and thus facilitate the transport of electrons and Li-ions.

Double role of silicon in improving the rate performance of LiFePO4 cathode materials

LiFeP1-xSixO4/(C + yLi2SiO3), (0 ≤ x ≤ 0.04, 0 ≤ y ≤ 2%) composites with a particle size of about 200 nm were synthesized via a simple solid state route. In this work, silicon plays double roles-silicon doping in lattice to enhance the electronic conductance of LiFePO4, meanwhile Li2SiO3 with high Li-ion conductivity and carbon hybrid coating can decrease the interface resistance between LiFePO4 grain and electrolyte. In this work, starch and TEOS were used as carbon source and the silicon source respectively. XRD, SEM, TEM, EIS and galvanostatic charge-discharge test are used to characterize these samples. The effects of the modification on the microstructure and electrochemical performances of LiFePO4 cathodes at 25 °C are systematically investigated. The results show the sample with x = 0.03 and y = 1.0 wt % exhibits excellent rate capacity and cycle performance among the investigated materials. Based on half-cell testing, the best sample achieves a high special discharge capacity of 119 mAh g−1 at 5C rate and a capacity of 94 mAh g−1 at the 10C rate with an excellent cycle life in the usual cycling voltage range 2.5–4.2 V. EIS result demonstrates that the Rct of the samples decrease greatly due to the modification of LiFePO4 via doping and coating.

Operando View on the Charge Transport through LiFePO4 Electrodes and the Phase Transformation in Individual LiFePO4 Grains Using Neutron Depth Profiling and Microbeam Synchrotron Diffraction

Here we present our recent findings using two novel in-situ techniques1,2. (1) The distribution of Li-ions in LiFePO4 electrodes under operando conditions reveals what charge transport mechanism is rate limiting under what conditions including the impact of particle size and carbon additives. (2) In operando micro beam diffraction is performed from slow to high cycling rates disclosing phase transformation mechanism in individual LiFePO4 for the first time. Neutron Depth Profiling (NDP) offers the possibility to see directly see lithium ions, or atoms, via the capture reaction of neutrons with the 6Li isotope to form an α-particle (4He) and a trition (3H) carrying the energy that is generated by the reaction according to conservation of energy and momentum. The 4He and 3H particle travel through the surrounding material during which they lose energy. By measuring the particle energy when it reaches the detector, the depth at which the 6Li atom was located can be determined. This allows to reconstruct a Li-atom density profile as a function of depth without significantly influencing the Li concentration due to the low neutron flux making this a non-destructive technique. Advancing the application of NDP from micro batteries3to conventional batteries we show the impact of particle size and charge rate on how the activity in the electrodes is distributed. This gives insight in what transport process dominates the internal resistance under what