Silica-nanoresin crosslinked composite polymer electrolyte for ambient-temperature all-solid-state lithium batteries

Yixi Kuai a, Feifei Wang ab, Jun Yang *a, Huichao Lu a, Zhixin Xu a, Xiaochuan Xu b, Yanna NuLi a and Jiulin Wang a
aShanghai Electrochemical Energy Devices Research Center, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. E-mail: yangj723@sjtu.edu.cn
bEvonik (Shanghai) Investment Management Co., Ltd, Shanghai 201108, China

Received 25th May 2021 , Accepted 29th June 2021

First published on 1st July 2021


Abstract

All-solid-state lithium batteries (ASSLBs) are in urgent demand for future energy storage. The basic problems are, however, low ambient-temperature ionic conductivity and narrow electrochemical windows of solid electrolytes as well as the abrupt lithium dendrite growth causing short-circuit. Herein, we demonstrate a novel poly(vinyl ethylene carbonate) based crosslinked composite polymer electrolyte (PVEC-NR20 CPE) with 20 wt% silica-nanoresin (NR) consisting of a nano-silica filler and trifunctional crosslinker. This electrolyte displays a high ionic conductivity of 1.65 × 10−4 S cm−1, a wide electrochemical window up to 5.3 V versus Li+/Li and a large lithium-ion transference number of 0.63 at 25 °C. Moreover, the resulting electrolyte membrane possesses a significantly high modulus of more than 6 GPa, which enables the suppression of lithium dendrite growth during repeated Li stripping/plating. The high-voltage all-solid-state NCM523/Li cells with the PVEC-NR20 CPE possess an outstanding rate performance (95.0 mA h g−1 at 2C) and cycling stability (79.4% after 200 cycles at 0.5C) at 25 °C. This work offers a new approach towards future high-voltage and ambient-temperature all-solid-state lithium batteries.


1. Introduction

Lithium-ion batteries (LIBs) with the virtues of high energy density and ensured safety are imperatively needed to meet the progressively growing demands for diverse emerging electronic applications such as cell phones, laptops and electric vehicles.1–3 Lithium metal as an anode material is considered to be the “Holy Grail” of batteries due to its extremely low electrochemical potential (−3.040 V vs. SHE) and high theoretical capacity (3860 mA h g−1).4,5 In addition, layered LiNixCoyMnzO2 (NCM) is regarded as one of the most promising cathode materials owing to its relatively high energy density, low cost and improved environmental benignity.6,7 However, the high-energy batteries using these electrode materials are quite unsafe if they are filled with flammable and explosive liquid electrolytes. What is more, propagation and growth of lithium dendrites during repeated charge–discharge cycling is likely to penetrate the fragile separator, causing short-circuit and eventually thermal runaway.8

One auspicious way to basically address the aforementioned dilemma of high energy density and safety is to employ solid-state electrolytes, including inorganic solid electrolytes (ISEs) and solid polymer electrolytes (SPEs).9–12 Despite the fact that ISEs possess good mechanical strength, high ionic conductivity and a wide electrochemical window, the unstable interfacial contact as well as the poor compatibility with the cathode or anode brings notoriously excessive interfacial impedance and retards their practical application.13–15 On the other hand, SPEs have attracted increasing attention due to their good interfacial compatibility and easy processing since Wright et al. first discovered the lithium salt/poly(ethylene oxide) (PEO) electrolyte in 1973.16 Nevertheless, application of semi-crystalline PEO based polymer electrolytes is limited by their low ionic conductivity (10−8 to 10−6 S cm−1 at 25 °C) and poor mechanical properties, especially at high operation temperature.17,18 In this way, many efforts have been made to boost the development of SPEs, such as the use of novel host polymers (e.g. PAN,19 PMMA,20 and PVDF21), in situ polymerization,22 3D crosslinked polymer electrolytes,23 composite polymer electrolytes with inert fillers24,25 or active fillers,26etc. Amorphous poly(vinyl ethylene carbonate) (PVEC) is considered to be a promising host polymer candidate for ambient-temperature all-solid-state batteries in view of its wide electrochemical window and high ionic conductivity.27 However, such a linear polymer electrolyte is not mechanically robust enough to suppress lithium dendrites. In addition, it is difficult to meet the high voltage requirements of NCM cathodes.

