Composite Solid Electrolytes with High Contents of Ceramics

Qiuyan Liu; Xuefeng Wang; Zhaoxiang Wang; Liquan Chen

doi:10.7536/PC200684

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (1) : 124-135 DOI: 10.7536/PC200684

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Composite Solid Electrolytes with High Contents of Ceramics

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Abstract

The all-solid secondary lithium batteries are characteristic of high energy density and high safety. Combining the high flexibility of the polymer electrolytes and the high mechanical strength and high Li-ion transference number of the ceramic electrolytes, the ceramic-polymer composite solid electrolytes with high ceramic contents(HCC) are expected to find applications prior to the other solid electrolytes in the all-solid secondary lithium batteries. Following a brief introduction on the composite solid electrolytes, the recent advances of the HCC ceramic-polymer composite electrolyte are reviewed in the general performances of the composite electrolytes, the fabrications of their membranes, the ceramic-polymer interfacial interactions and the resultant new ionic transport mechanisms. At the end of this review, we prospect the fundamental and applicable issues that the HCC composite electrolytes have to meet and propose the future research directions and possible solutions to these questions. We wish that this review could be of help for the R&D of the composite solid electrolytes of other ions as well.

Contents

1 Introduction

2 Composite solid electrolytes with high ceramic contents

2.1 Characteristics of high ceramic-contents composite electrolytes

2.2 Fabrication of high ceramic-content composite membranes

3 Ceramic-polymer interactions and extra Li-ion transport passage

3.1 Ceramic-polymer interfacial interactions

3.2 Optimization of ceramic-polymer composite electrolytes

4 Conclusion and perspectives

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Keywords

composite solid electrolyte / ionic conductivity / mechanism of ion transport / solid lithium battery / lithium metal

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Qiuyan Liu, Xuefeng Wang, Zhaoxiang Wang, Liquan Chen. Composite Solid Electrolytes with High Contents of Ceramics. Progress in Chemistry, 2021, 33(1): 124-135 DOI:10.7536/PC200684

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1 Introduction

Lithium-ion batteries are widely used in consumer electronics, power and energy storage due to their high operating voltage, high energy density, long cycle life and environmental friendliness^[1]. However, the development of electric vehicles and the like requires batteries with higher energy density, longer service life, and higher safety^[2]. Although the energy density of lithium-ion batteries using liquid electrolyte and high nickel oxide cathode/silicon-carbon composite anode system has reached the 300 Wh·kg^-1, it still can not meet the long endurance requirements of electric vehicles^[3]. Lithium metal has a high theoretical specific capacity (V vs SHE) and the lowest redox potential (-3.04 V vs SHE), which can effectively improve the energy density of batteries when used as an anode^[4]. However, the solid electrolyte interface (SEI) film formed by the reaction of electrolyte and lithium metal can lead to uneven deposition of lithium, low coulombic efficiency, dendrite growth and even short circuit in batteries, which affects the life and safety of batteries^[5]. In addition, non-protonic electrolyte solvents are mostly flammable organic compounds, which may leak, and the safety risk of batteries is high^[1]. Therefore, solid state electrolytes and solid state (lithium) batteries with higher safety have become a research hotspot in recent years.

Solid state electrolyte is a kind of material with ionic conductivity similar to that of molten salt or liquid electrolyte in solid state, which has been called fast ionic conductor and super ionic conductor in history. In a broad sense, solid electrolytes include both all-solid electrolytes and semi-solid electrolytes. Semisolid electrolytes contain liquid substances (solvents or liquid electrolytes, etc.), which can be classified as solid-liquid composite electrolytes (heterogeneous phase), but also include some gel polymer electrolytes (homogeneous phase). All-solid-state electrolytes include pure inorganic ceramic electrolytes, pure polymer electrolytes and composite electrolytes without liquid substances. Because these substances are not flammable, solid electrolytes have higher safety than non-protic liquid electrolytes.

According to the structure, ceramic electrolytes can be divided into perovskite type, anti-perovskite type, NASICON type, garnet type and sulfide type^[6]^[7]^[8]^[9]^[10]. Their common characteristics are high bulk conductivity (in the order of 10^-4S·cm^-1 at room temperature), high lithium ion transference number (theoretical value is 1.0), and high hardness (effectively preventing lithium dendrites from penetrating the electrolyte). However, ceramic electrolytes are difficult to be processed into films at room temperature and pressure, resulting in thick electrolyte sheets (~ 300 μm), poor flexibility, and high interfacial resistance with electrodes. Polymer electrolytes are usually formed by complexing a polymer with strong polarity and a metal salt through ion-dipole interactions. Compared with ceramic electrolytes, polymer electrolytes have the advantages of flexibility, good film-forming, viscoelasticity, stability and good contact with electrodes^[11]. However, the room temperature ionic conductivity of polymer electrolytes is very low, most of them need to be used at high temperatures, the lithium ion transference number is not high, and they are easily pierced by lithium dendrites. Polyethylene oxide (PEO) -based polymer electrolytes have poor oxidation resistance, and can not be used to assemble batteries with cathode materials with charge voltage above 4. 0 V^[3]. The organic-inorganic composite solid electrolyte composed of ceramic (electrolyte) and polymer (electrolyte) has better comprehensive performance and is expected to be used in solid-state lithium batteries before pure ceramic electrolyte or pure polymer electrolyte.

Croce et al. added a small amount of inert ceramic alumina (Al₂O₃) as a filler to the polymer electrolyte to form a high-polymer-electrolyte-content (HPC) composite electrolyte^[12]. The morphology of the ceramic filler is various, and the proportion of the ceramic filler in the composite electrolyte is about 20 wt%^{[13 ⇓~15]}. The main function of ceramic filler is to reduce the crystallinity of polymer phase and promote the movement of polymer segments. Ceramic fillers can also immobilize lithium salt anions through Lewis acid-base interactions^[16]. The HPC composite electrolyte retains almost all the advantages of the polymer electrolyte and significantly improves the performance of the polymer electrolyte. The conductivity of HPC composite electrolyte filled with ceramic electrolyte (with lithium ion conductivity) can be about two orders of magnitude higher than that of pure polymer electrolyte, and the lithium ion transference number can be increased by 2 to 3 times^[17]^[18]. The transport behavior of lithium ions in HPC composite electrolytes mainly depends on the transport behavior of lithium ions in polymer electrolytes due to the small proportion of ceramic fillers, but some studies suggest that lithium ions in HPC composite electrolytes mainly transport along the ceramic-polymer electrolyte interface^[19]^[20,21]. Compared with the pure polymer electrolyte, the oxidation voltage and mechanical properties of the composite electrolyte with ceramic electrolyte as filler are also slightly improved^[22]. However, the lithium ion transference number, oxidation resistance and mechanical modulus of these composite electrolytes are still low, and there is a risk of being pierced by lithium dendrites (especially when the polymer substrate is softened at high temperature)^[23]^[24,25].

