LiFePO4 Crystals Research Articles

Low‐Resistance LiFePO4 Thick Film Electrode Processed with Dry Electrode Technology for High‐Energy‐Density Lithium‐Ion Batteries

LiFePO4 emerges as a viable alternative to cobalt‐containing cathodes, such as Li[Ni1–x–yMnxCoy]O2 and Li[Ni1−x−yCoxAly]O2. As Fe is abundant in nature, LiFePO4 is a low‐cost material. Moreover, stable structure of LiFePO4 imparts long service life and thermal stability. However, the practical implementation of LiFePO4 cathode in energy storage devices is impeded by its low energy density and high ionic/electrical resistance. Herein, the LiFePO4 electrode with high active material loading and low ionic/electrical resistance through the dry process is reported for the first time. The dry process not only enables the uniform distribution of the polymeric binders and conductive additives within the thick electrode but also inhibits the formation of cracks. Furthermore, the bridge‐like connection of polytetrafluoroethylene facilitates the insertion and extraction of Li ions to the LiFePO4 crystal. Hence, the dry‐processed LiFePO4 electrode with high areal capacity (7.8 mAh cm−2) exhibits excellent cycle stability over 300 cycles in full‐cell operation. In addition, it is demonstrated that the estimated energy density of prismatic cell with the dry‐processed LiFePO4 electrode is competitive with state‐of‐the‐art Li[Ni1–x–yMnxCoy]O2‐based battery.

Atomistic Observation and Transient Reordering of Antisited Li/Fe Defects toward Sustainable LiFePO4

The Li/Fe disordering confined within the single unit cells of LiFePO4 (LFP) crystals has been notoriously plaguing their reversible Li+ storage, capacity, and thus service life, posing a grand challenge...

Theoretical investigation on niobium doped LiFePO4 cathode material for high performance lithium-ion batteries

In this work, the first principle study has been conducted to investigate the properties of LiFePO4 (LFP) cathode material for the Li-ion batteries, and the effects of doping on their crystalline structures with Nb atoms was explored. The obtained results showed that doping of the LFP structure with Nb at the Fe site significantly improved the conductivity of LFP and decreased the band gap from 3.502 to 0.658 eV, which is consistent with the acquired density of state data. Moreover, energy formation results proved that the stability of the system after doping was ideal. The geometrical analysis of the structure showed that the Nb doping leads to widening of the Li passage channel, increased bond length, and reduction in covalent strength among atoms which in turn mitigated energy barrier. Furthermore, our calculation proved that Nb doping has no adverse effect on the theoretical voltage of LFP and, its value remained constant at about 3.1 V. Then, the nudged elastic band method was examined for the different pathways of Li ion diffusion in the LFP crystal. The finding represented that energy barriers of Li migration along all paths were decreased through Nb doping. In addition, the diffusion coefficient of Li-ion in the doped systems was improved by four orders of magnitude compared to undoped LFP. Moreover, the effective mass of electron theory and theoretical voltage of pristine LFP and Nb-doped LFP were evaluated. Finally, the electron localization function theory was implemented, for in-depth study and illustration of the difficulty of Li-ion diffusion and bond characteristics.

Annealing Treatment: A Facile Approach to Enhance Transfer Kinetics for a LiFePO 4 /C Cathode

Nowadays, solid phase synthesis has been proven to be the most cost-effective and convenient approach to mass preparation of a LiFePO4/C cathode. Nevertheless, the LiFePO4/C cathode usually shows unsatisfactory electrochemical performance due to its high proportion of antisite defects and inefficient carbon coating. In this work, a post-synthesis annealing treatment method is used to improve the ionic and electron transfer kinetics of high-temperature solid phase synthesized LiFePO4/C. Owing to the annealing treatment, the proportion of Fe-Li antisite defects in the LiFePO4 crystal can be significantly reduced. The graphitization degree of carbon coating on LiFePO4 has been increased. This contributes to the enhancement of rapid transfer kinetics and suppressed polarization of the electrode.

