An inorganic membrane as a separator for lithium-ion battery

An Al₂O₃ inorganic separator is prepared by a double sintering process. The Al₂O₃ separator has a high porosity and good mechanical strength. After the liquid electrolyte is infiltrated, the separator exhibits quite high ionic conductivities, and even the conductivity reaches 0.78 mS cm⁻¹ at −20 °C. Furthermore, the inorganic separator has an advantage over the polymer separator in the electrolyte retention. The LiFePO₄/graphite cell using the Al₂O₃ inorganic separator shows higher discharge capacity and rate capability, and better low-temperature performance than that using the commercial polymer separator, which indicates that the Al₂O₃ separator is very promising to be applied in the lithium-ion batteries.     

The role of mechanically induced separator creep in lithium-ion battery capacity fade

Lithium-ion batteries are well-known to be plagued by a gradual loss of capacity and power which occur regardless of use and can be limiting factors in the development of emerging energy technologies. Here we show that separator deformation in response to mechanical stimuli that arise under normal operation and storage conditions, such as external stresses on the battery stack or electrode expansion associated with lithium insertion/deinsertion, leads to increased internal resistance and significant capacity fade. We find this mechanically induced capacity fade to be a result of viscoelastic creep in the electrochemically inactive separator which reduces ion transport via a pore closure mechanism. By applying compressive stress on the battery structure we are able to accelerate aging studies and identify this unexpected, but important and fundamental link between mechanical properties and electrochemical performance. Furthermore, by making simple modifications to the electrode structure or separator properties, these effects can be mitigated, providing a pathway for improved battery performance.     

Use of lithium-ion batteries in electric vehicles

An account is given of the lithium-ion (Li-ion) battery pack used in the Northern Territory University's solar car, Fuji Xerox Desert Rose, which competed in the 1999 World Solar Challenge (WSC). The reasons for the choice of Li-ion batteries over silver–zinc batteries are outlined, and the construction techniques used, the management of the batteries, and the battery protection boards are described. Data from both pre-race trialling and race telemetry, and an analysis of both the coulombic and the energy efficiencies of the battery are presented. It is concluded that Li-ion batteries show a real advantage over other commercially available batteries for traction applications of this kind.     

Overcharge investigation of lithium-ion polymer batteries

The overcharge performances of lithium-ion polymer batteries (LIPB) have been studied by monitoring their temperature variation and analyzing the generated heat during overcharge. The critical concentration of lithium in the cathode material is determined for the thermal runaway of the battery. Solutions against the thermal runaway are proposed based on these results.     

Anode properties of titanium oxide nanotube and graphite composites for lithium-ion batteries

Titanium oxide nanotube and graphite composites are prepared by adding graphite before and after a hydrothermal reaction to enhance the cyclic performance and high-rate capability of lithium-ion batteries. The composite powders, their anode electrodes, and lithium half-cells containing the anodes are characterized by means of morphological and crystalline analysis, Raman spectroscopy, cyclic voltammetry, impedance spectroscopy, and repeated discharge-charge cycling at low and high C-rates. Notably, the composite anode (R5G5-T) that concurrently uses natural graphite and rutile particles before the hydrothermal reaction shows superior high-rate capability and achieves a discharge capacity of ca. 70 mAh g⁻¹ after 100 cycles at 50 C-rate. This may be due to the high-rate supercapacitive reactions of the TiO₂ nanotube on the graphite surface caused by a diffusion-controlled or a charge-transfer process.     

Modeling stresses in the separator of a pouch lithium-ion cell

The stress in a separator is mainly caused by the lithium diffusion induced deformation in the electrodes and thermal expansion differential between the battery components. To compute the lithium concentration distribution and temperature change during battery operation, multi-physics models have been developed previously. In this work, a macro-scale model for a pouch cell was developed and coupled with the multi-physics models. In this model, the porous battery components were treated as homogenized media and represented with the effective properties estimated using the rule of mixtures. The stress analysis showed that the maximum stress in the separator always emerged at the area around the inner corner of the separator where it wrapped around the edge of an anode and when the lithium-ion battery was fully charged. Numerical simulations were also conducted to investigate the influences of some design adjustable parameters, including the effective friction, electrode particle radii and thickness of the separator, on the stresses in the separator. The results provided the reference conditions for the improvement of separator materials and the design of lithium-ion batteries.     

