Overcharge to Remove Cathode Passivation Layer for Reviving Failed Li–O 2 Batteries

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE22 Apr 2022Overcharge to Remove Cathode Passivation Layer for Reviving Failed Li–O2 Batteries Kai Chen, Dong-Yue Yang, Jin Wang, Gang Huang and Xin-Bo Zhang Kai Chen State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 School of Applied Chemistry and Engineering, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Dong-Yue Yang State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 School of Applied Chemistry and Engineering, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Jin Wang State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 Google Scholar More articles by this author , Gang Huang State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 School of Applied Chemistry and Engineering, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author and Xin-Bo Zhang *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 School of Applied Chemistry and Engineering, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202201876 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Prolonging the lifetime of batteries is a long-term pursuit, and it is also one of the prerequisites for the practical application of batteries. However, this endeavor is challenging for high-energy Li–O2 batteries due to their poor charge efficiency and cathode passivation-induced by-products accumulation. Here, we demonstrated that overcharging Li–O2 batteries could facilitate the decomposition of accumulated residue products and revive the cathode; thus, the battery lifespan could be significantly extended. This long battery lifetime not only made full use of the Li anode but also enabled the battery to recycle in a safer way without the risk of firing and explosion. Furthermore, overcharge could be used in Li–O2 batteries with high mass loading, high rate, and large capacity. This overcharge strategy simplified the cathode regenerating procedures and realized system-level efficient use of battery components, thereby prolonging the life of Li–O2 batteries to meet the requirements of practical applications. Download figure Download PowerPoint Introduction With a theoretical energy density 10 times higher than current Li–ion batteries (LIBs), Li–O2 batteries have received intensive research in the past two decades. A typical Li–O2 battery consists of a Li anode, an organic electrolyte, and a porous cathode and follows the electrochemical reaction of 2Li+ + O2 + 2e− ↔ Li2O2 with the formation and decomposition of Li2O2 during the discharge and charge processes. Though the reaction looks simple, several parasitic reactions occur during battery cycling, making the actual situation of the battery extremely complicated. Especially, the existence of superoxide intermediates, like O2− and LiO2, during battery operation is aggressive toward both the cathodes and electrolytes. Moreover, the discharge product, Li2O2, is not completely compatible with the cathode and electrolyte, inducing carbonate films formation at each contact interface.1 The situation is even worse considering the recently demonstrated singlet oxygen (1O2), which renders severe electrolyte decomposition.2 All these parasitic reactions generate hardly decomposed insulating side products on the cathode. The continuous accumulation of the side products, as cycling goes on, buries the active sites and blocks electron transfer, preventing the proceedings of the electrochemical reactions. Besides the cathode, the Li anode endures grievous corrosion by H2O and intermediates with the generation of a thick but porous LiOH layer. The degradation of the cathode and anode together limits the lifetime of Li–O2 batteries, typically <300 cycles, which is far from meeting the requirements of practical applications. Thus, developing methods to prolong the life of Li–O2 batteries is paramount for bringing Li–O2 batteries to application. Traditional methods to extend the life of Li–O2 batteries include cathode material engineering,3–5 electrolyte design,6–11 anode optimization,12,13 and interphase tuning.14–17 However, the performance of Li–O2 batteries is still far from satisfactory. In recent years, numerous conceptual works have been conducted to further boost the life of Li–O2 batteries, like changing the failed battery with a new cathode and regenerating the degraded cathode by acid wash and high-temperature pulse annealing.18,19 Although effective, these replacement and regeneration strategies involve tedious battery disassembly and re-assembly procedures, which