A “shuttle-relay” lithium metal battery enabled by heteroatom-based gel polymer electrolyte

Employing high-energy electrode couples and releasing the capacity of anions in the electrolyte are promising avenues to increase the energy density of existing lithium (Li)-based batteries. Herein, we develop a “shuttle-relay” Li metal battery (SRLMB) based on a hybrid Li-rich oxide cathode with graphite as conductive agent and a heteroatom-based gel polymer electrolyte (HGPE). The HGPE was facilely prepared by polymerizing diethyl allyl phosphate (DAP) monomer in-situ in an all-uorinated electrolyte, which features high ionic conductivity, high oxidation stability up to 5.5 V vs. Li/Li + , high safety, and superior compatibility with Li metal (a plating/striping Coulombic eciency of 99.7 %). When applied to SRLMBs, this quasi-solid-state electrolyte enables a reversible insertion of hexauorophosphate (PF 6− ) anions into the conductive graphite after the stripping of Li ions from Li-rich oxide, thus improving the overall energy density of batteries. Our ndings provide new insights into the upgrading of Li-based battery technology.


Introduction
Lithium (Li)-based batteries, particularly Li-ion batteries, have dominated the market of portable energy storage devices for decades 1 . However, the energy density of Li-ion batteries is approaching their theoretical limit (300 Wh kg − 1 ), making it di cult to satisfy the requirement for long-distance driving with a single charging of electric vehicles 2 .
To further increase the energy density of Li-based batteries, the upgrading of electrode and electrolyte materials is urgently desired. As for anode materials, Li metal has been regarded as the ideal candidate due to its ultrahigh speci c capacity (3860 mAh g − 1 ) and the lowest redox potential (-3.04 V versus standard hydrogen electrode) 3 . However, its real-world application has been severely hampered by uncontrollable Li dendrite growth during cycling 4 . It is well-recognized that the highly reactive Li metal is prone to react with the electrolytes and form a passivated solid electrolyte interphase (SEI) layer on the surface 5 . Nevertheless, the strength of such a SEI layer generally cannot withstand the repeated volume changes during Li deposition and striping, which results in surface defects and subsequent dendrite growth from these defects 6 . The resulting Li dendrites can not only pierce through the separator and trigger catastrophic safety hazards, but also constantly consume both active Li and electrolyte, giving rise to low Coulombic e ciency and degraded cycle life 7 .
On the cathode side, layered transition metal oxides, e. g. nickel-rich oxides (LiNi 1 − x M x O 2 , M = Co, Mn and Al) and Li-rich oxides (Li 1 + x M 1−x O 2 , M = Mn, Ni, Co), are desirable for high-energy Li-based batteries considering their combined merits in speci c capacity, working potential and cycling performance 8 .
However, for the layered transition metal oxide-based batteries, only the Li ions in the electrolyte participate in the electrochemical reactions based on a "rocking-chair" chemistry, while no extra capacity contribution is made by the anions in electrolytes. Therefore, unlocking the additional potential of anions in the electrolyte is a promising approach to further enhance battery energy density. Recently, dual-ion batteries (DIBs) based on graphitic cathode materials have attracted extensive attentions, in which anions (e.g. hexa uorophosphate (PF 6 − ) 9 , bis(tri uoromethanesulfonyl) imide (TFSI − ) 10 or bis( uorosulfonyl)imide (FSI − ) 11 ) reversibly intercalate into/deintercalated from graphite interlayers within the cathode during charge/discharge processes 12 . The operating voltage of these DIBs generally is about 5 V vs. Li/Li + , which is favorable for energy density improvement 13 . However, such a high intercalation voltage of graphite leads to severe oxidative decomposition of the electrolytes, and tends to construct a high-resistance cathode electrolyte interface (CEI) on the cathode surface 14 . This seriously impedes anion insertion, resulting in inferior reversibility and poor cycling stability 15 . Furthermore, the cointercalation of the solvent molecules into graphite cathode causes an exfoliation of graphite layers and the subsequent irreversible loss of active materials during cycling 16 . As for the electrolytes, the highly ammable solvents (e. g. organic carbonates and ethers) widely applied in Li-based batteries trigger serious safety concerns including re, explosion, and leakage of toxic electrolyte components 17 . All these drawbacks have brought great challenges for the development of high-energy Li-based batteries.
