PO4 Materials Research Articles

The Effect of Oxygen Vacancy Defects on the Structure and Electrochemical Behaviors of LiMn0.65Fe0.35PO4 Cathode

The presence of oxygen vacancy defects significantly impacts the crystal structure and electrochemical attributes of phosphate cathodes. In this investigation, LiMn0.65Fe0.35PO4 materials with varying levels of oxygen vacancy defects were synthesized via hydrogen plasma-induced reduction. It was observed that the content of oxygen vacancy defects on the crystal surface increased proportionately with the rise in hydrogen (H2) flow rate. Notably, the LMFP-3 sample, prepared with an H2 flow rate of 10 ml min−1, demonstrated superior electrochemical performance, characterized by a 159.7 mAh g−1 discharge capacity at 0.1 C and a remarkable 99.8% capacity retention at 5 C after 200 cycles. This enhancement in electrochemical performance is attributed to the improved intrinsic conductivity of the LiMn0.65Fe0.35PO4 material due to the presence of oxygen vacancy defects. However, it is important to note that an excessively high H2 flow rate can lead to the formation of Fe2P impurities, which hinder lithium ion (Li+) diffusion. Furthermore, theoretical calculations conducted using density functional theory provide a rational explanation for the observed improvement in electronic conductivity. The introduction of oxygen vacancy defects results in a significant reduction in the Band gap, which is highly beneficial for enhancing the intrinsic conductivity of the LiMn0.65Fe0.35PO4 materials.

First principles study on the effects of Li deintercalation on the structural properties of a novel mixed transition metal cathode material LiNi0.75Mn0.125Fe0.125PO4 and investigation of its electric properties

Identifying and developing new cathode materials for Lithium-ion batteries (LIBs) that can achieve the desired balance between performance and environmental requirements is very challenging. Some of the key factors being highly considered in those searches include energy density, voltage, structural stability, cost, and abundance. In this work, ternary doped cathode material LiNi0.75Mn0.125Fe0.125PO4 was synthesized using sol gel method. The structural analysis using Rietveld refinement showed an orthorhombic structure with Pnma space group. The DFT + U calculations were investigated to study the structural and electronic properties during Li deintercalation in Li1-xNi0.75Mn0.125Fe0.125PO4 (0 ≤x ≤ 1). It was found that the material remained structurally stable throughout the deintercalation process. During deintercalation, the Li1-xNi0.75Mn0.125Fe0.125PO4 showed a semiconducting nature for x = 0 and x = 0.25, respectively with Eg = 1.75 eV and 0.57 eV. Then, the material becomes semi-metal for x = 0.5 and x = 0.75 and returns to the semiconducting behavior for x = 1 with Eg = 0.38 eV. The formation energy calculations indicated that the material formed a solid solution throughout the Li concentration range (0 ≤x≤), with values ranging from −17 eV to −1 eV. The voltage decreases from 4.7 eV to 3.4 eV during deintercalation. The Nyquist plot showed the presence of different relaxation processes for LiNi0.75Mn0.125Fe0.125PO4 material. A semiconducting behavior was confirmed. The ac-conductivity as a function of frequency follows the double Jonscher power law. A low dissipation factor was observed, which also decreased with frequency. This shows that LiNi0.75Mn0.125Fe0.125PO4 material could be a good cathode for LIBs.

Dynamic phase transition behavior of a LiMn0.5Fe0.5PO4 olivine cathode material for lithium-ion batteries revealed through in-situ X-ray techniques

LiMn0.5Fe0.5PO4 has attracted great interest due to its good electrochemical performance and higher operating voltages. This has led to a greater than 30 percent higher energy density than for commercial LiFePO4 olivine cathodes. Understanding the phase transition behaviors and kinetics of this material will help researchers to design and develop next generation cathodes for Li-ion batteries. In this study, we investigated non-equilibrium phase transition behaviors in a LiMn0.5Fe0.5PO4 cathode material during charge–discharge processes by varying current rates (C-rates) using synchrotron in-situ X-ray techniques. These methods included wide angle X-ray scattering (in-situ WAXS) and X-ray absorption spectroscopy (in-situ XAS). The WAXS spectra indicate that the phase transition of LiMn0.5Fe0.5PO4 material at slow C-rates is induced by a two-phase reaction. In contrast, at a high C-rate (5C), the formation of an intermediate phase upon discharge is clearly observed. Concurrently, the oxidation numbers of the redox reactions of Fe2+/Fe3+ and Mn2+/Mn3+ were evaluated using in-situ XAS. This combination of synchrotron in-situ X-ray techniques gives clear insights into the non-equilibrium phase transition behavior of a LiMn0.5Fe0.5PO4 cathode material. This new understanding will be useful for further developments of this highly promising cathode material for practical commercialization.

