Surface Of Active Material Research Articles

Multiscale Simulation Platform Linking Lithium Ion Battery Electrode Fabrication Process with Performance at the Cell Level.

A novel multiscale modeling platform is proposed to demonstrate the importance of particle assembly during battery electrode fabrication by showing its effect on battery performance. For the first time, a discretized three-dimensional (3D) electrode resulting from the simulation of its fabrication has been incorporated within a 3D continuum performance model. The study used LiNi0.5Co0.2Mn0.3O2 as active material, and the effect of changes of electrode formulation is explored for three cases, namely 85:15, 90:10, and 95:5 ratios between active material and carbon-binder domains. Coarse-grained molecular dynamics is used to simulate the electrode fabrication. The resulting electrode mesostructure is characterized in terms of active material surface coverage by the carbon-binder domains and porosity. The trends observed are nonintuitive, indicating a high degree of complexity of the system. These structures are subsequently implemented into a 3D continuum model which displays distinct discharge behaviors for the three cases. The study offers a method for developing a coherent theoretical understanding of electrode fabrication that can help optimize battery performance.

Solvation and Dynamics of Lithium Ions in Carbonate-Based Electrolytes during Cycling Followed by Operando Infrared Spectroscopy: The Example of NiSb2, a Typical Negative Conversion-Type Electrode Material for Lithium Batteries

Conversion-type electrode materials show extremely interesting performance in terms of capacity, which is however usually associated with bad Coulombic efficiency. The latter is mainly the consequence of the relentless evolution of solid electrolyte interphase (SEI) formed and/or dissolved during conversion/back-conversion reactions on the continuously reshaping active material surface. The thorough comprehension of the dynamic processes occurring during cycling in a working electrochemical cell, such as solvation/desolvation of ionic species and formation/dissolution of the SEI at the electrode/electrolyte interface, is thus of utmost relevance in the study of electrochemical mechanism and performance of conversion-type electrode materials. Operando Fourier transform infrared (FTIR) spectroscopy, one of the methods of choice for the study of such phenomena, was applied to study the dynamic interfacial properties of NiSb2, a representative intermetallic conversion-type electrode material for Li batteries,...

General Method of Manipulating Formation, Composition, and Morphology of Solid-Electrolyte Interphases for Stable Li-Alloy Anodes.

Li-alloy-based anode materials are very promising for breaking current energy limits of lithium-ion battery technologies. Unfortunately, these materials still suffer from poor solid-electrolyte interphase (SEI) stability, resulting in unsatisfied electrochemical performances. The typical SEI formation method, electrochemical decomposition of electrolytes onto the active material surface, lacks a deliberate control of the SEI functions and structures. Here we propose a general method of manipulating the formation process, chemical composition, and morphology of the SEI for Li-alloy anodes, using Si and Ge nanoparticle anodes as the platform. The SEI was fabricated through a covalent anchoring of multiple functional components onto the active material surface, followed by electrochemical decomposition of the functional components and conventional electrolyte. Click reaction, serving as the covalent anchoring approach, allows an accurate control of the SEI composition and structure at the molecular level through tuning the chemical structure and amount of variety of functional components and provides an intimate contact between the SEI and the Li-alloy material surface contributed by the covalent bonding. The optimized Si nanoparticle SEI, functionalized by a unique combination of diverse components and containing a high concentration of organic components attributed to the preanchored functional components, presented a stable composition and durable morphology during cycling and led to an improved first cycle efficiency of Si nanoparticle anodes and its long cycle life in a full cell. This general method displays potential benefits to construct stable SEIs for other Li-alloy anodes.

Stabilization of Electrode-Electrolyte Interface By Conformal Passivating Layers on the Surface of Spinel Nanocrystals

Stable cycling performance with high power density in lithium ion battery (LIBs) is required to meet the criteria for the power source such as electric vehicles. But generally, high power density is hindered by slow diffusion of lithium ions in micrometric electrode materials. [1] From this perspective, nanoparticle electrode materials can enhance the rate of lithium ions because of shortened diffusion pathway of lithium ion. Besides, high surface area of electrode induces facile access of charge via large contact area with electrolyte and conducting additives like carbon. In contrast, chemical degradation at electrode-electrolyte interface is also facilitated by large contact area with nanoparticle electrode. Unfavorable interfacial reactions such as dissolution of cathode species and decomposition of electrolyte mainly occur through energetically unstable surfaces of the active material. [2] In order to minimize side reactions that can negatively affect electrochemical performance, introducing electrochemically inactive ions on the surface of active materials can improve the interfacial stability. However, this substitution should take place as thin passivating layers on individual particles to preserve storage capacity. [3] A strategy toward the stabilization of electrode-electrolyte interfaces has been devised by introducing core-shell nanocrystals, consisting of electroactive lithium transition metal oxide cores and ultra-thin inactive epitaxial oxide shells on the surface. Spinel Li1+xMn2-xO4 nanocrystals were used as a core component, with an Al-rich shell as passivation layers to minimize unfavorable reaction on the surface of cathode particles. The electrochemical performance shows that Spinel Li1+xMn2-xO4 nanoparticle with conformal shell reveals improved capacity retention and rate capability even at elevated temperature, compared to a bare counterpart. Reference 1. Isaac D. Scott, Yoon Seok Jung, Andrew S. Cavanagh, Yanfa Yan, Anne C. Dillon, Steven M. George, and Se-Hee Lee, Nano Letters, 414–418, 11 (2011). 2. Peter G. Bruce, Bruno Scrosati, and Jean-Marie Tarascon, Angew. Chem. Int. Ed, 2930-2946, 47 (2008). 3. Chunjoong Kim, Patrick J. Phillips, Linping Xu, Angang Dong, Raffaella Buonsanti, Robert F. Klie, and Jordi Cabana, Chem. Matter, 394-399, 27 (2014).

