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Synthesis, characterization and electrochemical studies on LiCoAsO4
Materials Research Bulletin 41 (2006) 601–607 www.elsevier.com/locate/matresbu
Synthesis, characterization and electrochemical studies on LiCoAsO4 M.V.V.M. Satya Kishore, U.V. Varadaraju * Materials Science Research Centre and Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, Tamil Nadu, India Received 5 April 2005; received in revised form 26 August 2005; accepted 2 September 2005 Available online 26 September 2005
Abstract LiCoAsO4 is synthesized by solid state reaction method and its crystal structure has been refined by the Rietveld method using powder X-ray diffraction data. LiCoAsO4 crystallizes in olivine structure with space group Pnma and orthorhombic lattice ˚ , b = 5.9970(1) A ˚ and c = 4.8866(1) A ˚ . Electrochemical studies reveal that in LiCoAsO4, lithium is parameters are a = 10.4614(2) A deintercalated and intercalated at high voltage 5.0 V. On the other hand, the compound can react with about 9Li on discharge to 0.05 V. A reversible capacity of 100 mAh/g is obtained in the voltage range 1.0–2.5 V. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; C. X-ray diffraction; D. Electrochemical properties
1. Introduction Materials with framework structures built up from polyanion groups such as SiO4, PO4 and AsO4 are considered potential electrode materials for lithium batteries. In particular, phosphates of the general formula LiM2+PO4 (M = Mn, Fe and Co) with olivine structure are investigated as positive electrode materials for secondary lithium batteries. Despite their poor electronic conductivity, phosphates show good reversible lithium extraction/insertion, especially in case of LiFePO4. LiFePO4 with a flat voltage curve at 3.5 V and a theoretical specific capacity of 170 mAh/g emerges as the most promising cathode material in the olivine family of compounds [1]. An improvement in the rate capability of LiFePO4 is achieved by decreasing the particle size and by an electronic conductive particle coating [2–4]. This motivated the search for other insulating compounds such as Li2FeSiO4 as cathode materials [5]. High discharge potentials at 4.1 and 4.8 V are observed for LiMnPO4 and LiCoPO4 compounds, respectively [6–8]. Another class of compounds which crystallizes in olivine structure is LiMAsO4, where M = Ni2+ and Mn2+ [9,10]. The Fe and Co analogues have not been reported hitherto. The crystal structure of LiNiAsO4 has been refined by using powder X-ray diffraction data and it is shown that the compound is isostructural with olivine LiNiPO4 [9]. Recently, transition metal containing network phosphates such as LiTi2(PO4)3 [11] and LiFePO4 [12] are explored as anode materials for Li-ion batteries. Reversible capacity of 300 mAh/g is observed for LiFePO4 in the voltage range 0.0–3.25 V with a large hysteresis during charge and discharge. The authors did not comment on the reaction mechanism of lithium with LiFePO4. In LiTi2(PO4)3, a reversible capacity of 130 mAh/g is observed in the voltage * Corresponding author. Tel.: +91 44 2257 4215; fax: +91 44 2257 0509. E-mail address: [email protected] (U.V. Varadaraju). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.09.005
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range 0.25–1.2 V. The large intake of lithium during initial discharge of the phosphates suggests that the reversible capacity is not due to the simple intercalation/deintercalation of lithium. To the best of our knowledge, lithium extraction/insertion in arsenates with olivine structure has never been explored. Thus, in the present study, LiCoAsO4 has been synthesized for the first time and its electrochemical properties are evaluated as cathode and anode. 