WO2002099919A1

WO2002099919A1 - Electrochemical cell electrolyte

Info

Publication number: WO2002099919A1
Application number: PCT/GB2002/002366
Authority: WO
Inventors: Fazlil Coowar; William James Macklin
Original assignee: Aea Technology Battery Systems Limited
Priority date: 2001-06-05
Filing date: 2002-05-21
Publication date: 2002-12-12
Also published as: GB0113544D0; TW544969B

Abstract

A lithium ion cell comprising an anode layer and a cathode layer each comprising respective lithium ion insertion materials, separated by a separator, in which the electrolyte comprises gamma-butyrolactone in the range 10-80% by volume, ethylene carbonate in the range 1-30% by volume, and at least one of either vinyl ethylene carbonate in the range 1-8% by volume or methoxyethyl methyl carbonate in the range 10-80% by volume. Such a cell has good electrical properties, and is comparatively safe if over-charged because the electrolyte components have high boiling point and high flash points.

Description

Electrochemical Cell Electrolyte

The present invention relates to an electrolyte for use in a lithium ion cell, and to electrochemical cells incorporating this electrolyte.

For many years it has been known to make cells with lithium metal anodes, and cathodes of a material into which lithium ions can be intercalated or inserted. Such cells may use, as electrolyte, a solution of a lithium salt in an organic solvent such as propylene carbonate, and a separator such as filter paper or polypropylene. In the case of secondary or rechargeable lithium cells, the use of lithium metal anodes is unsatisfactory as problems arise from dendrite growth, but the use of an intercalation material such as graphite has enabled satisfactory cells to be made. Such cells may be referred to as "lithium ion" cells, or "swing" cells, as lithium ions are exchanged between the two intercalation materials during charge and discharge. The electrical properties of the cell, especially as regards cycle life, are to a significant extent determined by the selection of the electrolyte solvent.

Gel or solid electrolytes may be made, as described by Gozdz et al (US 5 296 318) , with a copolymer of 75 to 92% vinylidene fluoride and 8 to 25% hexafluoropropylene as the polymer, this being dissolved in a low boiling- point solvent such as tetrahydrofuran along with a lithium salt and a plasticising electrolyte solvent such as ethylene carbonate/propylene carbonate mixture, and cast from solution. Such an electrolyte, but using homopolymer polyvinylidene fluoride (PVdF) with a very low melt flow index, is described in GB 2 309 703 B (AEA Technology) . It is also possible to make such a solid or gel polymer electrolyte by first making a porous film of the polymer material, and then immersing the film in a solution of lithium salt in an organic solvent so the electrolyte solution is absorbed by the polymer film, as described in EP 0 730 316 A (Elf Atochem) . These electrolytes, whether made by casting or by immersion, have the appearance of a gel or solid, and will be referred to hereafter as a separator. In this case too, the electrical properties of the cell are significantly affected by the selection of the electrolyte solvent.

It will be appreciated that there are many considerations in this selection process. The solvent must not react chemically with the dissolved lithium salt, nor must it react chemically or electrochemically with the electrodes. It desirably has a high dielectric constant, and a low viscosity. It should remain liquid over the expected operating range of the cell, but should have a high boiling point and a high flash point to enhance safety if the cell becomes hot as a result of overcharge. And it should not be too expensive. No one organic liquid has been found to be ideal in all respects .

According to the present invention there is provided an electrolyte for use in a lithium ion cell comprising an anode layer and a cathode layer each comprising respective lithium ion insertion materials, separated by a separator, the electrolyte comprising gamma- butyrolactone in the range 10-80% by volume, ethylene carbonate in the range 1-30% by volume, and at least one of either vinyl ethylene carbonate in the range 1-8% by volume or methoxyethyl methyl carbonate in the range 8- 80% by volume.

Although gamma-butyrolactone (gBL) provides good electrical properties it tends to react electrochemically with graphite. The ethylene carbonate (EC) can improve the charge-discharge efficiency, and helps in the formation of a passivating layer on the surface of the graphite (which may be referred to as a "solid/ electrolyte interface" or SEI) .. This passivating layer prevents subsequent side reactions such as reduction of the electrolyte . The use of a mixture of gBL and EC as solvent for a cell electrolyte is known for example from JP 10-312825 (Toshiba) , but cell properties can be enhanced by adding the other components specified in the present invention.

