Graphitic Carbon Anode Research Articles

(Invited) Advanced Electrolytes for Lithium Metal Electrode

Advanced electrolyte containing functional additives as well as dissolved lithium salts in organic solvents has greatly upgraded the performance and reliability of lithium-ion batteries (LIBs). For example, a small amount of vinylene carbonate (VC) or fluoroethylene carbonate(FEC) is still indespensable to state-of-the-art LIBs to control solid electrolyte interphase (SEI) upon graphitic carbon anodes. However, due to the energy density limitation of conventional LIBs, post LIBs having lithium metal anodes have been extensively investigated for lithium metal secondary batteries, Li/S, Li/Air, etc. during the last ten years. But, without controlling lithium dendrite formation, post LIBs may end in an empty talk. Thus, there are a variety of previous works to suppress the lithium dendrite formation by placing organic/inorganic protection layers upon lithium metal, modifying the surface morphology, or adding some functional additives. In particular, since the last approach is the most facinating method owing to simplicity, we cannot help finding new additives for electrolytes. Herein, we intoduce recent researches on advanced electrolytes for lithium metal electrode first. And then, we apply a few good candidates to micropatterned lithium metals, which could deliver much more stable cycling performance than bare ones. Finally, some optimized advanced electrolyte and micropatterned lithium metal systems are proposed in more detail.

High Performance Li-Ion Battery Anodes Based on Silicon-Graphene Self-Assemblies

With an ever-expanding application of energy storage systems, lithium-ion batteries (LIBs) have attracted a significant scientific and industrial attention during last decades. However, the current LIBs based on lithium metal oxides or phosphate cathodes and graphitic carbon anodes have intrinsic limitation in energy density. To further improve the specific and volumetric energy densities of current LIBs, graphite anode has to be replaced with a high capacity alternative. Thus, silicon has received a great attention due to its high theoretical capacity (3572 mAh/g at RT as a form of Li15Si4) [1] and relatively low working potential. However, the huge volume expansion (~300%) of silicon associated with lithiation and delithiation during cycling leads to electrode pulverization causing the common rapid capacity fading. Another issue associated with silicon is its low electrical conductivity. Therefore, composites of silicon with conducting carbons could be a practical solution. Among many classes of carbon, graphene has been identified as an attractive supporting material that may address the critical issues associated with Si anodes [2, 3]. For practical application of silicon-graphene assemblies in LIB anodes, it is desirable to form a dense particulate assembly which can give high volumetric capacity and be readily adaptable in industrial coating process [4, 5]. In this study, we employed an electrostatic self-assembly process between amine-functionalized SiNPs (Si-APTMS) and graphene oxide (GO) in an acidic aqueous solution (pH=2) followed by carbon coating process to form dense particulate Si/GS/C assemblies. For carbon coating, two different carbon sources such as poly-vinylidenefluoride (PVdF) or sucrose were employed to prepare the Si/GS/C-P and Si/GS/C-S samples, respectively. For comparison, a Si/GS was prepared by a direct carbonization of Si/GO assembly. The characteristic features of the Si/GS and Si/GS/C samples were assessed with elemental analysis (EA), FT-IR, TGA, SEM, TEM, XRD and XPS etc. The electrochemical responses of samples were obtained with galvanostatic cycling test, and cyclic voltammetry and impedance tests using CR-2032 coin-type cells with a Li-foil as the reference electrode. The TEM image in Fig. 1a shows that SiNPs are all attached on GO surface in the Si/GO assembly. Different from the Si/GS, the Si/GS/C-S formed dense particulate composites (Fig. 1b and c), in which SiNPs are well distributed between graphene sheets and the whole structure become dense with additional carbon coating. As compared in Fig. 1d, the Si/GS (80% Si) showed very poor cycling stability at 500 mA g-1. On the other hand, the Si/GS/C-P (60% Si) and Si/GS/C-S (50% Si) showed much enhanced cycling stability at 500 mA g-1. In particular, the Si/GS/C-S prepared by an in-situ carbon-gel coating in solution of Si/GO assembly delivered reversible capacities over 1300 mAh g-1 at 100 mA g-1 and over 900 mAh g-1 at 500 mA g-1 without capacity fading. The excellent cycling performance of the Si/GS/C-S can be attributed to its structural features. First, the volume variations of SiNPs are effectively damped by flexible and highly conducting graphene sheets encapsulating them. Second, additional carbon coating surrounding the Si/GS assemblies enhances the coherence between SiNPs and graphene in comparison to the Si/GS obtained by direct carbonization of Si/GO assembly without carbon coating. [1] M. N. Obrovac, L. Christensen, Electrochem Solid State Lett. 7, A93 (2004). [2] J. K. Lee, K. B. Smith, C. M. Hayner, and H. H. Kung, Chem. Commu., 46 ,2025-2027 (2010). [3] R. Raccichini, A. Varzi, S. Passerini, and B. Scrosati, Nat. Mater., 14, 271-279 (2015). [4] C. Chae, H.-J. Noh, J. K. Lee, B. Scrosati, and Y.-K. Sun, Adv. Funct. Mater,, 24, 3036-3042 (2014). [5] J. Kim, C. Oh, C. Chae, D.-H. Yeom, J. Choi, N. Kim, E.-S. Oh, and J. K. Lee, J. Mater. Chem. A., 3,18684-18695 (2015). Fig.1. TEM image of (a) Si/GO assembly and (b) Si/GS/C-S, (c) SEM image of dense particulate form of Si/GS/C-S, and d) cycling performances of Si/GS, Si/GS/C-P and Si/GS/C-S (Inset shows TGA profiles of samples). Figure 1