An application of lithium cobalt nickel manganese oxide to high-power and high-energy density lithium-ion batteries

Evolved gas analysis (EGA) by mass spectroscopy (MS) was carried out for the pyrolysis of Li_1−xCo_1/3Ni_1/3Mn_1/3O₂ (185 mAh g⁻¹ of charge capacity) and the results were compared with that of Li_1−xCoO₂ (140 mAh g⁻¹). Electrochemically prepared Li_1−xCo_1/3Ni_1/3Mn_1/3O₂ clearly shows that O₂ evolution begins at much higher temperature than Li_1−xCoO₂, suggesting that Li_1−xCo_1/3Ni_1/3Mn_1/3O₂ is superior to LiCoO₂ with respect to thermal stability. High-temperature XRD measurements of charged LiCo_1/3Ni_1/3Mn_1/3O₂-electrodes at 4.45 V were also carried out and shown that the decomposition product by heating was identified as a cubic spinel consisting of cobalt, nickel, and manganese. This indicates that phase change from a layered to spinel-framework structure plays a crucial role in the suppression of oxygen evolution from the solid matrix. In order to show practicability of the new material, lithium-ion batteries with graphite-negative electrodes are fabricated and examined in the R18650-hardware. The new lithium-ion batteries show high rate discharge performances, excellent cycle life, and safety together with high-energy density.     

Preparation and characterization of Ti-doped LiFePO4 cathode materials for lithium-ion batteries

Olivine structured LiFePO₄ (lithium iron phosphate) and Ti⁴⁺-doped LiFe_1−xTi_xPO₄ (0.01 ≤ x ≤ 0.09) powders were synthesized via a solution route followed by heat-treatment at 700 °C for 8 h under N₂ flowing atmosphere. The compositions, crystalline structure, morphology, carbon content, and specific surface area of the prepared powders were investigated with ICP-OES, XRD, TEM, SEM, EA, and BET. Capacity retention study was used to investigate the effects of Ti⁴⁺ partial substitution on the intercalation/de-intercalation of Li⁺ ions in the olivine structured cathode materials. Among the prepared powders, LiFe_0.97Ti_0.03PO₄ manifests the most promising cycling performance as it was cycled with C/10, C/5, C/2, 1C, 2C, and 3C rate. It showed initial discharge capacity of 135 mAh g⁻¹ at 30 °C with C/10 rate. From the results of GSAS refinement for the prepared samples, the doped-Ti⁴⁺ ions did not occupy the Fe²⁺ sites as expected. However, the occupancy of the doped Ti⁴⁺ ions are still not clear, and theoretical calculations are needed for further studies. From the variation of lattice parameters calculated by the least square method without refinement, it suggested that Ti⁴⁺-doped LiFePO₄ samples formed solid solutions at low doping levels while TiO₂ was also observed with TEM in samples prepared with doping level higher than 5 mol%.     

Research progress of separators for lithium-ion batteries

随着锂离子电池在动力和储能等新能源领域应用的不断拓展,传统锂离子电池的性能已经无法满足新兴领域的要求,作为影响锂离子电池性能的关键材料,隔膜制备技术急需深入研究和发展.目前,从组成材料和结构可以将锂电隔膜分成如下五类:① 聚烯烃微孔膜;② 改性聚烯烃微孔膜;③ 有机-无机复合膜;④ 纳米纤维膜;⑤ 固态电解质膜.本文介绍了新能源领域锂离子电池的发展对隔膜性能的严格要求,简要分析了聚烯烃隔膜的缺点,重点综述了各类型锂电隔膜的研究成果,讨论了改性聚烯烃隔膜,复合膜,纳米纤维膜及固态电解质膜的特点及应用情况,指出了安全性和均一性是下一代锂电隔膜的基本要求及关键性能,并展望了锂电隔膜的未来发展方向.     

Solar photovoltaic charging of lithium-ion batteries

Solar photovoltaic (PV) charging of batteries was tested by using high efficiency crystalline and amorphous silicon PV modules to recharge lithium-ion battery modules. This testing was performed as a proof of concept for solar PV charging of batteries for electrically powered vehicles. The iron phosphate type lithium-ion batteries were safely charged to their maximum capacity and the thermal hazards associated with overcharging were avoided by the self-regulating design of the solar charging system. The solar energy to battery charge conversion efficiency reached 14.5%, including a PV system efficiency of nearly 15%, and a battery charging efficiency of approximately 100%. This high system efficiency was achieved by directly charging the battery from the PV system with no intervening electronics, and matching the PV maximum power point voltage to the battery charging voltage at the desired maximum state of charge for the battery. It is envisioned that individual homeowners could charge electric and extended-range electric vehicles from residential, roof-mounted solar arrays, and thus power their daily commuting with clean, renewable solar energy.     