The vinyl ethylene carbonate (VEC) is preferably present at no more than 5% by volume; it is particularly effective at forming a passivating layer which is of low ionic impedance. The methoxyethyl methyl carbonate (MEMC) lowers the melting point of thev electrolyte, so enabling use of the cell at lower temperatures. It is also believed that the -OCH3 group has a role in attaching the solvent molecules during electrochemical reduction, enabling the efficient formation of the SEI as a compact film on the graphite surface.

The electrolyte may also comprise chlorodiethyl carbonate (CDEC) (i.e. 1-chloroethyl ethyl carbonate), preferably no more than 5% by volume; this also helps with formation of the passivating layer. One of the main reason for using this material is it allows graphitic materials to be cycled with gBL (and PC) . It also has the advantages of having a high boiling point (159-1S1°C) and a relatively high flash point (65°C) . The electrolyte may also contain tri-fluoro propylene carbonate (TFPC) , possibly in the range up to 80% by volume, which is more compatible with graphite and also less reactive with the charged cathode material. Carbon dioxide may also be dissolved in the electrolyte with advantage, as this also helps in the formation of the passivating layer.

The electrolyte may also contain a dicarbonate such as dimethyl dicarbonate, diethyl dicarbonate or di-tert- butyl dicarbonate, or 1,2 di-phenyl vinylene carbonate, in each case at no more than 10% by volume, preferably about 2%. These additives can also help protect the cell against damage due to overcharge, and also improve rate performance and storage. For example if the cell electrolyte contains 1,2 di-phenyl vinylene carbonate then even when overcharged (at the C rate) to cell voltages above 4.3 V there is no smoke or fire.

Such an electrolyte may be used in conjunction with a separator such as microporous polyethylene, or micropσrous vinylidene fluoride-based polymer, in the latter case forming a gel or solid electrolyte separator. As described in WO 01/48063, a microporous membrane may be cast from a solvent/non-solvent mixture, or from a latent solvent, sσ that the entire process can be carried out in the absence of water or humidity, reducing the risk of water being present in the final film or membrane (which would be detrimental to the properties of a lithium ion cell) . The non-solvent should not only dissolve in the solvent, but it should be miscible with the solvent in substantially all proportions. The boiling point of the non-solvent is preferably higher than that of the solvent, preferably about 20°C higher. For example the solvent might be dimethyl formamide or dimethyl acetamide, in which case a suitable non-solvent is 1-octanol which is soluble in those solvents and whose boiling point is about 194°C.

Some liquids suitable as solvents, and as latent solvents, for vinylidene fluoride-based polymers are listed in the Tables. It should however be appreciated that not all solvents are suitable for all grades of polymer.

Table 1

Table 2

The evaporation rate during drying must not be rapid, as rapid drying tends to produce macropores, and also may lead to formation of an impervious skin which prevents evaporation of underlying liquid. When using a latent solvent, the drying process should be carried out at a temperature below the dissolution temperature for the latent solvent. Consequently the polymer precipitates, and it is believed that two phases occur: a polymer-rich phase, and a polymer-poor phase. As the latent solvent evaporates the proportion of the polymer- rich phase gradually increases, but the remaining droplets of polymer-poor phase cause the formation of pores . The electrolyte of the invention is suitable for use with a range of different forms of graphite and carbon in the anode, and for a range of different materials in the cathode . It may for example be used with a cathode comprising the oxides LiCoθ₂ or LiNiθ₂, or the spinel oxide LiMn₂θ₄; the cathode may also contain conductive material such as carbon black. The electrolyte must have a salt dissolved in it to provide ionic conductivity, this salt for example being LiPFε, LiBF₄, lithium imide

(LiN(CF₃S0 )2) lithium methide (LiC (S0 CF₃) ₃) , or lithium bis-oxalatoborate (LiB (C₂O₄) ₂) _> or a mixture of such salts.

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

Figure 1 shows graphically the variation of voltage with charge for a cell of the invention, at different rates of discharge; and

Figure 2 shows graphically the variation of voltage with charge for another cell of the invention, at different rates of discharge