2. Experimental LiCoAsO4 was synthesized by high temperature solid state reaction from Li2CO3 (Merck, 99.5%), Co3O4 (Cerac, 99.5%) and As2O5 (Cerac, 99.5%). Stoichiometric amounts of the reactants were ground well and heated at 600 8C for 12 h. The resultant powder was ground well and sintered at 800 8C for 36 h in air with intermittent grindings. The compound was characterized by powder X-ray diffraction (Rich Seifert, P3000, Germany, Co Ka). The lattice parameters were obtained by least square fitting using AUTOX program. Structure refinement of LiCoAsO4 was carried out using powder X-ray diffraction data collected at room temperature. The data were collected in the 2u range 10–1208 in steps of 0.028 with a counting time of 8 s per step. The structure was refined by Rietveld method [13] using Fullprof program. Electrochemical lithium insertion/extraction tests were performed in Swagelok type cells. Electrodes were obtained by spreading the mixture of 45 wt% active material and 45 wt% acetylene black (Denka Singapore Pvt. Ltd.) with 10 wt% PVDF in NMP on a stainless steel foil. For anodic studies, electrodes were made by mixing active material, acetylene black and PVDF in a weight ratio of 70:20:10. Cell assembly was done in an argon filled glove box (mBraun, 120G, Germany), wherein the moisture content was less than 5 ppm. Teklon (Anatek, USA) was used as the separator and lithium metal (Aldrich, 99.9%) as counter electrode. The electrolyte used was 1 M LiPF6 in 1:1 EC + DMC (Chiel Industries Ltd., Korea). Galvanostatic charge/discharge cycling tests were carried out by using an Arbin battery cycling unit (Arbin Instruments, BT 2000, USA). Cyclic voltammetry (CV) test was performed at a scan rate of 0.1 mV/s, using an Autolab PGSTAT 30 instrument. Ex situ XRD patterns of the electrodes were recorded by covering the electrodes with mylar film. In situ XRD study during initial discharge of Li/LiCoAsO4 to 0.05 V was carried out by using an in-house developed in situ XRD cell as described previously by us [14]. 3. Results and discussion 3.1. Structural study Rietveld refinement of LiCoAsO4 is carried out by using the structural parameters of LiNiAsO4 [9]. Li3AsO4 is present as a small impurity and is not included in the Rietveld refinement. The refinement is performed in the space group Pnma. The final agreement factors reached are: Rp = 6.64; Rwp = 8.40; Rex = 6.73; x2 = 1.56; RF = 11.0; RBragg = 12.1. The observed, calculated and difference patterns obtained from the final refinement are shown in Fig. 1. ˚ , b = 5.9970(1) A ˚ and c = 4.8866(1) A ˚ . The atomic The refined orthorhombic lattice parameters are a = 10.4614(2) A coordinates with standard deviations are given in Table 1. LiCoAsO4 adopts olivine structure consisting of LiO6, CoO6 octahedra and AsO4 tetrahedra. The structure can be described as corner shared CoO6 octahedra cross-linked by AsO4 tetrahedra forming a network. Li+ ions present in the tunnels form a linear chain of edge shared octahedra. 3.2. Electrochemical studies 3.2.1. LiCoAsO4 as cathode The cyclic voltammogram of LiCoAsO4 carried out in the voltage window 3.0–5.0 V at a scan rate of 0.1 mV/s is shown in Fig. 2. LiCoAsO4 shows an oxidation peak at 5.0 V during anodic sweep and during reverse sweep a distinct peak centered at 4.5 V is observed. This indicates a reversible process of deintercalation and intercalation of lithium from LiCoAsO4. The charge and discharge curves of LiCoAsO4 electrode containing 45 wt% acetylene black in the voltage window 3.0–5.0 V at C/10 rate are shown in Fig. 3. During initial charge, a plateau at 5.0 V is observed with a capacity of 110 mAh/g. The theoretical specific capacity of LiCoAsO4 is 131 mAh/g. Thus, during initial charge, nearly 0.8Li is deintercalated. During discharge, a plateau at 4.5 V is observed with an initial discharge capacity of 38 mAh/g. The discharge capacity falls to 20 mAh/g after 25 cycles. The poor capacity retention can be
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Fig. 1. Observed, calculated and difference powder X-ray diffraction profiles of LiCoAsO4.
Table 1 ˚ 2) for LiCoAsO4 Atomic coordinates and isotropic temperature factors (A Atom
Wyckoff position
x
y
z
Biso
Occupancy
Li Co As O(1) O(2) O(3)
4a 4c 4c 4c 4c 8d
0 0.2746(2) 0.0947(2) 0.097(1) 0.452(1) 0.1635(7)
0 0.25 0.25 0.25 0.25 0.0356(7)
0 0.9945(2) 0.4337(2) 0.763(1) 0.186(1) 0.2813(7)
0.83(4) 0.82(4) 0.67(4) 0.31(4) 0.38(4) 0.41(4)
0.5 0.5 0.5 0.5 0.5 1
Fig. 2. Cyclic voltammogram of Li/LiCoAsO4 cell obtained at 0.1 mV/s between 3.0 and 5.0 V.
