A Review: Pre‐lithiation Strategies Based on Cathode Sacrificial Lithium Salts for Lithium‐Ion Capacitors

Similar to lithium‐ion batteries (LIBs), during the first charge/discharge process of lithium‐ion capacitors (LICs), lithium‐intercalated anodes (e.g., silicon, graphite, and hard carbon) also exhibit irreversible lithium intercalation behaviors, such as the formation of a solid electrolyte interface (SEI), which will consume Li+ in the electrolyte and significantly reduce the electrochemical performance of the system. Therefore, pre‐lithiation is an indispensable procedure for LICs. At present, commercial LICs mostly use lithium metal as the lithium source to compensate for the irreversible capacity loss, which has the demerits of operational complexity and danger. However, the pre‐lithiation strategy based on cathode sacrificial lithium salts (CSLSs) has been proposed, which has the advantages of low cost, simple operation, environmental protection, and safety. Therefore, there is an urgent need for a timely and comprehensive summary of the application of CSLSs to LICs. In this review, the important roles of pre‐lithiation in LICs are detailed, and different pre‐lithiation methods are reviewed and compared systematically and comprehensively. After that, we systematically discuss the pre‐lithiation strategies based on CSLSs and mainly introduce the lithium extraction mechanism of CSLSs and the influence of intrinsic characteristics and doping amount of CSLSs on LICs performance. In addition, a summary and outlook are conducted, aiming to provide the essential basic knowledge and guidance for developing a new pre‐lithiation technology.


Introduction
As the main energy storage equipment, LIBs have been successfully applied in people's daily life.However, due to the limitation of energy storage mechanism, they have a much lower power density than electrochemical capacitors (ECs).3][4][5][6][7][8][9][10] The first LIC, designed and reported by Amatuccietal [11,12] in 2001, used nanostructure Li 4 Ti 5 O 12 as the anode and activated carbon as the cathode.However, because of the high lithium intercalation potential of Li 4 Ti 5 O 12 anode (~1.5 V vs Li + /Li) and the potential of activated carbon hardly exceeding 4 V versus Li + /Li, the cutoff voltage of this LIC device was only 2.5 V.[15][16] Therefore, other lithium storage anode materials with low lithium intercalation potential are required.
Since the development of the first LIC device, many typical LIB-type anode materials (such as Si, TiSb 2 , Li 2 TiSiO 5 , Sn, SnO 2 , and P 2 Sn 3 and various carbon-based compounds) with low discharge potential, high capacity, excellent rate performance, and good cycle life have been proposed to improve the energy density, power density, and lifespan of LIC systems. [15, Howevr, during the first charge cycle, lithiumion loss is inevitable due to the irreversible electrochemical process and/or solid electrolyte interface (SEI) formation, especially when the anode potential is lower than 1 V versus Li + /Li.For the graphite anode, the irreversible capacity loss depends largely on the electrode surface area accessible to the electrolyte and generally increases approximately linearly with an increase in the area ratio of the non-basal surface.[48][49] An irreversible capacity loss means that some of the intercalated lithium cannot be deintercalated from the anodes, which results in insufficient lithium ions concentration in the electrolyte.Meanwhile, it can also lead to an irreversible adsorption of anions with the same mole number on the surface of cathode-activated carbon, which prevents the complete discharge of the cathode.[52] To address this problem, it is necessary to compensate lithium to the LICs system for its irreversible capacity loss (i.e., to perform pre-lithiation operation).
So far, various pre-lithiation technologies, including the ectopic electrochemical (EEC) method, in situ electrochemical (ISEC) method, and in situ chemical (ISC) method, have been used to resoundingly realize the pre-intercalation of lithium. [53]The EEC method involves preassembling a half-cell with lithium metal as the counter-electrode, while the ISEC method involves short-circuiting anode and auxiliary lithium electrode with an external circuit.Compared with the EEC method, the ISEC method involves fewer steps for battery reassembly; however, it uses a through-hole current collector, which increases the cost.Meanwhile, the ISC method involves contacting the lithium metal directly with the anode in the presence of the electrolyte, which has security risks due to the existence of lithium sheet/foil/powder.In addition, the lithium ions present in the relatively concentrated lithium salt electrolyte can be used as a supplementary lithium source, which can avoid the application of a lithium auxiliary electrode but increase the resistance of the electrolyte.Recently, a novel pre-lithiation strategy based on CSLSs has been proposed.In this method, a type of lithium-based compound is directly added to the cathode, which irreversibly supplies lithium ions to the anode during the first charge cycle, and is then inactivated, leaving AC as the sole active electrode material.It is hereinafter referred to as "sacrificial lithium salt."This method has the advantages of short time consumption, simple process, high safety factor, and controllable lithium doping level, which promote the commercialization of LICs.
The key of this pre-lithiation method is to identify sacrificial lithium salts with high capacity, high irreversibility in the first cycle, low lithium extraction voltage, and no residual "dead" materials after the first charge.Although many articles and patents have been published on the use of sacrificial lithium salts for pre-lithiation, no review has yet provided a detailed summary and exposition.Therefore, a timely and comprehensive summary of applying CSLSs to LICs is urgently required.
In this review, as shown in Figure 1, we first discuss the multiple roles of the pre-lithiation technology applied in LICs.Second, the various pre-lithiation methods are classified in detail (divided into five categories), and their advantages and disadvantages are compared.Then, the lithium supplement method based on CSLSs in LICs is emphatically introduced, including the lithium extraction mechanism, the influence of intrinsic characteristics (e.g., lithium removal potential, reversibility, and residue) and the amount of CSLSs added to the cathode on LICs performance is determined.Meanwhile, since beginners might confuse CSLS-based LICs with lithium-ion battery-type supercapacitors (LIBCs), this article also makes a distinction between them.Finally, a series of recommendations and prospects for future research are afforded in accordance with the current development status of CSLSs in LICs.

Roles of Pre-lithiation 2.1. Improving the Coulombic Efficiency and Lifespan of LICs
During the first charge/discharge cycle of LICs, the lithium loss caused by the irreversible electrochemical processes and/or SEI formation is inevitable, especially when the anode potential is lower than 1 V versus Li + /Li.As shown in Figure 2a, the irreversible capacity loss of graphite is 6.6% and that of hard carbon (HC) is 29.8%. [67]Consequently, these materials deliver relatively low first-cycle coulombic efficiency, directly resulting in poor energy density and cycle life of the LIC device.In addition to carbon materials, the initial coulombic efficiency of the Si anode is also low (50%-80%), which is triggered by the formation of an SEI layer.Nevertheless, as shown in Figure 2b, the pre-lithiation technologies are usually conducted before the normal cycling of the LIC device and enable the formation of a stable SEI layer on the HC surface through a complex chemical process, thus significantly mitigating the irreversible capacity of the anode and increasing the coulombic efficiency. [68]As shown in Figure 2c, Kumagai et al. [69] studied the influence of pre-lithiation degree on the coulombic efficiency, cycle stability, and internal resistance of an AC//HC system.The results showed that the system performs the best at a pre-lithiation degree of 71.1%, which is evidently better than the performance achieved when the pre-lithiation level is too high (89%) or in the absence of prelithiation.As shown in Figure 2c1, the coulombic efficiency of the system with pre-lithiation level of 71.7% after 100 cycles is approximately 100%, while that of the system without pre-lithiation declines to approximately 20% at the 20th cycle.This example proves that the prelithiation treatment can significantly improve the coulombic efficiency and lifespan of LIC systems.

