- Email: [email protected]
Effect of lithium content on the electrochemical properties of solid-state-synthesized spinel LixMn2O4
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RARE METALS Vol. 26, No. 3, Jun 2007, p . 280 E-mail: [email protected]
=‘;
ScienceDirect
Effect of lithium content on the electrochemical properties of solid-state-synthesizedspinel LixMnz04 LI Tao”, QIU Weihua’), W A O Hailei”, and LIU Jingjing” 1) General Research Insticute for Nonferrous Metals, Beumg 100088, China
2) School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China (Received 2006-02-10)
Abstract: Lithium-substituted Li,Mn204 (x = 0.98, 1.03, 1.08) spinel samples were synthesized by solid-state reaction. X-ray diffraction (XRD) patterns show that the prepared samples have a spinel stn~cturewith a space group of Fd 5 m. The cubic lattice parameter was determined from least-squares fitting of the XRD data. Li1.&n204 shows high capacity at both low and high current densities, while Lil.&n204 shows good cycling performance but relatively low capacity when cycled at both room and elevated temperatures. A variety of electrochemical methods were employed to investigate the electrochemical properties of these series of spinel Li.,Mn204. Key words: lithium manganese oxide: solid-state reaction; spinel; electrochemicalproperties [This work wasfinanciallv supported by the National Naturul Science Foundation of Chino (No. 50272012).]
1. Introduction Among the vast number of lihum manganese oxide compounds, LiMn204 had been singled out because of its attractive spinel-type structure and high potential of 4 V versus lithium metal [ 1-21. It has the spinel structure (A[B]&, space group F d j m ) with Li-ions occupying one-eighth of the tetrahedral sites ( 8 a sites ) and h4n-ions occupying half of the octahedral sites (16 d sites ) in a cubic close-packed may of oxygen ions (32 e sites). The unoccupied 16 c sites form a three-dimensional network which is the pathway for the transport of Li ions. Lithium can be extracted from or inserted into LiMnzO4 [3-41. Capacity fade in spinel LiMnz04 cells is a result of multiple processes, including those related solely to the spinel material and others involved in the interaction of the spinel with the electrolyte and negative electrode materials. Most fade mechanisms can be amibuted to the following three factors: Correspondingauthor: QIU Weihua
E-mail: qiuwh@vip,sina.com
(1) Dissolution of ~ n ’ + into the e~ectrolyteafter disproportionationof Li,Mn204. (2) Jahn-Teller distortion in discharged cells. (3) Instability of the electrolyte. The Jahn-Teller distortion can cause symmetry change and thus result in slow capacity loss. It is believed that there is a critical average oxidation state (AOS) of manganese (3.5) above which Jahn-Teller effect can be suppressed [4-51. An increase of lithium can lead to the increase of the AOS of manganese in Li,Mn204. The goal of this study was on the electrochemical properties, stability, and capacity retention of LiXMn2O4electrodes synthesized with different contents of lithium. X-ray diffraction (XRD), Rietveld refinement, and transmission electron microscope (TEM)were employed to obtain the structural information of Li,Mn204. The elemental analysis of the final products was carried out using the ICP-AES method. To study the electrochemical kinetic properties, constant current-charge and dis-
Li Z et ul., Effect of lithium content on the electrochemical properties of solid-state-synthesized... charge, AC-impedance and impedance-potential(IP) techniques were used.
