Lithium Iron Silicate Research Articles

Mechanistic study in sulfur-carbon co-modification on Li2FeSiO4 for lithium-ion batteries

Lithium iron silicate (Li2FeSiO4) has high theoretical capacity of 332 mAh∙g-1 upon full extraction of two lithium ions, which is accompanied by high delithiation potential and large volume change. Sulfur modification could be an effective approach for stabilizing Li2FeSiO4 cathodes. A series of carbon-coated Li2FeSiO4 samples were prepared through a hydrothermal and solid-state reaction route at a fixed sucrose content (14 wt% carbon) with varying amounts of sublimed sulfur at 5, 10, 15, 20 and 25 at%. The crystal structures, chemical compositions, microstructures and electrochemical performances of the differently sulfur-modified Li2FeSiO4/C were systematically investigated. The sulfur-carbon co-modifications and possible existing chemical states of sulfur are discussed in terms of the surface and near-bulk modifications. It was found that all the samples exhibited a monoclinic P21 structural Li2FeSiO4, while the sulfur contents over 10 at% formed impure phases such as iron sulfides (Fe1-xS) and lithium metasilicate (Li2SiO3). Furthermore, three types of sulfur states could be found when sulfur and carbon were added simultaneously into Li2FeSiO4. The surfaces were predominated by S–C and S–O–C, indicating the sulfur-carbon co-modification, which would reduce the resistance at the solid/electrode interface (SEI) but increase the charge transfer resistance. Near the bulk, the enhanced interactions between sulfur and iron S–Fe(−S) or sulfur and silicon S–Si(−O) may lead to the formation of possible impurities or oxygen substitution by sulfur. The former is detrimental as the effective sulfur amount is reduced, while the latter is beneficial to improve specific capacity and cycle stability. The overall performance of sulfur-carbon co-modified sample is influenced complicatedly by the sulfur content. The best initial discharge capacity of 152.3 mAh∙g-1 was obtained with 5 at% sulfur modification, while the best overall performance, showing a typical warm-up response and good cycle stability, was observed with 15 at% sulfur modification.

Hollow Hemispherical Lithium Iron Silicate Synthesized by an Ascorbic Acid-Assisted Hydrothermal Method as a Cathode Material for Li Ion Batteries.

High-capacity and high-voltage cathode materials are required to meet the increasing demand for energy density in Li ion batteries. Lithium iron silicate (Li2FeSiO4) is a cathode material with a high theoretical capacity of 331 mAh·g−1. However, its poor conductivity and low Li ion diffusion coefficient result in poor capability, hindering practical applications. Morphology has an important influence on the properties of materials, and nanomaterials with hollow structures are widely used in electrochemical devices. Herein, we report a novel hollow hemispherical Li2FeSiO4 synthesized by a template-free hydrothermal method with the addition of ascorbic acid. The hollow hemispherical Li2FeSiO4 consisted of finer particles with a shell thickness of about 80 nm. After carbon coating, the composite was applied as the cathode in Li ion batteries. As a result, the hollow hemispherical Li2FeSiO4/C exhibited a discharge capacity as high as 192 mAh·g−1 at 0.2 C, and the average capacities were 134.5, 115.5 and 93.4 mAh·g−1 at 0.5, 1 and 2 C, respectively. In addition, the capacity increased in the first few cycles and then decayed with further cycling, showing a warm-up like behavior, and after 160 cycles the capacities maintained 114.2, 101.6 and 79.3 mAh·g−1 at 0.5, 1 and 2 C, respectively. Such a method of adding ascorbic acid in the hydrothermal reaction can effectively synthesize hollow hemispherical Li2FeSiO4 with the enhanced electrochemical performance.

Approaching theoretical specific capacity of iron-rich lithium iron silicate using graphene-incorporation and fluorine-doping

Lithium iron silicate, Li2FeSiO4, is a promising cathode material for lithium ion batteries due to its high theoretical specific capacity, earth abundance, low cost, and environmental friendliness.

