Patent Publication Number: US-2022223832-A1

Title: Device for rolling a metal lithium band

Description:
TECHNICAL FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to a device for rolling a metal lithium band. 
     To date, the lithium-ion technology allows reaching energy densities in the range of 250 Wh/kg and the progress margin of this technology is low according to specialists. The so-called “all-solid-state” technologies seem to be the future of lithium-ion in particular with the use of metal lithium at the anode. Indeed, the mass and volume energy densities of this material are very high and, if these technologies develop, would allow considerably increasing the energy density of rechargeable batteries. 
     Hence, one of the major concerns relates to mastering of the process of manufacturing metal lithium bands, used in large amounts in metal lithium batteries to date, thereby limiting the energy densities. The targeted thicknesses, in the range of a few microns to a few tens of microns, are reached thanks to a rolling mill. 
     Metal lithium is a malleable, sticky and therefore very fragile material, the manufacture of bands of a few tens of m is then relatively complex. Hence, it is necessary to optimise all of the rolling parameters to master the process and thus produce bands of a few tens of μm. 
     In general, the rolling cylinders are made of metal. In order to avoid sticking of the lithium on the rolling cylinders, a polymer material film is interposed between each rolling cylinder and metal lithium. An example of such a manufacturing device is described in the document U.S. Pat. No. 3,783,666, wherein endless belts of polyvinyl or polyethylene are wound around the rolling cylinders. This device enables the manufacture of a metal lithium film in the range of 250 μm. 
     The document EP0692831 describes a device for rolling lithium bands allowing reaching thicknesses of a few tens of μm, however it requires the implementation of a rolling lubricant. 
     DISCLOSURE OF THE INVENTION 
     Consequently, it is an aim of the present invention to provide a device for dry rolling a metal lithium band, i.e. without any liquid lubricant input. 
     The above-mentioned aim is achieved by a rolling device including at least two rolling cylinders between which the metal lithium is intended to be rolled, means for feeding in a metal lithium film at the input of the rolling mill disposed upstream of rolling cylinders, means for storing a metal lithium film at the output of the rolling mill, said storage means being disposed downstream of the rolling cylinders. The device applies a tension on the lithium film at the output of the rolling mill so that it is mechanically tensioned. The device also includes means for circulating two plastic material films disposed so as to interpose a plastic material film between each rolling cylinder and the lithium film and to make said films run with the lithium film. 
     The inventors have determined that there was a range of detachment angle of the film and of the band allowing making a metal lithium film with a second thickness of a few tens of μm while reducing, and even suppressing, the risk of tearing of the film and without the addition of a liquid lubricant. This angle range is obtained by particular orientations of each plastic material film downstream of the rolling cylinders. 
     The particular orientation of the plastic films is such that the separation line between a plastic material film and the lithium film is located in an area beyond a horizontal plane passing through the axis of rotation of the rolling cylinder with which the plastic material film is in contact. 
     Lithium films with a thickness of a few tens of am, for example between 10 μm and 100 μm, without any tearing could be obtained, without the implementation of a lubricant that might have contaminated the metal lithium. 
     Advantageously, the separation line is located in an area located between the horizontal plane and a vertical plane passing through the axis of rotation of the cylinder. 
     Preferably, the specific tension applied to the plastic films is comprised between 0.1 N/mm and 0.6 N/mm, preferably this tension is comprised between 0.2 N/mm and 0.3 N/mm. The application of this specific tension, associated to the orientation of one or several plastic material film(s) with respect to the rolling roll(s) allows facilitating the detachment between the plastic films and the lithium film with a second thickness. 
     For example, the particular orientation of each plastic material film is obtained by a roll with an axis of rotation parallel to the axis of rotation of the rolling cylinder, the plastic material film passing between the rolling cylinder and the roll. 
     Advantageously, the plastic material film includes a silicone coating. 
