Electrochimica Acta, Vol. 38, No. 9, pp. 1221-1231, 1993 Printed in Great Britain. 0013-4686/93 56.6.00 + 0.W 0 1993. Pergamon Press Ltd THE Li 1 +,Mn204/C ROCKING-CHAIR SYSTEM : A REVIEW J. M. TARASCON and D. GUYOMARD Bellcore, Red Bank, NJ 07701, U.S.A. (Received 15 October 1992) Abstract-The new emerging rechargeable battery technology, called “rocking-chair” or “Li-ion”, that uses an intercalation compound for both the positive and negative electrodes is safer than the battery technology using pure Li metal. In this paper we review our study of the Li-ion type battery based on the spine1 Li, +xMn204 positive electrode and the negative carbon electrode. First we give a brief history of rocking-chair batteries, followed by a description of how to select and optimize intercalation materials for such a type of cell and then we present the main findings which allowed us to bring this battery system from concept to realization. Among the main findings are: (1) the use of the second Li intercalation plateau of the spine1 as a Li reservoir; (2) the discovery of a new electrolyte composition resistant against oxidation up to 5 V; and (3) the ability to reversibly inter&ate 0.9 Li ions into graphite at high rate with a small reversible loss. Based on these findings, AA research prototype cells with enhanced safety charac- teristics were constructed. We report on the performance of these ceils, compare our results with existing or emerging battery technologies such as NiCd or Ni metal hydrides, respectively, and discuss problems inherent to the Li-ion technology. Key words: intercalation, battery, Li-ion. INTRODUCTION For the last two decades an increasing interest in Li batteries (eg a battery that uses Li metal as the nega- tive electrode) has developed amongst battery manu- facturers. One reason for this is the high specific capacity (3800 Ah kg- ’ of lithium compared to only 260 Ah kg- ’ for lead). In addition, Li is also a highly electropositive metal, leading to high voltage cells. With this combination of properties, Li-based bat- teries are expected to have energy densities much larger than other systems. To date, only primary lithium cells (among them CuO/Li, MnO,/Li and FeS,/Li systems) have been commercialized. In contrast, the development of sec- ondary Li cells (eg cells that consist of an inter- calation compound as the positive electrode) has been slow because of safety problems associated with the use of Li metal as the negative electrode[l-41 and due to problems stemming from dendritic regrowth of Li upon cycling that might short-circuit the cell. A more advanced and inherently safe approach to rechargeable Li batteries, proposed a decade ago[S- 93, is to replace Li metal with a material able to reversibly intercalate Li ions. This has been called the “rocking-chair” battery because Li ions rock back and forth between the intercalation compounds during charge/discharge cycles (Fig. 1). The output voltage of such a cell is determined by the difference between the electrochemical potential of Li within the two Li intercalating electrodes. It is important to have as the positive and negative electrodes, cheap compounds which can reversibly inter&ate Li at high and low voltages, respectively. The Li inter- calation voltage for some intercalation compounds is shown in Fig. 2. Among the materials proposed for replacement of Li metal are LiAl, WO,, MoO,[lO], Mo,Se,[ll] or carbon[12-141, with the latter pro- viding the best compromise between large specific capacity and reversible cyling behavior. However, in rocking chair cells a price is paid in terms of average L&ion battery Cathode Charae Anode (Li4k@4) (carbon) Cathode Discharae Anode Wl.,Mn2O4) (carbon) Fig. 1. The charge and discharge mechanism for a Li-ion battery is shown (both the cathode and anode materials are intercalation compounds). 1221
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`1222 J. M. TARA~CON and D. GUYOMAIW output voltage and energy density when compared to a lithium metal cell; thus strongly oxidizing com- pounds (ie compounds which reversibly intercalate lithium above 4V) must be used as the positive elec- trode. So far, as indicated in Fig. 2, there are only three Li-based compounds, LiNiO,, LiCoO, and LiMn,O, that satisfy this requirement. These Li- bearing positive electrode materials are not moisture sensitive and can be handled in ambient atmospheres as can Li free carbon negative electrode materials. The Li-ion cell is assembled in its discharged state, where the output voltage is close to zero. As with the Ni-Cd cells, they need to be charged prior to use. Recently, Sony Energytec[15] announced the commercialization of a battery based on this concept. The Li-ion rechargeable battery developed by Sony used the layered LiCoO, intercalation com- pound as the positive electrode and a form of carbon (petroleum coke) as the negative intercalation elec- trode. Following up on this success, Moli Energy Ltd. (1990) is developing a battery using the same conceptC16, 171, but using the layered compound LiNiO, as the positive electrode instead of
