Low‐Enthalpy and High‐Entropy Polymer Electrolytes for Li‐Metal Battery

Ionic‐conductive solid‐state polymer electrolytes are promising for the development of advanced lithium batteries yet a deeper understanding of their underlying ion‐transfer mechanism is needed to improve performance. Here we demonstrate the low‐enthalpy and high‐entropy (LEHE) electrolytes can intrinsically generate remarkably free ions and high mobility, enabling them to efficiently drive lithium‐ion storage. The LEHE electrolytes are constructed on the basis of introducing CsPbI3 perovskite quantum dots (PQDs) to strengthen PEO@LiTFSI complexes. An extremely stable cycling >1000 h at 0.3 mA cm−2 can be delivered by LEHE electrolytes. Also, the as‐developed Li | LEHE | LiFePO4 cell retains 92.3% of the initial capacity (160.7 mAh g−1) after 200 cycles. This cycling stability is ascribed to the suppressed charge concentration gradient leading to free lithium dendrites. It is realized by a dramatic increment in lithium‐ion transference number (0.57 vs 0.19) and a significant decline in ion‐transfer activation energy (0.14 eV vs 0.22 eV) for LEHE electrolytes comparing with PEO@LiTFSI counterpart. The CsPbI3 PQDs promote highly structural disorder by inhibiting crystallization and hence endow polymer electrolytes with low melting enthalpy and high structural entropy, which in turn facilitate long‐term cycling stability and excellent rate‐capability of lithium‐metal batteries.


Introduction
[3] Efficiently designing one of the most important components-the electrolyte-into solid state is a promising strategy to improve the lifetime, energy density, and safety of Liion batteries. [4,5]Polymer solid-state electrolytes are considered to be the underlying strategy because of their higher safety, excellent flexibility, and higher energy density. [6,7]Despite many advances in polymer solid-state electrolytes, there are still remain some apparent drawbacks including poor ionic conductivity, low lithium-ion migration number, high internal concentration gradient, large interfacial impedance with electrodes, as well as the growth of lithium dendrites, which greatly restricts their practical application.To address the abovementioned issues, exploiting novel polymer electrolyte and disclosing fundamental ion-transfer kinetics hold the primary significance for advanced lithium-ion batteries.
It is generally believed that the ionic conduction mechanism is directly related to the segmental motion of polymers. [8]Therefore, the induced polymer chain scission is mainly controlled by increasing entropy-term-dependent ion-conduction. [9]In this case, increasing disorder degree of the polymer is significant because the chaotic complex can guarantee superior ionic conductivity through strong solvation of lithium cations and localization of anions. [10,11]Also, it is noteworthy that the degree of Li + ion transfer relies on the compromise between minimum steric hindrance and lattice size polarization. [12,13]Both of them are largely determined by the entropy and enthalpy-term-controlled ion dissociation, ion transfer, and the related charge accumulation effect.
[19] The first one is the suppression of crystallinity in polymer matrix that is conducive to ion transport; the second one is the formation of a fast ion transport path at the electrolyte/filler interface; and the last one is the generation of much more additional Li + ions through Lewis acid-base interaction between polymer chains and lithium salts. [20][23][24] These large-sized fillers make the transport path of Li + ions in polymer electrolytes too tortuous and destroy the structural integrity of the electrolyte membrane, and the ionic conductivity of decorated polymer electrolytes is also far behind the requirement of commercial criterion.

Results and Discussion
Herein we report on the design of a novel low-enthalpy and highentropy (LEHE) electrolyte that enables extremely long-term and high-rate cycling stability (Figure 1).In this study, CsPbI 3 perovskite quantum dots (PQDs) with optimal doping concentration largely break the polymer chain, suppress polymer crystallinity, tailor polymer molecular orientation, enhance polymer disorder degree, and hence endow PEO-LiTFSI-CsPbI 3 complexes low-enthalpy and high-entropy characteristics (Figure 1a).LEHE electrolytes intrinsically generate remarkably free ions and high mobility, enabling them to efficiently drive lithium-ion storage.Consequently, a high-rate cycling stability at 0.5 mA cm −2 with a low overpotential of about 95 mV and an extremely stable cycling >1000 h at 0.3 mA cm −2 can be delivered by LEHE electrolytes at 70 °C (Figure 1c).In contrast, as shown in Figure 1c, regular PEO-LiTFSI electrolyte generates a larger overpotential of about 185 mV and occurs the phenomenon of short circuit at 0.3 mA cm −2 , and worse still, it fails to cycle only 490 h at the same current density and presents the risk of short circuit at the very early stage (~20 h).
