EP0063129A1

EP0063129A1 - Lithium batteries

Info

Publication number: EP0063129A1
Application number: EP81902829A
Authority: EP
Inventors: Alan James Parker; Pritam Singh; Eric John Frazer
Original assignee: Anumin Pty Ltd
Current assignee: Anumin Pty Ltd
Priority date: 1980-10-22
Filing date: 1981-10-21
Publication date: 1982-10-27
Also published as: WO1982001442A1

Abstract

Des cellules electrochimiques ont une anode contenant du lithium comme materiau a action electrochimique, et une cathode, cette anode et cette cathode etant immergee dans un electrolyte sensiblement anhydre exempt d'acides de Lewis et d'anhydride sulfureux et contenant de 0,1 M jusqu'a saturation de LiAsF6 dissout dans un melange solvant de carbonate de propylene et d'acetonitrile dans lequel l'acetonitrile constitue de 10 a 80% du volume combine d'acetonitrile et de carbonate de propylene, ce melange solvant consistant en au moins 80% en volume d'acetonitrile et de carbonate de propylene, le reste etant constitue eventuellement par un materiau liquide inerte.Electrochemical cells have an anode containing lithium as an electrochemically active material, and a cathode, this anode and this cathode being immersed in a substantially anhydrous electrolyte free of Lewis acids and sulfur dioxide and containing from 0.1 M up to 'at saturation of LiAsF6 dissolved in a solvent mixture of propylene carbonate and acetonitrile in which the acetonitrile constitutes from 10 to 80% of the combined volume of acetonitrile and propylene carbonate, this solvent mixture consisting of at least 80% by volume of acetonitrile and propylene carbonate, the remainder possibly consisting of an inert liquid material.

Description

TITLE "LITHIUM BATTERIES" DESCRIPTION The present invention relates to lithium batteries. BACKGROUND AND PRIOR ART

Lithium batteries typically have an anode comprising lithium as the electro-chemically active component, a cathode and a liquid electrolyte solution in which the anode and cathode are immersed. It is known to have lithium batteries with a liquid electrolyte solution containing a mixture of lithium perchlorate or lithium hexafluoroarsenate and propylene carbonate. However such systems suffer from low conductance and high polarisation. Lithium is a reactive material and reacts readily with a wide range of organic materials. Those skilled in the art of lithium batteries are faced with a choice of solvents from an enormous number of solvents and solvent mixtures.

One solvent which is in principle desirable for lithium batteries is acetonitrile. Acetonitrile has a high dielectric constant, an extremely low viscosity (0.35 cp) and is a moderately good solvator of lithium cations. Many lithium salts dissolve in acetonitrile to give highly conducting solutions. However, acetonitrile reacts rapidly with lithium and previously known lithium batteries containing acetonitrile have proved totally impractical because of the reaction between the lithium and the acetonitrile.

It is true that in a general sense acetonitrile and mixtures thereof with other liquids has been suggested as an electrolyte for lithium batteries. For example, U.S. Patent No. 3658592 disloses cells containing a particular type of positive electrode, a negative electrode comprised of any of the light metals such as Li , Na , K , Ca , Mg and Al , said electrodes being disposed in an electrolyte comprising an organic solvent selected from the group consisting of tetrahydrofuran, N-nitroso, dimethyl amine, dimethyl sulphite, propylene carbonate, gamma butyrolactone, dimethyl carbonate, dimethoxy ethane, acetonitrile, dimethyl sulphoxide, dimethyl formamide and the mixtures thereof; and having dissolved therein soluble salts of the light metals; for example, the perchlorates, hexafluorophosphates, tetrafluoroborates, tetrachloroaluminates and hexafluoroasenates of lithium. Similar disclosures are found in U.S. Patents Nos. 3679844, 3681143, 3681144, 3808052, 3877988, 3945848, 3998658, 4057679 and 4085256.

In U.S. Patent 3098770 there is disclosed an electro chemical battery comprising a substantially ahydrous electrolyte, consisting of a highly conducting organic liquid in the form of a non-aqueous solvent in which an inorganic compound constituting a Lewis acid in relation to the solvent, has been dissolved, and a set of compatible positive and negative electrodes constituting an effective electrochemical couple. The anhydrous electrolyte may be acetonitrile or a mixture thereof with a ketone.