From this perspective, we have designed a PVEC based crosslinked composite polymer electrolyte (referred to as PVEC-NR CPE) using a silica-nanoresin (denoted as NR) as both an inert filler and a crosslinking agent. This NR is a versatile dispersion of 50 wt% silica nanoparticles ( 20 nm) in trifunctional polyether acrylate (ETPTA). Despite the high SiO2-content of 50 wt%, the NR is highly transparent with low viscosity (∼1000 mPa s at 25 °C) and shows no sedimentation due to the agglomerate-free dispersion of the nanoparticles in the acrylate. As shown in Fig. S1 (ESI), the Tyndall effect is observed in this NR dispersion. Fig. 1 schematically illustrates the fabrication process of the PVEC-NR composite polymer electrolyte. First, a highly porous cellulose membrane (∼30 μm thick) is adopted as a separator and supporting matrix for the polymer electrolyte due to its proper mechanical strength and chemical stability.28 Second, the porous cellulose membrane is wetted and filled by the precursor electrolyte dispersion, which contains four components including the Li salt (LiFSI), initiator (BPO), monomer (VEC) and NR. After the cell is assembled, the wetted cellulose further transforms into a PVEC-NR CPE and perfect interfacial contact is achieved through in situ free-radical polymerization under heating. Soft PVEC polymer segments are connected by the trifunctional ETPTA crosslinker to form an intersecting 3D structure with inserted rigid nano-sized silica around the cellulose membrane fibers, as vividly illustrated in the enlarged image. In addition, the shear modulus of silica (∼30 GPa) is six times as high as that of Li metal (∼4.8 GPa at 298 K), and the silica/polymer composite is expected to mechanically suppress interfacial roughening according to the Monroe model.29,30 Such a favorable structure coupling softness with hardness ensures great harmony between mechanical strength and ionic conductivity. The PVEC-NR composite polymer electrolyte with 20 wt% NR (PVEC-NR20 CPE) exhibits a high conductivity of 1.65 × 10−4 S cm−1 and a wide electrochemical window over 5.3 V (vs. Li+/Li) at room temperature. Moreover, it is compatible with the Li metal electrode and can suppress Li dendrite growth via its GPa-level modulus. Using this electrolyte system, excellent room temperature electrochemical performances have been achieved for high-voltage Li/NCM523 cells.


image file: d1qm00769f-f1.tif
Fig. 1 Schematic illustration of the fabrication process of the PVEC-NR CPE.

2. Experimental

2.1 Electrode preparation

The LiNi0.5Co0.2Mn0.3O2 (NCM523) electrode was prepared by the casting method. NCM523 powder (EASPRING, Beijing, China), PVDF and Super P conductive carbon were mixed with a mass ratio of 8[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 in NMP (99.99%, H2O ≤ 10 ppm, Capchem) to form a homogeneous slurry. The as-obtained slurry was cast uniformly onto an aluminum foil collector and was then dried under a vacuum at 80 °C for 6 h. After being cut into small disks of 12 mm in diameter, the cathode disks were weighed and dried under a vacuum at 120 °C for 12 h. The cathode mass loading is approximately 2.0 mg.