Here, for the rst time, we demonstrate that a reversible insertion/extraction of PF 6 − anions between graphite interlayers can be achieved in a heteroatom-based gel polymer electrolyte (HGPE), which was synthesized via in-situ co-polymerization of diethyl allyl phosphate (DAP) monomer and pentaerythritol tetraacrylate (PETEA) crosslinker in the presence of an all-uorinated electrolyte. This HGPE exhibited high safety (i. e. non-ammability and non-leakage), high ionic conductivity (3.7×10 − 3 S cm −

Results And Discussion
Mechanism of "shuttle-relay" Li metal battery. Fig. 1a illustrates the working mechanism of the quasisolid-state SRLMB, which is realized by the well-designed HGPE. Currently, Li-ion batteries extensively apply organic electrolytes containing cyclic carbonate solvents (e.g. ethylene carbonate (EC)) with high dielectric constant to dissolve lithium hexa uorophosphate (LiPF 6 ) salt, and linear carbonate solvents (e.g. ethylmethyl carbonate (EMC)) to reduce the electrolyte viscosity 11 . However, such carbonate-based electrolytes generally show poor compatibility with both Li metal anode and 5 V-class cathodes (e. g. LRO for "rocking-chair" chemistry and graphitic carbon for "dual-ion" chemistry). On the anode side, the electrolyte solvents cannot form stable SEI layer on the Li metal surface, leading to Li dendrite growth and low Columbic e ciency 18 . On the cathode side, the insu cient oxidation resistance of carbonate solvents triggers severe electrolyte decomposition and constructs a thick CEI with high-resistance, which dramatically degrades the battery performance (Fig. 1a, upper panels). The interfacial issues are even more severe for graphite cathodes, since the carbonate molecules tend to co-intercalate into graphite interlayers, giving rise to low reversible anions insertion/extraction capacity accompanying structural deterioration 16 .
To address these issues, we developed an all-uorinated electrolyte for high-voltage Li Fig. 1). Moreover, as a novel electrolyte component, the HTE not only functionsas a diluent to reduce the electrolyte viscosity, but also optimizes the localized solvation structure of cation/anion aggregates, thus further stabilizing the Li|electrolyte interfaces. On this basis, 3 wt% DAP monomer and 1.5 wt% PETEA crosslinker were in-situ polymerized in this uorinated electrolyte to form a HGPE, in which the threedimensional polymer matrix effectively improves the electrolyte safety by preventing liquid leakage ( Supplementary Fig. 2). To verify the effect of each electrolyte component, we performed density functional theory (DFT) calculation on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the solvent molecules. Based on molecular orbital theory, the HOMO energy correlates to the oxidative decomposition potential, while the LUMO energy is associated with the reductive decomposition potential 19 . As shown in Fig. 1b, the HOMO energy of uorinated solvents (i. e. FEC: -7.3278 eV, FEMC: -7.001 eV and HTE: -8.1353 eV) is much lower than those of EC (-6.8853 eV) and EMC (-6.4791 eV), demonstrating the superior oxidation resistance of uorine solvents owing to the strong electron-withdrawing effect of uorine atoms on the core of solvent molecules 20 . Meanwhile, DAP monomer presents the highest HOMO value (-6.2292 eV). As a result, the residual DAP monomer after polymerization acts as a CEI-forming additive in the HGPE to further inhibit the electrolyte oxidation and the co-intercalation of solvent molecules into graphite. Moreover, FEC and DAP exhibit the lowest LUMO energies of -0.8835 eV and -1.0021 eV, respectively. Consequently, FEC and the residual DAP monomer will be preferentially reduced on the Li anode to form a protective LiF-rich and phosphorus-containing SEI inhibiting dendrites growth. To evaluate the effect of HTE, we calculated the binding energy of electrolyte components with Li + cation and PF 6 anion (Fig. 1c). It is seen that the HTE shows the lowest absolute values of binding energy with both Li + and PF 6 -, indicating a weak interaction between HTE and ions. This is consistent with the lower solubility of LiPF 6 salt in HTE compared with other solvents (Supplementary Fig. 3). Therefore, the introduction of HTE enables formation of a highly concentrated electrolyte in local regions by increasing the ratio of ion: uorinated carbonate in the solvation structures. Such a unique solvation structure can minimize the excessive side reactions between electrolyte solvents and electrodes, thus improving the battery performance 21 .
When applied in SRLMBs with LRO as active materials and KS6 graphite as conductive agents, the welldesigned HGPE endows a novel "shuttle-relay" battery chemistry. As shown in the lower panels of Moreover, it is worth noticing that the polymer matrix in the HGPE effectively immobilizes the solvents and decreases their volatility, thus preventing the risk of liquid leakage ( Supplementary Fig. 5) 25 . Such high safety of the HGPE is a critical asset for the practical application of high-energy Li metal batteries.
