Experimental Study on the Deterioration of Nickel Cadmium Battery Based on External Characteristic Analysis

Although nickel cadmium batteries have good comprehensive performance and have been used in rail transit auxiliary power systems for a long time, battery degradation is inevitable during long-term use, which largely limits its power output capability and increases the cost of use. Unlike lithium batteries, the degradation study of nickel cadmium batteries has many shortcomings, mostly limited to a certain characteristic data for degradation prediction, and lack of systematic experimental study. Therefore, this article investigates the external characteristics of nickel cadmium battery degradation from multiple points, such as electrochemical impedance, heat production, thermal stability, gas production, etc. Through the experimental analysis, it can be seen that the degradation leads to an advance in the time of oxygen production, a decrease in the thermal stability, an increase in the heat production per unit time, an increase in the charge transfer impedance and a decrease in the interface double layer capacitance.


Introduction
Nickel cadmium (Ni-Cd) batteries have stable performance, convenient maintenance, no need for additional management electronic devices, strong abuse resistance, and good low-temperature performance.Therefore, they have been widely used in rail transit auxiliary power systems for a long time.However, battery degradation is inevitable during long-term use, and when the degree of degradation is severe, it can sometimes cause the vehicle to not start normally, thereby affecting driving safety.
For Ni-Cd battery, the anode material is nickel hydroxide (oxy-nickel hydroxide), and the cathode material is cadmium hydroxide (cadmium), doped with auxiliary conductive materials such as iron oxide and cobalt hydroxide.The electrolyte uses alkaline aqueous solutions such as potassium hydroxide, and the separator uses materials such as polypropylene or polyamide.Each component of the battery, including positive and negative active materials, separators, and electrolytes, undergoes various side reactions during long-term use.These side reactions directly or indirectly lead to battery degradation, which is reflected in external characteristics such as capacity decay, internal resistance rise, and heat generation increase.
The degree of battery degradation, also known as the State of Health (SOH), is divided into disintegration analysis and external characteristic analysis.The former is to disassemble the battery sample in a suitable environment to obtain the internal components, and to study the internal deterioration characteristics through material analysis or electrochemical testing.The application of disintegration analysis methods in the field of Ni-Cd batteries is relatively limited, and only Soge et al. [1] found through SEM testing that both nickel and cadmium electrodes tend to form large crystals

Experimental Platform
The used Ni-Cd batteries are from the auxiliary power system of a CRH380A high-speed Electric Multiple Units.The type of battery is composed of positive plate, negative plate, diaphragm, fasteners, battery slot and cover, liquid port bolt, and other components.It is a typical sintered Ni-Cd battery.The battery physical photo is shown in figure 1 (a).The interior of the battery consists of 32 negative plates and 30 positive plates stacked in sequence through separators, as shown in figure 1 (b).The main performance parameters of a single cell are shown in Table 1.The experimental platform, shown in Figure 2, mainly consists of CT-4002 battery tester, high and low temperature alternating humidity and heat test chamber used to provide a constant environmental temperature, GSD320 mass spectrometer used to real-time analyse the gas generated by the battery during operation, daily BT4560 impedance tester used to measure the AC impedance spectrum, and other equipment.The CT-4002 battery tester is used to set the charging and discharging conditions of

Electrochemical Impedance
The electrochemical impedance of Ni-Cd batteries can be represented by an electrochemical equivalent circuit model, as shown in Figure 3, including Ohmic resistance (R ohm ), charge transfer resistance (R ct ), double-layer capacitance (C dl ), and diffusion impedance (R w ) [4].The R ct is the main electrochemical model parameter used to describe redox reactions, reflecting the exchange between ions and electrons at the electrode interface.The deposition of ion charges at the electrode interface is offset by electronic charges at the electrode interface, forming a double-layer capacitor C dl .R ohm is mainly a solution resistance.R w mainly reflects the solid-state proton diffusion process.Figure 4 (a) presents the EIS results with different SOH.The EIS curve is considered to be composed of an arc (high frequency region) and a straight line (low frequency region).Usually, the curve of the inductive part is ignored, as shown in figure 4 (b).The high-frequency region is controlled by electrode reaction kinetics (charge transfer process), while the low-frequency region is controlled by the diffusion of solid phase protons.Fit the EIS to obtain the corresponding electrochemical parameters, as shown in figure 4 (c) and (d).It can be observed that the R ohm hardly changes as the degradation deepens.Normally, alkaline electrolyte may inevitably absorb carbon dioxide from the air to generate carbonate anions and decomposition of gas production side reactions during long-term use, which will probably lead to an increase in the solution resistance, but the experimental results are not the case, this is because of the batteries from time to time for water replenishment and thus effectively alleviate the deterioration of the electrolyte.During the degradation process, the R ct and R w increased significantly where the R ct increased by more than 53.8%, the R w increased by more than 44.0%, and the C dl decreased significantly by more than 32.1%.
The nickel hydroxide anode reaction is a solid-phase proton diffusion mechanism, i.e., protons are released during the process of being oxidized into, and protons diffuse through the hydrogen bonding between the interlayers or diffuse through the surface and combine with the interface to generate water [5]- [7].The charging and discharging process of the hydroxide anode is essentially a proton diffusion transfer process, and the diffusion of protons is the controlling step of the charging and discharging rate of the hydroxide anode.An increase in the diffusion resistance leads to a blockage of the proton diffusion transfer process, thus affecting the battery charging and discharging performance.An increase in charge transfer resistance means that the redox processes (interconversion of Ni 2+ and Ni 3+ and Cd and Cd 2+ ) need to overcome greater resistance, which leads to an increase in the heat of polarization and further accelerates the degradation of the cell.

