Raising the cycling stability of aqueous lithium-ion batteries by

ARTICLES PUBLISHED ONLINE: 8 AUGUST 2010 | DOI: 10.1038/NCHEM.763

Raising the cycling stability of aqueous lithium-ion batteries by eliminating oxygen in the electrolyte Jia-Yan Luo, Wang-Jun Cui, Ping He and Yong-Yao Xia* Aqueous lithium-ion batteries may solve the safety problem associated with lithium-ion batteries that use highly toxic and flammable organic solvents, and the poor cycling life associated with commercialized aqueous rechargeable batteries such as lead-acid and nickel-metal hydride systems. But all reported aqueous lithium-ion battery systems have shown poor stability: the capacity retention is typically less than 50% after 100 cycles. Here, the stability of electrode materials in an aqueous electrolyte was extensively analysed. The negative electrodes of aqueous lithium-ion batteries in a discharged state can react with water and oxygen, resulting in capacity fading upon cycling. By eliminating oxygen, adjusting the pH values of the electrolyte and using carbon-coated electrode materials, LiTi2(PO4)3/Li2SO4/LiFePO4 aqueous lithium-ion batteries exhibited excellent stability with capacity retention over 90% after 1,000 cycles when being fully charged/ discharged in 10 minutes and 85% after 50 cycles even at a very low current rate of 8 hours for a full charge/discharge offering an energy storage system with high safety, low cost, long cycling life and appropriate energy density.

A

review of the working mechanisms of commercially available aqueous battery technologies, such as nickel metal hydride (Ni-MH), nickel–cadmium (Ni-Cd) and lead-acid (Pb-acid) batteries has shown that none of them could offer long cycling stability. This should not be surprising because the alloy in Ni-MH is pulverized during the cycling process and Pb-acid and Ni-Cd batteries rely upon the dissolution/deposition process with Pb or Cd, which means that the electrodes are not fully reversible. The lithium-ion battery using two intercalated compounds (carbon anode and LiCoO2 cathode) in an organic solution electrolyte was commercialized by Sony in 1990 and is now widely used for cellular phones, notebook-size personal computers, video and digital cameras, and other electronics owing to its long cycling capability and high energy density. The lithium-ion battery has much better cycling stability than Ni-MH, Ni-Cd and Pb-acid batteries because the lithium ion can be reversibly intercalated into a lithium-accepting anode and deintercalated from a lithium-source cathode without destroying the structure of the electrode materials. However, despite the remarkable performance of these organicbased systems, they suffer from the use of highly toxic and flammable solvents, which can cause safety hazards if used improperly, such as overcharging or short-circuiting. Recently, numerous lithium-ion battery accidents causing fires and explosions have been reported. Furthermore, non-aqueous electrolytes generally have ion conductivities about two orders of magnitude lower than those of aqueous electrolytes, and the fabrication costs when using organic electrolytes are high. These drawbacks limit their application in large-scale batteries, which require low cost, high safety and long cycling life. An attractive approach to circumvent this problem is to use an aqueous electrolyte for lithium-ion batteries, which adopt a ‘rocking-chair’ concept similar to the organic lithium-ion battery. In 1994, Dahn’s group reported an aqueous lithium-ion battery based on the same technological concepts developed by Sony, in which VO2 was used as a negative electrode and LiMn2O4 as a positive electrode. The cell could operate at an average voltage close to 1.5 V with a specific energy density of 75 W h kg21 based on the

