Discharge Cut-off Voltage Research Articles

Reversible Capacity of Conductive Carbon Additives at Low Potentials: Caveats for Testing Alternative Anode Materials for Li-Ion Batteries

The electrochemical performance of alternative anode materials for Li-ion batteries is often measured using composite electrodes consisting of active material and conductive carbon additives. Cycling of these composite electrodes at low voltages demonstrates charge storage at the operating potentials of viable anodes, however, the conductive carbon additive is also able to store charge in the low potential regime. The contribution of the conductive carbon additives to the observed capacity is often neglected when interpreting the electrochemical performance of electrodes. To provide a reference for the contribution of the carbons to the observed capacity, we report the charge storage behavior of two common conductive carbon additives Super P and Ketjenblack as a function of voltage, rate, and electrolyte composition. Both carbons exhibit substantial capacities after 100 cycles, up to 150 mAh g−1, when cycled to 10 mV. The capacity is dependent on the discharge cutoff voltage and cycling rate with some dependence on electrolyte composition. The first few cycles are dominated by the formation of the SEI followed by a fade to a steady, reversible capacity thereafter. Neglecting the capacity of the carbon additive can lead to significant errors in the estimation of charge storage capabilities of the active material.

High performance stoichiometric Cu2S cathode on carbon fiber current collector for lithium batteries

In this study, stoichiometric Cu2S with monoclinic crystal structure with space group P21/c was prepared via spray pyrolysis with followed heat treatment. Its electrochemical properties were investigated after coating on a carbon fiber paper (CFP) current collector and cycling at various cut-off voltages. The reversible stable capacity of about 330mAhg−1 (98% of its theoretical capacity) exhibited by the cell for 20 cycles at 0.1C and slightly dropped to 311mAhg−1 after 50 cycles. At higher 1C rate, a discharge capacity of 280mAhg−1 was retained for 100 cycles with approximately 100% Coulombic efficiency between 1.0-3.0V. While the gradual capacity decrease is observed with the increase of the discharge cut-off voltage to avoid solid electrolyte interface (SEI) formation during the 1st discharge. The cells showed capacities of 307 and 280mAhg−1 after the 5th cycle at 0.1C when cycled between 1.1-3.0V and 1.2-3.0V, respectively, due to the lowered lithium ion diffusion. Ex situ XRD analysis of the recovered electrodes after cycling with near theoretical capacity allowed discussing the actual reaction mechanism of stoichiometric Cu2S with Li+.