Raman spectra was measured to characterize the coordination environment in the electrolyte. It is seen that in the mixture of FEC: FEMC (1: 6 by volume), peaks at around 730cm -1 were recorded (assigned to free FEC) and one at about 840 cm -1 corresponding to the free FEMC molecule ( Fig. 2d and Supplementary Fig. 6). After dissolving 1 M LiPF 6 into the mixture, the peak intensity of free solvent molecules diminished accompanying the appearance of new bands at about 849 cm -1 (Li + -coordinated FEMC), 921 and 745 cm -1 (Li + -coordinated FEC) 26 . With the addition of HTE, an extra peak of free HTE molecules was observed at approximately 706 cm -1 in the spectrum of LiPF 6 -FEC: FEMC: HTE electrolyte, meanwhile the peak intensity of Li + -coordinated carbonates increased, which veri es that more uorinated carbonate molecules are coordinated with Li ions in the solvation sheaths 27 . This is wellconsistent with the binding energy calculation results in Fig. 1c, which e ciently alleviates the excessive side reactions between free solvent molecules and Li metal.
Ionic conductivity is considered as an important property of electrolytes. Fig. 2e and Supplementary Fig. 7 show the temperature dependences of ionic conductivities for 1 M LiPF 6 Fig. 3c and Supplementary Fig. 13). The plating morphologies of Li on Cu substrates were examined by eld emission scanning electron microscope (FE-SEM). As shown in Fig. 3d Fig. 14).
In the Li|Cu cell employing the HGPE, for comparison, the plating Li showed a compact morphology as aggregated large particles, and the plated thickness (≈10.7 μm) was very close to the theoretical value (Fig. 3e). Such a dense Li deposition with a smaller surface/volume ratio effectively minimizes the parasitic reaction between metallic Li and electrolyte, and thus enables the high CE avg of Li|HGPE|Cu cells.
To analyze the microstructure of the SEI, 1 mAh cm -2 Li was repeatedly plated on and stripped off a Cu grid for 10 cycles at 0.2 mA cm -2 to obtain a vacant SEI shell for transmission electron microscopy (TEM) characterization. It is seen that a large amount of "dead Li" residues appeared on the Cu grid cycled in 1 M LiPF 6 -EC: EMC electrolyte, indicating an irreversibility of Li plating/striping (Fig. 4a, inset). The SEI was mainly composed of Li 2 O particles distributed in an amorphous matrix (Fig. 4a), and the composition was further identi ed as organic compounds (e. g. ROCO 2 Li, where "R" represents functional groups) originating from the decompositions of carbonate solvents 32 , and Li x PO y F z /Li 2 O as the decomposition products of LiPF 6 salt, respectively (see the in-depth X-ray photoelectron spectroscopy (XPS) results in Supplementary Fig. 15-18) 39 . The Young's modulus of this SEI layer was as low as 398 MPa (Fig. 4c). For comparison, the amount of residual inactive Li obviously decreased on the Cu substrates in uorinated electrolytes ( Supplementary Fig. 18, insets). Meanwhile, the proportion of LiF, mainly originating from the reduction of uorinated solvents, greatly increased in the SEI (Supplementary Fig. 15-18). It is well-known that LiF with high mechanical strength (i. e. a shear modulus of 55.1 GPa, almost 11 times higher than that of Li metal (4.9 GPa)) can signi cantly enhance the robustness and interfacial energy of SEI layers, thus blocking Li dendrite growth 24 Fig. 20-21). In the Cu grid retrieved from the cell employing the HGPE,a high Young's modulus up to 2768 MPa (Fig. 4d) has been achieved, owing to the co-existence of LiF and phosphorouscontaining compounds (i. e. P-O-C and P=O) derived from the residual DAP monomer in the SEI ( Fig. 4b and Supplementary Fig. 15-18). The robust SEI can e ciently suppresses the formation of Li dendrite and dead Li (Fig. 4b, inset) and leads to a smooth surface morphology of the cycled Cu grid (Fig. 4d, inset). The above results are well-consistent with the electrochemical behavior in traditional Li metal batteries.