`Herein we will review Bellcore’s work on rocking- chair type batteries using the Li,+,Mn,O, spine1 phase as the positive electrode and a form of carbon as the negative electrode[18-201. Our LiMn,O,/C rocking-chair system shows similar performances, to the Sony and Moli systems, with the exception that the specific capacity is about 5% smaller. However, the use of the spine1 man- ganese oxide material offers the following advan- tages: (1) a lower overall electrode cost coming from an easier synthesis (only one low temperature step) and the use of Mn which is naturally more abundant and cheaper than Co or Ni; and (2) the low toxicity, a well known characteristics of the widely used manganese-based oxide materials. A large number of procedures have already been developed for the recovery of MnO, from alkaline-zinc batteries[21] and there has been extensive work on the recovery of lithium metal from mineralsC22-J. Another advantage of the LiMn,O, system[23- 261 over the LiCoO, and LiNiO, systems is that Voltage vs Ll A 5 -- Voltage vs Lt A 4 -- liCOOi ~;%04 -- ‘,
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`0 Fig. 2. Electrochemical potential range of Li intercalation (as measured with respect to Li metal) for a wide variety of compounds are shown. LiMn,O, can intercalate a second lithium atom at an average voltage of 3V
`a potential at which Li-based materials should be barely moisture sensi- tive if synthesized under mild conditions). We will show how this second plateau can be used as a lithium reservoir to improve the specific capacity of these batteries by 10% (a property unique to the spine1 system) and detail a new way to synthesize Li,+.Mn20, powders of well defined composition, x. An problem inherent to rocking-chair technology, is the risk of electrolyte oxidation at high operating voltages (>4V). For instance, the voltage of the Li, +xMn,O,/Li couple is about 4.1 V and one would need to be able to charge the cell up to 4.5V to take full advantage of this redox system. This means that the electrolyte of choice for this rocking- chair system must be stable over a voltage window ranging from about 0 to SV. We have found a new electrolyte composition with such a range of stabil- ity. In this paper, we will illustrate and comment on the different problems faced in going from the idea of using the spine1 LiMn,O, as the positive electrode to the realization of rocking-chair prototypes based on this compound. EXPERIMENTAL The LiMn,O, powders were prepared by reacting a mixture of Li,CO, and MnO, powders in air, in an alumina crucible for three days, followed by two successive grinding and annealing sequences. An alternative route is the synthesis of LiMn,O, at lower temperatures ( < 600°C) via a sol gel route that consists in using LiOH and Mn acetates as precursors[27] leading to small particle sizes (< 2 pm). We have initially used petroleum coke and then graphite as intercalation materials for our nega- tive electrode. A one molar solution of LiClO_, salt dissolved in a SO/SO weight ratio of ethylene carbon- ate (EC) and diethoxyethane (DEE) was initially used as electrolyte. The above intercalation compounds are mixed with binders and highly conducting powders prior to use in electrochemical test cells. The composite elec- trodes are made by dissolving a small amount of binder, either ehtylene propylene diene monomer (EPDM) in a solution of cyclohexane, or polyvinyl- idene fluoride (PVDF) in a solution of cylopenta- none, and by adding the appropriate amounts of intercalation compound and black carbon to this solution. The resulting slurry is deposited onto an aluminum disk for the positive electrode, and onto a copper disk for the negative electrode and then dried at 150°C for 1-2 h. All the electrochemical measurements were carried out using Swagelock laboratory test cells. Depending upon the type of measurements performed, two or three electrode cells were used. A “Mac Pile” system[28] that can operate ether in a galvanostatic or potentiostatic mode was used to perform the mea- surements. In the galvanostatic mode, the output voltage of the cell was monitored while the cell delivered a constant current. From the elapsed time and the amount of current one can deduce the
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`RESULTS
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`The Li , +xMn,O,,/C rocking-chair system 1223 amount of intercalated species x in Li,Mn,O, or the capacity of the cell (a more meaningful number for battery manufacturers). In the potentiostatic mode, the current was monitored while the voltage was scanned at very low rate (lo-50 mV h- ‘).
