Charge Cutoff Research Articles

Consequences of Increasing Complexity in Anorectal Surgery Performed at an Academic Center

Anorectal surgery encompasses a wide range of procedures with varying complexity. The Accreditation Council for Graduate Medical Education Review Committee for Colon and Rectal Surgery recommends minimum case numbers (60) for 1-year specialty trainees in 6 categories of anorectal surgery, with definitions for procedural complexity. The purpose of this study was to assess the scope of anorectal procedures and propose a stratification of procedures based on a consensus of levels of difficulty, as well as to identify a predictive charge cutoff suggestive of procedural complexity. This was a retrospective review. The study was conducted at a tertiary academic center. Patients undergoing anorectal procedures between January 2011 and December 2014 identified by Current Procedural Terminology coding were entered into 6 categories. Codes were stratified as routine or complex based on an assessment of perioperative care and technical expertise required. Patients with an abdominal portion to any procedure were excluded. The study measured distribution of complexity in anorectal surgical procedures and procedural charge associated with differentiating routine from complex procedures. Seven colorectal surgeons performed 2483 anorectal procedures (mean = 620 per year). Mean age was 48 ± 16 years. Forty six (64%) of 71 procedures were classified as routine and 25 (36%) of 71 as complex. Most disease processes had subsets with routine or complex procedures, whereas all of the procedures performed for fecal incontinence or advanced anorectal techniques were considered complex. Fistula procedures and transanal excisions were most heterogeneous, with a high procedural complexity rate (37% and 50%). After a procedural complexity rating, intraclass correlation by 6 surgeons was 0.70, demonstrating good correlation. Receiver operating curve assessments of consensus categorization according to billing codes revealed $553 as the optimal cutoff between routine and complex procedures. This was a single-institution retrospective review. Colorectal residents may benefit from anorectal case stratification, because it serves as a dialogue for those interested in complex anorectal surgery during training. Surgeon categorization of procedures correlates well with a charge-based model of complexity. See Video Abstract at http://links.lww.com/DCR/A806.

Comparative study of Al2O3, SiO2 and TiO2-coated LiNi0.6Co0.2Mn0.2O2 electrode prepared by hydrolysis coating technology

In this work, Al, Si and Ti oxides are used to modify the surface of LiNi0.6Co0.2Mn0.2O2 (NCM622) electrode through the hydrolysis coating technology. SEM and TEM results revealed that three prepared oxide layers have different uniformity and morphology. Also, charge-discharge results showed different initial discharge capacity and cycle ability of three different oxide coatings. It is shown that when the temperature is increased from 25 to 50 °C, the capacity retention of Al2O3-coated NCM622 is reduced by only 4 %, what demonstrated the best ability of this oxide to restrain cycle deterioration. Additionally, when the charge cutoff voltage is increased to 4.6 V, Al2O3-coated NCM622 showed 74 % of capacity retention. As the number of charge-discharge cycles increases, the dissolution of some transition metal ions may be restrained by Al2O3 layer. Generally, the enhanced electrochemical performance of Al2O3-coated NCM622 could be ascribed to the suppression of mutual reaction between electrode and electrolyte and improvement of structural stability of the material by Al2O3 coating.

Surface Modification of the LiNi0.5Co0.2Mn0.3O2 Cathode by a Protective Interface Layer of Li1.3Ti1.7Al0.3(PO4)3

Solid-state batteries are the next step on the way of developing lithium power sources because of combination of high energy density (up to 500 W/kg and 1000 W/l) and better safety. Yet their major drawbacks are high interfacial resistance and poor interface contacts between the solid electrolyte and active material. One of possible solutions to the problem is to produce an active material whose particles would be coated with a solid electrolyte. Titanium-aluminum-lithium phosphate Li1,3Al0.3Ti1.7(PO4)3 (LATP) is a solid electrolyte which exhibits one of the highest conductivity (∼10−3−10−5 S/cm) and is chemically stable. Therefore, the aim of this study was synthesis and investigation of composite cathodes based on cathode materials LiNi0.5Co0.2Mn0.3O2 (NMC) with protective interface layer of solid state electrolyte LATP. The influence of the amount of LATP on the electrochemical properties of NMC was investigated. The composite NMC cathode material with protective interface layer of solid state electrolyte with content of LATP 1% exhibited high capacity (ca. 150 mAh g−1) and good cycle performance with a high charge cutoff voltage up to 4.5 V.

