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Advanced oxide and metal powders for negative electrodes in lithium-ion batteries
Powder Technology 128 (2002) 124 – 130 www.elsevier.com/locate/powtec
Advanced oxide and metal powders for negative electrodes in lithium-ion batteries T. Brousse a,*, O. Crosnier a, X. Devaux b, P. Fragnaud a, P. Paillard a, J. Santos-Pen˜a a, D.M. Schleich a a
Laboratoire de Ge´nie des Mate´riaux, Ecole Polytechnique de l’Universite´ de Nantes, La Chantrerie, rue Christian Pauc, BP 50609, 44306 Nantes Cedex 3, France b Laboratoire de Physique des Mate´riaux, UMR CNRS 7556, Ecole des Mines, 54042 Nancy Cedex, France
Abstract In this study, tin dioxide, tin and bismuth have been envisioned as possible candidate to replace graphite negative electrodes in Li-ion batteries. Tin dioxide thin films and nanoscaled tin and bismuth powders have been synthesized by different techniques. Their electrochemical behaviors have been compared to electrodes made with standard commercially available powders. In all cases, nanoscaled materials have shown enhanced electrochemical properties compared to standard powders. SnO2 thin films exhibit a longer cycle life than tin dioxide powder. The capacities measured on both bismuth and tin nanoscaled materials were more important than for the same electrodes prepared by commercially available powders. Moreover, the values are close to those expected from the theoretical reactions. However, the cycling life of tin or bismuth electrodes is still the weak point of these systems. Subsequently, an optimized matrix is required in order to prevent capacity loss upon cycling. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Lithium-ion batteries; Anodes; Tin; Tin dioxide; Bismuth
1. Introduction Lithium-ion batteries are now commercially available products for many applications (portable computers, mobile telephones, etc.). Actual developments aim at their use as power source for electric vehicle. However, these batteries still have safety problems due to the materials used. Lithium-ion cells are composed of lithiated transition metal oxides as the positive electrode and graphite as the negative electrode. The specific capacity of this latter compound is 372 mAh/g. Nevertheless, the reactivity of carbon with the liquid electrolytes leads to several problems, with a capacity loss between the first and the second cycle of more than 20%, and a slow fade in capacity upon cycling. Some metals, alloys and metal oxides have been recently proposed [1– 10] as possible negative electrodes in lithiumion batteries. The main property of the metals involved in
*
Corresponding author. Fax: +33-240683199. E-mail address: [email protected] (T. Brousse).
these compounds is that they can form binary alloys with lithium. For example, Fuji Film released a patent [1] where the carbonaceous anode was replaced by a tin oxide based compound. During the first cycle an irreversible reaction occurs with the reduction of SnO2 into Li2O and metallic tin, followed by the reversible formation of Li –Sn alloys. Despite the huge lithium loss due to the reaction with oxygen, the reversible capacities of Li– Sn alloys (more than 700 mAh/g) raise up a growing interest for tin based compounds. In these systems, the intercalation reactions that usually take place into a host structure are replaced by the formation of Li– M alloys which leads to a high volume expansion during the electrode reduction. Despite huge specific capacities, obtained with metallic tin for example (three times bigger than graphite), it seems that the electrochemical performance of the candidate material are strongly influenced by its microstructure. Thus the use of alternative methods to synthesize nanoscaled materials can be helpful to optimize the design of electrodes with improved cycling behavior compared to standard materials. In this study, tin oxide thin films, tin and bismuth nanoscaled powders have been synthesized by different
0032-5910/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 ( 0 2 ) 0 0 1 8 6 - 9
T. Brousse et al. / Powder Technology 128 (2002) 124–130
techniques and their electrochemical behaviors have been compared to electrodes made with standard commercially available powders.