Effects of Carboxylated Polythiophenes in Fe3O4 Li-Ion Battery Anodes

The high energy/power density of Li-ion batteries elicits intensive research efforts related to high capacity anode materials. The main obstacles which retard the practical employment of high-capacity electrochemically active particles including Si, Sn, metal oxide and their derivatives, stem from large volume changes associated with Li insertion/extraction and the resultant electrical contact loss, thereby leading to poor cycling performance. Efforts have been made to circumvent the breakdown of electron pathways through the introduction of electrical conducting functionalities to the surface of active materials, such as carbon coatings and conductive polymeric binders, or the fabrication of a stable battery anode with electrically inactive polymeric binders that enable the system to maintain its integrity. These approaches are reasonably effective, however, incorporation of a porous entity which helps ion transport, into a composite electrode is also essential for improved performance. In principle, electrochemical reactions that occur within the electrode, reveal the intrinsic energy capacity when a Li ion encounters an electron inside an active site. Electron and ion transport are both critical factors to determine the internal resistance of the electrodes, which in turn influences their electrochemical performance. Here we present how to improve both electronic and ionic transport in Li-ion battery electrodes using conjugated polymers. The approach includes poly[3-(potassium-4-butanoate)thiophene] (PPBT) — a water-soluble, carboxylate substituted polythiophene — as a binder component, and polyethylene glycol (PEG) as a surface coating on active material, namely, Fe3O4 nanoparticles. Additionally, carbon nanotubes (CNTs) are considered for use as the conducting networks to facilitate design of a light-weight, flexible web electrode. To enhance the electronic conduction in the electrodes, connection between active materials (PEG- Fe3O4) and conducting agents (or CNTs) through binding components is of importance. PEG coating and carboxylated polythiophenes play an important role in dispersing the Fe3O4 nanoparticles and conducting agents (or CNTs), respectively, in a water medium, which allow for well-developed and interconnected electrode structures. Furthermore, carboxylated polythiophenes (e.g. PPBT) can boost electronic conduction, based on their high conducting properties and through electrochemical doping during electrochemical testing. The presence of carboxylic moieties on the side chain of polythiophenes such as PPBT could facilitate the formation of stable electrodes via chemical interactions between PPBT moieties (COO-) and the Fe3O4 surface (–OH). The results will show that this methodical consideration of both ion and electron transport through introduction of a carboxylated PPBT component, can remarkably enhance the performance of Fe3O4 based high-capacity anodes.

Understanding Binary Interactions and Aging Effects in Catalyst Layer Inks for Controlled Manufacturing

Controlled manufacturing and processing of multi-material electrodes and catalyst layers for energy storage and conversion applications is important for achieving low-cost and durable systems. Typically electrodes are coated using solution-based or wet- methods to achieve high aspect ratio thin films. The processing conditions as well as the ink formulation affect the ultimate material arrangment and electrode properties. Currently, ink formulation primarily focuses on macro-scale process-specific optimization (i.e. viscosity and surface/interfacial tension) rather than understanding local and nano-scale interactions between in constituents in an ink. Herein, we investigate local interactions between carbon and polymer components in a carbon based ink. We specifically investigate the role of active material surface chemistry plays on aggregation, stability, and sedimentation dynamics in multi-component inks1,2. The material arrangement and organization in a cast thin film is discerned from both shear and elongational flow processing methods. Finally, we introduce novel direct and indirect (rheological ) techniques to probe shear induced transformationd in inks or viscoelastic materials3,4. [1] Holdcroft, S. (2013). Chemistry of Materials, 26(1), 381-393. [2] Takahashi, S., Shimanuki, J., Mashio, T., Ohma, A., Tohma, H., Ishihara, A.,& Miyazawa, A. (2017). Electrochimica Acta, 224, 178-185. [3] Hatzell, K. B., Eller, J., Morelly, S., Tang, M., Alvarez, N. J., & Gogotsi, Y. (2017). Faraday Discussions [4] Hatzell, K. B., Boota, M., & Gogotsi, Y. (2015). Chemical Society Reviews, 44(23), 8664-8687.

Improvements in the electrochemical performance of Li4Ti5O12-coated graphite anode materials for lithium-ion batteries by simple ball-milling

Li4Ti5O12 (LTO)-coated graphite anode materials for lithium-ion batteries with superior rate-capability and cycling performance were prepared by simple ball-milling in a short time. LTO particles, uniformly coated on the surface of graphite active materials, improved the kinetics and stability on the surface of graphite particles on the basis of their high Li-ion diffusivity and structural stability. As a result, the LTO-coated graphite, which was ball-milled for 5 min, presented a high initial discharge capacity (324 mAh g−1 at 0.2 C), superior rate-capability (>260 mAh g−1 at 5 C), and excellent cycling performance (∼94% after 100 cycles at 0.2 C).

