Surface Modification of the LiFePO4 Cathode for the Aqueous

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Surface modification of the LiFePO4 cathode for the aqueous rechargeable lithium ion battery Artur Tron, Yong Nam Jo, Si Hyoung Oh, Yeong Don Park, and Junyoung Mun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16675 • Publication Date (Web): 21 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017

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ACS Applied Materials & Interfaces

Surface modification of the LiFePO4 cathode for the aqueous rechargeable lithium ion battery Artur Tron,a Yong Nam Jo,b Si Hyoung Oh,c Yeong Don Park,a and Junyoung Muna* a

Department of Energy and Chemical Engineering, Incheon National University, 12-1, Songdo-

dong, Yeonsu-gu, Incheon 22012, Korea. E-mail: [email protected] b

Advanced Batteries Research Center, Korea Electronics Technology Institute, 68 Yatap-dong,

Bundang-gu, Seongnam, Gyeonggi-do, 13509, Korea c

Centre for Energy Convergence Research, Korea Institute of Science and Technology,

Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul, 02792, Korea KEYWORDS: Aqueous rechargeable lithium battery, Surface modification, LiFePO4, Coating, AlF3.

ABSTRACT: The LiFePO4 surface is coated with AlF3 via a simple chemical precipitation for aqueous rechargeable lithium ion batteries (ARLBs). During electrochemical cycling, the unfavorable side reactions between LiFePO4 and the aqueous electrolyte (1 M Li2SO4 in water) leave a highly resistant passivation film, which causes a deterioration in the electrochemical performance. The coated LiFePO4 by 1 wt.% AlF3 has a high discharge capacity of 132 mAh g−1 and a highly-improved cycle life, which shows 93% capacity retention even after 100 cycles,

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whereas the pristine LiFePO4 has a specific capacity of 123 mAh g−1 and a poor capacity retention of 82%. The surface analysis results, which include X-ray photoelectron spectroscopy and transmission electron microscopy results, show that the AlF3 coating material is highly effective for reducing the detrimental surface passivation by relieving the electrochemical side reactions of the fragile aqueous electrolyte. The AlF3 coating material has good compatibility with the LiFePO4 cathode material, which mitigates the surface diffusion obstacles, reduces the charge-transfer resistances and improves the electrochemical performance and surface stability of the LiFePO4 material in aqueous electrolyte solutions.

1. INTRODUCTION Lithium ion batteries (LIBs) have attracted much attention for various applications from conventional portable devices to electric vehicles (EVs) and energy storage systems (ESSs) because of their high energy/power density and credible cycle life.1 However, LIBs have critical limitations preventing their uses in large energy storage devices. One of the limitations is the high price of LIBs. The demand for expensive elements, such as Co ions in the cathode of LiCoO2 and LiNi0.33Co0.33Mn0.33O2, ultra-high purity electrolyte, and special manufacturing utilities to ensure the dry atmosphere, increases the cost of LIBs. Additionally, the carbonate electrolyte, which is used as an electrolyte of LIBs, has high safety concerns because it is flammable, harmful and toxic. LIBs have also been recognized as a great challenge because the safety issues of large energy storage devices become more important than before. In addition to the recent surge of fire accidents in mobile phones, customers have become increasingly concerned about the dangers of LIBs. Such anxiety will be accelerated because of the demand for


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large size batteries for EVs and ESSs from kWh to MWh.2-4 Therefore, the aqueous rechargeable lithium batteries (ARLBs) can be notably promising candidates to resolve the following two issues: high cost and safety concern because aqueous electrolytes are cheap enough, extremely safe, and environmentally friendly compared to the conventional organic electrolyte systems.5 Nevertheless, currently, the aqueous solutions as electrolytes for LIBs have not been considered the best candidates because of their poor electrochemical stability, which limits the operating potential of LIBs. At least, it is impossible to develop an ARLB with a higher energy density than that of conventional LIBs. Nonetheless, to accommodate as much energy as possible in the ARLBs, conventional cathode materials, such as LiCoO2, LiFePO4 and LiMn2O4, have been investigated for the cathodes of ARLBs.6-13 Fortunately, their operating potentials are in the electrochemical stability window of aqueous solutions. Additionally, new candidates with a high-energy density and long cyclability are not easy to find, except for conventional cathode materials. Among several candidates of cathode materials for ARLBs, we believe that olivineLiFePO4 is one of the most promising materials because Fe is an abundant element, which reduces the fabrication cost, and the phosphate anion is a notably robust anion to make safe LIBs with high theoretical capacity for high energy density and rate capability.14-19 However, their applications to ARLBs are also restricted because of the surface instability issues of LiFePO4, which are related to the electrochemically fragile aqueous electrolyte, which cause the capacity fading and poor rate capability.7,20 For conventional LIBs with carbonate organic electrolytes, many scientists have conducted studies to enhance the electrochemical performance of LiFePO4 by altering the morphology, doping and introducing coatings such as carbon, Al2O3 and AlF3.21-24 Among these approaches, coatings can be a notably effective method to solve the surface problem of LiFePO4 in ARLBs. Similarly, it was recently reported that “water-in-salt” systems,