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Fig. 3. Charge and discharge curves of Li/LiCoAsO4 cell cycled in the voltage window 3.0–5.0 V at C/10 rate.
accounted for electrolyte instability with repeated cycling at high voltage (5 V). The discharge potential of LiCoAsO4 is same as the value observed in case of analogous LiCoPO4. Thus, the redox potential of Co2+/Co3+ couple is not changed by varying the polyhedral group from PO4 to AsO4. The redox potential of a given Mn+/M(n+1)+ couple in compounds with Nasicon structure, LixM2(XO4)3 (M = transition metal, X = S, P and As) varies with the nature of the polyanion [15]. However, due to the nearly same covalency nature of As–O and P–O bonds, the redox potential of a given Mn+/M(n+1)+ couple is similar for arsenates and phosphates having same structure. This is in agreement with the report by Masquelier et al. [16], who showed that the position of redox couple Fe3+/Fe2+ in Li3Fe2(XO4)3 is not varied for X = P and As compounds. The ex situ XRD patterns of electrodes after initial charge to 5.0 V and after first discharge are shown in Fig. 4. The structural changes after charge confirm the deintercalation of lithium from LiCoAsO4. The deintercalation of lithium from LiCoAsO4 leads to splitting in the XRD peaks. Also, the presence of reflections corresponding to LiCoAsO4 phase indicates that lithium is not extracted completely during charge. The splitting in the XRD pattern of LiCoPO4 electrode after charge is attributed to the formation of CoPO4 phase [8,17]. Since LiCoAsO4 is isostructural with the related phosphate LiCoPO4, the splitting in the reflections observed in case of XRD pattern of charged electrode of LiCoAsO4 can also be attributed to the formation of CoAsO4 phase. Also, the reflections of CoAsO4 phase appear on the higher angle side with respect to original reflections of LiCoAsO4, indicating decrease of lattice parameters. After relithiation, the splitting in the peaks disappears with concomitant increase in
Fig. 4. Ex situ XRD patterns of LiCoAsO4 electrode: (a) before electrochemical reaction, (b) after initial charge to 5.0 V [(*) CoAsO4 phase] and (c) after initial discharge to 3.0 V.
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Fig. 5. Initial discharge and charge curves of Li/LiCoAsO4 cell at C/5 rate in the voltage range between 0.05 and 2.5 V.
intensities of reflections corresponding to LiCoAsO4 phase. This indicates that lithium intercalates into CoAsO4 phase and forms the original LiCoAsO4. 3.2.2. Reaction of lithium with LiCoAsO4 In the present study, LiCoAsO4 has been explored as an anode material. Li/LiCoAsO4 cell discharged initially to 0.05 V, i.e., initially lithium is reacted with LiCoAsO4, and subsequent extraction and insertion of lithium is carried out in the voltage window 0.05–2.5 V at C/5 rate. The corresponding discharge and charge curves are shown in Fig. 5. As soon as initial discharge started, voltage dropped rapidly from open circuit voltage (2.9 V) and a plateau at 1.3 V is
Fig. 6. In situ XRD patterns of LiCoAsO4 electrode during initial discharge to 0.05 Vat C/5 rate. Peaks marked with ‘*’ and ‘**’ are due to separator and Li metal, respectively.
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observed till the capacity reached a value of 200 mAh/g. On further discharge, the voltage decreases gradually to 0.05 V with a small step at 0.4 V. To obtain information on the reaction mechanism of lithium with LiCoAsO4, in situ X-ray diffraction measurement is carried out. During the discharge of Li/LiCoAsO4 cell, in situ XRD patterns are recorded at regular intervals of time and are shown in Fig. 6. The initial XRD pattern is characteristic of LiCoAsO4 with additional reflections due to a separator and lithium metal. The XRD pattern after 5 h discharge shows decrease in intensities of the peaks corresponding to LiCoAsO4 phase without any shift in the positions. It shows that there is no lithium intercalation into the olivine structure. The intensity of each peak gradually decreases with discharge. At 1.0 V, the diffraction peaks corresponding to parent phase disappear and no crystalline peaks are observed even on further discharge to 0.05 V. This indicates that after initial discharge, olivine structure of LiCoAsO4 is completely destroyed and irreversibly transformed into amorphous phases. Similar