NL7906294A - ELECTROLYTIC ELEMENT AND CATHOD FOR IT. - Google Patents

Description

4 *

Rudolph Roland Haering, North Vancouver, British Columbia, Canada *

James Alexander Robert Stiles, of North Vancouver, British Columbia, Canada Klaus Brandt, of Vancouver, British Columbia, Canada.

Electrolytic element and cathode therefor

The invention relates to accumulators and in particular to a material for use as a cathode of such an accumulator or secondary electrical element. The object of the invention is to provide a material which, when used as a cathode in an accumulator or secondary electric cell, allows a high degree of reversibility when the cell is repeatedly charged and discharged. In particular, the object of the invention is to provide a relatively inexpensive material that can be easily prepared and used as a cathode in an accumulator.

It has been found that a lithium molybdenum disulfide (Li ^ MoSg) compound when used as a cathode in a lithium anode accumulator goes through a number of different operating phases.

During discharge of a newly manufactured accumulator (hereinafter referred to as "cross-phase 1"), lithium cations enter inside the cathode, increasing the concentration of lithium in the cathode. It has been found that the voltage of such accumulator decreases during discharge to a certain point where a plateau is reached The plateau is an area where the voltage of the accumulator remains constant as the concentration of lithium in the cathode continues to increase Once a certain concentration of lithium in the cathode is reached, the accumulator will continue to discharge under the conditions hereinafter referred to as "working phase 2." In working phase 2, the voltage of the accumulator may be reversibly increased or decreased within certain limits as the concentration of lithium in cathode 25 decreases or rises correspondingly. The method 2 overlaps the plateau before the transition from phase 1 to phase 2 takes place, if meaning that in phase 2 concentrations aa n Lithium in the cathode are observed equal to those observed during the transition from phase 190 to phase 2 to phase 2, but at higher voltages than at which the transition takes place. The transition between phase 1 and phase 2 appears not to be reversible across the plateau. Stage 2 represents a preferred stage of operation because of the excellent reversibility observed with accumulators equipped with cathodes which are conditioned to operate in stage 2. As discussed in more detail below, although the first discharge up to phase 2 can be performed at room temperature (for example at about 20 ° C), if the conversion is relatively fast, generally preferred (and when using relatively thick cathodes it may even be necessary) at operate at a lower temperature (e.g. 0 ° C).

If the voltage of an accumulator operating in phase 2 is allowed to drop to a certain level, a second plateau 15 is reached on which plateau the concentration of lithium in the cathode increases with constant voltage of the accumulator, to a third phase. (working phase 3) is reached where the voltage of the accumulator can again be varied reversibly as the concentration of lithium in the cathode increases or decreases. In the working phase 3, the concentration of lithium in the cathode can be decreased so that it overlaps with the values for the lithium concentration in the cathode found in the phase 2 and in the plateau where the transition from phase 2 to phase 3 takes place.

An accumulator operating in phase 3 does not appear to be as reversible as an accumulator operating in phase 2 and tends to lose its capacity more quickly with repeated charging and discharging.

However, in some applications, working in phase 3 may be preferable to working in phase 2, because the energy density in phase 3 is considerably higher.

The term "reversible" as used herein does not mean perfect, 100% reversibility.

The invention is described below in part with reference to the figure, which shows a graph of representative properties of an accumulator with a cathode prepared by applying a molybdenum disulfide (MoSg) on an aluminum foil substrate, with an anode of lithium foil and with a 1M solution of LiCIOin 790 6 2 94 * 3 * propylene carbonate as electrolyte. The voltage of the accumulator (measured in V) is plotted on the ordinate against the value "x" on the abscissa, "x" indicating the concentration of lithium in the cathode of the secondary element, which has the general formula LixMoS2.

The figure shows typical properties of an accumulator or secondary element equipped with a lithium anode and with a cathode, manufactured by applying molybdenum disulfide (MoS2) on a substrate of aluminum foil. "X" denotes the concentration of lithium in the cathode, which concentration increases as lithium cations are incorporated into the cathode during discharge of the accumulator. As will become apparent hereinafter, although the figure represents typical results, in practice the properties shown may vary slightly depending on various parameters.