Quantifying Contributions to Reversible and Irreversible Capacities of Silicon Electrodes

Silicon has received much attention as a next-generation anode material for lithium-ion battery (LIB) because it can give a theoretical capacity of up to 4000 mAh/g. Typically, silicon electrodes show poor cycle performance, which is attributed to large volume expansion during lithiation, pulverization of the active material, continuous solid electrolyte interphase (SEI) layer growth etc. Though, there are hardly any studies that can tell how much of the measured capacity comes from each process. The aim of this study is to establish a method to understand and quantify the contributions to the reversible and irreversible capacities of silicon anode in order to develop strategies to improve the stability of the electrodes. Here, we report the use of an electrochemical approach – depth of discharge test – to separate the charge-discharge capacities of crystalline silicon electrodes into four contributions: (1) SEI formation and electrolyte decomposition, (2) lithium accommodation in carbon and binder, (3) lithiation and delithiation of the Si active material, and (4) capacity loss associated with particle cracking and detachment. Silicon electrodes using bulk silicon particles (Sigma Aldrich), acetylene black (AB) and carboxymethyl cellulose (CMC) binder were made with a ratio of 4:3:3, 6:2:2 and 8:1:1. The electrodes are assembled into 2032-type coin cells with Celgard separator and Li metal as the counter electrode. 1M LiPF6 in fluorinated ethylene carbonate (FEC) and diethyl carbonate (DEC) = 1:1 by volume is used as the electrolyte. For the depth of discharge test, multiple cells were made, and each of them is initially discharged to different cutoff capacity (250, 500, 1000, 1500, 2000, 2500, 3000 and 3500mAh/g) and charged back to 1V. Figure 1a shows a summary of the initial discharge-charge curves of different cells with a composition of Si:AB:CMC = 4:3:3. The respective first charge capacity vs. first discharge capacity is summarized in Figure 1b, which shows a linear behavior that is shifted from the origin. The slope of the line corresponds to the intrinsic first cycle efficiency of Si, which is about 90% and is independent on the degree of lithiation. This suggests that no matter how much lithium is initially inserted, 10% of them remains in the lattice and cannot be removed. This is attributed to partial irreversibility of the crystalline-to-amorphous transition of Si during lithiation, and is consistent with a core-shell model of lithiation during first discharge. The x-intercept of Figure 1b is non-zero, which corresponds to the irreversible capacity of the electrode. The x-intercept depends on the composition of the electrode (see inset of Figure 1b) – larger x-intercept for large amount of carbon and binder, which is due to irreversibility from these two components. Accounting for all of these, we estimate the capacity from SEI formation and electrolyte decomposition to be about 16 mAh/(g Si) for bulk Si (with BET = 1.6 m2/g). SEI formation is expected to depend on particle size and surface area. We verify this by performing the depth of discharge test with ballmilled Si (in Ar at 200rpm for 24 hours) with BET = 6.6 m2/g. The capacity due to SEI formation during 1st discharge is increased to about 68 mAh/(g Si), and scales with BET surface area. We estimate that the SEI formation in our system to be about 10 mAh per square metres of active material surface. This value would depend on the type of electrolyte used in the system. Mechanical issues and particle isolations are observed in fully discharged electrode when the amount of binder is less than 20%. This is represented in the bend in the depth of discharge curve for the CMC-811 electrode, as in Figure 1b. Our results suggest that there is a threshold to mechanical breakdown. In the case of the CMC-811, high reversibility is still obtained when the discharge capacity is below 2000 mAh/g. With larger discharge capacity, volume expansion is larger, leading to severe capacity fading. In subsequent cycles, cycle stability is governed by the coulombic efficiency (CE) of the electrode - the larger the coulombic efficiency, the better the stability. The irreversible capacity is not only due to continuous SEI formation, but also to Li trapping in the active material and mechanical breakdown of the electrode. CE can be increased by limiting capacity, reducing particle size, or changing to a stronger binder such as polyimide. Some of results can be found in our recent publication1. More results will be shown during the meeting. [1] P.-K. Lee, Y. Li, D. Y. W. Yu, J. Electrochem. Soc. 164, A6206 (2017). Figure 1

Improved electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode material by double-layer coating with graphene oxide and V2O5 for lithium-ion batteries

LiNi0.5Co0.2Mn0.3O2 cathode material synthesized by a sol-gel method was surface-modified by double-layer coating. The results of X-ray diffraction (XRD) confirm that the intrinsic structure was no change after surface modification. A double-layer structure consisting of an inner V2O5 (VO) layer and an outer conductive graphene oxide (GO) layer was coated on the surface of active material, as confirmed by transmission electron microscopy (TEM). The results of field emission scanning electron microscope (FE-SEM) equipped with an energy dispersive spectroscope (EDS) show that both graphene oxide and V2O5 uniformly covered LiNi0.5Co0.2Mn0.3O2 cathode material. The double-layer-coated LiNi0.5Co0.2Mn0.3O2 cathode material shows improved electrochemical performance with a capacity retention of 74.2% after 50 cycles in a range of 2.5–4.5V at 55°C, compared with only 67.8% capacity retention for the pristine material. In addition, the double-layer-coated LiNi0.5Co0.2Mn0.3O2 releases 116.6mAhg−1 under a high current rate, while the pristine material only remains at 105.7mAhg−1. The results can be ascribed to the double coating layer not only avoids the side reaction between electrolyte and active material but also promotes Li+ and electronic conductivity. Differential capacity (dQ/dV) and electrochemical impedance spectroscopy (EIS) measurements reveal that the double coating layer effectively suppresses the increase of the electrode polarization during cycling.

