Carbon-coated active particles and processes for their preparation
A process for the preparation of carbon-coated particles, where the particles include an electrochemically active material. The process includes the steps of emulsion polymerization, drying and thermally treating the polymer to obtain a nano-layer of carbon on the particles, where the carbon layer includes fibers and nitrogen-containing polyaromatics have a graphene-like structure. Also, the particles produced by the method as well as to electrode materials, electrodes and electrochemical cells including the particles.
1. A process for producing carbon-coated particles, the process comprising the steps of:
a. forming an emulsion by mixing particles, acrylonitrile monomers, and an aqueous solvent, said particles comprising an electrochemically active material;
b. polymerizing the acrylonitrile monomers in the mixture of step (a) by emulsion polymerization;
c. drying the particles from step (b) to form a nano-layer of poly(acrylonitrile) at the surface of the particles; and
d. thermally treating the dried particles of step (c) to form the carbon-coated particles, said carbon consisting in a nano-layer of carbon comprising fibers on the surface of the particles.
2. The process of claim 1 , wherein step (a) further comprises the addition of a polymerization initiator.
3. The process of claim 1 , wherein step (b) further comprises:
degassing the emulsion; and
heating the emulsion under inert atmosphere at a temperature of 50° C. to 90° C. and for a time period within the range of from 5 to 15 hours.
4. The process of claim 1 , wherein the emulsion is formed using sonication, high power stirring, or any high shear agitation technique.
5. The process of claim 1 , wherein the drying step (c) comprises spray-drying the particles.
6. The process of claim 5 , wherein the spray-drying is carried out at a temperature in the chamber above the boiling point of the solvent.
7. The process of claim 5 , wherein the spray-drying is carried out at an applied temperature of between 120° C. and 250° C.
8. The process of claim 1 , wherein the drying step (c) is carried out without any previous purification step.
9. The process of claim 1 , wherein the thermal treatment step (d) is a carbonization step comprising heating the particle at a temperature of at least 500° C.
10. The process of claim 9 , wherein the carbonization step comprises at least one temperature ramp.
11. The process of claim 9 , wherein the carbonization step comprises:
progressively heating the particles having a nano-layer of poly(acrylonitrile) at the surface using a ramp of temperature from a temperature close to room temperature up to at least 200° C., with an increase rate of between 3° C.min −1 and 10° C.min −1 ;
keeping the temperature to at least 200° C. for a period of 30 minutes to 2 hours; and
further heating the particles under inert atmosphere with an increase rate of between 3° C.min −1 and 10° C.min −1 up to a final temperature of at least 500° C.
12. The process of claim 11 , wherein the increase rate is of 5° C.min −1 .
13. The process of claim 11 , wherein the final temperature is of at least 600° C.
14. The process of claim 11 , wherein the inert atmosphere is selected from argon, nitrogen, carbon dioxide or a mixture thereof.
15. The process of claim 14 , wherein the inert gas is a mixture of argon and carbon dioxide having a ratio Ar/CO 2 of between about 60:40 to about 90:10.
16. The process of claim 1 , wherein the electrochemically active material comprises a material selected from the group consisting of titanates, lithium titanates, lithium metal phosphates, vanadium oxides, lithium metal oxides, and combinations thereof.
17. A process for producing carbon-coated LTO particles, the process comprising the steps of:
a. forming a Pickering emulsion comprising particles, acrylonitrile monomers, a polymerization initiator and an aqueous solvent, said particles comprising LTO as an electrochemically active material;
b. polymerizing the acrylonitrile monomers by emulsion polymerization to form poly(acrylonitrile) on a surface and inside pores of the particles;
c. spray drying the polymerized particles of step (b) to obtain dried particles having a nano-layer of poly(acrylonitrile) at their surface; and
d. carbonizing the dried particles of step (c) to form a carbon coating comprising carbon fibers on the surface of the particles.
18. Carbon-coated particles produced by the process of claim 1 , wherein the particles are coated with a nano-layer of carbon comprising carbon fibers and polyaromatics consisting of carbon and nitrogen atoms.