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`The first step in the process of assembling a Li-ion cell consists in optimizing the electrochemical behav- ior (cycling performance and rate capability) of each composite intercalation electrode with respect to lithium over a wide temperature range. A survey of several (s, t, U) mixtures, where s, r and u represent, respectively, the weights in % of intercalation com- pound, binder and black carbon was carried out. We found that a percentage weight ratio of [89:10:1] and [94:5:1] gives the best result for the positive and negative electrodes when EPDM is used and [90:5:5] and [80:5:5] when PVDF is used. Figure 3a illustrates the cycling behavior of LiMn,O,/LiClO, based electrolyte/Li cells both at room temperature and at 55°C. At room tem- perature the cell was cycled between 3.4 and 4.5V, but at 55°C the charge cut-off voltage was limited to 4.25V because of risks of electrolyte oxidation. The spine1 phase LiMn,O, can rapidly and reversibly intercalate 0.8 Li per formula unit both at room tem- perature and at 55°C. However, note that only 0.9 Li per formula unit can be removed from LiMn,O, during the first charge, while the nominal composi- tion is 1. The cycling behavior of a petroleum coke/LiClO, based electrolyte/Li cell using an optimized compos- ite electrode is shown in Fig. 3b. After a subsequent loss of reversibility during the first cycle (25%), the Specific Capacity (mAh/g) carbon electrodes were found to rapidly and reversibly intercalate 0.5 and 0.55 Li per six carbons (Li,,,,C,) (at room temperature and at 55”C, respectively). The range of reversibility of our pet- roleum coke electrode was increased to x = 0.7 when the discharge cut-off voltage was decreased to 2mV vs. Li, but this voltage is undesirable for safety reasons, because it is too close to those required for Li plating. Thus, we have fixed 0.02V vs. Li as the lowest voltage limit that our negative electrode can reach in order to rule out any risk of Li plating. The delithiation of the spine1 occurring during the charge of a LiMn,OJElectrolyte/Li electrochemical cell to 4.3V vs. Li results in a I-MnO, phase. This phase was reported to be metastable and to decom- Dose uoon heating in air into a non Li-intercalable -__ $-Mnd, phase[2!?]. Temperatures ranging from 60 to 300°C have been reoorted for this conversion. The .___ conversion of the spin\1 could limit the usefulness of the LiMn,O, phase in a practical battery if it occurs at low temperature, so we investigated this phase decomposition by means of both differential scan- ning calorimetry (DSC) and X-ray diffraction mea- surements. We found, as shown in Fig. 4, that this phase transformation occurs at 180°C but can also be obtained at lower temperatures if lower heating rates are used. In contrast to previous reports, we did not observe a direct phase conversion to fl- MnO, , but rather an evolution of the I-phase to the E and then finally to the /I-MnO, phase. Finally, powders left at 55°C for three months were still pure I-MnO, , as determined by X-ray analysis, ensuring that rocking-chair cells based on this material could operate to 55°C without risk of failure due to an electrode phase conversion. In a rocking-chair cell the amount of positive and negative electrode materials has to be adjusted in a Specific Capacity (mAh/g) 4.6 3.4 El = EC + DEE(5O:SO) + 1 MLiC104 3.0 x in Li, Mnz 04 x in Lix C6 Fig. 3. The typical cycling behavior at 25 and 55°C for a LiMn,OJEC + DEE(50:50) + 1 M LiClO& (a) and a C (petroleum coke)/EC + DEE(50:50) + 1 M LiClOJLi (b) is shown. The spinel-based cell was cycled at a C/l0 rate in contrast to C/20 for the carbon-based cell. EC = ethylene carbonate, DEE = diethoxyethane.
