Effect of TiS2 Additive on LiMnPO4 Cathode

Oct 15, 2010 - Revised Manuscript Received October 1, 2010. Incorporation of TiS2 additive by physical admixture into the LiMnPO4 cathode leads to ...
0 downloads 0 Views 4MB Size
Download PDF
Energy Fuels 2010, 24, 6193–6197 Published on Web 10/15/2010

: DOI:10.1021/ef101063h

Effect of TiS2 Additive on LiMnPO4 Cathode in Aqueous Solutions Manickam Minakshi,*,† Akanksha Pandey,† Mark Blackford,‡ and Mihail Ionescu§ † Faculty of Minerals and Energy, Murdoch University, Murdoch, WA 6150, Australia, ‡Institute of Materials Engineering, ANSTO, Menai 2234, NSW, Australia, and §Institute for Environmental Research, ANSTO, Menai 2234, NSW, Australia

Received August 9, 2010. Revised Manuscript Received October 1, 2010

Incorporation of TiS2 additive by physical admixture into the LiMnPO4 cathode leads to modification of the electrochemical performance of the cathode, such as an improved delithiation and lithiation mechanism. Cyclic voltammetry suggests that the TiS2 additive suppresses proton deinsertion/insertion mechanism and does not contribute directly to the reduction/oxidation reactions of the LiMnPO4 working electrode.

reported so far in the nonaqueous battery systems.7-10 The use of LiMnPO4 is attractive because of the position of the Mn3þ/Mn2þ redox couple which is 4.1 V vs Li/Liþ.5 The literature on olivine type LiMnPO4 is limited only to nonaqueous media, and studies in aqueous media are scarcely reported.11 The focus of our work is to investigate this material for use as aqueous electrolytes and to eventually develop a safe and low cost aqueous battery. The current work follows our preliminary work11 carried out on LiMnPO4 in aqueous solution where we established the electro-oxidation/reduction processes in aqueous LiOH electrolyte. Here, we extend our study to report on titanium disul fide (TiS2) modified LiMnPO4 cathodes that offer an improved delithiation and lithiation mechanism. Interestingly, the presence of TiS2 additive suppresses the proton deinsertion/insertion during redox reactions. Titanium disulfide is an attractive composite material to form intercalation compounds with lithium; however, the formation of such phases are not obviously seen in the discharged LiMnPO4 material. To the best of our knowledge, there has been no work reported on the effect of TiS2 additive to LiMnPO4 for battery application. In this paper, we have confined our results to only 3 wt % TiS2 addition. The available literature on olivine type cathodes used in lithium batteries is limited to non-aqueous electrolytes containing dissolved lithium ions. The information available on LiMnPO4 cathode in aqueous solutions is very scant. The proposed Zn-LiMnPO4 aqueous rechargeable battery with a specific cell capacity of 130 mAh/g offers immediate advantages over existing technologies with respect to cost, safety, and environmental considerations. The aqueous rechargeable cell has several advantages like (a) high ionic conductivity so thick electrodes can be used, (b) expensive lithium salts (LiClO4 þ ethylene carbonate) has been replaced by cheap LiOH as electrolyte, (c) improved cycleability, and (d) safety issues are solved with stringent cell assembly in a protective glovebox atmosphere is not required. However, it will be necessary to improve the power density and cycle life in order to make this technology competitive with the performance of current Li-based battery systems.