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where the aqueous electrolyte has an exceptionally high concentration of lithium salt (> 20 mol L-1), have an exceptional operating potential. The operating potential is significant because of the surface fluoride layer on the negative electrode, which is a product of the decomposition of the electrolyte salt.25-30 The surface film of inorganic species from the “water-in-salt” system improves the cyclability of Li4Ti5O12 beyond the electrochemical stability window of the aqueous electrolyte.31 We thought that the surface coating could also play a beneficial role in the surface stability of the LiFePO4 material by protecting the surface of the LiFePO4 material from the contact with the aqueous electrolytes. Metal fluorides with strong ionic bonding are notably electrochemically stable; therefore, they are widely investigated for the surface modification of different types of electrode active materials of LIBs.32 The preparation of fluoride coating requires a notably simple coating process with precipitation and a heat treatment. At least for conventional LIBs, the fluoride coating is known to have acceptable lithium ion conductivity not to block the lithium transportation during cycling.33-38 Thus, the AlF3 surface coating can also create positive operating conditions to improve the electrochemical performance. In this work, we applied and optimized the AlF3 coating for surface modification of the LiFePO4 electrode materials for advanced ARLBs. This approach mitigates the detrimental behavior of LiFePO4 cathode for being a promising candidate for advanced ARLBs.

2. EXPERIMENTAL SECTION Pristine LiFePO4 were obtained from Aldrich. To make the AlF3 coating on LiFePO4, first, the AlF3-precursors of NH4F (Aldrich, South Korea) and Al(NO3)3·9H2O (Aldrich) were consistently mixed with LiFePO4 powder. The molar ratio between NH4F and Al(NO3)3·9H2O was controlled to be 1:3. The amount of precursors was precisely controlled to prepare 0.5, 1, 3,


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and 5 weigh percent (wt.%) of AlF3 versus LiFePO4 assuming that all precursors were totally converted to AlF3 after the coating process. Additionally, water, which amounted to half of the LiFePO4 mass, was poured into the mixture and thoroughly mixed for 20 minutes using mortar and pestle. Then, the mixture was dried at 120°C for 1 hour and subsequently heat-treated at 400°C for 2 hours in an inert Ar atmosphere. The composite electrodes with pristine LiFePO4 or various amounts of AlF3-coated LiFePO4 powder were prepared using the conventional method of mixing the active materials, super-P and poly(tetrafluoroethylene) (PTFE, Aldrich) with a weight ratio of 80:10:10 on a square stainless mesh of 0.25 cm2 with mass loading of 4 mg cm-2. These composite electrodes were used for the working electrode in the cell with 3-electrode configuration, where the counter electrode (CE) was the activated carbon composite electrode, and the reference electrode was Ag/AgCl (RE-1B, ALS, 0.197 V vs. SHE). The counter electrode (CE) was prepared with activated carbon (RP-20, Kuraray Chemical) using the same method for the working electrode, except the stainless mesh with a large area of 1 cm2 was used. For the electrolyte, 1 M of Li2SO4 (Aldrich) was dissolved in de-ionized water with pH value of 7.39 40 mL of the aqueous electrolyte was used to make a cell with three electrodes. The repeated galvanostatic charging and discharging were performed in the potential range from -0.2 to 0.8 V vs. Ag/AgCl using WBCS-3000 (Wonatech). The electrochemical behaviors of the pristine and AlF3-coated LiFePO4 electrodes were investigated with cyclic voltammetry (CV) in the identical potential range at a scan rate of 0.1 mV sec−1. The electrochemical impedance spectroscopy (EIS) measurements were performed using the multichannel electrochemical workstation of Zive Lab MP1 (Wonatech) in the frequency range from 100 kHz


