Ballmilling Synthesis and Electrochemical Characterization of Ternary Lithium Nitrides
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Ballmilling Synthesis and Electrochemical Characterization of Ternary Lithium Nitrides Jun Yang,* ,z Ke Wang, and Jingying Xie* Energy Science and Technology Laboratory, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China Lithium ~transition! metal nitrides were synthesized by the ballmilling technique. The obtained Li 32x M x N (M 5 Co, Ni, and Cu! compounds exhibit a high electrochemical activity and reversibility. They could act as high capacity anode materials for lithium- ion batteries. The average capacity of Li 2.6 Co 0.4 N sample prepared in N 2 is ca. 880 mAh/g for the first 10 cycles. Li 2.5 Ni 0.5 N sample has the smallest capacity about 500 mAh/g with slow capacity fade upon cycling. 78% of its second cycle capacity can be retained at the 50th cycle. © 2002 The Electrochemical Society. @DOI: 10.1149/1.1528942# All rights reserved. Manuscript submitted April 8, 2002; revised manuscript received July 22, 2002. Available electronicallyDecember 13, 2002. It was found that part of lithium in lithium ~transition! metal nitrides Li 32x M x N (M 5 Co, Ni, and Cu! could be electrochemi- cally extracted and reintercalated with high reversibility. 1 They rep- resent a new class of anode materials for secondary lithium batter- ies. Of these compounds Li 2.6 Co 0.4 N demonstrated the largest reversible capacity more than 800 mAh/g. 2,3 This value is much higher than theoretical capacity of conventional graphite material (LiC 6 :372 mAh/g). Another advantage of the use of Li 32x M x N compounds as anode is the slightly more positive intercalation po- tentials vs. Li/Li 1 , which minimize any risks of lithium deposition on the electrode surface in the high rate charging process. Although some lithium alloys can also provide a comparable capacity like the above mentioned nitrides ~e.g., theoretical capacity of 990 mAh/g for Li 4.4 Sn) at a similar potential level, lithium storage metals, in general, undergo drastic volume changes for lithium insertion and extraction, leading to electronic disconnection of the active material or even electrode disintegration. From this point of view, lithium alloys as anode are inferior to Li 32x M x N compounds in the cycla- bility, especially under high lithium utilization. As mentioned in several published papers, 4-6 Li 32x M x N samples were synthesized by a traditional ceramic method, i.e., heating a pressed mixture of Li 3 N and metallic powder at 600-750°C under N 2 atmosphere. In this study, we adopt the ballmilling technique to synthesize the ternary lithium nitrides. Their charge and discharge characteristics are investigated via coin cell with lithium counter electrode. Experimental Planetary Mono Mill P-6 ~Fritsch, Germany! was used for the synthesis of Li 32x M x N compounds (M 5 Co, Ni, and Cu!. The starting materials were stoichiometric mixtures of Li 3 N powder from Aldrich and metallic powder ~200 mesh! of cobalt, copper, or nickel. The molar ratio of Li 3 N/M was 2.17 for cobalt and copper as precursor, and 1.67 for nickel as precursor. In glove box, a total of ca. 540 mg powder mixture was loaded into a hardened steel bowl with 80 mL volume, together with 15 hardened steel balls in 10 mm diam. The bowl was sealed under Ar atmosphere and then moved to outside of the glove box. This mill set allows that Ar gas in the bowl is replaced by other gases. Ballmilling was conducted at a speed of 500 rpm under N 2 ~99.99%! or Ar ~99.99%! atmosphere. After each 75 min milling there was a 15 min pause. The total milling time was 5 or 10 h. Powder X-ray diffraction ~XRD! patterns of the samples were obtained using automated powder diffractometer with Cu Ka radia- tion ~Type: D-max 2550 V!. A polyethylene ~PE! protection film was used to prevent the sample from contacting air. Electrodes were prepared in the glove box. A given weight of the nitride powder was mixed with acetylene black ~AB! conductor and polytetrafluoroethylene ~PTFE! binder and the mixture was pressed onto a foamed nickel disk ~1.2 cm in diam! by a pressure of 0.8 MPa. The electrode contained 75 wt % of the active powder, 20 wt % AB and 5 wt % PTFE. 2025 type coin cells with lithium counter