Polycyclic Aromatic Hydrocarbon-Enabled Wet Chemical Prelithiation and Presodiation for Batteries

: The current mainstream energy storage systems are in urgent need of performance improvements to meet novel application requirements. In pursuit of a higher energy density in Li-ion and Na-ion batteries, the conventional electrode materials have reached the upper limit of their theoretical speciﬁc capacities. Hence, facile methods of reducing irreversible lithium-ion/sodium-ion loss are developed to further boost the battery energy density. Herein, we review studies that use polycyclic aromatic hydrocarbons for wet chemical prelithiation and presodiation. The molecular structures of arenes and solvents used for solution-based prelithiation/presodiation have a substantial impact on the prelithiation/presodiation power and effectiveness. Multiple reports have already shown excellent initial Coulombic efﬁciency and streamlined processes by using this type of wet chemical prelithiation/presodiation strategy. This review article will cover how to select appropriate polycyclic aromatic hydrocarbon prelithiation/presodiation reagents for various materials/electrodes and provide possible directions and guidelines for future works. process, the preparation and handling of Li-PAH complex due to their chemical stability. By controlling the concentration of reagents, reaction time, and temperature, precise, uniform, and highly efﬁcient prelithiation can be achieved. The outcome of PAH-enabled prelithiation/presodiation is a battery with a near-perfect initial Coulombic efﬁciency (ICE), along with a preformed protective SEI layer on the anode surface that can promote cycle stability.


Introduction
Li-ion and Na-ion batteries are leading a new wave of the industrial revolution. After dominating the market of portable consumer electronics, new battery technologies now aim at supplying power to much bigger devices, such as grid-scale energy storage and vehicle electrification [1][2][3][4][5]. To significantly increase the energy density of batteries, replacing traditional electrode materials with next generation anode/cathode materials has yielded many remarkable results in academia research [6][7][8][9][10]. However, there are still many performance and manufacturing gaps that need to be bridged until these state-ofthe-art technologies can be adopted in the battery industry, and the progress is not fast enough to keep up with actual developmental needs for new technologies. In this regard, prelithiation/presodiation procedures are favorable for a quick energy boost with the same battery chemistry [11][12][13][14].
A typical scenario comparing Li-ion and Na-ion cells without and with prelithiation and presodiation is depicted in Figure 1. Before formation cycles, lithium/sodium ions are stored at the cathode side with a specific capacity of Q c in a conventional cell. During the first charge, alkali metal ions migrate to the anode side, but some of them are consumed irreversibly, whereas the rest of the ions are stored in the anode (Q a = Q c − IRR). The irreversible capacity (IRR) mainly results from the solid-electrolyte interface (SEI) formed on the surface of anode, which is an inevitable process to secure anode cycling stability due to the protection of SEI film [15][16][17]. A study by Winter's group pointed out that the to the protection of SEI film [15][16][17]. A study by Winter's group pointed out that the major part of irreversible capacity from the LiNi1/3Co1/3Mn1/3O2 (NCM) cathode can be recoverable by adjusting the discharge (lithiation) setting with a constant voltage step since the active ion loss is impeded kinetically but not thermodynamically [18]. Thus, the IRR loss here is derived mostly from the anode SEI formation, lithium ions that are unfavorably trapped in the structure, and unwanted electrolyte decomposition reactions. After the first discharge, the ions in the anode move to the cathode to complete the reversible electrochemical reaction (Qrev ≈ Qa = Qc − IRR). To compensate for the alkali ion loss during the first cycle, prelithiation/presodiation can be used. Additional active ions are introduced into the battery cell before the formation cycle (shown as the IRR block in green). In this case, the alkali ions in the cathode can be fully utilized when transporting to the anode during the first charge (Q'a = Qc), and thereby the reversible capacity of the prelithiated/presodiated cell is deemed as its maximum (Q'rev ≈ Q'a = Qc). After effective prelithiation/presodiation, all the active lithium or sodium ions are conserved for reversible redox reactions rather than consumed for anode SEI formation, and the anode and cathode capacities can be perfectly utilized to store active ions and then achieve a maximal energy density. Many prelithiation/presodiation methods for battery systems have been developed such as wet chemical, molten alkali metal, in situ and ex situ electrochemical, short-circuiting, and alkali metal additions, which are summarized in Table 1. Most of these prelithiation/presodiation techniques utilize pure alkali metal in the form of foil or powder. A unique commercial metallic lithium product called SLMP (stabilized lithium metal powder) was frequently used in the prelithiation of anode in Li-ion batteries for better handling [19][20][21]. With an external Li2CO3 coating layer, SLMP exhibits excellent dry-air stability, outperforming other pristine metallic lithium sources. Molten alkali metal is a prelithiation/presodiation method that is not easily used for electrode-level operations, and a raised processing temperature is required to liquify the alkali metals, making this approach relatively unsafe and questionable for large-scale manufacturing [22,23]. On the other hand, wet chemical and direct alkali metal addition routes are both capable of being applied in either material-or electrode-level prelithiation/presodiation. The binder/solvent system must be specially selected to be compatible with highly reactive prelithiated/presodiated materials. There are two types of electrochemical prelithiation/presodiation means, which are via in situ or ex situ courses, and both are controlled by an external circuit. The in situ electrochemical method is conducted by introducing an additional layer of alkali metal foil into the cell with porous current collectors, and then connecting the metal electrode with the anode to initiate the process [24]. The ex situ electrochemical Figure 1. Schematic drawing illustrating how the reversible capacity changes in cells without and with prelithiation/presodiation during the formation cycle. Q c : initial cathode specific capacity; Q a and Q' a : available anode capacity after the first charge; Q rev and Q' rev : reversible capacity after the first discharge.
