Charged Cathode Research Articles

Investigating local degradation and thermal stability of charged nickel-based cathode materials through real-time electron microscopy.

In this work, we take advantage of in situ transmission electron microscopy (TEM) to investigate thermally induced decomposition of the surface of Li(x)Ni(0.8)Co(0.15)Al(0.05)O2 (NCA) cathode materials that have been subjected to different states of charge (SOC). While uncharged NCA is stable up to 400 °C, significant changes occur in charged NCA with increasing temperature. These include the development of surface porosity and changes in the oxygen K-edge electron energy loss spectra, with pre-edge peaks shifting to higher energy losses. These changes are closely related to O2 gas released from the structure, as well as to phase changes of NCA from the layered structure to the disordered spinel structure, and finally to the rock-salt structure. Although the temperatures where these changes initiate depend strongly on the state of charge, there also exist significant variations among particles with the same state of charge. Notably, when NCA is charged to x = 0.33 (the charge state that is the practical upper limit voltage in most applications), the surfaces of some particles undergo morphological and oxygen K-edge changes even at temperatures below 100 °C, a temperature that electronic devices containing lithium ion batteries (LIB) can possibly see during normal operation. Those particles that experience these changes are likely to be extremely unstable and may trigger thermal runaway at much lower temperatures than would be usually expected. These results demonstrate that in situ heating experiments are a unique tool not only to study the general thermal behavior of cathode materials but also to explore particle-to-particle variations, which are sometimes of critical importance in understanding the performance of the overall system.

Thermal stability of lithium-rich manganese-based cathode

Thermal stability of a lithium-rich layered oxide cathode material with composition 0.5Li4/3Mn2/3O2–0.5LiNi1/3Co1/3Mn1/3O2 (LMO–NCM) is investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Investigated material shows higher thermal stability (higher onset temperature) than LiCoO2. The state of charge and previous cycling history of LMO–NCM, particularly activation of lithium manganese oxide (LMO) “component” during first charge, affect its thermal stability. Activation of LMO has an ambivalent effect on cathode thermal stability: on the one hand, the onset temperature of cathode decomposition is increased, probably due to the change in oxidation states of Ni, Co and Mn after the first cycle. On the other hand, the enthalpy of decomposition increases, presumably due to formation of unstable oxygen in the lattice. X-ray diffraction (XRD) analyses of cathodes before and after thermal decomposition at 600°C indicate differences in decomposition reactions of charged cathodes, depending on state of charge and cycling history.

Comparison of the Thermal Stability of FeOF Cathode in Na-Ion Batteries and Li-Ion Batteries

