The influence of the Li+ addition rate during the hydrothermal synthesis of LiFePO4 on the average and local structure

In hydrothermal synthesis of LiFePO4 a slower addition of Li+ to a mixture of Fe2+ + PO43- increased Fe2+ oxidation, defects within the LiFePO4 structure and iron impurities in the product. No link was seen between Li3PO4 formation and addition rate.

From (S = 1) Spin Hexamer to Spin Tetradecamer by CuO Interstitials in A2Cu3O(CuO)x(SO4)3 (A = alkali)

(Na,K)₂Cu₃O(SO₄)₃ compounds form structural chains of Cu₆ hexameric units with nominal S = 1 spins due to the interplay between inner strong antiferromagnetic and ferromagnetic exchanges. We show here that the lattice relaxation after the replacement of alkali by larger Rb and Cs ones is accompanied by the insertion of neutral CuO species into (Rb,Cs)₂Cu₃O(CuO)_x(SO₄)₃ phases. Structurally, interstitial CuO links the next two Cu₆ units in longer Cu₁₄ tetradecameric ones. For A = Cs (x = 0.5), the cationic ordering is perfect inside a double-cell superstructure. Magnetically, the original Cu₁₄ units consist of frustrated fragments of an S = ¹/₂ spin ladder, with ferromagnetic rung-like but antiferromagnetic leg-like and next-nearest neighbor couplings. It returns S = 1 Cu₁₄ spin clusters, effective around 100 K. Our density functional theory calculations and susceptibility fits also show that at low temperatures they interact in two-dimensional lattices, despite the existence of short inter-Cu-Cu distances between the next two clusters along pseudo-one-dimensional chains.

Iron oxidation to amplify the Na and Li storage capacities of nano-sized maricite NaFePO4

This study reports an effective approach to improve dramatically the electrochemical performance of nanosized NaFePO₄ with a maricite structure, which is commonly considered as electrochemically inactive due to the absence of structural channels for alkaline ion mobility. The approach is based on the complete oxidation under mild conditions (i.e. at low temperatures around 280 °C and traces of oxygen) of the nanosized maricite phase. It is prepared by the phosphate-formate precursor method and is additionally ball-milled with a carbon additive. The oxidation of Fe²⁺ proceeds at the nanoscale level within the maricite nanoparticles and causes a massive structural transformation of the maricite phase into a monoclinic NASICON phase Na₃Fe₂(PO₄)₃ with the preservation of the crystallinity. The oxidized maricite phase exhibits high specific capacities, cycling stability and rate capability when it is used as an electrode in both Na and Li half-cells. The effect of different sodium and lithium electrolytes on the storage performance is investigated as well. It is found that the highest specific capacity (of about 150 mA h g^-1) is achieved in Li half-cells using the LiPF₆ electrolyte, while in Na half-cells the electrolyte NaFSI/EC:DMC achieves a specific capacity of around 100 mA h g^-1. The rate capability is better in Na half-cells than that in Li half-cells. The mechanism of the reversible intercalation/deintercalation of Na⁺ and Li⁺ ions is studied by ex situ XRD and TEM analyses. The results show that the maricite is an electrochemically inactive phase, but through manipulation including oxidation or amorphization it becomes an active electrode material.

