Cresyl diphenyl phosphate effect on the thermal stabilities and electrochemical performances of electrodes in lithium ion battery

To improve the safety of lithium ion battery, cresyl diphenyl phosphate (CDP) is used as a flame-retardant additive in a LiPF₆ based electrolyte. The electrochemical performances of LiCoO₂/CDP-electrolyte/Li and Li/CDP-electrolyte/C half cells are evaluated. The thermal behaviors of Li_0.5CoO₂ and Li_0.5CoO₂-CDP-electrolyte, and Li_xC₆ and Li_xC₆-CDP-electrolyte are examined using a C80 micro-calorimeter. For the LiCoO₂/CDP-electrolyte/Li cells, the onset temperature of single Li_0.5CoO₂ is put off and the heat generation is decreased greatly except the one corresponding to 5% CDP-containing electrolyte. When Li_0.5CoO₂ coexists with CDP-electrolyte, the thermal stability is enhanced. CDP improves the thermal stability of lithiated graphite anode effectively and the addition of 5% CDP inhibits the decomposition of solid electrolyte interphase (SEI) films significantly. The electrochemical tests on LiCoO₂/CDP-electrolyte/Li and Li/CDP-electrolyte/C cells show that when less than 15% CDP is added to the electrolyte, the electrochemical performances are not worsen too much. Therefore, the addition of 5-15% CDP to the electrolyte almost does not worsen the electrochemical performance of LiCoO₂ cathode and graphite anode, and improves theirs thermal stability significantly; thus, it is a possible choice for electrolyte additive.     

Cresyl diphenyl phosphate as flame retardant additive for lithium-ion batteries

To improve the safety of lithium-ion batteries, cresyl diphenyl phosphate (CDP) was used as a flame retardant additive in a LiPF₆ electrolyte solution. The flammability of the electrolytes containing CDP and the electrochemical performances of the cells, LiCoO₂/Li, graphite/Li and the battery LiCoO₂/graphite with these electrolytes, were studied by measuring the self-extinguishing time of the electrolytes, the variation of surface temperature of the battery and the charge/discharge curve of the cells or battery. It is found that the addition of CDP to the electrolyte provides a significant suppression in the flammability of the electrolyte and an improvement in the thermal stability of battery. On the other hand, the electrochemical performances of the cells become slightly worse due to the application of CDP in the electrolyte. This alleviated trade-off between the flammability and thermal stability and cell performances provides a possibility to formulate a nonflammable electrolyte by using CDP.     

Application of cyclohexyl benzene as electrolyte additive for overcharge protection of lithium ion battery

The electrochemical characterization and overcharge protection mechanism of cyclohexyl benzene as an additive in electrolyte for lithium ion battery was studied by microelectrode cyclic voltammetry, Galvanostatic charge–discharge measurements and SEM observation on both the cathode and separator of the overcharged cells. It was found that when the battery is overcharged, cyclohexyl benzene electrochemically polymerized to form polymer between separator and cathode at the potentials lower than that for electrolyte decomposition. The polymer blocks the overcharging process of the battery. The additive causes a small capacity loss and impedance increase in a real cell, but that can be mitigated if the operating voltage is much lower than the polymerization voltage.     

The reductive mechanism of ethylene sulfite as solid electrolyte interphase film-forming additive for lithium ion battery

The reduction mechanism of ethylene sulfite (ES) in propylene carbonate (PC) based electrolyte is investigated using density functional theory in gas phase. Based on the electron affinity energy and lowest unoccupied molecular orbital (LUMO) energy, it can be known that free ES is reduced most easily compared with ES-Li⁺ and ES-Li⁺-PC, generating SO₂ and propanal. However, the binding energy of ES-Li⁺ and ES-Li⁺-PC is quite negative, indicating that both of them are more possible in electrolyte solution than the free ES. The reductive decomposition products of ES-Li⁺ and ES-Li⁺-PC are OSO₂Li, OSO₂Li-R and ethylene. OSO₂Li and OSO₂Li-R are the main compositions of the solid electrolyte interphase film on the anode of lithium ion battery, which inhibits the reductive decomposition of PC. These calculations provide a detailed explanation on the experimental phenomena.     