Synthesis of Fe2+ Substituted High-Performance LiMn1-xFexPO4/C (x = 0, 0.1, 0.2, 0.3, 0.4) Cathode Materials for Lithium-Ion Batteries via Sol-Gel Processes.

A series of carbon-coated LiMn1−xFexPO4 (x = 0, 0.1, 0.2, 0.3, 0.4) materials are successfully constructed using glucose as carbon sources via sol-gel processes. The morphology of the synthesized material particles are more regular and particle sizes are more homogeneous. The carbon-coated LiMn0.8Fe0.2PO4 material obtains the discharge specific capacity of 152.5 mAh·g−1 at 0.1 C rate and its discharge specific capacity reaches 95.7 mAh·g−1 at 5 C rate. Iron doping offers a viable way to improve the electronic conductivity and lattice defects of materials, as well as improving transmission kinetics, thereby improving the rate performance and cycle performance of materials, which is an effective method to promote the electrical properties.

A new route for green synthesis of LiFe0.25Mn0.75PO4/C@rGO material for lithium ion batteries

A novel eco-friendly synthesis of LiFe0.25Mn0.75PO4/C@ reduced graphene oxide (rGO) was carried out. The effects of different grinding-drying process and reduced graphene oxide (rGO) modification on the morphology and electrochemical performance of the synthesized LiFe0.25Mn0.75PO4 materials were studied in detail. The studies show that the sanding assistant spray drying process can effectively refine the particle size of LiFe0.25Mn0.75PO4/C materials with uniform dispersion. The uniform nano size combination of rGO and carbon coating significantly reduce the charge-transfer impedance of electrode materials, and improve the conductivity and electrochemical properties of the materials. The optimization results show that LiFe0.25Mn0.75PO4/C@rGO materials display the most favorable kinetic behavior and the best electrochemical reversibility, with the specific discharge capacity of 143.8, and 139.8 mAh g−1 at 1 C and 2 C. After 100 cycles at 1 C, the capacity retention maintains 97.64%, showing a promising application prospect for lithium-ion batteries.

Synthesis of carbon-coated LiMn0.8Fe0.2PO4 materials via an aqueous rheological phase-assisted solid-state method

Synthesis method is crucial to the improved electrochemical performances of LiMnPO4 materials with poor electronic and ionic conductivity for large-scale preparation. Here, carbon-coated LiMn0.8Fe0.2PO4 materials were synthesized through a rheological phase-assisted solid-state method combined with freeze-drying. Effect of different carbon sources on the structure, morphology, and electrochemical performance of as-prepared LiMn0.8Fe0.2PO4 materials was investigated. The results suggest that LiMn0.8Fe0.2PO4 materials with citric acid as carbon sources show a high discharge capacity with 164, 122, and 106 mA h g−1 at 0.1, 1, and 5 C, and a capacity retention of around 99.6% after 100 cycles at 1 C. This can be attributed to higher surface area, intense peak of sp2 N–C=N bonds, and uniform particle distribution resulting from the rheological phase formed using citric acid as carbon sources combined with freeze-drying. The rheological phase-assisted solid-state method is a promising route for production of LiMn0.8Fe0.2PO4 materials with excellent performances.

