Anand Durairajan

Studies on Capacity Fade of Lithium-Ion Batteries

The capacity fade of Sony 18650S Li-ion cells has been analyzed using cyclic voltammetry, impedance spectroscopy and electron . . . probe microscopic analysis EPMA . The surface resistance at both the positive LiCoO and negative carbon electrodes were found to 2 increase with cycling. This increase in resistance contributes to decreased capacity. Impedance data reveal that the interfacial resistance at . LiCoO electrode is larger than that at the carbon electrode. The impedance of the positive electrode LiCoO dominates the total cell 2 2 resistance initially and also after 800 charge-discharge cycles. EPMA analysis on carbon electrodes taken from the fresh and cycled cell show the presence of oxidation products in the case of cycled cells. No change in the electrolyte resistance is seen with cycling. q 2000 Elsevier Science S.A. All rights reserved.

Study of Sn-Coated Graphite as Anode Material for Secondary Lithium-Ion Batteries

Tin-graphite composites have been developed as an alternate anode material for Li-ion batteries using an autocatalytic deposition technique. The specific discharge capacity, coulombic efficiency, rate capability behavior, and cycle life of Sn-C composites has been studied using a variety of electrochemical methods. The amount of tin loading and the heating temperature have a significant effect on the composite performance. The synthesis conditions and Sn loading on graphite have been optimized to obtain the maximum reversible capacity for the composite electrode. Heating the composite converts it from amorphous to crystalline form. Apart from higher capacity, Sn-graphite composites possesses higher coulombic efficiency, better rate capability, and longer cycle life than the bare synthetic graphite. Current studies are focused on reducing the first cycle irreversible capacity loss of this material.

Studies on Capacity Fade of Spinel-Based Li-Ion Batteries

It is well known that the capacity of a lithium-ion battery de- creases during cycling and most of the loss can be associated with some unwanted side reactions that occur in these batteries during overcharge and over discharge conditions. 1 These reactions may cause electrolyte decomposition, passive film formation, active ma- terial dissolution, phase changes in the insertion electrode, and sev- eral other phenomena. Carbonaceous anode materials in lithium-ion rechargeable cells exhibit irreversible capacity loss in the first cycle, mainly due to reaction of lithium during the formation of passive surface films. 2 Passivation of the carbon electrode during the formation period and subsequent capacity loss are highly dependent on specific properties of carbon in use, such as degree of crystallinity, surface area, and so on. Positive electrode dissolution phenomena are both electrode and electrolyte specific and the factors that determine the positive elec- trode dissolution are structural defects in the positive active mate- rial, high charging potentials, and several other phenomena. 1 Oxy- gen defects in the electrode material may weaken the bonding force between the transition metal and oxygen resulting in the metal dis- solution. Previously, capacity fade studies were done on commercially available lithium-ion cells with LiCoO2 as the positive material. 3 These studies revealed that the positive electrode contributes more to the capacity fade of the lithium ion cells, when compared to the negative electrode and the increase in impedance of LiCoO2 elec- trode with cycling is the dominant factor for loss in capacity of the battery. In this paper an attempt was made to study the capacity fade of commercially available spinel-based lithium-ion batteries and also to optimize the charging current based on charging time and capacity fade. Commercially produced Li-ion cells include several features for safe operation under different conditions. During charging, to pre- vent electrolyte oxidation a potential limit ~charging to ultimate volt- age! is used with internal electrical circuitry ~cell voltage control and equalization circuit!. 4 However, different charging protocols lead to different charging times. Further, varying the charge protocol also affects the capacity fade during cycling. 1 One of the commonly used charging protocol for Li-ion cells is charging at constant cur- rent to a particular voltage and subsequently holding the potential constant. In this case, the total time for charging is held constant. One of the drawbacks of this process is that, since the total charging time is constant, the battery is held at a high constant voltage for longer than essential. In this case, holding the cell potential at high voltage can contribute to oxidation of the cathode leading to capac- ity decay during cycling. Optimization of charging protocol is essential to achieve superior performance for the Li-ion batteries. Objectives of this paper were to study the performance of lithium-ion batteries with spinel-based cathodes. First, we want to optimize the discharge capacity of the cell based on the charge current, end potential, and total charging time. Next, we compare the capacity fade of cells charged at differ- ent rates to a common end potential and discharged at the same current. The goal here is to minimize the capacity loss with cycling by choosing an optimum charging current. Finally, we study the causes for the capacity fade in spinel based Li-ion batteries.