observation of formation of amorphous phases is reported during the study of LiCoVO4 and LiNiVO4 as anode materials [18,19]. Since there is no formation of crystalline phases, in situ XRD study does not reveal the reaction mechanism of lithium with LiCoAsO4. After correcting for the capacity contribution due to acetylene black, the initial discharge capacity is 1230 mAh/g. This corresponds to reaction of 9.4Li. In case of LiMVO4 (M = Co or Ni) spinels, reaction of 7Li per formula unit is observed on initial discharge down to 0.00 V [18,19]. From X-ray absorption spectroscopy study, it has been suggested that on complete discharge vanadium is reversibly reduced to 2+ oxidation state and Co2+/Ni2+ transforms to metallic state [19,20]. Similar reaction mechanism is expected for LiCoAsO4, but unlike in the case of vanadium, the arsenic formed has the affinity towards lithium to form Li3As phase. Also, the number of lithium atoms consumed is higher in LiCoAsO4 compared to that of LiCoVO4 indicating the formation of Li3As. Thus, the overall mechanism of reaction of lithium with LiCoAsO4 can be written as follows: LiCoAsO4 þ 10Li ! Co þ Li3 As þ 4Li2 O The observed capacity (9.4Li) is nearly same as that of the theoretical specific capacity (10Li) indicating the occurrence of above reaction. The capacity during initial charge to 2.5 V is 370 mAh/g, corresponding to the extraction of 2.8Li. It indicates that the products formed during initial discharge are not reversible. The XRD pattern of completely charged electrode is still amorphous (not shown). A large polarization upon charge and discharge is observed which can be due to decomposition of active material into poorly conducting phases. Fig. 7 shows the discharge capacity versus cycle number of LiCoAsO4 cycled in the voltage window 0.05–2.5 V. Capacity fades quickly on cycling and reaches a value of 40 mAh/g after 15 cycles. To study the effect of voltage window on reversible capacity, the cycling of Li/LiCoAsO4 is carried out in the voltage window 1.0–2.5 V. The initial discharge capacity down to 1.0 V is 400 mAh/g, corresponding to reaction of 3Li. Partial reduction of As5+ and Co2+ is expected on discharge to 1.0 V. On charge, voltage increases smoothly to 2.5 V with a capacity of 140 mAh/g. The charge capacity observed can be attributed to the extraction of lithium from Li–O–Co matrix with the simultaneous oxidation of Co [21]. On further cycling in the voltage window 1.0–2.5 V, a reversible capacity of 100 mAh/g is observed (Fig. 7).
Fig. 7. Discharge capacity vs. cycle number of Li/LiCoAsO4 cell cycled in the voltage ranges between 0.05–2.5 and 1.0–2.5 V, corrected for acetylene black.
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4. Conclusions A new compound LiCoAsO4 with olivine structure has been synthesized successfully and the electrochemical performance versus lithium metal has been evaluated. The charge/discharge curves of Li/LiCoAsO4 in the voltage range 3.0–5.0 V reveal that the Co3+/Co2+ redox couple has a high potential of 5.0 V. The ex situ XRD study demonstrates a two phase reaction mechanism involving the formation of CoAsO4 phase with lithium deintercalation. It is observed that large amount of lithium can react with LiCoAsO4 on discharge to 0.05 V. The large intake is accounted for by invoking the formation of amorphous products, Li2O, Co and Li3As. Li/LiCoAsO4 cell cycled in the voltage window 0.05–2.5 V shows a rapid capacity fade, whereas a stable reversible capacity of 100 mAh/g is observed in the voltage window 1.0–2.5 V. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188. P.P. Prosini, M. Carewska, S. Scaccia, P. Wisniewski, M. Pasquali, Electrochim. Acta 48 (2003) 4205. H. Huang, S.-C. Yin, L.F. Nazar, Electrochem. Solid State Lett. 4 (2001) A170. F. Croce, A. D’Epifanio, J. Hassoun, A. Deptula, T. Olczac, B. Scrosati, Electrochem. Solid State Lett. 5 (2002) A47. A. Nyte´n, A. Abouimrane, M. Armand, T. Gustafsson, J.O. Thomas, Electrochem. Commun. 7 (2005) 156. G. Li, H. Azuma, M. Tohda, Electrochem. Solid State Lett. 5 (2002) A135. C. Delacourt, P. Poizot, M. Morcrette, J.-M. Tarascon, C. Masquelier, Chem. Mater. 