Mesoporous SiO 2 anode armour for lithium oxygen battery

We are reporting a new lithium (Li) anode protective strategy using mesoporous silicon dioxide (mSiO 2 ) to extend the operation lifetime for Li oxygen batteries (LOBs). The fabrication method is simple and easy for mass production. The mSiO 2 protective layer inhibits the growth of Li dendrites and prevents the intrinsic Li corrosion phenomenon that occurs in all Li-metal-based batteries during charge/discharge cycles, the porous structure of nanoparticles within the mSiO 2 also actively created parallel pathways for Li + transport thus enhancing the mass transfer. Propylene carbonate/Li with a 100 μ L drop amount of mSiO 2 protective layer (mPL100) showed excellent per-formance in the Li||Li battery charge/discharge cycle test, withstanding 2,276 h of continuous operation, almost 16 times longer than the battery without the protective layer


Introduction
Aprotic lithium oxygen batteries (LOBs) have received growing attention as energy storage media owning to their well-known ultra-high theoretical energy density (≈3,600 Wh kg − 1 ), but challenges for further development still remain, such as the slow decomposition kinetics of lithium peroxide (Li 2 O 2 ), or passivation of the electrode surface due to thin film-like products, decomposition of the carbon cathode, electrolyte cracking and multiple problems with the lithium anode [1][2][3][4][5][6][7][8][9][10][11][12][13][14].Lithium metal (Li) has a very high theoretical specific energy (3,860 mAh g − 1 ) and a low oxygen reduction potential (-3.04 V vs. standard hydrogen electrode), which makes it an ideal material for battery anode, but due to its own characteristics, Li is prone to have dendrite growth and change in volume during battery operation.This accelerates the consumption of Li and increases the internal resistance of the battery, leading to premature failure [15][16][17][18][19][20][21][22][23].
Li anode protection strategies have been researched for decades, with reports including (1) electrolyte modification, which has been extensively studied for its process, viability and economic feasibility.According to the Frontier molecular orbital theory, Li is chemically active in most electrolytes, however, the reaction products cause passivation that prevents further reactions between Li and the electrolyte.Therefore, Li can be effectively protected by using a well-designed electrolyte [24][25][26][27][28][29][30][31]; (2) Introducing skeleton structure into Li anode to alleviate Li volume expansion, also effectively inhibit Li dendrites growth and improve cycle life [32]; (3) Chemical pretreatment methods are used to fabricate thin-film on the top of Li surface through chemical reactions between Li and precursors.Gases (e.g.N 2 , CO 2 and O 2 ) introduced to the Li surface will cause a passivation effect and then form a protective layer.Koch et al. calculated the passivation effect from different gases and to Li surface in terms of finding the optimised density and porosity etc. properties [33].However, due to the high reactivity between the gas and Li, the preparation process requires precise control of the reaction time and temperature.(4) Compared with the chemical pretreatment method, the electrochemical pretreatment method can yield an interfacial protective film with very similar components and structure to the real solid-state membrane electrolyte, which includes a better protection effect on the Li surface.This protective film formed by adding 4-Fluoro-1,3-dioxolan-2-one (FEC) not only maintains the properties of the solid-state membrane electrolyte for a long period but also protects the Li anode in LOBs [34].However, the preparation process of the existing electrochemical pretreatment method is rather complicated and requires advanced pretreatment which involves other side reactions.(5) physical pretreatment, the physical pretreatment method is facile and suitable for large-scale operation, these include scraping, magnetron sputtering and atomic layer deposition etc.The materials used are mainly inorganic and organic and are used to simulate SEI film components, of which Al 2 O 3 has received much attention [35][36][37], and the approximately 2 nm Al 2 O 3 protective layer prepared by atomic layer deposition can lead to dendrite inhibition [38,39].Luo et al [40] developed a simple method to construct a thin layer of SiO 2 / graphene oxide (SiO 2 /GO) on the Li anode surface to prevent Li dendrites growth and Li corrosion.The uniform pore retention layer guides the Li + flux through the microstructure during dissolution/deposition to prevent Li dendrite growth, resulting in a significant improvement in LOB cycle life, but the addition of GO increases the ion resistance, but the non-porous SiO 2 itself cannot yet achieve the desired effect.
