AU731463B2

AU731463B2 - Method and anode for improving the power density of lithium secondary batteries

Info

Publication number: AU731463B2
Application number: AU57484/98A
Authority: AU
Inventors: Bent Hundrup; Dennis W. Nielsen; Franz W. Winterberg
Original assignee: Dilo Trading AG
Current assignee: Dilo Trading AG
Priority date: 1996-12-23
Filing date: 1997-12-19
Publication date: 2001-03-29
Anticipated expiration: 2017-12-19
Also published as: CA2275969A1; EP0948826A1; WO1998028807A1; IL130566A0; BR9714165A; KR20000062304A; RU2175798C2; DE19654057A1; DE19654057C2; JP2001506799A; AU5748498A

Description

Method and Anode for Improving the Power Density of Lithium Secondary Batteries The invention relates to a method and anode for improving the power density of lithium secondary batteries, especially those containing solid polymer solutions.

In cell arrangements, or other arrangements, an improved materials utilization factor is needed in order to be able to meet the market demands. If charge transport from anode material to cathode material in a secondary battery is effected preferably by an electrolyte, or an electrolyte solution respectively, this can be put down to the fact that any material will be transported by an existing potential, and as a consequence, also positively charged ions will be transported by electrolytes to the negatively charged electrode, which applies to anions vice versa.

The electrolyte's current density is expressed as follows: I LE Aiti j i) (1) in which LE means the conductivity of the electrolyte solution, A0 means the potential difference between anode and cathode materials, tir means the reduced transport index for transport species and Agi means the difference in chemical potential of species between anode and cathode materials.

Since all materials in the anode are diluted per definition, Ali will be close to zero, and for usual batteries as explained by way of illustration this will lead to an equivalent of the 1st Ohm's Law.

**There may be voltage values higher than allowed for the system. This may lead to risks or damages that should be avoided by way of prevention.

Finally, polymer bonds are considered to be disadvantageous, and anions are not immobilised. The lithium's transport capacity is insufficient.

One aim of the present invention is to provide an additive, which will improve the power density and, at the same time, secure operational reliability, in particular 2 when implemented in lithium secondary batteries. Advantageously, a preferred form of the invention should also achieve a positive deviation of the 1 st Ohm's Law, reduce the salt exhaustion, and increase the number of cycles, or the cycle strength, when implemented in a lithium secondary battery.

It would also be desirable to provide process steps in the manufacture of batteries whereby the improvements described above can be implemented in practice.

Accordingly, in a first aspect of the invention, there is provided a method for improving the power density of lithium secondary batteries, characterized in that boric acid esters and/or boric acid ester derivates, or compounds thereof, are employed as additives at the anode in the manufacture of lithium batteries.

Said additive will bring about a reduction of the so-called salt exhaustion, a high lithium transport capacity, and a positive derivation of the 1 st Ohm's Law. This also will lead to an increased cycle strength of the battery system as well as an improved power density for the potentials specified. Compare the attached figures 3 and 4 in particular, which show the advantages of a cell system in accordance with the invention.

Preferably, boric esters and/or boric ester derivates are used as lithium complex compounds such as H H O 0 l /P B Li+

R

2 0- C- R 2

H

I

L

i and/or R/R Li+ and/or Hm MB M Li+ where the partial groups R, and R 2 can be aromatic and/or aliphatic, and in formula Ill.- "M" is a transition metal, and the cyclopentadienyl groups may contain Fluor instead of H.; SO- .Per definition, transition metals are elements the atoms of which have incomplete d shells, or may form one or more cations with such incomplete d shells. Therefore, and according to the IUPAC recommended notation, transition metals include in-the 4' period the elements from oleo Sc through to Zn, having atomic numbers from 21 through to 30, in the 50' period the elements from Y through to Cd, having atomic numbers from 39 through to 48, in the 6 period the elements from La through to Hg, together with the lanthanoid series which have the .e 4f shell filled up, having atomic numbers from 57 through to 80, and finally in the 7 period the actinoid series through to Lr, having atomic numbers from 89 through to 103.

Preferably, boric acid esters will be employed.

The partial groups will cause electrochemical stability and solubility in organic solvents. Due to the large and voluminous partial groups the negative charge is distributed, and therefore, it is very unlikely to happen that Li* may form ion pairs or complex species, and the organic solvent contains the salt in dissolved or dissociated form.

The additives are added preferably on the anode side.

The additives are added in quantities from 0 to 20 per cent (by weight), preferably 5 to per cent (by weight).

The anode according to the invention, especially those as employed in lithium secondary batteries and those containing solid polymer solutions, contains in its immediate vicinity additives of boric acid and/or boric acid ester derivates, or compounds thereof.

This will cause a comparably high current to flow with a potential set low, and in particular bring about effects as a stable system and higher number of cycles, or increased cycle strength, respectively.