Inhibiting Li-Ion Battery Operation at High Temperature Using Responsive Polymers

In recent years, remarkable progress in the development of Li-ion battery technology has led to devices that deliver records in power and energy density. The thermal instability in such batteries, however, creates a major roadblock to implementing these devices in large-format systems to support emerging applications in transportation and energy storage from intermittent sources, such as wind and solar. Efforts to mitigate safety issues pertaining to flammability, reactivity, and thermal runaway typically involve the implementation of low conductivity solid-state materials or irreversible and destructive safety devices, which prove problematic for high-power, large-scale systems. In this presentation, we will discuss a new approach to achieve thermal stability in Li-ion batteries using an electrolyte system that contains a polymer that phase separates from solution above a specific temperature. The phase separation causes the solid polymer to coat the electrode and separator leading to an increase in internal resistance, which prevents the flow of current within a specified voltage range. This approach is advantageous over existing methods because the polymer phase separation is a local process, that is, if hot spots form within the battery, the polymer within that area will phase separate and coat the electrode while the rest of the device continues to operate. Two responsive electrolyte systems will be described: poly(ethylene oxide) (PEO) in ionic liquids (ILs) and poly(benzyl methacrylate) PBMA in ILs. Both systems exhibit Li-ion concentration dependent phase behavior, where ion concentration affects the temperature at which the phase transition occurs and the reversibility of the phase transition. Two distinct differences exist between these electrolyte systems. The PEO-IL system exhibits a change in solution conductivity and charge transfer resistance above the phase transition temperature, while the PBMA-IL system only exhibits an increase in charge transfer resistance, but to a much greater extent. Solution and ion transport resistances are measured using electrochemical impedance spectroscopy (EIS) on stainless steel electrodes to determine changes in solution resistance with increasing temperature, and on porous carbons to measure ion-transport resistances. The influence of ion and polymer concentration on the phase transition temperature will be discussed, along with the extent to which these properties can changed with temperature. Li-ion batteries were fabricated using two electrode configurations, each including a high temperature stable separator (Dreamweaver, International) soaked in the responsive electrolytes. In one setup, lithium titanate (LTO) anodes were used with lithium iron phosphate (LFP) cathodes due to the stability of each material in various ionic liquids. With this cell, we investigated key battery characteristics as a function of temperature using EIS and constant current charge-discharge measurements. With properly formulated electrolytes, we showed that battery operation was inhibited above the phase transition temperature of the electrolyte, which fell in the range of 120-150 °C. Figure 1 shows the discharge characteristics of the LTO-LFP cell at a 4C discharge rate at 60 °C and at 150 °C. Above the phase separation temperature, the internal resistance of the cell increases leading to a rapid decrease in potential as the cell tries to deliver the specified current. Open circuit voltage measurements confirm the change in internal resistance with temperature. In another cell configuration, we investigate the performance of responsive electrolytes with lithium cobalt oxide (LiCoO2) cathodes and graphitic carbon anodes. These batteries represent state-of-the-art cells with high operating voltages and high power capabilities. The performance and stability of responsive electrolytes in these systems will be discussed and compared to LTO-LFP systems, and conventional batteries comprising electrolytes with lithium salt in propylene carbonate. Electrolytes with temperature-responsive polymers provide a new approach to achieve thermal stability in Li-ion batteries. Results presented in this work will provide the foundation for the development of advanced materials that enable large-format and safe lithium batteries capable of delivering high-power. Figure 1

