into LiCoPO4 in Aqueous Batteries

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Lithium Extraction-Insertion from/into LiCoPO4 in Aqueous Batteries Manickam Minakshi,*,† Pritam Singh,† Neeraj Sharma,‡ Mark Blackford,§ and Mihail Ionescu|| †

Faculty of Minerals and Energy, Murdoch University, Murdoch, Western Australia 6150, Australia The Bragg Institute, ANSTO, Lucas Heights, New South Wales 2234, Australia § Institute of Materials Engineering, ANSTO, Lucas Heights, New South Wales 2234, Australia Institute for Environment Research, ANSTO, Lucas Heights, New South Wales 2234, Australia

)

‡

ABSTRACT: A novel 1 V battery composed of Sn-LiCoPO4 using aqueous lithium hydroxide electrolyte is described. Reversible extraction and insertion of lithium from and into the olivine-type LiCoPO4 is reported. The electrochemical behavior of the Sn-LiCoPO4 battery was analyzed using charge/discharge cycling and cyclic voltammetry. Sn-LiCoPO4 battery exhibited charge/ discharge voltages of 1.3 V/0.8 V versus Sn with a reversible capacity of 80 mAh/g. The structural and morphological changes of LiCoPO4 particles before and after electrochemical measurements were investigated by X-ray diffraction (XRD) and transmission electron microscopy. XRD data showed that extraction of lithium proceeds via at least a two-phase mechanism with LiCoPO4 and CoPO4 phases. Upon lithium reinsertion crystalline LiCoPO4 was formed. The cell voltage indicated these batteries were not completely charged, forming single-phase CoPO4 material. Energy-dispersive X-ray analysis coupled with transmission electron microscopy confirmed the chemical quality of the charged and discharged LiCoPO4 in terms of crystallinity and elemental distribution.

’ INTRODUCTION Since the demonstration in 19971 of reversible electrochemical lithium extraction-insertion for lithium iron phosphate (LiFePO4), which adopts an olivine-type structure, considerable effort has been devoted to identification of other metallophosphates LiMPO4 (M = Fe, Mn, Co, and Ni) that can be used as a cathode for lithium-ion batteries. The LiMPO4 compounds contain tetrahedral structural units (XO4)n- with strong covalent bonding, generating oxygen octahedra occupied by the other metal ions. Among the LiMPO4 family, LiFePO4 has been recognized as a promising cathode material. This is mainly due to its structural stability, environmental compatibility of iron (Fe), and reversible capacity of 125 mAh/g. However, there are several problems with the use of LiFePO4 as a cathode material, such as complex synthesis procedures, poor electronic conductivity, and low operating voltage which are widely reported.2-4 The operating voltage of LiFePO4 is limited to 3.4 versus Li in the discharge-charge processes.5 Different approaches for enhancing the electrochemical properties of LiFePO4 have also been reported.6-8 To overcome some of these difficulties, the electrochemical activity of pure LiMnPO4 was investigated and reports show enhanced conductivity and a higher operating voltage of 4.1 versus Li. However, lithium can be extracted from LiMnPO4 with a specific capacity of only 70 mAh/g,9 since the reversibility of these reactions is limited. Studies on other metallophosphates, such as LiCoPO4 and LiNiPO4 are scarce because the existing preferred nonaqueous electrolyte is found to be unstable in the high-voltage range (g4.8 V) where cobalt and nickel redox reactions can occur. However, efforts toward improving the nonaqueous electrolyte have made LiCoPO4 an attractive cathode to explore in the high charge voltage regime, up to 5 V. Amine et al.10 first showed that lithium could be extracted from LiCoPO4 (at 5 V versus Li), and during discharge, a capacity of 70 mAh/g is r 2011 American Chemical Society

