Electrochemical lithiation and delithiation of LiMnPO4: Effect of cation substitution

The electrochemical lithiation-delithiation reaction was examined for LiMnPO₄ in which different cations were substituted for part of Mn. The X-ray diffraction analysis indicated that LiMnPO₄ is tolerant, to some extent, to substitution of Mg²⁺, Ca²⁺ and Zr⁴⁺. The substitution of Mg²⁺ and/or Zr⁴⁺ led to an increased reversible capacity and a reduced polarization, whereas Ca²⁺ substitution had a detrimental effect on the electrochemical properties. The potential transient analysis showed that LiMn_0.88Mg_0.1Zr_0.02PO₄ has higher lithium diffusivities than pure LiMnPO₄, indicating facilitated diffusion kinetics in the substituted material. Upon the first charge-discharge cycle, LiMn_0.88Mg_0.1Zr_0.02PO₄ suffered less irreversible capacity loss when compared with LiMnPO₄, and smaller amounts of electrolyte salt-based species were detected on the electrode surface of LiMn_0.88Mg_0.1Zr_0.02PO₄.     

Synthesis and electrochemical characterization of LiCoxMn1−xPO4/C nanocomposites

LiCo_xMn_1−xPO₄/C nanocomposites (0 ≤ x ≤ 1.0) were prepared by a combination of spray pyrolysis at 300 °C and wet ball-milling followed by heat treatment at 500 °C for 4 h in 3% H₂ + N₂ atmosphere. X-ray diffraction analysis indicated that all samples had the single phase olivine structures indexed by orthorhombic Pmna. The lattice parameters linearly decreased with increasing cobalt content, which confirmed the existence of solid solutions. It was clearly seen from the scanning electron microscopy observation that the LiCo_xMn_1−xPO₄/C samples were agglomerates with approximately 100 nm primary particles. The LiCo_xMn_1−xPO₄/C nanocomposites were used as cathode materials for lithium batteries, and electrochemical performance was comparatively investigated with cyclic voltammetry and galvanostatic charge–discharge test using the Li?1 M LiPF₆ in EC:DMC = 1:1?LiCo_xMn_1−xPO₄/C cells at room temperature. The cells at 0.05 C charge–discharge rate delivered first discharge capacities of 165 mAh g⁻¹ (96% of theoretical capacity) at x = 0, 136 mAh g⁻¹ at x = 0.2, 132 mAh g⁻¹ at x = 0.5, 125 mAh g⁻¹ at x = 0.8 and 132 mAh g⁻¹ (79% of theoretical capacity) at x = 1.0, respectively. While the first discharge capacity increased with the cobalt content at high charge–discharge rates more than 0.5 C due to higher electronic conductivity of LiCoPO₄ in comparison with LiMnPO₄, the cycleability of cell became worse with increasing the amount of cobalt. The existence of Mn²⁺ seemed to enhance the cycleability of LiCo_xMn_1−xPO₄/C nanocomposite cathode.     

Cathodic performance of LiMn1−xMxPO4 (M = Ti, Mg and Zr) annealed in an inert atmosphere

To improve the cathodic performance of olivine-type LiMnPO₄, we investigated the optimal annealing conditions for a composite of carbon with cation doping. Nanocrystalline and the cation-doped LiMn_1−xM_xPO₄ (M = Ti, Mg, Zr and x = 0, 0.01, 0.05 and 0.10) was synthesized in aqueous solution using a planetary ball mill. The synthesis was performed at the fairly low temperature of 350 °C to limit particle size. The obtained samples except for the Zr doped one consisted of uniform and nano-sized particles. The performance of LiMnPO₄ was much improved by an annealing treatment between 500 and 550 °C with carbon in an inert atmosphere. A small amount of metal-rich phosphide (Mn₂P) was detected in the sample annealed at 900 °C. In addition, 1 at.% Mg doping for Fe enhanced the rate capability in our doped samples. The discharge capacity of LiMn_0.99Mg_0.01PO₄/C was 146 mAh/g at 0.1 mA/cm² and 125 mAh/g even at 2.0 mA/cm².     

