Nanostructures and lithium electrochemical reactivity of lithium titanites and titanium oxides: A review

Being inherently safe and chemically compatible with the electrolyte, titanium oxide-based materials, including both Li-titanites and various TiO₂ polymorphs, are considered alternatives to carbonaceous anodes in Li-ion batteries. Given the commercial success of the spinel lithium titanites, TiO₂ polymorphs, in particular in nanostructured forms, have been fabricated and investigated for the applications. Nanostructuring leads to increased reaction areas, shortened Li⁺ diffusion and potentially enhanced solubility/capacity. Integration with an electron-conductive second phase into the TiO₂-based nanostructures eases the electron transport, resulting in further improved lithium electrochemical activity and the overall electrochemical performance. This paper reviews structural characteristics and Li-electrochemical reactivity, along with synthetic approaches, of nanostructures and nano-composites based on lithium titanites and TiO₂ polymorphs that include rutile, anatase, bronze and brookite.     

Improved electrochemical performance of modified natural graphite anode for lithium secondary batteries

Modified natural graphite is synthesized by surface coating and graphitizing process on the base of spherical natural graphite. The modified natural graphite is examined discharge capacity and coulombic efficiency for the initial charge–discharge cycle. Modification process results in marked improvement in electrochemical performance for a larger discharge capacity and better coulombic efficiency. The mechanism of the enhancement are investigated by means of X-ray powder diffraction, scan electron microscopy, and physical parameters examination. The proportion of rhombohedral crystal structure was reduced by the heat treatment process. The modified natural graphite exhibits 40 mAh g⁻¹ reduction in the first irreversible capacity while the reversible capacity increased by 16 mAh g⁻¹ in comparison with pristine graphite electrode. Also, it has an excellent capacity retention of ∼94% after 100 cycles and ∼87% after 300 cycles.     

Synthesis of SnO2 nanorods and hollow spheres and their electrochemical properties as anode materials for lithium ion batteries

Abstract

SnO₂ nanorods and hollow spheres were conducted via a surfactant assisted hydrothermal reaction with the hydrothermal temperature. The crystalline structure and morphologies of the as prepared samples were characterised by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results indicate that the products are hollow spheres with diameters of approximately 400–800 nm and shell thicknesses of 60–70 nm via hydrothermal treating at 160°C for 42 h and rod-like nanostructures with diameters of ～30 nm and lengths of 100–300 nm via hydrothermal treating at 200°C for 42 h respectively. The as prepared samples were used as anode materials for lithium ion battery, whose charge–discharge properties and cycle performance were examined. The results show that the initial discharge capacities of SnO₂ hollow spheres and SnO₂ nanorods samples are 1303 and 1426 mA h g^?1 at 0·2C rate, and still retain charge capacities of 518 and 578 mA h g^?1 respectively. Its good cycling behaviour and charge capacities make it a promising cathode material for advanced electrochemical devices for lithium ion batteries.     

Characteristics of carbon-coated graphite prepared from mixture of graphite and polyvinylchloride as anode materials for lithium ion batteries

A carbon-coated graphite is investigated as the negative electrode for Li-ion batteries. The carbon-coated graphite particles are prepared by simple heat-treatment of mixtures of graphite and poly(vinyl chloride), PVC, at 800–1000°C in an argon flow. The carbon coating reduces significantly the initial irreversible capacity of the graphite in a propylene carbonate-based electrolyte, by suppressing the solvated lithium ion intercalation, and also improves the initial charge–discharge coulombic efficiency. By carbon coating, the specific surface area of graphite particles is greatly increased. These findings can be explained by assuming that a turbostratic structure of PVC-carbon resists irreversible side-reactions which are controlled predominantly by active, edge surface sites.     

Synthesis and electrochemical characterization of amorphous Li-Fe-P-B-O cathode materials for lithium batteries

Various types of amorphous Li-Fe-P-B-O were investigated for use as a cathode material. The samples were prepared by the melt quench technique with a single roll. The samples had monotonically decreasing charge-discharge profiles peculiar to amorphous materials, and were between 76 and 119 mAh g⁻¹. The electrochemical performance of these amorphous Li-Fe-P-B-O materials improved with increasing boron ratio, improving from 85 to 119 mAh g⁻¹. Although the redox potentials of amorphous Li-Fe-P-O did not shift with changing P/Fe ratio, those of amorphous Li-Fe-P-B-O did shift with changing B/P ratio. The redox potentials of the Li-Fe-P-B-O in the ratios of 2/1/2/0, 2/1/1/1, 2/1/0.5/1.5, and 2/1/0/2 were approximately 3.1, 2.9, 2.6, and 2.2 V vs. Li/Li⁺, respectively. This demonstrates that the redox potentials of amorphous polyanionic materials can be tuned by adjusting the electronegativity of the glass former, such as phosphorus and boron.     