Introduction Since the demonstration of reversible lithium intercalation between the layers of TiS2,1 considerable effort has been devoted to identification of other lithium-insertion compounds that can be used as cathode for a secondary battery system.2 To date, among the known Li insertion compounds are LiCoO2, LiNiO2, and LiMn2O4. However, because of the toxicity and high price of cobalt and nickel, and the fact that LiMn2O4 suffers from dissolution in the electrolyte leading to a capacity fading, alternative cathode materials are desirable. Since the original work of Padhi,3 lithium transition metal phosphates have appeared to be a potential candidate due to their lower toxicity, lower cost, and better chemical and thermal stability, when compared to their oxide counter parts. The theoretical capacity of the olivine-type LiMPO4 (M = Fe, Mn, Co, and Ni) based on the M3þ/M2þ one electron redox reaction is 170 mAh/g, and it exceeds the theoretical capacity of the oxide materials.4 The presence of strong covalent bonds in the (PO4)3- tetrahedral polyanions5 do not undergo structural rearrangements during lithiation and delithiation. Among the available olivine-type cathodes, LiFePO4 was investigated extensively because of its attractive features such as cost, toxicity, and environment pollution. The main drawback of LiFePO4 is its low electronic conductivity, because it is basically an electronic insulator.6 To overcome this problem, LiMnPO4 appears to be the best candidate among the LiMPO4 family despite quite poor electrochemical performances *To whom correspondence should be addressed. Telephone: þ61-89360-6784. Fax: þ61-8-9310-1711. E-mail: [email protected]; [email protected]. (1) Whittingham, M. S. J. Electrochem. Soc. 1976, 123, 315–320. (2) Ohzuku, T.; Takehara, Z.; Yoshizawa, S. Electrochim. Acta 1979, 24, 219–222. (3) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188–1194. (4) Okada, S.; Sawa, S.; Egashira, M.; Yamaki, J.; Tabuchi, M.; Kageyama, H.; Konishi, T.; Yoshino, A. J. Power Sources 2001, 97-98, 430–432. (5) Delacourt, C.; Poizot, P.; Morcrette, M.; Tarascon, J.-M.; Masquelier, C. Chem. Mater. 2004, 16, 93–99. (6) Huang, H.; Yin, S.-C.; Nazar, L. F. Electrochem. Solid State Lett. 2001, 4, A170–172. (7) Li, G.; Azuma, H.; Tohda, M. Electrochem. Solid State Lett. 2002, 5, A135–137. (8) Yamada, A.; Chung, S.-C. J. Electrochem. Soc. 2001, 148, A960–967. (9) Bramnik, N. N.; Ehrenberg, H. J. Alloys Compd. 2008, 464, 259–264. r 2010 American Chemical Society

(10) Bakenov, Z.; Taniguchi, I. Electrochem. Commun. 2010, 12, 75–78. (11) Minakshi, M.; Singh, P.; Thurgate, S.; Prince, K. Electrochem. Solid State Lett. 2006, 9, A471–474.

6193

pubs.acs.org/EF

Energy Fuels 2010, 24, 6193–6197

: DOI:10.1021/ef101063h

Minakshi et al.

Experimental Section The olivine LiMnPO4 (10 wt % carbon included) material used in this work was received from Tokyo Institute of Technology, and its cathode properties are reported in the literature.12 Zn foil (99.9%) from Advent research materials Ltd., reagent grade LiOH 3 H2O from Sigma Chemicals Company and zinc sulfate heptahydrate (ZnSO4 3 7 H2O) from Ajax Chemicals were used as received. Titanium disulfide (TiS2) was obtained from Alfa Aesar. For the electrochemical test, a pellet was prepared by mixing 72 wt % LiMnPO4 and 3 wt % TiS2 with 20 wt % acetylene black (A-99, Asbury) and 5 wt % poly(vinylidene difluoride) (PVDF, Sigma Aldrich) binder in a mortar and pestle. The experimental procedures for the galvanostatic and cyclic voltammetric and its standard cell configuration were similar to those reported earlier.13,14 For galvanostatic experiments, the cell was discharged/charged galvanostatically at 0.5 mA/cm2 by using an eight channel battery analyzer from MTI Corporation, 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 experiments. Hg/HgO purchased from Koslow Scientific Company served as the reference electrode. For X-ray diffraction analysis, a Siemens X-ray diffractometer using Cu-KR radiation was used. Transmission electron microscopy (TEM) was performed using a JEOL JEM 2010F (JEOL, Japan) equipped with a field emission gun (FEG) as the electron source and operated at 200 kV. The TEM was equipped with an energy dispersive X-ray (EDS) spectrometer and NORAN System SIX microanalysis system (Thermo Electron Corporation). For these experiments, the delithiated and lithiated powder specimens were prepared by scraping material from the charged and discharged pellets, dispersing it in ethanol, then depositing onto permeable carbon film. Delithiation of the light element (lithium) in the charged LiMnPO4 samples was quantified through the elastically forward scattered recoil atoms of the sample. This technique is named 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.