Recently ambient temperature sodium-ion batteries have drawn great attention particularly in large-scale electric energy storage applications for renewable energy and smart grid due to the low cost and abundant sodium resources. Up to now, lots of cathode and anode materials as well as electrolytes have been proposed for Na-ion batteries. However, little research about the safety issue of Na-ion batteries has been carried out. It is well known that Li-ion batteries have thermal risk under abuse or severe environment. Because Na-ion batteries have similar working mechanism as Li-ion batteries and sodium shows even higher reactivity against air, the study on thermal stability of Na-ion batteries is indispensable for its practical application.Iron oxyfluoride (FeOF) is a kind of iron based conversion type cathode material, which had been proposed as a cathode material for Li-ion batteries and obtained a large practical capacity due to conversion reactions [1, 2]. In this study, FeOF was also applied as the cathode material for Na-ion batteries. After electrochemical pretreatment in Na-ion or Li-ion batteries, the charged/discharged FeOF cathodes were mixed with corresponding electrolytes and thermally analyzed. By varying the ratio of cathode/electrolyte in the mixture, the heat generation mechanisms of FeOF cathodes in Na-ion and Li-ion batteries were discussed, and the thermal stability of Na-ion and Li-ion batteries were compared.FeOF was synthesized by a reaction of 1:2.33 molar ratio of Fe2O3 (Sigma-Aldrich) and FeF3 (Soekawa Chemicals Co., Ltd.) at a temperature over 1000 oC in a roll-quenching machine (Harddays Co. Ltd). The synthesized FeOF had been indexed as a tetragonal structure with P42/mnm space group. The obtained FeOF flake was ground with acetylene black using a planetary ball milling machine at a weight ratio of 70:25. 5 wt.% PVdF and polyacrylate binders were added in the cathodes for Li-ion batteries and Na-ion batteries, respectively. The electrochemical measurements were carried out with 2032 coin-type two-electrode cells. In Li-ion batteries, Li foil was used as a counter electrode and 1 mol/cm3 LiPF6/EC-DMC or 1 mol/cm3 LiClO4/PC as an electrolyte. In Na-ion batteries, Na foil was used as a counter electrode and 1 mol/cm3 NaClO4/PC as an electrolyte. The cells were cycled at constant current densities of 10 mA/g (0.02 mA/cm2). For thermal analysis, the cycled cathodes were taken out from the disassembled cells, soaked in PC or DMC, rinsed by DMC, and then vacuum dried. Finally, the cathode powder was packed into a stainless-steel pan together with some amount of corresponding electrolyte. During TG-DSC analysis, the hermetically sealed pan was heat up to 500 oC with a heating rate of 5 oC/min.Figure 1 shows the DSC curves of the mixtures of given amount of lithiated or sodiated FeOF cathode and 1 μl corresponding PC-based electrolytes. In both Li- and Na-ions batteries, charged FeOF cathodes showed large exothermic heat with an onset temperature of about 100 oC. This was attributed to reactions between expanded FeOF at the charged state and the electrolyte. Moreover, intercalated Na ions were found to induce an exothermic heat peaked at about 450 oC, while this peak was not observed in Li-ion batteries. Detailed discussion will be presented at the conference.

Structural Changes and Thermal Decomposition of Ni Based Cathode Materials for Li-Ion Batteries Studied By in-Situ XRD

