Protecting Al foils for high-voltage lithium-ion chemistries

Materials Today Energy 7 (2018) 18e26

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Protecting Al foils for high-voltage lithium-ion chemistries Han Gao a, Tianyuan Ma a, b, Thy Duong a, c, Li Wang d, Xiangming He d, Igor Lyubinetsky e, Zhenxing Feng e, Filippo Maglia f, Peter Lamp f, Khalil Amine a, **, Zonghai Chen a, * a

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne 60439, IL, United States Materials Science Program, University of Rochester, Rochester 14627, NY, United States c Virginia Polytechnic Institute and State University, Blacksburg 24061, VA, United States d Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China e School of Chemical, Biological, and Environmental Engineering, Oregon State University, Corvallis 97331, OR, United States f BMW Group, Munich 80788, Germany b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 October 2017 Received in revised form 30 November 2017 Accepted 1 December 2017

High concentrations of protons generated by the rapid oxidation of electrolyte solvent at high working potentials always lead to undesired reactions like the corrosion of Al current collectors. This issue is exaggerated with the recent effort in search for high-voltage cathode materials for a further increase on the achievable energy density of lithium-ion batteries. In this study, LiNi0.6Mn0.2Co0.2O2 (NMC 622) was investigated as a model high-voltage cathode to illustrate the potential impact of corrosion on the longterm performance of lithium-ion batteries. Both electrochemical measurements and X-ray photoelectron spectroscopy depth-profile studies confirmed that the AlPO4-coated Al foil was more resistive to corrosion. Meanwhile, an improvement on the capacity retention of NMC 622 was also observed by utilizing an AlPO4-coated Al foil as the current collector. It also implies that the corrosive environment generated at the relatively high working potential needs to be effectively mitigated to unlock the full potential of high-voltage cathodes. © 2017 Published by Elsevier Ltd.

Keywords: Lithium-ion batteries High-voltage cathodes Chemical corrosion Protective coatings Aluminum phosphate

Introduction With demands on longer-lasting consumer electronics and longer-range electric vehicles, one major milestone on the development of rechargeable Li-ion batteries is to reach an even higher energy density [1e5]. A significant enhancement in energy density not only can be obtained by increasing the specific capacity of electrode materials or reducing the “dead” weight/volume of inactive cell components, but also can be reached by maximizing the operational voltage window of a lithium-ion cell. This requires high-voltage cathodes to replace the conventional cathodes that normally work below 4.2 V vs. Liþ/Li [6,7]. However, the high working potential generally leads to undesirable parasitic reactions, including the direct electrochemical oxidation of solvent molecules [8] and the chemical reaction between the solvent molecules and the intermediate cathode materials [9], causing a performance decay. Drawing our particular

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Amine), [email protected] (Z. Chen). https://doi.org/10.1016/j.mtener.2017.12.001 2468-6069/© 2017 Published by Elsevier Ltd.

attention here is the direct electrochemical oxidation of solvents, whose rate increases exponentially with the working potential. The oxidation of solvents like ethylene carbonate (EC) generates shortlife organic radical cations on the surface of the positive electrode; these radical cations will quickly undergo a deprotonation reaction to stabilize themselves, creating a highly acidic chemical environment on the cathode surface [10,11] that can lead to the corrosion of aluminum current collector [8] and the cathode materials [12]. The impact of chemical corrosion of positive electrode was not given full attention in the open literature. This is primarily due to the relatively low potential of conventional cathodes and the technical difficulties in decoupling the impact of chemical corrosion from the bulk properties of battery materials. First of all, the traditionally used cathode materials mostly work within a potential window not higher than 4.2 V vs. Liþ/Li, which is far below the oxidation potential of carbonate-based solvents [10,13,14]. The rate of the direct electrochemical oxidations is extremely low. Thus, the impact of proton generation is overwhelmed by the bulk properties of battery materials, such as the moisture contained in the electrolyte [11,14,15] and the crystal structural stability of cathode materials [6,16]. The recent development of high-voltage cathodes inevitably