Figure 1. First (galvanostatic) discharge-charge cycle of ZnLiMnPO4 cell containing (0 and 3 wt % TiS2 additive) using aqueous LiOH electrolyte.

compared with TiS2 added LiMnPO4. The polarization strongly increases as a function of the depth of delithiation, and this could be due to kinetic limitation. Therefore, a larger amount of lithium was removed from the LiMnPO4 crystal in the presence of TiS2 as an additive. The very slow kinetic behavior observed for the plain LiMnPO4 was improved with the addition of TiS2. Elastic recoil detection analysis (ERDA), a powerful ion beam technique, has been used to quantify the presence of lithium without the need for any calibration standard. An ERDA spectrum of a plain and TiS2 added LiMnPO4 surface layer was compared with the sample before undergoing any electrochemical treatment in Figure 2. The concentration of lithium is much lower for the charged samples implying that the 75% of the lithium is deintercalated from the LiMnPO4 cathode. During the subsequent discharge (in Figure 1), the capacity for plain and TiS2 added LiMnPO4 was measured to be of 70 and 90 mAh/g, respectively. This shows that the extraction of lithium was greater than the insertion of lithium in the LiMnPO4 compound. The observed values are very close to the data reported for this olivine compound in the nonaqueous electrolyte.7-10 The separation between mid charge and discharge voltages are much closer for the TiS2 added LiMnPO4. In contrast, plain LiMnPO4 resulted in a lower capacity and an increased polarization. The discharge capacity as a function of cycle number is shown in Figure 3. For plain LiMnPO4, the capacity dropped 30% during the first few cycles after which the decrease was gradual. For TiS2 added LiMnPO4, cell performance was improved with 70 mAh/g after 20 cycles, losing 20% of the initial capacity. Cyclic Voltammetric Studies. To further gain a better understanding of the lithium extraction/insertion mechanism within these cathode materials, potentiostatic technique and ex situ diffraction experiments were conducted. Cyclic voltammogram (CV) of the multiple cycles for both plain and TiS2 added LiMnPO4 electrodes are given in Figures 4 and 5. In order to reveal most of the available electrochemical properties, the CV measurements were carried out by using a very low potential scan rate of 25 μV/s. For plain LiMnPO4, the two peaks (A1 and C1 shown in Figure 4) characteristic of both LiMnPO4 oxidation and MnPO4 reduction are assigned to 0.02 and -0.25 V, respectively, relative to the Hg/HgO reference. The two peaks (A1 and C1) are in accordance with the oxidation/reduction reactions of the Mn2þ/Mn3þ redox couple observed in the nonaqueous

Results and Discussion Effect of TiS2 Additive on Electrochemical Delithiation and Lithiation. Galvanostatic Studies. Figure 1 shows typical charge-discharge curves measured for TiS2 modified LiMnPO4 using a current density of 0.5 mA/cm2. This data is compared with that for plain LiMnPO4 cathode. The charge capacity for TiS2 modified cell was measured to be 130 mAh/g whereas for the plain cathode it was 90 mAh/g. The theoretical capacity for LiMnPO4 is reported as 170 mAh/g using nonaqueous electrolytes.5 On the basis of this, the amount of lithium deintercalated from LiMnPO4 with a TiS2 additive is at most 0.76 and only 0.52 for plain LiMnPO4. It was clear that Mn3þ/Mn2þ redox reaction was improved with the TiS2 addition. The amount of lithium extraction was enhanced with the amount of additive and reached a maximum value of 76% corresponding to 3 wt % additive. It can also be noted from Figure 1 that for the TiS2 added cell, the mid charged voltage is seen at 1.2 V while that for plain LiMnPO4 it is at 1.0 V. This implies that LiMnPO4 suffers from much higher polarization during the redox reactions as (12) Yonemura, M.; Yamada, A.; Takei, Y.; Sonoyama, N.; Kanno, R. J. Electrochem. Soc. 2004, 151, A1352–1356. (13) Minakshi, M. Electrochim. Acta 2010, DOI: 10.1016/j.electacta. 2010.09.011. (14) Minakshi, M. Electrochem. Solid State Lett. 2010, 9, A125–127.

6194

Energy Fuels 2010, 24, 6193–6197

: DOI:10.1021/ef101063h

Minakshi et al.

Figure 2. ERDA spectrum of the concentration of lithium in LiMnPO4 cathode (a) before any electrochemical treatment (b) 0 wt % and (c) 3 wt % TiS 2 additive charged. The depth profile analysis (thickness in atoms/cm2) of the lithium distribution is shown schematically.

