Surface structure evolution of cathode materials for Li-ion batteries

Lithium ion batteries are important electrochemical energy storage devices for consumer electronics and the most promising candidates for electrical/hybrid vehicles. The surface chemistry influences the performance of the batteries significantly. In this short review, the evolution of the surface structure of the cathode materials at different states of the pristine, storage and electrochemical reactions are summarized. The main methods for the surface modification are also introduced.     

Preparation and characteristics of Sb-doped LiNiO2 cathode materials for Li-ion batteries

Compounds LiNi_1−xSb_xO₂ (x=0, 0.1, 0.15, 0.2, 0.25) were synthesized by the two-step calcination method. The structural and morphological properties of the products were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD analysis confirms that the uniform solid solution has been formed in the as-prepared compounds without any impurities. It is shown that the crystal lattice parameters (a, c) of the Sb-doped compounds are bigger than those of pure LiNiO₂ and the Sb-doped compound with x=0.2 consists of spherical-like nanoparticles with a mean grain size of 50 nm. The electrochemical performances of as-prepared samples were studied via galvanostatic charge-discharge cycling tests. The compound with x=0.2 exhibits excellent capacity retention during the charge-discharge processes due to its reinforced structural stability, and a discharge capacity of 102.4 mAh/g is still obtained in the voltage range of 2.5-4.5 V after 20 cycles. Thermal analysis further confirms that the structural stability of LiNi_0.8Sb_0.2O₂ is superior to that of pure LiNiO₂.     

Pre-conditioned Li-rich layered cathode material for Li-ion battery

Solid solution material Li_1.2Ni_0.16Co_0.08Mn_0.56O₂ (0.5Li₂MnO₃?0.5LiNi_0.4Co_0.2Mn_0.4O₂) is obtained through rheological phase method and further treated in ammonium persulfate solution. The post-treatment significantly decreases the charging capacity above 4.5 V and enhances the columbic efficiency in the initial cycle. Along with the higher efficiency, the cycling stability and the rate capability both get improved. The improvement mechanism is investigated in terms of XRD, XPS, Raman spectrometry, and ICP-AES. The results confirm that (NH₄)₂S₂O₈ treatment leads to Li⁺ removal from Li₂MnO₃ component while the layered structure of the solid solution phase is well maintained. After being treated in 30% (NH₄)₂S₂O₈ solution, 95% columbic efficiency is observed on Li_1.2Ni_0.16Co_0.08Mn_0.56O₂ in the first cycle and it also shows a near 200 mAh g^?1 capacity at 4C current rate.     

Preparation and characterization of LiFePO4 /Ag composite for Li-ion batteries

Ag is introduced to the surface of LiFePO₄ prepared from liquid phase and heated at different temperatures. SEM, XRD and electrochemical characterization are used to compare the differences between LiFePO₄ and LiFePO₄/Ag. The results are discussed from the viewpoint of impedance of the cathode composite. With increase of the particle size, Ag shows stronger function as conductor in the composites.     

Synthesis and electrochemical properties of nanostructured Li2FeSiO4/C cathode material for Li-ion batteries

Nanostructured Li₂FeSiO₄/C was synthesized by high-energy ball-milling and the amorphous citrate-assisted techniques. Similar redox behaviour is observed for samples prepared by the amorphous citrate-assisted route followed by a 4 h heat treatment: 0.3 V polarization and more sloping behaviour was observed when cycling between 2.0 V and 3.7 V at 60 °C; lower capacity fade is also observed compared to Li₂FeSiO₄/C prepared by the solid-state reaction technique. A discharge capacity of 102 mA h g^− 1 is obtained for samples prepared by the high-energy ball-milling method, while capacities decrease from 95 to 77 mA h g^− 1 using the amorphous citrate method for heat-treatment times increasing successively from 4 h to 18 h.     

