Separator paper for electrochemical cells

Abstract

A paper suitable for use as separator in an electrochemical cell, including at least 60 wt. % of an aramid fibril and at least 1 wt. % of an aramid fiber. The paper has a grammage of 5 to 100 g/m2, and the aramid fibril has a Canadian Standard Freeness (CSF) in a wet phase of less than 300 ml and a specific surface area (SSA) after drying of less than 3 m2/g. It has been found that a paper with this composition combines good electrolyte absorption and ion permeability with high stability during use, leading to a long lifetime for the electrochemical cell.

Claims

1. A paper for a separator in an electrochemical cell, comprising: at least 60 wt. % of an aramid fibril; and at least 1 wt. % of an aramid fiber, wherein: the paper has a grammage of 5 to 100 g/m.sup.2; the aramid fibril has a Canadian Standard Freeness (CSF) in a wet phase of less than 300 ml and a specific surface area (SSA) after drying of 0.1 to 3 m.sup.2/g; and the aramid fibril is not obtained by fibrillating fibers to form a pulp.

2. The paper according to claim 1, wherein the paper comprises at most 90 wt. % of the aramid fibril.

3. The paper according to claim 1, wherein the CSF of the aramid fibril in the wet phase is less than 200 ml.

4. The paper according to claim 1, wherein the aramid fibril has a weight weighted length for particles having a length above 250 micron (WL0.25) of less than 1.2 mm.

5. The paper according to claim 1, wherein the paper comprises at least 5 wt. % and at most 38 wt. % of the aramid fiber.

6. The paper according to claim 1, wherein the aramid fiber has a number-average length in the range of 2-15 mm.

7. The paper according to claim 1, wherein the paper comprises at least 70 wt. % of meta-para aramid and para-aramid relative to a total weight of the paper.

8. The paper according to claim 1, wherein the grammage of the paper is 5-50 g/m.sup.2.

9. A method for manufacturing a paper according to claim 1, comprising: preparing a suspension comprising the aramid fibril, the aramid fiber, and optional further components; applying the suspension onto a porous screen so as to lay down a mat of randomly interwoven material onto the screen; removing water from the mat; drying the mat to form a paper; and optionally subjecting the paper to a calendaring step.

10. The method according to claim 9, further comprising manufacturing the aramid fibril by a process comprising: polymerizing an aromatic diamine and an aromatic dicarboxylic acid halide to produce an aramid polymer, in a mixture of N-methylpyrrolidone or dimethylacetamide and calcium chloride or lithium chloride, to obtain a dope wherein the polymer is dissolved in the mixture and the polymer concentration is 2 to 6 wt. %; converting the dope to fibrils by using a jet spin nozzle under a gas stream; coagulating the fibrils using a coagulation jet; and optionally subjecting the fibrils to shear forces.

11. An electrochemical cell, comprising a cathode, an anode, and an electrolyte between the cathode and the anode, wherein the cathode and the anode are separated by the separator paper according to claim 1.

12. The electrochemical cell according to claim 11, wherein the electrochemical cell is a lithium ion battery, and the electrolyte comprises lithium ions.

13. The lithium ion battery according to claim 12, wherein the anode comprises carbon, the cathode comprises a metal oxide or metal phosphate, and the electrolyte comprises a lithium complex in an organic solvent.

14. The paper according to claim 1, wherein the aramid fibril is a para-aramid fibril.

15. The paper according to claim 1, wherein the aramid fiber is a meta-para-aramid or para-aramid fiber.

16. The paper according to claim 1, wherein the SSA after drying of the aramid fibril is less than 1.5 m.sup.2/g.

17. The paper according to claim 1, wherein the aramid fiber has a linear density of less than 1.6 dtex.

18. The paper according to claim 1, wherein the paper comprises at least 60 wt. % of para-aramid relative to a total weight of the paper.

19. The paper according to claim 1, wherein the aramid fiber is non-fibrillated aramid fiber.

Description

EXAMPLE 1

(1) An aramid paper was manufactured from para-aramid fibrils and para-aramid fibers.

(2) The para-aramid fibrils had a Canadian Standard Freeness of 120 ml, a specific surface area after drying of 0.86 m2/g and a weight weighted length for particles having a length>250 micron of 0.93 mm. The fibrils were manufactured as described in WO2005/059211, with a further shear step being carried out before incorporating them into the paper.