need to be conducted by professionals. In application scenarios, detailed processes, like battery collecting, transferring, reassembling, and quality checks must be done before usable batteries could be realized, greatly increasing the time and cost, which might not be more economical than assembling new batteries (Figure 1a). Therefore, we investigated whether the cathode could be revived in situ without battery disassembly to aid in extending the life of Li–O2 batteries while reducing the cost. Since the side products that induce cathode degeneration mainly originate from the poor charge efficiency of Li–O2 batteries, we considered compensating for the low efficiency by introducing extra charge capacity. Figure 1 | Comparison of traditional regenerating method and overcharge method for Li–O2 batteries. (a) Two kinds of battery regeneration procedures. (b) Schematic illustration of overcharge method. (c) The comparison of properties of residue Li and LiOH in failed Li–O2 batteries. Download figure Download PowerPoint For LIBs, overcharge should be avoided due to the consequent negative impacts, including Li deposition on the anode, electrolyte decomposition, cathode structure changes, and capacity fading.20,21 More seriously, overcharge could lead to gas generation and heat accumulation in the closed system of LIBs.21,22 Consequently, swelling, rupture, thermal runaway, firing, and even explosion could happen, which pose serious threats to human and device safety. On the contrary, the semi-open nature of the Li–O2 battery made it possible to withstand the overcharge since the gases generated from side reactions could be liberated, and the flowing gas could eliminate heat and prevent its accumulation. Therefore, the safety of Li–O2 batteries could be ensured under abusive conditions, including nail penetration, crushing, and high-temperature tests.23,24 In addition, the semi-open system of Li–O2 batteries makes electrolyte addition easier than the closed system of LIBs or Li–metal batteries; thus, we could supplement fresh electrolytes to avoid degradation (capacity fading and battery failure) of Li–O2 batteries.25 Hence, the unique structure of the Li–O2 battery enabled it to endure overcharge in a secure manner and prevented the electrolyte from drying up. Herein, with Li–O2 battery as an object for detailed research, we demonstrate that overcharging metal–O2 batteries could prolong their lifetime significantly. The excess amount of Li anode used in the battery makes the culprit of the initial battery failure come from the cathode passivation, induced by continuous side products accumulation and poor charge efficiency. We have identified the main side product is Li2CO3 and confirmed its accumulation experimentally. These accumulated side products could be removed through conducting a simple overcharge step, leaving the buried active sites and blocked electron transfer pathways recovered for further proceedings of the electrochemical reactions; thus, the cathode life of Li–O2 batteries was greatly extended to 1316 cycles. When CO2 was introduced into the O2 reaction gas to protect the Li anode, the lifespan was further prolonged, reaching an ultrahigh value of 2714 cycles. Moreover, we checked the application of overcharge in Li–O2 batteries with high-mass loading cathodes [5 mg cm−2 Ru/CNT (Ru/CNT = ruthenium–carbon nanotube)] and still achieved 900 cycles at a high rate of 1 mA cm−2 and a capacity of 0.2 mAh cm−2. Furthermore, Li–O2 batteries with full discharge-charge suffer quick capacity fading; our overcharge strategy could obviously improve capacity retention. Importantly, this overcharge method is also applicable to other metal–O2 batteries such as Na–O2 and K–O2 batteries. The excellent performance improvement enabled by overcharge redefines its functions in batteries by prolonging the life of metal–O2 batteries in an easy, time-saving, and cost-effective way. Experimental Methods Cathode fabrication CNTs and polyvinylidene difluoride (PVDF) binder material were mixed at a mass ratio of 9:1 in N-methyl-2-pyrrolidone to form a homogeneous slurry by a thorough grounding in a mortar. The slurry was then sprayed on carbon paper, followed by overnight drying in a vacuum oven. After this, the dried carbon paper was punched into round pieces (12 mm in diameter) to create cathodes. Several batches of cathodes were prepared with mass loading ranging from 0.1 to 0.3 mg per cathode. The specific capacity was calculated based on the mass of active materials. For the Li–O2 batteries with a high mass loading (5 mg cm−2) on the cathode (Ru/CNT), the