Many prelithiation/presodiation methods for battery systems have been developed such as wet chemical, molten alkali metal, in situ and ex situ electrochemical, shortcircuiting, and alkali metal additions, which are summarized in Table 1. Most of these prelithiation/presodiation techniques utilize pure alkali metal in the form of foil or powder. A unique commercial metallic lithium product called SLMP (stabilized lithium metal powder) was frequently used in the prelithiation of anode in Li-ion batteries for better handling [19][20][21]. With an external Li 2 CO 3 coating layer, SLMP exhibits excellent dry-air stability, outperforming other pristine metallic lithium sources. Molten alkali metal is a prelithiation/presodiation method that is not easily used for electrode-level operations, and a raised processing temperature is required to liquify the alkali metals, making this approach relatively unsafe and questionable for large-scale manufacturing [22,23]. On the other hand, wet chemical and direct alkali metal addition routes are both capable of being applied in either material-or electrode-level prelithiation/presodiation. The binder/solvent system must be specially selected to be compatible with highly reactive prelithiated/presodiated materials. There are two types of electrochemical prelithiation/presodiation means, which are via in situ or ex situ courses, and both are controlled by an external circuit. The in situ electrochemical method is conducted by introducing an additional layer of alkali metal foil into the cell with porous current collectors, and then connecting the metal electrode with the anode to initiate the process [24]. The ex situ electrochemical approach is more complicated, requiring another electrochemical bath setup and implementing prelithiation/presodiation before battery reassembly [24]. If the prelithiation/presodiation process is accomplished via direct alkali metal contact (e.g., spreading SLMP, short-circuiting, etc.), external pressure has to be applied to activate the reaction. The protective film on SLMP can be cracked by the applied pressure to trigger the process, and an enhanced alkali metal/electrode contact leads to homogeneous and controllable ion distribution [11][12][13]22]. When solvents or electrolytes are used for the prelithiation/presodiation in wet chemical, electrochemical, and short-circuiting methods, a following washing step might be necessary to eliminate residual reagents and byproducts. Last but not least, scalability is always the key determining whether the process has commercial value. Most electrodelevel prelithiation/presodiation processes are compatible with roll-to-roll production design [25][26][27][28][29][30][31][32][33], indicating great potential for scaled-up manufacturing in the future. Among these various prelithiation/presodiation selections, the wet chemical method shows the flexibility to be used in both materials and electrodes, possesses minimal processing requirements (room temperature operation; no external circuit and pressure needed), and can be applied to a roll-to-roll and cost-effective battery production line as an add-on prelithiation/presodiation process.

Features of Polycyclic Aromatic Hydrocarbon-Enabled Wet Chemical Prelithiation/Presodiation
Early chemical prelithiation methods utilized a highly flammable and reactive chemical liquid n-butyl lithium (n-BuLi) to reduce the IRR of negative and positive electrodes [34,35]. However, the prelithiation reagent n-BuLi has a relatively high redox potential (≈1 V vs. Li/Li + ) and spontaneously combusts in air, making it unfavorable for commercialization due to its poor prelithiation power and safety. Tabuchi et al. developed another type of prelithiation reagent adopting a lithium-organic complex solution by dissolving metallic lithium and naphthalene (Naph) in butyl methyl ether (BME) or dimethoxyethane (DME) [36,37]. In fact, other polycyclic aromatic hydrocarbons (PAHs or arenes), such as biphenyl (BP) and 9,9-dimethylfluorene (DiMF), can replace naphthalene to make PAH complex solutions with alkali metals [38,39]. The PAH compounds form radical anions when an alkali metal is presented, and then a PAH complex molecule is formed containing alkali metal ions. When electrodes/materials react with the PAH complex solution, the radical PAH anion donates its electron and the alkali metal ion reduces, resulting in the completion of prelithiation/presodiation. Since the electron affinity of each PAH is different [39], individual PAH complexes shows differences in terms of prelithiation/presodiation power (i.e., different redox potentials) that will be discussed in the next section. Due to the much lower redox potential and process-friendly solution-type prelithiation/presodiation procedures, a lot of related studies using PAH complex solutions have been published since 2019 [25,32,33,.
The prelithiation potential of typical Li-PAH radical anions can be seen in Figure 2. Three main lithium-organic reagents are Li + [Naph] − , Li + [BP] − , and Li + [DiMF] − , which are ranked from high to low in terms of lithiation/de-lithiation potential [40]. Li + [DiMF] − offers better reduction power than that of Li + [Naph] − and Li + [BP] − for deep anode prelithiation. These arenes with simple structures are common organic compounds produced in industry, which can guarantee scalable and low-cost production. During the prelithiation process, the preparation and handling of Li-PAH complex solutions are quite safe due to their chemical stability. By controlling the concentration of reagents, reaction time, and temperature, precise, uniform, and highly efficient prelithiation can be achieved. The outcome of PAH-enabled prelithiation/presodiation is a battery with a near-perfect initial Coulombic efficiency (ICE), along with a preformed protective SEI layer on the anode surface that can promote cycle stability.
