Classification of Solid State Batteries

According to the different solid electrolytes used, solid-state batteries can be divided into three categories: polymers, oxides, and sulfides. Among them, oxides and sulfide solid electrolytes belong to inorganic ceramics. In addition, in order to integrate the advantages of each system, composite electrolyte design is often used to learn from each other’s strengths.

Classification of Solid State Batteries

Polymer Solid Electrolyte

Polymer electrolytes are composed of polar polymers (such as polyethylene oxide (PEO)) and lithium salts (such as lithium bistrifluoromethanesulfonimide (LiTFSI), lithium bisfluorosulfonimide (LiFSI) or ionic liquids, etc.) Complexation formation. For example, Armand et al. prepared an all-solid-state polymer lithium-ion battery based on PEO electrolyte in 1979. The main mechanism of its ionic conduction is the complexation and dissociation of Li+ and the ether oxygen group on the PEO chain, which is realized by the movement of the PEO chain segment. The migration of Li+, its transport mechanism is shown in Figure 1. PEO-based polymers are currently the mainstream polymer electrolyte system. In addition, polycarbonate-based and polysiloxane-based systems can also be used as polymer system matrices. The mechanism of conduction varies.

Classification of Solid State Batteries

Fig.1 Li+ transport mechanism in ether-based polymer solid electrolyte

Polymer solid-state electrolytes have a series of advantages, such as good mechanical properties and film-forming properties, and are easy to form a stable interface with lithium metal; in addition, polymer electrolytes with sufficiently high Young’s modulus can effectively prevent the formation of lithium dendrites, stable lithium anode.

The polymer matrix materials of polymer solid electrolytes mainly include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polymethyl methacrylate (PMMA), polyethylene oxide (PEO), aliphatic polycarbonate (APC) and polysiloxane, etc., but most of the ion conductivity at room temperature is relatively low, about 10-7S/cm, and it has a large dependence on temperature, generally requiring more High temperature is required for normal operation, which is far from meeting actual needs. Although the room temperature conductivity can be improved by modifying the polymer matrix or adding inorganic nanoparticles, it is still difficult to meet the practical requirements of high-performance secondary batteries.

The polymer-based electrolyte system has strong processability and good interface matching. It has made great progress in the field of academic research and is the most mature. Due to the elasticity and viscosity of the polymer film, it can be mass-produced in a roll-to-roll method with relatively mature technology, and the cost is low. Therefore, the polymer system is currently the solid-state battery with the strongest mass production capacity, and has achieved small-scale mass production. At present, the polymer solid-state battery material system suitable for mass production is mainly PEO-LiTFSI, which has high conductivity at room temperature and is easy to process, which has become the first technical direction to realize industrialization.

In view of the low upper limit of theoretical energy density and low conductivity at room temperature of solid-state batteries based on polymer electrolytes, combining them with other inorganic solid-state electrolytes to improve ionic conductivity is a potential development direction. Researchers have done a lot of modification work from the mechanism of ion transport of all-solid polymer electrolytes, including blending, copolymerization, development of single-ion conductor polymer electrolytes, high-salt polymer electrolytes, addition of plasticizers, and cross-linking, Development of organic/inorganic composite systems, etc. Especially for the organic-inorganic composite system, the research enthusiasm of the academic circle is very high. For example, Goodenough et al. blended garnet-type electrolyte LLZTO with PEO, prepared composite electrolyte by hot pressing method, and changed the dosage of LLZTO from 0 to 80wt%, and discussed two types of ceramic-in-polymer and polymer-in-ceramic respectively. The performance of the composite electrolyte, the research shows that both types of electrolytes exhibit good electrochemical performance, the highest ionic conductivity can exceed 10-4S/cm at 55°C, and the electrochemical window reaches 5V.

At present, the modification of polymer electrolyte mainly has the following methods:

1) Blending:

Through the blending of various polymers, the amorphous region of the polymer electrolyte is increased, and the segmental movement ability is improved. At the same time, the advantages of various polymers are also taken into account to improve the comprehensive performance of the electrolyte. For example, blending PEO with PMMA can improve the flexibility of PMMA, reduce brittleness, and increase the amorphous area of PEO. When the PEO content is 92wt%, the electrical conductivity is 1-2 times higher than that of pure PEO or PMMA. order of magnitude. After blending PEO and PCA and coating it on the cellulose membrane, the obtained electrolyte membrane has excellent comprehensive properties, mechanical properties and thermal stability, an electrochemical window of 4.6V, and good rate performance and interfacial stability.

2) Copolymerization:

Similar to blending, different monomers are copolymerized to obtain copolymers. Copolymerization can reduce the crystallinity of the polymer, improve the mobility of the chain segment, and play the functions of different blocks at the same time, thereby enhancing the performance of the polymer electrolyte. For example, PE and PEO blocks are used, PEO is used as a conductive block, and PE is used as a mechanical performance enhancing block to improve the electrical conductivity and mechanical properties of the electrolyte. The higher the PE content, the better the overall performance. When it reaches 80%, The performance is the best, and the conductivity at room temperature reaches 3.2×10-4S/cm.

3) Single-ion conductor polymer electrolyte:

Generally speaking, the polymer electrolyte is a double-ion conductor, and the cations coordinate with the polar atoms on the polymer chain, which will lead to faster and easier migration of anions, resulting in the migration of lithium ions If it is too low (less than 0.5), it will cause serious concentration polarization and affect the cycle performance of the battery. To reduce the polarization, it is an effective approach to develop single-ion conductor polymer electrolyte systems by covalently incorporating anions into the polymer backbone.