Enhancing the high voltage interface compatibility of LiNi0.5Co0.2Mn0.3O2 in the succinonitrile-based electrolyte

A stable electrolyte system with wide electrochemical window and excellent interface compatibility is the key to successfully obtaining higher power and energy density of lithium ion batteries. Here we report that cycle life and rate performance of LiNi0.5Co0.2Mn0.3O2 (NCM) at a high charge cutoff voltage of 4.7 V can be greatly extended by manipulating the electrode/electrolyte interphases with succinonitrile (SN) and fluoroethylene carbonate (FEC) simultaneously as solvents. Remarkably, NCM//Li cells can retain extraordinary capacity retention up to 73.6% after 100 stable cycles, which far surpasses the commercial electrolyte counterpart (35.5%). It also delivers a high discharge capacity of >80 mA h g−1 at 5 C, significantly higher than that of commercial electrolytes (20 mA h g−1). Meanwhile, the characteristics of interface layer on NCM are investigated in detail using scanning electron microscopy, transmission electron microscopy, electron diffraction images, X-ray diffraction, X-ray photoelectron spectroscopy with and without Ar ion etching technology, and electrochemical impedance spectroscopy. This is mainly attributed to a stable, uniform, N-containing and high ionic conductive electrolyte interphase layer on the surface of NCM positive material using this optimized electrolyte. So it reveals the importance of controlling electrode/electrolyte interphases through manipulation of electrolyte compositions, and opens a new avenue for developing NCM as high-capacity electrode materials for lithium ion batteries.

Fault diagnosis and quantitative analysis of micro-short circuits for lithium-ion batteries in battery packs

Micro-short circuit (MSC) of a lithium-ion battery cell is a potential safety hazard for battery packs. How to identify the cell with MSC in the latent phase before a thermal runaway becomes a difficult problem to solve. We propose a diagnosis method to detect the MSC according to the remaining charging capacity (RCC) variations between cells. When the charging cell voltage curve (CCVC) of the first fully charged cell is regarded as the benchmark, the RCC of each cell can be obtained by the CCVC transformation based on the uniform CCVC hypothesis. The accuracy of the RCC estimation method is validated under constant current and constant power charging experiments. The leakage current of the MSC cell can be obtained by the increase of the RCC after each charge, and it can be converted into the MSC resistance. We set up the battery pack system model with the MSC fault in Simulink® to verify the effectiveness of the method when the battery pack is charged up to different charge cutoff voltages. Through a series of experiments with external resistors, the adaptability of the MSC diagnosis method is further validated for the aged cell, multi-stage charging, and constant power charging.

Communication—Lithium Difluorophosphate as an Electrolyte Additive to Improve the High Voltage Performance of LiNi0.5Co0.2Mn0.3O2/Graphite Cell

Lithium difluorophosphate (LiDFP) is used as an electrolyte additive to improve the electrochemical performances of LiNi0.5Co0.2Mn0.3O2 (NCM523)/graphite cells operating at a high charging cutoff voltage. LiDFP additive generates a more uniform, extremely stable, and lower resistance passivation film on the cathode surface. This passivation film not only suppresses the subsequent decomposition of electrolyte, but also prevents the dissolution of transition metal ions from NCM523 particles, resulting in improved cyclic stability and discharge capability of NCM523/graphite cells. With electrolyte contained 1.0 wt% LiDFP, the 18650 batteries achieves a capacity retention of 88.2% after 300 cycles at 3.0∼4.4 V and 90.0% after 200 cycles at 3.0∼4.45 V, comparing to 77.9% and 66.3% of the batteries without additive.

Effects of spot parameters in pencil beam scanning treatment planning.

