A solid-state lithium-ion battery with micron-sized silicon anode operating free from external pressure

Applying high stack pressure (often up to tens of megapascals) to solid-state Li-ion batteries is primarily done to address the issues of internal voids formation and subsequent Li-ion transport blockage within the solid electrode due to volume changes. Whereas, redundant pressurizing devices lower the energy density of batteries and raise the cost. Herein, a mechanical optimization strategy involving elastic electrolyte is proposed for SSBs operating without external pressurizing, but relying solely on the built-in pressure of cells. We combine soft-rigid dual monomer copolymer with deep eutectic mixture to design an elastic solid electrolyte, which exhibits not only high stretchability and deformation recovery capability but also high room-temperature Li-ion conductivity of 2×10−3 S cm−1 and nonflammability. The micron-sized Si anode without additional stack pressure, paired with the elastic electrolyte, exhibits exceptional stability for 300 cycles with 90.8% capacity retention. Furthermore, the solid Li/elastic electrolyte/LiFePO4 battery delivers 143.3 mAh g−1 after 400 cycles. Finally, the micron-sized Si/elastic electrolyte/LiFePO4 full cell operates stably for 100 cycles in the absence of any additional pressure, maintaining a capacity retention rate of 98.3%. This significantly advances the practical applications of solid-state batteries.

The authors present the design of an elastic polymer mixed with N-methylacetamide-LIFSI solution as an electrolyte for Si|LFP batteries and demonstrate its operation under zero external pressure.Reducing external pressure is crucial for high energy density batteries, and this work shows decent electrochemical performance in the specific system.However, I have two major concerns.
Secondly, for high-energy silicon batteries, it is recommended to pair NMC with a Si-based anode.LFP is not an energy-dense option and lacks general interest (Science 2021, 373, 1494-1499).
Fig. 1: The function of the N-methylacetamide-LIFSI mixture is missing.Fig. 2b: Is N-methylacetamide chemically bonded/crosslinked on the polymer backbone?Please provide experimental evidence if so.Fig. 2d: How does the stretching recovery capability help with the exemption of external pressure?UV curing was used for the in-situ preparation of the silicon anode.The light was supposed to be blocked by the Si electrode.The bottom part of the electrode is not exposed to the light.How would the UV curing be processed?It is unclear if there are liquid electrolytes at the bottom of the Si electrode.Additionally, the Si loading of 0.5-0.7 mg cm-2 is low for practical applications."Fluoroethylene carbonate in 5wt% was added to the electrolyte to assist the formation of a stable SEI."This statement confuses me.It appears to be a liquid additive that was not mentioned in the main text.Furthermore, there seems to be liquid SEI formation in the system.On the cathode side, there is no polymer electrolyte present.How are Li ions conducted without an electrolyte?I believe liquid may diffuse into the cathode.Figure 5: The control samples of LiPSCl are not reasonable, as they consist of pure solidstate electrolyte, while the reported polymers contain a liquid phase.

Reviewer #2 (Remarks to the Author):
This work provides a solution to address the issue with the internal void formation in Si anode.And poor contact due to volume change by using an elastic solid electrolyte.A stable cycle performance was reported in a μm-Si anode without additional pressure.The mechanical design, which leverages microphase separation of soft and rigid phases, enables the enhanced stretchability, fracture strength, shape memory capacity, and energy dissipation properties.This manuscript is well organized and systematically studied, the innovation is good.There are some questions for authors to clarify before it can be accepted.
1.It has been previously reported that combining a silicon anode with high elastic and selfhealing polymer electrolytes enables stable cycling, despite the significant volume expansion and shrinkage of silicon particle (e.g., Q. Huang et al., Nat.Commun., 10, 5586 (2019); C. Wang et al., Nat.Chem., 5, 1042-1048 (2013)).It is advisable to reference these prior studies while also highlighting the originality of your own work in the Introduction section.
2. The microphase separation in copolymer to increase mechanical strength is typically achieved using block copolymer.Is there a specific reason why a random copolymer was used in this work?Additionally, what are the monomer reactivity ratios of AM and DMAM monomer in a random copolymerization?
3. The polymer electrolyte with high ionic conductivity of 2×10−3 S cm−1 and reinforced mechanical properties is expected to demonstrate improved Li plating and stripping performance at elevated current density, as well as superior Li deposition, in comparison to the results shown in Fig. 3 (0.1 mA cm−2, 0.1 mAh cm−2).The rate performance of should be provided to further highlight its properties.
4. In Fig. 3b, it seems that the interfacial resistance is larger compared to the bulk resistance in Li symmetrical cell.What could be the reason for this? 5.In Fig. 5a, the cycle performance of μm-Si half-cell was evaluated at 0.4 C with a lower cutoff voltage of 50 mV after the initial cycles, demonstrating a discharge capacity of 1039 mAh g−1.However, a higher capacity of ~2000 mAh g−1 can be expected with a lower cutoff voltage of 50mV and low overpotential.How would the cycle performance be affected by a reduced C-rate and a higher discharge capacity, considering more severe expansion and shrinkage of Si particle?6.The computational details of the finite element method to generate Figure 5b is not provided.Please add.

7.
What is the capacity ratio of LFP to μm-Si in an LFP/μm-Si full cell?This information is crucial as the utilization rate of Si anodes is considered to have a significant impact on the cycle characteristics of full cells.