It can be seen that the accumulator, when discharged from an initial voltage of more than 3V (3.3V is generally a typical value), traverses path AB. When discharged in this way, lithium cations are incorporated into the cathode as the voltage of the accumulator drops, increasing the concentration of lithium in the cathode 20 as shown in the figure. The path AB shows that the voltage drops from the initial value of approximately 3.3 V to a plateau (the path BD) while the value of x correspondingly increases from 0 to approximately 0.2. / This is a typical example of the behavior at room temperature. However, it has been found that at low temperatures (for example 0 ° C) the road AB starts to run much steeper than is shown in the figure, in which case the point B is located at a point where x is only slightly greater than 0. In some cases when discharged at room temperature, a point B was found which is at a value x = 0.5. However, it may be that in that case some electrolytic decomposition has taken place or some impurities were present.

The physical structure of a cathode involving a variation in the accumulator voltage at lithium concentrations in the cathode controlled by the path AB, where talk B is at a value of x ranging from a value slightly greater than 0 to x - 0.5, becomes referred to as "phase 1".

790 62 94 k

If the voltage of an accumulator operating in phase 1 is allowed to decrease along the road AB, a plateau, indicated in the figure by the road BD, is reached. In the figure, this plateau is at approximately 1.0 V - in practice, this plateau will generally be at room temperature at a voltage between 0.9 and approximately 1.1 V at room temperature 5, but it may even lie at very low temperatures at a voltage of approximately 0.7 V.

Although the plateau in the figure starts at x - 0.2, it will be clear from the foregoing that in practice the plateau can start at values for xdLe only slightly greater than 0.5 up to values of x * 0.5 .

The plateau BD ends in the figure at a point for which x is approximately 1.0. In practice, this endpoint can even occur with values for x of approximately 1.5. The reason for these variations is not entirely clear, but perhaps these variations are due to unknown impurities in the cathode. The plateau BD in the figure indicates an area where the accumulator operates at a relatively constant voltage of about 1.0 V, while the concentration of lithium in the cathode, represented by the value x, increases during the discharge of the accumulator.

As shown in the figure, if an accumulator operating in the plateau BD is further discharged, once a lithium concentration in the cathode is reached for which x = approximately 1.0, the accumulator is further discharged, the path DE 25 continues. The discharge of the accumulator can be stopped at any point along the road DE and the accumulator kancharna can be recharged in a practically reversible manner, traversing the road EC. The physical structure of a cathode giving a variation in the voltage of an accumulator when the lithium concentration of the cathode is determined by the pathway CE in the figure, indicated by "phase, 2" and the discharge of the accumulator with the pathway CE is referred to as "work phase 2". Once work phase 2 is reached, the accumulator will not return directly from phase 2 to the plateau BD.

It is also noted, however, that although Figure 35 is representative of the phenomena occurring, in practice 790 6 2 94 5 variations are observed. As shown in the figure, when following the path DE, the voltage drops from an initial value of about 1.0 to about 0.55 V while the value of x increases from about 1 to about 1.5. Likewise, according to the figure, when traveling through the road CE, the voltage drops from about 2.7 V to about 0.55 V as x increases from about 0.2 to about 1.5. In practice, for given voltages, the observed value of x is somewhat variable. For example, point C (voltage about 2.7 V) can range from the point where x is slightly greater than 0 (at low temperature) to the point where x is about 0.5.

10 Point D (voltage approximately 1.0 V) can lie between the point for which x is approximately 1.0 to the point for which x is approximately 1.6.

Point E (voltage approximately 0.55 V) may lie between the point for which x = approximately 1.3 and the point for which x 3 approximately 2.0. In all cases, however, the road CE slopes downwards to the right. The reason for the aforementioned variations is not clear, but even now it may be due to unknown impurities in the cathode. It is further noted that the said observations were made at room temperature.