Insights from Studying the Origins of Reversible and Irreversible Capacities on Silicon Electrodes

Silicon electrodes can give high capacity as anodes for lithium-ion batteries. However, there has not been much work quantifying the different contributions to the reversible and irreversible capacities. Here, we report the use of an electrochemical approach – depth of discharge test – to separate the charge-discharge capacities of crystalline silicon electrodes into four contributions: (1) SEI formation, (2) lithium accommodation in carbon and binder, (3) lithiation and delithiation of the Si active material, and (4) capacity loss associated with particle cracking and isolation. We find that the intrinsic coulombic efficiency for the crystalline-to-amorphous transition of Si during initial cycle is about 90%, which is independent of particle size. SEI formation is estimated to be about 10 mAh per square meters of active material surface and scales with BET surface area. Mechanical issues and particle isolation are observed in fully discharged electrode when the amount of binder is less than 20%. Capacity limitation prolongs lifetime of Si electrode, but the overall performance is governed by the coulombic efficiency (CE) during cycle. Low CE is due to continual SEI formation with cycling, increase utilization of Si upon cycling, trapping of Li in the material and mechanical failure of the electrode.

Robust solid/electrolyte interphase on graphite anode to suppress lithium inventory loss in lithium-ion batteries

Lithium inventory loss is the most important reason for capacity decay of commercial lithium ion batteries. To suppress lithium inventory loss and prolong the battery cycle-life, sodium maleate (SM) is coated onto the surface of graphite active materials and act as the starting material for in-situ growth of SEI film. Microscopic studies show that the SM salt is uniformly dispersed on the graphite particles. The SM coating favors the formation of robust solid electrolyte interphase (SEI) due to its abundant carboxyl group and improves the mechanical property due to the polymerization between the unsaturated bonds. With 3.0 wt% SM coating, the first columbic efficiency, cycling stability and rate capability of the graphite anode are simultaneously improved. Electrochemical impedance spectroscopy (EIS) studies and scanning electron microscope (SEM) observations show that continuous lithium deposit on the graphite surface arising from the SEI instability during long-term electrochemical cycles is effectively suppressed. Cycle-life of the full cell assembled with LiFePO4 cathode and 3.0 wt% SM coated graphite anode is thus significantly prolonged.

Electrochemical Properties of Nb-Based Oxides As Active Materials for the Negative Electrode of Lithium Secondary Batteries

During the past twenty years, much effort has been undertaken to obtain lithium secondary batteries with higher energy densities, and a great progress has been made. However, there is still an increasing demand for further lightweight and small-size rechargeable batteries for portable electronic devices. In addition, recent environmental concerns about global warming and the need to reduce CO2 emissions provide us a strong driving force for developing large-scale lithium secondary batteries with high energy densities for use in electric vehicles. One of the important requirements of negative electrode material to get a lithium secondary battery having a large energy density, the redox potential at the negative electrode should be as low as possible. In the present work, we report Nb-based oxides as active materials for the negative electrode of lithium secondary batteries. Composite electrodes were used for charge/discharge tests and cyclic voltammetry (CV). The test electrode was prepared by coating a mixture of the Nb-based oxide powder (80 wt.%) and a carbon black (10 wt.%) and a poly(vinylidene difluoride) binder (10 wt.%) on copper foil. The charge and discharge tests were carried using conventional three-electrode cells at a constant current of 18.1 mA g–1. The electrolyte solution was 1 mol dm–3 (M) LiClO4 dissolved in 1:1 (by volume) mixtures of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC + DEC) (Kishida Chemical Co., Lithium Battery Grade). The counter and reference electrodes were lithium foil. CV was performed between 3.0 and 30.0 V at a sweep rate of 0.5 mV s–1. All electrochemical measurements were carried out in an argon-filled glovebox with a dew point < –60 °C. All potentials are referred to as volts vs. Li/Li+.The electrochemical properties of Li1.1Nb0.9O2-x material was theoretically predicted by first principal method and experimentally tested by electrochemical method. The potential of Li1.1Nb0.9O2-x for lithium ion insertion into and extraction out of the material, as predicted through theoretical calculations, was quite low below 0.3 V vs. Li/Li+, giving high cell voltage as a negative electrode material for lithium secondary batteries. Li1.1Nb0.9O2-x was successfully synthesized by a solid state reaction and shown to have good cycle performance, although there is a large irreversible capacity in the first cycle. The main reversible reaction of the material is occurred at almost 0 V vs. Li/Li+. The low potential is originated from the electronic structure of the Li1.1Nb0.9O2-x; it acts as a favorable factor for an increase in energy density even though the possibility of an increase in electrolyte decomposition is present in initial charging process. The study on the electrochemical properties of Li1.1M0.9O2-x (M=Ti, Cr, Zr, and Mo) might be meaningful because those compounds present low potentials for lithium ion insertion as predicted by the first principal method and show high cell voltage as a negative electrode material for lithium secondary batteries.The NbO2 electrode showed a large irreversible capacity and small discharge capacity. The results of X-ray photoelectron spectroscopy indicate that the poor electrode performance of NbO2 may be caused by niobium pentoxide (Nb2O5) formed on the surface of active material. The Nb2O5 could be removed by chemical etching to some extent, thus improving the electrode performance. In addition, The electrochemical properties of niobium monoxide, NbO, were investigated as a negative electrode material for lithium-ion batteries. Lithium ions were inserted into and extracted from NbO material at potentials < 1.0 V versus Li/Li+. The process involved the formation of a solid electrolyte interface (SEI) on the NbO surface in the first cycle. The reversible capacity was ~67 mAh g–1 with a capacity retention of ~109% after 50 cycles. The magnitude of the charge transfer resistance was greatly decreased by ball-milling the pristine NbO, but the ball-milling had no effect on the SEI resistance. References1) K.T. Jacob, C. Shekhar, M. Vinay, Y. Waseda, J. Chem. Eng. Data, 55,4854 (2010).2) L.-F. Cui, Y. Yang, C.-M. Hsu, Y. Cui, Nano Lett., 9, 3370 (2009).