2. Experimental The spinel LixMn204 samples were prepared from Li2C03 and electrolytic Mn02 by solid-state reaction. The well-mixed starting materials with different contents of lithium (x = 0.98, 1.03, 1.08) were calcined at 600°C for 4 h and then at 830°C for 12 h. The cooling rate was selected as 1"C.min-'. After cooling, the samples were tempered at 600°C for 6 h. The three samples were labeled as Li098, Li103, and Lil08. The prepared samples were characterized by powder X-ray diffraction (XRD, D/max-rb dfiactometer, Rigaku, Japan) using Cu KQ (1= 0.154056 nm) radiation at room temperature. The particle morphology and the electron diffraction (ED) measurements were performed using a transmission electron microscope (EM-1OOCXII). Samples were dispersed in ethanol and collected on a holey micro-grid supported on a copper mesh. The elemental analysis of the final products was carried out using two types of methods: induced coupled plasma spectroscopy (ICP) measurement and oxidation-reduction titration. The Perkin-Elmer optima 3000 (ICP-AES) mass-spectrometer was used to determine the content of lithium in the samples. The content of manganese was measured by oxidation-reduction titration method using ammonium ferrous sulfate. For electrochemical measurement, the electrode was prepared by rolling a mixture of 85 wt.% active materials, 10 wt.% acetylene black, and 5 wt.% polytetra-fluoroethylene (PTFE). The electrodes were vacuum-dried at 120°Cfor over 12 h, and then the testing cells were assembled in an argon-filled glove box. Lithium metal was used as the counter electrode, and porous polypropylene (celguard2300) as the separator. The electrolyte was 1.0 M LiPF6 in a mixture of DMC, EC, and DEC, 1:1:1 v/v (Li-batterygrade, S w u n g ) . A computer-controlledLAND BTI-10 8-channel battery cycler was used for half-cell cycle test. The cells were tested using constant-charge and dis-
281
charge current at a voltage ranging between 3.3-4.35 V. The charge current density used was 70 mA.g-'. The discharge current densities were selected as 70, 140, 280, 420, 700 rnA.g-'. The cells were also tested at an elevated temperature of 55°C with a current density of 70 mA.g-'. All the electrochemicaltests were performed on a potentiostatic/galvanostaticsystem (CHI66OA electrochemical workstation, Chenhua instrument company, Shanghai). Electrochemical impedance spectroscopy (EIS) was measured at an open circuit after preliminary equilibration at desired Li-ion intercalation depth for a period of 3 h. The impedance spectra data was covered in the frequency range from 100 kHz to 0.01 Hz at an excitation signal of 5 mV. Impedance potential analysis was carried out at constant frequency from all the discharge potential range (4.5-3.5 V) with a scanning rate of 5 mV.s-'.
3. Results and discussion 3.1. Structural analysis Fig.1 shows the XRD patterns of Li,Mn204 with different lithium contents. No impurity peak can be detected in all the synthesized samples. All the peaks correspond to the spinel-type LiMn204 structure with space group Fd3 rn and can be refined with Rietveld program on the basis of the standard LiMn204 structure [6] using MDI Jadi 5.0 program. The lattice parameter a, of each sample is shown in Table 1. The lattice parameters of the L&&ln204 and Li1.03Mn204samples have little difference while
. 0 cd
.-
-ez
1111
311 400
,331
Standard I
20
60
40
,
80
28 I (01 Fig. 1. XRD patterns of Li,Mn204 synthesized with Merent Li contents.
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the parameter of the Lil,aMnz04sample shows a smaller value. The atomic ratio of Li/Mn and manganese valences from the ICP measurement and oxidation-reduction titration are summarized in Table 1. The relative atomic ratios of the composition were calculated with the atom number of 0 fixed at 4. Sample Li098 has the lowest manganese valences (< 3.5) among the three samples, which may cause the Jahn-Teller distortion. This phase transition results in a transformation from the cubic space group Fd 3 m to the tetragonal group I4,lamd. Related mechanisrns can cause strain and structural failure. result-
ing in electrically disconnected particles and capacity loss. The high manganese content of Sample Li098 indicates that there is some manganese occupying the octahedral sites. For Sample Li103, the contents of lithium and manganese are all smaller than the theoretical value. For Sample Li108, there are more lithium inserting in the spinel structure, and excess lithium may occupy the tetrahedral sites. Fig.2 shows the particle morphology and the electron diffraction of Samples Li098, Li103, and Li 108. All the three ED patterns agree well with the cubic face-centered lattice. The samples with various lithium contents all maintain a cubic structure.
Table 1. Lattice parameter r e M by Rietveld method and element compositionof synthesized LixMnz04 Sample
I-attice parameter / nm
L1098 Li103
82.451 82.453
Li 108
82.319
._
Startirig ratio of Li/Mn ICP value of LdMn
Manganese AOS
Composition
0.9802
0.9909
3.479
Li09%Nln201304
1.0302 1.080:2
1.021:2 1.044:2
3.599 3.593
Lio.%&h.94604
Lil O I N ~ ~0 I
2. 'EM micrographs of the Li,Mn204 samples: (a) Lio98, observed [012] zone ED pattern; (b) Li103, observed i i 231 zone ED pattern; (c) LilW, observed 111i 1 zone ED pattern. Fig.
3.2. Capacity properties Figs. 3 and 4 show the discharge capacity versus cycle number at room and elevated temperatures (55T) with a constant current density of 70 mA.g-'. At mom temperature, all the three samples show good cycling performances. But Samples Li098 and Li103 have relatively higher capacities than Sample Li 108. Sample Li 108 shows excellent cycling property but relatively low initial capacity. Sample Li098 has a relatively larger initial capacity, but a little worse cycling performance than those of Samples Li 103 and Li 108. At an elevated temperature, all the three samples show capacity fade with cycling. But
Fig. 3. Discharge capacity versus cycle number curves at room temperature (3.3-435 V).
