Patent Description:
Electrical double-layer capacitors (EDLCs, also known as a type of symmetric supercapacitors) have relatively low working voltages and energy densities thereof depending on potential windows of decomposition of electrolytes present therein. The dissimilar configuration of the positive and negative electrodes in supercapacitors forms the asymmetric supercapacitors which have improved working voltages. Considering that E=CV<NUM>/<NUM>, (where E is energy stored in a supercapacitor; C is cell capacitance of the supercapacitor; and V is cell voltage across the supercapacitor), use of various combinations of electrode materials to form asymmetric cells and use of various electrolytes might further enlarge the cell voltages, resulting in enhanced energy storage performances of the asymmetric supercapacitors.

In recent years, hybrid capacitors, such as lithium-ion capacitors (LiCs), have been proposed to improve energy densities of supercapacitors by using the asymmetric design (i.e., dissimilar electrode materials are used for the negative and positive electrodes in each of the hybrid capacitors).

Lithium-ion capacitors mainly adopt activated carbon as a positive electrode material, and materials that allow insertions/extraction (intercalation/ deintercalation) of lithium ions as the negative electrode materials. Examples of such negative electrode materials include graphite, hard carbon, soft carbon, and lithium titanate. In comparison with EDLCs, lithium-ion capacitors exhibit the charging-discharging curves with cell voltages significantly higher than <NUM> V, as a result of a non-faradaic current on activated carbon in positive electrodes thereof (i.e., adsorption/desorption of anions) and a faradaic reaction of lithium ions on negative electrodes thereof (i.e., intercalation/deintercalation of lithium ions which have a voltage close to <NUM> V vs. Li/Li+). Such asymmetric electrodes allow lithium-ion capacitors to have working voltages of approximately <NUM> V, which is much higher than those of EDLCs (approximately <NUM> V).

However, power densities and cycle life of such asymmetric structures depend on properties of negative electrode materials. The intercalation/deintercalation of lithium ions in the aforementioned negative electrode materials has a voltage close to <NUM> V vs. Li/Li+, and forms a solid electrolyte interphase (SEI) membrane on the surface of the negative electrode materials. Therefore, the negative electrode materials are subjected to a prelithiation process before being used as negative electrodes of the lithium-ion capacitors.

<CIT> discloses a pre-lithiated lithium ion supercapacitor cathode, a preparation method thereof and a lithium ion supercapacitor. The lithium ion supercapacitor cathode comprises a cathode material composed of a lithium-free active material and a metal lithium-frame carbon composite material; and the content of the metal lithium-frame carbon composite material, by mass percentage, is <NUM>%to <NUM> % of the total mass of the cathode material. During the shelving process of the lithium ion supercapacitor device, lithium metal in the metal lithium-frame carbon material in the cathode reacts with carbon in the lithium-free active material in the cathode, so that a lithium-carbon compound can be formed, and therefore, the loss of lithium ions of the device during operation can be compensated; and the frame carbon material provides a porous structure for the cathode, and therefore, the infiltration of an electrolyte can be ensured, and the energy density of the device can be improved.

<CIT> discloses a method for pre-lithiation and pre-sodiation of a negative electrode, including the steps of: interposing a separator between a lithium ion-supplying metal sheet and a negative electrode to prepare a first simple cell; dipping the first simple cell in an electrolyte for pre-lithiation; carrying out primary electrochemical charging of the first simple cell dipped in the electrolyte for pre-lithiation to carry out pre-lithiation of the negative electrode; interposing a separator between a sodium ion-supplying metal sheet and the pre-lithiated negative electrodeto prepare a second simple cell; dipping the second simple cell in an electrolyte for pre-sodiation; and carrying out secondary electrochemical charging of the second simple cell dipped in the electrolyte for pre-sodiation to carry out pre-sodiation of the negative electrode. A pre-lithiated and pre-sodiated negative electrode obtained by the method and a lithium secondary battery including the same are also disclosed.