Electrochemical and Electrocatalytic Behavior of Phosphate Family of Battery Insertion Materials: Few Case Studies

Exploring post Li-ion battery systems, various monovalent (Na+, K+) and multivalent (Mg2+, Ca2+, Zn2+, Al3+) chemistry have been intensely investigated over the last decade. Among them, sodium-ion based insertion compounds have emerged as front-runners owing to their operational similarity with Li-ion systems, low cost/ abundance and high rate kinetics [1]. Various oxides and oxyanionic compounds have been reported with great success delivering high capacity and/or redox potential leading to promising energy density. In addition, some of these systems can be exploited as electrocatalysts with bi-functional activity. Over the past two years, our group has investigated various phosphate (PO4 3-) based insertion compounds for electrochemical and electrocatalytic activity [2-9]. In the current work, we will summarize these work to give some insights on sodium intercalation properties and bifunctional (oxygen evolution/ reduction reaction) electrocatalytic activity of following PO4 materials. (i) Solution combustion synthesis was used to prepare phase-pure Na2FePO4F fluorophosphate compound involving low cost Fe(III) precursor. The target phase was obtained by annealing the intermediate complex in short duration of 1 minute. The electrochemical performance (3 V vs Na, 100 mAh/g) and one dimensional Na+ diffusional mechanism will be demonstrated. (ii) The electrochemical and diffusional activity of novel sodium metaphosphate [NaM(PO3)3] class of cathodes will be shown with a 2.8 V and 3.2 V (vs. Na) activity for Fe- and Co-based metaphosphate respectively. The electrocatalytic (oxygen reduction reaction) behavior of NaM(PO3)3 compounds will be demonstrated in comparison to Pt/C system. (iii) The bifunctional activity of various phosphate (PO4 3-) and pyrophosphate (P2O7 2-) materials (e.g. NaFePO4, KFePO4, NaCoPO4, Na2CoP2O7, K2CoP2O7) will be described and compared to Pt/C system. Most of these materials were found to be bifunctional in nature with potential application in Na-air batteries. (iv) The electrochemical and electrocatalytic performance of phosphate based alluaudite materials [NaMFe2(PO4)3] will be reported. An overall 3 V Na (de)intercalation was noticed with reversible capacity over 70 mAh/g along with efficient oxygen reduction reaction activity. (v) The entire work on phosphate based insertion materials will be shown using various experimental tools in synergy with computational investigation. It will demonstrate some examples of half-cell batteries, thin-film micro-batteries and metal-air batteries. Reference s : [1] N. Yabuuchi et al, S. Komaba, Chem. Rev. 114 (2014) 11636–11682. [2] L. Sharma et al, P. Barpanda, ACS Appl. Mater. Interfaces. 9 (2017) 34961–34969. [3] L. Sharma et al, P. Barpanda, Ionics. 24 (2018) In press. [4] C. Murugesan et al, P. Barpanda, ChemCatChem. 10 (2018) In press. [5] R. Gond et al, P. Barpanda, ChemElectroChem. 5 (2018) In press. [6] R. Gond et al, P. Barpanda, Inorg. Chem. 56 (2017) 5918–5929. [7] D. Dwibedi et al, P. Barpanda, ECS Trans. 80 (2017) 337–342. [8] D. Dwibedi et al, P. Barpanda, MRS Adv. (2018) In press. [9] D. Dwibedi et al, P. Barpanda, Electrochim. Acta. (2018) Submitted.

Improved electrochemical performance of LiFe0.65Mn0.35PO4 cathode material by using electrolytic manganese dioxide for lithium-ion battery

The olivine-type structure LiFe0.65Mn0.35PO4 materials are respectively synthesized via MnO2 and MnC2O4·2H2O as manganese resources by using solid-state reaction. The compound materials are characterized by scanning electron microscopies (SEM), transmission electron microscopy (TEM), X-ray photoelectronspectroscopy (XPS) and electrochemical test. The experimental results demonstrate that LiFe0.65Mn0.35PO4 prepared by MnO2 as manganese resource exhibits uniform particles with porous structure in SEM and TEM images. XPS data show the coexistence of Mn4+, Mn3+ and Mn2+ cations. Besides, this sample shows better discharge special capacity of 107.46 mA h g–1 at 5 C and capacitance conservation rate about 95.47% after 100 cycles at 1 C. The superior electrochemical capability is attributed to the coexistence of mixed-valence manganese cations in crystal and the uniform particles with porous structure.