H. Gao et al. / Materials Today Energy 7 (2018) 18e26

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solvent for producing slurries of cathode active materials to a water-based process may be desirable to reduce emissions of volatile organic compound and costs. But the use of aqueous-based slurries can lead to general and pitting corrosion of the Al current collector [26]. This also requires a stable passivation layer on the Al foil. Carbon coating of Al foils has been developed as the first generation of such surface modification [27e29], but our recent study [8] have shown that the carbon-coated Al actually increases the rate of electrolyte oxidation reaction significantly at the solid/ liquid interface even though it is resistive to chemical corrosion. On the other hand, we were attracted to aluminum phosphate (AlPO4) as the new generation of dual functional nano-coating material to suppress the oxidation of electrolyte solvents and to enhance the corrosion resistivity of Al current collector, leading to a reduced amount of leached Al3þ that compromises the long term performance of lithium-ion cells [30e32]. In this work, LiNi0.6Mn0.2Co0.2O2 (NMC 622) was used as a model material to study the impact of Al protection on the electrochemical performance of the high-voltage cathodes. Here we show the AlPO4 nano-coating can suppress severe Al corrosion in the acidic environment at high potentials. Microscopic and spectroscopic methods were used to characterize the difference in morphology and surface chemistry of the foils with and without protection. The relationship between the protection of Al and the electrochemical performance of NMC 622 was investigated.

push the upper cut-off potential of the new cathodes to 4.5 V vs. Liþ/Li, which is very close to the oxidation potential of carbonatebased solvents. Hence, the detrimental impact of protons originated from the deprotonation of organic radical cations can no longer be ignored. Moreover, the direct measurement of these parasitic reactions using high precision columbic efficiency [17,18] and high precision leakage current measurement [8,9,19] is still under continuous development, and these latest techniques are only available to very limited number of research teams. Therefore, the corrosion behavior of positive electrode at a relatively high potential deserves a systematic investigation for rational design of next generation lithium-ion chemistries. Fig. 1 shows a schematic diagram of the commonly believed corrosion process occur to Al current collectors. The presence of HF (either from the protons due to solvent oxidation [8] or the residual water) can react with Al2O3, forming AlF3 and H2O. Even though AlF3 is insoluble in organic solvents like ethylene carbonate (EC) and ethyl methyl carbonate (EMC), it is reasonably soluble with the presence of H2O, causing the breakdown of the film [20]. Together with the attack from protons, the corrosion of Al will occur when the electrolyte solution penetrates through the pores of the electrode. The dissolved Al3þ ions from Al corrosion (and its by-products) not only can interfere with the active material on the cathode but also can migrate to the counter anode and reductively deposit there, reducing the integrity of the cathode-electrolyte interphase (CEI) and solid electrolyte interphase (SEI). In addition, this process will also result in a loss of electrical contact between the electrode's active materials and the current collector, leading to an increase in impedance or even delamination of electrode materials. These lower the cell performance by simultaneously aggravating its energy density and cyclability. For example, Al corrosion were reported for cells using cathodes in the 4 V class, including LiMn2O4 [21] and LiNi0.8Co0.15Al0.05O2 (NCA) [22]. Devine et al. suggest capacity fade and power fade of the graphite/NCA cells should be at least partially attributed to the negative impact of Al corrosion [22]. They observed a substantial amount of corrosion pits and cracks in the Al foils in the graphite/NCA cells after galvanostatic cycling. In fact, Al corrosion occurs even on cycled Li-ion cells using LiFePO4 cathodes, which has a relatively low working potential of 3.5 V vs. Li/Liþ [21]. Since much more protons and protic species will be generated by the rapid oxidation of electrolyte solvent at higher working potentials [8], the rate of Al corrosion will also increase exponentially and lead to more severe damage to the performance of lithium-ion cells. In this regard, protection of Al current collector becomes a critical component for the utilization of high-voltage cathodes in the next-generation high energy density Li-ion batteries. Previous studies focused on the effect of various electrolytes with different salts and solvents on the stability of Al foils [21,23e25]. However a more efficient and economical alternative is to modify the surface chemistry of existing Al foils for improved interfacial properties. In addition, the transition from non-aqueous

Experimental section Preparation of electrodes and cells Two types of Al foils were investigated in this study: (i) batterygrade Al foil (thickness ¼ 20 mm, A1235-H18, DONG-IL Aluminium Co., Ltd.); and (ii) AlPO4-coated Al foil. The AlPO4-coated foil was prepared by first washing the commercial battery-grade Al foil with ethanol and dipping the foil into a 2 wt% AlPO4 solution until the foil losses its shining metallic surface and shows light white in color. The AlPO4 solution was prepared by mixing a trivalent aluminum source (e.g. Al(NO3)3) solution with a phosphate radicalcontaining (e.g. (NH4)2HPO4) solution. Finally, the coated Al foil was dried to remove the excess solvents. The same AlPO4-coated foil has been demonstrated in our pervious study [8]. Both Al metallic and NMC half-cells were constructed in a CR2032 coin cell configuration with lithium metal (MTI Corporation, USA) reference/counter electrodes. The electrolyte used was 1 M LiPF6 in EC-EMC (3:7 by weight, BASF LP-57). Celgard 2325 was used as the separator. For Al metallic cells, the punched (14 mm in diameter) Al foils were directly served as working electrodes after vacuum-drying. To prepare electrodes for NMC half-cells, Al foils with and without coating were used as substrates for laminating a NMC slurry with a doctor blade. The slurry was composed of 91.5 wt% NMC 622 (secured from an industrial partner), 4.4 wt% conductive agent (C45 carbon black), and 4.1 wt% polyvinylidene