total weight of both electrode materials1. With this combination, the safety problem arising from the use of organic electrolytes is fundamentally resolved, and the rigorous assembly conditions required for non-aqueous lithium-ion batteries can be avoided. However, the cycling life of the VO2 (B)/LiMn2O4 aqueous lithium-ion battery is poor (‘B’ designates a particular crystal form of VO2 (ref. 1)). Following this work, many aqueous lithium-ion batteries such as LiV3O8/LiNi0.81Co0.19O2 , LiV3O8/LiCoO2 , TiP2O7/LiMn2O4 and LiTi2(PO4)3/LiMn2O4 systems have also been reported2–4. However all these systems have poor cycling stability similar to that of VO2 (B)/LiMn2O4: the capacity retention is typically less than 50% after 100 cycles. Studies addressing the mechanism of capacity fading during cycling in aqueous lithium-ion batteries have been very limited, and mostly focus on the dissolution of electrode materials into the bulk electrolyte5–7. Even the battery systems consisting of insoluble electrode materials, such as LiMn2O4 positive and LiTi2(PO4)3 negative electrodes, have shown fast capacity fading4. Until now, the mechanism responsible for this severe capacity fading upon cycling has not been clarified, and little progress has been made during the past two decades. In the present work, we analysed the stability of electrode materials in aqueous electrolytes extensively, and for the first time we found that, the discharged state of lithium-ion intercalated compounds (LICs) of all negative electrode materials suitable for aqueous lithium-ion batteries reacts with water and O2 , with no dependence on the pH value of the electrolyte. The instability of the lithiated LIC is mainly responsible for the capacity fading of aqueous lithium-ion batteries during charge/discharge cycling. By eliminating O2 (using a sealed cell), adjusting the pH values of the electrolyte, and using carbon-coated electrode materials, a LiTi2(PO4)3/LiFePO4 aqueous lithium-ion battery in Li2SO4 aqueous electrolyte exhibits significant improvement in cycling stability.

Results and discussion Figure 1 gives the intercalation potential of some electrode materials that could possibly be used for aqueous lithium-ion batteries. The chemical/electrochemical processes of LIC electrodes in aqueous

Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200433, China. * e-mail: [email protected] 760

NATURE CHEMISTRY | VOL 2 | SEPTEMBER 2010 | www.nature.com/naturechemistry

© 2010 Macmillan Publishers Limited. All rights reserved.

NATURE CHEMISTRY

4.5

1.5 LiCoO2

3.5

LiMn2O4 LiV2O5

0.0 AC

ev H2

VO2

tio n

LiV3O8

–1.0 –1.5

LiNbO5

0

7 pH

3.0 2.5

olu

–0.5

NASICON

γ-FeOOH

Potential (V versus Li+/Li)

0.5

4.0 LiFePO4

n io ut ol

Potential (V versus NHE)

LiNiO2

ev O2

1.0

ARTICLES

DOI: 10.1038/NCHEM.763

2.0

Li(intercalated) + H2 O ⇔ Li+ + OH− + (1/2)H2 The calculated potential of the LIC V(x) in equilibrium with H2O at a particular pH is as follows:

1.5

14

rather than undergoing the electrochemical redox process. We speculate that, in the presence of O2 , the discharged state of the LICs of any negative electrode candidates for the aqueous lithium-ion batteries would react with H2O and O2 for any pH value of the electrolyte, resulting in capacity fading upon cycling. Therefore, aqueous lithium-ion batteries can not work sustainably in the presence of O2. In the absence of O2 , the lithium-ion intercalated compounds may react with H2O as in following reaction, which is similar to the result presented by Dahn8:

V(x) = 3.039 − 0.059 pH (V)

Figure 1 | The intercalation potential of some electrode materials that could possibly be used for aqueous lithium-ion batteries. Left: O2/H2 evolution potential versus NHE for different pH in 1 M Li2SO4 aqueous solution. Right: lithium-ion intercalation potential of various electrode materials versus NHE and Li/Liþ. Theoretically, an aqueous lithium-ion battery can be assembled by combining a lower potential lithium-accepting anode and a higher potential lithium-source cathode within the O2/H2 evolution potential range. AC, activated carbon; NASICON, materials with NASICON structure.

solutions are much more complicated than those in the organic electrolytes. Many side reactions are involved, such as electrode materials reacting with water or O2 , proton co-intercalation into the electrode materials parallel to the intercalation of lithium ions, H2/O2 evolution reactions, and the dissolution of electrode materials in water. It has been demonstrated that materials with a voltage versus Li/Liþ of greater than 3.3 V are basically stable7–10. When acting as negative electrodes for aqueous lithium-ion batteries, the intercalation potential of lithium-ions is generally below 3.3 V versus Li/Liþ. First, we considered the stability of the electrode materials in the presence of both H2O and O2, because the aqueous battery customarily operates in air. The following reaction may occur: Li(intercalated) + (1/4)O2 + (1/2)H2 O ⇔ Li+ + OH− The potential of a LIC, V(x), can be calculated with the following equation7,9,11: V(x) = −

