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On the role of water contamination in rechargeable Li batteries
Electrochimica Acta 45 (1999) 1135 – 1140 www.elsevier.nl/locate/electacta
Short communication
On the role of water contamination in rechargeable Li batteries D. Aurbach a,*, I. Weissman a, A. Zaban a, P. Dan b a
Department of Chemistry, Bar-Ilan Uni6ersity, Ramat-Gan 52900, Israel b Tadiran Batteries Ltd., P.O. Box 1, Kiryat-Ekron, Israel Received 30 March 1999; received in revised form 2 July 1999
Abstract Many cathode materials such as LiMnO2 can be highly hygroscopic and thus, introduce considerable water contamination into Li batteries. Water reacts with the Li anode, and this strongly affects its surface chemistry. In this work, we investigated some phenomena related to water contamination due to the cathode material in LiLix MnO2 cells containing 1–3 dioxolane/LiAsF6 solutions. When these cells contain cathodes which were exposed to air, their electrolyte solutions become contaminated with water, which reacts with lithium and thus, hydrogen gas is formed. We discovered that discharging cells containing wet cathodes stops this liberation of hydrogen. We explored several possible explanations for this phenomenon. It was concluded that lithiation of water-containing Lix MnO2 considerably inhibits the liberation of water into the electrolyte solution. The effect of the presence of water in solutions on the properties of the Li anode is discussed. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: Water contamination; Li batteries; Electrolyte solution
1. Introduction
within the surface films are reduced by the active metal. The following 3 reactions are possible:
It is well known that electrolyte solutions for Li batteries unavoidably contain water contamination. The best commercially available Li battery grade electrolyte solutions contain not less than 20 ppm water. Water at any concentration in solutions reacts with lithium. Most of the surface species covering Li electrodes in nonaqueous electrolyte solutions are highly hygroscopic (organic or inorganic insoluble Li salts) [1]. Hence, water in solutions hydrates the surface films on lithium electrodes and diffuses through the surface films towards the Li-film interface. The water molecules
H2O+ Lio LiOH+ 12H2
(1)
LiOH+ Li Li2O+ H2
(2)
* Corresponding author. Tel.: +972-3-531-8317; fax: + 972-3-535-1250. E-mail address: [email protected] (D. Aurbach)
o
H2 + Lio LiH
1 2
1 2
(3)
The presence of hydrated species, as well as the products of the above reactions in the surface films, obviously affects the behavior of the Li anode that is controlled by the surface films covering it [2]. The major source of water in Li batteries is not necessarily the electrolyte solutions. Many cathode materials may be highly hygroscopic, thus delivering a considerable amount of water into the batteries, and contaminating the electrolyte solutions with a large amount of water. In recent years, we studied intensively
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LiLix MnO2 3V battery systems in which the electrolyte solution was 1–3 dioxolane (DN)/LiAsF6 [3–7]. A few years ago this rechargeable battery system became a commercial product [5]. The behavior of Li electrodes in 1–3 dioxolane is unique. The surface films formed on the active metal in these solutions are composed of HCOOLi, ROLi species (e.g. CH3CH2OCH2OLi and possibly LiCH2CH2OCH2OLi) and oligomers of DN with OLi edge groups [5–7]. Salt reduction reaction also contributes to the Li surface chemistry and hence, the Li surface films also include species such as LiF and Lix AsFy (x=0–3, y=0–2) [5–7]. This matrix of surface species forms surface layers which induce highly smooth and uniform Li deposition dissolution, and thereby, Li cycling efficiency in these solutions is very high. Tributylamine at hundreds to thousands of ppm is an important ingredient in these solutions because it prevents the easy polymerization of DN (via an acidic mechanism) by trapping trace Lewis acids which are unavoidably present in solutions (e.g. AsF3) [6]. The behavior and structure of the Lix MnO2 cathodes was also extensively investigated. As already reported, it is produced by a thermal reaction between MnO2 and LiNO3 that produces Li0.3MnO2, which can insert lithium at around 3 V (Li/Li+) up to a stoichiometry of LiMnO2 [3]. This cathode material is hygroscopic and if it is not dried properly, it can introduce pronounced water contamination into the battery. Taking into account the high electrode surface/solution volume ratio in practical
batteries, wet cathodes can contaminate the electrolyte solutions in practical batteries by thousands of ppms of water which can react further with lithium, thus producing dangerous hydrogen. The goal of this work was to study possible routes of contamination of these Li battery systems by water through the cathode, and to investigate possible reactions of this water contamination.
