WO1991000624A1

WO1991000624A1 - Rechargeable lithium battery

Info

Publication number: WO1991000624A1
Application number: PCT/GB1990/001025
Authority: WO
Inventors: Clive Daniel Sidney Tuck
Original assignee: Alcan International Limited
Priority date: 1989-07-03
Filing date: 1990-07-03
Publication date: 1991-01-10
Also published as: GB8915252D0; CA2055471A1; EP0480988A1; JPH04506434A; AU6038790A

Abstract

A lithium battery comprises a cathode and an electrolyte and a lithium-based anode of aluminium or an aluminium alloy having an average grain size not greater than 5-10 microns. The anode may be of the class known as the 255 alloys or may be AlTiB grain refiner or a superplastic alloy. The anode may be annealed or chemically or electrochemically pretreated to remove a surface oxide layer.

Description

RECHARGEABLE LITHIUM BATTERY

European Patent Application 323888 is

published on 12 July 1989. This specification

describes lithium batteries comprising a lithium-based anode, an electrolyte, preferably organic, containing a dissolved lithium salt, and a cathode which may be an intercalation compound. The batteries described are rechargeable, in the sense that they are capable of being subjected to a series of charge-discharge cycles. One battery described has an anode of aluminium alloy containing three to twenty five volume percent of dispersed particles with an average size in the range 0.05 to 2 microns.

The present invention concerns lithium batteries of the kind generally described in the aforesaid European patent application. The invention provides a lithium battery comprising a cathode and an electrolyte and a lithium-based anode, wherein the anode is of aluminium or an aluminium alloy having an average grain size not greater than 10 microns.

The invention concerns primary or secondary batteries. Primary batteries are not rechargeable, or at least not designed to be recharged. Secondary batteries are designed to be rechargeable, and are capable of being subjected to repeated charge-discharge cycles without significant loss of performance.

Whether a particular battery is rechargeable or not depends to a substantial extent on the

cathode/electrolyte structure, and several embodiments are described:- a) Materials normally used as cathodes in rechargeable lithium batteries may be used in this invention. These are often intercalation compounds. Examples are TiS₂, MoS₂, MoS₃, FeS₂, ZrS₂, NbS₂, NiPS₃, and VSe₂. TiS₂ is particularly suitable as it

possesses a lamellar structure and the diffusion constant of Li into it is extremely high .

As electrolytes , organic electrolyte solutions are preferably used in which 1 , 2 or more electrolytes such as e . g . LiClO₄, LiPF₆, LiAsF₆ , LiBF . or LiB (C₆H₅) ₄ are dissolved in one or more of the lithium-conductive organic solvents commonly used in this type of cell, e.g. 1, 2-dimethoxyethane, 1, 2-diethoxyethane, propylene carbonate, acetonitrile, gamma-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolan and 4-methyl-1,3-dioxolan. Stabilisers such as hexamethylphosphoric triamide may be incorporated into the above organic electrolyte solutions in order to control the

decomposition of electrolytes which lack stability, such as LiPF₆. Alternatively, molten salt electrolytes containing Li may be used as is well known.

Primary lithium batteries may be split into three classes:- i) Those with a soluble cathode. These are SO ₂,

SOCl₂ or S₂OCl₂ as cathode material dissolved in electrolyte. A carbon electrode is used and reaction takes place only at its surface. Embodiments b) and c) below are in this class.

ii) Those with a solid cathode. These have a reactive cathode, most commonly (CF) or MnO₂. Less common cathode materials are CuS, CuO, FeS_x, Bi₂Pb₂O₅,

Ag₂CrO₄, CrO_x, V₂O₅. Embodiment d) below is in this class.

iii) Those with a solid electrolyte such as LiI, Pbl₂, LiBr as electrolyte. Embodiment e) is of this type.

b) The electrolyte is acetonitrile saturated with liquid sulphur dioxide. A carbon cathode is generally used as having high surface area and conductivity, but a metal or other cathode could be used instead. On discharge, lithium dithionite is formed in the

electrolyte. Batteries of this kind are normally rechargeable only if they have been charged and

discharged at a relatively slow rate.

c) The electrolyte is thionyl chloride or

S₂OCl₂. On discharge, this is converted to SO₂ and LiCl. The cathode may be as in b). Cell batteries of this kind are not rechargeable.

d) The cathode is the intercalation compound polycarbonmonofluoride. The electrolyte may be as described in a). On discharge, LiF is formed as a precipitate in the cathode. Batteries of this kind are normally not rechargeable.

e) The electrolyte is an organic iodine complex. The cathode may be as in b). On discharge, Lil is formed which coats the cathode and acts as a cell separator. Batteries of this kind are also not normally rechargeable.