Spot size σ (in air at isocenter), interspot spacing d, and spot charge q influence dose delivery efficiency and plan quality in Intensity Modulated Proton Therapy (IMPT) treatment planning. The choice and range of parameters varies among different manufacturers. The goal of this work is to demonstrate the influence of the spot parameters on dose quality and delivery in IMPT treatment plans, to show their interdependence, and to make practitioners aware of the spot parameter values for a certain facility. Our study could help as a guideline to make the trade-off between treatment quality and time in existing PBS centers and in future systems. We created plans for seven patients and a phantom, with different tumor sites and volumes, and compared the effect of small-, medium-, and large-spot widths (σ = 2.5, 5, and 10 mm) and interspot distances (1σ, 1.5σ, and 1.75σ) on dose, spot charge, and treatment time. Moreover, we quantified how postplanning charge threshold cuts affect plan quality and the total number of spots to deliver, for different spot widths and interspot distances. We show the effect of a minimum charge (or MU) cutoff value for a given proton delivery system. Spot size had a strong influence on dose: larger spots resulted in more protons delivered outside the target region. We observed dose differences of 2-13 Gy (RBE) between 2.5 mm and 10 mm spots, where the amount of extra dose was due to dose penumbra around the target region. Interspot distance had little influence on dose quality for our patient group. Both parameters strongly influence spot charge in the plans and thus the possible impact of postplanning charge threshold cuts. If such charge thresholds are not included in the treatment planning system (TPS), it is important that the practitioner validates that a given combination of lower charge threshold, interspot spacing, and spot size does not result in a plan degradation. Low average spot charge occurs for small spots, small interspot distances, many beam directions, and low fractional dose values. The choice of spot parameters values is a trade-off between accelerator and beam line design, plan quality, and treatment efficiency. We recommend the use of small spot sizes for better organ-at-risk sparing and lateral interspot distances of 1.5σ to avoid long treatment times. We note that plan quality is influenced by the charge cutoff. Our results show that the charge cutoff can be sufficiently large (i.e., 106 protons) to accommodate limitations on beam delivery systems. It is, therefore, not necessary per se to include the charge cutoff in the treatment planning optimization such that Pareto navigation (e.g., as practiced at our institution) is not excluded and optimal plans can be obtained without, perhaps, a bias from the charge cutoff. We recommend that the impact of a minimum charge cut impact is carefully verified for the spot sizes and spot distances applied or that it is accommodated in the TPS.

Impact of Microcrack Generation and Surface Degradation on a Nickel-Rich Layered Li[Ni0.9Co0.05Mn0.05]O2 Cathode for Lithium-Ion Batteries

To address the growing demand for energy density, the Ni-rich layered [Ni0.90Co0.05Mn0.05]O2 cathode has been synthesized and its electrochemical performance in lithium-ion cells has been benchmarked against a lower-Ni content Li[Ni0.6Co0.2Mn0.2]O2 cathode. Li[Ni0.90Co0.05Mn0.05]O2 delivers a high discharge capacity of 227 mA h g–1 compared to a capacity of 189 mA h g–1 for Li[Ni0.6Co0.2Mn0.2]O2 when cycled up to a lower cutoff voltage of 4.3 V, making it an appealing candidate for electric vehicles. With an increase in the charge cutoff voltage to 4.5 V, Li[Ni0.90Co0.05Mn0.05]O2 displays a capacity of 238 mA h g–1 compared to a capacity of 208 mA h g–1 for Li[Ni0.6Co0.2Mn0.2]O2. Although Li[Ni0.90Co0.05Mn0.05]O2 suffers during cycling from the usual rapid capacity fade in a manner similar to that of LiNiO2, 87 and 81% of the initial capacity could still be retained after 100 cycles even after cycling to higher cutoff voltages of 4.3 and 4.5 V, respectively. A comparison of Li[Ni0.90Co0.05Mn0.05]O2 and Li...

Preparation and electrochemical properties of Si0.8Sb/C nanofiber composite anode materials for lithium-ion batteries

The Si0.8Sb/C nanofiber composite anode materials were synthesized with the method of high-energy ball milling combined with electrospinning. The microstructures, morphologies, and electrochemical properties were investigated utilizing X-ray diffraction (XRD), scanning electron microscopy (SEM), constant current charge/discharge tests, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). The results indicated that the Si0.8Sb mutually insoluble alloy nanoparticles were coated by the carbon nanofiber. The carbon fiber exhibited high ductility which efficiently improved the cycle performance by reducing the shredding and powdering of anode materials during the volume expansion. The temperature, charge, and discharge cutoff voltage range and current density had an influence on the electrochemical properties and charge/discharge efficiency of Si0.8Sb/C electrode material. The electrochemical performance was optimal when the mass ratio of Si0.8Sb and polyacrylonitrile (PAN) was 3:5. Its initial charge/discharge was 1128.9 mAh/g, and the initial efficiency was 67.6%. After 100 cycles of charge and discharge, a capacity of 510.7 mAh/g and capacity retention of 66.9% were kept, displaying good electrochemical performance.