Further, the negative electrode may be lithium. U.S. Patent No. 3829330 discloses a cell comprising a lithium anode and a Mo03 cathode, and offers a very wide choice of solvents, salts and mixtures, of unspecified composition. One combination given as an example is acetonitrile and propylene carbonate saturated with SO₂ and having dissolved therein LiBr. Lithium hexafluoroarsenate is mentioned as an alternative to the lithium haiide. Here the SO₂ is essential and plays two roles, one as a depolariser, the other to protect the lithium against attack by solvent. The combination of SO₂ and acetonitrile is used in some lithium batteries and the SO₂ is essential to prevent reaction of acetonitrile with lithium.

Finally, U.S. Patent No. 3468716 discloses an electrochemical system comprising an anode, a cathode and a substantially anhydrous electrolyte for electrolytic conduction between said anode and cathode, said electrolyte comprising a pentacyclic ester and at least one solvent selected from the group consisting of aliphatic ethers, cyclic ethers, nitroparaffins, cyclic ketones and aliphatic nitriles. The electrolyte may comprise a solute to improve the conductivity thereof said solute being preferably a Lewis acid and may additionally comprise a lithium halide. The anode may be lithium. The pentacyclic ester may be propylene carbonate. Of all of the above references only U.S. Patents

3829330 and 3098770 disclose workable systems containing a lithium anode and acetonitrile. The former relies on the presence of SO₂ which makes the cells of that invention non-recyclable and the latter relies on the presence of Lewis acids.

The remaining references contain extremely general and speculative disclosures which contain some combinations of solvents which will definitely not work, often because of reaction of the solvent with a lithium anode as shown in Table 1 and none of them teaches the skilled addressee a particular solvent combination which will work.

SUMMARY OF THE INVENTION It has now been discovered that a specific solvent and solute combination is stable in the presence of lithium electrodes, particularly substantially pure lithium electrodes, and this combination produces a cell that does not contain Lewis acids or sulphur dioxide.

In accordance with the present invention there is provided an electrochemical cell having an anode containing lithium as the electrochemically active material, and a cathode, said anode and cathode being immersed in a substantially anhydrous electrolyte free from Lewis acids and sulphur dioxide and containing from 0.1M to saturation LiAsF₆ dissolved in a solvent mixture of propylene carbonate and acetonitrile said acetonitrile forming from 10 to 80% by volume of the combined volume of propylene carbonate and acetonitrile and the solvent mixture consisting of at least 80% by volume, preferably at least 90% by volume, more preferably at least 95% by volume, acetonitrile and propylene carbonate with the balance, if any, being in the form of inert liquid material

DESCRIPTION OF THE INVENTION Preferably the electrolyte contains as liquids only acetonitrile and propylene carbonate. Further the solvent preferably contains from 20 to 60% by volume acetonitrile and at least 0.5M LiAsF₆. Preferred cathodes are titanium sulphides, copper sulphides, niobium sulphides, titanium selenides, copper selenides, niobium selenides, vanodium oxides and molybdenum oxides. The lithium anode can contain lithium itself as active material or the lithium can.be alloyed with suitable other metals such as magnesium or aluminium in which case the alloy is the electrochemically active material. The lithium material is typically supported on an inert electrically conductive material such as nickel. Prior to the discovery of the present invention that lithium was stable (as will be discussed in more detail hereafter) at ambient temperatures in the presence of mixtures of propylene carbonate and acetonitrile containing LiAsF₆, lithium batteries containing acetonitrile as solvent were totally impractical unless large amounts of SO₂ were present. They were not considered by those skilled in the art because of the rapid reaction of lithium with acetonitrile. For example, lithium reacts with acetonitrile containing a variety of soluble lithium salts including LiNO₃, LiCl, LiClO₄ and LiAsF₆ and also in the absence of any salt. Acetonitrile reacts with lithium metal when mixed in equal proportions with propylene carbonate containing LiClO₄ and when mixed in equal proportions with tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxy ethane or dimethylformamide all containing 1M LiAsF₆.

However, it is found that the specific combination of at least 0.1M LiAsF₆ at least 20% by volume propylene carbonate and acetonitrile unexpectedly provides an electrolyte solution in which lithium is stable. These results are summarised below in Table -1.

propionitrile; PC is propylene carbonate; THF is tetrahydrofuran; 2-MeTHF is 2-methyltetrahydrofuran; DMSI is dimethylsulfite; MF is methylformate; DMF is dimethylformamide; DMSO is dimethylsulfoxide; HMPT is hexamethylphosphoramide; DME is 1:2 dimeth oxyethane. Numbers before symbols refer to percent by volume of first component. b. Salt was 0.5 M. c. Where changes are noted, lithium and solvent continued to react after period noted. d. Similar result for 0.1 M LiAsF₆.