2.2 Precursor preparation and cell assembly

All the steps were performed in an Ar-filled glovebox (MB-10 compact, O2 ≤ 0.01 ppm, H2O ≤ 0.01 ppm, MIKAROUNA). Vinyl ethylene carbonate (VEC, 99%, Rahwn), the silica-nanoresin (NR, product name: Nanocryl A 223, Evonik), benzoyl peroxide (BPO, AR, Aladdin) and lithium bis(fluorosulfonyl)imide (LiFSI, Xinzhoubang) were mixed and stirred at 25 °C for 0.5 h to form a transparent and homogeneous precursor electrolyte dispersion. The contents of BPO and LiFSI are 0.5 wt% and 15.7 wt% of the total mass of NR and VEC, respectively. The mass ratios of VEC/NR are 9[thin space (1/6-em)]:[thin space (1/6-em)]1, 8[thin space (1/6-em)]:[thin space (1/6-em)]2 and 7[thin space (1/6-em)]:[thin space (1/6-em)]3, corresponding to PVEC-NR10, PVEC-NR20 and PVEC-NR30, respectively. The PVEC precursor electrolyte dispersion was prepared in the same way but without the addition of the NR.

During the in situ polymerization process, the precursor electrolyte dispersion was added into a cellulose membrane (MPF30AC, NKK, Japan) sandwiched by an NCM523 cathode and Li foil (400 μm thick) anode in a CR2032-type coin cell. After being assembled, the cells were kept at 25 °C for 2 h to completely wet the cellulose membrane and the electrolyte/electrode interface, and then were heated at 80 °C for 24 h to complete the polymerization. Solidifying of the polymer electrolyte was confirmed as the CR2032 cell was disassembled.

2.3 Material characterization

The surficial and cross-sectional morphologies and EDS elemental mapping images were obtained using a field emission scanning electron microscope (Nova NanoSEM 450, FEI Company, USA). Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a FTIR spectrophotometer (EQUINOX 55, Bruker). Thermogravimetric analysis (TGA) was performed with a thermogravimetric analyzer (Spectrum 100, PerkinElmer, Inc., USA) under a nitrogen atmosphere. Differential Scanning Calorimetry (DSC) was performed on a differential scanning calorimeter (DSC 8000, PerkinElmer) under a nitrogen atmosphere. X-Ray diffraction (XRD) patterns were collected using an X-ray diffractometer (D8-Advance, Bruker Corp., Germany). Young's modulus mapping images were obtained by atomic force microscopy (AFM, MFP-3D, Asylum Research).

2.4 Electrochemical measurements

The ionic conductivity of the polymerized electrolyte was measured between two stainless-steel electrodes ( 16 mm) in a CR2032-type coin cell using AUTOLAB (PARSTAT 4000, Metrohm) using AC impedance with a frequency ranging from 100 kHz to 0.01 Hz at an amplitude of 5 mV. The ionic conductivity was calculated according to the following equation:
image file: d1qm00769f-t1.tif
where σ represents the ionic conductivity of the electrolyte, d is the thickness of the electrolyte, R is the bulk resistance of the electrolyte and S denotes the contact area of the electrolyte and electrode.

The electrochemical window of the polymerized electrolyte was measured between a stainless-steel electrode and Li foil in a CR2032-type coin cell using a CHI650E electrochemical workstation (Chenhua, Shanghai, China) using linear sweep voltammetry (LSV) from 3.0 V to 6.0 V at a sweep rate of 5 mV s−1. As the current density reached 0.01 mA cm−2, the onset potential was defined as the electrochemical window value.

The lithium-ion transference number (LTN) was measured and calculated with amperometric it curve (8000 s) and AC impedance measurements on a CHI650E electrochemical workstation by the Bruce–Vincent–Evans equation as follows:

image file: d1qm00769f-t2.tif
where V represents the applied voltage (10 mV), Io and Iss are the initial and stable currents and Ro and Rss are the initial and stable interfacial resistance.

The rate performance and cycling stability of Li/NCM523 cells were examined on a LAND-CT 2001A battery test system (Wuhan, China) in a voltage range of 2.7–4.3 V (vs. Li/Li+) at room temperature.

