The invention relates to a method for pre-lithiating an anode for an energy storage device.

Lithium-ion batteries are generally seen as the state-of-the-art energy storage devices and are very important in the transition towards a fossil-free society. They contain a positive electrode comprising a material which is able to release lithium ions, and a negative elec trode able to accept lithium ions, for example graphite, placed in a suitable electrolyte. When the lithium-ion battery is charged for the first time, some lithium ions will be lost due to formation of the solid-electrolyte interface, which will decrease the performance of the battery. To mitigate this loss, the anode is typically doped with lithium ions prior to assem- bly of the battery. This is referred to as pre-lithiation.

Lithium-ion capacitors are hybrid systems which integrate a lithium-ion battery negative electrode, for example graphite, and a supercapacitor positive electrode, typically activat ed carbon, together. Therefore, they exhibit a high specific power, a good cyclic stability, and a moderate specific energy, so they have a wide range of potential applications. However, pre-lithiation of the anode with lithium ions is a prerequisite step to lower the potential of the anode, thereby widening the operation voltage window and increasing the specific energy.

Various methods have been proposed for the pre-lithiation of the anode. They can be di vided into three groups, namely methods using lithium metal, lithium-containing com- pounds, or lithium ions. US 6862168 B2 discloses use of a sacrificial metallic lithium elec trode, which is partially or completely dissolved during the first charge. A drawback is that metal foils with penetrating holes, which are expensive, are required as current collectors to let the lithium ions pass through. Additionally, the pre-lithiation process is very slow.

Stabilized lithium metal particles have also been used for the pre-lithiation. Lithium car- bonate (Cao, W.J. and J.P. Zheng, Li-ion capacitors with carbon cathode and hard car- bon/stabilized lithium metal powder anode electrodes. Journal of Power Sources, 2012. 213: p. 180-185) or lithium hexafluorophosphate (US 2017/0062142 A1 and US 20 2014/0146440 A1) have been coated on the surface of lithium metal particles to prevent its reactivity with oxygen. However, a drying room is still required for handling stabilized lithium metal particles.

Lithium-containing compounds have also been utilized as lithium sources for the pre-lithiation of lithium-ion capacitors. Kim and co-workers (Park, M.-S., et al., A Novel Lithium-Doping Approach for an Advanced Lithium Ion Capacitor. Advanced Energy Mate rials, 2011. 1(6): p. 1002-1006.) utilized a lithium transition metal oxide mixed with activat- ed carbon as positive electrode, thereby providing lithium cations to the negative electrode during the first charge step. The transition metal oxide cannot be lithiated again during the following discharge process. The delithiated metal oxide will be left in the positive elec trode as electrochemical inactive materials. Therefore, the specific energy of the cell is reduced. Recently, F. Beguin and co-workers (Jezowski, P., et al., Safe and recyclable lithium-ion capacitors using sacrificial organic lithium salt. Nature Materials, 2017) employed a mix ture of sacrificial organic lithium salt and activated carbon as positive electrode. The lithi um salt is oxidized, and lithium cations are released to the negative electrode during the first charge. The oxidized salt will be dissolved into the electrolyte. However, the proposed salt is air-sensitive, which makes it difficult to handle.

Lithium salt in the electrolyte has also been considered as lithium sources for pre- lithiation. F. Beguin and co-workers employed a specific charging protocol to provide the negative electrode with lithium cations from the electrolyte (Khomenko, V., E. Raymundo- Pinero, and F. Beguin, High-energy density graphite/AC capacitor in organic electrolyte. Journal of Power Sources, 2008, 177(2): p. 643-651). Stefan et al. pre-lithiated the nega tive electrode by oxidizing the lithium salt in the electrolyte (US 2015/0364795 A1). Lithi um salts normally have a limited solubility in the organic solvent, so the conductivity of the electrolyte is reduced, and thereby also the specific power.

In US 2002/0122986 A1 it is disclosed to store lithium ions in a separator which is made with molecular sieves to compensate the lithium ions lost in lithium-ion battery, thus ex tending the lifetime of lithium-ion batteries. However, the cost is too high for commercial application, and the lithium ion storage capacity is also very limited.

Although all these approaches are effective or partially effective in pre-lithiating the nega- tive electrode of lithium-ion battery or capacitor, they all have their drawbacks. None of the known methods can meet the requirements of being efficient, having low cost, being safe to handle, and having no significant side effect at the same time.

