Discharging of Spent Cylindrical Lithium-Ion Batteries in Sodium Hydroxide and Sodium Chloride for a Safe Recycling Process

Battery discharging prior to size reduction is an essential treatment in spent lithium-ion battery recycling to avoid the risk of fire and explosion. The main challenge for discharging the residual charges by immersion in an electrolyte solution is corrosion because of electrolysis reactions occurring at the battery terminals. This study investigated the discharging process of 18650 cylindrical lithium-ion batteries (LiBs) in NaCl and NaOH solutions and the generation of corrosion products, with the aim of developing a safe and clean discharging system for practical applications. The results show that water electrolysis is the primary reaction during battery immersion in either NaCl or NaOH solutions. Different forms of corrosion occur in each solution. Unlike the NaCl solution, which severely corroded the positive terminal of the battery, resulting in significant amount of solid residue, build-up of fluoride ions, and chlorine gas evolution, in the NaOH solution, a darkened surface of the negative terminal was the only obvious solid product, with no solid residue in the bulk solution, while oxygen gas was evolved. The NaOH solution was found to reduce battery capacity to a residual capacity range of 0–25 mAH after immersing batteries in the solution for 20 h. This value puts the battery in a safe condition for subsequent mechanical treatment. The results indicated that NaOH creates a clean discharging system and can potentially be reused.


INTRODUCTION
Lithium-ion battery (LiBs) demand will grow by 81% of the expected global rechargeable battery market within 5 years (2019-2024). 1Many of these batteries are used in the electricity grid and electric vehicles.Electric vehicles typically use cylindrical 18650 or 21700 LiBs due to these batteries' consistency of quality and relatively low cost. 2 LiBs in electric vehicles have an average lifespan of 8 10 years, 3 and the use of the batteries in electric vehicles will therefore result in significant numbers of spent batteries in the near future. 4Recycling is the ultimate option for dealing with the spent batteries, as remanufacturing and repurposing the batteries only delay the recycling path. 3An efficient recycling process aims to recover valuable materials from the batteries for sustainable re-use, 5,6 and to reduce the impact of the process in terms of environmental pollution. 7he initial step in recycling LiBs is discharging the residual capacity of the batteries.This treatment aims to avoid potential safety issues such as fire and explosion during size reduction treatment, which are crushing, milling, or shredding. 8This mechanical treatment can create a short circuit, due to an unintentional connection between positivelyand negatively-charged materials of the battery if the plastic separator is damaged while the battery is not fully discharged. 9The internal short-circuiting leads to heat production and hazardous electrolyte combustion. 10A low state-of-charge (SOC) of batteries prior to crushing (SOC < 2% and (Received January 29, 2023; accepted August 21, 2023; published online August 30, 2023)  preferably $ 0%) is necessary to avoid this hazard. 11Thus, it is crucial to find an effective discharging method for LiB.
The conventional discharging methods involve intensive labor or a cryogenic environment.One standard method is to attach batteries to ceramic resistors 12 or battery dischargers. 13However, this method is impractical for an industrial-scale process, as connecting batteries to loads requires intensive manual handling.Another possible method is a cryogenic system.On a laboratory scale, this approach immerses batteries in liquid nitrogen before dismantling them to avoid a violent reaction between the released hydrogen gas and oxygen. 14,15Toxco used a cryomilling method in which liquid nitrogen cooled the waste batteries to -198°C prior to the batteries being mechanically shredded. 16The two methods can make the battery safe during crushing, ALthough neither of these cryogenic methods is an economically viable option.
Utilizing electrolytic reactions is a potential approach for discharging large quantities of spent batteries.This method immerses the batteries, including the electrodes, in a conductive electrolyte solution which transports charges between the electrodes and generates a current to discharge the batteries. 17Most of the research conducted to date [18][19][20] has used sodium chloride (NaCl) as the electrolyte, which produces an effective conducting system to discharge the LiBs within minutes.However, the use of NaCl solution results in corrosion to the casing terminal and produces sediments in the solution as solid waste.
There are reports in the literature of efforts to minimize corrosion by using different NaCl concentrations, adding metallic elements, or using reagents other than NaCl.However previous studies have reported that corrosion is observed even in low concentrations of NaCl. 21,22Also, the addition of metallic elements to the NaCl solution was not found to successfully reduce the corrosion reaction on the battery casing. 22Some studies used salt solutions other than NaCl, 22,23 but these solutions also generated a solid corrosion product.The significantly lower cost of NaCl to other salts, however, is seen as a compelling reason, and this salt deserves to be reconsidered as an alternative solution and examined in a consistent comparative manner with the NaOH.
Recent research 24 investigated discharging 18650 batteries from Sony by submersion in 26 different ionic solutes.An encouraging result was observed in the sodium hydroxide (NaOH) solution.The study reported that, although the solution attacked the interior of the rubber gasket in the battery, no significant visual damage was seen on the battery casing.
An electrolyte solution for battery discharging should meet some technical and environmental requirements. 23,24It should: (1) be conductive; (2) not lead to release of toxic gases; and (3) be non-reactive to the battery outer casing.NaOH has good conductivity and does not react strongly with stainless steel, which is widely used as a battery casing material for 18650 batteries.The properties of NaOH indicate that a further investigation of discharging LiBs in NaOH solution is warranted to understand the process involved and to determine if the by-products would be environmentally damaging.
Although published investigations have used several electrolytes to safely discharge LiBs by electrolytic reactions, previous studies used the voltage profile to assess the amount of discharge, which can result in a false reading. 8Also, the residual charge capacity of the battery after discharging has not been measured.Moreover, most studies did not report the discharging process in any detail.Thus, this study aims to investigate the discharging process for 18650 cylindrical batteries immersed in either NaCl or NaOH solutions and to determine whether the residual capacity would mean the batteries are safe for subsequent crushing, milling, or shredding.The research examines the process of LiBs discharging in the two electrolyte solutions using experimental setups that involve direct or indirect contact of the battery with the solution, to enable different aspects of the process to be investigated.

