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Elastomeric electrolytes for high-energy solid-state lithium batteries

Abstract

The use of lithium metal anodes in solid-state batteries has emerged as one of the most promising technologies for replacing conventional lithium-ion batteries1,2. Solid-state electrolytes are a key enabling technology for the safe operation of lithium metal batteries as they suppress the uncontrolled growth of lithium dendrites. However, the mechanical properties and electrochemical performance of current solid-state electrolytes do not meet the requirements for practical applications of lithium metal batteries. Here we report a class of elastomeric solid-state electrolytes with a three-dimensional interconnected plastic crystal phase. The elastomeric electrolytes show a combination of mechanical robustness, high ionic conductivity, low interfacial resistance and high lithium-ion transference number. The in situ-formed elastomer electrolyte on copper foils accommodates volume changes for prolonged lithium plating and stripping processes with a Coulombic efficiency of 100.0 per cent. Moreover, the elastomer electrolytes enable stable operation of the full cells under constrained conditions of a limited lithium source, a thin electrolyte and a high-loading LiNi0.83Mn0.06Co0.11O2 cathode at a high voltage of 4.5 volts at ambient temperature, delivering a high specific energy exceeding 410 watt-hours per kilogram of electrode plus electrolyte. The elastomeric electrolyte system presents a powerful strategy for enabling stable operation of high-energy, solid-state lithium batteries.

Main

Commercial applications of lithium (Li) metal batteries (LMBs) based on organic electrolyte systems have been hindered by safety concerns and the well documented challenges of Li metal anodes: uncontrolled Li dendrite growth, unstable thickening of the solid–electrolyte interphase (SEI) layer, ‘dead’ Li formation and substantial volume change of Li metal during cycling, all of which accelerate cell fading3,4. Thus, many research efforts have been devoted to resolving these issues using porous scaffolds, artificial SEI layers and solid-state electrolytes (SSEs)5,6,7,8,9,10,11. In particular, solid-state LMBs based on inorganic or organic SSEs have emerged as a promising candidate as they offer a substantial improvement in safety by eliminating the flammable organic solvents. Considering the compatibility with the current roll-to-roll-based manufacturing process of Li-ion batteries, solid polymer electrolytes (SPEs) have attracted great interest because of their low manufacturing cost, non-toxicity and relatively soft nature that enables the formation of a smooth interface with the electrodes5,8,12. Among the various polymers, poly(ethylene oxide) (PEO)-based SPEs have been the subject of intensive research; however, these polymers do not exhibit sufficient ionic conductivity and stability for the stable operation of LMBs5,13,14. A common approach to improve the ionic conductivity is to incorporate additives, such as organic and inorganic fillers, into the polymer matrix to form gel or hybrid SPEs10,15,16. However, the ionic conductivity and/or mechanical properties of these gel and hybrid SPEs should be further enhanced for viability in high-energy LMBs.

Elastomers, which are synthetic rubbers, are widely used in consumer products and advanced technologies (wearable electronics and soft robotics) owing to their superior mechanical properties17,18,19. Elastomers can provide an excellent matrix to disperse functional components while maintaining both mechanical elasticity and functionality. For example, the important functionalities of the blends, such as electrical and ionic conductivities, can be well maintained when dispersed components are three-dimensionally connected within an elastomer matrix20,21. Polymerization-induced phase separation (PIPS) is a process that controls the domain size and connectivity of phase-separated structures, allowing the formation of co-continuous nanostructures22,23,24. However, no attempt has been made to develop an ion-conducting phase within an elastomeric system using PIPS. Thus, we envision that excellent ionic and mechanical properties could be realized if ionic conducting materials can form a three-dimensional (3D) interconnected phase within a mechanically robust elastomer matrix through PIPS.

Here we report a different class of SPEs for high-energy LMBs, which is based on an in situ-formed elastomer with a 3D interconnected phase of ion-conductive plastic crystals. Co-continuous structures of plastic-crystal-embedded elastomer electrolyte (PCEE) are developed by PIPS between polymers and plastic crystals within the cell. The PCEE exhibits both superior mechanical properties and high ionic conductivity (1.1 mS cm−1 at 20 °C) with a high Li-ion transference number (t+) of 0.75. In addition, owing to its mechanical elasticity, the in situ-formed PCEE within the cells (hereinafter, ‘built-in PCEE’) effectively accommodates the substantial volume changes of Li during the fast charge–discharge cycling. Using built-in PCEE, we demonstrate the stable operation of an SPE-based solid-state LMB with a LiNi0.83Mn0.06Co0.11O2 (NMC-83) cathode at a high voltage of 4.5 V. This elastomeric electrolyte system presents a promising strategy for achieving high-performance and stable solid-state LMBs.

