Studies on the enhancement of solid electrolyte interphase formation on graphitized anodes in LiX-carbonate based electrolytes using Lewis acid additives for lithium-ion batteries

The new electrolyte systems utilizing one type of Lewis acids, the boron based anion receptors (BBARs) with LiF, Li₂O, or Li₂O₂ in carbonate solutions have been developed and reported by us. These systems open up a new approach in developing non-aqueous electrolytes with higher operating voltage and less moisture sensitivity for lithium-ion batteries. However, the formation of a stable solid electrolyte interphase (SEI) layer on the graphitized anodes is a serious problem needs to be solved for these new electrolyte systems, especially when propylene carbonate (PC) is used as a co-solvent. Using lithium bis(oxalato)borate (LiBOB) as an additives, the SEI layer formation on mesophase carbon microbeads (MCMB) anode is significantly enhanced in these new electrolytes containing boron-based anion receptors, such as tris(pentafluorophenyl) borane, and lithium salt such as LiF, or lithium oxides such as Li₂O or Li₂O₂ in PC and dimethyl carbonate (DMC) solvents. The cells using these electrolytes and MCMB anodes cycled very well and the PC co-intercalation was suppressed. Fourier transform infrared spectroscopy (FTIR) studies show that one of the electrochemical decomposition products of LiBOB, lithium carbonate (Li₂CO₃), plays a quite important role in the stablizing SEI layer formation.     

New electrolytes for lithium ion batteries using LiF salt and boron based anion receptors

The thermal and electrochemical stability, as well as compatibility with various bench mark cathode and anode materials of two new lithium fluoride salt (LiF) based electrolytes have been studied. These two new electrolytes are formed by using boron-based anion receptors, tris(pentafluorophenyl) borane (TPFPB), or tris(2H-hexafluoroisopropyl) borate (THFPB) as additives, which were designed and synthesized at Brookhaven National Laboratory (BNL), to dissolve the LiF salt in carbonate solvents. The transference number of Li⁺ for these electrolytes is as high as 0.7 and the room-temperature conductivity is around 2 × 10⁻³ S cm⁻¹. The electrolytes containing propylene carbonate (PC) show superior low-temperature conductivity properties. The electrochemical window is approaching 5.0 V. It was also found that the new electrolytes work well with LiCoO₂ or LiMn₂O₄ cathodes. However, when PC containing electrolytes were used, PC co-intercalation is still a problem for graphite anodes. The formation of a stable solid electrolyte interface layer on the surface of anode in this type of electrolyte needs to be studied further.     

Low-temperature solid oxide fuel cells with La1−xSrxMnO3 as the cathodes

La_1−xSr_xMnO₃ (LSM) has been widely developed as the cathode material for high-temperature solid oxide fuel cells (SOFCs) due to its chemical and mechanical compatibilities with the electrolyte materials. However, its application to low-temperature SOFCs is limited since its electrochemical activity decreases substantially when the temperature is reduced. In this work, low-temperature SOFCs based on LSM cathodes are developed by coating nanoscale samaria-doped ceria (SDC) onto the porous electrodes to significantly increase the electrode activity of both cathodes and anodes. A peak power density of 0.46 W cm⁻² and area specific interfacial polarization resistance of 0.36 Ω cm² are achieved at 600 °C for single cells consisting of Ni-SDC anodes, LSM cathodes, and SDC electrolytes. The cell performances are comparable with those obtained with cobalt-based cathodes such as Sm_0.5Sr_0.5CoO₃, and therefore encouraging in the development of low-temperature SOFCs with high reliability and durability.     