Thermal Stability
The thermal runaway experiment is a commonly used method for evaluating the thermal stability of batteries [8]- [10].This article will compare the thermal runaway of a deteriorated battery and a new battery after being fully charged, triggered by long-term overcharging at a high voltage of 1.9V, where the SOH of the deteriorated battery is 86.13%.Figure 5 shows the experimental results.New batteries and deteriorated batteries exhibit roughly the same trend of change during thermal runaway.The current rapidly decreases to point A in a short period of time, then slowly increases to point B, and rapidly increases after point B. The temperature began to slowly increase to point C, and rapidly increased after point C, causing the battery to enter thermal runaway.The difference is that when reaching point A, the new battery takes a shorter time and has a lower current.This is because the electrode activity of the new battery is higher, which can quickly increase the potential difference between the positive and cathodes under external voltage.In the AB segment, the average current growth rate of the deteriorated battery is 2.619 A•h -1 , and 1.287 A•h -1 for new battery.When reaching 45℃, the time taken by the deteriorated battery is 3.35h shorter, and the current is 15.85A higher.This indicates that before 45℃, the new battery has better resistance to high voltage and overcharging than the deteriorated battery.In the BD stage, the average current growth rate of the deteriorated battery is 13.9 A•h -1 , and 11.1 A•h -1 for the new battery.When reaching 60℃, the time used for the deteriorated battery is 0.16h shorter and the current is 26A higher.The above experiments indicate that the thermal stability of deteriorated batteries is reduced compared to new batteries.

Heat Production
Figure 6 shows the temperature changes of the new degraded batteries (SOH of 86.13%) in 0.5C and 1C discharging experiments, respectively.In the 0.5C discharging process, the discharge voltage of the degraded battery is higher than that of the new battery in the early stage, and the discharge voltage falls faster than that of the new battery in the late stage; the temperature rise rate of the degraded battery is higher than that of the new battery, which indicates that the degraded battery has a higher power of heat production; at the end of the discharge, the temperature of the degraded battery rises from 27℃ to 35.5℃, and from 27℃ to 32℃ for the new battery, and the discharging capacity of the degraded battery is lower than that of the new battery.The temperature of the degraded battery was significantly higher than that of the new battery, indicating that the heat production of the degraded battery increased.During the 1C discharging, the discharge voltage of the degraded battery is always lower than that of the new battery, indicating that the discharge capacity of the degraded battery decreases; the temperature of the degraded battery is higher than that of the new battery at the same time, but the temperature difference between the degraded battery and the new battery at the same moment is not as obvious as that of the 0.5C discharge; and at the end of the discharge, the temperature of the degraded battery rises from 26°C to 32.7°C, and 27°C to 34.3°C for the new battery, which is due to the significantly shorter discharge time of the degraded battery at 1C, and the reduction of total heat production compared with the new battery.The above experiments show that the degradation will lead to an increase in the heat production, which is related to the increase in the internal resistance and the increase in the side reactions, and the degradation has a small effect on the heat production of the battery at 1C, while the effect is significant at 0.5C.

