Reversible Capacity Loss Research Articles

Concentrated LiBF4/PC Electrolyte Solutions for 5 V LiNi0.5Mn1.5O4 Positive Electrode in Lithium-Ion Batteries

Introduction Lithium-ion batteries (LIB) are used for a wide range of purposes, such as portable devices and electric vehicles. The demand for high-energy-density LIB is ever-increasing. To improve energy density of LIB, positive electrode materials working at high potentials have been widely investigated. Spinel LiNi0.5Mn1.5O4 is one of the most promising active materials for high-energy-density LIB because it works at extremely high potentials (ca. 4.7 V vs. Li/Li+). However, almost all electrolyte solutions are oxidatively decomposed at such high potentials, which results in a loss of reversible capacity and lowers the Coulombic efficiency. Recently, highly concentrated electrolyte solutions have been reported to offer unique features, such as a wide potential window and high thermal stability [1]. We have already reported that concentrated LiPF6/PC effectively suppressed the decomposition of solvent molecules, while the polarization in charge/discharge reactions much increased [2]. This issue remained to be solved before practical application. In this work, highly concentrated LiBF4/PC electrolyte solutions are used and charge and discharge properties of Li|LiNi0.5Mn1.5O4cells were investigated. Experimental The active material of LiNi0.5Mn1.5O4 was mixed with Ketjenblack (10 mass%) as a conductive additive and poly(vinylidene fluoride) (10 mass%) as a binder as a binder using 1-methyl-2-pyrrolidone to form a slurry. The slurry was spread out onto an aluminum current collector, and then dried at 80oC for 18 h under vacuum. The electrode sheets were punched out into disks of 13 mm in diameter. Charge-discharge tests were performed with constant current mode (30 μA cm-2, C/10 rate) using two-electrode coin-type cells at 30oC. The test cells were assembled in a glove box filled with Ar. Li foil served as a counter electrode. The electrolyte solutions were used 0.83 and 7.25 mol kg-1 LiBF4 dissolved in propylene carbonate (PC), together with 0.83 and 4.27 mol kg-1 LiPF6 / PC for comparison. Results and discussion Figure 1 shows initial charge-discharge curves of Li|LiNi0.5Mn1.5O4 cells in concentrated electrolyte solutions. Two plateaus appeared at around 4.7 V during charging and discharging, which is very similar to those obtained for conventional electrolyte solutions with 0.83 mol kg-1 (ca. 1 M) LiPF6. When the electrolyte concentration was 0.83 mol kg-1, initial charge capacities far exceeded the theoretical capacity of 148 mAh g-1, as summarized in Table 1, which resulted in a poor Coulombic efficiency. These results suggest that irreversible reactions, such as the oxidative decomposition of the electrolyte solution, occurred at high potentials during charging. On the other hand, the initial charge capacities were reasonably low in 4.27 mol kg-1 LiPF6/PC and 7.25 mol kg-1 LiBF4/PC, and as a result high Coulombic efficiencies were achieved. These results clearly indicate that the oxidative decomposition of electrolyte solution was effectively suppressed by using concentrated electrolyte solutions, particularly for LiBF4/PC. In addition, the polarization in charge/discharge reactions, which was evaluated from the difference between the voltages at 50% state of charge (SOC) and 50% depth of discharge (DOD), was 33 mV in 7.25 mol kg-1 LiBF4/PC, which is much lower than that of 4.27 mol kg-1 LiPF6/PC (131 mV). In general, the large polarization occurs due to the poor ionic conductivity of the concentrated electrolyte solution with a high viscosity. However, the ionic conductivity of 7.25 mol kg-1 LiBF4/PC (0.14 mS cm-1) was lower than that of 4.27 mol kg-1 LiPF6/PC (0.40 mS cm-1). We will discuss it in our presentation. Acknowledgement This research is supported by the Super Cluster Program from MEXT and JST. Reference [1] S. Jeong et al., Electrochem. Solid-State Lett., 6, A13-A15 (2003).[2] R. Masuhara et al., 228thECS Meeting, #406, Phoenix, USA (2015). Figure 1

Insights on Calendar Aging of Lithium-Ion Batteries from Differential Voltage Analysis and Coulometry