2 Composite solid electrolyte with high ceramic content

Different from HPC composite electrolytes, high-ceramic-content (HCC) composite electrolytes can not only maintain the advantages of ceramic electrolytes, but also alleviate the problems of poor flexibility of ceramic electrolytes, high contact resistance with electrode materials and thick electrolyte sheets, and can work in a wide range of voltage and temperature. In view of the lack of a clear distinction between HPC and HCC composite electrolytes in the current literature, the composite electrolytes with a ceramic phase content higher than 50 wt% are defined as HCC composite electrolytes in this paper. Because many properties and ion transport mechanisms of ceramic-polymer composite electrolytes are related to the shape and density of ceramic particles and the ionic conductivity of ceramic materials, this classification is obviously rough. In addition, since there is generally no "composite electrolyte" in which neither the ceramic phase nor the polymer phase is capable of conducting lithium ions, at least one phase of the composite electrolyte described herein is capable of conducting lithium ions.

Skaarup et al. (Earlier than the work of Croce et al.) Were the first to compound lithium nitride (Li₃N) with 5 vol% -10 vol% PEO-based polymer electrolytes (EO: Li = 12:1). The conductivity of the composite electrolyte is 1000 times higher than that of the polymer electrolyte, and its activation energy is close to that of the Li₃N^[26]^[12]. They believe that lithium ions are mainly conducted in Li₃N, but the ion transport behavior of polymer electrolytes is also affected by Li₃N grains. Later, Inda et al. Compounded 75 wt% lithium ion conductor Li_1+x+yAl_xTi_2-xSi_yP_3-yO₁₂ micron particles with PEO-polypropylene oxide (PPO) -based electrolyte to prepare a composite electrolyte with a film thickness of 25 μm and a room temperature conductivity of 1.4×10^-4S·cm^-1^[27]. The organic-inorganic composite also reduces the impedance of the ceramic electrolyte-electrode interface. In addition to the composite electrolyte with high ceramic electrolyte content, Syzdek et al. filled the polymer electrolyte composed of polyethylene oxide dimethyl ether (PEODME) and lithium perchlorate (LiClO₄) into the porous ceramic sheet of Al₂O₃ without lithium ion conductivity, and the proportion of Al₂O₃ in the composite electrolyte reached 75 wt%^[28]. The room temperature ionic conductivity of the composite electrolyte is in the order of 10^-5S·cm^-1, which is significantly higher than that of the PEODME-based polymer electrolyte (~10^-6S·cm^-1). They believe that although lithium ions are mainly transported in polymers, the porosity and pore size of Al₂O₃ significantly affect the conductivity of composite membranes, and composite electrolytes with small pore structure and higher porosity of Al₂O₃ ceramic sheets have higher conductivity. Therefore, the change from HPC to HCC is not only the change of ceramic content in the composite electrolyte, but also the change of the main transport path and conduction mechanism of lithium ions. Therefore, composite electrolytes composed of ceramic phase and polymer phase are becoming a new research hotspot^[29].

2.1 Characteristics of Composite Solid Electrolyte with High Ceramic Content

HCC composite electrolytes not only have the flexibility of polymers and the hardness of ceramics, but also have higher lithium ion conductivity, lithium ion transference number and wider electrochemical window (from "ceramics in polymer" to ceramics in polymer ")^[30,31]^[30,32].

Since the lithium ion transference number of the ceramic electrolyte is close to 1, the lithium ion transference number of the HCC composite solid state electrolyte is also high. The lithium ion transference number of the PEO-based composite electrolyte prepared by Yang et al with 70 wt% of Li_1.3Al_0.3Ti_1.7(PO₄)₃ micron particles is 0.49; the lithium ion transference number of the HCC composite electrolyte prepared by Ramakrishnan et al with two kinds of layered perovskite structure ceramic electrolytes Li₂SrNb₂O₇ and Li₂CaTa₂O₇ compounded with PEO is 0.49 and 0.58 respectively^[32]^[35]. The lithium ion transference number of the HCC composite electrolyte can be further improved if it is matched with a polymer electrolyte with a higher lithium ion transference number, such as a poly (ethylene carbonate) (PEC) -based high-salt polymer electrolyte (the lithium ion transfer number can reach 0.41)^[36]. He et al. Compounded Li_6.25Al_0.25La₃Zr₂O₁₂ nanoparticles with PEC-based high-salt polymer electrolyte, and the lithium ion transference number of the composite electrolyte reached 0.82^[30]. The optimized HCC composite electrolyte maintains a high lithium ion transference number, and the room temperature conductivity can also reach the order of 10^-4S·cm^-1. Peng et al. filled PEO-based polymer electrolyte (LLZTO accounted for 90 wt% in the composite electrolyte) into the micropores of the Li_6.4La₃Zr_1.4Ta_0.6O₁₂, and the resulting composite electrolyte had a lithium ion transference number of 0.71 and an ionic conductivity of 2.61×10^-4S·cm^-1 at 25 ° C^[37]. A composite electrolyte composed of a sulfide electrolyte having a higher ionic conductivity and a polymer electrolyte also has a very high conductivity. Zhang et al. compounded a polymer electrolyte composed of 95 wt% sulfide (78Li₂S-22P₂S₅) and PEO with lithium bis (trifluoromethanesulfonyl) imide (LiTFSI), and the conductivity of the composite electrolyte reached the 5.31×10^-4S·cm^-1^[38].

