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.     

Thermal studies of charged cathode material (LixCoO2) with temperature-programmed decomposition-mass spectrometry

Thermal reactions of charged Li_xCoO₂ and electrolyte are investigated by means of temperature-programmed decomposition-mass spectrometry (TPD-MS), DSC, TGA, and XRD. The electrolyte is composed of ethylene carbonate, propylene carbonate, and dimethyl carbonate. Direct observation of gas species resulting from the reactions is beneficial in understanding the reaction mechanisms. From O₂ peaks in the TPD-MS spectra with weight loss in TGA and XRD results, it is obvious that the reduction of Li_xCoO₂ to LiCoO₂ and Co₃O₄ is triggered at 190 °C and completed at 400 °C. The initial stages of electrolyte combustion and decomposition are confirmed by H₂O and CO₂ peaks in the TPD-MS spectra along with exothermic peaks in DSC plots. The reaction of electrolyte with O₂ released from the reduction of Li_xCoO₂ begins at 240 °C; the decomposition starts at 290 °C.     

Ni3(PO4)2-coated Li[Ni0.8Co0.15Al0.05]O2 lithium battery electrode with improved cycling performance at 55 °C

To improve the cycling performance of LiNi_0.8Co_0.15Al_0.05O₂ at 55 °C, a thin Ni₃(PO₄) layer was homogeneously coated onto the cathode particle via simple ball milling. The morphology of the Ni₃(PO₄)₂-coated LiNi_0.8Co_0.15Al_0.05O₂ particle was characterized using SEM and TEM analysis, and the coating thickness was found to be approximately 10-20 nm. The Ni₃(PO₄)₂-coated LiNi_0.8Co_0.15Al_0.05O₂ cell showed improved lithium intercalation stability and rate capability especially at high C rates. This improved cycling performance was ascribed to the presence of Ni₃(PO₄)₂ on the LiNi_0.8Co_0.15Al_0.05O₂ particle, which protected the cathode from chemical attack by HF and thus suppressed an increase in charge transfer resistance. Transmission electron microscopy of extensively cycled particles confirmed that the particle surface of the Ni₃(PO₄)₂-coated LiNi_0.8Co_0.15Al_0.05O₂ remained almost undamaged, whereas pristine particles were severely serrated. The stabilization of the host structure by Ni₃(PO₄)₂ coating was also verified using X-ray diffraction.     

Improving the electrochemical properties of graphite/LiCoO2 cells in ionic liquid-containing electrolytes

The electrochemical performance of graphite/lithium cobalt oxide (LiCoO₂) cells in N-methoxymethyl-N,N-dimethylethylammonium bis(trifluoromethane-sulfonyl) imide (MMDMEA-TFSI)-containing electrolytes is significantly enhanced by the formation of a fluoroethylene carbonate (FEC)-derived protective film on an anode during the first cycle. The electrochemical intercalation of MMDMEA cations into the graphene layer is readily visualized by ex situ transmission electron microscopy (TEM). Moreover, differences in the X-ray diffraction (XRD) patterns of graphite electrodes in cells charged with and without FEC in dimethyl carbonate (DMC)/MMDMEA-TFSI are clearly discernible. Conclusively, the presence of FEC in MMDMEA-TFSI-containing electrolytes leads to a remarkable enhancement of discharge capacity retention for graphite/LiCoO₂ cells as compared with ethylene carbonate (EC) and vinylene carbonate (VC).     

Structural changes in a commercial lithium-ion battery during electrochemical cycling: An in situ neutron diffraction study

The structural response to electrochemical cycling of the components within a commercial Li-ion battery (LiCoO₂ cathode, graphite anode) is shown through in situ neutron diffraction. Lithuim insertion and extraction is observed in both the cathode and anode. In particular, reversible Li incorporation into both layered and spinel-type LiCoO₂ phases that comprise the cathode is shown and each of these components features several phase transitions attributed to Li content and correlated with the state-of-charge of the battery. At the anode, a constant cell voltage correlates with a stable lithiated graphite phase. Transformation to de-lithiated graphite at the discharged state is characterised by a sharp decrease in both structural cell parameters and cell voltage. In the charged state, a two-phase region exists and is composed of the lithiated graphite phase and about 64% LiC₆. It is postulated that trapping Li in the solid|electrolyte interface layer results in minimal structural changes to the lithiated graphite anode across the constant cell voltage regions of the electrochemical cycle.     