observed. Although the reversibility of lithium extraction-insertion from/into LiCoPO4 was demonstrated in a few reports,10-14 a severe capacity fading during cycling has limited the use of this material in practical application. During the delithiation process the material consisted of a three-phase mixture in which LiCoPO4, an intermediate phase Li0.7CoPO4, and CoPO4 coexisted. CoPO4 is unstable at room temperature, and sufficient quantities of this material, which often becomes amorphous, lead to a quick fade in capacity.15,16 As a result, less attention has been paid toward the development of LiCoPO4 cathode in lithium batteries using nonaqueous electrolytes. As stated earlier, poor electronic conductivity is another common problem which hampers the olivine-type materials. To overcome the difficulty of electrolyte instability, in the current work we demonstrated the use of LiCoPO4 electrode and the aqueous electrolyte with tin as an anode. The objective of the present work is to identify the effect of replacing nonaqueous electrolyte with aqueous LiOH electrolyte on the battery performance of LiCoPO4 versus Sn in order to build a safe and an inexpensive battery. Aqueous rechargeable batteries are principally very safe17 and cost effective, and this technology can compete with Ni-Cd and Pb-PbO2 batteries. All available literature on the olivine-type cathode LiCoPO4 is limited to nonaqueous electrolyte as the solvent. No studies in aqueous solutions using Sn as an anode with LiCoPO4 as cathode are reported. Hence, in the present work, we primarily investigated the LiCoPO4 cathode with Sn as an anode in aqueous LiOH electrolyte. In order to improve the reversible capacity and cyclic efficiency compared to the existing Received: November 9, 2010 Accepted: December 21, 2010 Revised: December 16, 2010 Published: January 14, 2011 1899

dx.doi.org/10.1021/ie102267x | Ind. Eng. Chem. Res. 2011, 50, 1899–1905

Industrial & Engineering Chemistry Research

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Zn-based anode researched by our group, Sn was investigated as the anode for aqueous secondary batteries. It was found that Sn exhibited better electrochemical performance compared to Zn-MnO2 batteries18 in which ZnO passivation limits the cell performance.

’ EXPERIMENTAL SECTION Olivine-type LiCoPO4 was synthesized by mixing stoichiometric amounts of Li2CO3, CoC2O4 3 2H2O, and NH4H2PO4 with ethanol in a mortar and pestle. After mixing the ingredients for 30 min, the mixture was transferred into an oven heated at 80 °C for 1 h. The dried powder was collected in a crucible and given a two-step heat treatment in a box furnace containing ambient atmosphere. In the initial step the powder was heated at 375 °C for 12 h and then crushed and ground. The ground powder was fired at 750 °C for 24 h and then cooled to room temperature. The resulting material consisted of a single-phase LiCoPO4. Sn powder (99.9%) was purchased from Ajax Chemicals, and analytical reagent-grade LiOH 3 H2O from Sigma Chemicals Co. was used as received. For the electrochemical test, a pellet was prepared by mixing 75 wt % LiCoPO4 with 15 wt % acetylene black (A-99, Asbury, USA) and 10 wt % poly(vinylidene difluoride) (PVDF, Sigma Aldrich) binder in a mortar and pestle and pressed at 78 MPa. An electrochemical cell (battery) was constructed with the disk-like pellet (8 mm in diameter, 0.5 mm in thickness, and weighing 0.10 g) as the cathode, metallic Sn as the anode (with identical dimensions to the cathode), and filter paper as the separator. The electrolyte used was a saturated solution of lithium hydroxide (LiOH) with a pH of 10.5. The experimental procedures for the galvanostatic and slow scan cyclic voltammetric studies were similar to those reported earlier.17 For galvanostatic experiments, the cell was discharged/ charged galvanostatically at 0.5 mA/cm2 using an 8 channel battery analyzer from MTI Corp., USA, operated by a battery testing system (BTS). An EG&G Princeton Applied Research Versa Stat III model was used to scan the potential at 25 μV/s in all cyclic voltammetric (CV) experiments. Hg/HgO purchased from Koslow Scientific served as the standard reference electrode. X-ray diffraction (XRD) data were collected on Siemens and Panalytical X’pert Pro X-ray diffractometers which use Cu KR radiation. Transmission electron microscopy (TEM) was performed using a JEOL JEM 2010F (JEOL, Japan) equipped with a field emission gun (FEG) electron source operated at 200 kV. The TEM was equipped with an energy-dispersive X-ray (EDX) spectrometer and NORAN System SIX microanalysis system (Thermo Electron Corp., USA). For these experiments, the charged and discharged powder specimens were prepared by scraping material from the charged and discharged pellets, dispersing it in ethanol, and then depositing onto holey carbon film. Extraction of lithium from the LiCoPO4 samples was quantified through the elastically forward scattered recoil atoms of the sample. This technique is referred to as elastic recoil detection analysis (ERDA). For ERDA analysis, the studies were performed on a 10 MV tandem ion beam accelerator using a 35 MeV Cl5þ ion beam with recoiled particles being detected at 45°. The forward-recoiled atoms were mass analyzed using a time-of-flight detection system. Both the TEM and the ERDA experiments were conducted using the facilities of the Australian Nuclear Science and Technology Organisation (ANSTO), Australia.