The stabilization effect of HGPE on the Li|electrolyte interface can be elucidated as follows. It is known that the ion transfer kinetics and Li deposition behavior are mainly determined by the composition and morphology of the SEI 32 . As shown in Fig. 4e for dissociating the Li ions from the solvation sheath (E a2 ) is similar to that for the traditional liquid electrolyte, the E a1 is signi cantly reduced to facilitate Li ion diffusion through the SEI. This is because the addition of HTE as diluent leads to a formation of localized highly concentrated regions in the electrolyte, in which uorinated carbonates and Li + -anion ion pairs participate in the solvation shell 32 . This solvation shell structure and residual DAP monomer in the HGPE endow a formation of an inorganic component (e. g. LiF-rich) SEI on the Li metal surface, which is highly robust to suppress dendrite formation and maintains low resistance throughout cycling (Fig. 4f). Additionally, the crosslinked DAP-PETEA matrix not only generates a relatively homogeneous Li + ux, but also effectively eases the volume changes upon Li deposition, thus inhibiting any incipient dendrite growth 7 . Such a stable Li|HGPE interface with low Li + diffusion energy barrier contributes to the superior performance of HGPE in Li metal batteries.
Electrochemical performances evaluation. Fig. 5a Fig. 25). When the current density was switched back to 0.1 C, the capacity retention of the HGPE-based cell was 98.2 % of the initial value, demonstrating that this battery system is highly robust and stable. In contrast, the capacity of Li||KS6 graphite cell with 1 M LiPF 6 -EC: EMC rapidly decreases to ≈0 at a current density of 3 C, indicating a sluggish Li ions diffusion kinetics at the graphite|electrolyte interface. and the capacity suddenly droped to 21.7 mAh g -1 at the 169 th cycle. This is probably caused by serious structure exfoliation and destruction of graphite originating from the co-intercalation of solvent molecules into the graphite interlayers, as well as the thickening of the CEI induced by the severe electrolyte oxidation 42 . The lifespan and reversible capacity of DIBs signi cantly increased with the adoption of uorinated solvents (Supplementary Fig. 26). The Li||KS6 graphite cell using the HGPE demonstrateda high initial discharge capacity of 89.8 mAh g -1 with a capacity retention of ≈93 % after 1000 cycles, and the Coulombic e ciency was maintained at ≈98.9 % except for the activation process in the rst 10 cycles (Fig. 5c). The above results were further corroborated by the small interfacial resistances (R sei and R ct ) of the HGPE-based cells, and the interfacial resistance changes were much smaller than in the cells using other electrolytes during cycling (inset of Fig. 5c, Supplementary Fig. 27 and Supplementary Note 6). This superior cycling stability is mainly because the HGPE effectively suppresses solvent co-intercalation and protects the structural integrity of graphite, thus allowing a highly reversible and durable insertion/extraction of anions into/from the KS6 graphite.
SRLMBs have been further developed by applying KS6 graphite as conductive agent in the cathode of the LRO|HGPE|Li cells. As shown in Fig. 5d, during the charging of LRO|HGPE|Li and Li|HGPE|LRO/graphite cells, a sloping potential below 4.5 V corresponded to Li ion extraction from LRO cathode 44 . For the hybrid LRO/KS6 graphite cathode an extra plateau at 4.9 V appears, which is ascribed to a "relay" intercalation step of PF 6 into the graphite. Supplementary Fig. 28 further validated that KS6 contributed ≈ 6.2 % of the areal capacity and ≈ 7.1 % of the areal energy density (Supplementary Note 7). Such a "shuttle relay" process was highly reversible in the subsequent discharging. The SRLMB delivered a discharge capacity of 205.3 mAh g -1 , based on the total mass of the cathode active material and conductive agent, which was higher than that of the Li|HGPE|LRO cell with Super P as the cathode conductive agent (191.1 mAh g -1 , Fig.5e). The cell can maintain a capacity of 188.0 mAh g -1 after 100 cycles at 0.2 C (1 C=250 mAh g -1 based on the mass of LRO) with a high capacity retention of 91.6 %.
This indicates that the PF 6 intercalation/deintercalation after Li extraction/insertion can increase the battery capacity without sacri cing its cycling stability. In addition, the Li||LRO/graphite cell applying traditional 1 M LiPF 6 -EC: EMC electrolyte suffered from a quick capacity fading during cycling, demonstrating a poor electrode|electrolyte compatibility ( Supplementary Fig. 29).