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. The invention is defined by the independent patent claims, while the dependent claims define advantageous embodiments of the invention.

In a first aspect, the invention relates more specifically to a method for pre-lithiating an anode for an energy storage device, wherein the method comprises the steps of: Assem bling an electrochemical cell comprising the anode, an activated carbon electrode, and a solution comprising a dissolved lithium salt; charging the activated carbon electrode to wards the anode; and disassembling the cell to obtain a pre-lithiated anode.

The anode may typically be a battery-type anode comprising graphite and/or silicon, pos- sibly combined with other carbon species such as hard carbon. A separator may be placed between the anode and the cathode. Pre-lithiation is triggered by charging the ac tivated carbon electrode towards the targeted anode, whereby anions are absorbed on the surface of the activated carbon electrode while lithium ions are intercalated into the an ode. The pre-lithiated anode may be used in an energy storage device such as a battery to mitigate the problem caused by the lithium ions lost in the solid-electrolyte interface or a lithium-ion capacitor to increase the energy storage capacity by increasing the cell volt age. The volume of the activated carbon electrode relative to the volume of the anode may be chosen based on the desired degree of pre-lithiation; a larger volume of activated carbon will have a larger surface area and be able to accommodate a larger amount of ions, whereby a larger amount of lithium ions will be able to be absorbed or intercalated into the anode.

An advantage of this method is that the solution comprising a dissolved lithium salt, for example LiPF ₆ , LiCI, UNO ₃ , LITFSI, or LiFSI, which could be cheap or air-stable, functions both as an electrolyte and a source of lithium ions. The method is therefore cost-effective, safety issues related to lithium metal are avoided, and the usage efficiency of the lithium source is increased.

The method may additionally comprise the step of drying the anode before further pro cessing. In one embodiment, the method may additionally comprise the step of treating the activat ed carbon electrode to desorb anions from the activated carbon electrode and obtain a reusable activated carbon electrode. The treatment may for example include heat treat ment or washing of the activated carbon electrode in a solution, which will accelerate the self-discharge process by releasing the adsorbed anions into the solution. The solution may for example be ethanol, DMC, and/or an aqueous solution and may comprise further additives as known by a person skilled in the art. An advantage of this washing step is that the activated carbon electrode can be re-utilized, whereby the method reduces waste. The treatment step may also include boiling the activated carbon electrode in an aqueous so- lution to release the anions, for example for 5 minutes to 24 hours depending on the prop erties of the activated carbon and the anions. Alternatively, or in addition, the treatment step may include heating the activated carbon electrode to e.g. 50-200 °C for example for 5 minutes to 24 hours depending on the properties of the activated carbon and the anions. The heating may be performed in an inert atmosphere. The method may thereby addition- ally comprise the step of repeating the pre-lithiation of the anode using the reusable acti vated carbon electrode. In this way the degree of the pre-lithiation may be increased.

In a second aspect, the invention relates to a method for pre-lithiating a plurality of an odes, wherein the method comprises repeating the steps of the method according the first aspect of the invention with the reusable activated carbon electrode and different anodes. In this way pre-lithiation of a plurality of anodes may be performed using only one activat ed carbon electrode and a solution containing a lithium salt, whereby the pre-lithiation will be cost-effective and produce little waste.

In the following is described examples of preferred embodiments illustrated in the accom panying drawings, wherein:

Fig. 1 shows an assembled electrochemical cell comprising an anode, an activat ed carbon electrode, and a solution comprising a dissolved lithium salt;

Fig. 2 shows the electrochemical cell of figure 1 in a charged state;

Fig. 3 shows the pre-lithiated anode from figure 2;

Fig. 4 shows the activated carbon electrode from figure 1 and 2 being washed;

Fig. 5 shows the voltage profiles of three cells (solid, dashed, and dotted lines) during pre-lithiation for example 1; Fig. 6 compares the cyclic stability of Li-ion battery with (solid line) and without

(dashed lines) pre-lithiation;

Fig. 7 shows the voltage profiles of three cells (solid, dashed, and dotted lines) during pre-lithiation for example 2; and Fig. 8 shows the voltage profiles of three cells (solid, dashed, and dotted lines) during pre-lithiation for example 3.