Improving electrochemical properties and structural stability of lithium manganese silicates as cathode materials for lithium ion batteries via introducing lithium excess

The serious capacity decay caused by structural amorphization is still a major issue for polyanion-type lithium manganese silicates (Li₂MnSiO₄) as cathode material for lithium ion batteries. In this work, a new strategy for alleviating the structural instability via the introduction of excess lithium into the host crystal lattice is provided. A comprehensive study demonstrates that the required energy for the extraction/insertion of lithium ions into host crystal lattice was decreased as a result of changed local environment of cations in the compound after the excess lithium occupancy in lattice. Importantly, it was found that Li-rich samples deliver higher reversible capacity and increased average potential than pristine sample, indicating the improved energy density of polyanion-type Li_{2 + 2x}Mn_{1 − x}SiO₄/C_. Additionally, the structure of Li2.2 sample was kept intact, while the Li2.0 sample was transformed to amorphous state at 200 mA h g⁻¹ during the initial charging process by controlling the charge cut-off potential. As expected, the introduction of a certain amount of excess lithium into Li₂MnSiO₄ is explored as a route to achieving increased capacity with more movable lithium, while maintaining its structural stability and cyclic stability.     

Effect of covalently bonded polysiloxane multilayers on the electrochemical behavior of graphite electrode in lithium ion batteries

Polysiloxane multilayers were covalently bonded to the surface of natural graphite particles via diazonium chemistry and silylation reaction. The as-prepared graphite exhibited excellent discharge–charge behavior as negative electrode materials in lithium ion batteries. The improvement in the electrochemical performance of the graphite electrodes was attributed to the formation of a stable and flexible passive film on their surfaces. It was also revealed that the chemical compositions of the multilayers exerted influence on the electrochemical behavior of the graphite electrodes. The result of this study presents a new strategy to the formation of elastic and strong passive film on the graphite electrode via molecular design. Owing to the diversity of polysilxoane multilayers, this method also enables researchers to control the surface chemistries of carbonaceous materials with flexibility.     

Surface structure and electrochemical characteristics of plasma-fluorinated petroleum cokes for lithium ion battery

Plasma-fluorination of petroleum coke and those heat-treated at 1860, 2300 and 2800 °C (abbreviated to PC, PC1860, PC2300 and PC2800) was conducted for 15, 30 and 60 min using CF₄ gas at 90 °C. Fluorine contents obtained by elemental analysis were negligible except PC fluorinated for 60 min (0.7 at.%). Fluorine concentration on the surface decreased with increasing heat-treatment temperature of petroleum coke, i.e. from PC to PC2800 when plasma-fluorination was made for 30 and 60 min. Transmission electron microscopic observation revealed that the closed edges of PC2800 were destroyed and opened by plasma-treatment. Plasma-fluorination increased surface disorder of heat-treated petroleum cokes, however, slightly reduced surface areas. These surface structure changes increased first coulombic efficiencies of PC2300 and PC2800 by 6–8 and 8–10% at both 60 and 150 mA g⁻¹, respectively.     

Electrochemical characteristics of graphite,coke and graphite/coke hybrid carbon as negative electrode materials for lithium secondary batteries

Electrochemical characteristics of various carbon materials have been investigated for application as a negative electrode material in lithium secondary batteries with long cycle life. Natural graphite electrodes show large discharge capacity in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC). However, their charge/discharge performance is largely influenced by electrolytes. There is a possibility that a rapid rise in the discharge potential of the natural graphite electrode at the end of the discharge would cause a side reaction such as decomposition of the electrolyte because of an unequal reaction over an electrode. In order to improve the cycle performance of natural graphite electrodes, mixtures of graphite and coke electrodes are prepared by adding coke to natural graphite. It is found that the mixture of graphite and coke electrode shows a better cycle performance than that of a natural graphite or coke electrode. The deterioration ratio of the mixture of graphite and coke negative electrode measured by using AA-type test cells is 0.057%/cycle up to the 500th cycle. The mixture of graphite and coke is a promising material for a negative electrode in long-life lithium secondary batteries for energy storage systems because of its excellent cycle performance and large discharge capacity.     