MnO/C nanocomposite prepared by one-pot hydrothermal reaction for high performance lithium-ion battery anodes

Among various candidates to replace the low capacity graphitic carbon anode in current lithium ion batteries (LIBs), manganese oxides possess the advantages of high lithium storage capacity, low cost, high intrinsic density, environmental friendliness and low lithium storage voltage, i.e., 0.5 V Li+/Li. Manganese oxides, however, have to be incorporated with conducting and porous matrix due to poor electrical conductivity and large volume expansions associated with conversion reaction upon cycling. In this study, a facile one-pot route was attempted for the synthesis of MnO/C nanocomposite for which Mn3O4 nanoparticles were grown in aqueous medium followed by carbon gel formation in a one-pot reactor. Thus obtained Mn3O4/C carbon gel was transformed into MnO/C nanocomposite by thermal annealing in an Ar flow. The MnO nanoparticles (60wt%) of 20–50 nm in diameter were well dispersed throughout the MnO/C composite. The MnO/C composite delivered reversible capacity of 541mAh g−1 with an excellent cycling stability over 100 cycles, while parent Mn3O4 lost most of its capacity in 10 cycles. The MnO/C composite also exhibited much higher rate capability than a commercial graphite anode. Hence, the MnO/C composite based on low cost materials and facile synthetic process could be an attractive candidate for large-scale energy storage applications.

Three dimensional studies of particle failure in silicon based composite electrodes for lithium ion batteries

Silicon based composite electrodes for lithium ion batteries are of significant interest because of their potential to be high capacity alternatives to the commonly used graphitic carbon anodes. A drawback to their use, however, is the Si particle debonding and fracture that occurs as a result of the volumetric expansion by the lithium host particles upon lithiation of the anode electrode. We use X-ray micro computed tomography to visualize the evolution of the internal microstructure of a silicon-based electrode before and after four lithiation steps during the first half cycle of the cell. We develop a novel threshold edge detect method to perform 3D volumetric measurements of silicon particle expansion. According to our results, 100% lithiation of the composite anode resulted in up to 290% volume expansion of individual Si particles. Furthermore, the global and localized image intensity histogram profiles from 3D data were used to analyze the silicon particle X-ray attenuation effects as a function of lithiation: a decreasing attenuation with lithiation and the propagation of the reaction front through a core–shell process between the original state and 25% lithiation of the silicon-based electrode have been observed.