The anode is of aluminium or an aluminium alloy having an average grain size not greater than 10 microns and preferably not more than 5 microns. When an aluminium anode is charged up, it appears that lithium may be deposited in the form of columnar cells, 1 to 5 or 1 to 10 microns in diameter, of a lithiumaluminium phase. It is believed that, if the grain size of the aluminium anode is similar to that of the columnar cells formed, then deposition of lithium on the anode surface, and removal of lithium from that surface, will proceed more rapidly and efficiently and with less mechanical damage to the surface.

A class of aluminium alloys that can be made with an average grain size below 10 microns, and preferably below 5 microns, is that known as the 255 alloys. These contain 3 - 25 volume % of dispersed particles with an average size in the range 0.05 - 2 microns. Preferably, the particles contain one or more of Fe, Mn, Ni, Si, Ca, Co and Zn. More preferably, the alloy contains Ni alone or two or more of Ni, Fe, Mn, Si and Co in amounts constituting between 20% less and 10% more than the value of a eutectic composition.

Preferably the alloy is a wrought eutectic aluminium alloy containing 5 - 20 volume % of dispersed

intermetallic particles in the form of unaligned intermetallic rods having an average diameter in the range 0.1 to 1.5 microns with no more than 2% of coarse primary inter-metallic particles. The term "wrought" is used to indicate that the aluminium alloy has been worked, preferably to effect at least 60% reduction in thickness. The most convenient method for producing wrought-like intermetallic phases in an aluminium mass is to cast a eutectic or near eutectic alloy,

incorporating elements which form intermetallic phases with aluminium on solidification, under selected casting conditions to produce so-called "coupled growth". That phenomenon is well known and is

explained in an article by J.D. Livingston in "Material Science Engineering" Volume 7 (1971) pages 61 - 70. The cast alloy is then worked to break up the rods into small dispersed particles.

Alloys of this kind are well known, and are described for example in British patent specification 1479429 and in US patents 4126487; 4126486; and

4483719.

Particularly preferred alloys of this kind, containing both Fe and Mn, may be chosen from the 8000 Series of the Aluminium Association Register. These wrought eutectic alloys give rise to advantages in two ways:

- On being subjected to repeated charge- discharge cycles, batteries having anodes based on these alloys show particularly high efficiency.

- On being charged up prior to incorporation in a battery, these alloys incorporate lithium at a faster rate than pure Al or Al alloys containing dissolved alloying constituents.

Lithium capacity of the 255-type aluminium alloys described above, is improved without loss of mechanical properties by annealing the material.

Preferred annealing conditions are 250°C to 550ºC for 3 to 5 hours. See Example 1.

Other aluminium alloys having grain sizes below 5 or 10 μm can be used. The anode may contain dispersed grain refining titanium diboride particles. AlTiB grain refining rod is particularly preferred.

The anode may be a spray deposit formed by co-spraying Al and a refractory such as SiC. The anode may be of one of the class of super-plastic alloys, for example having the composition of 2004 or 5083 or 8090 of the Aluminum Association Register. 8090 alloys contain Li as an alloying component; batteries with anodes of such alloys have the advantage of reaching maximum efficiency earlier in the repeated charge-discharge cycle than do other Al alloys.

Aluminium alloys carry a somewhat variable natural oxide layer on their surface and this may affect the rate and extent to which lithium is

deposited on the metal surface when the anode is charged up. To avoid this problem, the surface can be subjected to a pretreatment which may be either chemical or electrochemical. The procedure used in Example 2 below is to etch in 1.25M NaOH at 60°C for 1 minute. Other pretreatments in acid or alkaline electrolytes, to remove or make more even the

protective oxide layer, are possible.

The aluminium anode can be charged up, preferably by being electrochemically alloyed with lithium, prior to installation in the battery.

Alternatively, the battery can be assembled in the discharged state, with the required lithium present in the cathode, and charged up in the assembled state.

Such batteries are naturally rechargeable.

