Corrosion and protection of aluminum current collector in lithium-ion batteries

Aluminum (Al) current collector, an important component of lithium-ion batteries (LIBs), plays a crucial role in affecting electrochemical performance of LIBs. In both working and calendar aging of LIBs, Al suffers from severe corrosion issue, resulting in the decay of electrochemical performance. However, few efforts are devoted to the research of Al compared to anode and cathode materials, electrolyte, and even separators in LIBs. Here, the recent research advance in Al corrosion and protection is reviewed. We first briefly overview Al corrosion mechanism and its affecting factors. Then, the advanced technologies used to evaluate the electrochemical, morphology and chemical properties of Al are summarized in order to uncover the Al corrosion mechanism in LIBs. Next, we review the Al protection strategies in Al, electrolyte, and inhibitors with function mechanism, materials selection and their structural design. Finally, we outlook the future research direction in Al corrosion and protection. This review provides experimental and theoretical supports in understanding Al corrosion and development of Al anticorrosion, which will be beneficial to the research communities including corrosions, advanced materials, and energy storage devices.


INTRODUCTION
In 1991, Sony successfully commercialized lithium-ion batteries (LIBs). 1,24][5][6] LIBs enrich the modern society and improve the life quality of people.John Goodenough, Stanley Whittingham, and Akira Yoshino were awarded the Nobel Prize in 2019 in recognition of their great contributions to the development of LIBs.The advance of the electronic devices requires the high electrochemical performance LIBs, especially the energy density.9][30][31][32] These researches greatly facilitate the development of LIBs.
However, few researchers studied the field of cathode current collector Al in LIBs.During the charging and discharging processes, the electrolyte tends to be oxidized and raises the severe corrosion of Al, especially in the high working potential. 33,34][37] Once the oxide of current collector was damaged, the soluble products including Al 3+ would contaminate electrolyte, affect anode and cathode materials, whereas the non-soluble materials, such as AlF 3 , would induce the increase of resistance of batteries. 38,39What is worse, the corrosion pitting would have a serious impact on conductivity with the availability of Al current collector. 40,41The severe corrosion issue of Al current collector would deteriorate the electrochemical performance of LIBs. 42Therefore, reviewing the recent progress in regard to Al corrosion and its protection in LIBs becomes necessary and important.Herein, we propose the review briefly summarized the progress of Al current collector in four aspects, including overview of Al corrosion, failure analysis, protection strategy, and summary and perspective.We expect the reviewer would be beneficial to the development of advanced LIBs.

OVERVIEW OF AL CORROSION Corrosion in LIBs
Corrosion is a way that metal reacts with species in the environment through chemical or electrochemical reactions.In corrosion process, the key factors are metals and its environment.In LIBs, current collectors Al and Cu are the metals, and the electrolyte is the environment for cathode, anode, and current collectors.The corrosion is involved in the battery working processes and calendar aging.It includes the Al corrosion with or without external current/voltage, the cathode electrolyte interphase (CEI) with applied current/voltage, solid electrolyte interphase (SEI) with or without external current/voltage, and Cu corrosion with Li metal by galvanic corrosion. 43,44mong these corrosion processes, Al corrosion plays an important role in affecting the electrochemical performance of LIBs.Therefore, it is necessary to study Al corrosion in detail.
Al corrosion was influenced by many factors.Figure 1 illustrates the three major factors of Al, electrolyte, and environment in Al corrosion of LIBs.For Al, its surface covered by one dense oxide passive film of Al 2 O 3 , which can endure higher working potential than Al (1.4 V vs Li/Li + ). 40Once the working potential is over 3.8 V, Al 2 O 3 was decomposed and the bared Al metal was exposed to the electrolyte. 35,45The side reactions between Al and electrolyte would happen, resulting in the corrosion.The purity of Al is also crucial in the corrosion processes.The impurities, such as Fe and Si, would form intermetallic phase with Al to affect the corrosion rate. 46Hence, the Al current collector in LIBs is required to meet the high purity.Beyond the passive Al 2 O 3 layer and high purity of Al, the alloying and structure design of Al are effective ways to suppress corrosion behaviors.
For electrolyte, the different lithium salts (LiPF 6 or lithium bis(trifluoromethanesulfonyl) imide LiTFSI) and solvents (carbonates and ethers) strongly affect the corrosion properties due to the formation of passivation layer with different composition.They also affect the pH value of the electrolyte, resulting in the side reactions with Al. 47 The additives in electrolyte play an important effect on the corrosion of Al.For environment, the temperature is an important factor in affecting Al corrosion.With the increase of the temperature, the side reactions of electrolyte and Al were facilitated.The cutoff voltage was another factor that influences the Al corrosion.At high voltage, the electrolytes themselves are unstable, both their solvents and lithium salts are decomposed.In LiPF 6 -based electrolytes, the high potential facilitates the formation of HF. 48,49 HF would react with electrode materials, current collectors, and electrolytes, deteriorating the electrochemical performance of batteries.For Al current collectors, high-voltage corrosion would increase interface impedance and even detached electrode materials from Al due to the increased corrosion passivation layer and the corrosion pits.The higher the cut-off voltage is, the severer the Al corrosion is.