First-Principles Study: Tuning the Redox Behavior of Lithium-Rich Layered Oxides by Chlorine Doping

Lithium-rich layered oxides (LLOs) are promising cathode materials for next-generation lithium ion batteries with high energy density. However, the charge cutoff potential of 4.8 V constrains seriously the actual application of LLOs. Herein, using density functional theory (DFT) calculation, we investigated the tuning mechanism of chlorine doping on the redox potential and redox process in LLOs. The results showed that chlorine doping can decrease the charge potential, modulate the ratio of two redox couples of cation and anion, and lower the band gap of LLOs. These tunings were beneficial for the modification of the safety, cycling stability, and voltage decay of LLOs materials. This work opens up a new route in terms of performance improvement via tuning of redox behavior based on deep understanding of anion doping mechanism.

Changing Established Belief on Capacity Fade Mechanisms: Thorough Investigation of LiNi1/3Co1/3Mn1/3O2 (NCM111) under High Voltage Conditions

The further development of lithium ion batteries operating at high voltages requires basic understanding of the occurring capacity fade mechanisms. In this work, the overall specific capacity loss with regard to reversible and irreversible processes for LiNi1/3Co1/3Mn1/3O2 (NCM111)/Li half cells, cycled at a charge cutoff potential of 4.6 V vs Li/Li+, has been investigated in detail. By means of total X-ray fluorescence (TXRF) technique it was shown that specific capacity losses associated with the amount of dissolved transition metals are negligible, implying a still intact NCM111 active material after 53 cycles. It was demonstrated that the specific capacity fade during cycling at constant specific currents can be mainly attributed to the increase of the delithiation (charge) hindrance, whereas lithiation (discharge) hindrance is only present after a specific current increase, leading to apparent specific capacity losses and to decreased Coulombic efficiencies. This could be proven by the determination ...

Fluorinated Carbonate-Based Electrolyte for High-Voltage Li(Ni0.5Mn0.3Co0.2)O2/Graphite Lithium-Ion Battery

A stable electrolyte system at a charge voltage over 4.5 V is the key to successfully obtaining higher energy density by raising the charging cutoff voltage. We demonstrate a fluorinated electrolyte (1 M LiPF6 fluoroethylene carbonate (FEC) and methyl (2,2,2-trifluoroethyl) carbonate (FEMC) (FEC/FEMC = 1/9, v/v)) for a high-voltage LiNi0.5Mn0.3Co0.2O2/graphite system. The stability of the fluorinated electrolyte for the LiNi0.5Mn0.3Co0.2O2 (NMC532) cathode was investigated using scanning electron microscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy. The charge-discharge performance of the fluorinated electrolyte was superior to the corresponding non-fluorinated electrolyte system at a charging cutoff voltage of 4.7 V.

A Tutorial into Practical Capacity and Mass Balancing of Lithium Ion Batteries

In a lithium ion battery, balancing of active materials is an essential requirement with respect to safety and cycle life. However, capacity oversizing of negative electrodes is associated with decrease of specific energy/energy density. In this work, the required trade-off between maximized specific energy and minimized risk of lithium plating is thoroughly investigated by evaluating underlying potential/voltage curves. The adjustment of targeted state of charge (SOC) for both, positive and the negative electrode, can be achieved by intentional selection of only two parameters: negative/positive electrode active mass ratio and charge cutoff voltage. For investigation and controlling reasons, specific charge capacity reveals to be a simple but effective tool to indirectly predict electrode potentials. While cell kinetics/overvoltage are influenced by both electrodes, specific capacity losses are affected by a single electrode. The latter only correlate with negative electrode’s BET surface area as long as specific capacity losses of negative electrodes are higher compared to positive electrodes. Based on these insights, a more systematic performance and safety optimized handling of the trade-off between specific energy and safety risk can be realized.