Compatibility of Polymer Binders with Solvate Ionic Liquid Electrolytes in Lithium-Sulfur Batteries

The molten complex of tetraglyme (G4) and Li[TFSA] (TFSA: bis(trifluoromethanesulfonyl)amide), [Li(G4)][TFSA], is a solvate ionic liquid (SIL) composed of complex [Li(G4)]+ cation and [TFSA] anion.1 We reported that stable operation of Li-S batteries can be achieved using [Li(G4)][TFSA] electrolyte.2 The solubility of lithium polysulfides (Li2S m , 2 ≤ m≤ 8), which are reaction intermediates of the S cathode, is suppressed in the SIL, and the redox shuttle effect in the Li-S cell is prohibited, leading to the high coulombic efficiency of discharge/charge and long cycle life of the cell. In this study, we focus on the effects of the compatibility of polymer binders with SIL electrolytes on the electrochemical reactions of the S cathode. The S cathode is composed of sulfur, a conductive agent, and a polymer binder, and the composite cathode has a porous structure. In the porous composite cathode, the surface of active material is partially covered with the binder. Here we report that the electrochemical reaction mechanism, discharge capacity, and rate capability of S cathode change depending on the compatibility of polymer binder with the SIL electrolyte.[Li(G4)][TFSA] electrolyte was prepared by mixing Li[TFSA] and G4 in 1:1 molar ratio in an Ar-filled glove box. The ionic conductivity of [Li(G4)][TFSA] is 1.6 mS/cm at 30 °C. The S composite cathode was composed of elemental sulfur, carbon powder (Ketjenblack, specific surface area of 1270 m2/g), and a polymer. Poly(vinyl alcohol) (PVA-x) with different degrees of saponification (x%) were used as polymer binders.The mass ratio of sulfur, carbon, and polymer in the composite cathode was 60:30:10.The electrolyte uptake of PVA-x film was estimated. PVA-100 was negligibly swollen with [Li(G4)][TFSA] electrolyte. The electrolyte uptake of PVA-x increased on decreasing the degree of saponification. The weights of PVA-80 and PVA-66 films increased by 8 and 17%, respectively, owing to electrolyte uptake. [Li(G4)][TFSA] electrolyte was incorporated into the acetate moiety of the PVA-x polymer chain, and the PVA-x film was partially swollen with the electrolyte.3 Figure 1 shows the dependence of discharge capacity of Li-S cells on current density. As decreasing the saponification degree of PVA, the discharge capacities increased. The compatibility of binder also affects the charge-discharge rate capability of Li-S cells. The partial swelling of binder in the electrolyte facilitated Li+ ion conduction in the S composite cathode. This results in the high utilization of active material of cathode and the higher rate capability of the cell. 3 Acknowledgements This study was supported in part by Specially Promoted Research for Innovative Next Generation Batteries (SPRING) of the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST), and JSPS KAKENHI (No. 15H03874 and No. 15K13815) from the Japan Society for the Promotion of Science (JSPS). References K. Ueno et al., J. Phys. Chem. B, 116, 11323 (2012).K. Dokko et al., J. Electrochem. Soc., 160, A1304 (2013).T. Nakazawa, J. Power Sources, 307, 746 (2016). Figure 1

