Entropy change in lithium ion cells on charge and discharge

Open circuit voltage (OCV) was measured as a function of temperature and state of charge (SOC) for six kinds of lithium ion cells. The following cells were used: four kinds of commercial cell using a LiCoO₂ cathode and a graphite or hard carbon anode; a trial manufacture cell using a Li–Ni–Co complex oxide cathode and a graphite-coke hybrid carbon anode; and a trial manufacture cell using a LiMn₂O₄ cathode and a graphite anode. The entropy change in the cell reaction was determined by calculating the derivative of the OCV with temperature. Results were compared and discussed to determine the influence of the phase transition in the electrode materials due to cell reaction. It was clarified that the entropy change in cells using a LiCoO₂ cathode is negative except for the part of the SOC region where Li _x CoO₂ phase transition occurred. An endothermic reaction then occurs during discharge and an exothermic reaction during charge. In cells using LiCoO₂ cathodes, there was a fluctuation in the entropy change originating from the Li _x CoO₂ phase transition in the SOC range between 70% and 90%. This fluctuation was influenced by temperature and by additives or excess lithium in the cathode material. The entropy change in both cells using a Li–Ni–Co complex oxide cathode or a LiMn₂O₄ cathode was comparatively small.     

Cycling of lithium/metal oxide cells using composite electrolytes containing fumed silicas

The effect on cycle capacity is reported of cathode material (metal oxide, carbon, and current collector) in lithium/metal oxide cells cycled with fumed silica-based composite electrolytes. Three types of electrolytes are compared: filler-free electrolyte consisting of methyl-terminated poly(ethylene glycol) oligomer (PEGdm, M_w=250)+lithium bis(trifluromethylsufonyl)imide (LiTFSI) (Li:O=1:20), and two composite systems of the above baseline liquid electrolyte containing 10-wt% A200 (hydrophilic fumed silica) or R805 (hydrophobic fumed silica with octyl surface group). The composite electrolytes are solid-like gels. Three cathode active materials (LiCoO₂, V₆O₁₃, and Li_xMnO₂), four conducting carbons (graphite Timrex® SFG 15, SFG 44, carbon black Vulcan XC72R, and Ketjenblack EC-600JD), and three current collector materials (Al, Ni, and carbon fiber) were studied. Cells with composite electrolytes show higher capacity, reduced capacity fade, and less cell polarization than those with filler-free electrolyte. Among the three active materials studied, V₆O₁₃ cathodes deliver the highest capacity and Li_xMnO₂ cathodes render the best capacity retention. Discharge capacity of Li/LiCoO₂ cells is affected greatly by cathode carbon type, and the capacity decreases in the order of Ketjenblack>SFG 15>SFG 44>Vulcan. Current collector material also plays a significant role in cell cycling performance. Lithium/vanadium oxide (V₆O₁₃) cells deliver increased capacity using Ni foil and carbon fiber current collectors in comparison to an Al foil current collector.     

LiAlO2-coated LiNi0.8Co0.1Mn0.1O2 and chlorine-rich argyrodite enabling high-performance all-solid-state lithium batteries at suitable stack pressure

All-solid-state lithium batteries (ASSLBs), which are consisted of Li_5.5PS_4.5Cl_1.5 electrolyte, metal lithium anode and LiNi_0.8Mn_0.1Co_0.1O₂ (NCM811) cathode, are speculated as a promising next generation energy storage system. However, the unstable oxide cathode/sulfide-based electrolyte interface and the dendrite formation in sulfide electrolyte using the lithium metal anode hinder severely commercialization of the ASSLBs. In this work, the dendrite formation in sulfide electrolyte is investigated in lithium symmetric cell by varying the stack pressure (3, 6, 12, 24 MPa) during uniaxial pressing, and uniformly nanosized LiAlO₂ buffer layer was carefully coated on NCM811 electrode (LiAlO₂@NCM811) to improve the cathode/electrolyte interface stability. The result shows that lithium symmetrical cell has a steady voltage evolution over 400 h under 6 MPa stacking pressure, and the assembled LiAlO₂@NCM811/Li_5.5PS_4.5Cl_1.5/Li battery under the stack pressure of 6 MPa exhibits large initial discharge specific capacity and excellent cycling stability at 0.05 C and 25 °C. The feasibility of using the lithium metal anode in all-solid-state batteries (ASSBs) under suitable stack pressure combined with uniformly nanosized LiAlO₂ buffer layer coated on NCM811 electrode supply a facile and effective measures for constructing ASSLBs with high energy density and high safety.     