The following examples illustrate the invention. EXAMPLE 1 - The effect of Annealing

Strips of AlFeMn alloys 8006 and 8008 of thickness 58 microns and 75 microns respectively were taken in their fully worked condition and approximately 18 microns of lithium was deposited on both sides of them. This was accomplished by passing a charge of

13.4 coulombs/cm 2 at current density 0.2 mA/cm² using lithium strip counter electrodes on both sides of the aluminium in a solution of 1M LiAsF₆ in purified propylene carbonate with up to 50% by volume ethylene carbonate addition. At the end of the charge the strips were taken and bent around a 2.29 mm rod. The strips bent to the shape of the rod without breaking but showed evidence of brittleness and when bent back again began to break up. A second bend on the same area was not successful. (This problem can be

alleviated by restricting the Li deposit thickness on each side of the strip to one sixth that of the strip.)

An annealing treatment was carried out at 350ºC for five hours and again the sheets were

subjected to the same lithium deposition treatment and ductility tests. In this case the strips were

obviously more flexible and would withstand at least two bends exercised on the same area; straightening the area between tests. EXAMPLE 2

The effect of Precleaning the Aluminium Surface

AlFeMn alloys 8006 and 8008 of thickness 58 nicross and 75 microns were cut into strips measuring

42 not × 600 mm and approximately 18 microns of lithium vas deposited on both sides. This was accomplished by passing a charge of 13.4 coulombs/cm² at current density 0.2 mA/cm ² using lithium strips as counter electrodes on both sides of the aluminium separated by surgical lint soaked in 1 M LiBF. solution in propylene carbonate and clamped together in a press. At the end of the charge the surface of the strips was examined and they appeared to be rather patchy in appearance, some areas being darker than others. This indicated that the lithium was not deposited evenly over the surface.

In order to mitigate this situation, a chemical treatment of the aluminium strips was tried to enable a more uniform surface oxide to be present before lithium deposition was attempted. The treatment chosen was an etch in 1.25 M sodium hydroxide at 60ºC for 1 minute followed by a thorough rinsing in

deionised water and drying for 1 hour in a drying cabinet at 100°C. After this treatment, the lithium subsequently deposited on the surface showed a much more uniform appearance indicating the advantage of chemical pretreatment before deposition. The surface lost its metallic appearance, becoming grey in colour and rough in texture. X-ray diffraction identified the surface lithiated layer as the delta-Al-Li phase. EXAMPLE 3

Microscopic Observation of Lithium deposition

reaction with Aluminium

Observation of the surface of pure aluminium on which lithium was deposited from a solution of 1M Li

C10₄ in purified propylene carbonate showed that the lithium reacted with the aluminium to produce a

lithium-aluminium phase (δ-AlLi) which has a volume about twice that of the aluminium from which it was formed. This resulted in rounded protuberances appearing on the surface of about 1 -5 microns up to a maximum of about 10 microns in diameter and about 1 micron high. As the charging continued these

protuberances gradually covered the surface, there being total cover when approximately 1 0 coulombs/cm² had been achieved. Microscopic analysis of the internal structure of the material, carried out by fracturing the surface regions on bending showed that the surface protuberances were defining an internal columnar microstructure approximately 10 microns thick, each column being about 1-5 microns diameter and crowned by a surface protuberance of similar

dimensions. On discharge, as the lithium depleted from the aluminium, the columns were seen to separate, allowing easy access of solution inside the structure, hence facilitating lithium discharge. Recharging simply expanded the structure again, filling in the gaps around the separated columns so that a coherent whole was again achieved.

A further experiment was performed also using pure aluminium. On discharge, when approximately 30% of the lithium was lost from the aluminium, large cracks appeared in the surface (viewed in SEM). On further lithium depletion, the columns were seen to separate, allowing easy access of solution inside the structure hence facilitating lithium discharge but allowing greater lithium loss through electrolyte reaction and irreversible break up of the structure. If less than about 30% of the lithium had been

discharged, recharging simply expanded the structure again filling in the large cracks so that a coherent whole was again achieved. If more than about 30% of the lithium had been discharged, then the morphology on recharging was not the same and the structure exhibited greater damage.