Designing elastomeric electrolytes

Built-in PCEE was synthesized by polymerizing a homogeneous solution consisting primarily of butyl acrylate (BA), succinonitrile (SN) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at 70 °C in an assembled electrochemical cell (Fig. 1a, Extended Data Fig. 1a, c). SN, a representative plastic crystal, was selected as an ionic conductive material owing to its high ionic conductivity when complexed with Li salts25,26. For polymerization, azobisisobutyronitrile (AIBN; 0.5 mol%) and poly(ethylene glycol) diacrylate (PEGDA; 1 mol%) were used as the thermal initiator and cross-linking agent, respectively. In this polymerization process, BA/PEGDA produces in situ-formed polymers chemically cross-linked by PEGDA, eventually resulting in elastomer networks, whereas the SN–LiTFSI phase is partitioned into nanoscale domains. The synthesized PCEE showed a high mechanical elasticity owing to the elastomer matrix (Extended Data Fig. 1b). Three-dimensional tomography and scanning electron microscopy (SEM) images showed that PCEE had an interconnected network structure in which worm-shaped polymer ducts were three-dimensionally connected (Fig. 1b, c, Supplementary Fig. 2). In detail, the SN phase was well enclosed by the cross-linked polymer phase, and the interface between the SN phase and the elastomer matrix was smoothly connected, as revealed by transmission electron microscopy (TEM) (Fig. 1d). The electron energy-loss spectroscopy elemental mapping image confirmed that nitrogen (N) of the SN phase was embedded within oxygen (O) of the BA-based elastomer matrix (Fig. 1e). We attribute this unique structure to nanoscale phase separation during PIPS, which was driven by the increase in the molecular weight of polymerized BA/PEGDA and the enthalpic interactions between the SN and the emerging polymer23,27,28. Importantly, the coarsening of the resulting SN phase was effectively suppressed by the elastomer matrix, allowing the formation of the 3D interconnected SN phase as an effective ion-conducting pathway within the BA-based elastomer. In addition, differential scanning calorimetry, thermogravimetric analysis and Fourier transform infrared spectroscopy revealed that PCEE showed the characteristics of both the plastic crystal and the rubbery polymer phases (Supplementary Figs. 3–5). It should be noted that the unique structure of PCEE is completely different from that of a conventional blend (Extended Data Fig. 2). The simple blend of BA-based polymer (BA100) and SN without involving PIPS showed macroscopically separated morphologies. The ionic conductivity and tensile toughness of the blend are substantially inferior by factors of 32 and 47, respectively, compared with those of PCEE (Extended Data Fig. 2c, d).

Fig. 1: Design of plastic-crystal-embedded elastomer electrolyte.
figure 1

a, Schematic illustration of the design and structure for PCEE. The structure of PCEE shows that the 3D interconnected plastic crystal phase (cyan) is surrounded by the elastomer phase (orange). The plastic crystal phase supports a fast ion-conducting pathway, whereas the elastomer phase provides superior mechanical extensibility. b, A 3D tomography image of PCEE constructed by an X-ray microscope. c, d, SEM (c) and cross-sectional TEM (d) images of PCEE film. e, Electron energy-loss spectroscopy elemental mapping of the TEM image in d.

The interconnected nature of the SN phase in PCEE was further studied by temperature-dependent ionic conductivity measurements. Despite the low ionic conductivity of BA100 (about 10−6 mS cm−1 at 20 °C), PCEE exhibited a high ionic conductivity of 1.1 mS cm−1 at 20 °C, which was slightly lower than that of the SN–LiTFSI complex (SN100; 4.1 mS cm−1 at 20 °C; Fig. 2a, Supplementary Fig. 6). The activation energy (Ea) of the built-in PCEE was estimated to be 0.13 eV, which was comparable to that of SN100 (0.11 eV), but much smaller than that of BA100 (0.69 eV). Considering the volume fraction of the SN conducting phase in PCEE, the tortuosity of the continuous conducting phase was determined to be very low (1.9)29. This result suggests that the ion-conductive pathways of the continuous SN domains with high connectivity were successfully developed within the elastomer matrix of PCEE. In addition, X-ray diffraction results showed that the crystallinity of the SN phase within the elastomeric phase was well maintained, contributing to the high ionic conductivity of PCEE (Supplementary Fig. 7). The superior mechanical elasticity of PCEE was demonstrated by tensile tests. PCEE showed an excellent extensibility of about 300%, whereas the tensile property of SN100 could not be measured owing to its extreme brittleness (Fig. 2b). Overall, the results indicate that the embedded SN network in PCEE did not compromise the mechanical robustness of the elastomer matrix. Moreover, PCEE showed excellent flame retardancy (Supplementary Fig. 8).