2. Experimental SnO2 thin films have been deposited on stainless steel substrates (AISI 316) using a low pressure chemical vapor deposition (CVD) technique [2,3]. Tetrabutyl tin was used as a precursor and kept in a bubbler at 314 K to obtain an adequate vapor pressure. Oxygen was supplied both as the carrier gas as well as a supplemental oxidant. The total pressure was maintained at 670 Pa during deposition. The growth temperature used for the films was 773 K. Electrochemical tests were performed on the as-deposited SnO2 thin films without the addition of any binder neither conductive additive. The nanosized metallic tin and bismuth powders were obtained by using an arc-plasma discharge technique [11]. A tin or bismuth ingot was evaporated in a direct current (DC) arc plasma under an Ar – H2 gas flow at reduced pressure. The fine powder was collected and stored in a glove box under high purity argon atmosphere. SnO2 and Sn micrometric powders were obtained from Aldrich Chemical (99.9% purity). Except for the SnO2 thin films, the composite electrode was prepared by mixing together active material (metal oxide), graphite and a polymer (PVdF: polyvinylidene fluoride) as a binder at a weight ratio of 0.85:0.1:0.05. For the bismuth electrodes this ratio was varied as indicated in the related paragraph. Graphite was used to assume a good electronic conductivity throughout the electrodes and PVDF was used to keep the mechanical integrity. The mixture was deposited on copper substrates using a barcoater and dried for 12 h at 100 jC. Some 12mm diameter pieces of the composite electrode were cut and used as samples for both electrochemical tests and structural analysis. Electrochemical tests were performed in a simple twoelectrode cell using metallic lithium as the negative electrode. The electrolyte was a 1 M solution of LiPF 6 dissolved in EC/DMC 1:1 (Merck). A glass fiber paper was used as the separator and wetted with the electrolyte solution. The cells were assembled in a glove box filled with argon and cycled using a MacPile system (BioLogic) in potentiostatic or galvanostatic mode. The voltage limits and rate are indicated when required. Lithium uptake by the electrode will be referred as the discharge (reduction step) and lithium release as the charge of the electrode. One complete cycle corresponds to one discharge followed by one charge. Morphology and composition of the different materials were determined by means of a Leica Stereoscan 440 scaning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) analyser (Oxford Instrument).
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3. Results and discussion 3.1. SnO2 Fuji Film patent [1] concerning the use of SnO2 as possible anode in lithium-ion batteries (issued in 1994) highlights two major points: the possibility of reaching specific capacities three times bigger than the specific capacity of graphite, with a relatively low potential vs. lithium, and the use of oxides in an amorphous state. This latter point is illustrated in the patent by an XRD patterns of SnO2 without any well-defined peak. Even if the patent is related to many compounds and several substitutions, the basic material is always tin dioxide. SnO2 thin films prepared by CVD have been tested vs. a metallic lithium electrode. The observations by scanning electronic microscopy (not shown in here) reveal that the film is a compact structure of disordered grains 50 –200 nm wide [2]. The electrochemical behavior of a SnO2 thin film is shown on Fig. 1. It clearly indicates the irreversible reaction during the first discharge with a reduction peak at 0.85 V. During the following cycles, this peak disappears and only the peaks at low potential ( < 0.5 V) corresponding to the Li –Sn alloys formation are observed. The presence of SnO2 cassiterite is confirmed before cycling by the XRD patterns. At the end of the first charge (1.15 V), XRD experiments show that the cassiterite is no longer present in the electrode and instead metallic tin is detected [2,3]. These structural changes confirm the reduction of SnIV into Sn0 upon cycling. During this reaction, only four lithium are supposed to react with SnO2 according to the following reaction: SnIV O2 þ 4Liþ þ 4e ! 2Li2 O þ Sn0
ð1Þ
Then, during the following discharges, lithium reacts with metallic tin to form Li –Sn alloys according to reaction (2): Sn þ xLiþ þ xe ! Lix Sn
ð2Þ
The formation of the LixSn alloys (LiSn, Li7Sn3, Li5Sn2, Li13Sn5, Li7Sn2, Li22Sn5) has been studied in the early 1980s by Huggins et al. [4– 7]. The ‘‘global’’ reaction (2) is reversible and leads to the decomposition of the LixSn alloys during discharge, according to reaction (3): Lix Sn ! Sn þ xLiþ þ xe
ð3Þ
The highest capacities associated with reactions (1) and (2) are 711 and 783 mAh/g, respectively, and are in good agreement with the values determined by several studies [8,9]. This analysis highlights the advantages and drawbacks of the use of SnO2 instead of graphite in lithium-ion cells. First, the total specific capacity is 1494 mAh/g, four times bigger than for graphite. The associated volumetric capacity is 10383 mAh/cm3, 12 times bigger than for graphite. However, these values are calculated with the first
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Fig. 1. Cyclic voltamogram of CVD-SnO2 thin film plated on stainless steel substrate AISI 316 (cycling conditions: 10 mV steps/2 min; 0.05 V < U < 1.15 V; reference electrode: metallic lithium; electrolyte: LiPF6 dissolved in 1:1 EC/DMC mixture).