Mesoporous silica nanoparticles (mSiO 2 NPs) have adjustable morphology and particle size, controllable structure, large specific surface area, ease of preparation, low cost and have been widely used in the application of catalysis, chemisorption and environmental protection etc. [41,42].Herein, we prepared mSiO 2 protective layer adjacent to the Li surface with a parallel pore structure for LOBs, it has significantly improved the Li anode process and produced some optimization effects on the cathode operation process, significantly enhancing the LOB performance.The mSiO 2 has a hexagonal mesoporous structure that actively improved the mass transfer of Li + and the capacity, it also prevented the growth of Li dendrites and then protected the Li anode, this protective layer adsorbs/blocks intermediates (e.g.O 2 , LiO 2 reacted with traces of H 2 O, O 2 , etc.) induced corrosion dissolution/ deposition process, thus improved the cycle life.

Preparation oflithium silica protective layer
Fig. S1a shows the synthesis procedure of mSiO 2 NPs.201 mg CTAB, 10,170 mg anhydrous ethanol, 128,000 mg deionised water and 100 mg NaOH were added to a 250 mL round bottom flask in sequential order and under magnetic stirring at 600 rpm for 0.5 h, the temperature was maintained using a silicone oil bath at 30 • C. 1,000 mg TEOS was then added dropwisely to the mixture and the reaction continued for 5 h.After standing for 24 h, the mixture was centrifuged and washed, the precipitate was collected and dried in a blast oven at 80 • C and ground to a powder, then calcined in a muffle furnace at 550 • C for 2 h to obtain the mSiO 2 powder used in this paper.
Fig. 1a shows the preparation of mSiO 2 /PC/Li.The prepared mSiO powder was further heat-treated at 180 • C for 24 h in a vacuum drying oven to remove any adsorbed water from the surface of the mSiO 2 NPs.The mSiO 2 powder was then added to 0.1 mol/L LiClO 4 /PC solution, mixed using an ultrasonic sound bath for 0.5 h to make a solution with concentration of 10 mg mL − 1 .mSiO 2 /LiClO 4 /PC dispersion was added dropwisely onto the polished Li sheets with 10 μL, 30 μL, 50 μL, 70 μL, 100 μL, 120 μL, 150 μL, 200 μL, etc., which are denoted as mPLX in the text (X stands for 10, 30, 50, 70, etc.), followed by drying in an argon atmosphere glove box for 3 days to ensure the PC liquid on its surface has completely evaporated.3-5 pieces of the electrodes are placed between two flat and smooth glass sheets and pressed with a 750 g metal block for 0.5 h to bind the mSiO 2 NPs onto the Li surface, and then placed on a heating table to dry at 60 • C for 2 days, followed by drying at room temperature (25 • C) for 3 days for further use.