3. Results and discussion

Firstly, the NR content in this composite polymer electrolyte was optimized according to the physical state and basic electrochemical properties including the ionic conductivity and electrochemical window. The polymer electrolytes with different weight contents of NR (0 wt%, 10 wt%, 20 wt%, and 30 wt%) are named PVEC SPE, PVEC-NR10 CPE, PVEC-NR20 CPE and PVEC-NR30 CPE, respectively. Before in situ polymerization in coin cells, the physical state of these composite polymer electrolytes was examined by ex situ preparation without cellulose membranes in transparent bottles. Fig. 2a depicts the corresponding optical photographs before and after polymerization. Amazingly, no obvious agglomeration can be observed for the precursor electrolyte dispersion (up) or for the polymerized electrolyte (down) even with the addition of 30 wt% NR (that is, 15 wt% SiO2). It is well known that normal nano-silica tends to aggregate via van der Waals forces due to its strong specific surface energy and existence of many inorganic –OH groups on its surface, making it hard to disperse homogenously in the organic monomer solution.31 When we add traditional fumed silica nanoparticles ( 7–40 nm) at 10 wt% into the VEC monomer solution, the silica nanoparticles cannot be dispersed uniformly to form a gel-like state but bulky silica exists, as shown in Fig. S2 (ESI). In fact, we find that the maximum dispersible amount of this fumed silica is ca. 7 wt% in the VEC solution. In contrast, the highly transparent and low-viscosity NR can reach a homogeneous dispersion of a surprisingly high content of nano-silica in VEC or in the composite polymer electrolyte, which could promote the mechanical properties of the composite polymer electrolyte and transport of Li ions, and may also play an exceptional role in other fields. The excellent uniform dispersion of nano-silica in this article arises from the good compatibility between the SiO2 nanoparticles and ETPTA in the NR as well as between the NR and VEC monomer. It is manifested that all silica-nanoresin composite polymer electrolytes show higher ionic conductivity than the pure PVEC SPE (0.58 × 10−4 S cm−1) at room temperature, among which the PVEC-NR20 CPE provides the highest ionic conductivity of 1.65 × 10−4 S cm−1, as shown in Fig. 2b. The EIS plots and the related ionic conductivity associated with the different silica contents are presented in Fig. S3 and Table S1 (ESI), respectively. The intersecting network with conductive pathways constructed by PVEC polymer segments, the ETPTA crosslinker and nano-sized silica with high specific surface area can be assumed to promote Li+ migration.32,33 It is noticed that the ionic conductivity begins to decrease when the silica mass ratio exceeds 20 wt%. This is because the too many silica particles and the excessive crosslinked structure may hinder the ion mobility. The electrochemical stability window is another important feature parameter for the electrolyte system. Fig. 2c presents the electrochemical windows of the above-mentioned polymer electrolytes and a conventional liquid electrolyte (1 M LiPF6 in EC/DMC) obtained by sandwiching between two stainless-steel electrodes through linear sweep voltammetry (LSV). The liquid electrolyte starts to decompose oxidatively at ca. 4.3 V (vs. Li+/Li), and the pure PVEC SPE decomposes at about 4.5 V (vs. Li+/Li). To be sure, such electrochemical windows are insufficient for high-voltage NCM cathodes to exert their best performance. Fortunately, adding the NR into PVEC can improve the electrochemical stability. Specifically, the electrochemical window of the PVEC-NR10 CPE is notably widened to 5.0 V (vs. Li+/Li), and the PVEC-NR20 CPE and PVEC-NR30 CPE even remain stable up to 5.3 V (vs. Li+/Li). Here, the crosslinked structure and uniformly dispersed silica filler play an important role in the electrochemical stability. According to the primary evaluation of the ionic conductivity and electrochemical window, we choose the composite polymer electrolyte adding 20 wt% NR (PVEC-NR20 CPE) as the research target.
image file: d1qm00769f-f2.tif
Fig. 2 (a) Optical images of the PVEC-NR CPE with different mass ratios of NR (0 wt%, 10 wt%, 20 wt% and 30 wt%, from left to right) before and after polymerization. (b) Ionic conductivity and (c) electrochemical windows of different electrolytes at room temperature.