MATERIALS AND METHODS
This study used spent and new 18650 cylindrical LiBs.The spent batteries were from laptops supplied by a local WEEE processing facility (Total Green Recycling, Perth, Western Australia).The new batteries were 3.7 V Powertech LiBs, with a nominal capacity of 2600 mAH, and were obtained from Jaycar Electronics, Australia.
A battery analyzer (BA8; 5 V 3000 mA) was used to diagnose the battery condition prior to the discharging process by applying three cycles of discharging, charging, and rest stages.A constant current of 1000 mA was employed in the discharging stage to discharge the battery to 2.75 V, then the battery was rested for 3 min.The battery was then charged to 4.2 V using a fixed current of 1000 mA, after which the battery was rested again for 3 min.The diagnostic test result ascertains the capacity of the battery before the discharging process.
The NaCl and NaOH solutions used in the discharging process were from UNIVAR (Ajax Finechem, Australia).The NaCl solution concentration of 10 wt.% (1.71 M) was chosen based on previous research. 21,22The study reported that 10 wt.% of NaCl is the optimum concentration to discharge the batteries in terms of residence time and discharge efficiency, based on the voltage measurement.Doubling the concentration did not significantly affect those two parameters.Hence, a solution of NaOH at the same concentration (10 wt.%) was used for comparison.A lower concentration (5 wt.%) has been tested in a previous study. 24Hypothetically, the conductivity of the solution would increase with NaOH concentration, enhancing the electrolytic reaction rate.
The first discharging method involved immersing a battery in the NaCl or the NaOH solution.Insulated wires were connected to each battery terminal with a conductive adhesive (Chemtronic CW2400) to obtain voltage measurements over time on the submerged battery.The half-cell voltages at each terminal were also measured using Ag/AgCl (4 M KCl) reference electrodes, and the value was converted to the Standard Hydrogen Electrode (SHE) scale.A Datataker 505 series 2 data logger recorded potential variation during the discharging process.A schematic of the discharging process for an immersed battery is given in Fig. 1a.Visual observation of the battery and the solution during the immersion process was also conducted to provide information on corrosion products.
The second discharging method did not immerse the batteries in the solution and was designed to investigate the electrolytic reactions to determine the discharging process.The method avoided direct reactions of each of the two solutions (NaCl and NaOH) with the battery terminals, and, instead, two Pt wires (0.5 mm diameter, 99.95% purity) from A&E metals were connected to the battery positive (+ve) and negative (-ve) terminals, respectively and the other ends of the wires were immersed in the solution.The same data logger was used for recording the voltage and current.Individual electrode voltage was also measured by employing the same reference electrode, and the value was converted to the SHE scale.A combined pH probe monitored the solution pH during the discharging process, which was conducted at room temperature without stirring.Figure 1b shows the schematic representation of the discharging setup with the Pt electrodes.After the first set of experiments, the Pt wires were replaced with electrodes fabricated from the stainless steel battery casing to assess the electrolysis reaction more accurately on the battery terminal material.
The residual capacity of the battery after immersion in NaOH solution in 20 h was measured by applying a 0.3 mA discharge current to the battery using the same battery analyser used for the battery health diagnosis.The lowest discharge voltage limit of 0 V was employed to record the capacity.
After each battery immersion test, the NaCl and NaOH solutions were assayed for dissolved elements by inductively coupled plasma optical emission spectrometry using a Thermo Fisher Scientific iCAP 7600 Duo instrument.The solutions were also analyzed for non-purgeable organic carbon using an Aurora 1030W TOC Analyzer.The fluoride content in both solutions was measured by ion chromatography using a Dionex ICS-5000 from Thermo Scientific.The sediment formed in the NaCl solution during the discharging process was examined for mineral phases by x-ray diffraction (XRD; CuKa-Emma; GBC).