At lower temperatures, the measured voltages for a given value of x are generally somewhat lower. The voltage of 0.55 V which applies in the figure for point E (and for the path EG discussed below) is typical for the situation at room temperature, but in general the respective voltage may vary from approximately 0.4 V - up to approximately 0.6 V.

It was observed that the most reliable reversible action of the accumulator occurs when the accumulator has a cathode conditioned to operate in phase 2, while it was further found that the most reliable reversible action in phase 2 occurs along the CD It has been found that when an accumulator operating in phase 2 is discharged along the road CE, the reversibility deteriorates when the voltage of the accumulator drops below a value of about 1V. Since the reversibility of the accumulator in phase 2 decreases when the accumulator is discharged below a voltage of about 1V, it is preferred that operation in phase 2 be limited to the path CD, due to the voltage of the accumula-35 tor to recharge above a voltage of 790 62 94 * · 6 of approximately 2.7 V and avoid discharging the accumulator to a voltage of less than 1 V.

If the voltage of an accumulator with a cathode explored in phase 2 is dropped along the veg Ce to about 0.55 V 5 (each voltage as noted above is typical for this condition), a second plateau, indicated by the portion EG, is reached (at a lithium concentration in the cathode approximately x = 1.5 for the representative case shown in the figure) over which plateau a transition occurs from phase 2 to a third phase at a relatively constant potential while the concentration of lithium in the cathode is shown increases by x to a value of x = approximately 2.8. It is preferred to keep the value of x equal to or less than about 3 when practicing the invention.

If an accumulator operating on the plateau EG is allowed to continue discharging, once a lithium concentration in the cathode indicated by a value of x = about 2.8 has been reached, it is found that the accumulator is further discharged according to the path GH. Discharging the accumulator can be stopped at any point along the road. The accumulator can then be recharged and the path HF is traversed. The physical structure of a cathode giving a variation in the accumulator voltage at varying lithium concentrations in the cathode, which is determined by the path FH in the figure, is referred to as "phase 3" and the reversible charging and discharging of the accumulator where the road FH is passed is indicated as "working in phase 3".

Once the state of working in phase 3 has been reached, the accumulator will not return directly from phase 3 to the plateau EG.

Point H in the figure does not represent the lower limit for the accumulator discharge power. However, a substantial deterioration in the behavior of an accumulator was observed that was discharged in phase 3 below a voltage of about 0.3 V. It is believed that this deterioration in operation is related to a diffusion of lithium ions into 1st aluminum substrate of the cathode leading to the formation of a lithium aluminum alloy.

It is believed that working in phase 3 is not as reliably reversible as working in phase 2. Furthermore, applicants have failed in phase 790 62 94 7 to achieve accumulator voltages that are as high as the voltages achieved when working with the accumulator in phase 1 or phase 2. As said before, this does not mean that working in phase 3 is always undesirable. The energy density of an accumulator in phase 3 is significantly higher than the energy density of an accumulator in phase 2. In cases where energy density requirements are more important than better reversibility and better voltage properties can work in phase 3 are more desirable than working in phase 2.

It has been found that if an accumulator operating in phase 3 is charged slowly there will be a transition from phase 3 to phase 2 when an accumulator voltage of approximately 2.3 V is reached.

A deterioration in the reversibility of the accumulator when operating in phase 2 and in phase 3 was observed when the accumulator voltage was dropped below about 1 V. It is believed that this deterioration in properties may be due to decomposition of the electrolyte of the accumulator. It is believed that if the problem of electrolyte instability can be overcome, excellent reversibility will be observed throughout the phase 20 2 region and improved reversibility can be observed upon phase 3 exploration.

X-ray diffraction analysis shows that the structure of the cathode in phase 2 is a layered structure with a different crystal symmetry than the structure that occurs in phase 1.