electrode were assembled to evaluate the electrochemical cycling performance. All the cells contained an organic electrolyte of 1 M LiPF 6 dissolved in ethylene carbonate and dimethyl carbonate ~1:1 in volume! from Mitsubishi Chemical. Charge and discharge was performed at a constant current density of 0.4 mA/cm 2 . Results and Discussion The initial use of new stainless steel bowl and balls brought iron impurity in the milled powder. After test milling for two times, the iron impurity could be no longer detected. Figure 1 shows XRD patterns for Li 32x Co x N synthesized by ballmilling respectively un- der Ar and N 2 atmosphere for 5 h. The XRD responses of the sample prepared in N 2 are well in accordance with those of Li 2.6 Co 0.4 N recorded on JCPDS card 5-605. The peak width is slightly broader than that of Li 2.6 Co 0.4 N compound synthesized by a high tempera- ture method, 2 indicating that the sample has a smaller grain size ~or lower crystallinity! than the latter or that the material has a relatively wide of composition range ~e.g., Li 32x Co x N, x 5 0.3-0.5). On the other hand, an XRD pattern of the sample prepared in Ar is more complicated. 44.2, 51.5, and 76 ~2u! are attributed to metallic cobalt. A small peak near 32.5 ~2u! is probably associated with residual Li 3 N. This result suggests that the reaction is incomplete in Ar. In view of that Li 2.6 Co 0.4 N product has a higher N/Li ratio than Li 3 N precursor, N 2 gas may be not only an inhibitor for Li 3 N decompo- sition, but also a reaction participator for complete lithium utiliza- tion, probably via the formation of Li 3 N. Therefore, N 2 atmosphere is favorable for the synthesis of lithium ~transition! metal nitrides. Figure 2 exhibits voltage profiles for the first 1.5 cycles of Li 32x Co x N samples prepared under different gas atmospheres. The open-circuit voltage of the fresh electrode is 0.79 V vs. Li for the sample prepared in N 2 and 0.68 V vs. Li for that in Ar. There is a small capacity for the first lithium insertion, which is associated with some lithium vacancy sites arising from crystalline defects and the existence of Co 21 valence state. The charge and discharge volt- ages of the sample obtained in Ar shift about 0.1 V toward the negative direction. The stronger electronegativity is an indication of its Li-richer structure that may be produced by more decomposition of Li 3 N precursor under Ar atmosphere. Although the first lithium extraction until 1.4 V can deliver a similar capacity for both the electrodes, the sample prepared in Ar undergoes decomposition at this voltage upper limit and the electrode will lose electrochemical activity in the following cycle. When the upper voltage is controlled at 1.1 V, cycling is feasible. The reversible capacity is about 700 mAh/g in the first several cycles and remains 620 mAh/g at the 20th * Electrochemical Society Active Member. z E-mail: [email protected] Journal of The Electrochemical Society, 150 ~1! A140-A142 ~2003! 0013-4651/2002/150~1!/A140/3/$7.00 © The Electrochemical Society, Inc. A140 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 132.174.255.3 Downloaded on 2014-11-13 to IP
Transcript of Ballmilling Synthesis and Electrochemical Characterization of Ternary Lithium Nitrides
Journal of The Electrochemical Society, 150 ~1! A140-A142 ~2003!0013-4651/2002/150~1!/A140/3/$7.00 © The Electrochemical Society, Inc.
A140
Dow
Ballmilling Synthesis and Electrochemical Characterizationof Ternary Lithium NitridesJun Yang,* ,z Ke Wang, and Jingying Xie*
Energy Science and Technology Laboratory, Shanghai Institute of Microsystem and Information Technology,Chinese Academy of Sciences, Shanghai 200050, China
Lithium ~transition!metal nitrides were synthesized by the ballmilling technique. The obtained Li32xMxN (M 5 Co, Ni, and Cu!compounds exhibit a high electrochemical activity and reversibility. They could act as high capacity anode materials for lithium-ion batteries. The average capacity of Li2.6Co0.4N sample prepared in N2 is ca. 880 mAh/g for the first 10 cycles. Li2.5Ni0.5Nsample has the smallest capacity about 500 mAh/g with slow capacity fade upon cycling. 78% of its second cycle capacity can beretained at the 50th cycle.© 2002 The Electrochemical Society.@DOI: 10.1149/1.1528942# All rights reserved.
