Thermal and electrochemical behaviour of C/LixCoO2 cell during safety test

Thermal and electrochemical processes in a 1000 mAh lithium-ion pouch cell with a graphite anode and a Li_xCoO₂ cathode during a safety test are examined. In overcharge tests, the forced current shifts the cell voltage to above 4.2 V. This causes a cell charged at the 1 C rate to lose cycleability and a cell charged at the 3 C rate to undergo explosion. In nail penetration and impact tests, a high discharge current passing through the cells gives rise to thermal runaway. These overcharge and high discharge currents promote joule heat within the cells and leads to decomposition and release of oxygen from the de-lithiated Li_xCoO₂ and combustion of carbonaceous materials. X-ray diffraction analysis reveals the presence of Co₃O₄ in the cathode material of a 4.5 V cell heated to 400 °C. The major cathode product formed after the combustion process cells abused by forced current is Co₃O₄ and by discharge current the products are LiCoO₂ and Co₃O₄. The formation of a trace quantity of CoO through the reduction of Co₃O₄ by virtue of the reducing power of the organic solvent is also discussed.     

Investigation on the root cause of the decreased performances in the overcharged lithium iron phosphate battery

The overcharge of the lithium iron phosphate (LiFePO₄) batteries usually leads to the sharp capacity fading and safety issues, especially under low temperature environment. Thus, investigating their root cause originated from the electrode materials is critical for the safety performance optimization and market promotion of the LiFePO₄ batteries. In this work, the electrochemical/thermal behaviors of 18650 LiFePO₄ batteries are investigated after overcharge under room and low temperature of 25°C and ?20°C, respectively. The results demonstrate a decreased electrochemical performance and faster heating rate of the overcharged battery, particularly under harsh working environments such as high discharge rate and low temperature. Coupling with the analyses of the internal resistance, the crystal structure, and microstructure of the electrodes, the root cause is attributed to the damage of the crystal structure and microstructure, which reduce the electron/Li⁺ migrating capability and electrolyte diffusion/transfer efficiency.     

The electrochemical insertion and safety properties of the low-cost Li-ion active material, Li2FeS2

The electrochemical insertion properties and safety characteristics of the alternative Li-ion positive electrode material, lithium iron sulfide (Li₂FeS₂) are presented. The active material is synthesized by a low cost, proprietary solid-state method. In terms of specific energy, the Li₂FeS₂ material offers a significant advantage over conventional lithium-ion positive electrode materials. The fully de-lithiated (charged) Li_2−xFeS₂ phase also demonstrates outstanding thermal stability suggesting that it may represent an excellent choice for safe, large format Li-ion battery applications.     

Preparation and rate capability of Li4Ti5O12 hollow-sphere anode material

Spinel lithium titanate, Li₄Ti₅O₁₂, with novel hollow-sphere structure was fabricated by a sol–gel process using carbon sphere as template. The effect of the hollow-sphere structure as well as the wall thickness on the Li storage capability and high rate performance was electrochemically evaluated. High specific capacity, especially better high rate performance was achieved with this Li₄Ti₅O₁₂ hollow-sphere electrode material with thin wall thickness. It is believed that this macroporous hollow-sphere structure has shortened the Li diffusion distance, increased the contact area between Li₄Ti₅O₁₂ and electrolyte, and also led to better mixing of the active material with AB. All these factors have resulted in the good rate capability of the hollow-sphere structured Li₄Ti₅O₁₂ electrode material.     

Discharge characteristic of a non-aqueous electrolyte Li/O2 battery

Discharge characteristic of Li/O₂ cells was studied using galvanostatic discharge, polarization, and ac-impedance techniques. Results show that the discharge performance of Li/O₂ cells is determined mainly by the carbon air electrode, instead by the Li anode. A consecutive polarization experiment shows that impedance of the air electrode is progressively increased with polarization cycle number since the surfaces of the air electrode are gradually covered by discharge products, which prevents oxygen from diffusing to the reaction sites of carbon. Based on this observation, we proposed an electrolyte-catalyst “two-phase reaction zone” model for the catalytic reduction of oxygen in carbon air electrode. According to this model, the best case for electrolyte-filling is that the air electrode is completely wetted while still remaining sufficient pores for fast diffusion of gaseous oxygen. It is shown that an electrolyte-flooded cell suffers low specific capacity and poor power performance due to slow diffusion of the dissolved oxygen in liquid electrolyte. Therefore, the status of electrolyte-filling plays an essential role in determining the specific capacity and power capability of a Li/O₂ cell. In addition, we found that at low discharge currents the Li/O₂ cell showed two discharge voltage plateaus. The second voltage plateau is attributed to a continuous discharge of Li₂O₂ into Li₂O, and this discharge shows high polarization due to the electrically isolating property of Li₂O₂.     