reduction. The irreversible reaction (1) (reduction of SnIV into Sn0) decreases the specific capacity of the following cycles to 783 mAh/g, and severely limits the interest of this material. However, the reversible capacity is still higher than for graphite. The main drawback is the loss of capacity (711 mAh/g) between the first and the following cycles. In a ‘‘rocking chair’’-type battery, this means that nearly one half
of the lithium amount taken from the positive electrode material will be given to the system during reaction (1) on the first cycle and will be lost for the following cycles of the battery. The SnO2 electrode can be described at the end of the first discharge as a dispersion of LixSn alloys into a Li2O matrix. At this step of cycling, the electrode volume is four
Fig. 2. Specific capacity versus cycle number for (a) SnO2 thin film (thickness: 0.7 Am). Cycling conditions: 0.1 mA/cm2, 0.04 V < U < 1.15 V, reference electrode: metallic lithium, electrolyte: LiPF6 dissolved in 1:1 EC/DMC mixture, and (b) SnO2 electrode prepared with micrometric powders.
T. Brousse et al. / Powder Technology 128 (2002) 124–130
times bigger than for the initial state (Vdisch = 4.07Vini). At the end of the first charge, the decomposition of LixSn alloys leads to the formation of metallic tin dispersed into a Li2O matrix, two times bigger than the SnO2 initial volume, showing that the electrode will not recover its original volume. Despite these structural modifications with a ‘‘mac-
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roscopic’’ effect on the electrode (cracking of the electrode, etc.) SnO2 thin films have a reversible capacity of 400 – 500 mAh/g over more than 200 cycles (Fig. 2). However, when SnO2 electrodes are prepared in the standard way by mixing commercially available powders with a conductive material and an organic binder, it does not
Fig. 3. SEM observations of (a) commercially available tin powder (white bar on the left is 10 Am) and (b) nanosized tin prepared by arc-plasma technique (white bar is 300 nm).
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lead to the same results. An irreversible reaction is effectively observed, followed by the reversible formation/ decomposition of LixSn, but the reversible capacities determined in galvanostatic mode quickly fade below 100 mAh/g after a few cycles (Fig. 2). The difference between thin films prepared by CVD and SnO2 powders is essentially the initial crystallite size (20 – 40 nm for the thin films, 10 Am for the powders). It seems that the microstructure of the material has a leading role in obtaining interesting electrochemical properties. The films prepared by CVD show better or similar results to those reported on the Fuji patent with ‘‘amorphous’’ SnO2 powder. In this material the grain size is small enough to promote lithium diffusion within the whole volume of each grain. Nanodomains of Sn embedded in a Li2O matrix are formed, thus limiting the cracking of the electrode due to volume expansion during Li– Sn alloy formation. In the case of commercial powder, the volume expansion of the Sn grains during the formation of Li– Sn alloys takes place in a larger area than for the thin film, thus leading to the decrepitation of the active material and to the loss of electrical contact of some part of the electrode with the current collector.
We have previously shown that reaction (1) observed during the first discharge, corresponds to the formation of metallic tin and Li2O and leads to the loss of four lithium per formula unit. This problem remains even with the use of tin dioxide thin film. In order to cancel this irreversible part of the capacity, some authors have suggested to directly use metallic tin as electrode material [10]. Then the theoretical expected capacity will be 993 mAh/g. 3.2. Metallic tin The use of micrometric tin powders in a standard composite electrode composition leads to low capacity ( < 500 mAh/g) and to a quick fade in capacity which falls below 100 mAh/g after a few cycles. This poor cycle life of the electrode can be explained by two effects: the huge particle size of the initial powder and the lack of an adequate matrix to maintain the mechanical integrity of the electrode upon cycling. This role was held by Li2O in the case of SnO2 electrodes (Fig. 3). The particle size of the metallic tin powder was decreased by using an arc-plasma discharge technique [11]. This powder enables the electrode to reach specific
Fig. 4. Cyclic voltamograms of bismuth electrodes. Cycling conditions: 10 mV/2 min; 0.5 V < U < 1.2 V except 85% Bi 10 mV < U < 1.2 V. Reference electrode: metallic lithium, electrolyte: LiPF6 dissolved in 2:1 EC/DEC mixture, Start in reduction. The most intense peaks are related to the first cycle, then the peak intensities decrease upon cycling.