conditions. Zooming in on the phase transition reaction within single LiFePO4 grains has been achieved by applying a bright and sub-micron sized synchrotron beam. Under these conditions diffraction rings observed with powder diffraction fall apart in individual spots each representing individual LiFePO4 grains in the positive electrode. In this way the phase transition behavior through time of hundreds of individual 140 nm LiFePO4 grains is monitored at the same time while varying the electrochemical conditions ranging from slow (C/5) up to fast rates (2C). Following the phase transformation kinetics of individual grains reveals much slower transformation rates than expected based on the generally accepted mosaic or “Domino Cascade” transformation. In addition we show that the transformation rate of individual grains actually depends on the cycling rate. The appearance of many streaked reflections observed at low rates discloses that the majority of the grains is actively transforming via nanometer thin plate shaped domains that nucleate in specific crystallographic orientations. Also this observations defies the long believed mosaic or “domino cascade” transformation model. As the (dis)charge rate increases the number of these plate shaped domains decreases and their width increases, driving the local compositions of the coexisting phases towards each other. The observation of a diffuse interface in a single grain at high (dis)charge rates reveals the growth mechanism at high rates, consistent with recent operando powder diffraction probing the average crystalline state over all grains4,5. Thereby, a consistent mechanistic picture is revealed for the phase transformation in individual LiFePO4 grains under realistic operando conditions for the first time. Counter intuition, the existence of well-defined interfaces at low discharge rates, may cause a shorter cycle life as compared to the more diffuse interfaces at higher rates. The electrochemically driven higher transformation rates indicate phase transformation kinetics play a more important role at low rates as compared to higher rates where the diffuse interfaces appear to be responsible for the faster transformation. Consistent with the electrolyte transport limited reactions, a smaller faction of the electrode material appears actively transforming, in accordance to the NDP results, indicating this has to be the focus for high rate electrode improvement. (1) Zhang, X.; Verhallen, T. W.; Labohm, F.; Wagemaker, M. Advanced Energy Materials 2015, 15, 1500498. (2) Zhang, X.; van Hulzen, M.; Singh, D. P.; Brownrigg, A.; Wright, J. P.; van Dijk, N. H.; Wagemaker, M. Nat Commun 2015, 6. (3) Oudenhoven, J. F. M.; Labohm, F.; Mulder, M.; Niessen, R. A. H.; Mulder, F. M.; Notten, P. H. L. Advanced Materials 2012, 23, 4103. (4) Zhang, X.; van Hulzen, M.; Singh, D. P.; Brownrigg, A.; Wright, J. P.; van Dijk, N. H.; Wagemaker, M. Nano Letters 2014, 14, 2279. (5) Liu, H.; Strobridge, F. C.; Borkiewicz, O. J.; Wiaderek, K. M.; Chapman, K. W.; Chupas, P. J.; Grey, C. P. Science 2014, 344. Figure 1