Protection of LiFePO4 against Moisture

In this study, a carbon-coated LiFePO₄ (LFP/C) powder was chemically grafted with trifluoromethylphenyl groups in order to increase its hydrophobicity and to protect it from moisture. The modification was carried out by the spontaneous reduction of in situ generated 4-trifluoromethylphenyl ions produced by the diazotization of 4-trifluoromethylaniline. X-ray photoelectron spectroscopy was used to analyze the surface organic species of the modified powder. The hydrophobic properties of the modified powder were investigated by carrying out its water contact angle measurements. The presence of the trifluoromethylphenyl groups on the carbon-coated LiFePO₄ powder increased its stability in deionized water and reduced its iron dissolution in the electrolyte used for assembling the battery. The thermogravimetric and inductively coupled plasma atomic emission spectroscopy analyses revealed that 0.2–0.3 wt.% Li was deinserted during grafting and that the loading of the grafted molecules varied from 0.5 to 0.8 wt.% depending on the reaction conditions. Interestingly, the electrochemical performance of the modified LFP/C was not adversely affected by the presence of the trifluoromethylphenyl groups on the carbon surface. The chemical relithiation of the grafted samples was carried out using LiI as the reducing agent and the lithium source in order to obtain fully lithiated grafted powders.

Sputtered Porous Li-Fe-P-O Film Cathodes Prepared by Radio Frequency Sputtering for Li-ion Microbatteries

The increasing demands from micro-power applications call for the development of the electrode materials for Li-ion microbatteries using thin-film technology. Porous Olivine-type LiFePO₄ (LFP) and NASICON-type Li₃Fe₂(PO₄)₃ have been successfully fabricated by radio frequency (RF) sputtering and post-annealing treatments of LFP thin films. The microstructures of the LFP films were characterized by X-ray diffraction and scanning electron microscopy. The electrochemical performances of the LFP films were evaluated by cyclic voltammetry and galvanostatic charge-discharge measurements. The deposited and annealed thin film electrodes were tested as cathodes for Li-ion microbatteries. It was found that the electrochemical performance of the deposited films depends strongly on the annealing temperature. The films annealed at 500 °C showed an operating voltage of the porous LFP film about 3.45 V vs. Li/Li⁺ with an areal capacity of 17.9 µAh cm^-2 µm^-1 at C/5 rate after 100 cycles. Porous NASICON-type Li₃Fe₂(PO₄)₃ obtained after annealing at 700 °C delivers the most stable capacity of 22.1 µAh cm^-2 µm^-1 over 100 cycles at C/5 rate, with an operating voltage of 2.8 V vs. Li/Li⁺. The post-annealing treatment of sputtered LFP at 700 °C showed a drastic increase in the electrochemical reactivity of the thin film cathodes vs. Li⁺, leading to areal capacity ~9 times higher than as-deposited film (~27 vs. ~3 µAh cm^-2 µm^-1) at C/10 rate.

Mixed-Valence Iron Dumortierite Fe13.52.22+(As5+O4- x)8(OH)6 and Its Intricate Topotactic Exsolution at Mild Temperatures

The mixed-valent iron arsenate hydroxide Fe_13.5^2.22+(AsO_{4- x})₈(OH)₆, x = 0.25, was prepared using the reaction of iron metal with arsenate in aqueous solution and autogenous pressure. Its crystal structure reveals a dumortierite-like framework with mixed-valent Fe^2+/Fe³⁺ in double chains creating channel walls. Remarkably, hexagonal channels consist of chains of face-sharing Fe²⁺O₆ octahedra, 3/4th occupied, whereas AsO₄ tetrahedra occupy triangular ones with a single " up" orientation according to the polar P6₃ mc symmetry. We have analyzed the transformation of this phase upon heating, in which several chemical processes interact, including dehydroxylation, arsenate to arsenite reduction, and oxidative exsolution of a significant part of iron (ca. 15%) found at the surface as hematite and amorphous Fe-rich surficial layer. It leaves a strongly disordered composite structure between several Fe³⁺-based subunits, in which ∼80% of them is ordered in a complex supercell. Because of the high degree of disorder, the crystal chemistry of the individual subunits and their plausible imbrication were considered to unravel the most plausible ideal 3D model.