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`I-MnO,) sample (see text). The phase transformation occurs at 195°C. The inset shows the X-rays taken at position marked by asterisks on the DSC curve. The ambient was air and the temperature rate of 2°C per min. way that the reversible capacity of each electrode is equalized for both safety and optimum performance. In short, with this technology it is necessary to balance the weight of the two electrodes so that when LiMn,O, is fully charged (ie delithiated) the negative intercalation electrode has to be fully lithiated with its voltage close to 0.02V vs. Li. To determine accurately such a ratio, we have used a three electrode Swagelock cell (Fig. 5a) where we simultaneously monitor the voltage of each electrode vs. a Li reference electrode as well as the output voltage of the cell, From these measurements we deduced that the optimum ratio is 2.1 (Fig. 5b). For such a ratio, the LiMn,O, spine1 phase is fully deli- thiated when the voltage of the negative electrode vs. Li reaches 0.02V. The effect of changing the ratio r on both the safety and performance behavior of a rocking-chair cell is illustrated in Fig. 6. Ratios greater than 2.1 may result in Li plating that could jeopardize the cell safety. Ratios smaller than 2.1 result in poor use of the rocking-chair system and decreases its safety, since the potential can reach values above that of electrolyte decomposition of the electrolyte. This situation is illustrated in Fig. 7 where the cycling behavior of a rocking-chair with a ratio of 1.7 is shown. The overcharge cut-off voltage is limited to 4.5 V (4.6 V being the electrolyte decompo- sition potential at room temperature). However, note that on charge the voltage of the cell stabilizes at 4SV because of the electrolyte oxidation, by the positive electrode. (The potential at the positive elec- trode is 4.8V vs. Li.) On subsequent cycles, even if the cell is kept lower than 4.4V vs. Li, the voltage composition curve shows lower capacity and larger polarization than a cell that has not been over- charged. Most likely, the electrolyte decomposition results in the formation of film at the positive elec- trode that increases the internal cell resistance. This trend is enhanced by further cycling until the cell has no more capacity. These results indicate that for rocking-chair cells using LiClO, based electrolytes, once the oxidation of the electrolyte has been initi- ated, it cannot be stopped even by lowering the over- charge cut-off voltage on subsequent cycles. An optimized cell with a ratio of positive to nega- tive electrode of 2.1 was assembled and its first cycles are shown in Fig. 8. The cell is cycled at a rate of C/5 between 2 and 4.35 V. After the first cycle, the differ- ence in voltage between charge and discharge is small indicating a good rate capability. The cell capacity decreases rapidly from the first to the second cycle and then levels off after the fifth and sixth cycle. This cell can reversibly intercalate 0.32 Li atoms per Mn atom at an average output voltage of 3.7 V, yielding an effective energy of 250 Wh kg-i (both positive and negative masses included). This is 2.5 times greater than that obtained with the well known Ni-Cd batteries. The above results indicated that rocking-chair cells can indeed be made with LiMn,O, as the posi- tive electrode. However, before such a system can be used in a commercial product several problems need to be solved in order to enhance safety and improve performance. The use of a LiClO, based electrolyte is not a viable option with respect to safety because of the explosive nature of the LiC104 salt. In addition, the narrow voltage window between the end of the lithium deintercalation in the spine1 and the begin- ning of the electrolyte oxidation, 0.2V at room tem- perature and OV at 55°C limits the capacity of our system at 55”C, and does not provide a safety margin large enough in case of overcharge. When a LiMn,O,/LiClO, based electrolyte/C cell is discharged to OV, the voltage of the negative elec- trode is equal to that of the positive electrode nearly 4V vs. Li (a LiMn,O,/Electrolyte/Li has an open circuit voltage of 3.5V). A reaction may occur
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`Diffraction angle (2 0)
`Fig. 4. Differential scanning calorimetric measurements (DSC) for a delithiated LiMn,O,
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`The Li 1 +xMn,O.,/C rocking-chair system 122.5 Counter Electrode - h )pli@k+: g 2- / lst charge ISt discharge 2”d charge 2 2 - ! 0.4 mAkm2 4 ‘\ ‘\ /’ ‘\ “k. I’ /’ /?a ‘\ ‘1 . . _________________~~~________________________--___ ‘-.C-_- I’ 1 I I I 1 I I I I 1 11 0 20 40 60 60 100 Time (hours) Fig. 5. The schematic of a three-electrode cell is shown in (a) and the first cycles at room temperature of a LiMn,OJEC + DEE(50:50) + 1 M LiClOJC (petroleum coke) three-electrode cell, under constant current, are shown in (b). The voltage measured at the positions denoted 1,2 and 3 on the cell schematic is plotted as a function of time in the bottom plot, curves 1,2 and 3, respectively. 4.5 1 LiMn,O, 5 . F 1 4 2 0.2 e 8 g 0.1 S i -A a Carbon 4 b V
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`2.1 (b) and r = 1.7 (c). For these experiments the ma% of the carbon electrode was maintained constant (30mg), the electrolyte used was EC + DEE (50:50) + 1 M LiClO, and the current rate was 0.4mA cm-‘. between the copper current collector and the C elec- trode at voltages greater than 3.4V vs. Li. Thus, a requirement for this rocking-chair system is the pro- vision of an overdischarge protection allowing the cell to operate safely. El = EC + DEE(SO:SO) + 1 MLiClOh c 25’C I I I I 1 0.2 0.4 0.6 0.6
`V I. I 0 _ ___-..____________---------- Ef YoGP%-kJ 50 60 Time (hours) Fig. 6. Voltage profiles vs. Li (as measured from a three electrode cell) of LiMn,O, and carbon electrodes at the end of the first charge and the beginning of the first dis- charge with different ratios r = 3.1 (a),
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`r =
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`in Li,MnzO4 Fig. 7. The cycling behavior of a cell using a weight ratio (positive to negative electrode) of 1.7 (51/30mg) is shown. The cell was cycled at 0.5mAcm-2 between 0 and 4.5V during the first charge and then between 2.5 and 4.35 on subsequent cycles.