T. Brousse et al. / Powder Technology 128 (2002) 124–130
capacities close to the theoretical ones ( c 900 mAh/g). However, the cyclability of the electrodes prepared with such powder is poor, and there is still a need for an inactive matrix that can prevent the electrode from falling apart upon cycling. It is believed that the amount of polymer binder (PVDF) in the composite electrode is low to avoid the grinding of the active grain and subsequently to prevent the capacity fade. The amount of graphite should also be too low to promote a good electronic conductivity upon cycling, especially when metallic particles are cracking. In order to check this idea, different compositions (metal/binder/conductive additive) have been tested. 3.3. Bismuth In this perspective, small particle size bismuth powders prepared by the same arc-plasma technique [11] were tested as negative electrodes in lithium-ion batteries [12]. Bismuth was used instead of tin since the effects of the composition of the composite electrode were going to be investigated. In this case the amount of graphite can reach non-negligible value and subsequently can add an electrochemical contribution to tin at low potential. The relatively ‘high’ potential of Li – Bi formation (0.8 – 0.6 V vs. Li) prevents other components within the electrode, especially graphite, from being electrochemically active vs. lithium in this potential window. Several electrode compositions were tested while increasing the ratio of binder and conductive additive. Some results are shown on Fig. 4 and clearly indicate that the increase of binder and graphite leads to a better cyclability of the bismuth electrodes. Even with 25 wt.% of graphite and 25 wt.% of PVDF, the capacity of the electrode drastically fades on the five first cycles. The loss of capacity is hindered when the amount of active material is decreased down to 12% but there is still a huge capacity fade even if the active material content is very low (6% weight). Additionally, such low content of active material will be unacceptable in commercial batteries. It seems difficult to prepare composite electrodes with an active metal (Sn, Bi, etc.) with a cycling behavior as good as for electrodes prepared with oxide materials. Based on the results on tin dioxide, tin and bismuth based electrodes, it seems that using nanoscaled materials instead of micrometric powders enhances the electrochemical properties. Nanoscaled tin and bismuth powders enable the electrodes to reach specific capacities close to the theoretical ones. The role of nanoscaled materials is to help the fast diffusion of lithium in all the grains of active material and to limit electrochemical reactions in nanodomains which can handle the mechanical stress experienced upon formation of Li –Sn or Li– Bi alloys more easily than microsized powders. However, the nanoscaled materials have to be dispersed in an adequate matrix in order to prevent capacity fade upon cycling. In the case of SnO2, the Li2O matrix formed during
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the first discharge plays an important role during the cycle life of the electrode. This role is essentially to keep the active tin grain together since the Li2O matrix is electronically isolating and is a poor ionic conductor of lithium ions (10 12 S cm 1 at 293 K). The main drawback of SnO2 is the irreversible capacity loss in the first cycle due to the formation of Sn and Li2O. In the case of pure metals, theoretical reactions indicate no irreversible capacity loss but the absence of matrix leads to a quick fade in capacity. Thus the capacity fade upon cycling in the case of bismuth and tin electrodes is due to mechanical milling of the grains during charge– discharge processes. A part of the material is disconnected from the rest of the electrode (and subsequently from the current collector) and becomes inactive for the following cycles in the absence of an adequate matrix. Increasing the amount of binder and conductive additive in the composite electrode limits the capacity loss upon cycling but does not succeed in canceling it even with only small amount of active materials.
4. Conclusions Tin dioxide was first claimed to be the ‘‘miracle material’’ to replace standard graphite anodes in lithium ions cells, and it was immediately disclaimed as a «big consumer» of Li-ions, bringing an irreversible capacity loss to the battery which makes it commercially unacceptable. However tin dioxide is still the key point of the growing research on negative electrodes based on the formation of lithium-metal alloys instead of classical intercalation between graphite planes. Both volumetric and specific capacities of the Li-metal systems are several times greater than the usual carbonaceous materials used up to now in commercially available batteries. The decrease in the particle size has enhanced the cycling ability of SnO2 electrodes. The same modification of bismuth and tin powders has led to capacities which are bigger than for commercially available powders. However, the use of nanoscaled metal powders does not cancel the capacity fade upon cycling and further efforts on these materials should aim at the search of an adequate inactive matrix to disperse the active material. This matrix must be able to handle huge mechanical stress encountered by the electrode during charge–discharge processes.
References [1] Y. Idota, M. Mishima, M. Miyaki, T. Kubota, T. Miyasaka, US Patent, 5,618,640 (April 8, 1997). [2] R. Retoux, T. Brousse, D.M. Schleich, J. Electrochem. Soc. 146 (1999) 2472. [3] T. Brousse, R. Retoux, U. Herterich, D.M. Schleich, J. Electrochem. Soc. 145 (1998) 1. [4] C.J. Wen, R.A. Huggins, J. Solid State Chem. 35 (1980) 376.