Polytriphenylamine used as an electroactive separator material for overcharge protection of rechargeable lithium battery

An electroactive polytriphenylamine (PTPAn) was synthesized and used as separator material for providing a self-activating overcharge protection of rechargeable lithium batteries. The experimental results from the Li–LiFePO₄ cells demonstrated that the electroactive separator could transform from an electronically isolating state to a conductive state at overcharge, producing an resistive internal short circuit to maintain the cell's voltage at the safety value of ∼3.75 V. In addition, the electroactive PTPAn separator works reversibly and has no negative influences on the normal charge–discharge behaviors of the Li–LiFePO₄ cells.     

Influence of conductive additives and surface fluorination on the charge/discharge behavior of lithium titanate (Li4/3Ti5/3O4)

Effect of conductive additives and surface modification with NF₃ and ClF₃ on the charge/discharge behavior of Li_4/3Ti_5/3O₄ (≈4.6 μm) was investigated using vapor grown carbon fiber (VGCF) and acetylene black (AB). VGCF and mixtures of VGCF and AB increased charge capacities of original Li_4/3Ti_5/3O₄ and those fluorinated with NF₃ by improving the electric contact between Li_4/3Ti_5/3O₄ particles and nickel current collector. Surface fluorination increased meso-pore with diameter of 2 nm and surface area of Li_4/3Ti_5/3O₄, which led to the increase in first charge capacities of Li_4/3Ti_5/3O₄ samples fluorinated by NF₃ at high current densities of 300 and 600 mA g⁻¹. The result shows that NF₃ is the better fluorinating agent for Li_4/3Ti_5/3O₄ than ClF₃.     

Electrochemical behaviour of Heusler alloy Co2MnSi for secondary lithium batteries

Electrochemical lithiation of Co₂MnSi with a Heusler structure is investigated as a candidate negative electrode (anode) material for secondary lithium batteries. The electrode maintains a reversible discharge capacity of 112 mAh g⁻¹ for 50 cycles when cycled between 0.01 and 3 V. It is proposed that the lithiation mechanism consists of two steps. Co₂MnSi transforms to Heusler-type Li₂MnSi during the first charge cycle and subsequent charge–discharge cycles involve the formation of a solid solution in Li_xMnSi. The latter compound maintains its structural integrity throughout cycling to provide steady cycling behaviour. Magnetic measurements are also employed to substantiate further the structural changes during electrochemical cycling.     

In situ lithiation of TiS2 enabled by spontaneous decomposition of Li3N

We report on a novel method for in situ lithiation of lithium free TiS₂ using Li₃N in an all-solid-state battery configuration. This method was tested using a Li₃N-TiS₂-80Li₂S:20P₂S₅ composite positive electrode and an indium metal negative electrode. It is shown that approximately 37% of Li₃N spontaneously decomposes during composite preparation regardless of the composition. Solid-state battery cells built with a 3:1 stoichiometric ratio of Li:Ti demonstrated a high 1st cycle charge capacity of 287 mAh g⁻¹, 20% greater than the theoretical capacity of TiS₂ at 239 mAh g⁻¹. The difference provides an excess capacity in the indium metal negative electrode.     

Fused ring and linking groups effect on overcharge protection for lithium-ion batteries

The derivatives of 1,3-benzodioxan (DBBD1) and 1,4-benzodioxan (DBBD2) bearing two tert-butyl groups have been synthesized as new redox shuttle additives for overcharge protection of lithium-ion batteries. Both compounds exhibit a reversible redox wave over 4 V vs Li/Li⁺ with better solubility in a commercial electrolyte (1.2 M LiPF₆ dissolved in ethylene carbonate/ethyl methyl carbonate (EC/EMC 3/7) than the di-tert-butyl-substituted 1,4-dimethoxybenzene (DDB). The electrochemical stability of DBBD1 and DBBD2 was tested under charge/discharge cycles with 100% overcharge at each cycle in MCMB/LiFePO₄ and Li₄Ti₅O₁₂/LiFePO₄ cells. DBBD2 shows significantly better performance than DBBD1 for both cell chemistries. The structural difference and reaction energies for decomposition have been studied by density functional calculations.     