cathode was used as received. Battery assembly The batteries were assembled based on ECC-Air models (EL-cell GmbH, Hamburg, Germany) that could flow gas during battery operation. The lithium plate (diameter: 14 mm) was placed at the bottom, followed by the stacking of the glass fiber separator (thickness, 300 μm, diameter, 18 mm) and the prepared cathode (thickness, 200 μm) to obtain a fabricated battery of ∼900 μm thickness. The amount of the electrolyte used in the battery was 130 μL. The battery was sealed in the model with a spring. The 900 μm thickness battery had a spring load of 15 N, with a calculated pressure of 1.33 × 105 Pa; the flowing gas was 2 mL/min, and the gas pressure was ∼1 atm. After assembly, all prepared batteries were transferred to a thermotank (30 °C) and allowed to rest for at least 3 h to ensure a pure gas environment before cycling. Operando pressure test The quantitative pressure test during battery cycling was based on an ECC-Press system (EL-Cell GmbH, Hamburg, Germany) connected with a VMP-300 electrochemical workstation (Biologic, Seyssinet-Pariset, France). Before a test, the free volume of the battery system was determined by Boyle’s law. Then the system was purged with pure O2 to remove the residue gas to ensure a pure O2 environment. After that, the system was sealed quickly to avoid gas permeation. This test was conducted in a thermotank to minimize temperature fluctuation so that a smooth curve could be achieved. Characterizations Before characterization procedures, the cathodes were washed three times with acetonitrile to remove residual electrolytes, while the Li anodes were washed with 1,3-dioxolane (DOL) to remove residual glass fibers and electrolytes on their surface. Both the battery disassembling and electrode washing procedures were conducted in a glove box. After washing, the electrodes were dried in the air-lock chamber and then stored in the glove box. When testing the morphology of the electrodes, they were transferred to a sealed plastic bag to minimize air exposure. A scanning electron microscopy (SEM) test was conducted on a field emission Hitachi S4800 scanning electron microscope (Tokyo, Japan), and the X-ray diffraction (XRD) characterization was performed on a MiniFlex 600 X-ray diffractometer (Rigaku, Tokyo, Japan). Fourier transform infrared (FTIR) measurement was performed on a Nicolet 6700 spectrometer (Waltham, Massachusetts, United States). Battery cycling tests were conducted on a multi-channel battery testing system CT2100A (Wuhan, Hubei, China). Electrochemical impedance spectra (EIS) were performed on a VMP-300 electrochemical workstation. Results and Discussion The benefits of overcharge in Li–O2 batteries Figure 1b depicts the proposal of cathode regeneration by overcharge. A typical Li–O2 battery consists of a fresh Li anode and a carbon cathode. During battery cycling, the parasitic reactions and the poor rechargeability result in side products accumulation on the cathode. When the cathode is passivated to some extent, the discharge voltage will plummet to lower than 2.0 V, a sign of battery failure.4,26 The failed battery is considered useless in most previous investigations. However, at this stage, the anode is not fully utilized because of its excess amount. Disposing of the failed battery leads to safety problems owing to the active nature of the contained lithium and results in resource waste. Therefore, the failed batteries should be recycled to meet the demand for a sustainable and green society. Nevertheless, the traditional procedures for battery recycling are complicated, time-consuming, and costly (Figure 1a), thereby increasing the price of the renewed battery. In contrast, the battery regenerated by a simple overcharge process can prolong its lifespan and maintain a low cost simultaneously. As shown in Figure 1b, after overcharge, the passivation layer on the cathode can be removed, thus, providing free space for the deposition of the following discharge products and enabling the battery to cycle again. This overcharge strategy can be used many times until the exhaustion of active Li, which brings another benefit. If Li residue remains in the battery, risks of firing and explosion exist during reservation and transportation because of its active nature. However, LiOH is much safer and can be easily recycled without considering Li processing (Figure 1c). At this stage, the battery components realize system-level efficient use, and the failed battery is safer; therefore, easy to collect and recycle. Cathode passivation and overcharge to remove the passivation