Mechanism of Al corrosion
The Al corrosion includes both chemical and electrochemical corrosions which depend on the external current/voltage in LIBs. 44However, two reactions occur simultaneously make it difficult to distinguish them.Besides, the Al corrosion in different electrolytes shows different reaction mechanisms.LiPF 6 in carbonate solvent (such as 1 M LiPF 6 in ethylene carbonate (EC) and diethyl carbonate (DEC)) is the most successful electrolyte in commercial market.The Al corrosion mechanism in this electrolyte is exhibited in Figure 2A.9][50] HF not only damages the electrode materials, but also produces severe corrosion on Al. 51 In the corrosion process, HF reacts with Al 2 O 3 layer on Al to form AlF 3 as shown in Eq. 3. Once Al 2 O 3 was completely consumed, it still reacts with Al and also forms AlF 3 (Eq.4).AlF 3 forms a stable passive film on Al, preventing further corrosion, but increases the internal resistance of batteries, sacrificing their electrochemical performance. 39PF 6 → LiF + PF 5 (1) In LiTFSI electrolyte (such as 1 M LiTFSI in 1,3-dioxolane (DOL) and dimethoxy ethane (DME), there is no HF issue. 52,53However, the anions diffuse in the bulk electrolyte, and migrate between cathode and anode.When the anions arrive at Al surface, the anions react with Al 2 O 3 to form soluble byproducts when the working potential is over 3.5 V (Eq.5). 38Once Al 2 O 3 is completely destroyed, the uncovered bare Al would react with anions to form soluble complex ion in electrolyte (Eq.6).In this corrosion process, there is no valid way to form passivation layer. 33,54 Beyond LiPF 6 and LiTFSI, other lithium salts can also cause corrosion issues.LiFSI is expected to replace LiPF 6 as the next generation of commercial electrolytes. 55However, it corrodes Al, especially over 3.3 V, to form soluble byproducts rather than passive layer on Al. 56 Increasing LiFSI concentration can broaden the voltage range and form passive layer, such as LiF, on Al.
For LiBOB salt, it can react with Al to form passive layers with AlB 2 O 3 and/or B 2 O 3 . 57However, it is only used as additive in electrolyte to prevent Al corrosion due to its low ionic conductivity.For Al corrosion with these salts, LiPF 6 can generate passive layer (AlF 3 ) covered on Al with a high cut-off voltage of 4.3 V, whereas the corrosion product of LiTFSI is soluble and can no longer protect Al with a low cut-off voltage of 3.8 V. Normally, LiBOB is chosen as additives in electrolytes and LiFSI is used to prepare high concentration electrolyte (4 M) to broaden the working voltage, preventing the Al corrosion. 58owever, none of them can achieve the stable operation voltage over 5 V for Al current collector without corrosion.Besides, the calendar aging of batteries and the moisture from electrolyte, binders, electrodes, and current collectors in LIBs also influence the Al corrosion.All these factors work together to influence Al corrosion.The Al corrosion and its prevention are systematic issues in LIBs.

FAILURE ANALYSIS
The Al corrosion occurs during the entire life cycle of LIBs.It relates to the electrochemical and chemical properties of Al, and seriously affects the electrochemical performance of LIBs.Analyzing the corrosion process plays an important role in studying degradation mechanism of Al corrosion and guiding the protection of Al in LIBs.This review will briefly introduce the technolo-

REVIEW
The Innovation Materials 1(2): 100030, September 20, 2023 3 gies used in evaluating electrochemical, morphology and chemical properties of Al corrosion (Figure 3).The following will briefly introduce them.

Electrochemical property
There are the traditional and localized electrochemical analyses used to study Al corrosion in LIBs.In the traditional electroanalyses, the fundamental methods include linear sweep voltammetry (LSV), 59 open circuit potential (OCP), 60 cyclic voltammetry (CV), 61 chronoamperometry (CA), 62 chronopotentiometry (CP), 63 Tafel curves, 64 electrochemical quartz crystal microbalance (EQCM), 65 electrochemical impedance spectroscopy (EIS), 66 galvanostatic charge discharge (GCD). 67The corrosion phenomenon is reflected by monitoring the current, potential, and resistance.The most intuitive method of analysis is CV and LSV in three electrodes system or two electrodes system.In CV, the redox reactions are recorded from CV curves characterized with obvious peaks.The peak location on voltage stands anodic dissolution and passivation of Al.Meanwhile, the current value and peak location is shifted with the variation of the sweep rate.Besides, the electrochemical window of Al can be determined according to the increased current from LSV curves.CA and CP are usually used to observe corrosion condition by current value at a fixed potential or in reverse.The corrosion potential obtained from OCP. Tafel (Tafel extrapolation) is used to study the polarization about corrosion kinetics, which determines the corrosion current density and corrosion rate.EIS is used to evaluate the resistance in Al corrosion.From GCD experiments, the corrosion consequence related to the electrochemical performance of LIBs is revealed.Owing the quality variation from dissolution of Al 3+ or formation of passive layer, EQCM is used to study the corrosion in LIBs.
In localized electroanalytical studies, a series of test methods, such as scanning electrochemical microscopy (SECM), 68 scanning vibrating electrode technique (SVET), 69 scanning kelvin probe (SKP), 70 and localized electrochemical impedance spectroscopy (LEIS), 69 are developed to study the corrosion.Based on the microelectrodes, SECM is used to study the interface properties of Al with electrolyte at high spatial resolution.SVET utilizes the electrode to sense the corrosion current in the electrical field.SKP can obtain a surface topography maps by potential, in which the change of potential represents the corrosion behaviors.Different from the routine EIS, LEIS can observe the local information about pits in Al by calculation of the impedance.Based on these technologies, the electrochemical properties regarding the Al corrosion can be evaluated in LIBs.