Nano-CaCO3 templated mesoporous carbon as anode material for Li-ion batteries

Mesoporous hard carbon is obtained by pyrolyzing a mixture of sucrose and nanoscaled calcium carbonate (CaCO₃) particles. The microstructure of the carbon is characterized by N₂ adsorption/desorption, Hg porosimetry, field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and Raman spectroscopy. The electrochemical performances of the carbon as an anode material for lithium ion batteries are evaluated by galvanostatic charge/discharge and cyclic voltammetry tests. It is shown that this mesoporous carbon possesses high capacity, good cycling performance and rate capability, indicating the promising application of nano-CaCO₃ particle as template in massive fabrication of mesoporous carbon anode materials for lithium ion batteries.     

Polymer electrolyte lithium batteries rechargeability and positive electrode degradation: An AC impedance study

AC impedance measurements of polymer electrolyte-based, symmetrical composite cathode cells were used to probe the effects of the composite cathode composition and fabrication process upon its performance when used in polymer electrolyte-based thin film rechargeable lithium batteries. The relationship between cycling performance and AC impedance measurements were used to elucidate some of the reported failure mechanisms of rechargeable lithium polymer electrolyte batteries. The rapid initial capacity decay observed within the first few cycles of the polymer electrolyte/V₆O₁₃ based composite cathode is shown to be related to the properties of the composite cathode active material, while the slower capacity decay observed during subsequent cycles, under continuous cycling regimes, appears to be related to a loss of ionic and electronic contact in the composite cathode.     

改善锂硫电池循环性能的研究进展

综述了制约锂硫电池循环性能的因素和正极、负极、电解质对锂硫电池循环性能改善的影响。介绍了制约锂硫电池循环性能的主要因素：不可逆硫化锂的形成、硫正极多孔结构的失效和电解液组分与锂负极的副反应。分别介绍了改善锂硫电池循环性能的途径：合适的黏合剂、碳材料、正极制备工艺,锂负极保护技术,合理组分的电解质,电池结构与设计。并在此基础上对今后的发展趋势进行了展望。     

Flame retardant coated polyolefin separators for the safety of lithium ion batteries

A thermally stable and flame-retardant separator is proposed to improve the safety of lithium ion batteries. The separator is prepared by dip-coating both sides of a conventional tri-layer polyolefin separator with brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO). Significantly reduced thermal shrinkage and flammability are exhibited without decreases in the pore size or porosity of the conventional separator. By using the BPPO-coated separator, an electrochemical half-cell composed of a lithium metal anode and a LiCoO₂ cathode are successively tested. The resulting stable cycle performances are demonstrated. It is expected that the BPPO-coated separator can be a capable candidate as a separator for the safe of lithium ion batteries.     

Electrochemical properties of Si–Ge–Mo anode composite materials prepared by magnetron sputtering for lithium ion batteries

Si–Ge–Mo composites are prepared using an RF/DC magnetron sputtering method, and their potential use as anode materials for rechargeable lithium-ion batteries is investigated. The Si–Ge–Mo composite films present an amorphous structure. The reaction mechanism of the Si–Ge–Mo with Li is investigated by various analytical techniques. The fabricated Si_0.41Ge_0.34Mo_0.25 composite film shows excellent electrochemical properties, including a high energy density (1st charge: 1193 mAh g⁻¹), long cycleability (ca. 870 mAh g⁻¹ over 100 cycles), and good initial Coulombic efficiency (ca. 96%). Additionally, when coupled with a LiCoO₂ cathode, the Si_0.55Ge_0.22Mo_0.23 composite electrode used as an anode shows excellent cycleability with a high energy density. The excellent electrochemical properties demonstrated by the Si–Ge–Mo composite film electrode confirm its potential as an alternative anode material for lithium-ion batteries.     