Electrochemistry and safety of Li4Ti5O12 and graphite anodes paired with LiMn2O4 for hybrid electric vehicle Li-ion battery applications

A promising anode material for hybrid electric vehicles (HEVs) is Li₄Ti₅O₁₂ (LTO). LTO intercalates lithium at a voltage of ∼1.5 V relative to lithium metal, and thus this material has a lower energy compared to a graphite anode for a given cathode material. However, LTO has promising safety and cycle life characteristics relative to graphite anodes. Herein, we describe electrochemical and safety characterizations of LTO and graphite anodes paired with LiMn₂O₄ cathodes in pouch cells. The LTO anode outperformed graphite with regards to capacity retention on extended cycling, pulsing impedance, and calendar life and was found to be more stable to thermal abuse from analysis of gases generated at elevated temperatures and calorimetric data. The safety, calendar life, and pulsing performance of LTO make it an attractive alternative to graphite for high power automotive applications, in particular when paired with LiMn₂O₄ cathode materials.     

Development of trimetallic Ni–Cu–Zn anode for low temperature solid oxide fuel cells with composite electrolyte

Trimetallic alloys of Ni_0.6Cu_0.4−xZn_x (x = 0, 0.1, 0.2, 0.3, 0.4) have been investigated as promising anode materials for low temperature solid oxide fuel cells (SOFCs) with composite electrolyte. The alloys have been obtained by reduction of Ni_0.6Cu_0.4−xZn_xO oxides, which are synthesized by using the glycine–nitrate process. Increasing the Zn content x decreases the particle sizes of the oxides at a given sintering temperature. Fuel cells have been constructed using lithiated NiO as cathode and as-prepared alloys as anodes based on the composite electrolyte. Peak power densities are observed to increase with the increasing Zn addition concentration into the anode. The maximum power density of 624 mW cm⁻² at 600 °C, 375 mW cm⁻² at 500 °C has been achieved for the fuel cell equipped with Ni_0.6Zn_0.4 anode. A.c. impedance results show that the resistances dramatically decrease with increasing temperatures under open circuit voltage state. Both cathodic and anodic interfacial polarization resistances increase with the amplitude of applied DC voltage. Possible reaction process for H₂ oxidation reaction at anode based on composite electrolyte has been proposed for the first time. The stability of the fuel cell with Ni_0.6Cu_0.2Zn_0.2 composite anode has been investigated. The results indicate that the trimetallic Ni_0.6Cu_0.4−xZn_x anodes are considerable for low temperature SOFCs.     

Predicting electrical and thermal abuse behaviours of practical lithium-ion cells from accelerating rate calorimeter studies on small samples in electrolyte

An accelerating rate calorimeter (ARC) is used to measure the thermal stability of de-intercalated Li_1+xMn_2−xO₄ in LiPF₆ EC:DEC (33:67) electrolyte. Self-heating is detected well after the 80°C onset of self-heating measured for lithium intercalated mesocarbon microbead (MCMB) electrodes in LiPF₆ EC:DEC (33:67) electrolyte. As a result, the initial self-heating measured in a practical carbon/Li_1+xMn_2−xO₄ lithium-ion cell is caused by reactions at the anode. In previous work, we have proposed a model for the reactions that cause self-heating in MCMB electrodes in electrolyte. By assuming that a cell self-heats only because reactions occur at the anode, the model can be used to predict the power generated by the amount of MCMB in practical cells with an inert cathode. The calculated chemically generated power can be combined with power loss measurements, due to the transfer of heat to the environment, to predict the short-circuit behaviour and the oven exposure behaviour for a cell containing an MCMB anode and an inert cathode. The results agree qualitatively with short-circuit and oven exposure results measured on NEC Moli energy 18650 cells containing an Li_1+xMn_2−xO₄ cathode.     