Figure 4. A typical cyclic voltammogram of lithium manganese phosphate (LiMnPO4) in aqueous lithium hydroxide electrolyte (scan rate, 25 μV s-1; potential limit, 0.2 to -0.4 V and back). Cycle numbers are indicated in the figure.

Figure 3. Discharge capacity of Zn-LiMnPO4 cell containing (0 and 3 wt % TiS2 additive) versus cycle number using aqueous LiOH electrolyte.

Figure 5. A typical cyclic voltammogram of lithium manganese phosphate (LiMnPO4) containing 3 wt % of TiS2 additive in aqueous lithium hydroxide electrolyte (scan rate, 25 μV s-1; potential limit, 0.2 to -0.4 V and back). Cycle numbers are indicated in the figure.

lithium systems.15 It is to be noted that an additional oxidation peak labeled A2 is seen at -0.065 V vs Hg/HgO ascribed to proton deinsertion, which is not reversible. For TiS2 added LiMnPO4, the presence of two peaks (A1 and C1 shown in Figure 5) corresponding to LiMnPO4 oxidation and MnPO4 reduction are assigned at 0.02 and -0.3 V, respectively, vs Hg/HgO. The TiS2 added cell shows typical reversible cycles of lithium insertion and extraction behavior indicating a significant rise in current. This rise in current is expected due to the nature of TiS2 as an excellent conductor. The reduction peak observed for plain LiMnPO4 could be due to both lithium and proton insertion, whereas the presence of TiS2 suppresses the proton insertion and the peaks are shifted to -0.30 V ascribed mainly to lithium insertion, which is reversible. It is proposed that the reason for this negative potential shift is due to lithium insertion. The role of additives or cation substitution in the olivine LiMnPO4 in nonaqueous systems eases the Jahn-Teller lattice distortion,16,17 which

enhances the lithium transport. This mechanism is not observed in our aqueous systems. Consequently, on the basis of the galvanostatic and CV studies, authors propose that the effect of TiS2 aids to suppress the proton insertion/ deinsertion mechanism (see Figure 6c) with improved cell capacity. Materials Characterization. XRD Studies. The cathodic material formed during the oxidation was characterized by ex situ XRD. Figure 6 compares the XRD patterns of the oxidized LiMnPO4 cathodes in the absence and the presence of the additive. The simulated (calculated based on unit cell values shown for each compound, published in the ICSD crystal structure database) XRD spectra for the lithiated and delithiated compounds are compared in Figure 7. The X-ray diffraction pattern of the starting material LiMnPO4 in Figure 6a is in good agreement with the simulated pattern for LiMnPO4 shown in Figure 7a except for the peak shifts. These shifts are due to the difference in Cu (KR) and Co (KR) radiation used for experimental and simulation patterns. The spectra (in Figure 6b) of the material formed on electrooxidation of the product, i.e., oxidation of LiMnPO4 consist of peaks assigned to MnPO4 and LiMnPO4 in addition to δMnO2 (birnessite). The electrochemical oxidation of the LiMnPO4 compound proceeds in a two-phase reaction

(15) Martha, S. K.; Markovsky, B.; Grinblat, J.; Gofer, Y.; Haik, O.; Zinigrad, E.; Aurbach, D.; Drezen, T.; Wang, D.; Deghengi, G.; Exnar, I. J. Electrochem. Soc. 2009, 156, A541–552. (16) Yamada, A.; Kudo, Y.; Liu, K.-Y. J. Electrochem. Soc. 2001, 148, A747–754. (17) Lee, J.-W.; Park, M.-S.; Anass, B.; Park, J.-H.; Paik, M.-S.; Doo, S.-G. Electrochim. Acta 2010, 55, 4162.

6195

Energy Fuels 2010, 24, 6193–6197

: DOI:10.1021/ef101063h

Minakshi et al.

Figure 6. X-ray diffraction patterns of LiMnPO4 (a) before electrooxidation (b) 0 wt % and (c) 3 wt % electro oxidized in aqueous LiOH electrolyte.