锂离子电池负极硬炭／沥青复合材料的性能研究

通过高温固相法在硬炭表面包覆沥青热解炭,制备锂离子电池负极材料。SEM测试显示,硬炭包覆上沥青热解炭后表面形貌发生了明显变化。由例得到,硬炭表面包覆的沥青热解炭的平均厚度为300nm。当硬炭表面包覆上沥青热解炭后（硬炭与沥青的质量比为2：1）,首次库仑效率由55％提高到70％,可逆容量也有所提高。研究发现,硬炭在高倍率下的容量和循环稳定性比石墨好。     

High voltage stability of LiCoO2 particles with a nano-scale Lipon coating

For high-voltage cycling of rechargeable Li batteries, a nano-scale amorphous Li-ion conductor, lithium phosphorus oxynitride (Lipon), has been coated on surfaces of LiCoO₂ particles by combining a RF-magnetron sputtering technique and mechanical agitation of LiCoO₂ powders. LiCoO₂ particles coated with 0.36 wt% (∼1 nm thick) of the amorphous Lipon, retain 90% of their original capacity compared to non-coated cathode materials that retain only 65% of their original capacity after more than 40 cycles in the 3.0–4.4 V range with a standard carbonate electrolyte. The reason for the better high-voltage cycling behavior is attributed to reduction in the side reactions that cause increase of the cell resistance during cycling. Further, Lipon coated particles are not damaged, whereas uncoated particles are badly cracked after cycling. Extending the charge of Lipon-coated LiCoO₂ to higher voltage enhances the specific capacity, but more importantly the Lipon-coated material is also more stable and tolerant of high voltage excursions. A drawback of Lipon coating, particularly as thicker films are applied to cathode powders, is the increased electronic resistance that reduces the power performance.     

Microstructural development and electrochemical characteristics of lithium cobalt oxide powders prepared by the water-in-oil emulsion process

Lithium cobalt oxide (LiCoO₂) powders, utilized as cathode materials in lithium–ion secondary batteries, have been synthesized through the emulsion process. When the emulsion-derived precursors are calcined at elevated temperatures, the amounts of residual Co₃O₄ and organic species decrease and the crystallinity of LiCoO₂ gradually improves with a rise in the calcination temperature. After calcination at 800 °C for 1 h, monophasic LiCoO₂ powders with a R$\text{3}$m structure are successfully obtained. Compared to the traditional solid-state reaction, the emulsion process not only significantly curtails the reaction time to prepare LiCoO₂, but also reduces the particle size. In the electrochemical test, both the specific discharge/charge capacity and the coulomb efficiency of LiCoO₂ increase with a rise in the calcination temperature. Increasing the calcination temperature results in the formation of pure LiCoO₂ without residual reactants, and also enhances the crystallinity of LiCoO₂ as well as the ordering arrangement of lithium and cobalt cations in the layered structure, thereby facilitating the intercalation and deintercalation of lithium ions. As a result, the combination of the emulsion method with proper calcination processes is highly successful in producing LiCoO₂ powders having large specific capacity and good cyclic stability along with high coulomb efficiency.     

Performance evaluation of printed LiCoO2 cathodes with PVDF-HFP gel electrolyte for lithium ion microbatteries