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to 10 mHz at a voltage amplitude of 5 mV. The EIS data were obtained after the stabilization time, which was over an hour after the end of charge and discharge steps. To further investigate the surface stability of the LiFePO4 material, a self-discharge behavior was studied. After 2 cycles, at the elevated temperature of 60℃, the pristine and AlF3coated LiFePO4 were fully charged to 0.8 V vs. Ag/AgCl with a current density of 1 C. Then, the materials were rested for 6 hours in open-circuit condition at 60°C. After the rest time, these materials were discharged to -0.2 V at the 1 C rate. For the postmortem analyses, the electrodes were obtained by disassembling the cycled three-electrode cells. The crystal structure of the active materials before and after cycling was analysed by X-ray diffraction using a Rigaku SmartLab diffractometer with Cu Kα radiation in the 2θ range of 10–80° at 5° min-1. The surface morphologies of several electrodes were observed using field scanning electron microscopy (FE-SEM, JSM 7800F) with energy dispersive X-ray spectroscopy (EDS) and transmission electron micrographs (TEM, JOEL Oxford X-Max). The surface chemical states of the electrodes were also examined by X-ray photoelectron spectroscopy (XPS) measurements using a physical Electronics PHI5000 VersaProde II instrument with an X-rays Al anode as the source.

3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the LiFePO4 samples coated with 0, 0.5, 1, 3, and 5 wt.% of AlF3. For the XRD pattern of pristine LiFePO4, which is shown as a black line in Figure 1, all diffraction peaks match the pattern from the olivine structure of LiFePO4.40,41 The corresponding planes for each peak are described on top of each peak. The pattern of the olivine structure of LiFePO4 is found in the result of pristine LiFePO4 and every AlF3-coated sample.


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Thus, the original crystal structure of LiFePO4, which enables the electrochemically stable cycling through repeated insertion and extraction of lithium ions from the structure, is well preserved after the additional AlF3-coating process. In other words, the coating process does not strongly spoil the olivine structure of the LiFePO4 material. For the AlF3 coating material, the XRD peaks of AlF3 were not observed even in the 5-wt.%-AlF3-coated samples. It is thought that the AlF3 coating has a low amount, is thinly dispersed on the surface, and does not form a sufficiently long range order generating strong reflections in the XRD analyses. Meanwhile, the robust structural behavior of LiFePO4 even after the AlF3-coating process is consistent with previous reports.24,42

Figure 1. XRD patterns of the pristine and coated LiFePO4 with various content of AlF3 of 0.5, 1, 3 and 5 wt.% before cycling.

Additionally, to confirm the presence of the possible carbon coating on pristine LiFePO4, a thermo-gravimetric analysis (TGA) was performed in air. The mass of the sample during the


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TGA would decrease if the carbon on the surface was converted to form CO2 over 400°C. Instead of evidence for the carbon thermal reduction at 400°C, the pristine sample had an increase in weight of 4.5 wt.% during the TGA (Figure S1). This thermal weight gain is explained by the formation of Li3Fe2(PO4)3 and Fe2O3 from LiFePO4 above 600°C in air.43 Therefore, it is thought that pristine LiFePO4 did not have a lot of carbon to have a crucial effect on the performance of LiFePO4. To investigate the morphology of the prepared materials, FE-SEM, TEM and EDS analyses were performed. As shown in the FE-SEM images (Figure S2), the size of the round primary particles of pristine LiFePO4 was approximately 100 nm. The shape and size of AlF3-coated LiFePO4 were confirmed to be almost identical to those of pristine LiFePO4. Even after the coating process, it was not simple to check for the presence of AlF3 on the surface using only FE-SEM images. To determine the existence of the AlF3 coating material, the distribution of Al and F elements on the surface of LiFePO4 particles was investigated by the TEM and EDS analyses as shown in Figure 2. On the surface of pristine LiFePO4 powder, only O, P and Fe spectra were observed (Figure 2a). Unlike the pristine sample, the EDS mapping images of a 1wt.%-AlF3-coated LiFePO4 particle exhibit distinct Al and F spectra from AlF3 and Fe, P and O spectra from LiFePO4 (Figure 2b). Additionally, Al and F were found to be homogeneously spread on the particle surface, which indicates that the AlF3 coating fully covered the surface of LiFePO4 powder. Although there was AlF3 on the surface, the additional layer of AlF3 was not apparently distinguished in the TEM images. Therefore, we can conclude that the surface layer of AlF3 is not notably thick. This behavior highly corresponds to the absence of AlF3 in the XRD patterns and FE-SEM images.