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`1226 J. M. TARAKKBN and D. GUYOMARD
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`Specific Capacity
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`Fig. 8. Cycling behavior at room temperature of a LiMn,OJEC + DEE(S0: SO) + 1 M LiClO.,/C rocking- chair cell at a rate of C/10 with the mass of the positive electrode being 2.1 times that of the negative electrode. Finally, although the petroleum coke negative electrode gives the best compromise between capac- ity and cycling performance over other graphitic carbons it would be of tremendous benefit to improve the capacity of this negative electrode (ie using graphite instead of petroleum coke). The Li intercalation into graphite has been intensively studied for more than twenty years. Even the best results obtained so far show irreversible losses during the first cycle of 50% and poor rate capabil- ities, never exceeding C/30[15, 161. Such a large irre- versibility during the first cycle is not critical for cells having a Li negative electrode, however, in a rocking-chair type configuration where the amount of lithium is well defined by the amount of positive electrode, such a large loss is detrimental to per- formance. There is also a need to enhance the kinetics of the lithium intercalation process with graphite. In the next section we present our solutions to the above problems. Li reservoir within the spine1 LiMn,O, We have previously shown that our petroleum coke electrode exhibits a 25% loss in reversibility during the first cycle similar to all the rocking-chair systems using a carbon negative electrode. This implies that a large amount of Li from the positive electrode is lost during the first recharge of the rocking-chair cell. Our LiMn,Odpetroleum coke rocking-chair system has been optimized with a mass ratio of 2.1 which takes into account the 25% capac- ity loss at the carbon electrode during the first cycle. However, in these cells, an excess amount of positive electrode [(l + x)LiMn,Od material is used to provide the extra amount of Li that is lost on the carbon negative electrode during the first cycle. This excess reduces the gravimetric and volumetric capac- ities of these cells. A way to overcome this problem would be to use the spine1 structure with extra Li (Li 1 +.Mn,Od in such a way that the excess Li could be totally used to compensate for the 25% loss at the negative electrode. The construction of Li 1 +XMn,O,& instead of (1 + x)LiMn,O,/C would result in a lower mass ratio (<2.1), leading to enhanced gravimetric and volumetric capacities for the Li-Mn-0 based rocking-chair system. The spine1 LiMn,O, possesses a second reversible lithium intercalation plateauC23, 241 located at 3V and we propose to use this second lithium inter- cation plateau as a lithium reservoir[18]. Indeed, by increasing the amount of Li in Lil+,MnzOd, the value of x may be adjusted to compensate for the loss of Li associated with the negative electrode, barely affecting the weight of the positive electrode. To take advantage of this unique property of the spinel, it is necessary to prepare air-stable Li ,+xMn204 powders so that the manufacturing cost will remain unchanged. The synthesis of Li, Mn,O, has been reported[30] using n-BuLi as a reducing reagent and it was claimed that the reacting material was moisture sensitive. We identified the use of a strong reducing reagent such as n-BuLi as the source of the problem. We have tried a mild reducing reagent, such as LiI, and demonstrated the possibility of preparing air-stable Li,Mn,O, powders. Our chemical reaction simply involves refluxing LiMn,O, powders in an acetonitrile solu- tion of LiI for several hours. The key advantage of this chemical reaction is that it allows preparation of Li,+,MnzO., powders of well defined composition x as described in detail elsewhere. By fixing the amount of LiMn,O, powder and the volume of acetonitrile and by using the Nernst equation, the redox potential of the solution at any state of the reaction x, and the amount of LiI needed to form a Li 1+xMn204 powder of defined x may be calculated. Figure 9 illustrates this. Note that the reduction reaction can only occur when the redox potential of the solution is lower than the one of the LiMnzOJLi,Mn,O, plateau. Finally, it should be possible to scale this process up. Using the three electrode cell, we determined that the right amount of excess lithium in Li,+.Mn,O, to compensate for the loss associated with the nega- tive electrode during the first cycle is x = 0.1. The behavior of a rocking-chair cell using Li,.,MnlOd