The three-component system Li[NixCoyMnz]O2 has outstanding electrochemical properties due to advantage of the high capacity of LiNiO2, thermal stability and low cost of manganese in LiMnO2 and layered characteristics of LiCoO2. While capacity of Li[NixCoyMnz]O2 is similar to capacity of LiCoO2, it is a promising alternative in terms of performance, safety and cost. Various material combinations are being developed to achieve the higher capacity by increasing Ni content while retaining its advantages. Although higher Ni content contributes to better capacity in Li[NixCoyMnz]O2, but on the other hand, it tends to decrease the thermal stability of electrode material. Safety is one of the most important considerations in EV and HEV. First experimental condition is same state of charge with different Ni contents. Interesting phase transition behavior during heating is observed in Li0.66Ni0.5Co0.2Mn0.3O2 cathode material. Li0.66Ni0.5Co0.2Mn0.3O2 without electrolyte converts from layered (R-3m) structure to disordered LiM2O4-type spinel (Fd-3m) around 415 ºC. Upon further heating, M3O4-type spinel (Fd-3m) appears and co-exists with LiM2O4-type spinel (Fd-3m) from 497 ºC and remains in this structure up to 600 ºC. In the absence of electrolyte, no peaks of MO-type rock salt phase (Fm-3m) appear until 600 ºC whereas in the presence of electrolyte, further phase transition from M3O4-type spinel (Fd-3m) to MO-type rock salt phase (Fm-3m) takes place above 415 ºC. The electrolyte accelerates the thermal decomposition of charged cathode materials. The presence of electrolyte alters the paths of structural changes and lowers the onset temperatures of thermal decomposition reactions. In case of Li0.66Ni0.5Co0.2Mn0.3O2with electrolyte, more dramatic structural changes are observed in comparison with the one in the absence of electrolyte. The sample with electrolyte shows the structural decomposition to MO-type rock salt phase at 415 ºC, in the early stage of heating. Thermal behavior of Li0.33Ni0.6Co0.2Mn0.2O2 cathode material is similar to the one of Li0.66Ni0.5Co0.2Mn0.3O2. Li0.33Ni0.6Co0.2Mn0.2O2 without electrolyte converts from layered (R-3m) structure to disordered LiM2O4-type spinel (Fd-3m) around 374 ºC. Upon further heating, M3O4-type spinel (Fd-3m) co-exists with LiM2O4-type spinel (Fd-3m) from 497 ºC and maintains this structure up to 600 ºC. Presence of electrolyte initiates additional phase transitions in this cathode material. M3O4-type spinel (Fd-3m) starts to convert into MO-type rock salt phase (Fm-3m) at 374 ºC. Above 579 ºC, pure MO-type rock salt phase (Fm-3m) is observed. Second condition is same Ni contents in electrode material with different state of charge. The thermal behavior of Li0.66Ni0.5Co0.2Mn0.3O2 cathode material is mentioned above. Li0.33Ni0.6Co0.2Mn0.2O2 without electrolyte converts from layered (R-3m) structure to disordered LiM2O4-type spinel (Fd-3m) around 340 ºC. Upon further heating, M3O4-type spinel (Fd-3m) co-exists with LiM2O4-type spinel (Fd-3m) from 449 ºC and maintains this structure up to 600 ºC. Presence of electrolyte initiates additional phase transitions in this cathode material. M3O4-type spinel (Fd-3m) starts to convert into MO-type rock salt phase (Fm-3m) at 436 ºC. Above 560 ºC, pure MO-type rock salt phase (Fm-3m) is observed. In the presence of electrolyte, thermal induced phase transition of Li0.33Ni0.6Co0.2Mn0.2O2 shows conversion from LiM2O4-type spinel (Fd-3m) through M3O4-type spinel (Fd-3m) to MO-type rock salt phase (Fm-3m) step by step. On the other hand, thermal decomposition behavior of Li0.66Ni0.6Co0.2Mn0.2O2 shows phase transition to M3O4-type spinel (Fd-3m) and MO-type rock salt phase (Fm-3m) at the same time. This result of different thermal behavior of Li0.66Ni0.6Co0.2Mn0.2O2 and Li0.33Ni0.6Co0.2Mn0.2O2contributes towards better understanding for thermal stability of Ni-based cathode materials. Working on other samples and more detailed discussion will be presented at the time of meeting.Fig1. In-situ XRD patterns of the (a) 33% (b) 66% SOC LiNi0.6Co0.2Mn0.2O2 in the presence of electrolyte heated from 25℃ to 600℃

Principal Factors that Contribute to the Gas Formation in High Voltage Cathode Systems of Lithium Ion Batteries - a Discussion about the Interaction Between Carbon Black and the Electrolyte -