Therefore, an object of the disclosure is to provide a method for prelithiating a soft carbon negative electrode that can alleviate at least one of the drawbacks of the prior art.

According to an aspect of the disclosure, there is provided a method for prelithiating a soft carbon negative electrode according to claim <NUM>.

Other features and advantages of the disclosure will become apparent in the following detailed description of the embodiment(s) with reference to the accompanying drawings. It is noted that various features may not be drawn to scale.

It should be noted herein that for clarity of description, spatially relative terms such as "top," "bottom," "upper," "lower," "on," "above," "over," "downwardly," "upwardly" and the like may be used throughout the disclosure while making reference to the features as illustrated in the drawings. The features may be oriented differently (e.g., rotated <NUM> degrees or at other orientations) and the spatially relative terms used herein may be interpreted accordingly.

Different prelithiating processes are first discussed to evaluate elements that are to be included during prelithiating a soft carbon negative electrode. <FIG> and <FIG> show two different prelithiating processes of two identical soft carbon negative electrodes. For each of the prelithiating processes shown in <FIG> and <FIG>, firstly, the soft carbon negative electrode and a lithium metal piece are arranged to be spaced apart from each other such that a lithium-containing electrolyte is presented therebetween. Then, the soft carbon negative electrode and the lithium metal piece (serving as a Li/Li+ reference electrode) are subjected to a plurality of cycles of prelithiating/delithiating processes, thereby obtaining a respective prelithated soft carbon negative electrode. The prelithiating process corresponding to <FIG> includes, during each prelithiating, prelithiating the soft carbon negative electrode with a constant current (denoted as "CC"). The prelithiating process corresponding to <FIG> includes, during each prelithiating, prelithiating the soft carbon negative electrode with a constant current at a constant C-rate of <NUM> C, followed by a constant voltage (CV). That is, the prelithiating process shown in <FIG> may be represented by the expression: <MAT>.

In each of the aforementioned prelithiating processes, a voltage (the y-axis) of the soft carbon negative electrode (measured with reference to the Li/Li+ reference electrode) against a specific capacity thereof is shown but is not encompassed by the present invention. Before each prelithiating process, the soft carbon negative electrode may have an initial voltage greater than <NUM> V vs. Li/Li+ and not greater than <NUM> V vs. Li/Li+. <NUM> cycles of prelithiating/delithiating are shown in each of <FIG> and <FIG>. Delithiation capacities (Cs) and coulombic efficiencies of the two prelithiated soft carbon negative electrodes are shown in Table <NUM>.

Referring to Table <NUM> and <FIG> not according the invention, for the prelithiating process that includes only prelithiating the soft carbon negative electrode with the constant current (CC), an irreversible capacity loss of the soft carbon negative electrode is found in the <NUM>st and even after the <NUM>nd cycle; a delithiation capacity of the soft carbon negative electrode gradually decreases as a number of cycle increases, and a coulombic efficiency of the soft carbon negative electrode does not level until reaching the <NUM>th cycle. In addition, the delithiating curves for all <NUM> cycles do not overlap, especially at end potions of the delithiation curves. This shows that, formation of a solid electrolyte interphase (SEI) on a surface of the soft carbon negative electrode does not complete until approximately the <NUM>th, or the <NUM>th cycle of prelithiating/delithiating of the soft carbon negative electrode.

Referring to Table <NUM> and <FIG> not according to the invention, for the prelithiating process that includes prelithiating the soft carbon negative electrode at a constant C-rate of <NUM> C followed by a constant voltage (CV), an irreversible capacity loss of the soft carbon negative electrode is found merely in the <NUM>st cycle, and is hardly observed in each of the <NUM>nd, <NUM>rd, <NUM>th, <NUM>th cycles. Delithiation capacities of the soft carbon negative electrode of all <NUM> cycles are similar, and a coulombic efficiency of the soft carbon negative electrode levels at the <NUM>rd cycle. In addition, the delithiating curves for all <NUM> cycles overlap with one another and are stable. Therefore, formation of the SEI is completed at the <NUM>st cycle of prelithiating of the prelithiating process (in which the soft carbon negative electrode is prelithiated by a constant current (CC) mode followed by a constant voltage (CV) method), which is conducive for shortening a time period of the prelithiating process. Thus, a step of prelithiating a soft carbon negative electrode with a constant voltage (CV) is included in a method for prelithiating the soft carbon negative electrode in the following discussion.