2. Experimental 1 – 3 Dioxolane (Li battery grade) was obtained from Tomiyama Co. (Japan), and was further distilled over Li-benzophenone (blue solutions). LiAsF6 was obtained from Lithco Inc., and was used as received. LiAsF6/DN solutions were stabilized by tributylamine (500 – 1000 ppm). Lix MnO2 cathodes were prepared as already described [3]. LiLix MnO2 cells in a jelly-rolled configuration were prepared for this study. The electrodes were a Li foil (Foote Mineral Inc.) anode and a Lix MnO2 cathode, separated by a Celgard polypropylene separator (single electrode area was about 100 cm2). These cells were measured in special gasometers to which a solution was introduced after the cells were loaded. The gasometer described in Fig. 1 allowed the measuring of hydrogen evolution during electrochemical processes of the cells. Impedance spectroscopic studies of Li electrodes freshly prepared in solutions were already described [4]. All preparations for the spectroscopic studies, as well as the LiLix MnO2 cell measurements were carried out under highly pure argon atmosphere in VAC glove boxes (which included H2O and O2 trace gas absorbers from VAC). The amount of water in solutions was measured by Karl Fischer titration (Metrohm 562 CF coulometer). SEM measurements were performed using the JEOL JSM 840 microscope. Li samples were introduced from the glove box to the microscope using a transfer method which prevents their contamination with atmospheric components, as already described [6,7]. In brief, we built a transfer system in which Li samples are loaded and hermetically closed in the glove box. This transfer system is then connected to the electron microscope, and the samples are introduced into it after high vacuum (10 − 6 torr) is achieved.
3. Results and discussion Fig. 1. A sketch of the gasometer used in this work for studying H2 evolution during processes of LiLix MnO2 cells. (1) Glass vessel. (2) A rubber cap. (3) Lix MnO2 cathode/separator/Li anode cell in a jelly rolled configuration. (4) Solution. (5) Electrical contacts (Pt wires sealed in glass). (6) Gas bubbles and volume. (7) Solution level in the gasometer (liberated gas volume reading).
The first subject of this study relates to the impact of water contamination on the performance of the lithium anodes in terms of morphology, impedance and cycling efficiency. Especially important is knowing what concentrations of water could be considered as critical to the Li anode’s performance. As already reported [5 – 7],
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Fig. 2. SEM micrographs of Li electrodes after being cycled in wet DN/LiAsF6 1 M/TBA (1000 ppm) solutions. 10 galvanostatic charge-discharge cycles at 1.5 mA/cm2, 0.5 C/cm2 per process (0.55 C/cm2 per last half cycle), a, c, e — last process was Li dissolution= b, d, f — last process was Li deposition. a, b—100 ppm H2O, c, d — 600 ppm H2O, e, f — 2000 ppm H2O.
we measured extensively the behavior of lithium electrodes in dry and wet DN/LiAsF6/TBA solutions. When Li is freshly exposed to H2O contaminated solutions, water reduction profoundly influences its surface and the interfacial impedance increases considerably during storage. When a Li surface is freshly prepared in dry ( B30 ppm H2O) DN/LiAsF6 solutions, it reaches full passivation in less than 3 h and then, the interfacial impedance remains unchanged during prolonged storage (weeks). When DN/LiAsF6 solutions are contaminated with water after their passivation is reached, the impact of the presence of water is much less pronounced compared with Li electrodes freshly prepared in wet solutions. The critical water concentration in this respect is around 1000 ppm. Below this concentration, water reaction with Li is very moderate and is limited by a relatively slow H2O diffusion through the surface
films. When the water content in DN/LiAsF6 solutions exceeds 1000 ppm, the Li passivity breaks down. Then a pronounced reaction between Li and water occurs even when the water contamination appears after the Li electrodes are already covered by stable, passivating surface films (originating from reactions between the Li and solution species). The above results correlate well with morphological and electrochemical studies of Li electrodes (initially covered by surface films) in wet DN/LiAsF6 solutions. Fig. 2 shows SEM micrographs of Li electrodes cycled 10 times in wet DN/LiAsF6 1 M/TBA (1000 ppm) solutions. The charge involved in each process was 0.5 C/cm2, and the current density (galvanostatic process) was 1.5 mA/cm2. Micrographs a, c and e relate to electrodes whose last process was Li dissolution, while micrographs b, d and f relate to electrodes whose
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last process was Li deposition. Micrographs a and b relate to a solution containing 100 ppm of water. Micrographs c and d relate to 600 ppm water in solution, and 2e, f relate to 2000 ppm H2O in solution. These micrographs clearly show that the Li morphology upon cycling in these solutions was very smooth, even when the water concentration reached 600 ppm. This correlates well with Li cycling efficiency measurements in these solutions (performed as already described [6,7]). The cycling efficiency of electrodes prepared in dry LiAsF6/DN/TBA solutions followed by cycling in wet solutions was high ( \90%), even when the water content was as high as 600 ppm. Hence, as already discussed [5–7], the high Li cycling efficiency usually obtained in DN solutions can also be achieved in solutions contaminated by several hundred ppm of water, provided that the surface films that passivate the Li electrodes are formed initially in dry solutions. Such
Fig. 3. H2 evolution as a function of time of LiLix MnO2 cells in DN/LiAsF6/TBA solutions. One cell was stored at open circuit, the other cell (as indicated) was processed as follows: three hours of discharge at 1.2 mA/cm2, rest for 20 h at OCV, then 3 h of charging (1.2 mA/cm2) and rest at OCV. The cathodes were exposed to normal air (and thus absorbed humidity) before the cells were constructed. (The active material Li0.3MnO2 initially contained 1500–2000 ppm of water by weight). The upper picture shows the experiment during its entire time scale, while the lower one describes the first 80 h of the experiment.