Synthesis of single crystalline Li0.44MnO2 nanowires with large specific capacity and good high current density property for a positive electrode of Li ion battery

The fabrication of single crystalline Li_0.44MnO₂ nanowires for the positive electrode of lithium ion battery is reported. The single crystalline Li_0.44MnO₂ nanowires are obtained by lithium exchange reaction of Na_0.44MnO₂ nanowires with high aspect ratio. The Li_0.44MnO₂ nanowires indicate both the large specific capacity of around 250 mAh g⁻¹ (1.5-4.5 V vs. Li/Li⁺) and the good high current density property for the positive electrode of lithium ion battery.     

Investigation of positive electrodes after cycle testing of high-power Li-ion battery cells: I. An approach to the power fading mechanism using XANES

Cylindrical lithium-ion (Li-ion) cells with a nickel-cobalt oxide (LiNi_0.73Co_0.17Al_0.10O₂) positive electrode and a non-graphitizable carbon (hard carbon) negative electrode were degraded using cycle tests. The degraded cells were disassembled and examined; most attention was paid to the positive electrodes in order to clarify the origin of the power fade of the cells. X-ray absorption near-edge structure (XANES) analysis demonstrated that the crystal structure of the electrode at the surface changed from rhombohedral to cubic symmetry. Furthermore, a film of lithium carbonate (Li₂CO₃) covered the surface of the positive electrode after the cycle tests. Using a combination of X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and glow discharge optical emission spectrometry (GD-OES) measurements, a schematic model of the changes occurring in the surface structure of the positive electrode during the cycle tests was constructed. The appearance of both an electrochemically inactive cubic phase and lithium carbonate films at the surface of the positive electrode are important factors giving rise to power fade of the positive electrode.     

Power-ion battery: bridging the gap between Li-ion and supercapacitor chemistries

A 40 Wh/kg Li-ion battery using a Li₄Ti₅O₁₂ nanostructured anode and a composite activated carbon LiCoO₂ cathode was built using plastic Li-ion processing based on PVDF-HFP binder and soft laminate packaging. The specific power of the device is similar to that of an electrochemical double-layer supercapacitor (4000 W/kg). The high power is enabled by a combination of a nanostructured negative electrode, an acetonitrile based electrolyte and an activated carbon/LiCoO₂ composite positive electrode. This enables very fast charging (full recharge in 3 min). The effect of electrode formulation and matching ratio on energy, power and cycle-life are described. Optimization of these parameters led to a cycle-life of 20% capacity loss after 9000 cycles at full depth of discharge (DOD).     

Relationships between processing,morphology and discharge capacity of the composite electrode

The cycled capacity of Li_1.1V₃O₈ based positive electrodes varies between 100 and 250 mAh g⁻¹ (C/5 rate, 3.3–2 V) depending on the processing parameters. The initial volatile solvent concentration has a strong impact on the distribution of the electrode constituents. For a concentration below the optimal one, the mechanical energy available for mixing is insufficient to overcome viscosity forces and to reach a good dispersion of the constituents in the bulk of the electrode. Above the optimal concentration, settling of the Li_1.1V₃O₈ and carbon black particles in the low viscosity suspensions creates a concentration gradient. In these two cases the electrochemical performance are degraded. The viscosity of the electrode slurry must be systematically adjusted since the grain size and density depend on the active material.     

Investigation of the rechargeability of Li-O2 batteries in non-aqueous electrolyte

To understand the limited cycle life performance and poor energy efficiency associated with rechargeable lithium-oxygen (Li-O₂) batteries, the discharge products of primary Li-O₂ cells at different depths of discharge (DOD) were systematically analyzed using XRD, FTIR and Ultra-high field MAS NMR. When discharged to 2.0 V, the reaction products of Li-O₂ cells include a small amount of Li₂O₂ along with Li₂CO₃ and RO-(CO)-OLi in the alkyl carbonate-based electrolyte. However, regardless of the DOD, there is no Li₂O detected in the discharge products in the alkyl-carbonate electrolyte. For the first time it was revealed that in an oxygen atmosphere the high surface area carbon significantly reduces the electrochemical operation window of the electrolyte, and leads to plating of insoluble Li salts on the electrode at the end of the charging process. Therefore, the impedance of the Li-O₂ cell continues to increase after each discharge and recharge process. After only a few cycles, the carbon air electrode is completely insulated by the accumulated Li salt terminating the cycling.     