Characterization of mSiO 2
Fig. 1b shows the SEM images of mSiO 2 NPs with spherical and ellipsoidal morphologies.Fig. S2 further shows the long-axis particle size distribution of mSiO 2 NPs with an average particle size of 190 ± nm (N = 100).Fig. 1c and 1d show the high-resolution TEM images of the mSiO 2 NPs and the closely spaced parallel distribution of the pore structure.Fig. S1b shows the TG/DSC analysis of mSiO 2 particles, and the DTG curve in the result shows that the sample has a heat absorption peak in the range of 220-300 • C, indicating an endothermal process.The weight loss of the sample can be divided into three stages: 1) before ℃, the weight loss was about 10.35 %, which was caused by the removal of ethanol and adsorbed water on the surface of the NPs; 2) between 200 • C and 350 • C, a weight loss of ~ 34.51 % mainly attribute to the decomposition and desorption of the polymer template [43]; 3) above 350 • C, the weight loss of the sample is attributed to the desorption of the remaining template or the loss of water due to the condensation of silanol groups to form siloxane bonds [44].Fig. S1c shows the FTIR analysis of mSiO 2 particles before and after calcination, which shows that CTAB has been completely removed after calcination (both products show bending vibration peaks near 445 cm − 1 , symmetric stretching vibration peaks near 792 cm − 1 , and Si-O-Si anti-symmetric stretching vibration peaks near 1,065 cm − 1 [45], results indicate the presence of SiO 2 .Comparing the pre-calcination data (red line), it was found that the antisymmetric stretching vibration peaks and symmetric stretching vibration peaks of -CH 2 -are near to 2,924 cm − 1 and 2,852 cm − 1 and the stretching vibration peaks of H-C-H chains in CTAB is near to 1,473 cm − 1 that disappeared after calcination (cyan line), showing that the CTAB template agent was removed after calcination [46].Fig. S1d further confirms the presence of amorphous SiO 2 as shown at 23 • on XRD spectrum [47], and in the small-angle XRD analysis in Fig. S1e, the hexagonal ordering of the mesoporous structure is illustrated by XRD analysis of two peaks at 2 • -3 • and 4 • -5 • [48].Fig. S1f shows the typical type IV isotherm pattern in the nitrogen adsorption/desorption test, confirming the presence of mesoporous structures [49,50].Fig. S1g shows the pore distribution of the synthesised mSiO 2 particles with a narrow pore distribution and an average pore size of 3.343 nm calculated using the BJH model.The pore volume is 0.5672 cm 3 g − 1 and the BET specific surface area is 1,034.74m 2 g − 1 .
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Characterization of the mSiO 2 protective layer
Fig. 1e shows an SEM image of the PL surface, showing that the surface of Li soaked in 0.1 M LiClO 4 /PC is smooth with only a very thin passivation layer that is smoother than the Pristine Li surface (Fig. S3), suggesting that the 0.1 M LiClO 4 /PC soaked Li had some protective effect.Fig. 1f and 1 g show the cross-sectional SEM images of PL and mPL100, respectively.It can be seen mPL100 has a flat profile similar to that of unprotected PL, which guarantees uniformity of Li + flux, and Table S1 provides load statistics for mPL mSiO 2 NPs with different titrations.Fig. 1h shows the as prepared mPL100 surface that is completely covered by mSiO 2 NPs.Tiny voids can be observed surrounding the particles that facilitate electrolyte wetting and Li + transport, providing a multidimensional pathway for Li + flux to be Fig. 1.Protection of Li anode by mPLs (a); SEM (b), TEM (c) and (d) images of mSiO 2 ; SEM images of PL surface (e) and cross-section (f); SEM images of mPL100 cross-section (g) and surface (h) and element mapping of Si (i) and O (j); Nyquist plot (k) and resistance value (l) of mPLs with different loading amounts of mSiO 2 ; and the Li + transfer number (m).