Battery Capacity
Figure 2a, b, and c shows that new batteries and some spent batteries can be recharged to the same maximum voltage of 4.2 V, although some spent batteries cannot reach this voltage or have an untypical voltage profile, as reported in supplementary Fig. S-1.However, the most significant difference between a new and a spent battery is the capacity.In general, a spent battery has a smaller capacity (Fig. 2e and f) than a new battery (Fig. 2d), even though the battery has a similar voltage after recharging.In addition, the spent battery capacity can have a wide range of values, as reported in Table 1, and several spent batteries had unknown capacities as the capacity reading was abnormal.Thus, the battery analysis demonstrates that the measured voltage does not accurately reflect the battery capacity, and residual capacity should be used to determine the state of discharge of the battery.

Discharging Batteries by Immersion
Figure 3a shows the change in visual appearance over time of the new batteries and of the solution during NaCl and NaOH immersion, which can provide information on the relevant reactions involved in the discharging process.Gas evolution in both solutions commenced almost instantly upon submersion, and was more pronounced at the -ve than at the + ve terminal.Different intensities of gas generation at the battery terminals are most likely the result of water electrolysis producing hydrogen and oxygen in the ratio 2:1.In NaCl, cogeneration of chlorine and oxygen at the + ve terminal may occur, as reported in the literature. 22,23A qualitative examination detected chlorine gas during the first hour of the discharging process in NaCl.The qualitative analysis used saturated potassium iodide (KI) solution to wet a filter paper, which was then placed above the surface of the electrolyte solution.The observations of the change in color of the filter paper are reported in supplementary Table S-I.The reaction between the chlorine gas and the KI gave a brown color to the wetted filter paper due to the oxidation of iodide to iodine. 25The test result confirms that chlorine was evolved during the process.
In this study, the NaCl solution became cloudy after several minutes of battery submersion because of the production of reddish brown-colored sediments, which covered the battery over time.The metallic cap at the + ve terminal started corroding within 5 min after submersion and severely corroded within 6 h.However, in the NaOH solution, the battery experienced only mild corrosion at the + ve terminal (Fig. 3a), and the solution was clear with no sediments.The cause of the corrosion in NaCl solution is because the steel material in the + ve terminal of the battery may not be stable if exposed to chloride media. 26hlorides in the NaCl solution facilitate metallic corrosion, which corroded the + ve battery terminal.The oxygen generated by hydrolysis at this terminal then further oxidized the dissolved metals from the terminal, which formed pale yellow sediments that became brown over time (Fig. 3a).The color changes demonstrated the iron oxidation, which can be atributed to transformation from goethite (pale yellow) to ferrihydrite (dark brown). 27The XRD spectra of the sediments from the NaCl solution (Fig. 3b) corresponded to two dominant phases, aluminum hydroxide and poorly crystallized ferrihydrite.This is consistent with a study that reported dissolved Fe(II) precipitated in an oxidative environment as iron oxyhydroxide. 28The material from that study had the same phase as in this study.The sediments in the NaCl solution consisted of complex metal elements, mostly Fe, Ni, and Al, which were detected by the XRD analysis, as shown in Fig. 3b.The iron and nickel are from the positive battery cap, which is nickel-plated steel, 2 and the Al is likely to be from the burst disk of the current interruptive device (CID) underneath the + ve cap of the battery. 29e composition of supernatants from the NaCl and NaOH solutions and the sediment from the NaCl solution after the discharging process are shown in Table 2.These results corroborate the XRD analysis of the corrosion products in both solutions.The concentration of the dissolved elements Fe, Al, Cu, Ni, and Co are lower in the NaCl supernatant than in the NaOH supernatant, but the concentrations of F, Li, and Mn are higher.The results in Table 2 indicate that, in NaCl, most of the corroded metals were likely precipitated as sediments.In the NaOH supernatant, Al has the highest metal concentration, which demonstrates that the solution selectively corroded the CID material of the battery.It seems likely that a gasket seal broke during the immersion, as also reported in a previous study, 24 when the NaOH solution penetrated to the safety vent.Electrolyte leakage to the solution discussed below is further evidence of a damaged gasket.