Electrochemical Study of Nano-Iron/Iron Carbide As Rechargeable Electrodes

Fe–air system is considered to be of low-cost, environmentally friendly and mechanically stable among the available battery technologies. Iron electrodes can maintain more than 3000 cycles in Ni-Fe secondary batteries. However, the iron electrode possesses low charge efficiency and inferior rate capability. In order to circumvent the hydrogen evolution taking place during the reaction, which is the main contributor to the irreversibility, various additives both to the electrolyte and porous electrode have been employed to increase the charge/discharge performance (1, 2). A range of nano-iron materials were prepared to increase the surface area and thereby improve the utilization (3-5). Recently carbon grafted in iron as well as carbon/iron composite morphologies have shown increased discharge capacities (6, 7). In this work, we have studied the surface morphology evolution upon electrochemical cycling of core-shell structured samples, i.e. 2 wt% Cu-doped nano-iron coated with a thin layer of iron carbide (NanoFeC-Cu). The electrodes were prepared by homogenizing iron powder, high surface area carbon black, bismuth sulfide and PTFE dispersion in a high speed mixer. This paste was then rolled on a nickel wire mesh. After this step, the electrode was sintered in nitrogen atmosphere for 30 min.Electrochemical measurements were carried out in a three cell configuration with nickel mesh as counter electrode and an Hg/HgO as reference electrode in 6M KOH. The electrochemical formation of the iron electrodes took about 30 to 40 cycles. The discharge/charge rate for respective steps was limited to C/10 and C/7 and the discharge cut-off voltage was -0.75V. The Nano FeC-Cu achieved 350 mAh/g at ≈ 80% current efficiency, successfully running over 100 cycles as shown in Fig. 1.The electrode after galvanostatic and potentiodynamic polarization measurements showed Tafel slopes of 35 mV/decade and no passivation was observed at a current density of 40 mA/cm2 at -0.75V. SEM images of both the fresh and used electrode surfaces show that the nano particles are still intact with negligible particle agglomeration. The electrodes have shown stable performances with low capacity decay. The present core-shell structured material is, therefore, a promising anode candidate in alkaline-metal/air batteries. References (1). R. D. McKerracher, C. Ponce de Leon, R. G. A Wills, A. A. Shah, F. C. Walsh, ChemPlusChem, 80 (2014), 323–335. (2). A. K. Manohar, C. Yang, S. R. Narayanan, S. R., J. Electrochem. Soc., 162 (2015), A1864-A1872. (3). C. -Y. Kao, K. -S. Chou, J. Power Sources, 195 (2010), 2399-2404. (4). C.-Y. Kao, Y.-R. Tsai, K.-S. Chou, J. Power Sources, 196 (2011), 5746-5750. (5). K. C. Huang and K. S. Chou, Electrochem Commun., 9 (2007), 1907-1912. (6). A. Sundar Rajan, S. Sampath, A. K. Shukla, Energy Environ. Sci., 7 (2014), 1110. (7). B. T. Hang, M. Eashira, I. Watanabe, S. Okada, J.-I. Yamaki, S-H. Yoon, I. Mochida, J. Power Sources, 143 (2005), 256-264. Acknowledgement This work is funded by the Swedish Energy Agency. Figure 1

Large Polarization of Li4Ti5O12 Lithiated to 0 V at Large Charge/Discharge Rates.

The ability to enhance the specific capacity of Li4Ti5O12 (LTO) is of practical significance and offers the opportunity to increase the energy density of a full LTO-based battery. Widening the discharge cutoff voltage to 0 V is an effective way to increase the capacity of LTO at low current density. However, whether the specific capacity of LTO at large current rates can be enhanced remains largely unknown. Herein, intriguingly, we found that, when cycled down to 0 V (i.e., from 1.0-2.5 to 0-25 V), LTO exhibited greatly enhanced specific capacity at low rates (<5 C), while it showed a rapid capacity fading and a greatly increased charge/discharge potential gap at high rates (>10 C). The decreased lithium-ion diffusion caused by extra lithium-ion intercalation together with increased internal impedance significantly enhances the polarization and reduces the electrochemical reaction kinetics, which, in turn, hinders the lithiation reaction from LTO to Li7Ti5O12 and further to Li9Ti5O12.

Polyglutamate Binder for Silicon-Graphite Negative Electrode: Effects of Cut-Off Voltage and Electrolyte Additive