4) High-salt polymer electrolyte:

High-salt polymer electrolyte refers to a type of electrolyte with a lithium salt content (more than 50wt%) higher than that of the polymer matrix. By increasing the content of lithium salts, the number of carriers can be increased, and new ion transport channels can be created, thereby increasing the ion conductivity and lithium ion migration number.

5) Adding a plasticizer:

The addition of a plasticizer can increase the amorphous region of the polymer electrolyte, promote the movement of chain segments and the dissociation of ion pairs, thereby improving the ionic conductivity of the polymer electrolyte. Plasticizers can generally be divided into three categories, including low-molecular-weight solid organics, organic solvents, and ionic liquids. Succinic nitrile (SN) was used as a plasticizer in the PEO-LiTFSI-LGPS system. When the SN content was 10%, the conductivity reached 9.1×10-5S/cm (25°C), and the electrochemical window was 5.5V. In the LiFePO4/Li battery system, it shows excellent cycle rate performance, but when the SN content exceeds 10%, the excess SN will aggregate and hinder the transport of ions, resulting in a decrease in ionic conductivity.

6) Cross-linking:

By constructing a polymer electrolyte with a cross-network structure, the crystallization of the polymer matrix can be inhibited to a certain extent, and the mechanical properties of the polymer electrolyte can also be significantly improved. Cross-linking methods include physical cross-linking, chemical cross-linking or radiation cross-linking.

7) Organic/inorganic composite polymer electrolyte:

The organic/inorganic composite system usually refers to a composite system composed of some inorganic fillers added to the polymer electrolyte. Inorganic fillers can be divided into inert fillers and active fillers. Common inert fillers such as Al2O3, SiO2, and TiO2 do not directly participate in the process of ion transport, but through their Lewis acid-base interaction with the polymer matrix and lithium salts, they can Reduce the crystallinity of the polymer matrix, promote the dissociation of lithium salts, increase the number of free Li+ and the fast transport channel of Li+, thereby improving the ion conductivity. Active fillers usually refer to inorganic solid electrolytes (divided into oxides and sulfides), which can directly participate in ion transport, provide lithium sources, and further improve ion conductivity. At the same time, the organic/inorganic composite system can also combine the advantages of the two, and has a great advantage in the improvement of comprehensive properties (such as mechanical properties and interface properties). It has been reported that SiO2 inorganic fillers were synthesized in situ in polymer electrolytes to prepare composite electrolytes. Compared with the method of direct mechanical physical mixing, this method improves the dispersion of inorganic fillers, increases the effective surface area of the Lewis acid-base action of the filler, significantly improves the ion conductivity, and the electrochemical window reaches 5.5V. The assembled LiFePO4/CPE /Li batteries exhibit excellent performance.

Goodenough blended the garnet-type electrolyte LLZTO with PEO, prepared the composite electrolyte by hot pressing, and changed the amount of LLZTO from 0 to 80wt%, and discussed the two types of composites of ceramic-in-polymer and polymer-in-ceramic respectively. The performance of the electrolyte. Studies have shown that both types of electrolytes exhibit good electrochemical performance. The highest ionic conductivity can exceed 10-4S/cm at 55°C, and the electrochemical window reaches 5V. By assembling LiFePO4/Li solid-state batteries , found that both types of composite electrolytes can perform well, ceramic-in-polymer is more suitable for small flexible devices, and polymer-in-ceramic can be used in large batteries such as electric vehicles due to its better safety performance Take advantage of the system.

8) Gel polymer electrolyte:

In order to further improve the room temperature ionic conductivity of the solid electrolyte, the researchers introduced a gel polymer electrolyte, that is, adding a large amount of organic electrolyte to the polymer/lithium salt system for plasticization, through a certain The method makes polymer, plasticizer and lithium salt form a gel film with network structure. Generally speaking, the preparation methods of gel polymer electrolyte mainly include Bellcore method, phase inversion method, pouring method, casting method, screen printing method, electrospinning method, etc., and the principle is to use the interaction between molecular chains. Physical cross-linking is formed by force, and then the electrolyte is introduced to form a gel polymer electrolyte. However, when the temperature rises or the polymer electrolyte is left for a long time, the electrolyte overflows due to the weakening of the force between the molecular chains, and the performance of the battery deteriorates. requirements, and does not match the existing battery production line, which is not conducive to further expansion of production.

9) In-situ polymerization:

As a new type of polymer electrolyte molding process, in-situ polymerization in the secondary battery generates a polymer electrolyte, which can make the polymer secondary battery have better interface compatibility and significantly improve the battery life. performance. The principle of in-situ polymer electrolyte preparation is to mix polymer monomers, initiators (partial reactions do not require initiators) and lithium (sodium, magnesium) salts in a certain proportion and then assemble the battery. (such as thermal initiation, gamma ray, etc.) to initiate the polymerization reaction, and the three-dimensional skeleton structure electrolyte will be produced after the monomer is polymerized (gels also need to add electrolyte solution before initiating polymerization to uniformly solidify in the voids of the network structure) . Generally speaking, according to the mechanism of polymerization, the polymerization processes for in-situ generation of polymer electrolytes mainly include: free radical polymerization, cationic polymerization, anionic polymerization, gel factor-induced polymerization, thermochemical cross-linking polymerization without initiator, non-initiated Gamma ray-initiated polymerization of the agent and electrochemically-initiated polymerization without the initiator. Their respective characteristics are as follows:

① Radical polymerization generates polymer electrolyte in situ. The general initiator is a compound that can generate free radicals, such as azobisisobutyronitrile (AIBN), and the initiation condition is mainly heating;

② Cationic polymerization generates polymer electrolytes in situ. The initiators of cationic polymerization mainly include two categories, namely protonic acid and Lewis acid. The triggering conditions are determined by the catalyst reactivity and the difficulty of monomer polymerization, generally under heating and room temperature conditions;

③ Anionic polymerization generates polymer electrolytes in situ. Anionic polymerization and cationic polymerization belong to ionic polymerization, but the catalysts for this reaction are generally electron donors, such as alkalis, alkali metals and their hydrides, amides, metal organic compounds and Its derivatives and other nucleophilic catalysts. Catalytic conditions also depend on monomer and catalyst activity;

④ The gel factor triggers polymerization to form a polymer electrolyte. The mechanism of this reaction is different from the above-mentioned reactions. This reaction is a purely physical change process and does not involve chemical changes. Small molecular organic compounds (such as sugar derivatives, fatty acids and their derivatives, etc.) at very low concentrations (usually less than 1wt%) make organic solvents interact through hydrogen bonds, π-π bonds, etc., and assemble in situ into Three-dimensional network structure, so that the small molecules of the solvent are gelled to form a molecular gel electrolyte, and inorganic particles (such as silicon dioxide, titanium dioxide, etc.) can also realize the gelation of the organic electrolyte through the sol-gel process;

⑤Other in-situ polymerization processes include thermochemical cross-linking polymerization without initiators, gamma-ray-induced polymerization without initiators, and electrochemically-induced polymerization without initiators. The in-situ generation of polymer electrolytes in secondary batteries is different from the complex preparation processes of traditional polymer electrolytes such as scraping film, electrolyte soaking, swelling to form gels, etc., but adopts an in-situ integrated preparation process, which not only simplifies the preparation process, but also Improve the electrochemical performance of the secondary battery (such as improving the solid-solid contact resistance and interfacial compatibility; optimizing the composition of CEI and SEI, thereby stabilizing the positive and negative electrodes; effectively inhibiting the dissolution of intermediate transition products and the dissolution of transition metal ions, etc.), And it plays a vital role in improving the safety performance. After decades of hard work by scientific researchers, the basic research work of in situ generation of polymer electrolytes has made great progress.

Since the conductivity of the polymer electrolyte is low at room temperature (about 10-5S/cm or lower), the conductivity can barely rise to the level of 10-3S/cm when the temperature reaches 50-80°C; in addition, the PEO material The oxidation potential is 3.8V. Now it is only limited to matching LiFePO4 positive electrode. It is difficult to match the high energy density positive electrode material, making it difficult for the energy density of the polymer system to break through 300Wh/kg. In the long run, the advantages are not obvious, and various improvements are needed. Combination of means to improve the comprehensive performance of the polymer electrolyte as a whole.

Oxide solid electrolyte

Oxide solid electrolytes can be subdivided into crystalline electrolytes and glassy electrolytes according to their crystalline forms. x(PO4)3, Li1+xAlxGe2-x(PO4)3, etc.), and glassy electrolytes include anti-perovskite and LiPON thin-film battery electrolytes. The crystal structures of several oxide solid electrolytes are shown in Fig. 2. The outstanding features of oxide electrolytes are that they have considerable electrochemical and chemical stability for metal lithium negative electrodes and high-voltage positive electrodes, wide electrochemical windows, high ionic conductivity, and their insensitivity to water and oxygen also make large-scale preparation of powders materials made possible. Although the bulk particle conductivity of the oxide electrolyte is not satisfactory, the contact between the oxide electrolyte and the electrode interface is poor. Insufficient contact makes the interface resistance very high, and the large grain boundary resistance also leads to the total conductivity is often controlled by the grain boundary conductivity. The current density distribution at the interface is not uniform. Doping can improve the bulk phase conductivity, but the improvement of the grain boundary conductivity is not significant. It depends on the grain boundary modification, so it is difficult to further improve the ionic conductivity. In addition, perovskite materials are poorly compatible with metal lithium anodes, because metal lithium can reduce Ti4+ to Ti3+ and introduce electronic conductivity, resulting in poor interface stability.

According to the difference between glassy electrolyte and crystalline electrolyte, they are generally used in thin film type and non-thin film type oxide solid electrolyte batteries respectively. The thin-film oxide electrolyte is mainly LiPON amorphous oxide, which has good performance, but it is limited to consumer electronics products with relatively small capacity. The non-film type refers to crystalline or amorphous oxide electrolytes other than LiPON, including Li7La3Zr2O12, Li1.3Al0.3Ti1.7(PO4)3, Li0.5La0.5TiO3, etc., among which Li7La3Zr2O12 has excellent comprehensive properties and is Current hot material.

Classification of Solid State Batteries

1. Thin film solid electrolyte

The power supply requirements of microelectronic devices such as microchips, micro-electromechanical systems and micro-memory in low-energy fields make all-solid-state thin-film lithium batteries an important direction for future battery miniaturization technology and industrial development. All-solid-state thin-film lithium battery means that all electrodes, electrolytes, and current collectors in the battery unit are in the form of solid-state thin films (nano- and micron-scale thicknesses). Among them, all-solid-state thin-film metal lithium batteries refer to thin-film batteries that use metal lithium films as the negative electrode of the battery. The all-solid-state thin-film lithium-ion battery refers to a thin-film battery that uses thin-film materials other than metallic lithium as the negative electrode of the battery.