surface films possess a high degree of passivity against water diffusion, and a massive reaction between Li and water solutions is largely avoided. A pronounced reaction between H2O and lithium (which is accompanied by H2 evolution) occurs only when the Li surfaces are freshly exposed to H2O-contaminated solutions, or when a Li surface covered with passivating surface layers (formed in dry DN solutions) is in contact with wet solutions containing more than 1000 ppm of H2O. The next stage of this study was related to the behavior of LiLix MnO2 cells whose cathodes contained water. Li0.3MnO2 cathodes were stored for different periods of time in air, and were thus allowed to absorb water. Then, completed cells, in a jelly-rolled configuration (i.e. a Li anode, a cathode and a separator in between, rolled in a parallel plate configuration) were loaded into the gasometer (Fig. 1). LiAsF6(1 M)/DN/TBA (1000 ppm) solution was introduced and the amount of gas evolved was measured as a function of time and the electrochemical process conducted. The gas was identified as pure hydrogen by mass spectrometry. The amount of water contained in the wet cathode was estimated by wetting 10 cc of solution, followed by KF measurements. It appears that the wet cathodes (AA cell, 100 cm2, about 4 gr of Li0.3MnO2 per cell) introduced 650 ppm of water into 10 cc of solution. This corresponds to a water contamination of 1500 – 2000 ppm (by weight) in the cathode material. Since the amount of solution contained by the separator (between the Li and the cathode) is around 2 cc, the water contamination in solution which affects the lithium anode can be as high as 3000 ppms of water. It appears that cycling the cell (charge – discharge) at current densities ranging between 0.3 – 1.5 mA/cm2 stopped the gas evolution. Further studies revealed that the first discharge process of the cell in which the Li0.3MnO2 cathode is lithiated and Li dissolves from the anode, also considerably decreased the H2 evolution. Fig. 3 presents typical H2 evolution (gas volume versus time) for a cell stored at OCV, which is the reference measurement, and for a cell during discharge and charge at constant current. In the specific experiment described in Fig. 3, the cell was discharged for 3 h (1.2 mA/cm2), stored for 20 h at OCV, and then discharged for 3 h (1.2 mA/cm2) following storage at OCV. It is significant that for this cell, the initial rate of H2 evolution was higher than that of the cell stored at OCV (probably due to exposure of reactive lithium to the wet solution). However, as discharge proceeded, the gas evolution diminished considerably. We conducted several experiments similar to the one described in Fig. 3, at different current regimes and sequence of processes. There is no doubt that the discharge process alone was enough to stop massive gas evolution. These experiments imitate dangerous situations in Li batteries in which the cathodes may become a source of
D. Aurbach et al. / Electrochimica Acta 45 (1999) 1135–1140
water due to inefficient drying. This water may contaminate the solution at high concentration and thus can be involved in a pronounced reaction with Li, thus liberating a considerable amount of hydrogen. This may build up a pressure in the cell, which is dangerous by itself due to the flammability of the gas. Of particular importance was the fact that the discharge of these cells stopped the H2 evolution. This is a very interesting result, and we can offer a priori three possibilities to explain this effect: 1. The cell discharge involves an anodic process of the Li electrode (Li dissolution). This process exposes fresh, reactive lithium that reacts directly with the H2 formed by water reduction. LiH formation prevents evolution of hydrogen gas. In contrast, the water reduction in the cell stored at OCV occurs within existing surface films. Hence, there is no exposure of fresh Li, which is sufficiently reactive to react with H2. 2. The anodic process of lithium in these solutions changes the surface chemistry so that the new surface species formed are much less hygroscopic that the original ones, and they thus block H2O diffusion to the active metal. 