Interface reactions, especially those that lead to gas evolution, between carbon black (CB) and electrolyte in the positive electrode at potentials over 4.5 V have been studied. Key measures to suppress these undesirable reactions and sequential side reactions between the components of the cathode and the electrolyte will be discussed. In this study, we focused on the investigation of the behavior of CB at high voltages in combination with active material and binder, a topic that has not been subject to a lot of research so far. We believe that conductive agents of the positive electrode like CB are subject to anion intercalation, similar to what was observed for a graphite positive electrode [1]. The reactions between different CB and the electrolyte in the presence of active material was studied by a research team comprising of a carbon and an active material producer. The first part of the study will focus on the quantification of the gas that is formed due to the presence of the carbon material. Various conductive agents such as acetylene black (AB), furnace black (FB), specially customized AB as well as graphite (GR) were examined. The experiments revealed that in the high voltage system the gas evolution was induced by both, the cathode active material and the conductive agents. Mass spectroscopic analysis indicated that the source of the gas was oxidized electrolyte. The specific amount of gas that was generated due to the presence of the CB was found to be 8 to 15 times higher than the portion that originated from the cathode active material LiNi0.5Mn1.5O4(LNMO) itself, Fig. 1. In order to interpret the above findings, the electrochemical performance of the different carbon materials at high voltages was analyzed in more detail utilizing carbon positive electrode/Li-metal half cells (carbon cell). At a potential of 5.0 V vs. Li+/Li at 60 oC the amount of gas detected was directly proportional to the degree of reversibility of the cathodic/anodic current as summarized in Fig. 2. Furthermore, this reversibility tends to be higher when the anion intercalation/de-intercalation reaction is more dominant than the decomposition of some impurities, and we conclude that the anion intercalation is an important factor for the gas evolution at high voltages. In a second experiment, the carbon cells were discharged stepwise in the voltage region of 5.2 to 4.0 V in decrements of 0.2 V (CC followed by CV). Although the carbon cell was being discharged, most of the cells required the application of a charge current in order to keep the voltage at the desired level during the CV phase. In order to explain the above results an interaction between the carbon additive, the electrolyte and the cathode material is considered. We assume that the charged (= anion intercalated) CB in a high voltage cathode system is very unstable and tends to release the anions back to the electrolyte which can be considered as a “self-discharge”. As the discharged CB is in contact with the fully charged cathode material it can be recharged via this solid-solid interface and discharge itself at the same time again via the solid-liquid interface (local battery system within the cathode). This complex reaction that includes the active material reduction and electrolyte decomposition continues until the electrode potential reaches the 4.0V level where the anion intercalation is stopped. The study of a larger range of commonly used conductive agents allowed us also to examine the influence of specific material properties on the gas formation phenomena. Further details shall be presented at the meeting.Reference[1] J. A. Seel and J. R. Dahn, J. Electrochem. Soc., 147(2000) 892.

Electrochemical and thermal properties of P2-type Na2/3Fe1/3Mn2/3O2 for Na-ion batteries

P2-type Na2/3Fe1/3Mn2/3O2 was prepared by means of solid-state reaction. Its electrochemical properties as a cathode for sodium-ion batteries were investigated in the 1.5–4.3 V range vs. Na metal. This material delivers an initial discharge capacity as high as 193 mAh/g and maintains 153 mAh/g in 40 cycles. Such a high reversible capacity is achieved because both Mn3+/Mn4+ and Fe3+/Fe4+ become active in the studied voltage range, as confirmed by X-ray absorption spectroscopy (XAS). The thermal stability of charged cathodes was investigated by differential scanning calorimetry (DSC). Although the total heat generated by the fully charged Na0.25Fe1/3Mn2/3O2 is larger than that of the Li0.5CoO2 used as a reference, it is remarkably stable up to ca. 320 °C.

Ionic states associated with extraction/infusion of Li ions at LiFePO4 thin film cathode surface

A 100‐nm‐thick lithium iron phosphate (LiFePO4) film was deposited by sputtering, and charge–discharge cycle tests were performed using LiFePO4 films as cathodes in electrochemical cells. After removing the electrolyte, the charged and discharged LiFePO4 thin film cathodes were investigated using XPS. XPS results revealed that the peak area intensity of Fe2+ state increases for charged cathodes, while that of Fe3+ increases for discharged cathodes. In addition, the variation of Fe3+ and Fe2+ peak area intensities gradually attenuated as the number of charge–discharged cycles increased. The results reflect the performance of the battery and its degradation. Copyright © 2014 John Wiley & Sons, Ltd.

Thermally Stable Electrolyte for Lithium-Ion Batteries

The weight of gas formed by Li-ion cell components heated to 400°C was studied by DSC. Electrolyte released the most gas – over 70% of its original weight during the heat treatment, while fully charged cathode and anode released gas at 4.6% and 3.9% of their original weight, respectively. Thus, the weight of gas formed during heat treatment by various solvents, LiPF6, and electrolytes containing various solvents or lithium salts was studied. Solvent thermal stability was the key to reducing the gas formation of electrolyte at elevated temperature. Electrolyte containing g-butyrolactone (GBL) solvent delayed the venting of Li-ion cells by approximately 50 °C compared with conventional electrolyte containing carbonate solvents during a hot box test. Cell venting was observed to be due to gas pressure from electrolyte decomposition in the cell.