Fig. 2: Properties of the built-in PCEE.
figure 2

a, Arrhenius plots of the built-in PCEE and its components (BA100 and SN100) based on measurements of ionic conductivity (σ) versus temperature (T). b, Stress–strain curves of the built-in PCEE and BA100 at an extension rate of 100 mm min−1. The tensile property of SN100 could not be measured owing to its brittleness. c, Photo images and schemes showing the ex situ and the built-in PCEEs. d, Cross-sectional SEM image of the built-in PCEE on Li metal anode. e, Interfacial adhesion test between the built-in PCEE and the Cu foil. The red dotted line indicates the average force per width. f, Nyquist plots of as-prepared symmetric Li cells configured with various electrolytes. The solid lines correspond to the equivalent circuit fits.

A photographic image of the built-in PCEE that was formed on Li metal is compared with that of the ex situ-synthesized PCEE (hereinafter, ‘ex situ PCEE’) (Fig. 2c). The built-in PCEE had a seamless contact with the electrode, forming a more stable interface than the ex situ PCEE. A cross-sectional SEM image shows that the built-in PCEE seamlessly adhered to the Li metal anode (Fig. 2d), in which the solution that initially wet the rough electrode is in situ-polymerized along the surface8. An interfacial adhesion test shows that the built-in PCEE produced a strong adhesion with the electrode, resulting in a higher adhesion energy of 21.5 J m−2 than that of the ex situ PCEE (2.7 J m−2) (Fig. 2e, Supplementary Fig. 9). In general, adhesion energy above 5 J m−2 is considered to be a crucial requirement for creating a stable interface that is capable of withstanding mechanical stress during cell fabrication and operation30. In addition, electrochemical impedance spectroscopy (EIS) measurements showed that the overall resistance of the built-in PCEE was 122 Ω cm−2, which was lower than that of the ex situ PCEE (304 Ω cm−2) (Fig. 2f).

Investigating Li reversibility

Motivated by the exceptional mechanical and electrochemical properties of built-in PCEE, we performed Li plating and stripping tests in symmetric Li cells (Fig. 3a). At a high current density of 10 mA cm−2, the cell with SN100 lasted for only a few cycles. In contrast, the cell with the built-in PCEE showed excellent cycling performance with low polarization over 1,500 h, reaching a cumulative capacity of 7.5 Ah cm−2 (Supplementary Fig. 10). Specifically, an extremely low polarization of 13 mV at 10 mA cm−2 was observed for the built-in PCEE, which is considerably lower than that of the ex situ PCEE (63–111 mV). This low polarization can be attributed to the conformal coating of the built-in PCEE on Li metal, which can substantially reduce the interfacial resistance and generate a uniform Li-ion flux. It is noted that the ‘arch’ shape at the edge of the voltage profile, which is commonly observed with dendritic and dead Li accumulation, did not appear even after repeated plating and stripping of Li metal for 1,500 h (insets of Fig. 3a)31. These results far exceed the Advanced Research Projects Agency-Energy (ARPA-E) Integration and Optimization of Novel Ion-Conducting Solids (IONICS) goal based on current density, cumulative capacity and per-cycle areal capacity (Supplementary Fig. 11)32. The compatibility of PCEE with Li metal was further supported based on the stable interfacial resistances (175 Ω cm−2) for 30 days at the Li–PCEE (solid–solid) interface (Extended Data Fig. 3a). In addition, as the cycle increased, the overall resistance of the built-in PCEE significantly decreased (Extended Data Fig. 3b). These results demonstrate that the in situ-polymerized PCEE can effectively reduce the interfacial resistance at the solid–solid interfaces, enabling ultrastable Li plating and stripping cycles.