Performance improvement of lithium ion battery using PC as a solvent component and BS as an SEI forming additive

The electrochemical behavior of propylene carbonate (PC)-based electrolytes with and without butyl sultone (BS) on graphite electrode and the performance of lithium ion batteries with these electrolytes were studied with cyclic voltammetry (CV), energy dispersive spectroscopy (EDS), as well as density functional theory (DFT) calculation. It is found that the co-insertion of PC with lithium ions into graphite electrode can be inhibited to a great extent by adjusting the composition of solvent in electrolytes. With the application of PC in the electrolyte without any additive, the discharge capacity of lithium ion battery is improved under high temperature or low temperature, however it decays under room temperature compared with the battery without PC. This drawback can be overcome by using BS as a solid electrolyte interphase (SEI) forming additive. BS has a lower LUMO energy and can be more easily electro-reduced than other components of solvent in electrolyte on a graphite electrode, forming a stable SEI film. With the application of BS in the electrolyte, the discharge capacity and cyclic stability of lithium ion battery is improved significantly under room temperature.     

Diphenyloctyl phosphate as a flame-retardant additive in electrolyte for Li-ion batteries

The use of diphenyloctyl phosphate (DPOF) as a flame-retardant additive in liquid electrolyte for Li-ion batteries is investigated. Mesocarbon microbeads (MCMB) and LiCoO₂ are used as the anode and cathode materials, respectively. Cyclic voltammetry (CV), differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) are used for the analyses. The cell with DPOF shows better electrochemical cell performance than that without DPOF in initial charge/discharge and rate performance tests. In cycling tests, a cell with DPOF-containing electrolyte exhibited better discharge capacity and capacity retention than that of the DPOF-free electrolyte after cycling. These results confirm the viability of using DPOF as a flame-retardant additive for improving the cell performance and thermal stability of electrolytes for Li-ion batteries.     

Vinylene carbonate and vinylene trithiocarbonate as electrolyte additives for lithium ion battery

Vinylene carbonate (VC) and vinylene trithiocarbonate (VTC) are studied as electrolyte additives in two kinds of electrolytes: (1) propylene carbonate (PC) and diethyl carbonate (DEC) (1:2 by weight) 1 mol dm⁻³ LiPF₆; (2) ethylene carbonate (EC) and DEC (1:2 by weight) 1 mol dm⁻³ LiPF₆. Characterization is performed by cyclic voltammetry, impedance spectroscopy, scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and half cell tests. Cyclic life is better in either electrolyte with VC than either electrolyte with/without VTC. SEM shows VC and VTC both form well developed passivation films on the graphite anode, but the films with VTC are thicker than with VC. EIS shows the VTC films have significantly higher charge transfer resistance. The VTC film in PC fails to protect against exfoliation. XPS indicates VTC has different reaction pathways in PC relative to EC. In EC/DEC, VTC forms polymeric C-O-C-like components and sulfide species (C-S-S-C, S and C-S-C). In PC/DEC, VTC does not form polymeric species, instead forming a film mainly containing LiF and Li₂S. It appears that a thinner polymeric film is preferential. The specific data herein are of interest, and the general conclusions may help development of improved additives for enhanced Li-ion battery performance.     

Mathematical modeling of a lithium ion battery with thermal effects in COMSOL Inc. Multiphysics (MP) software

The existing lithium ion battery model in Multiphysics (MP) software (COMSOL Inc., Palo Alto, CA) is extended to include the thermal effects. The thermal behavior of a lithium ion battery is studied during the galvanostatic discharge process with and without a pulse.     