Silicon is one of the promising active materials for negative electrodes of Li-ion batteries (LIB) because of its larger reversible capacity (~3600 mAh g-1) through lithiation/delithiation process, compared with that of conventional graphite active material (~370 mAh g-1). However, it is difficult to maintain its superior discharge capacity for long-term cycles because large volume change of Si occurs during cycles, which triggers intensive electrode degradation. To overcome this drawback, binder polymer is thought to be a key factor for improvement of the performance. In particular, polyacrylate binder1,2) and natural polymer binder3) effectively suppress the degradation due to their superior mechanical strength, coatability for active materials, and effects of pre-formed solid electrolyte interphase (SEI)4). Recently, we have focused on polyglutamate (PGlu) as an efficent binder for the Si/graphite composite electrode of LIB. PGlu is known to be contained in Japanese traditional food “Natto,” which is fermented soy beans. In this study, effects of discharge cut-off voltage in the cycles and electrolyte additive on the performance of Si/graphite electrodes with PGlu binder are investigated from the viewpoint of surface chemistry. Figure 1 shows cycle performance of the Si/graphite electrodes with different discharge cut-off voltage. For additive-free cells, capacity retention was highly improved under the cycles with lower discharge cut-off voltage (1.0 V vs. Li/Li+). Furthermore, addition of fluoroethylene carbonate (FEC) to the electrolyte effectively suppressed deterioration of the performance and brought the best capacity retention, leading to discharge capacity of ~800 mAh g-1 even after 100 cycles. To clarify details of the effects, chemical components and thickness of the SEI formed on the electrode surface were examined by hard X-ray photoelectron spectroscopy (HAXPES, hν = 7.94 keV). Figure 2 shows the HAXPES C 1s and Si 1s spectra of Si/graphite electrodes before and after the first cycle with different discharge cut-off voltage. For the electrode with cut-off voltage of 1.0 V, the peak intensity at 284.6 eV is relatively lower than that of the 2.0 V cut-off electrode, which is assigned to sp 2 carbon bonding in graphite of the composite. Similarly, the peaks at 1839.5 eV and 1844 eV in the Si 1s spectra, which are assigned to Si0 and silicon oxide, respectively, are smaller for the case of lower cut-off voltage. These results indicate that thickness of SEI is affected by the discharge cut-off voltage. We presume that moderately-low discharge cut-off voltage of 1.0 V effectively brings thicker and stable SEI on the electrode surface by suppression of oxidative decomposition and dissolution of SEI, which probably occur at higher potential condition in delithiation process. In this presentation, other electrochemical and morphological characterizations of the electrode and superior effects of FEC for the cyclability will be also discussed.

Selection of Carbon Sources for Enhancing 3D Conductivity in the Secondary Structure of LiFePO4/C Cathode

Secondary LiFePO4/C microspheres (LFP) are synthesized with different carbon sources by the spray drying process. The carbon sources effect on the structures, morphologies, and 3D conductivity of the secondary structure are systematically investigated. LFP samples prepared with polyethylene glycol (PEG) and beta-cyclodextrin (β-CD) as mixing carbon sources possesses the loose structure with higher specific surface area, showing the best rate capability, cycling stability and low-temperature discharge characteristic. Additionally, the differences of 3.3V plateau performance at room temperature and 2.85V plateau performance at −20°C are investigated. It could be observed that the electronic and ionic conductivities are reduced gradually with the decrease of the discharge cut-off voltage, while the electronic conductivities are greater than ionic conductivities for the four LFP samples, indicating that the ionic transport is more difficult and the electrochemical reaction is more and more difficult with the increase of Li-ion intercalation. Li-ion diffusion coefficients at the cut-off voltage of 3.30V under room temperature and at the cut-off voltage of 2.85V under −20°C are both the highest for the LFP sample synthesized with PEG and β-CD, further indicating that PEG and β-CD as mixing carbon sources can decrease the charge transfer resistance and promote the 3D electronic/ionic conductivities and Li-ion diffusion coefficients in the secondary structure, thus greatly improve the rate capability, cycling stability and low-temperature capacity of LFP cathode.

Pitfalls in Li–S Rate-Capability Evaluation

Rate-capability tests are widely used to highlight advances in Li–S-system development. Here we show, by studying the individual effects of a number of cycling- and electrolyte-related parameters on a simple Li–S cell, that the rate-capability results are sensitive not only to the applied current but also to the cycling prehistory of the cell. On the one hand, the performance is affected by the order in which cycling rate is changed. Slow initial rates aggravate material loss and reduce the achievable capacity. On the other hand, the results of rate-capability tests are significantly different when the rates are varied only on the charge or only on the discharge. The charge rate does not directly affect the measured capacity, whereas the discharge rate does, especially when discharge cutoff voltages are high, due to slow discharge reactions. High charge rates, however, do affect the long-term stability, which is difficult to predict from usual rate-capability tests. Our findings also provide insights into the relative reaction rates and the influence of the general cycling procedures, underlining the importance of understanding the kinetics of Li–S systems, while designing them for high performance batteries.