In addition, electrochemical stability window tests were performed to determine the suitability of LEHE electrolyte for high-voltage cathode materials.Figure S1, Supporting Information, shows the electrochemical stability window of LEHE and PEO-LiTFSI electrolytes determined by linear sweep voltammetry.The PEO-LiTFSI electrolyte has obvious polarization at only about 4.3 V while LEHE electrolytes show much higher stable potential windows.And the highest value reaches up to 5.21 V for LEHE-1.5 electrolyte, indicating its better adaptation to high-voltage cathode materials.

LEHE Effects Eliminating Charge Concentration Gradient
To demonstrate the low-enthalpy and high-entropy effect boosts electrochemical performance through eliminating charge concentration gradient, we assembled a full cell with LiFePO batteries, demonstrating the improved electrochemical stability. [25,26]ore evidence that LEHE electrolytes possess smooth charge distribution is implied by the coulombic efficiency (CE).High Li reversibility and CE of 99.99% are achieved for the LEHE-1.5 electrolyte and retain a high value of 99.85% after 200 cycling at 0.5 C. When using the regular PEO-LiTFSI as the electrolyte, the Li | LEHE-1.5 | LiFePO 4 full cell shows an evident fluctuation of CE and follows by a quick drop of CE (only 24.8% after 110 cycling at 0.5 C).Therefore, the low-enthalpy and high-entropy characteristics of polymer electrolytes demonstrate very good potential for long-term cycling.29] After cycling, the impedance of the batteries was evaluated.As shown in Figure 2b, the interfacial resistance of Li | LiPEO-LiTFSI | LiFePO 4 batteries increases by 218% (from 110 Ω to 350 Ω) after cycling, while the interfacial resistance of Li | LEHE-1.5 | LiFePO 4 batteries only increases by 40% (from 50 Ω to 70 Ω, Figure 2c).[32][33] This stable solid-electrolyte interface (SEI) layer ensures long-term efficiency and secure operation of the battery.
And the stable SEI layer would also guarantee smooth Li + ions plating and efficiently suppress the growth of lithium dendrites as shown in Figure 2e.To verify the improved lithium plating/stripping process of Li | LEHE-1.5 | LiFePO 4 batteries, the cycled batteries were disassembled and the lithium sheet was observed by field-emission scanning electron microscopy (FESEM).Figure 2f is the FESEM image of the lithium sheet Energy Environ.Mater.2024, 7, e12514 before cycling.Figure 2g shows the lithium sheet after cycling with PEO-LiTFSI electrolyte.Obviously, its surface is very uneven and a large number of lithium dendrites are deposited.On the contrary, the lithium sheets in Li | LEHE-1.5 | LiFePO 4 batteries after cycling are relatively more uniform and flatter (Figure 2h), indicating an efficient suppressed charge concentration gradient in the LEHE electrolytes. [34]Therefore, the growth of lithium dendrites is inhibited and the excellent cycle stability of lithium metal batteries is ensured by LEHE electrolytes.

The Mechanism of LEHE Effects on Impacting Electrochemical Properties
[37][38][39] The excellent cycling stability and the suppressed lithium dendrites drive us to disclose the intrinsic factors of LEHE electrolytes on impacting the electrochemical properties.

The Even More Free Li + Ions
Firstly, we proved that the number of free Li + ions is largely increased in LEHE electrolytes.As illustrated in Figure 3a, CsPbI 3 PQDs enter into the polymer chains, form the inorganic/organic compound center, break into polymer chain, and restrain polymer crystallinity.These effects act on enhancing the chaotic and amorphous states, correlating to low enthalpy and high entropy.DC polarization curves (Figure 3b,  c) display that the lithium-ion transference number of PEO-LiTFSI electrolyte is only 0.19, while that of LEHE-1.5 electrolyte is significantly increased to 0.57.Noted that a very long static time of 6000 s was set to fully eliminate nonequilibrium state effect. [40]The insets of Figure 3b,c show the EIS plots of PEO-LiTFSI and LEHE-1.5 electrolytes, respectively.Apparently, the contact resistance (R b ) and the diffusion resistance (R b + R i ) determined by the fit of equivalent circuit model (Figure S4, Supporting Information) were much lower for LEHE-1.5 than PEO-LiTFSI electrolytes.