A facile chemical route to synthesize copper particles-modified LiFe0.95Mo0.05PO4 for lithium-ion batteries

In this work, a method of fabricating copper particles-modified LiFe0.95Mo0.05PO4 materials by a two-step chemical process is proposed. The structures and morphologies of the samples are characterized by XRD, SEM and XPS. The results demonstrate that Cu0 is the major valence of Cu element, and crystal structure of the electrode material isn’t destroyed after distributing copper particles well on the surface of LiFe0.95Mo0.05PO4. Compared with LiFe0.95Mo0.05PO4, the samples containing Cu exhibit better initial discharge capacity of 143.6mAhg−1 at 0.1C and capacity retention of 96.6% after 60 cycles. The results indicate that the enhanced kinetics of electron and lithium-ion transport in the samples is attributed to the superficial copper particles.

Graphene encapsulated spherical hierarchical superstructures self-assembled by LiFe0.75Mn0.25PO4 nanoplates for high-performance Li-ion batteries

the electrochemical performance of LiFe0.75Mn0.25PO4 material is further enhanced via synergistic strategies including crystal orientation growth, microspherical self-assembly and graphene encapsulation. The crystal orientation growth of LiFe0.75Mn0.25PO4 with plate-like morphology not only reduces the Li-ion transport length, but also enlarges the (010) surface, and leads to the improvement of Li-ion diffusion. The self-assembled spherical hierarchical superstructures can effectively prevent the plane-plane stacks of LiFe0.75Mn0.25PO4 nanoplates during spontaneously aggregating, providing more (010) surface for Li+ insertion/extraction. The graphene encapsulation can build a 3D conductive network as well as stabilize the microspherical aggregations, resulting in superior electronic conductivity and stability. As a consequence of the synergistic effects, the as-obtained LiFe0.75Mn0.25PO4 sample exhibits excellent rate capability (141.1mAhg−1 at 5C, 126.5mAhg−1 at 10C, 107.7mAhg−1 at 20C) and outstanding cyclability (25°C, 94.6% capacity retention after 500 cycles at 1C). The synergistic strategy involving crystal orientation growth, microspherical self-assembly and graphene encapsulation provides a fascinating candidate to obtain superior olivine-type cathode materials with excellent rate capability and cycling stability, and holds the potential to be extended to the controlled preparation of other electrode materials.

Lithium-Ion Battery Based on LiMn0.5Fe0.5PO4 Cathode and Lithium Alloying Anode

Polyanion framework olivine materials have attracted large attention as cathodes in LIBs. Recently, the attention of the scientific community has been focused on the substitution of iron by manganese, cobalt or nickel within the olivine lattice [1]. Indeed, Mn3+/Mn2+, Co3+/Co2+ and Ni3+/Ni2+ couples show increasing redox potentials compared to Fe3+/Fe2+, thus leading to an increased energy density. However, the latter two exhibit redox potential of about 4.8 and 5.2 V vs. Li+/Li, respectively, i.e. a value exceeding the conventional carbonate-based electrolytes stability limit [2]. LiMnPO4 has instead an operating voltage of about 4.1 V vs. Li+/Li, still within the working electrochemical window of conventional electrolytes, thus leading to a theoretical energy density of about 700 Wh kg-1 [3]. LiMnPO4 suffers from poor electric conductivity in comparison to LiFePO4 [4]. Several approaches have been studied to improve LiMnPO4 electrochemical activity in lithium cell. Incorporation of conductive additives, such as carbon, by post-synthesis ball milling or formation of a carbon layer on particle surface can increase the electronic conductivity [5, 6]. Significant decrease of the lithium diffusion pathway and consequent conductivity increase may be reached by reduction of particle size by proper synthetic procedures leading to nano-morphologies [7, 8]. Soft-chemistry methods allow more controlled tailoring of morphologies and size than the conventional solid-state approaches [9]. In particular, hydro- and solvo-thermal methods are simple techniques allowing the precipitation of olivines at low temperature and controlled crystal growth [10]. Further improvement of the cathode performance may be reached by mixed compositions, LiMn1-xFexPO4. Iron incorporation in the olivine lattice has proven to enhance the electrochemical activity of the Mn3+/Mn2+ redox reaction by increasing both electronic and ionic conductivity, and by reducing lattice strain due to Jahn-Teller active ion Mn3+ [11, 12].Herein we report the synthesis and characterization of LiMnPO4, LiFePO4 and mixed LiMn0.5Fe0.5PO4 materials through solvothermal approach. Careful analysis of the electrochemical properties by potentiodynamic cycling with galvanostatic acceleration and galvanostatic cycling techniques were carried out in order to investigate the features of the Fe3+/Fe2+ and Mn3+/Mn2+ redox couples in the olivine structure. In addition, an advanced lithium-ion battery formed by combining a LiMn0.5Fe0.5PO4 cathode with a Sn-C anode is reported. The figure shows the galvanostatic cycling results of a Sn-C/LiMn0.5Fe0.5PO4 cell.

