Theoretical Considerations for Improving the Pulse Power of a Battery through the Addition of a Second Electrochemically Active Material

Abstract

Porous electrode theory is used to conduct case studies for when the addition of a second electrochemically active material can improve the pulse-power performance of an electrode. Case studies are conducted for the positive electrode of a sodium metal-halide battery and the graphite negative electrode of a lithium "rocking chair" battery. The replacement of a fraction of the nickel chloride capacity with iron chloride in a sodium metal-halide electrode and the replacement of a fraction of the graphite capacity with carbon black in a lithium-ion negative electrode were both predicted to increase the maximum pulse power by up to 40%. In general, whether or not a second electrochemically active material increases the pulse power depends on the relative importance of ohmic-to-charge transfer resistances within the porous structure, the capacity fraction of the second electrochemically active material, and the kinetic and thermodynamic parameters of the two active materials. To accelerate an electric vehicle, an important requirement of a battery is the ability to deliver high power pulses at all depths of discharge.1-3 Nevertheless, a high power pulse can be difficult to achieve, especially at high depths of discharge (DoD). In some cases, high power is difficult to achieve at high DoD because of an increase in the ohmic resistance during discharge. The ohmic resistance increases due to the movement of the reaction fronts within the electrodes from more favorable (less resistive) to less favorable (more resistive) locations.4,5 For instance, this behavior has been documented in the positive electrode of sodium metal-halide batteries, where the low resistivity of the electrode (nickel and/or iron) and the higher resistivity of the electrolyte (sodium tetrachloroaluminate) cause the reaction front to move from the separator to the current collector during discharge. 6,7 At high DoD, the reaction front is far from the separator and the ionic path length is increased, which increases the overall ohmic resistance in the electrode.One way to improve the pulse-power performance of an electrode is through the addition of a second active material that only reacts at higher DoD.7,8 A schematic of this concept is shown in Figure 1. Part a) shows the discharge and pulse-power process of an electrode with one active material. In this case, the reaction front starts near the separator and moves deeper into the electrode as the cell is discharged. When the electrode is pulsed at the high DoD, the reaction occurs deep within the electrode and there are high ohmic losses due to the increased ionic path length. In contrast, part b) shows the discharge and pulsepower process for an electrode with two active materials. For this case, the same initial behavior is observed, whereby the reaction front moves from the separator to the current collector during discharge. However, during the initial discharge, the second active material does not react. Therefore, when the electrode is pulsed at a high DoD, the high ohmic losses are avoided b...

Vanadium-Substituted Tunnel Structured Silver Hollandite (Ag1.2VxMn8–xO16): Impact on Morphology and Electrochemistry

“…Silver plays an interesting covalent structural role in the α-MnO 2 framework, with decreasing tunnel size upon increasing silver ( x ) content . The reduction displacement mechanism of Ag + serves as an interesting mode to increase cathode electrical transport in situ; as example, the electrically insulating Ag 2 VO 2 PO 4 cathode , reduces to form a network of dispersed, conductive Ag 0 particles that support fast electron transport. , There have been previous attempts to apply the Ag + reduction displacement mechanism to silver hollandite. ,− However, the detectable formation of silver metal occurs at late stages of electrochemical lithiation (>0.5 electron equivalents (ee) per Mn formula unit, >4ee per Mn 8 O 16 formula unit) and moderate stages of electrochemical sodiation (<0.5ee/Mn) in nonaqueous electrolytes, and is electrochemically irreversible based on prior reports for Ag x Mn 8 O 16 . At low potentials, silver is known to undergo a size-dependent reversible electrochemical alloying process with lithium; , however, there are limited examples of reversible silver metal/silver ion electrochemistry at potentials suitable for a cathode material (i.e., >1.5 V versus lithium).…”

“…18 The reduction displacement mechanism of Ag + serves as an interesting mode to increase cathode electrical transport in situ; as example, the electrically insulating Ag 2 VO 2 PO 4 cathode 25,26 reduces to form a network of dispersed, conductive Ag 0 particles that support fast electron transport. 27,28 There have been previous attempts to apply the Ag + reduction displacement mechanism to silver hollandite. 24,29−35 However, the detectable formation of silver metal occurs at late stages of electrochemical lithiation (>0.5 electron equivalents (ee) per Mn formula unit, >4ee per Mn 8 O 16 formula unit) 18 and moderate stages of electrochemical sodiation (<0.5ee/Mn) 35 in nonaqueous electrolytes, and is electrochemically irreversible based on prior reports for Ag x Mn 8 O 16 .…”

Reduction of silver ions in molybdates: elucidation of framework acidity as the factor controlling charge balance mechanisms in aqueous zinc-ion electrolyte

“…Towards this goal, the development of materials containing Ag + is pursued for fundamental interest because the reduced product can contain a network of atomically dispersed, conductive Ag 0 particles that support fast electron transport throughout the bulk. 1,2 Minute amounts of dispersed Ag + are known to signicantly improve electronic transport; as example, cathodes of the silver-vanadium-phosphate-oxide type show percolation thresholds with 0.3% volume reduction of Ag + concurrently resulting in 10 6 decrease in electrical resistance. [3][4][5] Silver vanadium oxide (SVO) cathodes remain the dominant material in implantable cardiac debrillator batteries due to this reductive displacement mechanism.…”

Deliberately Designed Atomic-Level Silver-Containing Interface Results in Improved Rate Capability and Utilization of Silver Hollandite for Lithium-Ion Storage

“…As an alternate strategy, our group has demonstrated that electrical conductivity can be improved by taking advantage of reduction displacement mechanisms in silver-ion-containing materials. For example, the electrically insulating Ag 2 VO 2 PO 4 cathode − reduces to form a network of dispersed, conductive Ag 0 particles that support fast electron transport. , This Ag 0 network provides advantages which carbon alone cannot attain, as the Ag + precursor is controlled and distributed on a smaller atomic length scale as part of the inherent crystal structure; that is, Ag 0 is generated with immediate proximity to each active material particle.…”