In order to improve the discharge capacity in lithium ion microbatteries, a thick-film cathode was fabricated by a screen printing using LiCoO₂ pastes. The printed cathode showed a different discharge curves when the cell was tested using various (liquid, gel and solid-state) electrolytes. When a cell test was performed with organic liquid electrolyte, the maximum discharge capacity was 200 μAh cm⁻², which corresponded to approximately 133 mAh g⁻¹ when the loading weight of LiCoO₂ was calculated. An all-solid-state microbattery could be assembled using sputtered LiPON electrolyte, an evaporated Li anode, and printed LiCoO₂ cathode films without delamination or electrical problems. However, the highest discharge capacity showed a very small value (7 μAh cm⁻²). This problem could be improved using a poly(vinylidene fluoride-hexafluoro propylene) (PVDF-HFP) gel electrolyte, which enhanced the contact area and adhesion force between cathode and electrolyte. The discharge value of this cell was measured as approximately 164 μAh cm⁻² (≈110 mAh g⁻¹). As the PVDF-HFP electrolyte had a relatively soft contact property with higher ionic conductance, the cell performance was improved. In addition, the cell can be fabricated in a leakage-free process, which can resolve many safety problems. According to these results, there is a significant possibility that a film prepared using the aforementioned paste with screen printing and PVDF-HFP gel electrolyte is feasible for a microbattery.     

In situ composite of nano SiO2-P(VDF-HFP) porous polymer electrolytes for Li-ion batteries

Nano SiO₂-P(VDF-HFP) composite porous membranes were prepared as the matrix of porous polymer electrolytes through in situ composite method based on hydrolysis of tetraethoxysilane and phase inversion. SEM, TEM, DSC and AC impedance analysis were carried out. It is found that the in situ prepared nano silica was homogeneously dispersed in the polymeric matrix, enhanced conductivity and electrochemical stability of porous polymer electrolytes, and improved the stability of the electrolytes against lithium metal electrodes. The in situ composite method was found to be much better than the direct composite method in lowering the interfacial resistance between electrolyte and lithium metal electrode. Moreover, cycle test of lithium batteries using lithium metal as anode and sulfur composite material as cathode showed that the electrolyte based on in situ composite of silica presented stable charge-discharge behavior and little capacity loss of battery.     

Effect of the concentration of diphenyloctyl phosphate as a flame-retarding additive on the electrochemical performance of lithium-ion batteries

We selected diphenyloctyl phosphate (DPOF) as a flame-retardant and plasticizer, and studied the influence of different amounts of the DPOF additive on the electrochemical performance of lithium-ion batteries. The electrochemical cell performances of the additive-containing electrolytes in combination with a cell comprising an LiCoO₂ cathode and mesocarbon microbeads (MCMB) anode were tested in coin cells. The cyclic voltammetry (CV) results showed that the oxidation potential of the electrolyte containing DPOF in the concentration range from 10 to 30 wt.% is about 4.75-5.5 V versus Li/Li⁺. In the present work, a DPOF content of 10 wt.% in the 1.15 M LiPF₆/EC:EMC (4:6 by vol.%) electrolyte turned out to be the optimum condition for the improvement of the electrochemical cell performance, due to the decrease of the irreversible capacity during the first cycle and decrease of the charge-transfer resistance after 40 cycles.     

Effects of heteroatoms on electrochemical performance of electrode materials for lithium ion batteries

Recent studies of lithium ion batteries focus on improving electrochemical performance of electrode materials and/or lowering cost. Doping of active materials with heteroatoms is one promising method. This paper reviews the effects of heteroatoms on anode materials such as carbon- and tin-based materials, and cathode materials such as LiCoO₂, LiNiO₂, LiMn₂O₄ and V₂O₅. There are favorable and unfavorable effects, which depend on the species and physicochemical states of heteroatoms and the parent electrode materials. In the application of lithium ion batteries advantageous factors should be exploited, unwelcome side effects should be avoided as far as possible. Considerable gains towards improved electrochemical performance of the electrode materials have been achieved. Nevertheless, there are still problems needing further investigation including theoretical aspects, which will in the meanwhile stimulate the investigation for better electrode materials.     

Gel-based composite polymer electrolytes with novel hierarchical mesoporous silica network for lithium batteries

In the present work, novel gel-based composite polymer electrolytes for lithium batteries were prepared by introducing a hierarchical mesoporous silica network to the poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP)-based gel electrolytes. As compared with the PVDF-HFP-based gel electrolytes with/without conventional nano-sized silica fillers, the novel electrolytes have shown more homogeneous microstructure, higher ionic conductivity and better mechanical stability, which could be caused by the strong silica network and the effective interactions among the polymer, the liquid electrolytes and the silica. Moreover, the cell with this kind of electrolytes could achieve a discharge capacity as much as 150 mAh g⁻¹ at room temperature (LiCoO₂ as the cathode active material), with high Coulomb efficiency.     