The Influence of Platinum Doping on the Structure, Morphology and Electrochemical Properties of LiFePO4/C Cathode Material

In this work, a dual approach of metal doping and carbon coating was employed to improve the poor ion diffusion rate and low electrical conductivity of LiFePO4, respectively. LiFePO4 and LiFe0.96Pt0.04PO4 materials were synthesized using a sol-gel method followed by carbon coating with sucrose. The effect of platinum doping on the physical-chemical properties and the electrochemical performance of LiFePO4/C material were investigated by XRD, XPS, SEM, carbon analyzer, charge/discharge testing, cyclic voltammetry and electrochemical impedance spectroscopy. The electrochemical results showed that the specific discharge capacities of the material significantly increased by platinum doping. This improvement will be discussed considering the expansion of the lattice parameter, reducing the particles size with uniform distribution and pillar effect of platinum element for stabilizing the crystal structure.

Graphene oxide assisted facile hydrothermal synthesis of LiMn0.6Fe0.4PO4 nanoparticles as cathode material for lithium ion battery

Assisted by graphene oxide (GO), nano-sized LiMn0.6Fe0.4PO4 with excellent electrochemical performance was prepared by a facile hydrothermal method as cathode material for lithium ion battery. SEM and TEM images indicate that the particle size of LiMn0.6Fe0.4PO4 (S2) was about 80 nm in diameter. The discharge capacity of LiMn0.6Fe0.4PO4 nanoparticles was 140.3 mAh·g−1 in the first cycle. It showed that graphene oxide was able to restrict the growth of LiMn0.6Fe0.4PO4 and it in situ reduction of GO could improve the electrical conductivity of LiMn0.6Feo.4PO4 material.

Solvothermal synthesis of nano LiMn0.9Fe0.1PO4: Reaction mechanism and electrochemical properties

Solvothermal approach is used to synthesize LiMn0.9Fe0.1PO4 (LMFP) nanomaterial for Li-ion batteries (LIBs). Experimental parameters such as feeding sequences, reaction time and reaction temperature are discussed and the obtained LMFP are characterized by XRD, SEM and TEM. To understand the formation of LMFP, a reaction mechanism is proposed. The proposed mechanism indicates that the suitable concentration of MLi (M = Fe, Mn) antisite defect can improve the electrochemical performance of the material. The charge–discharge data of obtained LMFP shows that the LiMn0.9Fe0.1PO4 material synthesized at 180 °C for 4 h and then sintering with sucrose at 650 °C for 5 h under argon protection has the highest discharge capacity, which is 149.2 mAh g−1 at 0.1C rate.

Effects of Mg<sup>2+ </sup> and Ti<sup>4+</sup> Doping on Performance of LiFePO<sub>4</sub> for Lithium-Ion Batteries

Stoichiometric Mg2+ and Ti4+ with different proportions were doped to prepare the LiFe1-x-yMgxTiyPO4 cathode materials by high temperature solid state method. The samples were investigated with XRD, SEM and charge/discharge measurements. Results show that doping of Mg2+ and Ti4+ distinctly changes the paticle sizes and morphologies, which leads to a improvement of electrochemical performance. The mix-doped LiFe0.98Mg0.01Ti0.01PO4 material shows the best electrochemical performance due to its smaller crystalline particles and lower the polarization. At the discharge rate of 0.1C, the initial specific capacity of LiFe0.98Mg0.01Ti0.01PO4 is 105.78 mAh·g-1, its high-rate performance is also better.