In situ Electrochemical-AFM Study of LiFePO4 Thin Film in Aqueous Electrolyte.

Lithium-ion (Li-ion) batteries have been widely used in various kinds of electronic devices in our daily life. The use of aqueous electrolyte in Li-ion battery would be an alternative way to develop low cost and environmentally friendly batteries. In this paper, the lithium iron phosphate (LiFePO4) thin film cathode for the aqueous rechargeable Li-ion battery is prepared by radio frequency magnetron sputtering deposition method. The XRD, SEM, and AFM results show that the film is composed of LiFePO4 grains with olivine structure and the average size of 100 nm. Charge-discharge measurements at current density of 10 μAh cm−2 between 0 and 1 V show that the LiFePO4 thin film electrode is able to deliver an initial discharge capacity of 113 mAh g−1. Specially, the morphological changes of the LiFePO4 film electrode during charge and discharge processes were investigated in aqueous environment by in situ EC-AFM, which is combined AFM with chronopotentiometry method. The changes in grain area are measured, and the results show that the size of the grains decreases and increases during the charge and discharge, respectively; the relevant mechanism is discussed.

Low - voltage FIB/SEM Tomography for 3D Microstructure Evolution of LiFePO4/C Electrode

Li-ion batteries find widespread use in many electricity storage applications, from portable devices to electric vehicles [1-3], and LiFePO4 (LFP) is one of the most common cathodes because of its long durability and high safety [4, 5] but, since its low ionic and electronic conduction, it is always mixed with carbon black (CB) additives to increase electronic percolation in the electrode. Focused Ion Beam (FIB)/Scanning Electron Microscopy (SEM) Tomography is one of the most used techniques for the study of the three-dimensional microstructure of porous electrodes [6-8]. Imaging at low-kV has been shown to be an excellent technique for studying electron percolation in Ni-network in solid oxide fuel cells [9]. In this work we study the degradation process that occurs in a LFP/C electrode by Low-kV FIB/SEM Tomography, using the low-voltage percolation technique to identify compositional changes in the CB network in three-dimensions. FIB/SEM images of a fresh and degraded cathode are compared and LFP grains are seen to crack with cycling, resulting in the formation of secondary disconnected particles with increased ionic resistivity; CB particles are instead observed to agglomerate, reducing the electrochemically active surface area. Using low voltage imaging (1 kV) a significant fraction of the large carbon agglomerates found in the aged electrode show a higher secondary electron yield compared to the fresh CB particles at low accelerating voltage. This suggests that degradation occurs both due to morphological changes and due to amorphous-crystalline phase transitions in the carbon network, resulting in non-percolating CB agglomerates. Figure 1 shows lateral Everhart – Thornley (E-T) and in-lens detector images of fresh (a, b) and degraded cathode (c, d) collected after FIB slicing. White grains are LFP, black particles are CB additive and grey regions are pores infiltrated with silicon resin to improve phase contrast [7]. The CB network appears entirely dark in the in-lens detector image of the fresh electrode (Fig. 1b). It is possible to notice some charging effects from the insulating silicon resin. The in-lens detector image of the degraded electrode (Fig. 1d) is instead characterized by the presence of big carbon agglomerates (red rings) which are brighter because they charge as the electron beam hit them. This indicates a lower electric conductivity. 3D reconstruction of the entire network (Fig. 2) revealed that the carbon phase with lower electric conductivity accounted for approximately 25% of the volume of the total carbon in the sample. This of course affects electrode capacity since a reduced electron percolation in the CB network impedes (de)lithiation process of LFP particles. References M. Armand, J. M. Tarascon, Nature, 451, 652-657 (2008).B. Scrosati, J. Hassoun, and Y.K. Sun, Energy Environ. Sci., 4, 3287-3295 (2011).J.M. Tarascon, M. Armand, Nature, 414, 359-367 (2001).A.K. Padhi, K.S. Nanjundaswamy, and J. B. Goodenough, J. Electrochem. Soc., 144(4), 1188-1194 (1997).Y. Wang, P. He, and H. Zhou, Energy Environ. Sci., 4, 805-817 (2010).T. Hutzenlaub et al., Electrochemical and Solid-State Letters, 15 (3), A33-A36 (2012).M. Ender et al, J. Electrochem. Soc., 159(7), A972-A980 (2012).Z. Liu et al., J. Power Sources, 227, 267-274 (2013).K. Thydén, Y.L. Liu, and J.B. Bilde-Sørensen, Solid State Ionics, 178, 1984-1989 (2008). Figure 1

Engineered Electronic Contacts for Composite Electrodes in Li Batteries Using Thiophene-Based Molecular Junctions

Fourier transform infrared spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy experiments indicate that molecular junctions can be achieved between non-carbon-coated LiFePO4 (LFP) and multiwall carbon nanotubes (MWCNT) using a thiophene-based conjugated system which was designed to selectively functionalize these two different types of surfaces. The strategy enables the architecturing of the cathode electrode of lithium batteries, leading to a vast improvement in the component intermixing, which results in the individual MWCNT being nanocontacted at the surface of LFP grains. This advancement leads to much higher specific capacity, especially at high charge/discharge rates, for undensified electrodes of 2 mA h cm–2, for which the electronic wiring of the electroactive material is a critical issue. Furthermore, thanks to molecular junctions, better capacity retention comparable to that of carbon-coated LiFePO4 electrodes could be achieved. These results are exp...