In most battery applications, idle periods are considerably longer than operating periods, in which the batteries are charged or discharged. Thus, calendar aging during idle periods has a substantial impact on battery life in various applications, such as electric vehicles, power tools, and stationary storage systems. Experimental For different commercial 18650 lithium-ion cells with graphite anode and NMC, NCA, or LFP cathode, we investigated calendar aging at different temperatures, states-of-charge (SoC), and storage periods. In our study, we used a fine SoC resolution with 16 values from 0% to 100% to gain a thorough understanding of the impact of storage SoC on calendar aging. In total, more than 200 cells were examined. To analyze battery aging, we used conventional capacity and impedance measurements but also differential voltage analysis (DVA) and coulometry. Results As illustrated in Figure 1a for the NCA cells after a storage period of almost 10 months, the capacity loss from calendar aging is not increasing steadily with higher SoC. In fact, there are two plateau regions in which the capacity fade is rather independent from SoC. A high capacity fade is observed between 65% and 95% SoC and a medium capacity fade is observed between 30% and 55% SoC.Using DVA [1], a clear correlation between these plateaus and the different intercalation stages of graphite was identified for all cell types examined. For the NCA cells, Figure 1b shows a reconstruction of the full-cell differential voltage spectrum from half-cell data, highlighting the central graphite peak which separates different intercalation stages of the graphite anode. Analyzing differential voltage spectra over time confirmed that the capacity fade resulted predominantly from a loss of lithium inventory, since no or only minor degradation of the electrodes’ active materials was determined.Coulometry [2,3] enabled to localize loss of lithium inventory at the anode. Furthermore, side reactions causing reversible capacity losses were detected, especially for storage at high SoC. Conclusions All in all, there is a more complex dependence of calendar aging on SoC than generally assumed. Our aging study, comprising different cell chemistries, has revealed that lowering the SoC of a lithium-ion battery to reduce calendar aging during storage is not really effective as long as the SoC remains in the plateau region of high capacity fade. To avoid this SoC region, a storage SoC below 55%-70%, depending on the balancing of the graphite anode, is required. To maximize battery life in an electric vehicle, it is beneficial to keep the SoC at 50% or lower when the vehicle is not being used.

Microfibrillated Cellulose Based Ink for Eco-Sustainable Screen Printed Flexible Electrodes in Lithium Ion Batteries

Free organic solvent ink containing graphite, carboxymethyl cellulose and microfibrillated cellulose as active material, dispersing and binder, respectively, has been formulated to produce flexible and eco-sustainable electrodes for lithium ion batteries. Content ratio of components and dispersion protocol were tailored in order to have rheological properties suitable for a large and cheap manufacturing process as well as screen printing. The bio-sourced printed electrodes exhibit a high porosity value of 70% that limits the electrochemical performances. However, the calendering process enhances electrode performances by increasing the reversible capacity from 85 until 315 mAh/g and reducing porosity to an optimal value of 34%. Moreover the introduction of 2% w/w of monofluoro-ethylene carbonate in the electrolyte reduced their reversible capacity loss of 11% in the printed electrode.

A first-principles study on the effect of oxygen content on the structural and electronic properties of silicon suboxide as anode material for lithium ion batteries

Silicon suboxide is currently considered as a unique candidate for lithium ion batteries anode materials due to its considerable capacity. However, no adequate information exists about the role of oxygen content on its performance. To this aim, we used density functional theory to create silicon suboxide matrices of various Si:O ratios and investigated the role of oxygen content on the structural, dynamic, electronic properties and lithiation behavior of the matrices. Our study demonstrates that the O atoms interact strongly with the inserted Li atoms resulting in a disintegration of the host matrix. We found that higher concentration of oxygen atoms in the mixture reduces its relative expansion upon lithiation, which is a desirable quality for anode materials. It helps in preventing crack formation and pulverization due to large fluctuations in volume. Our study also demonstrates that a higher oxygen content increases the lithium storage capacity of the anode. However, it can also cause the formation of stable complexes like lithium silicates that might result into reversible capacity loss as indicated by the voltage–composition curves. The study provides valuable insights into the role of oxygen in moderating the interaction of lithium in silicon suboxide mixture in microscopic details.

