Synergetic Control of Li+ Transport Ability and Solid Electrolyte Interphase by Boron‐Rich Hexagonal Skeleton Structured All‐Solid‐State Polymer Electrolyte

High Li+ transference number electrolytes have long been understood to provide attractive candidates for realizing uniform deposition of Li+. However, such electrolytes with immobilized anions would result in incomplete solid electrolyte interphase (SEI) formation on the Li anode because it suffers from the absence of appropriate inorganic components entirely derived from anions decomposition. Herein, a boron‐rich hexagonal polymer structured all‐solid‐state polymer electrolyte (BSPE+10% LiBOB) with regulated intermolecular interaction is proposed to trade off a high Li+ transference number against stable SEI properties. The Li+ transference number of the as‐prepared electrolyte is increased from 0.23 to 0.83 owing to the boron‐rich cross‐linker (BC) addition. More intriguingly, for the first time, the experiments combined with theoretical calculation results reveal that BOB− anions have stronger interaction with B atoms in polymer chain than TFSI−, which significantly induce the TFSI− decomposition and consequently increase the amount of LiF and Li3N in the SEI layer. Eventually, a LiFePO4|BSPE+10% LiBOB|Li cell retains 96.7% after 400 cycles while the cell without BC‐resisted electrolyte only retains 40.8%. BSPE+10% LiBOB also facilitates stable electrochemical cycling of solid‐state Li‐S cells. This study blazes a new trail in controlling the Li+ transport ability and SEI properties, synergistically.


Introduction
Aiming to peak carbon emissions by 2030 and achieve carbon neutrality by 2060, China is forging ahead in clean and renewable energy development amid its transition to a low-carbon economy. [1]For this concern, battery technology, which exhibits zero-carbon emission and even significantly shapes much of our ordinary life, has unprecedentedly skyrocketed nowadays and over the past decades. [2]Among the numerous battery technologies, lithium-ion battery (LIB) is highly demanded and extensively popularized in portable electrical appliances, large power facilities, and transport sectors.However, conventional LIBs exploit graphite anode and flammable organic solvents, currently confronting restricted energy density and safety hazards originating from intercalation chemistry within liquid electrolytes. [3,4][13][14] Therefore, considerable research efforts have been devoted to design SSEs compatible with metallic Li, and by far SSEs are prevailingly classified into solid polymer electrolytes (SPEs) and solid inorganic electrolytes (SIEs).[17][18] SPEs with high chemical stability can physically block the contact between electrolyte and metallic Li, in favor of avoiding detrimental interface side reactions. [19,20]Moreover, the vexing growth of Li dendrite would also be suppressed by virtue of the high Young modulus of SPEs. [21,22]PEs normally have poor ionic conductivities (σ < 10 À5 S cm À1 ) [23,24] associated with low Li + transport abilities, [25] severely retarding their application and even commercialization.Hence, improving the ionic conductivity of SPE is of overwhelming importance, and sizable endeavors concentrate on these research topics, for instance, polymer molecular structure design, [26,27] polymer-inorganic composite, [28] gel electrolyte, [29] and polymer blending. [30]Importantly, it is worth noting that high Li + conductivity as the prerequisite preferentially determines the SPEs' performance; however, it has been usually overlooked in comparison to overall ionic conductivity.The Li + conductivity (σ Li

+
) in SPE hinges on the Li + transference number (t Li + ), which represents the contribution of Li + transport in SPEs according to the equation (t Li + + t anion À = 1). [31]Unfortunately, "free" anions of Lithium salts indeed contribute more to the overall ionic conductivity than Li + , resulting that t Li + apparently being far below 0.5 and the buildups of the anions at the electrode|electrolyte interface also causing concentration polarization. [32,33]Accordingly, designing the SPEs with high Li + transference number (t Li

+
) via immobilizing "free" lithium salt anions in polymers seems to be an intelligent strategy and highly desired for SSEs engineering.
Despite immobilization free anions in a polymer matrix is beneficial to Li + transport within the SPE layer, but on the contrary, this could cause serious Li|SPE interfacial issue involving unstable SEI generation due to the insufficient "available" anions.Moreover, to our knowledge, a stable Li|SPE interface is largely dependent on the ingredients of SEI that are derived from anions decomposition. [34,35]F-containing Li salts, e.g., LiTSFI, and LiFSI, are frequently used and instrumental to building fine Li|SPE interface on account of LiF-rich SEI formation. [36,37]Nevertheless, once these sole F-containing anions are dragged by the polymers, it would impair the stability of the Li|SPE interface.[40][41] In addition, the intermolecular interactions among the lithium salts, additives, and polymer hosts are still ambiguous, and the underpinning mechanism of their synergistic influence on SEI evolution remains an ongoing puzzle.
In this contribution, we fabricated a boron-rich hexagonal structured all-solid-state polymer electrolyte (BSPE+10% LiBOB) via in situ UV polymerization with a self-regulating anion-trapping boron-rich crosslinker (BC).The introduction of boron-rich domains into EO-based electrolytes can effectively enhance the Li + transport ability and increase Li + transference number due to the strong fastening effect of boronrich polymer on anion by the Lewis acid-base interaction.More importantly, the incorporation of LiBOB additive weakens the interaction between BSPE and TFSI À anions and favors the formation of TFSI Àderived SEI with an increased amount of LiF and Li 3 N, thereby optimizing a stable and high-conductive SEI layer on the Li anode.Finally, allsolid-state Li metal batteries including Li-LiFePO 4 and Li-S batteries based on BSPE+10% LiBOB both show promising long-term stability.The study promises opportunities for the design of a new class of allsolid-state polymer electrolytes combining the good Li + transport ability with the optimal SEI properties for high-stable Li metal battery applications.