Evaluation of Olivine LiFePO<sub>4</sub> Polyanionic Cathode Material Using Density Functional Theory

The first principles study on the LiFePO4 and FePO4 crystal has been evaluated using the density functional theory encrypted in the Cambridge Serial Total Energy Package (CASTEP) computer code. The structural properties, electronic properties, and intercalation voltage of the cathode material are presented. Without the Hubbard U parameter, the conventional functional of GGA-PBE and GGA-PBEsol unable to produce the experimental open-circuit voltage (OCV) and band gap (BG) of cathode material correctly. Generally, the value of the lattice parameter, OCV, and BG will increase as the U value is increased. For OCV, the suitable U value for the GGA-PBE and GGA-PBEsol is 3 eV, whereas, for BG, the appropriate U value for both functional is around 4.3 eV to 4.5 eV. Different Hubbard U value is needed for different functional. It is found that GGA-PBEsol + U is the best parameter to calculate the electrical properties of LiFePO4 ­­­­material.

First-Principles Study of Effects of Combined Ti Supervalent Cations and Lithium Ion Vacancies Doping on Crystal and Electronic Structures and Conductivity in LiFePO<sub>4</sub>

Olivine-type LiFePO4 is widely considered as a cathode for lithium-ion batteries owing to its environmental friendliness and low-cost, yet its applicability in the pristine state is limited due to poor electronic and ionic conductivity. To investigate the conductivity enhancement of LiFePO4, first-principles method under the GGA+U framework is implemented to study effects of doping with Ti4+ at Fe2+ sites under the lithium-deficient environment. LiFePO4 crystal and electronic structures as well as conductivity are investigated. Ti doping creates the impurity states at the acceptor level, which are normally degenerate states, but split into multiple states by the crystal field splitting. Doping under the lithium-deficient environment induces small hole polarons localizing at the Fe atoms and creates defect states located in the intermediate band. Both phenomena combine to facilitate charge carrier hopping. The climbing-image nudge elastic band (cNEB) calculation shows that Li hopping can be promoted by doping with high Ti concentration. This co-doping mechanism therefore can enhance both the electronic and ionic conductivities, which can be beneficial benchmark for cathode-material synthesis in the future.

A green process for phosphorus recovery from spent LiFePO4 batteries by transformation of delithiated LiFePO4 crystal into NaFeS2

Recovery of high-content and valuable elements including phosphorus (P) is critical for recycling of spent LiFePO4 battery, but P recovery is challengeable due to the poor solubility of lithium phosphate and iron phosphate. This study compared two strategies to recover P by adopting sulfide salt to induce P dissolution, i.e., recovery of P directly from LiFePO4, and step-by-step recovery of Li then P. The results revealed that the second strategy was more efficient because of the higher recovering efficiency and selectivity. Accordingly, an acid-free process to recover P was successfully demonstrated. Li-recovery efficiency of 97.5 % was reached at a leaching time of 65 min, and nearly 100 % P-recovery efficiency was reached at 5 h. Mechanism analysis revealed that the transforming of delithiated LiFePO4 crystal to NaFeS2 was mainly responsible for P dissolution. Thermodynamic analysis and density functional theory calculation further proved the transformation reaction, and a stepwise-transformation mechanism was proposed. In addition, P was reclaimed in the form of soluble phosphate salts. The process is especially appealing due to its environmental and economic benefits for recycling spent LiFePO4 batteries.