H. Mu et al. homogeneously distributed, and avoiding high local current density, thus inhibiting dendritic growth during the stripping/plating of Li.In addition, the mesoporous structure of mSiO 2 can leach electrolyte effectively, thus promoting the Li + conductivity, which inhibits the conduction of corrosive substances.Fig. 1i and 1j show the surface EDX mapping scan of mPL100.It can be seen from the distribution of Si and O elements in the diagram, mSiO 2 NPs are homogeneously distributed on the lithium surface with a thickness of 15 μm without obvious agglomeration.Fig. S4 shows the SEM and EDX analysis of the mPL surfaces with the increasing addition amount of mSiO 2 in 10 μL, 30 μL, 50 μL, 70 μL, 150 μL, 200 μL, as shown in the figure, the protective layer shows a gradual dense trend as the amount of mSiO 2 increases, and when the amount exceeds 150 μL, the mSiO 2 NPs appear agglomerated, which will affect the ion transport on the lithium surface.Therefore, it is concluded that the addition of 100 μL is the optimized amount.Fig. 1k-l show the Nyquist curves for all mPLs conductivity resistance and specific value.The intercept of Nyquist solid axis represents Li + impedance (Rs) from electrolyte solution diffusion to Li [28,51].Fig. 1l shows a narrower variation between different titration curves and a small difference in transverse coordinates, suggesting that Li + conduction resistance does not significantly increase with increasing titration to Li + conductivity to NPs.Calculated the ionic conductivity (1.93*10 -4 S cm − 1 ) and Li ion migration number (~0.6) of mPL100 (Fig. 1m).The higher ionic conductivity and Li ion migration number are attributed to the continuous distribution of mSiO 2 , its porous structure, and large internal surface area.These factors facilitate the interaction with anionic Lewis acids-bases effect in lithium salts, leading to a high degree of segregation of lithium [52,53].As shown in Fig. S5b-c, density functional theory (DFT) was used to calculate the most favourable interaction site (adsorption energy is − 0.57 eV) between the (0 0 1) surface of SiO 2 and Li atoms.The simulation results revealed the interaction between Li +/0 and SiO 2 , which means that the Li + solvation environment near the SiO 2 surface has changed, and the solvent has little effect on Li + coordination.Additionally, free Li + in the electrolyte will be adsorbed on the SiO 2 surface [54], According to theoretical calculations, mPL100 helps to reduce the influence of solvents on Li + and improve the efficiency of Li + transport.Fig. S5d-i

mPL100 and PL lithium symmetric (Li||Li) battery testing and characterization
Fig. 2a shows the Li||Li battery tests with mPL100 and PL, where the symmetric battery with mPL100 can withstand up to 2,276 h, while the Li||Li battery with PL can only run 144 h.The electric potential difference between the two batteries at different operating times also shows mPL100 can significantly reduce the overpotential.Fig. 2b-c show that the charge/discharge potential difference between the mPL100 symmetric cell is approximately 0.031 V over 0-8 h, compared to 0.142 V for the PL symmetric cell, which is 4.5 times greater than that of mPL100.Within 140-146 h, the potential difference was 0.046 V and 0.281 V, respectively, and the charge/discharge potential of the PL symmetric cell increased 6.1 times to the mPL100 Li||Li cell, which is similar to the result in Fig. 2a, further demonstrating the significant effect of the mSiO 2 protective layer in the lithium stripping/plating process.Fig. S6 presents the detailed cyclic performance of the mPL100 Li||Li cells at 200 h, 500 h, 800 h, 1000 h, 1500 h, and 2000 h of operation.Fig. S7 shows the discharge performance of Li||Li cells with PL and mPL100 at a current density of 0.3 mA cm − 2 .The Li||Li cells with PL fails after only 28 h of operation, whereas the mPL100 one can cycled for 450 h, indicating a significant improvement in Li ion insertion/extraction. Perez-Beltran