Figure 1. XRD pattern of LiCoPO4 (red crosses). The vertical marks show position calculated for Bragg reflections (vertical lines middle) and the Rietveld refined model (black line through the crosses). The plot of the difference of the observed pattern and calculated data is shown (bottom). The sharp reflection at 2θ = 27° corresponds to carbon black.

’ RESULTS AND DISCUSSION The Rietveld-refined model of LiCoPO4 and XRD data is shown in Figure 1. The solid-state reaction at 750 °C produced pure olivine-type LiCoPO410 with refined lattice parameters of a = 10.2044(9) Å, b = 5.9220(5) Å, and c = 4.6602(5) Å. The refined model or calculated pattern (solid black line) is in good agreement with the observed data (red cross marks) which confirm the product used for electrochemical tests was pure LiCoPO4. The statistics of the fit are profile factors, Rp = 2.09%, wRp = 3.69%, and goodness-of-fit term χ2 = 3.36. The noisy background and peak at 2θ = 27° can be attributed to carbon black and binder in the sample which was used for electrochemical tests, which also contributed to the χ2 value. I. Electrochemical Characterization

a. Charge/Discharge Tests. The first charge/discharge cycle evaluated galvanostatically at 0.5 mA/cm2 for LiCoPO4 cathode with Sn as an anode using aqueous LiOH electrolyte is shown in Figure 2. The lithium extraction and insertion during charge and discharge processes proceeded with a voltage difference of 0.5 V, indicating high polarization in this material. The charge and discharge capacity of LiCoPO4 was 82 mAh/g. The discharge and charge processes are controlled by time, unlike the voltage cutoff, corresponding to 0.5 mol of reversible Liþ extraction/ insertion with a reversible capacity of 82 mAh/g. The middischarge and charge potential for Sn-LiCoPO4 was observed to be 1.3 and 0.8 V, respectively. The discharge and charge profile with the appearance of one voltage plateau was consistent with the previously reported data for nonaqueous electrolytes.10 However, the striking difference seen while using nonaqueous electrolyte is the deliverable charge capacity which was 187 mAh/g, which exceeds the theoretical value (= 167 mAh/g).19,20 This implies that the excess capacity in nonaqueous electrolyte batteries might account for electrolyte decomposition at the higher charging voltage of 4.8 V.19 This high capacity value was reported to be irreversible, the capacity reduces to 60 mAh/g subsequent to successive cycles, and hence this material was found to be not suitable for battery applications when using nonaqueous organic solvents as electrolyte.12,14,19,20 The cycling performance for the Sn-LiCoPO4 battery is shown in Figure 3, clearly illustrating that the battery in aqueous media was rechargeable. The 1900

dx.doi.org/10.1021/ie102267x |Ind. Eng. Chem. Res. 2011, 50, 1899–1905


Figure 2. Typical first charge-discharge curves for Sn-LiCoPO4 battery using LiOH electrolyte at a current density of 0.5 mA/cm2.