3. Lithiation of the cathode forms materials, which are much more hygroscopic than the solution. Hence, water is trapped irreversibly in the lithiated cathode. Considering the first and second possibilities, we studied possible reactions of Li with H2 and DN/ LiAsF6 solutions contaminated with water using surface sensitive FTIR spectroscopy. The effect of anodic processes (Li dissolution in these solutions was also explored in this study [8]. We did find some evidence that LiH may be formed at ambient temperature when fresh lithium surfaces are exposed to hydrogen [8]. It is possible that fresh lithium is indeed exposed to hydrogen during Li dissolution in wet Li salt/non-aqueous solutions. In such a process, lithium is depleted by the anodic process in the interface of the active metal with the surface film, forming holes which expose fresh Li surfaces to solution species. In parallel to such a process, H2O is continuously reduced by lithium to form hydrogen. Hence, H2 can easily percolate to the holes formed by Li dissolution and hence, a reaction between freshly prepared Li and H2 can take place. However, our spectral studies revealed that even if LiH is formed, it is not a major surface species formed on lithium in wet ethereal solutions [8]. Hence, LiH formation can not explain the pronounced depletion of H2. In addition, our spectral studies clearly showed that the surface chemistry developed on lithium in wet ethereal solutions during Li dissolution does not differ much from the usual Li-surface chemistry at open circuit voltage. The surface films covering lithium in wet DN/ LiAsF6 solutions contain the same ROLi species, HCOOLi and LiF species formed in dry solutions, and,
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of course, Li2O and LiOH (and maybe also LiH), which are the products of water reduction. The latter products are even more hygroscopic than ROLi or HCOOH. This rules out the second possibility mentioned above. Hence, the only possible explanation for the depletion of H2 evolution during discharge of the LiLix MnO2 cells as described above (Fig. 3) is that water is irreversibly trapped in the cathodes when they are lithiated. Further structural studies of LiMnO2 cathodes during discharge (Li insertion) in wet solutions are in progress and are beyond the scope of this short communication. The last point examined in this work relates to the possibility that water in DN/LiAsF6 solutions is oxidized by the cathode (OCV =3.4 V versus Li/Li+). Such oxidation may form H+ which can either lead to polymerization of the solvent (which, as an acetal is highly sensitive to acids [9]), or can diffuse to the anode and react on the Li surface. Such a possibility was ruled out by comparing the composition of wet solutions stored over cathodes, or polarized to 3.4 V with noble metal electrodes, with that of the pristine wet solutions (by GCMS). The mass spectrometry of the treated solutions was nearly identical to that of the reference solution.
4. Conclusion We studied some aspects of the impact of water contamination on the behavior of Li battery systems. There is a big difference between a situation in which fresh Li is exposed to wet DN/LiAsF6 solutions and the exposure of lithium already covered with surface films to these solutions. In the former case, the impact of water on the Li surface chemistry is very pronounced at any concentration. As found in parallel studies, LiH appears as one of the possible products of the reaction between Li and trace water. However, when the solutions are contaminated with water after the Li is already covered by surface films, up to 1000 ppm, the impact of water on the Li surface chemistry is not pronounced, because the surface films formed in DN solutions passivate the active metal. Hence, water reactions are controlled by its slow diffusion through the surface films. When the water concentration exceeds 1000 ppm, the passivity of Li in solution is not steady enough to prevent a massive reaction of Li with water which liberates hydrogen. Such a situation may occur in practical Li batteries in which the cathode material (which is usually hygroscopic) accidentally absorbed water. Since the solution volume in practical batteries is usually small, the solutions can be contaminated by H2O above the critical concentration at which the passivity of the Li anode is broken down. Then, the cells can
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develop a considerable pressure of H2 (due to the reaction of Li with water). In the case of Li0.3MnO2 cathodes (whose structure has already been described [10]) which are hygroscopic, their lithiation (discharge) seems to diminish water liberation into the solutions, probably because the lithiated cathode material irreversibly traps hydrated water in its lattice. This work ruled our the possibility of water oxidation on Li0.3MnO2 cathodes (OCV :3.4V).
Acknowledgements This work was partially supported by Tadiran–Battery Division, the Belfer Fund for studies related to energy storage and conversion (Israel), and the Israeli Ministry of National Infrastructure and Energy.
References [1] D. Aurbach, A. Zaban, Y. Gofer, Y. Ein-Eli, I. Weissman, O. Chusid, O. Abramson, J. Power Sources 54 (1995) 76.
.
