Yaron S. Cohen

Prototype systems for rechargeable magnesium batteries.

The thermodynamic properties of magnesium make it a natural choice for use as an anode material in rechargeable batteries, because it may provide a considerably higher energy density than the commonly used lead–acid and nickel–cadmium systems. Moreover, in contrast to lead and cadmium, magnesium is inexpensive, environmentally friendly and safe to handle. But the development of Mg batteries has been hindered by two problems. First, owing to the chemical activity of Mg, only solutions that neither donate nor accept protons are suitable as electrolytes; but most of these solutions allow the growth of passivating surface films, which inhibit any electrochemical reaction. Second, the choice of cathode materials has been limited by the difficulty of intercalating Mg ions in many hosts. Following previous studies of the electrochemistry of Mg electrodes in various non-aqueous solutions, and of a variety of intercalation electrodes, we have now developed rechargeable Mg battery systems that show promise for applications. The systems comprise electrolyte solutions based on Mg organohaloaluminate salts, and MgxMo 3S4 cathodes, into which Mg ions can be intercalated reversibly, and with relatively fast kinetics. We expect that further improvements in the energy density will make these batteries a viable alternative to existing systems.

A short review of failure mechanisms of lithium metal and lithiated graphite anodes in liquid electrolyte solutions

Abstract Li electrodes in any relevant electrolyte solution (i.e., polar aprotic) are covered by surface films of a very complicated structure. It was found that even in cases where the surface films formed on lithium contain elastomers, or where the lithium metal reactivity is reduced by doping with elements such as N, As, Al, Mg, Ca, etc., it is impossible to achieve sufficient passivation with lithium electrodes and liquid solutions. Passivation is considerably worsened when Li electrodes are operated at high rates (especially at high charging, Li deposition rates). Thus, there is no way that rechargeable Li batteries can compete with Li-ion batteries in any application that requires high charging rates (e.g., in powering portable electronic devices). The electrochemical behavior of lithiated graphite electrodes also depends on passivation phenomena. The surface films formed on lithiated graphite are similar to those formed on Li metal in the same solutions. The volume changes of graphite electrodes during Li insertion–deinsertion are small enough to enable their reasonable passivation in a variety of electrolyte solutions. A critical factor that determines the stability of graphite electrodes is their morphology. It was found that the shape of graphite particles plays a key role in their application as active mass in anodes for Li-ion batteries.

New insights into the interactions between electrode materials and electrolyte solutions for advanced nonaqueous batteries

Abstract In this paper we review some recent work with Li metal and Li–graphite anodes and Li x MO y cathodes (M=transition metals such as Ni, Co, Mn). The emphasis was on the study of surface phenomena using in situ and ex situ FTIR spectroscopy, atomic force microscopy (in situ AFM), electrochemical quartz crystal microbalance (EQCM) and impedance spectroscopy (EIS). The performance of Li metal and Li–carbon anodes in secondary batteries depends on the nature of the surface films that cover them. The use of Li metal anodes requires the formation of highly uniform and elastic surface films. Thus, most of the commonly used liquid electrolyte solutions are not suitable for Li metal-based rechargeable batteries. In the case of Li–C-based batteries, the passivating films need not be elastic. Channeling the Li–C electrode surface chemistry towards the formation of Li 2 CO 3 surface films provides adequate passivation for these electrodes. This can be achieved through the use of EC-based solutions of low EC concentration (cosolvents should be less reactive than EC). An interesting finding is that the behavior of many commonly used cathodes also depends on their surface chemistry, and that their overall Li insertion processes include the step of Li ion migration through surface films. Their origin is discussed herein, as well as possible oxidation processes of the relevant solutions.