A modified Al2O3 coating process to enhance the electrochemical performance of Li(Ni1/3Co1/3Mn1/3)O2 and its comparison with traditional Al2O3 coating process

Al₂O₃-modified Li(Ni_1/3Co_1/3Mn_1/3)O₂ is synthesized by a modified Al₂O₃ coating process. The Al₂O₃ coating is carried out on an intermediate, (Ni_1/3Co_1/3Mn_1/3)(OH)₂, rather than on Li(Ni_1/3Co_1/3Mn_1/3)O₂. As a comparison, Al₂O₃-coated Li(Ni_1/3Co_1/3Mn_1/3)O₂ also is prepared by traditional Al₂O₃ coating process. The effects of Al₂O₃ coating and Al₂O₃ modification on structure and electrochemical performance are investigated and compared. Electrochemical tests indicate that cycle performance and rate capability of Li(Ni_1/3Co_1/3Mn_1/3)O₂ are enhanced by Al₂O₃ modification without capacity loss. Al₂O₃ coating can also enhance the cycle performance but cause evident capacity loss and decline of rate capability. The effect of Al₂O₃ coating and Al₂O₃ modification on kinetics of lithium-ion transfer reaction at the interface of electrode/electrolyte is investigated via electrochemical impedance spectra (EIS). The result support that the Al₂O₃ modification increase Li⁺ diffused coefficient and decrease the activation energy of Li⁺ transfer reaction but the traditional Al₂O₃ coating lead to depression of Li⁺ diffused coefficient and increase of activation energy.     

Charge–discharge and high temperature reaction of LiCoO2 in ionic liquid electrolytes based on cyano-substituted quaternary ammonium cation

The charge–discharge performance of LiCoO₂ positive electrode was observed in a mixed electrolyte system consisting of two ionic liquids: cyano-substituted quaternary ammonium bis(trifluoromethane)sulfoneimide (TFSI) and a same-anion salt of 1-ethyl-3-methyl imidazolium (EMI). The positive electrode exhibited a discharge capacity rather close to the theoretical one when N,N,N,N-cyanoethyl trimethyl ammonium salt was applied. Differential scanning calorimetry (DSC) studies revealed that these electrolytes exhibited exotherm only around 260 °C, 50 °C higher than conventional carbonate-based electrolytes. This is the first attempt to reveal the thermal stability of ionic liquid electrolyte under a practical situation.     

Cation ordering in the layered Li1+x(Ni0.425Mn0.425Co0.15)1 − xO2 materials (x = 0 and 0.12)

The layered Li_1+x(Ni_0.425Mn_0.425Co_0.15)_1 − xO₂ (x = 0 and 0.12) materials were prepared by a coprecipitation method. Their structure was investigated using the combination of X-ray and electron diffraction experiments. For both materials (x = 0 and 0.12), the electron diffraction patterns revealed an in-plane √3a_hex. × √3a_hex. superstructure in agreement with the ordering of the Li⁺, Ni²⁺, Ni³⁺, Mn⁴⁺ and Co³⁺ ions in the transition metal layers. The stoichiometry of these materials was not in agreement with an ideal ordering: the possible presence of point defects or of a domain microstructure was thus discussed. Electron diffraction also revealed that these ordered layers were slightly correlated along the c_hex. axis for both materials.     

Enhanced cycleability of LiCoO2 coated with vanadium oxides

Vanadium oxide (V₂O₅/V₂O₅ hydrates) sol–gel coatings on lithium cobalt oxide (LiCoO₂) are investigated as a means are prepared to improve the cycleability at a high-charge cut-off voltage. The V₂O₅ sol was prepared by a melt-quenching method. The crystal structure and morphology of the samples are examined by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The charge and discharge characteristics, including cycleability, are measured at room temperature, and the electrochemical properties of bare and coated LiCoO₂ are compared. Overall, a vanadium oxide coating on LiCoO₂ improves the cycleability at a high-charge cut-off voltage.     