Identifying main factors of capacity fading in lithium ion cells using orthogonal design of experiments

The aging rate under cycling conditions for lithium-ion cells is affected by many factors. Seven principal factors are systematically examined using orthogonal design of experiments, and statistical analysis was used to identify the order of principal factors in terms of strength in causing capacity fade. These seven principal factors are: the charge and discharge currents (i1,i2) during the constant current regime, the charge and discharge cut-off voltages (V1,V2) and the corresponding durations (t1,t2) during the constant voltage regime, and the ambient temperature (T). An orthogonal array with 18 test units was selected for the experiments. The test results show that (1) during the initial 10% capacity fading period, the capacity faded linearly with Wh-throughput for all the test conditions; (2) after the initial period, certain cycling conditions exacerbated aging rates, while the others remain the same. The statistical results show that: (1) except for t1, the other six principal factors significantly affect the aging rate; (2) the strength of the principal factors was ranked as: i1>V1>T>t2>V2>i2>t1. Finally, a multi-factor statistical aging model is developed to predict the aging rate, and the accuracy of the model is validated.

Highly Reversible Lithium‐ions Storage of Molybdenum Dioxide Nanoplates for High Power Lithium‐ion Batteries

Herein, MoO2 nanoplates have been facilely prepared through a hydrothermal process by using MoO3 microbelts as the intercalation host. The obtained MoO2 nanoplates manifest excellent electrochemical properties when the discharge cutoff voltage is simply set at 1.0 V to preclude the occurrence of conversion reactions. Its initial reversible capacity reaches 251 mAh g(-1), which is larger than that of Li4Ti5O12 , at a current rate of 0.2 C. The average capacity decay is only 0.0465 mAh g(-1) per cycle, with a coulombic efficiency of 99.5% (from the 50th cycle onward) for 2000 cycles at 1 C. Moreover, this MoO2 electrode demonstrates an outstanding high power performance. When the current rate is increased from 0.2 to 50 C, about 54% of the capacity is retained. The superior electrochemical performance can be attributed to the metallic conductivity of MoO2, short Li(+) diffusion distance in the nanoplates, and reversible crystalline phase conversion of the addition-type reaction of MoO2. The prepared MoO2 nanoplates may hopefully replace their currently used analogues, such as Li4Ti5O12 , in high power lithium-ion batteries.

Capacity Fading Mechanisms of Submicron-Sized Silicon Negative Electrode for Lithium Ion Batteries

Silicon, due to its large specific capacity, is regarded as one of the most promising negative electrode materials for lithium ion batteries despite critical drawbacks associated with large volume change upon lithiation/de-lithiation. Although nano-sized silicon particles have been employed to mitigate the failures and related techniques have achieved considerable improvements, current materials do not meet the demand of commercial applications. Thus, a systematical approach to elucidating failure mechanisms is necessary to further improve electrochemical performances of the silicon electrode. Here, the evolution of voltage profiles recorded during cycling was thoroughly analyzed and thereby the failure mechanisms are proposed. The main failure mechanism of the silicon electrode is related to an inability to fully de-lithiate due to resistance increases during discharging, which are mostly contact and SEI resistance. The capacity retention was significantly improved by lowering discharge cut-off voltage and by introducing electrolyte additives, in which reduce the resistances during discharging.

A structural study of solid electrolyte interface on negative electrode of lithium-Ion battery by electron microscopy

For the last decades, the performance of the lithium-ion battery (LIB) has been significantly improved and its applications have been expanding rapidly. However, its performance has yet to be enhanced.In the lithium-ion battery development, it is important to elucidate the electrode structure change in detail during the charge and discharge cycling. In particular, solid electrolyte interface (SEI) formed by decomposition of the electrolytes on the graphite negative electrode surface should play an important role for battery properties. Therefore, it is essential to control the structure and composition of SEI to improve the battery performance. Here, we conducted a scanning electron microscope (SEM) and transmission electron microscope (TEM) study to elucidate the structures of the SEI during the charge and discharge process using LiNi1/3Co1/3Mn1/3O2 [1] cathode and graphite anode. [2] Since SEI is a lithium-containing compound with high activity, it was observed without being exposed to the atmosphere. The electrodes including SEI were sampled after dismantling batteries with cutoff voltages of 3V and 4.2V for the charge process and 3V for the discharge process. Fig.1 shows SEM images of the graphite electrode surface during the charge and discharge process. The change of the SEI structure during the process was clearly observed. Further, TEM images showed that the SEI grew thicker during the charge process and becomes thinner when discharged. These results with regard to the reversible SEI structure could give a new insight for the battery development.jmicro;63/suppl_1/i21/DFU056F1F1DFU056F1Fig. 1.SEM images of the graphite electrode surface:(a) before charge process;(b) with charge-cutoff voltage of 3.0V; (c) with charge-cutoff voltage of 4.2V; (d) with discharge-cutoff voltage of 3.0V.