Reviewer #3 (Remarks to the Author):
The need for external pressure is a major obstacle to the application of all-solid-state batteries in practice, but the authors raised an effective strategy to alleviate this issue, with convincing demonstration using the micrometer-Si anode.The reviewer considers it as a highly important contribution to the all-solid-state battery community.However, before it can be accepted for publication, the following issues need further clarification.
1.The authors indicate that the synthesis of the elastic polymer electrolyte is conducted in Ar-filled glovebox.Is it unstable in ambient air?Which component in air will react with this material?Is it possible to improve its air stability in future studies?This directly influences the production cost, which is also rather important to the successful commercialization of allsolid-state batteries.
2. The authors claim that their elastic polymer electrolyte shows an oxidation potential of 4.5 V vs. Li/Li+, but the cell for cycling tests utilizes a 3 V-class material, LiFePO4, as the cathode.The reviewer would like to see the cycling performance of the cell where the elastic polymer electrolyte is paired with the 4 V-class cathodes like LiCoO2 or LiNi0.8Mn0.1Co0.1O2.
3. The authors specified the areal mass loading of micrometer-Si in their cells, but the areal capacity (in mAh cm-2) that can be actually achieved in the cell is more meaningful.The desired mechanical properties reported here could also be helpful in reaching higher areal capacity.Therefore, the reviewer suggests the authors adjust the mass loading of micrometer-Si and investigate the maximum areal capacity that the micrometer-Si anode can achieve in the μm-Si/elastic electrolyte/Li cell with decent cycling stability.

Responses to Reviewers' Comments
We would like to thank all reviewers for their constructive comments and suggestions, which has in our view significantly raised the quality of the manuscript (NCOMMS-23-50616-T).We have modified the manuscript accordingly, and listed the detailed corrections below point by point for each reviewer.All revised portions have been marked in yellow in the revised manuscript.The main corrections and the responses to the reviewers' comments are as follows.

Reviewer #1:
The authors present the design of an elastic polymer mixed with N-methylacetamide-LIFSI solution as an electrolyte for Si|LFP batteries and demonstrate its operation under zero external pressure.Reducing external pressure is crucial for high energy density batteries, and this work shows decent electrochemical performance in the specific system.However, I have two major concerns.
Secondly, for high-energy silicon batteries, it is recommended to pair NMC with a Si-based anode.LFP is not an energy-dense option and lacks general interest (Science 2021, 373, 1494-1499).
Response: We thank the reviewer for the approval of the importance of our work and the affirmative comment on electrochemical performances of the batteries in the manuscript.
Regarding to the first concern from the reviewer, please allow me to point out that the deep eutectic mixture (DEM) consisting of N-methylacetamide and LiFSI has great differences from conventional organic liquid electrolytes.The DEM forms as a result of the intermolecular interactions between the solid-state hydrogen bond donor (N-methylacetamide) and acceptor (LiFSI), which makes DEM have similar properties to molten salts including incombustibility and low vapor pressure 1,2 .Therefore, the elastic electrolyte proposed in our work possesses higher safety than the normal gel polymer electrolytes containing liquid solvents and presents nonflammability (see the combustion tests in Fig. 2j).Moreover, the elastic electrolyte features much superior mechanical properties (excellent stretchability and compressibility, desirable fatigue resistance, good deformation resiliency and unique energy dissipation characteristic) than gel electrolytes, which is crucial to realize the stable operation of the batteries without external pressure, especially those with large volume change.
It is well-acknowledged that there are trade-offs between the ionic conductivity and mechanical properties of polymer electrolytes.Although the adoption of liquid phase in gel electrolytes is beneficial to the Li + conduction, it usually comes at the cost of mechanical strength 3 .For this reason, despite of the fine ionic conductivity of gel polymer electrolytes, few works have succeeded in constructing well-performed μm-Si anode with gel electrolytes.There were some attempts using nanostructured Si pairing with gel electrolytes.For example, poly(ethylene glycol)diglycidylether crosslinked with diamino-poly(propylene oxide) immersing in 1 M LiPF6 solution was investigated as the gel electrolyte for nano-Si anode 4 .Triacrylate-based gel polymer electrolytes combined with mesoporous Si enabled the Li-Si battery to function for 100 cycles 5 .In these works, sophisticated nanostructure designs were still necessary to alleviate the volume change of Si due to the poor mechanical properties of the gel polymer electrolytes.However, the volumetric capacity of nano-Si is much smaller than that of μm-Si due to its lower tap density 6 .Besides, the tedious structural design greatly raises the cost.