Actually, ion transport in electrolytes is not only related to the mobility of polymer chains, but also to the concentration and migration paths of lithium ions.To gain insight into the mechanism of this enhanced ion transport performance, we performed FT-IR, Raman, and solid-state NMR spectroscopy characterizations to systematically investigate the coordination effect occurring within the LEHE electrolytes and their dissociation behavior of lithium salts.FTIR curves exhibit an asymmetric peak in the range of 755-725 cm −1 .By subpeak fitting, we can obtain one peak at ~745 cm −1 corresponding to ion pair for Li + and TFSI − and another at ~738 cm −1 corresponding to free Li + .And the relative ratio of these two peaks determines the degree of dissociation of LiTFSI salt.The relative free ion concentration (88.8%) in the LEHE-1.5 electrolyte is higher than PEO-LiTFSI electrolyte (79.6%), indicating that CsPbI 3 quantum strengthened effect promotes the dissociation of Li + cations while delocalizes TFSI − anions that will bring about the enhanced ionic conductivity.In addition, there is no signal of cyano group (-CN) in FTIR spectra, indicating that the solvent solution of acetonitrile molecules is completely removed or their residue is very few.
Figure S5, Supporting Information, is full Raman spectra (150-1350 cm −1 ) of the PEO, PEO-LiTFSI, and LEHE electrolytes and Table S2, Supporting Information, summarizes the characteristic peaks and the corresponding molecular vibration modes.The -SO 2stretching vibration s(SO 2 ) located at 1136 cm −1 and the -CF 3 stretching vibration s(CF 3 ) located at 1242 cm −1 were shifted to 1140 cm −1 and 1240 cm −1 , respectively.There is a strong peak band at 740 cm −1 , which is usually caused by the mixing vibration of the -SNS-symmetric stretching in the TFSI − anions.In addition, the -CF 3 rocking vibrations r(CF 3 ) located at 275 cm −1 and the -SO 2rocking vibrations r(SO 2 ) located at 311, 326, 342 cm −1 ; they shifted to 277 cm −1 and 309, 325, 338 cm −1 , respectively, with addition of CsPbI 3 PQDs.Clearly, the significant shifts of -CF 3 and -SO 2vibration modes with the addition of CsPbI 3 PQDs prove that there is a strong interaction between CsPbI 3 PQDs and LiTFSI, which enhances the Li + ion dissociation and provides a special channel for the rapid transport of lithium ions.Further, by subpeak fitting the characteristic peaks located at 720-760 cm −1 in the Raman spectrum, two peaks can be obtained (Figure 3f), respectively, at ~740 cm −1 (corresponding to free Li + ) and ~745 cm −1 (corresponding to the ion pair of Li + and TFSI − ).It can be confirmed by the area under the fitted peak in the spectrum that the free ion concentration (94.5%) in the LEHE-1.5 electrolyte is also significantly higher than that in the PEO-LiTFSI electrolyte (78.4%), which is consistent with the conclusion of the FT-IR spectrum.
Although the mobility of Li + ions is driven by the coordination interaction of oxygen atoms in PEO chains, too strong interactions are not always conducive to the migration of Li ions.We speculate that the introduction of CsPbI 3 PQDs weakens the coordination interaction and promotes the movement of lithium ions.For testing this hypothesis, we used solid-state NMR spectroscopy to further investigate the Energy Environ.Mater.2024, 7, e12514 chemical environment of Li + ions.As shown in Figure 3f and Figure S6, Supporting Information, the interaction between CsPbI 3 PQDs and Li + ions can be reflected by the change of 7 Li chemical shift.Compared with PEO-LiTFSI electrolyte, the 7 Li NMR resonance signal of LEHE-1.5 electrolyte is shifted to the lower field, indicating that the interaction between PEO and Li ions is weakened, reducing the confinement of Li + ion motion.By further fitting the 7 Li NMR spectral peaks, we obtained two different local Li + ion environments as marked by A1 and A2.The A2 part represents the more mobile Li + ions, while the A1 part represents Li + ions bound by the coordination of oxygen atoms in PEO.The introduction of CsPbI 3 PQDs increases the concentration of mobile Li + ions (part A2) by a ratio of 18.5%, indicating that the mobility of Li + ions in the LEHE-1.5 electrolyte is stronger and the ionic conductivity would be improved accordingly.
In short, LEHE electrolytes release even more free Li + ions because of the stronger interaction between PEO segments and Li + ions that is promoted by CsPbI 3 quantum strengthened effect.