High-throughput quantum chemistry and virtual screening for lithium ion battery electrolyte additives

Advances in the stability and efficiency of electronic structure codes along with the increased performance of commodity computing resources has enabled the automated high-throughput quantum chemical analysis of materials structure libraries containing thousands of structures. This allows the computational screening of a materials design space to identify lead systems and estimate critical structure-property limits which should prove an invaluable tool in informing experimental discovery and development efforts. Here this approach is illustrated for lithium ion battery additives. An additive library consisting of 7381 structures was generated, based on fluoro- and alkyl-derivatized ethylene carbonate (EC). Molecular properties (e.g. LUMO, EA, μ and η) were computed for each structure using the PM3 semiempirical method. The resulting lithium battery additive library was then analyzed and screened to determine the suitability of the additives, based on properties correlated with performance as a reductive additive for battery electrolyte formulations.     

Investigation and application of lithium difluoro(oxalate)borate (LiDFOB) as additive to improve the thermal stability of electrolyte for lithium-ion batteries

Lithium difluoro (oxalate) borate (LiDFOB) is used as thermal stabilizing and solid electrolyte interface (SEI) formation additive for lithium-ion battery. The enhancements of electrolyte thermal stability and the SEIs on graphite anode and LiFePO₄ cathode with LiDFOB addition are investigated via a combination of electrochemical methods, nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared-attenuated total reflectance (FTIR-ATR), as well as density functional theory (DFT). It is found that cells with electrolyte containing 5% LiDFOB have better capacity retention than cells without LiDFOB. This improved performance is ascribed to the assistance of LiDFOB in forming better SEIs on anode and cathode and also the enhancement of the thermal stability of the electrolyte. LiDFOB-decomposition products are identified experimentally on the surface of the anode and cathode and supported by theoretical calculations.     

Application of lithium ion phosphate battery in 110 kV substation DC power system

本项目选用了两种不同的电池管理模式对磷酸铁锂电池组进行管理,并将组装好的两套电池组应用于110 kV变电站直流系统的日常运行.运行结果表明,磷酸铁锂电池可以在变电站替代铅酸蓄电池使用,并且可以浮充运行;运行过程中,单体电池的电压会由于电池充电态的变化下降或上升;单体电池的内阻呈现先下降再上升的变化;电池组的放电容量随着运行时间的延长出现每年3%左右的衰减(浮充电压为3.6 V);合适的电池管理模式能将电池组内单体电池的电压差保持在较小的范围,有利于电池组长寿命的运行.     

Research progress on safety of lithium ion battery electrolyte

锂离子电池的能量密度及其安全问题是限制其在电动汽车应用中的主要障碍。随着能量密度的不断提升,当务之急是有效解决锂离子电池的安全性问题。锂离子电池安全问题本质上与当前电解液中使用的高挥发性、易燃的有机溶剂有关。因此,本文主要从电解液的燃烧性角度,介绍了电解液在锂离子电池材料安全性方面的研究现状,包括阻燃添加剂、不燃性氟代有机溶剂、高浓度电解液及固液混合电解质的应用等,分析其对安全性能提升的机理,并对电解液的发展方向进行了展望。     

Differential pulse effects of solid electrolyte interface formation for improving performance on high-power lithium ion battery

Solid electrolyte interface (SEI) formation is a key that utilizes to protect the structure of graphite anode and enhances the redox stability of lithium-ion batteries before entering the market. The effect of SEI formation applies a differential pulse (DP) and constant current (CC) charging on charge-discharge performance and cycling behavior into brand new commercial lithium ion batteries is investigated. The morphologies and electrochemical properties on the anode surface are also inspected by employing SEM and EDS. The electrochemical impedance spectra of the anode electrode in both charging protocols shows that the interfacial resistance on graphite anodes whose SEI layer formed by DP charging is smaller than that of CC charging. Moreover, the cycle life result shows that the DP charging SEI formation is more helpful in increasing the long-term stability and maintaining the capacity of batteries even under high power rate charge-discharge cycling. The DP charging method can provide a SEI layer with ameliorated properties to improve the performance of lithium ion batteries.     