Extending the Working Potential Window of the Cathode
Generally, LICs without pre-lithiation follow a single-step charging and discharging mechanism.As shown in Figure 3a, during the first charge cycle from open-circuit voltage (OCV) to the maximum voltage (I-II), PF 6 ions diffuse from the electrolyte to the cathode, while Li ions are intercalated in the anode at the same time, resulting in an increase in Li + concentration in the anode and a slight decrease in this concentration in the electrolyte.When the cell is discharged from the maximum voltage to OCV (II-III), PF 6 ions desorb from the cathode, while Li ions de-intercalate from the anode and enter the electrolyte, resulting in a decrease in Li + concentration in the anode and its recovery in the electrolyte to the initial state.In this process, the cathode in the LIC system without pre-lithiation exhibits an operating potential profile of 3.5-4.2V versus Li/Li + , which is always higher than the OCV.
The pre-lithiation treatment is capable of providing lithium sources for the cathode and introduces an additional potential variation window and a charge-discharge mechanism.Then, the cathode in the LIC system with pre-lithiation undergoes a two-step ion migration mechanism, one step below the OCV and the next step above the OCV.Hung et al. [70] explained the whole ion diffusion principle in an AC(+)// pre-lithiated HC(−) system by combining the theoretical analysis and experimental results.The charging/discharging process can be divided into two parts-the electrolyte consumption part above the OCV and the Li + diffusion part below the OCV-where the capacity is provided by the electrolyte and pre-lithiated lithium ions, respectively.Figure 3a, b is schematic diagrams of the charge-discharge voltage curve and in situ 7 Li nuclear magnetic resonance (NMR) of an AC (+)//prelithiated HC (−), respectively.When the cell is discharged from the maximum voltage to the OCV (II-III), PF 6 ions desorb from the cathode, while Li ions de-intercalate from the anode and enter the electrolyte, resulting in a decrease in Li + concentration in the anode and the recovery of ion concentration in the electrolyte to the initial state.When the voltage decreases from the OCV to the minimum voltage (III-IV), due to the pre-lithiation treatment, the anode continues to delithium to provide lithium ions for the system; thus, the Li + concentration in the anode continues to decrease, while that in the cathode increases rapidly.The final charging process from the minimum voltage to the OCV (IV-V) is completely opposite to the discharging process  [67] with permission from IOP Publishing.b) SEI formation on HC surface during pre-lithiation.Reproduced from Yao et al. [68] with permission from IOP Publishing.c) Example AC//HC systems with or without pre-lithiation treatment: the effects of the pre-lithiation levels on c1) the coulombic efficiency; c2) the cycling stability; and c3) the internal resistance.Reproduced from Kumagai et al. [69] with permission from Elsevier Ltd.
Energy Environ.Mater.2023, 6, e12506 from the OCV to the minimum voltage.The intercalation peaks (2.5-5.5 ppm) and double peaks (~0.6 and ~−2.75 ppm) in the in situ NMR are consistent with results of the voltage analysis with continuous variation.
Figure 3c shows the ideal potential distribution of the cathode and anode, where the pre-lithiation treatment increases the potential window and capacity.According to the experimental data of Zhang et al. [71] for AC (+)//mesocarbon microbead (−) systems with different pre-lithiation degrees, the cathode potential distribution of the system without pre-lithiation is 3.41-4.25V (Figure 3d1).However, the potential range is extended to 2.63-4.11V (Figure 3d2), and even 2.14-4.03][74] These results indicate that the pre-lithiation treatment introduces additional charge/discharge processes below the OCV, which extends the available potential window of the cathode.

Dropping the Initial Potential and Potential Distribution of the Anode
The anode material without pre-lithiation shows a high initial operating potential, mostly between 2.0 and 3.0 V (i.e., in a lithium-depleted state).Therefore, when assembling LIC devices using such anode with a high initial potential and AC cathode, the open-circuit potential of the device is relatively low.For example, without pre-lithiation, the initial potentials of both the anode and cathode electrodes of the HC//AC LIC are approximately 3 V, and the open-circuit potential of the HC//AC LIC is nearly zero.As shown in Figure 4a, the anode and cathode potentials increase from 0.75 to 1.9 V and 4.0 to 4.6 V versus Li/Li + , respectively.The HC anode potential oscillates significantly, and the AC cathode potential exceeds beyond the upper limit of the electrochemical stable potential, resulting in low energy density (~23 Wh kg −1 ), rapid capacitance degradation, and low cycle life of the HC//AC LIC. [54]evertheless, with an increase in the pre-lithiation degree, the anode potential gradually decreases from over 2.1 to 0.001 V; thus, the above HC//AC LIC systems provide a high open-circuit potential of 2.9 V and high energy density (85 Wh kg −1 ) after pre-lithiation.Figure 4b shows that the potentials of the HC anode and AC cathode swing within a relatively low and stable voltage range (from 0.47 to 0.12 V and 2.27 to 4.02 V versus Li/Li + , respectively).Here's another example.Figure 4c shows the effects of different pre-lithiation degrees on the anode potential in AC//LTO systems.The pre-lithiated anodes also show much lower potential distribution compared with the results obtained without pre-lithiation, which leads to a high-voltage output of the LIC system.][77] Figure 4d is a schematic diagram of the anode potential distribution and voltage distribution of LIC systems as a function of capacity, which shows that the working potential of the anode is in a  7 Li NMR spectra: b1) Front view of the stacked plot; b2) Side view from right; and b3) Top view.Reproduced with permission from Shellikeri et al. [70] Copyright (2016) American Chemical Society.c) the cathode potential window is enlarged and the capacity is increased by pre-lithiation treatment.d) The charge/discharge curves of AC//LMCMB systems through different degrees of pre-lithiation treatment: d1) The capacity of pre-lithiation is 0 mAh g −1 : LIC0; d2) The capacity of pre-lithiation is 100 mAh g −1 : LIC100; and d3) The capacity of pre-lithiation is 300 mAh g −1 : LIC300.Reproduced from Zhang et al. [71] with permission from Elsevier Ltd.
Energy Environ.Mater.2023, 6, e12506 low and relatively stable charge-discharge platform, which is conducive to the sufficient utilization of the AC cathode.Therefore, the reduction in the initial potential and potential distribution of the anode is a key advantage of the Li pre-lithiation technology, which widens the potential difference between the anode and cathode, resulting in a higher cell potential and energy density.

Increasing the Matching of Anode and Cathode
[80][81][82] Compared with the AC cathode, the anode always exhibits lower reaction kinetics and cycle stability, which inevitably results in poor power density and cycle life of the full device.To address the above problems, Jin et al. [78,83,84] proposed a practical and universal electrode matching approach for manufacturing LICs by taking an AC(+)//HC(−) system as an example.First, the discharge process and kinetic behavior of the anode at different potential windows were analyzed by in situ electrochemical impedance spectroscopy (EIS).Theoretically, the optimal operating potential range can be systematically evaluated and determined by EIS, [78,79,83] galvanostatic intermittent titration technique (GITT), [85] potentiostatic intermittent titration technique (PITT), [85] and galvanostatic charge and discharge (GCD) tests. [79,80,86,87]As indicated in Figure 5b, HC exhibits different electrochemical properties in four stages within 3.0-0.01V, and the optimal operating potential range of the anode with excellent reaction kinetic behavior that best matches the AC electrode is 0.1-0.7 V.The different pre-lithiation degrees result in different working potential ranges of the anode and different electrode matching states.The influence of the pre-lithiation degree on the matching states can be demonstrated by comparing various full cells at different pre-lithiation degrees.Then, the anode can be operated within the designed optimal working potential range by the desirable prelithiation degree, as shown in Figure 5c.After that, the anode with the desirable pre-lithiation degree and cathode are coupled to assemble LIC devices.The effective potential windows of the anode and cathode are monitored, and their effective specific capacity and optimal mass ratio are determined based on the monitoring results, thus maximizing the power density and cycle life of the device.Therefore, the pre-lithiation technology can effectively improve the matching of the anode and cathode.

Pre-lithiation Methods
According to the characteristics of pre-lithiation technology described in the literature, they can be divided into the following five categories, and their advantages and disadvantages are summarized in detail in this article.

EEC Lithium Supplement Method
As shown in Figure 6a, the EEC method is usually carried out by first pre-assembling a half-cell using the anode as the working electrode and lithium metal as the counter-electrode.The anode in the assembled half-cell reaches the required pre-lithiation degree by assigning predetermined cycle times or discharging to a predetermined potential.Then, the pre-lithiated anode is reassembled with the AC cathode to manufacture LICs.This method ensures that all Li ions consumed in the formation of SEI and/or other irreversible electrochemical processes originated from metallic lithium rather than the electrolyte.This  [50] with permission from Elsevier Ltd. c) Effect of different pre-lithiation amount on anode potential in AC/LTO systems.c1) 0; c2) 10; and c3) 80. Reproduced from Xu et al. [77] with permission from Elsevier Ltd. d) Schematic diagram of anode potential distribution and voltage distribution of LIC system as a function of capacity.Reproduced from Jin et al. [84] with permission from Elsevier Ltd.
Energy Environ.Mater.2023, 6, e12506 method is widely used in laboratories, and up to now, anode materials such as graphite, [88][89][90] HC, [73,91,92] SC, [93] mesocarbon microbeads, [71,76,94] graphene, [95][96][97] LTO, [77,88] and metal oxides [98,99] have been pre-lithiated using this method.The above results indicate that this method must meet two basic requirements: i) the number of cycles must be sufficient to stabilize the cell and eliminate the side reactions and ii) the pre-lithiation rate must be low.To accurately control the overall electrochemical reaction and lithiation level, it should be carried out at a relatively low current density (such as 0.01C, 0.02C, or 0.05C).Note that the smaller the current density, the more sufficient the Li intercalation reaction and the larger the lithium intercalation amount.Li deposition occurs as a reaction in parallel to intercalation, especially at a high current density.When the current density is high, the anode potential decreases rapidly, and when the potential exceeds a certain threshold, Li deposition instead of Li intercalation occurs, leading to fast degradation/corrosion and safety issues. [100,101]his method has the following two advantages: i) SEI formed during pre-lithiation is relatively uniform and stable and ii) the prelithiation degree is easy to accurately control, because the capacity (for those with a stable voltage platform) or cutoff voltage (for those without a voltage platform) of anode materials can be set.However, this method also has many shortcomings, which cannot be ignored.It not only adds additional procedures, such as assembly and disassembly of pre-lithiated cells, but also increases the process complexity caused by handling lithiated anodes, which are highly sensitive to air and moisture.Moreover, the anodes might be damaged due to uncontrollable factors in the reassembly process, which might lead to performance degradation. [30,69,71,80,88,102,103]Therefore, these defects considerably affect the feasibility of this method in large-scale production.In response to the pitfalls of this approach, Qu et al. [104] used a Cu mesh as the anode collector and placed the sacrificial Li-metal electrode on the "back" side of the anode, which provides advantages in that the cell does not involve the rearrangement step.Subsequently, to address the safety issues caused by the existence of Li metal, Zhou et al. [105] successfully pre-lithiated the Sibased anode in a Li-metal-free way using a new electrolytic cell, which was fabricated with a Cu pitting corrosion-type anodic half-cell in an aqueous electrolyte and a Li battery-type cathodic halfcell in a gel polymer electrolyte.