The Enhanced Transfer Ability of Free Li + Ions
Next, we will discuss that these free Li + ions also possess enhanced transfer ability.The ion-transfer performance of Li + ions inside the polymer solid electrolyte is closely related to the crystallinity of the latter.As schematically illustrated in Figure 4a, the crystallinity of PEO can be greatly inhibited in LEHE electrolytes.Since the CsPbI 3 PQDs have very smaller particle size (only 7.0 nm, Figure S7, Supporting Information), they have the tendency to enter into the polymer chain and hence break the semi-crystallized behavior of PEO.Moreover, the relatively strong electronegativity of iodine atoms will interact with Li + ions and oxygen atoms in PEO, which form ternary complexes.This action causes the strong entanglement of amorphous PEO and hence significantly increases its structural disorder.As shown in XRD patterns (Figure 4b), pure PEO electrolyte has two distinct strong diffraction peaks at 19.0°and 23.2°, proving the intrinsic semi-crystalline properties of PEO polymers.Although the peak positions remain almost unchanged after the addition of LiTFSI salt and CsPbI 3 PQDs, the intensity of the peaks was significantly weakened, indicating that the crystallinity of PEO is greatly decreased, which was due to the polar oxygen in the PEO chain caused by the coordination between atoms and lithium ions.Obviously, the crystallinity of the polymer solid electrolyte is minimized when the addition amount of CsPbI 3 PQDs is 1.5 wt%, and the decrease of the crystallinity of the PEO polymer is beneficial to ion migration, because the amorphous region of the PEO electrolyte can effectively improve ionic conductivity.
Further, the quantitative analysis of the inhibition of PEO segment crystallization by CsPbI 3 perovskite quantum dot fillers by differential scanning calorimetry (DSC), the thermodynamic data calculated according to the curve in Figure 4b, and the calculated data are summarized in Table S3, Supporting Information.It is evident that pure PEO electrolyte has a melting temperature (T m ) of 62.62 °C, a melting enthalpy (ΔH m ) of 156.69 J g −1 , and a crystallinity (χ c ) of 77.19%, which are all maximum values.However, after the addition of LiTFSI salt and CsPbI 3 PQDs, the thermodynamic data decreased to varying degrees.In particular, when the addition amount of CsPbI 3 PQDs was 1.5 wt%, the melting temperature was 45.04 °C, and the melting enthalpy of 34.07 J g −1 and the crystallinity of 16.79% are the lowest relative to pure PEO electrolytes, which proves that CsPbI 3 PQDs are beneficial to destroy the regularity of PEO polymer chain arrangement, thereby improving the motion of chain segment's ability.In addition, the thermal decomposition temperature of pure PEO electrolyte is about 360 °C and increases to a certain extent in LEHE electrolytes (Figure S8, Supporting Information).For LEHE-1.5, the thermal decomposition temperature reaches a maximum of about 400 °C.And the decomposition curves of LEHE electrolytes show an additional slope in the range of 440 °C-480 °C, which is originated from the quantum strengthened effect of CsPbI 3 perovskite QDs.The excellent thermal stability proves that LEHE solid-state electrolytes can guarantee high-safety lithium metal batteries.
Electrochemical technique including cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT) is direct and nondestructive method to quantify the ion-transfer capability.We firstly analyzed the lithium-ion diffusion coefficient by CV measurement.| LiFePO 4 cells at different scan rates, respectively.Through plotting the peak current at different scan rates against the square root of the scan rate, the slope of the anodic peaks can qualitatively compare the relative magnitude of the Li + ion diffusion coefficient (Equation 1). [41] Among them, I p is the peak current of the CV curve, n is the electron transfer number during the redox process, A is the area of the electrode, C 0 is the initial Li + ion concentration, D is the ion diffusion coefficient, and v is the CV scan rate.The calculated results are shown in Figure 4f.The good linear correlation in both LiTFSI and LEHE electrolytes indicates their typical diffusion-controlled behavior. [42,43]It can be seen that the slope of the full cell assembled with LEHE-1.5 electrolyte (4.11) is significantly larger than that of the full cell assembled with PEO-LiTFSI electrolyte (1.65), which proves that Li + ions are easily diffused in LEHE-1.5. [44]Apparently, LEHE electrolytes possess higher Li + ion diffusion parameter than PEO-LiTFSI electrolyte because of low-enthalpy and high-entropy facilitated segmental motion of PEO chains.The diffusion in the battery is faster, the charge-discharge capacity of the battery is better, and the high-rate cycling performance is also better.
Then, to gain insights into the LEHE effects on Li + ion-transfer dynamic behavior in LEHE electrolyte, we carried out the GITT measurement for the cells.Figure 4g,h  Since the diffusion overpotential and diffusion coefficient are closely related to the transport dynamics of Li + ions through electrodes, we can anticipate that LEHE electrolytebased batteries possess excellent ion conductivity and high-rate longcycling stability as discussed above.On the other hand, since the TFSI − anions (~0.325 nm) have much higher ion size in diameter than Li + ion (~0.152 nm), it is difficult to accelerate the motion of TFSI − in solid-state electrolytes.