The electrode materials of lithium batteries are all solid materials, which can be thin-filmed by thin-film technology. The birth of inorganic solid-state electrolytes enables electrolytes to be thin-filmed by thin-film technology, making the manufacture of all-solid-state thin-film batteries a reality. The performance of thin-film electrolytes significantly affects the cycle stability, safety, temperature resistance, and service life of thin-film batteries. Electrolytes with excellent performance have high ionic conductivity, high electronic resistivity, good contact with electrodes and good electrical conductivity. Basic characteristics such as stable chemical properties. The electrolyte is essentially a channel for ions, a barrier for electrons or neutral atoms. If there are pinholes and cracks in the electrolyte film, it is very easy to increase self-discharge, and even cause a short circuit between the positive and negative electrodes. Therefore, the electrolyte film should be dense and completely separate the positive and negative electrodes.

LiPON belongs to the nitrogen oxide type amorphous electrolyte material, and is currently the most mature electrolyte material for all-solid-state lithium (lithium-ion) thin film batteries. The γ-Li3PO4 not doped with N is crystalline and has a tetrahedral structure. The ionic conductivity of the film formed by it at room temperature is only 7×10-8S/cm. If N is doped, the obtained LiPON is in an amorphous state. The ionic conductivity has been improved, up to 3.3×10-6S/cm, the electronic conductivity is lower than 8×10-13S/cm, and the mechanical stability is high, which avoids dendrites, cracking, powdering etc. The increase in ionic conductivity after doping with N is generally believed to be due to the “nitrogen binding effect”. The coordination bond N(=N-) or nitrogen three-coordination bond N(-N<) structure increases the network crosslinking structure in the LiPON film to facilitate the transport of lithium ions. There is a certain relationship between the doping amount of N and the ionic conductivity of LiPON thin film.

Throughout the development of electrolyte thin films, the main line of research work is how to improve ionic conductivity and electrochemical stability. LiPON has excellent overall performance, but its low ionic conductivity still limits the further development of thin film batteries. With (1-x)Li3PO4-xLi2SiO3 as the target, LiSiPON thin film electrolyte was prepared by radio frequency magnetron sputtering. Studies have shown that the conductive activation energy decreases with the increase of silicon content, which leads to an increase in the ionic conductivity of the electrolyte film, which can reach 1.24×10-5S/cm. This improvement is attributed to the introduction of silicon that enhances the network cross-linking structure in the electrolyte and further increases the channels for lithium ion migration, namely the “hybrid network former effect”. Studies have found that the ionic conductivity of the electrolyte film can also be improved by introducing transition metals (Ti, Al, In, etc.) or non-metals (B, S, etc.) into LiPON, but the modification mechanism has not yet been fully agreed. The optimization of the preparation method can also play a role in adjusting the structure of the electrolyte. Adding 2 times the molar ratio of Li2O to the Li3PO4 target, the prepared LiPON film reduces the activation energy while maintaining the N/P ratio, and obtains an ion conductivity of 6.4×10-6S/cm, and there is no Li2O in the film residual. There is also new research progress in using other amorphous nitrogen oxides as electrolytes for all-solid-state thin-film lithium batteries. The LiSON solid electrolyte film was prepared by radio frequency magnetron technology under different gas components. Among them, the room temperature ionic conductivity of Li0.29S0.28O0.35N0.09 film can reach 2×10-5S/cm, and can be Electrochemical stability is maintained in the voltage range of 5.5V (vs Li/Li+). The LiBON thin film electrolyte was prepared by reactive sputtering method. The ion conductivity at room temperature was 2.3×10-5S/cm, and the decomposition voltage to Pt reached 5.0V. The Li/LiBON/LiCoO2 all-solid-state thin-film lithium battery prepared by using this electrolyte has a discharge specific capacity of 100mAh/g.

Anode materials for all-solid-state thin-film batteries generally choose light metals or light metal compounds with low electronegativity (the tendency for atoms to attract electrons in chemical bonds) and high electronic conductivity. Lithium metal has become a commonly used negative electrode material due to its low molecular weight and low electronegativity (thin film metal lithium batteries can be obtained by using lithium metal as the negative electrode). For anode materials other than metal lithium (thin-film lithium-ion batteries can be obtained), according to the reversible reaction mechanism, they are generally divided into three types: intercalation and deintercalation type, lithium alloy type and conversion type. Intercalation and deintercalation type negative electrode materials mainly include carbon materials (such as graphite), etc.; lithium alloy type negative electrode materials mainly include Si, Sn, Ge, etc.; transformation type negative electrode materials mainly include transition metal oxides, sulfides and phosphides materials, molecular formula It is MxOy, M=Fe, Co, Ni, Cu, Zn, etc.