ISEC Lithium Supplement Method
In this method, a lithium auxiliary electrode is intercalated into the LIC device and separated from the two electrodes, and the anode and auxiliary lithium electrode are short-circuited through an external circuit to complete the pre-lithium process.As shown in Figure 6b, during the pre-lithiation process, lithium metal is oxidized to release Li ions, which are reduced on the anode surface, and the anode voltage drops to close to 0 V (vs Li/Li + ).The metal lithium electrode is partially or completely dissolved during the first charge cycle.This method has the following characteristics.i) The pre-lithiation degree is controlled by the amount of auxiliary lithium.ii) The pre-lithiation rate and time can be modified by adjusting the external load resistance.iii) This method  [84] with permission from Elsevier Ltd.  [56] with permission from Elsevier Ltd. b) is reproduced from Tsuda et al. [66] with permission from IOP Publishing.c1, c2) are reproduced with permission from Cao & Zheng. [54]Copyright (2012) Elsevier and Cao et al. [65] Copyright (2014) Elsevier, respectively.
Energy Environ.Mater.2023, 6, e12506 uses special pre-punched aluminum and copper foils as the cathode and anode current collector, respectively.Kim et al. [106] proved that the Li ions present in an auxiliary lithium electrode can be transferred to the anode through the holes in the current collector by using the electrochemical method.The JM energy company of Japan [107,108] was the first to use this method to pretreat the anode.The cycle life of its LICs increased and the energy density reached approximately 10 Wh kg −1 .So far, this method is generally used in commercial LICs.
The advantage of this method is that the anode potential can be stabilized and the rearrangement step can be eliminated.In addition, because the pre-lithiation rate and time can be adjusted by the external load resistance, the pre-lithiation degree and rate can also be easily adjusted.However, this method has many disadvantages.111] ii) During the first charging step, the consumption of the lithium sacrificial electrode can lead to volume change inside the LIC cells, which can cause rupture between the electrodes and separators. [112]][115] iv) In the cell manufacturing process, it is difficult to accurately determine the required amount of lithium, because the loaded lithium metal cannot be completely intercalated into the anode material.v) The use of a pre-punched aluminum/copper foil increases the cost on account of the complexity of its manufacturing technology.vi) The lithiation time is too long, resulting in the reduction in efficiency.To address the problem of slow rate and long time of pre-lithiation, Tsuda et al. [116] optimized the above process by introducing a porous graphite anode with an opening area of 1% in 2016.Two types of graphite electrodes (a porous graphite anode with 1% opening area and an anode simply coated with graphite layer on the porous collector) were used to evaluate the dependence of lithiation rate on reaction temperature.The lithiation rate of the porous graphite electrode is seven times higher than that of the graphite electrode prepared on the porous collector at 25 °C.Limited by 20 h of pre-lithiation time, the lithiation rate of the porous anode electrode is twice as high as that of the graphite electrode prepared on the porous collector at 55 °C, as shown in Figure 7a.This confirms that the lithiation rate in the porous graphite electrode is much faster.In 2018, Tsuda et al. applied additional via holes on both the anode and cathode, which further increased the ion diffusion efficiency. [66]

ISC Lithium Supplement Method
This method is commonly mentioned in the literature.As shown in Figure 6c, the novel cell structure assembled with two electrodes requires the lithium metal to be in close contact with the graphite anode in the presence of the electrolyte, which can be achieved by pressing the lithium metal against the top of the graphite anode.However, the unused lithium remaining on the anode surface after the prelithium treatment can lead to lithium dendrite growth, which can affect the service life and safety of the equipment.Therefore, the maximum amount of lithium used cannot exceed the capacity of the anode. [117]s shown in Figure 7b, after the electrolyte is added, due to the establishment of the lithium metal/electrolyte interface, the lithium metal begins to electrolyze to generate Li ions and electrons (process A).Then, Li ions enter the electrolyte and undergo a solvation process  [66] with permission from IOP Publishing.b) Detailed process of pre-lithiation by ISC method.c) The 7 Li NMR spectra of the pre-lithiation of graphite using coated lithium metal.Reproduced from Holtstiege et al. [118] with permission from Elsevier Ltd. d) Diagram of four different lithium sources used ISC method: 1) SLMP; 2) thick Li strips; 3) thin Li film, and 4) thin Li film with pin holes.e) Corresponding to the change process of four different lithium sources immersed in electrolyte for 24 h in d diagram.f) Corresponding to the change in anode potential of four different lithium sources in d diagram during pre-lithiation.Reproduced from Shellikeri et al. [67] with permission from IOP Publishing.
due to the existence of solvent molecules (process B).When the anode reaches a specific potential (~0.8 V vs Li/Li + for graphite in most electrolytes), the reduction in electrolyte salts and solvents is initiated, and the reduction products are deposited on the surface of the carbon anode to form SEI. Because of its electronic insulation and ionic conductivity, the SEI film can prevent further reduction in the electrolyte, but it cannot prevent Li ions from being intercalated into the anode bulk.As a result, the solvated Li ions diffuse to the anode through the SEI and are dissolved on its surface (process C).Finally, lithium ions undergo charge transfer by combining with electrons at the SEI/electrode interface (process D).The driving force for this process mainly originates from the potential difference between graphite and the lithium metal.The reaction kinetics of the pre-lithium process based on this driving force are more difficult to monitor than the electrochemical prelithiation method, where the degree and partial rate of pre-lithiation can be adjusted by the externally set current.Florian et al. [118] used 7 Li NMR spectroscopy to monitor the apparent dissolution rate of lithium metal and the formation of the corresponding LiC x phase during the pre-lithiation process, to gain insight into the surface reaction kinetics during the pre-lithiation process, as shown in Figure 7c.
Stabilized lithium metal powder's benefits: i) Since SLMP is mostly tiny particles with a diameter of 20-50 μm, it can be uniformly coated on the electrode surface without damaging it.ii) SLMP with a high theoretical specific capacity of 3600 mAh g −1 exhibits better chemical stability than other lithium products. [136]It is suitable for large-scale production and can be safely stored for a long time at a wide temperature range of 25-55°C in a dry environment.iii) Because of its large specific surface area and easy uniform dispersion, it can accelerate the ionization rate of lithium and realize high-efficiency pre-lithiation.iv) The expensive copper mesh used to attach the lithium metal foil is not required. [137]SLMP's shortcomings: i) It is difficult to control the chemical lithiation process and SEI formation due to the direct contact between the lithium metal and anode material particles.ii) SLMP is more expensive than lithium foil and has relatively low purity (e.g., 97-98%), which might lead to high self-discharge and poor cycle life. [127,138]iii) During the first charging process, the non-lithium materials in SLMP and thermal runaway can cause serious side reactions and produce gaseous products, such as N 2 , CO, and CO 2 . [139]ithium strips'/films' merits: i) Because of their high purity (e.g., 99.9%), [140] the manufactured cells are more efficient and relatively safe to operate.ii) They have lower cost than SLMP.Lithium strips'/films' drawbacks: i) They can only partially cover the anode electrode surface and easily damage it due to the induced mechanical pressure.
ii) To ensure that the capacity of the lithium strip/film electrode is not higher than that of the anode, the lithium strips (thickness: 45 μm) should be cut into 5 mm or thinner strips.Thus, the cutting and loading of the narrow strips in mass production is costly and difficult to operate.iii) It takes as long as 18 h to complete the prelithiation process, which is too time-consuming.To shorten the prelithiation time, Yan et al. [134] used 15-20 μm ultra-thin lithium films as the lithium source.The results showed that, compared with lithium strips, the 200F LICs using 20 μm ultra-thin lithium films achieved not only higher capacitance (246F) and lower ESR (19 Ω) but also a high energy density of 26.9 WhL −1 and a high power density of 18.3 kWL −1 .
As shown in the previous section, although the traditional prelithiation method addressed the problem of supplementing lithium for the anode, there are still a great deal of defects, which cannot meet the practical application requirements.Therefore, two tentative novel methods were proposed, as detailed below.