1. Non-laminated cell

Making the porous membrane

Homopolymer PVdF (Solvay grade 6020) , which has a low value of melt flow index (less than 0.7 g/10 min at 10 kg and 230°C), is dissolved in N-methyl pyrrolidone (NMP) at a temperature of 45°C while stirring; 15 g of PVdF were dissolved in 85 g of NMP. A small quantity, 9 g, of 1-octanol is then added dropwise to the polymer solution, and carefully mixed during this addition to ensure the mixture is homogeneous. The quantity of 1- octanol must not be too large, or the solution will gel. The mixture is then mixed for a further period of 2 hours to ensure uniformity. The resulting ternary mixture is then cast, using a doctor blade over a roller, onto an aluminium foil substrate to form a layer initially 0.25 mm thick, and then passed through a 7 m long drying tunnel with two successive drying zones at temperatures of 65°C and 100°C respectively. It moves through the drying tunnel at 0.5 m/min. Within the drying zones the film is exposed to a dry air flow with a velocity of 14 m/s, to remove any solvent and non-solvent that evaporates. The dry air is obtained by passing air through a dehumidifier, so its dewpoint is -40°C.

During passage of the film through the drying tunnel, which takes 14 minutes, both the solvent and non- solvent gradually evaporate (although they are both well below their boiling points) , the solvent tending to evaporate more rapidly. A white polymer membrane is thereby obtained, of thickness about 20 μm, and analysis with a scanning electron microscope shows it to be microporous. The pores are of size in the range 0.5-2.0 μm, typically about 1 μm in diameter, at least at the surface. The membrane has been found to have a porosity of about 53%.

Making the electrodes

A cathode is made by making a mixture of LiCoθ₂

(from Nippon Chemical) , a small proportion of conductive carbon, and homopolymer PVdF 6020 as binder (as described above) in solution in N-methyl pyrrolidone (NMP) . The mixture is cast using a doctor blade onto an aluminium foil, and passed through a dryer with temperature zones at for example 80°C and 120°C to ensure evaporation of the NMP. This process is then repeated to produce a- - double-sided cathode. Removal of all the NMP is further ensured by subsequent vacuum drying.

An anode is made from a mixture of mesocarbon microbeads of particle size 10 μm, heat treated at 2800°C (MCMB 1028) , with a small amount of graphite, and homopolymer PVdF 6020 as binder in solution in NMP. This mixture is cast, onto a copper foil, in a similar fashion to that described in relation to the cathode.

Cell Assembly

A cell assembly is then wound with the porous membrane of thickness 20 μm separating the anode from the cathode. Each such cell assembly is enclosed in a sealed envelope of aluminium laminate, and then vacuum filled with a plasticising liquid electrolyte, for example 1 molar LiBF₄ in an solvent mixture comprising 60.83% gBL,

24.33% EC, 12.16% MEMC and 2.68% VEC (these being the percentages by volume) . After storing for 16 hours to ensure the electrolyte has been absorbed by all the cell components, it is then vacuum packed in a flexible packaging material .

The cell is then charged, aged for two weeks, and then subjected to five discharge and recharge cycles, with the charging and discharging current (in amps) at an estimate of the C/5 value (where C represents the cell capacity in amp hours) , in order to determine the cell capacity, C, this value being referred to as the rated cell capacity. These cells were found to have a rated capacity of 0.61 Ah. The cell is then discharged at a range of different discharge currents.

Referring now to figure 1, this shows graphically the variation of voltage during discharge of the cell at different discharge currents: C/5, C/2, C and 2C; the larger the discharge current, the lower the cell voltage. Although the cell does give a smaller capacity at higher discharge currents, nevertheless the capacity remains high (above 95%.) even at the highest discharge current 2C.

Alternative, laminated cell construction

Making thin copolymer layers

Two thin microporous copolymer layers^' are made by a similar process, a 12% by weight solution of a copolymer PVdF/6HFP (vinylidene fluoride and 6% by weight hexafluoropropylene) being made by dissolving 12 grams of the copolymer in 88 grams DMF. A small quantity of 1- octanol is then added dropwise to the copolymer solution, and carefully mixed during this addition to ensure the mixture is homogeneous, and is then kept stirred for 2 hours. The resulting ternary mixture is then cast, using a doctor blade over a roller, onto an aluminium foil substrate to form a layer initially 0.06 mm thick, and then dried exactly as described above.

This forms a microporous layer of thickness about 2 μm, the pores being similar to those in the membrane described above.

Making the electrodes The anode and cathode are made by the same process as described above, although in this case the LiCoθ₂ in the cathode was provided by FMC Corp. In both cases the electrodes are double-sided.

Cell Assembly

The cathode is sandwiched between two of the thin copolymer layers so that each surface is completely covered, and these components are laminated together by subjecting them, when placed between release papers, to compressive pressure in a press between rollers giving a compressive force of 20 N, at an elevated temperature of 120°C.

