The invention relates to a method for pre-lithiating a lithium-ion capacitor, wherein the lithium-ion capacitor comprises at least one bi-layer electrode stack, comprising a cathode and an anode; a lithium electrode; and an electrolyte. A rechargeable battery cell which could combine high specific power, specific energy, and good cyclic stability, would be highly desired. Li-ion batteries, which store electrical energy through chemical reaction in the bulk of the electrode material, exhibit a high specific en ergy but suffer from poor specific power and unsatisfactory cyclic stability due to the slow and poor reversibility of the chemical reactions and slow solid-state diffusion. On the other hand, supercapacitors, which store electrical energy through physical charge separation at the interface between the electrode and the electrolyte, could deliver a much higher specific power and show good cyclic stability. However, supercapacitors suffer from insuf ficient specific energy.

A lithium-ion capacitor, which employs a lithium-ion battery anode and typically a super- capacitor cathode, is a relatively new electrical energy storage device. This hybrid cell can combine the high specific energy and specific power in a single device because of its hy bridized energy storage mechanism. However, there is no lithium source in a lithium-ion capacitor cell, so a pre-lithiation step is normally required to lower the potential of the an ode and widen the operating voltage window, thereby enhancing the specific energy. Many pre-lithiation methods have been proposed to lower the potential of the anode. The existing methods can be classified into three groups according the lithium-ion precursor used. Firstly, lithium-containing compounds, such as lithium metal oxides (Advanced En ergy Materials, 2011. 1(6): p. 1002-1006.) and various lithium salts (Nature Materials, 2017. 17: p. 167., and US 9,136,066 B2), have been employed for pre-lithiation. However, for these compounds there are always some unwanted by-products left in the cell after pre-lithiation, for example inert components which reduce the specific energy of the cells. Alternatively, the by-products may be compounds which can react with other components in the cell, resulting in further side reactions which may shorten the cyclic stability of the cells. Secondly, lithium ions can also be utilized as lithium-ion sources for pre-lithiation. However, the pre-lithiation degree achieved by this method (US 2002/0122986 A1) is normally low. Thirdly, pure lithium metal has been used as lithium source for pre-lithiation (US 6,461,769 B1, US 6,740,454 B1, and US 9,183,994 B2). This method has the ad vantages of high pre-lithiation degree and no unwanted by-products after pre-lithiation. However, as lithium is very active towards water and oxygen, there are significant safety concerns connected with the use of lithium metal. Lithium metal can for example cause fire, or even explosion, if it is not handled properly. Despite these disadvantages, this is the only method that has been applied in the industry so far.

The third group, using pure lithium metal, can be further divided into two subgroups based on it use: In the first subgroup lithium is coated directly onto the surface of the anode (US 2017/0263388 A1), and in the second subgroup lithium is used as a third electrode in ad dition to the anode and the cathode (US 6,740,454 B1). For the first subgroup, lithium can be applied as foil, mesh, powder, or thin film on the surface of anode. For the second subgroup, the lithium electrode can be placed on top of or beside the anode and cathode. As the process within a lithium-ion capacitor is associated with the transfer of ions be tween the cathode and the anode, these electrodes are mostly shaped as parallel plates to have a large interfacial area. The plates are typically separated by a separator to avoid direct electrical contact. The combination of one or more anodes and cathodes separated by a separator is referred to as an electrode stack. A single pair of electrodes may be re ferred to as a bi-layer electrode stack, while a plurality of such pairs may be referred to as multi-layer electrode stack. A lithium-ion capacitor cell that can be charged and dis charged comprises an electrode stack and an electrolyte solution within a suitable casing. If the capacitor cell comprises a bi-layer electrode stack it may be referred to as a bi-layer cell, while it may be referred to as a multi-layer cell if it comprises a multi-layer electrode stack. The capacitance of the capacitor is increased by increasing the area of the elec trodes, either using a multi-layer electrode stack, wherein each bi-layer electrode stack is stacked next to each other, or using a single elongated bi-layer electrode stack rolled up as a so-called jelly roll. However, such stacking or rolling has the disadvantage that the accessibility of the anode is decreased, whereby the pre-lithiation time is increased. The lithium ions will then access the anode from the edges and diffuse slowly towards the cen ter. Due to the slow bulk diffusion of lithium ions in the solid anode plates of the electrode stack, such pre-lithiation process is very slow.