Surface depth-dependent and microstructural analyses of high performance lithium iron silicate with 15% vanadium modification

The surface, interface and microstructure studies were carried out for the nominal 15% vanadium modified carbon coated lithium iron silicate composite in an effort to understand the enhanced electrochemical performance and decayed cycle performance. The empirical formula was determined to be Li2.09Fe0.78V0.22Si1.29O6.29. It was confirmed that the surface of the as-prepared composite was coated by amorphous carbon with the thickness of approximately 50 nm. Furthermore, the predominant crystal phase changed from monoclinic P21 to orthorhombic Pmn21 after the vanadium modification. The simultaneous presences of multivalent iron (Fe) and vanadium (V) ions were identified by performing multi-peak fitting analyses in the depth-dependent X-ray photoelectron spectroscopic spectra of Fe2p and V2p. The detailed microstructure, interface and surface characterizations provided strong evidence that the partial substitution of divalent iron (Fe2+) with divalent vanadium (V2+) in the structure and the surface modification by vanadium might become possible, leading to 29.1% increase in total carbon content, and 35.8% and 41.3% decreases in the interfacial charge transfer resistance and lithium–ion diffusion coefficient, respectively. The faster interfacial charge transfer kinetics greatly enhanced the initial discharge capacity, while the slower lithium–ion diffusion process apparently degraded the cycle performance.

Silicate Nanocrystals: PEDOT Encapsulated and Mechanochemically Engineered Silicate Nanocrystals for High Energy Density Cathodes (Adv. Mater. Interfaces 13/2020)

In article number 2000226 Majid Rasool, George P. Demopoulos, and co-workers report the fabrication of a robust core-shell lithium iron silicate (Li2FeSiO4, LFS) cathode architecture, where nanocrystal's core is prepared via mechanochemical annealing and shell via PEDOT polymerization. As a result of the devised mechanochemical/interphasial engineering of the [email protected] nanocrystals, their Li-ion storage capacity is augmented to >1 Li, namely 200 mAh g−1 after 50 cycles or 1.2 Li+ units-the highest capacity reported for the Pmn21 LFS cathode.

Mechanochemical Engineering of Polymer-Coated Silicate Nanocrystal Cathodes

Lithium iron silicate has attracted a lot of attention due to its 330 mAh g-1 theoretical capacity (2 Li+ per formula unit). However, inherently it exhibits relatively poor Li-ion intercalation kinetics, interfacial reactivity and complex phase transitions resulting in lower than one Li+ storage and poor capacity retention. In this work, we report a core-shell strategy to overcome these obstacles making use of mechanochemical processing and polymer coating. Mechanochemical annealing has been sown recently [1] to lead to activation of Li-ion diffusion (DLi) by one order of magnitude enhancement. Subsequently, in situ conductive polymer coating was employed on the surface of nanocrystals via an ambient temperature chemical process. As a result of this integrated nanocrystal engineering approach the core-shell LFS@P (βII ) cathode material delivered >1.3 Li and substantially enhanced cycling stability.[1] Rasool, M.; Chiu, H. C.; Lu, X.; Voisard, F.; Gauvin, R.; Jiang, D.; Paolella, A.; Zaghib, K.; Demopoulos, G. P., Mechanochemically-tuned structural annealing: a new pathway to enhancing Li-ion intercalation activity in nanosized βII Li2FeSiO4. J. Mater. Chem. A 2019, 7, 13705-13713

Unusual Li-ion Intercalation Activation with Progressive Capacity Increase in Orthosilicate Nanocomposite Cathode

Lithium iron silicate Li2FeSiO4 (LFS) has received significant attention as a Li-ion intercalation cathode material. However, it exhibits relatively poor Li-ion intercalation kinetics. In this work...