Investigation of the Self-Discharge Behavior of Lithium-Sulfur Batteries

Lithium-Sulfur (Li-S) batteries represent a perspective energy storage technology, which reaches very high theoretical limits in terms of specific capacity, specific energy, and energy density. However, Li-S batteries are governed by the polysulfide shuttle mechanism, which causes fast capacity fade, low coulombic efficiency, and high self-discharge rate. The self-discharge is an important characteristic of Li-S batteries for both practical applications and laboratory testing, which is highly dependent on the operating conditions. Thus, to map and to understand the Li-S self-discharge behavior under various conditions, such as depth-of-discharge, temperature, and idling time, a set of experiments were performed in this work on 3.4 Ah Li-S pouch cells. The results are systematically presented in form of open-circuit voltages during idling and self-discharge separated into reversible and irreversible capacity loss. Furthermore, estimation of the actual high voltage plateau capacity based on a self-discharge constant was performed according to an earlier proposed methodology; however, the method needs further improvements in order to estimate this capacity accurately for all conditions.

Highly Concentrated Electrolytes for 5 V-Class Positive Electrodes in Lithium-Ion Batteries

Introduction There are great needs for lithium-ion batteries with higher energy density for realizing electric vehicles and hybrid electric vehicles. The energy density of lithium-ion batteries can be increased by using positive electrode materials with high capacities and/or high working potentials. Several kinds of active materials with working potentials higher than that of LiCoO2 have been reported. Among them, spinel LiN0.5Mn1.5O4 exhibits extremely positive potentials at ca. 4.7 V vs. Li/Li+, and is a promising positive electrode material. However, almost all electrolyte solutions oxidatively decompose at such high electrode potentials, which results in a loss of reversible capacity and lowers the Coulombic efficiency. A number of studies have been so far done on the surface coating of LiN0.5Mn1.5O4 with metal oxides. Li-rich solid-solution layered oxides, such as Li2MnO3-LiMO2 (LMNC, M=Ni, Co, Mn), are also promising active materials with high working potentials (ca. 4.8-2.0 V) and high specific capacity (ca. 320 mAh g-1). However, LMNC is known to deteriorate upon charge/discharge cycles due to the layered-to-spinel phase transformation, in addition to the irreversible electrolyte decomposition. In this work, highly concentrated electrolytes [1] were used for LiNi0.5Mn1.5O4 and LMNC positive-electrodes and the effects of lithium-salt concentration on the charge-discharge performance were studied. Experimental The active material of LiNi0.5Mn1.5O4 or LMNC (80 wt%) was mixed with acetylene black (10 wt%) as a conductive additive and poly(vinylidene fluoride) (10 wt%) as a binder as a binder using 1-methy-2-pyrrolidone to form a slurry. The slurry was spread out onto Al foil, and then dried at 80oC for 18 h under vacuum. The electrode sheets were punched out into disks of 13 mm in diameter. Charge-discharge tests were performed at a constant current mode (30 mA cm-2, C/10 rate) using two-electrode coin-type cells at 30 oC. The test cells were assembled in a glove box filled with Ar. Li foil served as a counter electrode. The electrolyte solutions used were 0.833 and 4.45 mol kg-1 LiPF6 dissolved in propylene carbonate (PC). Results and discussion Fig. 1 shows the initial charge-discharge curves for Li | LiNi0.5Mn1.5O4 cell with 0.833 mol kg-1 LiPF6/PC. The charge capacity (164 mAh g-1) was higher than the theoretical capacity (148 mAh g-1), and a high irreversible capacity (46 mAh g-1) was observed. The irreversible capacity was mainly due to the oxidative decomposition of the electrolyte solution. On the other hand, the initial charge capacity was 144 mAh g-1 when highly concentrated (4.45 mol kg-1) LiPF6/PC was used as an electrolyte (Fig. 2), which is close to theoretical capacity. The discharge capacity in the 1st cycle was 130 mAh g-1, and hence the irreversible capacity was 14 mAh g-1. The discharge capacity increased by 12 mAh g-1 higher, while the irreversible capacity decreased by 32 mAh g-1 by using the concentrated (4.45 mol kg-1) LiPF6/PC system. These results suggest that oxidative decomposition of the electrolyte solution was suppressed by increasing the concentration of Li salts. Similar suppression of the electrolyte decomposition was also observed for LMNC positive-electrodes. Therefore, the use of highly concentrated electrolytes is effective for the high-potential positive-electrodes in lithium ion batteries. However, the overpotential in charge/discharge reactions, which was estimated from the difference between the potentials at 50% state of charge (SOC) and 50% depth of discharge (DOD), was as high as 0.205 V for the concentrated electrolyte of 4.45 mol kg-1 LiPF6/PC, as shown in Fig.1. This value was much higher than that for 0.833 mol kg-1 LiPF6/PC (0.030 V). The large overpotential is likely due to a poor ionic conductivity of the concentrated electrolyte solution with a high viscosity. This issue remains to be solved before practical application, and is currently under investigation. Reference 1) S. Jeong et al., Electrochem. Solid-State Lett., 6(1) A13-A15 (2003). Figure 1