16 (2004) 93. K. Amine, H. Yasuda, M. Yamachi, Electrochem. Solid State Lett. 3 (2000) 178. J.L. Mesa, J.L. Pizarro, L. Lezama, J. Escobal, M.I. Arriortua, T. Rojo, J. Solid State Chem. 141 (1998) 508. M.G. Aranda, J.P. Attfield, S. Bruqeus, Mater. Sci. Forum 79 (1991) 827. G.X. Wang, D.H. Bradhurst, S.X. Dou, H.K. Liu, J. Power Sources 124 (2003) 231. N. Kalaiselvi, C.H. Doh, C.W. Park, S.I. Moon, M.S. Yun, Electrochem. Commun. 6 (2004) 1110. H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65. M.V.V.M. Satya Kishore, U.V. Varadaraju, B. Raveau, J. Solid State Chem. 177 (2004) 3981. K.S. Nanjundaswamy, A.K. Padhi, J.B. Goodenough, S. Okada, H. Ohtsuka, H. Arai, J. Yamaki, Solid State Ionics 92 (1996) 1. C. Masquelier, A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Solid State Chem. 135 (1998) 228. N.N. Bramnik, K.G. Bramnik, C. Baehtz, H. Ehrenberg, J. Power Sources 145 (2005) 74. J. Shirakawa, M. Nakayama, H. Ikuta, Y. Uchimoto, M. Wakihara, Electrochem. Solid State Lett. 7 (2004) A27. F. Orsini, E. Baudrin, S. Denis, L. Dupont, M. Touboul, D. Guyomard, Y. Piffard, J.-M. Tarascon, Solid State Ionics 107 (1998) 123. C. Rossignol, G. Ouvrard, E. Baudrin, J. Electrochem. Soc. 148 (2001) A869. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496.
Synthesis, characterization and electrochemical studies on LiCoAsO4 M.V.V.M. Satya Kishore, U.V. Varadaraju * Materials Science Research Centre and Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, Tamil Nadu, India Received 5 April 2005; received in revised form 26 August 2005; accepted 2 September 2005 Available online 26 September 2005
Abstract LiCoAsO4 is synthesized by solid state reaction method and its crystal structure has been refined by the Rietveld method using powder X-ray diffraction data. LiCoAsO4 crystallizes in olivine structure with space group Pnma and orthorhombic lattice ˚ , b = 5.9970(1) A ˚ and c = 4.8866(1) A ˚ . Electrochemical studies reveal that in LiCoAsO4, lithium is parameters are a = 10.4614(2) A deintercalated and intercalated at high voltage 5.0 V. On the other hand, the compound can react with about 9Li on discharge to 0.05 V. A reversible capacity of 100 mAh/g is obtained in the voltage range 1.0–2.5 V. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; C. X-ray diffraction; D. Electrochemical properties
1. Introduction Materials with framework structures built up from polyanion groups such as SiO4, PO4 and AsO4 are considered potential electrode materials for lithium batteries. In particular, phosphates of the general formula LiM2+PO4 (M = Mn, Fe and Co) with olivine structure are investigated as positive electrode materials for secondary lithium batteries. Despite their poor electronic conductivity, phosphates show good reversible lithium extraction/insertion, especially in case of LiFePO4. LiFePO4 with a flat voltage curve at 3.5 V and a theoretical specific capacity of 170 mAh/g emerges as the most promising cathode material in the olivine family of compounds [1]. An improvement in the rate capability of LiFePO4 is achieved by decreasing the particle size and by an electronic conductive particle coating [2–4]. This motivated the search for other insulating compounds such as Li2FeSiO4 as cathode materials [5]. High discharge potentials at 4.1 and 4.8 V are observed for LiMnPO4 and LiCoPO4 compounds, respectively [6–8]. Another class of compounds which crystallizes in olivine structure is LiMAsO4, where M = Ni2+ and Mn2+ [9,10]. The Fe and Co analogues have not been reported hitherto. The crystal structure of LiNiAsO4 has been refined by using powder X-ray diffraction data and it is shown that the compound is isostructural with olivine LiNiPO4 [9]. Recently, transition metal containing network phosphates such as LiTi2(PO4)3 [11] and LiFePO4 [12] are explored as anode materials for Li-ion batteries. Reversible capacity of 300 mAh/g is observed for LiFePO4 in the voltage range 0.0–3.25 V with a large hysteresis during charge and discharge. The authors did not comment on the reaction mechanism of lithium with LiFePO4. In LiTi2(PO4)3, a reversible capacity of 130 mAh/g is observed in the voltage * Corresponding author. Tel.: +91 44 2257 4215; fax: +91 44 2257 0509. E-mail address: [email protected] (U.V. Varadaraju). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.09.005