et al. suggested that lithium may present on SiO 2 surfaces via Si-O bond cracking with a ratio of 3.48 between Li/Si [56].The lithium surface is coated with mSiO 2 having a high specific surface area and the Li + flux is regulated by the interaction between Li + and mSiO 2 , thus homogenising the lithium stripping/plating.Fig. 2d-f show SEM images of the lithium surface of a PL symmetric cell after 100 h, 150 h and 200 h of operation.Fig. 2d shows that a number of branching crystals formed on the surface of the lithium sheet and produced localised bumps, which may be due to the non-uniform deposition of lithium at high localised current densities.In Fig. 2e, the surface of Li is clearly cracked and has separated from the Li, indicating that dead lithium has been produced.The surface cracking of Li in Fig. 2f is further exacerbated by signs of fragmentation and crushing.The phenomena in Fig. 2d-f also correspond to the changes in the PL symmetric cell cycle curve in Fig. 2a, where polarisation is severe and the charge/discharge potential difference gradually increases until the cell fails due to the heterogeneous stripping/plating of Li.Fig. 2g-h show the surface SEM of the mPL100 symmetric cell at 100 h, 150 h of operation respectively.Fig. 2g shows that the surface of Li is still smooth after 100 h, similar to the original Li.Fig. 2h shows a slight bulge on the Li surface, but the lithium sheet as a whole remains treated and flat.Fig. S8a-f show the surface morphology of Li after the mPL100 symmetric cell was run up to 200 h, 500 h, 800 h, 1,000 h, 1,500 h and 2,000 h, at which point some bulges and holes were produced on the Li surface, indicating that the local current density on the Li surface gradually increased, resulting in uneven Li + deposition and causing volume changes and local holes, but Li surface remained flat.The pores on the Li surface show a slow rise as the cycle progresses, but generally maintain their original flatness.Fig. 2i shows the surface morphology of Li after the mPL100 symmetric cell was run to 2,280 h, at which point surface cracks and bumps became more apparent and surface flatness decreased.These results suggest that the mSiO 2 protective layer contributes significantly to homogenising the lithium stripping/plating, inhibiting the generation of dendrites growth and dead lithium generation, and improving the morphological changes on the lithium surface.

mPL100 and PL lithium oxygen cell (LOBs) testing
Fig. 3a shows the cycling results of LOBs with mPL100, it has stable cycles and a charge/discharge potential difference of only about 0.5 V (at 500 mAh g − 1 ) in the first cycle, compared to 53 cycles for PL (Fig. 3b), with a potential difference of about 1.2 V in the first cycle, which is 2.4 times higher potential difference in the first cycle of the LOB with mSiO 2 protective layer, the difference between the two is only at the Li negative electrode, indicating that the optimisation of the negative electrode performance plays an important role in improving the LOB performance.Fig. 3c shows the discharge/charge open circuit potential for the mPL100 and PL LOBs.The more the charge/discharge potential deviates from the equilibrium potential value, the higher the polarisation of the cell, at which time some side reactions will occur within the LOBs, such as electrolyte cracking and polar decomposition of the carbon cathode, which in turn leads to corrosion of the Li anode by H 2 O, CO 2 or other by-products, resulting in a decrease in cell capacity; the increase in the charge potential also indicates that the cathode products accumulate severely after discharge and are difficult to decompose.Fig. S9 display the performance of mPL100 and PL LOBs with high MWNTs loading (0.7 mg cm − 2 ).The results show that, compared to the original loading, the cycle number fluctuates slightly but does not show a significant improvement with higher MWNTs loading.Fig. S10 demonstrates the cyclic performance