Table 2 shows the polarisations for cycling lithium between Li/Al alloys at 1 mA cm ^-2 in propylene carbonate (PC) and in solvents containing acetonitrile. In all cases the polarisations are less in the presence of acetonitrile than in pure PC, even after iR corrections.

Polarisations are least for LiAsF₆, in pure acetonitrile.

The advantages of lower polarisation are greater at

5 mA cm discharge. As expected from Table 1 , because of reactions of lithium with acetonitrile in most mixtures, cycling efficiencies are <90% in the presence of acetonitrile unless LiAsF₆is present together with propylene carbonate. Even with LiAsF₆. present, the efficiency is only 88% in pure acetonitrile because PC is needed also for high efficiency. With LiClO₄ as electrolyte, efficiencies are ca. 70% in the presence of acetonitrile for all solvents studied. versus Li/1M LiCIO₄ in PC reference electrode at discharge and charge. c . Abbreviations as in Table 1. d. 0.5M LiAsF₆. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: -

Figure 1 is a graph of the discharge of Li/MnO₂, Li/Cus, Li/NbSe₃ and Li/MoO₃ cells at 1 mA cm^-2 in various electrolyte solutions at ambient temperatures (discharge voltage versus utilization as coulombs per kg active cathodic material, electrolytes at 1M, mixtures are 50% by volume); Figure 2 is a graph of the discharge of Li/V2O₅ and Li/Cu₂S cells at 2 mA cm^-2 and of a Li/TiS₂ cell at 1 mA cm^-2 in various electrolyte solutions at ambient temperatures (discharge voltage versus utilisation as coulombs per kg of active cathodic material, electrolytes are 1M and mixtures are 50% by volume); Figure 3 is a graph of the discharge of Li/V₆ O _{1 3} cells at

1 mA cm ^-2 in LiAsF₆ solutions in propylene carbonate and in a 50/50 propylene carbonate/AN mixture at ambient conditions (discharge voltage versus utilisation as coulombs per kg of V₆O₁₃, electrolytes are 1M) ; and

Figure 4 is a graph of the discharge of Li/NbSe₃ cells at 1 mA cm^-2 in 1M LiAsF₆ in propylene carbonate and in 50/50 propylene carbonate/acetonitrile mixture at ambient conditions (expanded voltage scale over Figure 1) .

DETAILED DESCRIPTION OF THE DRAWINGS In the drawings, the cells used comprised a lithium anode pressed onto nickel gauze of area about 1 cm₂, the cellhaving a glass filter paper separator. The cathode was a powder of the active material (0.1 - 0.5 g) mixed with 5% teflon powder and 10% nickel powder and pressed between two nickel gauze discs. The cell was contained in a cylinder of polypropylene and electrical contact was via 2 brass screws.

Comparison of Figures 1 , 2 and 3 shows that the advantage of employing acetonitrile in a lithium battery is greater for some cathodes than others. Figure 1 shows the voltage of lithium batteries having cathodes containing various active materials which do not show especially great advantages in voltage when discharged at 1 mA cm^-2 of lithium anode in our new solvent mixture.

The utilisation of the cathodic material is recited as coulombs/kg and is better for some cathodes in the new solvent. The preferred electrolyte of the present invention of 1M to saturation LiAsF₆ in 50% V/V propylene carbonate-acetonitrile is compared with other electrolytes. Cathodes used in Figure 1 are Mn O₂, MoO₃, NbSe₃, and CuS. It can be seen that with the cathodes used in Figure 1 that LiAsF₆/PC/AN on discharge at 1mA cm^-2 does not provide a marked improvement in voltage discharge characteristics over certain other solvent mixtures. Indeed with a MnO₂ cathode it is an inferior electrolyte to LiClO₄ PC/DME or LiAsF₆/PC. However utilisation with MnO₂ and CuS as cathodes is better for the new electrolyte.

However, with cathodes in which the active material is Cu₂S, TiS₂ or V₂O₅ (Figure 2) or V₆O₁₃ (Figure 3) , the electrolyte of the present invention has clear advantages over LiAsF₆/PC or LiCIO₄. PC/DME when discharging at 1mA cm^-2 or 2mA cm^-2. The advantages include a markedly higher open current and discharge voltage with V₂O₅ and V₆O₁₃ and more effective utilisation of cathode material, especially with Cu₂S as cathode. The electrolyte of the present invention is particularly effective with V₆O₁₃, on all counts, as shown in Figure 3.