Lattice dynamics of Li2FeSiO4/C from 57Fe Mössbauer spectroscopy

Lithium Iron (II) Silicate, Li2FeSiO4, is a promising cathode material for Li-ion battery applications. Its carbon composite Li2FeSiO4/C was synthesized by a simple dry ball-milling process combined with high- temperature treatment, using citric acid as carbon source, and characterized by temperature-dependence. Powder X-ray diffraction shows that it crystallizes in a monoclinic structure with (P21/n) symmetry. 57Fe Mossbauer effect studies in the temperature range 16.5–298 K were carried out. The temperature dependence of the isomer shifts was used to estimate the effective vibrating mass, Meff, yielding a value of ~87 g mol−1 for Fe in the FeO42-- moiety. This is due to the significant covalency of the Fe-0 bond in the orthosilicate composite. The temperature dependence of the recoil-free fraction (spectral areas), when combined with the effective vibrating mass calculation, yields the “Mossbauer lattice temperature” of θM = 165 K Low temperatures reveals that paramagnetic Li2FeSiO4/C becomes antiferromagnetic at 16.5 K.

An Orthogonal Investigation of Sulfur Modified Li2FeSiO4 for Lithium Ion Batteries

Different sulfur modified carbon coated lithium iron silicate (Li2FeSiO4/C) composites were prepared by sol-gel precursor and solid-state reaction using 1-dodecanethiol as a sulfur source material. The effects of sucrose amount, sulfur content, sintering temperature and sintering time on crystal structure, surface morphology and electrochemical performance of sulfur modified Li2FeSiO4/C composites were investigated through orthogonal design experiments. The results of the orthogonal investigation indicated that the significance of the influence on the capacity performance followed the decreasing order as sulfur doping content > sucrose content > sintering temperature > sintering time. The optimized conditions were determined to be: sucrose content of 70%, sulfur doping content of 10%, sintering temperature at 600°C for 7 h.

Ethylenediamine-Enabled Sustainable Synthesis of Mesoporous Nanostructured Li2FeIISiO4 Particles from Fe(III) Aqueous Solution for Li-Ion Battery Application

Engineering of nanostructured lithium iron silicate (LFS) particles is pursued via a novel benign synthesis approach seeking to understand the crystalline particle formation process and its impact on energy storage capacity. Specifically, mesoporous Li2FeSiO4 nanostructured particles are synthesized via a novel dual-step process involving organic-assisted hydrothermal precipitation from concentrated Fe(III) (1 mol/L) aqueous solution followed by reductive (5 vol % H2) thermal transformation of the precipitate at 400 °C (LFS400) and 700 °C (LFS700). Scanning and transmission electron microscopy revealed the formation of secondary sub-micron-sized porous agglomerates of unitary primary nanocrystals (∼50 nm for LFS400 and ∼200 nm for LFS700). Both ethylene glycol and ethylenediamine are used as crystallization control additives. It is demonstrated that formation of LFS from Fe(III) precursor is made possible only by the action of ethylenediamine. The obtained LFS particles are found to be predominantly monoc...

An Orthogonal Investigation of Sulfur Modified Li2FeSiO4 for Lithium Ion Batteries

The electrochemical performance of lithium iron silicate (Li2FeSiO4) as a cathode material for lithium ion batteries would be remarkably enhanced by selective substitution of 25% oxygen with 25% sulfur to form a Li2FeSiO3S compound based on the previous first-principles calculation. However, the experimental verification has little studied. In this work, the sol-gel precursor and solid-state reaction route was employed to prepare a series of carbon coated Li2FeSiO4 (Li2FeSiO4/C) composites with different sulfur modifications using 1-dodecanethiol as a sulfur source material. The effects of important preparation parameters including sucrose amount, sulfur content, sintering temperature and sintering time on the crystal structure, surface morphology and electrochemical performance of sulfur modified Li2FeSiO4/C composites were investigated through orthogonal design experiments. The results indicated that the significance of the influence on the capacity performance in decreasing order followed sulfur doping content > carbon content > sintering temperature > sintering time. The optimized synthesis conditions were determined to be carbon content of 50% and sulfur doping content of 20% with the sintering temperature at 650°C for 12 h.