Degradation Studies on LiFePO4 cathode

Lithium-ion batteries are a promising technology for automotive application, but limited performance and lifetime is still a big issue. The aim of this work is to study and address degradation processes which affect LiFePO4 (LFP) cathodes - one of the most common cathodes in commercial Li-ion batteries.In order to evaluate how the LFP cathode is affected by C-rate a LFP working electrode, Lithium metal foil counter electrode and Lithium metal reference electrode was tested in a 3-electrode setup with a standard 1M LiPF6 in 1:1 EC/DMC electrolyte and glass fiber separator. The working electrode/counter electrode was subjected to several charge/discharge cycles between 3.0 V and 4.0 V at different discharge rates. Figure 1 shows the voltage profile of the LFP electrode (solid line) and full battery (dotted line) during charge/discharge process. It is seen that the higher the C-rate, the higher is the polarization furnished by the counter electrode which reduces the capacity.In Figure 2, the discharge capacity [mAh/g] is plotted vs the number of charge/discharge cycles. Series of 10 cycles at a given C-rate was applied to the battery. Each series was followed by a C/10 cycle (green points). A linear fit has been applied to the first series (omitting first two cycles where instability of the system is observed), in order to calculate the degradation rates.High C-rates are seen to affect the discharge capacity, but the capacity is almost completely recovered (green points) and only a limited degradation occurs. Impedance spectroscopy has been also applied to investigate the LFP cathode degradation. Figure 3 shows the imaginary part of the impedance measured at 50% State-of-Charge after each series of cycles. The relative increase in the impedance arc around 1 KHz (assumed to be associated with charge transfer resistance at the LFP particle surfaces) is seen to gradually decrease with increasing number of series. This indicates that more cycles per series is needed to establish a convincing relation between C-rate and degradation.The degradation studies will be coupled with FIB/SEM analysis in order to observe changes in the pore structure or micro cracks that would affect electronic percolation. Figure 4 displays an example of a fresh LFP cathode after FIB cutting. White particles are LFP grains while the black area contains carbon particles and pores, which are difficult to distinguish from each other. Substitution of the epoxy resin with a silicon resin increases the contrast between pores and carbon particles [1] and this will be used in the forthcoming FIB/SEM analysis.

Fabrication and characteristics of nano LiFePO4/C composites with high capacity and high rate using nano Fe2O3 as raw materials

Nano LiFePO4/C particles with high capacity and high rate were fabricated by one-step solid-state reaction using nano Fe2O3, LiH2PO4 and sucrose as raw materials. The effect of synthesis temperature on the morphology and the electrochemical performance was investigated. LiFePO4/C materials were characterized by differential scanning calorimetry (DSC) and thermogravimetry (TG), X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and the elemental analyzer. The results show that nano Fe2O3 can reduce and make uniform the LiFePO4 grain size (<100nm) effectively as well as accelerate the solid-state reaction kinetics which is a convenient way for industrialization. The precursor sintered at 740°C for 5h exhibits the highest electrochemical capacity (166.9mAh/g at 0.1C) and the best rate performance.

Synergistic Effect in Carbon Coated LiFePO4for High Yield Spontaneous Grafting of Diazonium Salt. Structural Examination at the Grain Agglomerate Scale

Molecular grafting of p-nitrobenzene diazonium salt at the surface of (Li)FePO4-based materials was thoroughly investigated. The grafting yields obtained by FTIR, XPS, and elemental analysis for core shell LiFePO4-C are found to be much higher than the sum of those associated with either the LiFePO4 core or the carbon shell alone, thereby revealing a synergistic effect. Electrochemical, XRD, and EELS experiments demonstrate that this effect stems from the strong participation of the LiFePO4 core that delivers large amounts of electrons to the carbon substrate at a constant energy, above the Fermi level of the diazonium salt. Correspondingly large multilayer anisotropic structures that are associated with outstanding grafting yields could be observed from TEM experiments. Results therefore constitute strong evidence of a grafting mechanism where homolytic cleavage of the N2(+) species occurs together with the formation and grafting of radical nitro-aryl intermediates. Although the oxidation and concomitant Li deintercalation of LiFePO4 grains constitute the main driving force of the functionalization reaction, EFTEM EELS mapping shows a striking lack of spatial correlation between grafted grains and oxidized ones.

ZnO incorporated LiFePO4 for high rate electrochemical performance in lithium ion rechargeable batteries