Towards Understanding Li-O2 Electrochemistry on Precious Metal Electrocatalyst Surfaces in Non-Aqueous Electrolytes

With expanding expectations for vehicle electrification, energy storage systems with high specific energy densities are attracting increased interest. In particular, Lithium-Oxygen (Li-O2) batteries are considered one of the best candidates for post lithium ion batteries due to their high specific energy density (2600Wh/kg-electrode mass). However, Li-O2batteries have several major issues including electrolyte decomposition[1] and carbon support corrosion[2] which inhibits cell reversibility. The reaction products and mechanism of product formation were investigated on various precious metal electrocatalyst surfaces using cyclic voltammetry (CV), rotating ring-disk electrodes (RRDE), in-situ electrochemical scanning probe microscopy (EC-SPM), and in-situ Raman spectroscopy.Gold, palladium, and platinum were investigated as Li-O2 oxygen reduction electrocatalysts under various operating conditions. Oxygenated 0.1M LiTFSI/DMSO and 0.1M LiClO4/DMSO were employed to investigate the influence of electrolyte salt on product formation. In order to obtain electrochemical characteristics of the various electrocatalyst surfaces, CV and RRDE were conducted using Li metal as the counter electrode and a Ag/Ag+reference electrode. For EC-SPM and in-situ Raman spectroscopy experiments, a two electrode configuration was utilized where Li metal served as both the counter and reference electrode. Additionally, constant current and voltage control modes were used case by case.Through our experiments, we have confirmed that the discharge product morphology and reversibility of the Li-O2 electrochemical reaction were influenced by the solvent, the electrolyte salt species, electrocatalyst metal, and electrochemical operating condition (current density and discharge cut-off voltage). The fact that discharge conditions can induce different Li2O2 product morphologies has been reported [3]. Once large Li2O2 particles form, decomposition becomes more difficult and larger charging overpotentials are observed. Additionally, under certain experimental conditions, a transition from Li2O2 formation via disproportionation to a 2nd electron transfer mechanism is observed, which also influences decomposition overpotentials. Finally, Li salts containing the perchlorate anion produce a uniform film-like morphology as opposed to a particle morphology produced by the TFSI anion containing electrolyte. Thus, a complex interaction between the solvent, electrolyte salt anion, LiO2 intermediate species, and operating condition appears to influence Li2O2formation mechanism and morphology and thus apparent reversibility. Further details of this investigation will be presented.[1] S.A. Freunberger, J.Am.Chem.Soc., 2011, 133, 8040–8047.[2] B.D. McCloskey, J. Phys. Chem. Lett. 2012, 3, 997-1001.[3] B.D. Adams, Energy Environ. Sci., 2013,6, 1772-1778.

Invited Presentation: Enhanced Cycleability of LiNi1/3Co1/3Mn1/3O2 Coated with Metal Oxides Under High Voltage Charge/Discharge Cycles

