Samsung Science & Technology Center

LMB and ASSB represent post-lithium-ion batteries (post-LIBs) owing to the superior energy densities. ASSB adds enhanced safety, which is critical in the e-mobility sector. Despite this promise, the performance and scaleup of these post-LIBs remain unclear, calling for solutions from fundamental diagnosis and research from researchers in academia.

In this introductory talk, I will briefly summarize the key issues in LMBs and ASSBs from both materials and cell perspectives along with the hope and frustration of the existing strategies. I will also introduce what have been accomplished in my research lab regarding both topics including electrolyte and binder designs. Finally, I will briefly present how the GTC was launched and how its program was developed.

Liquid electrolyte-based lithium metal battery is a promising option to break though the limitation of lithium-ion batteries in augmenting energy density.

In the past few years, significant advances have been made in understanding the behaviours of Li anodes in liquid electrolytes and in increasing the cycle life of Li anode. Recent strategies focusing on the regulation of Li deposit morphology have proved their efficacy in minimizing dead Li formation and Li corrosion, however, current LMB technology is not enough to achieve the target of 500 Wh kg-1. The remaining challenges to reach this goal are definitely to lower the amount of electrolyte used to a level comparable to LIB and to achieve ultra-thin Li electrode or anode-less free design. In this talk, the critical steps toward the goal of lithium metal battery will be described on the basis of previous understanding and recently found features of lean electrolyte and anode-free batteries. Furthermore, the recent results on the lean electrolyte and anode-free design from my group are shared and discussed. The importance of interfacial engineering will be highlighted.

Li metal is regarded as an ideal anode for the realization of high-energydensity batteries through conjunction with 4 V-class high-Ni layered oxide cathodes that can deliver a reversible capacity, outperforming electrode materials in current commercial products [1-2]. However, extreme volume fluctuations of Li metal anodes during Li plating and stripping reactions, dendritic Li growth, and severe decay in Coulombic efficiency decrease battery lifespan [3]. Ethers are often used as an electrolyte solvent for Li metal anodes because they improve the electrochemical reversibility of the Li metal anode and suppress dendritic Li growth [4]. However, their anodic application under high voltages that surpass the upper stability limit of etherbased solvents has been largely restricted because they do not develop a suitable interface structure for 4 V-class cathodes. In this presentation, I will review the progress achieved to date in the use of electrolyte additives and fluorinated solvents for the stabilization of electrodes in Li metal batteries, classifying the available stabilization techniques into clear-cut groups and presenting a detailed overview of each group [5].

Li metal electrodes have been considered the cutting-edge technology for high energy density rechargeable batteries, including all-solid-state batteries, Li metal-ion batteries, and Li-sulfur batteries. Li metal is one of ideal anode materials because Li metal has the lowest standard electrode potential of -3.04 V (vs. SHE) and the highest theoretical specific capacity of 3860 mA h g-1. However, current Li metal electrodes suffer from challenges derived from Li dendrites formation during cycling, including (i) continuous parasitic reaction between dendritic Li metal and electrolytes, (ii) formation of a dead Li layer due to isolation of Li dendrites, and (iii) safety accidents due to the penetration of dendritic Li through a separator. For these reasons, recent efforts have been devoted to suppressing Li dendrites formation and deciphering the plating and stripping mechanism of Li metal. However, the electroplating mechanism of Li metal was not fully clarified; thus, Li metal morphology was still uncontrollable during electroplating [1-3]. In this presentation, we clarify the principal conditions that determine Li metal morphology during an electroplating process. Moreover, we demonstrate the failure mode of Li metal in terms of macroscopic and microscopic behaviour [4].

Lithium-ion batteries have had a tremendous impact on our modern society owing to its eco-friendly characteristics of being rechargeable, and having high-capacity. However, current lithium-ion batteries using graphite have almost reached their theoretical capacity, and the need for a nextgeneration battery system has emerged. Among the considered candidates, systems using Li metal electrodes are deemed one of the most favorable setups due to its extremely high theoretical capacity (3860 mAh g-1) and lowest standard electrode potential (-3.04 V vs SHE). Nonetheless, serious problems occur in systems using Li metal as anodes; such as unfavorable growth of lithium dendrites along with the low Coulombic efficiency caused by these detrimental effects. Therefore, a precise control and suppression of dendrites is crucial in commercializing these systems. Up to this day, the exact mechanism of the lithium dendritic growth is still in debate, along with the conditions in which this unfavorable morphology is suppressed is being widely researched. In that account, we demonstrate the operando X-ray imaging of lithium plating behavior [1,2]. Furthermore, in correlation with various electrochemical techniques, we define the crucial conditions in which specific lithium dendritic growth is inhibited or accelerated [3].

The extensive development of high-energy lithium-ion batteries has accelerated the transition from fuel-driven to battery-driven electric vehicles. However, the intrinsic safety issues originating from the use of organic liquid electrolytes and limited energy density impede the widespread application of conventional lithium-ion batteries. Solidifying electrolytes with inorganic superionic conductor materials have the potential of stabilizing high-capacity electrode chemistries (e.g., Li metal anode) and enhancing safety. Thus, allsolid-state batteries (ASSBs) considered a promising alternative. Among various solid electrolyte (SE) candidates, sulfide materials are at the heart of extensive research and development as they are mechanically sinterable so that ASSBs can be fabricated based on simple cold-pressing. Moreover, state-of-the-art sulfide SEs show high ionic conductivities reaching 10-2 S cm-1, comparable to liquid electrolytes. However, when conventional layered oxide cathodes with suflide catholytes are tested, poor electrochemical performances have been obtained. This is explained by several issues, including 1) poor electrochemical stability, 2) poor chemical stability, and 3) space charge layer effect. Nevertheless, promising electrochemical reversibility has often achieved for cathodes using sulfide catholytes, even for high-voltage Li-excess cathodes. In this presentation, recent intriguing issues on interfacial evolution of ASSBs will be presented.