Development for 300Wh/Kg-Class High Energy Li-Ion Battery for EV

High energy density lithium-ion secondary batteries must be developed to extend the driving range of battery electric vehicles (BEV). We have been focusing on LiMO2-type layered oxides for the high capacity cathode, and Si-based materials for the high capacity anode. We manufactured a prototype 30 Ah-class cell using a cathode of Ni-rich layered oxide and an anode of Si-alloy mixed with graphite to prove the achievement of our development. The cell demonstrated an energy density of 335 Wh/kg and power density of 1600 W/kg when charged up to 4.4 V. The short cycle life shown by the cells was improved by coating on the cathode active material particles and optimized electrolyte. 1．Introduction Hitachi, Ltd. and Hitachi Automotive Systems, Ltd. have been participating in the national project supported by New Energy and Industrial Technology Development Organization (NEDO) of Japan: “Applied and Practical LIB Development for Automobile and Multiple Application”. Hitachi, Ltd. is in charge of developing cell chemistry and basic design of the single cell for the target of a 300 Wh/kg-class lithium-ion secondary battery. Such a high energy density battery naturally demands us to introduce high capacity density material both for the cathode and anode. Based on our preliminary study, we chose Ni-rich layered oxide and lithium-rich layered oxide for the cathode and Si-based anode. 2．Experimental The cathode was manufactured using a Ni-rich layered oxide, conductive carbon, and binder at a coating amount of 335 g/m2 and density of 3.0 g/cm3. The anode consisted of a mixture of Si-alloy and graphite and was coated with polyamide-imide binder to make the capacity ratio 1.1 and density 2.1 g/cm3. We assembled 30 Ah-class pouch cells, which were charged up to 4.4 V or 4.2 V. The cells charged up to 4.4 V and 4.2 V are denoted as Cell-4.4 and Cell-4.2. Thee capacity was measured by discharge current 1/20 CA and 1/3 CA down to 2.0 V. The direct current internal resistance (DC-IR) was calculated on the basis of the 10-second voltage drop and discharge current of 1/3 CA at 50% SOC. We also manufactured 1 Ah-class small cell denoted as Cell-imp. which has the same cell chemistry as the 30 Ah-class ones, but the active materials were treated with coating and the electrolyte was optimized. Cell-4.4, Cell-4.2, and Cell-imp. were cycled at 1/2 CA between 20 and 100% SOC. After the cycle test, the cells were disassembled and analyzed. 3．Results and Discus Figure 1 shows discharge curves of prototype 30 Ah-class cells, and Table 1 summarizes the results. The capacities of the Cell-4.4 and Cell-4.2 were 30.5 Ah and 27.7 Ah, which were almost the same as designed. Since the average voltages were 3.46 V and 3.43 V and the cell weights were as shown in the table, the energy densities were 335 Wh/kg in Cell-4.4 and 290 Wh/kg in Cell-4.2. DC-IR and OCV (Open circuit voltage) was used to calculate output power density. The results were 1,600 W/kg and 1,270 W/kg. The cycle life test of Cell-4.4, Cell-4.2 resulted in poor cycleability: they lost all the discharge capacity after 100 cycles. On the contrary, the Cell-imp. showed high capacity retention of 90% even after 100 cycles. Cell-4.4 and Cell-4.2 revealed growth of a cubic layer and SEI film on the surface of cathode active material, and detachment of the mixed materials layer of the anode, while the Cell-imp. showed reduced growth and detachment. Acknowledgement This work was supported by NEDO of Japan. Figure 1

Enhancing Electrochemical Performance By Coating Al2O3 on Na0.6Mn0.66Ni0.22Co0.11O2 cathode in Sodium-Ion Batteries

Rechargeable sodium-ion batteries (SIBs) have recently attracted much attention as an alternative electric energy storage device to lithium-ion batteries (LIBs) because of the availability and low cost of sodium. However, the cycle performance of SIBs is not comparable to LIBs due to the side reaction between electrode and electrolyte as well as the structural instability and transition metal dissolution into electrolyte. In order to solve these problems, various surface coating techniques have been used to coat oxides on the cathode active material surface before making the electrode. However, additional coating process causes the price increase, and they have not been applied for SIB cathodes. Herein, an Al2O3 thin film was directly deposited on the surface of Na0.6Mn0.66Ni0.22Co0.11O2(NaMNCO2) cathode by a radio-frequency magnetron sputtering. Only 4 min coating of Al2O3 upon the NaMNCO2 could improve the specific capacity from 134.6 to 146.4 mA h g-1 in a voltage range of 2.1 to 4.3 V. In addition, the cycle performance was also enhanced to deliver 49.7 mA h g-1 even after 200 cycles. In case of high voltage operation up to 4.5 V, the discharge capacities of pristine and Al2O3-coated electrodes are 150.6 and 160.1 mA h g-1, respectively. Acknowledgement This work was supported by the Human Resource Training Program for Regional Innovation and Creativity through the Ministry of Education and National Research Foundation of Korea (NRF-2014H1C1A1066977).

(Invited) Atomic Layer Deposition: Eye on the Future of Batteries

Over the past few years, as atomic layer deposition (ALD) has emerged as a promising technique to improve the performance of electrochemical energy storage devices. In conventional battery architectures and chemistries, ALD layers have been used to protect the surface of anodes1 and cathodes,2 resulting in improved electrode stability and enhanced cycling performance. Beyond conventional Li-ion batteries, ALD has allowed fabrication of ultra-high performance 3D nanoheterostructures that combine intimate contact between high electrical conductivity current collectors and thin layers of active material for incredible charge/discharge rate capabilities proportional to the active material surface area. Both anode3 and cathode4 nanoheterostructures exhibit significantly higher charge/discharge rate capabilities over their bulk electrode counterparts in ½ cell geometries. The culmination of this architecture uses the high degree of control inherent in the ALD process to fabricate arrays of opposing cylindrical heterostructures inside a nanoporous template, and can achieve nearly 50% theoretical capacity while charging/discharging at a rate of 150 C.5 However, use of liquid electrolytes present a number of challenges with high power electrodes. High charge/discharge rates, desirable for fast charging of plug-in hybrid vehicles and personal electronic devices exacerbate already present safety issues, particularly that of Li dendrite formation and subsequent cell failure. Increased electrode surface area may mitigate dendrite formation by lowering the local current density, but deleterious electrolyte degradation reactions can consume electrolyte and result in eventual cell failure. Use of solid state electrolytes, previously only suitable for niche applications due to their low inherent ionic conductivity and limitations in fabrication process, can now be expanded to new applications such as the previously described 3D batteries by utilizing ALD processes. A number of suitable ALD process for well-known solid electrolytes have been developed,6,7 allowing deposition of thin, conformal solid electrolyte layers at temperatures as low as 150˚C. The current status of 3D solid state batteries will be discussed, as will remaining challenges. ALD’s capability for very thin solid electrolyte layers opens the door to advanced electrode protection, and and recent results showcasing the utility of ALD solid electrolyte coatings for protection layers on 3D nanoheterostructured electrodes and next-generation Li metal anodes will be discussed. In the case of conversion electrode materials, ALD coatings can prevent electrolyte decomposition and solid electrolyte interphase formation, lowering overpotential required for charge and discharge. Additionally, ALD coatings mechanically constrain conversion electrode particles, preventing volume expansion and fracturing of the coatings upon cycling.8 ALD layers applied to Li metal anodes can prevent electrolyte decomposition and Li dendrite formation by chemical stabilization of the electrode surface, resulting in both higher capacities and longer cycling lifetimes in the case of the Li-S and Li-Air systems.9,10