Evaluation of Allylboronic Acid Pinacol Ester as Effective Shutdown Overcharge Additive for Lithium Ion Cells

Allylboronic acid pinacol ester (ABPE) was investigated as shutdown overcharge additive to increase the intrinsic safety of lithium ion cells during operation at elevated charge cutoff potentials up to 4.5 V vs. Li/Li+. It was demonstrated that the additive had no negative influence on the cycling performance of LiNi1/3Co1/3Mn1/3O2 (NMC-111)/graphite full cells operated in a standard operation voltage range between 2.5 V and 4.2 V. Electrochemical impedance spectroscopy was used to study the influence of the electrolyte additive on the impedance before and after overcharge and concomitant cell shutdown. Thereby, an immense increase in the charge transfer resistance after electrode shutdown was observed, thus, hinting to the formation of a lithium ion-insulating layer on the positive electrode surface. To further elucidate the working principle of the shutdown additive, surface investigations by means of scanning electron microscopy and X-ray photoelectron spectroscopy were carried out. The existence of a surface layer consisting mainly of polymeric species was proven. The presented results open up a new family of compounds for overcharge protection.

Aqueous Electrolyte Polyionic Batteries for Scaled Stationary Energy Storage

This presentation will cover the scaling and implementation of large-scale energy storage electrochemical batteries that use largely unexplored electrode interactions that exploit muti-cation interactions simultaneously, and have a high electrode mass to electrolyte mass ratio. The core (anode-limited) electrochemical couple uses a configuration wherein the active anode material consists of a blend of NaTi2(PO4)3 and activated carbon and the cathode is Li-inserted cubic spinel l-MnO2, and the electrolyte is a blend of Li+ and Na+ and hydrogen cation species (at some states of charge) solvated in an aqueous electrolyte with SO4 - and OH-(at some states of charge) as the countering anions. The basic function of the battery will be discussed, and comparisons between small bench top cells and fully scaled product-level devices will be drawn. Data will be presented showing that large scale industrially packaged batteries with over 3000 Wh in capacity have be produced and qualified. Further data will show that packs of these batteries in the multi-kWh range have been effectively implemented in field-testing around the world. These applications include support for both smaller off-grid applications with bus voltages in the in the 20 to 100 V range, as well as, grid compatible systems with bus voltages in excess of 1000 V. Key topics to be addressed include: (0) a description of the core electrochemical technology, including Na/Li displacement reactions in the anode, (1) a novel charge cutoff methodology that optimizes lifetime and function, (2) lifetime performance of this system through a range of temperatures, and (3) data from field tests in relevant applications showing the performance of our batteries under application specific load profiles.

Improvement and analysis of the hydrogen-cerium redox flow cell

The H2-Ce redox flow cell is optimized using commercially-available cell materials. Cell performance is found to be sensitive to the upper charge cutoff voltage, membrane boiling pretreatment, methanesulfonic-acid concentration, (+) electrode surface area and flow pattern, and operating temperature. Performance is relatively insensitive to membrane thickness, Cerium concentration, and all features of the (−) electrode including hydrogen flow. Cell performance appears to be limited by mass transport and kinetics in the cerium (+) electrode. Maximum discharge power of 895 mW cm−2 was observed at 60 °C; an energy efficiency of 90% was achieved at 50 °C. The H2-Ce cell is promising for energy storage assuming one can optimize Ce reaction kinetics and electrolyte.

Curbside parking pricing in a city centre using a threshold

How to set reasonable pricing for curbside parking, while balancing the demand for and the supply of parking spaces, is a troublesome problem for metropolitan areas such as Shanghai. This paper addresses this problem from the perspective of choice behaviour. Our research focuses on the parking charge cut-off point, which is the minimum or maximum acceptable value that a driver sets for an attribute. A multiple linear regression model reveals that older and inexperienced drivers are more likely to ignore the charge cut-off points they themselves have set. Discrete choice models incorporating charge cut-offs are further used to analyse charge implications for parking choice behaviour. Our results show that the precision of the conventional model is improved by including a cut-off. At the same time, parking charges, the time spent searching for a parking space, and walking time after finding the parking space, all have a significantly negative influence on parking choices. Finally, a pricing scheme is put forward to reduce the occupancy rates of curbside parking to 85%. This contention is based on parking pricing models with cut-offs. We find indications that not accounting for charge cut-off points, when they are in fact present, may lead to inaccurate willingness-to-pay and upwardly biased pricing schemes.