LEHE Effects Enabling Remarkable Ionic Conductivity
Finally, we demonstrate high-entropy effect in LEHE electrolytes and show how it may be rationalized to increase ionic conductivity by consideration of entropy-term determined activation.Figure 5a shows the ionic conductivities of LEHE electrolytes with different CsPbI 3 PQDs contents and also PEO-LiTFSI for comparing.The ionic conductivity of PEO-LiTFSI electrolyte is only 2 × 10 −5 S cm −1 at room temperature, while the ionic conductivity of all the electrolytes added with CsPbI 3 PQDs is significantly improved, among which as the addition amount of CsPbI 3 PQDs is 1.5 wt%, LEHE-1.5 possesses the highest room temperature ionic conductivity of 1.4 × 10 −4 S cm −1 .The reason probably attributes that there is an intermediate concentration to maximize the promoted effect.Typically, as the concentration is low, the electrochemical performance increases with an increasing concentration because of the well-dispersed CsPbI 3 PQDs in the SSEs.As the concentration is relatively high, the blocking effect caused by the agglomeration of the nanofillers in the SSE could cause a decrease in ion migration.
More direct evidence for high-entropy effect was obtained from calculation of the activation enthalpy, ionic conductivity, and the migration entropy.The activation enthalpy is calculated by combining the ionic conductivity at different temperatures with the Arrhenius equation (Equation 3).As shown in Figure 5b, LEHE-1.5 with the highest ionic conductivity also possesses the lowest activation enthalpy, indicating its fast ion transport.The observed curvature in Arrhenius plots of ionic conductivity can be attributed to the occurrence of a disordering transition in the mobile ion sublattice which in turn is a direct Energy Environ.Mater.2024, 7, e12514 consequence of the entropy of activation. [45]Because the addition amount of CsPbI 3 PQDs has a very important relationship with the ion transport of the LEHE electrolyte, as shown in Figure 5c, the ionic conductivity of the electrolyte does not further increase with increasing the addition amount of CsPbI 3 PQDs.As the addition amount of CsPbI 3 PQDs is 1.5 wt%, the effect is the optimum; as the addition amount is greater than this value, the effect becomes inferior probably due to the aggregation of CsPbI 3 PQDs.Highest room-temperature ionic conductivity is characterized by the lowest values of migration entropy. [46]he values of the migration entropy found for PEO-LITFSI and LEHE-1.5 electrolytes are about 7.7 k and 5.2 k (k is Boltzmann constant, 10.62 × 10 −23 and 7.18 × 10 −23 J K −1 ), respectively.This fact of smaller migration of entropy can be connected with the values of the room-temperature conductivity, which for LEHE-1.5 electrolyte was about 4.4 times higher.
Assuming the usual Arrhenius expression shown in Equation 3 for the temperature dependence of the ionic conductivity, the relationship of pre-exponential factor lgσ 0 with activation energy can be determined.As displayed in Figure 5d, the activation and the lgσ 0 of LEHE electrolytes are much lower than that of PEO-LiTFSI electrolyte.This dual-decline phenomenon implied that it became easier to an orderdisorder transition in polymer-chain segmental motion.Since the melting enthalpy is decreased by 42.5% in LEHE-1.5 electrolyte than PEO-LiTFSI as discussed in DSC results and the effective disordering temperature of the Li + ion mobility becomes lower as illustrated in Arrhenius plots, the disordered Li + ion localized PEO chains will accelerate their motion, which links up to form the conduction pathways.Because of this process is thermally activated, much lower activation energy and lgσ 0 will lead to much easier order-disorder transition and much higher Li + ion transfer capability, and hence thermodynamic-driven accelerated dynamics.
Low-enthalpy and high-entropy effect was also verified by AC conductivity (Figure 5e) that can separate the contributions made by the mobility and concentration of ions to DC conductivity. [47]Apparently, the ionic conductivity of LEHE-1.5 is much higher than that of PEO-LiTFSI at the whole measured frequency range, indicating the enhanced ion hopping rates of the LEHE-1.5 electrolyte.Therefore, the highentropy effect becomes evident and the contribution of migration terms to the activation energy conductivity increases accordingly.However, the concrete ion-hopping rate is difficult to determine because the ionic conductivity mechanism is still unclear in polymer-based electrolytes.Noted that a conductivity dispersion occurs in PEO-LiTFSI at high frequency, which is associated with electrode polarization effects, again indicating the rapid ionic response of the LEHE electrolytes.