Self-discharge behavior and its temperature dependence of carbon electrodes in lithium-ion batteries

Self-discharging characteristics of negative electrodes with different carbon materials have been investigated by monitoring the open circuit potential (OCP), the capacity loss and the ac impedance change during the storage at different temperatures. The OCP change with the storage time reflected state-of-charge (SOC), which depended on both the carbon material and the storage temperature. Higher specific surface area of the material and higher storage temperature lead to higher self-discharging rate. The activation energy for self-discharging was estimated from the temperature dependence of the self-discharging rate. Although small difference was observed among the materials, the value of the activation energy suggests that the self-discharging reaction at each electrode is controlled by a diffusion process. Changes in the interfacial resistance with the storage time reflected the growth of so-called Solid Electrolyte Interphase (SEI) at carbon surface. The rate of SEI formation at lower temperature does not depend on the carbon material, but at higher storage temperature the rate on spherical graphite was much higher than those on the other carbon materials.     

SnO2 nanoparticles@polypyrrole nanowires composite as anode materials for rechargeable lithium-ion batteries

The SnO₂@polypyrrole (PPy) nanocomposites have been synthesized by a one-pot oxidative chemical polymerization method. The structure, composition, and morphology of the as-prepared SnO₂@PPy nanocomposites are characterized by XRD, FTIR, TG, SEM, and TEM. Electrochemical investigations show that the obtained SnO₂@PPy nanocomposites exhibit high discharge/charge capacities and favorable cycling when they are employed as anode materials for rechargeable lithium-ion batteries. For the SnO₂@PPy nanocomposite with 79 wt% SnO₂, the electrode reaction kinetics is demonstrated to be controlled by the diffusion of Li⁺ ions in the nanocomposite. The calculated diffusion coefficiency of lithium ions in the SnO₂@PPy nanocomposite with 79 wt% SnO₂ is 6.7 × 10⁻⁸ cm² s⁻¹, while the lithium-alloying activation energy at 0.5 V is 47.3 kJ mol⁻¹, which is obviously lower than that for the bare SnO₂. The enhanced electrode performance with the SnO₂@PPy nanocomposite is proposed to derive from the advantageous nanostructures that allow better structural flexibility, shorter diffusion length, and easier interaction with lithium.     

Composite nonwoven separator for lithium-ion battery: Development and characterization

A new type composite nonwoven separator has been developed by combining a polyacrylonitrile (PAN) nano-fiber nonwoven and ceramic containing polyolefin nonwoven. The physical, electrochemical and thermal properties of the separator were investigated. The separator has mean pore size of about 0.8 μm as well as narrow pore-size distribution. Besides, the separator possesses higher porosity and air permeability than a conventional microporous membrane separator. The separator showed tensile strength of 46 N 5 cm⁻¹ at 10% elongation. Any internal short circuit was not observed for cells with the separator during charge-discharge test, and the cells showed stable cycling performance. Moreover, the cells showed better rate capabilities than cells with the conventional one. On a hot oven test at 150 °C, the composite nonwoven separator showed better thermal stability than the conventional one. In addition, internal short circuit by thermal shrinkage of separator was not observed for a cell with the separator at 150 °C for 1 h.     

Thermal stability and flammability of electrolytes for lithium-ion batteries

Safety is the key-feature of large-size lithium-ion batteries and thermal stability of the electrolytes is crucial. We investigated the thermal and flammability properties of mixed electrolytes based on the conventional ethylene carbonate-dimethyl carbonate (1:1 wt/wt)-1 M LiPF₆ and the hydrophobic ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr₁₄TFSI). The results of thermogravimetric analyses and flammability tests of mixed electrolytes of different compositions are reported and discussed. An important finding is that though the mixtures with high contents of ionic liquid are more difficult to ignite, they burn for a longer time, once they are ignited.     