The Effect of Some Nitriles as Electrolyte Additives in Li-Ion Batteries

The effect of the nitrile electrolyte additives, succinonitrile (SN), adiponitrile (AN) and pimelonitrile (PN) was studied using High Precision Charger at Dalhousie University, automated cell storage and AC impedance. Li[Ni1/3Mn1/3Co1/3]O2/graphite (NMC111/graphite) cells containing the nitrile electrolyte additives showed no significant effect on the cycling between 2.8 V and 4.2 V and storage at 4.2 V at 60. ± 0.1°C. However, when the high cutoff voltage increased to 4.5 V in Li[Ni0.4Mn0.4Co0.2]O2/graphite (NMC442/graphite) cells, the addition of 2 wt% SN and 2 wt% vinylene carbonate (VC) reduced the reversible capacity loss and gas generation during storage experiments at 60. ± 0.1°C compared to 2 wt% VC alone. The addition of 2 wt% SN also caused a large impedance growth during the 4.5 V storage period, but that could be suppressed by the addition of 1 wt% ethylene sulfite (ES) + 1 wt% tris(trimethlysilyl) phosphite (TTSPi) or 2 wt% trimethylene sulfate (TMS) + 2 wt% TTSPi. This work shows that SN leads to meaningful improvements in storage properties at voltages higher than 4.4 V. This also suggests that SN can enhance the long-term cycle life of the high voltage NMC/graphite cells by suppressing electrolyte oxidation at the positive electrode.

A novel and fast method of characterizing the self-discharge behavior of lithium-ion cells using a pulse-measurement technique

This publication presents a novel method for characterizing the self-discharge behavior of lithium-ion cells, a pulse-measurement technique. This not only yields the self-discharge in terms of capacity loss, but also uses changes in the open circuit voltage to determine an equivalent self-discharge resistance, Rself. This method is very useful; the required measurement time for characterizing self-discharge behavior is shortened by a factor of 60 (compared to a calendar test of 30 days). This measurement time improvement is also beneficial for investigating the temperature dependence of self-discharge in a lithium ion cell, enabling a quantitative evaluation. Best of all, the pulse-measurement technique can be easily implemented using commercially available measurement equipment. Nonetheless, storage experiments are still indispensable when separating reversible and irreversible capacity loss.

NH4PF6 as a Structural Modifier for Building a Robust Carbon‐Coated Natural Graphite Anode for Lithium‐Ion Batteries

AbstractTo improve the electrochemical performance of natural graphite anodes, we have designed a facile, scalable carbon‐coating process using coal‐tar pitch and ammonium hexafluorophosphate (NH4PF6) as a structural modifier. NH4PF6 effectively modifies the microstructure of amorphous carbon, coated on the surface of the natural graphite, leading to better, reversible Li+‐storage characteristics within a wide operating‐temperature range (from −20 to 60 °C). The COP linkage introduced as a result of the thermal decomposition of NH4PF6 assists the formation of open nanopores in the amorphous carbon layer. The integrated nanopores act as additional reversible Li+‐storage sites, which minimize the reversible capacity loss and improve the cycle performance of natural graphite. Moreover, the thermal stability of natural graphite can also be improved while maintaining the improved electrochemical performance. Thus, this approach is a step toward the practical use of natural graphite anodes for lithium‐ion batteries for electric‐vehicle applications.