For the second concern from the reviewer, it is undeniable that NMC is a promising cathode material considering its high theoretical specific capacity.Whereas, it is hard for NMC to deliver satisfying actual specific capacity in the long-term cycle in solid-state batteries.The specific capacity of the NMC811 is calculated to be ~80 mAh g -1 in the μm-Si/NMC cell in the mentioned reference, which is lower than that of LFP in our work (151.4 mAh g -1 at 0.5C for the long-term cycle test).
We agree with the reviewer that NMC is a more energy-dense cathode material and may raise general interest.We have applied the elastic electrolyte in the NMC cathode and conducted galvanostatic cycle tests on the NMC/elastic electrolyte/Li cells without external pressure.To fabricate the NMC electrode with the elastic electrolyte, the precursor solution of the elastic electrolyte was dripped onto the NMC cathode coated on the Al foil, followed by a vacuum infiltration process and then UV polymerization.As illustrated in Fig. R1, the cell possessed a specific capacity of 208.6 mAh g -1 with an initial coulombic efficiency of 82.6% and functioned normally for dozens of cycles, but the cycle stability still needed further improvements.It can be seen from Fig. R2 that NMC particles retained in tight contact with the elastic electrolyte after cycles, which ensured the rapid Li + transport inside the cathode.Consequently, it is reasonable to infer that the capacity decay was not caused by contact failure between the electrolyte and active materials, even though the cells are cycled with the exemption of external pressure.The capacity decay may be caused by the fact that the charge cutoff voltage of the cell (4.4 V versus Li + /Li) was quite close to the upper limit of the electrochemical stability window of the elastic electrolyte (4.5 V versus Li + /Li).In order to further enhance the electrochemical performances of the NMC/elastic electrolyte/Li cell, we employed 1 wt% LiPO2F2 as the additive in the elastic electrolyte in the cathode to construct a more stable cathode electrolyte interphase (CEI) 7 .The modified NMC/elastic electrolyte/Li cell working under no external stack pressure delivered a specific discharge capacity of 207.1 mAh g -1 with an increased initial coulombic efficiency of 85.4%.Moreover, the cycle stability was meliorated and the cell maintained a capacity retention of 77.4% after 90 cycles (Fig. R3).The improved electrochemical performances were attributed to formation of the more robust CEI.It is reasonable to speculated that optimizing the type and amount of the additive could further enhance the compatibility of the elastic electrolyte with NMC.Whereas, this work is meant to focus on using the elastic electrolyte to tackle the mechanical failure issue of batteries working without high stack pressure.Further optimization of the electrochemical compatibility of the elastic electrolyte with NMC cathode remains a future work.Response: We thank the reviewer for pointing this out.The N-methylacetamide-LiFSI mixture (DEM) played a role in conducting Li + and assisted with the formation of the phase-separate structure in the elastic electrolyte.Fig. 1 is a three-dimensional diagram describing the failure mechanism of solid-state batteries and the corresponding solutions.The function of Li + conduction has been indicated by arrows.However, for the sake of conciseness, detailed depiction of the molecular interactions between the DEM and the copolymer network, which lead to the phaseseparation structure, have not been included in Fig. 1., but in Fig. 2a.According to the reviewer's comment, we have redrawn Fig. 2a and added a schematic diagram in Supplementary Fig. 2b to show N-methylacetamide molecules in the elastic electrolyte, as well as to clearly demonstrate the molecular structure and intermolecular interactions of the N-methylacetamide-LiFSI mixture; please see Fig. 2a in the revised manuscript and Supplementary Fig. 2b in the revised Supplementary Information.Moreover, we add a more explicit description about the function of the DEM in the formation of the phase-separate structure; please see page 5 in the revised manuscript.It has been highlighted in yellow in the revised manuscript and copied here: "The rigid and soft phase copolymerize randomly in the electrolyte and a bicontinuous phase-separate network forms due to the disparate miscibility of poly-DMAM and poly-AM with DEM." Fig. 2b: Is N-methylacetamide chemically bonded/crosslinked on the polymer backbone?Please provide experimental evidence if so.
Response: We thank the reviewer for this comment.According to this comment, we conduct Fourier transform infrared (FTIR) and Raman measurements on the precursor solution of the elastic electrolyte and the polymer after UV polymerization.From the results of the FTIR and Raman spectra, it can be seen that N-methylacetamide was not bonded or crosslinked on the polymer backbone.As shown in Fig. R4, the C=O stretching vibration peak of N-methylacetamide at 1652 cm -1 did not shift or weaken after polymerization.Furthermore, the Raman peaks corresponding to N-methylacetamide remained unchanged after polymerization (Fig. R5a).Instead, the Raman peak at 1625 cm -1 belonging to the C=C in the monomers vanished due to the opening of the C=C (Fig. R5b).

