Reticular Elastic Solid Electrolyte Interface Enabled by an Industrial Dye for Ultrastable Potassium‐Ion Batteries

The solid electrolyte interface (SEI) is vital to the stability of alkali metal‐ion battery anodes. However, conventional SEIs that lack elasticity will be damaged during the anode's repeated volume expansion, such as in potassium‐ion battery anodes, ultimately resulting in cell failure. Herein, a low‐content additive (pigment green 7, 0.07 wt%) in a conventional carbonate electrolyte to create a reticular elastic SEI with excellent uniformity and good chemical stability is employed. As a result, long‐lasting K||K symmetric cell (over 1400 h), enhanced graphite anode (500 cycles with 97.9% capacity retention), and stabilized perylene‐3,4:9,10‐tetracarboxydiimide cathode (1200 cycles with 82.8% capacity retention) are achieved. Furthermore, the matched graphite||perylene‐3,4:9,10‐tetracarboxydiimide full cell operates stably for more than 200 cycles. This work provides a novel avenue into the rational design of elastic SEIs for advanced anodes.


Introduction
3][4][5][6][7] Nevertheless, the electrochemical performance of PIBs is plagued by the sluggish development of suitable electrolytes. [8][11][12] Specifically, it is due to the uneven and inelastic solid electrolyte interface (SEI) formed on the electrode surface. [13]16] As a result, the electrolyte gets constantly consumed with the continuous reformation of the SEI, ultimately leading to rapid cell failure. [17]umerous engineering strategies, such as artificial SEI, [18][19][20] separator decoration, [21,22] and high-concentration electrolytes, have been reported to improve SEI's properties. [23,24]However, all of them greatly increase costs; some even increase the construction steps, which are difficult to adapt to the needs of large-scale industrialization.Ideally, a straightforward and pragmatic method that naturally tunes the electrolyte composition forming in situ elastic SEI that would be stable during cycling is highly desirable. [25]t is known that an elastic SEI possesses excellent chemical stability and homogeneously covers the anode surface. [13]asing elasticity as an indicator, many elastic artificial SEIs have emerged, [26][27][28][29][30] which greatly improve the electrochemical performance.Besides, it is known that simply changing the electrolyte from a typical carbonate solvent to an ether solvent significantly elevates SEI's elasticity, thus enhancing battery performance. [31,32]At present, such strategies are only used to design artificial SEI or modify ether electrolytes effectively.However, carbonate electrolytes have great application prospects owing to their low cost and excellent oxidation stability.Hence, building elastic SEI using commercial carbonate electrolytes to improve SEI's electrochemical performance may be a good option, although several unknowns exist.
Herein, a novel infiltration strategy is developed by introducing planar macromolecular pigment green 7 (PG-7) as an additive to help naturally form a stable reticular elastic SEI, which covers the anode surface homogeneously and has excellent chemical stability.As a result, the K||K symmetric cell could cycle for over 1400 h at a low current density of 0.1 mA cm À2 .Furthermore, the K||perylene-3,4:9,10-tetracarboxydiimide (PTCDI) cell could stably operate 1200 cycles with 82.8% capacity retention and matched graphite||PTCDI full cell could operate for over 200 cycles.We also conducted a series of characterization tests to explore this SEI's properties.This study shows that traditional dilute carbonate electrolytes can significantly improve DOI: 10.1002/sstr.202300232 The solid electrolyte interface (SEI) is vital to the stability of alkali metal-ion battery anodes.However, conventional SEIs that lack elasticity will be damaged during the anode's repeated volume expansion, such as in potassium-ion battery anodes, ultimately resulting in cell failure.Herein, a low-content additive (pigment green 7, 0.07 wt%) in a conventional carbonate electrolyte to create a reticular elastic SEI with excellent uniformity and good chemical stability is employed.As a result, long-lasting K||K symmetric cell (over 1400 h), enhanced graphite anode (500 cycles with 97.9% capacity retention), and stabilized perylene-3,4:9,10-tetracarboxydiimide cathode (1200 cycles with 82.8% capacity retention) are achieved.Furthermore, the matched graphite||perylene-3,4:9, 10-tetracarboxydiimide full cell operates stably for more than 200 cycles.This work provides a novel avenue into the rational design of elastic SEIs for advanced anodes.
battery performance with a small addition of PG-7 (0.07 wt%) and a negligible cost increase (0.17%).

