Here in this review, we put a special focus on the fundamental issues about cathode-solid electrolyte interfaces in solid state lithium batteries based on diverse cathode-electrolyte materials. We hope to summarize the previous understandings and recent advances on the interface research.
Furthermore, we hope to shed light on the possible approach to the final understanding of interface phenomenon with advanced characterization techniques. In chapter 2, we present a brief overview on basic principle of battery operation and scientific issues relevant to interface layer. In chapter 3, the interfacial problems between cathodes and four kinds of prevailing solid electrolytes are specifically discussed, corresponding optimization methods are also introduced.
In chapter 4, advanced characterization techniques used for the investigation of solid-solid interface behavior are consolidated, corresponding advances and achievements are summarized. Finally, we give a comprehensive conclusion about the cathode-solid electrolyte issues and perspectives for building favorable interfaces. Solid state lithium batteries have three major components cathode, anode, and solid electrolyte. The cathode material herein refers to the same lithium-containing compound as the lithium ion battery. In this process, oxidation and reduction reactions take place at the cathode and anode sides, respectively.
A stable and intimate interface is necessary to ensure the above reaction steps proceed smoothly. Interface instability may derive from chemical or electrochemical problems, a most fundamental origin is the abrupt electrochemical potential change at electrode-electrolyte interface. While band bending and alignment of Fermi level will happen due to the formation of a heterojunction.
Hausbrand et al. In such cases, solid electrolyte decomposition and intermediate transition layer formation may take place at interface. In addition, conventional high temperature processing may further induce interfacial interdiffusion of TM transition metal elements and favor the formation of specific transition region. Copyright American Chemical Society].
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B Illustration of ionic and electronic structures of electrode and electrolyte before left and after contact right. Type 1 is a stable interface scenario with no electrolyte decomposition or chemical side reactions. This is the ideal interface, which seldomly appears in practical systems. Within this scheme, further interfacial side reactions can be suppressed and battery operation can be maintained. Type 3 is an undesirable but most common interface with mixed ionic and electronic conductivity. Depending on the intrinsic property of electrode and electrolyte, different types of interfaces will be built up, but only type 1 and 2 are accessible for practical applications.
By introducing proper buffer layers between cathode and electrolyte, a stable artificial layer can be constructed and convert interface from type 3 to type 2. Apart from the chemical stability of the interface, mechanical behavior also has a significant impact on battery performance. In conventional lithium ion batteries based on liquid electrolyte, cathode particles can be totally immersed in liquid electrolyte and passivation layer called solid electrolyte interphase SEI may form.
However, it is challenging to maintain intimate electrode-electrolyte interface in solid state lithium batteries, especially over many cycles Goodenough, The deficient contact in solid state lithium batteries may well-lead to low utilization of active particles, large polarization and even contact loss during cycle. Models of morphology at the interface between cathode-electrolyte: A The cathode particles are totally immersed in a liquid electrolyte and an interface layer will form. B Cathode particles are distributed in a Li-PEO binder with good contact while voids will generate upon cycling because of the interfacial reactions or pulverization of cathode particles.
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C Sulfide particles have favorable mechanical properties as ductility and deformability, which could change its shape to match with the rigid solid electrode. D Solid oxide electrolyte: Poor point-contact will form due to the rigid ceramic nature. Interface layer will form in all the aforementioned system if decomposition reactions or interdiffusion occurred at the interface. Due to the distinguished mechanical properties, there is distinct difference in contact behavior among various types of electrolytes.
Solid electrolytes can be generally classified into SPE and solid inorganic electrolyte, the latter can be further classified into solid oxide and solid sulfide electrolyte. Polymer electrolyte has moderate contact with cathode due to the elasticity and deformability of organic polymers. The effective contact area between cathode and polymer electrolyte will consequently reduce with battery cycle. Due to reasonable mechanical ductility, deformable sulfide particles could also change its shape to match with cathode particles.
While contact loss will also happen upon cycling along with the shrinkage and expansion of cathode particles Koerver et al.
The insufficient mechanical contact facilitates cathode particles completely isolated from solid electrolyte, i e. The poor solid-solid contact typically brings about large polarization and low capacity. To improve the interface contact, various strategies have been adopted, such as in-situ synthesis of solid electrolyte, interface buffer layer, cathode coating, gel system etc.
Based on different properties of various electrolytes, specific strategies will be adopted, and introduced specifically in Chapter 3. After Wright's discovery of alkali metal ions conductivity in poly ethylene oxide PEO in Fenton et al. PEO-based SPE is widely accepted as a most promising candidate for solid state lithium batteries owning to its advantages such as easy fabrication, low cost and excellent compatibility with lithium salt.
Considering the catalytic effect of transition metal oxides, PEO decomposition may well-be triggered at the interface region.
And improving the antioxidant properties of SPE to high-voltage range is essential to realize high energy density solid state lithium batteries based on PEO Fan et al. Diverse optimization strategies have been utilized for different SPE systems, as discussed below. The first one is the local current enhancement induced by cathode pulverization, which can be attributed to the solid-solid contact between electrolyte and electrode.