Manuscript submitted April 8, 2002; revised manuscript received July 22, 2002. Available electronicallyDecember 13, 2002.
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It was found that part of lithium in lithium~transition! metalnitrides Li32xMxN (M 5 Co, Ni, and Cu!could be electrochemi-cally extracted and reintercalated with high reversibility.1 They rep-resent a new class of anode materials for secondary lithium baies. Of these compounds Li2.6Co0.4N demonstrated the largesreversible capacity more than 800 mAh/g.2,3 This value is muchhigher than theoretical capacity of conventional graphite mate(LiC6 :372 mAh/g). Another advantage of the use of Li32xMxNcompounds as anode is the slightly more positive intercalationtentialsvs. Li/Li 1, which minimize any risks of lithium depositionon the electrode surface in the high rate charging process. Althosome lithium alloys can also provide a comparable capacity likeabove mentioned nitrides~e.g., theoretical capacity of 990 mAh/for Li4.4Sn) at a similar potential level, lithium storage metals,general, undergo drastic volume changes for lithium insertionextraction, leading to electronic disconnection of the active mateor even electrode disintegration. From this point of view, lithiualloys as anode are inferior to Li32xMxN compounds in the cyclability, especially under high lithium utilization.
As mentioned in several published papers,4-6 Li 32xMxN sampleswere synthesized by a traditional ceramic method,i.e., heating apressed mixture of Li3N and metallic powder at 600-750°C undN2 atmosphere. In this study, we adopt the ballmilling techniquesynthesize the ternary lithium nitrides. Their charge and dischacharacteristics are investigated via coin cell with lithium counelectrode.
Experimental
Planetary Mono Mill P-6~Fritsch, Germany! was used for thesynthesis of Li32xMxN compounds (M5 Co, Ni, and Cu!. Thestarting materials were stoichiometric mixtures of Li3N powderfrom Aldrich and metallic powder~200 mesh!of cobalt, copper, ornickel. The molar ratio of Li3N/M was 2.17 for cobalt and copper aprecursor, and 1.67 for nickel as precursor. In glove box, a totaca. 540 mg powder mixture was loaded into a hardened steel bwith 80 mL volume, together with 15 hardened steel balls in 10 mdiam. The bowl was sealed under Ar atmosphere and then moveoutside of the glove box. This mill set allows that Ar gas in the bois replaced by other gases. Ballmilling was conducted at a spee500 rpm under N2 ~99.99%!or Ar ~99.99%!atmosphere. After each75 min milling there was a 15 min pause. The total milling time w5 or 10 h.
Powder X-ray diffraction~XRD! patterns of the samples werobtained using automated powder diffractometer with Cu Ka radia-tion ~Type: D-max 2550 V!. A polyethylene~PE!protection film wasused to prevent the sample from contacting air.
* Electrochemical Society Active Member.z E-mail: [email protected]
address. Redistribution subject to ECS te132.174.255.3nloaded on 2014-11-13 to IP
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Electrodes were prepared in the glove box. A given weight ofnitride powder was mixed with acetylene black~AB! conductor andpolytetrafluoroethylene~PTFE!binder and the mixture was presseonto a foamed nickel disk~1.2 cm in diam!by a pressure of 0.8MPa. The electrode contained 75 wt % of the active powder, 20% AB and 5 wt % PTFE. 2025 type coin cells with lithium countelectrode were assembled to evaluate the electrochemical cyperformance. All the cells contained an organic electrolyte of 1LiPF6 dissolved in ethylene carbonate and dimethyl carbonate~1:1in volume! from Mitsubishi Chemical. Charge and discharge wperformed at a constant current density of 0.4 mA/cm2.