Mechanism study of enhanced electrochemical performance of ZrO2-coated LiCoO2 in high voltage region

In this study, the mechanism of enhanced performance of ZrO₂-coated LiCoO₂ especially at high potential range is systematically investigated. Firstly, when overcharging to 4.5 V (higher than 4.2 V, the normal cutoff charging potential), phase transformation from H1 to H2 takes place with less volume expansion for ZrO₂-coated LiCoO₂ (1.2% and 2.2% for as-received one). EIS analysis indicates the growth of interfacial impedance during charging/discharging can be effectively suppressed with ZrO₂ coating on the LiCoO₂ surface. It is demonstrated as well that cation mixing of the cycled LiCoO₂ caused by re-intercalation of dissolved Co²⁺ is inhibited with the ZrO₂ coating on the LiCoO₂. Therefore the ZrO₂-coated LiCoO₂ shows great enhancement in the electrochemical properties with 85% capacity retention after 30 cycles from 3 to 4.5 V at a rate of 0.5C. Nevertheless, under the same evaluation process, the as-received LiCoO₂ possesses only 21% capacity retention, which is resulted from the formation of polymeric layers by the electrolyte decomposition on its surface, the higher volumetric changes during charging/discharging and possible cation mixing by re-intercalation of the dissolved Co²⁺.     

Magnetism and lithium diffusion in LixCoO2 by a muon-spin rotation and relaxation (μSR) technique

Microscopic magnetism of the electrochemically Li-deintercaleted Li_xCoO₂ powders has been investigated by muon-spin rotation and relaxation (μ⁺SR) spectroscopy in the temperature (T) range between 10 and 300 K. Weak transverse-field μ⁺SR measurements indicate that localized moments appear in LiCoO₂ below 60 K, while both Li_0.53CoO₂ and Li_0.04CoO₂ are paramagnetic even at 10 K. Zero-field μ⁺SR measurements for the samples with x = 0.53 and 0.04 show that the field distribution width (Δ) due to randomly oriented nuclear magnetic moments of ⁷Li and ⁵⁹Co decreases monotonically with increasing T up to 250 K, and then it decreases steeper (increasing slope (dΔ/dT)) above 250 K. Because the muon hopping rate (ν) is almost T independent for Li_0.53CoO₂ below 300 K, the decrease in Δ suggests that the time scale of Li⁺ diffusion in Li_xCoO₂ is within a microsecond scale.     

Electrochemical performance and thermal stability of LiCoO2 cathodes surface-modified with a sputtered thin film of lithium phosphorus oxynitride

A lithium phosphorus oxynitride (LiPON) glass-electrolyte thin film is coated on a lithium cobalt oxide (LiCoO₂) composite cathode by means of a radio frequency (RF) magnetron sputtering method. The effect of the LiPON coating layer on the electrochemical performance and thermal stability of the LiCoO₂ cathode is investigated. The thermal stability of the delithiated LiCoO₂ cathode in the presence of liquid electrolyte is examined by differential scanning calorimetry (DSC). It is found that the LiPON coating, improves the rate capability and the thermal stability of the charged LiCoO₂ cathode. The LiPON film appears to suppress impedance growth during cycling and inhibits side-reactions between delithiated LiCoO₂ and the electrolyte.     

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.     

Investigation of cell parameters,microstructures and electrochemical behaviour of LiMn2O4 normal and nano powders

Nano materials are usually difficult to prepare. This work presents a simple way of preparing LiMn₂O₄ nano powders using the high-energy ball milling method. This method has the advantage of producing pure, single-phase and crystalline nano powders. The milling method is carefully controlled to avoid unwanted chemical reactions that may change the stoichiometry of the material. Nano powders of between 30 and 50 nm are obtained. Structural studies of the nano powders, as well as the more conventional micron-sized LiMn₂O₄, are made using X-ray diffraction and neutron diffraction methods. Electrochemical evaluation of the materials is undertaken with a three-probe cyclic voltammetry technique and galvanostatic charge–discharge measurements. Structural studies reveal that not only are the crystallites of the nano powders much reduced in size from the normal powders, but their cell parameters are also smaller. The performance characteristics of the nano material show an improvement over that of the micron-sized material by about 17% in the 1st cycle and 70.6% in the 5th cycle, at which the capacity is 132 mAh g⁻¹. The normal material suffers from severe capacity fading but the nano material shows much improved capacity retention.     