The HCC composite solid state electrolyte also has higher mechanical strength than the polymer electrolyte or the HPC composite electrolyte, is not easily pierced by lithium dendrites, and is more suitable for lithium secondary batteries^[30,32,33]. Huo et al. respectively compounded Li_6.4La₃Zr_1.4Ta_0.6O₁₂ powder with PEO-based electrolyte and assembled lithium-lithium symmetric batteries^[39]. At 55 ℃ and 0.2 mA·cm^-2, the battery with 80 vol% LLZTO composite electrolyte could cycle stably for 600 H, while the battery with 20 vol% LLZTO composite electrolyte could only cycle for 183 H. Yang et al. Used a PEO-based composite electrolyte containing 70 wt% Li_1.3Al_0.3Ti_1.7(PO₄)₃ microparticles in a lithium-lithium symmetric battery, and the battery could also cycle stably for more than 333 H at 0.2 mA·cm^-2 current density^[32]. On the other hand, the flexibility of HCC composite electrolyte is better than that of pure ceramic (electrolyte), and the interface contact with the electrode is better^[30,40,41]. Li et al. Filled the three-dimensional skeleton of Li₇La₃Zr₂O₁₂ with PEO-based polymer electrolyte, and the polarization voltages of lithium-lithium symmetric batteries with composite electrolyte were 0.015, 0.02 and 0.1 V at room temperature, 0.1, 0.2 and 0.5 mA·cm^-2 current densities, respectively, and all of them could be stably cycled for more than 500 H^[40]. In the absence of solvent, Huang et al. Made a composite electrolyte by mixing Li_6.4La₃Zr_1.4Ta_0.6O₁₂ powder with molten PEO-based polymer electrolyte. The corresponding lithium-lithium symmetric battery had a polarization voltage of about 0.05 V at 60 ℃ and 0.1 mA·cm^-2 current density, and could be stably cycled for 1200 H^[41].

In addition, electrolytes that easily react with lithium metal (such as Li_1+xAl_xTi_2-x(PO₄)₃(LATP) and sulfide electrolytes) usually require surface protection treatment for lithium metal electrodes before they can be used. After being compounded with the polymer electrolyte, LATP is wrapped by the polymer electrolyte and does not directly contact with metal lithium, so the interface stability of the composite electrolyte and the metal lithium electrode is better than that of the ceramic electrolyte and the metal lithium electrode. On the other hand, the polymer electrolyte coated with the ceramic electrolyte is in direct contact with the cathode, so it is limited by the electrochemical window of the polymer electrolyte. Zhang et al. Coated Li₆PS₅Cl sulfide electrolyte with polymer PEO, and the obtained composite electrolyte was stable in contact with metal lithium, but limited by the oxidation voltage of the coated PEO polymer, even if it was applied to the cathode of LiNi_0.8Co_0.1Mn_0.1O₂, it could only be charged to about 4 V^[42]. Therefore, this kind of HCC composite electrolyte is generally only suitable for cathode materials with relatively low working voltage, such as LiFePO₄^[32]. When the composition is appropriate, some HCC composite electrolytes can also be matched with higher voltage cathode materials. HCC composite electrolytes can be used for cathode materials with higher operating voltage by reducing the contact area between the polymer with poor oxidation resistance and the cathode material, or by improving the oxidation resistance of HCC composite electrolytes through the interaction between ceramic and polymer electrolytes (see "3.1 Interaction between Ceramic and Polymer Phases" in this paper)^[33,34,43]. Bae et al. filled the three-dimensional skeleton of the Li_6.28La₃Zr₂Al_0.24O₁₂ with PEO polymer electrolyte, and Cha et al. prepared a PEO-based composite electrolyte with a Li₇La₃Zr₂O₁₂ content of 70 wt%. The oxidation voltage on the inert electrode reached 5.0 V, which was higher than the oxidation potential (4.3 V) of pure PEO electrolyte on the inert electrode^[33]^[43]. The oxidation potential of the composite film of the gadolinium-based electrolyte (40 wt%) prepared by Piana et al. Also reached 4.5 V on the carbon black electrode^[34]. Kim et al. applied the composite electrolyte of gadolinium-based electrolyte (30 wt%) to the cathode of LiNi_0.4Co_0.2Mn_0.4O₂, and the battery could cycle stably in the voltage range of 3.0 ~ 4.1 V at 70 ℃ and 0.1 C^[44].

In addition, the high ceramic content of HCC composite electrolyte makes it have better high temperature stability and flame retardancy. The HCC composite electrolyte obtained by Bae et al. By combining PEO-based electrolyte with Li_6.28La₃Zr₂Al_0.24O₁₂(LLZO) can still maintain its structure at 400 ℃ for 2 H, while the composite electrolyte with lower ceramic content (10 wt% LLZO) has curled deformation^[33]. The PEC based HCC composite electrolyte containing Li_6.25Al_0.25La₃Zr₂O₁₂ nanoparticles prepared by He et al. Can still rely on the ceramic skeleton to maintain its shape after the polymer part is burned out, and there is no open fire in the whole process^[30]. Because of its high safety and good flexibility, solid-state lithium batteries constructed with HCC composite electrolytes can be used under harsh conditions such as folding and shearing batteries^[45].

The above HCC composite electrolytes are "all-solid" electrolytes without any liquid components (electrolyte, solvent for lithium salt or polymer, liquid plasticizer, etc.). However, these "dry" composite electrolytes have a high contact resistance with the solid electrodes. In order to improve the electrode-electrolyte interface contact, in recent years, a small amount of electrolyte has been added to the electrolyte-electrode interface as a plasticizer for the polymer to reduce the electrode-electrolyte interface resistance^[46]^[47,48]. Because the addition of electrolyte is small and limited to the vicinity of the electrolyte surface, the addition of a small amount of electrolyte does not affect the lithium ion transport mechanism in the solid electrolyte.

2.2 Preparation of High Ceramic Content Composite Electrolyte Membrane

The most common preparation method of the HCC composite electrolyte membrane is a solution casting method^{[30,32,49,50]}. The polar solvent used dissolves both the polymer and the lithium salt and disperses the ceramic phase, and generally does not react with either component. Polar solvents, such as acetonitrile (C₂H₃N), should have a low boiling point to facilitate later drying^[50]. However, too rapid solvent evaporation may leave holes in the electrolyte membrane^[51]. The thickness of the composite electrolyte membrane prepared by the casting method can be as thin as 30 μm^[30]. Choi et al. Added Li₇La₃Zr₂O₁₂(LLZO) powder, PEO and LiClO₄ into acetonitrile solution, coated on glass plate and dried to obtain composite electrolytes with different LLZO contents^[52]. 3D printing method has also been used to prepare composite electrolytes^[53].