Structural and electrical studies of LiMnVO4 cathode material for rechargeable lithium batteries

The lithium manganese vanadate (LiMnVO₄) cathode material was synthesized by using sol?Cgel method. The thermal behavior of the material has been examined by thermogravimetric and differential thermal analysis. The structure of LiMnVO₄ compound was studied by the Rietveld refined X-ray diffraction technique. Raman spectra showed that the local environment including VO₄ tetrahedra and LiO₆ octahedra as vibrational local units. X-ray photoelectron spectroscopy studies of synthesized LiMnVO₄ powder indicate that the oxidation states of manganese and vanadate are +2 and +5, respectively. The ionic conductivity of the sample is found to be 2.7?×?10^?5 Scm^?1 at 300?°C. The temperature dependent conductivity was conformed from the Arrhenius relation and the activation energy is found to be 0.3?eV. Dielectric spectra showed the decrease in dielectric constant with increase in frequency. Dielectric loss spectra reveal that dc conduction contribution predominates in the compound.     

Preparation of Li3V2 (PO4)3/LiFePO4 composite cathode material for lithium ion batteries

A novel process was attempted for synthesis of Li₃V₂ (PO₄)₃/LiFePO₄ composite cathode material via loading nano-LiFePO₄ (LFP) powders onto the outside of micrometer-size spherical Li₃V₂ (PO₄)₃ (LVP). The precursor of nano-LFP and LVP were synthesized via “controlled crystallization” and “spray drying” techniques, respectively. The X-ray diffraction characterization, scanning electron microscopy, and electrochemical performance measurements were studied. The results indicated that the prepared Li₃V₂(PO₄)₃/LiFePO₄ (LVP/LFP) composite material exhibited better discharging capacity at high C rate and at low temperature than that of LFP and bulk LVP/LFP. This can pave an effective way to improve the performance of LFP at high C rate and at low temperature.     

New-type low-cost cathode materials for Li-ion batteries: Mikasaite-type Fe2(SO4)3

In the present work, a new-type low-cost lithium ion battery cathode material, the Mikasaite-type iron sulfate, has been studied. It can be prepared by heating the water-containing iron sulfate raw chemicals in air atmosphere. The experimental results have shown that the oxidation and the reduction peaks are 3.92 and 3.37 V in the cyclic voltammogram, respectively, when the scanning rate is 0.05 mV s^?1. The galvanostatic measurements have explored that the voltage plateau during charging is slightly less than 3.70 V and the discharge voltage plateau is 3.40 V for the first cycle and 3.50 V for the following cycles at 0.1 C rate. The discharge capacity in the first cycle can reach 116 mAh g^?1, about 87 % of the theoretical capacity (134 mAh g^?1). It is believed that the product in the fully discharged state is Li₂Fe₂(SO₄)₃. However, the insertion reaction is reversible only for the second lithium ion. During cycling, the reversible capacity remains about 60 mAh g^?1. Further capacity fade is not found in the 20 discharge–charge cycles. The electrochemical impedance measurements have shown that there are two compressed semicircles in the Nyquist plots and a Warburg impedance in the low-frequency domain. The high-frequency semicircle is related with the electrode’s structural factor and the intermediate-frequency semicircle corresponds to the charge-transfer process.     

Mössbauer study on LiFePO4 cathode material for lithium ion batteries

Crystalline LiFePO₄ has been synthesized using solid-state, spray pyrolysis, and wet chemical methods. The crystal parameters were obtained from Rietveld’s refinement methods of the X-ray diffraction patterns. A detailed investigation of the Fe valency carried out using Mössbauer spectroscopy at room temperature indicates that Fe is predominantly present in its bivalent state.     

Synthesis of nanosized Si composite anode material for Li-ion batteries

A simple method was proposed to prepare nanosized Si composite anode materials for lithium-ion (Li-ion) batteries. The preparation started with the shock-type ball milling of silicon in liquid media of polyacrylonitrile (PAN)/dimethylformamide (DMF) solution, forming slurry where the nano-Si particles were uniformly dispersed, followed by the drying of the slurry to remove DMF. The nanosized Si composite anode material was obtained after the pyrolysis of the mixture at 300 °C where the pyrolyzed PAN provided a conductive matrix to relieve the morphological change of Si during cycling. As-prepared composite presented good cyclability for lithium storage. The proposed process paves an effective way to prepare high performance Si, Sn, Sb and their alloys based composite anode materials for Li-ion batteries.     

