【Background】
Solid-state lithium-ion batteries (SSLIBs) have become a hot spot in battery research and industrialization due to their potential high energy density and high safety. The main difference between solid-state lithium batteries and liquid lithium-ion batteries is that they replace the liquid electrolyte with a solid-state electrolyte, resulting in many types of solid-state interfaces that are unique to them. Compared with liquid electrolytes, solid electrolytes do not have fluidity, so it is difficult to penetrate into the interface between the entire electrode and separator, resulting in a series of problems caused by poor solid-solid interface contact, such as the slow transport of ions/electrons at the interface affecting the charge-discharge rate, the rigid contact at the heterogeneous interface makes the volume expansion/shrinkage inconsistency in the electrochemical cycling process, causing local stress cracks that affect the stability of the battery, and the electrode/ Poor electron transport at the electrolyte interface can induce chemical side reactions, forming a passivated interphase, resulting in high interface resistance, interface inhomogeneity, and local current density gradients, leading to lithium dendrite growth and inducing battery short circuits. In summary, a wide variety of interface problems have seriously hindered the large-scale industrialization of solid-state lithium batteries (as shown in Figure 1). Since the interface in SSLIBs involves complex carrier (i.e., lithium ion and electron) transport mechanisms, it is important to understand and control the influence of interface structure on carrier transport to further improve the performance of SSLIBs.
【Job Profile】
鉴于此，北京大学新材料学院潘锋/杨卢奕、英国萨里大学杨凯、江苏大学宋永利合作，基于他们过去5年对SSLIBs界面结构和性能的调控研究取得了系统性的研究进展 (Adv. Mater. 2018, 30, 1704436; Nano Energy 2019, 62, 844; Adv. Energy Mater. 2019, 9, 1900671; J. Mater. Chem. A 2020, 8, 342; Small 2020, 16, 1906374; Mater. Today 2021, 49, 145; Adv. Funct. Mater. 2021, 31, 2104830; Nano Energy 2021, 79, 105407; Nano-Micro Lett. 2022, 14, 191; Adv. Funct. Mater. 2023, 33, 2210845; Adv. Energy Mater. 2024, 14, 2303422; Nano Energy 2024, 125, 109617; J. Am. Chem. Soc. 2024, 146, 18535) ，对此进行总结与展望。 相关成果以“Tailoring Interfacial Structures to Regulate Carrier Transport in Solid-State Batteries”为题发表在Advanced Materials上。 邓志康和陈诗名为本文第一作者。
Figure 1. Various interfaces and problems faced by solid-state lithium batteries.
【Content Description】
1. Internal interface of solid electrolyte
Unlike liquid electrolytes, solid electrolytes (SEs) are typically heterogeneous with grain boundaries (GBs) that tend to exhibit suboptimal properties such as low ionic conductivity, high electronic conductivity, and poor mechanical strength. The voids and gaps between the grains add additional barriers to the transport of Li+, resulting in a decrease in ionic conductivity and allowing Li+ to aggregate at these boundaries. In addition, some SEs, such as Li7La3Zr2O12 (LLZO), have high electronic conductivity at the grain boundaries, which allows the electrons to bind to Li+ along the boundaries, resulting in the reduction of Li+ to Li0, which severely disrupts the electronic structure and initiates the growth of lithium dendrites along the grain boundaries.
Figure 2. (A) The case of two space charge layers formed at grain boundaries (GBs), where Φ denotes the Galvani potential. (B) Changes in energy barriers and Li+ concentrations within grain boundaries. The activation energies of grains and grain boundaries are expressed as Ea and EGB, respectively.
2. The interface inside the electrode
The composite electrode of SSLIBs is usually composed of an active material, an electronic conductive agent, and SEs. Poor contact between solid particles leads to an increase in carrier transport impedance, which is further exacerbated by cracks due to local stresses during the electrochemical cycle. In addition, Li+ is transported in a network of active materials and solid electrolytes, while e- is transported in a network of active materials and conductive agents. The gap between the electron transport network and the ion transport network hinders the cooperative transport of carriers. It is important to note that the chemical and electrochemical stability of the interface will be discussed in detail in the next section. Therefore, the internal interface in the composite electrode needs to be properly manipulated to create an efficient carrier transport network between these particles.
2.1 The internal interface of the positive electrode
The coexistence of cathode active materials (CAMs), electronically conductive carbon, and SEs in the composite cathode forms a large number of solid-solid interfaces (i.e., CAMs/SEs interface, CAMs/carbon interface, and SEs/carbon interface), resulting in a large charge transport impedance (Figure 3A). In addition, local strains at the interface between CAMs and SEs can lead to mechanical failures, such as crack formation and delamination, due to phase transitions during cycling (Figure 5B). Especially in the cathode with a high area capacity (>5 mAh cm-2), this problem is further exacerbated by the large contact impedance caused by the point contact between the particles. Therefore, it is critical to ensure close physical contact and sufficient contact area between CAMs and carrier conductors such as SEs and conductive carbon. In addition, the (electro)chemical stability between CAMs, SEs, and electronically conductive carbon must be ensured, otherwise side reactions may occur in the cathode.