Single-layer SRLMB pouch cells with 50 μm-thick Li foil as anodes were assembled to further evaluate the battery performance under abuse conditions ( Supplementary Fig. 30a). The Li|HGPE|LRO/graphite pouch cell not only showed excellent cycling performance ( Supplementary Fig. 31), but also exhibited superior exibility (i. e. consistently powering a red light-emitting diode (LED) under atted, bent, or even clustered states (Fig. 5f, lower panels and Supplementary Video 5). Whereas the cell using traditional liquid electrolyte losing power supply ability in bent or clustered states (Fig. 5f, upper panels and Supplementary Video 6). This veri es that the electrode|HGPE interfaces can maintain tight adhesion under signi cant shape deformations. Moreover, when aging the fully charged cells at 130 o C, an Li||LRO/graphite pouch cell with 1 M LiPF 6 -EC: EMC liquid electrolyte suffered from severe swelling and bulging due to the sever volatilization and thermal decomposition of the liquid electrolyte (Fig. 5g, inset), and the open circuit potential suddenly dropped to ≈0 V at 1964 s, illustrating a contact failure inside the cell (Fig. 5g). In sharp contrast, owing to the high thermal stability of uorinated solvents and leakagefree property of the gel, the shape and open circuit voltage of Li||LRO/graphite pouch cell with HGPE did undergo not change at the 130 o C test (Fig. 5g). Meanwhile, the temperature excursion of a fully charged Li|HGPE|LRO/graphite pouch cell was lower than that of a Li|1 M LiPF 6 -EC: EMC|LRO/graphite pouch cell during the nail penetration safety tests (Supplementary Fig. 30b). All these enable a highly safe operation of SRLMBs in practical applications.
Electrochemical mechanism of PF 6 intercalation/deintercalation in the HGPE. In-situ X-ray diffraction (XRD) tests were conducted to further investigate the operation mechanism of PF 6 intercalation/deintercalation in the presence of different electrolytes at 0.05 C. The in-situ XRD patterns and charge/discharge curves during the initial cycle are shown in Supplementary Fig. 32, and the corresponding intensity contour maps are presented in Fig. 6a and  where θ is the diffraction angle between the incident X-rays and the corresponding crystal plane, and λ is the X-ray wavelength (i.e., 0.15406 nm). The increase of graphite d (002) interplanar spacing from 0.335 nm at 3.0 V to 0.370 nm at 5.0 V is consistent with the intercalation of the PF 6 anions into the graphite interlayer ( Supplementary Fig. 33). In the subsequent discharge process, however, no distinct position change of the graphite (002) peak was observed, indicating a constant graphite d (002) spacing caused by the blocked PF 6 anion stripping from the graphite host (Fig. 6a). This has been further con rmed by the TEM image of the graphite cathode after cycling, in which the lattice spacing from XRD (0.370 nm) is well consistent with the calculated value from the TEM (Fig. 6c, inset). The thickness of the CEI derived from the oxidative decomposition of carbonate solvents is as high as 7.8 nm in 1 M LiPF 6 -EC: EMC liquid electrolyte, which strongly hinders the PF 6 stripping and causes the irreversibility during cycling (Fig. 6c).
In sharp contrast, in the Li|HGPE|graphite cell, the graphite (002) diffraction peak gradually shifted to 24.1°(i. e. interlayer spacing of 0.370 nm) during the charge process, and reversibly reverted to 26.50° (i. e. interlayer spacing of 0.336 nm) when discharged back to 3.0 V (Fig. 6b). The TEM image of the cycled graphite cathode exhibited lattice stripes with a spacing of 0.336 nm (Fig. 6d, inset), which is in accordance with the in-situ XRD results and almost the same as that of the pristine graphite powder (0.335 nm, Supplementary Fig. 34). This validates a highly reversible PF 6 intercalation into/deintercalation from the graphite without structural deterioration. Moreover, the thickness of the CEI formed in the HGPE is only 1.4 nm, indicating a suppressed electrolyte oxidation with reduced interfacial resistance.
In-depth XPS measurements were performed on graphite cathodes cycled in various electrolytes to further  Fig. 36). These results suggest that in 1 M LiPF 6 -EC: EMC liquid electrolyte, the CEI on the graphite cathode is mainly composed of abundant alkyl carbonate (e. g. ROLi) and polycarbonate as oxidation products of carbonate solvents (as further veri ed in the C 1s spectrum in Supplementary Fig. 37, and Li x PO y F z and LiF originated from the decomposition of LiPF 6 . More importantly, a large amount of intercalated PF 6 anions remained in the graphite interlayers, and the intensity of ROCO 2 -from carbonates abruptly increased at a depth of 20 nm (i. e. a sputtering time of 240 s). These demonstrate the inhibited stripping of PF 6 anions from the graphite host and the cointercalation of solvent molecules, which causes the irreversibility of batteries at 0.05 C.