The fluoride and lithium ions in the supernatants are from the battery electrolyte (LiPF 6 ) decomposition products.Sharp voltage drops were observed during submersion of the LiBs (the results for the NaOH solution are shown in Fig. 4), and these indicate that an internal short circuit occurred, which may have increased the internal temperature of the battery. 30The internal heat likely generated CO 2 and hydrocarbon gases from carbonate solvents with low flash points (< 33°C). 31Electrolyte decomposition into LiF and PF 5 gases could also occur due to the internal short circuit.The gas accumulation during the discharging process could increase the internal pressure of the cell. 32However, batteries have a safety vent system to relieve the internal pressure and prevent the battery case from rupturing. 29The vented gases could then have reacted with water, with the reaction rate potentially higher in NaCl than in NaOH due to the presence of corrosion products.Further reaction of water with intermediate products such as POF 3 from PF 5 hydrolysis could then generate HF. 33 This process series could explain the higher F concentration in the NaCl supernatant than in the NaOH supernatant, as reported in Table 2.The fluoride detection in this study is consistent with the previously reported comment that electrolytes from inside the battery can leak into the discharging electrolyte solution during submersion of a 18650 battery in NaOH solution. 24owever, the molar concentration ratio of the fluoride and chloride in the NaOH solution was > 2.
As reported in a study, 34 this ratio results in corrosion inhibition to steel surfaces due to the production of a semi-soluble film by fluoride and metallic ions.The process restrains the chloride ions migration, which in this study only affected a minor part of + ve terminal tip of the submerged battery in NaOH, as shown in Table 3.Thus, the fluoride ions in the NaOH solution from the leaking electrolyte do not lead to battery corrosion.The supernatants from both the NaCl and NaOH solutions contained similar total organic carbon (TOC) concentrations.The organic carbon was most likely from the vented hydrocarbon gases produced by carbonate solvent vaporization, as discussed earlier.Xiao et al. 17 also reported the leakage of electrolytes, namely NaCl, KCl, MnSO 4 , and MgSO 4 , during discharging of 18650 batteries.However, the organic content that leaked from the batteries did not take part in the corrosion reaction.These results suggest that leakage of carbonates in the battery electrolyte into the discharging solution is inevitable during discharging, but does not increase the environmental risk due to corrosion products.
A magnified image (Table 3) of the + ve terminal after submersion in the NaOH solution shows a small roughening of the surface and a reddishbrown section.As chloride impurities (0.015%) were listed in the chemical specifications for the NaOH pellet (supplementary Table S-II), the roughening most likely resulted from corrosion of the stainless steel in the presence of these low concentrations of chloride.The corroded product formed soluble chloro species, as reported in a previous study. 34he mild corrosion of the submerged battery in the NaOH solution was evidenced by the gradual increase of the iron concentration in the solution over the first 90 min of submersion (supplementary Table S-III).Iron hydroxide formed as positively charged colloidal particles, which moved to the negative electrode, lost their charges, and then precipitated onto the electrode.The decreased iron concentration after 90 min of battery submersion indicated iron precipitation as iron oxides, such as Fe(OH) 3 and Fe 3 O 4, which were detected in XRD analysis of the dark-colored material that coated the surface of the -ve terminal of the battery after a few hours of submersion (Fig. 3a).Thus, the solution remained clear without secondary sediment generation during the submersion.
The concentration of Al ions in the supernatant of the NaOH solution at various sampling times (supplementary Table S-III) increased after 2 h of submersion.The Al ions are most likely a product of anodic corrosion of the Al metal alloy of the CID system with the process mentioned earlier.The selective reaction of NaOH with Al has been well known since the invention of the Bayer process. 35t is worth noting that the corrosion reactions in the tests with NaOH and NaCl did not affect the battery case.Table 3 shows that the case appeared in good condition after being taken out of each solution.The battery case effectively functioned as a -ve terminal (cathode) which was cathodically protected by the + ve terminal, and provided immunity to corrosion.
The discharging of both new and spent batteries in the NaOH solution is shown by the decrease in voltage with time in Fig. 4. Although, as noted earlier, this does not give an accurate measurement of final battery capacity, it can still be used to indicate the occurrence of discharging.The battery voltage of the new battery (Fig. 4a) steadily decreased for about 6 h and then rapidly dropped to almost zero, indicating exhaustion of the battery power after being discharged.Figure 4b shows the individual voltage contributions of each terminal to the battery voltage (Fig. 4a) during battery discharging.
The voltage of a spent battery shown in Fig. 4c had the same profile as that of the new battery, except that the spent battery was completely discharged in a much shorter time.This is consistent with the fact that the spent battery's initial charge was smaller than that of the new battery.The individual electrode profiles (Fig. 4d) show that the battery discharge is limited by the active material of the + ve terminal.The + ve terminal voltage drops rapidly to its rest potential when the current becomes zero.
A similar trend of gradually decreasing voltage was observed for the spent battery submerged in NaCl solution over 2 h (supplementary Fig. S-2).The voltage measurement could not be continued beyond this as the metallic cap of the + ve electrode experienced severe corrosion, which resulted in a hollow section, as shown in Table 3.At that stage, a voltage measurement was no longer viable, and the residual capacity could not be accurately measured.