The improvement of the energy storage performance of all-solid-state thin-film lithium-ion batteries not only requires the negative electrode film to have good “intercalation/deintercalation” characteristics, but also requires the positive electrode film to be able to provide sufficient lithium ions. The positive electrode film requires a stable structure during the electrode change process. In order to transfer lithium ions quickly and effectively, the positive electrode must have high electronic conductivity, high diffusivity, and ion insertion capacity. The higher the open circuit voltage of the positive electrode relative to the negative electrode, the higher the working voltage of the battery. Among them, the transition group metal lithium oxides LiCoO2, LiMn2O4 and their corresponding doped Ni materials have the best energy density and open circuit voltage, while the performance of TiO2, V2O5 and MoO3 is slightly lower.

The thin-film solid electrolyte adopts a new preparation method. Through the coating technology, the material is vaporized and deposited in the form of atoms or molecules to form a film, which effectively solves the problem of poor solid-solid interface contact. The main advantages are: ①The electrode/electrolyte interface is in close contact and the electrolyte layer is extremely thin, which can realize fast charge and discharge; ②The electrode material is denser, which can achieve higher energy density and lower self-discharge rate (<1% /year), and has a long cycle life (reported in the literature up to 40,000 times, with a capacity of 95%); ③The battery has higher designability, small size, matches with the semiconductor production process, and can be integrated in the electronic chip .

In 1969, Liang et al. first reported an AgI-LiI-Li thin-film all-solid-state primary battery, which was difficult to achieve wide application due to its low capacity and inability to charge. In 1983, Hitachi Corporation of Japan reported a TiS2/Li3.6Si0.6P0.4O4/Li film-type all-solid-state rechargeable lithium battery with a thickness less than 10 μm. However, the power of the battery was too low to drive the electronic equipment of the time. In 1993, Bates of Oak Ridge National Laboratory in the United States developed the amorphous electrolyte LiPON, and prepared a series of thin-film lithium batteries with good performance based on the LiPON electrolyte film, which greatly promoted the commercialization of thin-film all-solid-state lithium batteries. process. Since then, the preparation process and analysis technology of thin-film all-solid-state lithium batteries using LiPON as electrolyte have become more and more mature, and its advantages such as high safety, long cycle life, and high energy density have been widely recognized by the industry.

Although there are many studies on the characteristics of electrolyte materials, positive electrode materials, and negative electrode materials of all-solid-state thin-film lithium (lithium-ion) batteries at home and abroad, there is still a certain distance from wide application. The main reason is that the thin-film lithium battery is a multi-film layer system, which is a six-layer film system of “(negative electrode collector/negative electrode/electrolyte/positive electrode/positive electrode collector) protective film”. In the process of manufacturing thin-film batteries, Involving the functional characteristics of a single film layer, as well as the interface characteristics and matching of each film and film layers, this makes the electrochemical reaction mechanism of the entire solid-state thin film battery extremely complicated. In addition, when the metal lithium film is used as the negative electrode, because the metal lithium easily reacts with air, the manufacturing conditions and packaging requirements of the thin film lithium battery are extremely strict, and the low melting point (180.7°C) of metal lithium limits its application in integrated circuits. Application (reflow soldering temperature is about 260°C), subject to the limitation of coating process, the thickness of thin-film electrodes is usually in the order of microns, and the specific capacity per unit area is low.

2. Non-thin film solid electrolyte

Non-thin-film products have excellent comprehensive performance, mainly due to the wide electrochemical window of the oxide electrolyte, which can reach more than 5V, and the thermal stability is also relatively good. Among them, the crystalline electrolyte mainly includes perovskite type, NASICON type, LISICON type and garnet type. At present, the methods to improve the conductivity are mainly element replacement and doping with asymmetric elements. For example, Li7La3Zr2O12 is doped with elements such as Si and Ge at the A site, and the M site is replaced with Al, which can obtain a conductivity of 10-6 to 10-5 S/cm at room temperature. Here we focus on lithium lanthanum titanyl oxide and lithium lanthanum zirconium oxide.

Lithium lanthanum titanyl oxide (Li3xLa2/3-xTiO3, LLTO) belongs to the perovskite-type oxide-based solid electrolyte. The perovskite structure (ABO3) has a cubic face-centered structure, connected by common vertices of BO6 octahedrons, and the A-site cations occupy the positions of the cubic vertices. In 1993, Inaguma et al. reported that the room temperature conductivity of LLTO electrolytes could reach 10-3 S/cm, which aroused widespread concern. However, the impedance of LLTO grain boundaries is large, so its total ionic conductivity is low, which limits the application of LLTO in solid-state batteries.

In order to improve the ionic conductivity of LLTO, researchers doped and modified the Li site, La site or Ti site, but the effect was not obvious, because the doping modification can only improve the bulk phase conductivity of LLTO, but cannot improve its crystallographic conductivity. Boundary conductivity. Therefore, in order to improve the conductivity of the grain boundary, the researchers chose to modify the grain boundary of LLTO. Introducing amorphous SiO2 into the LLTO matrix can effectively eliminate the anisotropy of the outer layer of LLTO grains, increasing its ionic conductivity to 10-4S/cm at room temperature. However, LLTO will react with lithium metal, which can reduce Ti4+ in LLTO to Ti3+. In order to prevent the reaction between LLTO and metal lithium due to direct contact, a layer of PEO polymer electrolyte is coated on the surface of LLTO electrolyte, and the all-solid-state battery with LiMn2O4 as the positive electrode and metal lithium as the negative electrode has excellent cycle performance.