     One of the objects of the present application is a device for rolling a metal lithium band including a rolling area comprising at least two rolling cylinders between which a lithium band with a first thickness is intended to pass, each rolling cylinder rotating about an axis of rotation, a feed-in area comprising means for feeding in the rolling area with the lithium band with a first thickness, means for feeding in two plastic material films so that each plastic material film is interposed between the lithium band with a first thickness and a rolling cylinder, called associated rolling cylinder, and a storage area including means for collecting a lithium band with a second thickness having a second thickness smaller than the first thickness. The device also includes means for tensioning the lithium band with a second thickness and means configured to ensure a separation of each plastic material film off the surface of one of the rolling cylinders on the storage area side in a separation area located beyond a horizontal plane passing through the axis of rotation of said rolling cylinder and located opposite the other rolling cylinder, said separation area including said horizontal plane (PH). 
     Preferably, the means applying a tension to the lithium band with a second thickness are configured to apply a tension per unit surface area of the cross-section of the film with a second thickness between 1 N/mm 2  and 1.4 N/mm 2 . 
     Still preferably, the rolling device includes means configured to apply a tension to the plastic material films at least in the storage area, said means being configured to apply a tension per unit width of the plastic material films between 0.1 N/mm and 0.6 N/mm. 
     Advantageously, the separation means include, for each plastic material film, a roll whose axis of rotation is parallel to that of the associated rolling cylinder and whose axis of rotation is located in the separation area. 
     Advantageously, the position of the roll in the separation area is modifiable. 
     The plastic material films may include a silicone coating at least over the face intended to be in contact with lithium. 
     For example, the plastic material films are made of poly(ethylene terephthalate). 
     The plastic material films may be in the form of endless belts, their tension being set by a tensioner roll. 
     According to an additional feature, the rolling device includes in the storage area a spindle around which the lithium band with a second thickness is wound and means for simultaneously winding a plastic material film so as to avoid sticking of the lithium band with a second thickness on itself. 
     According to an additional feature, the rolling device includes two actuators for driving in rotation each rolling cylinder separately and a control unit configured to control the two actuators so that the two rolling cylinders rotate at different speeds (V 1 , V 1 ). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be better understood based on the following description and the appended drawings wherein: 
         FIG. 1  is a schematic representation of a device for rolling a lithium film by a rolling process, 
         FIG. 2  is a detail view of a device for rolling a metal lithium film according to one embodiment, 
         FIG. 3  is a detail view of a device for rolling a metal lithium film according to another embodiment. 
     
    
    
     DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS 
     In  FIG. 1 , is shown a device for rolling a lithium film, also called band, including an area ZL for rolling the lithium, an area ZA for feeding in lithium upstream of the rolling area ZL, an area ZS for storing the lithium downstream of the rolling area ZL. 
     In the present application, the upstream-downstream direction should be understood from the feed-in area towards the storage area. 
     In the present application, the expressions “lithium film” and “metal lithium film” refer to a film made of lithium or of a lithium alloy, comprising for example and without limitation magnesium and/or aluminium . . . . 
     The rolling area includes at least two rolling cylinders  2 . 1 ,  2 . 2  opposite each other, the opposing external surfaces of the cylinders being separated by the desired thickness of the lithium film. Several pairs of rolling cylinders disposed one after another from upstream to downstream may be implemented for example to progressively lower the thickness of the lithium film. The rolling cylinders are driven in rotation by actuators, such as electric motors. 
     The plane in which the lithium film runs between the two rolling cylinders will be called “rolling plane”. 
     The feed-in area ZA includes a dispenser  4  of a metal lithium film FL 1  with a first thickness e 1 . For example, the film is obtained by extrusion and as for example a thickness in the range of 200 μm. The film is wound on the dispenser. Rolls and guides generally implemented in rolling mills ensure guidance of the film FL 1  with a first thickness up to the rolling area ZL. 
     The feed-in area ZA also includes a dispenser  6  of a first plastic material film FP 1 , a portion of the film FP 1  being intended to be interposed between the rolling cylinder  2 . 1  and the lithium film FL 1  with a first thickness and to be in contact with the rolling cylinder  2 . The feed-in area ZA also includes a dispenser  8  of a second plastic material film FP 2 , the film FP 2  being intended to be interposed between the rolling cylinder  2 . 2  and the lithium film FL 1  with a first thickness. Rolls and guides may be implemented to guide the plastic material films FP 1  and FP 2  up to the rolling area. 