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`2.0 ’ I I I I I I 0
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`1 /I I. I3 III”‘111 ‘%o 0.2 0.4 0.9 04 I.0 ,.a I .4 x in Li,+JJaCI Fig. 9. A calculation of the Nernst equation at 82°C for the reaction: LiMn,O, + 3x/2 LU -+ Li, +,Mn204 + x/2 LiI, has been done in order to determine the amount of Lil needed lo prepare Li,+,Mn,O, powders of any wanted lithium excess (x). The amount of LiMn,O, has been fixed at 4g, while nx4g of LiI is used in 100cm3 of acetonitrile. The dashed curve represents the voltage of the LiMn,OJLi,Mn,O, phase transformation measured at 82°C.
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`The Li
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`The Li , +Jvfn,O,/C rocking-chair system 1221
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`composition
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`reservoir I I I I I 1 I ’ 1 0 20 40 60 80 ,Time (hours) Fig. 10. The first cycles at room temperature of a Li,,,Mn,OJEC + DEE(50:50) + 1 M LiClOJC (petrol- eum coke) cell cycled between 4.35 and 2.3 V, as measured with the three-electrode configuration. The solid curve rep resents the output voltage of the cell, while the dashed and the dot-dashed curves represent, respectively, the voltage vs. Li of the LiMn,O, and the carbon electrode. powder as the positive electrode is shown in Fig. 10. We observed the removal of 0.1 Li at 3V during the first cycle, and subsequent cycles without any further loss in capacity. The use of the Li reservoir results in an increase in the cell capacity by 10%. In addition, this extra lithium will limit the voltage of the nega- tive electrode to 3V (vs. Li) when discharging the rocking-chair cell to OV, and thereby acts as a safety switch for these cells in case of overdischarge. New electolyte
`We showed that 1 M LiC104 in a 50: 50 mixture of ethylene carbonate (EC) and diethoxyethane (DEE) electrolytes begins to undergo oxidation at 4.5V, and 4.3V at 55°C. The electrolyte oxidation was found to result in solid and gaseous products, with the solid products forming a film that increases the polarization of the cell and decreases its output voltage, while the gaseous products increase the risk of explosion. Thus, to increase the cycle life of these batteries and to improve their temperature per- formance it is necessary to minimize the electrolyte oxidation. This search is purely empirical since the catalytic activity of the positive electrode material against electrolyte oxidation cannot be predicted. Numerous electrolyte compositions containing various ratios of salts, solvents and cosolvents were investigated for their ionic conductivity and resist- ance to oxidation. From this study a new LiClO,- free electrolyte composition, resistant against oxida- tion to 5 V, compatible with carbon (both petroleum coke and graphite) and with high ionic conductivity over a wide temperature range was discovered. The oxidation current for this proprietary new electrolyte under highly oxidizing conditions (up to voltages of 5 V) is compared (in Fig. 11) to the oxidation current of our previous electrolyte under similar test condi- tions. In both plots, the peaks located at 4.0 and 4.15V correspond to the removal of Li while the large increase of the current at 4.5V at room tem- -3.0 4 4.2 4.4 4.6 4.6 5 Voltage (V) 0.14 0.12 ? E 0. I z 0.08 r 0.06 5 ” 0.04 0.02 0 3.8 4 4.2 4.4 4.6 4.6 5 Voltage (V) 1s 1 :c I I ; i = 10 i /i‘L I t : /; : i h: E 5. / t 2 0 0 :.dM/ j .