Lithium lanthanum zirconium oxide (Li7La3Zr2O12, LLZO) belongs to the garnet-type oxide-based solid electrolyte with the general formula A3B2C3O12, in which La3+ occupies the A site, Zr4+ occupies the B site, and Li+ occupies the C site. Its crystal structure is a three-dimensional network structure composed of [LaO8] dodecahedron and [ZrO6] octahedral shared edges. There are a large number of gaps in this structure, and Li+ is distributed in these gaps. LLZO has two crystal structures of cubic phase and tetragonal phase. The difference lies in the distribution of Li+. In the cubic structure, Li+ is in a disordered state, and there are a large number of randomly distributed Li+ vacancies in the lattice, so the migration of Li+ is relatively easy. On the contrary, in the tetragonal structure, Li+ is highly ordered, so there will be longer lithium vacancies in an orderly distribution, and the migration of Li+ will show the characteristics of simultaneous migration of multiple ions. The ion migration is relatively difficult. The ionic conductivity of tetragonal LLZO is very low, only 10-7S/cm, which cannot meet the requirements of solid-state batteries. At present, the process of preparing LLZO is relatively complicated, and requires long-term high-temperature sintering. At the same time, during the sintering process, the Li element will volatilize, and it is easy to form La2Zr2O7 heterophase. The methods for preparing LLZO mainly include high-temperature solid-phase method, sol-gel method, and field-assisted sintering method.

Although the ion conductivity of cubic phase LLZO is higher than that of tetragonal phase, pure cubic phase structure LLZO is not stable under high temperature sintering, and it is easy to convert into tetragonal phase LLZO. The change of crystal structure is suppressed by doping substitution, so as to obtain cubic structure LLZO with higher conductivity. Rettenwander et al. added Fe2O3 to the raw materials for the synthesis of LLZO, and synthesized Li6.43Fe0.19La3Zr2O12 by a high-temperature solid-state method. Replacing Li in LLZO with Fe can make the cubic structure of LLZO more stable, thereby improving its ionic conductivity; Dumon et al. Substituting the La site with alkaline earth metal Sr, doped LLZO was prepared by the traditional solid-state synthesis method, and its ionic conductivity was as high as 4.95×10-4S/cm, while the room temperature ionic conductivity of undoped LLZO was only 2.1×10-4S/cm; lithium lanthanum zirconium tantalum oxide (Li6.4La3Zr1.4Ta0.6O12) is actually obtained by replacing Zr in the LLZO structure with Ta, and its ionic conductivity is as high as 1.6×10-3S/cm. In general, the garnet-type solid electrolyte LLZO has low electronic conductivity and high ionic conductivity at room temperature, and has good stability with metal lithium anode, does not react with metal lithium, and is electrochemically stable. The window is high and the cost is low, and it has a good development prospect in the all-solid-state lithium-ion battery.

Sulfide Solid Electrolyte

With the deepening of people’s research on sulfide solid electrolytes, the conductivity of sulfide as a solid electrolyte is also increasing. In 2011, the Kanno project of Tokyo Institute of Technology combined a new type of inorganic solid state with conductivity comparable to that of liquid electrolytes. The electrolyte material – Li10GeP2S12, has a conductivity of up to 10-2S/cm at room temperature, which greatly promotes the development of inorganic solid electrolytes and their industrial applications. Compared with the other two types of electrolytes, the most notable feature of the sulfide solid electrolyte is that it has a high ionic conductivity comparable to that of the liquid electrolyte, which is due to the larger ionic radius of S, strong polarization ability, and the ability to build larger The lithium ion transport channel; the electronegativity of S is weaker than that of O, which weakens the bonding between Li+ and the lattice, and free lithium ions increase. The sulfide is relatively stable and will not react with the metal lithium negative electrode, and has good chemical and electrochemical stability. Its good flexibility also makes this type of electrolyte easy to process into a film.

Sulfide solid electrolytes can be divided into crystalline, glass and glass-ceramic solid electrolytes. The crystalline electrolyte is mainly Li10GeP2S12 (LGPS), and the conductivity at room temperature can reach the level of 10-2S/cm; the glassy electrolyte mainly includes Li2S-SiS2 and Li2S-P2S5 according to the composition, and the ionic conductivity is relatively low, about 10-8 ~10-6S/cm. The glass-ceramic phase formed by crystallization can significantly improve the conductivity of this type of electrolyte; in addition, doping halogen into the glassy electrolyte in the form of halide raw materials (LiCl, LiI, etc.) can also increase the conductivity of this type of material to 10 -3 S/cm.

The sulfide solid electrolyte has the same level of conductivity as the liquid electrolyte, and also has good mechanical properties. Disadvantages, such as poor thermal stability, easy to absorb moisture, expensive raw material Li2S, reacting with metal lithium to form an impedance layer makes the cycle performance worse, so the electrochemical and chemical properties are unstable, and the requirements for the production environment are strict. Mass production There are problems in the process, and the technical difficulty is high, which restricts its further development.

A typical sulfide solid electrolyte is thio-LISICON, which was discovered by Professor Kanno of Tokyo Institute of Technology earlier in the Li2S-GeS2-P2S system. The chemical composition is expressed as Li4-xGe1-xPxS4, and the conductivity at room temperature can reach 10-3S/ cm. The glassy electrolyte is usually composed of P2S5, SiS2, B2S3 and other substrates plus Li2S modified substances to form a conductive network. The composition range is wide and the conductivity is high at room temperature. It has outstanding advantages in high-power and high-low temperature solid-state batteries. It is a great potential. electrolyte material.