     Each plastic material film FP 1 , FP 2  is associated to a rolling cylinder  2 . 1 ,  2 . 2  respectively. The storage area ZS includes a spindle  10  on which the metal lithium film FL 1  with a second thickness smaller than the first thickness is wound, a spindle  12  on which the plastic material film FP 1  is wound and a spindle  14  on which the plastic material film FP 2  is wound. Rolls and guides may be implemented to guide the lithium films FL 1  with a second thickness, the plastic material films FP 1  and FP 2  from the rolling area towards their storage spindle. 
     The dispensers and the spindles are driven in rotation by actuators, such as electric motors. 
     The elements of the feed-in area and of the storage area, in particular the dispenser  4  and the dispensers  6 ,  8  and the spindles  12 ,  14 , are controlled so that the lithium film at the input of the rolling mill and the plastic material films FP 1 , FP 2  have close or equal speeds of displacement. The speed of the spindle  10  of the lithium film at the output of the rolling mill is different from the speed of the dispensers  4 ,  6  and  8  and of the spindles  12 ,  14 . 
     The axes of the spindles, rolls, cylinders are parallel to each other and normal to the direction of displacement of the films. 
     Advantageously, a plastic material film is wound at the same time as the film FL 1  with a second thickness on the spindle  10  so that the windings of the film FL 1  with a second thickness are separated by a plastic material film, for example made of polypropylene. 
     As example for a lithium film comprised between 60 mm and 100 mm, the tightening force exerted on the lithium strip by the rolling cylinders is for example comprise between 10 kN and 23 kN. For example, this force is exerted by the rolling cylinder  2 . 1 . 
     In  FIG. 2 , is shown a detail view of a device D 1  for manufacturing a metal lithium film according to a first embodiment. 
     In addition to the elements of the device of  FIG. 1 , the device D 1  comprises separation means  16  allowing setting the separation area between the film FP 1  and the external surface of its associated rolling cylinder  2 . 1 , and separation means allowing setting the separation area between the film FP 2  and its associated rolling cylinder  2 . 2 . 
     The means for separating the films FP 1 , FP 2  being similar or identical, only the means  16  will be described in detail. 
     The separation area is materialised by a separation line LS 1  that is formed by a generatrix of the rolling cylinder. In  FIG. 2 , this line is symbolised by a point. 
     Indeed, the inventors have determined that by setting the position of the separation line in a given area, a lithium film FL 1  with a second thickness with a thickness of a few tens of μm, for example between 50 μm and 30 μm, could be obtained while limiting the risks of tearing of the film FL 1  with a second thickness, and without the need for bringing in liquid at the level of the rolling cylinders. 
     The inventors have determined that a separation line lying in an area located above a horizontal plane PH passing through the axis of rotation X 1  of the rolling cylinder  2 . 1  allowed facilitating the detachment between the plastic material film and the lithium film, thereby substantially reducing the risk of tearing of a lithium film FL 1  with a second thickness with a thickness of a few tens of μm. The horizontal plane PH is parallel to the rolling plane. 
     Advantageously, the area is also delimited by a vertical plane PV passing through the axis of rotation of the rolling cylinder  2 . 1 . Preferably, the separation line is located in an area delimited between a plane directed at 45° with respect to the vertical plane and the vertical plane in the counterclockwise direction. 
     In the represented example and advantageously, the means  16  include a roll  20  disposed between the rolling cylinder  2 . 1  and the spindle  12  on which the plastic material film FP 1  is wound, and whose axis of rotation X 2  and in the horizontal plane PH. The roll  20  is disposed on the route of the film FP 1  between the rolling cylinder and the spindle  12  so that the separation line LS 1  is in the plane PH. It is considered that the separation line LS 1  and the axis of rotation of the roll  20  are substantially in the same plane containing the axis of rotation of the rolling cylinder. 
     The plastic material film FP 1  runs between the rolling cylinder  2 . 1  and the roll  20 . 
     Alternatively, the spindle  12  and the rolls and guides of the plastic material film FP 1  in the storage area are disposed with respect to the rolling cylinder  2 . 1  so that the separation line LS 1  is in the plane PH. 
     The implementation of a roll  20  allows setting the position of the line LS 1  more accurately. 
     For example, the distance separating the surface of the rolling cylinder and the surface of the roll is comprised between 1 mm and 4 mm. 