Fig. 3a, c and e exhibit the surface morphologies of the cellulose membrane, PVEC SPE and PVEC-NR20 CPE, respectively, and Fig. 3b, d and f present the corresponding cross-sectional morphologies. The cellulose membrane with a thickness of 30 μm is composed of numerous randomly intersecting fibers in diameters from 0.2 to 2 μm, which construct a highly porous skeleton for a high uptake of the precursor dispersion. After in situ polymerization, the pores in the cellulose membrane are basically filled with polymer electrolyte and a continuous structure enabling transporting Li+ ions is formed. The PVEC SPE is about 33 μm thick and large cracks can be clearly observed on its surface which can be attributed to the lack of interacting force between single PVEC polymer chains. This problem can be solved by the use of the NR as shown in a low-magnification SEM image (Fig. S4, ESI), since the ETPTA crosslinker in the NR connects the soft PVEC polymer segments and forms an integral 3D crosslinked structure. As a comparison, the composite polymer electrolyte with 7 wt% traditional fumed silica nanoparticles (Fig. S5a, ESI) shows agglomeration of silica (dotted circles). Besides, some parts are not fully polymerized (dotted box), and the thickness is ca. 43 μm as seen from the cross-sectional image (Fig. S5b, ESI). This is because it is difficult for the gel-like monomer dispersion with high viscosity to spread evenly on the cellulose membrane and fill in the pores of it. In addition, the PVEC-NR20 CPE membrane is translucent and mechanically flexible, as shown vividly in Fig. S6 (ESI). The EDS elemental mapping images (Fig. 3g–i) of the surface of the PVEC-NR20 CPE exhibit homogeneously distributed C, O and Si elements, proving the uniform distribution of the polymer and nano-silica. This is favorable for the mechanical strength of the membrane and the interfacial property.


image file: d1qm00769f-f3.tif
Fig. 3 SEM images of the surficial (left) and cross-sectional (right) morphologies of the (a and b) cellulose membrane, (c and d) PVEC SPE and (e and f) PVEC-NR20 CPE. (g–i) EDS elemental mapping of the PVEC-NR20 CPE.

Fig. 4a shows the FTIR spectra of the NR, LiFSI, VEC and the PVEC-NR20 CPE without cellulose. The peaks at 988 cm−1 for VEC and the NR are assigned to the out-of-plane bending vibration of C[double bond, length as m-dash]C, and the related peak disappears in the PVEC-NR20 CPE, which demonstrates the successful polymerization. For the composite polymer electrolyte, the most characteristic peak at 1784 cm−1 is C[double bond, length as m-dash]O stretching and the other two peaks at 1058 cm−1 and 1176 cm−1 belong to C–O–C stretching, which originated from the 5-membered cyclic carbonate structure of PVEC.34 The peak of C[double bond, length as m-dash]O assigned to ETPTA in the NR shifts from 1723 cm−1 to 1751 cm−1 in the PVEC-NR20 CPE after polymerization because the conjugated double bond disappears. The peak at 1100 cm−1 can be ascribed to the Si–O–Si asymmetric stretching mode in the silica of the NR.31 In addition, the typical peaks at 1736 cm−1 and 1214 cm−1 resulting from the stretching vibration of O[double bond, length as m-dash]S[double bond, length as m-dash]O and –CF3 in the FSI anion can also be observed in the PVEC-NR20 CPE, suggesting the successful introduction of Li salt.35,36 On the other hand, the X-ray diffraction (XRD) patterns of both the pure PVEC polymer (Fig. S7a, ESI) and PVEC-NR20 composite polymer (Fig. S7b, ESI) exhibit broad hump characteristics, which prove that the addition of the NR can maintain the amorphous nature, favoring the ionic conduction. In addition, the increased intensity of the broad peak at ca. 22 degrees is accounted for by the amorphous nano-silica (JCPDF card no. 76-0938) in the NR.


image file: d1qm00769f-f4.tif
Fig. 4 (a) FT-IR spectra of the NR, LiFSI, VEC and the PVEC-NR20 CPE (without cellulose). (b) Temperature dependent ionic conductivity of the PVEC SPE and PVEC-NR20 CPE from 25 to 70 °C. It curves and AC impedance spectra (inset) of the (c) PVEC SPE and (d) PVEC-NR20 CPE in Li–Li symmetric cells.