Figure 3. Sn-LiCoPO4 battery using LiOH electrolyte illustrating the cyclability.

discharge capacity fell from 82 to 75 mAh/g for the second cycle and reached a stable value of 70 mAh/g after the fifth cycle. The lithium extraction and insertion process was found to be 85% reversible, and the LiCoPO4 cathode in aqueous batteries was found to be a potential candidate for low-voltage battery application. After 25 cycles, the discharge capacity was observed to be 70 mAh/g, while for nonaqueous media it was reported to be less than 50 mAh/g.21,22 The observed improved capacity in our current study was in part attributed to the stability of the LiOH electrolyte in the safe operating voltage window (1.3-0.8 V) versus Sn. b. Cyclic Voltammetric Studies. To gain a better understanding of the lithium extraction/insertion mechanism and its redox behavior within these cathodes, the potentiostatic technique and an ex-situ XRD experiments were conducted. Figure 4 shows the CV curves for multiple cycles of LiCoPO4 recorded in the voltage range from 0.2 to -0.3 V vs Hg/HgO (as a standard electrode) with a slow scan rate of 25 μV/s. The CV curve for LiCoPO4 exhibited one oxidative peak (A1) at 60 mV during an anodic sweep corresponding to extraction of lithium from LiCoPO4, producing CoPO4 phase with a charge compensation of Co2þ/3þ. During the reverse cathodic sweep at an identical scan rate one distinct cathodic peak (C1) at 267 mV was observed. The reduction peak (C1) corresponds to insertion of lithium into CoPO4 with a

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Figure 4. Typical cyclic voltammogram of lithium cobalt phosphate (LiCoPO4) in aqueous lithium hydroxide electrolyte (scan rate = 25 μV s-1; potential limit from 0.2 to -0.3 V and back). Cycle numbers are indicated in the figure.

Figure 5. XRD patterns of LiCoPO4 (a) before and (b) after charge, (c) subsequently discharged, and (d) discharged after the 25th cycle in aqueous LiOH electrolyte. Peaks are indexed as a pure olivine structure.

charge compensation of Co3þ/2þ, resulting in the original LiCoPO4. The observed anodic and cathodic peaks are consistent with the obtained charge and discharge plateau (in Figure 2) corresponding to the cobalt redox couple. As can be seen in Figure 4, during the second and fifth cycle the oxidation and reduction peak decreased but beyond that the peaks were stabilized and maintained constant, with virtually full rechargeability until the 25th cycle. Thus, it was demonstrated that the lithium extraction and insertion mechanism is versatile and reversible using aqueous LiOH electrolyte. II. Materials Characterization a. Ex-Situ XRD Studies. Figure 5 compares the XRD patterns of the LiCoPO4 cathode material. XRD was carried out on the LiCoPO4 cathode before and after charge, subsequent discharge, and at the discharged state of the 25th cycle. The XRD results were used to qualitatively identify the structural stability and versatility of the cathode after lithium extraction and insertion. Figures 1 and 5a show the XRD pattern of the synthesized LiCoPO4 before any electrochemical treatment, both with (in Figure 1) and without (in Figure 5a) carbon black. For the charged cathode (in Figure 5b), the XRD pattern showed the formation of a second phase identified by the splitting/appearance 1901



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Figure 6. TEM bright field image (a), SAED pattern (b), and EDS spectrum (c) of LiCoPO4 as-synthesized sample. The SAED pattern confirms the sample has a high degree of crystallinity. The EDS spectrum contains peaks due to O, P, and Co in the LiCoPO4 sample as indicated. The Cu peaks are due to the grid used to support the sample.

of a second set of reflection peaks (indicated by “o”). This is in agreement with previous reports made on LiCoPO4 for nonaqueous electrolyte.10,19 The appearance of newly revealed reflections can be assigned to the lithium-extracted composition of “CoPO4”.15 Similar to other olivine-type cathodes like LiFePO4 and LiMnPO4 the extraction of lithium results, in the majority of cases, in a two-phase mixture, e.g., LiFePO4 and FePO411,17 and LiMnPO4 and MnPO4,23 respectively. However, upon complete extraction (charged state) all of the LiFePO4 or LiMnPO4 is transformed to FePO4 or MnPO4, whereas in our case at the charged state the parent LiCoPO4 material is still present. Therefore, in the charged state in aqueous LiOH a coexistence of at least two crystalline phases crystalline LiCoPO4 and CoPO4 was observed.15 Note, the resolution of these XRD data is not sufficient to differentiate between the delithiated LiCoPO4 (LizCoPO4) and fully lithiated LiCoPO4. There is not much information