The voltage measurements and visual observations of batteries during direct submersion in the
NaCl and NaOH solutions demonstrate that discharging of the batteries occurred.Gas evolution and corrosion reactions are evidence of the batteries' discharging process, and these stopped when the discharging process was essentially complete.
The discharging performance of a battery in the NaOH solution was evaluated more accurately by determining the battery's residual capacity after the discharging process.The residual capacity of a new battery was 25 mAH from an initial capacity of 2546 mAh (Table 1).Figure 5 shows the discharging capacity over time measured using the battery analyzer.The plateau value on the graph was detected within 60 s at the end of the measurement, confirming that there was no current flowing beyond that point.Three of the tested spent batteries in Table 1 had zero residual capacity after immersion.The residual discharge capacities demonstrate that the batteries were discharged and safe for further mechanical processing.

Battery Discharging by Electrolysis Through Pt Electrodes Immersed in NaCl and NaOH Solutions
The setup in Fig. 1b, in which only the Pt electrodes connected to the battery terminals are submerged in the discharging solution, eliminates any plausible reaction other than water electrolysis.In other words, this setup was used to investigate the battery discharge independently of battery corrosion reactions at the terminals.The battery acted as a power source to drive electrolysis reactions at the Pt electrodes.Gases evolved at both the -ve and the + ve Pt electrodes during the discharging process and stopped being produced when the battery was exhausted.In all the tests, the solution remained clear without any sedimentation.Figure 6 shows the voltage and current profiles when a battery underwent discharge via electrolysis reactions occurring at the Pt electrodes in each of the two solutions.In NaCl (Fig. 6a), the voltage decreased as the electrolysis proceeded.The current initially dropped rapidly, then steadily decreased for a short period before decreasing more rapidly.The current profiles (Fig. 6a) indicate two distinct electrolysis reactions, chlorine and oxygen evolution, occurring.The shape at the -ve voltage profile in Fig. 6b shows electrolytic reaction changes.
The NaOH solution test in Fig. 6c displays a significant voltage drop after 5 h.The voltage continued to decrease to a value of 2.3 V after 20 h, after which the voltage did not support the relevant reaction, resulting in a nearly zero current.The reactions at the + ve and -ve terminals equally controlled the discharging process, as indicated by the individual voltage profiles, which initially decreased and then remained constant (Fig. 6d).
The electrode surface plays an important role in the reactions during discharging, and therefore further tests were carried out with the Pt electrodes replaced by electrodes derived from battery-casing material.This replacement was to assess whether the surface characteristics of the Pt electrodes in any way influenced the electrolysis reaction.The results are shown in Fig. 7 for the NaOH solution.The current from the battery shown in Fig. 7a dropped abruptly within the first hour after connecting the battery to electrodes fabricated from stainless-steel casing material immersed in a NaOH solution.The large surface area (225 mm 2 ) of the electrodes affected the result, which led to a large current density.As a result, the voltage also dropped rapidly from the initial 3.3 V to around 2.3 V, which is the rest potential of the battery, and remained unchanged beyond that point, as essentially no current was flowing.Individual voltages at both terminals shown in Fig. 7b displayed similar trends to the voltages in the tests with the Pt electrodes, which indicates that the Pt and the battery case material behaved identically in the electrolysis reaction.The results from both the Pt and the casing electrodes demonstrated that the potential cannot go lower than 2.3 V as this is the minimum potential value to cover the water electrolysis reaction and the overpotential of the reactions in the NaOH solution.