solvent oxidation residual water H+ porous electrode

+PF6-

H2O

Al3+

Al Al2O3 AlF3 Porous electrode Corrosion products

Continuous cycle

HF Al2O3 on Al

Al3+ H+

AlF3 & Al2O3 on Al

Corrosion on Al

Fig. 1. A schematic diagram showing the corrosion of Al current collectors used in a typical Li-ion cell.

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H. Gao et al. / Materials Today Energy 7 (2018) 18e26

fluoride (PVdF, Solvay chemicals) binder. After laminating, the electrodes were first dried at 75  C in air overnight and subsequently and punched (14 mm in diameter). All punched electrodes were vacuum-dried at ca. 110  C overnight prior to cell assembly. The NMC mass loading on each electrode was ca. 20 mg and all cells were assembled in an air and moisture free glovebox filled with Ar. Material characterizations Scanning electron microscopy (SEM) images of the Al foils were obtained from a Hitachi S-4700II field emission scanning electron microscope. Fourier transform infrared (FTIR) spectra of the Al foils were recorded on a Thermo Scientific Nicolet iS5 spectrometer with iD5 attenuated total reflection (ATR) module at room temperature inside the glovebox. X-ray photoelectron spectroscopy (XPS) measurements were performed with a PHI 5600 XPS system using monochromatized Al Ka radiation (photon energy ¼ 1486.6 eV) with a 200 mm spot size and a 45 emission angle. Charge compensation was accomplished using a low-energy electron beam coincident on the sample. The depth-dependent composition data were obtained after the samples were sputtered with 3 keV Ar-ions. The sputtering rate of the ion gun has been first calibrated on the standard SiO2 film followed by adjustment for the studied sample for corresponding sputtering yields [33]. The XPS data were fitted using CasaXPS software. The binding energies were corrected to the C 1s aliphatic carbon peak at 284.8 eV. In particular, the Al 2p peaks were fit with a doublet, namely Al 2p3/2 and Al 2p1/2, with a separation of 0.44 eV for the spin-orbit splitting. The Al 2p area ratio was fixed to 2:1 for 2p3/2 and 2p1/2 [34]. Electrochemical characterizations Discharge-charge tests were conducted on the CR2032-type half-cells on a MACCOR series 4000 battery tester at room temperature (ca. 25  C). A home-build high-precision leakage current measuring system (based on Keithley 2401 source meters) was used to investigate the reactions between the working electrodes and the electrolytes before and/or after galvanostatic cycling. The working electrodes were held at each specific potential over time to reach an equilibrium state at 30  C. Cyclic voltammetry (CV) measurements were recorded on a Solartron 1470E multi-channel potentiostat/galvanostat. Electrochemical impedance spectroscopy (EIS) spectra were recorded on a Solartron 1400 A frequency response analyzer interfaced with the Solartron 1470E from 100 kHz to 0.01 Hz with 10 mV amplitude. Results and discussions To confirm the successful coating of AlPO4 on Al foils, the morphology of the bare foil (Fig. 2a) and the chemically treated foil (Fig. 2b) were characterized by SEM. The bare Al foil sample showed a relatively clean surface without pits and cracks. The presence of the striped patterns is an evidence of rolling process during the production of Al foils from slabs. In contrast, the coated foil exhibited a completely different morphology. Even though the rolling patterns can still be observed, the coated foil exhibited a rougher surface (i.e. the light and dark areas) with a uniform film on top of the Al foil as expected. Fig. 2c shows the ATR-FTIR spectra of the two foils. Characteristic vibrational bands of the fundamental vibrating unit (PO3 4 ) were identified in the coated foil: P]O 1 stretching at 1380 cm1, PO3 4 asymmetric stretching at 1172 cm , 1 and PO3 symmetric stretching at 975 cm . 4 XPS was used to study the surface chemistry of the two foils. Fig. 2d compares the O 1s, Al 2p, and P 2p spectra of the two samples. For O 1s spectra, the bare Al showed the Al2O3 peak at