The prelithiation potential of typical Li-PAH radical anions can be seen in Figure 2. Three main lithium-organic reagents are Li + [Naph] − , Li + [BP] − , and Li + [DiMF] − , which are ranked from high to low in terms of lithiation/de-lithiation potential [40]. Li + [DiMF] − offers better reduction power than that of Li + [Naph] − and Li + [BP] − for deep anode prelithiation. These arenes with simple structures are common organic compounds produced in industry, which can guarantee scalable and low-cost production. During the prelithiation process, the preparation and handling of Li-PAH complex solutions are quite safe due to their chemical stability. By controlling the concentration of reagents, reaction time, and temperature, precise, uniform, and highly efficient prelithiation can be achieved. The outcome of PAH-enabled prelithiation/presodiation is a battery with a near-perfect initial Coulombic efficiency (ICE), along with a preformed protective SEI layer on the anode surface that can promote cycle stability. It is worth noting that a couple of reports indicating the ambient-air processibility of Li-PAH complex solutions and prelithiated anodes, displayed in Figure 3. Qu et al. found that a higher concentration of Li-BP in tetrahydrofuran (THF) gave good air stability for at least two weeks (Figure 3a) [62]. Prelithiation reagents such as Li-P powder, Li-Sn powder, and a low-concentration Li-BP/THF (0.01 M) solution rapidly changed color after the exposure under ambient atmosphere [62]. The 1 M Li-BP/THF complex solution also showed great stability and still maintained its dark-blue color when being stirred in air, which turned colorless when water was added (Figure 3b), highlighting that the solution was still active until it reacted with water [62]. The authors claimed that the Li-BP/THF complex solution was functionally reactive even after 9 days of storage, yielding similarly effective prelithiation results in either glovebox or ambient air [62]. They also proposed that electrode prelithiation can be carried out under ambient air. After the completion of prelithiation using Li-BP/THF, the residual complex does not need to be removed by electrode washing, and the Li-BP layer can therefore protect the prelithiated electrode ( Figure  3c). The additional BP remaining on the electrode can be removed later by its dissolution in carbonate-based electrolytes [62]. Wang   It is worth noting that a couple of reports indicating the ambient-air processibility of Li-PAH complex solutions and prelithiated anodes, displayed in Figure 3. Qu et al. found that a higher concentration of Li-BP in tetrahydrofuran (THF) gave good air stability for at least two weeks ( Figure 3a) [62]. Prelithiation reagents such as Li-P powder, Li-Sn powder, and a low-concentration Li-BP/THF (0.01 M) solution rapidly changed color after the exposure under ambient atmosphere [62]. The 1 M Li-BP/THF complex solution also showed great stability and still maintained its dark-blue color when being stirred in air, which turned colorless when water was added (Figure 3b), highlighting that the solution was still active until it reacted with water [62]. The authors claimed that the Li-BP/THF complex solution was functionally reactive even after 9 days of storage, yielding similarly effective prelithiation results in either glovebox or ambient air [62]. They also proposed that electrode prelithiation can be carried out under ambient air. After the completion of prelithiation using Li-BP/THF, the residual complex does not need to be removed by electrode washing, and the Li-BP layer can therefore protect the prelithiated electrode ( Figure 3c). The additional BP remaining on the electrode can be removed later by its dissolution in carbonate-based electrolytes [62]. Wang et al. utilized a Li-DiMF complex solution to prelithiate aluminum foil to form LiAl alloy with an alumina-containing artificial SEI layer [40]. Figure 3d exhibits the air stability of different lithiated anodes (c-LiAl: wet chemically prelithiated Al; e-LiAl: electrochemically prelithiated Al; e-Li/Cu: electrochemically deposited Li on Cu). After 30 min of exposure to air, only e-Li/Cu showed significant oxidation, verified by the color change. In terms of capacity retention, the Li-DiMF prelithiated c-LiAl, at over 91% of the initial capacity, which significantly outperformed the other two electrode samples lithiated via the electrochemical method [40]. The preformed SEI generated during wet chemical prelithiation plays an important role in resisting the corrosion induced by oxygen and moisture. Figure 3e shows another example of dry-air-stable prelithiated electrodes processed by tailored Li-BP complex solutions [32].