Synthesis Procedure, Structure and Morphology of Electrolytes
As shown in Figure 1a, a boron-rich hexagonal structured all-solidstate polymer electrolyte (BSPE+10% LiBOB) was composed of PEO, tetraethylene glycol dimethyl ether (TEGDME), three-branched boronrich cross-linker (BC), LiTFSI salt, and LiBOB additive.It can be observed from its cross-sectional SEM that the thickness of the BSPE+10% LiBOB membrane is about 120 μm (Figure 1b).BC as a boron moiety supplier is synthesized by substitution reaction using poly(ethylene glycol) methacrylate (PEGMA) with trimethyl borate (TMB) (Scheme S1, Supporting Information), which not only contains Li + conductive segments but also possesses cross-linkable methacrylate groups.The 1 H NMR analysis verifies the successful preparation of BC (Figure 1c), and the conversion value of BC can be calculated as 87% on the basis of the Formula S2, Supporting Information.Under UV irradiation, the methacrylate groups of three-branched BC can be crosslinked through the self-polymerization of C=C bonds to form a hexagonal skeleton (marked by yellow dotted lines in Figure 1b), and the schematic of the formation of a regular boron-rich hexagonal crosslinked skeleton structure formed by BC is shown in Figure S1, Supporting Information.The cross-linking reactions were confirmed by the Fourier transform infrared spectroscopy (FTIR) analysis (Figure 1d and Figure S2, Supporting Information).The absorption peak at 1638 cm À1 assigned to the characteristic vinyl (C=C) in BC clearly disappears after UV irradiation, which proves that the self-polymerization of C=C bonds is successfully completed.This unique hexagonal skeleton structure formed by the self-polymerization of C=C bonds of threebranched BC can be clearly observed in SEM images (Figure 1b).Concurrently, the protons from the methylene group of PEO and TEGDME are captured from the photoinitiator, and thus produce free radical chains with active domains.The random connections of these active sites facilitate the random cross-linking of PEO and TEGDME, which will be uniformly fixed inside the regular boron-rich hexagonal support skeleton.The SEM result shows that the SPE membrane without BC exhibits a disorderly wrinkled texture (Figure S3, Supporting Information), while the unique hexagonal skeleton structure constructed by the three-branched BC can regulate the uniformity of the polymer electrolytes to form a highly uniform and ordered network structure (Figure 1b and Figure S4a,b, Supporting Information).The uniformity of electrolytes is beneficial to form a well-contacted Li/SPE interface, and the highly ordered network structure contributes to uniform Li deposition.In addition, this highly ordered network structure increases the mechanical properties of the membranes, which to some extent inhibits Li dendrite growth.The corresponding energy dispersive spectroscopy (EDS) mapping images of BSPE and BSPE+10% LiBOB membranes can confirm the even distribution of BC, LiTFSI and LiBOB in modified electrolytes (Figure 1b and Figure S4c,d, Supporting Information).The as-prepared boron-rich polymer electrolytes present Lewis acids properties because of the existence of sp2 hybrid boron atoms in BC, which have strong interactions with TFSI À and BOB À anions as Lewis bases.However, the various structures of TFSI À and BOB À may induce the various degrees of trapping force of BC on them, which have an impact on the chemical environments of the electrolytes even their Li + transport abilities and the derived SEI properties.Some discussions have been conducted in the following sections.