The anode is also sandwiched between two thin copolymer layers, and laminated together in the same way.

A cell assembly is then wound with a microporous polyethylene membrane of thickness 16 μm separating the anode from the cathode, this membrane being supplied by Tonen Chemical Corp. Each such cell assembly is enclosed in a sealed envelope of aluminium/plastic laminate, and a small quantity such as 0.5 g of acetone is injected into the envelope. The envelope containing the cell assembly is then held at a temperature of 30°C for a period of at least 5 minutes. This elevated temperature enhances the solvation of the surface of the copolymer layer by the acetone.

After cooling to ambient temperature, the cell assembly is removed from the envelope and is then vacuum dried at 60°C for 3 hours to ensure removal of any traces" of acetone. The cell assembly is thenxun vacuum filled with the the four-component plasticising li .iquid electrolyte described above, that is to say 1 molar " LiBF₄ in an solvent mixture comprising 60.83% gBL, 24.33% ^■ EC, 12.16% MEMC and 2.68% VEC. After storing for 16 houxmurs to ensure the electrolyte has been absorbed 1 by all the cell components, it is then vacuum packed in a - flexible packaging material .

It has been found that thrihe anode and cathode are both laminated to the porous mmmembrane. It is apparent that this is because the copoll'lymer layers, when partially solvated by the acetone at 30° ° °C, are sufficiently tacky that they adhere to the porousa.s membrane. Because the lamination occurs without appll'lication of external pressure there is no risk of pqperforation of the porous membrane. Surprisingly the pas^artial salvation does not affect the porosity of the copq'polymer layers, and the overall process does not affeco:ct the porosity of the membrane, so that the cell hass.s good electrical properties after addition of the plastici±'ising liquid electrolyte.

Cells made in this way wes^rere charged, and aged for two weeks, before their capaci±'ity was measured as described above. Referring toσo figure 2, this shows the variation of voltage with capas-acity for a laminated cell, for various different rates of3>f discharge, the cell being charged and discharged betweenπn voltages of 2.75 V and 4.25 V. The rated cell capacityty in this case was about 0.66 Ah for this particular cesell. As with the cell described above, the capacity ^• decreases slightly as the rate of discharge increases; bctbut even at a discharge rate of 2C the available capacity i is about 95% of the rated capacity. It will be appreciated that a laminated cell of this type may incorporate a different microporous separator in place of the polyethylene membrane, for example a microporous homopolymer PVdF membrane of 6020 PVdF or 1015 PVdF, made as described in relation to the non- laminated cell described above. Both these homopolymer PVdFs have very low melt flow indexes: typical values obtained at 230°C and 21.6 kg being: SOLEF 1015: between 2.8 and 4.6 g/10 min and SOLEF 6020: <= 2 g/10 min; this latter measurement is close to the detection limit. The molecular weights of the homopolymers are 240 000 and 300 000 respectively. The^' microporous separator must be of a material that is not significantly solvated by the acetone, as it is important that its porosity is not affected.

The benefits of the present invention are (i) high boiling and high flash point electrolyte necessary for safety issues, and (ii) low swelling of cells when this electrolyte is used. It is clear from the discharge graphs in the figures that both the non-laminated and laminated cells have good electrical properties.

Claims

1. An electrolyte for use in a lithium ion cell comprising an anode layer and a cathode layer each comprising respective lithium ion insertion materials, separated by a separator, the electrolyte comprising gamma-butyrolactone in the range 10-80% by volume, ethylene carbonate in the xange 1-30% by volume, and at least one of either vinyl ethylene carbonate in the range 1-8% by volume or methoxyethyl methyl carbonate in the range 8-80% by volume.

2. An electrolyte as claimed in claim 1 also comprising chlorodiethyl carbonate.

3. An electrolyte as claimed in claim 1 or claim 2 also comprising tri-fluoro propylene carbonate.

4. An electrolyte as claimed in any one of the preceding claims also comprising a dicarbonate.

5. An electrolyte as claimed in claim 4 wherein the dicarbonate is dimethyl dicarbonate, diethyl dicarbonate or di-tert-butyl dicarbonate.

6. An electrolyte as claimed in any one of the preceding claims also compxising 1,2 di-phenyl vinylene carbonate at no more than 10% by volume.

7. A lithium ion cell including an electrolyte as claimed in any one of the preceding claims .

8. A lithium ion cell substantially as hereinbefore described, with reference to figure 1 or figure 2 of the accompanying drawings.