Crystallization of LiMn2O4 observed with high temperature X-ray diffraction

We investigated the formation of LiMn₂O₄ phases by calcinating a stoichiometric mixture of Li₂CO₃ and various manganese compounds with high temperature X-ray diffraction (HT-XRD) technique to understand the influence of starting materials on the electrochemical performance. XRD measurements were carried out during heating processes from room temperature to 700 °C. In case of Li₂CO₃/electrolytic manganese dioxide and Li₂CO₃/MnCO₃ mixtures used as starting materials, Li_0.33MnO₂ phase and low crystalline phase, respectively, appeared as intermediate products during heating process followed by the crystallization into the spinel. HT-XRD observation confirmed that the LiMn₂O₄ phase was directly formed from starting Li₂CO₃/Mn₂O₃ and Li₂CO₃/Mn₃O₄ mixtures. The reactivity of the mixture, meant by the lower reaction temperature between Li and Mn compounds and the faster evolution of Li–Mn–O phase, depended on manganese compounds. The purity and stoichiometry of spinel type LiMn₂O₄ was not achieved only by the higher reactivity. From these results, the dependence of reversible capacities and cycleability of synthesized LiMn₂O₄s on the formation process which varied with the starting materials was discussed.     

Electrochemical AFM study of LiMn2O4 thin film electrodes exposed to elevated temperatures

Surface morphology changes of LiMn₂O₄ thin film positive electrodes in lithium-ion batteries after repeated potential cycling or storage at elevated temperatures were observed by in situ atomic force microscopy (AFM) to elucidate the origin of capacity fading of LiMn₂O₄. After repeated potential cycling in the overall potential range from 3.50 to 4.30 V at elevated temperatures, the entire thin film surface was covered with small round-shaped particles accompanied by capacity fading of the electrode, while no significant changes were observed at 25 °C. The discharge capacity decreased more significantly when cycled in the lower potential range (3.81–4.07 V) than when cycled in the higher potential range (4.04–4.30 V). After storage at elevated temperatures at a depth of discharge (DOD) of 75%, which is located in the lower potential range, similar surface morphology changes were observed. In addition, discharge capacity markedly decreased, and the crystallinity of the LiMn₂O₄ thin film was lowered after storage. Hence, the observed changes in morphology at elevated temperatures are closely related to capacity fading of the LiMn₂O₄ thin film. The loss of crystallinity was caused by the formation of small particles on the LiMn₂O₄ surface, which would be accelerated on the LiMn₂O₄ surface in contact with an electrolyte solution through some kind of dissolution/precipitation reaction.     

Synthesis of LixMnO2 by chemical lithiation in an aqueous media

Li_xMnO₂ (x = 0.302) was synthesized by chemical lithiation, using a formaldehyde reducing agent and a LiOH lithium source in an aqueous media. The electrochemical properties and structural stability of the product were characterized by X-ray diffraction and charge-discharge measurements. The chemically lithiated Li_xMnO₂ had a first charge and discharge capacity of 86.2 and 265 mAh/g, respectively, with good cycling behavior. Based on the electrochemical results of the first charge, a two-step mechanism of Li_xMnO₂ lithiation is proposed. γ-MnO₂ is first oxidized by formaldehyde, and then Li diffuses into the γ-MnO₂ lattice.     

Pyroxene LiVSi2O6 as an electrode material for Li-ion batteries

Lithium vanadium metasilicate (LiVSi₂O₆) with pyroxene structure has been exploited as an electrode material for Li-ion batteries. Galvanostatic charge and discharge tests show that LiVSi₂O₆ is able to deliver a capacity of 85 mAh g⁻¹ at 30 °C, and a high capacity of 181 mAh g⁻¹ at 60 °C. The high capacity is believed to be due to the reactions of V³⁺/V⁴⁺ and V²⁺/V³⁺redox couples, accompanied by the excess 0.42 Li⁺ insertion into the lattice forming a Li-rich phase Li_1.42VSi₂O₆. High-energy synchrotron XRD combined with the Rietveld refinement analysis confirms that the electrochemical delithiation-lithiation reaction proceeds by a single phase redox mechanism with an overall volume variation of 1.9% between LiVSi₂O₆ and its delithiated state, indicating a very stable framework of LiVSi₂O₆ for Li⁺ ions extraction-insertion.     