The pre-lithiation process can be accelerated if the electrodes consist of porous metal mesh current collectors as disclosed in US 6,740,454 B1 and US 6,461,769 B1. In this way the lithium ions may diffuse through the electrode stack in the electrolyte, so that a shorter diffusion distance in the solid anode material is required. However, the metal mesh required in this process as current collector is much more expensive than the more com monly used metal foil. Additionally, due to a slow diffusion in the electrode stack, it can still take up to 20 days to achieve a uniform pre-lithation of the anode.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art. The object is achieved through features which are specified in the description below and in the claims that follow. More specifically, it is an object of the invention to provide a method for pre-lithiating a lithium-ion capacitor, wherein the method is safe and cheap, is capable of providing a uni form and high degree of pre-lithiation, does not leave any unwanted by-products, can be used with different electrolyte/electrode system, and/or is efficient to increase productivity. The invention is defined by the independent patent claim, and the dependent claims de fine advantageous embodiments of the invention. KR101179629B1, US2012042490A1, JP2008311363A, and US2018301289A1 disclose methods for pre-lithiating a lithium-ion capacitor.

More specifically, the invention relates to a method for pre-lithiating a lithium-ion capacitor, wherein the lithium-ion capacitor comprises: at least one bi-layer electrode stack compris ing a cathode and an anode; a lithium electrode; and an electrolyte, wherein the method comprises the steps of: discharging the cathode towards the lithium electrode to adsorb lithium ions on the cathode; and charging the cathode towards the anode. The electrolyte allows the transfer of lithium-ions between the electrodes and typically contains lithium ions. The two steps may be performed with either of them first. If required, the steps may be repeated a plurality of times to increase the degree of pre-lithiation of the anode. This will still be faster than prior art pre-lithiation methods.

The method exploits that diffusion of lithium ions in the cathode is a surface-redistribution process, which is much faster than the solid-state diffusion process in the anode. Addi tionally, as mentioned above, the anode and cathode are often configured such that they have a large interfacial area, typically by having plate-like shapes and being separated by a suitable separator. Therefore, the lithium ions cannot diffuse freely from the electrolyte to the surface region of the anode which is directed toward the cathode, since the separa tor and cathode restrict this volume. The cathode is therefore used as a helper for lithium- ion distribution to achieve a faster and more uniform pre-lithiation process. If the step of discharging the cathode towards the lithium electrode is performed as the first step, lithium ions will adsorb onto the edges of the cathode and rapidly be redistribut ed across the surface. Surface redistribution of the lithium ions may be faster than the restricted diffusion in the electrolyte. The lithium ions may be transferred with substantially the same rate from the electrolyte to the cathode as from the lithium electrode to the elec trolyte, whereby the overall concentration of lithium ions in the electrolyte may remain substantially constant. The electrolyte typically comprises lithium ions. By subsequently charging the cathode towards the anode after a suitable amount of lithium ions have been adsorbed, the surface-adsorbed lithium ions will rapidly move across the separator and into the anode. Anions form the electrolyte may adsorb on the cathode during this step. The lithium ions will in this way diffuse and intercalate into the plate-shaped anode from the direction normal to the plate instead of from the edges of the plate, such that the re quired length for the ions to diffuse is shorter. This will increase the pre-lithiation uniformity and reduce the diffusion time while still using cheaper foil as electrodes. If the step of charging the cathode towards the anode is performed as the first step, lithi um ions will diffuse from the electrolyte to the anode, in this way depleting the electrolyte of lithium ions. The anode will typically not be saturated with lithium ions at this first step before the potential of the cathode achieves the electrolyte oxidation limit. Simultaneously, anions from the electrolyte may adsorb onto the cathode. When the cathode is subse- quently discharged towards the lithium electrode, lithium ions will be transferred from the lithium electrode to the electrolyte, and further onto the cathode if the pre-lithiation step is to be repeated and the discharge voltage is sufficient. The lithium ions may primarily ad sorb at the edges which are close to the lithium electrode but will rapidly be distributed across the surface of the cathode. Anions adsorbed on the cathode may be released into the electrolyte at the beginning of this step.