[2] E. Peled, in: J.P. Gabano (Ed.), Lithium batteries, Academic Press, London, 1983, p. 43. [3] E. Levi, E. Zinigrad, H. Teller, M.D. Levi, D. Aurbach, E. Mengeritsky, E. Elster, E. Granot, H. Yamin, J. Electrochem. Soc. 144 (1997) 4133. [4] D. Aurbach, E. Zinigrad, A. Zaban, J. Phys. Chem. 100 (1996) 3089. [5] E. Mengeritsky, P. Dan, I. Weissman, A. Zaban, D. Aurbach, J. Electrochem. Soc. 143 (1996) 2110. [6] D. Aurbach, Y. Gofer, M. Ben-Zion, J. Power Sources 39 (1992) 163. [7] (a) O. Youngman, Y. Gofer, A. Meitav, D. Aurbach, Electrochim. Acta, 35 (1990) 625. (b) D. Aurbach, O. Youngman, P. Dan, Electrochim. Acta, 35 (1990) 639. [8] D. Aurbach, I. Weissman, On the Possibility of LiH Formation on Li Surfaces in Wet Electrolyte Solutions, Electrochem. Commun. 1 (1999) 324. [9] P.G. Glugla, in: Proceedings of the Symposium on Power Sources for Biomedical Implantable Applications and Ambient Temperature Lithium-Batteries. PV 80-4, The Electrochemical Society Proceedings Series, Pennington, NJ (1980) pp. 407 – 416. [10] E. Levi, E. Zinigrad, H. Teller, M.D. Levi, D. Aurbach, J. Electrochem. Soc. 145 (1998) 3440.
Short communication
On the role of water contamination in rechargeable Li batteries D. Aurbach a,*, I. Weissman a, A. Zaban a, P. Dan b a
Department of Chemistry, Bar-Ilan Uni6ersity, Ramat-Gan 52900, Israel b Tadiran Batteries Ltd., P.O. Box 1, Kiryat-Ekron, Israel Received 30 March 1999; received in revised form 2 July 1999
Abstract Many cathode materials such as LiMnO2 can be highly hygroscopic and thus, introduce considerable water contamination into Li batteries. Water reacts with the Li anode, and this strongly affects its surface chemistry. In this work, we investigated some phenomena related to water contamination due to the cathode material in LiLix MnO2 cells containing 1–3 dioxolane/LiAsF6 solutions. When these cells contain cathodes which were exposed to air, their electrolyte solutions become contaminated with water, which reacts with lithium and thus, hydrogen gas is formed. We discovered that discharging cells containing wet cathodes stops this liberation of hydrogen. We explored several possible explanations for this phenomenon. It was concluded that lithiation of water-containing Lix MnO2 considerably inhibits the liberation of water into the electrolyte solution. The effect of the presence of water in solutions on the properties of the Li anode is discussed. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: Water contamination; Li batteries; Electrolyte solution
1. Introduction
within the surface films are reduced by the active metal. The following 3 reactions are possible:
It is well known that electrolyte solutions for Li batteries unavoidably contain water contamination. The best commercially available Li battery grade electrolyte solutions contain not less than 20 ppm water. Water at any concentration in solutions reacts with lithium. Most of the surface species covering Li electrodes in nonaqueous electrolyte solutions are highly hygroscopic (organic or inorganic insoluble Li salts) [1]. Hence, water in solutions hydrates the surface films on lithium electrodes and diffuses through the surface films towards the Li-film interface. The water molecules
H2O+ Lio LiOH+ 12H2
(1)
LiOH+ Li Li2O+ H2
(2)
* Corresponding author. Tel.: +972-3-531-8317; fax: + 972-3-535-1250. E-mail address: [email protected] (D. Aurbach)
o
H2 + Lio LiH
1 2
1 2
(3)
The presence of hydrated species, as well as the products of the above reactions in the surface films, obviously affects the behavior of the Li anode that is controlled by the surface films covering it [2]. The major source of water in Li batteries is not necessarily the electrolyte solutions. Many cathode materials may be highly hygroscopic, thus delivering a considerable amount of water into the batteries, and contaminating the electrolyte solutions with a large amount of water. In recent years, we studied intensively
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LiLix MnO2 3V battery systems in which the electrolyte solution was 1–3 dioxolane (DN)/LiAsF6 [3–7]. A few years ago this rechargeable battery system became a commercial product [5]. The behavior of Li electrodes in 1–3 dioxolane is unique. The surface films formed on the active metal in these solutions are composed of HCOOLi, ROLi species (e.g. CH3CH2OCH2OLi and possibly LiCH2CH2OCH2OLi) and oligomers of DN with OLi edge groups [5–7]. Salt reduction reaction also contributes to the Li surface chemistry and hence, the Li surface films also include species such as LiF and Lix AsFy (x=0–3, y=0–2) [5–7]. This matrix of surface species forms surface layers which induce highly smooth and uniform Li deposition dissolution, and thereby, Li cycling efficiency in these solutions is very high. Tributylamine at hundreds to thousands of ppm is an important ingredient in these solutions because it prevents the easy polymerization of DN (via an acidic mechanism) by trapping trace Lewis acids which are unavoidably present in solutions (e.g. AsF3) [6]. The behavior and structure of the Lix MnO2 cathodes was also extensively investigated. As already reported, it is produced by a thermal reaction between MnO2 and LiNO3 that produces Li0.3MnO2, which can insert lithium at around 3 V (Li/Li+) up to a stoichiometry of LiMnO2 [3]. This cathode material is hygroscopic and if it is not dried properly, it can introduce pronounced water contamination into the battery. Taking into account the high electrode surface/solution volume ratio in practical
batteries, wet cathodes can contaminate the electrolyte solutions in practical batteries by thousands of ppms of water which can react further with lithium, thus producing dangerous hydrogen. The goal of this work was to study possible routes of contamination of these Li battery systems by water through the cathode, and to investigate possible reactions of this water contamination.