Fig. 3: Built-in PCEE in symmetric Li and asymmetric Li||Cu cells.
figure 3

a, The cycling performance of the symmetric Li cells with various electrolytes. Insets: enlarged voltage profiles for different times. bd, Top-view SEM images of the Li metal anodes with SN100 after cell failure (b), ex situ PCEE (c) and built-in PCEE (d) after 100 cycles at a current density of 10 mA cm−2 with a capacity of 10 mAh cm−2. Insets: magnified top-view SEM images. e, Coulombic efficiencies for the asymmetric Li||Cu cells with the built-in PCEE at current densities of 2 mA cm−2 and 5 mA cm−2 with capacities of 4 mA cm−2 and 10 mAh cm−2, respectively. f, Corresponding enlarged voltage profiles. gi, Cross-sectional SEM images of pristine (g), Li-plated (h) and Li-stripped (i) Cu electrodes assembled with the built-in PCEE. j, Schematic illustration of Li plating and stripping with different electrolytes.

The morphologies of the Li metal anodes with different electrolytes were compared after 100 cycles at 10 mA cm−2. For SN100, Li metal showed a porous structure containing mossy and dead Li after only a few cycles (Fig. 3b), explaining the sudden failure of the Li cell (Fig. 3a). In contrast, the Li anodes with both the ex situ and the built-in PCEEs showed a dense and uniform Li structure without dendritic Li after 100 cycles (200 h) (Fig. 3c, d). However, after removing the attached PCEE film, Li metal with the built-in PCEE showed a much smoother surface (Fig. 3d) compared with that with the ex situ PCEE, which showed a wrinkled surface with cracks (Fig. 3c). The stability of the Li cell with the built-in PCEE was further investigated under higher current conditions of 20 mA cm−2 and 20 mAh cm−2 (Extended Data Fig. 3c). After voltage fluctuations in the earlier cycles (<50 h), the Li cell showed a stable polarization of 25–29 mV for 500 h. It is noted that the voltage hysteresis of the Li cells with the built-in PCEE at various current densities was significantly lower than those of previously reported polymer-based electrolytes (Extended Data Fig. 3d). Next, the t+ value of the built-in PCEE was determined by using the Bruce–Vincent method (Extended Data Fig. 3e, f, Supplementary Fig. 12)33. Notably, the built-in PCEE showed a high t+ value of 0.75, which is substantially higher than the t+ values of conventional organic liquid electrolytes (t+ ≈ 0.4)34 and PEO-based polymer electrolyte (t+ < 0.5)35. This excellent stability at high-rate Li plating and stripping cycles and high t+ value are crucial to enable fast charging of LMBs. In addition, an inorganic–organic hybrid SEI layer was found in the built-in PCEE, which is beneficial for interfacial stability, whereas the SEI layer derived from SN100 was mainly composed of organic compounds (Extended Data Fig. 4).

The built-in PCEE also showed excellent stability and cyclability in asymmetric Li||Cu cells (Fig. 3e). At 0.5 mA cm−2 with 1 mAh cm−2, the Coulombic efficiency (CE) of the Li||Cu cell with SN100 fluctuated and faded out after 35 cycles (Supplementary Fig. 13). In stark contrast, the cell with the built-in PCEE showed a CE of 100.0% with a small polarization below 8 mV after 500 cycles (Extended Data Fig. 5). The Li||Cu cells with PCEEs further maintained a 100.0% CE at higher current densities of 2 mA cm−2 and 5 mA cm−2 with capacities of 4 mA cm−2 and 10 mAh cm−2 for 430 and 200 cycles, respectively (Fig. 3e, f). To better understand the cycling behaviour, we investigated the morphological changes of the Li||Cu cells with the built-in PCEE during the Li plating and stripping process. The cross-sectional SEM image showed that PCEE was conformally coated onto the copper (Cu) foil before electroplating (Fig. 3g). After the first Li plating at 0.5 mA cm−2 with 1 mAh cm−2, densely and uniformly deposited Li on the Cu foil was observed (Fig. 3h). After subsequent Li stripping, the Li deposited on the Cu foil disappeared completely (Fig. 3i). These results indicate that PCEE with high elasticity and strong adhesion successfully accommodated large volume changes during Li plating (Fig. 3j). In addition, the resilience of the built-in PCEE allowed the system to return to its original state without having any deformation after the stripping process. Thus, the ultrastable cycling process of the cells with PCEE stems from its superior mechanical and interfacial properties.