Morphology
The morphology characterization is necessary to display the surface variation of Al in corrosion.The advanced technologies, such as scanning elec-tron microscope (SEM), 71 transmission electron microscope (TEM), 72 X-ray computed tomography (CT), 73,74 ultrasonic testing (UT), 75 atomic force microscope (AFM), 70 laser scanning confocal microscope (LSCM), 76 electron backscatter diffraction (EBSD), 77 scanning tunneling microscope (STM), 78 are benefitcial for analyzing the corrosion mechanism.SEM and TEM can obviously exhibit the corrosion morphologies of Al.With the assistance of focused ion beam, the surface and depth information of Al can be extracted on morphologies.The application of non-destructive testing technology, such as UT and CT, can obtain the damage situation of Al corrosion.For example, when the X-ray penetrates the LIBs, different element displays various absorption content, indicating the contrast in images.The corrosion defects in Al can be exhibited.During the corrosion process, the surface of sample generates defects or passivation layer.The AFM and LSCM can distinguish the roughness, protrusions and pits on the surface of Al.Besides, EBSD is the technology that can determine the local crystal structure and crystal orientation with visualization and quantization.STM can obtained atomic manipulation of surface by an atomically sharp tip.Based on the above technologies, the morphology of Al corrosion can be recorded and analyzed for research.

Chemical property
The chemical property of Al corrosion is characterized by X-ray diffraction (XRD), 79 Raman, 80 Fourier transform infrared spectroscopy (FTIR), 81 X-ray photo electron spectroscopy (XPS), 40 time-of-flight secondary ion mass spectrometer (TOF-SIMS), 82 inductively coupled plasma-atomic emission spectrometry (ICP-OES), 83 nuclear magnetic resonance (NMR), 84 and X-ray absorption near-edge spectroscopy (XANES). 85The change of atomic structure can be recorded during the corrosion process by XRD.Raman and FTIR can be used to identify the molecular structure on the surface of Al.XPS can obtain the chemical composition and is widely used to explore the passive layer after corrosion.The depth chemical composition can be obtained with Ar+ etching.TOF-SIMS is a technology to image mass spectrometry and obtain inside chemical composition information with ion beam.Another application used in corrosion science is to apply isotopic labeling to distinguish the corrosion mechanism.The dissolved ion can be detected by ICP-OES after corrosion, which help to analyze the corrosion products.NMR can provide the environment of element nucleus by NMR parameters especially 27 Al.XANES can detect the surface film of corrosion by X-rays.Based on these technologies, the chemical properties related to Al corrosion would be evaluated in LIBs.

PROTECTION STRATEGY
Inhibiting Al corrosion is an important step in developing high-performance LIBs.According to the mechanism of Al corrosion, great efforts have been devoted on preventing Al corrosion in the direction of Al surface, electrolyte, and inhibitor, and have achieved promising advance (Figure 4).We will briefly review these achievements in the following.