Enhanced electrochemical stability of Al-doped LiMn2O4 synthesized by a polymer-pyrolysis method

LiAl_xMn_2−xO₄ samples (x = 0, 0.02, 0.05, 0.08) were synthesized by a polymer-pyrolysis method. The structure and morphology of the LiAl_xMn_2−xO₄ samples calcined at 800 °C for 6 h were investigated by powder X-ray diffraction and scanning electron microscopy. The results show that all samples have high crystallinity, regular octahedral morphology and uniform particle size of 100-300 nm. The electrochemical performances were tested by galvanostatic charge-discharge and cyclic voltammetry. The results demonstrate that the Al-doped LiMn₂O₄ can be very well cycled at an elevated temperature of 55 °C without severe capacity degradation. In particular, the LiAl_0.08Mn_1.92O₄ sample demonstrates excellent capacity retention of 99.3% after 50 cycles at 55 °C, confirming the greatly enhanced electrochemical stability of LiMn₂O₄ by a small quantity of Al-doping.     

Enhanced electrochemical performance of LiFePO4/C via Mo-doping at Fe site

A series of Mo-doped LiFe_1−3xMo_xPO₄/C (x = 0.000, 0.025, 0.050, 0.100, 0.150) cathode materials are synthesized by sol–gel method. XRD, ICP and Rietveld refinement results reveal that Mo doped in the crystal lattice and probably occupied Fe site. The structure benefits the transportation of Li⁺ and the diffusion of Li⁺ in the doped materials are enhanced remarkably than that of the undoped one, which leads to excellent electrochemical performance. The doped sample with x = 0.025 exhibits the best electrochemical performance, with the initial discharge capacity of 162.3 mAh g⁻¹ at 0.1 C rate.     

Aluminothermal synthesis and characterization of Li3V2−xAlx(PO4)3 cathode materials for lithium ion batteries

Monoclinic Li₃V_2−xAl_x(PO₄)₃ with different Al³⁺ doping contents (x = 0, 0.05, 0.08, 0.10 and 0.12) have been prepared by a facile aluminothermal reaction. Aluminum nanoparticles have been used as source for Al³⁺ and nucleus for Li₃V_2−xAl_x(PO₄)₃ nucleation as well as reducing agent in the aluminothermal strategy. The products were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and electrochemical methods. The XRD results show that the as-obtained Li₃V_2−xAl_x(PO₄)₃ has a phase-pure monoclinic structure, irrespective of the Al³⁺ doping concentration. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) results reveal that the charge-transfer resistance of the Li₃V₂(PO₄)₃ is reduced and the reversibility is enhanced after V³⁺ substituted by Al³⁺. In addition, The Li₃V_2−xAl_x(PO₄)₃ phases exhibit better cycling stability than the pristine Li₃V₂(PO₄)₃.     

Optimum synthesis of Li2Fe1−xMnxSiO4/C cathode for lithium ion batteries

Li₂Fe_1−xMn_xSi0₄/C cathode materials were synthesized by mechanical activation-solid-state reaction. The effects of Mn-doping content, roasting temperature, soaking time and Li/Si molar ratio on the physical properties and electrochemical performance of the Li₂Fe_1−xMn_xSi0₄/C composites were investigated. The materials were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), charge-discharge tests and AC impedance measurements. SEM images suggest that the morphology of the Li₂Fe_1−xMn_xSi0₄/C composite is sensitive to the reaction temperature. Samples synthesized at different temperatures have different extent of agglomeration. Being charged-discharged at C/32 between 1.5 and 4.8 V, the Li₂Fe_0.9Mn_0.1Si0₄/C synthesized at the optimum conditions shows good electrochemical performances with an initial discharge capacity of 158.1 mAh g⁻¹ and a capacity retention ratio of 94.3% after 30 cycles. AC impendence investigation shows Li₂Fe_0.9Mn_0.1SiO₄/C have much lower resistance of electrode/electrolyte interface than Li₂FeSiO₄/C.     