2. Experimental 1 – 3 Dioxolane (Li battery grade) was obtained from Tomiyama Co. (Japan), and was further distilled over Li-benzophenone (blue solutions). LiAsF6 was obtained from Lithco Inc., and was used as received. LiAsF6/DN solutions were stabilized by tributylamine (500 – 1000 ppm). Lix MnO2 cathodes were prepared as already described [3]. LiLix MnO2 cells in a jelly-rolled configuration were prepared for this study. The electrodes were a Li foil (Foote Mineral Inc.) anode and a Lix MnO2 cathode, separated by a Celgard polypropylene separator (single electrode area was about 100 cm2). These cells were measured in special gasometers to which a solution was introduced after the cells were loaded. The gasometer described in Fig. 1 allowed the measuring of hydrogen evolution during electrochemical processes of the cells. Impedance spectroscopic studies of Li electrodes freshly prepared in solutions were already described [4]. All preparations for the spectroscopic studies, as well as the LiLix MnO2 cell measurements were carried out under highly pure argon atmosphere in VAC glove boxes (which included H2O and O2 trace gas absorbers from VAC). The amount of water in solutions was measured by Karl Fischer titration (Metrohm 562 CF coulometer). SEM measurements were performed using the JEOL JSM 840 microscope. Li samples were introduced from the glove box to the microscope using a transfer method which prevents their contamination with atmospheric components, as already described [6,7]. In brief, we built a transfer system in which Li samples are loaded and hermetically closed in the glove box. This transfer system is then connected to the electron microscope, and the samples are introduced into it after high vacuum (10 − 6 torr) is achieved.
3. Results and discussion Fig. 1. A sketch of the gasometer used in this work for studying H2 evolution during processes of LiLix MnO2 cells. (1) Glass vessel. (2) A rubber cap. (3) Lix MnO2 cathode/separator/Li anode cell in a jelly rolled configuration. (4) Solution. (5) Electrical contacts (Pt wires sealed in glass). (6) Gas bubbles and volume. (7) Solution level in the gasometer (liberated gas volume reading).
The first subject of this study relates to the impact of water contamination on the performance of the lithium anodes in terms of morphology, impedance and cycling efficiency. Especially important is knowing what concentrations of water could be considered as critical to the Li anode’s performance. As already reported [5 – 7],
D. Aurbach et al. / Electrochimica Acta 45 (1999) 1135–1140
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Fig. 2. SEM micrographs of Li electrodes after being cycled in wet DN/LiAsF6 1 M/TBA (1000 ppm) solutions. 10 galvanostatic charge-discharge cycles at 1.5 mA/cm2, 0.5 C/cm2 per process (0.55 C/cm2 per last half cycle), a, c, e — last process was Li dissolution= b, d, f — last process was Li deposition. a, b—100 ppm H2O, c, d — 600 ppm H2O, e, f — 2000 ppm H2O.