In contrast, for the surface of the graphite cathode from the cycled HGPE-based cell, two new O 1s peaks at about 533 eV and 531 eV are assigned to P-O-C and O-P=O as oxidative decomposition products of DAP 45,46 , which is consistent with the P 2p spectra in Supplementary Fig. 36. Additionally, the peak intensities of PF 6 -, Li x PO y F z and LiF in F1s spectrum and the ROCO 2 -from the C 1s spectrum signi cantly decreased. The above results suggest that phosphorus-containing substances in the CEI (i.e., P-O-C and O-P=O) can suppress the decomposition of electrolytes and form a thin CEI to ensure a reversible PF 6 deintercalation ( Fig. 5d and 6d). Considering allyl groups can easily undergo polymerization 45 , such a CEI lm may originate from polyphosphoesters generated by the electropolymerization of residual DAP monomer on the carbon-oxygen rich graphite surface ( Supplementary Fig. 39). Moreover, in the XPS depth pro les of the Li|HGPE|graphite cell, no noticeable peaks were observed from the carbonate solvents or their decomposition products. This con rms that solvent molecule co-intercalation can be effectively inhibited by such protective CEI, which preserves the cathode against structure destruction and facilitates the superior electrochemical performance of the HGPE-based DIBs and SRLMBs.

Conclusion
In conclusion, for the rst time, we showcased a heteroatom-based gel polymer electrolyte facilitating highly reversible insertion/extraction of PF 6 anion into/from graphite interlayers. The HGPE prepared via a facile in-situ thermally initiated polymerization possesses high ionic conductivity (3.7 ×10 -3 S cm -1 ) and safety (i. e. non-ammable and free of liquid leakage). The synergistic effect of uorinated solvents, polymer matrix and the residual DAP monomer in the HGPE contributes to highly stable electrode|HGPE interfaces, thus endowing superior oxidative stability up to 5.5 V vs. Li/Li + , high Li deposition/stripping Coulombic e ciency of 99.7 % and excellent cycling stability of graphite cathodes with 93% capacity retention after 1000 cycles. Utilizing this HGPE, as a proof-of-concept, we developed a quasi-solid-state After designated cycling tests, the cells were dissembled in an Ar-lled glove box and repeatedly rinsed with DMC before post-mortem analysis. The air-sensitive electrode samples were rapidly transferred into the vacuum chambers for in-depth XPS and TEM before subsequent tests. dQ/dV curves were calculated from the discharge/charge pro les with a set voltage interval of 10 mV. CVs of the assembled DIBs were tested using the VMP3 electrochemical working station at a scanning rate of 0.5 mV s -1 , while EISs were examined in a frequency of 10 -2 to 10 5 Hz by applying a disturbance amplitude of 5 mV. For the in-situ XRD experiments, Li||graphite cells were assembled applying beryllium foils as both cathode current collector and X-ray window. The graphite cathodes herein were composed of 90 wt% KS6 graphite and 10 wt% PVDF to exclude the effect of conductive agent in XRD patterns. The in-situ XRD patterns were characterized on a Rigaku D max 2500 diffractometer with Cu Kα radiation (λ = 1.5418 Å).
Theoretical Calculations. All the spin-polarized calculations were performed using a Vienna ab initio simulation package (VASP), which was a plane-wave density functional code. The electron-electron exchange and correlation interactions were described by using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional form. The projector augmented-wave (PAW) method was employed to describe the interaction between the core and valence electrons. To better describe the interactions between molecules, van der Waals (vdw) interactions were included by the DFT-D3 method of Grimme.

Declarations
Data availability. The data that support the ndings of this study are available from the corresponding author upon reasonable request. Figure 1 a Schematic illustration of the mechanisms of a Li|1 M LiPF6-EC: EMC|LRO "rocking-chair" battery (upper panels) and a "shuttle-relay" battery with a hybrid LRO cathode using graphite as conductive agent, a Li metal anode, and a HGPE (lower panels). b The LUMO and HOMO energy values of the solvent molecules. The molecular structures and corresponding visual LUMO and HOMO geometry structures are shown as insets. Brown, white, red, purple, and blue balls represent carbon, hydrogen, oxygen, phosphorus, and uorine atoms, respectively. c Binding energies of FEC, FEMC, HTE and DAP for a Li+ cation and a PF6-anion.      Fig. 6c, d; 1 nm in the inset of Fig. 6c, d.