Additional proof that the charge-transfer kinetics are much faster of Li + ion transfer in LEHE electrolytes can be seen from the Tafel plots.As shown in Figure 5f, the exchange current density (I 0 ) of PEO-LiTFSI is 0.13 mA cm −2 and tremendously increases to 0.59 mA cm −2 in LEHE-1.5 electrolyte.The larger I 0 means rapid ion-transfer dynamics during the deposition process of Li + ions, [48,49]

Conclusion
In conclusion, the CsPbI 3 PQDs quantum strengthened effect endows the PEO polymer electrolytes low-enthalpy and high-entropy (LEHE) Energy Environ.Mater.2024, 7, e12514 characteristics.LEHE effect promotes structural disorder degree of PEO, dissociation of Li salts, and generation of even more free Li + ions in thermodynamic side.And in dynamic side, it facilitates rapid ion transfer, smooth charge distribution, and free lithium dendrites.Consequently, LEHE-1.5 electrolyte has the highest room temperature ionic conductivity of 1.4 × 10 −4 S cm −1 , the largest Li + ion transference number of 0.57, and the widest electrochemical potential window of 5.2 V. Accordingly, Li | LEHE | Li symmetric cell can be cycled stably for over 1000 h at 0.3 mA cm −2 , and Li | LEHE | LiFePO 4 full cell retains a capacity retention rate of 92.4% after cycling stably for 200 cycles at a rate of 0.5 C with a Coulombic efficiency of >99.9%.
Preparation of perovskite quantum dots: CsPbI 3 PQDs are prepared by a hot injection method as reported elsewhere. [20]Firstly, the cesium oleate precursor solution was prepared by dissolving 0.271 g cesium carbonate (Cs 2 CO 3 , 0.8 mmol), 0.83 mL oleic acid (OA, 2.18 mmol), and 10 mL 1-octadecene (ODE) in a 50-mL three-necked flask at 120 °C for 1 h under vacuum atmosphere, followed by introduction of high-purity Ar gas, and further heated to 160 °C to obtain a transparent and clear precursor solution.Similarly, the PbI 2 precursor solution was prepared by similarly dissolving 0.4 mmol (i.e., 0.1844 g) PbI 2 and 24 mL ODE into a 50-mL three-neck flask at 120 °C under vacuum for 1 h under vacuum atmosphere followed by the addition of 3 mL of oleylamine (OAm) with the protection of Ar gas.Then, the temperature was raised to 180 °C prior to obtaining a clear solution.At 180 °C, extract 2 mL of the cesium oleate precursor solution was quickly injected into the PbI 2 solution to synthesize CsPbI 3 PQDs.After a very short reaction time (~5 s), the chemical reaction was terminated by an ice bath treatment.The aboveobtained solution was first centrifuged at 11 044 × g for 5 min and the precipitates dispersed in n-hexane again with an appropriate amount of ethyl acetate for centrifugation for three times.Finally, the CsPbI 3 PQDs was dispersed in n-hexane for further use.As CsPbI 3 PQDs was used to decorate PEO-LiTFSI electrolytes, they are dried in a vacuum oven at 80 °C for 24 h to remove organic solvent.
Preparation of LEHE electrolytes: Low-enthalpy and high-entropy electrolytes are composed by CsPbI 3 PQDs, PEO, and LiTFSI.Firstly, CsPbI 3 PQDs with mass ratio ranging from 0.7 w.t. to 5.0 w.t.%, EO with a ratio of EO: Li = 16: 1 and LiTFSI were dissolved in anhydrous acetonitrile at 60 °C for stirring 24 h to obtain a uniform slurry.Then, the mixed solution was poured slowly and uniformly on polytetrafluoroethylene (PTFE) panels using the solution casting method.In order to obtain the composite polymer electrolyte membrane, it was quickly transferred to a vacuum oven at 60 °C for drying for 24 h after standing and volatilizing in a glove box for 12 h.Then, the composite polymer electrolyte membranes were carefully peeled off from the PTFE mold.And a circular electrolyte membrane with a diameter of 16 mm was formed using a manual punching machine.The final polymer electrolytes are denoted as PEO-LiTFSI (without CsPbI 3 PQDs) and LEHE (with CsPbI 3 PQDs), respectively.