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Figure 2. The EDS analysis of the (a) pristine and (b) coated LiFePO4 by 1 wt.% AlF3.

As shown in Figures 3a and b, all CV results from the pristine and 1-wt.%-AlF3-coated LiFePO4 have a single pair of reduction and oxidation current peaks of approximately 0.2 V vs. Ag/AgCl, which is related to the de-lithiation and lithiation during 5 charge/discharge cycles. These results imply that the well-known stable redox processes of the two-phase transition between LiFePO4 to FePO4 are successful and maintained during electrochemical cycling.16,22 In detail, in terms of polarization, the difference in the positions of the two peaks in the results of pristine LiFePO4 and the coated LiFePO4 were 0.25 and 0.18 V, respectively. The width of the peaks from AlF3-coated LiFePO4 was also smaller than that of the pristine sample. Thus, the AlF3 coating has a positive effect on the fast kinetics of intercalation/deintercalation of lithium


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ions into/from the LiFePO4 structure. Hence, the coating material can play a beneficial role in improving the overall electrochemical performances of LiFePO4 in aqueous solution instead of the effect of negative resistance.

Figure 3. Cyclic voltammograms of the (a) pristine and (b) coated LiFeO4 by 1 wt.% AlF3 at a scan rate of 0.1 mV s−1 in the potential range of -0.2–0.8 V, (c) Cyclic performance of the pristine LiFePO4 and coated LiFePO4 with various content of AlF3 of 0.5, 1, 3 and 5 wt.% at charge/discharge rate of 1 C, Voltage profiles of the (d) pristine and coated LiFePO4 with various content of AlF3 of (e) 1- and (f) 5-wt.% at charge/discharge rate of 1 C after cycling for 50th cycles in 1 M Li2SO4 aqueous electrolyte solution.

Figure 3c shows the cycling performance of the cells with the pristine sample and those coated with various AlF3 contents (0.5, 1, 3, and 5 wt.%) under the charge/discharge rate of 1 C in 1 M Li2SO4 aqueous electrolyte solution. The initial discharge capacity of the pristine LiFePO4 material was 130 mAh g−1, which is lower than the theoretical capacity of 170 mAh g-1. Because of the small nanosize particles of LiFePO4, it is believed that the incomplete crystalline


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structure exhibits a low capacity.44 Even after the coating of 0.5, 1 and 3 wt.% AlF3, the initial discharge capacity increased to 126, 132 and 130 mAh g-1, respectively, which is similar to the value of the pristine sample. For the samples with more coating layers than 3 wt.%, the initial capacity began to decrease. The 5-wt.%-AlF3 coating layer on LiFePO4 had a smaller initial specific capacity (110 mAh g-1) and significantly poorer cyclability. Thus, the large amount of AlF3 coating can be unfavorably resistive for lithium transportations for subsequent cycles and the 1st cycle. Because AlF3 is not ionically conducting material, a large amount of AlF3 coating can cause high polarization by hindering lithium ion transportation through surface. The uncoated sample has better cyclability than the 5-wt.%-AlF3-coated sample, but the capacity of the uncoated sample dropped sharply during the initial 10 charge/discharge cycles and were well maintained in the latter cycle numbers. All coated samples had better cyclabilities than the pristine electrode, except the sample coated by 5 wt.% AlF3. Thus, the optimized amount of AlF3 coating is important to balance the effect of relieving polarization from the side reactions and the sluggish behavior of lithium transportation through the AlF3 layer. The surface of pristine LiFePO4 is considered fragile to the aqueous electrolyte and continues to cause the side reaction and a growth of resistance. However, the AlF3-coated samples appear to successfully prevent the side reaction that causes kinetic hindrances. Figures 3d-f show the voltage curves of the pristine and 1- and 5-wt.%-AlF3-coated LiFePO4 for the 1st, 2nd, 5th, 10th, 20th, 30th, 40th and 50th cycles, respectively. In every voltage curve, there is a pair of distinct plateaus at approximately 0.15 V vs. Ag/AgCl during charging and discharging, which is consistent with the results in Figures 3a and 3b. For the 1st cycle charge in Figure 3d, the pristine sample has a sloping curve at the potential above 2 V and a specific charge capacity of 143 mAh g-1. Considering that the 1st discharge specific capacity is