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range 0.25–1.2 V. The large intake of lithium during initial discharge of the phosphates suggests that the reversible capacity is not due to the simple intercalation/deintercalation of lithium. To the best of our knowledge, lithium extraction/insertion in arsenates with olivine structure has never been explored. Thus, in the present study, LiCoAsO4 has been synthesized for the first time and its electrochemical properties are evaluated as cathode and anode. 2. Experimental LiCoAsO4 was synthesized by high temperature solid state reaction from Li2CO3 (Merck, 99.5%), Co3O4 (Cerac, 99.5%) and As2O5 (Cerac, 99.5%). Stoichiometric amounts of the reactants were ground well and heated at 600 8C for 12 h. The resultant powder was ground well and sintered at 800 8C for 36 h in air with intermittent grindings. The compound was characterized by powder X-ray diffraction (Rich Seifert, P3000, Germany, Co Ka). The lattice parameters were obtained by least square fitting using AUTOX program. Structure refinement of LiCoAsO4 was carried out using powder X-ray diffraction data collected at room temperature. The data were collected in the 2u range 10–1208 in steps of 0.028 with a counting time of 8 s per step. The structure was refined by Rietveld method [13] using Fullprof program. Electrochemical lithium insertion/extraction tests were performed in Swagelok type cells. Electrodes were obtained by spreading the mixture of 45 wt% active material and 45 wt% acetylene black (Denka Singapore Pvt. Ltd.) with 10 wt% PVDF in NMP on a stainless steel foil. For anodic studies, electrodes were made by mixing active material, acetylene black and PVDF in a weight ratio of 70:20:10. Cell assembly was done in an argon filled glove box (mBraun, 120G, Germany), wherein the moisture content was less than 5 ppm. Teklon (Anatek, USA) was used as the separator and lithium metal (Aldrich, 99.9%) as counter electrode. The electrolyte used was 1 M LiPF6 in 1:1 EC + DMC (Chiel Industries Ltd., Korea). Galvanostatic charge/discharge cycling tests were carried out by using an Arbin battery cycling unit (Arbin Instruments, BT 2000, USA). Cyclic voltammetry (CV) test was performed at a scan rate of 0.1 mV/s, using an Autolab PGSTAT 30 instrument. Ex situ XRD patterns of the electrodes were recorded by covering the electrodes with mylar film. In situ XRD study during initial discharge of Li/LiCoAsO4 to 0.05 V was carried out by using an in-house developed in situ XRD cell as described previously by us [14]. 3. Results and discussion 3.1. Structural study Rietveld refinement of LiCoAsO4 is carried out by using the structural parameters of LiNiAsO4 [9]. Li3AsO4 is present as a small impurity and is not included in the Rietveld refinement. The refinement is performed in the space group Pnma. The final agreement factors reached are: Rp = 6.64; Rwp = 8.40; Rex = 6.73; x2 = 1.56; RF = 11.0; RBragg = 12.1. The observed, calculated and difference patterns obtained from the final refinement are shown in Fig. 1. ˚ , b = 5.9970(1) A ˚ and c = 4.8866(1) A ˚ . The atomic The refined orthorhombic lattice parameters are a = 10.4614(2) A coordinates with standard deviations are given in Table 1. LiCoAsO4 adopts olivine structure consisting of LiO6, CoO6 octahedra and AsO4 tetrahedra. The structure can be described as corner shared CoO6 octahedra cross-linked by AsO4 tetrahedra forming a network. Li+ ions present in the tunnels form a linear chain of edge shared octahedra. 3.2. Electrochemical studies 3.2.1. LiCoAsO4 as cathode The cyclic voltammogram of LiCoAsO4 carried out in the voltage window 3.0–5.0 V at a scan rate of 0.1 mV/s is shown in Fig. 2. LiCoAsO4 shows an oxidation peak at 5.0 V during anodic sweep and during reverse sweep a distinct peak centered at 4.5 V is observed. This indicates a reversible process of deintercalation and intercalation of lithium from LiCoAsO4. The charge and discharge curves of LiCoAsO4 electrode containing 45 wt% acetylene black in the voltage window 3.0–5.0 V at C/10 rate are shown in Fig. 3. During initial charge, a plateau at 5.0 V is observed with a capacity of 110 mAh/g. The theoretical specific capacity of LiCoAsO4 is 131 mAh/g. Thus, during initial charge, nearly 0.8Li is deintercalated. During discharge, a plateau at 4.5 V is observed with an initial discharge capacity of 38 mAh/g. The discharge capacity falls to 20 mAh/g after 25 cycles. The poor capacity retention can be
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Fig. 1. Observed, calculated and difference powder X-ray diffraction profiles of LiCoAsO4.