we measured extensively the behavior of lithium electrodes in dry and wet DN/LiAsF6/TBA solutions. When Li is freshly exposed to H2O contaminated solutions, water reduction profoundly influences its surface and the interfacial impedance increases considerably during storage. When a Li surface is freshly prepared in dry ( B30 ppm H2O) DN/LiAsF6 solutions, it reaches full passivation in less than 3 h and then, the interfacial impedance remains unchanged during prolonged storage (weeks). When DN/LiAsF6 solutions are contaminated with water after their passivation is reached, the impact of the presence of water is much less pronounced compared with Li electrodes freshly prepared in wet solutions. The critical water concentration in this respect is around 1000 ppm. Below this concentration, water reaction with Li is very moderate and is limited by a relatively slow H2O diffusion through the surface
films. When the water content in DN/LiAsF6 solutions exceeds 1000 ppm, the Li passivity breaks down. Then a pronounced reaction between Li and water occurs even when the water contamination appears after the Li electrodes are already covered by stable, passivating surface films (originating from reactions between the Li and solution species). The above results correlate well with morphological and electrochemical studies of Li electrodes (initially covered by surface films) in wet DN/LiAsF6 solutions. Fig. 2 shows SEM micrographs of Li electrodes cycled 10 times in wet DN/LiAsF6 1 M/TBA (1000 ppm) solutions. The charge involved in each process was 0.5 C/cm2, and the current density (galvanostatic process) was 1.5 mA/cm2. Micrographs a, c and e relate to electrodes whose last process was Li dissolution, while micrographs b, d and f relate to electrodes whose
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last process was Li deposition. Micrographs a and b relate to a solution containing 100 ppm of water. Micrographs c and d relate to 600 ppm water in solution, and 2e, f relate to 2000 ppm H2O in solution. These micrographs clearly show that the Li morphology upon cycling in these solutions was very smooth, even when the water concentration reached 600 ppm. This correlates well with Li cycling efficiency measurements in these solutions (performed as already described [6,7]). The cycling efficiency of electrodes prepared in dry LiAsF6/DN/TBA solutions followed by cycling in wet solutions was high ( \90%), even when the water content was as high as 600 ppm. Hence, as already discussed [5–7], the high Li cycling efficiency usually obtained in DN solutions can also be achieved in solutions contaminated by several hundred ppm of water, provided that the surface films that passivate the Li electrodes are formed initially in dry solutions. Such
Fig. 3. H2 evolution as a function of time of LiLix MnO2 cells in DN/LiAsF6/TBA solutions. One cell was stored at open circuit, the other cell (as indicated) was processed as follows: three hours of discharge at 1.2 mA/cm2, rest for 20 h at OCV, then 3 h of charging (1.2 mA/cm2) and rest at OCV. The cathodes were exposed to normal air (and thus absorbed humidity) before the cells were constructed. (The active material Li0.3MnO2 initially contained 1500–2000 ppm of water by weight). The upper picture shows the experiment during its entire time scale, while the lower one describes the first 80 h of the experiment.
surface films possess a high degree of passivity against water diffusion, and a massive reaction between Li and water solutions is largely avoided. A pronounced reaction between H2O and lithium (which is accompanied by H2 evolution) occurs only when the Li surfaces are freshly exposed to H2O-contaminated solutions, or when a Li surface covered with passivating surface layers (formed in dry DN solutions) is in contact with wet solutions containing more than 1000 ppm of H2O. The next stage of this study was related to the behavior of LiLix MnO2 cells whose cathodes contained water. Li0.3MnO2 cathodes were stored for different periods of time in air, and were thus allowed to absorb water. Then, completed cells, in a jelly-rolled configuration (i.e. a Li anode, a cathode and a separator in between, rolled in a parallel plate configuration) were loaded into the gasometer (Fig. 1). LiAsF6(1 M)/DN/TBA (1000 ppm) solution was introduced and the amount of gas evolved was measured as a function of time and the electrochemical process conducted. The gas was identified as pure hydrogen by mass spectrometry. The amount of water contained in the wet cathode was estimated by wetting 10 cc of solution, followed by KF measurements. It appears that the wet cathodes (AA cell, 100 cm2, about 4 gr of Li0.3MnO2 per cell) introduced 650 ppm of water into 10 cc of solution. This corresponds to a water contamination of 1500 – 2000 ppm (by weight) in the cathode material. Since the amount of solution contained by the separator (between the Li and the cathode) is around 2 cc, the water contamination in solution which affects the lithium anode can be as high as 3000 ppms of water. It appears that cycling the cell (charge – discharge) at current densities ranging between 0.3 – 1.5 mA/cm2 stopped the gas evolution. Further studies revealed that the first discharge process of the cell in which the Li0.3MnO2 cathode is lithiated and Li dissolves from the anode, also considerably decreased the H2 evolution. Fig. 3 presents typical H2 evolution (gas volume versus time) for a cell stored at OCV, which is the reference measurement, and for a cell during discharge and charge at constant current. In the specific experiment described in Fig. 3, the cell was discharged for 3 h (1.2 mA/cm2), stored for 20 h at OCV, and then discharged for 3 h (1.2 mA/cm2) following storage at OCV. It is significant that for this cell, the initial rate of H2 evolution was higher than that of the cell stored at OCV (probably due to exposure of reactive lithium to the wet solution). However, as discharge proceeded, the gas evolution diminished considerably. We conducted several experiments similar to the one described in Fig. 3, at different current regimes and sequence of processes. There is no doubt that the discharge process alone was enough to stop massive gas evolution. These experiments imitate dangerous situations in Li batteries in which the cathodes may become a source of