531 eV, while the other peak was due to the organic C]O bonds (from slight carbon contamination on the sample surface). To reduce the signal from contaminations, the surface of the coated Al sample was slightly sputtered by Ar-ion (~1 nm) to reveal its true surface chemistry. In this case, the coated foil exhibited a O 1s peak at 532.8 eV, which can be assigned to AlPO4 [35]. Further analysis was performed on Al 2p spectra, the bare Al showed one peak at 75.5 eV corresponding to the Al oxide on Al foil, while the small peak with a lower binding energy (~72.5 eV) is the signal from metallic Al. In contrast, the Al 2p signal in the coated sample showed a peak at 74.4 eV, agreeing with the reported value for AlPO4 [36]. The P 2p spectra are also compared. It is not surprising that no P 2p signals was obtained for the bare foil, while its coated counterpart showed a broad peak at 134 eV from AlPO4 [36]. Results from XPS and FTIR confirmed the successful coating of AlPO4 on the Al foil. To estimate the thickness of surface layers and their elemental concentrations, XPS depth profiling of the two foils was performed by using Ar-ion sputtering. Fig. 2e and f show the atomic concentration of the measured elements as a function of sputtering depth. For the bare Al (Fig. 2e), a significant amount of C 1s (i.e. carbon contaminations) was found on the surface (Figure S1a), leading to apparent initial decrease of the concentration of other elements. Upon further sputtering, both the Al 2p (oxide) and O 1s peak decreases, while the Al 2p (metal) increases. Based on this observation, the thickness of the Al2O3 layer on the Al foil was about 5e7 nm. Similarly, the coated sample also showed carbon contaminations on the surface (Figure S1b), leading to apparent initial decrease of the concentration of other elements. Further increase in sputtering depth revealed a reduction in the Al 2p (oxide and phosphate) and an increase in Al 2p (metal). On the other hand, the P 2p signals from AlPO4 was relatively constant throughout the sputtering depth, which implies the thickness of the AlPO4 layer was greater than the sputtering depth. This also suggests that the reduction in the Al 2p (oxide and phosphate) signal was caused by the reduction in the oxide content. Metallic half-cells were constructed using the coated and bare Al as working electrodes. The effect of the AlPO4 nano-coating on the suppression of Al corrosion was evaluated by CV measurements. The OCV of the newly assembled cells in the electrolyte was between 2.8 and 3.0 V vs. Li/Liþ. Fig. 3a shows the current response of the bare and the coated samples to the applied potential under a sweep rate of 10 mVs1. For both samples, the initial current values were small and were mostly contributed by the electrical doublelayer capacitance at the solid/liquid interface. However a quick increase in current density was observed for the bare sample when the potential reached ~3.3 V vs. Li/Liþ. This variation in the response current can be related to the conversion of the natural Al2O3 layer on Al into the AlF3 film and the corrosion of Al for the non-AlF3 covered regions. The peak current of this coupled electrochemicalchemical process was ca. 3.7 V vs. Li/Liþ. In contrast, this peak was not observed in the AlPO4-coated sample. Instead, the response current exhibited a relatively constant value, as expected for charging a double-layer capacitor. It should be noted that the current response of the bare Al was much higher than the AlPO4coated one even after the peak at 3.7 V vs. Li/Liþ, suggesting the continuously dissolution of Al was more severe for the bare sample. Further investigations on these two passivated foils (i.e. after the CV measurement) were conducted on the time dependence of their leakage current at different polarization potentials. In this measurement, the leakage current values are quantitative indicators of the rate of the charge transfer reaction (i.e. the oxidation of electrolyte solvent) occurred at the electrode/electrolyte interface. Fig. 3bed show the leakage currents of the bare and AlPO4-coated samples at 3.7, 4.0, and 4.5 V vs. Li/Liþ. Both metallic cells exhibited

H. Gao et al. / Materials Today Energy 7 (2018) 18e26

Absorbance (a.u.)

Bare Al AP-coated Al

HPO42- and PO43- PO43P=O Stretching

100

Atomic concentration (%)

(c)

(a) Bare Al

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(e) Bare Al Al 2p (metal) Al 2p (oxide) O 1s

90 80 70 60 50 40 30 20 10 0

30 μm

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(d) O 1s

bare

100

coated

90

Counts (a.u.)