4
Li II: et aZ., Effect of lithium content on the electrochemicalproperties of solid-state-synthesized...
283
current densities of 140, 280, 420, 700, 70 mA.g-' the capacity stability was enhanced as the content of while all the samples are charged at a constant curlithium increased. rent density of 70 d+-'. Sample Li103 shows The doping of lithium resulted in a large increase good cycling stability at high current densities durin the M I I ~ & : + r a t i o(increasing the average ~n ing all current densities. Sample Li108 shows exceloxidation state), so there are little Mn3+available for lent cycling property and relatively high capacity at the transition of Mn3++ Mn6 oxidation necessary a current density of 700 mA.g-'. The increasing of for lithium removal [7]. This results in low initial lithium content can enhance the capacity stability of capacities of Samples Li103 and Li108. Moreover, the LixMn204sample at high current density. there is less dissolution of Mn2+into the electrolyte after disproportionation of LixMnzOo( 2 ~ n ~ -+ + ( ~ ~ ~ ) 33. Electrochemical impedance spectroscopy fi2+(hquid) + Mn6(,hd,), especially at an elevated The typical impedance response recorded in the temperature. And increasing the Mn valence can also LiMn20&i half cell is shown in Fig. 6, which is suppress the Jahn-Teller distortion both at room and composed of two depressed semicircles and a spike elevated temperatures. Hence, Samples Li108 and in the low frequency range. Li103 show improved elevated temperature cycling (1) The semicircle in the high frequency range performance compared with Sample Li098. arises from the impedance of the interfacial film on Fig.5 shows the room temperature discharge cathe electrode. pacities versus cycle number at different discharge (2) The semicircle in the medium frequency -, range results from the charge-transferprocess. cQ4", 120 (3) The spike at the low frequency end indicates E the long range diffusion of Li-ion [8-151. ' 100 h
-.-
2oor
x *
!? z
x 5
80 60
40
150
5 10 Cycle number, n
0
Li098 -0-LiO98-cal -A- Li 103 -0-LilO3-cd +Li108 +-LilO8-cal -m-
m Li098 o Li103 A Li108
15
Fig. 4. Discharge capacity versus cycle number curves at an elevated temperature of 55OC (3.3-4.35 V).
0
50
100
150
200
Z'J R
2
v
'I .G 100 0
8
80
3 .-x
60 400
%
Fig. 6. Nyquist plots for LixMn204samples at an open-circuit voltage of 3.9 V. Inset: the equivalent circuit for the impedance data, -cal represents the simulated curves.
1201
5
MM
0 Li103 A Li108
20 25 Cycle number, n 10
15
30
Fig. 5. Discharge capacity versus cycle number curves at room temperature (331435V).
The EIS data were fitted using the analysis software Zsimple (Echemical software, USA), and the equivalent circuit is shown in Fig. 6. Re represents the electrolyte resistance, Rht the interfacial film resistance, RCtthe charge-transferresistance, and W the Warburg impedance that includes diffusioncontrolled process in the solids [ 151.
RARE METALS, Vol. 26, No. 3, Jun 2007
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According to Table 2, the impedances of the electrolyte and the interfacial film are almost similar. But the charge-transfer resistances have very large differences. As a small polaron semiconductor, the electronic conduction of the LiMn204sample occurs via hopping of electrons between eg orbitals on adjacent Mn"h41-1~cations [16]. The electrons associated with Li' deintercalation cannot be removed from a delocalized band, but must be removed from (or must result in) localized molecular orbitals surrounding the lithium site [ 161. The doping of lithium may change the electronic structure of Li,Mn204, and may influence the charge-transfer resistances of the three samples. Sample Li103 has the lowest charge-transfer resistance among the three samples, about 16 R. The charge-transfer reaction became easy as the charge-transfer resistance decreased, which is favorable for battery operation. Table 2. Simulated data and calculated DLiaccording to the Nyquist plots
Samples
~
R, I R Rm,l Q R,, / R
/ (crn'.s-')
Li098
7
10
Li103
7
9
22 16
4.84~
Li108
7
10
47
1.25 x lo-'*
3.39 x lo-'*