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[5] B.A. Boukamp, G.C. Lesh, R.A. Huggins, J. Electrochem. Soc. 128 (1981) 725. [6] C.J. Wen, R.A. Huggins, J. Electrochem. Soc. 128 (1981) 1181. [7] A. Anani, S. Crouch-Baker, R.A. Huggins, J. Electrochem. Soc. 134 (1987) 3098. [8] I.A. Courtney, J.R. Dahn, J. Electrochem. Soc. 144 (1997) 2045.
[9] I.A. Courtney, J.R. Dahn, J. Electrochem. Soc. 144 (1997) 2943. [10] J. Yang, M. Winter, J.O. Besenhard, Solid State Ionics 90 (1996) 281. [11] F. Brochin, X. Devaux, J. Ghanbaja, H. Scherrer, Nanostruct. Mater. 11 (1999) 1. [12] O. Crosnier, X. Devaux, T. Brousse, P. Fragnaud, D.M. Schleich, J. Power Sources 94 (2001) 169.
Advanced oxide and metal powders for negative electrodes in lithium-ion batteries T. Brousse a,*, O. Crosnier a, X. Devaux b, P. Fragnaud a, P. Paillard a, J. Santos-Pen˜a a, D.M. Schleich a a
Laboratoire de Ge´nie des Mate´riaux, Ecole Polytechnique de l’Universite´ de Nantes, La Chantrerie, rue Christian Pauc, BP 50609, 44306 Nantes Cedex 3, France b Laboratoire de Physique des Mate´riaux, UMR CNRS 7556, Ecole des Mines, 54042 Nancy Cedex, France
Abstract In this study, tin dioxide, tin and bismuth have been envisioned as possible candidate to replace graphite negative electrodes in Li-ion batteries. Tin dioxide thin films and nanoscaled tin and bismuth powders have been synthesized by different techniques. Their electrochemical behaviors have been compared to electrodes made with standard commercially available powders. In all cases, nanoscaled materials have shown enhanced electrochemical properties compared to standard powders. SnO2 thin films exhibit a longer cycle life than tin dioxide powder. The capacities measured on both bismuth and tin nanoscaled materials were more important than for the same electrodes prepared by commercially available powders. Moreover, the values are close to those expected from the theoretical reactions. However, the cycling life of tin or bismuth electrodes is still the weak point of these systems. Subsequently, an optimized matrix is required in order to prevent capacity loss upon cycling. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Lithium-ion batteries; Anodes; Tin; Tin dioxide; Bismuth
1. Introduction Lithium-ion batteries are now commercially available products for many applications (portable computers, mobile telephones, etc.). Actual developments aim at their use as power source for electric vehicle. However, these batteries still have safety problems due to the materials used. Lithium-ion cells are composed of lithiated transition metal oxides as the positive electrode and graphite as the negative electrode. The specific capacity of this latter compound is 372 mAh/g. Nevertheless, the reactivity of carbon with the liquid electrolytes leads to several problems, with a capacity loss between the first and the second cycle of more than 20%, and a slow fade in capacity upon cycling. Some metals, alloys and metal oxides have been recently proposed [1– 10] as possible negative electrodes in lithiumion batteries. The main property of the metals involved in
*
Corresponding author. Fax: +33-240683199. E-mail address: [email protected] (T. Brousse).
these compounds is that they can form binary alloys with lithium. For example, Fuji Film released a patent [1] where the carbonaceous anode was replaced by a tin oxide based compound. During the first cycle an irreversible reaction occurs with the reduction of SnO2 into Li2O and metallic tin, followed by the reversible formation of Li –Sn alloys. Despite the huge lithium loss due to the reaction with oxygen, the reversible capacities of Li– Sn alloys (more than 700 mAh/g) raise up a growing interest for tin based compounds. In these systems, the intercalation reactions that usually take place into a host structure are replaced by the formation of Li– M alloys which leads to a high volume expansion during the electrode reduction. Despite huge specific capacities, obtained with metallic tin for example (three times bigger than graphite), it seems that the electrochemical performance of the candidate material are strongly influenced by its microstructure. Thus the use of alternative methods to synthesize nanoscaled materials can be helpful to optimize the design of electrodes with improved cycling behavior compared to standard materials. In this study, tin oxide thin films, tin and bismuth nanoscaled powders have been synthesized by different
0032-5910/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 ( 0 2 ) 0 0 1 8 6 - 9
T. Brousse et al. / Powder Technology 128 (2002) 124–130
techniques and their electrochemical behaviors have been compared to electrodes made with standard commercially available powders.