The SiO x electrode lost some ICE after 5 min of dry-air exposure, but stabilized thereafter and still gave more than 100% of ICE. Additionally, no significant overpotential was found in the electrode after exposure to dry air [32]. These process-friendly features make the wet chemical PAH-based prelithiation/presodiation potential candidates to be integrated into the large-scale battery cell manufacturing process, which is always conducted in dry rooms.
prelithiate aluminum foil to form LiAl alloy with an alumina-containing artificial SEI layer [40]. Figure 3d exhibits the air stability of different lithiated anodes (c-LiAl: wet chemically prelithiated Al; e-LiAl: electrochemically prelithiated Al; e-Li/Cu: electrochemically deposited Li on Cu). After 30 min of exposure to air, only e-Li/Cu showed significant oxidation, verified by the color change. In terms of capacity retention, the Li-DiMF prelithiated c-LiAl, at over 91% of the initial capacity, which significantly outperformed the other two electrode samples lithiated via the electrochemical method [40]. The preformed SEI generated during wet chemical prelithiation plays an important role in resisting the corrosion induced by oxygen and moisture. Figure 3e shows another example of dry-air-stable prelithiated electrodes processed by tailored Li-BP complex solutions [32]. The SiOx electrode lost some ICE after 5 min of dry-air exposure, but stabilized thereafter and still gave more than 100% of ICE. Additionally, no significant overpotential was found in the electrode after exposure to dry air [32]. These process-friendly features make the wet chemical PAHbased prelithiation/presodiation potential candidates to be integrated into the large-scale battery cell manufacturing process, which is always conducted in dry rooms.  [40]. Copyright 2020, The Royal Society of Chemistry. (e) Initial Coulombic efficiency (black, top), OCV (pink, top), and voltage profiles (bottom) of the prelithiated SiOx anode exposed to dry air for different durations. Reproduced with permission from ref. [32]. Copyright 2020, Wiley-VCH.  [40]. Copyright 2020, The Royal Society of Chemistry. (e) Initial Coulombic efficiency (black, top), OCV (pink, top), and voltage profiles (bottom) of the prelithiated SiO x anode exposed to dry air for different durations. Reproduced with permission from ref. [32]. Copyright 2020, Wiley-VCH.

Adjusting Prelithiation/Presodiation Power and Rate of PAH Complex Solutions
Several research groups have identified the redox potentials of various PAH radical anions, predominantly for Li-ion battery applications. Wang et al. screened various PAH molecules to obtain a low enough redox potential and a high enough lowest unoccupied molecular orbital (LUMO) energy in order to prelithiate Al foil (Figure 4a) [40]. DiMF, calculated to have a LUMO energy of −0.758 eV using density functional theory (DFT), was found to have a quasi-reversible redox couple with a redox potential of 0.22 V vs. Li/Li + , showing the best prelithiation power compared to Naph, BP, and other PAH with more aromatic rings [40]. Lee et al. modified the molecular structures of BP by introducing electron-donating methyl groups into the benzene ring, and the electron density can be increased while the electron affinity and reduction potential are decreased [32]. Once methyl groups are added to the BP molecule, all derivatives exhibit a lower redox potential and a higher LUMO energy (Figure 4b) [32]. Either one methyl substitution at the ortho position (2-methyl BP; blue molecule in Figure 4b) or four methyl groups at both the meta and para positions (3,3 ,4,4 -tetramethyl BP; orange molecule in Figure 4b) offer a great prelithiation power close to 0.1 V vs. Li/Li + , enabling an effective prelithiation of Si/SiO x anodes [32]. Instead of grafting methyl groups on BP, Qian et al. engineered the substituent of naphthalene with different functional groups onto the alpha position to tune the prelithiation power [53]. Figure 4c lists a series of Li-Naph molecules, which offer different prelithiation power that can be distinguished from their unique cyclic voltammetry curves [53]. Among them, 1-cyanonaphthalene (CNaph) not only improves the ICE of SiO electrode to over 100%, but also promotes the formation of a rectified SEI containing nitrile-rich outer layer and an inorganic LiF-rich inner layer [53]. However, not all the modified PAH molecules can perform prelithiation/presodiation. After reacting with metallic lithium, 2-fluorobiphenyl (BP grafted with a fluorine atom), even possessing suitable LUMO energy (Figure 4d), does not have much prelithiation power because LiF and defluoriated BP are the dominated products rather than reactive BP-based radical anions [54]. Although some PAH complexes with a higher redox potential (>0.8 V vs. Li/Li + ) cannot be used to prelithiate anodes, they are good relithiation reagents for spent cathodes with deficient lithium ions [52]. Pyrene (Py) and perylene (Per) can regenerate recycled LiFePO 4 (LFP) and LiCoO 2 (LCO) cathodes, respectively, since they both have sufficiently higher redox potentials to avoid cathode decomposition (Figure 4e) [52]. As a result, the prelithiation/presodiation potential of PAH radical anions is flexibly tunable to be applied in a variety of situations, making them facile and universal prelithiation/presodiation reagents.