Results and Discussion
The initial use of new stainless steel bowl and balls brought iimpurity in the milled powder. After test milling for two times, thiron impurity could be no longer detected. Figure 1 shows XRpatterns for Li32xCoxN synthesized by ballmilling respectively under Ar and N2 atmosphere for 5 h. The XRD responses of the samprepared in N2 are well in accordance with those of Li2.6Co0.4Nrecorded on JCPDS card 5-605. The peak width is slightly broathan that of Li2.6Co0.4N compound synthesized by a high temperture method,2 indicating that the sample has a smaller grain size~orlower crystallinity! than the latter or that the material has a relativewide of composition range~e.g., Li32xCoxN, x 5 0.3-0.5). On theother hand, an XRD pattern of the sample prepared in Ar is mcomplicated. 44.2, 51.5, and 76~2u! are attributed to metallic cobaltA small peak near 32.5~2u! is probably associated with residuaLi3N. This result suggests that the reaction is incomplete in Ar.view of that Li2.6Co0.4N product has a higher N/Li ratio than Li3Nprecursor, N2 gas may be not only an inhibitor for Li3N decompo-sition, but also a reaction participator for complete lithium utiliztion, probably via the formation of Li3N. Therefore, N2 atmosphereis favorable for the synthesis of lithium~transition!metal nitrides.
Figure 2 exhibits voltage profiles for the first 1.5 cyclesLi32xCoxN samples prepared under different gas atmospheres.open-circuit voltage of the fresh electrode is 0.79 Vvs. Li for thesample prepared in N2 and 0.68 Vvs. Li for that in Ar. There is asmall capacity for the first lithium insertion, which is associatwith some lithium vacancy sites arising from crystalline defects athe existence of Co21 valence state. The charge and discharge vages of the sample obtained in Ar shift about 0.1 V towardnegative direction. The stronger electronegativity is an indicationits Li-richer structure that may be produced by more decomposiof Li3N precursor under Ar atmosphere. Although the first lithiuextraction until 1.4 V can deliver a similar capacity for both thelectrodes, the sample prepared in Ar undergoes decompositiothis voltage upper limit and the electrode will lose electrochemiactivity in the following cycle. When the upper voltage is controlleat 1.1 V, cycling is feasible. The reversible capacity is about 7mAh/g in the first several cycles and remains 620 mAh/g at the 2
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cycle. With increasing cycle number, the difference in the chaand discharge voltages will reduce for both the electrodes.lower voltage character will gradually disappear for the sample ppared in Ar. Figure 3 presents discharge and charge profiles foLi2.6Co0.4N prepared in N2 . The electrode supplies an average cpacity of about 880 mAh/g in the first 10 cycles and the faradyield is about 99%. If the cell operation starts from lithium extration, the first faradaic yield of extraction to insertion is about 86%means that original lithium vacancy sites are still retained for adtional lithium insertion. The voltage trend of the first lithium extration is greatly different from that of the followed cycles. Thisbecause the first extraction until 1.4 V results in the structure chafrom crystalline to amorphous state. A detailed discussion onstructure property in the different charge state has been presentpublished papers.1,2
Li 2.6Cu0.4N and Li2.5Ni0.5N were also prepared by ballmillingFigure 4 shows XRD patterns for the two samples synthesized
Figure 1. XRD patterns of Li32xCoxN samples prepared by ballmilling for 5h under different gas atmospheres.~a! in N2 , ~b! in Ar.
Figure 2. Voltage profiles for the first 1.5 cycles of the samples shownFig. 1.
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milling in N2 for 10 h. All the Li32xMxN (M 5 Co, Ni, and Cu!compounds prepared in N2 exhibit a very similar XRD charactewith a hexagonal structure, in which part of lithium in Li3N is sub-stituted by Co, Ni, or Cu. However, obvious XRD response at 3and 51~2u! for the Li2.5Ni0.5N suggests there may be more residuLi3N inside than in the other samples. Figure 5 exhibits dischaand charge profiles for Li2.6Cu0.4N and Li2.5Ni0.5N electrodes. Thereversible capacity is about 600 mAh/g for Li2.6Cu0.4N in the voltagerange of 0/1.3 V and about 500 mAh/g for Li2.5Ni0.5N in the voltagerange of 0/1.4 V. Reducing the milling time to 5 h has a smallinfluence on the cycle capacity.
Li 2.6Cu0.4N shows an obvious single-plateau character, whLi2.5Ni0.5N and Li2.6Co0.4N have two voltage plateaus in the firs
Figure 3. Discharge and charge profile for the Li2.6Co0.4N sample preparedin N2 .
Figure 4. XRD patterns of Li2.6Cu0.4N and Li2.5Ni0.5N samples prepared byballmilling for 10 h in N2 .