Table 1 ˚ 2) for LiCoAsO4 Atomic coordinates and isotropic temperature factors (A Atom
Wyckoff position
x
y
z
Biso
Occupancy
Li Co As O(1) O(2) O(3)
4a 4c 4c 4c 4c 8d
0 0.2746(2) 0.0947(2) 0.097(1) 0.452(1) 0.1635(7)
0 0.25 0.25 0.25 0.25 0.0356(7)
0 0.9945(2) 0.4337(2) 0.763(1) 0.186(1) 0.2813(7)
0.83(4) 0.82(4) 0.67(4) 0.31(4) 0.38(4) 0.41(4)
0.5 0.5 0.5 0.5 0.5 1
Fig. 2. Cyclic voltammogram of Li/LiCoAsO4 cell obtained at 0.1 mV/s between 3.0 and 5.0 V.
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Fig. 3. Charge and discharge curves of Li/LiCoAsO4 cell cycled in the voltage window 3.0–5.0 V at C/10 rate.
accounted for electrolyte instability with repeated cycling at high voltage (5 V). The discharge potential of LiCoAsO4 is same as the value observed in case of analogous LiCoPO4. Thus, the redox potential of Co2+/Co3+ couple is not changed by varying the polyhedral group from PO4 to AsO4. The redox potential of a given Mn+/M(n+1)+ couple in compounds with Nasicon structure, LixM2(XO4)3 (M = transition metal, X = S, P and As) varies with the nature of the polyanion [15]. However, due to the nearly same covalency nature of As–O and P–O bonds, the redox potential of a given Mn+/M(n+1)+ couple is similar for arsenates and phosphates having same structure. This is in agreement with the report by Masquelier et al. [16], who showed that the position of redox couple Fe3+/Fe2+ in Li3Fe2(XO4)3 is not varied for X = P and As compounds. The ex situ XRD patterns of electrodes after initial charge to 5.0 V and after first discharge are shown in Fig. 4. The structural changes after charge confirm the deintercalation of lithium from LiCoAsO4. The deintercalation of lithium from LiCoAsO4 leads to splitting in the XRD peaks. Also, the presence of reflections corresponding to LiCoAsO4 phase indicates that lithium is not extracted completely during charge. The splitting in the XRD pattern of LiCoPO4 electrode after charge is attributed to the formation of CoPO4 phase [8,17]. Since LiCoAsO4 is isostructural with the related phosphate LiCoPO4, the splitting in the reflections observed in case of XRD pattern of charged electrode of LiCoAsO4 can also be attributed to the formation of CoAsO4 phase. Also, the reflections of CoAsO4 phase appear on the higher angle side with respect to original reflections of LiCoAsO4, indicating decrease of lattice parameters. After relithiation, the splitting in the peaks disappears with concomitant increase in
Fig. 4. Ex situ XRD patterns of LiCoAsO4 electrode: (a) before electrochemical reaction, (b) after initial charge to 5.0 V [(*) CoAsO4 phase] and (c) after initial discharge to 3.0 V.
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Fig. 5. Initial discharge and charge curves of Li/LiCoAsO4 cell at C/5 rate in the voltage range between 0.05 and 2.5 V.
intensities of reflections corresponding to LiCoAsO4 phase. This indicates that lithium intercalates into CoAsO4 phase and forms the original LiCoAsO4. 3.2.2. Reaction of lithium with LiCoAsO4 In the present study, LiCoAsO4 has been explored as an anode material. Li/LiCoAsO4 cell discharged initially to 0.05 V, i.e., initially lithium is reacted with LiCoAsO4, and subsequent extraction and insertion of lithium is carried out in the voltage window 0.05–2.5 V at C/5 rate. The corresponding discharge and charge curves are shown in Fig. 5. As soon as initial discharge started, voltage dropped rapidly from open circuit voltage (2.9 V) and a plateau at 1.3 V is
Fig. 6. In situ XRD patterns of LiCoAsO4 electrode during initial discharge to 0.05 Vat C/5 rate. Peaks marked with ‘*’ and ‘**’ are due to separator and Li metal, respectively.