The thermal stability of the polymer electrolytes has been examined by TGA and DSC. The TGA plots in Fig. S8 (ESI) do not show appreciable weight loss within 100 °C in the enlarged inset for both the polymers. After 200 °C, PVEC decreases its weight rapidly. However, this rapid weight loss does not occur for PVEC-NR20 until 290 °C. Furthermore, when the temperature goes up to 800 °C, the PVEC-NR20 composite polymer still possesses 10% of its weight due to the existence of nano-silica (10 wt%). On the other hand, the PVEC-NR20 composite polymer shows a glass transition temperature (Tg) of approximately −26.2 °C (Fig. S9, ESI), lower than those of previously reported polycarbonate based electrolytes such as poly(propylene carbonate) (PPC, 24.4 °C)37 and poly(propylene carbonate) (PVCA, 18.9 °C).38 Such a low Tg indicates the rapid segmental motion of polymer chains at room temperature, which promotes Li+ transport in the CPE.

Fig. 4b displays the temperature-dependent ionic conductivity of the PVEC SPE and PVEC-NR20 CPE in the temperature range from 25 °C to 70 °C. Obviously, the ionic conductivity of the PVEC-NR20 CPE is higher than that of the PVEC SPE in the entire temperature range, and the plots of both electrolytes fit well with the Arrhenius equation. The activation energy (Ea) calculated from linear fitting of the PVEC-NR20 CPE is 0.093 eV, which is lower than that of the PVEC SPE (0.142 eV). The decreased Ea means enhanced Li+ motion dynamics due to the lowered energy barrier.39

The lithium-ion transference number (LTN) is a critical parameter evaluating a polymer electrolyte, and generally a high LTN value can meet the requirement of high power for lithium batteries. The LTN values of the PVEC SPE and PVEC-NR20 CPE can be calculated according to the amperometric it curves in symmetrical Li–Li cells and the corresponding EIS plots in the initial state and stable state after 8000 s, as shown in Fig. 4c and d. Through the Bruce–Vincent–Evans equation, the lithium-ion transference number of the PVEC-NR20 CPE is 0.63, much higher than that of the pure PVEC SPE (0.38). Actually, the existence of nano-silica is an important reason for the enhanced LTN, because oxygen vacancies on the silica surface could act as Lewis acid sites and interact with the O atoms of PVEC chains and FSI anions, hence releasing more free Li+ and giving prominence to its conductivity.40,41 Apart from that, it can be clearly observed that the semi-circle of the Li–Li cell using the PVEC-NR20 CPE is smaller than that of the Li–Li cell using the PVEC SPE, indicating smaller interfacial impedance as well as better interfacial compatibility.