Physicochemical and Electrochemical Characterizations of the Modified Electrolytes
A comprehensive survey of the physicochemical and electrochemical properties of all-solid-state polymer electrolytes is of great significance to their practical application.X-ray diffraction (XRD) analyses are performed to study the crystallinity of pure PEO and the modified electrolytes (Figure S5, Supporting Information).The XRD pattern of pure PEO electrolyte exhibits two intense typical characteristic peaks of the crystalline PEO (2θ = 19 and 23°).In sharp contrast, after UV-polymerization, Energy Environ.Mater.2024, 7, e12648 the corresponding crystalline peaks disappear, which confirms that cross-linked network structures can destroy the crystalline phase and facilitate the formation of the homogeneous amorphous phase in the modified electrolytes.Meanwhile, there are no apparent diffraction peaks of LiTFSI and LiBOB, which imply that they are completely dissolved in the modified electrolytes.Furthermore, the effect of the cross-linking structure on the crystallization behavior of pure PEO and modified electrolytes are investigated by differential scanning calorimetry (DSC).As shown in Figure S6a, Supporting Information, after UV polymerization, the melting points of the modified polymer electrolytes are below 25 °C, which are reduced to varying degrees compared to the melting point of pure PEO up to 68 °C.This result signifies that the polymer segment mobilities of the modified polymer electrolytes have been greatly improved at room temperature.In addition, the glass transition temperatures (T g ) of the modified electrolytes can be obtained as À46.54 °C, À46.42 °C, À46.67 °C and À46.60 °C, respectively, which testifies that the amorphous and flexible regions are greatly enhanced (Figure S6b, Supporting Information).The above test results preliminarily prove that the formation of an ordered cross-linked network structure effectively reduces the crystallinities of the polymer electrolytes, which can promote the movement of polymer segments, thereby facilitating Li + transport in the polymer matrix.The benign thermal stabilities and mechanical properties of the allsolid-state polymer electrolytes are the prerequisites for the safety of the battery during cell packaging and charge/discharge cycling in commercial applications.The thermogravimetric analyses (TGA) are taken for all modified electrolytes to evaluate their thermal stabilities.As shown in Figure S7a, Supporting Information, that all modified electrolytes exhibit negligible weight loss until the temperature reaches 125 °C, demonstrating superior thermal stabilities at high operating temperatures.Figure S7b, Supporting Information, shows the tensile stress-strain curves for all modified electrolytes.The tensile strength of SPE is 0.88 MPa with a tensile fracture value of 490%, while the tensile strength and fracture of the BSPE membrane increase simultaneously on account of the support of the hexagonal skeleton formed by BC and the formation of the random cross-linking network between PEO and TEGDME.As shown in Figure S8a, Supporting Information, the introduction of an appropriate amount of BC can improve the tensile strength and fracture strength of the electrolyte membrane to a certain extent.However, the tensile fracture of the electrolyte membranes gradually decreases with increasing of LiBOB (Figure S8b, Supporting Information) because BOB À can combine with active domains of the free radical chains produced by PEO and TEGDME under UV illumination, and the complete interaction between sufficient lithium salt and the polymer matrix will lead to the maximized proportion of amorphous regions, which deteriorate their mechanical properties.Nonetheless, the BSPE+10% LiBOB membrane can be bended several times along the glass rod (diameter = 5 mm) without any cracking (Figure S9, Supporting Information), showing great potential for application in flexible batteries.
The ionic conductivities of all-solid-state electrolytes are one of the decisive factors that affect the electrochemical performance of ASSLMBs.In order to explore the optimal ratios of BC and LiBOB, the various mass ratios of BC were added first to the SPE membrane and the corresponding ionic conductivities were measured (Figure S10a, Supporting Information).It is worth noting that when BC is added to the SPE membrane, the ionic conductivities of the membranes containing different mass ratios are all reduced, owing to the strong fastening effect of boron-rich polymer on anions.The electron cloud density distribution of the BC molecular model was simulated, and the result further confirms that the sp 2 hybrid boron atom in BC is positively charged and can act as a Lewis acid to produce a strong attraction to anions (Figure S11, Supporting Information).Comparing SPE membranes with different mass ratios of BC, BSPE with 10 wt% BC shows the highest ion conductivity of 4.10 × 10 À4 S cm À1 at 30 °C.After introducing LiBOB, BSPE+10 wt% LiBOB displays the best ionic conductivity as high as 1.22 × 10 À3 S cm À1 at 30 °C (Figure S10b, Supporting Information).According to the above investigation on the optimal mass ratios of BC and LiBOB, the modified electrolyte with 10 wt% BC and 10 wt% LiBOB (BSPE+10% LiBOB) is selected for subsequent research.To further investigate the effect of BC and LiBOB in electrolyte membranes, the ionic conductivities of the modified electrolytes in the temperature range of 30-90 °C were measured to reveal their temperature dependences (Figure S12, Supporting Information).As displayed in Table S1, Supporting Information, the ionic conductivities of all modified electrolytes increase with the increasing of temperatures because higher temperature promotes the movement of chain segments and thus improves ions movements.Besides, the linear relationship between ionic conductivity (σ) and temperature is in accordance with the typical Arrhenius-type behavior, and the calculated activation energy (Ea) values of SPE, SPE+10% LiBOB, BSPE, BSPE+10% LiBOB is 22.28, 17.90, 31.54 and 23.19 kJ mol À1 , respectively (Figure 2a).The lower activation energies of SPE+10% LiBOB and BSPE+10% LiBOB indicate that the introduction of LiBOB in electrolytes can reduce the energy barrier for Li + diffusion.Moreover, density functional theory (DFT) calculations validate the fastening effects of BC on TFSI À and BOB À , respectively (Figure 2d).The binding energy of BC with BOB À anion is À94.97 kJ mol À1 , lower than that of BC with TFSI À anion (À68.31kJ mol À1 ), which manifests that BC is more inclined to bind with BOB À anions.When TFSI À and BOB À anions coexist, the smaller binding energy of À110.98 kJ mol À1 is obtained and both B-O distances become larger due to steric hindrance.This result demonstrates that the incorporation of BOB À weakens the interaction between BC and TFSI À anions, which is consistent well with the above results for ion conductivity and activation energy.
From the above, it can be seen that the addition of BC decreases the ionic conductivities (σ) of bulk electrolytes, but actually, the Li + conductivity (σ Li + = σ × t Li