In addition to the PAHs with modified molecular structures, the solvent in the PAH complex solution plays a critical role in electrochemical reactions during prelithiation/presodiation. DME and THF are frequently used solvents to dissolve PAH compounds along with alkali metals. Ai et al. adopted a modified THF solvent, 2-methyl tetrahydrofuran, with a higher tendency to donate electrons (2-Me-THF), yielding better results with regard to graphite anode wet chemical prelithiation [55]. By investigating DFT calculations, assuming one lithium ion coordinating with two oxygen atoms of the solvent, Li-BP in 2-Me-THF gives the largest highest occupied molecular energy (HOMO), which means the lowest reduction potential in comparison with other Li-BP-solvent complexes (Figure 5a) [55]. The Li-BP/2-Me-THF system offers a much lower redox potential (0.08 V) than those of Li-BP/THF (0.26 V) and Li-BP/DME (0.30 V), indicating a better lithiating power for anode materials with a low redox potential (e.g., graphite) [55]. Wu et al. discovered that Li-PAH solution kinetically changes its conductivity as the duration of the lithiation increases (Figure 5b) [54]. In the beginning, 4,4-dimethyl BP (4,4-DMBP) is dissolved in THF, followed by the addition of metallic lithium to form the complex solution. Then, the THF solvent molecules solvate lithium ions to form Li(THF) n + , and the radical anions (4,4-DMBP) •− are simultaneously formed by accepting electrons from the lithium [54]. Plenty of solvent-separated ion pairs (SSIPs) and contact ion pairs (CIPs) generate after this stage, leading to a quick conductivity boost. Afterwards, the conductivity of solution gradually decays due to the association of Li(THF) n + with (4,4-DMBP) •− as aggregates [54].
The authors found that the associated active species may have a higher prelithiation activity than dissociated ion pairs, but the prelithiation uniformity of the aggregates, formed by resting the PAH complex solution for a longer time, is not comparable to the freshly made one [54]. The changing conductivity of the Li-PAH complex solution implies that the lithiating power could change with the lithiation time, making the prelithiation control somewhat challenging. Lee et al. examined various solvents in the PAH complex solutions and ranked their solvation power as tetrahydropyran (THP) < 2-Me-THF < THF < DME [56,63,64]. When a strong solvent (e.g., DME) is used, the formation of SSIPs dominates, leading to the poor desolvation of lithium ions and thereby the co-intercalation of solvent molecules can occur [56]. This behavior makes DME incapable of being used to prelithiate graphite anodes due to the solvated lithium ions (Li(DME) n + ) exfoliating and degrading the graphite structure [55,56,65,66]. Figure 5c,d compare the energies between BP decoordination and solvent molecule desolvation in different solvent systems. When there are two DME solvent molecules around Li + (BP) •− , the formation of free (BP) •− anions and solvated Li(DME) 2 + is favored (shown as a pink box in Figure 5c), generating unwanted SSIPs and resulting in solvent co-intercalation [56]. On the contrary, in a weak solvent system such as THP, the desolvation of THP dominates the process and CIPs are major products no matter how many solvent molecules are surrounding the lithium ion ( Figure 5d) [56]. As a result, an appropriate solvent selection for the PAH complex solution can adjust the prelithiation/presodiation power, activity, uniformity, and solvation/coordination behaviors for better wet chemical prelithiation/presodiation performance.  In addition to the PAHs with modified molecular structures, the solvent in the PAH complex solution plays a critical role in electrochemical reactions during prelithiation/presodiation. DME and THF are frequently used solvents to dissolve PAH compounds along with alkali metals. Ai et al. adopted a modified THF solvent, 2-methyl tetrahydrofuran, with a higher tendency to donate electrons (2-Me-THF), yielding better re- wanted SSIPs and resulting in solvent co-intercalation [56]. On the contrary, in a weak solvent system such as THP, the desolvation of THP dominates the process and CIPs are major products no matter how many solvent molecules are surrounding the lithium ion ( Figure 5d) [56]. As a result, an appropriate solvent selection for the PAH complex solution can adjust the prelithiation/presodiation power, activity, uniformity, and solvation/coordination behaviors for better wet chemical prelithiation/presodiation performance.

Anode Prelithiation Using PAH Complex Solutions
Due to the high reducing ability of the anode in lithium-ion batteries, the SEI generated by the decomposition of electrolyte consumes a lot of active lithium. This makes anode prelithiation very attractive to efficiently improve the energy density of battery. Most of the prelithiation studies using PAH complex solutions focused on raising the ICE of

Anode Prelithiation Using PAH Complex Solutions
Due to the high reducing ability of the anode in lithium-ion batteries, the SEI generated by the decomposition of electrolyte consumes a lot of active lithium. This makes anode prelithiation very attractive to efficiently improve the energy density of battery. Most of the prelithiation studies using PAH complex solutions focused on raising the ICE of anode to close or over 100%, summarized in Table 2. Not only conventional graphite anodes [55,56], but also high-power hard carbon anodes [41,44,54] and high-capacity silicon monoxide anodes [32,33,36,43,53,56] (both ICE < 80%) have been demonstrated to be compatible for the prelithiation with various lithium arene radical anions. Carbon-based anodes can be prelithiated within a very short time, ranging from just 0.5 to 5 min. SiO x anodes with a higher capacity (>1000 mAh g −1 ) would require a longer duration of prelithiation (10-30 min) to compensate for their large IRR. Some