Invited Presentation: Electrochemically Induced Stresses in Energy Related Materials

The volume changes that are associated with composition changes in a solid can induce significant stresses, when these expansions or contractions are physically constrained. These phenomena are important in a variety of energy-related materials, where they lead to complex interactions between electrochemical and mechanical driving forces. We have employed in situ measurements of these stresses along with a variety of other in situ and ex situ characterization methods, to obtain critical information about how the relevant mechanisms operate in different types of materials. One example is ceria electrolytes in solid oxide fuel cells, where oxygen potential gradients across the electrolyte can lead to large stresses. These stresses, caused by differences in the oxygen content of the ceria, are significantly enhanced by grain boundary contributions that can be interpreted with space charge effects. In Li ion battery (LIB) electrodes, Li insertion and removal can also lead to very large stresses and complex deformation behavior. One area of interest here is the solid-electrolyte interphase (SEI) layer, where we have recently shown that substantial near-surface stresses occur during SEI formation on graphitic carbon anodes. These results imply that stresses can be engineered during SEI formation, to enhance the stability of these critical passivation layers. Another concern in is the mechanical integrity of interfaces in composite electrode structures. Here, we have used patterned thin film structures to understand the response of bi-material interfaces during Li cycling. The integrity of these interfaces is critical for cycling stability and capacity retention in rechargeable Li ion batteries.

High‐Throughput Computational Screening of New Li‐Ion Battery Anode Materials

AbstractWe use density functional theory (DFT) in conjunction with grand canonical linear programming (GCLP), a powerful automated tool for analyzing ground state thermodynamics, to exhaustively enumerate the 515 thermodynamically stable lithiation reactions of transition metal silicides, stannides and phosphides, and compute cell potential, volume expansion, and capacity for each. These reactions comprise an exhaustive list of all possible thermodynamically stable ternary conversion reactions for these transition metal compounds. The reactions are calculated based on a library DFT energies of 291 compounds, including all transition metal silicides, phosphides and stannides found in the Inorganic Crystal Structure Database (ICSD). We screen our computational database for the most appealing anode properties based on gravimetric capacity, volumetric capacity, cell potential, and volume expansion when compared with graphitic carbon anodes. This high‐throughput computational approach points towards several promising anode compositions with properties significantly superior to graphitic carbon, including CoSi2, TiP and NiSi2.

The Impacts of Graphene Nanosheets and Manganese Valency on Lithium Storage Characteristics in Graphene/Manganese Oxide Hybrid Anode

Graphene nanosheets (GNS) with attached MnOx nanoparticles are studied in regard to their structure and morphology. The relationship between the lithium storage performances and GNS contents as well as manganese valency was investigated. Experimental results showed that the specimen with 44 wt% GNS and high content of MnO delivered high reversible capacity (over twice of that in graphitic carbon anode), good cycling stability (0.8% fading per cycle), and high rate capability (67% at the 800 mA/g), which are dramatically better than pure Mn3O4. The improvement is attributed to the presence of GNS which provides continuous networks for fast electronic conduction and mechanical flexibility for accommodating the large volume change. The MnOx/GNS hybrid material has the added advantages over pure GNS, benefiting from its lithium storage potential of around 0.5 V which not only ensures high rate capability but also reduces the risk of metallic lithium formation with its safety hazard.

Variable temperature performance of intermetallic lithium-ion battery anode materials