Mn influence on the electrochemical behaviour of Li3V2(PO4)3 cathode material

The role played by the substitution of Mn on the electrochemical behaviour of Li₃V₂(PO₄)₃ has been investigated. Independently of the synthesis route, the Mn doping improves the electrochemical features with respect to the undoped samples. Different reasons can be taken into consideration to explain the electrochemical enhancement. In the sol–gel synthesis the capacity slightly enhances due to the Mn substitution on both the V sites, within the solubility limit x = 0.124 in Li₃V_2−xMn_x(PO₄)₃. In the solid state synthesis the significant capacity enhancement is preferentially due to the microstructural features of the crystallites and to the LiMnPO₄ phase formation.     

Improving electrochemical performance of LiMnPO<Subscript>4</Subscript> by Zn doping using a facile solid state method

Olivine structure LiMnPO₄/C as cathode materials for Li-ion batteries were synthesized via a simple solidstate reaction. Improvement of the electrochemical performance of LiMnPO₄/C cathode material was realized significantly by the method of doping Zn. The obtained LiMn_0.95Zn_0.05PO₄/C electrode material was studied by the measurements of X-ray diffraction pattern, scanning electronic microscopy, electrochemical impedance spectroscopy and electrochemical performance. The results indicate that the LiMn_0.95Zn_0.05PO₄/C materials exhibit discharge specific capacity of 140.2 mA h g⁻¹ at 0.02 C rate and better rate capability. These excellent results are elucidated by EIS test, which showed that there was the decrease of charge transfer resistance and faster lithium-ion diffusion in LiMnPO₄/C cathode materials after Zn doping.     

Effects of neodymium aliovalent substitution on the structure and electrochemical performance of LiFePO4

LiFe_1−xNd_xPO₄/C (x = 0-0.08) cathode material was synthesized using a solid-state reaction. The synthesis conditions were optimized by thermal analysis of the precursor and magnetic properties of LiFePO₄/C. The structure and electrochemical performances of the material were studied using XRD, FE-SEM, EDS, electrochemical impedance spectroscopy and galvanostatic charge-discharge. The results show that a small amount of aliovalent Nd³⁺ ion-dopant substitution on Fe²⁺ ions can effectively reduce the particle size of LiFePO₄/C. Cell parameters of LiFe_1−xNd_xPO₄ (x = 0.04-0.08) were calculated, and the results showed that LiFe_1−xNd_xPO₄/C had the same olivine structure as LiFePO₄. LiFe_0.4Nd_0.6PO₄/C delivers the discharge capacity of 165.2 mAh g⁻¹ at rate of 0.2 C and the capacity retention rate is 92.8% after 100 cycles. Charge-transfer resistance decreases with the addition of glucose and Nd³⁺ ions. Poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) was synthesized and PZS nanorods were used as a carbon source to coat LiFePO₄. All of the results show that aliovalent doping substitution of Fe in LiFePO₄ is well tolerated.     

Solid solutions of mixed metal Mn3−xMgxFe4(PO4)6 orthophosphates: Colouring performance within a double-firing ceramic glaze