Cycle life and utilization studies on cobalt microencapsulated AB5 type metal hydride

Abstract LaNi 4.27 Sn 0.24 alloy was microencapsulated with cobalt by electroless deposition. The coated material has a higher capacity compared to the bare alloy due to the faradaic reaction of cobalt during discharge. This additional capacity has been studied using various material and electrochemical characterization techniques. The capacity due to cobalt varies depending on the amount of active material available for reaction. An increase in utilization is seen with decrease in thickness of the coating. Active surface area and the transport process within the film control the amount of cobalt utilized. Finally, cobalt coated alloys are seen to cycle ten times more than bare LaNi 4.24 Sn 0.27 with constant capacity.

Electrochemical Characterization of Cobalt-Encapsulated Nickel as Cathodes for MCFC

Abstract The stability of the NiO cathodes in molten carbonate fuel cell (MCFC) has been improved through microencapsulation of the NiO cathode with nanostructured Co. Cobalt was deposited on the NiO cathode using an electroless deposition process. The electrochemical oxidation behavior of the Co-coated electrodes is similar to that of the bare NiO cathode. The cobalt-coated electrodes have a lower solubility in the molten carbonate melt when compared to bare nickel oxide electrodes in the presence of cathode gas. The solubility decreased more than 50% due to microencapsulation with cobalt. The thermal oxidation rate was also lower in case of the cobalt-encapsulated electrode. Impedance data from the modified electrode indicate that the oxygen reduction reaction depended inversely on the CO 2 and directly on the oxygen partial pressures respectively suggesting a similar reaction mechanism to that of nickel oxide. The results indicated that cobalt-encapsulated NiO is a viable solution in the development of alternate cathodes for MCFC applications.

Characterization of Hydrogen Permeation Through a Corrosion-Resistant Zinc-Nickel-Phosphorus Alloy

Abstract Hydrogen permeation characteristics of a new Zn-Ni-P alloy were studied and compared with that of a Zn-Ni alloy. The Zn-Ni-P alloy was deposited from an acid sulfate bath containing 0.5 M ...

Development of a New Electrodeposition Process for Plating of Zn‐Ni‐X (X = Cd, P) Alloys: I. Corrosion Characteristics of Zn‐Ni‐Cd Ternary Alloys

Cadmium has been used extensively as a corrosion resistant coating in aerospace, electrical, and fastener industries owing to its excellent corrosion resistance and engineering attributes. 1 Cadmium deposition is done from cyanide baths, which are subject to stringent regulations. 2 Alternate baths for cadmium plating are also undesirable due to the toxicity of the metal and its salts. 3 Further during cadmium deposition, large amounts of hydrogen are introduced into the underlying metal. 4 This increases the risk of hydrogen embrittlement failure in the structure. Hence environmental concerns and performance criteria mandate the search for alternatives to cadmium coatings. 5 Various zinc and zinc alloy coatings show promise in this regard. Electrodeposited zinc has been widely used for the protection of steel from corrosion. These electrogalvanized coatings have been shown to be more effective when alloyed with metals such as nickel, iron, and cobalt. Zinc-nickel alloy coatings have been suggested in the literature as replacement for cadmium coating because this alloy provides good corrosion protection on steel, 6-8 superior formability, and improved weldability. 9-11 Zn-Ni alloys containing 15-20 wt % nickel has been shown to possess four times more corrosion resistance than cadmium-titanium deposit. 12 However, due to the high zinc content in the deposit, these alloys are more negative than cadmium and hence dissolve rapidly in corrosive environments. Although Ni is a more noble metal than Zn, the codeposition of Zn-Ni is anomalous and a higher percent of Zn is present in the final deposit. The mechanism for this preferential deposition has been discussed extensively in the literature. 13,14 Typical nickel composition in the alloy is approximately 510%, and any further increase in nickel composition is based on using a higher-than-predicted Ni/Zn ratio in the bath. 15,16 An enhancement in the nickel composition would lead to more anodic open-circuit potential, which in turn will reduce the driving force for the galvanic corrosion. Also the barrier properties associated with nickel-rich deposits are superior compared to other coatings. Several researchers have attempted to decrease the anomaly and increase the nickel content by either introducing inert species in the bath or by developing a ternary alloy. 17-21 Nonyl phenyl polyethylene oxide (NPPO) has been used to reduce this anomaly and to produce uniform deposits. 20,21 NPPO inhibited zinc electrodeposition