Surface structure and electrochemical properties of surface-fluorinated petroleum cokes for lithium ion battery

Surface structure and electrochemical behavior of surface-fluorinated petroleum coke samples (original petroleum coke and those heat-treated at 1860, 2300 and 2800 °C, abbreviated to original PC, PC1860, PC2300 and PC2800, respectively) have been investigated. Surface fluorination of petroleum cokes by elemental fluorine reduced surface oxygen. Surface areas of fluorinated petroleum cokes were nearly the same as those of non-fluorinated ones or only slightly increased by fluorination except original PC fluorinated at 300 °C. Total meso-pore volumes of fluorinated samples showed the same trend. The charge capacity of non-fluorinated petroleum coke was increased by heat-treatment at 2300 and 2800 °C. However, the first coulombic efficiency was the highest, 90–89% in PC1860, decreasing to 72–70 and 65–64% for PC2300 and PC2800, respectively. It is noted that first coulombic efficiencies were increased by 12–18% for PC2300 and PC2800 fluorinated at 300 °C.     

Interfacial reactions between graphite electrodes and propylene carbonate-based solutions: Electrolyte-concentration dependence of electrochemical lithium intercalation reaction

This study examines the electrochemical reactions occurring at graphite negative electrodes of lithium-ion batteries in a propylene carbonate (PC) electrolyte that contains different concentrations of lithium salts such as, LiClO₄, LiPF₆ or LiN(SO₂C₂F₅)₂. The electrode reactions are significantly affected by the electrolyte concentration. In concentrated solutions, lithium ions are reversibly intercalated within the graphite to form stage 1 lithium–graphite intercalation compounds (Li–GICs), regardless of the lithium salt used. On the other hand, electrolyte decomposition and exfoliation of the graphene layers occur continuously in the low-concentration range. In situ analysis with atomic force microscopy reveals that a thin film (thickness of ∼8 nm) forms on the graphite surface in a concentrated solution, e.g., 3.27 mol kg⁻¹ LiN(SO₂C₂F₅)₂/PC, after the first potential cycle between 2.9 and 0 V versus Li⁺/Li. There is no evidence of the co-intercalation of solvent molecules in the concentrated solution.     

Preparation and electrochemical characteristics of quaternary Li-Mn-V-O spinel as the positive materials for rechargeable lithium batteries

Quaternary Li-Mn-V-O spinels were prepared by heating mixtures of MnCO₃, V₂O₅ and LiNO₃ at 700 °C for 36 h in air. The spinel oxides were characterized by X-ray diffraction, FT-IR spectroscopic, density and electrochemical measurements. The unit cell volume in a cubic cell increased with an increase in V content in LiV_xMn_{2 − x}O₄ (x = 0–0.2), while the amounts of lithium intercalated into the spinels in a high potential region around 4 V versus Li/Li⁺ decreased considerably with an increase in V content. Furthermore, the thermodynamics and kinetics of the lithium intercalation process into the spinel Li_nV_xMn_{2 − x}O₄ were studied. The standard free energies of lithium intercalation into the spinels were −291 kJ/mol for x = 0 and −264 kJ/mol for x = 0.05 at n = 0–1 and 25 °C. The chemical and self-diffusion coefficients for lithium in Li_nV_xMn_{2 − x}O₄ spinels were measured as functions of the n and x values by a current-pulse relaxation method. The diffusion coefficients in the spinel with x = 0.05 in the n-value range from 0.3 to 1 were about one order of magnitude lower than that in Li_nMn₂O₄.     

Factors affecting the electrochemical reactivity vs. lithium of carbon-free LiFePO4 thin films

The electrochemical stability of various current collector materials such as Si, Pt, 304 stainless steel, Ti, Al exposed to the most common lithium-ion electrolyte salts (LiPF₆, LiBF₄, LiAsF₆, LiTFSI, LiClO₄) have been herein investigated. For applied potentials greater than 3 V, the acidic fluorine-based electrolytes were shown to be the most corrosive. Consequently, aqueous and non-aqueous electrolytes (1 M LiNO₃/H₂O vs. 1 M LiClO₄/EC-DMC) were successfully applied to study the electrochemical properties of C-free LiFePO₄ thin films whose redox potential is near 3.5 V vs. Li⁺/Li⁰. Using aqueous electrolyte has resulted in a lowering of both cell resistance and interfacial charge transfer resistance by almost one order of magnitude, hence enabling to considerably increase the electrochemical capacity of our LiFePO₄ thin films. Besides, we unravel the importance of the mechanical strains at the substrate/LiFePO₄ thin film interface on the film textural, structural modification and electrochemical stability upon cycling.     