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extraction process. This difference may arise from the differentdation state variation of cobalt, nickel, and copper ions. For coponly redox couple Cu11/Cu12 is stable. But for cobalt and nickevarious redox couples such as M11/M12 and M12/M13 could exist.Furthermore, it is noticed that Li2.6Co0.4N has a smaller hysteresibetween charge and discharge than Li2.5Ni0.5N and Li2.6Cu0.4N in theinitial cycles. Taking the second cycle as an example, the voltdifference between charge and discharge isca. 0.54, 0.72, and 0.8 Vrespectively, for Li2.6Co0.4N, Li2.5Ni0.5N, and Li2.6Cu0.4N, if the halfcapacity voltage is taken for a statistic value. The hysteresis is donated by the inherent property of the material, not by the kinpolarization. Although these compounds exhibit the same struccharacter, the difference in the microstructure~e.g., crystallinity andshort range ordering! and the phase stability upon cycling still exists. Fairly overlapped charge and discharge curves at the 2nd15th cycles in Fig. 5 suggest that the amorphous structureLi2.5Ni0.5N and Li2.6Cu0.4N is quite stable. On the other hand, itobservable from Fig. 3 that the voltage trend of lithium extractfrom Li2.6Co0.4N changes apparently from cycle to cycle. A smbut distinct voltage plateau near 0.95 V emerges at the 10th cycseems that some structure rearrangement of the Li2.6Co0.4N takesplace during cycling.
The cycle performance of Li32xMxN (M 5 Co, Ni and Cu!samples is compared in Fig. 6. The Li2.6Co0.4N electrode prepared inN2 presents the largest capacity, however, its capacity retentiorelatively poor. In spite of the smallest cycle capacity, the Li2.5Ni0.5Npossesses a fairly good cycling stability. 78% of the second ccapacity can be retained at the 50th cycle. Moreover, it is notedthis capacity value is much higher than that of the Li2.5Ni0.5N syn-thesized by the high temperature method~below 240 mAh/g!.2,6 Thedifferent preparation method may create a different microstrucof Li2.5Ni0.5N, which, in turn, influences the mobility and electrochemical activity of lithium in the compound. On the other hand,ballmilled powder product has generally a fine grain size and a la
Figure 5. Discharge and charge profile for Li2.6Cu0.4N and Li2.5Ni0.5Nsamples.
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specific surface area, which is favorable for speeding interfacharge transfer and shortening lithium diffusion length within tparticles. The improved kinetics could reduce the voltage polartion and thereby enhance the cycle capacity at a certain extent. Fthe XRD pattern@the strongest peak~100!# and the Scherrer equation, it is calculated that the crystallite size of the milled Li2.5Ni0.5Nsample is about 13 nm. For milled Li2.6Co0.4N sample, it is about 18nm, one seventh of that of the sample prepared by high temperatechnique.
Also, some quadruple lithium nitrides could be synthesizedballmilling. So far we have succeeded in preparing single phLi2.6Co0.2Cu0.2N sample under N2 atmosphere. The related resulwill be reported elsewhere.
Conclusions
High energy ballmilling provides an effective method for thsynthesis of lithium ~transition! metal nitrides. The obtainedLi32xMxN (M 5 Co, Ni, and Cu!compounds show a high electrochemical activity and cycling reversibility. The average capacityLi2.6Co0.4N sample prepared in N2 is ca. 880 mAh/g for the first 10cycles. The sample prepared in Ar exhibits lower charge andcharge voltages than that prepared in N2 in the initial cycles. Thevoltage difference will gradually reduce with progressive cyclinLi2.5Ni0.5N sample has good cycling stability with a capacity abo500 mAh/g, which is much larger than reported value for Li2.5Ni0.5Nsynthesized by the high temperature method. This may be attribto favorable microstructure and larger specific area of the prodsynthesized by ballmilling. The test results indicate that thesenary lithium nitrides are promising anode materials for rechargealithium batteries.
Shanghai Institute of Microsystem and Information Technology assiin meeting the publication costs of this article.
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Figure 6. Li extraction capacityvs. cycle number.~a! Li2.6Co0.4N, 0/1.4 V;~b! Li2.6Co0.4N prepared in Ar, 0/1.1 V;~c! Li2.6Cu0.4N, 0/1.3 V; ~d!Li2.5Ni0.5N, 0/1.4 V.
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