Abstract—The Lithium-Ion battery at the end of life represents a valuable source of secondary raw materials such as lithium, nickel, cobalt etc. Deep discharging, as a part of battery recycling, is a time-consuming process in which the battery’s thermal dependency on the discharge parameters and voltage recovery effect is manifested. Adjusting the discharging process adequately to address those two phenomena leads to a safety increase and discharging time decrease. This paper treats two aspects. It is observed the effect of the constant and variable discharging current along with the depth of the discharge in the form of discharge end voltage parameter on the maximal cell temperature reached during the process. The second aspect is the battery recovery voltage trend after the discharging process and its dependency on the same parameters. The impact of these parameters is demonstrated experimentally on two battery cell types.

Keywords—Li-ion battery, battery discharge, voltage recovery, voltage rebound, recycling, battery deep discharge

                                                                                                                Introduction

Several factors have been catalyzing the surge of the tremendous popularity, use and application of the Lithium-Ion (Li-Ion) batteries. Firstly, Li-Ion batteries are characterized by high energy density accompanied by a fast response. This fact placed this battery type on the pedestal of various industrial applications. One of the highest demand-rising trends is in the automotive industry. This is evident from the global market of Li-Ion batteries where automotive Li-Ion battery demand increased by approximately 65% to 550 GWh in 2022, from 330 GWh in 2021 [1]. Following this trend, it is predicted that the total number of electric vehicles (EVs) on the roads may exceed 300 million globally by 2030, compared to the 10 million EVs on the roads globally by 2020 [2].

The history of the Li-Ion battery can be traced back to the early 1970s when a team of scientists at Exxon were developing a new type of battery that would be more efficient and safer than the lead-acid batteries that were commonly used. Even the fact that the Nobel Prize for Chemistry in 2019 for the innovative development of the Li-Ion technology has been awarded to John B. Goodenough, Akira Yoshino and M. Stanley Whittingham, speaks for itself. Thus, Li-Ion batteries besides the application for EVs can be found in many other niches, even in satellite applications [3]. Due to the upsurge in Li-Ion battery utilization, especially concerning EVs, it is estimated that in Europe alone, 1.16 million batteries will be at their end of life by 2023 [4]. Additionally, it is reported a total of 65000 tons of Li-Ion batteries were consumed between the year 2013-2014.

However, only about 1900 tons were recycled at the same time [4].  Materials such as lithium, cobalt and nickel are critical to the operation of Li-Ion batteries, but they are also relatively scarce [5]. This implies the imperative of safe, reliable and efficient recycling methods and approaches The recycling process consists of several challenging stages. One of the stages is the deep discharging of the batteries considering the mechanical recycling approach [6]. The deep discharging begins at 2.5 V per cell. However, it has been observed that the 2.5 V per cell is too high for the safe crushing stage since the potential between the anode and cathode is high enough to cause thermal runaway. Thus, there is a tendency for the deep discharge to be performed to the range [0,1] V.

During the deep discharge of the battery, special attention should be paid to the cell temperature. The discharge close to 0 V yields the thermal runaway which can potentially lead to fire or explosion of the battery. Also, the thermal effect has a tremendous impact on the reusability of the materials which are recovered in other recycling stages [7]. It has been demonstrated that the optimal temperature range for Li-Ion batteries is from 15°C to 35°C  [8], [9]. Therefore, during the discharge stage, ideally, the battery cells temperature should remain withing these values. Additionally, the discharging stage duration and efficiency are affected by the voltage recovery effect, also known as the battery voltage rebound (BVR)in electrical engineering or voltage relaxation in electrochemistry [10]. Thus the temperature and the rebound effect are two components which should be addressed for the sake of the discharge safety and performance.

A battery converts the chemical energy stored in electrochemical cells to electrical energy [3]. A cell consists of an anode and a cathode separated by an electrolyte. When a load is connected, the anode undergoes oxidation, releasing electrons to the cathode through the external load. These electrons are accepted at the cathode, causing a reduction reaction. In a fully charged state, electroactive species on both electrodes are evenly distributed across the electrode–electrolyte interface. During discharge, species near the electrode are consumed first and replaced by those farther away, a process influenced by the diffusion constant. Prolonged continuous discharge can lead to rapid depletion without complete consumption of all species. Allowing time between discharges permits additional species to participate in redox reactions, extending battery life—a phenomenon known as the recovery effect. Different battery chemistries, shapes, and sizes exhibit varying degrees of recovery due to differences in diffusion rates and distance to the electrode [20]. During the discharging process, the

battery temperature rises with the increase of discharge C-rate due to ohmic heating. This increases the degradation of the Li-Ion batteries [21]. These phenomena highly affect the efficiency of the battery discharge as a part of the battery recycling process.