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only 130 mAh g-1, which is less than 143 mAh g-1, and the associated curve relatively disappears during the subsequent charge at the potential above 2 V, the sloping curve of the 1st charge is considered an irreversible reaction such as an electrolyte side reaction. This sloping curve completely disappears in the 1-wt.%-AlF3-coated sample in Figure 3d. Thus, the coating layer successfully inhibits the electrolyte side reaction in the initial cycle. However, the observed voltage curve in the sample with a 5 wt.% AlF3 coating shows a much lower initial charge capacity of 81 mAh g-1 (Figure 3f). Nonetheless, it was confirmed that the capacity obtained upon charging was completely reversible during the subsequent discharge, and the electrolyte side reaction appeared successfully reduced. In terms of Coulombic efficiency, it is prominent that the 1 wt.% AlF3 coating strongly improves the initial Coulombic efficiency compared to pristine LiFePO4. For the 1st cycle and subsequent cycles, the average Coulombic efficiencies over 50 cycles of the cells with AlF3-coated LiFePO4 were 99.7, 99.8, 99.6, and 99.8% for the 0.5, 1, 3, and 5 wt.% AlF3 coating, respectively. Those values are higher than the average Coulombic efficiency of 99.4% of pristine LiFePO4.7,17,18 These results also support that the surface coating effectively reduces the irreversible electrolyte decomposition during the 1st cycle. In addition, a long plateau of the 1st discharge curve from the pristine sample after one charge begins at 1.2 V in Figure 3c, whereas the 1-wt.%-AlF3-coated sample begins at 0.9 V in Figure 3d. Even in the 5 wt.% coated sample, the associated plateau is observed at 1.0 V (Figure 3f). Hence, the AlF3 coating has a beneficial effect on the reduction of resistance by inhibiting the side reaction of the electrolyte. However, a large amount of AlF3 such as 5 wt.% hinders the surface lithium diffusion on the surface during the charging and discharge process. We expected that the surface changed because of the side reactions of aqueous electrolyte decompositions. To investigate in detail, the postmortem SEM analyses after 100 cycles were


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conducted by SEM (Figure 4). Notably tiny round particles under 100 nm are considered the carbon conducting agent of super-P, and wire-type materials are considered PTFE binders. Except for these materials, all AlF3-coated cathode materials maintained their initial state well over 100 cycles (Figures 4b-d), whereas the uncoated samples showed an additional rough passivation on the surface (Figure 4a).

Figure 4. SEM images after cycling from the (a) pristine and coated LiFePO4 with various content of AlF3 of (b) 0.5, (c) 1 and (d) 3 wt.% in 1 M Li2SO4 aqueous electrolyte solution.

To identify the chemical species in the aforementioned surface film on the active materials, XPS analyses were performed on the electrochemically used electrodes after 100 cycles. The Al 2p, F 1s, O 1s, and Fe 2p spectra from the cycled pristine and 1-wt.%-AlF3-coated LiFePO4 are shown in Figure 5. The Al 2p and F 1s peaks at 74.4 eV and 685.2 eV, which are related to Al3+ and F− elements, are only observed on the AlF3-coated sample (Figures 5a-d). It is confirmed that the AlF3 coating significantly maintained even after a long cycling condition. In addition, the


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Fe 2p spectra of both samples have the main peak (711.4 eV for Fe 2p3/2 and 724.6 eV for Fe 2p1/2) and its satellites (711.2 eV for Fe 2p3/2 and 724.1 eV for Fe 2p1/2), which are assigned to Fe2+ in the LiFePO4 phase and Fe3+ ions in the FePO4 phase, respectively (Figures 5e and f).