Concentrated Lithium Salt Electrolyte Lithium Supplement Method
In the concentrated lithium salt electrolyte (CLSE) method, the lithium source is a relatively concentrated lithium salt electrolyte, such as lithium hexafluorophosphate (LiPF 6 ) or lithium bis (trifluoromethylsulfonyl) imide (LiTFSI). [141,142]As shown in Figure 6d, the Li ions from the electrolyte are supplied to the graphite anode to form the graphite intercalation compound by applying approximately 10 successive charge/self-discharge pulses.Béguin et al. [141,142] used 2 M (2 mol L −1 ) high-concentration lithium salt of LiTFSI dissolved in 1:1 ethylene carbonate/dimethyl carbonate as the electrolyte to compensate for the lithium loss caused by lithium intercalation.First, the Li ions provided at such a high concentration can satisfy the amount required to produce SEI.Second, the high concentration of lithium ions in the electrolyte can satisfy multiple current pulses separated by the rest period, allowing the lithium ions to further intercalate lithium into graphite.Finally, the system achieves a high energy density of up to 80 Wh kg −1 (based on the total mass of active substances only).The advantage of this method is that it changes the complex structure required for the use and removal of the auxiliary lithium electrode in traditional LICs.However, the disadvantage is that the high concentration of lithium salt can seriously affect the ionic conductivity and viscosity of the electrolyte.In addition, the Li ions present in the electrolyte fluctuate significantly during the cycle, resulting in a medium cycle life of the device.Thus, this method is not conducive to commercialization and will not be reviewed in the subsequent part of this article.

Cathode Sacrificial Lithium Salt Lithium Supplement Method
In the cathode sacrificial lithium salt (CSLS) method, as shown in Figure 6e, sacrificial lithium salt is added to the cathode as a lithium supplement material.The basic working principle of this method is that the sacrificial lithium salt irreversibly releases Li ions into the electrolyte during the first charge cycle, and these ions are then intercalated into the anode to achieve pre-lithiation.After this, AC is the only active material left in the cathode, and the residue of the sacrificial lithium salt serves only as a "dead" material in subsequent cycles.From a process viewpoint, this method can also achieve in situ pre-lithiation in the initial cycle, which is relatively more desirable than the anode prelithiation method.This method has the following advantages.i) It is technically feasible.As only a sacrificial lithium salt is added to the cathode in the electrode preparation, the tedious pre-lithiation procedures are avoided and the pre-lithiation time is considerably shortened.ii) The aforementioned safety problems are reduced by replacing lithium with CSLS.iii) There are no additional requirements for environmental conditions.iv) A stable SEI can be formed by following a pre-lithiation process of setting a constant current density of 0.1C (C: theoretical capacity of CSLS) or setting a potential above the de-lithiation plateau in the first cycle or previous cycles.v) Excellent controllability can be achieved by adjusting the amount of sacrificial lithium salt added to the cathode.This method has the following disadvantages.i) The cost is determined by the price of the sacrificial lithium salt, but so far, there is no ideal candidate material for evaluation.ii) The biggest bottleneck of this method at present is that the residual "dead" materials in the cathode increase the total weight of the device and reduce its mass-energy density.

Performance Comparison of Different Methods
According to their assembly methods and working principles, we compare the performances of different pre-lithiation methods and enumerate their benefits and drawbacks in detail.From the perspective of safety, the CSLS lithium supplementation method is safer than the other methods because it does not use a pure lithium metal.In terms of cost, traditional pre-lithiation methods are costly because they require reassembly steps (EEC), or use pre-punched current collectors, or relatively expensive lithium sources.However, the cost of the CSLS lithium supplement method mainly depends on the sacrificial materials, and most sacrificial lithium salts are relatively inexpensive.From the perspective of large-scale production, as compared to the EEC method, the ISEC, ISC, and CSLS methods can easily achieve industrialization.In terms of energy density, the CSLS lithium supplementation method is not dominant, which is the major bottleneck it has encountered so far.In terms of controllability of the pre-lithiation degree, which is difficult to control, except for the ISE method, all other methods can control the degree with the amount of lithium source or anode potential (with discharge plateau) or anode capacity (without discharge plateau).In terms of the requirements for the operating environment, due to the instability of the lithium metal, it cannot be operated directly in air but in a glovebox or a dry room.In contrast, most of the CSLSs are relatively stable and can be directly added to the cathode when preparing the electrodes under ambient conditions.Although several pre-lithiation methods have been developed, and some of them have even been industrialized, the cost, time consumption, safety, and industrial production difficulties cannot be underestimated.For a more clear and intuitive understanding, various pre-lithiation methods (except the CLSE method) are compared in Table 1 and Figure 8 from diverse perspectives.

Types of Sacrificial Lithium Salt Materials
A batch of CSLSs has been developed and successfully used to achieve anode pre-lithiation.The research results show that the pre-lithiation method based on CSLS can achieve large-scale application.To date, the pre-lithiation behavior of materials ranging from inorganic (lithium-containing transition metal oxides, binary compounds, and composites with synergistic effect) to organic has been systematically investigated.
Binary compounds, such as Li 3 N [61,148,149] and Li 2 S, [60] have also been studied.However, Li 3 N is generally harsh on the operating environment, reacts violently with water under ambient conditions, reacts with most commonly used organic solvents (such as Nmethy-2-pyrrolidone (NMP), dimethylacetamide (DMAC), and dimethyl sulfoxide (DMSO)) during pulping, and generates nitrogen after decomposition. [150][153] Due to the complexity of the reaction between Li 2 S and AC, the in situ lithiation process and LIC assembly process need to be further optimized in terms of, for example, electrode composition, cathode/anode mass or capacity ratio, electrolyte formula, and dosage.Moreover, Li 2 O, [154][155][156] Li 2 O 2 , [157] and LiF, [158] which have achieved in situ pre-lithiation in LIBs, have been extensively studied.They can also be applied to LICs. [159,160]owever, the additives Li 2 O and LiF suffer from the problem of strict preparation conditions, and the biggest obstacle that hinders Li 2 O 2 is that it is not easy to decompose and a catalyst is required for its activation. [161,162]n addition, composites, [59] such as pyrene and Li 3 PO 4 complexes, can also serve as excellent CSLS.[168][169] Then, Li 3 PO 4 (pKa = 12.2) acts as a lithium source to capture protons [167] and release an equal amount of Li ions, thus ensuring the pre-lithiation of the anode.The proposal of such complexes addresses the main and common weakness of the current pre-lithiation methods: both the lithium source and electron source are individual lithiated compounds.
In the following, this article provides an in-depth interpretation of CSLS lithium supplement method based on previously published articles.Table 2 lists the characteristics of all CSLSs and the performances of devices based on them.

Mechanism of Lithium Extraction in CSLS
In this study, the lithium extraction mechanism of CSLS is analyzed and demonstrated to take account of several specific cases of lithium transition metal oxides (LTMOs), to provide an across-the-aboard understanding.The LTMOs are irreversibly oxidized in the first charge step of the device, during which lithium ions are irreversibly released from the LTMOs and diffused to the anode electrode to form SEI or graphite intercalation compounds.Taking LiFeO 4 as an example, according to Equation (1), Li + is released during the first charge cycle, accompanied by the partial oxidation of Fe 3+ (to Fe 4+ ) and O 2− (to O 2 ) and the formation of oxygen vacancies. [179] Thus, the mechanism of lithium extraction of LTMOs is related to the valence change in transition metals during the extraction of lithium.Park et al. [62] determined the chemical state of Li 2 MoO 3 during the first charge using ex situ X-ray photoelectron spectroscopy (XPS).As shown in Figure 9a, when the cutoff voltage is increased from 4.3 to 4.7 V (vs Li/Li + ), Li ions are extracted from the host material to form the delithiated phase Li 2-x MoO 3 (0 < x < 1.7), accompanied by the oxidation of Mo 4+ to Mo 6+ , which can be confirmed by the shift of Mo 3d binding energy peak.The Mo 4+ /Mo 6+ redox reaction allows Li + extraction, while the oxygen stays intact in the oxygen 2p orbital even in a fully charged state.Lim et al. [143] investigated the de-lithium mechanism of Li 5 FeO 4 .As shown in Figure 9b, two distinct peaks appear in the first cycle due to the oxidation of Fe 3+ to Fe 4+ , corroborating to the two-step Li + extraction (two Li ions extracted at each step) from Li 5 FeO 4 under a given voltage window, during which the Li 5 FeO 4 provides a capacity of up to 700 mAh g −1 .Although these articles provide explanation for the extraction mechanism of lithium sacrifice extraction, they are not exhaustive enough.
On the basis of various experiments, combined with theoretical calculations, Cho et al. [63] performed a detailed study on the extraction mechanism of Li 6 CoO 4 .In the experimental study, as shown in Figure 10a, the change in the valence state of Co governs the Li + extraction and insertion into the host structure.Figure 10a2 shows the peak of 779.8ev corresponding to Co 2+ before lithium ions are extracted.When Li ions are extracted from Li  10b, where lithium occupies two positions.In terms of the Wyckoff positions, Li(1), Li (2), Co, and O occupy 8f, 4d, 2a, and 8 g sites, respectively. [180,181]ince the bond length and average net charges affect the local Coulomb interaction energy in the geometry of LiO 4 tetrahedron, [182] as shown in Figure 10c, the authors analyzed the changes in the bond length (Li and O) and the average net charge (Li and O) and found that the lithium extraction potential is strongly dependent on the position of lithium ions in the Li 6 CoO 4 structure.Taking these factors into account, the average de-lithiation potentials for Li (1) and Li (2) sites were calculated as 3.5 and 3.8 V (vs Li/Li + ), respectively, which is consistent with the experimental results (Figure 10d).
The relative formation energies of Li 6-x CoO 4 need to be calculated to ensure its structural stability.Figure 11a shows the formation energies of possible Li + configurations in each de-lithiation form of Li 6- x CoO 4 . [183,184]When the extraction of Li ions in Li 6-x CoO 4 is increased to x = 2.0, it is mainly controlled by the oxidation from Co 2+ to Co 4+ .When 2.0 < x ≤ 3.5, the oxidation of O is mainly responsible for further extraction of Li + .Figure 11b-e shows that less than 3.5 mol of Li ions can be extracted from Li 6 CoO 4 without significant structural degradation, which is mainly related to O 2 release.However, at x > 3.5, the material structure evidently collapses.
In conclusion, the mechanism of lithium extraction from LTMOs is not only governed by the change in the valence state of transition metals but might also be accompanied by the oxidation of O, and the amount of lithium extraction considerably influences the structural stability of lithium-removed phase.Therefore, the extraction mechanism of lithium needs to be analyzed accurately through experimental study and theoretical calculation.