layer To confirm the assumptions above, we first verified that the cathode passivation resulted in the initial battery failure rather than the Li anode. As shown in Figure 2a, the newly assembled Li–O2 battery could run for 100 cycles at a current density of 44.2 μA cm−2 and a fixed capacity of 44.2 μAh cm−2. After battery failure (discharge voltage dropped to 2.0 V), the battery was still wetted by the electrolyte. Then the cathode and anode were decoupled and paired with a fresh anode and cathode, respectively, to assemble new batteries and check their viability. From Figure 2b, it is apparent that the battery with the new cathode is able to run for 10 cycles without a drop in the discharge voltage, indicating good cooperation of the new cathode and the remnant of active Li in the old anode. However, even in the first cycle, the battery with the used cathode and fresh Li plate could hardly work, displaying a steep discharge voltage drop to 1.0 V (Figure 2c). As expected, overcharging the battery at 17.7 μA cm−2 for 10 h enabled it to deliver a higher discharge voltage in the subsequent cycles even though the voltage decreased gradually. This performance recovery was attributed to the overcharge-induced decomposition of side products on the cathode. Furthermore, the status of the cathode and anode in the initially failed battery was checked using XRD. The XRD patterns of the Li anode showed strong peaks of LiOH, but Li peaks still existed (Figure 2d). The relatively weak signals for Li were due to the upper thick LiOH shielding layer (∼131 μm; Supporting Information Figure S1). This meant that only 30% active Li was consumed after the first battery failure, and the residue Li could be a safety threat if not handled properly during battery recycling procedures. For the cathode, the pristine CNTs with a clean surface were fully covered by thick side products ( Supporting Information Figure S2), which were in amorphous states since there were no characteristic peaks from Li2O2, Li2CO3, or other species (Figure 2e). These results unambiguously aided in identifying that the cause of the initial battery failure originated from the cathode passivation, supporting the assumption of reviving the failed battery by following the procedures in Figure 1b. Figure 2 | Identification of the failure reason for the Li–O2 battery. (a) Cycling performance of a Li–O2 battery at 500 mA g−1. (b) The newly assembled battery with the used anode and a new cathode. (c) The newly assembled battery with the used cathode and a new anode. (d) XRD pattern of the anode from the failed battery in (a). (e) XRD patterns of the pristine and failed cathodes from the battery in (a). Download figure Download PowerPoint So the question is, “what causes the side products accumulation on the cathode?” To check the reason, the cathode morphologies were examined after discharge and charge at 0.05 mA for 0.1, 0.2, 0.4, and 0.8 mAh ( Supporting Information Figure S3). When the fixed capacity was small (0.1 or 0.2 mAh), the cathode structure could be recovered, with most products removed after the subsequent recharge process ( Supporting Information Figures S3a and S3b). Even though the undecomposed product was negligible in these small capacities, it was a challenging problem in high-capacity cycling. When the capacity was increased to 0.4 mAh, a passivation layer was observed on the cathode after the charge ( Supporting Information Figure S3c). Further, enlarging the capacity to 0.8 mAh, we observed that, except for the passivated film, particle agglomeration emerged ( Supporting Information Figure S3d), indicating that the amount of side products increased with the enlargement of the discharge capacity. The poor charge efficiency of Li–O2 batteries is also reflected in the discharge and charge profiles. In Supporting Information Figure S4a, when the charge capacity reached the discharge value (0.39 mAh), the voltage began to experience a slight increase, followed by a quick rise at 0.45 mAh due to the decomposition of the electrolyte. However, for the battery with a much higher discharge capacity of 2.74 mAh, almost no voltage change occurred when charged at the same capacity ( Supporting Information Figure S4b); the sudden death at 2.74 mAh was caused by the low electron and Li+ conductivity of large-sized Li2O2. Even at a 3.22 mAh charge, only a weak voltage increase emerged, implying that the reactions between 2.74 and 3.22 mAh were not significantly different from those before 2.74 mAh. This meant that cycling the battery at capacity-limited mode, and high discharge depth led to