(3) The para-aramid fibers were para-aramid shortcut with a length of 6 mm and a linear density of 1.1 dtex. The paper contained 70 wt. % para-aramid fibrils and 30 wt. % para-aramid fiber.

(4) The paper had the following properties:

(5) Grammage: 18 g/m2

(6) Thickness: 30 micron

(7) Density: 0.6 g/cc (calculated from grammage and thickness)

(8) Porosity: 60% (calculated from grammage and thickness)

(9) Gurley: 3 seconds

(10) Tensile strength: 5 N/cm

(11) Average pore size: 6 micron

(12) The paper was subjected to cycle testing in an electrochemical cell in accordance with the procedure described below. Celgard 2325 (standard commercially available polyolefin material) was also tested.

(13) Separator papers were tested in pouch cells with a maximum capacity of 80 mAh.

(14) Commercial NMC cathode and a graphite anode were used. Electrolyte used was 1M LiPF6 in EC:EMC (3:7% wt), 2% wt VC was used as additive.

(15) The results are given in Table 1, and in FIG. 1.

(16) TABLE-US-00001 TABLE 1 Capacity retention after after after after 200 cycles 300 cycles 500 cycles 900 cycles Celgard 93% 91% 87% 80% Aramid 97% 97% 99% 97% separator

(17) As can be seen from the table and the FIGURE, the paper according to the invention shows a much higher capacity retention than the commercially available product. After 900 cycles cells containing Celgard separator have reached the end of life (80% capacity) whilst cells containing aramid separator still show 97% capacity retention. This means that an electrochemical cell comprising the paper according to the invention will have a longer lifetime than an electrochemical cell comprising the comparative product.

EXAMPLE 2

(18) To show the effect of the present invention, comparative papers were manufactured with equivalent compositions, grammage (19 g/m2), thickness (30 microns), and density (0.6 g/cc) as the paper according to the invention, but wherein fibrils with different properties were used. All papers were manufactured on a Rapid Koethe (RK) handsheet former according to the method of ISO 5269-2.

(19) The composition and properties of the papers are given in Table 2.

(20) TABLE-US-00002 TABLE 2 Recipe % Highly % Refined % Fibrils refined pulp pulp Fibers Invention 70 30 CE1 70 30 CE2 70 30

(21) In the paper according to the invention para-aramid fibrils were used with a specific surface area after drying of 0.42 m2/g, a Canadian Standard Freeness of 50 ml, and a weight weighted length for particles having a length>250 micron of 0.70 mm. This material was obtained by jet-spinning polymer solution as described in WO2005/059211, followed by a further shear process step.

(22) In comparative example 1 highly refined para-aramid pulp was used, with a specific surface area after drying of 14 m2/g, a Canadian Standard Freeness of 90 ml, and a weight weighted length for particles having a length>250 micron of 0.84 mm. This pulp was obtained by subjecting fibers to a number of high shear refining steps.

(23) In comparative example 2 commercial Twaron 1094 para-aramid pulp was used, with a specific surface area after drying of 13 m2/g, a Canadian Standard Freeness of 170 ml, and a weight weighted length for particles having a length>250 micron of 1.46 mm. This pulp was obtained by subjecting fibers to a number of high shear refining steps.

(24) The para-aramid fibers were para-aramid shortcut with a length of 6 mm and a linear density of 1.1 dtex.

(25) To simulate conditions of use, the papers were kept for 20 hours in demineralised water, wherein the water is a model for the electrolyte in the cell. The tensile strength of the papers after this aging step was determined on the wet papers, and compared with the original strength of the papers. The results are given in Table 3.

(26) TABLE-US-00003 TABLE 3 TS after wet Tensile strength TS dry aging retention after N/cm N/cm wet aging Invention 3.98 2.17 54% CE1 1.02 0.22 21% CE2 1.33 0.15 12%

(27) Tensile strength (TS) was determined in accordance with ISO 1924-2.

(28) As can be seen from Table 3, the paper according to the invention shows a tensile strength retention after wet aging which is much higher than the tensile strength retention of the comparative papers. This means that the paper according to the invention will show less degradation during use, leading to an electrochemical cell with a longer lifetime. Additionally, the absolute value of the strength after aging of the paper according to the invention is also higher than the strength after aging of the comparative papers, which is an additional advantage. The same goes for the dry strength.