Galvanostatic Intermittent Titration Study of the Positive Electrode of a Na|Ni(Fe)-Chloride Cell

Self Cite

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...

Section: Separatorsupporting

confidence: 87%

“…The specific surface area was calculated using the following equation: [24] with an average particle radius (r p,k ) of 660 μm for both materials. 16 Note that this electrode satisfies the assumption outlined in Eq. 1 that the ohmic resistance in the solid electrode is negligible compared to the ohmic resistance in the electrolyte.…”

Section: Separatormentioning

confidence: 99%

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

Knehr

West

2016

Self Cite

“…1A, as the OCP changes at a rather low level. The stable OCP (0.991 V) is dictated by the reversible Nernst potential, E r , of reaction 1: 14,15…”

Section: Open Circuit Potential and Voltammetric Features Of The Ni-niclmentioning

confidence: 99%

“…Considerations of nucleation sites.-In general, nucleation and island formation (Volmer-Weber type growth) of a deposited species (D) on a substrate (S) is subject to meeting the condition: 60 γ DS + γ D > γ S [14] where γ S and γ D are the surface energies of D and S, respectively; γ DS is the deposit-substrate interaction energy. For homo-epitaxial deposition, γ DS = 0, and γ D = γ S , where the condition for nucleation is not met.…”

Section: Examination Of Nucleation Mechanism Using Chronoamperometric...mentioning

confidence: 99%

Nucleation Controlled Mechanism of Cathode Discharge in a Ni/NiCl₂Molten Salt Half-Cell Battery

Rock

Simpson

Türk

et al. 2016

The cathode of the Na-NiCl2 battery discharges through a complex process of NiCl2 reduction coupled with microstructural growth of Ni particulates. The electrochemical mechanism of this process is a leading determinant of the battery's power rating and cyclability. The present work investigates this cathode discharge mechanism by employing a half-cell test system, and combining scanning electron microscopy (SEM) with cyclic voltammetry (CV) and chronoamperometry. The charge/discharge characteristics of a Ni cathode are probed in a chloro-basic melt of NaCl-saturated NaAlCl4 at 280°C. The CV and chronoamperometric data are fitted to established models of electrodeposition kinetics. From these analyses, the rate determining step of cathode discharge is identified as instantaneous nucleation and growth of electrodeposited Ni. SEM results indicate that, NaCl-decorated sites of the cycled cathode surface facilitate this nucleation mechanism. Scan rate dependent CV indicates that, the charge transfer step of the nucleation reaction involves electro-reduction of NiCl42−. The CV data also reveal the equilibrium concentration and the diffusion coefficient of this dissolved species of NiCl2. The oxidation of Ni to NiCl2 (cathode-charge step) under potentiostatic conditions follows a modified Cottrell equation for interfacial diffusion of Cl−.

“…The method of a two electrode impedance spectroscopy has already been utilized by Wang et al 8 using a standard charge and discharge cycle to investigate the power output characteristics of a battery cell at 300 °C. GITT was also utilized by Zhu et al 14 investigating, interactions between nickel and iron. O'Sullivan et al 15 parameterized a Na/NiCl 2 cell model using GITT as parameter extraction profile.…”

mentioning

confidence: 99%

Internal Resistance Analysis of Na/NiCl₂ Cells using Electrochemical Impedance Spectroscopy

Büttner

Skadell²,

Schüßler³

et al. 2022

A pulsed discharge regime (GITT) was used to investigate the ohmic internal resistance of Na/NiCl2 cells, also known as ZEBRA Cells. Two methods were chosen to determine the internal resistance. On the one side the voltage response of the corresponding current enables a calculation. On the other side the potentiostatic electrochemical impedance spectroscopy was used to measure cell spectra including the ohmic internal resistance. The cells were investigated within five different operating temperatures (300°C – 180°C with 30 K steps). Results show that the ohmic internal resistance of a Na/NiCl2 is increasing when the operating temperature is decreasing, mainly due to the decrease of ionic conductivity of β^'' separator and the secondary electrolyte NaAlCl4. As a direct consequence, there is a significant capacity loss. Therefore, the operating temperature can be identified as capacity limiting factor.