Fig. 2d: How does the stretching recovery capability help with the exemption of external pressure?
Response: We thank the reviewer for this comment.As depicted in Fig. 1 in the manuscript, the elastic electrolyte surrounds Si particles and fills the internal holes of the fabricated μm-Si electrode.During the lithiation process, the elastic electrolyte filling in between Si particles is subjected to compression due to the volumetric expansion of Si, whereas the elastic electrolyte adhering to the surface of Si undergoes stretching.Conversely, Si granules shrink during delithiation and the deformed elastic electrolyte restores accordingly.Therefore, both the stretching and compressing recovery capability of the elastic electrolyte have an impact on the structural stability of the μm-Si electrode working without extra stack pressure.By introducing an elastic electrolyte with outstanding stretching and compressing recovery capability, the intimate contact between the electrolyte and the active materials can be readily maintained despite of the volume fluctuation of the active materials.In this way, rapid Li + transport can be realized dispensing with external pressure.
UV curing was used for the in-situ preparation of the silicon anode.The light was supposed to be blocked by the Si electrode.The bottom part of the electrode is not exposed to the light.How would the UV curing be processed?It is unclear if there are liquid electrolytes at the bottom of the Si electrode.Additionally, the Si loading of 0.5-0.7 mg cm -2 is low for practical applications.
Response: We thank the reviewer for this comment.2-hydroxy-4'-(2-hydroxyethoxy)-2methylpropiophenone (Irgacure 2959) used in our work is a free radical photoinitiator that can generate the primary radicals via α-cleavage after UV exposure 8 .The primary free radicals can trigger the opening of C=C bonds 9 in the monomers to form CC radicals, thereupon then a chain reaction is initiated until the completion of polymerization.The UV light serves only as a triggering condition for the start of the chain reaction.Therefore, even though the bottom part of the electrode was not directly exposed to the light, the polymerization of monomers could be realized successfully through the free radical chain reaction.In response to the reviewer's comment, we add a statement about the UV initiated chain polymerization in the "Cell assembly and Electrochemical tests" part in Methods; please see page 16 in the revised manuscript.It has been highlighted in yellow in the revised manuscript and copied here: "Upon UV exposure, the photoinitiator generated the primary radicals via α-cleavage and triggered the opening of the C=C bonds in the monomers to form CC radicals, thereupon then a chain reaction is initiated until the completion of polymerization." We are in full agreement with the reviewer that active material loading is one of the key indicators for practical batteries.The Si loading of 0.5-0.7 mg cm -2 corresponds to theoretical areal capacity of 1.8-2.5 mAh cm -2 , and the actual capacity reached 2.1 mAh cm -2 (3413.6 mAh g -1 , Fig. 4i) in our work.Such capacity (or lower capacity) has been frequently-used in innovation researches for lithium-ion batteries [10][11][12][13][14] , considering that high loading is not the main emphasis of these studies.Nevertheless, in response to the comment from the reviewer, we conduct galvanostatic discharge and charge tests on solid-state μm-Si with higher loading of 1.3 mg cm -2 under no external stack pressure.The μm-Si/elastic electrolyte/Li cell was tested at 0.2C and 0.3C (1C=3579 mA g -1 ).As illustrated in Fig. R6, the cell delivered an initial discharge capacity of 1377.8 mAh g -1 with the coulombic efficiency of 83% at 0.2C (i.e.0.9 mA cm -2 ).With an elevated current density to 0.3C (i.e.1.4 mA cm -2 ), the reversible specific capacity decreased to 757 mAh g -1 .The reduced specific capacity may be caused by the enlarged overpotential of the Li counter electrode.It can be seen from the rate performance of the Li/elastic electrolyte/Li symmetric cell that the overpotential increased with the current density (Fig. R7).After 50 cycles, the μm-Si/elastic electrolyte/Li cell maintained 631.3 mAh g -1 , corresponding to a capacity retention of 83.4%.
Although the cycle life of the μm-Si/elastic electrolyte/Li cell is curtailed with the increase of mass loading, it can be seen that the proposed elastic electrolyte remains effective to enhance the stability of the high-loading solid-state μm-Si electrode operating without external pressure."Fluoroethylene carbonate in 5wt% was added to the electrolyte to assist the formation of a stable SEI."This statement confuses me.It appears to be a liquid additive that was not mentioned in the main text.Furthermore, there seems to be liquid SEI formation in the system.
Response: We thank the reviewer for this comment.Fluoroethylene carbonate has been widely employed as a sacrificial additive in the electrolyte to form a stable SEI on the Si anode 15 .The reductive decomposition of fluoroethylene carbonate occurs at a high potential of 1.3 V versus Li + /Li 16 , which is higher than the lithiation potential of Si.That means the liquid additive fluoroethylene carbonate will decompose prior to the lithiation of the Si anode.The main decomposition products of fluoroethylene carbonate are solid-state species including inorganic LiF and organic polycarbonates and CHF-OCO2 compounds [17][18][19] .In our work, a very small amount of fluoroethylene carbonate was used to assist the formation of robust SEI on the Si anode and it decomposed completely during the first few cycles.That was why the coulombic efficiency of the μm-Si/elastic electrolyte/Li cell was 87.1% at the first cycle and rose to higher than 99.5% within 10 cycles.Moreover, the addition of fluoroethylene carbonate itself does not necessarily lead to a long-term stability of the Si anode, as the reported Li-Si cells with 2-10 wt% fluoroethylene carbonate showed degradation within 50 cycles 19,20 .Therefore, it is reasonable to attribute the superior cycle stability of the Si anode (300 cycles with 90.8% capacity retention) to the outstanding mechanical properties of the elastic electrolyte.
On the cathode side, there is no polymer electrolyte present.How are Li ions conducted without an electrolyte?I believe liquid may diffuse into the cathode.
Response: We thank the reviewer for the careful reading of our manuscript.The elastic electrolyte was integrated into the cathode using the same method as in the Si anode.To be specific, 50 μL precursor solution of the elastic electrolyte was dropped on the μm-Si electrode.After the full infiltration of the precursor solution, the electrode was exposed to UV light for 5 minutes for the polymerization of the electrolyte.We have added a detailed description of the cathode fabrication in the "Cell assembly and Electrochemical tests" part in Methods; please see page 16-17 in the revised manuscript.It has been highlighted in yellow in the revised manuscript and copied here: "The homogenous slurry was coated on the Al foil by doctor blade with the LFP loading of around 6 mg cm -2 .Then 50 μL precursor solution of the elastic electrolyte was dropped on the LFP electrode, followed by the vacuum infiltration process and then UV polymerization."Figure 5: The control samples of LiPSCl are not reasonable, as they consist of pure solid-state electrolyte, while the reported polymers contain a liquid phase.
Response: We thank the reviewer for this comment.The worry from the reviewer is completely understandable because the deep eutectic mixtures (DEM) of N-methylacetamide and LiFSI is liquid due to the strong intermolecular interactions, although both N-methylacetamide and LiFSI are solid-state at ambient temperature.However, the DEM has great differences from conventional organic liquid electrolytes.It forms as a result of the intermolecular interactions between the solidstate hydrogen bond donor (N-methylacetamide) and acceptor (LiFSI), which makes DEM have similar properties to molten salts including incombustibility and low vapor pressure 1,2 .
In addition, the DEM is not used independently as the electrolyte in our work.In the fabricated elastic electrolyte, the copolymer molecular chains crosslink through hydrogen bonds, while the DEM is constrained within the crosslinked polymer network, rendering it unable to flow like a free liquid.The elastic electrolyte appeared characteristics of solids including stretchability and compressibility, high fracture strength and deformation recovery capability.Even under abuse conditions (such as uniaxial tensile and compressive tests and the cutting tests), no leakage of liquid from the elastic electrolyte was detectable (see Fig 2d, 2g, 2i, Supplementary Fig. 10 and Supplementary Video).Therefore, although the elastic electrolyte contains DEM, it is rational to regard it possessing the characteristics of solid-state electrolytes.The selection of batteries with the LPSCl electrolyte as the control samples was out of the above consideration.