Al surface
Aluminum is the main substrate of corrosion, and its surface properties have a profound impact on the corrosion.Modifying the surface properties of Al is the most common method to prevent corrosion.The following will summarize the progress of Al surface modification for corrosion inhibition.
The purity Al.The purity of Al is related to its corrosion in LIBs.The impurities, such as Fe and Si, in Al would form intermetallic particles, which accelerate the pitting corrosion or galvanic corrosion. 46The existed impurity would induce a galvanic cell with anode of Al.Once the Al matrix was depleted, the corrosion pitting occurs on its surface.The impurities would dissolve into electrolyte and deposit onto the anode of LIBs.Therefore, the higher purity Al is essential to avoid the galvanic corrosion during calendar aging and working of LIBs.
Al 2 O 3 layer.It is well known that the Al 2 O 3 layer in Al displays anti-corrosion property.The thickness of Al 2 O 3 layer in Al would affect the corrosion of Al. 86,87 Martin Winter have studied the thickness effect of Al 2 O 3 in Al from 100 to 950 nm on its corrosion in different lithium salt and lithium solvent (Figure 5A). 37It was found that the thicker Al 2 O 3 layer in Al exhibited the better anticorrosion effect.Other groups also found the similar results.The commercial Al was treated by KMnO 4 with the presence of K 2 ZrF 6 and surfactant of sodium dodecyl sulfate to form a new rough and thick compact Al 2 O 3 layer in Al. 88 The pretreated Al as the current collectors displayed excellent electrochemical performance, especially the ultra-long cycles.Another strategy was 480 o C heat-treatment of commercial Al at 24 hours in air to form the thick Al 2 O 3 layer. 38The experiments indicated that the thick passive layer can greatly suppress the Al corrosion.Another work used direct current anodizing method to prepare a new dense and thick Al 2 O 3 layer on Al, in which its original Al 2 O 3 layer was completely etched.The new Al current collectors had high adhesion with electrode materials and suffered low corrosion current.It had higher stability than Al in full battery with LCO cathode materials. 87These results proved that thick Al 2 O 3 is benefit for the anti-corrosion of Al current collectors.However, on one hand, the thick Al 2 O 3 layer on Al had a high electrical resistance, reducing the electron transfer rate between the current collector and the electrode materials. 87On the other hand, the Al current collectors suffer severe corrosion would also increase interface resistance between Al and electrode materials. 89There is a tradeoff between the reduced electron transfer in the increased thickness of Al  59 CV, 61 CA, 62 EQCM, 65 EIS, 66 OCP, 60 GCD, 67 Tafel, 64 SVET, 69 LEIS, 69 SKP, 70 SECM. 68The morphology characterization: SEM, 71 TEM, 72 CT, 74 UT, 75 AFM, 70 LSCM, 76 EBSD, 77 STM 78 .The chemical characterization: XRD, 79 XPS, 40 Raman, 80 FTIR, 81 ToF-SIMS, 82 ICP-OES, 83 NMR, 84 XANES. 85repared by the electrolyte additives fluorinated carbamates in LiTFSI based electrolyte (Figure 5C). 90To achieve 5 V class LIBs in LiTFSI-based electrolyte, one ultra-thin Al doped ZnO layer was prepared on Al and proved 120 times anti-corrosion than commercial Al current collectors. 91The hybrid passive layer of MWCNT and Ni x Al alloy on Al was designed and prepared in two steps (Figure 5D). 92

Electrolyte in Lithium-ion battery
The electrolyte consists of lithium salt, solvent, additive and contaminant.It is the environment of Al corrosion.Great effort was devoted to the development of functional electrolyte to suppress Al corrosion.This part introduced the advance of electrolyte in the compositions modification to avoid Al corrosion.
Lithium salt.Lithium salt is one of the most important compositions in electrolyte.It directly affects the properties of the electrolyte, which influence Al corrosion.Therefore, the lithium salt in electrolyte was extensively studied.The effect of lithium salt in electrolyte on Al corrosion was studied in three aspects, including the types of lithium salt, mixture of lithium salt, and the concentration of lithium salt.
(1) Types of lithium salt: Based on previous works on Al corrosion, the passivation behaviors of different types of lithium salt in EC based electrolyte was compared and their passive sequence in Al corrosion is lithium tetrafluoroborate (LiBF 4 ), LiDFTFSI, LiPF 6 , LiFTFSI, LiDFOB, lithium bis(oxalato) borate (LiBOB), LiTFSI, and LiFSI. 93Among these lithium salts, LiPF 6 and LiTFSI were two most used in electrolytes according to their good properties beyond the corrosion issue.From the corrosion mechanism, the outstanding passive layer on Al is one of the most important factors enhancing Al anti-corrosion.Hence, modifying the lithium salt, especially LiTFSI, was attempted recently.