Although a variety of cathode and electrolyte materials have been studied and commercialized over the past two decades, nearly all commercial cells have used a graphitic carbon anode. Several reasons exist for this choice—including cost, low insertion voltage, and ease of use in the cell manufacturing process. However as uses for lithium-ion batteries expand, alternative anodes that may offer better energy and power capability are being explored. For transportation-oriented purposes, anodes based on simple lithiated Zintl compounds, e.g. Li 17Sn 4, or intermetallic insertion anodes offer significant advantages in capacity (volumetric and gravimetric) and stability in the cell environment that make them attractive candidates for future cell chemistries. Within this context, little however is known about how these alternative anode materials perform as a function of temperature, which is important for applications where operation at temperatures as low as −30 °C can be expected. In this study we evaluated a series of intermetallic insertion anodes that operate by a simple metal displacement mechanism. We have found that for Cu 6Sn 5, Ag/Cu 6Sn 5, and Cu 2Sb, the drop-off in performance with temperature is in line with that observed for a commercial graphite-based anode and indicates that additional variables such as cation diffusion through the electrode passivation film or the electrochemical double layer may be playing an important role that is independent of the underlying anode material. We additionally characterized the NiAs-type mineral Sorosite (CuSn 0.9Sb 0.1), as various literature reports had indicated that substitution of antimony for tin eliminated the need for interstitial copper, however powder X-ray diffraction studies of samples made by annealing or high energy ball milling indicated mixed phase samples.

(Invited) Electrolytes, SEI and Charge Discharge Kinetics in Li-Ion Batteries

Charge discharge kinetics of Li-ion batteries is dominated by the lithium ion (Li+) charge transfer kinetics, which involves the process of transporting the solvated Li+ in the electrolyte to the insertion of Li+ and the accepting of an electron at the same time in the electrode active materials. The importance of electrolytes and recent studies of Li+ charge transfer kinetics were briefly reviewed. Using 3-electrode cells and a DC Pulse Current Impedance method, we examined the charge discharge kinetics of the anode and the cathode in the same electrolyte at the same time. We observed a slower kinetics at the graphitic carbon anode as indicated by higher activation energy than that at the lithium nickel cobalt aluminum mixed oxide (LiNi0.80Co0.15Al0.05O2) cathode. While desolvation is a dominating step as concluded in recent studies on the Li+ charge transfer kinetics, this study suggests that the nature of SEIs and electrode materials play crucial roles on Li+ charge discharge kinetics.

Surface Modifications of Electrode Materials for Lithium Ion Batteries

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Surface modifications of electrode materials for lithium ion batteries

Since the birth of the lithium ion battery in the early 1990s, its development has been very rapid and it has been widely applied as power source for a lot of light and high value electronics due to its significant advantages over traditional rechargeable battery systems. Recent research demonstrates the importance of surface structural features of electrode materials for their electrochemical performance, and in this paper the latest progress on this aspect is reviewed. Electrode materials are either anodic or cathodic ones. The former mainly include graphitic carbons, whose surfaces can be modified by mild oxidation, deposition of metals and metal oxides, coating with polymers and other kinds of carbons. Through these modifications, the surface structures of the graphitic carbon anodes are improved, and these improvements include: (1) smoothing the active edge surfaces by removing some reactive sites and/or defects on the graphite surface, (2) forming a dense oxide layer on the graphite surface, and (3) covering active edge structures on the graphite surface. Meanwhile, other accompanying changes occur: (1) production of nanochannels/micropores, (2) an increase in the electronic conductivity, (3) an inhibition of structural changes during cycling, (4) a reduction of the thickness of the SEI (solid-electrolyte-interface) layer, and (5) an increase in the number of host sites for lithium storage. As a result, the direct contact of graphite with the electrolyte solution is prevented, its surface reactivity with electrolytes, the decomposition of electrolytes, the co-intercalation of the solvated lithium ions and the charge-transfer resistance are decreased, and the movement of graphene sheets is inhibited. When the surfaces of cathode materials, mainly including LiCoO 2, LiNiO 2 and LiMn 2O 4, are coated with oxides such as MgO, Al 2O 3, ZnO, SnO 2, ZrO 2, Li 2O⋅2B 2O 3 glass and other electroactive oxides, the coating can prevent their direct contact with the electrolyte solution, suppress the phase transitions, improve the structural stability, and decrease the disorder of cations in the crystal sites. As a result, side reactions and the amount of the heat production during cycling are decreased. Simultaneously, other effects are observed such as: (1) suppression of the dissolution of Mn 2+, (2) higher conductivity, and (3) removal of HF from the electrolyte solution. Consequently, after the above-mentioned effective coating, marked improvements in the electrochemical performance of the electrode materials including the reversible capacity, the coulombic efficiency in the first cycle, the cycling behavior, and the high rate capability have been achieved. However, many surface science issues are still remaining open, e.g., mechanisms of these coatings and different actions of different coatings, and some further directions are suggested for the surface modification of the electrode materials.