This work provides a solution to address the issue with the internal void formation in Si anode. And poor contact due to volume change by using an elastic solid electrolyte. A stable cycle performance
was reported in a μm-Si anode without additional pressure.The mechanical design, which leverages microphase separation of soft and rigid phases, enables the enhanced stretchability, fracture strength, shape memory capacity, and energy dissipation properties.This manuscript is well organized and systematically studied, the innovation is good.There are some questions for authors to clarify before it can be accepted.
Response: We sincerely appreciate the reviewer for this positive comment on the systematicness and originality of our work.

It has been previously reported that combining a silicon anode with high elastic and self-healing polymer electrolytes enables stable cycling, despite the significant volume expansion and shrinkage of silicon particle (e.g., Q. Huang et al., Nat. Commun., 10, 5586 (2019); C. Wang et al., Nat. Chem., 5, 1042-1048 (2013)). It is advisable to reference these prior studies while also highlighting the originality of your own work in the Introduction section.
Response: We thank the reviewer for the thoughtful suggestion.The references mentioned by the reviewer reported elastic and self-healing polymers as the protective layer on the Si anode working with organic liquid electrolytes.The electrochemical performances of these Si electrodes were greatly enhanced due to the circumvention of the structural collapse during cycling.Thus, the effectiveness of using elastic media to alleviate the Si electrode degradation was validated.Unfortunately, flammable organic liquid electrolytes were used in these previous works.In our work, nonflammable solid-state elastic electrolyte was designed and served as both the mechanical cushion and the electrolyte simultaneously.In response to the reviewer's advice, the suggested relevant references have been added as the 17 th and 18 th reference and we have added the relevant discussion in the Introduction section; please see page 2-3 in the revised manuscript.It has been highlighted in yellow in the revised manuscript and copied here: "Nevertheless, previous works have validated the effectiveness of using elastic media to alleviate the Si electrode degradation in the cells with liquid electrolytes 17,18 .Therefore, it is reasonable to speculate that even though the elastic solid electrolyte cannot completely prevent such deformations and fractures, it can effectively encase the active material when volume changes occur, thereby maintaining the efficient Li-ion transport."

The microphase separation in copolymer to increase mechanical strength is typically achieved using block copolymer. Is there a specific reason why a random copolymer was used in this work? Additionally, what are the monomer reactivity ratios of AM and DMAM monomer in a random copolymerization?
Response: We thank the reviewer for this comment.As the reviewer points out, the microphase separation inside the elastic electrolyte is the key to its superior mechanical properties.The reason for the selection of a random copolymer is that different segments play distinct roles.To be more specific, the AM-rich segments contain abundant hydrogen bonds (as shown in Fig. 2a) which contribute to high strength and energy dissipation property; while the DMAM-rich segments feature weak interchain interactions and thus induce high deformability.We have added the relevant discussion in the manuscript and it has been highlighted in yellow; please see page 7 of the revised manuscript.
In addition, the monomer reactivity ratios of AM and DMAM is approximately 0.78 and 1.11, respectively 21 .Both of the reactivity ratios are close to 1, therefore contributing to a random copolymerization behavior of the polymer network 22 .
3. The polymer electrolyte with high ionic conductivity of 2×10 -3 S cm -1 and reinforced mechanical properties is expected to demonstrate improved Li plating and stripping performance at elevated current density, as well as superior Li deposition, in comparison to the results shown in Fig. 3 (0.1 mA cm -2 , 0.1 mAh cm -2 ).The rate performance of should be provided to further highlight its properties.
Response: We thank the reviewer for the insightful comment.According to the reviewer's suggestion, we provide the rate performance of the Li/elastic electrolyte/Li symmetric cell without external stack pressure and the Li plating and stripping performance with a higher current density.As demonstrated in Fig. R8a, the overpotential of the symmetric cell increased with the elevated current density.Nonetheless, no indication of shortage arose even when the current density was up to 1 mA cm -2 .Furthermore, the symmetric cell realized a stable operation for over 350 hours at 0.2 mA cm -2 , 0.2 mAh cm -2 (Fig. R8b).We have added these data and the relevant discussion in the revised manuscript; please see page 8 and Supplementary Fig. 13.