The interfacial compatibility of the Li electrode with the as-obtained PVEC SPE and PVEC-NR20 CPE was also evaluated by long-time electrochemical Li stripping/plating cycles. Fig. 5a shows the polarization voltage–time profiles of symmetric Li/Li cells by repeatedly charging for 1 h and discharging for 1 h under a fixed current density of 0.10 mA cm−2 at room temperature. The Li/PVEC-NR20 CPE/Li cell shows a stable polarization voltage of approximately 60 mV lower than that of Li/PVEC SPE/Li cells (∼100 mV), which indicates its smaller interfacial resistance. What is more, the voltage of the Li/PVEC SPE/Li cell suddenly drops to zero at 1050 h as shown in the right inset, which is an indication of short-circuit. By contrast, no obvious voltage fluctuation and short-circuit are observed for the Li/PVEC-NR20 CPE/Li cell up to 1500 h. The short-circuit can be attributed to Li dendrite growth and penetration through the PVEC SPE membrane during cycling, as illustrated in Fig. 5b and c. However, the surface of cycled Li in the PVEC-NR20 CPE appears to be even and compact after the cycling test (Fig. 5d). This could be explained by the fact that the crosslinked polymer electrolyte reinforced by heavily and uniformly intercalated nano-sized silica, as illustrated in Fig. 5e, can impede undesirable Li dendrite growth. Young's modulus mapping based on AFM measurements was adopted to confirm the crucial mechanical properties of the PVEC-NR20 CPE. The resulting Young's modulus in Fig. 5f shows a minimum of ca. 5.97 GPa which is related to the cellulose fiber part and a maximum of ca. 11.5 GPa which is related to the pure composite electrolyte filled in the pores of the cellulose membrane. This confirms that the heavily and uniformly filled nano-sized SiO2 with high modulus can indeed lead to the mechanical reinforcement of the membrane. Prior theoretical predictions have documented that the strength of a coating layer needs to be of a magnitude of GPa to enable the suppression of Li dendrites,29,42 and apparently the PVEC-NR20 CPE could fully meet such a requirement. Besides the adequate mechanical strength, the perfect interfacial contact of the PVEC-NR20 CPE with Li metal is also important for suppressing Li dendrites. Fig. 5g shows the impedance spectra of the Li/PVEC-NR20 CPE/Li cell after different storage times at room temperature. The interfacial resistance between Li metal and the PVEC-NR20 CPE increases from 353 Ω at the beginning to 429 Ω at the 4th day, which might be related to the formation of a resistive SEI layer. Then, this resistance changes slightly and turns to be steady at about 450 Ω after 12 days, suggesting that a stable interfacial SEI layer has been formed.


image file: d1qm00769f-f5.tif
Fig. 5 (a) Galvanostatic cycling voltage profiles of Li–Li symmetric cells using the PVEC SPE and PVEC-NR20 CPE at 0.10 mA cm−2. (b) Schematic illustration of Li dendrite penetration into the PVEC SPE. SEM images of the Li metal surfaces of (c) Li/PVEC SPE/Li and (d) Li/PVEC-NR20 CPE/Li cells after cycling of 300 h. (e) The schematic reinforced structure of the PVEC-NR20 CPE and (f) its Young's modulus mapping. (g) AC impedance spectra of Li/PVEC-NR20 CPE/Li after different storage times.

To further explore the electrochemical performance for potential application, the PVEC SPE and PVEC-NR20 CPE were evaluated in coin cells using a Li metal anode coupled with a high-voltage NCM523 cathode at room temperature. Fig. 6a shows the rate performance of the NCM523/Li cells using the PVEC SPE and PVEC-NR20 CPE from 0.1 C to 2 C. There is almost no difference in the specific discharge capacity of the two cells until the current rate rises to 0.5C. However, the specific discharge capacities of NCM523/PVEC-NR20 CPE/Li at 1C and 2C are 117.1 mA h g−1 and 95.0 mA h g−1, respectively, exceedingly higher than those of NCM523/PVEC SPE/Li (107.5 mA h g−1 at 1C and 48.8 mA h g−1 at 2C). Furthermore, the incorporation of the NR significantly reduces the voltage polarization between charge and discharge, especially at high current rates (Fig. 6b). The average discharge voltage of NCM523/PVEC-NR20 CPE/Li is ca. 3.67 V at 2C, against ca. 3.39 V for the NR-free cell. This improved rate performance is in good consistency with the previous result of the enhancement in ionic conductivity and lithium-ion transference number of the proposed composite electrolyte. Furthermore, the influence of different electrolytes on the cycling stability at 0.5C and room temperature has been also investigated. As shown in Fig. 6c, both the cells present a similar trend in specific capacity in the beginning, but the capacity of NCM523/PVEC SPE/Li drops quickly after the 50th cycle. Finally, the cell with the PVEC-NR20 CPE displays much higher capacity retention of 79.4% at the 200th cycle than that with the PVEC SPE (39.3%). To further understand the excellent cycling stability, NCM523/Li cells were firstly charged to 4.3 V and then electrochemical impedance spectra (EIS) were measured in the initial state (0th) and after the 50th cycle. Fig. 6d shows that there is only one semi-circle before cycling, which can be assigned to the bulk resistance (Rs) at the highest frequency and the charge transfer resistance (Rct) at middle frequency. Nevertheless, the plots after the 50th cycle consist of two semi-circles, where the newly appeared one at high frequency represents the SEI resistance (Rf). Table 1 exhibits the resistance values via fitting based on simulated equivalent circuits in Fig. S10a and b (ESI). The cell with the PVEC-NR20 CPE shows a much smaller Rct (405 Ω) than the cell with the PVEC SPE (647.8 Ω) before cycling. Interestingly, after 50 cycles, the Rct values of both the cells with the PVEC-NR20 CPE and PVEC SPE decline drastically to 133.5 Ω and 487.9 Ω, respectively. Moreover, Rf of the former (177.2 Ω) is also significantly smaller than that of the latter (517.1 Ω). The significant resistance reduction could be explained by the better compatibility of the NCM cathode or Li anode with the PVEC-NR20 composite polymer electrolyte, which is also a reason for the suppression of Li dendrite formation in Fig. 5.43 On the whole, the remarkable improvement in the cycle life and rate performance by the use of the NR can be rationalized by the combined effect of the wider electrochemical window, faster Li+ transport, better interfacial compatibility and mechanical properties of the proposed composite electrolyte.