carbon-enhanced composite anodes such as tin oxide [42] and phosphorus [45,62] also showed much better ICEs after adopting 1 M Li-BP/THF as the prelithiation reagent. Generally, the reaction time was manipulated to control the level of prelithiation in previous reports. There have been no systematical studies of how the concentration and Li/PAH ratio of complex solutions may affect the prelithiation power and solution stability, which require further investigation. Recent research mainly used the concentration from 0.2 M to 1 M and the Li/PAH ratio from 1 to 8 to prepare the Li-PAH complex solutions, but how these recipes were designed remains unknown. An added benefit of using the PAH-enabled wet chemical prelithiation is that the preformed SEI may be more uniform, more ionically conductive, mechanically stronger, and chemically more stable than the in situ-formed SEI during the formation cycles. Figure 6a,b compare the SEI formed in the pristine and prelithiated hard carbon anodes after the initial cycle [41]. The prelithiated anode can promote the growth of a thinner and denser SEI, where LiF builds up after the preformed Li-OR-rich SEI reacts with the electrolyte during the first lithiation reaction [41]. Wu et al. discovered that the Li-PAH complex solution prepared in a shorter time (10 min vs. 60 min) has a slower prelithiation rate (80 s vs. 60 s) but can generate a more homogeneous preformed SEI layer, leading to smaller overpotential and electrochemical impedance (Figure 6c) [54]. Qu et al. compared two different procedures (lithiation first vs. delithiation first) after cell assembly for the Li-PAH prelithiated hard carbon anodes [44]. They proposed that the anode SEI be chemically formed under the lithiation-first procedure and electrochemically formed under the delithiation-first procedure. Although the reversible capacity was higher with the delithiation-first setting, the cell gave a lower ICE and never reached over 92.6% (Figure 6d) [44]. In contrast, when the prelithiated anode was first lithiated in the cell, the ICE can be lifted up to 186.8%. Nevertheless, both methods exhibited similar cycle stability values. Guo et al. made a head-to-head comparison between the SLMP and Li-BP prelithiated SiO x anodes, as shown in Figure 6e, where the latter outperformed the former in terms of capacity retention [43]. Figure 6f,g display the cross-section of the SLMP and Li-BP prelithiated SiO x anodes after cycling, respectively. A more homogeneous Li x SiO y distribution, the major lithiated product of SiO x [67], can be found in the Li-BP prelithiated sample, resulting in better structural stability under repeated swelling and shrinkage during cycling. Consequently, the PAH-enabled wet chemical prelithiation can not only increase the ICE of anodes, but also modify the composition and uniformity of artificial SEI for better electrochemical performance. The mechanical properties of the SEI layer need to be investigated in detail in the future to further confirm which prelithiation method could produce a stronger SEI on the anode. jor lithiated product of SiOx [67], can be found in the Li-BP prelithiated sample, resulting in better structural stability under repeated swelling and shrinkage during cycling. Consequently, the PAH-enabled wet chemical prelithiation can not only increase the ICE of anodes, but also modify the composition and uniformity of artificial SEI for better electrochemical performance. The mechanical properties of the SEI layer need to be investigated in detail in the future to further confirm which prelithiation method could produce a stronger SEI on the anode.

Cathode Prelithiation Using PAH Complex Solutions
Other than anode prelithiation, the strategy of utilizing prelithiated cathodes can enhance the battery energy density as well by compensating the active lithium loss from the anode. Sun et al. used a Li-Naph/THF system to create a core-shell structure for LCO cathode materials [47]. After prelithiation, the outer nanoshell of LiCoO2 is converted into Li2O/Co, which offers additional delithiation capacity and forms a Co3O4 shell during charging (Figure 7a) [47]. Qian et al. tried to develop a suitable PAH complex solution to relithiate the spent LCO and LFP cathode materials [52]. Nevertheless, using Li-Naph (Eredox = 0.37 V vs. Li/Li + ) will decompose both LCO (Eredox = 1.20 V vs. Li/Li + ) and LFP (Eredox = 0.59 V vs. Li/Li + ), leading to irreversible phase change (Figure 7b). Therefore, Li-Per (Eredox = 1.28 V vs. Li/Li + ) and Li-Py (Eredox = 0.82 V vs. Li/Li + ) are good candidates to lithiate LCO and LFP, respectively, without damaging the cathode material. Figure 7c illustrates how this mild wet chemical relithiation process can restore the aged LFP cathode material structure with lithium loss. A Li-enriched gradient interface (~20 nm) can be generated on Ni-rich layered oxide cathode materials through prelithiation in Li-Naph/THF [50]. Figure  7d shows that a higher concentration of lithium can be found close to the material surface, Figure 6. TEM images of (a) pristine hard carbon anode and (b) prelithiated hard carbon anode after initial cycle. Reproduced with permission from ref. [41]. Copyright 2020, Wiley-VCH. (c) Electrochemical impedance spectra of two half-cells assembled with hard carbon anodes prelithiated by Li-DMBP (10 min) and Li-DMBP (60 min), with a similar degree of prelithiation. Reproduced with permission from ref. [54]. Copyright 2021, ACS Publications. (d) OCV values and prelithiation capacities of hard carbon anodes with different procedures. Reproduced with permission from ref. [44]. Copyright 2020, ACS Publications. (e) Cycling performance of the Li-BP and SLMP prelithiated SiO x anodes. Top-view and cross-sectional SEM images of the (f) SLMP prelithiated and (g) Li-BP prelithiated anodes after 200 cycles. Reproduced with permission from ref. [43]. Copyright 2020, ACS Publications.