Ceramic powder, polymer monomer, lithium salt and a small amount of polymerization initiator are uniformly mixed and then heated to initiate polymerization, which is also a method to prepare the composite membrane, in which the ceramic phase is uniformly distributed^[54]. The method can form the composite electrolyte in situ inside the battery, so that the physical contact between the electrode and the electrolyte is closer, and the electrode-electrolyte interface resistance of the battery is reduced^[55].

The above methods all take advantage of the good affinity between ceramics and polymers^[49]. However, the ceramic content in the composite electrolyte prepared in this way should not be too high, the molecular weight of the polymer should not be too large, and the ceramic powder with small particle size is easy to agglomerate^[56]. When the content is more than 80 wt%, the ceramic will not be completely dispersed in the mixed solution^[50]. To prepare a composite electrolyte membrane containing a 70 wt%Li_1.3Al_0.3Ti_1.7(PO₄)₃, Yang et al. Added 15 wt% boronated polyethylene glycol (BPEG) to the polymer electrolyte portion to enhance the adhesion of the polymer^[32].

Hot pressing and cold pressing are also common preparation methods of composite electrolyte membrane^[31,34,57]^[58]. When the composite electrolyte membrane is prepared by this method, the composition of the composite electrolyte is hardly limited. Li et al. compressed the 99 wt%Li_1.5Al_0.5Ge_1.5(PO₄)₃ and 1 wt% polymer electrolyte to obtain a composite membrane of 70 μm^[59]. Cheng et al. prepared a composite membrane with a Li_1.5Al_0.5Ge_1.5(PO₄)₃ content of 80 wt% by cold pressing^[58]. It is reported that the conductivity of the composite membrane obtained by cold pressing is higher than that of the composite membrane with the same composition prepared by casting method, which may be due to the fact that the agglomeration of ceramic particles is not easy to occur during cold pressing. Even so, it is difficult to make a flexible film when the ceramic content is high and the viscosity of the polymer portion is insufficient^[60]. In addition, if the concentration of lithium salt in the polymer electrolyte is too high to be completely dissolved in the polymer, the lithium salt crystallized out will passivate the surface of the ceramic phase, resulting in a decrease in the conductivity of the composite electrolyte^[61].

3 Interactions between Ceramic and Polymer Phases and New Channels for Ion Transport

As described above, in addition to the ceramic electrolyte having lithium ion conductivity, the composite solid state electrolyte often has a material that does not have lithium ion transport capability as a main component of the ceramic phase, and the interaction between the two ceramics and the polymer phase is similar^[28]. Borodin et al. calculated PEO-lithium tetrafluoroborate (LiBF₄)- titanium dioxide (TiO₂) and other systems by molecular dynamics (MD) method, and believed that there was a three-layer structure around the TiO₂^[62]. Both the polymer and the ions behave differently in the three-layer structure, with the ionic conductivity in the first layer being an order of magnitude higher than that of the bulk polymer phase. Zhang et al. Filled PEO-based polymer electrolyte in a Al₂O₃ disk filled with vertically connected nano-holes, and the room temperature ionic conductivity of the composite electrolyte reached 1.79×10^-4S·cm^-1^[63]. The conductivity of these composite solid electrolytes composed of polymer electrolytes and non-electrolyte ceramics (inert ceramics) exceeds the conductivity of polymer electrolytes themselves, indicating that there is a strong interaction between the ceramic phase and the polymer phase, which changes the transport channel and mechanism of lithium ions in the electrolyte. In the ceramic-based HCC composite solid state electrolyte, the ceramic-polymer interface is very large, and the interaction between them becomes non-negligible and significantly affects the performance of the composite electrolyte^[56,59]^[64].

3.1 Interactions between ceramic and polymer phase

In the solid state composite electrolyte, the nanoparticles of inert ceramics such as Al₂O₃, silicon dioxide (SiO₂), TiO₂, etc. Can change the crystallization kinetics of the polymer, increase the proportion of amorphous state in the polymer, and make the polymer chain segments move more easily, which is conducive to the transport of lithium ions in the polymer^{[31,50,65 ⇓~67]}. When the interaction between these ceramic fillers and the polymer is considered, the ceramic-polymer interface may become another fast ion channel in the composite electrolyte^[68,69]. According to Croce et al., the Lewis acidic groups (such as — OH) on the surface of acidic Al₂O₃ particles interact with anions in lithium salts and polar groups in polymers, promoting the dissociation of lithium salts and reducing the crystallinity of surrounding polymers, so that lithium ions can be rapidly transported along the -Al₂O₃ interface of polymer electrolytes^[70]. The neutral Al₂O₃ has few surface functional groups and weak interaction with lithium salt or polymer, so the addition of neutral Al₂O₃ has limited effect on the improvement of ionic conductivity of polymer electrolyte, while the addition of basic Al₂O₃ particles has little effect. According to the existing literature, there is no objection that Al₂O₃ with Lewis acidity on the surface can improve the ionic conductivity of polymer electrolytes as fillers. Chen-Yang et al. Found that there is an interaction between the ether oxygen in the polymer and the Lewis acid group — OH on the surface of the Al₂O₃ particles, in which the acidic Al₂O₃ has the greatest promoting effect^[71]. Acidic Al₂O₃ can not only interact with the surrounding polymer to increase its amorphous degree, but also fix the anion of lithium salt, promote the dissociation of lithium salt, improve the concentration of lithium ions at the interface, and increase the number of lithium ion transference of electrolyte. Zhang et al. Calculated that the room temperature ionic conductivity of the Al₂O₃- polymer electrolyte interface with an acidic surface is in the order of 10^-3S·cm^-1, which is much higher than the bulk conductivity of the polymer 10^-5S·cm^-1^[63]. However, Dissanayake et al. believed that Lewis basic oxygen and acidic — OH on the surface of Al₂O₃ would interact with cations and anions of electrolyte salts, respectively, providing additional lithium ion transfer sites and forming fast ion conduction channels^[69]. Jayathilaka et al. Pointed out that lithium ions and basic oxygen on the surface of Al₂O₃ particles would form temporary and weak transient bonds, similar to the transient coordination formed by lithium ions and ether oxygen in polymers, providing additional sites for lithium ion migration^[72]. Nuclear magnetic resonance (NMR) spectroscopy is very sensitive to the local chemical environment of ions and can be used to distinguish anions or cations in different environments in composite electrolytes. Wuellen et al. measured ⁷Li-{¹H} cross-polarization magic angle spinning (CPMAS) NMR of poly [ (bis (2-methoxy-ethyl) amino) _1.6 (propylamino) _0.4] polyphosphazene (BMEAP) -based composite electrolytes filled with Al₂O₃ particles and found that new signals appeared in electrolytes containing Al₂O₃ when the exchange contact time was long^[73]. The dependence of the new signal on contact time is very similar to the lithium signal in the binary sample of the Al₂O₃/ lithium salt, but quite different from the lithium signal in the bulk phase of the polymer. This signal was not observed in the pure polymer electrolyte without the Al₂O₃. Therefore, they believe that this new signal corresponds to lithium ions associated with the surface of the Al₂O₃. They also mixed lithium trifluoromethanesulfonimide (LiNTf₂) with Al₂O₃ with neutral, acidic and basic surfaces, respectively.The results of magic angle spinning nuclear magnetic resonance (¹⁹F-MAS-NMR) show that a new peak of ¹⁹F appears in all three samples, which corresponds to the $NT_{2}^{-}$ on the surface of the Al₂O₃^[74]. Therefore, an additional path for lithium ion transport is created around either the surface acidic or basic Al₂O₃ particles. We found that the effect of nanofiller Al₂O₃ on polyacrylonitrile (PAN)-LiClO₄ electrolyte is similar to a "solid solvent", which can promote the dissociation of lithium salts and help to produce more free lithium ions. It should be noted that the surface acidity and alkalinity of the filler Al₂O₃ are not involved here^[75].