Using the above observations on super-purity aluminium it can be postulated that the small grain sizes (1-5 microns) and large volume fraction of second phases characteristic of 255-type aluminium alloys make their microstructural components smaller than the columnar δ-AlLi (1-10 microns diameter) which results from depositing lithium on aluminium. This imposed microstructure acts both to assist the diffusion of lithium in and out of the material and also to hold or anchor the structure together on repeated lithium cycling. This is indeed the case, as on full lithium discharge only the large surface cracks occur and the structure is obviously far less damaged than that of pure aluminium in similar circumstances.

it is believed that a mechanism such as this is the key to successful cycling of lithium in

aluminium and the overriding characteristic of the material is a fine microstructure, the grain size less than 10 microns, preferably less than 5 microns.

Reference is directed to the accompanying drawing which shows a possible mechanism of lithium charge/discharge. The drawing comprises diagrams arranged in two columns, of which the left hand relates to a conventional aluminium alloy, and the right hand to an Al alloy of the 255 type. The state of the metal surface is shown after charging with 1, 3 and 10 coulombs/cm of electricity, and after partial

discharge .

The aluminium alloy surface is indicated as 10, growing boundaries are indicated as 12 and second phase particles are indicated as 14. Note that the grain size in the right hand column is about 5 microns, while the grain size in the left hand column is much greater than 10 microns.

Lithium is present largely as δ-AlLi in the hatched regions 16. In successive diagrams in each column these comprise:

- isolated protuberances on the metal

surface;

- continuous protuberances covering the whole metal surface;

- continuous protuberances overlying columns extending perpendicular to the metal surface;

- (during or after discharge) separate columns .

When 255-Alloy is used, these columns are bound together by second phase particles which gives the material of the anode added mechanical stability.

EXAMPLE 4

This example reports on a number of controlled experiments with different aluminium alloys of grain sizes less than 6.5 μm, these alloys being of various types (255 types, super-plastic alloys, metalmatrix composite and grain refining material) and thickness mostly around 100 μm. The grain sizes of the alloys were measured by transmission electron

microscopy, some of which being varied by the annealing treatment normally given (350°C for 2 hrs). The lithium cycling efficiency of the alloys was measured using the standard procedure outlined in EPA 0323888 using either a Type I electrolyte (1M LiBF₄. in

propylene carbonate) or a Type II electrolyte (50% propylene carbonate, 50% ethylene carbonate with 1M LiAsF₆).

The following Tables 1 and 2 give grain size measurements and cycling efficiencies. It will be noticed that the best results have been obtained with Al 5% Ti 1 % B (grain refiner rod). This alloys shows 100% efficiency after about 13 cycles if it has a grain size of 0.5 μm but this drops considerably if the sample is annealed using the normal practice. However, if ansealing is only carried out for 15 mins, the cycling efficiency drops only slightly. This suggests a dependence of cycling efficiency on grain size.

It is apparent that, amidst a fair amount of scatter, there is general pattern of behaviour. For the first cycle, the trend is for the cycling

efficiency to increase on an increase of grain size. This may be explained by the consideration that on this cycle the lithium is reacting as previously at the grain boundaries and a greater number of grain

boundaries per unit area would give a higher surface area for reaction with the lithium. The structure does not break up significantly during lithium discharge, but the lithium becomes trapped which contributes to the initial loss of efficiency.

As the cycling proceeds the diffusion paths available for the lithium and presence of heterogeneous sites for δ-AlLi phase nucleation become established and the cycling efficiency becomes inversely

proportional to the grain size. This relationship diminishes in magnitude as the number of cycles increases, presumably as the internal structure becomes more heterogeneous, allowing a high volume fraction of δ-AlLi nucleation sites.

The 8090 alloy investigated contained 2-3% Li, which is approximately the saturation level at room temperature. The efficiency of this material rose to 100% earlier than the other materials (after 6 or 7 cycles).

Addition of Li to AA 8008 (NL-218, 1.1% Li added to 8008) improves the initial cycling

performance. Initial cycling efficiency is high probably due to the formation of precipitates of δ'-AlLi or δ-AlLi before cycling which enable more reversible growth and association of δ-AlLi during subsequent cycling of lithium.

EXAMPLE 5

Alloys with a small grain size are able to produce a stable microstructure after the initial five cycles. SEM observation of the degree of cracking which occurs on discharging lithium from the alloy indicates that the cracking level is stabilised after 5 - 10 cycles. The proportion of uncracked surface undergoes a similar pattern after cycle 5 and the SEM shows very little change to occur.