The anode consists of a substance that is able to intercalate lithium ions and/or lithium and conducting salts dissolved in solvents and/or in a polymer binder and/or a conducting carbon black and/or the additive.

o **o *i o* o* *o Especially suitable are those anodes which contain as an additive lithium compounds of boric acid esters and/or boric acid ester derivates in form of complexes such as 0

B

-'N

H

I.

O--C-R,

I

-M

O-C RI

H,

Li

R

2 -C--0

I

and/or

RI

Li+ a a a a it and/or Co 0

H

Under consideration of expediency, such additives should be contained in the anodes in quantities over 0 up to 20 per cent by weight, preferably from 5 up to 15 per cent by weight.

The following is a more detailed explanation of the invention, with references to the attached drawings. The drawing show the following:- Figure I a schematic cross section of a battery, for example a lithium ion battery LiC/PEO, lithium salt ILi Mn 2 O, without salt exhaustion, with extremely low electric current flows in a very short period of time (ideal case) Figure 2 a schematic cross section of the same system, but in contrast to figure 1 the curves show the behaviour when bigger current flows are used Figure 3 ****again a schematic cross section of the same system, the graphs here, however, show the behaviour under small and big current flow conditions, without salt exhaustion to happen Figure 4 trends shown as curves for smaller, medium and bigger current flow conditions Figure curves as shown in figure 4, but as ideal case, with immobilised anions Figure 6 schematic illustrative curves on how cycle strength can be increased in case of using polyethylenoxid (PEO) Figure 7 Curves to illustrate the positive derivation of the 1st Ohm's Law when additives are used, compared with graphs having no positive derivation Figure 8 a schematic illustration on anodes electrolyte cathodes in case of additives used not used Figure 9 current voltage diagram to illustrate the results from the examples used In arrangements as schematically illustrated in figure 1, there will be no salt exhaustion in case of extremely low current flows in short time cycles. This applies in particular to the lithium ion batteries, outlined as sketch infigure 1, where under ideal conditions the anions are not immobilised. For that reason, only small current flows can be drawn without gradient.

Figure 2 illustrates these condition in case of bigger current flows for the same system on the basis of a lithium ion battery, where local salt exhaustion will occur. As the lithiium ions move under the influence of a mass equilibrium the concentration thereof is constant, by approximation The anions move towards the electrolyte at the positive electrode. As there is no anions dispensed by the electrodes a concentration gradient will build up. According to Kohlrausch's Law, the ion conductivity depends on the electrolyte concentration the lower the concentration, the lower the conductivity. Furthermore, a conductivity gradient builds up as a concentration gradient develops, and local electrolyte resistance increases with decreasing electrolyte conductivity. As a consequence, the increase in the local electrolyte resistance entails a potential drop As shown in figure 3, the anions are now immobilised within the polymer matrix of the elec- S* trolyte, according to the invention.

O*l* In this way, both big and small current flows can be used without having any problems with *salt exhaustion effects and following potential drops, as also shown in figure 5 as trend of graphs in an ideal case with immobilised anions.

o*oo As far as medium and bigger current flows are concerned, the referenced trends are summarised in the graphs shown in figure 4.

The ideal case with immobilised anions as illustrated in figure 5 shall be taken as an example to give a more detailed explanation below.

The anions are not mechanically immobilised but their transport capacity is very small compared with the lithium's one.

In case the anions are mechanically immobilised, the complex constant is very high, and thd lithium transport capacity drops. Also, the overall conductivity drops as the complex constant between anions and lithium is high.

In case the anions are chemically immobilised, the complex constant between Li* and anion is very high, in contrast to the overall conductivity which is very low. If, however, the anion transport is very small compared with the Li* transport capacity, then there will be no significant complexes between anions and cations, which will lead to a high conductivity value.

Figure 6 is based on the fact that if a bigger current flow is needed a higher potential must be used. High potentials result in an only small number of cycles, or a rather qualified cycle strength. This is shown in figure 6, taking PEO solvent as an example.

In addition, it can be seen from figure 6 that according to the invention current levels can be kept constant with decreased potentials, the sizes of which are so that the PEO solvent is stable. Using substances according to the invention, the cycle capability could be improved by decreasing potentials and current flows kept on a constant level. The reduced potential **brings about a higher number of cycles, or a better cycle strength, respectively. Based on a lithium ion battery system, as used as illustrative example in figure 1, it could be shown that, S according to the invention, after adding substances, as detailed in the patent claims, to the electrolyte binder material in the anode the potential could be reduced without reducing current flow density, as drawn as an example in figure 6, which is considered a special advantage of the present invention.

ao..

,o In test series, the power density could be increased, and corresponding proof established. In this context, figure 7 is a schematic diagram of the so-called positive derivations of the 1' Ohm's Law together with graphs showing the normal course of the 1 Ohm's Law for ordinary batteries in lithium ion battery systems as described.