Overcharge reaction of lithium-ion batteries

Overcharge reaction was studied in detail using 650 mAh prismatic hermetically sealed lithium-ion batteries with LiCoO 2 cathodes, graphitic carbon anodes and ethylene carbonate/ethyl methyl carbonate (EC/EMC) electrolytes. Several varieties of gases (CO 2, CO, H 2, CH 4, C 2H 6 and C 2H 4) were evolved in the overcharge reaction. The amount of gas increased with the increase in the cell temperature and rose rapidly at the end of the overcharge. In particular, the amount of CO 2 gas produced by the oxidation of the electrolyte at the cathode increased markedly. The exothermic oxidation reaction of the electrolyte was accelerated at the temperature above 60 °C, causing the cell temperature to increase rapidly thereafter. The heating tests of the overcharged anode samples enclosed in cylindrical cell cases with EC/EMC electrolytes resulted in thermal runaways. In contrast, the overcharged cathodes tested in the same manner showed no thermal runaway. The thermal runaway reaction during overcharge was caused by the violent reaction between the overcharged anode (deposited lithium) and the electrolyte solvent at high temperature that was the result of the rapid exothermic reaction of the delithiated cathode and the electrolyte.

Additives-containing functional electrolytes for suppressing electrolyte decomposition in lithium-ion batteries

Several olefinic compounds such as vinyl acetate, divinyl adipate and allyl methyl carbonate were studied as additives for propylene carbonate (PC)-based electrolytes in lithium-ion battery, which kind of electrolytes always exfoliate graphitic carbon and decompose drastically to liberate organic gas. Three kinds of graphitic carbons commonly used in lithium-ion batteries, namely, natural graphite, MCMB 6-28 and MCF were chosen to test the decomposition-suppressing ability of additives. The effects of the type of graphitic anodes and the structure of additives on the electrolyte decomposition have been investigated in the terms solid electrolyte interface (SEI) formation, which was characterized by cyclic voltammetry (CV), ac impedance, SEM, XPS analyses, and auger electron spectroscopy (AES). The electrochemical performance of the additives-containing electrolytes in combination with LiCoO 2 cathode and graphitic carbon anode was also tested in coin cells.

In situ characterization of the SEI formation on graphite in the presence of a vinylene group containing film-forming electrolyte additives

Acrylic acid nitrile (AAN) is introduced as a novel example out of the large class of vinylene groups containing film-forming additives for lithium-ion batteries. The electrochemical behaviour, especially the electrolyte additive reduction and the associated film formation in the presence of this compound is investigated with the in situ methods of Fourier transform infrared (FT-IR) spectroscopy and electrochemical quartz crystal micro balance (EQCMB). The results clearly point at a solid electrolyte interphase (SEI) formation mechanism, which proceeds via the cathodically induced polymerization of AAN. We suggest that the electro-polymerisation of vinylene groups is a main electrolyte reduction mechanism for a vinylene group containing electrolyte additives. The outstanding filming properties of vinylene compounds such as AAN allow the use of graphitic carbon anodes in PC-based electrolytes even when only 1% of the additive is present in the electrolyte.