Demonstrating high-energy Li batteries

We investigated the use of the built-in PCEE with various cathodes, including LiFePO4 (LFP), LiNi0.6Mn0.2Co0.2O2 (NMC-622) and NMC-83 cathodes, for solid-state LMB applications. Before battery performance testing, an electrochemical floating experiment of PCEE was performed to strictly define the feasible electrochemical window (Extended Data Fig. 6a)8,36. The measured leakage current was less than 20 μA up to 4.6 V, which is consistent with the linear-sweep voltammetry results showing an oxidation wave at 4.75 V (Supplementary Fig. 14). Thus, the excellent oxidative stability of PCEE allows for stable operation (rate capability and cycling) with NMC-622 cathodes at a high voltage of 4.5 V (Extended Data Fig. 6b, c). It is noted that the use of SPEs with high-voltage cathodes has been limited to a voltage of 4.3 V (refs. 8,12). Thus, this study demonstrates stable operation of SPE-based all-solid-state LMBs at a high voltage of 4.5 V at ambient temperature. In addition, the full cell with an LFP cathode maintained a discharge capacity of 93 mAh g−1 at 1C without significant capacity fading (0.005% per cycle) over 1,000 cycles at 20 °C (Extended Data Fig. 7).

Towards high-energy LMBs, there is a strong demand for full cells with a limited source of Li metal anode, a high-loading cathode and a thin solid electrolyte. We further performed electrochemical tests of the full cell with a low negative/positive capacity (N/P) ratio of 3.4 (35-μm-thick Li anode; 25-μm-thick built-in PCEE; high-loading NMC-83 (>10 mg cm−2)). The full cell delivered a high capacity of 2.1 mAh cm−2 during the initialization cycles at 0.1 mA cm−2 (Extended Data Fig. 8a). Thereafter, the cell maintained a capacity of 1.1 mAh cm−2 (88% capacity retention) with a CE of 99.4% at 0.5 mA cm−2 after 100 cycles at 30 °C (Fig. 4a). In addition, the rate capability of the cell was evaluated at various current densities from 0.1 mA cm−2 to 3 mA cm−2 (Fig. 4b). The cell exhibited a capacity of 1.3 mAh cm−2 at a high rate of 1 mA cm−2 (60% of its capacity at 0.1 mA cm−2), demonstrating excellent rate capability (inset of Fig. 4b). We further investigated the temperature-dependent voltage profile of the cell from 60 °C to 0 °C (Extended Data Fig. 8b). The full cell showed a negligible capacity drop from 60 °C to 40 °C (99%), and retained 92% of its capacity at 20 °C and 57% at 0 °C (inset of Extended Data Fig. 8b). The excellent rate and low-temperature performances can be attributed to the exceptionally high ionic conductivity and t+ of PCEE. The specific energy and power of all-solid-state LMBs at ambient temperature were calculated based on the weight of the anode, cathode and solid electrolyte using a Ragone-type plot (Fig. 4c, Extended Data Table 1)37. The full cell with the NMC-83 cathode exhibited high specific energy exceeding 410 Wh kg−1 (791 Wh kgNMC-83−1) and maintained a high specific energy of 235 Wh kg−1 at 184 W kg−1 at ambient temperature. These performances of the full cell with thin PCEE are much higher than any of the previously reported cells using ceramic, polymer or composite electrolytes tested at ambient temperature (Fig. 4c, d)8,16,37,38,39,40,41,42,43,44. We expect that the specific energy of the cells can be further increased by optimizing the cathode structures or stacking cells multiple times.

Fig. 4: Towards a high-energy all-solid-state LMB with elastomeric electrolyte.
figure 4

a, Thc cycling performance of the full cell as a function of cycle number in the voltage range of 2.7–4.3 V. The cell was initially cycled at a current density of 0.1 mA cm−2 for 3 cycles before cycling at 0.5 mA cm−2. b, Rate capability of the full cell in the voltage range of 2.7–4.5 V at equal current densities. Inset: the capacity utilization at different areal current densities. c, Ragone plot for the full cells at ambient temperature (20–30 °C). Specific energy and power in the respective references are depicted within the light-shaded regions (blue, polymer/composite electrolyte; grey, inorganic electrolyte). Specific energy and power are normalized by the mass of the cells (anode, solid electrolyte and cathode), excluding the weight of the cell casing and current collectors. d, Schematic illustration of all-solid-state LMB that comprises a thin Li metal anode, thin built-in PCEE and high-loading NMC-83. The full cell with the built-in PCEE shows higher specific energy than the typical full cells listed in Extended Data Table 1. All full cells were configured with 35-μm-thick Li anode, 25-μm-thick built-in PCEE and high-loading NMC-83 (>10 mg cm−2).