The study of capacity fading processes of Li-ion batteries: major factors that play a role

In this work, we studied the impact of some factors on the behavior of practical electrodes of Li-ion batteries. These included elevated temperatures (45–80 8C), prolonged storage of Li-ion cells, and additives in the electrolyte solution. The Li-ion battery systems studied included negative electrodes (anodes) comprising of mesocarbon microbeads (MCMB) and mesocarbon fibers (MCF), and LixCoO2 positive electrodes (cathodes) in an ethylene carbonate (EC)/ethyl-methyl carbonate (EMC) (1:2)/LiPF6 1 M solution. Vinylene carbonate (VC) and a Li-organo-borate complex (Li-OBC) were tested as additives. It is shown that the electrochemical response of Li–C negative electrodes depends on the structure of the surface films controlling their behavior, which change upon storage, temperature, and cycling. We established that impedance of these electrodes increased with storage time due to the enrichment of the surface films by LiF and other fluorine-containing species. The capacity fading of the LixCoO2 electrodes in cycling/storage processes at elevated temperatures relates mostly to surface phenomena, whereas the bulk structural characteristics of the electrodes do not change. # 2003 Elsevier Science B.V. All rights reserved.

Attempts to Improve the Behavior of Li Electrodes in Rechargeable Lithium Batteries

In this work we studied properties of modified lithium electrodes in an attempt to improve the high rate performance of rechargeable Li (metal) batteries containing liquid electrolyte solutions. Li (metal)-Li 0.3 MnO 2 AA batteries with solutions containing 1,3-dioxolane (DN), LiAsF 6 , and a basic stabilizer became commercial several years ago but failed to compete with Li-ion battery technology because of a very limited cycle life at high charging rates. The problem relates to intensive reactions between Li deposited at high rates and the electrolyte solutions, which dry the batteries. The lithium-solution reactivity was modified through several approaches. Li anodes doped by Li 3 N, Al, and Mg were tested, as well as solutions containing derivatives of DN that are expected to be less reactive toward lithium than DN. It was concluded that reduction of the Li anode-solution reactivity by these approaches cannot solve the problem, because it is impossible to modify the rough morphology, high surface of lithium electrodes when charging (Li deposition) rates are high (>1 mA/cm 2 ). Since there is no hermetic passivation of any Li surface in liquid electrolyte solutions, the high-surface-area Li deposits react with solution components. Therefore, upon charge-discharge cycling of practical Li (metal) batteries, the electrolyte solution is consumed in these reactions. Hence, the future of Li (metal) rechargeable batteries lies either in the use of solid electrolyte matrices instead of the liquid solutions, or in applications where low charging rates are tolerable.

The effect of pressure on the electroanalytical response of graphite anodes and LiCoO2 cathodes for Li-ion batteries

The effect of the application of pressure during the preparation of composite flaky synthetic graphite anodes and LiCoO2 cathodes on their electrochemical behavior in Li insertion and de-insertion processes was studied using voltammetry, chronopotentiometry, electrochemical impedance spectroscopy (EIS), and ex situ AFM imaging. Unpressurized graphite electrodes reach a higher capacity and have faster kinetics than the same electrodes compressed or rolled at 5 × 10 3 kg cm − 3 . In contrast, the performance of rolled or compressed LiCoO2 electrodes in terms of capacity and kinetics was better than the performance of the unpressed electrodes. AFM imaging of pristine and cycled electrodes demonstrated a pronounced effect of pressure on the morphology of graphite electrodes, whereas the impact of pressure on the morphology of LiCoO2 electrodes was found to be much less pronounced. It was concluded that compressing graphite electrodes has an adverse effect on the contact between the active mass and ions in solution, while compressing LiCoO2 electrodes does not adversely affect the contact between solution species and the active mass, but rather, improves inter-particle electrical contact.

The study of lithium insertion–deinsertion processes into composite graphite electrodes by in situ atomic force microscopy (AFM)

Li insertion–deinsertion into composite graphite electrodes, comprising synthetic graphite flakes (6 μm average size), polyvinylidene difluoride binder (PVdF), and copper current collectors, in commonly used alkyl carbonate solutions were studied by in situ atomic force microscopy (AFM). In this study, we were able to probe by in situ AFM the behavior of practical, composite graphite electrodes in ethylene carbonate–dimethyl carbonate (EC–DMC) solutions containing salts such as LiAsF6 and LiPF6 during entire lithiation–delithiation cycles. These in situ micro/nanomorphological studies could probe surface film formation on the graphite particles, as well as periodic volume changes in the graphite flakes during Li insertion–deinsertion cycles. These cyclic volume changes can explain the capacity fading of graphite electrodes upon prolonged cycling, in Li-ion batteries. While the overall morphology of these electrodes remains steady upon cycling in the appropriate solutions (in which the Li–C electrodes are efficiently passivated), there is a continuous problem in the extent of accommodation of the small volume changes in the graphite particles upon lithiation–delithiation, by the surface films. It is suggested that graphite electrodes fail during prolonged cycling due to small scale, continuous reactions of the active mass with solution species, which gradually increase their impedance and decrease the content of the lithium stored in the electrodes.