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water due to inefficient drying. This water may contaminate the solution at high concentration and thus can be involved in a pronounced reaction with Li, thus liberating a considerable amount of hydrogen. This may build up a pressure in the cell, which is dangerous by itself due to the flammability of the gas. Of particular importance was the fact that the discharge of these cells stopped the H2 evolution. This is a very interesting result, and we can offer a priori three possibilities to explain this effect: 1. The cell discharge involves an anodic process of the Li electrode (Li dissolution). This process exposes fresh, reactive lithium that reacts directly with the H2 formed by water reduction. LiH formation prevents evolution of hydrogen gas. In contrast, the water reduction in the cell stored at OCV occurs within existing surface films. Hence, there is no exposure of fresh Li, which is sufficiently reactive to react with H2. 2. The anodic process of lithium in these solutions changes the surface chemistry so that the new surface species formed are much less hygroscopic that the original ones, and they thus block H2O diffusion to the active metal. 3. Lithiation of the cathode forms materials, which are much more hygroscopic than the solution. Hence, water is trapped irreversibly in the lithiated cathode. Considering the first and second possibilities, we studied possible reactions of Li with H2 and DN/ LiAsF6 solutions contaminated with water using surface sensitive FTIR spectroscopy. The effect of anodic processes (Li dissolution in these solutions was also explored in this study [8]. We did find some evidence that LiH may be formed at ambient temperature when fresh lithium surfaces are exposed to hydrogen [8]. It is possible that fresh lithium is indeed exposed to hydrogen during Li dissolution in wet Li salt/non-aqueous solutions. In such a process, lithium is depleted by the anodic process in the interface of the active metal with the surface film, forming holes which expose fresh Li surfaces to solution species. In parallel to such a process, H2O is continuously reduced by lithium to form hydrogen. Hence, H2 can easily percolate to the holes formed by Li dissolution and hence, a reaction between freshly prepared Li and H2 can take place. However, our spectral studies revealed that even if LiH is formed, it is not a major surface species formed on lithium in wet ethereal solutions [8]. Hence, LiH formation can not explain the pronounced depletion of H2. In addition, our spectral studies clearly showed that the surface chemistry developed on lithium in wet ethereal solutions during Li dissolution does not differ much from the usual Li-surface chemistry at open circuit voltage. The surface films covering lithium in wet DN/ LiAsF6 solutions contain the same ROLi species, HCOOLi and LiF species formed in dry solutions, and,
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of course, Li2O and LiOH (and maybe also LiH), which are the products of water reduction. The latter products are even more hygroscopic than ROLi or HCOOH. This rules out the second possibility mentioned above. Hence, the only possible explanation for the depletion of H2 evolution during discharge of the LiLix MnO2 cells as described above (Fig. 3) is that water is irreversibly trapped in the cathodes when they are lithiated. Further structural studies of LiMnO2 cathodes during discharge (Li insertion) in wet solutions are in progress and are beyond the scope of this short communication. The last point examined in this work relates to the possibility that water in DN/LiAsF6 solutions is oxidized by the cathode (OCV =3.4 V versus Li/Li+). Such oxidation may form H+ which can either lead to polymerization of the solvent (which, as an acetal is highly sensitive to acids [9]), or can diffuse to the anode and react on the Li surface. Such a possibility was ruled out by comparing the composition of wet solutions stored over cathodes, or polarized to 3.4 V with noble metal electrodes, with that of the pristine wet solutions (by GCMS). The mass spectrometry of the treated solutions was nearly identical to that of the reference solution.
4. Conclusion We studied some aspects of the impact of water contamination on the behavior of Li battery systems. There is a big difference between a situation in which fresh Li is exposed to wet DN/LiAsF6 solutions and the exposure of lithium already covered with surface films to these solutions. In the former case, the impact of water on the Li surface chemistry is very pronounced at any concentration. As found in parallel studies, LiH appears as one of the possible products of the reaction between Li and trace water. However, when the solutions are contaminated with water after the Li is already covered by surface films, up to 1000 ppm, the impact of water on the Li surface chemistry is not pronounced, because the surface films formed in DN solutions passivate the active metal. Hence, water reactions are controlled by its slow diffusion through the surface films. When the water concentration exceeds 1000 ppm, the passivity of Li in solution is not steady enough to prevent a massive reaction of Li with water which liberates hydrogen. Such a situation may occur in practical Li batteries in which the cathode material (which is usually hygroscopic) accidentally absorbed water. Since the solution volume in practical batteries is usually small, the solutions can be contaminated by H2O above the critical concentration at which the passivity of the Li anode is broken down. Then, the cells can
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develop a considerable pressure of H2 (due to the reaction of Li with water). In the case of Li0.3MnO2 cathodes (whose structure has already been described [10]) which are hygroscopic, their lithiation (discharge) seems to diminish water liberation into the solutions, probably because the lithiated cathode material irreversibly traps hydrated water in its lattice. This work ruled our the possibility of water oxidation on Li0.3MnO2 cathodes (OCV :3.4V).
Acknowledgements This work was partially supported by Tadiran–Battery Division, the Belfer Fund for studies related to energy storage and conversion (Israel), and the Israeli Ministry of National Infrastructure and Energy.
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