+
) and the Li + transference number (t Li

+
) in the electrolyte are both the key factors to judge the Li + transport capabilities.According to Sand's time Equation S1, Supporting Information, the closer Li + transference number (t Li

+
) is to 1, the slower the growth rate of Li dendrites.The calculated t Li + of BSPE and BSPE+10% LiBOB is 0.83 and 0.76, respectively, which is much higher than the samples without BC (SPE: 0.24, SPE+10% LiBOB: 0.13) (Figure 2b and Figure S13, Supporting Information).The above results show that although the fixation effect of BC on the anions reduces the ionic conductivities of bulk electrolytes, it can effectively improve the t Li + of the electrolytes.Additionally, by calculating the Li + conductivity (Table S2, Supporting Information), the BSPE+10% LiBOB owns the highest Li + conductivity of 4.05 × 10 À4 S cm À1 at 30 °C, which is conducive to the development of the rate performance of the solid-state battery.Linear sweep voltammetry (LSV) is commonly used to estimate the electrochemical stability of the electrolyte.As shown in Figure 2c, the BSPE+10% LiBOB exhibits improved oxidative stability below 4.94 V (vs Li + /Li).According to frontier molecular orbital theory (Figure 2e), the redox reaction activities of the electrolytes are heavily dependent on the energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO).For the component with a higher HOMO energy, the electron in the outermost layer has higher energy and tends to be lost more easily under high voltage, resulting in the oxidization of the component at a lower voltage. [42,43]he HOMO energy level of BC basically unchanges when LiTFSI is introduced, and goes up a little bit when LiBOB is added.This phenomenon is consistent well with the result of LSV, implying that the introduction of LiTFSI and LiBOB has no significant effect on the oxidation resistance of electrolytes.For the component with a lower LUMO energy, the innermost unoccupied electron orbital is lower, and foreign electrons are more likely to occupy this orbital, resulting in the reduction of the component at a higher voltage.The LUMO energy level of BC is significantly reduced after adding LiBOB.The above proves that LiBOB addition can be applied as a film forming agent and decomposed before LiTFSI to form SEI on the surface of the Li anode.