Stabilization of High Energy Cathodes By Introducing Conformal Passivating Shells on the Surface

Combined cycling stability at high energy density is required for lithium ion battery to meet the criteria for use in electric vehicles. In generally, material utilization and rate capability are enhanced at small particle sizes. [1] Reduced size of electrode materials can enhance the rate of lithium ions because of shortened diffusion pathway and increases surface area to induces facile access by the electrolyte. In contrast, chemical degradation at electrode-electrolyte interface is also facilitated by large contact area with nanoparticle electrode. Unfavorable interfacial reactions such as dissolution of active materials and decomposition of electrolyte mainly occur because the surface of active materials is energetically unstable. [2] In order to minimize side reactions that can negatively affect to electrochemical performance, replacing electrochemically inactive ions on the surface of active materials can improve the interfacial stability. However, this substitution should take place as thin passivating layers on individual particles to preserve storage capacity. [3] Herein, we demonstrate a strategy toward the stabilization of interfaces by introducing core-shell type of nanocrystals that is composed of electroactive transition metal oxide in core and ultra-thin inactive epitaxial oxide shell on the surface. To prove this concept, we introduce layered LixCoO2 nanocrystals as a core component, with Al-rich shells as passivation layer to minimize side reaction with electrolyte. The resulting materials shows stable cycling curves as well as higher capacity retention at high rate of charging and discharging reaction compared to bare LixCoO2.

Development of Lithium-Ion Battery with New Shutdown System Using Redox-Shuttle Compound

Lithium ion batteries (LIB) have been adopted for the power supply of electric vehicles, and higher energy density and larger capacity cells are now in demand. Complying with these demands means that cells under abnormal conditions could become less safe. In particular, the amount of heat due to cathode decomposition could increase if overcharged. Therefore, technology is necessary to improve the safety in overcharging conditions. A redox-shuttle compound is known as an effective protection technology for overcharging cells. A redox-shuttle compound can be reversibly oxidized and reduced at a defined potential slightly higher than the full-charge potential of the surface of cathode active materials. The oxidized redox-shuttle molecules that diffuse through the electrolytes would be reduced to their neutral state on the surface of anode active materials. Neutral molecules would then diffuse back to the cathode and be oxidized again. This shuttle reaction can convert excess energy during overcharging into heat, thereby protecting the cells by limiting the potential of the cathode to less than or equal to the oxidation potential of the shuttle molecules. However, when the overcharge current is bigger than the maximum rate of the shuttle reaction, the redox-shuttle compound cannot fully consume the current, and cathode active materials will be damaged. We designed and studied a new shutdown system that is a combination of a redox-shuttle compound and shutdown separator. In this system, the redox-shuttle compound will protect cathode active materials from overcharging, and at the same time, the separator will shut down because of the heat generated by the redox-shuttle reaction. First of all, we studied the rate of redox reaction and the temperature of the cell heated by the reaction of the redox-shuttle compound. We selected 1,4-di-tert-butyl-2,5-dimethoxybenzene as the redox-shuttle compound. The reaction potential of this material is about 3.9 V vs. Li/Li+ (From the point of view of the principle verification, we selected the redox-shuttle compound of a lower reaction potential to reduce the extra reaction while the redox-shuttle compound was reacting). The redox-shuttle compound was dissolved up to 0.15 M with a common electrolyte, EC/DEC (3/7 by volume)/ 1.0 M LiPF6 and used as a redox-shuttle electrolyte. Graphite and a mixture of lithium manganese oxide and lithium nickel oxide were used as the anode and cathode active material, respectively. Porous polypropylene (PP) and PP/polyethylene (PE)/PP stacked film were used as the separator. The melting points of PP and PE are 160°C and 130°C, respectively. Cells that varied in capacity from 3.5 to 10 Ah were prepared by changing the number of stacked layers (the size of the electrode was fixed). Each cell was charged at the rate of 0.1 to 1 C, and the cell voltage and the temperature on the surface of the cell were measured. As a result, as the cells were charged, a plateau was observed around 3.8 V of cell voltage, and the temperature of the cells increased at the same time. These results indicate that the redox-shuttle compound consumed current and converted it into heat. The increase in temperature of the cells heated by the redox-shuttle compound had a linear relationship with the charged current value (See figure 1). From this relational expression, when the charge current is higher than 10 A, the temperature of the cell can exceed 160°C (higher than the melting point of PP and PE), and ion diffusion will be protected. These results and equation suggest that the new shutdown system is capable of keeping the high energy density cells safe. On the meeting day, we will report the details of the new shutdown system and the results of the experiment using 10-Ah class cells. In particular, we will focus on the influence of the redox-shuttle reaction rate and the properties of the electrode and separator in this system.Fig. 1 Relationship between the charging current and the highest and equilibrium temperature on the surface of the cell. Figure 1