1 int uLi (x) − u0Li e

(1)

where uint Li (x) is the chemical potential of intercalated Li in LIC, u0Li is the chemical potential of Li in Li metal and e is the magnitude of the electron charge. The potential of a LIC, V(x), in equilibrium with O2 and H2O at a particular pH can be calculated with equation 2. (The details for the calculation process are shown in the Supplementary Information.) V(x) = 4.268 − 0.059 pH (V)

(2)

Notably in the presence of O2 , no materials can be used as negative electrodes for aqueous lithium-ion batteries regardless of the pH of the electrolyte, according to equation 2. The lithium-ion intercalation potential of the negative electrodes for aqueous lithium-ion batteries is generally below 3.0 V versus Li/Liþ, whereas the equilibrium voltage is 3.85 V at pH 7 and 3.50 V at pH 13. This means that the reduction state of all negative electrode materials would theoretically be chemically oxidized by the O2 and H2O

(3)

H2O may also chemically oxidize the reduction state of some negative materials for aqueous lithium-ion batteries. Theoretically, we can determine whether or not the lithium-ion intercalation at a particular pH of electrolyte is stable, and we can also adjust the pH of the electrolyte to guarantee the stability of the electrodes. For example, as the lithium-ion intercalation potential in LiTi2(PO4)3 is 2.45 V versus Li/Liþ, it is theoretically not stable in pH 7 aqueous solutions (2.626 V versus Li/Liþ of equilibrium voltage). Chemical stability can however be obtained in the aqueous solution in the absence of O2 by adjusting the pH of the aqueous solution electrolyte to more than 10—for example at pH 13, the equilibrium voltage is 2.272 V versus Li/Liþ (see Supplementary Fig. S1 and Table S1). Second, the positive electrode materials are generally stable in water. However, protons may be co-intercalated into the electrode materials parallel to the intercalation of lithium ions in the aqueous solution electrolyte. The proton intercalation depends on both the crystal structure and the pH of the electrolyte. It has been reported that spinel Li12xMn2O4 , and olivine Li12xFePO4 do not encounter such proton insertion, whereas delithiated layered Li12xCoO2 , Li12xNi1/3Mn1/3Co1/3O2 , and so on, show a significant concentration of protons in the lattice during deep lithium extraction with a low pH electrolyte12,13. However, this can be addressed easily by modulating the potential for proton intercalation through adjusting the pH of the electrolyte; for example, in LiCo1/3Ni1/3Mn1/3O2 stable lithium-ion intercalation is possible in a solution over pH 11, and in LiCoO2 over pH 9 (ref. 14). It should be noted that LiFePO4 decomposes in strong alkaline solutions, but the decomposition can be slowed down by carbon coating the material15. The LiFePO4 used here contains 15 wt% of coating carbon and shows excellent stability (see later discussion). Third, the H2/O2 evolution reaction in aqueous electrolyte is a basic factor that needs to be considered, because the capacity of the electrode materials should be used as much as possible before electrolyte decomposition. Thermodynamically, the aqueous electrolyte shows an electrochemical stability window of 1.23 V. Kinetic effects may expand the stability limit to 2 V. For instance, Pb-acid batteries can have an output voltage of 2.0 V. In principal, the use of most electrode materials depends on the pH value of the electrolyte. Lithium ions in LiMn2O4 can be fully extracted at pH 7, but only half of the lithium ions can be extracted at a pH greater than 9 before O2 evolution. LiFePO4 can be used over a broader pH range from 7 to 14. Fourth, the electrode materials should be insoluble in water. The dissolution scales mainly with surface area. For example, the vanadium oxides VO2 , LiV3O8 , LiV2O5 , and so on, are typically prepared at low temperature with a relatively large surface area and hence they are not good choices as electrode materials in



761

ARTICLES

NATURE CHEMISTRY b

4 2

4 2

Current (mA)

Current (mA)

a

0 –2 –4

0 –2 –4

–6

–6 –1.0

–0.8

–0.6

–0.4

–1.0

Voltage (V versus SCE)

–0.8

–0.6

–0.4


d

4 2

4 2

Current (mA)