Introduction In order to enhance cycleability of positive electrode materials, coating of metal oxides [1], phosphates [2], and fluorides [3] have been reported to be effective on depressing degradation of positive electrode materials. However, so far, mechanisms of surface modification by coatings have not been clarified well. In recent, we have reported that Al oxides coating significantly suppressed the increase of cracks in LiNi1/3Co1/3Mn1/3O2 particle, and that both capacity and area specific impedance retentions of LiNi1/3Co1/3Mn1/3O2during charge-discharge cycles were improved by the coating [4]. In this paper, effect of coating of various metal oxides on electrochemical characteristics and cycleability enhancement of LiNi1/3Co1/3Mn1/3O2 particles during high voltage charge higher than 4.5 V (vs. Li+/Li) have been examined. Furthermore, effects of coating of various metal oxides on crystal structure deformation at the surface of LiNi1/3Co1/3Mn1/3O2during the charge-discharge cycles were investigated by s-TEM/EELS. Experiments Precursors of metal oxides (Al oxides, Zr oxides, etc.) were coated on LiNi1/3Co1/3Mn1/3O2 particles by sol-gel methods. The precursors coated LiNi1/3Co1/3Mn1/3O2 particles were calcined at 500°C to obtain metal oxides coated LiNi1/3Co1/3Mn1/3O2. Coat layers of metal oxides and surface area of LiNi1/3Co1/3Mn1/3O2 were observed by high resolution s-TEM. Electrochemical characteristics of bare and metal oxides coated LiNi1/3Co1/3Mn1/3O2 were examined in pouch cells with using Li metal counter electrode. The electrolyte used was 1M-LiPF6/EC+DMC +EMC (1:1:1 by volume). The cells were cycled between discharge cutoff voltage of 2.5 V and charge cutoff voltages of 4.5, 4.6 and 4.7 V. Results and discussion Figure 1 shows an s-TEM image and EDX mappings (Al, Ni, and Co) of cross section of Al oxides coated LiNi1/3Co1/3Mn1/3O2. The high resolution s-TEM/EDX observation revealed that coated Al oxides exist as a solid solution (LiTMO2-LiAlO2) from the surface to depth of several nm. Figure 2 shows effect of Al oxides coating on capacity retention of LiNi1/3Co1/3Mn1/3O2 during the charge-discharge cycles. Bare LiNi1/3Co1/3Mn1/3O2 electrodes displayed clear capacity fading during the charge-discharge cycles under the high voltage charge conditions. On the other hand, LiNi1/3Co1/3Mn1/3O2 coated by Al oxides of 0.5 w/o showed enhanced cycleability even in charge cutoff voltages of 4.5 and 4.6 V, while capacity fading of the Al coated LiNi1/3Co1/3Mn1/3O2was not negligible in charge cutoff voltage of 4.7 V. AC impedance analysis showed that capacity fading of LiNi1/3Co1/3Mn1/3O2 was attributed to increases of polarization resistance of LiNi1/3Co1/3Mn1/3O2 electrodes. Since crystal structure deformation was observed at the surface of degraded LiNi1/3Co1/3Mn1/3O2 particles by s-TEM/EELS, increase of polarization is thought to be attributed to this crystal structure deformation. Al oxides coating effects on the crystal structure deformation at the surface of LiNi1/3Co1/3Mn1/3O2will be discussed in comparison with Zr oxides coating effects. Acknowledgements This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING) project of NEDO, Japan. References 1) L. A. Riley et al., J. Power Sources, 196, 3317 (2011).2) J. Y. Shi et al., J. Power Sources, 195, 6860 (2010).3) S.-U. Woo et al., J. Electrochem. Soc., 154, A1005 (2007).4) K. Araki et al., J. Power Sources, to be submitted.

Battery available power prediction of hybrid electric vehicle based on improved Dynamic Matrix Control algorithms

The core technology to any hybrid engine vehicle (HEV) is the design of energy management strategy (EMS). To develop a reasonable EMS, it is necessary to monitor the state of capacity, state of health and instantaneous available power of battery packs. A new method that linearizes RC equivalent circuit model and predicts battery available power according to original Dynamic Matrix Control algorithm is proposed. To verify the validity of the new algorithm, a bench test with lithium-ion battery cell and a HEV test with lithium-ion battery packs are carried out. The bench test results indicate that a single RC block equivalent circuit model could be used to describe the dynamic and the steady state characteristics of a battery under testing conditions. However, lacking of long time constant of RC modules, there is a sample deviation in the open-circuit voltage identified and that measured. The HEV testing results show that the battery voltage predicted is in good agreement with that measured, the maximum difference is within 3.7%. Fixing the time constant to a numeric value, satisfactory results can still be achieved. After setting a battery discharge cut-off voltage, the instantaneous available power of the battery can be predicted.

