Method for the purification of raw carbon nanotubes

Abstract

A process for purifying raw carbon nanotubes to obtain a content in metallic impurities of between 5 ppm and 200 ppm. The process includes an increase in the bulk density of the raw carbon nanotubes via compacting to produce compacted carbon nanotubes. The process further includes sintering the compacted carbon nanotubes by undergoing thermal treatment under gaseous atmosphere in order to remove at least a portion of the metallic impurities contained in the raw carbon nanotubes, and consequently producing purified carbon nanotubes. These purified carbon nanotubes are directly usable as electronic conductors serving as basis additive to an electrode material without requiring any subsequent purification step. The electrode material can then be used to manufacture an electrode destined to a lithium-ion battery.

1 . A process of purification of raw carbon nanotubes comprising metallic impurities, the process comprising the following steps:
 compacting the raw carbon nanotubes to produce compacted raw carbon nanotubes having a bulk density higher than that of the raw carbon nanotubes;   adding an inorganic base in aqueous solution or adding water before, during or after the compacting of the raw carbon nanotubes;   drying the compacted raw carbon nanotubes in order to remove moisture therefrom; and   sintering the compacted raw carbon nanotubes by subjecting the compacted raw carbon nanotubes to a thermal treatment under gaseous atmosphere, to remove at least a portion of the metallic impurities and produce purified carbon nanotubes.   
     
     
         2 . The process according to  claim 1 , wherein the compacting of the raw carbon nanotubes comprises an agglomeration of the raw carbon nanotubes to produce agglomerated carbon nanotubes. 
     
     
         3 . The process according to  claim 2 , wherein the bulk density of the agglomerated carbon nanotubes is comprised between 0.1 and 0.8 g/cm 3 . 
     
     
         4 . The process according to  claim 1 , wherein the compacting comprises at least one of the following steps:
 an agglomeration of the raw carbon nanotubes to produce agglomerated carbon nanotubes;   a briquetting of the raw carbon nanotubes to produce briquettes of carbon nanotubes; and   an extrusion of the raw carbon nanotubes to produce granules or pellets of extruded raw carbon nanotubes.   
     
     
         5 . The process according to  claim 4 , wherein the compacting only comprises the agglomeration when the bulk density of the raw carbon nanotubes is of at least 0.2 g/cm 3 . 
     
     
         6 . The process according to  claim 1 , wherein the gaseous atmosphere during thermal treatment comprises chlorine gas, and wherein the metallic impurities are removed by a pulse vacuum system after chlorination of the metals by the chlorine gas. 
     
     
         7 . The process according to  claim 1 , wherein the gaseous atmosphere during thermal treatment comprises nitrogen gas and wherein the metallic impurities are removed by vaporization. 
     
     
         8 . The process according to  claim 1 , wherein the thermal treatment is performed at a sintering temperature comprised between 1,200° C. and 3,000° C. 
     
     
         9 . The process according to  claim 1 , wherein the thermal treatment is performed in a pusher continuous kiln. 
     
     
         10 . The process according to  claim 1 , wherein the thermal treatment is performed in a batch kiln. 
     
     
         11 . The process according to  claim 1 , wherein the inorganic base in aqueous solution is LiOH, NaOH, KOH, Na 2 CO 3 , or any combination thereof. 
     
     
         12 . The process according to  claim 1 , wherein the aqueous solution of inorganic base or the water is added at a temperature between 20 and 80° C. 
     
     
         13 . The process according to  claim 1 , comprising a step of conditioning the purified carbon nanotubes in order to produce conditioned purified carbon nanotubes for a direct application. 
     
     
         14 . The process of  claim 13 , wherein the step of conditioning comprises the dispersion of the purified carbon nanotubes to reduce the bulk density thereof. 
     
     
         15 . The process according to  claim 14 , wherein the bulk density after dispersion is comprised between 0.21 and 0.25 g/cm 3 . 
     
     
         16 . The process according to  claim 13 , wherein the step of conditioning comprises packaging the purified carbon nanotubes. 
     
     
         17 . The process according to  claim 1 , wherein the raw carbon nanotubes are of multi-walled type presenting a length/diameter ratio superior to 10, a specific surface area comprised between 100 and 300 m 2 /g, and a bulk density comprised between 0.02 and 0.5 g/cm 3 . 
     
     
         18 . The process according to  claim 17 , wherein the purified carbon nanotubes have a metallic impurities content between 5 ppm and 200 ppm. 
     
     
         19 . The process according to  claim 18 , wherein the metallic impurities content of the purified carbon nanotubes is comprised between 5 ppm and 50 ppm. 
     
     
         20 . The process according to  claim 18 , wherein the metallic impurities comprise iron.