The method may typically comprise or be followed by a formation step for creating a solid electrolyte interface layer in the surface region of the anode and maturing the capacitor cell. Finally, the lithium-ion capacitors will typically be opened for degassing and for re moving the lithium electrode before final thermal sealing. The anode and cathode may be substantially solid plates to keep the interfacial area large and the cost low. The lithium-ion capacitor may comprise a multi-layer electrode stack or a bi-layer electrode stack rolled up into a jelly roll to increase the surface area of the elec trodes. In this case the method for pre-lithiation of the lithium-ion capacitor may be espe cially advantageous, since the diffusion of the ions in the electrolyte is limited on both sides of the plate-shaped anodes.

The lithium electrode may be lithium metal, such as foil, mesh, powder coated on metal foil, or free-standing lithium foil. The lithium electrode may also comprise a lithium com pound such as lithium metal oxide, lithium sulphide, lithium nitride, lithium oxide, lithium organic salt, lithium carbonate, or combinations thereof. It is sufficient to place the lithium electrode at one place in the lithium-ion capacitor, for example on one side of an electrode stack, but placing it on more sides may increase the pre-lithiation process even further.

The anode may be a battery anode. Examples of anode materials are graphite, silicon, hard carbon, soft carbon, tin, metal oxide, or combinations thereof. The cathode can be a supercapacitor cathode, but it may also be a hybrid cathode, which includes both battery cathode materials and supercapacitor cathode materials. Examples of lithium-ion battery cathode materials are lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, and other lithium metal oxides, or combinations thereof. Examples of super- capacitor cathode materials are activated carbon, graphene, carbide-derived carbon, car bon aerogel, carbon fiber cloth, carbon nanotubes, carbon cone, or combinations thereof.

External pressure can be applied on the cell during pre-lithiation, which may make the distance between the different electrodes in the electrode stack more uniform. The pre- lithiation process can be carried out at any desired temperature, e.g. room temperature, without its efficiency being affected to any significant degree.

Additional electrolyte can be introduced into the cell during pre-lithiation, which may be removed after pre-lithiation. Additional electrolyte may effectively increase the contact area between the lithium electrode and the electrode stack. As the lithium electrode is typically not used after the pre-lithiation, it can be removed to decrease size and weight of the capacitor.

In the following is described examples of embodiments illustrated in the accompanying drawings, wherein:

Fig. 1 shows a lithium-ion capacitor assembled for being pre-lithiated using the method according to the invention; Fig. 2 demonstrates the basic principle of behind the invention; and Fig. 3 shows flow diagrams of the preparation of a lithium-ion capacitor using the claimed method for pre-lithiation.

In the drawings, the reference numeral 1 indicates a lithium-ion capacitor. The drawings are presented in a simplified manner, and the features therein are not necessarily drawn to scale.

Figure 1 shows a lithium-ion capacitor 1 assembled for being pre-lithiated using the meth od according to the invention. The lithium-ion capacitor 1 comprises a multi-layer elec trode stack 2. The lithium-ion capacitor 1 is viewed from the direction normal to the plate shaped electrodes in the multi-layer electrode stack 2. The electrodes in the electrode stack 2 are indicated by a dashed line as the electrodes would otherwise not be visible in this view, since the electrode stack 2 is located within an aluminum pouch 7 for keeping the electrolyte within the capacitor 1. An anode tab 5 and a cathode tab 6 protrudes from each side of the pouch 7 to allow electrical connection of the electrode stack 2. A lithium electrode 3 is located on each side of the electrode stack 2 for providing the lithium ions for the pre-lithiation process. The lithium electrodes 3 each comprises a tab 4 protruding from the pouch 7 for electrical connection. Sealant tape 8 ensures that the electrolyte does not leak from the openings in the pouch 7 around the electrode tabs 4, 5, 6.