Differential scanning calorimetry material studies: implications for the safety of lithium-ion cells

Differential scanning calorimetry (DSC) has been demonstrated to be a useful means of studying the reactivity of materials used in lithium-ion cells. In the present study, the reactivity of three most commonly used transition metal oxides. i.e. Li_xNiO₂, Li_xCoO₂, Li_xMn₂O₄, in the presence of electrolyte were investigated by DSC. The most commonly used negative electrode material, carbon (Li_xC₆) was also studied. In the paper, relative levels of reactivity of the materials as a function of stoichiometry are reported. The implications of the results on the safety of lithium-ion cells are discussed,     

Thermal stability of LiPF6-based electrolyte and effect of contact with various delithiated cathodes of Li-ion batteries

Thermal stability of LiPF₆-based electrolyte (1 M LiPF₆/EC + DMC) was studied by in-situ FTIR spectroscopy and C80 calorimetry, which indicated that the electrolyte underwent furious polymerization and decomposition reactions and sharp heat flow was generated below 225 °C. The thermal stability of the electrolyte in contact with various delithiated cathodes (Li_xCoO₂, Li_xNi_0.8Co_0.15Al_0.05O₂, Li_xNi_1/3Co_1/3Mn_1/3O₂, Li_xMn₂O₄, Li_xNi_0.5Mn_0.5O₂, Li_xNi_0.5Mn_1.5O₄ and Li_xFePO₄) was also investigated by C80 calorimetry. The results show that the cathode materials except for Li_xFePO₄ usually have an enhancement effect on the decomposition of the electrolyte, but Li_xFePO₄ exhibits a suppression effect on the reactions of the electrolyte. Li_xFePO₄ is found to be with excellent thermal stability. Among the other cathodes, Li_xCoO₂, Li_xNi_0.8Co_0.15Al_0.05O₂, Li_xNi_0.5Mn_0.5O₂ and Li_xNi_0.5Mn_1.5O₄ promote the decomposition of electrolyte by releasing oxygen and thus considered not favorable for safety, but Li_xNi_1/3Co_1/3Mn_1/3O₂ with a lesser reaction heat and Li_xMn₂O₄ with even less heat flow in the low temperature range (50-225 °C) are believed as promising cathodes for better safety. By comparing X-ray diffraction (XRD) patterns of these cathode materials at room temperature and those heated to 300 °C in the presence of the electrolyte, we have found that Li_xFePO₄ only has experienced tiny structure change, which is greatly different from the other cathode materials.     

Electrochemical performance and kinetics of Li1+x(Co1/3Ni1/3Mn1/3)1−xO2 cathodes and graphite anodes in low-temperature electrolytes

Lithium-ion batteries have started replacing the conventional aqueous nickel-based battery systems in space applications, such as planetary landers, rovers, orbiters and satellites. The reasons for such widespread use of these batteries are the savings in mass and volume of the power subsystems, resulting from their high gravimetric and volumetric energy densities, and their ability to operate at extreme temperatures. In our pursuit to further enhance the specific energy as well as low-temperature performance of Li-ion batteries, we have been investigating various layered lithiated metal oxides, e.g., LiCoO₂, LiNi_0.8Co_0.2 and LiNi_0.8Co_0.15Al_0.05O₂, as well as different low-temperature electrolytes, including ternary and quaternary carbonate mixtures with various co-solvents. In this paper, we report our recent studies on Li_1+x(Co_1/3Ni_1/3Mn_1/3)_1−xO₂ cathodes, combined with three different low-temperature electrolytes, i.e.: (1) 1.0 M LiPF₆ in EC:EMC (20:80), (2) 1.2 M LiPF₆ in EC:EMC (20:80) and (3) 1.2 M LiPF₆ in EC:EMC (30:70). Electrical performance characteristics were determined in laboratory glass cells at different discharge rates and different temperatures. Further, individual electrode kinetics of both Li_1+x(Co_1/3Ni_1/3Mn_1/3)_1−xO₂ cathodes and MCMB graphite anodes were determined at different temperatures, using dc micropolarization, Tafel polarization and electrochemical impedance spectroscopy (EIS). Analysis of these data has led to interesting trends relative to the effects of solvent composition and salt concentration, on the electrical performance and on the kinetics of cathode and anode.     