Lithium-enriched polypyrrole as a prospective cathode material for Li-ion cells

Lithium substitution in polypyrrole can be accomplished by a variety of approaches and the present work introduces one of the cost-effective techniques using a relatively less expensive lithium salt, n-butyllithium in hexanes (n-BuLi), as the dopant. Chemical oxidative polymerization method is employed to synthesize polypyrrole (PPy) using anhydrous ferric chloride as the oxidant and it is dedoped using NH₄OH solution in the fully reduced state. The dedoped polypyrrole is treated with n-butyllithium in hexanes (n-BuLi) in an argon-filled glove box to get the lithiated form of polypyrrole (PPyL) and the concentration of n-BuLi is varied to improve metalation. The lithiated PPy is characterized by FTIR spectroscopy, XRD, FESEM, and TEM techniques to understand the structural and the morphological details. The lithium content in the lithiated samples is estimated using ICP-AES analysis. The thermal studies using the TGA technique show that the lithiated polypyrrole has good thermal stability. Coin cells are assembled in the argon-filled glove box using Li-substituted polypyrrole as the cathode, lithium metal foil as the anode, and lithium hexafluorophosphate (LiPF₆) as the electrolyte. The assembled cells are electrochemically characterized using cyclic voltammetry and charge–discharge cycling techniques and it is seen that the Li-substituted polypyrrole-based Li-ion cells are electrochemically active.     

Sonochemical synthesis of LiNi0.5Mn1.5O4 and its electrochemical performance as a cathode material for 5 V Li-ion batteries

LiNi_0.5Mn_1.5O₄ was synthesized as a cathode material for Li-ion batteries by a sonochemical reaction followed by annealing, and was characterized by XRD, SEM, HRTEM and Raman spectroscopy in conjunction with electrochemical measurements. Two samples were prepared by a sonochemical process, one without using glucose (sample-S1) and another with glucose (sample-S2). An initial discharge specific capacity of 130 mA h g⁻¹ is obtained for LiNi_0.5Mn_1.5O₄ at a relatively slow rate of C/10 in galvanostatic charge–discharge cycling. The capacity retention upon 50 cycles at this rate was around 95.4% and 98.9% for sample-S1 and sample-S2, respectively, at 30 °C.     

Electrochemical performance of surfactant-processed LiFePO4 as a cathode material for lithium-ion rechargeable batteries

This study focuses on the effect of addition of surfactant as a dispersing agent during vibratory ball milling of LiFePO₄ (LFP) precursor materials on the electrochemical performance of solid-state reaction synthesized LFP for lithium-ion battery cathode material. LFP particles formed after calcinations of ball milled LFP precursors (Li₂CO₃, FeC₂O₄, and NH₄H₂PO₄) showed better size uniformity, morphology control, and reduced particle size when anionic surfactant (Avanel S-150) was used. The specific surface area of LFP particles increased by approximately twofold on addition of surfactant during milling. These particles showed significantly enhanced cyclic performance during charge/discharge due to a reduced polarization of electrode material. Electrodes fabricated from LFP particles by conventional milling process showed a 22 % decrease in capacity after 50 cycles, whereas the performance of electrode prepared by surfactant processed LFP showed only 3 % loss in capacity. The LFP particles were characterized using XRD, FE-SEM, particle size distribution, density measurement, and BET-specific surface area measurement. Electrochemical impedance spectra and galvanostatic charge/discharge test were performed for the electrochemical performance using coin-type cell.     

Preparation and characterization of Li3V2(PO4)3/MWCNTs cathode material for lithium-ion batteries

The Li₃V₂(PO₄)₃/multiwalled carbon nanotubes (LVP/MWCNTs) composite is successfully synthesized by a sol?Cgel route using oxalic acid as the chelating reagent. Its structure and physicochemical properties are investigated using X-ray diffraction, field-emission scanning electron microscopy, and electrochemical methods. LVP particles are well mixed with MWCNTs, and most of them are around 100?nm. The galvanostatic charge?Cdischarge tests show that LVP/MWCNTs electrode owns an initial discharge capacity of 126?mAh?g^?1 at 0.5 C with capacity retention of 94% during the 100th cycle in the voltage range of 3.0?C4.3?V. A superior rate capability is also achieved, e.g., exhibiting discharge capacities of 75 and 58?mAh?g^?1 at high C rates of 10 and 15 C, respectively.     