Cathode Prelithiation Using PAH Complex Solutions
Other than anode prelithiation, the strategy of utilizing prelithiated cathodes can enhance the battery energy density as well by compensating the active lithium loss from the anode. Sun et al. used a Li-Naph/THF system to create a core-shell structure for LCO cathode materials [47]. After prelithiation, the outer nanoshell of LiCoO 2 is converted into Li 2 O/Co, which offers additional delithiation capacity and forms a Co 3 O 4 shell during charging (Figure 7a) [47]. Qian et al. tried to develop a suitable PAH complex solution to relithiate the spent LCO and LFP cathode materials [52]. Nevertheless, using Li-Naph (E redox = 0.37 V vs. Li/Li + ) will decompose both LCO (E redox = 1.20 V vs. Li/Li + ) and LFP (E redox = 0.59 V vs. Li/Li + ), leading to irreversible phase change (Figure 7b). Therefore, Li-Per (E redox = 1.28 V vs. Li/Li + ) and Li-Py (E redox = 0.82 V vs. Li/Li + ) are good candidates to lithiate LCO and LFP, respectively, without damaging the cathode material. Figure 7c illustrates how this mild wet chemical relithiation process can restore the aged LFP cathode material structure with lithium loss. A Li-enriched gradient interface (~20 nm) can be generated on Ni-rich layered oxide cathode materials through prelithiation in Li-Naph/THF [50]. Figure 7d shows that a higher concentration of lithium can be found close to the material surface, where more reduced nickel (Ni 2+ ) is generated simultaneously. This Li-rich cathode structure can supply extra lithium to resolve the IRR issue from the anode and thereby boost the energy density of the full cell.
Most conversion-type cathode materials are Li-free, so making use of them is very challenging [68,69]. However, in order to improve the energy density of batteries, it is necessary to use new cathode materials to replace traditional intercalation-type cathode materials. In this field, the excellent energy density and low cost of sulfur cathodes have attracted tremendous attention in recent years [70,71]. Although the conversion cathode can pair with metallic lithium to make a full cell, a prelithiated cathode could be used with a lithium-free anode to build a high-energy-density battery, leading to a higher possibility of new battery chemistries. In the past, active Li 2 S or lithium polysulfides (Li 2 S x ; 6 ≤ x ≤ 8) for Li-S batteries were obtained commercially or made by co-dissolving Li 2 S and sulfur in solvents [72,73]. Figure 7e exhibits another way to synthesize Li 2 S by utilizing a Li-Naph complex solution to prelithiate sulfur [74]. For constructing a Li-S cell without pairing with an unstable metallic lithium metal anode, sulfurized polyacrylonitrile (S-PAN) cathode materials are considered a better alternative to sulfur/carbon composites because of their compatibility with carbonate-based electrolytes [75][76][77]. Shen et al. built a lithium metal-free Si/S battery by prelithiating both the silicon anode and S-PAN cathode (Figure 7f) [48]. A high specific energy (710 Wh kg −1 ) and ICE (93.45%) were obtained in this cell configuration design adopting a Li-Naph complex solution [48]. Zuo et al. used the prelithiated product of S-PAN, Li 2 S-PAN, as the prelithiation reagent to prelithiate LFP electrodes, as illustrated in Figure 7g [46]. Here, Li-BP cannot be directly used to prelithiate LFP due to its low working voltage, which may result in LFP decomposition, as described in the previous paragraph [52].
can pair with metallic lithium to make a full cell, a prelithiated cathode could be used with a lithium-free anode to build a high-energy-density battery, leading to a higher possibility of new battery chemistries. In the past, active Li2S or lithium polysulfides (Li2Sx; 6 ≤ x ≤ 8) for Li-S batteries were obtained commercially or made by co-dissolving Li2S and sulfur in solvents [72,73]. Figure 7e exhibits another way to synthesize Li2S by utilizing a Li-Naph complex solution to prelithiate sulfur [74]. For constructing a Li-S cell without pairing with an unstable metallic lithium metal anode, sulfurized polyacrylonitrile (S-PAN) cathode materials are considered a better alternative to sulfur/carbon composites because of their compatibility with carbonate-based electrolytes [75][76][77]. Shen et al. built a lithium metal-free Si/S battery by prelithiating both the silicon anode and S-PAN cathode ( Figure  7f) [48]. A high specific energy (710 Wh kg −1 ) and ICE (93.45%) were obtained in this cell configuration design adopting a Li-Naph complex solution [48]. Zuo et al. used the prelithiated product of S-PAN, Li2S-PAN, as the prelithiation reagent to prelithiate LFP electrodes, as illustrated in Figure 7g [46]. Here, Li-BP cannot be directly used to prelithiate LFP due to its low working voltage, which may result in LFP decomposition, as described in the previous paragraph [52].  Reproduced with permission from ref. [46]. Copyright 2021, ACS Publications.