For the investigation, the potential had been fixed to a constant level. Then, the additive complexes, resp. the substances found, were added and a positive derivation of the 1 s Ohm's Law determined. This means that a higher current flow is achieved compared with that one achievable under normal graph courses according to the 1' Ohm's Law. Thus, the power density of the system is improved.

From the equation in figure 8 as well as from the sketch, it can be seen that the transport capacity of the anions comes close to zero. the chemical potential difference has no impact at all on the current flow density.

If a lithium compound of a boric acid ester derivate is added, the partial excess energy of lithium ions becomes permanently positive. This fact is based on an increase in the current flow density as well as an increase in the lithium transport capacity; i.e., 8t' 8 x 0 and RT (S In x u 8x) 0 and tr' e Ap 0 Volt (2) This will lead to a positive derivation of the 1' Ohm's Law.

"For predetermined cell designs and potentials, a bigger current may be drawn into an outer circuit, if the system shows a positive deviation from the 1" Ohm's Law, this is equivalent to an increase in power density.

Example 1 (comparative example) Formulation without Lithium-bis[1, 2 -benzenediolato(2-)-O,O'Iborate(1-)(LiBSE) Active substance content in (by weight) :Graphite (KS6 type) 90.29 Conducting carbon black (super P type) 4.74 F6 TEFLON binder 4.97 lectrode gross weight: 13.9 mg; weight of active KS6: 12.55 mg (equivalent to 4.67 mAh) t0 Example 2 Formulation with Lithium-bis[1,2-benzenediolato(2-)-O,O']borate(1-)(LiBSE) Active substance (by weight) Graphite (KS6 type) 82.08 Conducting carbon black (super P type) 4.30 TEFLON binder 4.53 LiBSE 9.09 Electrode gross weight: 11.3 mg; weight of active KS6: 9.3 mg (equivalent to 3.46 mAh) For both examples, measurements were made in a lithium half cell, with an active surface of approx. 1 cm 2 (standard electrolyte LP 30: EC DMC 1 m LiPF 6 chart speed: 0.1 mV/s).

For preparing the electrodes, corresponding active substances were mixed together in a mortar, and pressed onto the nickel mesh.

For both the formulations, a cyclovoltamogram was taken using a triggerable potentiostate, as shown in figure 9 (voltage-current diagram). In addition, figure 9 leads to the conclusion that both cathode and anode current is increased, which means that the LiBSE system ca- ,pacity (example 2) is increased, compared to the system w/o LiBSE (example 1).

6 6 o* C o 6 0

Claims

1. Method for improving the power density of lithium secondary batteries, characterized in that boric acid esters and/or boric acid ester derivates, or compounds thereof, are employed as additives at the anode in the manufacture of lithium batteries.

2. Method according to claim 1, characterized in that it is performed in the manufacture of lithium secondary batteries containing solid electrolyte polymer solutions.

3. Method according to claim 1 or 2, characterized in that the boric acid esters and/or boric acid ester derivates are present in the form of lithium compounds having a formula selected from the group consisting of I, II, III or combinations thereof, wherein H H I I R-C-O O-C-R 1 I is B Li+ R2-C-O O-C-R 2 I I H H II is R B RI Li *0 *O O* *o• and III is M B M Li H H and wherein the partial groups R 1 and R 2 can be aromatic and/or aliphatic, and in formula III M is a transition metal, and the cyclopentadienyl groups may contain Fluor instead of H.

4. Method according to claim 3, characterized in that the additives are added in quantities from 0 to 20 per cent by weight relative to the anode.

Method according to claim 4, characterized in that the additives are added in a quantity of 5 to 15 per cent by weight relative to the anode.

6. Anode for lithium polymer batteries, characterized in that boric acid esters and/or boric acid ester derivates, or the compounds thereof, are contained as additives in the anode.

7. Anode according to claim 6, characterized in that the additives are lithium compounds of boric acid esters and/or boric acid ester derivates having a formula o selected from the group consisting of I, II, III or combinations thereof, wherein D o* H H I I** R-C -O O-C-R I is B Li+ R 2 -C-O O-C-R 2 I I H H ooooo: 13 IIis R RB Li O O H HH and III is M B M Li CO OC-( H- H and wherein the partial groups R 1 and R 2 can be aromatic and/or aliphatic, and in formula III M is a transition metal, and the cyclopentadienyl groups may contain Fluor instead of H.

8. Anode according to claim 6 or 7, characterized in that the additives are contained in quantities from 0 to 20 per cent by weight relative to the anode.

9. Anode according to claim 8, characterized in that the additives are S contained in quantities from 5 to 15 per cent by weight relative to the anode.

10. Lithium secondary battery containing solid electrolyte polymer solutions and incorporating an anode according to any one of claims 6 to 9. *o 14 DATED this 8th day of January 2001 DILO TRADING AG WATERMARK PATENT TRADEMARK ATTORNEYS UNIT 1 THE VILLAGE RIVERSIDE CORPORATE PARK NORTH RYDE NSW 2113 AUSTRALIA P1 031 6AUOO