SEI Formation Mechanism Analysis in Li||Li Cells
In addition to the appropriate Li + conductivity and high Li + transference number, the compatibilities between the electrolytes and Li metal anodes are critical to maintaining the long-term cycling stabilities of solid-state cells.Therefore, galvanostatic Li plating/stripping of Li||Li Energy Environ.Mater.2024, 7, e12648 symmetric cells with the modified electrolytes are performed, whereby 1h charge and discharge intervals under fixed currents are adopted.As shown in Figure 3a, a large and irreversible voltage drop for Li|SPE|Li cell appears at the 102nd cycle (204 h) at 0.1 mA cm À2 , which is ascribed to a typical of short-circuit caused by Li dendrite growth.In contrast, the short-circuits of Li|SPE+10% LiBOB|Li and Li|BSPE|Li cells emerge at the 220th (440 h) and 330th cycle (660 h), respectively.Oppositely, the collaborative advantages of BC and LiBOB enabled Li| BSPE+10% LiBOB|Li cell exhibits superior resistance to premature cell failure resulting from dendrites growth, which is reflected in the smooth voltage profile with stable cycling for more than 500 cycles (1000 h) at 0.1 mA cm À2 .To further evaluate the Li anode interface stability of BSPE+10% LiBOB with current change, we tested the reversibility of Li deposition with current changes ranging from 0.05 mA cm À2 to 0.1 mA cm À2 and then back to 0.05 mA cm À2 , and excellent dendrite resistance is demonstrated for BSPE+10% LiBOB regardless of current variation (Figure 3b).The galvanostatic Li plating/stripping of Li||Li symmetric cells at various current densities with BSPE+10% LiBOB were performed and shown in Figure S14, Supporting Information.At 30 °C, a large polarization voltage appears at 0.3 mA cm À2 due to the limited kinetics of a solid-state polymer electrolyte.In contrast, at 50 °C, Li||Li symmetric cell a stable and low overpotential plateau of only about 25 mV at 0.1 mA cm À2 .Moreover, a flat overpotential plateau can also be maintained when the current density increases to 1.0 mA cm À2 .Besides, the interfacial impedance buildup of Li||Li symmetric cell comprised of BSPE+10% LiBOB is recorded during cycling by electrochemical impedance spectroscopy (EIS), and the corresponding fitting equivalent circuit is given in Figure S15, Supporting Information.As shown in Figure 3c, the interfacial impedance of Li|BSPE+10% LiBOB|Li symmetric cell reduces from 1290 to 1043 Ω after 100 cycles and stabilizes at about 970 Ω after 200 cycles, verifying the stability of Li|| BSPE+10% LiBOB interface further.
The morphology evolution of Li metal surface cycled in various Li||Li symmetric cells after 100 cycles at 0.1 mA cm À2 is examined by SEM and AFM, as shown in Figure 3d,e and Figure S16, Supporting Information.The Li anode surfaces morphologies formed in cycled Li|SPE|Li and Li|SPE+10% LiBOB|Li cells are both very rough with the massive dead Li (Figure S16a,b, Supporting Information).However, a relatively flat surface without obvious Li dendrite and dead Li of the cycled Li|BSPE|Li cell is obtained, arising from the uniform Li + deposition on the surface of the Li anode.After the incorporation of LiBOB, the Li anode surface morphology formed in cycled Li|BSPE+10% LiBOB|Li cell has a very smooth surface with no apparent dendrites or defects (Figure 3d).The top-view AFM images of Figure 3e and Figure S16, Supporting Information, provide more detailed evidences for surface morphology information of the cycled Li anodes.For the Li anode surfaces of Li|SPE|Li and Li|SPE+10% LiBOB|Li cells, both height variations are larger, which inevitably induce inferior Li electrodeposition performances and the faster occurrence of short-circuiting.As a contrast, the Li anode surfaces in the Li|BSPE|Li and Li|BSPE+10% LiBOB|Li cells are relatively even with both height variations of no more than 50 nm.These results manifest that Li tends to deposit uniformly on the Li anode surface in the Li||Li symmetric cells equipped with BC-assisted electrolytes, which can induce an even current distribution, suppress dendritic Li accumulation, and ultimately raise the safety of their applications.More significantly, when applying LiBOB to BSPE, the stronger Lewis acid-base interaction between BOB À and BC results in the Energy Environ.Mater.2024, 7, e12648 abundant free TFSI À decomposition to participate in the construction of stable electrolyte||Li interface.
Having determined the anion-immobilized polymer electrolytes conducing to reversible Li deposition/stripping, then their effects on SEI properties are disclosed.X-ray photoelectron spectroscopy (XPS) is collected to analyze the chemical compositions of SEI in these modified electrolytes (Figure 3f and Figure S17, Supporting Information).The SEI layers in all modified electrolytes have similar organic compositions.As revealed by the C1s spectra at 284.8 (C-C), 286.5 (C-O), 289.9 (O-C=O), and 293.0 eV (-CF 3 ), which are typical organic compositions in EO-based polymer electrolytes as reported in previous publications. [44,45]Particularly, C=O (C1s: 288.4 eV) is detected in the SEI layers of both BSPE and BSPE+10% LiBOB on account of the existence of BC.Nevertheless, they differ significantly in the relative amounts of LiF (F1s: 684.7 eV) and Li 3 N (N1s: 397.4 eV) inorganic components (Figure 3g).SEI formed in BSPE electrolyte has a high Li 3 N content of over 42%, while the SEI layers in SPE and SPE+10% LiBOB counterparts possess only 22% and 18% Li 3 N.This result illustrates the formation of Li 3 N-rich high conductive SEI for BSPE by the reason of that S-N bond in TFSI À is more easily to break ascribing to the strong absorption between O atom in TFSI À and centra B atom in a polymer.Furthermore, the detailed F1s analysis suggests that a less amount of LiF (11%) is presented in the SEI for BSPE, resulting from the abstemious TFSI À reduction.Unsatisfyingly, low LiF content in the SEI is insufficient to prevent electron tunneling and regulate Li + uniform deposition.Noteworthy, the SEI in BSPE+10% LiBOB shows a relatively high LiF content of 26% since the introduction of LiBOB additive can weaken the interaction between boron-rich polymer and TFSI À .These phenomena agree well with the theoretical calculation results above where the BOB À as a "shield" can modulate the decomposition degree of TFSI À in strongly anion-fastening electrolytes.To sum up, BSPE with a high Li + transference number boosts homogenous Li + flow before reaching the Li surface, whereas an incomplete and labile SEI derived from the less TFSI À reduction is formed, resulting in irreversible Li deposition/stripping.In comparison, the LiBOB additive promotes the formation of intact and stable (LiF and Li 3 N-rich) SEI in BSPE+10% LiBOB, which is beneficial for inhibiting dendrites growth and prolonging cycling life.