In situ constructed (010)-oriented LiFePO4 nanocrystals/carbon nanofiber hybrid network: Facile synthesis of free-standing cathodes for lithium-ion batteries

A free-standing LiFePO4/carbon nanofiber hybrid (LFP/CF) electrode with (010)-oriented LiFePO4 nanocrystals was in-situ constructed by a one-pot electrospinning method. It is found that poly (vinylpyrrolidone) (PVP) can regulate the growth of (010)-face-orientated LiFePO4 crystals partly. As a model material, the optimal (010)-manifest LiFePO4/carbon nanofibers cathode exhibits a discharge capacity of 152 mA h g−1 at 0.5C after 500 cycles with capacity retention of 98.2% and superior rate performance of 108.6 mA h g−1 at 5C. The excellent performance can be ascribed to the well-designed 3D conductive network, fast Li-ion diffusion path, and porous structure for electrolyte penetration. This route provides an efficiency strategy with low cost and one-step process for the preparation of free-standing (010)-manifest LiFePO4/carbon nanofiber cathode material, and can also be extended to other olivine materials.

Selectively peeling of spent LiFePO4 cathode by destruction of crystal structure and binder matrix for efficient recycling of spent battery materials

Impurity Fe could severely damage the performance of resynthesized cathode material, and therefore, LiFePO4 cathode should be removed from the mixed spent LIBs for materials recycling. In this research, a non-hydrometallurgy method has been developed to separate LiFePO4 by selectively peeling-off the LiFePO4 cathode material and the peeling-off process was well explained by theoretical modeling. The peeling-off efficiency of LiFePO4 was approximate 100 % and that of LiMn2O4/LiCoO2/Li(Ni, Co, Al)O2/Li(Ni, Mn, Co)O2 was only 0.08 %. That is, the separating selectivity was 1250. Mechanism study revealed that the peeling-off was achieved through selective destruction of the LiFePO4 crystal and the matrix of polyvinylidene fluoride (PVDF) binder. Particularly, the crystal structure of LiFePO4 was firstly destructed by sulfide, thus LiFePO4 particles were detached from the matrix of PVDF binder. Then, the PVDF binder without LiFePO4 particles filling were more susceptible to be brittlely peeled off by the micro-explosion force of hydrogen from the reaction of Al foil with water due to the weakened mechanical strength. The process is suitable for recycling varied types of spent LIBs, having a strong potential for industrial application.

Controllable synthesis and morphology evolution of hierarchical LiFePO4 cathode materials for Li-ion batteries

Hierarchical LiFePO4 crystals self-assembled with subunits were successfully synthesized via a simple hydrothermal method. The X-ray diffraction (XRD) curve indicates the highly crystalline nature of the prepared samples. The field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images reveal that with increased KOH concentrations, the LiFePO4 products evoluted from microspheres self-assembled with microwires to spindle-like structures self-assembled with micro-needles and eventually to flower-like hierarchical nanostructures self-assembled with nanorods. The crystal growth mechanism could be explained by the synergetic effect of pH value and the surface-adsorbed K+ ions. The OH– anions represent pH value dominate the hierarchical morphology under both low and high KOH concentration, while K+ ions play the major role in synthesizing monodispersed LiFePO4 nanorods under medium KOH concentration. Moreover, charge–discharge curves and CV measurements demonstrated good reversible capacities and stable cycle performances of the LFP/C samples.