The relevant reactions during the discharging process can be determined by considering the cations and anions involved in the process.The primary cations in both the NaCl and the NaOH solutions, Na + , are associated with a hydrogen evolution reaction (HER).The reduction of Na + to Na is not thermochemically feasible in aqueous solutions.The relevant reaction depends on the pH of the solution and the potential available to support it.Initially, the pH, which was neutral in NaCl and alkaline in NaOH, allowed water reduction at theve terminal, producing hydrogen gas and hydroxyl ions (OH À ).However, chloride anions (Cl À ) from the NaCl and hydroxyl anions from the NaOH resulted in different reactions at the + ve electrode.
Table 4 lists the electrochemistry data for the relevant reactions in the NaCl and NaOH solutions.The difference between the individual recorded voltages (Figures 6b, d and 7b) and standard potentials is the minimum voltage required to cover the electrolytic reaction and the activation potential (overpotential).The measured voltage at each terminal represents three components: (1) the equilibrium potentials of the half-cell reactions; (2) the ohmic resistance of the solution; and (3) the activation potential of the reactions at the electrode.The two half-reactions have fixed thermochemical electromotive force values.The ohmic voltage loss would be minor because NaCl and NaOH are conductive solutions.HER at the -ve electrode involved water reduction in both solutions, which depends on the pH of the solution.The increasing pH in the NaCl during electrolysis, from pH 7 to 11 over 40 min, suggests the dominance of hydroxyl ion (OH À ) production at the -ve electrode.Hydrogen gas evolved together with the OH -production.The pH then dropped to 9 in the next 50 min, indicating an increased H + ion concentration due to water oxidation at the + ve electrode.
An oxygen evolution reaction (OER) occurred in both solutions at the + ve electrode, but the NaCl solution generated chlorine in the initial stage (supplementary Table S-I).The shape of the current profile in Fig. 6a shows that two oxidation reactions occurred.A semi-quantitative analysis of the free chlorine in the solution was undertaken using a pool water test strip from Aquacheck.Supplementary Table S-I shows that the first test (13 min after the start of the experiment) gave a brown shade to the KI-wetted paper that corresponded to 20 ppm of free chlorine.The light brown color in the second test with the KI-wetted paper (after 67 min) represented 5-10 ppm of free chlorine.These results demonstrate that the decreasing battery voltage restricted chlorine generation.
In the NaOH solution and using an electrode made from Pt or battery-casing material, the primary reaction at the + ve terminal is hydroxyl ion oxidation (Table 4).The presence of gas bubbles at the + ve electrode is evidence that the process of OER took place at the electrode in the NaOH solution.
The decreasing voltage after the batteries were connected to either solution through wires shows that the electrolysis reactions in both solutions consumed the batteries' charges.Gas evolution at both electrodes was evidence of the charges' consumption.The steady voltage corresponding to the rest potential at the end of the process (Fig. 6b and  d) indicates that the battery cannot deliver a charge