of LOBs with PL and different mPL loadings (mPL10, 30, 50, 70, 150, and 200) and with 0.1 mg cm − 2 MWNTs loading.The cyclic numbers are 53, 89, 146, 188, 260, 382, 202 and 156 cycles respectively at current density 1 A g − 1 .Fig. 3d shows the LOBs multiplicative performance of mPL100 and PL.At current densities of 3A g − 1 and 5A g − 1 and fixed specific capacities of 1,000 mAh g − 1 , the LOBs of mPL100 and PL were tested for multiplicative performance.mPL100 cycled 181 times and 153 times respectively, while the LOB of PL cycled only 34 times and 26 times, an almost 4-fold improvement in LOB multiplicative performance using the LOB of mPL100 compared to the LOB of PL.Fig. S11 shows the cyclic performance at 3 times and 5 times rates as presented in Fig. 3d, along with the performance for mPL100 and PL at a 10 times rate.The cycle numbers for mPL100 and PL at 10 times rate are 68 and 3, respectively.Fig. 3e shows a 17-fold increase in full discharge capacity of 56,902 mAh g − and 3,008 mAh g − 1 for mPL100 and PL, respectively, in the oxygen environ -ment of the LOBs, while in argon environment (19 mAh g − 1 , mAh g − 1 , respectively，in Fig. S12).This is because the mSiO 2 protective layer provides sufficient and uniform ion channels to avoid excessive local current density during the discharge process, thus increasing the dispersion of the positive product and preventing premature passivation of the cathode electrode, thus increasing the discharge capacity.Fig. 3f displays the cycling test, rate performance and full discharge profiles of LOBs using mPL100, SiO 2 /GO/Li, pre-activation/  Li, and mSiO 2 colloidal electrolyte with the same electrolyte, separator, membrane, and cathode material.Compared to the other three systems, LOB with mPL100 has significantly enhanced the cycle performance (1A g − 1 ) by 34, 92, and 54 cycles, respectively.Compared to SiO 2 /GO/Li, LOB with mPL100 exhibits a substantial increase in rate performance using 3 A g − 1 and 5 A g − 1 current densities, reaching 98 and 97 cycles, respectively, while the pre-activation/Li showed only 9 and 7 cycles, respectively.However, LOB with mPL100 shows a full discharge capacity of 56,902 mAh g − 1 which is significantly higher than than SiO 2 / GO/Li (25,200 mAh g − 1 ), pre-activation/Li (6,240 mAh g − 1 ), and mSiO 2 colloidal electrolyte (47,600 mAh g − 1 ).

LOB characterization and analysis of PL and mPL100
Fig. 4a-c 4f (bottom spectra) shows the fine spectrum of O1s orbit, with peaks at 531.6 eV and 533.1 eV corresponding to Li 2 CO 3 * and RCO*CO 2 Li, respectively, and data show the absence of LiOH on the surface of LOBs lithium treated with mPL100 after 10 cycles, suggesting inhibition of Li corrosion production.Fig. S13 shows the detailed spectrum of Si2p orbit before and after the LOBs cycle of mPL100 for the corresponding Li sample XPS in Fig. 4 Based on XPS analysis of Li surface, the mSiO 2 protective layer absorbs or blocks corrosive substances for Li damage, effectively protects Li, and provides basic protection for LOBs long cycle.
Fig. 5a shows the anode surface morphology of PL after 1 cycle of LOB, where dead lithium have been observed.Fig. S14a shows the anode surface of PL after 15 cycles, there are clear protrusions and dendrites forming on the lithium surface.Fig. 5b shows the anode surface morphology of PL after 30 cycles, where there is a large amount of dead lithium and cracks have developed and severe volume deformation has occurred.Fig. 5c shows the anode surface morphology of PL after 53 cycles, at which point the lithium flakes have fragmented and pulverised.Fig. 5g, S14b and 5 h show the anode surface morphology of mPL100 LOB after 1 cycle, 15 cycles and 30 cycles.After 1 cycle and 15cycles the anode surface maintains a similar flatness compared to uncycled, and after 30 cycles the anode surface are also flat, till after 100 cycles (Fig. S14e) only a small amount of cycle negative products (e.g.LiOH) remained on the lithium