Similarly, fillers (active ceramics) with lithium ion conduction activity can also interact with polymer electrolytes (including polymer and lithium salt) through surface acidic groups. Liu et al. showed that in Li_1.3Al_0.3Ti_1.7(PO₄)₃(LATP) ceramics infiltrated with polymer electrolyte, the Lewis acid interaction between the ceramic and polymer promotes the dissociation of lithium salts, and the amorphous polymer on the ceramic surface forms a fast ionic conduction channel along the polymer electrolyte-LATP interface^[76]. Xu et al. Suggested that the interaction between the F^- of the TFSI^- in the composite electrolyte Li_3/8Sr_7/16Ta_3/4Zr_1/4O₃(LSTZO)-PEO-LiTFSI and the Ta⁵⁺ of the LSTZ surface improves lithium ion migration at the ceramic-polymer interface^[77]. Zheng et al. measured ⁶Li magic angle spinning nuclear magnetic resonance (⁶Li MAS NMR) on a composite electrolyte consisting of a 70 wt%Li₁₀GeP₂S₁₂(LGPS) and 30 wt% PEO-based polymer electrolyte^[78]. Compared with pure polymer electrolyte and pure LGPS electrolyte, the new lithium peak in the composite electrolyte is attributed to the interface between the polymer phase and the ceramic phase. Romero et al., using confocal Raman spectroscopy, concluded that La_0.57Li_0.3TiO₃ nanoparticles promoted the dissociation of lithium salts and increased the concentration of free lithium ions^[79]. However, the spatial resolution of confocal Raman spectroscopy is limited (265 nm), which affects the accuracy of the information obtained about the ceramic-polymer interface.

The ceramic can not only interact with the lithium salt in the polymer electrolyte to promote the dissociation of the lithium salt, increase the concentration of free lithium ions near the ceramic-polymer interface, fix anions and increase the number of lithium ion transference, but also interact with the polymer chain segment. Sun et al believed that there was an interaction between the Li_6.75La₃Zr_1.75Nb_0.25O₁₂(LLZNO) nanowires and the O—CH₃ on the polymethyl methacrylate (PMMA) chain through infrared spectroscopy^[80]. The close interaction between LLZNO nanowires and polymer chains not only forms more amorphous regions, but also provides a larger free volume for the thermal movement of polymer chains, which facilitates the transport of free lithium ions through polar groups on polymer chains. In addition, the vacancies on the surface of LLZNO nanowires also provide a special conductive channel for lithium ion migration. However, they did not provide evidence of the source of vacancies on the surface of LLZNO nanowires and how vacancies interact with polymer electrolytes to promote ionic transport at the interface. Zhang et al. Dispersed Li_6.4La₃Zr_1.4Ta_0.6O₁₂(LLZTO) nanoparticles in pure PEO, and the room temperature conductivity of LLZTO-PEO composite electrolyte reached the order of 10^-4S·cm^-1^[81]. They believe that PEO "sucks" lithium ions out of LLZTO, forming lithium vacancies on the surface of LLZTO, which is conducive to rapid lithium ion transport. However, whether PEO can "suck" lithium ions from LLZTO needs further study. Bae et al. filled PEO-based polymer electrolytes into the three-dimensional framework of Li_0.35La_0.55TiO₃(LLTO) and TiO₂ respectively, and found that the conductivity of LLTO composite electrolyte was one order of magnitude higher than that of TiO₂ composite electrolyte^[82]. They believe that this is due to the fact that LLTO is a lithium ion conductor, and the existence of lithium vacancies on the surface may also provide a jumping point for lithium ions. However, none of these studies indicate whether the lithium vacancies are inherent in the ceramic electrolyte or are newly generated after the ceramic electrolyte contacts the polymer. We note that although many works have mentioned the positive effect of lithium vacancies on the surface of ceramic electrolytes on lithium ion transport in the whole system,However, there is a lack of experimental evidence for the characterization of lithium vacancies on the surface of ceramic electrolytes, the formation of extrinsic lithium vacancies, the interaction between polymer phases and ceramics through lithium vacancies, and the promotion of lithium ion transport by negatively charged lithium vacancies as Lewis bases^{[80 ⇓~82]}. In fact, doping or heat treatment can also form lithium vacancies on the surface of the ceramic electrolyte^[83].