In Fig. 3b, it seems that the interfacial resistance is larger compared to the bulk resistance in Li symmetrical cell. What could be the reason for this?
Response: We thank the reviewer for this comment.In the Li symmetric cell, the bulk resistance is usually determined by the ionic conductivity of the electrolyte, while the interfacial resistance depends on the Li + transportation through the solid electrolyte interface (SEI).The elastic electrolyte possesses a high ionic conductivity of 2×10 -3 S cm -1 at ambient temperature.Therefore, the bulk resistance of the Li symmetric cell is as low as 11.6 ohm cm 2 .In contrary, the SEI consists of decomposition products of the electrolyte, which generally have much lower ionic conductivities than the bulk electrolyte.That means it is much more difficult for Li + to pass through the SEI than transport in the bulk electrolyte.As a result, the interfacial resistance is larger than the bulk resistance, which is a common phenomenon in Li symmetrical cells 23,24 .Fortunately, SEI between Li and the elastic electrolyte becomes stable after 50 hours (Supplementary Fig. 14) and hinders further progress of the interface reaction, so that the Li symmetrical cell displays a long-term stability for 1800 hours.
5. In Fig. 5a, the cycle performance of μm-Si half-cell was evaluated at 0.4 C with a lower cutoff voltage of 50 mV after the initial cycles, demonstrating a discharge capacity of 1039 mAh g -1 .However, a higher capacity of ~2000 mAh g -1 can be expected with a lower cutoff voltage of 50mV and low overpotential.How would the cycle performance be affected by a reduced C-rate and a higher discharge capacity, considering more severe expansion and shrinkage of Si particle?
Response: We thank the reviewer for this valuable comment.We have conducted galvanostatic discharge-charge experiment with a reduced C-rate on the μm-Si/elastic electrolyte/Li cell without external pressure.As illustrated in Fig. R9, the fabricated cell achieved a high reversible specific capacity of 2909.7 mAh g -1 at 0.1C (1C=3579 mA g -1 ) and maintained 2543.2 mAh g -1 after 35 cycles.This result indicates a good stability of the μm-Si with the elastic electrolyte, even though it underwent more severe volume fluctuation.

The computational details of the finite element method to generate Figure 5b is not provided. Please add.
Response: We thank the reviewer for this thoughtful reminder.We have provided the computational details of the finite element simulations in Method section; please see page 17 in the revised manuscript.It has been highlighted in yellow in the revised manuscript and copied here: "The stress distribution and evolution in the μm-Si electrode with Li6PS5Cl and the elastic electrolyte were simulated through the finite element method.Firstly, models with randomly distributed Si spheres were established.The models were constructed based on the linear elasticity assumption, of which the relevant parameters of Si during lithiation can be found in the previous work 53 .The implicit algorithm method was used and gradually iterated to the maximum expansion of Si to avoid convergence difficulties."

What is the capacity ratio of LFP to μm-Si in an LFP/μm-Si full cell? This information is crucial as the utilization rate of Si anodes is considered to have a significant impact on the cycle characteristics of full cells.
Response: We thank the reviewer for this insightful comment.The loading of LFP was approximately 6 mg cm -2 .Therefore, the capacity ratio of μm-Si to LFP was 2.4 based on their theoretical specific capacities (170 mAh g -1 for LFP and 3579 mAh g -1 for μm-Si).According to the reviewer's suggestion, we have information about the loading of LFP in the "Cell assembly and Electrochemical tests" part in Methods; please see page 16-17 in the revised manuscript.It has been highlighted in yellow in the revised manuscript and copied here: "As for the coin-type μm-Si/elastic electrolyte/LFP cells, LFP cathode was prepared by mixing LFP, acetylene black and PVDF in a weight ratio of 8:1:1 in N-methylpyrrolidone.The homogenous slurry was coated on the Al foil by doctor blade with the LFP loading of around 6 mg cm -2 ."

Reviewer #3:
The need for external pressure is a major obstacle to the application of all-solid-state batteries in practice, but the authors raised an effective strategy to alleviate this issue, with convincing demonstration using the micrometer-Si anode.The reviewer considers it as a highly important contribution to the all-solid-state battery community.However, before it can be accepted for publication, the following issues need further clarification.
Response: We would like to appreciate the reviewer for recognizing and highly praising the importance of our work.
1.The authors indicate that the synthesis of the elastic polymer electrolyte is conducted in Ar-filled glovebox.Is it unstable in ambient air?Which component in air will react with this material?Is it possible to improve its air stability in future studies?This directly influences the production cost, which is also rather important to the successful commercialization of all-solid-state batteries.
Response: We thank the reviewer for this comment.The elastic electrolyte is stable in dry air.In fact, the preparation of μm-Si/elastic electrolyte/LFP pouch cells as well as the measurements of mechanical properties of the elastic electrolyte were completed in the dry room with a dew point of -40°C.No decomposition or performance degradation occurred to the elastic electrolyte after storing in the dry room.However, in ambient air where contains much moisture, the elastic electrolyte tends to absorb water due to the hydrophily of the copolymer network and LiFSI.The narrow electrochemical stability window of H2O and its high reactivity toward electrodes and may lead to the degradation of batteries 25 .This is a common issue confronting the lithium-ion battery electrolytes 26 .Fortunately, dry rooms have been extensively used in the lithium-ion battery manufacturing industry and the cost has been controlled effectively.In future studies, constructing hydrophobic interphase on the electrode materials to improve their stability toward moisture 27 or adopting moisture-tolerating component such as ionic liquids in the electrolyte 25 can be potential solutions.
2. The authors claim that their elastic polymer electrolyte shows an oxidation potential of 4.5 V vs. Li/Li + , but the cell for cycling tests utilizes a 3 V-class material, LiFePO4, as the cathode.The reviewer would like to see the cycling performance of the cell where the elastic polymer electrolyte is paired with the 4 V-class cathodes like LiCoO2 or LiNi0.8Mn0.1Co0.1O2.
Response: We thank the reviewer for this insightful comment.We have applied the elastic electrolyte in the NMC cathode and conducted galvanostatic cycle tests on the NMC/elastic electrolyte/Li cells without external pressure.To fabricate the NMC electrode with the elastic electrolyte, the precursor solution of the elastic electrolyte was dripped onto the NMC cathode coated on the Al foil, followed by a vacuum infiltration process and then UV polymerization.As illustrated in Fig. R10, the cell possessed a specific capacity of 208.6 mAh g -1 with an initial coulombic efficiency of 82.6% and could function normally for dozens of cycles, but the cycle stability still needed further improvements.It can be seen from Fig. R11 that NMC particles retained in tight contact with the elastic electrolyte after cycles, which ensured the rapid Li + transport inside the cathode.Consequently, it is reasonable to infer that the capacity decay was not caused by contact failure between the electrolyte and active materials, even though the cells were cycled with the exemption of external pressure.The capacity decay may be caused by the fact that the charge cutoff voltage of the cell (4.4 V versus Li + /Li) was quite close to the upper limit of the electrochemical stability window of the elastic electrolyte (4.5 V versus Li + /Li).In order to further enhance the electrochemical performances of the NMC/elastic electrolyte/Li cell, we employed 1 wt% LiPO2F2 as the additive in the elastic electrolyte in the cathode to construct a more stable cathode electrolyte interphase (CEI) 7 .The modified NMC/elastic electrolyte/Li cell working under no external stack pressure delivered a specific discharge capacity of 207.1 mAh g -1 with an increased initial coulombic efficiency of 85.4%.Moreover, the cycle stability was meliorated and the cell maintained a capacity retention of 77.4% after 90 cycles (Fig. R12).The improved electrochemical performances were attributed to formation of the more robust CEI.It is reasonable to speculated that optimizing the type and amount of the additive could further enhance the compatibility of the elastic electrolyte with NMC.Whereas, this work is meant to focus on using the elastic electrolyte to tackle the mechanical failure issue of batteries working without high stack pressure. ) that can be actually achieved in the cell is more meaningful.The desired mechanical properties reported here could also be helpful in reaching higher areal capacity.Therefore, the reviewer suggests the authors adjust the mass loading of micrometer-Si and investigate the maximum areal capacity that the micrometer-Si anode can achieve in the μm-Si/elastic electrolyte/Li cell with decent cycling stability.