surface.Fig. S14g-i show the LOB of mPL100 after 250, 300 and 350 cycles, respectively, mPL100 has significant dead lithium formation on the lithium surface after 250 cycles.Fig. S15 presents the mPL100 surface morphology after 370 cycles.There are intermediates observed on the surface, but it still remains relatively smooth without significant protrusions or cracks.This indicates that the mSiO 2 protective layer maintains good stability even after the cyclic test.Fig. 5i shows the LOB of mPL100 after cycling up to 382 cycles.Despite the presence of some dead lithium and cracks, the Li surface remains intact and no significant powder appears.It can be concluded that the presence of the mSiO 2 lithium protective layer has a significant role in homogenising the lithium peel/coat and preventing corrosive substances from damaging the lithium surface.Fig. 5m shows that the peak LiOH intensity of the Li sheet with mSiO 2 protection was significantly lower than that of the Li sheet without mSiO 2 protection after the 53rd cycle, and the LiOH peak intensity on the Li surface with mSiO 2 protection layer was still lower than that of the Li sheet without mSiO 2 protection layer until after the 382nd cycle.The cross-sectional thickness of the original Li sheet at approximately 349 μm.Fig. S19 shows the surface morphology of the pristine multi-walled carbon nanotubes (MWNTs) cathodes, at which point the carbon nanotubes are clearly visible and there are many voids between the carbon nanotubes.Fig. 6a show the surface morphology of the carbon nanotubes cathode after LOBs 1st discharge of PL, at which point the outer surface of the nanotubes is covered with many blocky products.Fig. 6b shows the surface morphology of the carbon nanotubes cathode after the first recharge of the LOBs for PL, at which point there were undecomposed lumps on the outside of the nanotubes, and by the 53rd discharge (Fig. 6c), the carbon nanotubes had been completely coated and the cathode had largely passivated.recharge of mPL100 respectively, and the surface morphology of the carbon nanotubes cathode after charge testing shows an increase in the discharge products compared to after the first cycle, but gaps remain between the carbon nanotubes, at which point most of the charged products decompose and only a small amount remains.Fig. 6h-i show the 100th discharge/recharge of mPL100 LOBs respectively.The surface morphology of the carbon nanotube cathode after the charging test shows that the discharge product increases gradually with the increasing number of cycles.Fig. 6j-k show the 200th discharge/recharge of mPL100 LOBs respectively, the surface morphology of the MWNTs cathode after charging test.Fig. 6l shows the mPL100 of LOBs on the surface of the MWNTs after the 382nd discharge, at which point the MWNTs were essentially coated with the product and the battery failed.XRD analysis (Fig. 6m) showed that the product was primarily LiOH.XRD data from Fig. 6m showed that LOBs using PL had a positive LiOH peak at 53rd cycle, indicating a larger pair of inverse pairs, while mPL100 had a more pronounced LiOH peak at the 382nd cycle, compared to which the side reaction of the battery was significantly reduced.From the morphological changes of MWNTs cathodes and product XRD analysis, it is concluded that mSiO 2 lithium protective layer promotes uniform dissolution and deposition of Li + on Li surface.This effect also has a positive effect on the formation and decomposition of Li 2 O 2 cathode products.Due to the uniformity of Li + flux, cathode products tend to form dispersed particles with smaller blocks, which are easier to be decomposed than larger ones.Since the carbon nanotubes are completely encased at a slower rate, this ensures both high discharge capacity and low charge voltage rise.This reduces the complex reaction within the cell due to high polarization, which is consistent with the pattern described above regarding the thickness of Li sheets and the GF diaphragm.