All-solid-state rechargeable lithium batteries with Li2S as a positive electrode material

The Li₂S–Cu composite electrode materials were prepared by mechanical milling and applied to all-solid-state lithium cells using the Li₂S–P₂S₅ glass–ceramic electrolyte. The addition of Cu and the mechanical activation improved the electrochemical performance of Li₂S in all-solid-state cells. The In/Li₂S–Cu cells were charged and then discharged at room temperature, suggesting that Li₂S was utilized as a lithium source. The cells exhibited high discharge capacity of about 490 mAh g⁻¹ at the 1st cycle. The SEM and EDX analyses suggested that the amorphous Li_xCuS domain was partially formed by milling, and the domain played an important role in the improvement of capacity. The electrochemical reaction mechanism of the Li₂S–Cu composites was discussed on the basis of the mechanism of the S–Cu composite electrode.     

Optimization of MoO3 nanoparticles as negative-electrode material in high-energy lithium ion batteries

Highly uniform MoO₃ nanoparticles, created using a unique hot-wire chemical vapor deposition (HWCVD) system, were studied as active material for negative electrodes in high-energy lithium ion batteries. Transmission electron microscopy (TEM), surface area analysis (BET), and X-ray diffraction (XRD) were utilized for powder characterization. Electrodes were fabricated from a slurry of MoO₃, acetylene black (AB), and polyvinylidene fluoride (PVDF) binder deposited on copper foil. Electrochemical performance was optimized as a function of pre-annealing temperature and AB:PVDF ratio. Temperature programmed desorption (TPD) and Fourier transform infrared (FTIR) spectroscopy indicated both water removal and binder decomposition during heat treatment. However, melting binder rich electrodes appeared to redistribute the conductive additive and create a uniform coating that lead to improved durability. An optimized reversible high capacity of ∼1050 mAh g⁻¹ was obtained for an electrode fabricated from 70:10:20 active material:AB:PVDF with a 250 °C pre-heat treatment.     

Improvement of cycle behaviour of SiO/C anode composite by thermochemically generated Li4SiO4 inert phase for lithium batteries

A new anode composite material is prepared by thermal treatment of a blend made of silicon monoxide (SiO) and lithium hydroxide (LiOH) at 550 °C followed by ball milling with graphite. X-ray diffraction pattern confirms the presence of Li₄SiO₄ in the thermally treated (SiO + LiOH) material. The electrode appears to be smooth and glassy as evident from observation with a scanning electron microscope (SEM), possibly due to the presence of nano-silicon and Li₄SiO₄ particles, and exhibits superior performance with a charge capacity of ∼333 mAh g⁻¹ at the 100th cycle with a low-capacity fade on cycling. Cyclic voltammograms of the electrode predict high power capability. On the other hand, the electrode comprising of only SiO and C prepared through ball milling, devoid of Li₄SiO_4, shows hard crust particulates in the electrode exhibiting low charge–discharge capacities with cycling.     

Development of all-solid lithium-ion battery using Li-ion conducting glass-ceramics

We have developed a high performance lithium-ion conducting glass-ceramics. This glass-ceramics has the crystalline form of Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ with a NASICON-type structure, and it exhibits a high lithium-ion conductivity of 10⁻³ S cm⁻¹ or above at room temperature. Moreover, since this material is stable in the open atmosphere and even to exposure to moist air, it is expected to be applied for various uses. One of applications of this material is as a solid electrolyte for a lithium-ion battery. Batteries were developed by combining a LiCoO₂ positive electrode, a Li₄Ti₅O₁₂ negative electrode, and a composite electrolyte. The battery using the composite electrolyte with a higher conductivity exhibited a good charge–discharge characteristic.     

Diphenylamine: A safety electrolyte additive for reversible overcharge protection of 3.6 V-class lithium ion batteries

A polymerizable monomer, diphenylamine (DPAn), is reported to act as a safety electrolyte additive for overcharge protection of 3.6 V-class lithium ion batteries. The experimental results demonstrated that the DPAn monomer could be electro-polymerized to form a conductive polymer bridging between the cathode and anode of the battery, and to produce an internal current bypass to prevent the batteries from voltage runaway during overcharge. The charge–discharge tests of practical LiFePO₄/C batteries indicated that the DPAn additive could clamp the cell's voltage at the safe value less than 3.7 V even at the high rate overcharge of 3 C current, meanwhile, this monomer molecule has no significant impact on the charge–discharge performance of the batteries at normal charge–discharge condition.