Figure 5. XPS spectra of (a, b) Al 2p, (c, d) F 1s, (e, f) Fe 2p and (g, h) O 1s of the pristine and coated LiFePO4 by 1 wt.% AlF3 after 100 cycles at the charge/discharge rate of 1 C in 1 M Li2SO4 aqueous electrolyte solution.

The O 1s spectra in Figures 5g and h were deconvoluted with peaks from PO43- (531.8 and 531 eV), OH (533.0 eV) and Fe2O3 (529.5 eV). Basically, all spectra from the pristine and AlF3-


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coated samples have peaks from PO43- because there are PO43- polyanions in the active material of LiFePO4. Contrary to the behavior of photoelectrons from PO43-, the peaks from –OH and Fe2O3 are more prominent in the uncoated sample. The peak of pristine LiFePO4 at 529.5 eV can be attributed to Fe2O3 for the pristine sample, which is related to the side reaction components, whereas these peaks of the coated LiFePO4 material are assigned during cycling. Therefore, for the coated LiFePO4 material, its surface does not change after cycling, whereas the pristine LiFePO4 material significantly changes the surface as shown in Figure 4. We can conclude that the surface structure of AlF3-coated LiFePO4 does not significantly alter during cycling, and the capacity loss of pristine LiFePO4 in Figure 3c indicates that the surface resistance is caused by the side reactions on the surface of LiFePO4 particles in the aqueous electrolyte solution during cycling. Therefore, based on the XPS results, we can conclude that the surface coating of AlF3 successfully relieves the negative surface passivation from the side reaction of the aqueous electrolyte solution.45,46 Figure 6 shows the TEM images of the cycled pristine and 1-wt.%-AlF3-coated LiFePO4 after 100 cycles. Consistent with the XPS results, on the surface of pristine LiFePO4 particles, many additional aggregates are easily observed and attributed to Fe(OH)2 and Fe2O3. Meanwhile, for the coated LiFePO4 particles, those passivated layers are not observed. It should be noted that the similar mechanism formation of side reaction components on the surface of pristine LiFePO4 particles was reported to cause the poor cyclability, particularly for aqueous electrolyte solutions.18,47 The surface passivation of various forms of hydroxides and oxides by iron ion dissolution can result in the poor cycle performance of the LiFePO4 material. Thus, the TEM, XPS and SEM analyses were compared to support the positive effect of the AlF3 coating in improving the electrochemical performance of LiFePO4 material as follows: the AlF3 coating


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prevents the formation of side reaction components on the surface of LiFePO4 material during cycling in the aqueous electrolyte solution.

Figure 6. TEM images of the (a) pristine and (b) coated LiFePO4 by 1 wt.% AlF3 after 100 cycles at the charge/discharge rate of 1 C in 1 M Li2SO4 aqueous electrolyte solution.

To unveil the change in resistance behavior of the cell during cycling, AC impedance analyses were performed. Figures 7a and b show the Nyquist plots with one semicircle in the region of high to medium frequency, which corresponds to the surface resistance against lithium transportation through surface film and charge transfer between the aqueous electrolyte solution and the electrode materials. The Nyquist plots also show an inclined line in the low-frequency region, which relates to the diffusion of lithium ions into/from the structure of LiFePO4.20 The impedance spectra were fitted by the equivalent circuit as shown in the inset of Figure 7a. Re is the resistance of the aqueous electrolyte solution, and Rsurface is the surface ion transportation resistance, which is associated with the redox reaction of lithium ions into/from the structure of LiFePO4. In addition, we used a constant-phase element (CPE) instead of a pure capacitor, where CPE1 is placed parallel to the surface resistance (Rsurface) and in a series with another Warburg


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impedance. The AlF3-coated LiFePO4 material has a smaller resistance (Rsurface, 82 and 103 Ohm) than the pristine LiFePO4 material (151 and 187 Ohm) after the 1st and 15th charge and discharge cycles, respectively. The AlF3-coated LiFePO4 material has a smaller resistance than the pristine LiFePO4 material after the charge and discharge cycling. The AlF3 coating on the surface of LiFePO4 particles relieves the resistance increase to make the lithium reactions fast. Unlike the case of organic electrolytes, the conventional SEI layer is not generally formed in the aqueous electrolyte.20,48 However, the surface resistance (Rsurface) can occur through the electrolyte / electrode interface because of the passivation layer of FeOH and Fe2O3, which were detected in the XPS and TEM results. In addition, these results are consistent with the improved cyclability of the AlF3-coated LiFePO4 material as shown in Figure 3c.