Figure 2 demonstrates the basic principle of the invention. In the first step, the cathode 11 is discharged towards the lithium electrode 3, whereby lithium ions 14 are transferred from the lithium electrode 3 and adsorbed onto the cathode 11. The fast surface-redistribution process of the lithium ions 14 on the cathode 11 causes the lithium ions 14 to be distribut ed rapidly over the surface of the cathode 11, also in the center of the plate-shaped cath ode 11. In the subsequent step, the cathode 11 is charged towards the anode 13, where by the lithium ions 14 are transferred from the cathode 14 through a separator 12 to the anode 13. The cathode 11 thereby functions as a helper to distribute the lithium ions 14 uniformly in the anode 13.

Figures 3a and 3b show flow diagrams of the preparation of a lithium-ion capacitor using the claimed method for pre-lithiation. First, an electrode stack is packaged together with a lithium electrode, for example in a laminated aluminum pouch. Then, in the process shown in figure 3a, the cathode is discharged towards the lithium electrode to adsorb lithi um ions on the cathode, followed by charging of the cathode towards the anode to trans fer the lithium ions to the anode. In the process shown in figure 3b, these two pre-lithiation steps are reversed. After the pre-lithiation steps, the degree of pre-lithiation of the anode is evaluated by measuring the potential of the anode. If a sufficient degree is obtained, the process continues to the formation step, and if not, the pre-lithiation steps are repeated.

The following examples demonstrate pre-lithiated anodes and electrochemical perfor mance of pre-lithiated capacitors prepared using the methods described herein. These examples are only for illustrative purposes and are not intended to limit the scope of the invention.

Example 1

Activated carbon electrodes were produced by industrial-scale slot die coating of com mercially available activated carbon (YEC-8B purchased from Fujian Yihuan, China) on etched aluminum foil. Carboxymethylcellulose (CMC) and Styrene-butadiene rubber (SBR) were utilized as binder while carbon black (Timcal Super C65) was employed as conductive additive. The mass ratio of activated carbon: CMC: SBR: Super C65 was 86.5:1.5:4.0:8.0. A cold calendaring was followed to densify the electrode and enhance the adhesion of the activated coating layer on the metal foil. Graphite electrodes were produced by industrial scale slot die coating of commercially available graphite (BFC-18 purchased from BTR, China) on copper foil. CMC and SBR were utilized as binder while Timcal Super C65 and graphite (Imerys graphite SFG-6L) were employed as conductive additive. The mass ratio of graphite: CMC: SBR: Super C65: SFG-6L was 90:1.5:3.5:3.2:1.8. A cold calendaring was followed to densify the electrode and enhance the adhesion of activated coating layer on metal foil.

A lithium-ion capacitor cell comprising a bi-layer electrode stack was assembled by sand wiching a layer of separator in the form of a cellulose diaphragm (TF4030, NKK) between the anode and the cathode. The plate area of both anode and cathode were 59 by 79 mm. A third lithium electrode was fabricated by stick lithium strip on the surface of copper foil and ultrasonic welder the copper foil on a nickel tab. 2 pieces of lithium electrode were positioned on each side of the electrode stack and separated by a layer of separator. Then, the electrode stack was sealed inside of a laminated Al foil case through hot seal ing. Finally, 3 ml commercial lithium-ion battery electrolyte was injected into the cell before final vacuum sealing. The cell was inserted into a testing fixture before pre-lithiation. Firstly, the cathode was discharged towards the lithium electrode at 0.23 mA to 2.0 V, which was followed by dis charging at 2.0 V for another 16 hours. Then, the cathode was charged towards the anode to lithiate the anode electrode at 4.6 mA to a cell voltage of 3.8 V. The process was re- peated until the potential of the anode decreased down to 0.01 V vs Li/Li ⁺ . Finally, the formation was performed by cycling the cells at C/10 for 2 cycles and C/5 for another 2 cycles between 2.0 and 4.0 V.