Cells Performances of Modified Electrolytes in Li||LiFePO 4 and Li||S
The feasibilities of the modified electrolytes are further evaluated by assembling a full cell using Li metal as an anode and LiFePO 4 as the cathode.First, the cells based on BSPE electrolyte membranes with different proportions of BC and BSPE electrolyte membranes with different proportions of LiBOB are assembled.The cell based on the electrolyte membrane (BSPE+10% LiBOB) with 10 wt% BC and 10 wt% LiBOB shows the best cycle stability and capacity retention rate (Figures S18 and S19, Supporting Information), which is consistent with the optimal ratio of BC and LiBOB selected by the ionic conductivity above.Therefore, the electrochemical performances of the assembled full cells with BSPE+10% LiBOB are characterized in detail.Figure 4a shows the rate performances at various current densities based on various modified electrolytes, with a voltage range of 2.5-3.8V.It can be clearly seen that the discharge specific capacities of all electrolytes are reduced with the increasing of current densities due to the kinetic limitations.Remarkably, the Li|BSPE+10% LiBOB|LiFePO 4 cell can render relatively better performances at different current densities.It shows the higher specific capacities of 159.9, 155.7, 142.6, 118.0 mAh g À1 at 0.1, 0.2, 0.5, and 1 C (Figure S20, Supporting Information), which is mainly attributed to the synergistic effect of uniform Li deposition and stable SEI generated on Li anode surface.Moreover, the long-term cycling performances of the Li||LiFePO 4 cells based on the different modified electrolytes are further explored.At a rate of 0.2 C, the Li|SPE| LiFePO 4 cell displays a severe capacity fade after 170 cycles (Figure 4b), since the accumulations of dead Li originate from the side reaction between Li and SPE (Figure S16a, Supporting Information).The capacity retentions of the Li|SPE+10% LiBOB|LiFePO 4 and Li|BSPE|LiFePO 4 cells are only 40.8% and 61.4% after 400 cycles, respectively, which suggest that the presence of either BC or LiBOB alone is not sufficient to maintain a stable long cycle.In sharp contrast, the cycle life of Li| BSPE+10% LiBOB|LiFePO 4 cell is extended significantly, which maintains a capacity retention rate of 96.7% and an average coulomb efficiency of nearly 100% after 400 cycles (Figure 4b).Additionally, the Li||LiFePO 4 cell with BSPE+10% LiBOB exhibits prominent cycling stability even at a high rate of 0.5 C.After being cycled for over 450 cycles, the discharge capacity after stabilization is 139.9 mAh g À1 and the capacity retention rate and coulombic efficiency are both nearly 100% (Figure 4c and Figure S21, Supporting Information).Such high and stable capacity retention exceeds that of most all-solid-state polymer electrolytes (Table S3, Supporting Information).
Inferior Li anode compatibility is also a major performance drawback limiting the real application of Li-S cell.Therefore, the BSPE+10% LiBOB membrane is assembled into the all-solid-state Li-S cell to probe its electrochemical performances.The highest discharge capacity of Li| BSPE+10% LiBOB|S cell can be reached at 864.4 mAh g À1 after four cycles, and retain a capacity retention of 71.1% and a coulomb efficiency of 97.6% after 30 cycles at 0.1 C (Figure 4d,e).Its good cycling stability can be summarized as follow: i) solid polymer electrolyte does not dissolve polysulfides, which can avoid the polysulfide shuttle effect; ii) the high Li + transference number of BSPE+10% LiBOB is conducive to the uniform Li metal deposition; and iii) the stable and highly conductive SEI can inhibit Li dendrites growth, which is a favor to the performance of Li-S batteries.However, the specific capacity of all-solidstate Li-S cell still presents an attenuation, which may be related to the poor contact between solid polymer electrolyte and active S cathode.Future researches would focus on solving the interfacial issues of cath-ode|electrolyte to realize the higher energy density and the longer cycle stability of all-solid-state Li-S cell.All in all, synergetic control of high Li + transference number and excellent SEI properties can enable the BSPE+10% LiBOB electrolyte-supported cell with the best comprehensive performance, which can be intuitively observed from the radar diagram (Figure 4f).