available in the literature or JCPDS (Joint Committee on Powder Diffraction Standards) database for the existence of CoPO4; hence, the exact delithiation mechanism cannot be determined. The existence of two phases in Figure 5b also suggests that the extraction mechanism was not uniform or not complete at that stage, and hence, the transformation process from LiCoPO4 to CoPO4 was not complete. In other words, only a percentage of LiCoPO4 was transformed to CoPO4. After discharge, the XRD patterns returned to solely the identified parent LiCoPO4 olivinetype structure, reflecting that the transformation process proceeded in a reversible fashion. The observed pattern (Figure 5c) was similar to the starting material (seen in Figures 5a and 1). The peaks corresponding to CoPO4 disappeared at the end of discharge, implying that this material was electrochemically active and hence the efficiency of the battery is fully rechargeable from a structural point of view with XRD data. It must be noted 1902



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Figure 7. TEM bright field images (a) and SAED pattern (b) of LiCoPO4 samples after charging.

Figure 8. TEM bright field images (a) and SAED pattern (b) of LiCoPO4 samples after subsequent discharging.

that XRD data is sensitive to crystalline phases in the sample that are in sufficient concentration to be detected. There are possibilities that some metastable phases might have reacted by the time ex-situ XRD was conducted and some amorphous content was present. Figure 5d shows the XRD pattern of the discharged material after the 25th cycle, from which similar observations can be made to that of the first discharge, the presence of LiCoPO4 and virtually no evidence of CoPO4. The observed peaks indicated that lithium can be extracted and inserted reversibly for a number of cycles without structural deterioration. Furthermore, the peaks in Figure 5d remained very sharp, indicating that the LiCoPO4 maintained its crystallinity even after the 25th cycle and hence may be suitable for low-voltage battery applications. Note, these data suggest that the drop in observed capacity between the 1st and 25th cycle may not necessarily be due to structural reasons associated with the LiCoPO4 electrode, i.e., other side reactions might have occurred. b. TEM, SAED, and EDS Analysis. The morphology of the assynthesized LiCoPO4 powder was investigated by TEM. Bright field images and selected area electron diffraction (SAED) patterns indicated the presence of well-defined crystalline particles (Figure 6a and 6b). The SAED pattern was a series of diffraction spots reflecting the reciprocal space of the LiCoPO4 crystalline lattice. Polycrystalline nanosized (or smaller) or semiamorphous material would result in SAED ring patterns due to the random orientation of individual crystals. The corresponding EDS spectrum (Figure 6c) from the region marked “sp4” exhibits characteristic peaks of O, P, and Co which are well identified in the sample. These compositions were found to be uniform across the

sample examined. X-ray peaks corresponding to Cu were from the grid used to support the sample. It is not possible to detect the very low energy X-rays from lithium in the LiCoPO4 sample due to absorption in the sample and EDS detector window. These results are in agreement with XRD observations that indicated formation of highly crystalline particles. The nature of the LiCoPO4 samples after charging and subsequent discharging was analyzed and compared in Figures 7 and 8. The charged sample showed changes in both morphology and SAED pattern (Figure 7a and 7b, respectively), indicating a transformation from a highly crystalline single-phase material to a diffuse and multiphase composition with presumably smaller crystallite sizes and possibly indicating the three-phase mixture of CoPO4, LiCoPO4, and LizCoPO4 as suggested in ref 15. Lithium extraction from LiCoPO4 cathode was supported by ERDA analysis (Figure 9). ERDA analysis showed that the concentration of lithium was much lower for the charged sample, implying that lithium was extracted during the charge analysis. Sample morphology and crystal properties underwent further changes as a result of subsequent discharging. The bright field image (Figure 8a) and SAED pattern (Figure 8b) show changes in morphology and a remaining multiphase material. Note these multiple phases were not evident in the qualitative analysis of XRD data that was presented earlier. This could be due to small crystallite sizes of these other phases in agreement with the rings observed in SAED patterns. If these phases do show small particle sizes they produce broad peaks in the XRD patterns which are difficult to differentiate from the background in XRD data but are clearly seen in SAED patterns and TEM images which are 1903



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’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ61-8-9360-6784. Fax: þ61-8-9310-1711. E-mail: minakshi@ murdoch.edu.au; [email protected].