Impact of active material surface area on thermal stability of LiCoO2 cathode

Thermal stability of charged LiCoO2 cathodes with various surface areas of active material is investigated in order to quantify the effect of LiCoO2 surface area on thermal stability of cathode. Thermogravimetric analyses and calorimetry have been conducted on charged cathodes with different active material surface areas. Besides reduced thermal stability, high surface area also changes the active material decomposition reaction and induces side reactions with additives. Thermal analyses of LiCoO2 delithiated chemically without any additives or with a single additive have been conducted to elaborate the effect of particle size on side reactions. Stability of cathode–electrolyte system has been investigated by accelerating rate calorimetry (ARC). Arrhenius activation energy of cathode decomposition has been calculated as function of conversion at different surface area of active material.

Thermal instabilities of organic carbonates with charged cathode materials in lithium-ion batteries

Thermal instabilities of organic carbonates with charged cathode materials in lithium-ion batteries

Oxygen-Release-Related Thermal Stability and Decomposition Pathways of LixNi0.5Mn1.5O4 Cathode Materials

The thermal stability of charged cathode materials is one of the critical properties affecting the safety characteristics of lithium-ion batteries. New findings on the thermal-stability and thermal-decomposition pathways related to the oxygen release are discovered for the high-voltage spinel LixNi0.5Mn1.5O4 (LNMO) with ordered (o-) and disordered (d-) structures at the fully delithiated (charged) state using a combination of in situ time-resolved X-ray diffraction (TR-XRD) coupled with mass spectroscopy (MS) and X-ray absorption spectroscopy (XAS) during heating. Both o- and d- LixNi0.5Mn1.5O4, at their fully charged states, start oxygen-releasing structural changes at temperatures below 300 °C, which is in sharp contrast to the good thermal stability of the 4V-spinel LixMn2O4 with no oxygen being released up to 375 °C. This is mainly caused by the presence of Ni4+ in LNMO, which undergoes dramatic reduction during the thermal decomposition. In addition, charged o-LNMO shows better thermal stability than...

Enhanced electricity generation by triclosan and iron anodes in the three-chambered membrane bio-chemical reactor (TC-MBCR)

A three-chambered membrane bio-chemical reactor (TC-MBCR) was developed. The stainless steel membrane modules were used as cathodes and iron plates in the middle chamber served as the anode. The TC-MBCR was able to reduce fouling, remove triclosan (TCS) from a synthetic wastewater treatment and enhance electricity generation by ~60% compared with the cell voltage before TCS addition. The TC-MBCR system generated a relatively stable power output (cell voltage ~0.2V) and the corrosion of iron plates contributed to electricity generation together with microbes on iron anode. The permeation flow from anode to cathode chamber was considered important in electricity generation. In addition, the negatively charged cathode membrane and Fe(2+)/Fe(3+) released by iron plates mitigated membrane fouling by approximately 30%, as compared with the control. The removal of COD and total phosphorus was approximately 99% and 90%. The highest triclosan removal rate reached 97.9%.

In Situ Energy Dispersive X-ray Diffraction Study of Prototype LiMnO4- and LiFePO4-Based Coin Cell Batteries

This in-situ study demonstrates the feasibility of using energy dispersive x-ray diffraction (EDXRD) to characterize the internal structure and chemical composition of coin cell type Li-ion batteries. CR2032 coin cell batteries containing Li1-3xCrxFePO4 (with x = 0.015) or LixMn1.42Ni0.42Co0.16O4 cathode were studied by EDXRD measurement. EDXRD patterns with 50 to 70 scans were made between top and bottom of charged or discharged coin cells of about 3.2 mm thickness. The seven different layers of materials contained in the Li1-3xCrxFePO4 coin cell can be clearly identified from the measured 50 in-situ EXRD patterns. For the LixMn1.42Ni0.42Co0.16O4 cell, the Bragg lines shift to higher energy in the charged cathode consistent with a lattice contraction with decreased Li content. The charged spectrum Bragg lines are broadened due to two phase behavior. The detailed electrochemical phases of the cathode materials and other layers of materials in this prototype LiMn2O4-based coin cell have been profiled, using EDXRD, as a function of state of charge and as a function of z-position in the interior of the cell. Our results demonstrate that in-situ EDXRD characterization of the crystal structures and phase distribution of coin cell batteries in space and time is feasible under different charge/discharge conditions.