The authors specified the areal mass loading of micrometer-Si in their cells, but the areal capacity (in mAh cm
Response: We thank the reviewer for this comment.In response to the suggestion from the reviewer, we conduct galvanostatic discharge and charge tests on solid-state μm-Si with higher loading of 1.3 mg cm -2 under no external stack pressure.The μm-Si/elastic electrolyte/Li cell was tested at 0.2C and 0.3C (1C=3579 mA g -1 ).As illustrated in Fig. R13, the cell delivered an initial discharge capacity of 1377.8 mAh g -1 with the coulombic efficiency of 83% at 0.2C (i.e.0.9 mA cm -2 ).With an elevated current density to 0.3C (i.e.1.4 mA cm -2 ), the reversible specific capacity decreased to 757 mAh g -1 .The reduced specific capacity may be caused by the enlarged overpotential of the Li counter electrode.It can be seen from the rate performance of the Li/elastic electrolyte/Li symmetric cell that the overpotential increased with the current density (Fig. R14).
After 50 cycles, the μm-Si/elastic electrolyte/Li cell maintained 631.3 mAh g -1 , corresponding to a capacity retention of 83.4%.Although the cycle life of the μm-Si/elastic electrolyte/Li cell is curtailed with the increase of mass loading, it can be seen that the proposed elastic electrolyte remains effective to enhance the stability of the high-loading solid-state μm-Si electrode operating without external pressure.However, our work in this paper differs significantly from the above situation.Nmethylacetamide (NMA), serving as the solvent, is initially solid at room temperature and liquifies when mixed with LiFSI to form a deep eutectic material (DEM).This DEM is indeed liquid in its free state.Still, after binding with the polymer formed by copolymerization with dimethyl acrylamide (DMAM) and acrylamide (AM), it is constrained and coordinated by the large molecular chains inside the polymer.The surrounding may slightly deviate the DEM from its deep eutectic point.This interaction renders the deep eutectic electrolyte no longer possessing the physical properties of free solvent molecules, but rather exhibiting a tendency towards solid-state characteristics.This interaction also imparts excellent mechanical properties to the elastic electrolyte mentioned in this work.Evidence for this phenomenon can still be obtained through thermal gravimetric experiments on the material.We can observe the solvent evaporation characteristics of the free-state and polymer-bound DEM through thermal gravimetric experiments.As shown in Figure R2, the NMA solvent in the free-state deep eutectic electrolyte evaporates in the temperature range of 82.2 to 220 ℃.The LiFSI salt undergoes decomposition between 255 and 325 ℃.However, when the deep eutectic electrolyte is combined with the specific polymer electrolyte to form the elastic electrolyte reported in this work, all transition temperatures experience a significant increase.Below 200 ℃, there is almost no liquid evaporation.NMA solvent evaporation occurs only when the temperature rises to the range of 204 to 320 ℃.The decomposition temperature of LiFSI salt also increases to the range of 325 to 400 ℃.The significant increasements in the evaporation temperature of NMA and decomposition temperature of LiFSI can be attributed to the effects of confinement and coordination of the eutectic mixture inside the copolymer framework, which makes it distinct from conventional free-state liquid additives in gel electrolytes.
The phenomenon of solidification characteristics observed in this deep eutectic electrolyte after interacting with specific polymers is a significant discovery emphasized in this paper.Based on this phenomenon, the elastic electrolyte also exhibits excellent mechanical properties.The mass ratio of the solvent N-methylacetamide (NMA) in the practical elastic electrolyte is close to 40%.However, contrary to conventional definitions that consider any addition of liquid solvent to a polymer as constituting a gel electrolyte, and relying on the traditional approach of defining the degree of solidification based on the amount of added liquid electrolyte, we believe these conventional definitions are not suitable for our work.Therefore, this elastic electrolyte can still be considered a solid-state electrolyte.Of course, we also do not refer to it as an all-solid-state electrolyte.
General question 2: Is the low specific capacity attributed to poor electron or ion transfer?Can the polymer electrolyte reported in this work lead to a higher capacity utilization of NMC?
Response: Thank you for this comment.In the mentioned reference (Science 2021, 373, 1494-1499), the specific capacity of the NMC811 is calculated to be only ~80 mAh g -1 in the μm-Si/NMC cell.It is reasonable to attribute the low specific capacity to the sluggish ion transfer inside the cathode for the following reasons.Firstly, the poor point-to-point contact between Li6PS5Cl electrolyte particles and NMC811 may fail to provide sufficient ion transport sites.In addition, the high Young's modulus of the electrolyte (20-30 GPa for Li6PS5Cl, cited from J Power Sources 483 (2021)) makes it difficult to deform and create intimate interfacial contact with NMC811.By contrast, the elastic electrolyte can provide sufficient ion transport inside the cathode, thus lead to a higher specific capacity of NMC811 (207.1 mAh g -1 with an initial coulombic efficiency of 85.4%, as shown in Fig. R3).This is because that the precursor solution of the electrolyte can fully infiltrate the porous electrode by a vacuum infiltration process and the elastic electrolyte with high ionic conductivity tightly surrounds the active materials after UV polymerization.Furthermore, as shown in Fig. R4, NMC particles retained in compact contact with the elastic electrolyte after cycles, which ensured the rapid Li + transport inside the cathode.In this way, the high capacity of the NMC811 can be fully released.Response: Thank you for your comment.The reasons we categorize the elastic electrolyte as solidstate (not all-solid-state) have been explained above.We sincerely hope that you are satisfied with the response.