Figure 7. Nyquist plots of the (a) pristine and (b) coated LiFePO4 by 1 wt.% AlF3 in 1 M Li2SO4 aqueous electrolyte solution after the 1st, 2nd, 5th, 10th and 15th cycles.

Figure 8 shows the voltage profiles of the pristine and coated LiFePO4 by AlF3 materials in the self-discharge test at the elevated temperature of 60oC. During the storage, the electrochemical side reaction between the electrolyte and the electrode was accelerated and


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decreased the charge capacity. It is found that the 0.2 V polarization of the pristine LiFePO4 is much higher than 0.1 V of the 1-wt.%-AlF3-coated LiFePO4. It is thought that the accelerated surface failure caused a high polarization for the pristine LiFePO4. After the storage test, the Coulombic efficiency of the pristine LiFePO4 was low (83.5%) because the discharge capacity was even less than the charge capacity. In contrast, the 1-wt.%-AlF3-coated LiFePO4 almost maintained the charged capacity even after 6 hours of storage based on the high Coulombic efficiency of 98.1%. These results confirm that the electrochemical side reaction from the aqueous solution is notably severe on LiFePO4. This detrimental behavior is significantly mitigated by the surface modification of AlF3.

0.8 pristine LiFePO4

Potential / V vs Ag/AgCl

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AlF3 - 1 wt.%

0.6

0.4

0.2

0.0

-0.2 0

20

40

60

80

100

120

140

-1

Specific Capacity / mAh g

Figure 8. Voltage profiles of the pristine and coated LiFePO4 by 1 wt.% AlF3 after storage for 6 hours at 60oC.

The rate capabilities of pristine and AlF3-coated LiFePO4 under various current densities of 1, 2, 5, 10, and 20 C are summarized in Figure 9. The pristine LiFePO4 exhibited a specific


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capacity of approximately 123 mAh g-1 under the current condition of 1 C. The measured specific capacities suddenly decreased to 52 and 3 mAh g-1 when the current condition increased to 5 and 10 C, respectively. The ionic conductivity of the aqueous electrolyte is as high as 100 mS cm-1, which is much higher than 20 mS cm-1 of the conventional electrolyte, but the rate capability of the ARLB with pristine LiFePO4 was poor. It is strongly believed that the lithium diffusion through the surface is notably hindered by the undesired surface films. Contrarily, all coated samples in Figure 9 exhibit better rate capability than pristine LiFePO4.11-13,48-50 Among them, the 1-wt.%-AlF3-coated LiFePO4 showed the best rate capability at various current densities compared to the pristine and 0.5- and 3-wt.%-AlF3-coated LiFePO4 materials. Again, this surface failure is highly relieved by the surface stabilization via AlF3 coatings. On the other hand, the 3 wt.% of AlF3 exhibited a poorer rate capability than the other coated samples owing to highly non-ionically conducting behavior of thick AlF3 coating. Once again, it can be seen that optimization of the amount of AlF3 coating should be made to improve electrochemical performance.


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Figure 9. Cyclic performance of the pristine and coated LiFePO4 by AlF3 of 0.5, 1 and 3 wt.% at charge/discharge current densities of 1, 2, 5, 10 and 20 C rates.

Based on the above experimental results and discussion, we proposed a schematic figure of the failure and improvement mechanism for pristine and AlF3-coated LiFePO4 materials in aqueous electrolyte solutions before and after cycling, as shown in Figure 10. First, before cycling, for the pristine and AlF3-coated LiFePO4 materials, the bulk particles remain unchanged and maintain a good electrical contact among the active material, conducting agent and current collector. During cycling, the LiFePO4 surface has electrochemical side reactions, which result in a high polarization of the surface layer via insulating phenomena for ionic and electronic transfers. In addition, the coated particles of the LiFePO4 material have a stable surface because of the AlF3 coating, which enables the maintenance of structural integrity and correlates with its excellent electrochemical behavior and cycling performance (Figure 3c). In addition, the


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decrease in capacity during a long cycle life for the pristine material can be related to factors such as the destruction of the electrode, loss electric contact among the particles, low electronic conductivity of these materials, and formation of the side reaction in the aqueous medium. In contrast, the protective layer on the surface of LiFePO4 material favors fast kinetics and helps to reduce the formation of the side reaction in the aqueous medium. Based on these results, we conclude that the surface of the cathode active material is notably crucial to support a long cycle life and a high rate capability in aqueous electrolyte solutions. Thus, we suggest that the obtained results demonstrate the stable cycling performance of LiFePO4 materials because of the thin film of the AlF3 coating material on the surface of LiFePO4 particles in aqueous electrolyte solutions.