Figure 3 summarizes the development history of sulfide solid electrolytes. In 2011, Professor Kanno proposed that Li10GeP2S12 has a conductivity of 10-2S/cm at room temperature, and the electrochemical window of the electrolyte was measured to be greater than 5V. Therefore, a large number of researches on LGPS materials have appeared. In 2012, Professor Ceder carried out the stability calculation of sulfide solid electrolytes and found that the electrochemical window of LGPS materials is actually extremely narrow (1.7~2.1V), so academic researchers began to work on improving their electrochemical stability. In 2013, Professor Liang Chengdu first synthesized β-Li3PS4 by liquid phase method, and successfully synthesized layered Li4SnS4 in 2014. In 2016, Professor Kanno synthesized
Li9.54Si1.74P1.44S11.7Cl0.3, with a conductivity of 2.5×10-2S/cm at room temperature, is the sulfide solid electrolyte with the highest ion conductivity reported so far. The ionic conductivity of different types of sulfide solid electrolytes varies with temperature and their order is shown in Figure 4. It can be seen that the Li7P3S11 electrolyte has the highest ionic conductivity.

Classification of Solid State BatteriesClassification of Solid State Batteries

1. Binary sulfide solid electrolyte

The Li2S-P2S5 sulfide system is the most studied system. The P2S5-based sulfide solid electrolyte has good electrochemical stability, wide electrochemical window, high ionic conductivity, low electronic conductivity, and good compatibility with traditional negative electrode graphite materials. , and has good application prospects in all-solid-state lithium batteries. However, there are still some problems in the current Li2S-P2S5 electrolyte material. The lithium ion conductivity is still low, the chemical stability is slightly poor, the activation energy is high, and the preparation cost is high, making it difficult to realize industrial production and application. The corresponding modification methods mainly include adding lithium salts, increasing the content of network modifiers in an appropriate amount, doping oxides or forming glass-ceramic composite electrolytes, etc.

In Li2S-SiS2 sulfide solid electrolyte, SiS2 is a covalent compound and also a network modifier. Among them, the SiS2 glass macromolecule is a network structure composed of [SiS4] tetrahedra, which can generate more gaps for lithium ion migration and improve electrical conductivity. Li2S is an ionic compound. When SiS2 is added to it for a chemical reaction, the [SiS4] tetrahedral chain structure is broken, and many lithium ions bonded by ionic bonds are added. Li2S-SiS2 has high electrical conductivity and glass transition temperature, good thermal stability and electrochemical performance, and is easy to prepare, so it has become one of the hotspots of researchers. Although this system has high electrical conductivity, it has poor compatibility with graphite anode materials in industry, and it will affect the intercalation of lithium ions in the graphite layer during charge and discharge. In addition, Li2S and SiS2 are easy to absorb moisture and have poor chemical stability, so further improvement of electrochemical stability has attracted more and more attention. Doping modification is a common method to improve the performance of solid-state electrolytes, which can change the size of voids and channels, weaken the framework and the force of migrating ions, thereby increasing the conductivity. If O element is introduced, the activation energy of the system is reduced and the electrical conductivity is improved. The introduction of Li3MO3 (M=B, Al, etc.) increases the conductivity to 10-3S/cm at room temperature, and the stability of the system is also improved.

Li2S-GeS2 sulfide solid electrolyte is not strong in water absorption in the air, so it can reduce technical difficulties when applied to all-solid-state batteries. But the electrical conductivity of this system is not high, and the doping of oxide is an effective method to improve the electrical conductivity. If GeO2 is introduced, when the molar content reaches 5%, the electrical conductivity of the system can reach 10-4S/cm.

2. Ternary sulfide solid electrolyte

The binary sulfide solid electrolyte system has more or less problems such as low conductivity, poor electrochemical stability or poor chemical stability, which limit its application in industry, which makes adding a third sulfide network Forming agent has become a research hotspot.

(1) Li2S-GeS2-P2S5 sulfide solid electrolyte

Li10GeP2S12 (LGPS) synthesized in 2011 has attracted people’s attention. The crystal structure of LGPS is a three-dimensional network structure consisting of four basic units: (Ge0.5P0.5)S4 tetrahedron, PS4 tetrahedron, LiS4 tetrahedron, and LiS6 octahedron. The conductivity of the electrolyte of the system synthesized by ball milling, mechanical grinding, and high-temperature quenching has repeatedly reached new highs. However, the expensive price of Ge has greatly increased the cost, and many studies have been devoted to finding new elements to replace Ge.

(2) Other ternary systems

Including the introduction of MxOy (M=Fe, Zn, Bi) nanoparticles on the basis of Li2S-P2S5 to improve the electrochemical window of the sulfide solid electrolyte. After experiments, it was found that the amount of H2S gas produced by these electrolyte samples containing MxOy additives decreased in the order of Fe2O3>ZnO>Bi2O3. Among them, Bi2O3 has the best elimination effect, but its electrochemical window is relatively narrow. The electrical conductivity of glassy 90Li3PS4·10ZnO is greater than 10-4S/cm, and the electrochemical window is greater than 5V. The results of charge and discharge experiments show that the In/SE/LiCoO2 all-solid-state lithium battery assembled with 90Li3PS4·10ZnO has excellent cycle performance, and its first discharge specific capacity is 90mAh/g. After 70 charge and discharge cycles, the specific capacity of the battery It can still be maintained at 90mAh/g, and the Coulombic efficiency is 100%.