At a low frequency, when the diffusion process of Li ion in the cathode is considerably slower than that in an electrolyte solution, a linear portion is formed. The chemical diffusion coefficients of Li-ion, DL,in LiMn204 electrode were estimated using an ac-impedance method as described in Ref. [8]. The data of D L are ~ shown in Table 2. Eq. 1 is adopted to estimate the D L ~in LiMn204 electrode:
served in complex plane diagrams. According to Table 2, Samples Li098 and Li103 have relatively larger Li-ion diffusion coefficients than that of Sample Li108. There may be two reasons: first, Samples Li098 and Li103 have relatively larger lattice parameters, and the Li ion can diffuse easily in the electrode; second, there are more vacant sites of Li in Samples Li098 and Li103, which is favorable for Li' diffusion. 3.4. Impedance potential analysis
To observe the change of the Li-ion diffusion resistance and charge-transfer resistance at different potentials, the impedance potential analysis was carried out in the discharge potential range (3.5-4.5 V) at different frequencies with the scanning rate of 5 mV.s-'. The impedance at medium frequency represents the charge-transfer resistance. The frequency (at about 200 Hz) was selected from the elech-ochemical impedance spectroscopy (Fig. 6). According to Fig. 7, from all the potential range during Li-ion inserting process, the charge-transfer resistance distributes from 20 R to 48 R. The charge-transfer resistances of Sample Li103 are much lower than those of the other two samples. At about 4 V potential, the charge-transfer resistance of Sample Li103 is about 8 R smaller than those of Samples Li098 and LilO8. This may be why Sample Li103 shows good electrochemical properties. 48 45 42
c:
'N
0 Li098
39 36
33 30
where CL, (moVcm3)is calculated from the volumes and the qualities of Li storage of the LiMn204electrode; R the gas constant; n the number of transferred electrons; F the Faraday constant; and A the active surface area. The values of 0 were calculated from the slope of Re and I , vs. w-"' in the frequency range of the section with a slope of 45" ob-
27 24 21 3.4
3.6
3.8
4.0
4.2
4.4
4
Potential f V
Fig. 7. Impedancepotential plots for LiMn204samples obtained at about 200 Hz.
Li I: et aZ., Effect of lithium content on the electrochemical properties of solid-state-synthesized...
4. Conclusions LiXMn2O4samples with different Li contents were prepared by solid-state reaction. The lattice parameter of Sample Li108 is smaller than those of Samples Li098 and Li103. Samples Li103 and Li098 show good rate capability and cycling performance because the two samples have bigger lattice parameters. The Li ions can diffuse easily into/from the spinel structure. Sample Li108 shows excellent cycling property both at room and elevated temperatures because of the Li doping which can avoid the Jahn-Teller distortion. According to the electrochemical impedance spectroscopy, Sample Li103 has a relatively larger Li-ion di%sion coefficient and smaller chargetransfer resistance than those of the other two samples. The relatively high Li vacancies in Sample Li103 should be responsible for the increase in its Li' diffusion coefficient. Because of Li doping, the charge-transfer reaction becomes easy and the charge-transfer resistance decreases for Sample Li103. Hence, Sample Li103 shows good electrochemical properties.
References [l] Schilling 0. and Dahn J.R., Thermodynamic stability of chemically delithiated Li(LiXMn2-J04, J. Electrochem. SOC.,1998,145(2): 569. [2] Li T., Qiu W., Zhao H., and Liu J., Effect of cooling rate on electrochemical properties of spinel LiMn2O4 synthesized by solid-state reaction, Matel: LRtt., 2006, 60(9-10): 1251. [3] Thackeray M.M., Manganese oxides for lithium batteries, Prog. SolidStute Chem.,1997,s (1-2): 1. [4] Miaojun W. and Alexandra N., Thermochemistry of Lil+,Mnz_,o, (0 < x I 1/3) spinel, J. Solid State Chem., 2005,178 (4):1182. [5] Yamada A., Miura K., Hinokuma K., and Tanaka M., Synthesis and structural aspects of L i M n 2 0 ~as a cathode for rechargeable lithium batteries, J. Electrochem. SOC.,1995,142(7): 2149.