Infiltration Strategy for the Additive into the SEI
One of the critical goals of this study is to innovate a method that engineers carbonate-based electrolytes [33][34][35] to form a stable SEI naturally.Accordingly, we begin with a straightforward and efficacious method using a suitable additive in the electrolyte, [36][37][38] and explore which additive type would be ideal.Previous investigations showed that electrolyte additives could be broadly divided into two categories: one is that in which a stable inorganic-rich SEI [39,40] is formed by promoting an anion's participation in the construction of SEI; the other is that in which the additive itself participates in the construction of SEI. [41]Here, we report a novel infiltration strategy for the additive to promote reticular elastic SEI formation effectively.
First, we selected an additive that was more likely to react with the electrode than solvent molecules and sufficiently participate in the construction of SEI.Second, aromatic components' dimensional rigidity and mechanical strength were also considered. [25]Accordingly, we picked phthalocyanine-type compounds with planar structures.Finally, as the maximum electron affinity is for elemental chlorine (-348.6 kJ mol À1 ), we selected a high chloride phthalocyanine derivative PG-7 that supersedes the electron affinity of hydrogen (-72.8 kJ mol À1 ).
We chose 1 M potassium bis(fluorosulfonyl)imide (KFSI) salt in an isopyknic solvent mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) electrolyte [38,42] as the test electrolyte.As illustrated in Figure 1a, for the test electrolyte without additive (Figure 1a), loose and uneven SEI is formed on the electrode surface due to the instability of reaction products.Conversely, the planar molecule PG-7 favorably interacts with the graphite electrode, leading to a uniform SEI (Figure 1b).In other words, the favorable adsorption of PG-7 increases the contact area between solvent molecules and graphite, resulting in compact and even SEI formation on the anode surface.We prepared electrolytes with different additive contents and obtained the optimal additive content (0.07 wt%) by comparing the performance of matched K|| Prussian blue analogues (PBA) batteries (Figure S1, Supporting Information).Moreover, the addition of PG-7 has a minimal cost incremental ratio (0.17%); thus, the cost of the test electrolyte with PG-7 is the least compared to previous reports (Figure 1c, see Table S1, Supporting Information, for specific data).
On this basis, combined with long-range van der Waals interactions, we used dispersion-corrected total energy density functional theory approximations (DFT-D4) method [43][44][45] to obtain the adsorption energy between each solvent and slat with graphite (Figure 1d and Figure S2, Supporting Information).Compared to the adsorption of EC, DEC, and FSI À (which all have stereo structures), it is evident that the adsorption energy [40] for PG-7 with graphite (3.48 eV) is the highest, indicating that adsorption between PG-7 and graphite is most stable.Consequently, PG-7 molecules will favorably interact with the graphite electrode, consistent with the feasibility of the additive infiltration strategy.
The test electrolyte with PG-7 showed a downshift in its 1 H nuclear magnetic resonance (NMR) spectroscopy (Figure 1e), signifying an attenuated shielding between K þ and solvent molecules, [46] making it easier for the K þ -solvent cluster in the electrolyte to bond.This deduction is also consistent with Raman spectroscopy (Figure 1f ) which showed that has an obvious weakening of the shoulder (730-750 cm À1 ) in S-N stretching after adding the PG-7 additive, indicating variations of K þ -FSI À coordination in the electrolytes.Furthermore, the Raman bands corresponding to EC and DEC have not shifted, indicating that the binding of K þ to these two solvent molecules has not changed significantly.A new Raman band appears at 1216 cm À1 (Figure 1f ) due to the PG-7 molecule's participation in the solvation structure. [47]Thus, combined with its excellent adsorption energy on graphite, PG-7 has become a major contributor to SEI construction.