Solid solutions of mixed metal Mn_3−xMg_xFe₄(PO₄)₆ orthophosphates (x = 0, 0.5, 1, 1.5, 2, 2.5, and 3) were prepared for the first time (from coprecipitate powders calcined up to 1000 °C) and characterized by thermal analysis, XRD, SEM/EDX, UV-vis-NIR spectroscopy and colour measurements (CIE-L*a*b*). Orthophosphate (Mn,Mg)_3−yFe_4+z(PO₄)₆ solid solutions isotypic to Fe₇(PO₄)₆ structure (triclinic P-1(2) spatial group) formed successfully as the major crystalline phase within the studied range of compositions, accompanied only by variable quantities of α- and/or β-Mg₂P₂O₇ diphosphates as secondary phases. Noteworthy, the obtained solid solutions were tested as potential ceramic dyes and exhibited an interesting interaction upon enamelling within a double-firing ceramic glaze: considerable amounts of Fe segregated from the solid solutions to be stabilized as hematite particles (α-Fe₂O₃) in the ceramic glaze and conferring the glaze an intense dark-brown colouration, which was almost independent of the amount of Mg doping. Thus, the obtained solid solutions with a minimized Mn content (especially Mg₃Fe₄(PO₄)₆ composition, without Mn) could serve as low-toxicity Fe reservoirs to stabilize hematite in double-firing glazes and produce an interesting dark-brown colouration, being an alternative to other brown ceramic pigments containing hazardous metals (i.e. Cr, Ni, Zn, or Sb).     

Effects of tin doping on physicochemical and electrochemical performances of LiFe1−xSnxPO4/C (0 ≤ x ≤ 0.07) composite cathode materials

Nanocrystalline LiFe_1−xSn_xPO₄ (0 ≤ x ≤ 0.07) samples are synthesized using SnCl₄·5H₂O as dopant via an inorganic-based sol–gel method. The dependency of the physicochemical and electrochemical properties on the doping amount of tin are systemically worked out and regular changes are revealed. In the whole concentration range, the chemical valence of Fe²⁺ is not basically changed whereas tin is found in two different oxidation states, namely +2 and +4. The replacement of Fe²⁺ by supervalent Sn⁴⁺ would lead to electron compensation. Under the synergetic effects between the charge compensation and the crystal distortion, the electrical conductivities for the bulk samples first increase and then decrease with the increasing amount of Sn doping. Upon the doping amount, the apparent lithium-ion diffusion coefficient and the electrochemical performance also display the similar trends. The doping is beneficial to refine the particle size and narrow down the size distribution, however optimizing the doping amount is necessary. Compared with other samples, the sample with a doping amount of about 3 mol% delivers the highest capacities at all C-rates and exhibits the excellent rate capability due to the high electrical conductivity and the fast lithium-ion diffusion velocity.     

A facile strategy for recovering spent LiFePO4 and LiMn2O4 cathode materials to produce high performance LiMnxFe1-xPO4/C cathode materials

To successfully recycle spent LiFePO₄ and LiMn₂O₄ batteries and simultaneously produce high performances nano-LiMn_xFe_1-xPO₄/C powders, a mechanical activation-assisted method was effectively applied in the recycling process. The technique consists of the separation and purification of spent cathode materials, high-energy mechanical mixing of raw materials and a commonly used heat treatment processes. The structural and morphological characterization results indicate that nano-sized LiMn_xFe_1-xPO₄/C composites with uniform amorphous carbon coatings were successfully synthesized from spent LiFePO₄ and LiMn₂O₄ batteries. The electrochemical performance testing results show that the recovered nano-LiMn_xFe_1-xPO₄/C cathodes possess promising Li-ion storage properties. LiMn_0.5Fe_0.5PO₄/C displays high capacities of 143.2, 138.1, and 127.6 mAh g⁻¹ at 0.1, 1, and 2C rates, respectively. The obtained cathodes also show outstanding cycling stabilities of 98.47% and 97.58% for LiMn_0.5Fe_0.5PO₄/C and LiMn_0.8Fe_0.2PO₄/C after 100 cycles, respectively. This work indicates that recycling spent LiFePO₄ and LiMn₂O₄ to produce high-performance LiMn_xFe_1-xPO₄/C is a promising strategy for recycling spent LiFePO₄ and LiMn₂O₄ based lithium ion batteries (LIBs) in an efficient and environmentally friendly manner.     