Electrochemical properties of the LaNi4.5Co0.25Al0.25 hydrogen storage alloy in wide temperature range

The effect of temperature on the overall electrochemical properties of LaNi4.5Co0.25Al0.25 hydrogen storage alloy has been studied systematically. The results show that temperature has a striking effect on the overall electrochemical properties. The LaNi4.5Co0.25Aln0.25 alloy electrode has a maximum capacity of 324.4mAh/g at 253K after first activation at room temperature. The cycling stability and capacity retention S50 increase with the decreasing temperature from 333K to 253K, while the reversible capacity loss (RCL) and irreversible capacity loss (ICL) both increase with elevation of testing temperature. The high-rate discharge ability (HRD) at room temperature is superior to that of other temperatures.

The Effect of Battery Potential and Charge Rate on the Decomposition Reaction on the Anode Electrode of Lithium Ion Polymer Battery

The formation of the solid electrolyte interface (SEI) layer on the surface of the anode electrode prevents further electrolyte decomposition reaction. However, certain battery operating parameters initiate electrolyte solvent and salts decomposition reaction with lithium on the surface to the anode electrode. This decomposition reaction results in the formation of an insoluble reaction products that is non-uniform, electronically insulating, and unstable on the surface of the graphite particles. The loss/consumption of recyclable lithium ions at the anode by this passivation layer is one of the causes of major reversible capacity loss in the lithium ion battery. Understanding the potential and the charge rate that initiates the formation of the surface film layer is critical to effectively controlling/limiting these operational parameters in order to extend the calendar life performance of the battery. This paper seeks to investigate the effect of the state of charge and charge rate on the electrolyte decomposition reaction resulting in the formation of the surface film deposit. An image based quantitative analysis of the thickness of the surface film will be conducted to collaborate the observed changes in the electrochemical properties of the battery.

Capacity Fade Diagnosis of Lithium Ion Battery Pack in Electric Vehicle Base on Fuzzy Neural Network

The lithium ion battery pack, which is filled with cells, is an important part in electric vehicles (EVs), also the main fault source. The inconsistent cells or the design and assembly fail of the pack could affect its performance and life or even endanger vehicles security in extreme situation, which makes the early fault diagnosis is essential. For further analysis, we introduce an equivalent circuit model (ECM) to identify the cell characteristics parameters, which supports the fault diagnosis by simulating the fault battery performance in dynamic cycle. According the battery working mechanism and the practical experience, via collecting data and preprocessing the typical data, a diagnostic method and model based on fuzzy neural network is proposed to discover the battery pack fault related to irreversible or reversible capacity loss.

Impact of storage on the LiNi0.4Mn1.6O4 high voltage spinel performances in alkylcarbonate-based electrolytes

The two main degradation mechanisms of the LiNi0.4Mn1.6O4/electrolyte interface are already known to be the electrolyte oxidation and the dissolution of transition metal ions. The impact of these two phenomena on the performances during cycling and after prolonged storage of accumulators is evaluated by modifying the cell configuration. The aim is to understand the mechanisms leading to the decrease in performances of the electrochemical cells. Storage of LiNi0.4Mn1.6O4/Li and LiNi0.4Mn1.6O4/Li4Ti5O12 accumulators confirms that there is no correlation between the ability of alkylcarbonate-based electrolytes to resist to their own oxidation at high potential and the decrease in cell electrochemical performances. This is mainly due to the transition metal ion dissolution from Li1-xNi0.4Mn1.6O4. The use of LiNi0.4Mn1.6O4/Li accumulators for storage tests is not suitable over a long period, owing to the reactivity of reducing lithium, particularly at high temperatures, hindering distinguishing differences between electrolytes. The use of the LiNi0.4Mn1.6O4/Li4Ti5O12 cells reveals that a temperature of 30°C with the EC/EMC electrolyte improves the cell performances after prolonged storage. However, the use of such cells induces Mn2+ and Ni2+ in larger amounts than in the LiNi0.4Mn1.6O4/Li cells, resulting in a minimal reversible capacity loss of 20% related to the total disappearance of the electrochemically active manganese (Mn+IV→Mn+III).