In addition to the Lewis acid-base interaction, some researchers believe that when the two phases in the composite electrolyte contact, the difference of electrochemical potential between the two phases will lead to the redistribution of charge carriers at the interface and the formation of space charge layer. The space charge layer affects the ionic conductivity by changing the defect concentration in the region, which provides a new dynamic path for ion conduction^[84]. The space charge layer effect was first proposed by Liang to explain the higher ionic conductivity of LiI-alumina (LiI-Al₂O₃) system than that of pure LiI^[85]. At present, the space charge layer effect is mostly used to explain the increase of conductivity of multiphase electrolytes formed by the recombination of ionic conductors and insulators, as well as the soggy sand electrolyte.The total conductivity of a solid-liquid composite electrolyte (the liquid phase is a conductive phase, the solid phase is an insulating phase, and the solid phase accounts for a large proportion) is greatly improved due to the high conductivity of a space charge layer, and can be generalized to the improvement of the conductivity of an inert ceramic filled polymer electrolyte^[86]^[87]. At present, there are also some studies on the composite of active ceramics and polymers without lithium salts (insulating phase), and it is believed that there is a space charge layer at the interface between them^[81,84]. However, there is little research on the space charge layer between ceramic electrolyte and polymer electrolyte. Although there are articles mentioned, they are lack of in-depth research^[88,89].

In summary, it is generally believed that the ceramic-polymer interface in composite electrolytes is a fast channel for ion transport. Without considering the ionic conductivity of the ceramic phase, Bae et al. Calculated that the ionic conductivity of the ceramic-polymer interface in HCC composite electrolytes at room temperature can reach the order of 10^-4S·cm^-1, while Liu et al. Calculated that the ionic conductivity at the ceramic-polymer interface can reach the order of 10^-2S·cm^-1^[82]^[90]. In HCC composite electrolytes, because of the large interfacial area between ceramic and polymer, even the formation of continuous interfacial channels, interfacial transport becomes an important or even the main way of ion conduction^[56,64,82]. However, in the actual operation, the impurities on the ceramic surface and particle agglomeration will seriously affect the continuity and properties of the ceramic-polymer interface, hinder the rapid ion transport channel at the interface, and lead to the low conductivity of HCC composite electrolyte. In addition to the fast ion channel at the ceramic-polymer interface, the ceramic electrolyte phase can also form a continuous ion channel to transport lithium ions in the HCC composite electrolyte. As previously mentioned, nuclear magnetic resonance (NMR) spectroscopy is very sensitive to the local chemical environment of lithium ions and can be used to distinguish the polymer phase, ceramic phase and lithium at the interface of the two phases in composite electrolytes, so it is the most commonly used method to study the ion transport path in composite electrolytes. Zheng et al. Constructed a lithium-lithium symmetric battery by sandwiching a composite electrolyte of Li₇La₃Zr₂O₁₂(LLZO)-PEO electrolyte (mass ratio 1:1) between two pieces of lithium metal (⁶Li), and found that the ⁶Li in the lithium metal electrode would replace the ⁷Li in the electrolyte LLZO during cycling^[91]. Therefore, they believe that lithium ions in the composite electrolyte are mainly transported through the LLZO phase. Kato et al. Measured the impedance of the HCC composite electrolyte between two lithium electrodes, and saw the grain boundary resistance between the ceramic electrolyte particles from the impedance spectrum, so they believed that lithium ions were transported through the ceramic electrolyte^[92]. Similarly, the impurities and clusters on the ceramic surface seriously affect the transport of ions through the ceramic phase, which leads to the low conductivity of HCC composite electrolyte. The conductivity of the HCC composite electrolyte can be improved by the optimization method discussed next.

As mentioned in the previous section, the commonly used PEO will oxidize and decompose at high voltage (> 4.0 V), the polymer electrolytes such as polypropylene carbonate (PPC) and PEC have slightly higher oxidation and decomposition potentials, and the commonly used lithium salts (such as LiTFSI) have higher oxidation and decomposition potentials^[93]. Therefore, the low oxidation potential of polymer is the main obstacle to improve the oxidation resistance of polymer electrolyte. The oxidation stability of the composite electrolyte can be improved by the interaction between the ceramic and the polymer electrolyte in the composite electrolyte^{[94 ⇓~96]}. Part of the reason for the improvement in oxidation stability is the reduction in the area of direct contact of the polymer electrolyte with the electrode material. However, this can only alleviate the oxidative decomposition of the polymer electrolyte. The more important reason is that the interaction between the ceramic and the polymer electrolyte can stabilize the polymer electrolyte and inhibit its oxidative decomposition. According to Park et al., the interaction between the Al₂O₃ on the acidic surface and the lithium salt anions can stabilize the anions, increase the oxidation and decomposition potential of the anions, and enhance the oxidation resistance of the polymer electrolyte^[97]. These inferences are based on the idea that the oxidation potential of polymer electrolytes is limited by the irreversible oxidation of anions. However, as mentioned above, in many polymer electrolytes, lithium salts have stronger oxidation resistance than polymers, and polymers are the limiting factor to improve the oxidation resistance potential of composite electrolytes. Therefore, the interaction mechanism between polymer electrolytes and inorganic additives and the influence of these interactions on the oxidation stability of polymer electrolytes still need to be further studied. Compared with pure polymer electrolyte or HPC composite electrolyte, the oxidation stability of HCC composite electrolyte containing a large amount of ceramic phase is significantly improved^[98,99]. The relationship between this phenomenon and the polymer-ceramic interaction within the composite electrolyte has not been studied in depth.