Figure 4 shows the behaviour of an Li/NbSe₃cell in PC and in 50% PC/AN containing 1M LiAsF₆. The electrolyte of the present invention again has some advantage but it is not as marked as with V ₂O₅ and V₆O₁₃.

Also, the new electrolyte has extremely advantageous conductance at 25° C as shown below: -

Electrolyte Conductance at 25° C 1M LiAsF₆ in 50% PC/AN 21.8 ohm^-1 cm^-2 mol ^-1 1M LiClO₄ in 50% PC/AN 15.5 ohm^-1 cm^-2 mol ^-1 Electrolyte Conductance at 25° C 1M LiAsF₆ in PC 6.4 ohm ^-1 cm^-2 mol ^-1

From the above it can be seen that preferred cathodes for use in the present invention are Cu ₂S, V₂O₅, V₆O₁₃, TiS₂ or NbSe₃. Acetonitrile is an extremely good solvator of copper (I) ions, and, while we do not wish to be bound by any theory, it is believed that the acetonitrile assists the cathodic process at a Li (I) /Cu₂S cathode. Acetonitrile is a small and mobile molecule and it is believed that intercalation of acetonitrile solvated Li (I) has less polarisation at certain intercalation cathodes such as TiS₂, V₂O₅ and V₆O₁₃ than with large solvent molecules such as propylene carbonate.

Thus, the advantages of acetonitrile over most solvents i.e. low viscosity, high dielectric onstant and good lithium solvating power are particularly advantageous in conjunction with certain cathode types, e.g. Cu₂S, TiS₂, V₂O₅ and V₆O₁₃. The higher conductance of LiAsF₆ PC/AN mixtures and the stability of lithium mean that the new electrolyte is in general preferable to LiClO₄/PC for all lithium batteries, e.g. Li/MoO₃, Li/NbSe₃) at high current density, but as noted, batteries with the preferred cathodes exhibit special advantages when the new electrolyte is used.

All the cells shown in Figures 1, 2, 3 and 4 after approximately 50% discharge, were charged at 0.5 mA cm^-2 at a constant voltage over at least one hour. Discharge then continued satisfactorily at 1mA cm^-2. All the cells are recyclable, but some, such as Li/CuS are recyclable over only a few cycles.

Claims

1. An electrochemical cell having an anode containing lithium as the electro-chemically active material, and a cathode, characterised in that said anode and cathode are immersed in a substantially anhydrous electrolyte free from Lewis acids and sulphur dioxide and containing from 0.1M to saturation LiAsF₆ dissolved in a solvent mixture containing propylene carbonate and acetonitrile, said acetonitrile forming from 10 to 80% by volume of the combined volume of propylene carbonate and acetonitrile and the solvent mixture consisting of at least 80% by volume acetonitrile and propylene carbonate with the balance, if any, being in the form of inert liquid material.

2. An electrochemical cell according to Claim 1, characterised in that the acetonitrile forms from 20 to 60% by volume of the combined, volume of propylene carbonate and acetonitrile.

3. An electrochemical cell according to Claim 2, characterised in that the acetonitrile forms from 30 to 60% by volume of the combined volume of propylene carbonate and acetonitrile.

4. An electrochemical cell according to any one of the preceding claims, characterised in that the electrolyte contains at least 0.5M LiAsF₆.

5. An electrochemical cell according to any one of the preceding claims, characterised in that the electrochemically active material of the cathode is a titanium sulphide, a copper sulphide, a niobium sulphide, a copper selenide, a titanium selenide, a niobium selenide, a vanodium oxide or a molybdenum oxide.

6. An electrochemical cell according to claim 5 , characterised in that the electrochemically active material of the cathode is TiS₂, MoO₃ or NbSe₃.

7. An electrochemical cell according to claim 5, characterised in that the electrochemically active material of the cathode is Cu₂5.

8. An electrochemical cell according to claim 5 , characterised in that the electrochemically active material of the cathode is V₂O₁₃.

9. An electrochemical cell according to claim 5, characterised in that the electrochemically active material of the cathode is V₂O₅.

10. An electrochemical cell according to any one of the preceding claims, characterised in that the lithium is in the form of the free, unalloyed metal.