<FIG> shows another prelithiating process similar to that shown in <FIG>, except that for the prelithiating process shown in <FIG>, the constant current is applied at a constant C-rate of <NUM> C. The prelithiating process shown in <FIG> may be represented by the expression: <MAT>.

From the results shown in <FIG>, it is known that prelithiating the soft carbon negative electrode with a constant voltage (CV) allows complete formation of the SEI. However, referring to Table <NUM> and <FIG>, prelithiating the soft carbon negative electrode at a higher constant current (i.e., prelithiating the soft carbon negative electrode with a higher C-rate, followed by prelithiating with the constant voltage) does not help in completing prelithiation of the soft carbon negative electrode in a shorter time period. The coulombic efficiency of the soft carbon negative electrode prelithiated at <NUM> C remains fluctuating for a few cycles, and prelithiation of the soft carbon negative electrode does not complete. This could be due to that during prelithiating the soft carbon negative electrode at a constant C-rate (i.e., during the voltage reducing from <NUM> V to <NUM> V), in a relatively low potential range (e.g., <NUM> V to <NUM> V), the lithium-ion intercalation capacity of the soft carbon negative electrode is relatively high and lithium ions migrate into the soft carbon negative electrode at a relatively slow rate. Therefore, when the soft carbon negative electrode is prelithiated at a relatively high constant C-rate in a relatively high potential range, followed by being prelithiated at a relatively low constant C-rate in the relatively low potential range, the prelithiation of the soft carbon negative electrode could be completed, and the time period for prelithiation of the soft carbon negative electrode could be shortened.

Based on the above, in accordance with some embodiments of the present disclosure, the following paragraphs provide a method for prelithiating a soft carbon negative electrode in a shorter time period. The method includes a prelithiating process. The prelithiating process may include steps a) to d). <FIG> illustrates a system <NUM> for performing the method in accordance with some embodiments of the disclosure. Please note that the elements in the system <NUM> are merely for schematic illustration and not drawn to scale.

In step a), in the system <NUM>, the soft carbon negative electrode <NUM> and a lithium metal piece <NUM> are spaced apart from each other such that a lithium-containing electrolyte (not shown) is presented therebetween. The lithium metal piece <NUM> may serve as a Li/Li+ reference electrode simultaneously. The system <NUM> is connected to an external power supply (not shown) so as to prelithiate the soft carbon negative electrode <NUM> in the following steps.

Referring to <FIG>, in some embodiments, the system <NUM> includes the soft carbon negative electrode <NUM>, the lithium metal piece <NUM>, a separator <NUM> soaked with the lithium-containing electrolyte (not shown), a pad <NUM>, a spring <NUM>, an upper cap <NUM> and a lower cap <NUM>.

The soft carbon negative electrode <NUM> and the lithium metal piece <NUM> are spaced apart from each other by the separator <NUM>. The separator <NUM> is soaked with the lithium-containing electrolyte and allows the lithium-containing electrolyte to pass therethrough, and avoids physical contact between the soft carbon negative electrode <NUM> and the lithium metal piece <NUM>.