2.2 Internal interface of the negative electrode
The sharp rise in lithium mine prices and serious safety concerns of lithium metal anodes have hindered the development of SSLIBs. In addition to lithium anodes, silicon-based anode materials are considered to be promising anode materials for the next generation of SSLIBs due to their ultra-high theoretical specific capacity (1000∼4200 mAh g-1), abundant resources (the second most abundant content in the earth's crust) and environmental friendliness. However, due to the large volume changes (100∼300%) that occur during lithiation/delipigration, it can lead to the pulverization of the anode particles (known as mechanical pulverization) and cause disruption of the electron/ion transport environment in the anode active materials (AAMs) (known as conductive environment attenuation). At the same time, the freshly exposed surface of the anode particles reacts with electrolytes (e.g., sulfide SEs) to form an unstable and thick solid electrolyte interface (SEI) layer, which continues to react with SEs with carbon additives (Figure 3C). In addition, the contact area between AAMs and SEs is limited in the composite anode, hindering the transport of electrons and lithium ions. Therefore, the construction of a robust carrier conduction network in AAMs can be regarded as an effective strategy to bridge the gap between academic research and practical application.
Figure 3. (A) Schematic diagram of the microstructure of the composite cathode. (B) Schematic diagram of the failure mechanism of the composite cathode. (C) Schematic diagram of the failure mechanism of composite silicon anode.
3. The apparent interface between the electrode and the solid electrolyte
In conventional lithium-ion batteries, the liquid electrolyte penetrates the planar interface between the electrode and the separator, forming a fast and stable Li+ delivery channel. However, due to the limitations of manufacturing technology, it is difficult to directly assemble the integrated SSLIBs, which means that the interface between the electrode and the SEs is inherently present. Therefore, a deep understanding of the design of the apparent interface between the electrode and the SEs is essential to construct complete SSLIBs. Insufficient mechanical and electrochemical stability at the electrode/SE interface inevitably leads to undesirable side effects or poor physical contact, resulting in poor rate performance and capacity retention.
3.1 Apparent interface between the composite cathode and the solid electrolyte
Interfacial stability issues can cause CAMs and solid-state electrolytes to decompose at high voltages. Poor electron transport between the composite cathode/SEs interface can trigger chemical reactions that form inactive decomposition interlayers (e.g., La2O3 and Li2S), and even phase transitions and structural disorders of CAMs, which increase the interface resistance (Figure 4A). Therefore, a stable and orderly carrier transport interface between the composite cathode and SEs is crucial for the excellent performance of SSLIBs.
3.2 Apparent interface between the composite anode and the solid electrolyte
Unlike cathode materials, in addition to electrochemical stability, the mechanical properties of the composite anode/SEs interface are particularly important. On the one hand, alloy-type anodes, such as silicon and tin, are accompanied by huge anisotropic expansion during lithiation, leading to stress evolution and fracture at the interface, and ultimately to the degradation of the ion conduction network (Figure 4B). On the other hand, unstable interfacial contact, growth of lithium dendrites, and violent side reactions with SEs can shorten the lifetime of SSLIBs-based lithium metal anodes (Figure 4C). Essentially, the transport and lithium deposition of Li+ occurs at the lithium/SEs interface during cycling, which requires a deep understanding of its kinetics and failure mechanisms. The apparent interface between the ideal composite electrode and SEs (Figure 4D) needs to be properly tuned to create a selective carrier transport network that inhibits electron transport and accelerates lithium-ion transport.
Figure 4. (A) Schematic diagram of the interface between the composite cathode and the solid electrolytes (SEs). (B) Schematic diagram of the interface between lithium metal anodes and SEs. (C) Schematic diagram of the interface between the composite silicon anode and SEs. (D) Schematic diagram of the ideal interface between the electrodes and the SEs.
[Summary and outlook]
Compared to conventional liquid lithium-ion batteries, SSLIBs stand out due to their potentially high energy density and superior safety. As mentioned earlier, the multiscale solid-solid interface plays a key role in SSLIBs, controlling the transport of carriers. These solid-solid interfaces, which include three internal interfaces (composite cathode, solid electrolyte, and composite anode) and two composite cathode/apparent interfaces between the negative electrode and the solid electrolyte, often serve as the bottleneck of overall carrier transport, and their characteristics are very different from those of permeable liquid-solid interfaces. For example, GBs in solid-state electrolytes often hinder the transport of lithium ions and even lead to a decrease in mechanical properties. Unstable point contact at the solid-solid interface leads to large interface resistance and significant capacitance loss. In an ideal internal interface, lithium ions and electrons are transported in synergy in the composite electrode. Conversely, the ideal solid-state electrolyte should exhibit high ionic conductivity and extremely low electronic conductivity, even insulation. It is important to note that electron transport should be inhibited at the electrode/solid electrolyte interface to avoid side reactions. It is critical to customize and build robust carrier conduction networks in SSLIBs through interface engineering.