536 535 534 533 532 531 530 529 528

Al 2p

bare coated

80 79 78 77 76 75 74 73 72 71 70

P 2p

bare coated

30 μm 140

138

136

134

132

130

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128

126

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(b) AlPO4-coated Al

2

3

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(f) AlPO4-coated Al Al 2p (metal) Al 2p (oxide & phosphate) O 1s P 2p

80 70 60 50 40 30 20 10 0 0

1

2

3

4

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Fig. 2. SEM images of (a) the bare Al foil; (b) the AlPO4-coated Al foil; (c) overlay of the ATR-FTIR spectra of the two foils; (d) O 1s, Al 2p, and P 2p XPS spectra of the bare and coated Al; and XPS depth profile of (e) bare foil and (f) the AlPO4-coated foil.

a constant leakage current after the initial charge current at 3.7 V (Fig. 3b) and 4.0 V (Fig. 3c). On the other hand, the leakage current values did not reach a steady state with the same holding time when the potential was raised to 4.5 V (Fig. 3d). Nevertheless, the difference in the leakage currents between these two cells at each potential was mainly caused by the presence of the AlPO4 coating since both cells shared the same electrolyte chemistry. The smaller leakage current of the coated sample suggests that the AlPO4 nanocoating successfully suppressed the oxidation of electrolyte solvent. After the polarization test, corrosion pits were observed on the surface of the bare foil but not on the AlPO4-coated one (Figure S2). The severe corrosion of the bare sample could due to its faster oxidation of solvent since more reactive protons and protic species were generated. In addition, the AlPO4 coating can not only lowered the oxidation of solvent, but it may also process a higher chemical stability against corrosion. To examine the corrosion resistivity of the coated foil, XPS analysis with Ar-ion sputtering was also performed on the two passivated foils (i.e. after the CV measurement) to reveal any change in the surface chemistry. Fig. 4 shows the Al 2p, O 1s, Li 1s, and P 2p spectra for a typical passivated Al foil, while Fig. 5 shows the same set of elements for a passivated AlPO4-coated foil. In Fig. 4a, the presence of AlF3 on the passivated Al film surface was confirmed via the finger print peak at ~77 eV in the Al 2p spectra [37]. This broad peak started to shift towards lower binding energy and eventually reached the binding energy of Al2O3 at ~76 eV. This suggested the thickness of the AlF3 layer was ca. 3 nm. Correspondingly, an increase in the metallic Al peak at 72.5 eV was also observed. For O 1s spectra (Fig. 4b), the initial peak appeared at

532.5 eV and shifted to 531.1 eV. The latter one was originated from the Al2O3, while the former one at a higher binding energy was assigned to PeOeF compounds, such as POF3, POF2(OH), and/or POF(OH)2, which are the products from the hydrolysis of LiPF6 (generating HF) [38]. In addition, the Li 1s spectra (Fig. 4c) showed one single peak at 58.5 eV, which is different from the finger print value for LiF (~57 eV) [39]. This Li 1s peak may be resulted from the by-products from Al corrosion such as LixAlyFz due to the high affinity between Liþ, F and Al3þ [25,40]. In Fig. 4d, the P 2p spectra of the same sample showed an initial peak at ca. 137.5 eV, which is attributed to a PeF bond such as in a LixPFy compound [41]. This peak vanished with sputtering but another peak at ~134 eV appeared. This peak at 134 eV may correspond to the AlePeF compounds from the corrosion of Al. Similar investigations were performed on the passivated AlPO4coated Al foil. Although the Al 2p (Fig. 5a) and O 1s (Fig. 5b) signals from AlPO4 and Al2O3 overlapped in the spectra and were not deconvoluted, their overall trend was very similar to the bare film (Fig. 4a and b). Due to the presence of AlPO4 coating, Al2O3 was not converted into AlF3 as confirmed by the absence of the AlF3 peak. In addition, the Al metal peak was much weaker compared to the bare film shown in Fig. 4a, suggesting the phosphate/oxide content still dominated throughout the investigated sputtering depth. What is more important is that no Li 1s signals were obtained as shown in Fig. 5c, implying the absence of the LixAlyFz compound and the suppression of Al corrosion. On the other hand, the P 2p spectra (Fig. 5d) provided a good indication for AlPO4 because of its characteristic peak at 134 eV. This peak was observed throughout the sputtering depth. This agrees with the finding on the pristine

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H. Gao et al. / Materials Today Energy 7 (2018) 18e26

Fig. 3. (a) initial cyclic voltammograms of two metallic cells using bare and coated Al as working electrodes under a sweep rate of 10 mVs1; and leakage current curves measured during the potentiostatic hold of the two cells at (b) 3.7 V vs. Li/Liþ, (c) 4.0 V vs. Li/Liþ, and (d) 4.5 V vs. Li/Liþ after the CV measurement.