Electrochemical Property Analysis
According to previous reports (Figure 2a), many dendrites are generated during cycling in traditional organic-rich SEI systems, resulting in battery failure. [25,48]To some extent, the dendrites are strikingly diminished in inorganic-rich SEI systems. [49]evertheless, introducing the highly aromatic chloride phthalocyanine derivative PG-7 into the electrolyte as an additive can form a reticular elastic SEI with a uniform structure.
The electrochemical performance of electrolytes with and without PG-7 was tested.With a current density of 0.1 mA cm À2 and an areal capacity of 0.2 mAh cm À2 , the K||K symmetric cell (Figure 2b) could stably cycle for over 1400 h in the test electrolyte with PG-7, which is significantly higher than 138 h for the test electrolyte without PG-7.We matched K||Cu cells to evaluate the efficacy of additives (Figure S3, Supporting Information); it can be seen that at a current density of 0.1 mA cm À2 , the battery becomes more stable with the addition of PG-7. Figure 2c and Figure S2, Supporting Information, depict the discharge-charge profiles of K||graphite cells at a current density of 100 mA g À1 , proving that the capacity and stability of the test electrolyte with PG-7 are much better than without PG-7.Consequently, the cell with PG-7 cycled 500 times with a capacity retention of 97.9% (Figure 2d), while the capacity decayed by half after 80 cycles in the test electrolyte without PG-7 (Figure S4, Supporting Information).In addition, the K|| graphite cell in with PG-7 also exhibited superior performance at a current density of 200 mA g À1 (Figures S5, Supporting Information).According to the charge-discharge profiles of K||PTCDI cells (Figure 2e and Figure S6, Supporting Information), with the addition of PG-7, the cell exhibits a significant improvement in polarization and a slow capacity decay.Figure 2f shows that the capacity retention is still over 82.2% after K|| PTCDI cell cycles more than 1200 cycles.The cell's initial Coulombic efficiency (ICE) [50] increased from 78.1% to 89.1%, indicating that the addition of PG-7 improved the compatibility of the electrolyte with the PTCDI electrode.Moreover, the rate performance of the cell also improved. [51]When the current density was 2000 mA g À1 , the capacity of the comparative sample was almost reduced to 0 mAh g À1 , while the discharge capacity could still maintain 70.86 mAh g À1 in the test electrolyte with PG-7 (Figure 2g).Finally, we compared the electrochemical performances of the PTCDI electrodes for potassium-ion cells (Figure 2h, see Table S2, Supporting Information for specific data).Clearly, the test electrolyte performance with PG-7 reported in this study is much better than previous reports, with significantly improved cyclability and the least cost (Table S1, Supporting Information).Linear sweep voltammetry (LSV, Figure S7, Supporting Information) measurements were conducted by using K||Al asymmetric cells at a scan rate of 0.1 mV s À1 .With the addition of PG-7, the decomposition voltage of the electrolyte increased from 4 to 4.5 V, indicating in the oxidation stability of the system has increased. [52]Correspondingly, we matched the K||PBA cells with higher working potentials (1.7%3.9V, Figure S8, Supporting Information), and found that the addition of PG-7 enabled the battery to operate more stably.
Besides, we also investigated the influence of PG-7 on Li-ion batteries (LIBs, Figure S9, Supporting Information).Unlike the traditional 1 M lithium bis(fluorosulfonyl)imide in EC/DEC system, the Li||Li symmetric cells (Figure S9a, Supporting Information), with the addition of PG-7, stably cycled for over 1000 h.Both Li||graphite (Figure S9b,d, Supporting Information) and Li||LiFePO 4 (Figure S9c,e, Supporting Information) also stably cycled with the addition of PG-7.Notably, after running for 1400 cycles at a current density of 1000 mA g À1 , the Li||LiFePO 4 cell still maintained a capacity retention of 90.9%.

