An in-situ polymerization strategy for gel polymer electrolyte Si||Ni-rich lithium-ion batteries

Coupling the Si-based anodes with nickel-rich LiNixMnyCo1−x−yO2 cathodes (x ≥ 0.8) in the energy-dense cell prototype suffers from the mechanical instability of the Li-Si alloys, cathode collapse upon the high-voltage cycling, as well as the severe leakage current at elevated temperatures. More seriously, the cathode-to-anode cross-talk effect of transitional metal aggravates the depletion of the active Li reservoir. To reconcile the cation utilization degree, stress dissipation, and extreme temperature tolerance of the Si-based anode||NMC prototype, we propose a gel polymer electrolyte to reinforce the mechanical integrity of Si anode and chelate with the transitional cations towards the stabilized interfacial property. As coupling the conformal gel polymer electrolyte encapsulation with the spatial arranged Si anode and NMC811 cathode, the 2.7 Ah pouch-format cell could achieve the high energy density of 325.9 Wh kg−1 (based on the whole pouch cell), 88.7% capacity retention for 2000 cycles, self-extinguish property as well as a wide temperature tolerance. Therefore, this proposed polymerization strategy provides a leap toward the secured Li batteries.

Notably, the CTP with high softening point (250 ℃) was chosen, which possess adequate toluene insoluble (TI), quinoline insoluble (QI) contents and less volatile species (Fig. S1c). 69  It is noteworthy that Si/C-7 with 7 µm CTP exhibit the caking effect, which is detrimental to the dispersion of the Si NPs and the electrode processing (Fig. S3c).In stark contrast, the Si/G and Si/G@C composites exhibit aggregated Si NPs distributed on the smooth surface of Gr spheres, where the Si NPs may penetrate the scanty carbon coating layer (Fig. S5).The Raman spectra of composites were compared in Fig. S7.The peak at around 518 cm −1 is originated from crystal structure of Si.The D band at ~ 1350 cm −1 represents the defect-induced vibrations (disordered carbon).The G band at ~ 1580 cm −1 represents the sp2 bonded carbon atom vibration, corresponding to the first order scattering of the E2g mode.Noted that the ID/IG ratio of Si/G/@C (1.18) is higher than From the full survey spectrum of X-ray photoelectron spectroscopy (XPS) analysis in Fig. S8a, Si, C, and O elements can be detected in the Si/C-5 composite.Fig. S8b shows the high-resolution XPS spectrum of C 1s, which can be resolved into three individual peaks: C=C at the binding energy of 284.4 eV, C-C at 285.5 eV, and C=O at 286.8 eV.Fig. S8c shows the high-resolution XPS spectrum of Si 2p, which could be deconvoluted into four peaks.The distinct peak positioned at 98.9 eV is ascribed to the Si-Si bond; while the peak located at 103.5 eV is assigned to Si−O bond due to the partly interfacial oxidation of the Si NPs.In addition, two small peaks are observed at          for 30 cycles and superior average CE value (98.9%) from the 3 rd cycle onwards.For comparison, the Si|LE displayed the initial ICE value of 81.6% and 0.7% CR value for 30 cycles.Obviously, the difference in performance is determined by the different characteristics of the electrolyte.In the Si|GPE anode, PVCM-GPE encapsulation acts as a stress buffering layer to alleviate the volume expansion of silicon, which ensures the mechanical stability of the active particles during the lithiation, thus mitigating the loss of active material.Furthermore, this structural stability also avoids active Li+ depletion due to repeated generation of SEI (Fig. S15b).From the photograph in Fig. S22, the Si/C@C-Gr 550|GPE electrode was flat, without any notable fractures on the surface.On the contrary, the mechanical strain of anode composite induces microcracks in Si/C@C-Gr 550|LE electrode, which also demonstrated 19% increase in the thickness at the lithiated state over 100 cycles.Worst of all, the Si/G@C electrode exhibits a 38% increase in thickness owing to the volume expansion of the aggregated Si without adequate reversed compression, leading to obvious electrode delamination upon cycling.Table S3 The calculation results of the gravimetric energy density of the Si/C@C-Gr 550|GPE|NMC pouch cell.