’ ACKNOWLEDGMENT M.M. acknowledges the Australian Research Council (ARC). This research was supported under the Australian Research Council’s Discovery Projects funding scheme (DP1092543). M.M. would like to thank the Australian Institute of Nuclear Science and Engineering (AINSE) for providing financial assistance (AINGRA Award 10053, P1492) for access to TEM and ERDA facilities at ANSTO. Figure 9. ERDA spectrum of the elemental concentration in charged LiCoPO4 cathode. The depth profile analysis (thickness in atoms/cm2) of the elemental distribution is shown schematically.

more sensitive to materials of smaller particle size. The spacing of rings in the SAED pattern also showed that these other phases are different, indicating either changes in the crystal structure of the phases identified earlier or other completely different phases are present, e.g., cobalt oxides or delithiated LiCoPO4 (LizCoPO4). This also agreed with the EDS spectrum collected from the discharged sample (not shown) that featured a different intensity distribution for O, C, and Co peaks, suggesting the presence of an oxide/hydroxide or carbonate of an undetected element, most probably Li, indicating the atmospheric CO2 formed as a layer of Li2CO3 on the cathode material. These analyses suggests that further high-resolution, preferably in-situ, XRD, TEM, and SAED data need to be collected and accurately modeled in order to determine the exact species present in this battery at various stages of the charge process and to determine an accurate picture of the mechanism.

’ CONCLUSIONS The olivine-type LiCoPO4 is reported widely in the literature using nonaqueous electrolyte, but it is limited to few charge/ discharge cycles. The reported cell capacity for nonaqueous cell is significantly reduced from 160 to 60 mAh/g by the second cycle. In this study we report the investigation of LiCoPO4 in aqueous electrolyte using Sn as an anode, providing structural and cycling stability for over 25 cycles. On the basis of the galvanostatic and cyclic voltammetric data, after the fifth cycle the efficiency of the aqueous battery was found to be close to 100%, delivering a discharge capacity of 70 mAh/g. The difference seen in nonaqueous and aqueous media is explained in terms of a safe operating voltage range. TEM study evidenced that the synthesized LiCoPO4 exhibited a particulate (crystalline) nature. TEM images and SAED patterns for the charged/discharged samples confirmed particle size and crystal-chemical differences between the pristine, charged, and discharged states of LiCoPO4 electrode. ERDA analysis supports the lithium extraction hypothesis. The main characteristic feature from XRD data of the charged LiCoPO4 is the coexistence of crystalline LiCoPO4 and CoPO4, suggesting complete charging does not occur, which seems to be beneficial for the reversibility of these batteries. However, the CoPO4 phase by qualitative XRD analysis was completely reversible, leading to formation of LiCoPO4 during the reduction process.

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(17) Minakshi, M. Lithium intercalation into amorphous FePO4 cathode in aqueous Solutions. Electrochim. Acta 2010, 55, 9174. (18) Minakshi, M. Sn-MnO2 Aqueous Rechargeable Battery. Electrochem. Solid-State Lett. 2010, 13, A125. (19) Kishore, M. V. V. M. S.; Varadaraju, U. V. Influence of isovalent ion substitution on the electrochemical performance of LiCoPO4. Mater. Res. Bull. 2005, 40, 1705. (20) Deniard, P.; Dulac, A. M.; Rocquefelte, X.; Grigorova, V.; Lebacq, O.; Pasturel, A.; Jobic, S. High potential positive materials for lithium-ion batteries: transition metal phosphates. J. Phys. Chem. Solids 2004, 65, 229. (21) Wolfenstine, J.; Lee, U.; Poese, B.; Allen, J. L. Effect of oxygen partial pressure on the discharge capacity of LiCoPO4. J. Power Sources 2005, 144, 226. (22) Bramnik, N. N.; Bramnik, K. G.; Buhrmester, T.; Baehtz, C.; Ehrenberg, H.; Fuess, H. Electrochemical and structural study of LiCoPO4 - based electrodes. J. Solid State Electrochem. 2004, 8, 558. (23) Chen, G.; Richardson, T. J. Solid Solution Phases in the Olivine-Type LiMnPO4/MnPO4 System. J. Electrochem. Soc. 2009, 156, A756.

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