In summary, we report a class of SPEs based on an in situ formation of an elastomer electrolyte containing a 3D interconnected plastic crystal phase, which successfully combines the advantages from both elastomer and plastic crystal, including high ionic conductivity, superior mechanical properties, electrochemical stability, low interfacial resistance and high Li-ion transference number. The built-in PCEEs enabled excellent cycling performances in the symmetric Li and asymmetric Li||Cu cells with low voltage hysteresis below 26 mV and 100.0% CEs. Finally, under the constrained conditions of a limited Li source and a high-loading NMC cathode (N/P ratio <3.4), we have demonstrated the stable operation of a PCEE-based solid-state LMB with high specific energy and power at ambient temperature. We expect that this elastomeric electrolyte system can be widely applied to the operation of various post-metal (for example, sodium, potassium, zinc, magnesium and aluminium) batteries, including metal–air and metal–sulfur batteries, because of its excellent mechanical properties and high ionic conductivity.

Methods

Preparation of electrolytes

For PCEE fabrication, electrolyte preparation and cell assembly were implemented in an argon gas-filled glove box where the concentration of O2 and water (H2O) was below 0.1 ppm. The BA-based solutions were prepared by dissolving 1 mol% PEGDA (Sigma-Aldrich), 0.5 mol% AIBN (Sigma-Aldrich) and 1 M LiTFSI powder (≥99%; Sigma-Aldrich) in BA liquid (Sigma-Aldrich). The BA-based solutions were polymerized at 70 °C for 2 h to compare BA-based elastomer (BA100) with PCEE. The SN-based solutions (SN100) were made by mixing SN (Sigma-Aldrich) with 1 M LiTFSI powder and 5 vol% fluoroethylene carbonate additive (Sigma-Aldrich) to protect against the side reaction of SN with Li metal45 at 50 °C. Each prepared liquid solution was homogeneously mixed in a volume ratio of 1:1 at 50 °C to fabricate the built-in PCEE (Supplementary Fig. 1). After injecting the prepared solution into the cells (2032-type coin cell in this work), the assembled cell was heated at 70 °C for 2 h to generate built-in PCEEs (Extended Data Fig. 1). A homemade mould was used to prepare the free-standing ex situ PCEEs.

Material characterization

Fourier transform infrared spectroscopy spectra were measured using a Bruker ALPHA-P spectrometer in an attenuated total reflectance setup. Thermogravimetric analysis testing (TA Instruments Q500) was measured from room temperature to 700 °C at a heating rate of 10 °C min−1 under a nitrogen atmosphere. Differential scanning calorimetry curves were obtained using a TA Instruments Q200 under a nitrogen atmosphere at a heating and cooling rate of 10 °C min−1. The morphologies of the built-in PCEE were observed by SEM (Hitachi SU-5000) and TEM (Talos F200X). Cross-sectional TEM samples were prepared by microtoming with a diamond knife (RMC PowerTomeX). Electron energy-loss spectroscopy elemental mapping was performed using TEM with high-angle annular dark-field imaging. The 3D tomography image was constructed by an X-ray microscope (Zeiss Xradia 520 versa). Mechanical tensile-stress and interfacial adhesion-strength measurements were carried out using a universal testing machine (Lloyd Instruments LR5K). Adhesion energies were calculated using a previously reported method46. X-ray photoelectron spectroscopy (XPS; Thermal Scientific K-alpha XPS instrument) was used to investigate the compositions of the SEI formed on the Li metal anodes after 100 cycles in the symmetric Li cells with the built-in PCEE and SN100 (Extended Data Fig. 4). The cycled cells were disassembled in an argon-filled glove box. The cycled Li metal anodes were then transferred to the XPS using a vacuum transfer vessel to avoid contamination or side reactions with ambient oxygen and moisture. For comparison, the top surfaces of the cycled Li metal anodes were etched using argon-ion sputtering until the atomic ratio of Li in the SEI components of the built-in PCEE and SN100 reached approximately 30%. High-resolution XPS Li 1s, C 1s, O 1s, N 1s and F 1s spectra were deconvoluted using XPSPEAKS 4.1.