Conclusion
In summary, we fabricated a boron-rich all-solid-state polymer electrolyte (BSPE+10% LiBOB) with improved Li + transport ability by introducing a three-branched boron-containing cross-linker (BC).Threebranched BC enables an ordered hexagonal skeleton structure for polymer, and reinforces the mechanical strength and thermal stability of polymer electrolyte, which would be beneficial for the safety and cycle life of the assembled ASSLMBs.Moreover, due to the strong Lewis acid-base interaction between the BC-supported polymer host and TFSI À counter anions, the Li + transference number (t Li + = 0.83) in BSPE is conspicuously enhanced.Most significantly, the addition of LiBOB can moderately weaken the anion-trapping effect of a boron atom in polymer to TFSI À , and thereby promote the decomposition of TFSI À , ultimately favoring a LiF, Li 3 N-rich SEI layer on Li anode.Based on collaborative advantages, a LiFePO 4 ||Li cell employing BSPE+10% LiBOB delivers a high discharge capacity of 139.9 mAh g À1 at 0.5 C without obvious capacity fading during 450 cycles, as well as a good performance with S cathodes.To pursue high-power density ASSLMBs, future research will be motivated to improve the rate capability of solid polymer electrolytes at room temperature, without compromising their safety advantages.
Synthesis of three-branched boron-containing cross-linker (BC)-The threebranched boron-containing cross-linkers (BC) were synthesized via substitution reaction using poly(ethylene glycol) methacrylate (PEGMA) with trimethyl borate (TMB).The detailed synthesis process of BC is depicted in the Supporting Information.
Preparation of the all-solid-state polymer electrolyte membranes-All modified electrolytes were prepared through in situ UV-cured polymerization method.Taking boron-rich hexagonal structured all-solid-state polymer electrolyte (BSPE+10% LiBOB) as an example, first, BC, TEGDME, and PEO were mixed at a mass ratio of 1:10:11, and then a certain amount of Li salts (LiBOB: LiTFSI = 1:10 as mass ratio) were dissolved in the mixed solvents to obtain the precursor.The weight ratio of PEO to the total weight of TEGDME and BC is 1:1.In the meanwhile, LiTFSI and LiBOB were added with -EO-: Li molar ratio of 20:1.The precursor solution was stirred by magnetic agitation at 30 °C for 12 h.Then, 3 wt % MBP (relative to the total mass) as the photoinitiator was added into the precursor with continuous magnetic stirring for another 1 h.Subsequently, the mixed precursor was successively placed on the PTFE disk with a lid to prevent it from reacting with water and oxygen in the air, heat for about 2 h at 100 °C.Afterward, the obtained heated sticky substance was pressed at the appropriate stress (P ≈ 2 MPa) using a polyethylene terephthalate (PET) film on it.Finally, the precursor was exposed to a UV-lamp (500 W, with a wavelength of 365 nm and an irradiation energy of 150 mW cm À2 ) for 15 min and then the self-standing membrane was gained.As comparison samples, SPE (without BC and LiBOB), SPE+10% LiBOB (without BC), and BSPE (without LiBOB) were prepared by the Energy Environ.Mater.2024, 7, e12648 8 of 10 same method as above.The thicknesses of all modified electrolyte membranes were about 120 AE 10 μm.
The pure PEO membrane as a comparison sample was prepared by PEO and LiTFSI salt with -EO-: Li molar ratio of 20:1, which was mixed and stirred for 12 h in anhydrous acetonitrile (ACN) until the solution become homogeneous.The final solution was casted onto a PTFE plate and then dried for 12 h in the vacuum chamber at 50 °C.
Preparation of cathode-LiFePO 4 cathode: LiFePO 4 , PVDF, and Super P (8:1:1 by mass) were dispersed in NMP and stirred vigorously for 24 h.The resulting slurry was scraped onto Al foil and vacuum dried at 120 °C for 24 h to remove NMP.Finally, they were cut into 13 mm diameter wafers to obtain the cathodes, and the active materials loadings of LiFePO 4 cathodes were 1.5 AE 0.2 mg cm À2 .
S cathode: The 140 mg 350G and 60 mg S powder were mixed uniformly and sealed in a 50 mL autoclave and heated to 155 °C for 20 h to prepare 350G/S composite material (78 wt.% S loading).350G/S, super P, and PVDF were mixed with NMP in a mass ratio of 8:1:1 to make a slurry.The slurry was then scraped onto Al foil with a spatula.After the NMP evaporation at 50 °C overnight in a vacuum oven, the sulfur cathode was thus obtained.The active sulfur loading for each cathode is about 0.6 AE 0.1 mg cm À2 .
Materials characterization-The 1 H NMR (Bruker Avance NEO 500 MHz) spectra were conducted to determine the reaction products by using deuterated chloroform (CDCl 3 ) as solvent.The crystallinities of all samples were identified by XRD (Smartlab, Rigaku).Differential scanning calorimetry (DSC) curves were recorded to investigate the effect of the cross-linking structure on the crystallization behavior by using DSC Q20 V24.10 Build 122 at a heating rate of 20 °C min À1 under an N 2 atmosphere from À80 to 100 °C.The UV curing reaction of the modified electrolytes was estimated using an FT-IR spectrometer (Nicolet 6700, Thermo Fisher Scientific), which was recorded with a Magna 560 spectrometer (American Nicolet) using the KBr pellet technique.The microstructures and morphologies of the samples were tested by field emission scanning electron microscope (FESEM) (SU8010, Hitachi) equipped with an energy-dispersive X-ray (EDX) detector for elemental analysis and mapping.Thermogravimetric analysis (TGA) was taken to evaluate the thermal stability at a heating rate of 20 °C min À1 under an N 2 atmosphere from 25 to 600 °C.The mechanical properties of the membranes were measured by using a universal tensile testing machine (LR 10 K, Lloyd) at the strain rate of 20 mm min À1 .Surface morphologies of Li metals are revealed by atomic force microscopy (Asylum Research Cipher ES) at a scan rate of 1 Hz in peak force tapping mode.XPS (ESCALab 250Xi, Al-Kα, hν = 1486 eV, Thermo Scientific) was applied to explore the SEI components.The binding energy was calibrated with the C1s peak at 284.8 eV.The XPS profiles were fitted using a Shirley background subtraction and a Gaussian/Lorenzian (70%/30%) peak shape.
Density functional theory (DFT): The Gaussian 09 package was used to carry out all the DFT calculations, and the calculation is adopted to the B3LYP functional and 6-31G (d, p) as the basis set.This function is the choice to perform calculations on account of its reliability in studying the structures, interactions of molecular systems, redox properties, and interface reactions of electrolytes.The geometries optimization and frequency calculations of molecules are performed at the same theory level to ensure the absence of an unstable state (imaginary frequency).
Electrochemical measurement: The potentiostatic EIS of electrolytes were estimated with symmetric Stainless Steel coin cells (SS|electrolytes|SS) on the electrochemical workstation (PARSTAT 4000) with a frequency range from 1 MHz to 1 Hz with the amplitude of 10 mV.Ionic conductivities (σ) of electrolytes at different temperatures in the range 30 ≤ T ≤ 90 °C were calculated by the following formula: R is the total resistance of the electrolyte, and L and S are the thickness and surface area of the electrolyte membrane, respectively.All EIS measurements under different temperatures were conducted in an MQ-DT(H)30F-2 high-low temperature chamber (ZhongkeMeiqi (Beijing) Technology Co, Ltd.).The Li + transference numbers (t Li