more undecomposed products accumulation on the cathode, and the poor charge efficiency was amplified. As cycling continued, the constant accumulation of the undecomposed product would passivate the cathode completely and cause battery death. The morphology and composition evolution of the undecomposed product after different cycles were then characterized. After the 10th discharge, the discharge products covered the CNT cathode (Figure 3a), and their composition was confirmed by FTIR spectra with the existence of Li2O2 (500–600 cm−1) and Li2CO3 (880 and 1400–1520 cm−1) ( Supporting Information Figure S5). Although most of the discharge products decomposed and a clear CNT structure was observed during the subsequent charge process (Figure 3b), some undecomposed Li2O2 remained, and Li2CO3 connected the CNTs. After 50 cycles, more products were deposited on the discharged cathode (Figure 3c), almost exhibiting no decomposition after the charging process (Figure 3d), and was accompanied by peaks from Li2CO3, Li2O2, HCOOLi, and CH3COOLi ( Supporting Information Figure S5). The results revealed continuous accumulation of these products on the cathode. After confirming the composition of the undecomposed products, we then checked if the reactions were occurring during the overcharge process by monitoring the pressure of the battery to quantify the change of gas. Supporting Information Figure S6a shows the cycling performance of a Li–O2 battery at 0.1 mA with a fixed capacity of 0.4 mAh. At the 17th cycle, the discharge voltage dropped to 2.0 V, and then the battery was charged for 12 h at 0.1 mA. The pressure change of the battery at this cycle is shown in Supporting Information Figure S6b. The discharge process showed a 2.09 e/gas reaction. Considering O2 is the main component of the reaction gas; theoretically, 2.0 e/O2 corresponded to the formation of Li2O2 during discharge; thus, the 2.09 e/gas reaction suggested that Li2O2 was the dominant discharge product. However, during the overcharge process, the coefficient was much higher than 2.0, reaching 4.01 e/gas. This indicated that generating a gas molecule needed to consume more electrons on average, suggesting that severe side reactions had occurred. These reactions might include the decomposition of previously accumulated products (Li2CO3/HCOOLi/CH3COOLi), generation of aggressive 1O2 and superoxide, and degradation of electrolytes induced by the high voltage (>4.5 V). Importantly, the coefficient value was constant during the entire overcharge process, suggesting similar electrochemical reactions occurred continuously. After overcharge, the battery typically worked with a discharge plateau around 2.6 V ( Supporting Information Figure S6b). To identify the gas evolution species, we conducted differential electrochemical mass spectrometry (DEMS) test during the charge process ( Supporting Information Figure S7). At the initial 4 h of charge, the dominant evolved gas was O2 from Li2O2 decomposition, while a proportion of CO2 was also generated at the end of charge (3.5–4 h). However, the amount of CO2 was very limited compared with the generated O2, not to mention the O2 in the entire headspace. Therefore, there was only a slight deviation from the theoretical value during the discharge process. In addition, weak signals with mass to charge ratios (m/z) of 29 and 15 appeared after 1 h of charge and were sustained until the end of overcharge, indicating slight side reactions involving electrolyte decomposition, even though the electrolyte was stable up to 4.6 V ( Supporting Information Figure S7c). During the overcharge process (>4 h), the amount of CO2 increased significantly because of the decomposition of residues (Li2CO3/HCOOLi/CH3COOLi) and the occurrence of other side reactions mentioned above. This confirmed that overcharge could remove the residual products on the cathode to revive it for further cycling. Figure 3 | Accumulation of discharge products on cathodes. Morphologies of cathodes after (a) 10th discharge, (b) 10th charge, (c) 50th discharge, and (d) 50th charge. Scale bar, 2 μm. (e) Quantification of the products accumulated on cycled cathodes using the acid-base titration method. (f) EIS of Li–O2 battery at different stages. Download figure Download PowerPoint Since we demonstrated that the initial battery failure was caused by the cathode passivation, we inferred that the battery revival by overcharge could be attributed to the removal of the accumulated products on the cathode, which was visualized, as shown in the Supporting Information Figure S8. After 15 cycles, the cathode