Investigation of the gas evolution in lithium ion batteries: effect of free lithium compounds in cathode materials

The evolution of gas in lithium ion batteries (LIBs) was investigated. The large amount of gas emission related to a charged cathode has been a critical issue because it causes deformation and performance degradation of LIBs. This study examined the effect of free lithium compounds such as Li2CO3 or LiOH on gas generation, which revealed several different features comparing with gas generation related to the cathode active materials themselves: CO2 was the main gas generated, chain-structured carbonate solvents such as dimethyl carbonate or ethyl methyl carbonate generated more gas than cyclic-structured ethylene carbonate, and the gas generation did not occur without LiPF6 in the electrolyte solution. These were found to be the main reason for the different gas-generating behaviors between LiCoO2 (LCO) and LiNi0.85Co0.12Al0.03O2 (NCA) cathodes. For LCO, which has a very small amount of free lithium compounds on the surface, the gas was generated mainly by a reaction between delithiated LCO itself and the electrolyte solution, whereas a considerable amount of gas was generated by surface free lithium for NCA. Therefore, the removal of free lithium compounds is essential, particularly for NCA, to prevent the swelling of LIBs.

Thermal stability of charged LiNi0.5Co0.2Mn0.3O2 cathode for Li-ion batteries investigated by synchrotron based in situ X-ray diffraction

Structural changes for LiNi0.5Co0.2Mn0.3O2 cathode material of lithium-ion battery with and without electrolyte during heating from 25 to 600°C are investigated using synchrotron based in situ X-ray diffraction. LiNi0.5Co0.2Mn0.3O2 without electrolyte first converts from a layered structure to disordered LiM2O4-type spinel and M3O4-type spinel phase as the temperature increases, then two different types of disordered spinel phases are co-existed up to 600°C and no further decomposition to MO-type rock salt phase is presented at all. The electrolyte accelerates the thermal decomposition of the charged cathode materials. The presence of the electrolyte alters the paths of the structural changes and lowers the onset temperatures of the thermal decomposition reactions. In the case of LiNi0.5Co0.2Mn0.3O2 with electrolyte, more dramatic structural changes are observed compared with LiNi0.5Co0.2Mn0.3O2 without electrolyte and MO-type rock salt phase and metallic phase are presented at the end of the heating.

Correlating Structural Changes and Gas Evolution during the Thermal Decomposition of Charged LixNi0.8Co0.15Al0.05O2 Cathode Materials

In this work, we present results from the application of a new in situ technique that combines time-resolved synchrotron X-ray diffraction and mass spectroscopy. We exploit this approach to provide direct correlation between structural changes and the evolution of gas that occurs during the thermal decomposition of (over)charged cathode materials used in lithium-ion batteries. Results from charged LixNi0.8Co0.15Al0.05O2 cathode materials indicate that the evolution of both O2 and CO2 gases are strongly related to phase transitions that occur during thermal decomposition, specifically from the layered structure (space group R3m) to the disordered spinel structure (Fd3m), and finally to the rock-salt structure (Fm3m). The state of charge also significantly affects both the structural changes and the evolution of oxygen as the temperature increases: the more extensive the charge, the lower the temperature of the phase transitions and the larger the oxygen release. Ex situ X-ray absorption spectroscopy (XA...