Figure 7. X-ray diffraction simulated patterns of (a) LiMnPO4 and (b) MnPO4: orthorhombic (PNMA). Their lattice parameters in angstroms are shown in their insets.

between delithiated phase MnPO4 and lithiated phase LiMnPO4.5,18 It is seen from Figure 6b that LiMnPO4 peak move toward a higher 2θ value as the lithium is extracted from the sample. The relative amount of delithiated phase increased in intensity for the TiS2 added LiMnPO4 in Figure 6c. However, the reflection corresponding to the birnessite phase was weak. This reflects that the presence of TiS2 leads to strong delithiation, i.e., removal of Li and the formation of MnPO4. The simulated pattern for MnPO4 (Figure 7b) is in agreement with the experimental pattern shown in Figure 6c. Thus, TiS2 addition facilitates lithium transport while suppressing the unwanted birnessite phase to enhance the electrochemical properties. TEM and EDS Studies. In order to get a better overview on the morphologies of the particles obtained after electrooxidation and subsequent electro-reduction, TEM coupled with EDS were carried out on the prepared powders and their typical results are presented in Figures 8 and 9. These observations fully agreed with the XRD data supporting the delithiation and lithiation mechanism. Bright field TEM images of the charged LiMnPO4 particles in the absence and the presence of TiS2 additive are shown in Figure 8. The term “sp” on figures indicates the position from which EDS spectra were collected. From the EDS analysis, dark regions in the images represent LiMnPO4 particles and the light

Figure 8. TEM bright field images of the LiMnPO4 cathode. Charged sample containing (a) 0 wt and (b) 3 wt % TiS2 additive. Discharged sample containing (c) 0 wt and (d) 3 wt % TiS2 additive.

region corresponds to carbon. Figure 8a shows a partially delithiated image still containing LiMnPO 4 crystallites, while Figure 8b shows a fully delithiated morphology that did not contain any distinguishable trace of LiMnPO4 crystallites. The corresponding EDS spectra shown in Figure 9a, b exhibits characteristic peaks of O, Mn, P, and Zn which are well identified in the sample. The peaks due to Cu from the supporting grid can also be seen. The contribution of Zn is from the electrolyte. The higher amount of lithium extracted from the TiS2 added cell resulted in decreased Mn peak intensities (Figure 9b) as compared to the plain cell (Figure 9a). This is possibly caused by the surface area of the sample being larger for the delithiated MnPO4. On subsequent electroreduction, the TiS2 added cell (Figure 8d) is uniform and composed of agglomerated primary LiMnPO4 particles. The plain LiMnPO4 images in Figure 8c showed a different texture

(18) Chen, G.; Richardson, T. J. J. Electrochem. Soc. 2009, 156, A756–762.

6196

Energy Fuels 2010, 24, 6193–6197

: DOI:10.1021/ef101063h

Minakshi et al.

Figure 9. EDS analyses of the LiMnPO4 cathode. Charged sample containing (a) 0 wt and (b) 3 wt% TiS2 additive. Discharged sample containing (c) 0 wt and (d) 3 wt % TiS2 additive.

the proton insertion rather mitigating the Jahn-Teller distortion in the olivine structure. The study of lithium extraction and insertion mechanism is supported by ERDA, X-ray diffraction, and TEM associated with EDS studies. The two phase reaction of olivine type LiMnPO4 and delithiated MnPO4 is observed during charging and discharging. Improving material properties by locating suitable additives to attain the theoretical capacity of 170 mAh/g will be a necessary step to promote this olivine compound as a potential cathode material for aqueous rechargeable batteries.

indicating the process was not totally reversible. The large Mn peak intensity in the EDS spectrum (Figure 9d) for the TiS2 added cell compared with the EDS spectrum (Figure 9c) for the plain cell suggests the lithium insertion process is reversible. This confirms that the presence of TiS2 additive influenced the lithium transport during both the delithiation and lithiation process. Conclusions The depth of delithiation and lithiation observed from galvanostatic and potentiostatic measurements on TiS2 modified LiMnPO4 cathode accounts for a strong correlation between the proton suppression and paving the way for lithium transport that led to enhanced electrochemical properties. The electrochemical extraction and insertion of lithium is observed while using aqueous lithium hydroxides as electrolytes which is similar to that for nonaqueous media. However, the role of TiS2 additive in aqueous solutions is found to suppress

Acknowledgment. The author M.M. wishes to acknowledge the Australian Research Council (ARC). This article was produced as an outcome of the ARC Discovery Project (Grant DP1092543). M.M would like to thank the Australian Institute of Nuclear Science and Engineering (AINSE) for providing financial assistance (AINGRA Award 10053) for access to the TEM and ERDA facilities at ANSTO.

6197