Electrode preparation

NMC-622, NMC-83 and LFP cathodes were prepared using a slurry casting technique. Active material, Super P carbon as a conductive additive, SN–LiTFSI with a molar ratio of 20:1 and polyvinylidene fluoride were dissolved in N-methyl-2-pyrrolidone with a weight ratio of 7:1:1:1 to make a slurry and then coated onto a current collector of aluminium foil. The cathodes were dried in a vacuum oven at 55 °C for 24 h. The active loading density of the LFP cathode was 1.5 mg cm−2. The active loading densities of the NMC-622 cathode were 2.1 mg cm−2 and 9.8 mg cm−2. The active loading density of the NMC-83 cathode was in the range of 10.3–10.6 mg cm−2.

Electrochemical measurements

The electrochemical performances of all cells were tested with 2032-type coin cells assembled using Li foil as the anode in an argon-filled glovebox (M. Braun, O2 and H2O <0.1 ppm). Linear-sweep voltammetry was carried out using Li||stainless steel (SS) asymmetric cells from 1.5 V to 6 V versus Li/Li+ at a scan rate of 1 mV s−1. EIS (BioLogic VMP3) of the PCEEs was carried out from 100 Hz to 105 Hz using a 10-mV peak voltage at an open-circuit voltage. The ionic conductivity of the electrolytes was measured using EIS with SS||electrolyte||SS symmetric cells in an environmental chamber (MC-812R, Espec) at the desired temperatures. Considering the compatibility with the current roll-to-roll-based Li-ion battery manufacturing, polypropylene or glass fibre as a separator was applied to prevent a short circuit in the liquid precursors of the PCEEs for the full cells in this work. The cycling test of the 35-μm-thick Li||25-μm-built-in PCEE||high-loading NMC-83 cell was performed in the voltage range of 2.7–4.3 V with three initialization cycles at a current density of 0.1 mA cm−2 before cycling at a current density of 0.5 mA cm−2 without any voltage holding. The galvanostatic charge/discharge test of the 35-μm-thick Li||25-μm-thick built-in PCEE||high-loading NMC-83 cell was carried out in the voltage range of 2.7–4.5 V at equal current densities of 0.1–3 mA cm−2 (Arbin battery tester). For the temperature-dependent test, the 35-μm-thick Li||25-μm-built-in PCEE||high-loading NMC-83 cell was charged and discharged at equal current density (0.1 mA cm−2) and temperatures (0 °C to 60 °C) in an environmental chamber. All specific and areal capacities were normalized using the weight of the active material in the electrodes and the area of the electrodes, respectively.

Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request. Source data are provided with this paper.

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Acknowledgements

This work was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (ECCS-2025462). This work was partially supported by KRICT core project (BSF20-242). J.H., Y.J.L. and B.J.K. acknowledge the support from the National Research Foundation of Korea (NRF-2019R1A2B5B03101123, 2017M3D1A1039553 and 2020RlA4A1018516).

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S.W.L., B.J.K., M.J.L. and J.H. conceived the ideas and designed the experiments. K.L., Y.J.L., B.G.K. and K.-N.J. were involved with the methods and characterizations of materials. S.W.L., B.J.K., M.J.L. and J.H. co-wrote the manuscript. All authors reviewed the manuscript.

Corresponding authors

Correspondence to Bumjoon J. Kim or Seung Woo Lee.

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S.W.L., B.J.K., M.J.L. and J.H. have filed a US provisional patent application (63/209,140) covering the materials and lithium metal battery application described in this paper.

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Nature thanks Enrique Gomez and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer review reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Fabrication process of in situ-polymerized PCEE within the electrochemical cell.

a, Digital photo images of a homogeneous solution consisting of BA, SN, LiTFSI, PEGDA and AIBN for built-in polymerization (left) and haze-coloured PCEE on the bottom of a glass vial after polymerization at 70 °C for 2 h (right). b, Photo image of PCEE showing mechanical elasticity. c, Schematic illustration of the built-in polymerization process. The solution was injected into the electrochemical cells and then heated in an oven for built-in polymerization.