Al surface
Electrolyte Inhibitor  89 (C) Schematic illustration of the surface layer from organic carbonates to fluorinated carbamates. 90(D) The preparation of NiAl alloy as current collector. 92ue to the strong corrosion of LiTFSI with Al and its poor formed passivation layer, Zhang group used one H atom to replace one F atom in one CF 3 group in LiTFSI, forming new LiDFTFSI (Figure 6A). 93When LiDFTFSI was used in electrolyte, it facilitated the insoluble AlF 3 and LiF formation on Al.They worked as passivation layer in Al greatly suppressed Al corrosion, resulting in the enhanced electrochemical performance of LIBs.Similarly, the lithium fluorinated sulfonamide family of Li[(FSO 2 )(RFSO 2 )N] (RF = n-C m F 2m+1 , m = 0 (LiFSI), 1 (LiFTFSI), 2 (LiFPFSI), and 4 (LiFNFSI)) was designed to adapt the corrosion in LIBs. 94It was found that the value of m is larger, the better the passivation behaviors, reducing the further Al corrosion in LIBs.For instance, LiFNFSI significantly improves the passive layer on Al, enhancing the electrochemical performance LIBs.
(2) Mixture of lithium salt: Different lithium salts have different advantages in LIBs, mix two or more lithium salts into solvents to prepare electrolyte can take full use of their advantages, boosting the electrochemical performance. 95,96LiFSI as a novel lithium salt owns higher conductivity and transference number than LiPF 6 , but suffers from serve corrosion above 3.3 V with Al, which is much worse than that of LiPF 6 .Therefore, mixing them in carbonate solvent forms the electrolyte, which exhibits improved performance of LIBs in both room and high temperature. 97 x capacity retention of 84.3%. 66The similar ternary-salt electrolyte system by using their synergistic effect is one promising solution to improve the electrochemical performance of LIBs.
(3) Concentration of lithium salt: Researchers have found that the high concentration of lithium salt in electrolyte can facilitate to form stable passive layer on Al. 33,[98][99][100] For example, Suo studied the effect of LiFSI concentration in FEC on Al corrosion and found that the working potential window of electrolyte was expanded from 4.5 to 5.0 V without Al corrosion with the increase of LiFSI concentration. 101In another electrolyte of LiTFSI in EC/EDC, the concentration of LiTFSI increased from 0.8 to 1.8 M resulted in the reduced oxide current and corrosion pitting, proved by stable LiF passivation layer on Al current collector in XPS depth experiments. 53Similarly, it is a feasible way increasing LiFSA concentration in electrolyte tended to form dissolution passive layer without free solvents, leading to better electrochemical performance of LIBs (Figure 6B). 102These results indicate that concentration of lithium salt in electrolyte can adjust the components of passive layer on Al current collectors, suppressing its corrosion.
Solvent.The solvent in electrolyte plays a crucial role in the passivation of Al current collector.The carbonate and ether are two major classes of solvents in practical applications.The most important property of solvent is the dielectric constant, which is related to oxidation stability of Al.In carbonate-based electrolyte, the basic solvent includes linear molecular (DEC, DMC, ethyl methyl carbonate (EMC), EC) and cyclic molecular (EC, propylene carbonate (PC), vinylene carbonate (VC)).In general, the dielectric constant of cyclic molecular is higher than that of linear molecular. 103The low dielectric constant solvent induced the formation of electrolyte insoluble corrosion product, a good passivation layer materials, to suppress the further corrosion of Al. 38 Take EC as example, EC is employed in state-of-art electrolyte mainly due to its good compatibility with graphite.Nonetheless, ECbased electrolyte suffers from the limitation of working voltage because of the cyclic cleavage, such as the oxidation potential of 4.4 V (1M LiPF 6 in EC/DMC). 104Thereby, increasing the working potential window without Al corrosion issue becomes important.Yamada modified EC with cyclic phosphate to form new solvent TFEP.0.95M LiFSI dissolved into TFEP and FEMC mixture forming the electrolyte (Figure 6C). 104The sulfonamide-based elec-trolyte has tremendous advantage on the CEI of cathode and SEI of lithium anode than commercial carbonate electrolytes.Moreover, the new electrolyte facilitated the formation of passive layer of AlF 3 and Al 2 O 3 on Al, making it stable at 4.9 V.
In ether-based electrolyte, the ether solvents (DOL, DME and DEE) have low viscosity.They also have relatively low working voltage window and tend to corrosion Al when in practical applications under high working potential in LIBs.Therefore, increasing the ether solvents working potential window is becoming the key task.One solution is to increase lithium salt concentration in electrolyte.Another solution is to design new structure and composition of the ether solvent. 105,106For example, the methoxy groups in DME was substituted by ethoxy groups to from new solvent of 1,2-diethoxyethane (DEE) in Figure 6D.The DEE was preferred to form passivation layer on Al, suppressing the Al corrosion. 58With the increased concentration of lithium salt, the passive layer becomes much thicker, leading to a smooth surface after being held at 5.5V.DEE was further modified via monofluoro substituent to form a new solvent of monofluoride bis(2-fluoroethyl) ethers (BFE), which was evaluated in Li//Al batteries. 107The BFE-based electrolyte exhibited higher electrochemical stability window of 4.7 V than that of DME (3.8V) and DEE (4.4V).From the analysis of Al after cycling test, the passivation layer composited with LiF and AlF 3 is formed on Al and can effectively improve the corrosion resistant.
Additive.The additives in electrolyte have a large effect on the formation of passive layer on Al current collector, and greatly improve the electrochemical performance of LIBs. 108The additive can be classified into two types of inorganic compounds and organic compounds.For the inorganic compounds additives, one is HF.A little of HF was added into LiCF 3 SO 3 /PC electrolyte, the anodic current was deceased without obvious pitting corrosion on Al. 109 The reason is that the HF helps Al to form a stable passivation layer with new composition.Fumed silica was another inorganic additive. 110It can help the electrolyte to form the gel and enhance the corrosion of Al resistance.Lithium salt additives, such as LiBOB, can form a stable B 2 O 3 passive layer on Al to increase their anti-corrosion working potential. 38,52The most common used LiPF 6 can also be used as additives to enhance other type lithium salt electrolytes.For example, Zhang group chose 0.05 M LiPF 6 as additive to add into LiTFSI-LiBOB electrolyte. 111The new electrolyte exhibited excellent rate and cycling performance in LIBs thanks to the LiPF 6 induced passive layer on Al (Figure 6E).Organic compounds were another type of additives.Trimethylsilyl (trimethylsiloxy) (bis-TMSA) acetate was added into the electrolyte to remove H 2 O and suppress the formation of HF, which is beneficial to reduce Al corrosion. 112Succinonitrile (SN) and Fluoroethylene carbonate (FEC) were two types of electrolyte additives. 113When they were added into electrolyte at the same times, they can consume part of HF and increase the working potential window, resulting in the enhancement of Al corrosion resistance.Based on the above results, it can be safely claimed that additive is an effective strategy to enhance the ability of Al anti-corrosion.
In summary, the electrolyte has effects on the whole battery system.It is the corrosive agent, leading to the corrosion in the battery system, including the Al current collectors.All the components of electrolyte, lithium salt, solvents, and functional additives can influence the corrosion and passivation processes of Al current collectors (Table 1).In the view of electrolyte, preventing corrosion of Al current collectors can be realized by modifying the components of electrolytes in the three directions.The first is to prevent the corrosive agent formation in the electrolyte.The second is to consume the corrosive agent in the electrolyte, reducing the Al correction.The last is to form stable passivation layer to avoid the further corrosion in Al.The compatibility between electrolyte and current collectors, electrode materials, and separators is the precondition and should be kept in mind when the electrolyte is modified.