Comprehensive Study of Oxygen Storage in YbFe2O4+x (x ≤ 0.5): Unprecedented Coexistence of FeOn Polyhedra in One Single Phase

The multiferroic LuFe^2.5+₂O₄ was recently proposed as a promising material for oxygen storage due to its easy reversible oxidation into LuFe³⁺₂O_4.5. We have investigated the similar scenario in YbFe₂O_4+x, leading to a slightly greater oxygen storage (OSC) capacity of 1434 μmol O/g. For the first time, the structural model of LnFe₂O_4.5 was fully understood by high-resolution microscopy images, and synchrotron and neutron diffraction experiments, as well as maximum entropy method. The oxygen uptake promotes a reconstructive shearing of the [YbO₂] sub-units controlled by the adaptive Ln/Fe oxygen coordination and the Fe^2/3+ redox. After oxidation, the rearrangement of the Fe coordination polyhedra is unique such that all available FeO_n units (n = 6, 5, 4 in octahedra, square pyramids, trigonal bipyramids, tetrahedra) were identified in modulated rows growing in plane. This complex pseudo-ordering gives rise to short-range antiferromagnetic correlation within an insulating state.

Topochemical Approach for Transition-Metal Exchange Assisted by Copper Extrusion: from Cu2FeBO5 to Fe3BO5

All fields of today's technology are concerned with multifunctional materials, the subject of constantly expanding research. Among them, metal transition oxides occupy a strategic place because of the properties directly correlated with metal valence linked with oxygen stoichiometry. To enhance or induce new properties, knowledge of the relationships between the structural and physical characteristics is of prime importance, but a design at low scale also appears to be a powerful tool to increase the chemical reactivity and stabilize new compounds. Herein, an unexpected reaction is reported that associates the exchange of copper and iron in a 3D ludwigite lattice with the huge release of oxygen (14% by weight) at moderate temperature (<450 °C). Annealing of Cu₂FeBO₅ in a reducing atmosphere leads to the extrusion of copper and the formation of Fe₃BO₅, the micro/nanostructural state of which facilitates the partial Cu₂FeBO₅ recovery associated with the capture of oxygen in oxidizing conditions.

Microwave-Assisted Synthesis of NaCoPO4 Red-Phase and Initial Characterization as High Voltage Cathode for Sodium-Ion Batteries

Transition metal-containing polyanion compounds are attractive for use as cathode materials in sodium-ion batteries (SIB) because they possess elevated higher intrinsic electrochemical potentials versus oxide analogs given the same M^n+/(n+1)+ redox couple, which leads to higher energy densities. NaMPO₄ (M = transition metal) compounds have a driving force to form into the electrochemically inactive maricite phase when using conventional methods. Herein we report on the synthesis of a NaCoPO₄ (NCP) polymorph ("Red"-phase) by a microwave-assisted solvothermal process at 200 °C using tetraethylene glycol as the solvent. Ex situ XRD, XANES, and electrochemical data are used to determine the reversibility of the Co^2+/3+ redox center.

A density functional theory study of the carbon-coating effects on lithium iron borate battery electrodes

Density functional theory modelling shows that carbon coatings on a LiFeBO₃ cathode material does not impede the Li transport in a Li-ion battery.

Porous NASICON-Type Li3Fe2(PO4)3 Thin Film Deposited by RF Sputtering as Cathode Material for Li-Ion Microbatteries

We report the electrochemical performance of porous NASICON-type Li3Fe2(PO4)3 thin films to be used as a cathode for Li-ion microbatteries. Crystalline porous NASICON-type Li3Fe2(PO4)3 layers were obtained by radio frequency sputtering with an annealing treatment. The thin films were characterized by XRD, SEM, and electrochemical techniques. The chronoamperometry experiments showed that a discharge capacity of 88 mAhg(-1) (23 μAhcm(-2)) is attained for the first cycle at C/10 to reach 65 mAhg(-1) (17 μAhcm(-2)) after 10 cycles with a good stability over 40 cycles.

In situ formation of bromobenzene diazonium ions and their spontaneous reaction with carbon-coated LiFePO4 in organic media

In this study, the diazotization of bromoaniline in the presence of carbon-coated LiFePO₄ and the grafting of bromobenzene moieties are investigated.