大容量锂离子电容器用锂化硬炭负极

采用硬炭与锂源自放电这种简单的预锂化方法可使锂嵌入硬炭,而后以预锂化硬炭和活性炭分别为负极和正极组装了锂离子电容器,研究了负极预锂化时间对锂离子电容器比容量的影响,结果表明随着预锂化时间的延长,比容量先增大后减小,15 h为最适宜预锂化时间.经过15 h预锂化的锂离子电容器具有最高的能量密度(97.2 Wh·kg^-1)和功率密度(5 412 W·kg^-1)、最小的阻抗和良好的循环性能(1 A·g^-1的电流密度下循环1 000次后,能量保持率为91.2%).三电极数据表明锂离子电容器优异的电化学性能源于正负极材料各自处于合适的工作电压区间.     

Characteristics of an aqueous rechargeable lithium battery (ARLB)

Electrochemical performance of an aqueous rechargeable lithium battery (ARLB) containing a LiV₃O₈ (negative electrode) and LiCoO₂ (positive electrode) in saturated LiNO₃ aqueous electrolyte was studied. These two electrode materials are stable in the aqueous solution and intercalation/deintercalation of lithium ions occurs within the window of electrochemical stability of water. The obtained capacity of this cell system is about 55 mAh/g based on the mass of the positive electrode, which is lower than the corresponding one in the non-aqueous lithium ion battery. However, its specific capacity can be compared with those of the lead acid and Ni-Cd batteries. In addition, initial results show that this cell system is good in cycling.     

Correlation of the irreversible lithium capacity with the active surface area of modified carbons

Negative electrodes of lithium-ion batteries are generally based on graphite. Higher storage capacities can be obtained with disordered carbons, however they demonstrate a noticeable hysteresis and irreversibility, which can preclude a practical application. In this paper, the main parameters which may affect the irreversible capacity are analyzed and we show that the irreversible lithium consumption which occurs at the negative carbon electrode during the first charge (C_irr) is proportional to the active surface area (ASA). Composites with a reduced ASA have been obtained after coating a hard carbon or milled graphite samples with a thin film of pyrolytic carbon. Deactivating the surface by pyrolytic carbon deposition allows the irreversible capacity to be noticeably reduced, being lower than in graphite, while the reversible capacity is 50% higher than in graphite. The electrochemical properties of this new C/C composite are investigated by galvanostatic cycling at various current densities and by impedance spectroscopy. The main effect of the dense carbon coating is to hinder the diffusion of solvated lithium ions to the active sites of the carbon host during the first discharge, giving rise to a moderate development of the Solid Electrolyte Interphase (SEI).     

Effects of the structural characteristics of carbon cathode on the initial voltage delay in Li/SOCl2 battery

The study investigates electrochemical behaviors of Li/SOCl₂ batteries based on structural features of carbon cathodes to find out solutions of initial voltage delay, a natural problem of the batteries. The structural features of the carbon cathodes in the Li/SOCl₂ batteries are directly related to the transient minimum voltage (TMV) and the initial voltage delay, the inevitable fault in the batteries, and the study confirms possibilities to solve inherent problems of the batteries by structurally adjusting carbon cathodes. Low density of carbon cathodes inhibits the TMV increase, the initial voltage delay more than high density. The operating voltage of the battery may rise with increasing electronic conductivity of the carbon cathode, however, it fails to improve the TMV and the delay. The study shows that expanding carbon cathode volume shrinks a gap between the lithium and the carbon cathode, pressurizing and destroying passivation films, and improving the TMV and the initial voltage delay. Based on this, it is expected to manufacture excellent Li/SO₂Cl₂ batteries by improving the initial voltage delay as adjusting aging lapse and pressed density.