Inhibitor
When LIBs work under low working potential, such as 4.3 V, the passiva-tion layer can endure the corrosion for Al.However, when the working voltage is over 4.3 V, it exceeds the limitation by the passivation protection on Al.What is worse, Al current collector suffers a huge shock from current and voltage, the interface strength problem is magnified with the increasing voltage.][116][117][118] Some typical inhibitor materials, including graphene and graphene-like materials, inorganic materials, and conductive polymer layer, were briefly introduced in the following.

Graphene and graphene-like materials.
][121] Graphene as an inhibitor can effectively prevent the corrosion, but the challenge is how to prepare high quality graphene film on Al. 61,80 Insitu growing graphene in Al is preferred because of their ideal contact and high coverage between graphene and Al. 61Plasma enhanced chemical vapor deposition was used to grow multilayer graphene on Al foil as shown in Figure 7A.In the CV test, graphene modified Al showed increased peaks at higher voltage, indicating that graphene can effectively prevent Al exposure in corrosion environment.Another in-situ preparing method is to use laser directly reduced polyimide into graphene on Al (Figure 7B). 81The prepared graphene exhibits number of pores by gas release which is responsible for adhesion strength and interface resistance.The sample showed superior electrochemical performance compared to bare Al in both rate performance and cycling stability.
Different from graphene, graphene oxide (GO) was beneficial to dispersion in aqueous solution, and was often used to prepare the graphene inhibitors on Al. 40 The GO solution can be electro-sprayed on Al following thermal reduction under inert atmosphere to form the reduced graphene oxide on Al (rGO/Al). 122,123rGO/Al exhibits comparative anti-corrosion ability in both LiPF 6 and LiTFSI-based electrolyte.Beyond graphene itself coated on Al, other carbon materials, such as carbon black, can be introduced into the system, which compensated the low interlayer conductivity and increased the electrical contact. 117The protected Al current collector exhibited huge advantage compared to bare Al in cycling and rate performance of LIBs.
Graphene-like materials have similar ant-corrosion ability because of twodimensional property.For instance, the graphene-like carbon coated Al exhibits excellent anti-corrosion property and obtains high cycling and rate performance in Li-S batteries in. 85Another graphene-like 2D material of MXene armored Al displays the property of oxidation resistant (Figure 7C). 70i group chooses MXene armored Al 2 O 3 to block anion due to abundant functional groups, layered structure and conductivity.Through self-assembly method, the thin MXene layer was coated on Al current collector.The MXene protected Al exhibited small voltage fluctuation, indicating MXene possessed the resistant oxidation property.The cycling and rate performance of batteries with MXene-Al current collector are boosted at cut-off voltage of 4.5V.
Inorganic layer.The ceramic coating on Al is an efficient strategy to prevent corrosion. 125The chromium nitride (Cr x N) was coated on Al by magnetron sputter. 126The Al dissolution test was applied in LiTFSI with ethyl methyl sulfone and carbonate electrolytes, respectively.The prefabricated ceramic coating can partially solve the poor passive layer issue.Chromate conversion coating on Al was prepared and evaluated in LiTFSI electrolyte. 127he chromate conversion coating not only improved the anticorrosion ability, but also released the chromate to repair corrosion.Similarly, molybdate conversion coating on Al was proposed to block the corrosion in LiPF 6 based electrolyte.The Mo was reduced and produced a homogeneity passive layer on Al, which prevented the Al corrosion and boosted the electrochemical performance of batteries. 68onductive polymer layer.Conductive polymer is used to protect Al current collector thanks to its anti-corrosion, good conductivity and rich functional groups property. 128,129The PEDOT coated Al was prepared by the insitu oxidant EDOT (Figure 7D). 128The prepared sample displayed higher discharge capacity than bare Al in both room temperature and 40 ℃, indicating the great effect of PEDOT in anti-corrosion.Polymer can also be combined with other materials to prepare the hybrid coating layers on Al in order to enhance its anti-corrosion ability. 129For example, polyaniline (PANI) and Ni-Pc are combined together to coat Al, in which PANI blocks the contact between Al and electrolyte, and Ni-Pc fills the space hindering the movement of corrosive source. 130Carbon was also introduced into polymer inhibitor layers on Al and was studied their anticorrosion ability in both LiTFSI and LiPF 6 based electrolytes. 45Whether in terms of electrochemical performance or the data of corrosion pitting, the coated Al exhibited advantage compared to bare Al.
In short, corrosion inhibitor prevents the corrosion between Al current collectors and electrolyte via physical or chemical methods.It is the last guarantee of Al current collector to prevent corrosion.The major role of Al current collector is the conductive carrier, so the electrical conductivity of corrosion inhibitor is becoming very important.Carbon-based materials, featured with high electrical conductivity and good corrosion resistance, are the promising corrosion inhibitor materials.