A comparison between the electrochemical behavior of reversible magnesium and lithium electrodes

Abstract This paper describes briefly the difference between reversible lithium and magnesium electrodes. In the case of lithium, the active metal is always covered by surface films. Li dissolution–deposition is reversible only when the surface films contain elastomers and are flexible. Hence, they can accommodate the morphological changes of the electrode during the electrochemical processes without breaking down. In an ideal situation, lithium is deposited beneath the surface films, while being constantly protected in a way that prevents reactions between freshly deposited lithium and solution species. In contrast to lithium, magnesium electrodes are reversible only in solutions where surface film free conditions exist. Mg does not react with ethers, and thus, in ethereal solutions of Grignard reagents (RMgX, where R=alkyl, aryl, X=halide) and complexes of the following type: Mg(AlX 4− n R n ′ R n ″ ′) 2 , R and R′=alkyl groups, X=halide, A=Al, 0 n n ′+ n ′′= n , magnesium electrodes behave reversibly. However, it should be noted that the above stoichiometry of the Mg salts does not reflect the true structure of the active ions in solutions. Mg deposition does not occur via electron transfer to simply solvated Mg 2+ ions. The behavior of Mg electrodes in these solutions is discussed in light of studies by EQCM, EIS, FTIR, XPS, STM and standard electrochemical techniques.

A new approach for the preparation of anodes for Li-ion batteries based on activated hard carbon cloth with pore design

We demonstrate herein the possibility to prepare carbon anodes for Li-ion batteries using simple carbonized polymeric precursors such as cotton and phenolic cloths. Activation by controlled oxidation forms highly porous carbons whose electrochemical activity in Li salt solutions is mostly an irreversible reduction of solution species and double layer charging. Treating these porous carbons by chemical vapor deposition (CVD) of carbon on their surfaces, closes the pores in a way that they can insert Li-ions, but not solution species. These general carbon engineering processes form new carbons with nanoscopic, selectively closed pores, which can serve as highly reversible anode materials for Li-ion batteries, with relatively low irreversible capacity. The capacity of these electrodes depends on the nature of the carbon CVD process. This paper describes the scheme for carbon engineering, gas adsorption measurements that demonstrate the impact of the carbon CVD process, and the relevant changes in the structure of the pores and some preliminary electrochemical measurements in non-aqueous Li salt solutions.

Electrodeposition of Granular Cu-Co Alloys

Electrodeposition characteristics of Cu-Co films were studied for the formation of heterogeneous alloys for giant magnetoresistance applications. In situ scanning tunneling microscopy. Auger electron spectroscopy (AES), and high resolution scanning electron microscopy studies showed that rough lilms with a low concentration of cobalt [Cu92.5-Co7.5] (atom %) were deposited mainly due to a higher deposition rate of copper than of cobalt toward the end of the deposition process, and due to the formation of copper grains after the electrodeposition process by chemical exchange between copper ions in the solution and with the cobalt in the deposit, AES analysis resealed that the Cu-Co film is not homogeneous: the bulk of the film is richer in Co while the surface and the bottom of the film are Co-poor. X-ray diffraction showed that the electrodeposition is a topotaxial crystallization process and that the as-deposited film is composed of two phases, a solid solution of face centered cubic Cu-Co with preferred orientation of {111} plane, and a hexagonal close packed Co phase, Scanning electron microscopy micrographs and energy dispersive spectroscopy indicated the segregation of cobalt grains resulting from thermal treatments, according to the phase diagram of Cu-Co.