Thin-film rechargeable lithium batteries

Thin-film rechargeable batteries with a lithium metal anode, an amorphous inorganic electrolyte, and cathodes of amorphous V₂O₅ and crystalline and amorphous Li_xMn₂O₄ have been fabricated and characterized. The performance of the thin-film cells was evaluated at different current densities and, in the case of LiV₅, at several temperatures. Electrical measurements show that the current density of the thin-film cells is limited by the lithium-ion mobility in the cathodes. The resistance of LiLi_xMn₂O₄ cells with crystalline cathodes is about two orders of magnitude lower than that of LiV₅ cells with amorphous cathodes.     

A model lithium-ion system based on the insertion properties of the spinel phase LixMn2O4, (0 < x < 2)

A model lithium-ion system, based on the manganese oxide, Li_xMn₂O₄, has been used to study the reversibility of the lithium insertion processes in both the approximate 3 V and 4 V versus Li/Li⁺ voltage ranges. For the 3 and 4 V ranges, symmetrical cells with the configuration: Li_1+xMn₂O₄/electrolyte/Li_1−xMn₂O₄ were investigated, while asymmetrical cells, Li_xMn₂O₄/electrolyte/Li_1−xMn₂O₄, were used to investigate the electrochemical properties of the 4 V range only. The relatively large voltage hysteresis for the symmetrical cell was considered to be due to the concurrent cubic to tetragonal symmetry change occurring for the electrode cycling over the lower voltage range. Impedance measurements confirm that the Li_1+xMn₂O₄ electrode appears to possess significantly inferior electrochemical properties to the electrode covering the higher voltage range. The 4 V range studied in the asymmetrical configuration showed excellent reversibility towards lithium insertion and extraction.     

Implications of the first cycle irreversible capacity on cell balancing for Li2MnO3-LiMO2 (M = Ni, Mn, Co) Li-ion cathodes

Much of the research on lithium-ion cathodes consisting of layered solid solutions of Li₂MnO₃-LiMO₂ (M = Mn, Co, Ni) has focused on identifying the causes of the irreversible capacity loss on the first cycle. However, a key issue that must be addressed is whether the high irreversible capacity observed seen on the first cycle is associated with intercalated lithium at the anode, or if it is associated with irretrievable capacity (i.e., film formation, and/or decomposition reactions). To this end, we have quantified the amount of utilizable lithium that is made available for the anodes when employing Li₂MnO₃-LiMO₂ as cathodes. Using a MoS₂ anode lithiation plateau transition as a reference point to the amount of lithium transferred to the anode during charge, it has been shown that almost none of the cathode irreversible charge capacity resulted in lithiation of the anode. Further, by reacting charged graphitic anodes that were retrieved from C anode-Li_1.2Ni_0.175Co_0.1Mn_0.52O₂ cathode cells with water to generate H₂ gas to measure the active amount of lithium in the anode, we confirmed the results with the MoS₂ titration experiments, demonstrating that lithium released from the cathode during the first charge is not proportionate to the cathode charge capacity.     

Ni-Sm2O3 cermet anodes for intermediate-temperature solid oxide fuel cells with stabilized zirconia electrolytes

Ni-LnO_x cermets (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd), in which LnO_x is not an oxygen ion conductor, have shown high performance as the anodes for low-temperature solid oxide fuel cells (SOFCs) with doped ceria electrolytes. In this work, Ni-Sm₂O₃ cermets are primarily investigated as the anodes for intermediate-temperature SOFCs with scandia stabilized zirconia (ScSZ) electrolytes. The electrochemical performances of the Ni-Sm₂O₃ anodes are characterized using single cells with ScSZ electrolytes and LSM-YSB composite cathodes. The Ni-Sm₂O₃ anodes exhibit relatively lower performance, compared with that reported Ni-SDC (samaria doped ceria) and Ni-YSZ (yttria stabilized zirconia) anodes, the state-of-the-art electrodes for SOFCs based on zirconia electrolytes. The relatively low performance is possibly due to the solid-state reaction between Sm₂O₃ and ScSZ in fuel cell fabrication processes. By depositing a thin interlayer between the Ni-Sm₂O₃ anode and the ScSZ electrolyte, the performance is substantially improved. Single cells with a Ni-SDC interlayer show stable open circuit voltage, generate peak power density of 410 mW cm⁻² at 700 °C, and the interfacial polarization is about 0.7 Ω cm².     