THE OTHERS OF AL CURRENT COLLECTORS
With the advance of the energy storage technologies, the Al current collectors were developed in two directions.The first is the functionalization of Al current collectors.The second is the application of Al current collectors beyond LIBs.

The functionalization of Al current collectors
The functionalization of Al current collectors includes fire-extinguishing, safety, and flexibility.For the fire-extinguishing, a novel Al current collector was designed and prepared via coating Al on triphenyl phosphate and polyimide formed substrate. 131In the structure of novel Al current collector, triphenyl phosphate plays the role of preventing combustion via releasing gas formed its decomposition (Figure 8A).Polyimide works as the substrate to load triphenyl phosphate and Al.Al is used to increase the electrical conductivity of the novel Al current collector.The synergetic effects among them enhance the ability of fire-extinguishing in LIBs.For safety, polyethylene terephthalate (PET) was used to prepare the composite current collectors of Al-PET (Figure 8B).The prepared Al-PET did not have obvious thermal runway and short circuited behavior when the nail penetrated the pouch cells with charged to 4.2 V, resulting from that deformable PET cut off the conductive circuit and warped the protrusion in the penetrated process. 132For flexibility, graphene, featured with excellent chemical stability, high conductivity and flexibility, and lightweight (58% Al), was used to prepare a film worked as current collector instead of Al. 133 Thanks to the better interface between electrode and current collector, the new current collector displays low contact resistance, resulting in the excellent performance in cycling and rate stability of the full battery using LiFePO4 as cathode and graphite as anode (Figure 8C).

The application of Al current collectors in other batteries
Al foil can also be used as current collectors in sodium ion batteries (SIB), potassium ion batteries (PIB), aqueous LIBs, solid-state batteries, and super- capacitors.In all these batteries, the Al corrosion issue still exists.It should be studied in order to eliminate its negative effect on the electrochemical performance of batteries.
In SIB and PIB, Al foil is used as both cathode and anode current collectors, their corrosion would affect the batteries' performance. 134Laida Otaegui etc. studied the influence of different solvents in electrolytes and the temperature on Al current collectors in SIB. 135They found that the corrosion degree of Al in NaFSI based electrolyte with PC solvent is more serious than that in ionic liquid.Moreover, the Al corrosion rate is increased with the temperature increase.In order to reduce the Al corrosion, Al particles were coated on Al current collectors to tune its wettability with electrolyte, resulting in the improved cycling stability of batteries. 136In aqueous LIBs, the anode current collector of Al is prone to corrosion with aqueous electrolyte. 137When it was coated by graphene, the corrosion resistant of Al was enhanced.In solidstate batteries, the Al current collectors also react with the solid state electrolyte.Take halide-based solid state electrolyte for example, it reacted with Al to form AlCl 3 , which was confirmed by using XPS and ToF-SIMS. 82The corrosion reaction between Al and the electrolyte can be suppressed via graphene-like carbon protecting Al current collector (Figure 8D).For supercapacitors, Al foil is also the commonly used current collectors.Increasing the working voltage is preferred to improve energy storage ability, but the high working voltage would raise the corrosion reaction with Al, deteriorating the electrochemical performance of devices. 138Based on the above analysis, it can be found that Al has wide applications in different energy storage technologies and also suffers similar corrosion issues as LIBs.The similar effective strategies used to suppress the Al corrosion in LIBs is also useful for these technologies.

The challenge of Al current collectors in high working voltage batteries
In order to further increase the energy density of LIBs, developing high voltage batteries, such as 4.6 V LiCoO 2 batteries, 4.5 V LiNi x Co y Mn (1-x-y) O 2 batteries, and 4.9 V LiNi 0.5 Mn 1.5 O 4 batteries, becomes a trend in LIBs.Obviously, the high working voltage would deteriorate the Al corrosion issues with electrolyte, even Al itself confronts the dissolution issue (Figure 9A).The corrosion mechanism of Al in high working voltage needs to be clarified.New electrolytes are necessary to be explored.The studied protection strategies for Al current collectors would confront challenges in the high working voltage, but they are still the developing direction for the Al anti-corrosion (Figure 9B).