Figure 10. Schematic diagram of the reaction mechanism for the (a) pristine and (b) coated LiFePO4 by AlF3 in the aqueous electrolyte solution before and after cycling.

4. CONCLUSIONS We prepared AlF3-coated LiFePO4 to study the failure mechanism of the surface of LiFePO4 in 1 M Li2SO4 aqueous electrolytes for aqueous rechargeable batteries and solve the related problems. The pristine LiFePO4 forms a high-resistance surface film because of the


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surface side reaction in the aqueous electrolyte solution, The AlF3-coated LiFePO4 material has a highly extended cyclability, an excellent rate capability and a decreased surface resistance. Systematic analyses with the XRD, SEM, EDS, XPS and TEM methods were used to confirm that the AlF3 coating prevented the side reaction components, which can occur on the surface of LiFePO4 materials in aqueous electrolyte solutions. The pristine and coated LiFePO4 with 1 wt.% AlF3 materials in 1 M Li2SO4 aqueous electrolyte solution deliver the initial discharge capacities of 128 and 132 mAh g−1, respectively, and 109 and 123 mAh g−1 after 100 cycles at 1 C, respectively. Meanwhile, the coated LiFePO4 by the 1-wt.% AlF3 material has better capacity retention (93% after 100 cycles) than the pristine LiFePO4 material (82%). In addition, the 1wt.%-AlF3-coated LiFePO4 has higher Coulombic capacity and rate capability during cycling than the pristine LiFePO4 material. The electrochemical performance of the LiFePO4 material in the aqueous electrolyte solution were improved because of the AlF3 coating material on the surface of the LiFePO4 material. Thus, the AlF3 coating material is a notably promising approach to enhance the electrochemical performances of electrode materials for ARLBs. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: ##########. Experimental section including details about TGA curve of the pristine LiFePO4 measured from room temperature to 800oC in air at heating rate of 10oC min−1 (Figure S1), SEM images of the pristine and coated LiFePO4 with various content of AlF3 of 0.5, 1, 3 and 5 wt.% before cycling (Figure S2) (PDF). AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Post-Doctor Research Program (2015-2016) through the Incheon National University (INU), Incheon, Republic of Korea. The authors also acknowledge the support by the KIST Institutional Program (Project No. 2E26292). REFERENCES (1) Palacin, M.R. Recent Advances in Rechargeable Battery Materials: A Chemist’s Perspective, Chem. Soc. Rev. 2009, 38, 2565–2575. (2) Tarascon, J.M.; Armand, M. Review Article, Issues and Challenges Facing Rechargeable Lithium Batteries, Nature 2001, 414, 359–367. (3) Wang, G.J.; Fu, L.J.; Zhao, N.H.; Yang, L.C.; Wu, Y.P.; Wu, H.Q. An Aqueous Rechargeable Lithium Battery with Good Cycling Performance, Angew. Chem. Int. Ed. 2007, 46, 295–297. (4) Wang, G.J.; Zhao, N.H.; Yang, L.C.; Wu, Y.P.; Wu, H.Q.; Holze, R. Characteristics of an Aqueous Rechargeable Lithium Battery (ARLB), Electrochim. Acta 2007, 52, 4911–4915. (5) Li, W.; Dahn, J.R.; Wainwright, D.S. Rechargeable Lithium Batteries with Aqueous Electrolytes, Science 1994, 264, 1115-1118. (6) Ruffo, R.; Wessells, C.; Huggins, R.A.; Cui, Y. Electrochemical Behavior of LiCoO2 as Aqueous Lithium-Ion Battery Electrodes, Electrochem. Commun. 2009, 11, 247–249.


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Table of Contents Graphic 31x11mm (300 x 300 DPI)


Surface Modification of the LiFePO4 Cathode for the Aqueous

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