An analysis of rechargeable lithium-ion batteries after prolonged cycling

We report herein on extensive post-mortem analysis of rechargeable Li-ion batteries after prolonged cycling at 25 and 40 °C. The 18650-type batteries produced by standard procedure at LG Chem. Inc. were comprised of graphitic carbon anodes, LiCoO 2 cathodes, and a commercial, Li-battery grade ethylene carbonate, ethyl–methyl carbonate (EC-EMC)/LiPF 6 solution. The batteries were dismantled under highly pure inert atmosphere after prolonged cycling by a specially designed system, followed by analysis of both electrodes by SEM, XRD, FTIR, and electrochemical methods (voltammetry, impedance spectroscopy). The residual solution was analyzed by gas chromatography–high-resolution mass spectrometry (GCMS), and by Karl Fischer titration (for trace H 2O content). It was found that the structure of both electrodes is basically retained during prolonged cycling. However, they developed surface films that considerably increase their impedance, thus badly affecting the battery performance after a few hundred charge–discharge cycles. It also appears that part of the cathode's active mass became electronically isolated from the current collector. Hence, the impedance of both electrodes increases considerably upon cycling. The residual solutions contained 10–15 times more water than the pristine solution, as well as products of trans-esterification of the alkyl carbonates, and some fluorinated and phosphorous-containing organic species. The surface chemistry of both electrodes seems to be dominated primarily by LiF formation. Capacity fading mechanisms of Li-ion batteries are discussed.

Reconsideration of SEI stability: reversible lithium intercalation into graphite electrodes in trans-2,3-butylene carbonate

Lithium ion batteries with graphitic carbon anodes and LiCoO 2 cathodes are cycled reversibly in electrolytes based on trans-2,3-butylene carbonate ( t-BC), even in the absence of ethylene carbonate. While the poor interfacial film (the solid electrolyte interface (SEI)) on the lithium electrode can be readily explained in terms of previous models of its stability, this highly reversible behavior of graphite is hard to account for. To explain this profound difference in the SEI stability of the two electrodes, we have taken into account the influence that the nature of the electrode (lithium metal versus graphite) and the type of the reaction site (basal plane versus edge sites) exert on the solvent reduction pathways.

Development of high energy density Li-ion batteries based on LiNi 1− x− yCo xAl yO 2

Electrochemical characterisation of LiNi 1− x− y Co x Al y O 2 cathode material was conducted and a high reversible capacity of 184 mAh g −1 was obtained with good capacity retention. Evaluation of LiNi 1− x− y Co x Al y O 2 performance with a graphitic carbon anode was undertaken in several alternative high energy density Li-ion cell configurations. These included a D-size (33,600) cell design, a flat, “soft-packaged” 10 Ah cell design and a high capacity 75 Ah metal case cell design. Energy densities of 160, 150 and 130 W h kg −1 were achieved, respectively, for these cell designs. Further optimisation of these designs is envisaged to lead to projected energy densities, respectively, of 175, 180 and 150 W h kg −1.

Comparative study of thermal behaviors of various lithium-ion cells

Heat generation behaviors during charge and discharge are examined by calorimetric measurement for several lithium-ion cells that are commercially available. Heat flow from the cell shows interesting variation reflecting the characteristics of the electrode reactions. Heat generation of the cell with graphitic carbon anode draws extremely complicated curve caused by variation of stage structure of lithium intercalated graphite. The cell with hard carbon anode dissipates excess heat resulting in a voltage hysteresis between charge and discharge. In the LiCoO 2 cathode, thermal behaviors due to the crystal phase transition are clearly observed.

High Capacity Anode Materials for Rechargeable Sodium‐Ion Batteries

Electrochemical techniques have been used to study the reversible insertion of sodium into hard‐carbon host structures at room temperature. In this paper we compare these results with those for lithium insertion in the same materials and demonstrate the presence of similar alkali metal insertion mechanisms in both cases. Despite the gravimetric capacities being lower for sodium than lithium insertion, we have achieved a reversible sodium capacity of 300 mAh/g, close to that for lithium insertion in graphitic carbon anode materials. Such materials may therefore be useful as anodes in rechargeable sodium‐ion batteries. © 2000 The Electrochemical Society. All rights reserved.