Recent progress in hybrid cathode materials for lithium ion batteries

Various binary composite cathode materials for lithium ion batteries are summarized and discussed.

Selective Metal Exsolution in BaFe(2-y)M(y)(PO4)2 (M = Co(2+), Ni(2+)) Solid Solutions

The 2D-Ising ferromagnetic phase BaFe(2+)2(PO4)2 shows exsolution of up to one-third of its iron content (giving BaFe(3+)1.33(PO4)2) under mild oxidation conditions, leading to nanosized Fe2O3 exsolved clusters. Here we have prepared BaFe(2-y)M(y)(PO4)2 (M = Co(2+), Ni(2+); y = 0, 0.5, 1, 1.5) solid solutions to investigate the feasibility and selectivity of metal exsolution in these mixed metallic systems. For all the compounds, after 600 °C thermal treatment in air, a complete oxidation of Fe(2+) to Fe(3+) leaves stable M(2+) ions, as verified by (57)Fe Mössbauer spectroscopy, TGA, TEM, microprobe, and XANES. The size of the nanometric α-Fe2O3 clusters coating the main phase strongly depends on the yM metal concentration. For M-rich phases the iron diffusion is hampered so that a significant fraction of superparamagnetic α-Fe2O3 particles (100% for BaFe(0.5-x)Co(1.5)(PO4)2) was detected even at 78 K. Although Ni(2+) and Co(2+) ions tend to block Fe diffusion, the crystal structure of BaFe(0.67)Co1(PO4)2 demonstrates a fully ordered rearrangement of Fe(3+) and Co(2+) ions after Fe exsolution. The magnetic behaviors of the Fe-depleted materials are mostly dominated by antiferromagnetic exchange, while Co(2+)-rich compounds show metamagnetic transitions reminiscent of the BaCo2(PO4)2 soft helicoidal magnet.

Increasing the Affinity Between Carbon-Coated LiFePO4/C Electrodes and Conventional Organic Electrolyte by Spontaneous Grafting of a Benzene-Trifluoromethylsulfonimide Moiety

The grafting of benzene-trifluoromethylsulfonimide groups on LiFePO4/C was achieved by spontaneous reduction of in situ generated diazonium ions of the corresponding 4-amino-benzene-trifluoromethylsulfonimide. The diazotization of 4-amino-benzene-trifluoromethylsulfonimide was a slow process that required a high concentration of precursors to promote the spontaneous grafting reaction. Contact angle measurements showed a hydrophilic surface was produced after the reaction that is consistent with grafting of benzene-trifluoromethylsulfonimide groups. Elemental analysis data revealed a 2.1 wt % loading of grafted molecules on the LiFePO4/C powder. Chemical oxidation of the cathode material during the grafting reaction was detected by X-ray diffraction and quantified by inductively coupled plasma atomic emission spectrometry. Surface modification improves the wettability of the cathode material, and better discharge capacities were obtained for modified electrodes at high C-rate. In addition, electrochemical impedance spectroscopy showed the resistance of the modified cathode was lower than that of the bare LiFePO4/C film electrode. Moreover, the modified cathode displayed superior capacity retention after 200 cycles of charge/discharge at 1 C.

Very high power and superior rate capability LiFePO4 nanorods hydrothermally synthesized using tetraglycol as surfactant

Small polarizations, i.e. sufficiently good electronic and ionic conductivity is indispensible for high power lithium iron phosphate, especially for its applications to large current power supplies.