In some embodiments, the lithium-containing electrolyte includes lithium hexafluorophosphate (LiPF<NUM>) dissolved in a solvent. The solvent may include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and combinations thereof. In addition, the lithium-containing electrolyte may also include vinylene carbonate (VC) as an additive. In certain embodiments, the lithium-containing electrolyte includes <NUM> of LiPF<NUM> dissolved in the solvent including EC, EMC and DMC in a volume ratio of <NUM>:<NUM>:<NUM>, and <NUM> wt% of VC based on <NUM> wt% of the lithium-containing electrolyte. In other embodiments, the lithium-containing electrolyte includes <NUM> of LiPF<NUM> dissolved in the solvent including EC and DMC in a volume ratio of <NUM>:<NUM>. In some other embodiments, the lithium-containing electrolyte includes <NUM> of LiPF<NUM> dissolved in the solvent including EC and DEC in a volume ratio of <NUM>:<NUM>.

The pad <NUM>, the spring <NUM>, the upper cap <NUM> and the lower cap <NUM> may be made of any suitable materials, so as to facilitate prelithiation of the soft carbon negative electrode <NUM>.

In step b), the soft carbon negative electrode <NUM> is prelithiated with a first constant current at a first constant C-rate until a voltage of the soft carbon negative electrode <NUM> is reduced to a first predetermined voltage that is not greater than <NUM> V vs. Li/Li+ (voltages disclosed in the following disclosure are measured with reference to the Li/Li+ reference electrode, and the soft carbon negative electrode <NUM> may have an initial voltage greater than <NUM> V vs. Li/Li+ and not greater than <NUM> V vs. Li/Li+). In some embodiments, the first constant C-rate is not greater than <NUM> C. In other embodiments, the first constant C-rate ranges from <NUM> C to <NUM> C. The first constant C-rate is not less than <NUM> C so as to ensure a reasonable time period for performing step b). The first constant C-rate is not greater than <NUM> C so as to ensure an effective prelithiation of the soft carbon negative electrode <NUM>. In certain embodiments, the first predetermined voltage ranges from <NUM> V to <NUM> V vs. Li/Li+. In other words, in step b), the soft carbon negative electrode <NUM> may be prelithiated at the first constant C-rate ranging from <NUM> C to <NUM> C until the first predetermined voltage ranging from <NUM> V to <NUM> V vs. Li/Li+ is reached.

In step c), after step b), the soft carbon negative electrode <NUM> is prelithiated with a second constant current (smaller than the first constant current) at a second constant C-rate until the voltage of the soft carbon negative electrode <NUM> is reduced to a second predetermined voltage that is lower than the first predetermined voltage. In some embodiments, the second constant C-rate is not greater than <NUM> C and is less than the first constant C-rate. In other embodiments, the second constant C-rate ranges from <NUM> C to <NUM> C. In certain embodiments, the second predetermined voltage is not less than <NUM> V vs. Li/Li+.

In step d), after step c), the soft carbon negative electrode <NUM> is prelithiated at a prelithiation constant voltage which is not greater than the second predetermined voltage, thereby completing prelithiation of the soft carbon negative electrode <NUM>. In certain embodiments, the prelithiation constant voltage is the same as the second predetermined voltage. In some embodiments, step d) is conducted for a time period ranging from <NUM> hours to <NUM> hours, thereby completing prelithiation of the soft carbon negative electrode <NUM>. In other embodiments, the soft carbon negative electrode <NUM> is prelithiated at the prelithiation constant voltage by a third (variable) current at a third (variable) C-rate. In some other embodiments, if the second constant C-rate is not less than <NUM> C, step d) is terminated when the third C-rate is less than <NUM> C, thereby completing prelithiation of the soft carbon negative electrode <NUM>. In yet other embodiments, if the second constant C-rate is lower than <NUM> C, step d) is terminated when the third current is less than <NUM>% of the second constant current, thereby completing prelithiation of the soft carbon negative electrode <NUM>.

In some embodiments, if necessary, the method further includes a delithiating process after the prelithiating process. The prelithating/ delithiating process may be repeated for a desired number of cycles, if necessary. In such case, step a), i.e., set up of the system <NUM> is omitted in the prelithating process of the repeated cycles (cycles performed after the first cycle).