Stable Operation of Lithium Metal Batteries by the Formation of Transient High Concentration Electrolyte Layer during Fast Discharge

Lithium (Li) metal anode has an ultrahigh theoretical capacity of 3860 mAh/g and the lowest electrochemical potential at -3.040 V vs. standard hydrogen electrode, which makes the rechargeable Li metal batteries as the “holy grail” of energy storage systems.1 However, the development and application of rechargeable Li metal batteries has been hindered in the past decades mainly due to two major problems¾the Li dendrite growth on Li anode during repeated charge/discharge processes and the low Coulombic efficiency. Although several approaches have been reported to suppress Li dendrite growth on Li metal, including forming Li alloys, adding electrolyte additives, using blocking electrolyte membranes, and to improve Coulombic efficiency, the enhancement on Li protection so far is not satisfactory. Recently, highly concentrated ether-based electrolyte has been reported to be more compatible with Li metal anode because high concentration of Li+ ions in these electrolytes facilities the fast Li deposition/stripping even at high current density conditions.2,3 However, ether-based electrolytes may not be suitable for use at charge cutoff voltage higher than ~4.0 V due to their poor stability against oxidation. In this work, we report for the first time the significantly improved performances of Li metal batteries using a LiNi1/3Mn1/3Co1/3O2 (NMC) cathode and a conventional carbonate solvent-based electrolyte by an appropriate cycling protocol to generate a transient, highly concentrated Li+ ion solution layer in the vicinity of Li metal surface. The highly concentrated electrolyte formed in the vicinity of Li anode surface facilitates the formation of an SEI layer with considerably enhanced stability. It is demonstrated that a high capacity retention >80% after 500 cycles can be achieved for the moderately high areal-capacity Li metal batteries at an optimized charging/discharging process. Based on the results of morphology observation and the composition characterization of the cycled Li metal anodes, a more profound understanding of the underlying mechanism behind the improved performances has been obtained. The fundamental findings of this work provide new insights for the further development of high energy density and long cycle life Li metal batteries. Details of the investigations will be reported and discussed in the presentation. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, the Advanced Battery Materials Research Programs of the U.S. Department of Energy (DOE). The microscopy and spectroscopy measurements were performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. References A. Zhanmu, G. Chen, C. Liu, D. Neff, Q. Fang, Z. Yu, W. Xiong, Y. Wang, X. Wang, and B. Z. Jang, Energy Environ. Sci., 5, 5701 (2012). L. Suo, Y.-S. Hu, H. Li, M. Armand, and L. Chen, Nat. Commun., 4, 1481 (2013). J. Qian, W. A. Henderson, W. Xu, P. Bhattacharya, M. Engelhard, O. Borodin, and J.-G. Zhang, Nat. Commun., 6, 6362 (2015).

Interactions Between Positive and Negative Electrodes in Li-Ion Cells Operated at High Temperature and High Voltage