was covered by a passivation layer ( Supporting Information Figure S8a). It was clear that overcharging the battery for 4 h could remove most of the previously deposited products, leaving only minor aggregates ( Supporting Information Figure S8b). When the overcharge time was extended to 8 h, the deposited products were utterly removed ( Supporting Information Figure S8c), recovering the cathode to its fresh state, with exposed active sites and ample free space. To quantify the deposited products, we conducted acid-base titration (see Methods section). After 15 cycles, the residues coated on CNT ( Supporting Information Figure S8a) were titrated to be 33 μmol. With 4 h of overcharge, the amount of residual products reduced from 33 μmol to 18 μmol, which corresponded to the vanishment of the passivation layer in Supporting Information Figure S8b. In addition, after 4 h of overcharge, only minor particles were detected; thus, another 4 h of overcharge did not reduce the residues totally(from 18 μmol to 16 μmol, Supporting Information Figure S8d); there were still 16 μmol of residues left even though the surface of CNT was clear. We presumed that these residues came from the surface passivation of the CNT structure due to its relatively large surface area (>200 m2/g). Thanks to the overcharge function of removing the passivation layer, the impedance of the battery decreased with a longer overcharge time ( Supporting Information Figure S9). The above discussions have verified that the gradual increase of the passivation products with prolonged cycling number was due to the negative impact of the poor charge efficiency of Li–O2 batteries. Hence, we quantified the amount of the accumulated products after different cycles. As shown in Figure 3e, the Li-containing products increased as the cycling continued, although the increasing trend was not linear. This confirmed the accumulation of residual products on the cathodes during cycling. The EIS in Figure 3f also showed a higher impedance at the 10th cycle than at the 1st cycle. When the battery failed at the 85th cycle, the impedance increased further and was much higher than the initial value owing to the significant amount of undecomposed products accumulation on the cathode. However, after overcharging for 10 h, the impedance recovered to a normal value, enabling the battery to continue cycling for another 14 cycles. Notably, the strategy of reviving the battery by overcharge could be repeated several times until the active Li was used up. Overcharge to realize long-life Li–O2 batteries Following the demonstration that overcharge could indeed remove undecomposed products on the cathode and revive the battery for further cycling, the long cycling performance of Li–O2 battery with overcharge deserved testing to validate its advantages fully. As shown in Figures 4a and 4b, the Li–O2 battery failed after 180 cycles, and a 4-h overcharge enabled it to work normally for extra 10 cycles before the voltage dropped to 2 V at 387 h (Figure 4b). Every time the battery failed, an overcharge was conducted, and the process could be repeated many times. It is worth mentioning that for a Li–O2 battery, the electrolyte was constantly consumed during the cycling process because of side reactions and volatilization. Adding electrolyte to the battery could no doubt help extend the battery life as well, and this was possible due to the semi-open structure of the Li–O2 battery that could be achieved readily without the requirement of battery disassembly like LIBs. Therefore, when an overcharge could not revive a battery, about 150 μL of electrolyte was added (Figure 4c) to check the cathode limit after overcharging for regeneration, which is not common in practical scenarios. After resting 2 h for electrolyte wetting, the battery could run again, and the discharge voltage plateau recovered. In the following cycles, 4-h-overcharge was adopted when the battery failed. After 624 cycles (1320 h), the anode was exhausted (Figure 4f), then a new anode was utilized to couple with the original cathode to reassemble the battery, with added electrolyte to wet the anode. Surprisingly, this battery could keep running, sustaining a stable discharge plateau (Figure 4d). The overcharge, electrolyte addition (twice), together with the refreshment of the Li anode finally enabled the Li–O2 battery to deliver a long life of 1316 cycles (Figure 4e). Knowing that the cathode could be regenerated continuously by overcharge and easy addition of electrolyte to the battery, the life of the Li–O2 battery could only be limited by the durability