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[6] Berg H., Thomas J.O., Liu W., and Farrington GC., A neutron diffraction study of Ni substituted LiMn2O4, Solid State Zonics, 1998,112 (1-2):165. [7] Amatucci GG, Pereira N., Zheng T., and Tarascon J.M., Failure mechanism and improvement of the elevated temperature cycling of LiMn2O4 compounds through the use of the LiAlxMn2-X04zFzsolid solution, J. Electrochem. SOC.,2001,148 (2): A171. [8] Lin C., Qiu W., and Liu Q., Electrochemical studies of lithium intercalation into graphite film electrode for Li+-ionbatteries, J. Univ. Sci. Technol. B, 2000,7 (1): 48. [9] Mohamedi M., Takahashi D., Uchiyama T., Itoh T., Nishizawa M., and Uchida I., Explicit analysis of impedance spectra related to thin films of spinel LiMn204,J. Power Sources,2001,93 (1-2): 93. [lo] Xia Y, Zhou Y., and Yoshio M., Capacity fading on cycling 4V Li/LiMn2O4 cells, J. Electrochem. Soc., 1997,144(8): 2593. [111 Funabiki A., Inaba M., Ogumi Z., Yuasa S., Otsuji J., and Tasaka A., Impedance study on the electrochemical lithium intercalation into natural graphite powder, J. Electrochem. SOC.,1998,145(1): 172. [ 121 Zhang S., Ding M., Xu K., Allen J., and Jow, T.R., Understanding solid electrolyte interface film formation on graphite electrodes, Electrochem Solid St,, 2001,4 (12): A206. [13] Xu K., Zhang S., and Jow R., Electrochemical impedance study of graphitelelectrolyte formed in LiBOB/PC electrolyte, J. Power Sources, 2005, 143 (1-2): 197. [14] hkko K., Nishizawa M., Mohamedi M., Umeda M., Uchida I., Akimoto J., Takahashi Y., Gotoh Y., and Mizuta S., Electrochemical studies of Li-ion ext[ilction and insertion of LiMn2O4 single crystal, Electrochem. Solidst., 2001,4 (9): A151. [15] h r a P., Popov B.N., and White R.E., Electrochemical investigations of cobaltdoped LiMn2O4 as cathode material for lithium-ion batteries, J. Electrochem. SOC., 1998,145(3):807. [16] Lee Y.J., Wang F., Mukerjee S., McBreen J., and Grey C.P., 6Li and 7Li magic-angle spinning nuclear magnetic resonance and in situ X-ray diffraction studies of the charging and discharging of LixMn204 at 4 V, J. Electrochem. SOC.,2000,147 (3): 803.
RARE METALS Vol. 26, No. 3, Jun 2007, p . 280 E-mail: [email protected]
=‘;
ScienceDirect
Effect of lithium content on the electrochemical properties of solid-state-synthesizedspinel LixMnz04 LI Tao”, QIU Weihua’), W A O Hailei”, and LIU Jingjing” 1) General Research Insticute for Nonferrous Metals, Beumg 100088, China
2) School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China (Received 2006-02-10)
Abstract: Lithium-substituted Li,Mn204 (x = 0.98, 1.03, 1.08) spinel samples were synthesized by solid-state reaction. X-ray diffraction (XRD) patterns show that the prepared samples have a spinel stn~cturewith a space group of Fd 5 m. The cubic lattice parameter was determined from least-squares fitting of the XRD data. Li1.&n204 shows high capacity at both low and high current densities, while Lil.&n204 shows good cycling performance but relatively low capacity when cycled at both room and elevated temperatures. A variety of electrochemical methods were employed to investigate the electrochemical properties of these series of spinel Li.,Mn204. Key words: lithium manganese oxide: solid-state reaction; spinel; electrochemicalproperties [This work wasfinanciallv supported by the National Naturul Science Foundation of Chino (No. 50272012).]
1. Introduction Among the vast number of lihum manganese oxide compounds, LiMn204 had been singled out because of its attractive spinel-type structure and high potential of 4 V versus lithium metal [ 1-21. It has the spinel structure (A[B]&, space group F d j m ) with Li-ions occupying one-eighth of the tetrahedral sites ( 8 a sites ) and h4n-ions occupying half of the octahedral sites (16 d sites ) in a cubic close-packed may of oxygen ions (32 e sites). The unoccupied 16 c sites form a three-dimensional network which is the pathway for the transport of Li ions. Lithium can be extracted from or inserted into LiMnzO4 [3-41. Capacity fade in spinel LiMnz04 cells is a result of multiple processes, including those related solely to the spinel material and others involved in the interaction of the spinel with the electrolyte and negative electrode materials. Most fade mechanisms can be amibuted to the following three factors: Correspondingauthor: QIU Weihua
E-mail: qiuwh@vip,sina.com
(1) Dissolution of ~ n ’ + into the e~ectrolyteafter disproportionationof Li,Mn204. (2) Jahn-Teller distortion in discharged cells. (3) Instability of the electrolyte. The Jahn-Teller distortion can cause symmetry change and thus result in slow capacity loss. It is believed that there is a critical average oxidation state (AOS) of manganese (3.5) above which Jahn-Teller effect can be suppressed [4-51. An increase of lithium can lead to the increase of the AOS of manganese in Li,Mn204. The goal of this study was on the electrochemical properties, stability, and capacity retention of LiXMn2O4electrodes synthesized with different contents of lithium. X-ray diffraction (XRD), Rietveld refinement, and transmission electron microscope (TEM)were employed to obtain the structural information of Li,Mn204. The elemental analysis of the final products was carried out using the ICP-AES method. To study the electrochemical kinetic properties, constant current-charge and dis-
Li Z et ul., Effect of lithium content on the electrochemical properties of solid-state-synthesized... charge, AC-impedance and impedance-potential(IP) techniques were used.