Stress-Controlled Three-Dimensional Phase Evolution in Lithium Iron Phosphate

Many battery compounds undergo first-order phase transformations induced by ion (de)intercalation during battery operation, and their rate capability is often controlled by the kinetics of two-phase boundary movement. LiFePO4, a commercial cathode materials for Li-ion batteries, is a model system for studying the unique aspects of phase transitions in battery intercalation compounds. In a recent in-situ scanning transmission X-ray microscopy (STXM) study [Ohmer et al. Nature Communication 6, 6045 (2015)], Li deintercalation and phase evolution in microsized LiFePO4 single crystal upon delithiation are found to be highly non-uniform. FePO4 phase was observed to form [010]-aligned filaments growing into the LiFePO4 crystal, which is postulated to be dominated by elastic effects. In this work, we performed three-dimensional phase-field simulation to examine in the details phase evolution in LiFePO4 and the effect of misfit strain energy arising from the lattice match between FePO4 and LiFePO4. Our simulation reproduces the filamentary growth morphology of FePO4 phase (Figure 1) as observed in experiment, and confirms the dominating role of misfit stress in causing such behavior. We show that phase evolution in LiFePO4 goes through two steps during delithiation. In the first step, phase separation occurs in LiFePO4 via surface-mode coherent spinodal decomposition and results in the formation of individual FePO4 domains on particle surface. Next, FePO4 grows into the LiFePO4 matrix as wedge shaped domains that coarsen at the same time. The filamentary morphology of FP phase within (001) plane is mainly a result of the anisotropic misfit strain between LiFePO4 and FePO4, which favors the elongation of FP domains along the [010] axis to reduce elastic strain energy. Simulation reveals that Li diffusion anisotropy also significantly influences the phase growth morphology. The existence of three-dimensional Li diffusion in LiFePO4, which results from the presence of antisite defects, is found to be essential for the development of filamentary FePO4. The excellent agreement between the phase-field simulation and in-situ STXM experiment suggests that misfit stress plays an important role in triggering non-uniform Li (de)intercalation especially in large electrode particles, which is detrimental to battery performance and life and deserves close attention. Figure 1

The Effect of Reduced Graphene Oxide (rGO) Coating on Electrical Conductivity of Lithium Ferro Phosphate (LFP) as an Alternative Cathode for Li-Ion Battery

In this study an investigation has been conducted on the effect of reduced graphene oxide (rGO) coating on increasing the value of Lithium Ferro Phosphate (LFP) electrical conductivity. This coating process uses a variation of the mass ratio of LiFePO4/rGO by 90%:10%, 70%:20%, and 67%:33%. The LiFePO4 precursor was prepared using the sol-gel rute from the main commercial materials, namely Li2CO3 powder as a source of lithium ions, FeCl2.4H2O as a source of iron and NH4H2PO4 powder as a phosphate source. As for the coating used rGO extracted from coconut shell waste. The samples were calcined with temperature variations of 600°C, 650°C and 700°C in an argon environment for 10 hour. The phase purity and crystal structure of LiFePO4 were analyzed using XRD. The analysis of data from XRD was done using the the Match!, Rietica, and MAUD software. Based on the results of XRD analysis, LiFePO4 with high purity and good crystallinity was obtained when the sample was calcined at temperature of 700°C. The results of the MAUD analysis show that the best size of LiFePO4 crystal is 86,54 nm. LiFePO4/rGO nanocomposite was successfully synthesized by mechanical ultracentrifugation method. The characterization of the value of electrical conductivity, carried out using a four-point probe. The results show that the greater the percentage of rGO, the higher the value of electrical conductivity. The mass ratio of 67% LiFePO4 and 33% rGO shows an increment in good conductivity values, from the original order of 10-8 S/cm to the order of 10-4 S/cm.

Acid-Free and Selective Extraction of Lithium from Spent Lithium Iron Phosphate Batteries via a Mechanochemically Induced Isomorphic Substitution.

Lithium (Li) is the most valuable metal in spent lithium iron phosphate (LiFePO4) batteries, but its recovery has become a challenge in electronic waste recovery because of its relatively low content and inconsistent quality. This study proposes an acid-free and selective Li extraction process to successfully achieve the isomorphic substitution of Li in LiFePO4 crystals with sodium (Na). The method uses low-cost and nontoxic sodium chloride (NaCl) as a cogrinding reagent via a mechanical force-induced solid-phase reaction. X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX), and X-ray photoelectron spectroscopy (XPS) characterizations demonstrated the evidence of Li/Na isomorphic substitution, while XPS spectra of Fe 2p, P 2p, and O 1s revealed that the coordination environment of these elements was not significantly altered. Density functional theory calculations further provided evidence that Na and Li share similar outer electron arrangements and coordination environments, favoring Na over Fe as a replacement for Li in LiFePO4. Additionally, the regeneration of NaCl and the recovery of precipitated Li2CO3 were simultaneously achieved with Na2CO3 as the sole reagent. This concise and efficient acid-free mechanochemical process for Li extraction is a promising candidate for feasible recycling technology of Li from spent LiFePO4 batteries. The proposed process is particularly appealing because of its high selectivity, considerable economic advantages, and environmental benefits.