In the method of the present disclosure, there are three different stages of prelithiating the soft carbon negative electrode <NUM>, so as to force lithium ions to migrate into the soft carbon negative electrode <NUM>, thereby completing prelithation of the soft carbon negative electrode <NUM>. The three stages are respectively steps b), c), and d) as discussed in the foregoing. In steps b) and c), the soft carbon negative electrode <NUM> is first prelithiated by a higher constant current (higher C-rate), followed by a lower constant current (lower C-rate), and is then prelithiated by a prelithiation constant voltage in step d). By including numerous stages of prelithiating, the method of the present disclosure is capable of obtaining a prelithiated soft carbon negative electrode in a significantly short time period. In some embodiments, the prelithiation of the soft carbon negative electrode <NUM> is completed by performing the three stages of prelithiating once. That is, by performing one-time prelithiating that includes the aforementioned three stages, the prelithiation of the soft carbon negative electrode <NUM> is completed.

The aforementioned prelithiated soft carbon negative electrode <NUM> may be used as a soft carbon negative electrode of a lithium-ion battery (not shown), or a soft carbon negative electrode <NUM> of an asymmetric lithium-ion supercapacitor <NUM> shown in <FIG>. Referring to <FIG>, the asymmetric lithium-ion supercapacitor <NUM> includes the soft carbon negative electrode <NUM>, a positive electrode <NUM> spaced apart from the soft carbon negative electrode <NUM>, an electrolyte <NUM> and a membrane <NUM>. The electrolyte <NUM> is disposed between the soft carbon negative electrode <NUM> and the positive electrode <NUM>. The membrane <NUM> is disposed between the soft carbon negative electrode <NUM> and the positive electrode <NUM>, and allows the electrolyte <NUM> to pass therethrough. In addition, the positive electrode <NUM> may include one of an activated carbon and an alkaline-activated soft carbon. The asymmetric lithium-ion supercapacitor <NUM> including the prelithiated soft carbon negative electrode <NUM> and the activated carbon-containing positive electrode <NUM> may be operated at a relatively high working voltage, such as approximately <NUM> V. In some cases, the asymmetric lithium-ion supercapacitor <NUM> may have an energy density that is at least twice as that of a symmetrical capacitor.

Examples of the disclosure will be described hereinafter. It is to be understood that these examples are exemplary and explanatory and should not be construed as a limitation to the disclosure.

A prelithiation system was prepared as described with reference to <FIG>. The prelithaition system included a soft carbon negative electrode, a lithium metal piece, a separator soaked with an electrolyte, a pad, a spring, an upper cap and a lower cap.

The soft carbon negative electrode was formed by coating a slurry that includes a soft carbon, a carboxymethyl cellulose and a carbon-based conductive material (Vulcan XC-<NUM>) over a copper foil so that the soft carbon on the coated copper foil had a weight of <NUM> per square centimeter. The lithium metal piece was formed by cutting a lithium metal sheet (purchased from Ubiq Technology Co. ) into a disc having a diameter ranging from <NUM> to <NUM>. The separator was formed by soaking a membrane with the electrolyte. The electrolyte was prepared by dissolving <NUM> of lithium hexafluorophosphate in a solvent including ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate in a volume ratio of <NUM>:<NUM>:<NUM>, and <NUM> wt% of vinylene carbonate based on <NUM> wt% of the electrolyte.

The prelithiation system was subjected to a plurality of cycles of prelithiation/delithiation within a voltage ranging from of <NUM> V to <NUM> V. The soft carbon negative electrode had a voltage of <NUM> V vs. Li/Li+ before prelithiating. During each delithiating, the soft carbon was delithiated at a rate of <NUM> C. During each prelithiating, the soft carbon negative electrode was, in stage (i), prelithiated at a first constant C-rate of <NUM> C (i.e., prelithiated at a first constant current) until reaching a first predetermined voltage of <NUM> V vs. Li/Li+ (voltage values stated hereinafter are measured with reference to the Li/Li+ reference electrode), and then in stage (ii), prelithiated at a second constant C-rate of <NUM> C (i.e., prelithiated at a second constant current) until reaching a second predetermined voltage of <NUM> V vs. Li/Li+, followed by in stage (iii), being prelithiated at a prelithiation constant voltage of <NUM> V for <NUM> hours, thereby completing prelithation of the soft carbon negative electrode, and obtaining a prelithiated soft carbon negative electrode of E1. The stages (i), (ii), and (iii) are represented by the following expressions:.