Preparation of LMB cathode: The dried lithium iron phosphate (LiFePO 4 ) and conductive carbon black (Super P) powder and polyvinylidene fluoride binder solution (PVDF, the solution concentration is 4 wt%) were weighed according to the mass ratio of 8.5:1.0:0.5 and added to the ball milling tank, and then an appropriate amount of N-methylpyrrolidone solution was added as a solvent for ball milling at a speed of 400 rpm for 8 h.Then, the uniformly mixed slurry was coated on the aluminum current collector foil with an automatic film coater.The coated films were transferred to a vacuum oven for drying at 80 °C for 12 h to initially remove most of the solvent and then dried at 120 °C for another 12 h to completely remove the residual solvent.The aluminum foil was then cut into desired electrode platelets with a diameter of 12 mm using an ultraviolent cold laser processing.The loading mass of the active material is about 2.65 mg cm −2 .
Characterizations: XRD spectra with the range of 5-60°were recorded on a PANalytical instrument with Cu K α radiation (PANalytical Corporation, Dutch).Field-emission SEM imaging was performed on a JEOL JSM-7800F instrument.FT-IR spectra were probed with a Nicolet iS10 infrared instrument, and Raman spectra were recorded using a HORIBA Jobin-Yvon XploRA PLUS confocal Raman microscope with 532 nm laser.DSC curves ranging from −60 to 100 °C were measured on a Waters 2500 (USA) instrument, and the heating rate was set to be 10 °C min −1 .NMR characterization was carried out on an AVANCE III HD instrument (Bruker, Germany).
AC impedance test (EIS): Ionic conductivity is used to evaluate the ionconducting ability of electrolytes and is the primary factor for evaluating electrolyte performance.Generally, the impedance value R is obtained by testing and fitting through alternating current impedance spectroscopy (EIS), and the ionic conductivity σ of the electrolyte under certain conditions is obtained by Equation 2.
where σ is the ionic conductivity (S cm −1 ), L is the thickness of the electrolyte membrane (cm), R is the resistivity (Ω), and S is the area of the electrolyte membrane (cm 2 ).The test cell was assembled by sandwiching the electrolyte membrane between two stainless steel gaskets and encapsulating it in a coin-typed cell.The test frequency ranges from 10 6 Hz to 10 −2 Hz and test temperature from 25 °C to 100 °C with an interval of 10 °C.The relationship between ionic conductivity and activation energy is calculated according to the Arrhenius equation as following: where σ(T) is the ionic conductivity, A is the pre-exponential factor, E a is the activation energy of ion conduction, and R is the ideal gas constant.

Linear sweep voltammetry (LSV):
The electrochemical stability window determines the stable working voltage range of the electrolyte material.LSV was used to determine the electrochemical potential window.The test cell was assembled by sandwiching the electrolyte membrane between a stainless-steel gasket and a lithium sheet and encapsulating it in a coin cell.The sweep voltage ranges from 0 to 6 V with sweep rate of 1 mV s −1 .
DC polarization test: DC polarization test combined with AC impedance test was used to determine the lithium-ion transference number and calculated through Equation 4. [50] τ þ Li ¼ where τ þ Li is the lithium-ion transference number, ΔV is the polarization voltage (10 mV), and the initial current (I 0 ) and steady state current (I ss ) are determined by DC polarization, and bulk resistance (R 0 b and R ss b ) and interface resistance (R 0 i and R ss i ) are tested by AC impedance.The test battery was assembled by sandwiching the electrolyte membrane between two lithium sheets and encapsulating it in a coin-typed cell.
Galvanostatic current charge and discharge test: The galvanostatic chargedischarge test is mainly to evaluate the dynamic stability of the electrolyte to lithium under the condition of continuous lithium plating/stripping.The test battery was assembled by sandwiching the electrolyte membrane between two lithium sheets and encapsulating it in a button cell.The current densities are ranging from 0.1 to 0.5 mA cm −2 .
GITT test: In GITT test, the cells were first charged or discharged at a constant current pulse of 0.1 C for 30 min, followed by an equal duration relaxation of 10 h, allowing the equilibrium potential of Li + ion storage at different points to be probed in the whole voltage window.
Cycling performance of all-solid-state LMBs: The electrolyte membrane is sandwiched between the lithium sheet and the LiFePO 4 electrode sheet and is packaged in a coin-typed cell.Different current rates from 0.1 to 5 C were set to evaluate the cycling lifetime.
AC conductivity and Tafel plot tests: AC conductivity was measured on electrichemical workstation (CHI660E, Chenghua, Shanghai) at a constant Energy Environ.Mater.2024, 7, e12514 temperature of 303 K.And the frequency was set to ~100 to 10 6 Hz.Tafel plots were measured with the voltage changing from −0.1 to 0.1 V.