CONCLUSIONS
The results of this study indicate that the battery discharging process in both the NaCl and NaOH solutions involved electrolysis, and that, when the battery was submerged in either solution, a corrosion reaction was also involved which occurred simultaneously with the water electrolysis.Corrosion reactions in both NaCl and NaOH solutions during the immersion process resulted in different impacts on the battery's terminals.In NaCl, chlorine was generated, the + ve battery terminal was severely corroded, creating a hollow section and generating ferrihydrite sediments in the solution.The corroded opening led to hydroxide infiltration attacking the Al content of the CID system, which producing Al(OH) 3 precipitates.The infiltrated hydroxide also likely damaged the safety vent and freed the decomposed electrolyte to the solution, in which high fluoride ions built up.The lack of electrolyte in the battery affected the electron transport between the + ve and the -ve terminals.Thus, within minutes, the voltage measurement of discharged battery in NaCl solution is not reliable after the corrosion product generation.In NaOH, a different corrosion process selectively attacked the Alunder the + ve terminal, which was likely because of a broken seal gasket.In addition, chloride impurities listed in the chemical specifications for the NaOH pellet likely led to corrosion and the production of colloidal iron particles, which were then deposited as an oxide film on the -ve terminal.However, no secondary sediments were produced.The thin film production likely consumed the residual battery's charges, resulting in a desired residual capacity (0-25 mAH).The NaOH solution was also not reactive to the battery-casing material.The molar ratio of fluoride and chloride (> 2) in the NaOH solution inhibited the chloride migration, which restrained the corrosion and resulted in minor corrosion at the + ve battery terminal.Although organic carbon leakage occurred during battery submersion in both electrolyte solutions, this did not result in corrosion by-products and did not appear to impede battery discharging.
The discharging time of the cylindrical batteries varied significantly between new and spent batteries because it depends on the battery's charge at the start of the process.However, the tests show that battery immersion in NaOH solution can fully discharge spent batteries after between 1 and 10 h, depending on the initial charge.This result means that the discharged batteries are safe to be further processed for recycling.It is recommended to have a further study on the discharging time for a bulk battery discharging application.Quantitative measurement of gas collection during battery submersion and electrochemical data collection related to corrosion are suggested for future studies to thoroughly understand the corrosion mechanism.

Fig. 1 .
Fig. 1.The electrical circuit used for battery discharging (a) by submerging the battery and (b) by submergiung Pt electrodes or stainless-steel battery-casing electrodes connected to the battery terminals.

Fig. 2 .
Fig. 2. Analysis of battery condition based on voltage and capacity of a new battery (a, d), a spent battery 1 (b, e), and a spent battery 2 (c, f) using a battery analyzer.The order of the capacity reading is 1 discharging, 2 Charging, and 3 rest.

Fig. 3 .
Fig. 3. (a) Visual observations on the discharging process in NaCl and NaOH solutions over time; (b) XRD pattern of filtered sediment in the NaCl solution (Color figure online).

Fig. 4 .
Fig. 4. Voltage profiles and individual voltages at both terminals of a new battery (a, b) and a spent battery (c, d) submerged in NaOH solution.

Fig. 5 .
Fig. 5. Recorded discharged capacity from a discharged new battery by applying a fixed current of 0.3 mA.

Fig. 6 .
Fig. 6.Profiles of current and total voltage for a spent battery with Pt wires connected to the terminals submerged in (a) NaCl solution and (c) NaOH solution, and individual voltages at both terminals for the tests in (b) NaCl solution and (d) NaOH solution.

Fig. 7 .
Fig. 7. (a) Profiles of total voltage and current of a spent battery connected to an electrode made of casing material submerged in the NaOH solution, and (b) individual voltages at both terminals of a spent battery connected to casing material submerged in the NaOH solution.

Table 1 .
The battery capacity of new and spent batteries

Table 2 .
The composition of the supernatants from the NaCl and NaOH tests and the sediment from the NaCl tests after battery discharging by submersion

Table 3 .
The visual appearance of the battery after the submersion discharging process (Color figure online)

Table 4 .
Comparison of electrolysis reactions during battery discharging using NaCl and NaOH, with the corresponding standard potentials (E°(V)) Based on the total standard potential, both solutions could discharge the battery to a similar rest potential of around 1.23 V.This means that both solutions can discharge batteries, but other factors, such as toxic gas generation from NaCl solution, most likely overlook its advantage (reasonable price).