Synthesis and crystal structure analysis of complex hydride Mg(BH4)(NH2)

Complex hydride Mg(BH₄)(NH₂), which consists of double anion BH₄⁻ and NH₂⁻, was synthesized and the crystal structure was analyzed by synchrotron X-ray diffraction. The mixture sample of Mg(BH₄)₂ + Mg(NH₂)₂ prepared by ball milling was reacted and crystallized to Mg(BH₄)(NH₂) by heating at about 453 K. This crystal phase transforms into amorphous phase above 473 K and subsequently the dehydrogenation begins. The crystal structure of Mg(BH₄)(NH₂) was determined from measurement data at 453 K (chemical formula: Mg_0.94(BH₄)₁(NH₂)_0.88, crystal system: tetragonal, space group: I4₁ (No.80), Z = 8, lattice constants: a = 5.814(1), c = 20.450(4) Å at 453 K). Mg(BH₄)(NH₂) is ionic crystal which the cation (Mg²⁺) and the anions (BH₄⁻ and NH₂⁻) are stacking alternately along the c-axis direction. Two BH₄⁻ and two NH₂⁻ tetrahedrally coordinate around Mg²⁺ ion.     

Influence of surface modification of LiCoO2 by organic compounds on electrochemical and thermal properties of Li/LiCoO2 rechargeable cells

LiCoO₂ is the most famous positive electrode (cathode) for lithium ion cells. When LiCoO₂ is charged at high charge voltages far from 4.2 V, cycleability of LiCoO₂ becomes worse. Causes for this deterioration are instability of pure LiCoO₂ crystalline structure and an oxidation of electrolyte solutions LiCoO₂ at higher charge voltages. This electrolyte oxidation accompanies with the partial reduction of LiCoO₂. We think more important factor is the oxidation of electrolyte solutions. In this work, influence of 10 organic compounds on electrochemical and thermal properties of LiCoO₂ cells was examined as electrolyte additives. As a base electrolyte solution, 1 M (M: mol L⁻¹) LiPF₆-ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (mixing volume ratio = 3:7) was used. These compounds are o-terphenyl (o-TP), Ph-X (CH₃)_n (n = 1 or 2, X = N, O or S) compounds, adamantyl toluene compounds, furans and thiophenes. These additives had the oxidation potentials (Eox) between 3.4 and 4.7 V vs. Li/Li⁺. These Eox values were lower than that (6.30 V vs. Li/Li⁺) of the base electrolyte. These additives are oxidized on LiCoO₂ during charge of the LiCoO₂ cells. Oxidation products suppress the excess oxidation of electrolyte solutions on LiCoO₂. As a typical example of these organic compounds, o-TP (Eox: 4.52 V) was used to check the fundamental properties of these organic additives. Charge-discharge cycling tests were carried out for the Li/LiCoO₂ cells with and without o-TP. Constant current charge at 4.5 V is mainly used as a charge method. Cells with 0.1 wt.% o-TP exhibited slightly better cycling performance and lower polarization than those without additives. Lower polarization arises from a decrease in a resistance of interface between electrolyte solutions and LiCoO₂ by surface film formation resulted from oxidation of o-TP. Oxidation products were found by mass spectroscopy analysis to be mixture of several polycondensation compounds made from two to four terphenly monomers. Thermal stability of LiCoO₂ with electrolyte solutions did not improve by addition of o-TP. Slightly better charge-discharge cycling properties were obtained by using organic modifiers. However, when industrial applications were considered, drastic improvements have not been obtained yet. One of reasons may be too large influence of the deterioration of stability of pure LiCoO₂ structure at high voltage charging for industrial use. We hope to realize the tremendous improvements of high energy, long cycle life and safe lithium cells by the combination of both LiCoO₂ with more stable structure such as LiCoO₂ treated with MgO and new organic additives with molecular structure more carefully designed.