Example I

An accumulator or secondary electric cell was constructed in the following manner:

The cathode consisted of a piece of aluminum foil of about 2 cm 2 on which 3 mg / cm MoS 2 was deposited. The anode was a piece of lithium foil of similar dimensions. The electrodes were separated by a polypropylene separator made with an electrolyte consisting of 0.7 M LiBr in propylene carbonate (PC). The accumulator was discharged through phase 1 with a current of 1mA and further discharged through a first voltage plateau at approximately 1.1 V until the cathode passed into phase 2, then the accumulator became more than 790 62 94

If

X

8 hundred times charged and discharged in phase 2 with a current of 10mA, intermediate voltages up to 2.7 V corresponding to a phase 2 cathode fully charged and a voltage of 1.0 V. The capacity of the accumulator came corresponds to 1 electron per MoS ^ -5 molecule (Δχ = 1).

Example II

A secondary electrolytic element similar to that described in Example 1 was provided, except that the cathode was now only covered with 0.3 mg / cm @ 10 MoSg. The cathode was brought into the phase 3 state by discharging the element both through the first voltage plateau into phase 2 and further through a second voltage plateau at approximately 0.55 V until phase 3 was reached. The discharge current was 1 mA. The element was then charged and discharged more than a hundred times between a voltage of 2.1 V (which coincides with a fully charged cathode in phase 3) and 1.0 V, with a current of 1 mA. The capacity of the element corresponded to 1.5 + 0.2 electrons per MoSg molecule (Δχ ci 1.5 + 0.2)

The element was charged and discharged several times between a voltage of 2.1 V and 0.5 V. The capacitance of the element in this case corresponded to 2 + 0.2 electrons per MoSg molecule (Δχ * 2 + 0, 2). Example III

The element of Example II was slowly charged (with a current of about 100 mp A) to 2.7 V. It was found that after charging in this manner, the cathode was converted back to the phase 2-25 state.

Example IV

A secondary electrical element was built up in the following manner.

2

The cathode consisted of 1.3 cm aluminum foil on which was deposited 0.5 mg / cm MoSg. The anode consisted of a strut of lithium foil of the same dimensions printed on an expanded nickel grid. The electrodes were suspended in an electrolyte consisting of 0.7 M LiBr in PC contained in a 50 ml beaker. An argon atmosphere was introduced into the beaker via a neoprene stopper. This element was conditioned by first discharging with a current of 100 µm A and then charged twenty-eight times in the phase 2 state and discharged between a voltage of 2.7 V and a voltage of 1V, with an amperage of 100 jik. The element was then further discharged until it entered phase 3 where it was subjected to a charge and discharge cycle ten times between a voltage of 2th V and 0.5V.

Although the first discharge to achieve the desired conditioning in the previous examples was performed at room temperature (about 20 ° C) to obtain good results, it is generally considered beneficial to cool an element for the conditioning discharge. Otherwise, problems with electrolyte decomposition can occur. The cathodes in the previous examples were relatively thin and it was possible to perform the conditioning discharge at room temperature relatively quickly without developing an essential temperature gradient in the cathode. However, with thicker cathodes it becomes more desirable to cool. At 10 mg / cm MoSg, cooling appears to be essential. Although cooling temperatures as low as -20 ° C have been used, a temperature of 0 ° C has been found to be quite satisfactory for cathodes with a coating layer in a low weight up to about 20 mg / cm MoSg.

Phase action can also be achieved if the MoSg is partially oxidized to MoOg. Partial oxidation to MoOg can improve conductivity without a serious loss of capacity. Example V

A secondary electrical element was prepared in the following manner with a cathode containing MoSg partially oxidized to MoOg.

(a) MoSg powder with an average particle diameter of about 20 µm. was mixed 1: 1 by volume with propylene glycol 30 and a layer of the resulting suspension was applied to a substrate of aluminum foil.

(b) graze the substrate with the applied layer in an atmosphere containing about 0.1 mol% oxygen in nitrogen baked at 58 ° C for about 10 minutes, after which a cathode was obtained containing about 20 mol% MoOg and about 80 mol% MoSg contained.