(b) O 1s

(c) Li 1s

80 78 76 74 72

534 532 530 528

63

(d) P 2p

Counts (a.u.)

(a) Al 2p

60

57

54

141 138 135 132 129

Binding energy (eV) Fig. 4. (a) Al 2p, (b) O 1s, (c) Li 1s, and (d) P 2p XPS spectra for a typical passivated Al foil (i.e. after the CV measurement) with Ar-ion sputtering (depth from top to bottom: 0 nm, 1 nm, 3 nm, 5 nm, 7 nm, 11 nm).

H. Gao et al. / Materials Today Energy 7 (2018) 18e26

(b) O 1s

(c) Li 1s

(d) P 2p

Counts (a.u.)

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23

78

76

74

72

534 532 530 528

63

60

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138 135 132 129

Binding energy (eV) Fig. 5. (a) Al 2p, (b) O 1s, (c) Li 1s, and (d) P 2p XPS spectra for a typical passivated AlPO4-coated Al foil (i.e. after the CV measurement) with Ar-ion sputtering (depth from top to bottom: 0 nm, 4 nm, 6 nm, and 10 nm).

AlPO4-coated foils of Fig. 2f. Also shown in Fig. 5d is another peak at a much lower binding energy of 130 eV, which may be assigned to PeOeF compounds generated from the hydrolysis of LiPF6. In summary, XPS characterizations together with Ar-ion sputtering confirmed: (i) part of the Al2O3 layer was converted to AlF3 in the passivated Al foils; (ii) even with this fluoride layer, the Al corrosion still occurred, resulting in LieAleF and AlePeF compounds; (iii) the thin layer nano-coating of AlPO4 was retained after passivation in the AlPO4-coated Al foil; and (iv) this layer successfully suppress the corrosion of Al. It has been demonstrated that the corrosion of Al can impact the performance of Li-ion cells [21,22]. In this study, we employed NMC 622 as a model material to study the impact of Al corrosion on the electrochemical performance of the high-voltage cathodes. Laminated NMC 622 electrodes were prepared using coated and bare Al foils. In general, a higher operation voltage deteriorates the Al current collector more severely during extensive cycling. Here we focused on an upper cut-off potential of 4.5 V vs. Li/Liþ for these NMC 622 cells. Before going into the cycling tests, some basic electrochemical performance of the NMC 622 cathodes were extracted. Fig. 6a shows the voltage profiles of the NMC 622/Li cell with the bare and AlPO4-coated Al current collectors during the initial cycling between 2.8 and 4.5 V with a constant current of C/10 (1 C ¼ 195 mAg1). Both cells delivered an initial charge capacity of 222 mAhg1, and an initial discharge capacity of 196 mAhg1, leading to an initial irreversible capacity loss of ~26 mAhg1 and 88% CE. At the fourth cycle (Fig. 6b), the irreversible capacity loss is barely visible with a specific capacity of 197 mAhg1. The overlapping voltage profiles as well as the differential capacity profiles (Fig. 6c) suggest the AlPO4 coating on the Al did not influence the initial charge-storage behavior of the NMC electrodes. In addition, the rate performance of the NMC 622 cathodes with bare the coated Al is shown in Fig. 6d. At high current densities, the cell with the AlPO4-coated Al showed slightly lower capacity compared to the one with bare Al at high current densities due to the additional resistance contributed by the AlPO4 coating, but the overall performance was very similar.

To investigate more on the impact of the AlPO4 coating, NMC 622/Li half-cells with bare and AlPO4-coated Al current collectors were cycled between 2.8 and 4.5 V vs. Li/Liþ under a current of C/10. EIS measurements were conducted at different potentials during charging. Battery surrogates with the same cell cable configuration and connection fixture with a four-terminal connection scheme were used to maximize the accuracy of EIS measurements. Fig. 7a and b show the Nyquist plots of the cells during the 1st cycle, while Fig. 7d and e show the Nyquist plots of the same set of cells during the 10th cycle. In general, the impedance spectra of the cells showed two time constants with two overlapping semicircles and a “tail”. The semicircle at the high-mid frequency window display the charging of double-layer on the materials and the contribution of total surface film resistances, while the semicircle at the mid-low frequency represents the rate of the charge transfer reaction. The “tail” at the low frequency region was affected by the Li ion diffusion within the electrode materials, which was not included in the data fitting in this study. An equivalent circuit is shown in Fig. 7a, where Rs is the equivalent series resistance, Rf is surface film resistance, Rct is charge-transfer resistance, Qf and Qct are the constant phase elements to take into account of the non-ideal capacitive behavior in a porous electrode. This model was applied to the frequency region before the appearance of the “tail” in order to deconvolute the surface film resistance and the charge-transfer resistance during cycling. The extracted resistance values are shown in Fig. 7c for the 1st cycle and Fig. 7f for the 10th cycle. Since the cathode electrodes are always porous to increase the cathode/electrolyte interfacial area, electrolytes can be trapped in microscopic crevices formed between the porous cathode and the current collector. Therefore it should be emphasized that the measured impedance spectra always contained information of both cathode/electrolyte and current collector/electrolyte interfaces. As shown in Fig. 7c, both cells showed an increase in Rf and a reduction in Rct with increasing potential during the 1st charging process. The increase in Rf may be due to the creation of surface layers on the electrodes and both cells showed similar Rf values. In