Visualization of the secondary phase in LiFePO4 ingots with advanced mapping techniques

AbstractLiFePO4 has been widely used as a cathode material in lithium ion batteries. However, some impurity phases existing in LiFePO4 have a significant influence on its electrochemical performance. Therefore, detection of the impurity phases is necessary for the manufacturer in order to improve the quality of LiFePO4. In particular, visualization of the impurity and secondary phase distributions immersed in the bulk LiFePO4 crystal can help to understand the origin of the impurity and secondary phases, providing clear guidance towards the synthesis of high purity LiFePO4. The results show that with the combination of Raman and EDS, we are capable of identifying the low melting lithium phosphate phase in LFP ingot. Through micro XRF mapping, more detailed information about the LFP materials after carbon coating are observed. The secondary phases are easily defined due to the high sensitivity of this technology.

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.

Atomic‐Scale Observation of LiFePO4 and LiCoO2 Dissolution Behavior in Aqueous Solutions

AbstractUnderstanding the atomic structure variation at the surface of electrode materials in contact with an electrolyte is an essential step toward achieving better electrochemical performance of rechargeable cells. Different types of water‐based aqueous solutions are suggested as alternative electrolytes to the currently used flammable organic solvents in Li‐ion batteries. However, most research on aqueous rechargeable Li‐ion cells has largely focused on the synthetic processing of materials and resulting electrochemical properties rather than in‐depth atomic‐level observation on the electrode surface where the initial charge transfer and the (de)intercalation reaction take place. By using LiFePO4 and LiCoO2 single crystals, serious P and Co dissolution from LiFePO4 and LiCoO2 into aqueous solutions without any electrochemical cycling is identified. Furthermore, both strong temperature‐dependent behavior of P dissolution in LiFePO4 and very unusual occupancy of Co in the tetrahedral interstices in LiCoO2 are directly demonstrated via atomic‐scale (scanning) transmission electron microscopy. Ab initio density functional theory calculations also reveal that this tetrahedral‐site occupation is stabilized when cation vacancies are simultaneously present in both Li and Co sites. The findings in this work emphasize the significance of direct observation on the atomic structure variation and local stability of the cathode materials.

In Situ Electron Diffraction Tomography Using a Liquid-Electrochemical Transmission Electron Microscopy Cell for Crystal Structure Determination of Cathode Materials for Li-Ion batteries.

We demonstrate that changes in the unit cell structure of lithium battery cathode materials during electrochemical cycling in liquid electrolyte can be determined for particles of just a few hundred nanometers in size using in situ transmission electron microscopy (TEM). The atomic coordinates, site occupancies (including lithium occupancy), and cell parameters of the materials can all be reliably quantified. This was achieved using electron diffraction tomography (EDT) in a sealed electrochemical cell with conventional liquid electrolyte (LP30) and LiFePO4 crystals, which have a well-documented charged structure to use as reference. In situ EDT in a liquid environment cell provides a viable alternative to in situ X-ray and neutron diffraction experiments due to the more local character of TEM, allowing for single crystal diffraction data to be obtained from multiphased powder samples and from submicrometer- to nanometer-sized particles. EDT is the first in situ TEM technique to provide information at the unit cell level in the liquid environment of a commercial TEM electrochemical cell. Its application to a wide range of electrochemical experiments in liquid environment cells and diverse types of crystalline materials can be envisaged.

Electrospun LiFePO₄/C Composite Fiber Membrane as a Binder-Free, Self-Standing Cathode for Power Lithium-Ion Battery.