2. Experimental SnO2 thin films have been deposited on stainless steel substrates (AISI 316) using a low pressure chemical vapor deposition (CVD) technique [2,3]. Tetrabutyl tin was used as a precursor and kept in a bubbler at 314 K to obtain an adequate vapor pressure. Oxygen was supplied both as the carrier gas as well as a supplemental oxidant. The total pressure was maintained at 670 Pa during deposition. The growth temperature used for the films was 773 K. Electrochemical tests were performed on the as-deposited SnO2 thin films without the addition of any binder neither conductive additive. The nanosized metallic tin and bismuth powders were obtained by using an arc-plasma discharge technique [11]. A tin or bismuth ingot was evaporated in a direct current (DC) arc plasma under an Ar – H2 gas flow at reduced pressure. The fine powder was collected and stored in a glove box under high purity argon atmosphere. SnO2 and Sn micrometric powders were obtained from Aldrich Chemical (99.9% purity). Except for the SnO2 thin films, the composite electrode was prepared by mixing together active material (metal oxide), graphite and a polymer (PVdF: polyvinylidene fluoride) as a binder at a weight ratio of 0.85:0.1:0.05. For the bismuth electrodes this ratio was varied as indicated in the related paragraph. Graphite was used to assume a good electronic conductivity throughout the electrodes and PVDF was used to keep the mechanical integrity. The mixture was deposited on copper substrates using a barcoater and dried for 12 h at 100 jC. Some 12mm diameter pieces of the composite electrode were cut and used as samples for both electrochemical tests and structural analysis. Electrochemical tests were performed in a simple twoelectrode cell using metallic lithium as the negative electrode. The electrolyte was a 1 M solution of LiPF 6 dissolved in EC/DMC 1:1 (Merck). A glass fiber paper was used as the separator and wetted with the electrolyte solution. The cells were assembled in a glove box filled with argon and cycled using a MacPile system (BioLogic) in potentiostatic or galvanostatic mode. The voltage limits and rate are indicated when required. Lithium uptake by the electrode will be referred as the discharge (reduction step) and lithium release as the charge of the electrode. One complete cycle corresponds to one discharge followed by one charge. Morphology and composition of the different materials were determined by means of a Leica Stereoscan 440 scaning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) analyser (Oxford Instrument).
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3. Results and discussion 3.1. SnO2 Fuji Film patent [1] concerning the use of SnO2 as possible anode in lithium-ion batteries (issued in 1994) highlights two major points: the possibility of reaching specific capacities three times bigger than the specific capacity of graphite, with a relatively low potential vs. lithium, and the use of oxides in an amorphous state. This latter point is illustrated in the patent by an XRD patterns of SnO2 without any well-defined peak. Even if the patent is related to many compounds and several substitutions, the basic material is always tin dioxide. SnO2 thin films prepared by CVD have been tested vs. a metallic lithium electrode. The observations by scanning electronic microscopy (not shown in here) reveal that the film is a compact structure of disordered grains 50 –200 nm wide [2]. The electrochemical behavior of a SnO2 thin film is shown on Fig. 1. It clearly indicates the irreversible reaction during the first discharge with a reduction peak at 0.85 V. During the following cycles, this peak disappears and only the peaks at low potential ( < 0.5 V) corresponding to the Li –Sn alloys formation are observed. The presence of SnO2 cassiterite is confirmed before cycling by the XRD patterns. At the end of the first charge (1.15 V), XRD experiments show that the cassiterite is no longer present in the electrode and instead metallic tin is detected [2,3]. These structural changes confirm the reduction of SnIV into Sn0 upon cycling. During this reaction, only four lithium are supposed to react with SnO2 according to the following reaction: SnIV O2 þ 4Liþ þ 4e ! 2Li2 O þ Sn0
ð1Þ
Then, during the following discharges, lithium reacts with metallic tin to form Li –Sn alloys according to reaction (2): Sn þ xLiþ þ xe ! Lix Sn
ð2Þ
The formation of the LixSn alloys (LiSn, Li7Sn3, Li5Sn2, Li13Sn5, Li7Sn2, Li22Sn5) has been studied in the early 1980s by Huggins et al. [4– 7]. The ‘‘global’’ reaction (2) is reversible and leads to the decomposition of the LixSn alloys during discharge, according to reaction (3): Lix Sn ! Sn þ xLiþ þ xe
ð3Þ
The highest capacities associated with reactions (1) and (2) are 711 and 783 mAh/g, respectively, and are in good agreement with the values determined by several studies [8,9]. This analysis highlights the advantages and drawbacks of the use of SnO2 instead of graphite in lithium-ion cells. First, the total specific capacity is 1494 mAh/g, four times bigger than for graphite. The associated volumetric capacity is 10383 mAh/cm3, 12 times bigger than for graphite. However, these values are calculated with the first
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T. Brousse et al. / Powder Technology 128 (2002) 124–130
Fig. 1. Cyclic voltamogram of CVD-SnO2 thin film plated on stainless steel substrate AISI 316 (cycling conditions: 10 mV steps/2 min; 0.05 V < U < 1.15 V; reference electrode: metallic lithium; electrolyte: LiPF6 dissolved in 1:1 EC/DMC mixture).