SEI Structural Characterization
As discussed above, batteries could achieve excellent cyclability and stability by introducing the PG-7 additive into traditional electrolytes.This phenomenon inevitably raises questions about how a small amount of additive greatly enhances the operation of batteries.
First, we characterized the initial and recycled graphite electrodes (five cycles) through X-ray photoelectron spectroscopy (XPS) [53] to explore SEI's composition and protection mechanism.From the C 1s spectrum (Figure 3a), it is seen that recycled graphite electrodes exhibit peaks of C═C/C─C (284.6 eV), C─O (265.2 eV), and C═O (288.1 eV).The difference is that a C─Cl (286.5 eV) peak [54] derived from PG-7 could be observed in the test electrolyte with PG-7.Accordingly, in addition to the K-Cl peak (198.2 eV), [55] the C─Cl peak (200.5 eV) [54] can also be found in the Cl 2p spectrum (Figure 3c).These XPS features indicate that the addition of PG-7 brings SEI with aromatic components that can enhance its dimensional rigidity and mechanical strength and provides it with a stable inorganic component KCl.Although the peaks in the F 1s spectrum (Figure 3b) in these two systems are congruent, the areal proportion of the KFSI peak (685.3 eV) in the test electrolyte without PG-7 is visibly higher, which is consistent with the different solvation structure in these two electrolyte systems.
Subsequently, we conducted atomic force microscopy (AFM)based nanoindentation test to obtain force versus displacement curves of SEIs formed in these two electrolytes to test their elasticity. [25,46]As the AFM probe applies pressure to the sample, two stress curves labeled "approach" and "retract" are obtained. [31]rom point 1 to point 2 in the approach segment of Figure 3d, the AFM probe moves toward the sample surface driven by the extension of the piezo.The negative peak force is due to the attractive force (van der Waals force) between the sample surface and the AFM probe. [31].Under the action of the downward force, the sample undergoes continuous deformation (point 2 to point 3).The AFM probe retracts after reaching the maximum preset load (reflected as the retract segment in Figure 3d).In order to reduce the impact of the substrate on testing, we used the same batch of graphite manufactured for all batteries used for testing, and conducted the same indentation test on the graphite substrate as the sample (Figure S10, Supporting Information).For the organic-rich SEI (without PG-7, the top panel in Figure 3d and Figure S11a, Supporting Information), the SEI is severely deformed, and the "approach" and "retract" curves do not overlap at all, indicating its poor elasticity. [56]For reference, the corresponding data of the inorganicrich SEI formed in 3 M KFSI in EC/DEC electrolyte is shown in Figure S12, Supporting Information.During PG-7's participation in SEI construction, it not only added inorganic components such as KF and KCl that can increase modulus, but also brought organic components that can enhance elasticity to SEI.Correspondingly, the reticular SEI formed in the test electrolyte with PG-7 (the lower panel of Figure 3d and Figure S11b, Supporting Information) exhibits a conspicuous overlap in its "approach" and "retract" curves, indicating its excellent elasticity.In short, a reticular SEI with excellent elasticity has been formed in the test electrolyte with PG-7.
We also explored the SEI morphology of the graphite electrodes after five cycles for more intuitive exploration.The surface morphology of the SEI was obtained through scanning electron microscopy (SEM) (Figure 3e and Figure S13, Supporting Information).Without PG-7, the SEI exhibits obvious cracks, which are not present in the initial graphite (Figure S14, Supporting Information).In contrast, in the presence of PG-7 the SEI does not exhibit cracks and presents a complete block structure.The results of transmission electron microscopy (TEM) (Figure 3f and Figure S15, Supporting Information) show that the SEI formed in the presence of PG-7 is compact and uniform, while loose and nonuniform boundaries are present in the SEI in the absence of PG-7.One of the reasons for this result is that the loose and inelastic SEI (cf., top panel in Figure 3d) cannot accommodate volume changes during cycling. [16]The other is the inevitable side reactions caused by the contact between the electrolyte and the electrode at the interface.
In order to further study the composition of SEI, we conducted energy-dispersive spectroscopy (EDS) elemental mapping on the recycled graphite electrodes (Figure 3g).It can be seen that the Cl signature in the SEI is absent when the test electrolyte does not contain PG-7, while it is present when the test electrolyte contains PG-7, consistent with the additive infiltration during the SEI formation.Thus, we clearly understand the elemental analysis data of these two SEIs because we added high chloride phthalocyanine derivative PG-7 into the 1 M KFSI in EC/DEC test electrolyte.Due to changes in the solvation structure in the presence of PG-7, the type and proportion of inorganic components in SEI increased.Ultimately, the test electrolyte with PG-7 formed a more robust reticular elastic SEI, thus enhancing the electrochemical performance and cycle stability of the PIBs and LIBs.

Full-Cell Performance
We assembled a full cell to investigate the utility of the test electrolyte with PG-7.The stable graphite anode was matched with the PTCDI cathode.From the normalized curve diagram of the full and half batteries (Figure 4a) and the overlapping charge and discharge curves (Figure 4b), it can be concluded that the matched full cell has good cycling stability.Meanwhile, the matched full cell has capacity retention of 71.2% after more than 200 stable cycles (Figure 4c).We also compared the rate performance of graphite||PTCDI full cells (Figure S16, Supporting Information); the graphite|PG-7|PTCDI full cell not only operates more stably, but also improves its rate performance.
We also made further efforts to conduct an exchange experiment [57,58] to verify the compatibility of this formed SEI with graphite anode.In the first instance, we cycled the K||graphite cell in the test electrolyte with PG-7 for 30 cycles to form a stable SEI.Next, the cell was disassembled in the glove box, and the cycled graphite anode was used in another cell containing the test electrolyte without PG-7 and tested for cycling.Because of the uniform SEI formed in the first step above, the cell without PG-7 operated stably (Figure 4d) with almost no capacity degradation after the second step.At the same time, the cell using the test electrolyte without PG-7 suffered substantial fluctuations in capacity and swift attenuation (Figure S17, Supporting Information).In addition, due to the formation of a stable SEI in the first step above, the cell's ICE also improved from 61.91% to 99.15%, indicating that the SEI formed in the presence of PG-7 is extremely stable and has good compatibility with graphite electrode. [58]Overall, due to the addition of PG-7, the electrolyte's conductivity (Table S3, Supporting Information), the SEI's elasticity, and the electrochemical performance have all been enhanced (Figure 4e).