Encapsulation of LiNi0.5Co0.2Mn0.3O2 with a thin inorganic electrolyte film to reduce gas evolution in the application of lithium ion batteries

Emission of gas at charged states of lithium ion batteries (LIBs) is a significant problem because it causes swelling and deformation of LIBs. In this study, the gas generation mechanism is investigated and preventative measures are developed. Decomposition of the electrolyte solution related to residual lithium compounds on the surface of LiNi0.5Co0.2Mn0.3O2 (NCM523) and to the structural change in the cathode active material is investigated as two main mechanisms of gas generation in LIB cells. NCM523 particles are encapsulated in a continuous lithium lanthanum titanium oxide (LLTO) thin layer to inhibit gas-generating reactions. The LLTO layer fixes free lithium of the cathode surface and effectively suppresses side reactions between the charged cathode active material and the electrolyte solution, resulting in substantial reduction of the gas generation. In addition, LLTO-coated NCM523 shows improved capacity retention without any loss of capacity or rate performance because LLTO is a good conductor of Li ions.

Energy Dispersive X-ray Diffraction Profiling of Prototype LiMn2O4-Based Coin Cells

This study demonstrates the feasibility of using energy dispersive X-ray diffraction (EDXRD) to characterize the internal structure and chemical composition of coin cell type Li-ion batteries. CR2032 coin cell batteries containing Li1–xMn1.42Ni0.42Co0.16O4 cathode were studied and profiled by collecting 70 EDXRD patterns collected at equal spatial intervals between top and bottom of charged and discharged coin cells of about 3.2 mm thickness. Diffraction profiling of the discharged cell manifests the spinel structure with essentially constant Li content across the entire cathode. In contrast, the Bragg lines of the charged cathode cell shift to higher energy, consistent with a lattice contraction with decreased Li content. The charged-cell spectrum Bragg lines are also broadened due to two phase behavior and exhibit a distinct Li-content gradient across the cathode. The detailed electrochemical phases of the cathode materials and other layers of materials in these prototype LiMn2O4-based coin cells have been profiled, using EDXRD, as a function of z-position in the interior of the cells. Our results demonstrate that EDXRD characterization of the crystal structures and phase distribution of coin cell batteries in space and under different charge/discharge conditions should be routinely feasible.

An electron energy loss spectroscopy and electron diffraction study of the Pmnb polymorph of Li2MnSiO4

The Mn valency and the crystallinity of Li2MnSiO4 cathodes (Pmnb form) were examined with electron energy-loss spectroscopy (EELS) and selected area electron diffraction (SAED) both before and after electrochemical lithium extraction. A decrease in the crystallinity of the delithiated charged cathode particles compared to the as-prepared material was observed. The decrease in crystallinity varied from particle to particle. EELS analysis showed that the non-uniform decrease in crystallinity was due to a non-uniform extraction of lithium from the particles. The observed decrease in discharge capacity of the Pmnb polymorph of Li2MnSiO4 with cycling was attributed to the progressive loss of crystallinity and the structural collapse of Li diffusion pathways.

Investigation on suppressed thermal runaway of Li-ion battery by hyper-branched polymer coated on cathode

A polymeric material consisting of self-terminated oligomers with hyper branched architecture, also known as STOBA, is used as surface coating of Li(Ni0.4Co0.2Mn0.4)O2 cathode particles, and it is shown to effectively suppress thermal runaway and slow down voltage drop of graphite/Li(Ni0.4Co0.2Mn0.4)O2 18650-battery during nail-penetration test. In situ synchrotron X-ray diffraction study indicates that the STOBA coating does not affect the layer-to-spinel phase transformation, suggesting that the suppressing mechanism is operative below the temperature of oxygen release from the oxide particles. Data of electrochemical and differential scanning calorimetry (DSC) analyses suggest that STOBA coating passivates the surface activity of charged cathode particles so that formation of solid-electrolyte-interphase (SEI) under high potential is suppressed and that heat-generation rate arising from SEI break-down and electrolyte decomposition by the charged cathode surface upon heating is significantly reduced. The combination of the reduced joule-heating and diminished heat generation by the electrolyte-oxide interfacial reaction may contribute to the suppression of thermal runaway as a result of the STOBA coating.