Fig. S1
Fig. S1 (a) PSD curves of Si NPs upon the sand-milling process from 1.3 h ~ 2.5 h.(b) FESEM image of Si NPs after 2.5 h sand-milling process.(c) TG spectra of CTP with different softening point.(d) HRTEM image of the carbon layer of the Si/C-5 composite.

Fig
Fig.S2shows the XRD patterns of Si/C-5 composite.It can be easily concluded that diffraction peaks can be well indexed to the (111), (220) and (311) planes of cubic Si structure (ICSD number 01-077-2111).

Fig. S3
Fig. S3 The cross-sectional image of the (a) Si/C-0.3,(b) Si/C-3, and (c) Si/C-7 composites and corresponding EDS elemental maps of C and Si.(d) TEM image of the Si/C-5 composite and corresponding EDS elemental maps of C and Si.
Fig.S6TG profiles of the Si/G, Si/G@C, and Si/C composites.
Fig. S8 (a) The full XPS spectrum, (b) High-resolution C 1s core-level spectrum, and (c) Highresolution Si 2p core-level spectrum of the Si/C-5 composite.
100.5 eV and 101.6 eV corresponding to the Si-C bond and Si−O−C bond, respectively.The strong interfacial bonds (Si-C and Si−O−C) would not only enhance the interactions between graphite and Si NPs but also increase the electrical conductivity of the electrode materials.

Fig
Fig. S11 PVCM-GPE was experienced combustion experiment for three times.

Fig. S12
Fig. S12 Chemomechanical modeling of stress distribution during lithiation.Stress distribution modeling across the anodes for (a) Si/G@C|GPE and (b) Si/C@C|LE at different lithiation states, and stress distribution of the single composite particle at the deep lithiation state.Stress changes of Si NPs and carbon layer of (c) Si/G@C and (d) Si/C@C at 100% lithiation state.(e) Stress comparison of the Si and carbon layer species from two models.

Fig. S13
Fig. S13 (a) The voltage-capacity profiles of the third cycles at 0.5 C and (b) Rate performance of Si/C@C|GPE, Si/C@C|LE and Si/G@C|LE anodes.

Fig. S14
Fig. S14 TEM image of the (a) Si/C@C and (c) Si/G@C composites and corresponding EDS elemental maps of C and Si after 100 cycles.(b) SEAD pattern of the Si/C@C.

Fig. S15
Fig. S15 (a) The cyclability values of the Si|GPE|Li and Si|LE|Li at 200 mA g −1 .(b) Schematic illustration of Si|GPE and Si|LE half-cell model.

Fig. S15 summarized
Fig.S15summarized the electrochemical performance of pure Si electrodes obtained in half-cells as paired with GPE or LE.The cycle behaviors of the Si|GPE and Si|LE anodes were compared at 200 mA g −1 with the similar areal capacity loading of ~ 2 mg cm −2 (Fig.S15a).The first discharge and charge capacities of Si|GPE were documented as 3554.6 mA h g −1 and 3016.6 mA h g −1 , rendering a satisfactory initial CE (ICE) of 84.9%.Meanwhile, Si|GPE renders better capacity retention (CR) of 44.1%

Fig
Fig. S16 (a) TEM images and (b) corresponding EDS elemental maps of F and B of Si/C@C-Gr in GPE after 100 cycles.(c) TEM images and (d) corresponding EDS elemental maps of F and B of NMC811 in GPE after 100 cycles.

Fig. S20
Fig. S20 Comparison of leakage current holding at different voltages for the Si/C@C-Gr 550|GPE|NMC and Si/C@C-Gr 550|GPE|NMC.
Fig. S24 Schematic illustration of operando XRD measurement of the Si-based anode||NMC full cell.

Table S1
Parameters used in the simulations.

Table S2
The calculation parameters of the main components and results of the gravimetric/volumetric energy density of the Si/C@C-Gr 550|GPE|NMC pouch cell.