4 as the positive electrode and Li metal as the negative electrode.Comparing with Li | PEO-LiTFSI | LiFePO 4 batteries, Li | LEHE | LiFePO 4 batteries possess much better ratecapability at all rates (Figure S2, Supporting Information).Specifically, Li | LEHE-1.5 | LiFePO 4 batteries has the best specific capacity of 159 mAh g −1 at 0.1 C (1 C = 170 mAh g −1 ) and maintain a high ratio of 64.2% (102 mAh g −1 ) at 5 C. As returned to 0.1 C, it still recovers to a high level of 97.5% (155 mAh g −1 ) of the initial value.Figure S3, Supporting Information, shows the typical charge-discharge curves of Li | LEHE-1.5 | LiFePO 4 batteries at different rates.All the curves exhibit a flat discharge plateau and high reversibility indicating its low polarization characteristics.To further verify the cycling stability of the batteries, we carried out a long-term cycling test at 0.5 C. As shown in Figure 2a, the initial specific capacity of Li | LEHE-1.5 | LiFePO 4 is 160.7 mAh g −1 .After 200 cycles, the Li | LEHE-1.5 | LiFePO 4 batteries still deliver a specific capacity of 148.4 mAh g −1 , a 92.4% retention of the initial specific capacity.In contrast, the specific capacity of Li | Li-TFSI | LiFePO 4 batteries continuously declines with prolonging cycles and basically fails after only about 100 cycles.In addition, the charge-discharge curves under different cycles (Figure S3, Supporting Information) of Li | LEHE-1.5 | LiFePO 4 batteries are more stable and smoother than that of Li | Li-TFSI | LiFePO 4

Figure 1 .
Figure 1.Low-enthalpy and high-entropy (LEHE) polymer electrolyte enables lithium-metal batteries extremely long-term stability.a) Schematic illustration of LEHE electrolyte with disorder PEO chain and even more free ions through CsPbI 3 quantum strengthening PEO-LITFSI complexes.b) Polarized voltage versus measured time of Li | LEHE | Li and Li | PEO-LiTFSI | Li cells at different current densities.c) Comparison of long-term cycling performance of Li | LEHE | Li and Li | PEO-LiTFSI | Li cells at a current density of 0.3 mA cm −2 .

Figure 3 .
Figure 3. LEHE polymer electrolyte delivering even more free lithium ions.a) Schematically illustrating LEHE electrolyte guarantees much more free Li + ions through promoted dissociation of LiTFSI salts.b) The Li + ion transference number (τ þ Li ) of LEHE-1.5 electrolyte determined by DC polarization test.c) τ þ Li of PEO-LiTFSI determined by DC polarization test.d) FT-IR spectra of pure PEO, PEO-LiTFSI, and LEHE-1.5 electrolytes.The generated even more free ions of LEHE-1.5 electrolyte proved by e) FT-IR characterization; f) Raman characterization; and g) NMR characterization.

Figure 4 .
Figure 4.The enhanced Li + ion transfer capability of LEHE electrolytes.a) Schematic illustration of CsPbI 3 PQDs promotes the disorder degree of PEO polymer.b) XRD patterns of LEHE electrolytes with different contents of CsPbI 3 PQDs and PEO-LiTFSI.c) DSC curves of PEO, PEO-LiTFSI, and LEHE electrolytes.d) CV curves of PEO-LiTFSI electrolyte at different scan rates.e) CV curves of LEHE-1.5 electrolyte at different scan rates.f) The slope curves of anodic peak currents depend on the square root of scan rates for LEHE and PEO-LiTFSI electrolytes.GITT and Li + ion diffusion parameter of g) PEO-LiTFSI electrolyte, and h) LEHE-1.5 electrolyte.
which promotes longterm cycling stability of Li | LEHE | Li symmetric cells and Li | LEHE | LiFePO 4 full cells as we have discussed in Figures 1c and 2a,b.

Figure 5 .
Figure 5. High-entropy effect endows excellent ionic conductivity of LEHE electrolytes.a) Temperature dependence ionic conductivity of LEHE and PEO-LiTFSI electrolytes.b) The fitted ionic conductivity of LEHE-1.5 and PEO-LiTFSI electrolytes.c) The percolation curve of ionic conductivity correlated with the content of CsPbI 3 PQDs.d) Pre-exponential factor of ionic conductivity changes with the activation energy.e) Frequency-dependent ionic conductivity of LEHE-1.5 and PEO-LiTFSI electrolytes, the solid lines are fitted by nonlinear method.f) Tafel plots of LEHE-1.5 and PEO-LiTFSI electrolytes.