CONCLUSIONS AND PERSPECTIVES
Focused on the Al corrosion in LIBs, this review highlighted the advance made recently in Al corrosion mechanism, characterization technologies, and protection strategies.We briefly overview the Al corrosion mechanism and its affecting factors in LIBs.Then, the advanced technologies used to characterize Al properties in corrosion process in order to well uncover the Al corrosion mechanism was discussed.Finally, we emphasized the protection strategies related to function mechanism and materials selection and construction, structural design.Despite great advance has been made so far, there are still many mists and challenges that require sustained research in the following directions.
(1) Explore Al corrosion mechanism in LIBs: The Al chemical corrosion process is clear, whereas the electrochemical corrosion process in organic electrolyte does not uncover completely.At high working potential in LIBs, the corrosion mechanism under the coupling effect of chemical and electrochemical corrosions has not been conducted.It is very necessary to explain Al corrosion mechanism through a combination of theoretical calculation and experimental experiments.
(2) Develop in-situ characterization technologies to well explore Al corrosion mechanism in LIBs: Using various characterization techniques reveals the changes in chemical, electrochemical, and morphological properties of Al involved in the corrosion process, however almost all of them are not the insitu characterization techniques.Developing in-situ characterization technologies is conducive to real-time and accurate study of the corrosion processes.It can help us better reveal Al corrosion mechanisms in LIBs.
(3) Develop new high-performance materials for inhibitors, electrolyte and Al: The composition and structure of materials used in protection strategies of Al, electrolyte and inhibitors play the key role in determining the protection effect.Developing new materials for these protection strategies is important in order to enhance Al anticorrosion ability.
(4) Explore Al corrosion issues of LIBs in extreme harsh working condition: The practical applications of LIBs confront many extreme conditions, such as high temperature (≥ 55 o C) or low temperature (≤ -20 o C).The harsh working condition would affect the Al corrosion processes, and then influence the electrochemical performance of batteries.There is almost no discoveries in this direction.This is the frontier for Al corrosion issues in LIBs.
(5) Clarify the effect of nitrile and sulfone solvent based high voltage electrolyte on Al corrosion: The high working potential is one direction of developing LIBs with high energy density.The common used electrolytes are based on carbonate and/or ether solvents, which could not endure high working potential.Nitrile and sulfone solvents are stable at high working potential.They are the possible solution for the high potential electrolytes.Therefore, exploring the Al corrosion in these electrolytes becomes important.

Figure 1 .
Figure 1.Factors of corrosion and corresponding protection strategy.

Figure 2 .
Figure 2. Corrosion mechanism for LiPF 6 and LiTFSI based electrolytes (A) The Al current collector (left) and Al with passivation layer current collector (right) with LiPF 6 based electrolyte in LIBs.(B) The Al current collector (left) and Al current collector and solution production (right) with LiTFSI based electrolyte in LIBs.

Figure 3 .
Figure 3. Multiscale characterization for corrosion and protection including of electrochemical property, morphology and chemical property The electrochemical characterization: LSV,59 CV,61 CA,62 EQCM,65 EIS,66 OCP,60 GCD,67 Tafel,64 SVET,69 LEIS,69 SKP,70 SECM.68The morphology characterization: SEM,71 TEM,72 CT,74 UT,75 AFM,70 LSCM,76 EBSD,77 STM78 .The chemical characterization: XRD,79 XPS,40 Raman,80 FTIR,81 ToF-SIMS,82 ICP-OES,83 NMR,84 XANES.85 Ni x Al alloy was prepared onto Al by electrodeposition and thermal annealing method, and then MWCNT was in-situ growing on Ni x Al alloy at 650 o C, forming the hybrid payer.The hybrid layer protected Al exhibited excellent electrochemical performance in aqueous electrolyte.The above results indicated that the passive layer could be effectively suppress the Al corrosion in LIBs.In summary, the nature of Al current collector plays a basic role in anticorrosion in LIBs.Tuning the grade of Al, the composite of Al (alloyed Al), and the passivation layer of Al (Al 2 O 3 , AlF 3 , AlPO 4 , etc.) can affect the Al corrosion process.They are the major solutions to enhance the anti-corrosion of Al current collectors.

Figure 4 .Figure 5 .
Figure 4.The protection strategies of Al corrosion in three aspects of Al surface condition, the recipes of electrolyte, and inhibitors in LIBs.

Figure 6 .
Figure 6.The composition of electrolyte recipes (lithium salt, solvents, additive) (A) The passivation behavior of LiDFTFSI on Al current collector. 93(B) The schematic of passivation layer on Al with different lithium salts and concentration. 102(C) The XPS spectra of Al after CA test (down) and SEM images of Al current collector after CA test (up) in different electrolyte. 104(D) SEM (up) and XPS (down) of Al current collector after CA test with LiFSI salts.58(E) Schematic illustration on Al corrosion with LiBOB additive in electrolyte.111

Figure 7 . 128 REVIEW
Figure 7.The inhibitor for Al current collector (A) The schematic illustration of graphene armored Al current collector. 61(B) Preparation process of the PI derived graphene. 81(C) Schematic illustration the protection of by MXene. 70(D) Schematic illustration of the preparation of PEDOT inhibitors on Al current collector. 128EVIEW

Figure 8 .
Figure 8.The others of Al current collectors (A) The schematic illustration of composite Al current collector. 131(B) The pouch battery with Al and Al-PET current collector.132(C) Preparation process of the GAF current collector.133(D) The passivation structure of Al and GLC Al by TOF-SIMS in solid-state battery.82

Figure 9 .
Figure 9.The schematic of Al corrosion and protection strategies (A) Al corrosion process with the increased working potential.(B) The Al protection strategies related with Al, electrolyte and inhibitor.

Table 1 .
Summary of corrosion results with different electrolytes.