Fig. R1 .
Fig. R1.(a) The charge-discharge curve of the 1 st , 5 th , and 10 th cycle and (b) the cycling stability test of the NMC/elastic electrolyte/Li cell without external pressure.

Fig. R2 .
Fig. R2.Scanning electron microscopy images of the NMC electrode with the elastic electrolyte after cycles.

Fig. R3 .
Fig. R3.(a) The charge-discharge curve of the 1 st , 5 th , and 10 th cycle and (b) the cycling stability test of the Li/elastic electrolyte/NMC cell with 1 wt% LiPO2F2 as the additive in the cathode tested without external stack pressure.

Fig. R4 .
Fig. R4.FTIR spectroscopy of the N-methylacetamide-LiFSI mixture, the precursor solution of the elastic electrolyte and the polymer after UV polymerization.

Fig
Fig. R5.(a) Raman spectra of the N-methylacetamide, the N-methylacetamide-LiFSI mixture and the elastic

Fig. R6 .
Fig. R6.The galvanostatic discharge and charge test on the μm-Si/elastic electrolyte/Li cell with a μm-Si loading of 1.3 mg cm -2 at 0.2C and 0.3C without external stack pressure.

Fig. R7 .
Fig. R7.Rate performance of the Li/elastic electrolyte/Li symmetric cell with an areal capacity of 0.1 mAh cm -2 under no external stack pressure.

Fig. R8 .
Fig. R8.(a) Rate performance of the Li/elastic electrolyte/Li symmetric cell with an areal capacity of 0.1 mAh cm - 2 .(b) Galvanostatic Li plating and stripping profiles of the Li/elastic electrolyte/Li symmetric cell at 0.2 mA cm -2 , 0.2 mAh cm -2 .The symmetric cells were tested without external stack pressure.

Fig. R9 .
Fig. R9.(a) The charge-discharge curve of the 1 st , 2 nd , 15 th , and 25 th cycle and (b) the cycling stability test of the μm-Si/elastic electrolyte/Li cell at 0.1C without external stack pressure.

Fig. R10 .
Fig. R10.(a) The charge-discharge curve of the 1 st , 5 th , and 10 th cycle and (b) the cycling stability test of the NMC/elastic electrolyte/Li cell without external pressure.

Fig. R11 .
Fig. R11.Scanning electron microscopy images of the NMC electrode with the elastic electrolyte after cycles.

Fig. R12 .
Fig. R12.(a) The charge-discharge curve of the 1 st , 5 th , and 10 th cycle and (b) the cycling stability test of the Li/elastic electrolyte/NMC cell with 1 wt% LiPO2F2 as the additive in the cathode tested without external stack pressure.

Fig. R13 .
Fig. R13.The galvanostatic discharge and charge test on the μm-Si/elastic electrolyte/Li cell with a μm-Si loading of 1.3 mg cm -2 at 0.2C and 0.3C without external stack pressure.

Fig. R14 .
Fig. R14.Rate performance of the Li/elastic electrolyte/Li symmetric cell with an areal capacity of 0.1 mAh cm -2 under no external stack pressure.

Fig. R3 .
Fig. R3.(a) The charge-discharge curve of the 1 st , 5 th , and 10 th cycle and (b) the cycling stability test of the Li/elastic electrolyte/NMC cell with 1 wt% LiPO2F2 as the additive in the cathode tested without external stack pressure.

Fig. R4 .
Fig. R4.Scanning electron microscopy images of the NMC electrode with the elastic electrolyte after cycles.

Fig. 2b
Fig.2bSince no crosslinking was seen, the electrolyte system is composed of a polymer and deep eutectic mixture.I prefer gel instead of solid-state.