Studies on the standard of lithium ion battery electrolyte

电解液是锂离子电池关键技术之一,在正负极之间起着输送和传导电流的作用,是连接正负极材料的桥梁。它影响着电池的工作电压、能量密度和安全性能等。近年来随着正负极材料的技术进步,电解液为了实现与新材料的匹配,在组分上出现了很大变化,现有的锂电池电解液标准及相应的测试方法也需要进一步更新,才能实现产品的检验规范化和质量标准化。本文介绍了电解液的现状及发展趋势,分析了我国现有电解液相关标准的情况,并对新的电解液国标提出了建议。     

A novel flame retardant and film-forming electrolyte additive for lithium ion batteries

Allyl tris(2,2,2-trifluoroethyl) carbonate (ATFEC) was synthesized as a bi-functional additive of flame retardant and film former in electrolytes for lithium ion batteries (LIBs). The flame retardancy of the additive was characterized with differential scanning calorimetry (DSC) and self-extinguishing time (SET). It is shown that adding 1 vol.% ATFEC in 1 M LiPF₆/propylene carbonate (PC) can effectively enhance the thermal stability of the electrolyte and suppress the co-intercalation of PC into the graphitic anode. Further evaluation indicates that the additive hardly affect the conductivity of electrolyte. These support the feasibility of using ATFEC as an additive on formulating an electrolyte with multiple functions such as film-forming enhancement, high thermal stability and high ionic conductivity.     

Nanosized tin anode prepared by laser-induced vapor deposition for lithium ion battery

Nanosized tin powder was prepared by laser-induced vapor deposition and studied as an alternative anode material for lithium ion batteries. The nano tin particles are spherical, and their size varies from 5 nm to 80 nm. The prepared powder consists of two compounds: a major amount of Sn and a minor amount of SnO. Results of cyclic voltammograms (CVs) indicate that SnO is deoxidized to Sn almost completely in the first cycle. The reaction of tin with lithium proceeds in two steps. At first, a Li-deficient phase is formed, later a Li-rich phase. Reaction kinetics are controlled by a diffusion step, and the diffusion coefficient of lithium ions in the anode is calculated to be 4.15 × 10⁻⁸ cm² s⁻¹. The initial charge capacity is nearly to the theoretical reversible capacity of lithium insertion into tin, resulting in Li₂₂Sn₅.     

Effect of Cu2O coating on graphite as anode material of lithium ion battery in PC-based electrolyte

Cuprous oxide-coated graphite was synthesized by a polyol reduction process and analyzed by scanning electron microscopy, charge–discharge measurements and cyclic voltammetry. Cu₂O exists at the surface of graphite in the form of nanoparticles and nanorods. The coated cuprous oxide layer acts as a protective layer separating graphite from the propylene carbonate (PC)-based electrolyte solution, and greatly suppresses PC decomposition and graphite exfoliation in PC-based electrolyte systems.     

Preparation of hierarchical porous carbon and its rate performance as anode of lithium ion battery

A hierarchical porous carbon with low oxygen content has been prepared by using polystyrene (PS) spheres as template and its structure, composition and performances as anode of lithium ion battery are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, element analysis (EA), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge test. The results obtained from SEM, TEM, XRD, FTIR, and EA indicate that the prepared sample has a well-interconnected pore structure with a pore size of 170 nm and has an oxygen content of 3.3 ± 0.2 wt.%. The low oxygen content of the prepared sample can be ascribed to the low decomposition temperature of the template that was determined by thermal analysis. EIS shows that the prepared sample has lower electrochemical impedance for the lithium insertion/de-insertion than commercial natural graphite and charge/discharge tests show that the battery using the prepared sample as anode exhibits better rate performance than that using the graphite.     

A study of lithium ion intercalation induced fracture of silicon particles used as anode material in Li-ion battery

The fracture of Si particles due to internal stresses formed during the intercalation of lithium ions was described by means of a thermal analogy model and brittle fracture damage parameter. The stresses were calculated following the diffusion equation and equations of elasticity with an appropriate volumetric expansion term. The results were compared with the acoustic emission data from the experiments on electrochemical cycling of Li ion half-cells with silicon electrodes. A good correlation between experiment and prediction was observed. The results of computations with different particle sizes show the existence of a critical size below which fracture during the lithiation is not expected to occur. Such a critical size appears to be within micrometer scale.