Introduction When NMC/graphite Li-ion cells are operated at high temperature and a cut-off potential higher than 4.2 V, generated gaseous products and oxidized species which could passivate the positive electrode and/or dissolve in the bulk electrolyte could reach the negative electrode and be reduced there.1,2 In order to study electrode/electrode interactions, pouch bags were used to separate the two electrodes, so that interactions between two electrodes would be impossible. Pouch bags containing only the charged negative or positive electrode can be used as a simple tool to obtain valuable information concerning the consequences of the absence of an interaction with the other charged electrode. Experimental Before electrolyte filling, dry (no electrolyte) NMC442/graphite (240 mAh) pouch cells were cut below the seal and vacuum dried at 80°C for 14 h. The pouch cells were filled with 0.9 g of 1M LiPF6 in EC:EMC (3:7 by weight) (control electrolyte) plus 2 % vinylene carbonate (VC), 2% prop-1-ene-1,3-sultone (PES) or 2% pyridine-boron trifluoride (PBF) in an argon-filled glove box and vacuum sealed in the same glove box. All the additives were added by weight percentage. After electrolyte filling, cells were placed in a temperature box at 40. ± 0.1°C and held at 1.5 V for 24 h. They were then charged to 3.8 V at C/20, and transferred to a glove box for degassing (cut open below the seal and re-sealed under vacuum). After degassing, they were charged to either 4.4 V, then discharged to 2.8 V and charged back to the same charge cutoff voltage until the holding period for the NMC442/graphite cells reached 30 hours. Four cells either at 4.2 or 4.4 V were moved to a 60°C temperature box for storage. The other cells were transferred to the glovebox and dissembled there. The delithiated NMC442 electrodes collected from the pouch cell were inserted into pouch bags with 0.3 g EMC. The addition of 0.3 g EMC to the pouch bags created a similar electrolyte environment as in the original pouch cells, because EC and LiPF6 were still left in the electrode and only about 0.3 g of EMC evaporated from the positive electrodes. Figure 1 shows photographs to illustrate the process used to make and test pouch cells and pouch bags. Results and discussion Figure 2 shows gas evolution for pouch bags containing lithiated graphite electrodes which were taken from pouch cells with control electrolyte, electrolyte with 2% PES, 2% VC and 2% PBF. No volume changes were detected for these pouch bags during approximately 500 h storage at 60°C. This suggests that parasitic reactions between the lithiated graphite and electrolyte do not contribute to gas generation at 60 °C. Figure 3 shows the volume change versus time for pouch cells with control electrolyte, or electrolyte with 2% PES, 2% VC or 2% PBF and pouch bags containing charged positive electrodes taken from brother pouch cells during a 500 h storage period at 60°C. Figure 3 shows that the volume of the pouch cells and pouch bags containing the charged positive electrodes increased with time. Figure 3 also shows that the pouch bags containing the delithiated NMC442 electrodes produced almost two times more gas than the corresponding pouch cells during the storage period. This suggests that some gaseous products generated at the positive electrode are consumed at the negative electrode in a full cell. Pouch bags with the delithiated NMC442 electrode taken from pouch cells at 4.4 V produced more gas than those taken from pouch cells at 4.2 V. This means, unsurprisingly, that delithiated NMC442 electrodes at a higher potential oxidize electrolyte at a higher rate. Pouch bags containing the washed charged NMC442 electrode (DMC washing) + EC/EMC produced gas at a higher rate than those containing a charged NMC442 electrode without washing. This suggests that the presence of LiPF6 can affect the parasitic reactions at the positive electrode that cause gaseous products. The above results suggest that oxidized species such as gaseous products generated at the positive electrode can be consumed at the negative electrode leading to a smaller volume expansion in pouch cells than pouch bags. The “clean-up” of other oxidized species by the lithiated graphite electrode resulting in a lower rate of impedance increase at the positive electrode will be discussed.

On electrochemistry of Al2O3-coated LiCoO2 composite cathode with improved cycle stability

LiCoO2-based cathode does still have a powerful competition in high-end mobile electronics due to its relatively high true density (about 5.2 g/cm3). When the operation potential range is extended, the improvement in its cycle stability has attracted more attention. The extension of its operation potential can be realized by partial replacement of Co by Ni and Mn or by surface modification. However, Ni and Mn replacing partial Co results in decreased true density; for example, the true density of LiNi0.5Mn0.3Co0.2O2 is about 4.6 g/cm3. In this case, the increase in its practical energy density is impossible. As a result, the surface modification technology becomes very important to extend its operation potential range. In this article, an Al2O3-coated LiCoO2 cathode was synthesized. X-ray diffraction test did not show any impurity. Scanning electron spectroscopy measurements showed that the basic microstructure of pristine LiCoO2 grain is sustained after coating Al2O3. The surface characteristic of pure and Al2O3-coated LiCoO2 was also analyzed using an X-ray photoelectron spectroscopy (XPS) technique. Unusual XPS peaks of O 1s, Al 2p, and Co 2p binding energy were found and may be caused by the possible H existence in crystal structure. The electrochemical behavior was systematically investigated, and the cathode was cycled at different charge cutoff voltages (4.30∼4.60 V). The charge-discharge and cyclic voltammetry measurements showed an obviously improved cyclic performance after coating Al2O3. The electrocatalytic activity is not clearly changed before and after coating Al2O3. From our systematical investigation, it could be concluded that the Al2O3-coated LiCoO2 cathode is suitable for practical application in the potential range of 3.70∼4.50 V vs. Li/Li+.