Synthesis and characterization of carbon-coated Li3V2(PO4)3 cathode materials with different carbon sources

The carbon-coated monoclinic Li₃V₂(PO₄)₃ (LVP) cathode materials were synthesized by a solid-state reaction process under the same conditions using citric acid, glucose, PVDF and starch, respectively, as both reduction agents and carbon coating sources. The carbon coating can enhance the conductivity of the composite materials and hinder the growth of Li₃V₂(PO₄)₃ particles. Their structures and physicochemical properties were investigated using X-ray diffraction (XRD), thermogravimetric (TG), scanning electron microscopy (SEM) and electrochemical methods. In the voltage region of 3.0-4.3 V, the electrochemical cycling of these LVP/C electrodes all presents good rate capability and excellent cycle stability. It is found that the citric acid-derived LVP owns the largest reversible capacity of 118 mAh g⁻¹ with no capacity fading during 100 cycles at the rate of 0.2C, and the PVDF-derived LVP possesses a capacity of 95 mAh g⁻¹ even at the rate of 5C. While in the voltage region of 3.0-4.8 V, all samples exhibit a slightly poorer cycle performance with the capacity retention of about 86% after 50 cycles at the rate of 0.2C. The reasons for electrochemical performance of the carbon coated Li₃V₂(PO₄)₃ composites are also discussed. The solid-state reaction is feasible for the preparation of the carbon coated Li₃V₂(PO₄)₃ composites which can offer favorable properties for commercial applications.     

FTIR spectroscopy of a LiMnPO4 composite cathode

A Li_xMnPO₄ (x = 1.0–0.15) composite cathode was investigated by Fourier-transform infrared spectroscopy at different states of charge. Significant spectral changes of the PO₄³⁻ vibrations, which are correlated with the Jahn–Teller distortion of Mn³⁺ in MnPO₄ and the 3rd ionization potential of Mn, were observed upon electrochemical delithiation of LiMnPO₄. The presence of two sets of peaks observed in the series of delithiated Li_xMnPO₄ spectra is consistent with a two-phase process for delithiation. These results provide insight into the structural changes that occur during lithium extraction and insertion in LiMnPO₄.     

Synthesis of spinel LiMn2O4 with manganese carbonate prepared by micro-emulsion method

Micro-spherical particle of MnCO₃ has been successfully synthesized in CTAB-C₈H₁₈-C₄H₉OH-H₂O micro-emulsion system. Mn₂O₃ decomposed from the MnCO₃ is mixed with Li₂CO₃ and sintered at 800 °C for 12 h, and the pure spinel LiMn₂O₄ in sub-micrometer size is obtained. The LiMn₂O₄ has initial discharge specific capacity of 124 mAh g⁻¹ at discharge current of 120 mA g⁻¹ between 3 and 4.2 V, and retains 118 mAh g⁻¹ after 110 cycles. High-rate capability test shows that even at a current density of 16 C, capacity about 103 mAh g⁻¹ is delivered, whose power is 57 times of that at 0.2 C. The capacity loss rate at 55 °C is 0.27% per cycle.     

Studies on electrical properties of La0.8Sr0.2Ga0.8Mg0.2O2.80 (LSGM) and LSGM-SrSn1−xFexO3 (x = 0.8; 0.9) composites and their chemical reactivity

We report the electrical conductivity properties of solid-state synthesized perovskite-like La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.80 (LSGM) and LSGM-SrSn_1−xFe_xO₃ (x = 0.8; 0.9) composites. LSGM exhibits both bulk and grain-boundary contribution in the ac impedance plots. The grain-boundary conductivity (σ_gb) is slightly (≤half-order of magnitude) higher than that of the bulk oxide ion conductivity (σ_bulk). Powder XRD study reveals that no chemical reaction occurs between LSGM and SrSn_1−xFe_xO₃ (1:1 wt.%) at 1000 °C (48 h) and forms a single-phase perovskite-like compound at 1300 °C (48 h) in air, while in hydrogen atmosphere, at 800 °C for 48 h, a growth of LaSrGaO₄ and LaSrGa₃O₇ impurity phases and formation of metallic Fe was observed. The LSGM-SrSn_1−xFe_xO₃ (x = 0.8; 0.9) composites show a single or part of semicircle in air at low-temperature regime. The electrical conductivity of the composites were found to be much higher compared to pure LSGM and lower about an order of magnitude than those of pure Sn-doped SrFeO₃ perovskite.     