Structural, magnetic and redox properties of a new cathode material for Li-ion batteries: the iron-based metal organic framework

The iron-based metal organic framework (MOF) presently studied is the first example of MOF showing a reversible electrochemical Li insertion with a very good cycling life. Its potential application as a cathode material in Li-ion battery is nevertheless curbed by a rather poor capacity of 70 mAh/g. To figure out the origin of this limited insertion, first-principles density functional theory (DFT)+U calculations were performed. The results show that Fe^III{OH(BDC)} is a weak anti-ferromagnetic charge transfer insulator at T = 0 K with iron in the high-spin S = 5/2 state. In agreement with the absence of electronic de-localisation along the inorganic chains, lithium insertion leads to the stabilisation of a Fe^II/Fe^III mixed-valence state of class I or II in the Robin–Day classification, whatever the Li sites considered in the calculations. Among these Li sites, the most probable site I (OH-Li) and site II (O=CO-Li) are shown to induce incompatible structural changes on the reduced Li_0.5Fe{OH(BDC)} form that could be at the origin of the small capacity measured for this compound. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

First principles study on Mn-doped LiFePO4 as cathode material for rechargeable lithium batteries

The electronic structure and diffusion energy barriers of Li ions in pure and Mn-doped LiFePO4 have been studied using density functional theory （DFT）. The results demonstrate clearly that Fe - O covalent bond is weaker than P- O covalent bond. Pure LiFePO4 has band gap of 0.56 eV and diffusion energy barrier of 2.57 eV for Li ions, while the dopant has small band gap of 0.25 eV and low diffusion energy barrier of 2.31 eV, which indicates that the electronic and ionic conductivity of LiFePO4 have been improved owing to doping.     

First principles study on Mn-doped LiFePO4 as cathode material for rechargeable lithium batteries

The electronic structure and diffusion energy barriers of Li ions in pure and Mn-doped LiFePO4 have been studied using density functional theory(DFT).The results demonstrate clearly that Fe-O covalent bond is weaker than P-O covalent bond.Pure LiFePO4 has band gap of 0.56 eV and diffusion energy barrier of 2.57 eV for Li ions,while the dopant has small band gap of 0.25 eV and low diffusion energy barrier of 2.31 eV,which indicates that the electronic and ionic conductivity of LiFePO4 have been improved owing to doping.     

Nanoscopic scale studies of LiFePO4 as cathode material in lithium-ion batteries for HEV application

We present a review of the structural properties of LiFePO₄. Depending on the mode of preparation, different impurities can poison this material. These impurities are identified and a quantitative estimate of their concentrations is deduced from the combination of X-ray diffraction analysis, Fourier transform infrared spectroscopy, Raman spectroscopy, and magnetic measurements. An optimized preparation provides samples with carbon-coated particles free of any impurity phase, insuring structural stability and electrochemical performance that justify the use of this material as a cathode element a new generation of lithium secondary batteries.     

Ex situ FTIR spectroscopy study of LiVPO4F as cathode material for lithium-ion batteries

The LiVPO₄F as cathode material for lithium-ion batteries was synthesized through two steps of solid-state reactions and investigated by ex situ Fourier transform infrared (FTIR) spectroscopy for the initial charge and discharge cycle. The characterization of the effect on the structure of the LiVPO₄F in the process of lithium-ion insertion/extraction at a molecular level by ex situ FTIR spectroscopy is helpful for the mechanism research for lithium-ion insertion/extraction and the improvement of the performance of lithium-ion batteries. In the process of the initial cycle, new bands of VPO₄F appear in the charge and the featured bands of LiVPO₄F reappear in the discharge. In this paper, ex situ FTIR spectra indicates that the structure of the LiVPO₄F in the process of lithium-ion insertion/extraction is almost not affected, which clearly states that the LiVPO₄F possesses stable structure as cathode material. Consequently, the LiVPO₄F might be expected as a potential cathode replacement for commercial lithium-ion batteries.