In the drawings, the reference numeral 1 indicates an electrochemical cell. The drawings are illustrated in a schematic manner, and the features therein are not necessarily drawn to scale. Figure 1 shows an assembled electrochemical cell 1 comprising an anode 3, an activated carbon electrode 5, and a solution 7 comprising a dissolved lithium salt of lithium ions 11 and anions 9. In figure 2, the activated carbon electrode 5 is being charged with the anode 3 as counter and working electrode. This results in intercalation of lithium ions 11 between adjacent graphite layers 13 to pre-lithiate the anode 3, while anions 9 are adsorbed on the surface of the porous activated carbon 15. Upon dissembling of the electrochemical cell 1, a pre-lithiated anode 3 is obtained as shown in figure 3 for further processing manufactur ing of an energy storage device such as a battery or lithium-ion capacitor. An advantage of this pre-lithiation method is that the activated carbon electrode 5 may be reused by treating the activated carbon electrode 5, typically by washing it with an aqueous solution, whereby the anions 9 will desorb from the activated carbon surface 15 as shown in figure 4. By reusing the activated carbon electrode 5, the waste produced by this method will be greatly decreased compared to other pre-lithiation methods.

Examples

Example 1 Three coin cells were assembled with components supplied by Hohsen (Japan) while ac tivated carbon as cathode and graphite as anode. The activated carbon electrode was manufactured through a dry process by Unigreen (China). The activated carbon electrode was cut into disk with a diameter of 15 mm. The graphite electrode was homemade by a roll-to-roll slot die coating process with a single side areal capacity of 1.74 mAh/cm ² . The graphite electrode was cut into disk with a diameter of 16 mm. A cellulose paper provided by NKK (Japan) was utilized as separator, while 50 pL conventional lithium-ion battery electrolyte was utilized. These coin cells were charged at 0.14 mA for 2 hours after rested for 12 hours. This is equivalent to 10% pre-lithiation degree. The voltage profiles of the cells are presented in Fig. 5 (solid, dotted, and dashed lines). The cells were disassembled immediately after pre-lithiation. Three new coin cells were assembled with the pre-lithiated graphite as an- ode and Lithium Nickel Manganese Cobalt Oxide (NMC532) as cathode. NMC532 cath ode with areal capacity of 1.6 mAh/cm ² was purchased from Univercell (Germany). The received electrode was cut into a disk with a diameter of 15 mm. The electrolyte and sep arator were the same as the previous cells. Another three coin cells with fresh graphite electrode and fresh NMC electrode were assembled as well to demonstrate the benefit of pre-lithiation. The cyclic stabilities are presented in Fig. 6 of cells with (solid line) and without (dashed line) pre-lithiation. The pre-lithiation can obviously improve the cyclic sta bilities.

Example 2

The activated carbon electrodes used in example 1 were recovered by rinsing with DMC followed by boiling in water for 2 hours. Three new coin cells were assembled with the recovered activated carbon electrode as cathode and fresh graphite electrode as anode. The electrolyte and separator were the same as in example 1. The coin cells were charged at 0.14 mA for 4 hours after rested for 12 hours. This is equivalent to 20% pre- lithiation degree. The cells were disassembled immediately after pre-lithiation. The voltage profiles of the cells during pre-lithiation are presented in Fig. 7 (solid, dotted, and dashed lines). It can be observed that the initial voltages are close to 0 V, which indicates the ani on has been fully desorbed. Additionally, the final voltage is about 3.8 V, which is still with in the electrolyte stability window.

Example 3 The used activated carbon electrode in example 2 were rinsed with DMC first and then dried at 150 °C for 2 hours inside of Ar-filled glove box. Three new coin cells were assem bled with the recovered activated carbon electrode as cathode and fresh graphite elec trode as anode. The electrolyte and separator were the same as in Example 1. The coin cells were charged at 0.14 mA for 4 hours after rested for 12 hours. This is equivalent to 20% pre-lithiation degree. The cells were disassembled immediately after pre-lithiation.

The voltage profiles of the cells during pre-lithiation are presented in Fig. 8 (solid, dotted, and dashed lines). It can be observed that the initial voltages are close to 0 V, which indi- cates the anion has been fully desorbed. Additionally, the final voltage is about 3.8 V, which is still within the electrolyte stability window.

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.