Current (mA)

c

DOI: 10.1038/NCHEM.763

0 –2 –4

0 –2 –4

–6

–6 –1.0

–0.8

–0.6

–0.4

–1.0


–0.8

–0.6

–0.4


Figure 2 | Cyclic voltammograms of LiTi2(PO4)3 at the 1st, 2nd, 3rd, 5th and 10th cycles. a, pH 7 in the absence of O2. b, pH 13 in the absence of O2. c, pH 7 in the presence of O2. d, pH 13 in the presence of O2. Scan rate ¼ 0.3 mV s21. Fast capacity fading upon cycling was detected in both pH 7 and pH 13 aqueous solutions in the presence of O2. However, in the absence of O2 , good stability of the LiTi2(PO4)3 electrode was observed (the arrows indicate the direction of cycle 1 to cycle 10).

a

0.0

0.0

b

In the absence of O2

In the presence of O2

–0.2



In the absence of O2

–0.4

–0.6

In the presence of O2

–0.2

–0.4

–0.6

–0.8

–0.8 0

200

400

600

800

Time (s)

0

1,000

2,000

3,000

4,000

Time (s)

Figure 3 | Typical charge/discharge curves of LiTi2(PO4)3 at 4C and 1C charge/discharge rates in the presence/absence of O2. a, At a 4C rate, the coulombic efficiency of LiTi2(PO4)3 in the aqueous electrolyte was 99% in the absence of O2 and 92% in the presence of O2. b, This discrepancy in coulombic efficiency became more obvious when cycled at a 1C rate—the coulombic efficiency was 98% in the absence of O2 versus 77% in the presence of O2. This confirms that the reduction state Li32xTi2(PO4)3 can be chemically oxidized.

aqueous lithium-ion batteries; electrode materials with small surface areas are preferred. Considering all the factors above, we chose LiTi2(PO4)3 as the negative electrode in our aqueous lithium-ion battery because it has relatively satisfactory capacity, excellent operating potential in an aqueous electrolyte (about –0.5 V versus natural hydrogen electrode (NHE)) and a flat voltage plateau. Moreover, it has a relatively small surface area of about 0.7 m2 g21 because it is normally prepared at high temperature5. The cyclic voltammograms of LiTi2(PO4)3 at various operating conditions are shown in Fig. 2. One pair of redox peaks was observed for LiTi2(PO4)3 tested under all conditions, which agrees with the lithium-ion intercalation/de-intercalation process in an organic electrolyte. In the presence of O2 , the fast capacity fading upon cycling was detected in both pH 7 and pH 13 aqueous solutions. However, in the absence of O2 , LiTi2(PO4)3 undergoes a different process. Even though there is some irreversibility in the first cycle, which is mainly due to the residual O2 in the testing 762

cells, the good overlap of the anodic and cathodic peaks in the subsequent cycles indicates that the LiTi2(PO4)3 electrode is stable. This is consistent with the calculations. The reason for the capacity fading of LiTi2(PO4)3 in the aqueous electrolyte in the presence of O2 is that the reduction state Li32xTi2(PO4)3 is chemically oxidized by the O2 instead of undergoing the electrochemical oxidation process. This can be illustrated directly with the charge/discharge coulombic efficiency of LiTi2(PO4)3 in the aqueous electrolyte in the presence and absence of O2. It can be seen from Fig. 3 that when cycled at a rate of 4C (where 1C corresponds to complete discharge in 1 hour), the charge/discharge coulombic efficiency of LiTi2(PO4)3 in the aqueous electrolyte in the absence of O2 was 99%, which was higher than that in the presence of O2 (92%). This discrepancy became more obvious when LiTi2(PO4)3 was cycled at a rate of 1C: the coulombic efficiency was 98% versus 77%. This result confirmed that the reduction state Li32xTi2(PO4)3 can be chemically oxidized to LiTi2(PO4)3 and other impurities by O2 during the discharging of LiTi2(PO4)3. NATURE CHEMISTRY | VOL 2 | SEPTEMBER 2010 | www.nature.com/naturechemistry



–0.5

ARTICLES

DOI: 10.1038/NCHEM.763

(i)

(ii)

–0.6

–0.7

–0.8 0

50

100

150

200

250

Time (h)

Figure 4 | The typical self-discharge curves of LiTi2(PO4)3 in an aqueous electrolyte at pH 13. (i) In the presence of O2 , the open circuit voltage could only be sustained at the equilibrium potential for about 15 hours. (ii) In the absence of O2 , the open circuit voltage could last for about 10 days at equilibrium. This result strongly confirmed that the reduction state Li32xTi2(PO4)3 can be chemically oxidized by O2 during the discharge process of LiTi2(PO4)3.