High Performance LiNi0.5Mn1.5O4 Cathode Material for Lithium Ion Batteries

As applications of Lithium ion batteries increasingly expand into the plug hybrid electric vehicles (PHEV), or even electric vehicles (EV), the existing battery technology established for portable electronics has faced challenges to meet the criteria for automotive application, especially in terms of energy density. One strategy of increase energy density of batteries is to increase the voltage. Thus, spinel LiNi0.5Mn1.5O4 has received tremendous research efforts due to its high discharge voltage (~ 4.7 V), low cost and environmentally benign properties. However the high operational potential will easily lead to the decomposition of electrolyte and salt, forming a thick solid-electrolyte interface (SEI) layer on anode side, which not only increases the cell impedance but also traps cyclable Li resulting in the capacity fade. Another challenge of LiNi0.5Mn1.5O4 is that Mn dissolution can be accelerated by HF arising from the reaction between trace of water with electrolyte. In addition, many studies have shown that the Mn is more readily dissolved in electrolyte at high potential leading to the loss of active materials. The generated Mn cations can be easily reduced to deposit on anode side trapping more Li and increasing the cell impedance during cycling. The combination of these factors leads to the dramatic capacity decay of the cells based on LiNi0.5Mn1.5O4, then limiting its practical application. Thus, development of LiNi0.5Mn1.5O4 materials with stable long-term cyclablity is crucial but challenging. It is known that in lithium batteries, most of the electrochemical reactions happen on the solid-liquid interface followed a mass transport. For LiNi0.5Mn1.5O4, electrolyte decomposition and Mn dissolution most likely initialize on its surface, especially at high potential. Thus, the interface property between LiNi0.5Mn1.5O4 and electrolyte is believed to be a key factor affecting the electrochemical performance. To establish a more stable interface to minimize the unwanted reactions, coating a protective layer on LiNi0.5Mn1.5O4 surface has been demonstrated as an efficient approach to improve its electrochemical performance. In this study, we demonstrate that cyclability of LiNi0.5Mn1.5O4 can be significantly enhanced by applying a Li3PO4 coating layer on LiNi0.5Mn1.5O4 particles. For half cell performance, Li3PO4/LiNi0.5Mn1.5O4 can retain 80 % of initial capacity in 650 cycles. On the contrary, non-coated LiNi0.5Mn1.5O4 lost most of the capacity in 345 cycles. Li3PO4/LiNi0.5Mn1.5O4 materials also exhibit a superior cycling stability in full cells without obvious fading in 250 cycles. The substantial improvement of cycling performance is attributed to Li3PO4 coating, which can significantly minimize the electrolyte decomposition on active material surface by separating the active material from the electrolyte and preventing the Ni2+/Ni3+ or Ni3+/Ni4+ redox couple from catalyzing electrolyte decomposition. Thus, by demonstrating that Li3PO4 coating layer can enable the cycling of high voltage LiNi0.5Mn1.5O4 materials with regular electrolyte, we believe that Li3PO4 coating can be applied to other functional materials for lithium-ion batteries with the desire of greater stability and cyclability. Figure 1

Optimizing Energy Density of Li-Ion Batteries Using Thick Electrodes

Low-cost, high energy density lithium-ion batteries (LIBs) have been consistently pursued in consumer electronics, electrical vehicles (EVs) and stationary (grid) energy storage. Modern Li-ion cells can have an energy density of up to 550 Wh/L, compared to only 200 Wh/L in the late 1990s [1]. The gradual energy density improvement over the last 15-20 years was mostly due to the improved engineering of the electrode coatings and, to some extent, some improvement in active material development and manufacturing. The former has significantly increased the volume ratio of active materials (which provides the capacity of the battery) from ~20% at early stages to ~45% in the state-of-art LIBs [2,3]. Thickening the cathode and anode is one effective approach to further increase the active material content, enabled from reducing current collector and separator layers in the stack, which further improves the energy density and lowers the cost of LIBs. It has been estimated that significant reduction in pack cost can be realized when compared to conventional manufacturing if electrode thickness is doubled and aqueous slurry processing is utilized [4]. However, thicker electrodes lead to poor kinetics and electrolyte salt depletion and thus underutilization of active materials. The major concern of thick coatings is electrolyte salt depletion, which results in fewer lithium ions available in the liquid phase for reaction at the active material surface. Therefore, there is an optimum thickness for maximum energy and power density. To maximize energy and power density, the effect of various coating parameters on the reaction kinetics and thus battery performance must be considered during electrode design. The operation of LIBs follows porous electrode theory and electrochemical reaction thermodynamics, and the governing equations have been summarized by Newman et al. [5]. Thus, the porous electrode model is used in the present work to investigate the effect of manufacturing parameters such as electrode thickness and porosity. In the present study, the following will be discussed: (1) The effect of thickness on active material utilization, areal capacity, voltage and energy density. (2) The effect of porosity on active material utilization, areal capacity, voltage and energy density. (3) Possible solutions to further increase the energy density of thick-electrode battery. Acknowledgement This research at Oak Ridge National Laboratory (ORNL), managed by UT Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy Vehicle Technologies Office (VTO) Applied Battery Research (ABR) subprogram (Program Managers: Peter Faguy and David Howell).