Irreversible Capacity, de-Lithiation, and Re-Lithiation Potential
In CSLS selection, the theoretical specific capacity should be calculated first, and then, it should be determined whether a small amount of this material can meet the pre-lithiation requirements.The theoretical specific capacity can be expressed as follows: where n is the mole numbers of Li in sacrificial lithium salts, N A is the Avogadro constant, e is the electric charge of an electron, F is the Faraday constant, and M is the relative molecular mass of sacrificial lithium salt.Materials with large theoretical capacity are more likely to become excellent CSLS, because of the small dose required.However, there is still a gap between the theoretical capacity and practical application capacity, and the actual capacity is closely related to the cutoff potential of lithium extraction.The previously developed lithium metal oxides, such as Li 2 MoO 3 and Li 5 FeO 4 , need to be operated at a high potential (above 4.7 V vs Li/Li + ) to supply sufficient Li ions to the anode. [62,143,185]There is a controversial claim that if the operating potential of CSLS is excessively increased (>4.3 V vs Li/Li + ), it will inevitably lead to side reactions associated with the decomposition of the electrolyte and the performance deterioration caused by the blockage of AC pores.In addition, the gas released by the decomposition of the electrolyte endangers safety.However, Park et al. [143] took a different view, arguing that these materials only reach the high potential of 4.7 V (vs Li/Li + ) during the first charge cycle, after which LIC operates at a voltage window below 3.9 V and the de-lithiated Li 5 FeO 4 no longer participates in the reaction.Thus, the effect of high lithium extraction potential is not as serious.In fact, more researchers believe the first view that the de-lithiation potential of CSLS should not be too high and be in accordance with the operating potential range of LICs.
To explore CSLS with low extraction potential of Li ions, Jeżowski et al. [144] [62] with permission from Willy periodicals.b) Cyclic voltammograms of AC-Li 5 FeO 4 electrode.Reproduced from Park et al. [143] with permission from Wiley-VCH.c) Requirement for de-lithiation and re-lithiation potentials of CSLS.Reproduced from Park et al. [146] with permission from Elsevier Ltd.
Energy Environ.Mater.2023, 6, e12506 LIC cathode.In 2016, they also studied the performance of Li 5 ReO 6 as the CSLS, and found that the irreversible capacity of this CSLS was 410 mAh g −1 (close to the theoretical capacity of 423 mAh g −1 ) at a potential lower than 4.3 V versus Li/Li + , which can significantly reduce its dose ratio in the LIC cathode. [64]or developing materials with low de-lithiation potential, attention has been turned to organic lithium salts.For example, AC-Li and Li 2 C 4 O 4 have specific capacities of 349.9 and 375 mAh g −1 , respectively, when charged to 4.2 V (vs Li/Li + ) for the first time. [57,177]Moreover, Li 2 DHBN achieves a specific capacity of 320 mAh g −1 at a preintercalated lithium cut-off potential of 3.2 V (vs Li/Li + ). [56]Recently, the Li 3 N studied by Sun et al. [61] yielded a practical specific capacity of as high as 1379 mAh g −1 at a low potential of 4.1 V (vs Li/Li + ), which considerably reduced the amount of sacrificial lithium salt in the cathode.As seen from the above results, the de-lithiation potential of the organic sacrificial lithium salt is lower than that of the inorganic sacrificial lithium salt.However, the preparation conditions of the organic sacrificial lithium salt are harsh, and the organic sacrificial lithium salt is difficult to be purified due to a large number of by-products.
In general, as an ideal CSLS, in addition to high irreversible capacity, their potential of de-lithiation and re-lithiation is also strictly required.As shown in Figure 9c, the de-lithiation potential of the CSLS is required to be lower than the upper operating limit potential of AC, while the re-lithiation potential of the CSLS is required to be either lower than the lower potential limit of AC (type 1) or irreversible (type 2).The purpose of this is not only to prompt Li ions released from the CSLS in the normal operating range of LICs to form an SEI and be intercalated into the anode material but also to ensure that AC serves as the sole electroactive material in subsequent cycles.

Reversibility and Residual
Whether CSLS is reversible or not is closely related to the success of prelithiation, since the irreversible lithium loss during the first cycle can only be compensated when CSLS is irreversible, thus ensuring the anion-cation balance in the electrolyte.The sacrificial lithium salts can be evaluated in detail by designing (sacrificial lithium salts/Li) half-cells and testing their galvanostatic voltage curves and in situ XRD patterns.Park et al. [62] used Li 2 MoO 3 with a hexagonal structure and R-3 m space group as sacrificial lithium salt and found that two evident voltage plateaus appeared at 3.6 V (vs Li/Li + ) and 4.4 V (vs Li/Li + ) in the galvanostatic voltage plots during the first charge cycle, which corresponded to the Li ions extracted from the Li layer and transition metal layer in the Li 2 MoO 3 structure (because it has a cubic close-packed oxide ion lattice with basal planes of octahedral interstices filled alternately with lithium and lithium/molybdenum (1:2 molar ratio)), [186,187] respectively.This Li 2 MoO 3 sacrificial lithium salt has an initial charge capacity exceeding 250 mAhg −1 when charged to 4.7 V (vs Li/Li + ) and a discharge capacity of approximately 75 mAhg −1 when subsequently discharged to 2.5 V (vs Li/Li + ), indicating an irreversible capacity of more than 70% (meaning that <30% of lithium ions were recovered to sacrificial lithium  [63] with permission from Royal soc chemistry. Energy Environ.Mater.2023, 6, e12506 salt during the discharge process).The in situ XRD pattern shows that the peaks are at 18.0°and 36.5°, which corresponded to the (003) and (006) planes of Li 2 MoO 3 in the original state, respectively, gradually decreased until the voltage reached 4.7 V (vs Li/Li + ) when Li ions were extracted, and then did not fully recover during the subsequent discharge process (Figure 12a1).Afterwards, in 2012, Park et al. [143] used Li 5 FeO 4 , an orthogonally symmetric anti-fluorite structure with the Pcab (a = 9.214 Å, b = 9.206 Å, c = 9.169 Å) space group, [188,189] as the sacrificial lithium salt.It was found that under the cutoff voltage of 4.7 V (vs Li/Li + ), 4 mol Li + was extracted twice, and the initial charge capacity was up to 678 mAhg −1 .The irreversible capacity of this material was as high as 84%, which was also confirmed by the in situ XRD pattern (Figure 12a2).In 2015, Lim et al. [63] discovered a new tetragonal Li 6 CoO 4 with an anti-fluorite structure as a sacrificial lithium salt.They found that the Li + concentration was up to 6 mol in a given structure, and when the cutoff voltage reached 4.3 V (vs Li/Li + ), approximately 4 mol Li + could be electrochemically extracted from the structure; thus, the initial charge capacity was approximately 630.2 mAhg −1 .Finally, the irreversible capacity of this material was calculated to be as high as 98% (Figure 12a3).Furthermore, the reversibility of Li 1-x Ni 1 + x O 2 depends on the amount of Li + deficiency, and a highly reversible cathode material for LIBs can be obtained when the Li + deficiency is <0.2.In contrast, when the Li + deficiency is higher than 0.2, Li + can be irreversibly extracted from the material and lose their electrochemical activity during further circulation, so the material can be used as a good sacrificial lithium salt for LIBs.
Nonetheless, the de-lithiated states of the above materials are electrochemically inert after the initial charge and discharge cycles.Thus, the residues after de-lithiation in the cathode become the so-called "dead materials," which will only increase the total weight and reduce the energy density of the device.Similarly, Li 5 ReO 6 , [64] Li 0.65 Ni 1.35 O 2 , [144] and Li 2 CuO 2 [145] become "dead materials" after de-lithiation.Jin et al. [55] estimated the effect of the dead materials on the energy density of LICs using the lithium foil as a reference.The results showed that the energy density loss is acceptable only when the capacity of the CSLS is sufficiently high (>1000 mAhg −1 ).Therefore, it is urgent in the LIC field to develop new advanced CSLS materials with high specific capacity or no dead materials after de-lithiation.
Based on the above requirements, Jezowski et al. [56] developed Li 2 DHBN, which not only has high irreversible capacity at a low lithium extraction potential but the residual material can also be completely dissolved in the electrolyte.In addition, the practical specific capacity of the lithium-based nitride Li 3 N is up to 1379 mAhg −1 at a  [63] with permission from Royal soc chemistry.
Energy Environ.Mater.2023, 6, e12506 cutoff potential of 4.1 V versus Li/Li + , which can significantly reduce the amount added in the cathode.Moreover, the residual obtained after the de-lithiation of Li 3 N is N 2 (2Li 3 N = 6Li + +6e − + N 2 ), which can be removed by cell opening, so there is no "dead" mass in the cathode, and thus, the energy density of the device is not reduced.
In conclusion, CSLS is the key material for lithium supplementation and should meet the following requirements.i) High specific capacity.To provide enough Li ions to achieve high-efficiency pre-lithiation, they should have the ability to achieve a high practical specific capacity.ii) There are also requirements for the de-lithiation and re-lithiation potentials of CSLS.The extraction potential of Li + in the sacrificial lithium salt is lower than the AC's high limiting potential, so as to avoid electrolyte decomposition, poor cycling performance, and safety problems; the re-lithiation should be lower than the AC's low operation limiting potential.iii) High irreversibility.During charging and discharging, the sacrificial lithium salt is highly irreversible or substantially irreversible (i.e., with low first coulombic efficiency).iv) No residue.
After lithium removal, sacrificial lithium salt is almost completely decomposed or its residue can be dissolved in the electrolyte, which will not cause mass burden on the cathode, thus improving the volumetric and gravimetric energy densities of the device.v) Low operating environment requirements.For the requirements of production and processing environment, its stability can meet the requirements of treatment in an atmospheric environment.