* \/ \ / ‘;, : V’ -5 I 4.2 4.4 4.6 1.8 5 5.2 Valt6g6 (V) Fig. 11. Comparative plots of cell current against charging voltage at both room temperature (a) and 55°C (b) for two Lit +,Mn,O,/Li cells using our prior LiCIO, based electro- lyte (dashed curve) and our new LiCO, free electrolyte (solid curve). The voltammogram in the high voltage range 4.3 and 5.1 V for a Li 1 +xMn,OJLi cell using the new elec- trolyte is shown in (c). perature (Fig. 1 la) and at 4.3 V at 55°C (Fig. 1 lb) for our LiClO,-based electrolyte corresponds to its oxi- dative decomposition. In contrast, this rapid increase of the current upon increasing voltage is not observed for our new electrolyte above 4.3V. Strik- ingly, the oxidizing current at 5 V remains very small even at 55”C, indicating an increased stability of this electrolyte against oxidation. However, several weak features can be observed on the I vs. V curve at volt- ages greater than 4.3V. These anomalies become more pronounced in Fig. llc where this part of the curve has been enlarged. This new set of reversible oxidation-reduction peaks, observed for the first time, corresponds to the removal of 0.06 lithium ion from the spinel. This result shows why it is difficult to remove more than 0.9Li from the spinel. The two capacity peaks are related to local structural defects
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`(ie
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`differ- ent environments for the Li ions) and their ampli- tude is extremely sensitive to the conditions of synthesis of LiMn,O, . The electrochemical per- formance of LiMn,O, was found to be closely tied to the relative amplitude of these two peaks. In short, they can simply be used as fingerprints for pre- paring LiMn,O, powders which will give the best electrochemical performance when used as positive electrode in a rocking-chair cell. A complete study
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`TARASCON
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`1228 J. M.
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`and D. GUYOMARD dwelling on the importance of these two peaks with respect to the optimization of the Li 1 +.Mn,O,/Electrolyte/C system will be described elsewhere[31]. Besides allowing a better understanding of the importance of the synthesis conditions on the elec- trochemical behavior of Li, +,Mnz04 with respect to lithium, this newly discovered electrolyte has also increased the cycle-life of these rocking-chair bat- teries. Figure 12 shows the capacity plotted as a function of the cycle-number both at room tem- perature and at 55°C. It is remarkable to note that the cell capacity of a Li 1 +XMn,OdElectrolyte/C cell, charged and discharged at a rate of C/2 between 2 and 4.7V, is still higher than 60% of the initial capacity after 2300 cycles. Even at 55°C when cycled under similar conditions, the loss in cell capacity is less than 20% of the initial capacity measured after the fifth cycle.
`
`Carbon vs. graphite
`
`The effective use of graphite as an intercalation material in a Li battery has been prevented by the inability of finding an electrolyte that: (1) minimizes the large irreversibility of the Li intercation process into graphite during the first cycle; and (2) allows for a fast intercalation rate. The best results so far were reported by J. Dahn
`(Fig. 13a) which shows successful intercalation of two lithiums per six carbons (“Li&“) during the first cycle and one reversibly on subsequent cycles at a rate of C/40. He suggested that the formation of a superficial passiva- tion layer on carbon (resulting from electrolyte reduction) controls the kinetics of the Li inter-
`
`et aI.[32]
`
`-0
`
`0.5
`
`1 .o
`x in Li,C,
`
`1.5
`
`2.0
`
`0
`
`0.2
`
`0.8
`0.6
`0.4
`in Li C_
`1 x
`
`~3w?--,c - 200 0 e ”
`
`100 -
`
`120
`
`100
`
`80
`
`60
`
`40
`
`20
`
`0
`
`0
`
`20°C
`
`500
`
`1000
`
`1500
`
`2000
`
`2500
`
`Discharge number P 0
`
`20
`
`0’ : : : : : : 1 0 0.5 1.0 1.5 2.0 2.5 Cycle Rate (hr..‘) Fig. 13. The previous state of the art for the electrochemi- cal intercalation of Li into graphite is shown in (a). In (b) the typical cyling behavior at 25°C of a graphite/new electrolyte/Li cell charged and discharged at a rated of C/15 is shown with (c) the capacity as a function of various discharging rates for the same cell. calation process into carbon, limiting the rate capa- bility of the C/Electrolyte/Li cell. In spite of the low rate capability, the 50% loss during the first cycle is not tolerable with the present rocking-chair tech- nology, because the amount of available lithium is defined by the amount of Li in the positive electrode. Thus, electrolytes that are compatible w