Despite significant achievements in interface issues, the road to commercialization of SSLIBs remains bumpy. Based on the above, Figure 5 summarizes the optimization strategies and design principles of SSLIBs. Fundamentally, sufficient physical contact between each solid component, such as active material particles, electronic conductive agents, and solid electrolytes, is the cornerstone of carrier transport and battery operation. Second, the construction of a solid chemical anchor between the multiscale solid-solid interface to maintain physical contact during cycling is essential for battery systems with large volume variations, especially with large volume variations. On this basis, a deep understanding of the transport of lithium ions and electrons, and the customization of selective carrier transport networks throughout SSLIBs, is essential to address the universal problems associated with interfaces, which involve complex physical, (electro)chemical, and electrochemical-mechanical processes. The following paragraphs describe the future challenges and research directions needed to accelerate the practical application of SSLIBs.
(1) Increasing the proportion of active material in the actual battery through rational electrode design shows great potential to achieve higher energy density of SSLIBs. However, increasing the electrode thickness results in longer carrier transport distances, resulting in poor charge-discharge dynamics and ion concentration gradients along the longitudinal axis. This can be improved by introducing solid polymer electrolytes, applying multi-dimensional conductive additives such as carbon nanotubes and graphene, and matching the size of the cathode material and electrolyte.
(2) Scalable and cost-effective manufacturing technologies are essential for the commercialization of SSLIBs. Traditional slurry casting electrode fabrication techniques often lead to cracking or delamination of thick electrodes, and high tortuosity due to the random arrangement of particles, hindering fast carrier transport. An effective solution is to construct thick electrodes with vertical channels by template method or self-assembly strategy, reducing the transmission distance of Li+/e-. In addition, solvent-free dry film technologies, such as powder coating and binder fibrosis, deserve more attention in the future. These methods are not limited by the thickness of the electrode, and can suppress the delamination of various electrode components, and build a favorable transmission network. Designing robust multi-dimensional current collectors is another way to improve Li+ transmission throughput.
(3) The design and synthesis of SEs with high ionic conductivity and wide electrochemical stability is essential for the development of high-performance SSLIBs. Oxide-based SEs are challenged by high GB resistance and the material's inherent hard properties, resulting in poor contact. In this regard, halide and sulfide-based SEs are more promising because of their better ionic conductivity and ductility. In addition, the development of thin electrolyte manufacturing technology is also very important for the actual manufacturing of SSLIBs.
(4) In terms of cathodes, serious physical contact problems and high voltage unstable conditions are major issues that need to be further considered. The development of soft and conductive coatings for CAMs and the adjustment of the 3D structure of the electrodes may alleviate these problems. For the negative electrode, more attention should be paid to how to inhibit the growth of lithium dendrites and inhibit volume changes during cycling. The construction of multi-layered and dynamically stable interfaces, the use of self-healing and ionic-electron-conductive binders, and the design of external pressure applicators for pouch cells have the potential to address these challenges.
(5) Advanced in-situ characterization techniques (such as in-situ X-ray photoelectron spectroscopy, in-situ NMR spectroscopy, and in-situ transmission electron microscopy) should be developed to gain a deep understanding of the complex evolution mechanism of interfacial composition and structure, and to guide the rational design of the interfacial structure of SSLIBs. For example, X-ray computed tomography in operation has been used to observe the growth of lithium dendrites in real time. However, these advanced characterization strategies have only been studied for a limited number of electrode materials and require specific experimental equipment.
(6) Theoretical calculations and simulations (e.g., density functional theory calculations, molecular dynamics simulations, and finite element simulations) are powerful tools for screening promising SEs and analyzing physicochemical processes in SSLIBs. By calculating the properties of SEs, such as electrochemical windows and ion transport, large computational material databases can be created. In addition, it is important to develop multi-scale simulation models, from materials to batteries, for predicting electrochemical behavior and stress distribution in SSLIBs.
Figure 5. A look forward to future SSLIBs design guidelines.
Through this review, we hope to inspire research in these directions to a certain extent, so as to further achieve high-performance SSLIBs. Once a selective and stable carrier transport network is established throughout solid-state batteries, the commercialization of SSLIBs will be just around the corner.
【Literature Details】
Zhikang Deng, Shiming Chen, Kai Yang,* Yongli Song,* Shida Xue, Xiangming Yao, Luyi Yang,* and Feng Pan*, Tailoring Interfacial Structures to Regulate Carrier Transport in Solid-State Batteries, 2024, Advanced Materials.
https://doi.org/10.1002/adma.202407923
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