The Microstructure of Sintered Silver Electrodes

Abstract

Ag electrodes were charged at the 20-hr rate of constant current and discharged at the 1-hr rate in 35% KOH solution at 25~Cross sections of samples cut from the electrodes during charges at cycles 1-30 were examined. Almost all Ag particles had a visible coating of Ag20 when an electrode was charged for half the capacity accepted at the Ag/AgeO potential plateau. The electrode surface had the same structure as the interior. In a cycled electrode approximately 60% of the Ag had been oxidized to Ag20 when the potential peak separating the Ag/Ag20 and Ag20/AgO plateaus was reached.The present paper describes structural changes in Ag electrodes as Ag20 was formed in an alkaline solution. The Ag electrodes were of the type used in Ag-Zn and Ag-Cd storage batteries.There have been two previous papers in this series concerning the microstructure of sintered Ag electrodes. The first paper described the structures present as charged electrodes were being reduced during discharges (1). The second described the structures present at the end of a series of complete discharges (2). The formation of AgO will be the subject of a later paper. The electrodes in this series of investigations were always discharged at the 1-hr rate and charged at the 20-hr rate of constant current in 35% KOH solution.When Ag electrodes are discharged in a KOH solution at a high current density for a series of cycles the electrode capacity is m a i n t a i n e d at a higher value than when a low discharge C.D. is used. This difference in capacity results from changes in the microstructure of the electrodes. The microstructure of a cycled electrode depends partly on the discharge C.D. that is used. A discharge at the relatively fast 1-hr rate results in formation of small Ag particles, m a n y having a size on the order of 1~. Particles of this size oxidize readily during the next charge. When the slow 20-hr discharge rate is used, however, most of the Ag particles that form are 10-25~ (3).Particles of Ag larger than 4 or at most 5~ do not oxidize completely during a charge. Oxidation usually stops before an oxide layer reaches a thickness of 2~. As a result, the central portion of large particles remains as metallic Ag at the end of a charge instead of being oxidized. Once large particles form in an electrode during a slow discharge, the original discharge capacity is not regained readily. When slow discharges are used, the capacity decreases so rapidly that by cycle 3 the electrodes always discharged at the 20-hr rate give less capacity than electrodes always discharged at the 1-hr rate. This occurs despite the fact that approximately 10% of the oxide remains at the end of a fast discharge, and almost all of the oxide is reduced during a slow discharge. Experimental ProcedureOnly a brief account of the experimental procedure will be given here, because it has already been described (1). Sintered Ag electrodes were cycled individually in an excess of 35% KOH solution at 25~ The Ag electrodes were charged using constant current at the 20-hr...

The Microstructure of Sintered Silver Electrodes

“…Charged electrode at cycle 15.--One of the most noticeable structural changes as an electrode was cycled was the gradual clumping of the active material. This change was evident in both discharged and charged electrodes (2,3). The AgO in a charged electrode was in the form of small angular particles at cycle 2, with small voids in the AgO (Fig.…”

“…At the end of a charge, the surface of a wellcycled electrode contained smaller AgO crystals and more Ag20 than the interior. Manuscript submitted Aug. 3,1970; revised manuscript received ca. Jan. 28, 1971.…”

“…The structure of silver electrodes of the type used in storage batteries is of interest, because knowledge of the structure helps in explaining the capacity changes observed when cells containing Ag electrodes are cycled. Earlier papers in this series have described changes in structure during discharges (1) and at the end of discharges (2), as well as formation of Ag.~O during charges (3). Electrodes in this work were cycled using the relatively fast 1 hr rate of discharge and the 20 hr rate of charge.…”

Fabrication of a High‐Performance Flexible Silver–Zinc Wire Battery

“…The particle size is determined by the rate at which battery is discharged with smaller particles formed at faster discharge rates. [46,47] Smaller particles are desirable as they can be fully oxidized on the consequent charge, while oxidation in the larger particles will stop at the thickness determined by ionic transport limitations (Figure S5 B). [46,47] In order to determine the optimum discharge rate, silver-zinc batteries were discharged at slow (0.05C and 0.1C) and fast (0.5C and 0.25C) rates between 1.8 V and 1 V (Figure S4 C,D).…”

“…[46,47] Smaller particles are desirable as they can be fully oxidized on the consequent charge, while oxidation in the larger particles will stop at the thickness determined by ionic transport limitations (Figure S5 B). [46,47] In order to determine the optimum discharge rate, silver-zinc batteries were discharged at slow (0.05C and 0.1C) and fast (0.5C and 0.25C) rates between 1.8 V and 1 V (Figure S4 C,D). The corresponding cycling profiles show that optimum discharge rate is >0.5 C. Therefore, in order to maintain stable capacity, batteries presented in this work were discharged at the rates 0.5C and above.…”