These results were further confirmed by the open-circuit voltage test in Fig. 4. The process was carried out as follows. LiTi2(PO4)3 was first discharged to –0.85 V at a current rate of 1C. The open circuit voltage of Li32xTi2(PO4)3 was then measured as a function of time. It was clearly observed that if the Li32xTi2(PO4)3 was exposed to O2 the open circuit voltage could only be sustained at the equilibrium potential for about 15 hours. In the absence of O2 , however, it could last for about 10 days. A similar phenomenon was also found in other electrode materials, such as LiV3O8 (see Supplementary Fig. S2). This result again strongly confirmed that the reduction state Li32xTi2(PO4)3 can be chemically oxidized by O2 during the discharge process of LiTi2(PO4)3. This was also confirmed by the charge/discharge test of LiTi2(PO4)3/LiFePO4 in an open system at very small current rate (C/8, 8 h rate). It was found that the cell was not rechargeable in the presence of O2 as the speed of the oxidation process for the reduced state Li1þxTi2(PO4)3 by O2 and H2O is much faster than that of the lithium intercalation (see Supplementary Fig. S3). It is worth mentioning that even though the reduction potential of water at pH 7 (2.63 V versus Li/Liþ) is slightly higher that the potential of LiTi2(PO4)3 (2.45 V versus Li/Liþ), LiTi2(PO4)3 did not show obvious capacity fading because the potential discrepancy DE between the water reduction potential and the lithium intercalation potential LiTi2(PO4)3 is only about 0.2 V, which is much lower than the DE of 1.4 V between the O2 reduction potential (3.85 V versus Li/Liþ) and the lithium intercalation potential LiTi2(PO4)3 (2.45 V versus Li/Liþ) at pH 7. We speculate that the reaction between the reduction state Li32xTi2(PO4)3 and O2 is the primary cause of the capacity fading, whereas the reaction between the reduction state Li32xTi2(PO4)3 and H2O is secondary. As for the positive electrodes for aqueous lithium-ion batteries, layered Li12xCoO2 , Li12xNi1/3Mn1/3Co1/3O2 , and so on, may encounter proton insertion at deep lithium extraction; spinel Li1-xMn2O4 cannot be used in an electrolyte with a high pH because of its relatively high potential. Olivine Li12xFePO4 is therefore a comparatively suitable positive electrode material owing to its appropriate potential, high capacity, low cost, safety, low environmental impact and high reversibility in aqueous electrolytes. The cyclic voltammograms show that the carbon-coated LiFePO4 can have a long stability in aqueous electrolyte (see Supplementary Fig. S4). Thus an aqueous lithium-ion battery consisting of a LiFePO4 positive electrode and a LiTi2(PO4)3 negative electrode in a

1 M Li2SO4 solution electrolyte in the absence of O2 is expected to exhibit excellent electrochemical performance, especially cycling ability. Figure 5 shows the typical charge/discharge curves of an aqueous lithium-ion battery consisting of a Li12xFePO4 positive electrode and a LiTi2(PO4)3 negative electrode in a 1 M Li2SO4 solution electrolyte at pH 13 cycled between 0 and 1.4 V at a current rate of 1C. The balancing mass ratio of anode and cathode materials was set as 1:1 by using the specific capacity of 110 mA h g21 for both electrodes. The aqueous lithium-ion battery shows a flat voltage profile centred at 0.9 V and delivers a capacity of 55 mA h g21 and a specific energy of 50 W h kg21 based on the total weight of the active electrode materials (including both the positive and negative electrodes). Typically, the electrode material comprises about 60% of the total weight of the large-size practical batteries1. Thus, a practical specific energy of the current cell of close to 30 W h kg21 can be expected, which is competitive with Pb-acid and Ni-Cd technology but is also associated with much a higher power density and longer cycling life. The aqueous lithium-ion battery has a good rate capability, for example, at 10C, it retains about 80% of the reversible capacity at 1C (see Supplementary Fig. S5). Figure 6a shows the cycling profile of the aqueous lithium-ion battery between 0 and 1.4 V under various conditions. In the absence of O2 , the battery exhibited 10% capacity loss after 1,000 cycles at 6C under extreme cycling conditions without relaxation between cycles. We also carried out the cycling life test of LiTi2(PO4)3/LiFePO4 aqueous batteries at a very low rate (C/8, 8 h rate) in both the presence and absence of O2. The battery cannot be cycled in the presence of O2; however, in the absence of O2 it exhibits good cycling stability, keeping 85% of maximum capacity after 50 cycles even at very low current rate of C/8. The improvement in the cycling stability was also confirmed in LiTi2(PO4)3/LiNi1/3Co1/3Mn1/3O2 (See Supplementary Fig. S6). The results in Fig. 6a indicate that cycling stability at a low current rate is inferior to cycling at a high rate. The cycling data in Fig. 6a is also plotted as capacity retention versus time shown in Fig. 6b. The battery shows the same capacity fading tendency at both high and low current rates. This is because the battery performance is mainly dependent on the stability of LiTi2(PO4)3 , which as discussed above, is critically depended on its charge/discharge rate. All of the above results show that the aqueous lithium-ion battery exhibits a significant improvement in cycling stability in the