2. Experimental The spinel LixMn204 samples were prepared from Li2C03 and electrolytic Mn02 by solid-state reaction. The well-mixed starting materials with different contents of lithium (x = 0.98, 1.03, 1.08) were calcined at 600°C for 4 h and then at 830°C for 12 h. The cooling rate was selected as 1"C.min-'. After cooling, the samples were tempered at 600°C for 6 h. The three samples were labeled as Li098, Li103, and Lil08. The prepared samples were characterized by powder X-ray diffraction (XRD, D/max-rb dfiactometer, Rigaku, Japan) using Cu KQ (1= 0.154056 nm) radiation at room temperature. The particle morphology and the electron diffraction (ED) measurements were performed using a transmission electron microscope (EM-1OOCXII). Samples were dispersed in ethanol and collected on a holey micro-grid supported on a copper mesh. The elemental analysis of the final products was carried out using two types of methods: induced coupled plasma spectroscopy (ICP) measurement and oxidation-reduction titration. The Perkin-Elmer optima 3000 (ICP-AES) mass-spectrometer was used to determine the content of lithium in the samples. The content of manganese was measured by oxidation-reduction titration method using ammonium ferrous sulfate. For electrochemical measurement, the electrode was prepared by rolling a mixture of 85 wt.% active materials, 10 wt.% acetylene black, and 5 wt.% polytetra-fluoroethylene (PTFE). The electrodes were vacuum-dried at 120°Cfor over 12 h, and then the testing cells were assembled in an argon-filled glove box. Lithium metal was used as the counter electrode, and porous polypropylene (celguard2300) as the separator. The electrolyte was 1.0 M LiPF6 in a mixture of DMC, EC, and DEC, 1:1:1 v/v (Li-batterygrade, S w u n g ) . A computer-controlledLAND BTI-10 8-channel battery cycler was used for half-cell cycle test. The cells were tested using constant-charge and dis-
281
charge current at a voltage ranging between 3.3-4.35 V. The charge current density used was 70 mA.g-'. The discharge current densities were selected as 70, 140, 280, 420, 700 rnA.g-'. The cells were also tested at an elevated temperature of 55°C with a current density of 70 mA.g-'. All the electrochemicaltests were performed on a potentiostatic/galvanostaticsystem (CHI66OA electrochemical workstation, Chenhua instrument company, Shanghai). Electrochemical impedance spectroscopy (EIS) was measured at an open circuit after preliminary equilibration at desired Li-ion intercalation depth for a period of 3 h. The impedance spectra data was covered in the frequency range from 100 kHz to 0.01 Hz at an excitation signal of 5 mV. Impedance potential analysis was carried out at constant frequency from all the discharge potential range (4.5-3.5 V) with a scanning rate of 5 mV.s-'.
3. Results and discussion 3.1. Structural analysis Fig.1 shows the XRD patterns of Li,Mn204 with different lithium contents. No impurity peak can be detected in all the synthesized samples. All the peaks correspond to the spinel-type LiMn204 structure with space group Fd3 rn and can be refined with Rietveld program on the basis of the standard LiMn204 structure [6] using MDI Jadi 5.0 program. The lattice parameter a, of each sample is shown in Table 1. The lattice parameters of the L&&ln204 and Li1.03Mn204samples have little difference while
. 0 cd
.-
-ez
1111
311 400
,331
Standard I
20
60
40
,
80
28 I (01 Fig. 1. XRD patterns of Li,Mn204 synthesized with Merent Li contents.
RARE METALS, Vol. 26, No. 3, Jun 2007
282
the parameter of the Lil,aMnz04sample shows a smaller value. The atomic ratio of Li/Mn and manganese valences from the ICP measurement and oxidation-reduction titration are summarized in Table 1. The relative atomic ratios of the composition were calculated with the atom number of 0 fixed at 4. Sample Li098 has the lowest manganese valences (< 3.5) among the three samples, which may cause the Jahn-Teller distortion. This phase transition results in a transformation from the cubic space group Fd 3 m to the tetragonal group I4,lamd. Related mechanisrns can cause strain and structural failure. result-
ing in electrically disconnected particles and capacity loss. The high manganese content of Sample Li098 indicates that there is some manganese occupying the octahedral sites. For Sample Li103, the contents of lithium and manganese are all smaller than the theoretical value. For Sample Li108, there are more lithium inserting in the spinel structure, and excess lithium may occupy the tetrahedral sites. Fig.2 shows the particle morphology and the electron diffraction of Samples Li098, Li103, and Li 108. All the three ED patterns agree well with the cubic face-centered lattice. The samples with various lithium contents all maintain a cubic structure.