Effect of Doping Amount of CSLS in the Cathode on LIC Performance
The doping amount of sacrificial lithium salts in the cathode plays a pivotal role in the electrochemical performance of LICs.The effect of CSLS on the electrochemical performance of the cathode can be evaluated by testing the charge-discharge characteristics of the composite cathode/Li half-cells with different CSLS contents.Park et al. [62] [143] with permission from Willy periodicals.a2, c) are reproduced from Lim et al. [63] with permission from Wiley-VCH.a3) is reproduced from Jezowski et al. [144] with permission from Royal soc chemistry.d) is reproduced from Zhang [60] with permission.Copyright (2017) Royal Society of Chemistry.
Energy Environ.Mater.2023, 6, e12506 investigated in detail the influence of Li 2 MoO 3 on the electrochemical performance of the cathode by charging the composite cathode/Li half-cells to 4.7 V at a current density of 0.1C.As seen from Figure 12b, the charging capacities of composite cathodes with different Li 2 MoO 3 contents (0, 5, and 10 wt%) were estimated to be 137.2,156.1, and 184.5 mAh g −1 , respectively.It was found that the charge capacity of the cathode was proportional to the amount of Li 2 MoO 3 in the cathode.Thereafter, they tested the galvanostatic charge-discharge performance of the cathodes with different Li 5 FeO 4 contents (0, 3, 5, and 10 wt%) in the voltage range of 2.5-4.7 V (vs Li/Li + ).The results showed that, first, the charge capacity increased proportionally to the Li 5 FeO 4 content, but there was no significant difference in the discharge capacity, as shown in Figure 12c1.By investigating the cycling performance, energy, and power density of the (10.3 wt% Li 5 FeO 4 + AC)/HC system, at the current density of 10C within the voltage range of 2.5-3.9V, the capacity of LFOcontaining LICs at the initial cycling was approximately 315.2 mF, which was lower than that of the LICs with metallic lithium (overpotential due to the addition of LFO), as shown in Figure 12c2.However, after 1000 cycles, the former maintained 87.4% (275.5 mF) of the initial capacity, while the latter maintained only 71.1% (239.6 mF), indicating that the former exhibited better rate performance.The Ragone plots in Figure 12c3 show that the energy and power density of LFO-containing LICs were notably better than those of ones with metallic lithium under the same circumstances.Compared with the metal lithium supplement method, the LICs based on CSLS lithium supplement method exhibited better rate performance, energy, and power density.Recently, Zhang et al. [60] assembled three half-cells using activated carbon cathodes containing different Li 2 S contents (0, 15, and 30 wt%) and tested their galvanostatic chargedischarge performance.The results showed that with increasing Li 2 S content, the charging capacity increased significantly, but the discharging capacity did not have any evident difference (Figure 12d1).Then, they prepared (Li 2 S + AC)/graphite hybrid LIC equipment.
According to Figure 12d2-d4, the AC cathode with 15 wt% Li 2 S loading delivered a specific capacity of 71 mAh g −1 (128 F g −1 ) in the potential range of 2.0-4.0V (vs Li/Li + ).No matter in cyclic performance or rate capability has a certain amelioration.
All above results confirm that the charging capacity of the cathode can be controlled by CSLS, but the discharging capacity is not affected, which further confirms that CSLS can irreversibly supply lithium ions to the LIC anode.For this reason, the pre-lithiation level of LIC devices can be controlled by adjusting the CSLS content in the cathode, so that the device performance is improved.
The irreversible capacities of CSLSs as well as the reversible capacity of graphite need to be taken into account when calculating the amount of CSLS that should be doped in the cathode.The lithium intercalation of graphite is divided into 100% intercalation (reversible capacity C G = 372 mAh g −1 ) and 60% intercalation (reversible capacity C G = 223 mAh g −1 ).Taking Li 2 DHBN as an example, when the cutoff potential is 4 V (vs Li/Li + ), its irreversible capacity C Li2DHBN is 365 mAh g −1 .Therefore, to achieve 100% lithium intercalation of graphite, the mass ratio of Li 2 DHBN (m Li2DHBN ) to graphite (m G ) is calculated by the following formula: mLi2DHBN mG ¼ CG CLi2DHBN ¼ 372 mAh g À1 365 mAh g À1 ¼ 1:02.Regarding the AC cathode, for optimizing the energy and power of the system, its mass should be balanced with that of graphite; that is, the charge passing though the cathode (Q AC ) and anode (Q G ) should be equal. [141]According to Q ¼ C Â ΔE Â m, where C is the specific capacitance, ΔE is the potential range for the charge/discharge process, and m is the mass of the electrode, mAC mG The calculation method for 60% intercalation graphite is the same as above.Sun et al. [145] optimized the ratio of cathode active material (AC), CSLS (Li   [148] with permission from Elsevier Ltd.
Energy Environ.Mater.2023, 6, e12506 Consequently, LICs with different mass ratios of AC, SC, and Li 3 N are designed, which are named LICs-x41.Figure 13b shows the potential curves of initial charging to 4.1 V of LICs-x41 full cell, anode, and cathode under different ratios (LIC-441, LIC-841, and LIC-1241).The minimum potential of SC is 0.26, 0.04, and −0.09V, and the maximum potential of AC can reach 4.37, 4.14, and 4.01 V, respectively.It can be inferred that excessive Li + intercalation caused by a large ratio of cathode to anode can promote the growth of lithium dendrites, which can damage the cycling properties.In contrast, if the amount of lithium ions intercalation is insufficient because of a small cathode-to-anode ratio, the maximum cathode potential will increase and the electrolyte will decompose.Table 3 lists the specific capacity and energy density of the three, from which it is concluded that the proportion of LICs-841 is the best.The LIC-841 cell packs are secure, light, and efficient, along with excellent performance with maximum energy and power densities of 74.7 Wh kg −1 and 12.9 Kw kg −1 (Figure 13c1,c2), respectively, and the capacity retention of 91% after 10 000 cycles.Meanwhile, a comparison of the energy and power density of this system with those of the systems based on other sacrificial lithium salts (Figure 13c3) indicates that this system is successfully optimized.In summary, the appropriate mass ratio of the cathode, anode, and sacrificial lithium salts significantly influences the LIC performance.