Example 2 The electrodes and bi-layer cell were prepared in the same way as in Example 1.

The cell was inserted into a testing fixture before pre-lithiation. Firstly, the cathode was discharged towards the lithium electrode at 0.46 mA to 2.2 V, which was followed by dis charging at 2.2 V for another 24 hours. Then, the cathode was charged towards the anode to lithiate the anode electrode at 4.6 mA to a cell voltage of 3.8 V. The process was re- peated until the potential of the anode decreased down to 0.01 V vs Li/Li+. Finally, the formation was performed by cycling the cells at C/10 for 2 cycles and C/5 for another 2 cycles between 2.0 and 4.0 V.

Example 3

The electrodes and bi-layer cell were prepared in the same way as in Example 1. The cell was inserted into a testing fixture before pre-lithiation. Firstly, the cathode was charged towards the anode at 4.6 mA to a cell voltage of 3.8 V. Then, the cathode was discharged towards lithium electrode at 0.23 mA to 2.0 V, followed by discharging at 2.0 V for 16 hours. The process was repeated until the potential of the anode decreased to 0.01 V vs Li/Li+. Finally, the formation was performed by cycling the cells at C/10 for 2 cycles and C/5 for another 2 cycles between 2.0 and 4.0 V.

Example 4

The electrodes were prepared in the same way as in Example 1. The cell was also pre pared in the same way as in Example 1, but with 9 layers of double side coated cathodes and 10 layers of double side coated anodes. The cell was inserted into a testing fixture before pre-lithiation. Firstly, the cathode was discharged towards the lithium electrode at 2.3 mA to 2.0 V, which was followed by dis charging at 2.0 V for another 16 hours. Then, the cathode was charged towards the anode to lithiate the anode electrode at 46 mA to a cell voltage of 3.8 V. The process was re peated until the potential of the anode decreased down to 0.01 V vs Li/Li ⁺ . Finally, the formation was performed by cycling the cells at C/10 for 2 cycles and C/5 for another 2 cycles between 2.0 and 4.0 V.

Control Example 1

The electrodes and bi-layer cell were prepared in the same way as in Example 1. This cell was pre-lithiated through external short circuit of the anode and the lithium electrode un- der room temperature for 10 days to finish the pre-lithiation. It was rested for another 10 days to let the lithium ions be distributed uniformly in the anode. Finally, the formation was performed by cycling the cells at C/10 for 2 cycles and C/5 for another 2 cycles between 2.0 and 4.0 V.

Control Example 2 The electrodes were prepared in the same way as in Example 1. The anode was pre- lithiated by attaching a piece of lithium foil on the surface of electrolyte impregnated elec trode for 1 hour. Then, a bi-layer cell was assembled in the same way as previous exam ples, but without any lithium electrode besides. Finally, the formation was performed by cycling the cells at C/10 for 2 cycles and C/5 for another 2 cycles between 2.0 and 4.0 V. Control Example 3

The electrodes were prepared in the same way as in Example 1. The anode was electro- chemically pre-lithiated by discharging at C/10 coupled with a lithium electrode to 80% depth of charge. Then, a bi-layer cell was assembled in the same way as previous exam ples but without lithium electrode besides. Finally, the formation was performed by cycling the cells at C/10 for 2 cycles and C/5 for another 2 cycles between 2.0 and 4.0 V.

Table 1: Resistance and capacitance of Li-ion capacitors produced with different pre- lithiation technologies. The resistance and capacitance of the cells were evaluated after formation. The AC re sistances were tested at 1 kHz at 100% state of discharge. DC resistance and capaci tance were obtained per IEC 62391-1. The data are summarized in Table 1. In summary, the disclosed pre-lithiation method shows similar electrochemical performance to electro- chemical pre-lithiation method, but better than other methods. Compared to electrochemi cal pre-lithiation method, the disclosed method is practical and scalable.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodi ments without departing from the scope of the appended claims. In the claims, any refer- ence signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.