A vein-like nanoporous network of Nb2O5 with a higher lithium intercalation discharge cut-off voltage

A novel morphology of a criss-cross vein-like nanoporous network of Nb2O5 produced using a simple electrochemical anodization method is presented as a superior electrode for safe lithium-ion batteries. Scanning electron microscopy (SEM) observations demonstrate that the synthesised Nb2O5 is made of a continuous and highly packed vein-like nanoporous network with many lateral interconnections, which provides excellent channels for the fast transfer of both Li+ ions and electrons. Even without surface coating or cation doping, the porous Nb2O5 electrode could deliver durable capacity within the operating voltage window of 1.0–3.0 V vs. Li/Li+, with a reversible capacity of 201 mA h g−1 after 300 cycles at a current density of 0.4 A g−1. At the higher discharge cut-off voltage window of 1.2–3.0 V, the reversible capacity decreased to 175 mA h g−1. The first cycle Coulombic efficiency was above 94% for both operating voltage windows with a negligible capacity fading up to 300 cycles. The porous Nb2O5 electrode demonstrates several advantages as an anode including: (i) Improved cell safety due to a higher, V ≥ 1.0, discharge cut-off voltage which reduces dangerous high-temperature reactions; (ii) low level of irreversibility in the first cycle by preventing the formation of a solid electrolyte interface layer; (iii) high Coulombic efficiency due to sufficient infiltration of the electrolyte and fast diffusion of Li+ ions and (iv) high rate capability. Moreover, the synthesis method reports a novel smart design of nanostructured anode electrode materials capable of overcoming the existing limitations.

Improved lithium–sulfur cells with a treated carbon paper interlayer

A simple, low-cost modification of lithium-sulfur (Li-S) cells by placing a treated carbon paper between the sulfur electrode and the separator has been investigated to significantly improve the performance of Li-S cells. The treated carbon paper was prepared by an alcohol-alkaline/thermal treatment of a commercial Toray carbon paper, introducing hydroxyl functional groups and micro-cracks on the carbon fibers in the carbon paper, which enhances the hydrophilicity and increases surface areas of the carbon paper matrix. The modified Li-S cells deliver a higher initial capacity of 1651 mAh g(-1) at 1.5-2.8 V at a rate of C/5 compared to the cells without any interlayer or with an untreated carbon paper interlayer. The cells with the treated carbon paper offer additional improvement in performance when the discharge cut-off voltage is raised to 1.8 V: 1057, 1002, and 929 mAh g(-1) after 100 cycles, respectively, at C/5, C/2, and 1 C rates. The improved cell performance is attributed to the 3D architecture of the carbon paper interlayer, serving as a conductive skeleton for trapping and depositing dissolved sulfur-containing active materials, as confirmed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The novel configuration presented here offers a low-cost approach to overcome the persistent problems of Li-S cells.

Effect of Discharge Cutoff Voltage on Reversibility of Lithium/Sulfur Batteries with LiNO3-Contained Electrolyte

LiNO3 is unique in suppressing the redox shuttle of lithium polysulfides in Li/S battery. In this paper we evaluate the effect of discharge cutoff voltage on the reversibility of Li/S battery with a LiNO3-contained electrolyte. Based on a study of the liquid Li/S cells with a 0.05 m Li2S9 solution as the catholyte, we find that the highly reversible discharge and charge cycling of Li/S cells occurs at higher than 1.8 V vs. Li/Li+, which corresponds to a multistage redox process of the sulfur cathode between elemental sulfur and insoluble Li2S2. At lower potentials, not only the Li2S2 is reduced into poorly reversible Li2S, but also the LiNO3 is reduced irreversibly. Moreover, the irreversible reduction of LiNO3 further reduces the reversibility of the poorly reversible Li2S, resulting in a significant slowdown in the electrode reaction kinetics and a permanent loss in the reversibility of the Li/S cell. This work suggests that deep discharge must be avoided for a long cycle life Li/S battery when LiNO3 is used as an additive or a co-salt of the electrolyte.