790 6 2 94 10

An electric cell was fabricated using two stainless steel flanges separated by a neoprene rubber sealing O-ring. The anode consisted of a 6 cm lithium 2 plate. A piece of the 6 cm surface cathode prepared in the manner described above (to which approximately b3 mg of the partially oxidized MoSg was applied) was used as the cathode for the element. A polypropylene porous separator plate saturated with a 1 m solution of lithium perchlorate in propylene carbonate was placed between the anode and the cathode.

The newly manufactured element was conditioned by first discharging with a current of 4 mA to a lower cut-off voltage of approximately 0.85 V. During this initial discharge, the cell voltage dropped to a plateau at approximately 1 V in approximately 20 min. 15 then gradually declined linearly in an additional 2 hours to approximately 0.85 V.

The element so manufactured and conditioned was subjected to 66 discharge-charge cycles using a charge and discharge current of U mA and charged and discharged between a minimum voltage of about 0.85 V and a maximum voltage of about 2.7 V.

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Claims (15)

An electrolytic element comprising a lithium anode, a non-aqueous electrolyte and a cathode, characterized in that the cathode comprises a material of the chemical formula wherein 0 <x £ 3, and the cathode is conditioned to be suitable for reversible charging and discharging, by discharging the element to a first voltage plateau, further discharging the element over that first voltage plateau and further discharging the element to a voltage below the voltage value of the first voltage plateau but not less than 0.6 V . Electrolytic element according to claim 1, characterized in that the first voltage plateau is at a voltage between 0.7 V and 1.1 V. Electrolytic element according to claim 1, 15 or 2, characterized in that the first voltage plateau is between 0.9 V and 1.1 V. Electrolytic element according to one of the preceding claims, characterized in that the element is charged after conditioning to a maximum voltage of approximately 2.7 V. Electrolytic element according to any one of claims 1 to 3, characterized in that the discharge took place while the element was at a low temperature below room temperature. 6. Electrolytic element according to claims 1-3, characterized in that the discharge took place at a temperature between 25 ° C and -20 ° C. 7. Electrolytic element according to claims 1-3, characterized in that after discharge to the first voltage plateau the cathode was further conditioned by discharging the element to a second voltage plateau located below the first voltage plateau and the element was further discharged over that second voltage plateau and further was discharged to a voltage below the value of the second voltage plateau but not less than 0.3 V. Electrolytic element according to claim 7, characterized in that the first voltage plateau is at a voltage of between 0.7 V and 1.1 V and that the second voltage plateau is at a * 7 s s 790 62 94 .9 voltage between 0.4 V and 0.6 V. 9. Electrolytic element according to claim 7 or 8, characterized in that the first voltage plateau is at a voltage between 0.9 V and 1.1 V and the second voltage plateau is at a voltage between 0.45 V and 0.55 V. Electrolytic element according to claim 7, 8 or 9, characterized in that the discharge took place while the element was at a low temperature below room temperature. Electrolytic element according to claim 7, 10. or 9, characterized in that the discharge took place while the element was at a temperature between 0 ° C and -20 ° C. Electrolytic element comprising a lithium anode, a non-aqueous electrolyte and a cathode, characterized in that the cathode comprises a material of the formula Li Li MoS, wherein 0 <x <2, which cathode reversibly charges and discharges X ύ - allows the element to be between a voltage of about 2.7 V and 0.8 V, where x reaches a value of less than 2 greater than 1 when the element is discharged to a voltage of about 0.8 V. 13. Electrolytic element comprising a lithium anode, a non-aqueous electrolyte and the cathode, characterized in that the cathode comprises a material of the formula Li MoS ™ in which 0 <x <3, and the cathode reversibly charges and discharges the element between the voltages of 2.4 V and 0.5 V, where x reaches a value close to or approximately equal to 3 when discharging the element to a voltage of about 0.5 V. Electrolytic element according to claims 1 to 6 or 12, characterized in that the MoS 1 is partially oxidized to MoC 2 Electrolytic element according to claims 7-11 or 13, characterized in that the MoS 2 is partially oxidized to MoO 2. 30 Q \ 790 6 2 94