Presodiation Using PAH Complex Solutions
Similar to prelithiation, presodiation is an effective means to boost the output energy of Na-ion energy storage devices [78,79]. Table 3 summarizes the presodiated anode systems for Na-ion batteries. Basic PAH compounds such as Naph and BP were applied, paired with DME or THF as the complex solution for the presodiation [57][58][59][60][61]. Most studies utilized a low concentration of 0.1 M to 0.5 M for the preparation of a Na-PAH complex solution, and the improved ICE can approach 100% after 1~15 min of presodiation [59][60][61]. In fact, as the concentration of Na-Naph/DME increased from 3 M to 5 M, the irreversible capacity of the Na 2 Ti 6 O 13 anode significantly decreased (Figure 8a), demonstrating the controllability of wet chemical presodiation [57]. Sun et al. developed a solution-spraying presodiation route to enhance the ICE of hard carbon anode electrodes [58]. Unlike routine wet chemical presodiation via the immersion of electrode in a Na-PAH solution for a certain time duration, the removal of volatile THF (b.p. = 66 • C) and low boiling point Naph (b.p. = 218 • C) was implemented by simple drying after the solution spraying process, leading to the possible recycling of the reagents (Figure 8b). No inactive residues remained in the presodiated hard carbon anode, which guarantees the high energy density of full cell [58]. The SEI was preformed after the presodiated anode was in contact with the liquid electrolyte during cell assembly [58,59,80,81]. Liu et al. used Na-BP/DME as the presodiation source to improve the ICE of high-capacity Sb anode [60]. High reversible capacity (540 mAh g −1 ), high ICE (100%), and great capacity retention (85% over 300 cycles at 4C) were achieved in the presodiated Sb anode [60]. A chemically inert, mechanically strong, and electrically insulating NaF-rich SEI layer formed by the selective reaction between the presodiated active material and the FEC solvent in the electrolyte (Figure 8c) can contribute to the cycle stability of anode [60,[82][83][84]. The SEI formed on the presodiated Sb shows a highly homogenous structure (Figure 8d) to resist volume expansion, outperforming that on the pristine Sb (Figure 8e) [60]. Using the PAHs-based wet chemical presodiation to improve the ICE of Na-ion batteries is still emerging and under development, and recent approaches were mostly applied to anodes. There are also considerable opportunities for Na-PAH complexes to be used in other high-capacity anodes and even cathodes in the foreseeable future.  Reproduced with permission from ref. [57]. Copyright 2019, The Royal Society of Chemistry. (b) Schematic illustration showing the presodiation mechanism of the hard carbon anode using a Na-Naph solution as the presodiation reagent. Reproduced with permission from ref. [58]. Copyright 2019, Wiley-VCH. (c) Schematic illustration showing the presodiation mechanism of the Sb anode in the Na-BP/DME solution and the subsequent SEI formation process in a Na-ion battery electrolyte (1 M NaPF6 EC/DEC with 5% FEC). TEM images of the SEI layer on the (d) presodiated Sb and (e) pristine Sb anodes after one cycle. Reproduced with permission from ref. [60]. Copyright 2021, The Royal Society of Chemistry.

Summary and Outlook
In comparison with other methods, PAH-enabled prelithiation and presodiation show great potential to be used in Li-ion and Na-ion batteries. This scalable process is low-cost, safe, precise, uniform, efficient, and air-stable, which can facilitate the reduction in irreversible active ion loss during initial cycles and formation of an artificial protective SEI layer. Many PAH derivatives and solvent combinations have been carefully investigated, pointing out explicit guidance for different applications. The prelithiation/presodiation power and efficiency can be altered by the selections of PAH and solvents, concen- Figure 8. (a) Comparison of the specific capacity and Coulombic efficiency after the anode presodiation with different concentrations of the Na-Naph solution. Reproduced with permission from ref. [57]. Copyright 2019, The Royal Society of Chemistry. (b) Schematic illustration showing the presodiation mechanism of the hard carbon anode using a Na-Naph solution as the presodiation reagent. Reproduced with permission from ref. [58]. Copyright 2019, Wiley-VCH. (c) Schematic illustration showing the presodiation mechanism of the Sb anode in the Na-BP/DME solution and the subsequent SEI formation process in a Na-ion battery electrolyte (1 M NaPF 6 EC/DEC with 5% FEC). TEM images of the SEI layer on the (d) presodiated Sb and (e) pristine Sb anodes after one cycle. Reproduced with permission from ref. [60]. Copyright 2021, The Royal Society of Chemistry.

Summary and Outlook
In comparison with other methods, PAH-enabled prelithiation and presodiation show great potential to be used in Li-ion and Na-ion batteries. This scalable process is low-cost, safe, precise, uniform, efficient, and air-stable, which can facilitate the reduction in irreversible active ion loss during initial cycles and formation of an artificial protective SEI layer. Many PAH derivatives and solvent combinations have been carefully investigated, pointing out explicit guidance for different applications. The prelithiation/presodiation power and efficiency can be altered by the selections of PAH and solvents, concentration, and aging time of complex solutions. Recent studies have shown significant ICE improvements in prelithiated/presodiated anodes and cathodes for Li-ion and Na-ion batteries.
In-depth research related to the impact from the molar ratio between alkali metal and PAH molecule to the prelithiation and presodiation power and efficiency remains to be carried out in the future. Applications such as prelithiating/presodiating various electrodes for Na-ion batteries, Li-ion capacitors, and Na-ion capacitors are yet to be fully developed, leaving plenty of room for perspective research projects. Other conversiontype cathodes besides sulfur are also potential targets to be prelithiated/presodiated for high-energy-density alkali metal-free batteries.