The all-solid-state battery (ASSB) has become one of the most promising next-generation battery systems, since the aspect of safety has emerged as a crucial criterion for new large-scale applications such as in electric vehicles. Despite the recent remarkable progress in the performance enhancement, the real-world implementation of the ASSB still requires full comprehension/evaluation of its properties and performance under various practical operational conditions. Unlike batteries employed in conventional electronic devices, those in electric vehicles—the major application that the ASSB is expected to be employed—would be exposed to wide temperature variations (-20 to ~70 °C) at various states of charges due to their outdoor storage and irregular discharge/rest/charge conditions depending on vehicle drivers' usage patterns. Herein, we investigate the reliability of a Li6PS5Clbased ASSB system in practically harsh but plausible storage conditions and reveal that it is vulnerable to elevated-temperature storage as low as 70 °C, which, in contrast to the common belief, causes significant degradation of the electrolyte and consequently irreversible buildup of the cell resistance.

It is unraveled that this storage condition induces the decomposition of Li6PS5Cl in contact with the cathode material, involving the SOx gas evolution particularly at charged states, which creates a detrimental porous cathode/ electrolyte interface, thereby leading to the large interfacial resistance. Our findings indicate that the stability of the solid electrolyte, which has been believed to be failsafe, needs to be carefully revisited at various practical operational conditions for actual applications in ASSBs.

Recently, solid-state batteries (SSBs) employ sulfide solid electrolytes (SSEs) to replace flammable LE to their thermal stability and high ionic conductivity up to 10-2 ~ 10-3 S cm-1. Nevertheless, SSBs usually show poor ionic and electronic conductivity on the composite cathode, due to their poor interparticle connectivity and dispersion uniformity in the microstructure.

For the cathode of an all-solid-state battery to have a high energy density at the LIB level, ionic and electronic conductivity must be optimized. However, ion conduction via physical contact between an active material and a solid electrolyte is not as effective. Additionally, the use of conductive additives for electron conduction induces a side reaction with the SSEs, necessitating a different strategy. To improve the high utilization of Ni-rich cathode active materials (CAMs) in the composite cathode, specifically high-rate capability, conducting agents are indispensable to improve the electronic conductivity of electrodes is essential, and it is highly required to prevent degradation of ionic conductivity in the composite cathode, such as side reaction of SSEs and blocking of the ion conduction network due to the conducting agent. In this review, the all-solid-state battery cathode should have a specific value by design when compared to LIB, and what kind of research has been done for this, as well as how to study it in the future, will be discussed.

All-Solid-State-Batteries (ASSBs) have garnered a lot of attention as nextgeneration energy storage devices because of their higher energy density and superior safety to that of conventional lithium-ion batteries, which use liquid electrolytes. They, however, suffer from many unresolved problems and have not yet successfully proven their full potential. First, the capacity decay is severe and cycle life is still not satisfactory. Second, poor lithium-ion mobility and transfer kinetics in electrode–electrolyte interface or interphase limit the power density. Among several such issues, improving electrochemical stability and (electro)chemo-mechanics of the cathode–electrolyte interface is crucial for improving the performance of ASSBs. The cyclic expansion and contraction of electrodes induce local stresses and fracture formation/ propagation, and even delamination of the interface. Such issues are complicated in ASSBs compared with those in conventional liquid electrolytes because electrode strain therein only affects hydrostatic pressure, which dissipates homogeneously and ionic contact is maintained in liquid. Thus, it is crucial to develop direct characterization technologies for ASSBs to elucidate the complexity of the buried interphase, strain, lithium pathway, stress, etc. Here, I will discuss our recent results that investigate lithium-ion transport, strain evolution, and stress distribution at the single-particle level and will provide essential electro-chemo-mechanic insights for engineering ASSBs. I will also discuss how the fundamental limit of a lifetime and rate-capability is governed by the electrochemical performance of individual electrode particles.

All-solid-state batteries (ASSBs) offer a fundamental solution to resolve the safety and reliability issues of conventional lithium-ion batteries with flammable liquid electrolytes. To realize high-performance ASSBs, it is necessary to develop robust and stable solid electrolyte materials as well as the controlled fabrication methods of the composite electrodes with intimate solid-solid contacts, (electro)chemically stable interfaces, and optimized electronic/ionic conduction pathways. This talk deals with multiple reaction kinetics at solid-state composite electrodes, which provides a critical scientific foundation for developing ASSBs. At first, this talk presents AC-impedance models based on multi-rail transmission lines to describe the unique features of the electrode reactions at ASSBs. Then, exemplary experimental/analytical studies are introduced to show the interfacial and charge-transport impedances of composite electrodes with specific focus on (i) the effects of electrode formulations and electrolyte chemistry (sulfide/ halide) and (ii) performance degradation during charge-discharge cycling. In particular, the performance-determining factors of ASSBs are presented based on the impedance analyses, which can provide an insight into the design and construction of advanced ASSLBs with high rate-capability and cycling stability.