A LiFePO4/C composite fiber membrane was fabricated by the electrospinning method and subsequent thermal treatment. The thermal decomposition process was analyzed by TG/DSC, the morphology, microstructure and composition were studied using SEM, TEM, XRD, Raman, respectively. The results indicated that the prepared LiFePO4/C composite fibers were composed of nanosized LiFePO4 crystals and amorphous carbon coatings, which formed a three dimensional (3D) long-range networks, greatly enhanced the electronic conductivity of LiFePO4 electrode up to 3.59× 10-2 S · cm-2. The 3D LiFePO4/C fiber membrane could be directly used as a binder-free, self-standing cathode for lithium-ion battery, and exhibited an improved capacity and rate performance. The LiFePO4/C composite electrode delivered a discharge capacity of 116 mAh·g-1, 109 mAh·g-1, 103 mAh·g-1, 91 mAh·g-1, 80 mAh·g-1 at 0.1 C, 0.5 C, 1 C, 3 C, 5 C, respectively. And a stable cycling performance was also achieved that the specific capacity could retain 75 mA·g-1 after 500 cycles at 5 C. Therefore, this LiFePO4/C composite fiber membrane was promising to be used as a cathode for power lithium ion battery.

Concentration-controlled morphology of LiFePO4 crystals with an exposed (100) facet and their enhanced performance for use in lithium-ion batteries

A study of the morphology-controlled growth of olivine LiFePO4 particles by varying the precursor concentration via a facile solvothermal synthesis was carried out. The influences of the solvent product on the structure, morphology and electrochemical performance were systematically investigated by X-ray diffraction, scanning/transmission electron microscopy and charge-discharge tests. It was found that as the precursor concentration increased from 0.15 M to 0.90 M, the morphology of the LiFePO4 particles changed from spindle-shape to plate-type and then to a hierarchical club-shaped structure, and the predominantly exposed facet of all LiFePO4 samples was (100). The shape of LiFePO4 particles remained as nanoplates while synthesized at different reaction temperatures and time with a precursor concentration of 0.30 M. This in turn confirms that the precursor concentration plays an important role in controlling the morphology of LiFePO4 particles. To illustrate the phenomenon caused by variations in the concentration, a possible morphological transformation mechanism was demonstrated. With the optimal precursor concentration of 0.3 M, the obtained LiFePO4 shows square nanoplates with uniform particle size and carbon coating, resulting in excellent electrochemical performance: a discharge capacity of 157.3 mAh/g at 1 C with 95% capacity retention after 1000 cycles and a high-rate capacity of 140.8 mAh/g at 10 C. This facile and precursor concentration-controlled solvothermal method is anticipated to be a guide for large-scale cathode material manufacturing and can fulfil the requirements for its application to high-power lithium-ion batteries.

Synthesis of LiFePO4 nanocrystals and their properties as cathodic material for lithium ion batteries

The method of lithium iron phosphate (LiFePO4) synthesis in a melt mixture of choline chloride and diethylene glycol (DEG) at 230 ?C is proposed. Powders with lamellar morphology consisting of LiFePO4 crystals (size ~30 nm) with olivine structure were synthesized. The size of crystals increased to ~60 nm during the annealing process carried out to obtain a carbon coating on the grain surface of LiFePO4. The charge-discharge curves of the electrode prepared from these powders have a horizontal portion at the potential of 3.4 and 3.5 V, corresponding to the intercalation/deintercalation of lithium in the structure of olivine. The specific discharge capacity of the LiFePO4/C is 133 mA h g-1 for a discharge current of 0.1C. The dependence of the anodic (cathodic) voltammetric current peaks, Ip, on the potential scan rate indicates the diffusion nature of the lithiating step. For the anodic and cathodic processes the averaged diffusion coefficient values are 1.3?10-10 and 1.5?10-10 cm2 s-1, respectively.