1.2

Voltage (V)

NATURE CHEMISTRY

0.8

0.4

0.0 0

10

20

30

40

50

60

Capacity (mA h g–1)

Figure 5 | Typical charge/discharge curves of the LiTi2(PO4)3/LiFePO4 aqueous lithium-ion battery. The cell was charged/discharged at a current rate of 1C in 1 M Li2SO4 aqueous solution electrolyte with pH 13 in the absence of O2. The capacity was calculated based on the total weight of the active electrode materials (including both the positive and negative electrodes). The balancing mass ratio of anode and cathode materials was designed as 1:1 using the specific capacity of 110 mA h g21 for both electrodes.



763

ARTICLES

NATURE CHEMISTRY

a

Capacity retention (%)

100

4 3

2

80

60

40 1 20

0

200

400 600 Cycle number

800

1,000

Capacity retention (%)

b 120 100 3 2

80

330 h

60 40 20

1

0

100

200

300

400 500 Time (h)

600

700

800

Figure 6 | Cycling life test of the LiTi2(PO4)3/LiFePO4 aqueous lithium-ion battery. a, Capacity versus cycle number for cells cycled at different conditions. Line 1: in the presence of O2 , pH 13; line 2: absence of O2 , pH 13 at low rate of C/8; line 3: absence of O2 , pH 7; line 4: absence of O2 , pH 13 at 6C rate. The battery showed better stability. The battery showed better stability in the absence of O2 with a capacity retention over 90% after 1,000 cycles under 6C conditions, and of 85% after 50 cycles even at a very low current rate of C/8 under extreme cycling conditions without relaxation between cycles. b, The data in a was plotted as capacity retention versus time. Line 1: in the presence of O2 at 6C rate; line 2: absence of O2 , at C/8 rate; line 3: absence of O2 at 6C rate in pH 13 electrolyte. The battery shows the same capacity fading tendency at both high and low current rates, but the cycling stability is critically depend on the cycling duration.

absence of O2 regardless of current density. They also demonstrate an improvement in cycling life compared with previous reports1–4. For example, previous results show that VO2(B)/LiMn2O4 has a cycling life of only 25 cycles, LiV3O8/LiNi0.81Co0.19O2 is reduced to 40% after 100 cycles, LiV3O8/LiCoO2 to 36% after 100 cycles, and TiP2O7/LiMn2O4 and LiTi2(PO4)3/LiMn2O4 to 85% and 75% after 10 cycles, respectively (see Supplementary Table S2) (refs 1–4). It should be noted that the difference in the cycling life reported by different groups may partly result from different measurement conditions. As demonstrated above, by adjusting the pH of the electrolyte and eliminating O2 (using a sealed cell) to create feasible conditions for the negative electrode, and selecting a suitable positive electrode with an appropriate potential, an aqueous lithium-ion battery with good cycling ability has been theoretically and practically demonstrated. The difficulty for long-life aqueous lithium-ion battery mainly lies in the control of O2 evolution during the charge/ discharge process as O2 evolution occurring during the overcharge process (up to 1.8 V) causes capacity fading (see Supplementary Fig. S7); therefore the charge cut-off voltage of the positive electrode needs to be controlled. Because the discharge curve of LiTi2(PO4)3 shows a fast drop after full discharge, it is 764