Auxiliary Reaction for Gas Production
When charging a Ni-Cd battery, the anode active substance Ni(OH) 2 loses its electrons and is oxidized to NiOOH.The electrons transfer to the external circuit through the conductive network and current collector in the anode; At the same time, cadmium hydroxide undergoes a reduction reaction on the cathode to generate metal cadmium.In order to ensure the normal operation of Ni-Cd batteries under various working conditions, the ratio of the capacity of the cathode to the anode plate is usually 1.5~2 during design, which allows the cathode to have excess discharge protection capacity and overcharging protection capacity, effectively avoiding the generation of hydrogen gas under mild overcharging conditions [11].However, in practical use, due to the malfunction of the charger and inconsistent internal resistance, the voltage of the single battery is too high, which can cause severe overcharging and hydrogen evolution reaction of the battery.Due to the low oxygen evolution potential and strong competition with the anode oxidation reaction, even designing overcharge protection cannot avoid the generation of oxygen.
This article first studies the oxygen evolution reaction during constant current charging, and the experimental results are shown in Figure 7.The gas generated is fed into a mass spectrometer through a Peek tube, which ionizes the gas molecules and analyzes the gas changes through the current generated by ionization.The experimental results indicate that the oxygen ionization current of the new battery begins to rapidly increase around 13600s (SOC 85.13%) and decreases around 16000s (SOC 99.8%), and rapidly increases around 6300s (SOC 46.7%) and decreases around 13500s (SOC 100%) for the deteriorated battery.This is because at the end of charging, oxygen begins to form at the Ni electrode or electrolyte and is transported through the gas phase to the Cd electrode.Here, oxygen dissolves back into the electrolyte and diffuses to the cadmium electrode/electrolyte interface, followed by a composite reaction at the Cd electrode/electrolyte interface [12].However, the transport and composite reaction of oxygen take some time, resulting in a phenomenon where the oxygen ionization current rapidly increases and then decreases.Compared with new batteries, deteriorated batteries have an earlier oxygen generation time.This is because deterioration can reduce the oxygen evolution potential of the anode or weaken the competition between the oxidation reaction and oxygen evolution reaction of the deteriorated battery, but the oxygen composite reaction will not advance.As a result, the duration of oxygen evolution reaction in the constant current charging process of the deteriorated battery increases, and oxygen production increases and accumulates inside the battery, leading to an increase in battery pressure.
Figure 8 shows the hydrogen ionization current generated by a Ni-Cd battery during constant voltage overcharging.The hydrogen ionization current of the deteriorated battery under constant voltage overcharging of 1.7V and 1.9V is significantly higher than that of the new battery, indicating that the deteriorated battery will produce more hydrogen during overcharging, which is related to the loss of cathode capacity and the decrease in hydrogen evolution potential of the deteriorated battery.Due to the more severe overcharging behavior of the battery during the 1.9V constant voltage charging process, the rate and amount of hydrogen generated will also increase.The hydrogen changes during constant voltage charging at 1.46V for the two experimental units in the figure are almost identical, and there is almost no hydrogen generation compared to other voltages.By amplifying the hydrogen change curve at 1.46V, it can be seen that the amount of hydrogen shows a decreasing trend, indicating that the battery hardly produces hydrogen during the constant voltage charging process at 1.46V.This is because the overcharging capacity of the battery at 1.46V is small and still within the protection range of the cathode's overcharging capacity.Long-term operation in a high-temperature environment will accelerate battery degradation.With a discharging rate (0.2C), 25℃ ambient temperature, after 460 times of shallow charging and discharging (discharge depth of 10%) cycle, the SOH is 78%; while with the 40℃ ambient temperature, SOH has been lower than 70%.The increase of discharging rate also accelerates the battery degradation, and the capacity decay rate under 0.2C is 20.63%, while the capacity decay rate under 0.5C reaches 31.72%under 25℃ ambient temperature and the same cycle number.The degradation resulted in a lower discharging voltage plateau, which is mainly related to the decrease of the positive equilibrium potential and the attenuation of the reactive polarization.
During the degradation process, the charge transfer resistance and diffusion resistance increased significantly, with the charge transfer resistance increasing by more than 53.8%, the diffusion resistance increasing by more than 44.0%, and the double-layer capacitance decreasing significantly by more than 32.1%.
Deterioration leads to lower thermal stability, from the beginning of overcharging to entering thermal runaway, the time taken by the deteriorated battery is shorter than that of the new one by 3.35h.When reaching the warning of 60°C, the time taken by the deteriorated battery is shorter than that of the new one by 3.51h, and the current is higher than that of the new one by 26A.
At 1C discharging rate, degradation has a small impact on battery heat generation; Under 0.5C, the effect is significant, and the thermal power and heat production of the deteriorated battery will significantly increase.
Deterioration leads to the advancement of oxygen precipitation time of the battery, the new battery starts to produce oxygen at about 85.13% of SOC, and the deteriorated battery produces oxygen at about 46.7% of SOC, which is due to the weakening of polarization of the positive reaction, which makes it difficult for oxidation reaction to compete with the oxygen precipitation side reaction.Hydrogen precipitation time during overcharging of the degraded battery was advanced and hydrogen production increased exponentially due to the following two reasons: capacity loss due to the migration of cadmium from the cathode, and the decrease in the hydrogen precipitation potential.
This article investigates the effects of degradation of Ni-Cd batteries on open circuit voltage, internal resistance, electrochemical equivalent circuit, heat and gas production.The main conclusions are as follows:

Table 1 .
6M100B Single Cell Main Performance Parameters