     The lithium film FL 1  with a second thickness is kept under mechanical tension. Preferably, a specific tension is between 1 N/mm 2  of lithium film section and 1.4 N/mm 2  of lithium film section is applied. A specific tension in the range of 1 N/mm 2  is preferably applied to a pure or almost pure lithium film, and a specific tension in the range of 1.4 N/mm 2  is preferably applied to a lithium alloy film. 
     For example, for a lithium film having a section of 3 mm 2  (for example 60 mm wide and 50 μm thick), the applied force is comprised between 3 N (for a specific tension of 1 N/mm 2 ) and 4.2 N (for a specific tension of 1.4 N/mm 2 ). 
     The mechanical tension applied to the plastic material films FP 1  and FP 2  downstream of the rolling area depends on the width of the films. Preferably, the specific tension is comprised between 0.1 N/mm and 0.6 N/mm, preferably this tension is comprised between 0.2 N/mm and 0.3 N/mm, which facilitates even more the detachment and contributes to the suppression of the apparition of tearing of the lithium film FL 1  with a second thickness. 
     For example, the tension of the film FL 1  with a second thickness and the tensions of the plastic material films downstream of the rolling area are set by adjusting the speed and the torque of motors. 
     Quite advantageously, the plastic material films include, at least over a face intended to come into contact with the lithium film, a silicone coating reducing the risks of sticking between the lithium and the plastic material film. For example, the coating has a thickness of 100 nm. 
     Silicone has the advantage of leaving no residue after rolling. Indeed, no silicone trace has been detected on the film FL 1  with a second thickness. 
     For example, the plastic material films are made of poly(ethylene terephthalate) covered with a solid silicone layer, for example of a few tens of nanometres, over at least one face. For example, the thickness of the plastic material films is comprised between 15 μm and 100 μm. 
     Advantageously, the roll  20  has a modifiable position with respect to the rolling cylinder in particular its angular position, allowing adapting the device easily to the manufacture of films with different thicknesses while limiting the risks of tearing. 
     Alternatively, the plastic material films consist of closed loops forming endless belts, which allows avoiding having to manage the plastic material film length remaining in the feed-in area. For example, the tension of the plastic material films is set by means of a tensioner roll. 
     The invention allows reaching lithium film thicknesses of 30 μm without any tearing. In contrast, it has been noticed that by placing the roll  20  underneath the horizontal plane PH, films having at least a thickness of 60 μm were torn. 
     In  FIG. 3 , is shown another example of a device for rolling a lithium film which differs from that of  FIG. 2 , in that the separation line of the plastic material film and of the associated rolling cylinder is located substantially in a vertical plane passing through the axis of rotation of the rolling cylinder. 
     In the represented example and advantageously, the means  116  include a roll  120  which is disposed between the rolling cylinder  2 . 1  and the spindle  12  on which the plastic material film FP 1  is wound, and whose axis of rotation X 102  is in the vertical plane PV. The roll  120  is disposed on the route of the film FP 1  between the rolling cylinder and the spindle  12  so that the separation line LS 101  is substantially in the plane PV. It is considered that the separation line LS 101  and the axis of rotation of the roll  120  are substantially in the same plane containing the axis of rotation of the rolling cylinder. 
     The separation line of each of the plastic material films could be located beyond the vertical plane when considering the counterclockwise direction. The location of the separation line could depend on the relative arrangement of the different elements of the rolling device and on the available space. 
     In the examples of  FIGS. 2 and 3 , the positionings of the separation rolls for the two plastic material films are symmetrical with respect to the rolling plane. 
     Alternatively, their positioning could be asymmetrical, and the separation lines could also be located asymmetrically with respect to the rolling plane. 
     Also, different means for setting the location of the separation line between the plastic material film and the associated rolling cylinder could be different between the plastic material films. 
     In a particularly advantageous operating mode, the actuators of each of the rolling cylinders are driven independently of each other so as to make them rotate at different speeds, in order to create a speed difference between them, which allows promoting sticking of the lithium film on the upper or lower plastic film. 
     Preferably, the difference between the speed V 1  of the upper rolling cylinder  2 . 1  and the speed V 2  of the lower rolling cylinder  2 . 2  is comprised between: 
       −30%≤( V 2− V 1)/ V 1≤30%.