High performance Li3V2(PO4)3/C composite cathode material for lithium ion batteries studied in pilot scale test

Li₃V₂(PO₄)₃/C composite cathode material was synthesized via carbothermal reduction process in a pilot scale production test using battery grade raw materials with the aim of studying the feasibility for their practical applications. XRD, FT-IR, XPS, CV, EIS and battery charge-discharge tests were used to characterize the as-prepared material. The XRD and FT-IR data suggested that the as-prepared Li₃V₂(PO₄)₃/C material exhibits an orderly monoclinic structure based on the connectivity of PO₄ tetrahedra and VO₆ octahedra. Half cell tests indicated that an excellent high-rate cyclic performance was achieved on the Li₃V₂(PO₄)₃/C cathodes in the voltage range of 3.0-4.3 V, retaining a capacity of 95% (96 mAh/g) after 100 cycles at 20C discharge rate. The low-temperature performance of the cathode was further evaluated, showing 0.5C discharge capacity of 122 and 119 mAh/g at −25 and −40 °C, respectively. The discharge capacity of graphite//Li₃V₂(PO₄)₃ batteries with a designed battery capacity of 14 Ah is as high as 109 mAh/g with a capacity retention of 92% after 224 cycles at 2C discharge rates. The promising high-rate and low-temperature performance observed in this work suggests that Li₃V₂(PO₄)₃/C is a very strong candidate to be a cathode in a next-generation Li-ion battery for electric vehicle applications.     

Microwave solid-state synthesis and electrochemical properties of carbon-free Li3V2(PO4)3 as cathode materials for lithium batteries

Monoclinic Li₃V₂(PO₄)₃ can be rapidly synthesized at 750 °C for 5 min (MW5m) by using microwave solid-state synthesis method. The refined cell parameters and atomic coordination of the sample MW5m show some deviations compared with those of the sample synthesized in conventional solid-state synthesis method, especially the coordinate of Li atoms. Compared with the electrochemical properties of the carbon-coating sample Li₃V₂(PO₄)₃, the carbon-free sample MW5m presents well electrochemical properties. In the cut-off voltage of 3.0-4.3 V, MW5m sample presents a specific charge capacity of 132 mAh g⁻¹, almost equivalent to the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹), and specific discharge capacity of 126.4 mAh g⁻¹. In the cut-off voltage of 3.0-4.8 V, MW5m shows an initial specific discharge capacity of 183.4 mAh g⁻¹ at 0.1 C, near the theoretical discharge capacity. In the cycle performance, the capacity fade of Li₃V₂(PO₄)₃ is dependent on the cut-off voltage and the preparation method, more capacity lost at relatively higher charge/discharge voltage. The reasons for the excellent electrochemical properties of Li₃V₂(PO₄)₃ rapidly synthesized in microwave field are discussed in detail.     

Three-dimensional (3D) LiMn0.8Fe0.2PO4 nanoflowers assembled from interconnected nanoflakes as cathode materials for lithium ion batteries

Three-dimensional (3D) olivine LiMn_0.8Fe_0.2PO₄ nanoflowers constructed by two-dimensional (2D) nanoflakes have been successfully synthesized through an easy liquid phase method. Hierarchical LiMn_0.8Fe_0.2PO₄/C could be easily formed via a liquid coating technology and subsequent calcination treatment. When acting as cathode materials for lithium ion batteries, the LiMn_0.8Fe_0.2PO₄/C nanoflowers show excellent rate performance and cycle stability. The unique flower-like hierarchical structured LiMn_0.8Fe_0.2PO₄ and thin carbon coating outside make this composite a promising candidate as cathode materials for lithium ion batteries.