Pulverization and Corrosion Studies of Bare and Cobalt-Encapsulated Metal Hydride Electrodes

Electrochemical impedance spectroscopy was used as an in situ technique to determine the average particle size of metal hydride electrodes. Using this, the pulverization of bare and cobalt-encapsulated LaNi Sn alloy was studied as a function of charge-dis- 4.27 0.24 charge cycles. In the case of bare alloy, pulverization causes an exponential decay in particle size with cycling. Cobalt-encapsulated alloys do not undergo much pulverization with cycling. Bode responses obtained for bare alloy electrodes indicate the increase in particle to particle resistance with cycling. Alloy oxidation, which is responsible for the increase in particle to particle resistance is absent in the case of cobalt encapsulated alloy. Surface analysis indicates the presence of alloy segregation for bare LaNi Sn . Decrease in particle 4.27 0.24 size and increase in bare alloy resistance is accompanied with severe decay in electrode discharge capacity. q 2000 Elsevier Science S.A. All rights reserved.

Capacity fade studies on spinel based Li-ion cells

The performance of Cell-Batt/sup (R)/ Li-ion cells using nonstoichiometric spinel as the positive electrode material has been studied at different charging rates. The capacity of the cell was optimized based on varying the charging current and the end potential. Subsequent to this, the capacity fade of these batteries was studied at different charge currents. For all charge currents, the resistance of both the electrodes does not vary significantly with cycling. Comparison of cyclic voltammograms of spinel and carbon electrode before and after 800 cycles reveals a decrease in capacity with cycling. Low rate charge-discharge studies confirmed this loss in capacity. The capacity loss was approximately equally distributed between both electrodes. On analyzing the XRD patterns of the spinel electrode that were charged and discharged for several cycles, it can be seen that apart from the nonstoichiometric spinel phase, an additional phase slowly starts accumulating with cycling. This is attributed to the formation of defect spinel product /spl lambda/-MnO/sub 2/ according to a chemical reaction, which also leads to MnO dissolution in the electrolyte. EDAX analysis of the carbon samples shows an increase in Mn content with cycling. These studies indicate that capacity fade of spinel based Li-ion cells can be attributed to: (i) structural degradation at the cathode; and (ii) loss of active materials at both electrodes due to electrolyte oxidation.

Capacity fade of Li-ion cells: comparison of DC and ENREV charging protocols

Sony 18650S Li-ion cells have been cycled using direct current and ENREV pulse charging (CC-CV) protocols. The influence of the charging protocol on the capacity fade of these batteries has been analyzed using cyclic voltammetry, galvanostatic charge-discharge and impedance spectroscopy. Batteries with different cycle numbers charged using ENREV pulse charging protocol showed superior charge transfer performance and rate capabilities and more than 90% retention of the initial capacity after 800 cycles compared with 64% retention of capacity observed for batteries charged using DC charge protocol.