Discrimination Between LIBCs and CSLS-Based LICs
Both CSLS-based LICs and LIBCs exhibit an internal parallel structure; that is, the cathode is composed of capacitance-type AC material and battery-type cathode material, while the anode is composed of Li + intercalation battery-type material.Because researchers might have conceptual confusion when encountering them for the first time, this article provides a detailed distinction betwseen the two systems.I) In the internal parallel structure.The cathode of LICs based on CSLS is composed of a capacitance-type material (AC) and a sacrificial lithium salt, and the anode is formed of a battery-type lithium storage material, which is not pre-lithiated in advance.In contrast, LIBCs can be divided into two classes: i) the cathode is a type of composite electrode composed of capacitance-type materials and lithium-containing materials (i.e., LiNi x Co y Mn z O 2 (NCM) ternary materials, LiFePO 4 , or LiMn 2 O 4 ), [190][191][192] and the anode is formed of pre-lithiated or without pre-lithiated battery-type materials.In the initial stage of charging, both the cathode and anode potentials of the LIBCs in which the anode is not pre-lithiated change rapidly, and at this time, the anode generates certain irreversible capacity.ii) The cathode is a type of composite electrode composed of capacitance-type materials and lithium-free materials (such as Na 0.76 V 6 O 15 ). [193]In this case, a pre-lithiated battery-type anode material must be used to keep the lithium ions stable in the electrolyte.II) The properties of lithium-containing materials added to the cathode are different.CSLSs in the LIC cathode are required to have high specific capacity, high irreversibility (Figure 14a), and less or preferably no residue in the first cycle.In contrast, the battery-type materials in the LIBC cathode must be able to undergo reversible re-lithiation and de-lithiation (Figure 14b).III) The purpose of adding battery cathode materials in the cathode is distinct.The addition of CSLS in the LIC cathode is intended to provide adequate lithium ions for pre-lithiation of the anode, while the introduction of batterytype materials in the composite cathodes of LIBCs aims to improve the specific capacity of the cathode and then the energy density of the device. [194]f the mass ratio of the capacitive material to the battery material is appropriate, LIBCs can obtain excellent rate performance and retain more capacity at a high current density.The optimal mass ratio of the capacitor material to battery material in LIBCs can be determined from three aspects: the specific capacity of the composite cathode, energy density, and rate performance of the system.Sun et al. [75,195] conducted a detailed study on the performances of the (AC + LiNi 0.5- Co 0.2 Mn 0.3 O 2 )//HC system at different mass ratios r (r ¼ mNCM mNCM þ mAC ¼ 0-1).As shown in Figure 14c1, when r = 0.25, the capacitive behavior of activated carbon is dominant.At this time, the discharge-specific capacity is low, but the rate capability is better.The cathode presents an almost entirely electric double-layer mechanism and the redox peaks corresponding to lithium removal and intercalation of NCM cannot be observed in the dQ=dV curves.This indicates that when the NCM content is small, the ternary NCM materials are mainly consumed as the lithium supplementary materials in the first cycle of charging.In this case, although the NCM materials play the role of CSLS, they cannot be called CSLS in the strict sense.To verify this claim, they prelithiated HC in advance.As shown in Figure 14d1, the redox peaks of NCM materials can be observed in the dQ=dV curves and have good repeatability in the subsequent cycles, indicating that NCM is not consumed but works in collaboration with AC.When 0.25 < r < 1, with an increase in r (i.e., the content of NCM increases), the redox peaks in the dQ=dV curves become evident.In addition, the specific capacity and energy density increase gradually and the rate performance decreases gradually.Among them, when r ≥ 0:5, the LIBC devices exhibit capacitive behavior in the voltage region below 3.0 V and battery behavior in the high-voltage region.Moreover, under the same conditions, with an increase in the NCM content in the cathode, the cutoff potentials of both the cathode and anode decrease during charging, and the fluctuation range of the anode potential becomes wider.When r = 1, the system is equivalent to an LIB with relatively high specific capacity but poor rate performance.Therefore, after quality control, a mass ratio of r = 0.75 with best comprehensive performance out of the three is finally obtained.

Summary and Outlook
Compared with SCs and LIBs, LICs are currently the state-of-the-art energy storage devices with both high energy density and power density.However, due to the irreversible capacity loss of the lithiumintercalated anode material during the first charge cycle, anode prelithiation has become an indispensable step in LICs assembly.In this article, the roles of pre-lithiation are described in detail, and then, the principle, advantages, and disadvantages of the existing pre-lithiation methods are analyzed and summarized.Considering the safety, cost, operation difficulty, and controllability of the LICs performance, it is found that adding CSLSs directly to the cathode is the most promising lithium supplement method.Therefore, we carefully studied the  We make a bold prediction that the lithium supplement method using CSLSs will become the mainstream pre-lithiation method.However, with respect to the current landscape of development, there are still scientifically critical issues and challenges to be addressed for this approach.In this review, the prospects of CSLSs based on LICs are predicted from the following three aspects.
I Performance optimization of existing CSLSs.Although different types of CSLSs have been explored, including lithium-rich transition metal oxides, organic lithium salts, and lithium sulfides and nitrides, these materials are still insufficient to meet the commercial requirements of LICs.For example, the high de-lithium potential (>4.3 V vs Li/Li + ) can lead to electrolyte decomposition, the low specific capacity can increase the CSLS dose in the cathode, and the residual "dead" materials in the cathode can increase the total weight of the device and reduce its mass-energy density.Some CSLSs, such as Moreover, some CSLSs suffer from poor electronic and/or ionic conductivity, [197] produce side reactions on cycling, [196,198] sensitivity to the operating environment, or incompatibility with common solvents.All these problems need to be further explored and solved, and the following  [146] with permission from Elsevier Ltd. b1) Cyclic voltammograms of NCM electrode.b2) voltage profile of NCM/Li cell.c) Voltage profiles and the corresponding dQ/dV curves of LIBCs for the first three cycles.c1) r = 0.25; c2) r = 0.5; c3) r = 0.75.d) Voltage profiles and the corresponding dQ/dV curves of pre-lithiated LIBCs with different NCM contents.d1) r = 0.25; d2) r = 0.5; d3) r = 0.75.Reproduced from Florian et al. [196] with permission from Elsevier Ltd.
Energy Environ.Mater.2023, 6, e12506 improvement measures have been suggested.i) The existing CSLSs can be nanosized using different treatment processes or employing electron-donating effects as well as strongly coupled interface strategies to regulate the highest occupied molecular orbital and the band structure of the sacrificial agents, which, for example, can be combined with highly conductive materials (e.g., graphene, conductive carbon black, acetylene black, and conductive graphite), thereby reducing the lithium extraction potential and promoting lithium salt decomposition.ii) For the sacrificial lithium salts that have strict requirements on the environment, such as lithium nitride, their practical applicability can be increased by selectively coating a protective layer, direct in situ preparation on the cathode, or exploring new organic solvents.iii) For the by-products produced.The negative effect of CO 2 can be eliminated by treating the graphite anode, adding an electrolyte additive that has a synergistic effect with CO 2 (for O 2 − by-products, fluorinated ether additive with the specific ability to synergistically neutralize the O 2 − species can be added) [155] or by integrating the cathode material with a sacrificial lithium salt. [178,199]In the actual production process, the first charge cycle is carried out in an open system, followed by the release of O 2 before sealing.][202][203] Although these conversionreaction-based nanocomposites exhibit excellent lithium supplementing performance, inactive metal oxides, fluorides, and sulfides still exist after the first lithium supplement, which can reduce the energy density of the LIC device to a certain extent. [204,205]I Explore new CSLSs.The application of CSLSs to LICs is still in the research stage.It is another task for scientists to develop new candidate materials after understanding all the conditions required to develop ideal CSLSs.Valid recommendations are provided for inorganic and organic lithium salts.i New inorganic lithium-containing compounds.First, highperformance inorganic CSLSs can be developed by adjusting the structure.Taking the lithium-containing transition metal oxides as an example, the material structure impacts the difficulty/ease of lithium release, and the rhomboidal symmetry is more prone to releasing lithium than the monoclinic symmetry phases.In addition, the main and common weakness of all pre-lithiation methods currently in existence is that the lithium source and the electron source are attributed to a single lithiated compound (n-type).This can be addressed by the cascade reaction (i.e., the first reaction provides the electron source, the second reaction provides the lithium source, and vice versa).As a result, the search for composite materials that can serve as lithium and electron sources (including bi-inorganic, bi-organic, and organicinorganic materials) has become a top priority.Next, considering the problem of incomplete decomposition of CSLSs, new pre-lithiation strategies (self-pre-lithiation methods) can be explored.Without the need for adding CSLSs, the same materials can play the dual role of CSLSs and active species.ii New organic lithium-containing compounds.In recent years, to develop CSLS materials with low de-lithiation potential, zero residue, and no other metal elements, organic lithium salt compounds with structural diversity and redox activity at the molecular level have also become the focus of attention.The physicochemical properties of organic sacrificial lithium salts, especially the de-lithiation potential, are related to their structures (main skeleton and substituents).However, these cognitions are only based on the summary of external experimental phenomena, and there is a lack of detailed understanding on the structureactivity relationship between the structure and properties of such materials.A precise regulation of the de-lithiation potential (related to the electronic structure of organic lithium salts) and the trade-off of the de-lithiation capacity (related to the molecular weight of organic lithium salts and the number of lithium-containing moles) remain challenging issues.Therefore, by systematically studying the influence of the intrinsic structures of organic sacrificial lithium salts on their de-lithiation performance (delithiation potential, de-lithiation capacity, reversibility of de-lithiation, and residue), the structure-activity relationship between the material structure and de-lithiation performance can be mastered.Then, guided by the obtained structure-activity relationship, organic lithium salts that meet all the requirements of the concept of ideal CSLSs can be designed and synthesized.III In-depth study of the detailed evolution mechanism of CSLS lithium supplement method.Pre-lithiation is a crucial link in the manufacturing technology of LICs.Therefore, how to realize its high-efficiency lithium supplementation to LICs after obtaining high-performance CLSLs is also an essential scientific issue.Thus, a thorough study of the mechanism of the prelithiation process is required.However, the current research is mainly empirical, and the detailed evolution mechanism of the pre-lithiation process has been rarely studied.In-depth studies are required to evaluate the effects of the pre-lithiation conditions (the amount of lithium intercalated and the current density of pre-lithiation) of CSLSs on SEI films (composition, morphology, uniformity, stability, and ionic conductivity/dielectric properties), the dynamic performance and effective operating voltage window of the anode, and the kinetic matching of the cathode and anode.Integrating the electrochemical test results and in situ and ex situ characterization results with theoretical calculations to reveal and understand the highefficiency lithium supplementation mechanism of CSLSs on LICs from a multiple-perspective microscopic level can provide reliable experimental evidence and theoretical guidance for promoting the application of novel CSLSs for high-performance LICs.In addition, there has been no clear research conclusion on the decay mechanism after pre-lithiation.
In conclusion, the applications of CSLSs in LICs have progressed notably over the past decade and have been widely accepted.However, the development of CSLSs is still in its infancy, as highlighted by the diversity of sacrificial lithium salt materials, the tunability of their Energy Environ.Mater.2023, 6, e12506 structures, and the unknowns about their exploration.More research is required for the continued maturity of the application of CSLSs to LICs through a clear perception of CSLSs and by developing the desirable sacrificial materials.