These results indicate that the mechanical property of SPE and the Li salt selection are both essential. Ma et al. Copyright Nature]. Improving the antioxidative capability of PEO is another critical aspect to promote high voltage interface stability. Copolymerization, branching, and crosslinking are common polymer modification methods, which also favor designing more antioxidative polymers Tong et al.
UV-induced co polymerization can promote effective interlinking between polyethylene oxide PEO chains plasticized by tetraglyme. Hereby, Porcarelli et al. It is clear that electrolyte creates conformal coating by following the contours of active particles, which leads to improved active materials utilization. Similar optimized SPE have also been achieved by a PEO and liquid-crystalline copolymers with small molecular liquid crystals as fillers.
High ionic conductivity, lithium ion transference number combined with wide electro-chemical stability window of the copolymer facilitated a good electrochemical performance Tong et al. Except for developing PEO derivatives, exploiting other antioxidative polymer electrolyte has also attracted much attention Zhang et al. Chai et al. From in-situ polymerization of PVCA, polymer electrolyte can be even incorporated into the porous cathodes and the effective contact area can be much increased as a result.
Owing to the good contact and compatibility, the battery exhibited high discharge capacity and excellent cycling performance. Thus, gel polymers possess both cohesive properties of solids and the diffusive property of liquids Manuel Stephan, which makes hybrid solid liquid electrolyte lithium batteries have unique advantages Huang et al. The long terms cycling stability was also improved obviously Zewde et al. By phase inversion technique, Deng et al. Solid composite electrolyte combined the virtues of both polymer and ceramic, exhibiting excellent mechanical stability, high ionic conductivity, wide electrochemical stability window, intimate contact performance, and etc.
Relevant researches will be specifically introduced in Chapter 3. Apart from solid electrolyte modification, cathode surface modification is another effective way to mitigate the interface degradation. Consequently, surface modification on cathode material becomes another way to enable PEO operation at high voltage. Yang et al. Oxide-based solid electrolytes exhibit good chemical stability against air and compatibility with high voltage cathodes. Solid oxide electrolyte is a most competitive choice for solid state lithium batteries Chen et al. However, there are two major challenges for solid oxide electrolytes.
The first one is the generally low ionic conductivity, which is lower than sulfide electrolytes. Despite the phenomenally low intrinsic bulk conductivity, recent investigations point to the high interface polarization that restrains battery dynamics. The second challenge is the rigid ceramic nature, which causes poor point-contact at electrode-electrolyte interface, as discussed above. Solid oxide electrolytes have a key advantage of intrinsic wide electrochemical window. In the following discussion, we mostly take garnet LLZO as a typical example to discuss the interfacial problems solid oxide electrolyte faced with, other systems are briefly mentioned at the end.
Tremendous efforts have been devoted to lower the sintering temperature of solid oxide electrolytes to mitigate the interdiffusion problem, while very finite progress has been achieved so far. In , Park et al. It was further proved that interface modification with Li 3 BO 3 can reduce chemical cross-contamination and improve physical bonding. By radio frequency RF magnetron sputtering, Kato et al. In-situ synthesis of LiCoO 2 by PLD guaranteed an intimate contact between cathode and solid electrolyte, while introducing Nb layer improved the interface performance by forming Li-Nb-O amorphous region.
Kazyak et al.
Journal of the Optical Society of America B
Low melting compounds were also employed for good interfacial contact in solid state lithium batteries. Liu et al. Corresponding interface resistance reduced dramatically and electrochemical performance improved significantly. Yoshima et al. LLZO coated with polyacrylonitrile PAN -based gel was prepared as electrolyte sheet, which reduced internal resistance of the whole battery.
Very recently, Han et al. Owing to the rigid ceramic nature, most solid oxide electrolytes face similar interfacial challenges when paired with solid cathode. The aforementioned interface modifying strategies can also be applied to diverse solid oxides electrolytes, except that typical solid oxide electrolyte are further hindered by other factors.
As a result, most works on LLTO focus on improving ionic conductivity and chemical stability vs. Li anode Chen and Amine, ; Kotobuki et al. In these solid oxide electrolytes related research, interface softening, and in-situ synthesizing have also been carried out Kim et al. Solid sulfide electrolytes are the derivatives of solid oxide electrolytes by substituting oxygen with sulfur.
Another attractive feature of solid sulfide electrolyte is their mechanical property. These materials exhibit plastic deformation under mechanical pressure, and this softness makes it possible to prepare densely packed interface Koerver et al. According crystallinity difference, the two kinds of SSEs can be further divided into glass, glass ceramic, and ceramic form solid electrolyte, which exhibit different performance in terms of ionic conductivity, chemical stability, and contact with solid electrode.
However, SCL will remain at electrolyte side due to the single ionic conductivity of electrolyte. SCL was firstly proposed by Wagner and extensively investigated on conduction type and conductivity change of composite materials, polycrystalline and heterojunctions Liang, ; Maier, ; Bhattacharyya and Maier, With theoretically calculation, Haruyama et al.