Novel precursor of Mn(PO3(OH))·3H2O for synthesizing LiMn0.5Fe0.5PO4 cathode material

A brand new method for synthesizing Mn(PO3(OH))·3H2O is attained in this paper. During this process, pure flake-like Mn(PO3(OH))·3H2O precipitate is prepared using C2H5OH as initiator. Besides that, LiMn0.5Fe0.5PO4/C is successfully synthesized from the Mn(PO3(OH))·3H2O precursor at 650 °C for the first time. Thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) are applied in the characterization of the Mn(PO3(OH))·3H2O precursor and LiMn0.5Fe0.5PO4/C. High-resolution transmission electron microscopy (HRTEM) is also used to investigate the morphology of LiMn0.5Fe0.5PO4/C. X-ray photoelectron spectroscopy (XPS) and galvanostatic charge and discharge test are employed to characterize the Mn(PO3(OH))·3H2O precursor and LiMn0.5Fe0.5PO4 material, respectively. The as-prepared LiMn0.5Fe0.5PO4/C material exhibited a reversible capacity of 131 mAh g−1 at 0.05 C. It can be confirmed that the incorporation of Fe into LiMnPO4 can significantly improve the electrochemical properties for improving the conductivity of the material and facilitating the Li+ diffusion. In addition, a capacity of 120 mAh g−1 is still delivered at 0.05 C rate with a capacity retention of about 91% after 25 cycles, and reversible capacity can reach 105 mAh g−1 at 1 C.

Co-precipitation synthesis of micro-sized spherical LiMn0.5Fe0.5PO4 cathode material for lithium batteries

It is important to increase the energy density of olivine-type cathode materials utilizing micro-sized particles for rechargeable lithium batteries. However, according to the literature, micro-sized LiMn1−xFexPO4 compounds have limited specific capacity because they exhibit lower electrical conductivity than nano-sized materials: isolation of the inner part of the particles from the electrolyte results in an electrochemically inactive area during cycling, which hinders proper Li+ transport that causes the failure of the micron-sized particles. To improve the electrical conductivity of micro-sized LiMn1−xFexPO4, we designated micro-sized C-LiMn0.5Fe0.5PO4 materials composed of nanopores in the micro-sized particles synthesized viaco-precipitation. The resulting morphology was spherical, showing a tap density of 1.27 g cm−3, and uniform carbon coating layers on the primary particles with nanopores were observed in the secondary particles. The as-synthesized micro-sized C-LiMn0.5Fe0.5PO4 exhibited outstanding cyclability at a C-rate of 0.5, retaining 97% of its capacity at room temperature and 85% at 55 °C, due to the synergetic effect of the presence of a uniform carbon coating layer on the primary particles with nanopores in the secondary particles.

Strong crystalline field at the Fe site and spin rotation in olivine LiNi0.99 F57e0.01 PO4 material by Mössbauer spectroscopy

The crystal structure of LiNi0.99 F57e0.01 PO4 compound has been determined to be orthorhombic by Rietveld refinement method. Temperature dependence of magnetization M shows an anomalous antiferromagnetic behavior. A sudden change in both the magnitude of magnetic hyperfine field and its slope below 11 K suggests that magnetic phase transition related to the spin ordering takes place abruptly. From the result of Mössbauer measurement, it is shown that a strong electric crystalline field of octahedral symmetry including the contribution of spin-orbit coupling and magnetic hyperfine field by space-modulated spin structure is acted to the sites of Fe2+ ions simultaneously at low temperature.

Synthesis and Electrochemical Characterization of Carbon-Free Li1.05FePO4 and Li1.05Fe0.997Cu0.003PO4 Materials

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Combustion Synthesis of Nanometric Powders of LaPO4 and Sr-Substituted LaPO4

La(1-x)Sr(x)PO4 materials are very stable at high temperature and water vapor partial pressure and they show protonic conductivity; thus, they could be used as electrolytes in HTPFCs (high-temperature protonic fuel cells), hydrogen sensors, and hydrogen separation technology. This work describes the combustion synthesis of La(1-x)Sr(x)PO4 powders (x = 0, 0.025, 0.050) from mixtures of La(NO3)3·6H2O and Sr(NO3)2 as cation precursors, (NH4)2HPO4 as anion precursor, and urea as fuel. The characterization of the as-prepared combustion products showed that combustion synthesis provides a straightforward method for the achievement of solid solutions of these materials as nanometric powders. Furthermore, dilatometric studies reveal that dense ceramics can be obtained at low sintering temperature (1300 °C), in air, from the synthesized nanometric powders.