Figure 2 .
Figure 2. a) Example of the first cycle irreversible capacities loss of carbonaceous anode: a1) graphite/Li half-cell; and a2) HC/Li half-cell.Reproduced from Shellikeri et al.[67] with permission from IOP Publishing.b) SEI formation on HC surface during pre-lithiation.Reproduced from Yao et al.[68] with permission from IOP Publishing.c) Example AC//HC systems with or without pre-lithiation treatment: the effects of the pre-lithiation levels on c1) the coulombic efficiency; c2) the cycling stability; and c3) the internal resistance.Reproduced from Kumagai et al.[69] with permission from Elsevier Ltd.

Figure 3 .
Figure 3. a) Ions transfer in different charging/discharging stages.b) Time-resolved in situ7 Li NMR spectra: b1) Front view of the stacked plot; b2) Side view from right; and b3) Top view.Reproduced with permission from Shellikeri et al.[70]Copyright (2016) American Chemical Society.c) the cathode potential window is enlarged and the capacity is increased by pre-lithiation treatment.d) The charge/discharge curves of AC//LMCMB systems through different degrees of pre-lithiation treatment: d1) The capacity of pre-lithiation is 0 mAh g −1 : LIC0; d2) The capacity of pre-lithiation is 100 mAh g −1 : LIC100; and d3) The capacity of pre-lithiation is 300 mAh g −1 : LIC300.Reproduced from Zhang et al.[71] with permission from Elsevier Ltd.

Figure 4 .
Figure 4. Galvanostatic charge/discharge curves of HC//AC LICs a) without pre-lithiation and b) with pre-lithiation.Reproduced from Paled[50] with permission from Elsevier Ltd. c) Effect of different pre-lithiation amount on anode potential in AC/LTO systems.c1) 0; c2) 10; and c3) 80. Reproduced from Xu et al.[77] with permission from Elsevier Ltd. d) Schematic diagram of anode potential distribution and voltage distribution of LIC system as a function of capacity.Reproduced from Jin et al.[84] with permission from Elsevier Ltd.

Figure 5 .
Figure 5. a) Schematic diagram of LIC and unbalanced performance.b) the reaction mechanism scheme image of HC anode.c) Optimal anode potential range with excellent reaction kinetics matched with AC electrode.Reproduced from Jin et al.[84] with permission from Elsevier Ltd.

Figure 7 .
Figure 7. Traditional and novel pre-lithiation methods.a) Dependence of lithiation rate of two graphite electrodes on reaction temperature.Reproduced from Tsuda et al.[66] with permission from IOP Publishing.b) Detailed process of pre-lithiation by ISC method.c) The7 Li NMR spectra of the pre-lithiation of graphite using coated lithium metal.Reproduced from Holtstiege et al.[118] with permission from Elsevier Ltd. d) Diagram of four different lithium sources used ISC method: 1) SLMP; 2) thick Li strips; 3) thin Li film, and 4) thin Li film with pin holes.e) Corresponding to the change process of four different lithium sources immersed in electrolyte for 24 h in d diagram.f) Corresponding to the change in anode potential of four different lithium sources in d diagram during pre-lithiation.Reproduced from Shellikeri et al.[67] with permission from IOP Publishing.

Figure 8 .
Figure 8. Schematic diagram of comparing different pre-lithiation methods in terms of safety, cost, application scale, controllability, influence on energy density, and environmental conditions of operation.(Note: expanding to the outer circle represents the better benefit.)Reproduced with permission from ref [55].Copyright (2022) Royal Society of Chemistry.
6-x CoO 4 to x = 1.0, a C O 3+ peak of 780.4ev can be observed, indicating that the redox reaction of C O 2+ /C O 3+ is the main reason for the initial Li + extraction.When Li ions continue to be extracted until x = 2.0, the C O 3+ peak decreases, while the peak at 781.4ev corresponding to Co 4+ increases, indicating that the redox reaction of C O 3+ /C O 4+ is mainly responsible for the intermediate Li + extraction.However, Co 4+ cannot be further oxidized beyond x = 2.0, as proved by the Co 2p3/2 spectrum at the end of the charge cycle (x = 3.75).As shown in Figure 10a3, when x > 2.0, further Li + extraction can cause structural changes.Consequently, to provide a theoretical basis for practical application, the authors conducted a further theoretical study on the Li + extraction mechanism and structural stability of Li 6-x CoO 4 .A model of tetragonal Li 6 CoO 4 is shown in Figure developed Li 0.65 Ni 1.35 O 2 materials.Although lithium ions in Li 0.65 Ni 1.35 O 2 can be completely extracted at 4.2 V (vs Li/Li + ), the low irreversible capacity (120 mAh g −1 ) increases its dosage ratio in the

Figure 9 .
Figure 9. a) Ex situ XPS spectra for Mo 3d binding energy of Li 2 MoO 3 obtained at different cut-off potentials.Reproduced from Park et al.[62] with permission from Willy periodicals.b) Cyclic voltammograms of AC-Li 5 FeO 4 electrode.Reproduced from Park et al.[143] with permission from Wiley-VCH.c) Requirement for de-lithiation and re-lithiation potentials of CSLS.Reproduced from Park et al.[146] with permission from Elsevier Ltd.

Figure 11 .
Figure 11.a) Comparison of relative formation energy of de-lithium phase Li 6-x CoO 4 .b) Formation energy of Li 6-x CoO 4 with different Li contents.c) O 2p partial density of state of Li 6-x CoO 4 with different Li contents; c1) x = 0, c2) x = 1.5, c3) x = 3.5.d) Total amount of electrons formed by oxidation of Co and O. e) O vacancy formation energy of Li 6-x CoO 4 with respect to Li contents (x < 4.0) during the first charge.Reproduced from Lim et al.[63] with permission from Royal soc chemistry.
3 N), and anode soft carbon (SC) more accurately through experiments.Li/Li 3 N and Li/SC half-cells were assembled and tested to examine the mass ratio of SC to Li 3 N, and the potential curves are shown in Figure 13a1,a2.The initial charge capacity of SC is Q SC = 334 mAh g −1 , and the actual specific capacity of Li 3 Nis Q Li3N = 1379 mAh g −1 ; thus, the optimal ratio is m SC : m Li3N ¼ 4 : 1.
development and application of CSLSs on LICs in the past 10 years and summarize the lithium extraction mechanism of CSLSs and the influence of intrinsic characteristics and doping amount of CSLSs on the electrochemical performance of the LICs in detail.In addition, this article carefully distinguishes LICs and LIBCs, which can be easily confused, to reduce the mistakes caused by unclear conceptual understanding in the future research.

Table 1 .
Comparison of characteristics of four pre-lithiation methods for LICs Note: In the column of ISC, only the various characteristics of the LICs device based on SLMP as the lithium supplement source are listed.Energy Environ.Mater.2023, 6, e12506

Table 3 .
Specific capacity and energy density of LICs at various ratios Energy Environ.Mater.2023, 6, e12506 Li 2 C 2 O 4 and Li 2 O 2 , can generate CO 2 , O 2 , and O 2