reduction. The irreversible reaction (1) (reduction of SnIV into Sn0) decreases the specific capacity of the following cycles to 783 mAh/g, and severely limits the interest of this material. However, the reversible capacity is still higher than for graphite. The main drawback is the loss of capacity (711 mAh/g) between the first and the following cycles. In a ‘‘rocking chair’’-type battery, this means that nearly one half
of the lithium amount taken from the positive electrode material will be given to the system during reaction (1) on the first cycle and will be lost for the following cycles of the battery. The SnO2 electrode can be described at the end of the first discharge as a dispersion of LixSn alloys into a Li2O matrix. At this step of cycling, the electrode volume is four
Fig. 2. Specific capacity versus cycle number for (a) SnO2 thin film (thickness: 0.7 Am). Cycling conditions: 0.1 mA/cm2, 0.04 V < U < 1.15 V, reference electrode: metallic lithium, electrolyte: LiPF6 dissolved in 1:1 EC/DMC mixture, and (b) SnO2 electrode prepared with micrometric powders.
T. Brousse et al. / Powder Technology 128 (2002) 124–130
times bigger than for the initial state (Vdisch = 4.07Vini). At the end of the first charge, the decomposition of LixSn alloys leads to the formation of metallic tin dispersed into a Li2O matrix, two times bigger than the SnO2 initial volume, showing that the electrode will not recover its original volume. Despite these structural modifications with a ‘‘mac-
127
roscopic’’ effect on the electrode (cracking of the electrode, etc.) SnO2 thin films have a reversible capacity of 400 – 500 mAh/g over more than 200 cycles (Fig. 2). However, when SnO2 electrodes are prepared in the standard way by mixing commercially available powders with a conductive material and an organic binder, it does not
Fig. 3. SEM observations of (a) commercially available tin powder (white bar on the left is 10 Am) and (b) nanosized tin prepared by arc-plasma technique (white bar is 300 nm).
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lead to the same results. An irreversible reaction is effectively observed, followed by the reversible formation/ decomposition of LixSn, but the reversible capacities determined in galvanostatic mode quickly fade below 100 mAh/g after a few cycles (Fig. 2). The difference between thin films prepared by CVD and SnO2 powders is essentially the initial crystallite size (20 – 40 nm for the thin films, 10 Am for the powders). It seems that the microstructure of the material has a leading role in obtaining interesting electrochemical properties. The films prepared by CVD show better or similar results to those reported on the Fuji patent with ‘‘amorphous’’ SnO2 powder. In this material the grain size is small enough to promote lithium diffusion within the whole volume of each grain. Nanodomains of Sn embedded in a Li2O matrix are formed, thus limiting the cracking of the electrode due to volume expansion during Li– Sn alloy formation. In the case of commercial powder, the volume expansion of the Sn grains during the formation of Li– Sn alloys takes place in a larger area than for the thin film, thus leading to the decrepitation of the active material and to the loss of electrical contact of some part of the electrode with the current collector.
We have previously shown that reaction (1) observed during the first discharge, corresponds to the formation of metallic tin and Li2O and leads to the loss of four lithium per formula unit. This problem remains even with the use of tin dioxide thin film. In order to cancel this irreversible part of the capacity, some authors have suggested to directly use metallic tin as electrode material [10]. Then the theoretical expected capacity will be 993 mAh/g. 3.2. Metallic tin The use of micrometric tin powders in a standard composite electrode composition leads to low capacity ( < 500 mAh/g) and to a quick fade in capacity which falls below 100 mAh/g after a few cycles. This poor cycle life of the electrode can be explained by two effects: the huge particle size of the initial powder and the lack of an adequate matrix to maintain the mechanical integrity of the electrode upon cycling. This role was held by Li2O in the case of SnO2 electrodes (Fig. 3). The particle size of the metallic tin powder was decreased by using an arc-plasma discharge technique [11]. This powder enables the electrode to reach specific
Fig. 4. Cyclic voltamograms of bismuth electrodes. Cycling conditions: 10 mV/2 min; 0.5 V < U < 1.2 V except 85% Bi 10 mV < U < 1.2 V. Reference electrode: metallic lithium, electrolyte: LiPF6 dissolved in 2:1 EC/DEC mixture, Start in reduction. The most intense peaks are related to the first cycle, then the peak intensities decrease upon cycling.