This result consistently explained SCL at atomic-scale and clearly indicated the effect of buffer layers. Even though solid sulfide electrolyte has moderate physical deformability, electrochemically driven mechanical failure also contributes to interfacial resistance increase and capacity fading. Koerver et al. Results suggest that the majority of passivating layer is developed during the first charge and present slow growth upon further cycling. It was further found that electrode-electrolyte contact lose occurs in first charging due to electrochemical contraction.
In order to achieve and sustain intimate interface contact, different methods, and strategies have been developed and investigated. Sticking the supercooled liquid state of electrolyte on active material particles combined with a hot press was used to achieve an intimate electrode-electrolyte interface Kitaura et al.
Yao et al. The obtained intimate contact contributed to an excellent rate capability and cycling stability. Similar intimate contacts could be achieved by sulfide electrolyte coating onto active materials to form a favorable interface Ito et al. By Mixing LiCoO 2 particles with different grain sizes during the electrolyte coating process, higher packing density pellets with less voids were obtained both before and after cycling which ensured fine networks of ionically conductive pathways.
Moreover, Oh et al. The decomposition layer could also isolate the delithiated LixNi 0. The research demonstrates that suitable conductive additive and sulfide solid electrolyte are crucial to overcome the poor cycle performance of high-voltage solid state lithium batteries. Yoon et al. Solid composite electrolyte is a subset of polymer electrolytes by dispersing electrochemically inert fillers, such as Al 2 O 3 and TiO 2 nanoparticles or inorganic solid electrolyte into polymer electrolyte.
Weston and Steele, ; Croce et al. Due to the absence of liquid components and interfacial stabilizing action from dispersed fillers, composite electrolyte offers wide electrochemical stability window Croce et al. With advantages such as high ionic conductivity, wide electrochemical stability window, favorable interface mechanical properties, composite electrolytes have attracted extensive attention. Lin et al.
Zhao et al. In , Lin et al. At the same time, electrochemical stability window can be largely extended up to 5. The improvement of electrochemical stability indicates that the adsorption effect on anion is much stronger in in-situ CPE, which suppresses anodic decomposition at high potential Park et al. A Schematic figures showing the procedure of in situ hydrolysis and interaction mechanisms among PEO chains and MUSiO 2 up and the electrochemical stability windows curves of three kinds of solid electrolyte.
The curves refer to first three galvanostatic charge and discharge curves.
Recently, Chen et al. The interface between composite cathode and composite electrolyte layers may keep its structural integrity albeit the large volumetric change during cycling Chen et al. Since the synergetic-composite electrolyte combines the virtues of two components, compositing stands a chance in building favorable interfaces, and further realizing high energy density solid state lithium batteries.
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Chen et al. The composite solid electrolyte possesses high self-standing and flexibility, which exhibits electrochemical stability window up to 5. Hence, by varying the composition of composite electrolyte, different properties will be obtained for special applications. In anodic process, in-situ prepared electrolyte shows no anodic current until 5.
The differences in ionic conductivity and stability may originate from more homogeneous dispersion of fillers in PEO by in-situ preparation than by mechanical-mixing. Interfacial challenges exist in cathode-solid electrolyte systems according to the different characteristics of the four types of solid electrolytes and the corresponding solutions, recent advances and limitations still exist. As discussed above, the interface behaviors play an important part in determining the final solid state lithium battery parameters and performances, including internal resistance, kinetic response, and cycle stability etc.
However, the buried solid-solid interfaces in solid state lithium batteries are extremely difficult to investigate directly, and present knowledge on interfacial reactions and interfacial kinetics is still deficient. As a result, it is increasingly important and urgent to develop novel characterization techniques for more detailed understanding into the interface behavior Hu et al. Kim, Ruslan V. Kapra, Andrey A. Fedyanin, Mitsuteru Inoue, Anatoliy F.
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Allow All Cookies. B 22 , Not Accessible Your account may give you access. Abstract Magnetization-induced effects in the nonlinear optical response of magnetic media, such as magnetization-induced-second-harmonic generation MSHG , led to very strong and novel nonlinear magneto-optical effects that appear to be very sensitive to magnetic interface properties.
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Magnetization-induced second-harmonic generation of light by exchange-coupled magnetic layers L. Quasiparticle dynamics 1. Nonlinear terahertz studies of ultrafast uasiparticles dynamics in semiconductors 2. Higher order photoemission from metal surfaces 3.
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Electron dynamics in image potential states at solid surfaces 4. Relaxation dynamics in image potential states at solid interfaces 5. Spin-dependent relaxation of hot electrons on ferromagnetic surfaces 7. Electron-phonon interaction at interfaces II. Collectice excitations 8. Low-energy collective electronic excitations at metal surfaces 9. Low-dimensional plasmons in atom sheets and atom chains Excitation and time-evolution of coherent optical phonons Photo-induced coherent nuclear motion at surfaces Coherent excitations at lanthanide surfaces III.
Heterogeneous electron transfer Electron transfer investigated by x-ray spectroscopy Exciton formation and decay at surfaces and interfaces Electron dynamics at polar molecule-metal interfaces: Competition between localization, solvation, transfer IV.
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