Methods for improving low temperature performance of flow batteries

 

The efficiency of liquid flow batteries will be significantly reduced at low temperatures, and divalent vanadium ions will precipitate in vanadium electrolytes at low temperatures, seriously affecting battery performance and life. The main factors causing poor performance of liquid flow batteries at  low temperatures are:

1. At low temperatures, the viscosity of the electrolyte increases, resulting in reduced conductivity;

2. At low temperatures, the charge transfer impedance at the electrode/diaphragm interface increases;
At low temperatures, the migration rate of active substances in the electrolyte decreases, and the electrode polarization increases.
Effective methods to improve the low-temperature performance of flow batteries are proposed mainly from the aspects of electrodes, electrolytes, and operating parameters.

1. Electrodes

As the reaction site of active substances, the electrode's activity, conductivity, compressibility, porosity, permeability and other performance parameters are directly related to the performance of the battery stack. Among them, the electrode activity is most significantly affected by temperature. In  vanadium flow batteries, the poor activity of the negative electrode is the main factor restricting the further improvement of the performance of vanadium flow batteries. At present, most flow battery electrodes use graphite felt as electrodes. Graphite felt is a porous material.

As an electrode, it can increase the specific surface area of ​​the electrode and can also be used as a diffusion layer. Ordinary graphite felt has poor electrode activity. Even after high-temperature treatment, the electrode activity cannot be well exerted. At present, the main research work is focused on electrode modification and modification, especially the activity of the negative electrode needs to be improved.

The main way to improve the electrochemical activity of negative electrode materials at low temperatures is to modify the electrodes. The  electrodes are modified with catalysts (such as TiN nanowires, TiC, MnO2, OTiB2, TixOy), and the electrode activity is improved by surface coating and deposition on the electrodes, which reduces the electrochemical polarization of the battery and the side reactions of the battery at the end of charge and discharge.

2. Electrolyte

As the storage place for active substances in flow batteries, the electrolyte is the capacity unit of flow batteries. The conductivity of the electrolyte increases with increasing temperature, and the viscosity decreases with increasing temperature. The comprehensive performance of the battery can be optimized by increasing the comprehensive valence state of the electrolyte and increasing the volume of the negative electrode electrolyte. See the influence of the state of the negative electrode electrolyte of the flow battery on the battery performance.

For vanadium battery electrolytes, the negative electrode electrolyte is easy to precipitate at low temperatures, the stability of the negative  electrode electrolyte is poor at low temperatures, and the viscosity increases and the conductivity decreases. At present, the main ways to improve low-temperature performance through electrolytes are: 

1) Optimizing the solvent composition, by optimizing the sulfuric acid-vanadium ion concentration ratio, to improve the stability of the negative electrode electrolyte at low temperatures; 
2) Developing mixed acid electrolytes, such as hydrochloric acid-sulfuric acid system vanadium electrolytes.
3) Additives, such as inorganic salts, organic acids, etc., through additives, destroy the precipitation/precipitation mechanism to increase the  precipitation/precipitation barrier.

3. Operation parameters
The operation parameters of flow batteries mainly include charge and discharge mode, electrolyte flow rate, temperature, SOC, etc. The performance of flow batteries can be improved by adjusting and optimizing the operation parameters. For example, at the end of charge and discharge, the  electrolyte flow rate is increased to increase the battery capacity and electrolyte utilization.

The main ways to improve the performance of flow batteries at low temperatures by optimizing the operation parameters are:

1) Increase the electrolyte flow rate, actively increase the diffusion of the electrolyte, reduce the concentration polarization of the electrolyte on the electrode, reduce the diffusion impedance, and improve the performance of the flow battery;

2) Control the SOC and operate the flow battery at a low SOC as much as possible. At low temperatures, the stability of the negative electrode electrolyte deteriorates. By reducing the concentration of divalent vanadium ions at the negative electrode, the risk of divalent vanadium ion precipitation is reduced;

3) Reduce the charge and discharge density (power). At low temperatures, the electrochemical performance decreases, and there is a risk that the liquid flow battery cannot operate normally under high density. By controlling the charge and discharge mode, high-power (density) charging and discharging is not performed at low temperatures. After running for a period of time, the heat generated is used to increase the temperature before high-power charging and discharging.