Synthesis and characterizations of Li[CrxLi(1−x)/3Mn2(1−x)/3]O2 (0.15 ≤ x ≤ 0.3) cathode materials in rechargeable lithium batteries

A series of Li[Cr_xLi_(1−x)/3Mn_2(1−x)/3]O₂ (0.15 ≤ x ≤ 0.3) cathode materials was prepared by citric acid-assisted, sol–gel process. Sub-micron sized particles were obtained and the X-ray diffraction (XRD) results showed that the crystal structure was similar to layered lithium transition metal oxides (R-3m space group). The electrochemical performance of the cathodes was evaluated over the voltage range 2.0–4.9 V at a current density of 7.947 mA g⁻¹. The Li_1.27Cr_0.2Mn_0.53O₂ electrode delivered a high reversible capacity of up to 280 mAh g⁻¹ during cycling. Li[Cr_xLi_(1−x)/3Mn_2(1−x)/3]O₂ yielded a promising cathode material.     

Cresyl diphenyl phosphate effect on the thermal stabilities and electrochemical performances of electrodes in lithium ion battery

To improve the safety of lithium ion battery, cresyl diphenyl phosphate (CDP) is used as a flame-retardant additive in a LiPF₆ based electrolyte. The electrochemical performances of LiCoO₂/CDP-electrolyte/Li and Li/CDP-electrolyte/C half cells are evaluated. The thermal behaviors of Li_0.5CoO₂ and Li_0.5CoO₂-CDP-electrolyte, and Li_xC₆ and Li_xC₆-CDP-electrolyte are examined using a C80 micro-calorimeter. For the LiCoO₂/CDP-electrolyte/Li cells, the onset temperature of single Li_0.5CoO₂ is put off and the heat generation is decreased greatly except the one corresponding to 5% CDP-containing electrolyte. When Li_0.5CoO₂ coexists with CDP-electrolyte, the thermal stability is enhanced. CDP improves the thermal stability of lithiated graphite anode effectively and the addition of 5% CDP inhibits the decomposition of solid electrolyte interphase (SEI) films significantly. The electrochemical tests on LiCoO₂/CDP-electrolyte/Li and Li/CDP-electrolyte/C cells show that when less than 15% CDP is added to the electrolyte, the electrochemical performances are not worsen too much. Therefore, the addition of 5-15% CDP to the electrolyte almost does not worsen the electrochemical performance of LiCoO₂ cathode and graphite anode, and improves theirs thermal stability significantly; thus, it is a possible choice for electrolyte additive.     

Analysis of Pt/C electrode performance in a flowing-electrolyte alkaline fuel cell

We characterize the performance of Pt/C-based electrodes under alkaline conditions using a microfluidic H₂/O₂ fuel cell as an analytical platform. Both anodes and cathodes were investigated as a function of electrode preparation procedures (i.e., hot pressing, acclimatization) and fuel cell operating parameters (i.e., electrolyte composition) via chronoamperometric and electrochemical impedance analyses. X-ray micro-computed tomography was employed to link electrode structure to performance. In addition, the flowing electrolyte stream is used to study the effects of carbonates on individual electrode and overall fuel cell performance. Our studies provide direct evidence that the performance of hydrogen-fueled room-temperature alkaline fuel cells (AFCs) is limited by transport processes to and from the anode primarily due to water formation. Furthermore, the presence of carbonate species in the electrolyte appears to impact only anode performance whereas cathode performance remains unchanged.     