Table 1

255-TYPES

LITHIUM CYCLING EFFICIENCY

Grain ElectroNo. of

Size lyte Resul ts

(μ») Averaged

8006 255 - Type 1.6 Fe 0.4 Mn

Cycle No 1 5 10 15 20

Efficiency (%)

1 I NON-ANNEALED 54 57 94 96 97 4 3.5 I ANNEALED (90 μm) 53 81 93 95 96 4 6.5 ANNEALED (450 μm) 83 77 91 93 94 3

8008 255 - Type 1.1 Fe 0.7 Mn

Cycle No. 1 5 10 15 20

Efficiency (%)

2 I NON-ANNEALED 44 80 91 94 94 4 3.5 I ANNEALED 49 80 91 96 98 1

81151 255 - Type 1.1 Fe 0.14 Mn Cycle No. 1 5 10 15 20 30 40

Efficiency (%)

2 1 ANNEALED 70 82 93 95 96 - - 4

2 II ANNEALED 71 87 96 98 99 99 99 3

A16X Ni 255 - Type (SP-Base)

Cycle No. 1 5 10 15 20 30 40

Efficiency (%)

0.8 II NON-ANNEALED 65 92 99 98 97 96 956 2 1.5 II ANNEALED 60 95 97 96 95 94 2

NL-218 1.1 Fe 0.7 Mn 1.1 Li (8008 + 1.1 Li)

Cycle No. 1 5 10 15 20 30 40

Efficiency (%)

2 II NON-ANNEALED 81 88 98 100 100 99.2 99.5 Table 2

SMALL GRAIN TYPES

LITHIUM CYCLING EFFICIENCY

Grain EϊectroNo. of Size lyte Resul ts

Averaged

8090 Super Plastic 2.3 Li 1.0 Mg 0.25 Zn 1.0 Cu

Cycle No. 5 10 20 30 40

Effi ciency (%)

4 .0 II NON-ANNEALED 46 97 102 100 97 95 95 2

AlTiB Grain Refiner Al 5% Ti 1% B

Cycle No. 1 5 10 15 20 30 40

Efficiency (%)

0 .5 II NON-ANNEALED 63 82 95 100 100 100 99 2

4 .0 II ANNEALED 73 82 93 99 99 99 - 2

8585 Cospray 25 with Si C Parti cles

Cycl e No . 1 5 10 15 20 30 40

Efficiency (%)

1 .5 II NON-ANNEALED 74 92 96 97 96 96 96 2 * II ANNEALED 74 91 94 96 96 96 97 2

2004 Superplastic

Cycle No. 1 5 10 15 ?o 30 40

Efficiency (%)

4 .0 NON-ANNEALED 30 89 100 98 96 93 93

* ANNEALED 94 92 99 95 94 94 -

* Not Determined

Claims

1. A lithium battery comprising a cathode and an electrolyte and a lithium-based anode, wherein the anode is of aluminium or an aluminium alloy having an average grain size not greater than 10 microns.

2. A lithium battery as claimed in Claim 1, wherein the anode has an average grain size not more than 5 microns.

3. A lithium battery as claimed in Claim 1 or Claim 2, wherein the anode contains dispersed grainrefining titanium diboride particles.

4. A lithium battery as claimed in any one of Claims 1 to 3, wherein the anode contains 3 - 25% by volume of dispersed particles with an average size of 0.05 - 2 microns.

5. A lithium battery as claimed in Claim 3, wherein the anode is a wrought aluminium alloy

containing Ni alone or two or more of Ni, Fe, Mn, Si and Co in amounts constituting between 20% less and 10% more than a eutectic composition.

6. A lithium battery as claimed in any one of Claims 1 to 5, wherein the anode is of an Al-Li alloy.

7. A lithium battery as claimed in any one of Claims1 to 6, wherein the anode is an aluminium alloy which has been annealed.

8. A lithium battery as claimed in any one of Claims 1 to 7, wherein the anode has been subjected to a chemical or electrochemical pretreatment to remove or make more even the protective oxide surface layer.

9. A lithium battery as claimed in any one of Claims 1 to 8, wherein the anode has been charged up by being electrochemically alloyed with lithium.

10. A lithium battery as claimed in any one of Claims 1 to 9, wherein the battery is rechargeable.