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observed till the capacity reached a value of 200 mAh/g. On further discharge, the voltage decreases gradually to 0.05 V with a small step at 0.4 V. To obtain information on the reaction mechanism of lithium with LiCoAsO4, in situ X-ray diffraction measurement is carried out. During the discharge of Li/LiCoAsO4 cell, in situ XRD patterns are recorded at regular intervals of time and are shown in Fig. 6. The initial XRD pattern is characteristic of LiCoAsO4 with additional reflections due to a separator and lithium metal. The XRD pattern after 5 h discharge shows decrease in intensities of the peaks corresponding to LiCoAsO4 phase without any shift in the positions. It shows that there is no lithium intercalation into the olivine structure. The intensity of each peak gradually decreases with discharge. At 1.0 V, the diffraction peaks corresponding to parent phase disappear and no crystalline peaks are observed even on further discharge to 0.05 V. This indicates that after initial discharge, olivine structure of LiCoAsO4 is completely destroyed and irreversibly transformed into amorphous phases. Similar observation of formation of amorphous phases is reported during the study of LiCoVO4 and LiNiVO4 as anode materials [18,19]. Since there is no formation of crystalline phases, in situ XRD study does not reveal the reaction mechanism of lithium with LiCoAsO4. After correcting for the capacity contribution due to acetylene black, the initial discharge capacity is 1230 mAh/g. This corresponds to reaction of 9.4Li. In case of LiMVO4 (M = Co or Ni) spinels, reaction of 7Li per formula unit is observed on initial discharge down to 0.00 V [18,19]. From X-ray absorption spectroscopy study, it has been suggested that on complete discharge vanadium is reversibly reduced to 2+ oxidation state and Co2+/Ni2+ transforms to metallic state [19,20]. Similar reaction mechanism is expected for LiCoAsO4, but unlike in the case of vanadium, the arsenic formed has the affinity towards lithium to form Li3As phase. Also, the number of lithium atoms consumed is higher in LiCoAsO4 compared to that of LiCoVO4 indicating the formation of Li3As. Thus, the overall mechanism of reaction of lithium with LiCoAsO4 can be written as follows: LiCoAsO4 þ 10Li ! Co þ Li3 As þ 4Li2 O The observed capacity (9.4Li) is nearly same as that of the theoretical specific capacity (10Li) indicating the occurrence of above reaction. The capacity during initial charge to 2.5 V is 370 mAh/g, corresponding to the extraction of 2.8Li. It indicates that the products formed during initial discharge are not reversible. The XRD pattern of completely charged electrode is still amorphous (not shown). A large polarization upon charge and discharge is observed which can be due to decomposition of active material into poorly conducting phases. Fig. 7 shows the discharge capacity versus cycle number of LiCoAsO4 cycled in the voltage window 0.05–2.5 V. Capacity fades quickly on cycling and reaches a value of 40 mAh/g after 15 cycles. To study the effect of voltage window on reversible capacity, the cycling of Li/LiCoAsO4 is carried out in the voltage window 1.0–2.5 V. The initial discharge capacity down to 1.0 V is 400 mAh/g, corresponding to reaction of 3Li. Partial reduction of As5+ and Co2+ is expected on discharge to 1.0 V. On charge, voltage increases smoothly to 2.5 V with a capacity of 140 mAh/g. The charge capacity observed can be attributed to the extraction of lithium from Li–O–Co matrix with the simultaneous oxidation of Co [21]. On further cycling in the voltage window 1.0–2.5 V, a reversible capacity of 100 mAh/g is observed (Fig. 7).
Fig. 7. Discharge capacity vs. cycle number of Li/LiCoAsO4 cell cycled in the voltage ranges between 0.05–2.5 and 1.0–2.5 V, corrected for acetylene black.
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4. Conclusions A new compound LiCoAsO4 with olivine structure has been synthesized successfully and the electrochemical performance versus lithium metal has been evaluated. The charge/discharge curves of Li/LiCoAsO4 in the voltage range 3.0–5.0 V reveal that the Co3+/Co2+ redox couple has a high potential of 5.0 V. The ex situ XRD study demonstrates a two phase reaction mechanism involving the formation of CoAsO4 phase with lithium deintercalation. It is observed that large amount of lithium can react with LiCoAsO4 on discharge to 0.05 V. The large intake is accounted for by invoking the formation of amorphous products, Li2O, Co and Li3As. Li/LiCoAsO4 cell cycled in the voltage window 0.05–2.5 V shows a rapid capacity fade, whereas a stable reversible capacity of 100 mAh/g is observed in the voltage window 1.0–2.5 V. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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