Facile synthesis of hierarchical porous Li2FeSiO4/C as highly stable cathode materials for lithium-ion batteries

Lithium iron silicate has caught tremendous attentions as an appealing cathode for future lithium-ion batteries due to high capacity, low cost, and environmental friendliness, and its drawback of extremely low conductivity can be overcome efficiently through nanoarchitecture building. However, the construction of nanostructures always involves with expensive surfactants and complicated synthetic processes, which restrict these methods from large-scale production. In this paper, we develop a simple synthetic route to prepare hierarchical porous Li2FeSiO4/C. XRD, SEM, TEM, Raman, and N2 adsorption-desorption are employed to investigate its physical properties. Electrochemical tests reveal that the composite delivers a high specific capacity of 243.5 mAh g−1, superior rate capability, and excellent cycling performance with capacity retention of 95.2% after 200 cycles. The excellent electrochemical performance should be attributed to the unique structure in which hierarchical pores provides fast transport channels for lithium ions and interconnected carbon coating builds up conductive networks to enhance the conductivity of Li2FeSiO4. In addition, electrochemical impedance spectroscopy, ex situ SEM, and TEM are conducted to demonstrate its structural stability upon long-term cycling. In addition, the route described in this work is facile, cheap, and easily scaled-up, which allows for extension to the fabrication of other energy storage materials.

Charge-Discharge Property of Non-Stoichiometric Lithium Iron Silicate

Lithium iron silicate, Li2FeSiO4 exhibits a high theoretical capacity; however, the reversible charge-discharge reaction is proceeded actually only between the divalent and the trivalent of iron, and the excess lithium ion is not involved. In this study, we attempted to maximize the amount of iron redox couple by synthesizing non-stoichiometric compound compounds. In the highly non-stoichiometric composition, the charge-discharge capacity decreased due to the impurity phase was observed, but in the case of a small amount of lithium deficient composition the capacity increase and the improvement of rate capability were observed.

Preparation of 2% Sulfur Doped Li2FeSiO4/C with High Performance for Lithium Ion Batteries

Capability of obtaining high capacity by exchanging more than one lithium ion has made lithium iron silicate (Li2FeSiO4) very attractive to be an ideal cathode material for lithium ion batteries. However, high deintercalation voltage and large deformation are expected during the delithiation of the second lithium ion. In this work, the nominal 2% sulfur doped carbon coated Li2FeSiO4 composite was prepared by sol-gel precursor and solid-state reaction method. The results revealed that a well crystallized Pmn21 phase was formed, and no common impurities including iron oxide and lithium silicate were identified. The initial discharge capacity of 229.2 mAh·g-1 was obtained at 0.1C and room temperature, which corresponds to 1.38 moles lithium ions per formula unit. The capacity fluctuated to 185.6 mAh·g-1 after 50 cycles, achieving the capacity retention of 81%.

An Orthogonal Investigation in Structures and Performances of Vanadium Modified Li2FeSiO4

A series of different vanadium modified carbon coated lithium iron silicate (Li2Fe1-xVxSiO4/C) composites were prepared by sol-gel precursor and solid-state reaction using ammonium metavanadate (NH4VO3) as a vanadium source material. The effects of vanadium content, sucrose amount, sol-gel time and type of carboxylate acid on crystal structure, surface morphology, elemental composition and electrochemical performance of Li2Fe1-xVxSiO4/C composites were investigated through orthogonal design experiments. The initial discharge capacity of the nominal 35% vanadium modified Li2FeSiO4/C, prepared under the optimized conditions (sucrose amount of 80% and sol-gel time of 2 h using citric acid as a pH regulator), reached as high as 216.7 mAh/g (corresponding to 1.30 moles per formula unit) at 0.1C and room temperature. The composite exhibited a mixed Pmn21 and P21 phase with the coexistences of metallic iron (Fe) and lithium silicate (Li2SiO3) as the main impure phases.