Table 1. Lattice parameter r e M by Rietveld method and element compositionof synthesized LixMnz04 Sample
I-attice parameter / nm
L1098 Li103
82.451 82.453
Li 108
82.319
._
Startirig ratio of Li/Mn ICP value of LdMn
Manganese AOS
Composition
0.9802
0.9909
3.479
Li09%Nln201304
1.0302 1.080:2
1.021:2 1.044:2
3.599 3.593
Lio.%&h.94604
Lil O I N ~ ~0 I
2. 'EM micrographs of the Li,Mn204 samples: (a) Lio98, observed [012] zone ED pattern; (b) Li103, observed i i 231 zone ED pattern; (c) LilW, observed 111i 1 zone ED pattern. Fig.
3.2. Capacity properties Figs. 3 and 4 show the discharge capacity versus cycle number at room and elevated temperatures (55T) with a constant current density of 70 mA.g-'. At mom temperature, all the three samples show good cycling performances. But Samples Li098 and Li103 have relatively higher capacities than Sample Li 108. Sample Li 108 shows excellent cycling property but relatively low initial capacity. Sample Li098 has a relatively larger initial capacity, but a little worse cycling performance than those of Samples Li 103 and Li 108. At an elevated temperature, all the three samples show capacity fade with cycling. But
Fig. 3. Discharge capacity versus cycle number curves at room temperature (3.3-435 V).
4
Li II: et aZ., Effect of lithium content on the electrochemicalproperties of solid-state-synthesized...
283
current densities of 140, 280, 420, 700, 70 mA.g-' the capacity stability was enhanced as the content of while all the samples are charged at a constant curlithium increased. rent density of 70 d+-'. Sample Li103 shows The doping of lithium resulted in a large increase good cycling stability at high current densities durin the M I I ~ & : + r a t i o(increasing the average ~n ing all current densities. Sample Li108 shows exceloxidation state), so there are little Mn3+available for lent cycling property and relatively high capacity at the transition of Mn3++ Mn6 oxidation necessary a current density of 700 mA.g-'. The increasing of for lithium removal [7]. This results in low initial lithium content can enhance the capacity stability of capacities of Samples Li103 and Li108. Moreover, the LixMn204sample at high current density. there is less dissolution of Mn2+into the electrolyte after disproportionation of LixMnzOo( 2 ~ n ~ -+ + ( ~ ~ ~ ) 33. Electrochemical impedance spectroscopy fi2+(hquid) + Mn6(,hd,), especially at an elevated The typical impedance response recorded in the temperature. And increasing the Mn valence can also LiMn20&i half cell is shown in Fig. 6, which is suppress the Jahn-Teller distortion both at room and composed of two depressed semicircles and a spike elevated temperatures. Hence, Samples Li108 and in the low frequency range. Li103 show improved elevated temperature cycling (1) The semicircle in the high frequency range performance compared with Sample Li098. arises from the impedance of the interfacial film on Fig.5 shows the room temperature discharge cathe electrode. pacities versus cycle number at different discharge (2) The semicircle in the medium frequency -, range results from the charge-transferprocess. cQ4", 120 (3) The spike at the low frequency end indicates E the long range diffusion of Li-ion [8-151. ' 100 h
-.-
2oor
x *
!? z
x 5
80 60
40
150
5 10 Cycle number, n
0
Li098 -0-LiO98-cal -A- Li 103 -0-LilO3-cd +Li108 +-LilO8-cal -m-
m Li098 o Li103 A Li108
15
Fig. 4. Discharge capacity versus cycle number curves at an elevated temperature of 55OC (3.3-4.35 V).
0
50
100
150
200
Z'J R
2
v
'I .G 100 0
8
80
3 .-x
60 400
%
Fig. 6. Nyquist plots for LixMn204samples at an open-circuit voltage of 3.9 V. Inset: the equivalent circuit for the impedance data, -cal represents the simulated curves.
1201
5
MM
0 Li103 A Li108
20 25 Cycle number, n 10
15
30
Fig. 5. Discharge capacity versus cycle number curves at room temperature (331435V).