Electrochemical performance and low discharge cut-off voltage behavior of ruthenium doped Li4Ti5O12 with improved energy density

Pristine Li4Ti5O12 (LTO) and Ru-doped Li4Ti5O12 with the composition of Li4Ti4.95Ru0.05O12 (Ru-doped LTO) are synthesized by solid-state reaction. Ru doping into the lattice of LTO affects the electronic structure of LTO, leading to the modification of optical properties and improvement in electrochemical performance. The variations between pristine LTO and Ru-doped LTO in optical properties are investigated by UV–vis and Raman spectroscopy. In addition, the related microstructure is characterized by XRD, SEM and TEM. The enhancement in electronic conductivity of Ru-doped LTO can provide the discharge capacities of 222, 183 and 132mAh/g at 1C, 5C and 10C, respectively, during the voltage window of 0.01–2.5V. Furthermore, the capacity retentions are 95, 92 and 86% for 1C, 5C and 10C rates, respectively, at 100th cycles. The significant improvement in electrochemical performance demonstrates that ruthenium doped lithium titanate is promising as a high rate anode for lithium ion batteries.

Enhanced lithium storage in a VO 2(B)-multiwall carbon nanotube microsheet composite prepared via an in situ hydrothermal process

A novel VO 2(B)-multiwall carbon nanotube (MWCNT) composite with a sheet-like morphology was synthesized by a simple in situ hydrothermal process. The morphology and structural properties of the samples were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). FE-SEM observations demonstrated that the nanosheets are frequently grown together in the form of bundles composed of numerous nanosheets, each with a smooth surface and a typical length of 300–500 nm, width of 50–150 nm, and thickness of 10–50 nm. Electrochemical measurements were carried out using different discharge cut-off voltages. Electrochemical tests show that the VO 2(B)–MWCNT composite cathode features long-term cycling stability and high discharge capacity (177 mAh g −1) in the voltage range of 2.0–3.25 V at 1 C with a capacity retention of 92% after 100 cycles. The electrochemical impedance spectra (EIS) indicate that the VO 2(B)–MWCNT composite electrode has very low charge-transfer resistance compared with pure VO 2(B), indicating the enhanced ionic conductivity of the VO 2(B)–MWCNT composite. The enhanced cycling stability is attributed to the fact that the VO 2(B)–MWCNT composite can prevent the aggregation of active materials, accommodate the large volume variation, and maintain good electronic contact. We strongly believe that the VO 2(B)–MWCNT composite can be considered as a potential cathode material for lithium-ion batteries.

Air stable Al2O3-coated Li2NiO2 cathode additive as a surplus current consumer in a Li-ion cell

Highly air stable Al2O3-coated Li2NiO2 cathode additive is prepared by coating with Al iso-propoxide on Li2NiO2, obtained from firing of a physical mixture of pure Li2O and NiO at 600 °C for 10h under N2 atmosphere. An as-prepared coated cathode has first charge and discharge capacities of 420 mAh/g and 310 mAh/g, respectively, between 4.3V and 1.5V showing an irreversible capacity ratio of 26%. However, when the discharge cut-off voltage increases to 2.75V (2.85V vs. graphite), its discharge capacity decreases to 120 mAh/g, which corresponds to an irreversible capacity ratio of 71%. Owing to such a high irreversible capacity, it can effectively compensate for the irreversible capacity of the Li-ion cell using LiCoO2 and natural graphite as cathode and anode materials, respectively, in spite of only 4wt% addition to the LiCoO2 cathode. In addition, the additive prevents the 12V overcharge, thereby preventing the explosion of the cell. We believe that Li2NiO2decomposition consumes the surplus current during the overcharging to 12V, and therefore the voltage is not increased until the complete decomposition of the Li2NiO2.