Extended Data Fig. 2 Comparison of morphology, ion conductivity and mechanical property between polymerization induced-PCEE and blend systems.

a, Morphology of PCEE. SEM image of PCEE shows the continuously-connected SN phases within the elastomeric matrix that was uniformly developed over a large area through PIPS. b, Morphology of blend consisting of elastomeric polymer (cross-linked poly(butylacrylate) and PEGDA) and plastic crystal (SN) with LiTFSI. A blend is prepared from a mixture of elastomeric polymers and SN–LiTFSI in chloroform, followed by a drying process. The same weight ratios of BA, SN, PEGDA and LiTFSI are used for constructing the PCEE and blend systems. SEM image of blend shows a macrophase separation with a length-scale of over μm. c–d, Comparison of ionic conductivity (c) and toughness (d) between the PCEE and blend systems.

Source data

Extended Data Fig. 3 Electrochemical characterization for the symmetric Li cells with built-in PCEE.

a, Time-dependent Nyquist plots of the symmetric Li cells configured with built-in PCEE. b, Nyquist plots of the symmetric Li cells configured with built-in PCEE after 25, 75, and 100 cycles. c, Cycling performance of the symmetric Li cells configured with built-in PCEE at different current densities. d, Voltage hysteresis of Li plating/stripping for built-in PCEE compared with previously reported literature data8,12,16,38,39,47,48,49,50,51,52. e, Nyquist plots of the symmetric Li cells before and after polarization of 10 mV. f, Steady-state current measurement of the symmetric Li cells under 10 mV polarization for 10 h. EIS was measured at open-circuit voltage in the range of 105 to 10° Hz with an amplitude of 10 mV.

Source data

Extended Data Fig. 4 Characterization of the SEI components on the cycled Li metal anodes with built-in PCEE and SN100 by XPS.

The high-resolution Li 1s, C 1s, O 1s, N 1s, and F 1s XPS spectra of the Li metal anodes were measured after 100 cycles of the symmetric Li cells with built-in PCEE and SN100 at a current density of 1 mA cm–2 with a capacity of 1 mAh cm–2.

Source data

Extended Data Fig. 5 Li plating and stripping behaviour of built-in PCEE on bare Cu.

a, Cycling performance of the asymmetric Li||Cu cells at current densities of 0.5 and 1 mA cm–2, respectively. b–c, Li stripping and plating profiles for built-in PCEE at a current density of 0.5 mA cm–2 with a capacity of 1 mAh cm–2 (b), and a current density of 1 mA cm–2 with a capacity of 2 mAh cm–2 (c).

Extended Data Fig. 6 Electrochemical stability of built-in PCEE paired with high-voltage NMC-622 cathode.

a, Electrochemical floating experiment was performed using Li||NMC-622 with built-in PCEE. The cell was charged to 4.2 V at 0.2C (1 C = 180 mA g–1) and then held at gradually higher voltages for 10 h up to 4.7 V. b, Rate capability of the full cell (35-μm-thick Li anode; 25-μm-thick built-in PCEE; high-loading NMC-622 (9.7 mg cm–2) in the voltage range of 2.7–4.5 V at equal current densities. (Inset: the capacity utilization at different areal current densities). c, Cycling performance of the full cell (excess Li; 25-μm-thick built-in PCEE; NMC-622 (2.1 mg cm–2)) as a function of cycle number in the voltage range of 2.7–4.5 V. The cell maintained a high capacity of ~140 mAh g–1 (82% capacity retention) with high CEs of 99.5% for 100 cycles, confirming the stable operation at high voltage. Cells were performed at 20 °C.

Extended Data Fig. 7 Cycling performance of the Li||LiFePO4 cell at 1 C without voltage holding.

a, Capacity and Coulombic efficiency as a function of cycle number. b, Corresponding voltage profiles. 1 C = 170 mA g–1.

Extended Data Fig. 8 Electrochemical performances of the full cells with high-voltage NMC-83 cathode.

a, The charge and discharge profiles of the full cell in the voltage range of 2.7–4.3V at 0.1 mA cm–2. b, Temperature-dependent voltage profiles of the full cell charged/discharged at equal temperatures (60 to 0 °C) in the voltage range of 2.7–4.5 V. (Inset: the capacity utilization at different temperatures). All full cells were configured with 35-μm-thick Li anode; 25-μm-thick built-in PCEE; high-loading NMC-83 (>10 mg cm–2).

Extended Data Table 1 Comparison of battery performance with previously reported solid-state LMBs

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Lee, M.J., Han, J., Lee, K. et al. Elastomeric electrolytes for high-energy solid-state lithium batteries. Nature 601, 217–222 (2022). https://doi.org/10.1038/s41586-021-04209-4

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