A Consideration of Electrolyte Additives for LiNi0.5Mn1.5O4/Li4Ti5O12 Li-Ion Cells

LNMO/LTO cells represent an excellent vehicle with which to test the impact of electrolyte additives on cell performance due to the high potential of LNMO which leads to electrolyte oxidation. Additives that prevent this oxidation are desired. Electrolyte additive researchers normally compare the capacity retention versus cycle number of cells with and without additives. If the capacity retention is improved, the additive is “good”. In this paper, this logic is shown to be flawed. One additive, LiO-t-C4F9, which does improve capacity retention versus cycle number, is shown to do so by increasing electrolyte oxidation at the positive electrode. This causes increased charge end point capacity slippage and increased self discharge rates. As such, these additives, like many promoted in the literature, are not “good”, but are “bad”. On the other hand, one additive, Al(HFiP)3, is shown to reduce parasitic reactions at the LNMO electrode but does not lead to improved capacity retention. A full understanding of the impact of the additive on the parasitic reaction rates at both positive and negative electrodes is required before a sound judgment about the impact of an additive can be made. Therefore, additive researchers must pay attention to coulombic efficiency, capacity end point slippage, reversible and irreversible capacity loss during storage as well as voltage drop during storage in addition to the long-time cycling performance for cells to get an overall evaluation of electrolyte additives.

Solid Electrolyte Interphase on Lithium-Ion Carbon Nanofiber Electrodes by Atomic and Molecular Layer Deposition

Carbon nanofibers were coated with Al2O3 by atomic layer deposition (ALD) or with an alumina-organic hybrid thin film layer by molecular layer deposition (MLD) to produce an artificial solid electrolyte interphase (SEI) prior to use as a lithium-ion battery electrode. The elemental composition of the materials was investigated using energy dispersive X-ray spectroscopy (EDX) and inductively coupled plasma mass spectrometry (ICP-MS). A coating of ten Al2O3 layers reduced the lithium lost to the SEI formation from 359 to 291 mAh/g (24%) during the first charge. These same cells possessed 370 mAh/g of stable reversible capacity when tested at low current density (25 mA/g), similar to uncoated material. At increased currents, Al2O3 films of either ten or twenty layers lowered the capacity retention when compared with uncoated materials. When compared to the ALD material, films deposited by MLD resulted in less improvement to reversible capacity and a greater loss of reversible capacity. These results indicate the use of ALD to create a new electrode surface and mitigate the Li losses to SEI formation may be a viable method of addressing the challenges associated with high-surface area electrode materials.

On the limited performances of sulfone electrolytes towards the LiNi0.4Mn1.6O4 spinel

Cycling after storage of LiNi0.4Mn1.6O4/Li4Ti5O12 cells evidences lower total capacity losses for EMS-, TMS- and MIS-based electrolytes as compared to EC-based at 20 °C. The shuttle-type mechanism induced by the electrolyte oxidation is mainly present in the accumulators at this temperature, as compared to those due to the Mn(2+) and Ni(2+) dissolution. At 30 and 40 °C, EC is responsible for the polymer film formation on the LiNi0.4Mn1.6O4 surface, which limits the transition metal ion dissolution. This results in lower reversible capacity losses compared to sulfones, but are still important: 45% at 30 °C and 70-75% at 40 °C. XPS spectra reveal that EMS does not contribute to the surface film formation on the LiNi0.4Mn1.6O4 spinel, regardless of the cycling conditions and temperature. Only the EMC decomposition at high potential in sulfone/EMC electrolytes is responsible for an organic layer formation, which is composed of low passivating oligomers that comprise the C-O and C=O functional groups. Sulfones are promising compounds to be used in high voltage Li-ion batteries thanks to their non-reactivity towards the LiNi0.4Mn1.6O4 cathode. However, this does not allow the deposition of surface films that would have enabled stopping the Mn(2+) and Ni(2+) dissolution in the electrolyte. This is responsible for degraded performances of LiNi0.4Mn1.6O4/Li4Ti5O12 cells as compared to EC-based electrolytes over ambient temperatures, especially at 30 °C.