+
) of electrolytes were determined at 30 °C by the combination of AC impedance measurement and chronoamperometry of a polarization (ΔV = 10 mV) using symmetric Li|electrolyte|Li cells and were calculated by the following formula: where I 0 and Is are the initial and steady-state currents, and R 0 and R S are the initial and steady-state interfacial resistances between electrolytes and Li electrodes, respectively.The electrochemical stability windows of electrolytes were measured with linear sweep voltammetry (LSV), which performed on a SS as a working electrode and Li metal as a counter and reference electrode at a scan rate of 0.1 mV S À1 over a voltage range from 0 to 6 V.
Symmetric Li|electrolytes|Li coin cells were assembled with the modified electrolytes to investigate the electrochemical kinetic processes at the interfaces on the Li anode side by EIS measurements.All of the EIS measurements were conducted on the electrochemical workstation (PARSTAT 4000) with a frequency range from 1 MHz to 0.1 Hz under a voltage amplitude of 10 mV.
Electrochemical tests were carried out using coin-type cells (CR2025) assembled by LiFePO 4 or sulfur cathodes, as-prepared electrolytes, and Li metal anodes.All assembly of batteries was carried out in an air-filled glove box (H 2 O and O 2 < 0.1 ppm).Galvanostatic charge-discharge tests of Li||LiFePO 4 cells were carried out at 30 °C overpotential range of 2.5-3.8V (vs Li + /Li) on a battery test system (NEWARE, China).The galvanostatic discharge/charge measurements of the assembled Li||S cells were conducted on a battery test system (NEWARE, China) at a galvanostatic current with cut-off voltages of 1.6-2.8V (vs Li + /Li) at 30 °C.

Figure 1 .
Figure 1.a) Schematic diagram of BSPE+10% LiBOB membrane.b) SEM images with different magnifications, cross-sectional SEM and EDS mappings of BSPE+10% LiBOB.c) 1 H NMR spectrum of BC. d) FTIR spectra of BC and the modified electrolytes.

Figure 2 .
Figure 2. a) Arrhenius plots of the ionic conductivities of the modified electrolytes.b) Chronoamperometry curve of BSPE+10% LiBOB.Inset shows the complex impedance response before and after the DC.polarization.c) Linear sweep voltammetries (LSV) of the modified electrolytes.d) The Lewis acid-base interactions of BC-TFSI À , BC-BOB À , BC-TFSI À -BOB À and their binding energies.e) HOMO-LUMO orbital distributions and energy level diagrams for BC, BC + LiTFSI, BC + LiBOB and BC + LiTFSI + LiBOB.

Figure 3 .
Figure 3. a) The voltage profiles of Li|modified electrolytes|Li symmetric cells at 0.1 mA cm À2 with a deposition capacity of 0.1 mA h cm À2 at 30 °C.b) The voltage profiles of Li||Li symmetric cells with BSPE+10% LiBOB at different current densities at 30 °C.c) Impedance monitoring of Li|BSPE+10% LiBOB|Li symmetric cell during cycling.d) Top-view SEM image of Li-metal surface obtained from Li|BSPE+10% LiBOB|Li cell after 100 cycles.e) The top-view AFM images of Li metal surface obtained from Li|BSPE+10% LiBOB|Li cell after 100 cycles.f) XPS spectra of C1s, F1s, N1s, and B1s for Li anode retrieved from Li| BSPE+10% LiBOB|Li cell after 30 cycles.g) The corresponding peak area ratios retrieved from the F1s and N1s spectra of SEI formed in Li||Li symmetric cells containing the modified electrolytes after 30 cycles, and schematic illustrations of the LiF and Li 3 N in SEI formed on the Li anode of Li||Li symmetric cells with BSPE and BSPE+10% LiBOB.

Figure 4 .
Figure 4. a) Rate capabilities of LiFePO 4 ||Li cells using modified electrolyte membranes.b) Cycling performances of LiFePO 4 ||Li cells using modified electrolytes at 0.2 C. c) Cycling performance of LiFePO 4 ||Li cell using BSPE+10% LiBOB electrolyte at 0.5 C. d) Representative charge/discharge curves of Li| BSPE+10% LiBOB|S cell.e) Cycling performance of Li||S cell using BSPE+10% LiBOB electrolyte at 0.1 C. f) Radar plots of the characteristics for the modified electrolytes.All electrochemical tests were performed at 30 °C.