Development and characterization of a bilayer separator for lithium ion batteries

A battery separator is placed between the positive and negative electrodes to prevent electric contact of the electrodes while maintaining good ionic flow. The most commonly used separators for lithium-ion batteries are porous polyolefin membranes. However, they generally do not have good dimentional stability at elevated temperatures. In this study, a bilayer separator has been formed directly on an anode. This bilayer separator comprised a ceramic layer and a porous polyvinylidene fluoride (PVDF) layer. Coin cells with this type of separators showed stable cycling performance at room temperature. They also showed significantly improved rate capabilities compared to the reference cell with a conventional polyolefin separator. An oven test has been used to characterize the cells thermal stability. Charged cells were kept in an oven at 150 °C and their voltage drop was recorded. The reference cell with a conventioal separator failed within about 50 min, while no noticeable voltage drop was observed for the cells with the new bilayer separator within the measured 2 h.     

Preparation and characterization of electrospun poly(acrylonitrile) fibrous membrane based gel polymer electrolytes for lithium-ion batteries

Electrospun, non-woven membrane of high molecular weight poly(acrylonitrile) (PAN) is demonstrated as an efficient host matrix for the preparation of gel polymer electrolytes for lithium-ion batteries. Electrospinning process parameters are optimized to get a fibrous membrane of PAN consisting of bead-free, uniformly dispersed thin fibers with diameter in the range 880-1260 nm. The membrane with good mechanical strength and porosity exhibits high uptake when activated with the liquid electrolyte of 1 M LiPF₆ in a mixture of organic solvents and the gel polymer electrolyte shows ionic conductivity of 1.7 × 10⁻⁵ S cm⁻¹ at 20 °C. Electrochemical performance of the gel polymer electrolyte at 20 °C is evaluated in lithium-ion cell with lithium cobalt oxide cathode and graphite anode. Good performance with a low capacity fading on charge-discharge cycling is demonstrated.     

Enhanced coulomb counting method for estimating state-of-charge and state-of-health of lithium-ion batteries

The coulomb counting method is expedient for state-of-charge (SOC) estimation of lithium-ion batteries with high charging and discharging efficiencies. The charging and discharging characteristics are investigated and reveal that the coulomb counting method is convenient and accurate for estimating the SOC of lithium-ion batteries. A smart estimation method based on coulomb counting is proposed to improve the estimation accuracy. The corrections are made by considering the charging and operating efficiencies. Furthermore, the state-of-health (SOH) is evaluated by the maximum releasable capacity. Through the experiments that emulate practical operations, the SOC estimation method is verified to demonstrate the effectiveness and accuracy.     

Li4Ti5O12/Sn composite anodes for lithium-ion batteries: Synthesis and electrochemical performance

Li₄Ti₅O₁₂/tin phase composites are successfully prepared by cellulose-assisted combustion synthesis of Li₄Ti₅O₁₂ matrix and precipitation of the tin phase. The effect of firing temperature on the particulate morphologies, particle size, specific surface area and electrochemical performance of Li₄Ti₅O₁₂/tin oxide composites is systematically investigated by SEM, XRD, TG, BET and charge-discharge characterizations. The grain growth of tin phase is suppressed by forming composite with Li₄Ti₅O₁₂ at a calcination of 500 °C, due to the steric effect of Li₄Ti₅O₁₂ and chemical interaction between Li₄Ti₅O₁₂ and tin oxide. The experimental results indicate that Li₄Ti₅O₁₂/tin phase composite fired at 500 °C has the best electrochemical performance. A capacity of 224 mAh g⁻¹ is maintained after 50 cycles at 100 mA g⁻¹ current density, which is still higher than 195 mAh g⁻¹ for the pure Li₄Ti₅O₁₂ after the same charge/discharge cycles. It suggests Li₄Ti₅O₁₂/tin phase composite may be a potential anode of lithium-ion batteries through optimizing the synthesis process.     

A review of the patents of lithium-ion battery polyolefin multilayer separator

从专利角度梳理了聚烯烃多层锂电池隔膜的技术输出国和申请人分布,并总结归纳了聚烯烃多层锂电池隔膜的技术手段和技术功效分布。结果表明,日本、中国、韩国和美国是目前全球聚烯烃多层锂电池隔膜的主要技术来源国,日本的东丽、旭化成、住友,韩国的LG公司以及中国的深圳星源材质和中国科学院是主要申请人。采用特定的层组分、改性层的设计以及拉伸和造孔工艺的调控,以改进锂电池隔膜的透气性、孔隙率、孔隙均匀性,耐热性、热收缩性,热关闭性能,机械强度和离子渗透性是主要的研究方向。