Electrode/Electrolyte Interfacial Behaviors of LiCoO2/Mixed Graphite Li-Ion Cells during Operation and Storage

Interfacial properties of Li-ion cells containing LiCoO2 and a mixture of natural and artificial graphite in a LiPF6-based nonaqueous electrolyte were studied during storage and operation under various temperatures. The graphite/electrolyte interface showed a higher activation energy of Li-ion transfer than the LiCoO2/electrolyte interface for a cell with 100% state of charge. Li-plating occurred on graphite in charging at low temperature, where the charge transfer resistance of graphite electrode became dominating in the fully charged cell. The instability of solid electrolyte interphase (SEI) film on graphite during storage and cycles led to a decrease in impedance and capacity loss. Mechanisms of reversible capacity loss of an aged cell during storage were discussed.

Effects of Al- and Mn-contents in the negative MH alloy on the self-discharge and long-term storage properties of Ni/MH battery

The charge retention and long-term storage properties of AB5-based Ni/MH batteries were studied by partial/complete replacements of Al- and Mn-contents by Ni in the AB5 alloys, separately. A high-temperature accelerated testing scheme (60°C and 60-days) was proven to be capable of predicting the status of batteries after long-term storages at room temperature (6-months and 293-days). Charge retention and impedance measured from the accelerated scheme can be correlated to the open-circuit voltage and impedance after long-term storage, respectively. While both Al and Mn were important to maintain good charge retention, Al, with its good oxidation resistance to slow down the oxidation/corrosion of the MH alloy, contributed to a lower internal impedance, Mn, on the other hand, slowed down the pulverization rate and reduced the irreversible capacity loss, however, contributed to a higher reversible capacity loss. The addition of Y2O3 in the positive electrode was found to raise the open-circuit voltage and reduce both reversible capacity loss and impedance increase during long period storage by slowing down the oxidation/corrosion of the MH alloy in the negative electrode.

Calendar aging of a graphite/LiFePO4 cell

Graphite/LFP commercial cells are stored under 3 different conditions of temperature (30°C, 45°C, and 60°C) and SOC (30%, 65%, and 100%) during up to 8 months. Several non-destructive electrochemical tests are performed at different storage times in order to understand calendar aging phenomena. After storage, all the cells except those stored at 30°C exhibited capacity fade. The extent of capacity fade strongly increases with storage temperature and to a lesser extent with the state of charge. From in-depth data analysis, cyclable lithium loss was identified as the main source of capacity fade. This loss arises from side reactions taking place at the anode, e.g. solvent decomposition leading to the growth of the solid electrolyte interphase. However, the existence of reversible capacity loss also suggests the presence of side reactions occurring at the cathode, which are less prominent than those at the anode. The analyses do not show any evidence about active-material loss in the electrodes. The cells do not suffer substantial change in internal resistance. According to EIS analysis, the overall impedance increase is 70% or less.

Influence of Fe (II) Species in Electrolyte on Performance of Graphite Anode for Lithium-Ion Batteries

Iron dissolution into an electrolyte from the olivine LiFePO4 in the lithium-ion cell has been recently investigated. In order to study the influence of the dissolved Fe2+ species in electrolyte on the lithium intercalation/deintercalation into an artificial graphite electrode, the electrochemical behavior of graphite was investigated in 1 mol L−1 LiPF6 electrolyte solution containing Fe2+ species. The cells containing Fe2+ species showed larger reversible capacity loss than the Fe-free cells. Electrochemical impedance spectroscopy and ex-situ techniques, such as scanning electron microscopy (SEM), Raman spectroscopy, Fourier Transform Infrared Spectrometer (FTIR) were employed to investigate the aging mechanism. The Fe2+ species were reduced on the graphite anode and formed iron deposits. The iron deposits affected the performance of graphite on two approaches. On the one hand, the deposits hindered intercalation of Li ions into graphite. On the other hand, the deposits accelerated decomposition of electrolyte and aggravated surface structural disorder in graphite, resulting in thicker solid-electrolyte interface SEI layer. Both effects caused capacity fading upon cycling.