Recently, the conduction mechanism of a polyvinylidene fluoride (PVDF) -based composite electrolyte containing a small amount of solvent has attracted much attention and controversy. Unlike the aforementioned semi-solid electrolyte in which a small amount of electrolyte does not affect the lithium ion transport mechanism in the solid electrolyte, this composite electrolyte will change the structure and performance of the polymer (electrolyte). Zhang et al compounded PVDF, LiClO₄(PVDF∶LiClO₄=1∶3 (mass ratio) and Li_6.75La₃Zr_1.75Ta_0.25O₁₂(LLZTO) particles with a small amount of N, N-dimethylformamide (DMF) solvent, and the conductivity of the composite electrolyte (25 ℃) reached 5×10^-4S·cm^-1^[100]. They analyzed the results of infrared spectroscopy, Raman spectroscopy and ¹H nuclear magnetic resonance (¹H NMR), and concluded that the coexistence of DMF and LLZTO formed an alkaline environment, which made PVDF remove hydrogen fluoride (HF) to form a double-bond structure. PVDF with double bond structure interacts with lithium salt and LLZTO to form solid electrolyte. There are also some researchers who add a large number of ceramic electrolytes to PVDF-based composite electrolytes to form new composite electrolytes. The PVDF-based (PVDF∶LiClO₄=1∶3, mass ratio) composite electrolyte designed by Zhao et al with 75 wt% Li_6.4La₃Zr₂Al_0.2O₁₂(LLZAO) nanofibers has a room temperature conductivity of 1.16×10^-4S·cm^-1, which is higher than many PEO-based HCC composite electrolytes^[101]^{[32 ⇓~34,43]}. They believe that the high conductivity comes from the transport of lithium ions through LLAZO nanofibers and ceramic-polymer interfaces. However, the above researchers did not mention whether the (residual) solvent participated in the lithium ion transport and contributed to the total conductivity besides promoting the deHF of PVDF. According to Callegari et al., the electrolyte composed of residual solvent DMF and lithium salt is the main way of lithium ion transport in composite electrolyte^[102]. Therefore, the interaction between the components of the electrolyte and the mechanism of lithium ion conduction still need to be further studied.

3.2 Performance Optimization of Composite Electrolyte with High Ceramic Content

The conductivity of the HCC composite electrolyte depends largely on the conductivity of the ceramic electrolyte or the ceramic-polymer interface. However, the actual situation is that although some ceramic electrolytes and ceramic-polymer interfaces have high ionic conductivity through experiments and calculations, the room temperature ionic conductivity of ceramic-polymer composite solid electrolytes composed of them is still not ideal, mostly ^[~34] in the order of 10^-5S·cm^-1. It is considered that the low conductivity of the composite electrolyte at room temperature may be due to the surface impurities of the ceramic phase, the agglomeration of ceramic particles, and the poor contact between ceramic particles and between ceramic and polymer^[50,103]^[56]^[50].

During the storage of electrolyte Li₇La₃Zr₂O₁₂(LLZO), water and carbon dioxide in the environment will react with LLZO to form lithium carbonate (Li₂CO₃) with low conductivity on its surface, which will hinder the transport of lithium ions in the composite electrolyte. Huo et al. compounded the Li_6.4La₃Zr_1.4Ta_0.6O₁₂ before and after heat treatment at 600 ℃ with PEO-based electrolyte (ceramic content 80 wt%), and found that heat treatment increased the conductivity of the composite electrolyte from 7.2×10^-6S·cm^-1 to 2.4×10^-5S·cm^-1(25℃)^[103]. Ruan et al. used phosphoric acid (H₃PO₄) to treat the Li₇La₃Zr₂O₁₂ ceramic electrolyte with Li₂CO₃ on the surface. In addition to forming a uniform lithium phosphate (Li₃PO₄) layer to increase the lithium ion concentration on the ceramic surface and promote the transport of lithium ions at the ceramic-polymer interface, it can also enhance the chemical interaction between the ceramic and polymer, which is conducive to improving the conductivity of the composite electrolyte^[104]^[105].

Particle agglomeration is also one of the reasons that often occurs in HCC composite electrolyte and seriously affects its conductivity. Particle agglomeration not only reduces the contact area between the ceramic phase and the polymer phase, but also reduces the chance of adding additional lithium ion transport channels through the interface effect.It also makes it difficult to form percolation channels through the connection of ceramic particles, and lithium ions need to migrate longer distances in polymer electrolytes with lower conductivity, which significantly reduces the conductivity of composite electrolytes^[54,55]. In addition, the agglomeration of ceramic particles will also reduce the film forming performance and mechanical strength of the composite electrolyte. Huang et al. compounded Li_6.4La₃Zr_1.4Ta_0.6O₁₂ nanoparticles (LLZTO content 80 wt%) modified by polydopamine (PDA) with PEO-based electrolyte by solution casting method. The conductivity of the composite electrolyte reached 1.1×10^-4S·cm^-1 at 30 ℃, which was one order of magnitude higher than that of the unmodified composite electrolyte^[106]. The PDA coating improves the contact between LLZTO and PEO due to the good wettability of PDA to both inorganic and organic substances, and also prevents the aggregation of LLZTO nanoparticles by reducing their surface energy. Yan et al. mixed silane-modified Li_6.28La₃Al_0.24Zr₂O₁₂(LLAZO) nanofibers (content 70 wt%) with polyethylene glycol acrylate (PEGDA) monomer and LiTFSI, and then polymerized to form a composite electrolyte membrane^[54]. Due to the bonding between the surface modified LLAZO and the PEGDA monomer, the nanofibers in the composite membrane are uniformly distributed, and the room temperature conductivity of the composite electrolyte reaches a 4.9×10^-4S·cm^-1. Cai et al. Filled PEO-based electrolyte into the three-dimensional skeleton of Li_6.4La₃Zr₂Al_0.2O₁₂, and the room temperature conductivity of the composite electrolyte reached 2.51×10^-4S·cm^-1, and the lithium ion transfer number was 0.53^[107]. Zhai et al. filled PEO-based electrolyte (LATP, 40 vol%) between the vertically aligned $Li_{1+x}Al_{x}Ti_{2-x}(PO_{4})_{3}(LATP)$ nanoparticles. The room temperature conductivity of the composite membrane reached 0.52×10^-4S·cm^-1, which was close to the room temperature conductivity of the pure LATP ceramic sheet at 1×10^-4S·cm^-1, while the composite membrane with the same content but randomly distributed LATP ceramic particles as filler had only 1.5×10^-5S·cm^-1^[108]. If LATP is replaced by Li₇La₃Zr₂O₁₂ with higher conductivity at room temperature, the conductivity of the composite film at room temperature can reach 3×10^-4S·cm^-1. Wang et al. Prepared vertically aligned Li_1.5Al_0.5Ge_1.5(PO₄)₃(LAGP,40 vol%) and combined them with PEO-based electrolyte to form a composite electrolyte, and the room temperature conductivity of the composite membrane reached 1.67×10^-4S·cm^-1^[109]. According to the Arrhenius equation, the activation energy was calculated, and they believed that lithium ions were mainly transported in the LAGP phase. However, the vertically aligned LATP and LAGP particles are prepared by the ice template method, and the interfacial impedance between particles and between particles and polymer electrolyte is large, so a certain amount of low molecular weight polyethylene glycol (PEG) is needed to be added as a plasticizer. Kim et al. prepared a composite electrolyte containing $86 vol\%Li_{1+x}Ti_{2-x}Al_{x}(PO_{4})_{3}(LATP)$ and 11 vol% epoxy polymer (the remaining space was occupied by pores in the composite electrolyte) by firing and sintering the slurry-cast composite powder film^[110]. The LATP particles in the composite electrolyte form a continuous ion channel, and the particles are connected with each other, so the impedance is low. The lithium ion conductivity of the composite electrolyte is comparable to that of the pure LATP electrolyte. The performance of the composite electrolyte can be improved by properly designing the shape and distribution of the ceramic and polymer phases in the composite electrolyte.