Review of doping and surface coating of cathode materials for lithium ion batteries

随着国家政策对电动汽车的支持力度不断加大,锂离子电池的电化学性能瓶颈愈发凸显。本文综述了锂离子电池正极材料钴酸锂、锰酸锂、磷酸铁锂及三元材料在掺杂和表面包覆两种工艺对电池电化学方面的影响,并展望了掺杂和表面包覆两种工艺未来的研究方向。     

Effects of electrode particle morphology on stress generation in silicon during lithium insertion

We study the effects of particle morphology and size on stress generation during Li insertion into Si particles using a fully coupled diffusion-elasticity model implemented in a finite element formulation. The model includes electrochemical reaction kinetics through a Butler-Volmer equation, concentration-dependent material properties, and surface elasticity. Focusing on two idealized geometries (hollow spheres and cylinders), we simulate stresses during Li insertion in Si. These systems describe a wide variety of morphologies that have been fabricated and studied experimentally, including particles, nanowires, nanotubes, and porous solids. We find that stresses generated in solid particles during Li insertion decrease as particle radii decrease from μm-scale, but reach a minimum at about 150 nm. Surface stresses then begin to dominate the stress states as the particle size continues to decrease. The minimum occurs at larger radii for hollow particles. We also find that hollow particles experience lower stresses than solid ones, but our results suggest that there is not a significant difference in maximum stress magnitudes for spherical and cylindrical particles. Studying the influence of concentration-dependent elastic moduli we find that while they can significantly influence stress generation for potentiostatic insertion, their role is minimal when surface reaction kinetics are considered.     

Preparation of carbon-coated Sn powders and their loading onto graphite flakes for lithium ion secondary battery

Carbon-coated Sn powders were prepared from the powder mixtures of thermoplastic precursor PVA, SnO₂ and MgO. The characterization of composite powders synthesized was carried out by XRD, TG, TEM, SEM and anodic performance measurement. SnO₂ was reduced to metallic Sn by heating with PVA, and its particle size in carbon shell was around 30–100 nm. MgO existence hindered the agglomeration of molten metallic Sn and made the dispersion of metallic Sn as fine particles possible. They showed high anodic performance in lithium ion batteries; high charge capacity as 500 mAh g⁻¹ even after tenth cycle and stable cyclic performance. The spaces left in carbon shell by MgO after its dissolution were supposed to absorb a large volume expansion of Sn metal particle by Li alloying during discharging. When carbon-coated Sn loaded onto graphite flakes, metallic tin contributed to the increase in capacity.     

Preparation and electrochemical characterization of tin/graphite/silver composite as anode materials for lithium-ion batteries

The tin/graphite/silver (Sn/G/Ag) composite was prepared by high-energy mechanical milling (HEMM) for the first time. The composite powders consisted of electrochemically active Sn, Ag₄Sn phases which were uniformly distributed on the surface of the graphite particles. The formation of Ag₄Sn alloy phase and the uniform distribution of the active particles could accommodate the large volume changes during cycling. X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM) and scanning electron microscopy (SEM) were used to determine the phases obtained and to observe the microstructure and dispersion of particles. In addition, cyclic voltammetry (CV) and galvanostatic discharge/charge tests were carried out to characterize the electrochemical properties of the composite. The composite electrodes exhibited an initial capacity of 1154 mAh g⁻¹ and maintained a reversible capacity of above 380 mAh g⁻¹ for more than 100 cycles.     

太阳能电池背表面钝化的研究

利用PC1D模拟不同少子寿命的电池效率与背表面复合速率的关系,采用氮化硅和及其与二氧化硅薄膜的叠加层作为背面钝化膜,通过丝网印刷的方法形成条形局域背接触和局域背面点接触,条形接触的面积为背表面的25%,背面点接触孔径为250μm,间距2mm。经过RTP处理之后,两种不同的接触方式存在相同的问题,串联电阻大,并联电阻小,而利用腐蚀浆料的方法形成背面点接触,在电性能参数有少许改善。结果表明,在正常的烧结状态下,常规铝浆很难完全穿透氮化硅薄膜及其叠加层背面钝化层。而利用腐蚀浆料的方法形成背面点接触,在电性能参数有少许改善。