Prelithiation of the soft carbon negative electrode of E2 was generally similar to that of E1, except that during each prelithiating, in stage (i), the first predetermined voltage was set at <NUM> V vs. Li/Li+. The stages (i), (ii), and (iii) are represented by the following expressions:.

The procedures of CE1 differs from those of E1 in that there were only two stages in CE1. The two stages of prelithiating with two different constant currents in E1 (i.e., stages (i) and (ii) in E1), were replaced by a single stage of prelithiating with only one constant current in CE1 (i.e., stage (i) in CE1). Stage (ii) of CE1 remained the same as stage (iii) of E1. The stages (i), and (ii) of CE1 are represented by the following expressions:.

It should be noted that if coulombic efficiencies of the <NUM>nd, <NUM>rd, <NUM>th and <NUM>th cycles for the soft carbon negative electrode are high with little fluctuation, completion of the <NUM>st cycle prelithiating of the soft carbon negative electrode was regarded as completion of prelithiation of the soft carbon negative electrode. Time periods for completion of the <NUM>st cycle prelithiating of the soft carbon negative electrodes of E1, E2 and CE1 are shown in Table <NUM>.

It is found that, the <NUM>st cycle prelithiating of the soft carbon negative electrodes of E1 and E2 was completed earlier than that of CE1. In addition, the <NUM>st cycle prelithiating of the soft carbon negative electrode of E2 was completed earlier than that of E1. Prelithiating/delithiating curves of the soft carbon negative electrode of E2 are shown in <FIG>. Based on the three stages performed to prelithiate the soft carbon negative electrode of E2, further investigations are performed to achieve optimization of parameters of each of the three stages. Details thereof are discussed in the following paragraphs.

Three samples of soft carbon negative electrodes, each of which was prelithiated in a similar manner as that of E2, except that stage (iii) was conducted for different time periods, i.e., <NUM> hour, <NUM> hour, and <NUM> hours, respectively. The delithiation capacities and coulombic efficiencies of the prelithiated soft carbon negative electrodes of the three samples and E2 are shown in Table <NUM>.

Since coulombic efficiencies of the <NUM>nd, <NUM>rd, <NUM>th and <NUM>th cycles for each sample shown in Table <NUM> are high with little fluctuation, completion of the <NUM>st cycle prelithiating of the soft carbon negative electrode in each sample shown in Table <NUM> was regarded as completion of prelithiation of the soft carbon negative electrode. As shown in Table <NUM>, prelithiation of the soft carbon negative electrodes (of all three samples and E2) completes when stage (iii) was conducted for a time period ranging from <NUM> hour to <NUM> hours. In addition, the time period ranging from <NUM> hour to <NUM> hours is sufficient for completion of stage (iii), and thus completing prelithiation of the soft carbon negative electrodes. In the following discussion, the aforesaid sample that was subjected to stage (iii) for <NUM> hours (the median among <NUM> hour, <NUM> hour, <NUM> hours and <NUM> hours) were taken as Example <NUM>-<NUM> (E2-<NUM>). Prelithiating/delithiating curves of the soft carbon negative electrode of E2-<NUM> are shown in <FIG>. The stages (i), (ii), and (iii) performed in prelithiation of the soft carbon negative electrode of E2-<NUM> are represented by the following expressions:.