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capacities close to the theoretical ones ( c 900 mAh/g). However, the cyclability of the electrodes prepared with such powder is poor, and there is still a need for an inactive matrix that can prevent the electrode from falling apart upon cycling. It is believed that the amount of polymer binder (PVDF) in the composite electrode is low to avoid the grinding of the active grain and subsequently to prevent the capacity fade. The amount of graphite should also be too low to promote a good electronic conductivity upon cycling, especially when metallic particles are cracking. In order to check this idea, different compositions (metal/binder/conductive additive) have been tested. 3.3. Bismuth In this perspective, small particle size bismuth powders prepared by the same arc-plasma technique [11] were tested as negative electrodes in lithium-ion batteries [12]. Bismuth was used instead of tin since the effects of the composition of the composite electrode were going to be investigated. In this case the amount of graphite can reach non-negligible value and subsequently can add an electrochemical contribution to tin at low potential. The relatively ‘high’ potential of Li – Bi formation (0.8 – 0.6 V vs. Li) prevents other components within the electrode, especially graphite, from being electrochemically active vs. lithium in this potential window. Several electrode compositions were tested while increasing the ratio of binder and conductive additive. Some results are shown on Fig. 4 and clearly indicate that the increase of binder and graphite leads to a better cyclability of the bismuth electrodes. Even with 25 wt.% of graphite and 25 wt.% of PVDF, the capacity of the electrode drastically fades on the five first cycles. The loss of capacity is hindered when the amount of active material is decreased down to 12% but there is still a huge capacity fade even if the active material content is very low (6% weight). Additionally, such low content of active material will be unacceptable in commercial batteries. It seems difficult to prepare composite electrodes with an active metal (Sn, Bi, etc.) with a cycling behavior as good as for electrodes prepared with oxide materials. Based on the results on tin dioxide, tin and bismuth based electrodes, it seems that using nanoscaled materials instead of micrometric powders enhances the electrochemical properties. Nanoscaled tin and bismuth powders enable the electrodes to reach specific capacities close to the theoretical ones. The role of nanoscaled materials is to help the fast diffusion of lithium in all the grains of active material and to limit electrochemical reactions in nanodomains which can handle the mechanical stress experienced upon formation of Li –Sn or Li– Bi alloys more easily than microsized powders. However, the nanoscaled materials have to be dispersed in an adequate matrix in order to prevent capacity fade upon cycling. In the case of SnO2, the Li2O matrix formed during
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the first discharge plays an important role during the cycle life of the electrode. This role is essentially to keep the active tin grain together since the Li2O matrix is electronically isolating and is a poor ionic conductor of lithium ions (10 12 S cm 1 at 293 K). The main drawback of SnO2 is the irreversible capacity loss in the first cycle due to the formation of Sn and Li2O. In the case of pure metals, theoretical reactions indicate no irreversible capacity loss but the absence of matrix leads to a quick fade in capacity. Thus the capacity fade upon cycling in the case of bismuth and tin electrodes is due to mechanical milling of the grains during charge– discharge processes. A part of the material is disconnected from the rest of the electrode (and subsequently from the current collector) and becomes inactive for the following cycles in the absence of an adequate matrix. Increasing the amount of binder and conductive additive in the composite electrode limits the capacity loss upon cycling but does not succeed in canceling it even with only small amount of active materials.
4. Conclusions Tin dioxide was first claimed to be the ‘‘miracle material’’ to replace standard graphite anodes in lithium ions cells, and it was immediately disclaimed as a «big consumer» of Li-ions, bringing an irreversible capacity loss to the battery which makes it commercially unacceptable. However tin dioxide is still the key point of the growing research on negative electrodes based on the formation of lithium-metal alloys instead of classical intercalation between graphite planes. Both volumetric and specific capacities of the Li-metal systems are several times greater than the usual carbonaceous materials used up to now in commercially available batteries. The decrease in the particle size has enhanced the cycling ability of SnO2 electrodes. The same modification of bismuth and tin powders has led to capacities which are bigger than for commercially available powders. However, the use of nanoscaled metal powders does not cancel the capacity fade upon cycling and further efforts on these materials should aim at the search of an adequate inactive matrix to disperse the active material. This matrix must be able to handle huge mechanical stress encountered by the electrode during charge–discharge processes.
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