image file: d1qm00769f-f6.tif
Fig. 6 (a) Rate performance and (b) corresponding charge–discharge curves of NCM523/PVEC-NR20 CPE/Li and NCM523/PVEC SPE/Li from 0.1C to 2C in a voltage range of 2.7–4.3 V. (c) Cycling stability of NCM523/PVEC-NR20 CPE/Li and NCM523/PVEC SPE/Li cells at 0.5C in a voltage range of 2.7–4.3 V. (d) AC impedance spectra of uncycled cells (0th) and after 50 cycles (50th) at 0.5C. Temperature: 25 °C.
Table 1 EIS values of NCM523/Li cells using the PVEC SPE and PVEC-NR20 CPE
Electrolytes R s (Ω) R f (Ω) R ct (Ω)
0th 50th 0th 50th 0th 50th
PVEC SPE 5.925 12.75 517.1 647.8 487.9
PVEC-NR20 CPE 5.194 5.472 177.2 405 133.5


4. Conclusions

In conclusion, we have successfully prepared a novel PVEC-NR crosslinked composite polymer electrolyte with high content and dispersal uniformity of nano-sized silica by use of a dual-component NR containing the ETPTA crosslinker, and assembled the related cells via facile in situ free-radical polymerization. The PVEC-NR20 CPE with 20 wt% NR (i.e., with 10 wt% SiO2 nanoparticles) presents a superior ionic conductivity of 1.65 × 10−4 S cm−1, wide electrochemical window up to 5.3 V (vs. Li+/Li) and greatly enhanced lithium-ion transference number of 0.63 at 25 °C. Meanwhile, Li dendrite growth is effectively suppressed during repeated charge/discharge processes in Li–Li symmetric cells by the robust composite electrolyte membrane with its GPa-level modulus and the good interfacial compatibility between Li and the electrolyte. The high-voltage NCM523/PVEC-NR20/Li cells exhibit excellent rate performance and long-term cycling stability at 0.5 C compared to those with the pure PVEC SPE. Overall, the PVEC-NR composite polymer electrolyte could be a very promising candidate for high-performance all-solid-state lithium batteries.

Author contributions

Yixi Kuai: methodology, investigation, data curation, writing – original draft, visualization. Feifei Wang: project administration, validation, data curation. Jun Yang: conceptualization, resources, supervision, funding acquisition. Huichao Lu: formal analysis, writing – review & editing. Zhixin Xu: validation, writing – review & editing. Xiaochuan Xu: writing – review & editing. Yanna NuLi: resources. Jiulin Wang: resources.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21773154) and the National Key Basic Research Program of China (No. 2014CB932303).

Notes and references

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm00769f

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