0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6

cycle 4

Capacity retention (%)

Spec. Diff. Cap. (mAhV-1mg-1)

4.6 (a) cycle 1 4.4 4.2 4.0 3.8 3.6 3.4 3.2 Bare Al 3.0 AlPO4-coated Al 2.8 0 50 100 150 200 Specific capacity (mAhg-1) 1.4 Bare Al 1.2 (c) AlPO4-coated Al 1.0

Potential vs. Li/Li+ (V)

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Potential vs. Li/Li+ (V)

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4.6 (b) cycle 4 4.4 4.2 4.0 3.8 3.6 3.4 3.2 Bare Al 3.0 AlPO4-coated Al 2.8 0 50 100 150 200 Specific capacity (mAhg-1) 100

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+

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Fig. 6. (a, b) voltage profiles of the NMC 622 electrodes on bare Al and AlPO4-coated Al for 1st and 4th charge-discharge cycles; (c) differential capacity profile, and (d) rate performance of the NMC 622/Li half-cells.

-45 -30

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Z'' (ohm)

-75

Bare Coated

(f)

Rf Rct

40 30

Cycle 10

20 10 0

0

10

20 30 Z' (ohm)

40

50

3.8

4.0

4.2

4.4 +

Potential vs. Li/Li (V)

Fig. 7. Nyquist plots of the NMC 622/Li half-cells with (a, d) bare Al and (b, e) AlPO4-coated Al during 1st cycle and 10th cycle; (c, f) the extracted surface film resistance (Rf) and charge-transfer resistance (Rct) as a function of potential.

H. Gao et al. / Materials Today Energy 7 (2018) 18e26

CE (%) Capacity (mAhg-1)

addition, a reduction in Rct was observed for both cells. This trend of lower Rct may be contributed by the activation of the electrode (e.g. the irreversible loss of oxygen creating vacancies in the crystal structure of the oxide [42]). Comparing the Rct values between the two samples, the cell with coated Al showed an almost linear reduction in Rct with increasing potential, while a change in the slope of the Rct vs. potential was observed at 4.0 V for the cell with bare Al. It is speculated here that the behavior of the latter cell is closely related to the corrosion of Al current collectors and passivation of the cathode material with the dissolved Al3þ ions. Eventually both cell showed almost the same Rct value at 4.4 V vs. Li/Liþ. Due to the corrosion of bare Al during cycling, this situation was changed after ten cycles (Fig. 7f). Since the corrosion products formed on the surface of the electrodes can impede the chargetransfer reaction, the cell with only bare Al demonstrated higher Rct than the one with the AlPO4. On the other hand, the Rf values of both cells increased compared to the values obtained in the 1st cycle due to the growth of the surface layer during cycling. It is interesting to note the cell with the coated Al exhibited larger Rf values, which may due to the presence of electrochemically/ chemically stable AlPO4 layer. To understand the effects of AlPO4 nano-coating on Al corrosion and on the performance of NMC 622 at the high working potential, cycling tests were conducted on the formed NMC 622 electrodes with an upper cut-off potential of 4.5 V vs. Li/Liþ. Fig. 8a shows the specific discharge capacity as well as CE of the cells with bare and coated Al over cycling at C/10. Both cells showed a similar specific