In the niche of battery recycling deep discharging, it is interesting that very few papers in surveyed literature address this phenomenon. Considering a wider scope and application, some authors investigate this phenomenon for battery modelling purposes [11], [12], [13], phenomena treatment from the safety point of view [14] or as a parameter that should be monitored with a battery management system. However, there is a wide scope of literature addressing this phenomenon from the electrochemical point of view [15]. Additionally, there is a research focus on the comparison of the BVR effect among different battery chemistries [16]. Furthermore, the thermal aspect during the discharging process has been extensively observed in relevant literature. Most of the published papers address the electrochemical perspective of the thermal aspect [17], [7]. On the other hand, authors pursue to investigate the temperature dependency on discharging at the multiple C-rates [18], [19].

The contribution of the method and obtained results presented in this manuscript have the scientific, industrial as well as indirect economic perspective. It has been demonstrated the effect of the constant and variable current discharge process on battery temperature. Additionally, those changes are reflected in the battery recovery voltage after the discharging process. Those dependencies are crucial to providing a safe discharging process for the spent batteries prior to the crushing stage and chemical treatment.

The manuscript consists of IV sections. Section I provides a overview of the research introduction and state-of-the-art. Section II describes the process of discharging and the utilized equipment in the experimental section. The experimental validation and obtained results are presented in Section III. Final remarks are given in Section IV.

                                                                                                                Discharge process

To investigate the dependency of the battery thermal behaviour and BVR on the discharging current, it is crucial to define the discharging mode. At the market, most of the controlled discharging devices perform discharging using constant discharging current, constant resistance or constant power. Each of the stated modes has advantages and disadvantages according to the battery type being discharged. For the constant power mode, the discharging current has the lowest value at the beginning of the discharge and increases as the battery voltage drops in order to maintain constant power output. In the constant resistance mode, the current during the discharge follows the drop in the battery voltage. However, these are not optimal scenarios for the purpose and initial premise of this research. According to the previous, for the experiment, the constant current discharge mode has been chosen. To investigate the impact of the discharging parameters on the battery temperature and BVR, the testing strategy with three different options has been selected. The first discharge test consists of discharging the battery with a constant current to an end voltage value of 0 V. The second test consists of discharging the battery with the constant current to the negative end voltage value. The third test alters the approach of the previous two tests and consists of discharging the

battery with the variable discharge current related to the measured temperature of the battery. For each test, after the discharging phase, the battery is short-circuited followed by the resting period where the BVR has been observed. The experimental battery discharge strategy is shown in Fig. 1.

Fig. 1.   Battery discharging experiment strategy

                                                                                                                Experimental Results

The proposed discharging methods have been validated by performing the discharge of two different cylindric cell types using industrial field testing equipment.

A.    Experimental setup

The test have been conducted on two different battery cells. Both battery cell types are Li-Ion batteries with nickel-rich cathode chemistry. This type of battery chemistry typically offers higher energy density, improved performance and longer lifespan compared to traditional lithium-ion chemistries with cobalt-rich cathode [22]. The ambient temperature during the experiments was 23 °C and the ambiental humidity was 55 %. The specifications of the test objects are given in Table I.

Table I: Test Object Specifications

A specimen of each battery cell type used in the experimental segment is shown in Fig. 2.

As the discharge devices, the battery capacity tester BLU700C and zero voltage discharge module ZVD80 manufactured by the DV Power along with the software suite DV-B Win have been used [23]. The BLU700C device has a maximal discharge current of 260 A and a maximal discharge power of 42 kW. The ZVD80 is a specially designed module enabling total discharge required before recycling.

The BLU700C and ZVD act as a single controlled discharging system. This system is chosen for the experimental segment of the research due to its ability to select discharge with the constant current as well as define the discharge current profiles.

Fig. 2.   Test object type 1 (right) and Test object type 2 (left)

Fig. 3.   Test setup wiring diagram

The wiring diagram is shown in Fig. 3. where the test leads are connected between the test object and the ZVD80 while the BLU700C and ZVD80 are interconnected to act as a single discharging system. The laboratory test setup used for the experiments is shown in Fig. 4.

      The discharge devices are selected for their compact size and ability to perform discharge up to the voltage levels which are less than 0 V. During the discharging process, the following parameters are recorded: cell voltage, discharge current and cell temperature. The experiments are conducted according to the diagram given in Fig. 1. The samples of those parameters are recorded with a sampling time of 1 s. The signal presentation and analysis have been performed with the DV Power DV-B Win software and Matlab R2023 environment.