Fig. 7 shows the in-situ Raman spectra of the LOBs with PL and mPL100 after one discharge/charge cycle tested for 12 h.In Fig. 7a, the Raman spectra of the LOB with PL during the discharge of the batteries.There is a clear Li 2 O 2 peak (785 cm − 1 ) appears at 9 h, which reaches its Fig. 6.SEM image (a-l) and product XRD spectra (m) analysis of LOB on the surface of circulating MWNTs cathode, SEM images of LOBs with PL after the 1st discharge, the 1st recharge and the 53rd discharge (a-c); SEM images of LOBs with mPL100 after the 1st discharge, the 1st discharge, the 53rd discharge, the 53rd recharge, the 100th discharge, the 100th recharge, the 200th discharge, the 200th recharge, the 382nd discharge after failure (d-l); and XRD analysis of MWNTs surface after the cycles aforementioned (m).maximum intensity at 12 h, along with the appearance of LiOH (485 cm − 1 ) [57,58], indicating the presence of side reactions during discharge.In Fig. 7b, during charging of the LOB PL cell, the Li 2 O 2 peak gradually diminishes and almost disappears at the 9th hour, while the LiOH peak persists until 12 h, suggesting that LiOH, as a byproduct, is not easily decomposed.Similarly, in Fig. 7c, LOB with mPL100 cell shows a distinct Li 2 O 2 peak (785 cm − 1 ) at 9 h, which reaches its maximum intensity at 12 h during discharge.However, there is no observable LiOH peak, side reactions did not happen during discharge.Fig. 7d shows the Li 2 O 2 peak gradually diminishes and significantly weakens at 7 h, completely disappearing by 9 h during charging of the LOB mPL100 cell, suggesting a full decomposition of Li 2 O 2 .It can be seen that the uniform Li ion pathways in mPL100 facilitate the homogeneous deposition of discharge products on MWCNTs.Additionally, the improved Li ion transport properties of mSiO 2 lead to the generation of reversible products, suppression of Li dendrite growth, and mitigation of Li corrosion.On the cathode electrode side, mPL100 induces the homogeneous deposition of discharge products, promoting the formation of more reversible Li 2 O 2 thus reducing side reactions and contributing to an overall improvement in battery performance.

Conclusion
In summary, we describe a simple and easily scalable strategy to enhance the performance of LOBs in aprotic electrolytes by preparing a mSiO 2 protective to lithium anodes to precent dendrites growth and reduce the corrosion.mSiO 2 NPs have a large number of voids between them and a parallel distribution of through-hole structures inside the particles, thus providing sufficient pathways for Li + transport.Compared to unprotected PL, the Li||Li cell with mPL has an extended cycling time of 2,276 h, that is a significant increase to the unprotected LOB from 53 to 382, together with a substantial increase in multiplicative performance and full discharge capacity.Electrochemical and morphological characterisation showed that the mSiO 2 NPs layer significantly increased Li conductivity compared to non-porous NPs, uniformly stripped/plated Li during cell cycling, reduced local current density on the Li surface, and the protection against charging/discharging by-products, reducing negative capacity loss.The uniform Li + flux also promotes a uniform distribution of cathode products, thus reducing the overcharge potential and ultimately achieving a long battery life.

Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Kun Luo reports was provided by National Natural Science Foundation of China.Zhihong Luo reports financial support was provided by Guangxi Natural Science Foundation.Xiaoteng Liu reports financial support was provided by Engineering and Physical Sciences Research Council.

Fig. 3 .
Fig. 3. Charge/discharge profiles in the LOBs with mPL100 (a) and PL (b); cut-off voltages at different cycles of the LOBs with mPL100 and PL (c); rate performance of the LOBs with mPL100 and PL (d); full discharge capacities (e); and comparison with previous literature (f).

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Fig.
Fig. 5j-k show the LOB cross-sectional thicknesses of 307 μm and 280 μm after 1, and 30 cycles of mPL100, respectively, depleted by approximately 12 % and 20 %.The data show that the mPL100 LOB still has 44 % (155 μm) capacity after 382 cycles, while the PL without mSiO 2 protection has almost zero capacity remaining after 53 cycles, which further demonstrates the pattern of lithium surface changes.Figs.S16-S18 show the surface morphology of PL and mPL100 on LOBs cycled glass fibre diaphragms (GF), product XRD and comparison of the stability of lithium-nickel foam (Li-NF) cells under oxygen conditions

Fig. 5 .
Fig. 5. Surface and cross-sectional SEM images of the Li anodes with PL (a-f) and mPL100 (g-l) at different cycles; XRD analysis of the product on Li anodes after different cycles (m).

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Fig. 7 .
Fig. 7. In-situ Raman analysis of cathode products of the LOBs with PL (a,b) and mPL100 (c,d).

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