The EIS data were fitted using the analysis software Zsimple (Echemical software, USA), and the equivalent circuit is shown in Fig. 6. Re represents the electrolyte resistance, Rht the interfacial film resistance, RCtthe charge-transferresistance, and W the Warburg impedance that includes diffusioncontrolled process in the solids [ 151.
RARE METALS, Vol. 26, No. 3, Jun 2007
284
According to Table 2, the impedances of the electrolyte and the interfacial film are almost similar. But the charge-transfer resistances have very large differences. As a small polaron semiconductor, the electronic conduction of the LiMn204sample occurs via hopping of electrons between eg orbitals on adjacent Mn"h41-1~cations [16]. The electrons associated with Li' deintercalation cannot be removed from a delocalized band, but must be removed from (or must result in) localized molecular orbitals surrounding the lithium site [ 161. The doping of lithium may change the electronic structure of Li,Mn204, and may influence the charge-transfer resistances of the three samples. Sample Li103 has the lowest charge-transfer resistance among the three samples, about 16 R. The charge-transfer reaction became easy as the charge-transfer resistance decreased, which is favorable for battery operation. Table 2. Simulated data and calculated DLiaccording to the Nyquist plots
Samples
~
R, I R Rm,l Q R,, / R
/ (crn'.s-')
Li098
7
10
Li103
7
9
22 16
4.84~
Li108
7
10
47
1.25 x lo-'*
3.39 x lo-'*
At a low frequency, when the diffusion process of Li ion in the cathode is considerably slower than that in an electrolyte solution, a linear portion is formed. The chemical diffusion coefficients of Li-ion, DL,in LiMn204 electrode were estimated using an ac-impedance method as described in Ref. [8]. The data of D L are ~ shown in Table 2. Eq. 1 is adopted to estimate the D L ~in LiMn204 electrode:
served in complex plane diagrams. According to Table 2, Samples Li098 and Li103 have relatively larger Li-ion diffusion coefficients than that of Sample Li108. There may be two reasons: first, Samples Li098 and Li103 have relatively larger lattice parameters, and the Li ion can diffuse easily in the electrode; second, there are more vacant sites of Li in Samples Li098 and Li103, which is favorable for Li' diffusion. 3.4. Impedance potential analysis
To observe the change of the Li-ion diffusion resistance and charge-transfer resistance at different potentials, the impedance potential analysis was carried out in the discharge potential range (3.5-4.5 V) at different frequencies with the scanning rate of 5 mV.s-'. The impedance at medium frequency represents the charge-transfer resistance. The frequency (at about 200 Hz) was selected from the elech-ochemical impedance spectroscopy (Fig. 6). According to Fig. 7, from all the potential range during Li-ion inserting process, the charge-transfer resistance distributes from 20 R to 48 R. The charge-transfer resistances of Sample Li103 are much lower than those of the other two samples. At about 4 V potential, the charge-transfer resistance of Sample Li103 is about 8 R smaller than those of Samples Li098 and LilO8. This may be why Sample Li103 shows good electrochemical properties. 48 45 42
c:
'N
0 Li098
39 36
33 30
where CL, (moVcm3)is calculated from the volumes and the qualities of Li storage of the LiMn204electrode; R the gas constant; n the number of transferred electrons; F the Faraday constant; and A the active surface area. The values of 0 were calculated from the slope of Re and I , vs. w-"' in the frequency range of the section with a slope of 45" ob-
27 24 21 3.4
3.6
3.8
4.0
4.2
4.4
4
Potential f V
Fig. 7. Impedancepotential plots for LiMn204samples obtained at about 200 Hz.
Li I: et aZ., Effect of lithium content on the electrochemical properties of solid-state-synthesized...
4. Conclusions LiXMn2O4samples with different Li contents were prepared by solid-state reaction. The lattice parameter of Sample Li108 is smaller than those of Samples Li098 and Li103. Samples Li103 and Li098 show good rate capability and cycling performance because the two samples have bigger lattice parameters. The Li ions can diffuse easily into/from the spinel structure. Sample Li108 shows excellent cycling property both at room and elevated temperatures because of the Li doping which can avoid the Jahn-Teller distortion. According to the electrochemical impedance spectroscopy, Sample Li103 has a relatively larger Li-ion di%sion coefficient and smaller chargetransfer resistance than those of the other two samples. The relatively high Li vacancies in Sample Li103 should be responsible for the increase in its Li' diffusion coefficient. Because of Li doping, the charge-transfer reaction becomes easy and the charge-transfer resistance decreases for Sample Li103. Hence, Sample Li103 shows good electrochemical properties.
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