200 195 190 185 180 175

25

capacity of ca. 197 mAhg1 and a CE of 99.3% at the beginning. Generally, graduate capacity fade is expected with long-term cycling. In this case, the cell with the coated Al showed better capacity retention and higher CE than the one with only bare Al. This difference in capacity retention and CE may be caused by: (i) the corrosion products formed on the NMC electrode increased mass transfer limitations for Liþ ion intercalation; (ii) the dissolved Al3þ ions migrate to the counter Li electrode and reductively deposit there, and (iii) the reduction of active electron conducting pathways between the electrode and the current collector. This clearly demonstrated the beneficial effect of the AlPO4 layer and the negative influence of the Al corrosion on the performance of the high-voltage cathode materials. Two sets of cells with bare and coated Al current collectors were disassembled after the cycling test. A significant amount of corrosion products together with electrode delamination were observed for the cell with bare Al but not for the one with coated Al (Figure S3). Cathode materials were removed from the bare Al current collector by first soaking the electrode in n-methyl pyrrolidinone (NMP) solution overnight and then by gently wiping the materials off the current collector with a cotton swab. SEM images of the recovered bare Al confirmed that the Al current collector experienced severe corrosion during cycling (Fig. 8b). To further demonstrate the benefits of using the AlPO4-coated Al for high-voltage cathodes, cycling tests at a more severe condition were conducted for the NMC 622/Li half-cells. Fig. 8c and d shows the cycling of the formed cells with coated and bare Al under a high temperature environment of 45  C with a low current density of C/

(a) room temperature, C/10

(b)

Bare Al AlPO4-coated Al

100.0 99.6 99.2 98.8 98.4 98.0

30 μm

280 240 200 160 120 80 40 0 100

20

30

Cycle number

40

50

CE (%) Capacity (mAhg-1)

CE (%) Capacity (mAhg-1)

10

(c) 45 °C, C/10

Bare Al AlPO4-coated Al

95 90 85 80

280 240 200 160 120 80 40 0

(d) 45 °C, C/3

Bare Al AlPO4-coated Al

100 95 90 85 80

10

20

30

Cycle number

40

50

10

20

30

40

50

Cycle number

Fig. 8. (a) the cycling performance of the NMC 622/Li half-cells with bare and AlPO4-coated Al at room temperature; (b) SEM images showing the corrosion of the recovered bare Al after cycling of the NMC 622/Li half-cells; high temperature cycling of the NMC 622/Li half-cells with bare and AlPO4-coated Al at 45  C with (c) a low current density of C/10 and (d) a higher current density of C/3.

26

H. Gao et al. / Materials Today Energy 7 (2018) 18e26

10 (Fig. 8c) and a higher current density of C/3 (Fig. 8d). In both cases, the cell with the AlPO4 nano-coating showed better capacity retention and higher CE compared to the one with only bare Al, agreeing with the results shown in Fig. 8a. Also shown in Fig. 8c and d are the more significant capacity fades together with much lower CE values after 30 cycles for the cells with only bare Al. This may be due to the degradation in the mechanical integrity of electrodes caused by the corrosion of Al. This certainly revealed the importance of Al protection and the positive effects of the protective AlPO4 layer on the Al current collector. Conclusions The rapid oxidation of electrolyte solvent always generates high concentrations of protons, leading to the corrosion of Al current collectors. In this study, we demonstrated that the traditional approach by converting the native Al2O3 film on Al into AlF3 in the presence of non-aqueous electrolyte is not enough to sustain the working potentials of high-voltage cathodes. A thin-layer of AlPO4 was coated on Al as an artificial barrier to suppress corrosion. Both electrochemical characterizations and XPS studies confirmed the higher stability of the coated foil in the non-aqueous electrolyte. This functional nano-coating can successfully suppress the oxidation of electrolyte solvents and enhance the corrosion resistivity of Al current collector. The impact of this protection layer on the electrochemical performance of high-voltage lithium-ion chemistries was studied using NMC 622 as a model cathode material. The cell with the protected current collector exhibited consistently smaller chargetransfer resistance, better capacity retention, and higher CE over cycling, especially under high temperature and high current conditions. Finally, it is worth mentioning that, although the enhancements were demonstrated using NMC 622, it is believed that this protection of Al by AlPO4 can provide the same beneficial effects for all other high-voltage cathodes with different chemistries. The findings in this work also imply that the corrosion reaction occurs to Al current collector at relatively high potential can also occurs to the positive electrode, leading to the leaching of transition metal cations into the electrolyte. Therefore, a corrosion resistance coating on high-voltage cathodes is also highly desired to fully unlock their potential. Acknowledgments This research was supported by BMW Corporation. Argonne National Laboratory is operated for the U.S. Department of Energy by UChicago Argonne, LLC, under contract DE-AC02-06CH11357. Support from Tien Duong of the U.S. DOE's Office of Vehicle Technologies Program is gratefully acknowledged. H. Gao would also like to acknowledge the NSERC Canada Postdoctoral Fellowships Program. Z. Feng also thanks the Callahan Faculty Scholar Endowment Fund from Oregon State University. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.mtener.2017.12.001.

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