In order to improve the electrical conductivity of LiFePO4 (LFP) cathode, ZnO/Carbon was incorporated in the cathode by wet vibratory ball milling ZnO, Polyethylene glycol (PEG) and LFP particles together. Herein, polyethylene glycol (PEG) was used as both dispersant during ball milling and carbon source after calcination process. The uniformly dispersed carbon and ZnO on the surface of LFP led to a good electronic contact between the LFP grains. The charge/discharge rate in the range of C/10 to 10C and cycle performance at C/10 for 50 cycles was tested at room temperature. The LFP/ZnO/Carbon composite cathode showed a high capacity of 158.9mAhg−1, displayed excellent high rate and cyclic performance due to high amounts of graphitic carbon transformed by ZnO used as a catalyst during calcination process. The decrease in capacity was within 31% at a discharge rate of 10C and less than 3% after 50 cycles at C/10 rate. The LFP/ZnO/Carbon composite was characterized using X-ray diffraction (XRD), Scanning electron microscopy (SEM), high resolution Transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), Micro-Raman, and specific surface area. Electrochemical properties were measured using electrochemical impedance spectroscopy (EIS), potentiostatic intermittent titration technique (PITT) and galvanostatic measurements.

High tap-density and high performance LiFePO4/C cathode material synthesized by the combined sol spray-drying and liquid nitrogen quenching method

The cathode material LiFePO4/C is synthesized by a combined sol spray-drying and liquid nitrogen quenching method. Combining the advantages of sol–gel, spray-drying, and quenching, the well crystallized LiFePO4/C sample possesses both high tap-density and excellent electrochemical performance. The sphere-like particles are of the size 0.1–6μm, and the tap-density is as high as 1.53gcm−3. TEM results show that there are a number of defects in LiFePO4 crystals, and a uniform carbon layer (∼2nm) exists on the LiFePO4 grain surface. Electrochemical tests reveal that quenching can greatly improve the rate capability and cycle performance of LiFePO4/C. The quenched sample shows the initial discharge capacities of 162.5, 154.7 and 117.8mAhg−1 at 0.1C, 1C and 10C rate, respectively, and after the cell cycled 110 times at different rates, the capacity can recover to 99.4% of its original value when the rate reverses back to 0.1C.

Kinetics of lithium deintercalation from LiFePO4

We have studied the kinetics of lithium deintercalation from lithium iron phosphate in a cathode material for batteries. The main contribution to the resistance of the cell is made by interfaces and the resistance of LiFePO4 grains. The FePO4 solubility in LiFePO4 is 4.0%. The lithium deintercalation process can be described in terms of a heterogeneous grain model and its rate is controlled by the lithium diffusion across the layer of the forming product (FePO4).

Stability of LiFePO4 in water and consequence on the Li battery behaviour

The stability of LiFePO4 in water was investigated. Changes upon exposure to water can have several important implications for storage conditions of LiFePO4, aqueous processing of LiFePO4-based composite electrodes, and eventually for utilisation in aqueous lithium batteries. A Li3PO4 layer of a few nanometers thick was characterised at the LiFePO4 grains surface after immersion in water, accompanied by an increase of FeIII percentage in the grains. For first charge–discharge cycles in a lithium battery, no effect was observed on electrochemical performances for a sample of LiFePO4 immersed for 24 h at a concentration of 50 g L−1 without any pH modification. To limit the aging of LiFePO4 during aqueous electrode processing, it is advised to reduce the immersion duration, to concentrate the LiFePO4 suspensions, and not to modify the pH. In addition, since immersion in water mimics an accelerated exposure to air humidity, LiFePO4 should be stored in a dry atmosphere.

Characterization of the carbon coating onto LiFePO4 particles used in lithium batteries

While nanosized ferromagnetic particles could poison the performance of the Li batteries containing phospho-olivine, the carbon-film coating the LiFePO4 particles has a beneficial effect on cycling life of the cells. In this paper, we present the properties of the carbon layer deposited at the surface of the LiFePO4 grains. Characteristics of the carbon layer are analyzed using scanning electron microscopy, high-resolution transmission scanning electron microscopy, Fourier transform infrared, and Raman scattering (RS) spectroscopy. The carbon deposit characterized by RS spectroscopy is hydrogenated with very small hydrogen/carbon ratio, so that it belongs to the family of the amorphous graphitic carbon. The carbon deposit is similar to that obtained by pyrolysis technique at high temperature. It is expected to have the same properties (small hardness, high electronic conductivity) that favor both the Li diffusion from the LiFePO4 bulk and the charge-discharge rate of the cell. A model for the Li-ion trans...