DOI: 10.1038/NCHEM.763

possible to partially control the charge cut-off voltage by adjusting the ratio of positive and negative electrode materials (excessive positive material) or adding an oxygen eliminator such as a storage alloy. The latter method has been widely used in Ni-MH batteries where oxygen can be more easily generated, but requires further study. In conclusion, we have demonstrated both experimentally and theoretically that, in the presence of oxygen, the discharged-state LICs of any negative electrode candidates for aqueous lithium-ion batteries would react with water and oxygen at any pH value of the electrolyte, and this is the primary cause of the capacity fading upon cycling. By eliminating O2 (using a sealed cell), adjusting the pH values of the electrolyte, and using carbon-coated electrode materials, an aqueous lithium-ion battery consisting of a LiFePO4 cathode and a LiTi2(PO4)3 anode in a Li2SO4 solution was assembled and showed excellent cyclic capability. The aqueous lithium-ion battery adopts the ‘rocking-chair’ concept similar to organic lithium-ion batteries, in which the lithium ion can be reversibly intercalated into a lithium-accepting anode and deintercalated from a lithium-source cathode without destroying the structure of the electrode materials. We expect that the aqueous lithium-ion battery will show the longest cycling life among all aqueous electrolyte-based secondary batteries. The energy density of the aqueous lithium battery cannot compete with that of its organic counterpart for portable electronics. However, in terms of the high safety, long cycling life, high power, low cost and low toxicity, aqueous lithium-ion batteries should be a very suitable energy storage and conversion device for short-distance city-bus and stationary power sources that store energy from sustainable sources, such as wind and solar power. It should be noted that, even though we have made a big step in improving the cycling life of aqueous lithium-ion batteries, there is still a long way before its practical application. For example, the dissolution of active materials, and oxygen/hydrogen evolution during the overcharge/discharge process must first be solved.

Methods Synthesis of LiTi2(PO4)3 and LiFePO4 electrode materials. LiTi2(PO4)3 was prepared according to a previous report5. An aqueous precursor containing Li2CO3 , NH4H2PO4 and TiO2 was blended with 100 ml of 2 wt% polyvinyl alcohol solution. The mixture was stirred at a constant temperature of 80 8C until the water was evaporated and a white solid formed. The product was placed in a porcelain boat and heated at 900 8C for 12 h at a rate of 10 8C min21 under N2 flow in a tube furnace, the amount of carbon in the composite was 3% by weight. The LiFePO4 was synthesized by the sol-gel process6. CH3COOLi, (CH3COO)2Fe, and NH4H2PO4 salts were mixed in stoichiometric amounts with a carbon gel formed from the polymerization of resorcinol-formaldehyde (after aging, the gel was washed twice with acetone prior to use, to extract water and any carbonate impurities), and the mixture was heated at 350 8C for 5 h, then at 700 8C for 10 h under a toluene vapour carried by nitrogen gas at a flow rate of 1 l min21 to ensure the product was well coated. The amount of carbon in the composite was 15% by weight. LiV3O8 was prepared by solid-state reaction at 650 8C of stoichiometric ground mixture of Li2CoO3 and V2O5 for 10 h in air. The layered structure LiNi1/3Co1/3Mn1/3O2 was the commercial product from Superhoo. Electrochemical measurements. The working electrode was fabricated by compressing a mixture of the active materials (LiTi2(PO4)3 or LiFePO4), the conductive material (acetylene black, AB), and the binder (polytetrafluoroethylene, PTFE) in a weight ratio of active materials/AB/PTFE ¼ 16:3:1 onto a stainless steel grid at 10 MPa. The electrodes were punched in the form of disks typically with a typical diameter of 12 mm. The typical mass load of the active material is about 10 mg cm22. The electrodes were dried at 120 8C for 12 h before assembly. A 1 M Li2SO4 solution electrolyte with pH 7 was prepared by dissolving Li2SO4 in distilled water. A 1 M Li2SO4 solution electrolyte with pH 13 was prepared by dissolving Li2SO4 into a 0.1 M LiOH solution. Cyclic voltammograms were obtained using a three-electrode cell with or without N2 flow, in which the active carbon and saturated calomel electrode (SCE; 0.242 V versus NHE) were used as counter and reference electrodes, respectively. Measurements were performed using a Solarton Instrument Model 1287 electrochemical interface. The battery test used CR2016-type coin cell (sealed and unsealed). In the sealed cell, the dissolved oxygen in the electrolyte is consumed during the first several cycles, but cannot be further replenished. The capacity was determined using only the weight of the active materials. NATURE CHEMISTRY | VOL 2 | SEPTEMBER 2010 | www.nature.com/naturechemistry