The procedures in prelithiations of the soft carbon negative electrodes of E2-<NUM>, E2-<NUM>, and E2-<NUM> were generally similar to those of E2-<NUM>, except that during each prelithiating, in stage (i), the soft carbon negative electrode was prelithiated at different C-rate, i.e., <NUM> C, <NUM> C, and <NUM> C, respectively. The delithiation capacities and coulombic efficiencies of the prelithated soft carbon negative electrodes of E2-<NUM>, E2-<NUM>, E2-<NUM>, E2-<NUM> are shown in Table <NUM>. Time periods for completion of different stages (in terms of prelithiating with different voltage level) of the <NUM>st cycle prelithiating of the soft carbon negative electrodes of E2-<NUM>, E2-<NUM>, E2-<NUM>, E2-<NUM> and CE1 are shown in Table <NUM>.

As shown in Tables <NUM> and <NUM>, in the prelithiation of the soft carbon negative electrode of E2-<NUM>, in which stage (i) is performed by prelithiating the soft carbon negative electrode at <NUM> C, and stage (ii) is performed by prelithiating the soft carbon negative electrode at <NUM> C, the <NUM>st cycle prelithiating is completed within the shortest time period, and is approximately <NUM>% of the time period of the two-stage method (i.e., the prelithiation performed in CE1 using only a single stage of prelithiating with only one constant current, followed by prelithiating with one constant voltage). Prelithiating/delithiating curves of the soft carbon negative electrode of E2-<NUM> are shown in <FIG>. The stages (i), (ii), and (iii), performed in prelithiation of the soft carbon negative electrode of E2-<NUM>, are represented by the following expressions:.

The procedures in prelithiations of the soft carbon negative electrodes of E2-<NUM>-<NUM>, and E2-<NUM>-<NUM> were generally similar to those of E2-<NUM>, except that during each prelithiating, in stage (ii), the soft carbon negative electrode was prelithiated at different C-rate, i.e., <NUM> C and <NUM> C, respectively. The delithiation capacities and coulombic efficiencies of the prelithated soft carbon negative electrodes of E2-<NUM>-<NUM>, and E2-<NUM>-<NUM> are shown in Table <NUM>. Time periods for completion of different stages (in terms of prelithiating with different voltage level) of the <NUM>st cycle prelithiating of the soft carbon negative electrodes of E2-<NUM>-<NUM>, and E2-<NUM>-<NUM> are shown in Table <NUM>.

As shown in Tables <NUM> and <NUM>, in the prelithiation of the soft carbon negative electrode of E2-<NUM>-<NUM>, in which stage (i) is performed by prelithiating the soft carbon negative electrode at <NUM> C, and stage (ii) is performed by prelithiating the soft carbon negative electrode at <NUM> C, the <NUM>st cycle prelithiating is completed within the shortest time period. Prelithiating/delithiating curves of the soft carbon negative electrode of E2-<NUM>-<NUM> are shown in <FIG>. The stages (i), (ii), and (iii), performed in prelithiation of the soft carbon negative electrode of E2-<NUM>-<NUM>, are represented by the following expressions:.

Claim 1:
A method for prelithiating a soft carbon negative electrode (<NUM>), comprising the steps of:
a) disposing the soft carbon negative electrode (<NUM>) and a lithium metal piece (<NUM>) spaced apart from each other such that a lithium-containing electrolyte is present therebetween;
b) prelithiating the soft carbon negative electrode (<NUM>) at a first constant C-rate until a voltage of the soft carbon negative electrode (<NUM>) is reduced to a first predetermined voltage that is not greater than <NUM> V vs. Li/Li+, the first constant C-rate being not greater than <NUM> C; characterized in that it comprises the steps of
c) after step b), prelithiating the soft carbon negative electrode (<NUM>) at a second constant C-rate until the voltage of the soft carbon negative electrode (<NUM>) is reduced to a second predetermined voltage that is lower than the first predetermined voltage, the second constant C-rate being not greater than <NUM> C and being less than the first constant C-rate; and
d) after step c), prelithiating the soft carbon negative electrode (<NUM>) at a prelithiation constant voltage which is not greater than the second predetermined voltage, thereby completing prelithiation of the soft carbon negative electrode (<NUM>).