Redox centers evolution in phospho-olivine type (LiFe0.5Mn0.5 PO4) nanoplatelets with uniform cation distribution

In phospho-olivine type structures with mixed cations (LiM1M2PO4), the octahedral M1 and M2 sites that dictate the degree of intersites order/disorder play a key role in determining their electrochemical redox potentials. In the case of LiFexMn1-xPO4, for example, in micrometer-sized particles synthesized via hydrothermal route, two separate redox centers corresponding to Fe(2+)/Fe(3+) (3.5 V vs Li/Li(+)) and Mn(2+)/Mn(3+) (4.1 V vs Li/Li(+)), due to the collective Mn-O-Fe interactions in the olivine lattice, are commonly observed in the electrochemical measurements. These two redox processes are directly reflected as two distinct peak potentials in cyclic voltammetry (CV) and equivalently as two voltage plateaus in their standard charge/discharge characteristics (in Li ion batteries). On the contrary, we observed a single broad peak in CV from LiFe0.5Mn0.5PO4 platelet-shaped (∼10 nm thick) nanocrystals that we are reporting in this work. Structural and compositional analysis showed that in these nanoplatelets the cations (Fe, Mn) are rather homogeneously distributed in the lattice, which is apparently the reason for a synergetic effect on the redox potentials, in contrast to LiFe0.5Mn0.5PO4 samples obtained via hydrothermal routes. After a typical carbon-coating process in a reducing atmosphere (Ar/H2), these LiFe0.5Mn0.5PO4 nanoplatelets undergo a rearrangement of their cations into Mn-rich and Fe-rich domains. Only after such cation rearrangement (via segregation) in the nanocrystals, the redox processes evolved at two distinct potentials, corresponding to the standard Fe(2+)/Fe(3+) and Mn(2+)/Mn(3+) redox centers. Our experimental findings provide new insight into mixed-cation olivine structures in which the degree of cations mixing in the olivine lattice directly influences the redox potentials, which in turn determine their charge/discharge characteristics.

Linking local environments and hyperfine shifts: a combined experimental and theoretical (31)P and (7)Li solid-state NMR study of paramagnetic Fe(III) phosphates

Iron phosphates (FePO(4)) are among the most promising candidate materials for advanced Li-ion battery cathodes. This work reports upon a combined nuclear magnetic resonance (NMR) experimental and periodic density functional theory (DFT) computational study of the environments and electronic structures occurring in a range of paramagnetic Fe(III) phosphates comprising FePO(4) (heterosite), monoclinic Li(3)Fe(2)(PO(4))(3) (anti-NASICON A type), rhombohedral Li(3)Fe(2)(PO(4))(3) (NASICON B type), LiFeP(2)O(7), orthorhombic FePO(4)·2H(2)O (strengite), monoclinic FePO(4)·2H(2)O (phosphosiderite), and the dehydrated forms of the latter two phases. Many of these materials serve as model compounds relevant to battery chemistry. The (31)P spin-echo mapping and (7)Li magic angle spinning NMR techniques yield the hyperfine shifts of the species of interest, complemented by periodic hybrid functional DFT calculations of the respective hyperfine and quadrupolar tensors. A Curie-Weiss-based magnetic model scaling the DFT-calculated hyperfine parameters from the ferromagnetic into the experimentally relevant paramagnetic state is derived and applied, providing quantitative finite temperature values for each phase. The sensitivity of the hyperfine parameters to the composition of the DFT exchange functional is characterized by the application of hybrid Hamiltonians containing admixtures 0%, 20%, and 35% of Fock exchange. Good agreement between experimental and calculated values is obtained, provided that the residual magnetic couplings persisting in the paramagnetic state are included. The potential applications of a similar combined experimental and theoretical NMR approach to a wider range of cathode materials are discussed.

Recent atomistic modelling studies of energy materials: batteries included

Advances in functional materials for energy conversion and storage technologies are crucial in addressing the global challenge of green sustainable energy. This article aims to demonstrate the valuable role that modern modelling techniques now play in providing deeper fundamental insight into novel materials for rechargeable lithium batteries and solid oxide fuel cells. Recent work is illustrated by studies on important topical materials encompassing transition-metal phosphates and silicates for lithium battery electrodes, and apatite-type silicates for fuel cell electrolytes.