Charge-Discharge Mechanism of Nonstoichiometric Lithium Iron Silicate

Li2FeSiO4 is one of interesting cathode material for lithium ion batteries. The two lithium composition per one iron atom and one poly-anion unit, in principle, exhibits a multi-electron charge transfer with Fe2+/Fe4+redox couple, which enables much high theoretical capacity of 331 mAhg–1. This value is approximately twice as high as the commercialized cathode materials, such as LiCoO2 and LiFePO4. However, the accessible charge-discharge capacity of Li2FeSiO4 was limited to one-electron reaction with Fe2+/Fe3+ redox couple in the early research 1 , 2 ). Recently, some research groups have reported more than one-electron reaction by using nanostructured materials3-6 ). Unfortunately, their high capacity is not stable during charge-discharge cycle and there is a large polarization at high voltage reaction. Therefore, the utilization of Fe2+/Fe3+ redox couple in this system is preferred for the stable battery operation. To maximize this Fe2+/Fe3+ redox reaction in lithium iron silicate system, we investigate composition dependency of Li x Fe2+ (4-x)/2SiO4 on charge-discharge capacity. Although the nonstoichiometric lithium iron silicate has been reported in the previous conference by the other group7 ), we cannot access enough data to discuss the possibility of the nonstoichiometric system. We synthesized various Li x Fe2+ (4-x)/2SiO4 samples and the charge-discharge measurements were performed. Their reaction mechanisms are discussed by using X-ray absorption spectroscopic data. Carbon-coated Li x Fe2+ (4-x)/2SiO4 samples were synthesized by the solid-state reaction. A given amounts of SiO2, FeC2O4・2H2O and Li2CO3 powders were weighed and 10 wt% of carbon (acetylene black) was added prior to mixing. These powders were mixed in a planetary ball mill at 400 rpm for 6 hours. The mixture was calcined at 700°C for 6 hours with a fixed Ar flux. The products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). For electrochemical measurements, Li x Fe2+ (4-x)/2SiO4 samples, carbon black, and polyvinylidene fluoride were mixed at a ratio of 80:10:10 with 1-methyl-2-pyrrolidone. The slurry was coated onto an aluminum foil current collector and dried in a vacuum oven at 80°C. For the charge-discharge measurements, the prepared electrode, lithium metal, and electrolyte-soaked separator (Celgard #2500) were constructed into a stainless steel flat cell. The electrolyte was a 1 mol dm–3 solution of LiPF6 in ethylene carbonate/ethyl methyl carbonate (3:7 volume ratio, Kishida). The cell construction process was performed in an Ar-atmosphere glove box. Galvanostatic charge-discharge measurements were performed at 55°C. For the prepared Li x Fe2+ (4-x)/2SiO4 samples, their crystal structures are almost similar from XRD measurements. Considering the Fe2+/Fe3+ redox reaction in Li x Fe2+ (4-x)/2SiO4 system, the maximum charge-discharge capacity is 203 mAh g-1 in Li1.33Fe1.335SiO4. However, the observed discharge capacity was smaller than the stoichiometric system. The diffusion path of lithium ion might be blocked by the occupation of iron ion in the lithium site. We will discuss the charge-discharge mechanism in this system by using X-ray absorption spectroscopy data. 1) A. Nyten, A. Abouimrane, M. Armand, T. Gustafsson, J.O. Thomas, Electrochem. Commun., 7, 156-160 (2005). 2) A. Nyten, S. Kamali, L. Haggstrom, T. Gustafsson, J.O. Thomas, J. Mater. Chem., 16, 2266-2272 (2006). 3) D. Rangappa, K.D. Murukanahally, T. Tomai, A. Unemoto, I. Honma, Nano Lett., 12, 1146-1151 (2012). 4) Z.X. Chen, S. Qiu, Y.L. Cao, J.F. Qian, X.P. Ai, K. Xie, X.B. Hong, H.X. Yang, J. Mater. Chem. A, 1, 4988-4992 (2013). 5) D.P. Lv, J.Y. Bai, P. Zhang, S.Q. Wu, Y.X. Li, W. Wen, Z. Jiang, J.X. Mi, Z.Z. Zhu, Y. Yang, Chem. Mat., 25, 2014-2020 (2013). 6) T. Masese, C. Tassel, Y. Orikasa, Y. Koyama, H. Arai, N. Hayashi, J. Kim, T. Mori, K. Yamamoto, Y. Kobayashi, H. Kageyama, Z. Ogumi, Y. Uchimoto, J. Phys. Chem. C, 119, 10206-10211 (2015). 7) K. Kam, A. Liivat, D. Ensling, T. Gustafsson, J. Thomas, ECS Meeting s, MA2009-02, 390 (2009).

