Separator paper for electrochemical cells

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 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 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 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 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.