However, the current problems are that the electrochemical properties of sulfide solid electrolytes are unstable, the voltage window is narrow, and it is difficult to match the positive and negative electrodes. The main idea to improve the stability of its interface is to make the interface decomposition products electronically insulative and play the role of “surface passivation” through the regulation of doping element types and components, thereby improving its electrochemical stability; in addition, it can also be used in solid electrolytes. A coating material is added at the interface with the electrode material to form a physical protective layer and inhibit the decomposition reaction of the electrolyte.

In order to improve the shortcomings of sulfide solid electrolytes, researchers have done in-depth research on the preparation and synthesis methods, doping modification and composite modification of sulfide solid electrolyte materials. The preparation method is no longer limited to the traditional solid-state material preparation method, but replaced by sintering, ball milling and liquid phase method, which are relatively cheap, convenient and quick engineering methods. Low activation energy, improve system conductivity. As a typical representative of inorganic electrolyte materials, sulfide solid electrolyte materials have attracted much attention due to their excellent ionic conductivity, wide electrochemical window and stable electrochemical performance. By reducing the cost of synthetic electrolytes, simplifying synthesis steps, and introducing alternative elements, it is the future development direction of sulfide solid-state electrolytes to give full play to the performance and coordination of each element.

Composite Electrolyte

Based on the above, a solid electrolyte often has a single function, and its performance has advantages and disadvantages. In order to learn from each other’s strengths, composite electrolytes came into being. Comparing with pure polymer solid electrolytes, composite solid electrolytes have lower melting temperature and glass transition temperature. The presence of the filler can improve the ionic conductivity and mechanical properties of the electrolyte, and the electrolyte is stable and compatible with the lithium negative electrode. The main types of fillers are: inorganic inert fillers, inorganic active fillers and organic porous fillers.
1. Inorganic inert filler composite solid electrolyte

The addition of nanoparticles to polymer solid electrolytes affects how lithium ions are transported. Nanoparticles can inhibit the crystallization of polymers, increase the number of free chain segments and accelerate the movement of chain segments. As far as PEO-based electrolytes are concerned, fillers can reduce the recrystallization of PEO and enhance the mobility of polymer chains. The surface of the nanoparticles serves as the cross-linking site between the PEO chain segment and the lithium salt anion, forming a lithium ion transport channel. The acidic surface of the filler is easy to adsorb anions, which enhances the solubility of lithium salts, and the corresponding cations become freely mobile conductive ions. When the mass fraction of nanoparticles reaches 10%-15%, the ionic conductivity of the electrolyte increases significantly; when the amount of nanoparticles added is higher than the optimal amount, due to the reduction of the conductive polymer content, the ion transport path is blocked, and the ions in the composite solid electrolyte The conductivity decreases instead.

2. Inorganic active filler composite solid electrolyte

Compared with inert fillers, active fillers have the advantage of directly providing Li+, which can not only increase the concentration of free Li+, but also enhance the surface transport ability of Li+. The ion transport mechanism of the active filler mainly has the following four types: ① the interaction between the active filler and the ion pair, promote the dissociation of the ion pair, and increase the number of conductive ions; ② in the composite electrolyte, the interaction between the lithium salt and the nano filler, the nano The surface of the filler adsorbs the moving cations, which increases the ion transmission channel; ③The surface of the filler adsorbs anions, which reduces the activity of anions, promotes the dissociation of ion pairs, and increases the activity of cations; ④The presence of fillers promotes the cross-linking of EO segments and anions, Change the structure of the polymer chain at the interface to provide a more convenient channel for Li+ transport.

3. Organic porous filler composite solid electrolyte

Organic fillers not only have good compatibility with the electrolyte matrix, but also the pore structure of macromolecules provides a natural channel for Li+ ion transport. Therefore, the addition of organic porous fillers has also become a current research hotspot. Nano-metal-organic frameworks are used to prepare composite solid-state electrolytes, and the assembled all-solid-state batteries have good electrochemical performance. Goodenough et al. prepared a nano-mesoporous organic filler, which was combined with a PEO matrix to obtain a solid electrolyte. The presence of the porous filler can adsorb small molecules at the interface and improve the interface stability between the electrolyte and the electrode. All these provide a new design idea for the development of new solid electrolytes.

For example, polyacrylonitrile (PAN)/lithium aluminum titanium phosphate (LATP) nanocomposite fibers are used as the three-dimensional framework, and PEO-based polymer electrolyte is used as the filled composite solid electrolyte membrane. Polymer fibers can insulate LATP from lithium metal, achieving an electrode/electrolyte interface with high stability. Due to the introduction of the three-dimensional composite fiber network skeleton, the tensile strength, electrochemical window and thermal stability of the composite electrolyte membrane have been effectively improved compared with the pure polymer electrolyte. Some studies have used a polymer/ceramic/polymer sandwich structure to coat polyethylene glycol methacrylate on the surface of LLZO ceramic sheets. The polymer coating avoids direct contact between LLZO and lithium metal. The surface of the material provides a soft contact interface while achieving uniform deposition of lithium ions and inhibiting the growth of lithium dendrites.

Designing a composite solid electrolyte with uniform multiphase composition, stable interface of each phase, and high lithium ion conductivity can improve the physical contact of the solid-solid interface, reduce the interface impedance, and achieve stable compatibility between the solid electrolyte and the positive and negative electrode materials.

Classification of Solid State Batteries

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