An Orthogonal Investigation in Structures and Performances of Vanadium Modified Li2FeSiO4

A series of vanadium modified carbon coated lithium iron silicate (Li2Fe1-xVxSiO4/C) composites were prepared by sol-gel precursor and solid-state reaction using ammonium metavanadate (NH4VO3) as a vanadium source material. The effects of vanadium content (x), sucrose amount (y), sol-gel time (t) and type of carboxylate acid on crystal structure, surface morphology, elemental composition and electrochemical performance of Li2Fe1-xVxSiO4/C composites were investigated through orthogonal design experiments. The preparation parameters were optimized to be x=35%, y=80%, t=2 h and use of citric acid as a pH regulator. The initial discharge capacity of 216.7 mAh/g (corresponding to 1.30 moles per formula unit) was achieved at 0.1C and room temperature with the Li2Fe0.65V0.35SiO4/C composite synthesized under the abovementioned optimized condition. A mixed Pmn21 and P21 phase was identified as the main structure, while metallic iron and lithium silicate (Li2SiO3) as the impure phases. The elemental analysis result revealed that 10.7% carbon was introduced. The elemental vanadium was detected by performing energy dispersive X-ray spectroscopic (EDX) analysis.

Preparation of 2% Sulfur Doped Li2FeSiO4/C with High Performance for Lithium Ion Batteries

Capability of obtaining high capacity by exchanging more than one lithium ion has made lithium iron silicate (Li2FeSiO4) very attractive to be an ideal cathode material for lithium ion batteries. However, high deintercalation voltage and large deformation of Li2FeSiO4 are expected during the delithiation of the second lithium ion. In this work, the nominal 2% sulfur doped carbon coated Li2FeSiO4 composite was prepared by sol-gel precursor and solid-state reaction method. A well crystallized Pmn21 and P21 mixed phase was formed. The initial discharge capacity of 229.2 mAh/g was obtained at 0.1C and room temperature, which corresponds to 1.38 moles per formula unit. The capacity fluctuated to 185.6 mAh/g after 50 cycles, achieving the capacity retention of 81%.

Search of high-capacity cathode materials based on lithium–iron silicate compounds

The crystal structures of 197 lithium–silicate compounds have been analyzed using the method of crystal chemistry analysis (TOPOS software package). The compounds whose structures are characterized with a combination of high values of such parameters as the channel radius, stability, gravimetric capacity, and capacity per volume unit have been revealed: LiFeSiO4 (R3), Li4Fe2Si3O10 (C2/c), Li2FeSiO4 (Pc21n), and Li2FeSiO4 (C2221). It has been demonstrated that lithium–iron silicates of the monoclinic syngony have high values of the capacity per volume unit, as compared to those of the rhombic syngony. The structural stability of the Li2FeSiO4 (Pc21n) framework has been corroborated using the method of computer simulation within the scopes of the electron density functional theory. The obtained information could facilitate creation of novel cathode materials of high capacity and specific accumulated energy.

Carbon nanotube directed three-dimensional porous Li2FeSiO4 composite for lithium batteries

Lithium iron silicate (Li2FeSiO4) is capable of affording a much higher capacity than conventional cathodes, and thus, it shows great promise for high-energy battery applications. However, its capacity has often been adversely affected by poor reaction activity due to the extremely low electronic and ionic conductivity of silicates. Here, we for the first time report on a rational engineering strategy towards a highly active Li2FeSiO4 by designing a carbon nanotube (CNT) directed three-dimensional (3D) porous Li2FeSiO4 composite. As the CNT framework enables rapid electron transport, and the rich pores allow efficient electrolyte penetration, this unique 3D Li2FeSiO4-CNT composite exhibits a high capacity of 214 mAh·g−1 and retains 96% of this value over 40 cycles, thus, outstripping many previously reported Li2FeSiO4-based materials. Kinetic analysis reveals a high Li+ diffusivity due to coupling of the migration of electrons and ions. This research highlights the potential for engineering 3D porous structure to construct highly efficient electrodes for battery applications.