A total of 10 cells of both Test Object 1 and 2 types have been discharged in the series of experiments. The selected results are presented in the following subsection.

Fig. 4.   The test setup for the battery discharge experiment – battery discharge unit, zero voltage discharge unit and test object

A.    Results – Thermal Effect

In order to demonstrate the effects of the discharge parameters on the BVR effect and the temperature, several experiments have been conducted.

The following parameters during the discharging process have been considered: discharge voltage , discharge current , discharge end voltage , discharging time , short circuit time . It has been observed their impact on the rebound voltage  value reached 30 minutes after disconnecting the short circuit connection. The obtained results of the voltage rebound values are presented in Table II and Table III.

• Rebound Voltage dependency on discharge parameters – Test Object 1

• Rebound Voltage dependency on discharge parameters – Test Object 2

It is evident from the obtained data that for both test objects, the voltage rebound effect is significantly reduced by discharging to 0 V. However, the rebound voltage reduction is even better by discharging the battery to the negative end voltage values. The numerically presented values in previous tables are visualized in the Fig. 5.

Fig. 5.   Dependency of BRV on discharge and short circuit time – Test object 1

If the main objective is to minimize the BVR, the Test 3 group which utilizes battery discharge with the negative end voltage significantly outperform Test 1 and Test 2 groups comparing the BRV of 0.8 V, 5.20 V and 1.70 V, respectively. Even though the number and graph indicate prolonged discharging and short-circuit time, that time extension is justified by the minimization of the BVR.

A.    Results – Thermal Effect

During the testing process, it has been observed how the discharge parameters affect the battery temperature trend. This is crucial since the temperature affects the quality of the black mass which needs to be recovered during the recycling process. Thus, to minimize the temperature and the voltage rebound effect, the experiment has been conducted on the same battery models, observing the maximal battery temperature . The first experiment consists of discharging with the constant current mode. The results are presented in Table IV.

Firstly, batteries have been discharged with the end voltage limit of 0 V (Test 1, Table IV). Depending on the discharging time, the highest temperature reached has been 52.5 °C. The discharging to the negative end voltage parameter (Test 2, Table IV) yields even higher temperatures, up to 67 °C. Similar behaviour has been recorded during the experiment on Test Object 2, as shown in Table V.

• Temperature dependency on discharging parameters – constant current – test object 1

• Temperature dependency on discharging parameters – constant current – test object 2

Evidently, those temperatures are too high to preserve black mass for later exploitation. Since there is a high increase in the temperature when the battery voltage level is in the range [1, 1.4] V, it is a premise that by controlling the discharge current by the temperature value, the temperature values presented in Table IV and Table V might be minimized. The results of the second experiment are presented in Table VI and Table VII.

• Temperature dependency on discharging parameters – variable current – test object 1

• Temperature dependency on discharging parameters – variable current – test object 2

Comparing the results presented in Table IV and Table V with the ones presented in Table VI and Table VII,  it is clear that there is a reduction in the highest battery temperatures reached during the discharging process. In both cases, discharging to 0 V and to the negative end voltage, the maximum temperature reached has been decreased. Considering Test object 1 and comparing the results presented in Table IV and Table VI, the highest temperature value is reduced by approximately 19.5 °C in Test group 1. A similar conclusion is derived by comparing the Test 1 in Tables V and VII. Additionally, it is shown that the discharging time has been reduced by utilizing the variable discharge current approach. The example of the variable current discharge controlled by the temperature recorded

during the discharging process of Test Object 2 is shown in Fig. 6. including the short-circuit time and BVR.

Fig. 6.   Discharing process with variable current including the battery short-circuit time and voltage rebound time – Test object

                                                                                                                 Conclusion

Two phenomena during the battery discharge are covered in this paper: the impact of the discharge current on the thermal behaviour of the battery and the discharge current, discharging duration and the short-circuit duration impact on the BVR. It shows that those manifestations might be well controlled by the discharging parameters in order to provide a faster discharging process as a part of the battery recycling process. However, there is a wide area where the presented research and results might be expanded. As a future work, it should be considered the following:

• Investigate the dependency of the discharging parameters on cell temperature and BVR of different battery cell types,
• Define the coefficient for relating the cell temperature and discharge current during controlled discharge.

To download the .pdf format of this article, please log in and visit the following link. Literature and copyrights are stated in the original document.

July 19, 2024