Another aspect of improving the electrochemical performance of the ceramic-polymer composite electrolyte is to improve the conductivity of the polymer electrolyte and broaden its electrochemical window. Generally speaking, the conductivity of polymer electrolytes increases with the increase of lithium salt concentration (up to 10^-4S·cm^-1 at room temperature) within the range of lithium salt content in which the polymer can be dissolved, and the electrochemical window is widened and the lithium ion transference number is increased because the unstable polar groups on the polymer are surrounded by lithium salt^[111]. This requires the polymer to have strong lithium salt dissociation ability (high enough dielectric constant), good stability, and functional groups on the chain that can bind anions^[112]. PPC, PEC and bi-grafted polysiloxane (bi-grafted BPSO) are common polymers used in high-salt electrolytes. Li et al. Composed high-salt polymer electrolytes (lithium salt content up to 80 wt%, polymer conductivity 6.26×10^-4S·cm^-1 at 30 ℃, lithium ion transference number 0.75) with PPC and lithium bis-fluorosulfonyl imide (LiFSI), and formed composite electrolytes with Li_1.5Al_0.5Ge_1.5(PO₄)₃ (content 80 wt%) by blade coating method^[60]^[113]^[114]^[60]. And that conductivity of the composite electrolyte reach the 2.1×10^-4S·cm^-1 at 25 deg C.

4 Conclusion and prospect

HCC solid state composite electrolytes have higher conductivity, lithium ion mobility and wider electrochemical window than polymer electrolytes. HCC composite electrolyte also has high mechanical strength, which can effectively prevent the short circuit in the battery caused by lithium dendrite and improve the safety of solid-state batteries. Better flexibility and processability also contribute to the preferential application of HCC solid state composite electrolytes in solid state batteries. However, due to the high ceramic-polymer interface resistance and the poor dispersion of ceramic particles, the conductivity of most HCC composite electrolytes is still much lower than that of pure ceramic electrolytes at room temperature. The contact interface between the solid composite electrolyte and the electrode also needs to be improved. In view of this, we look forward to the future research direction of solid composite electrolytes for HCC.

(1) Interfacial interactions and ion transport pathways between ceramics and polymers. Due to the high content of ceramics and the large contact area between ceramics and polymers, the interaction between ceramics and polymers will have a greater impact on the electrochemical and mechanical properties of composite electrolytes, regardless of whether ceramics or polymers are lithium ion conductors. In addition to the two "natural" ion transport paths of polymer electrolyte and ceramic electrolyte, this interaction may also provide a third transport path for the composite electrolyte, that is, the ion transport along the two-phase interface, and its contribution to the conductivity of the composite electrolyte is not necessarily smaller than that along the traditional path. In addition, the interfacial interaction between the ceramic and polymer phases can also affect the ion transference number and the ion conduction mechanism of the composite electrolyte. Nuclear magnetic resonance (NMR), especially isotopic NMR, which is sensitive to chemical environment, and infrared spectroscopy (FTIR) and Raman spectroscopy, which are especially sensitive to organic molecules and have spatial resolution, are effective methods to study lithium ion transport channels in solid electrolytes. Atomic force microscopy (AFM) based ion and electron transport imaging techniques and electrochemical strain atomic force microscopy (ECAFM) have also recently been used to study ion transport in composite electrolytes. The combination of experiments and first-principles calculations will deepen our understanding of the interaction between ceramics, polymers and lithium salts, and screen the appropriate combination of ceramics, polymers and lithium salts to improve the conductivity of composite electrolytes.

(2) Interface compatibility of ceramic-polymer. Although the addition of the polymer component significantly reduces the interfacial resistance between the ceramic electrolyte and the electrode, the introduction of ceramics may also cause problems with the stability of the composite electrolyte-electrode interface, such as the use of sulfide solid electrolytes. Therefore, in order to meet the special needs of high ceramic-polymer electrolytes, the appropriate modification of the electrode surface or the appropriate screening of the properties of ceramic electrolytes must be considered before the application of such composite electrolytes in solid-state batteries. In addition, limited by the low oxidation and decomposition potential of polymer electrolyte components, there are few reports on the application of HCC solid state composite electrolyte in lithium battery systems above 4.0 V. It is also one of the research directions to explore polymer electrolytes with high oxidation and decomposition potential matching with ceramic electrolytes or to enhance the high voltage stability of polymers by increasing the concentration of lithium salts.

(3) New mechanism polymer electrolyte. Solvents used in the preparation of traditional polymer electrolytes usually do not react chemically with polymers or electrolyte salts or ceramics, and generally do not intentionally leave any liquid solvents in the solid electrolyte. Polymers such as PVDF are only used as the skeleton material of polymer electrolytes or the binder material of powder electrodes. However, some recent reports suggest that DMF, as a solvent for lithium salt and polymer, can react with PVDF to form a new PVDF/lithium salt polymer electrolyte. Considering the relatively strong oxidation resistance of PVDF and the considerable conductivity reported (assuming that the conductivity contribution of the common liquid electrolyte composed of DMF and lithium salt or the gel electrolyte composed of DMF and lithium salt is excluded), this may be a new type of polymer electrolyte through a new lithium salt-solvent-polymer interaction mode and a new lithium ion conduction mechanism. Therefore, we may continue to pay attention to or study this kind of electrolyte system with an open attitude.

It should be pointed out that although this paper mainly introduces the solid state electrolyte conducting lithium ions for solid state lithium batteries, some basic principles (conduction mechanism, function and improvement strategy) involved in this paper are also of reference and inspiration for the study of solid state composite electrolytes conducting other ions and the development of corresponding solid state batteries.

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