Effect of an organic additive on the cycling performance and thermal stability of lithium-ion cells assembled with carbon anode and LiNi1/3Co1/3Mn1/3O2 cathode

A small amount of thiophene or ethylene dioxythiophene (EDOT) is introduced into the liquid electrolyte of lithium-ion cells as an additive. These organic additives are electrochemically oxidized to form a thin conductive polymer film on the surface of the cathode at high potential. With the liquid electrolyte containing different additives, the lithium-ion cells composed of carbon anode and LiNi_1/3Co_1/3Mn_1/3O₂ cathode are assembled, and their cycling performances are evaluated. Adding small amounts of thiophene or EDOT to the liquid electrolyte is found to reduce the interfacial resistance in the cells and thus the cells containing an organic additive exhibit less capacity fading and better high-rate performance. Differential scanning calorimetric studies show that the thermal stability of the charged Li_1−xNi_1/3Co_1/3Mn_1/3O₂ cathode is also enhanced in the presence of an organic additive.     

Some new facts on electrochemical reaction mechanism for transition metal oxide electrodes

Transition metal oxides represent a new type of anode materials for lithium-ion batteries. Due to their high capacity (usually above 700 mAh g⁻¹) and excellent cycleability, they have attracted much attention in recent years. Regarding the electrochemical reaction mechanism for this type of electrode, the conversion reaction mechanism proposed by Tarascon and co-workers is widely accepted, i.e. MO_x + 2xLi ? M + xLi₂O. Nevertheless, in our recent explorations, we have found some new phenomena which may help us to further understand the electrode reaction mechanism, and even pose a necessity to modify the current conversion reaction mechanism. These new phenomena can be summarized as electrochemical milling, capacity rise and no-metal-formation effects.     

Performances of LnBaCo2O5+x-Ce0.8Sm0.2O1.9 composite cathodes for intermediate-temperature solid oxide fuel cells

Double-perovskite oxides, LnBaCo₂O_5+x (LnBCO) (Ln = Pr, Nd, Sm, and Gd), are prepared using a solid-state reaction as cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The performances of LnBCO-Ce_0.8Sm_0.2O_1.9 (SDC) composite cathodes were investigated for IT-SOFCs on La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) electrolyte. The thermal expansion coefficient can be effectively reduced in the case of the composite cathodes. No chemical reactions between LnBCO cathodes and SDC electrolyte, and LnBCO and LSGM are found. The electrochemical performances of LnBCO cathodes and LnBCO-SDC composite cathodes decrease with decreasing Ln³⁺ ionic radii, which is closely related to the decrease of the electrical conductivity and fast oxygen diffusion property. The area specific resistances of the LnBCO cathodes and LnBCO-SDC composite cathodes on LSGM electrolyte are all lower than 0.13 Ω cm² and 0.15 Ω cm² at 700 °C, respectively. The maximum power densities of single-cell consisted of LnBCO-SDC composite cathodes, LSGM electrolyte, and Ni-SDC anode achieve 758-608 mW cm⁻² at 800 °C with the change from Ln = Pr to Gd, respectively. These results indicate that LnBCO-SDC composite oxides are candidates as a promising cathode material for IT-SOFCs.     

Surface structural disordering in graphite upon lithium intercalation/deintercalation

We report on the origin of the surface structural disordering in graphite anodes induced by lithium intercalation and deintercalation processes. Average Raman spectra of graphitic anodes reveal that cycling at potentials that correspond to low lithium concentrations in Li_xC (0 ≤ x < 0.16) is responsible for most of the structural damage observed at the graphite surface. The extent of surface structural disorder in graphite is significantly reduced for the anodes that were cycled at potentials where stage-1 and stage-2 compounds (x > 0.33) are present. Electrochemical impedance spectra show larger interfacial impedance for the electrodes that were fully delithiated during cycling as compared to electrodes that were cycled at lower potentials (U < 0.15 V vs. Li/Li⁺). Steep Li⁺ surface-bulk concentration gradients at the surface of graphite during early stages of intercalation processes, and the inherent increase of the Li_xC d-spacing tend to induce local stresses at the edges of graphene layers, and lead to the breakage of C-C bonds. The exposed graphite edge sites react with the electrolyte to (re)form the SEI layer, which leads to gradual degradation of the graphite anode, and causes reversible capacity loss in a lithium-ion battery.