NATURE CHEMISTRY

ARTICLES

DOI: 10.1038/NCHEM.763

Received 28 September 2009; accepted 26 May 2010; published online 8 August 2010

References 1. Li, W., Dahn, J. R. & Wainwright, D. Rechargeable lithium batteries with aqueous-electrolytes. Science 264, 1115–1118 (1994). 2. Kohler, J., Makihara, H., Uegaito, H., Inoue, H. & Toki, M. LiV3O8: characterization as anode materials for an aqueous rechargeable Li-ion battery system. Electrochim. Acta. 46, 59–66 (2000). 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. 46, 295–297 (2007). 4. Wang, H. B., Huang, K. L., Zeng, Y. Q., Yang, S. & Chen, L. Q. Electrochemical properties of TiP2O7 and LiTi2(PO4)3 as anode materials for lithium ion battery with aqueous solution electrolyte. Electrochim. Acta. 52, 3280–3286 (2007). 5. Luo, J. Y. & Xia, Y. Y. Aqueous lithium-ion battery LiTi2(PO4)3/LiMn2O4 with high power and energy densities as well as superior cycling stability. Adv. Funct. Mater. 17, 3877–3884 (2007). 6. Huang, H., Yin, S. C. & Nazar, L. F. Approaching theoretical capacity of LiFePO4 at room temperature at high rates. Electrochem. Solid State Lett. 4, A170–A172 (2001). 7. Li, W., McKinnon, W. R. & Dahn, J. R. Lithium intercalation from aqueoussolution. J. Electrochem. Soc. 141, 2310–2316 (1994). 8. Li, W. & Dahn, J. R. Lithium-ion cells with aqueous electrolytes. J. Electrochem. Soc. 142, 1472–1746 (1995). 9. Zhang, M. J. & Dahn, J. R. Electrochemical lithium intercalation in VO2(B) in aqueous electrolyte. J. Electrochem. Soc. 143, 2730–2734 (1996). 10. Dahn, J. R., Von Sacken, U., Juzkow, M. W. & Al Janaby, H. Rechargeable LiNiO2 carbon cells. J. Electrochem. Soc. 138, 2207–2211 (1991).

11. McKinnon, W. R. & Haering, R. R. in Mordern Aspects of Electrochemistry (eds White, R. E., Bockris, J. O’M. & Conway, B. E.) No. 15 (Plenum Press, 1983). 12. Choi, J., Alvarez, E., Arunkumar, T. & Manthiram, A. Proton insertion into oxide cathode during chemical delithiation. Electrochem. Solid State Lett. 9, A241–A244 (2006). 13. Choi, J. & Manthiram, A. Chemical and structural instabilities of lithium ion battery cathode. J. Power Sources 159, 249–253 (2006). 14. Wang, Y. G., Luo, J. Y., Wang, C. X. & Xia, Y. Y. Hybrid aqueous energy storage cells using activated carbon and lithium-ion intercalated compound II. Comparison of LiMn2O4 , LiCo1/3Ni1/3Mn1/3O2 , and LiCoO2 positive electrodes. J. Electrochem. Soc. 153, A1425–A1431 (2006). 15. Yu, D. Y. W. et al. Impurities in LiFePO4 and their influence on material characteristics. J. Electrochem. Soc. 155, A526–A530 (2008).

Acknowledgements We acknowledge the support of the National Natural Science Foundation of China (20633040, 20925312), the State Key Basic Research Program of PRC (2007CB209703), and Shanghai Science & Technology Committee (09XD1400300, 08DZ2270500).

Author contributions J.L., W.C., P.H. and Y.X. conceived and designed the experiments, analysed and discussed results and commented on the manuscript. J.L. and W.C. performed the experiments and analysed the data. J.L. and Y.X. co-wrote the paper.

Additional information The authors declare no competing financial interests. Supplementary information accompanies this paper at www.nature.com/naturechemistry. Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/. Correspondence and requests for materials should be addressed to Y.Y.X.



765