Conclusions
In summary, we demonstrated an infiltration strategy for planar macromolecule PG-7 into the SEI, which proved effective for constructing a stable reticular elastic SEI in a carbonate-based electrolyte.The rationale behind this infiltration strategy is that PG-7 alters the solvation structure in the test electrolyte favorably and participates in the construction of the SEI.With the addition of PG-7, the long-lasting symmetric K||K cells (over 1400 h), stabilized graphite anodes, and PTCDI cathodes (1200 cycles with 82.8% capacity retention) were achieved.Furthermore, the matched full cell operated stably for over 200 cycles.The theoretical calculations and characterizations provided collective evidence for our infiltration strategy and the properties of this SEI.Notably, the improvement in the electrochemical performance outweighs the incremental cost ratio of only 0.17% due to the addition of PG-7 in the test electrolyte.These findings provide a novel strategy through the rational design for elastic SEI and are expected to motivate further enhancements in the performance of rechargeable batteries.

Figure 1 .
Figure 1.Test electrolyte (1 M KFSI in EC/DEC) properties with and without the PG-7 additive.Schematic diagram illustrating SEI's morphology a) without PG-7 and b) with PG-7.c) Cost comparison for KFSI in different electrolytes.The inset shows the incremental cost ratio when PG-7 is used in the three lowest cost electrolytes.The cost of solvents was calculated based on Macklin.d) The DFT calculation of the adsorption energy between EC molecule, DEC molecule, PG-7 molecule, and graphite.e) 1 H NMR and f ) Raman spectroscopy of the test electrolytes.

Figure 2 .
Figure 2. The electrochemical performances of the test electrolytes with and without PG-7.a) Schematic of dendrites growth for three types of SEIs.b) Comparison of the K||K symmetric cells' performance at a current density of 0.1 mA cm À2 with the test electrolyte in the presence and absence of PG-7.c) Charge-discharge voltage profiles of graphite anode in the test electrolyte with PG-7.d) Cycling performance of K||graphite cells with Coulombic efficiency in the two electrolytes at a current density of 100 mA g À1 .e) Charge-discharge voltage profiles of PTCDI cathode in the test electrolyte with PG-7.f ) Cycling performance of K||PTCDI cells and their Coulombic efficiencies in the two electrolytes at a current density of 100 mA g À1 .g) Rate performance of K||PTCDI cells with the two electrolytes after one formation cycle at 100 mA cm À2 (keeping charge and discharge at the same current density).h) Comparison of current density, cycle number, and capacity retention of K||PTCDI cells cycled in different electrolytes.

Figure 3 .
Figure 3. SEI structural analysis: a) K 2p þ C 1s XPS; b) F 1s XPS; c) Cl 2p XPS.d) The AFM-based nanoindentation tests of graphite electrodes without PG-7 and with PG-7 in the test electrolyte after five cycles.e) The SEM characterizations of the graphite electrodes in the test electrolyte without PG-7 and with PG-7 after five cycles.f ) The TEM characterizations of the SEI formed on graphite electrodes in the test electrolytes without PG-7 and with PG-7 after five cycles.g) The EDS elemental mapping images of graphite electrodes after five cycles in the test electrolyte without PG-7 and with PG-7.

Figure 4 .
Figure 4.The construction of a full cell.Full cell based on a graphite anode and a PTCDI cathode in the test electrolyte with PG-7.a) Normalized charge/discharge curves at a current density of 100 mA g À1 .b) Cycle stability at a current density of 100 mA g À1 .c) Discharge-charge curves at a current density of 100 Ma g À1 .d) Electrolyte exchange experiment described in the text for a K||graphite coin cell.e) Comparison of the properties and performances of the test electrolyte with and without PG-7.