Bonho Koo’s research while affiliated with Seoul National University and other places

Degradation of electrode particles during cycling is chemo-mechanically coupled and needs to be systematically investigated for the Li-ion battery development. The effect of Li transport pathway on internal stress and crack formation during cycling remains elusive. The opposite effect of cracked, newly exposed active surface on Li transport pathway alteration, accelerating the chemo-mechanical degradation, also remains poorly known. Using operando scanning transmission X-ray microscopy on [100]-oriented LiFePO 4 single particles, we demonstrated that lithium insertion from the edge of the non-cracked particle dynamically generate tensile stress within the particle, thereby initiating crack formation during lithiation. The exposed crack surface acts as a Li (de)insertion hotspot, redirecting Li transport pathway and internal stress-fields. Delithiation process induces the Li-poor phase around the crack, creating tensile stress, propagating the crack, and subsequently exposing fresh surface which serves as active hot-spot. On the other hands, lithiation process induces the Li-rich phases around the crack, creating compressive stress and suppressing crack propagation. Phase-field simulations demonstrate chemo-mechanically interactive loop that lithium (de)insertion pathway determines dynamic tensile/compressive stress distribution, which recursively determines (de)insertion pathway. This study offers insights that can help develop high-performance and long-lasting batteries, as well as cycling protocols that suppress crack formation. Figure 1

Understanding the dynamics of lithium diffusion within solid-state electrodes is pivotal for advancing high-performance batteries. Conventionally, lithium diffusion within solid-solution battery particles has been assumed to be solely driven in the direction of minimizing the concentration gradient, resulting in a monotonous lithium distribution. However, employing operando scanning transmission X-ray microscopy, this study has revealed the presence of non-monotonous dense and dilute concentration domains of lithium within individual single-crystalline LiNi 1/3 Mn 1/3 Co 1/3 O 2 particles at the nanoscale during charging and discharging processes. Our findings advocate that the formation of Li-dense and -dilute domains is associated with nanoscopic non-uniform strain fields, challenging conventional solid-solution lithium diffusion models that rely solely on the concentration gradient as the driving force. Bragg coherent X-ray diffraction imaging verified such non-uniform nanoscopic intraparticle strain fields, which may cause the direction of lithium diffusion to deviate from the direction of the concentration gradient. Moreover, we have identified that Li-dilute domains near the surface could be manipulated in situ to enhance rate-capability. This study paves a new avenue for understanding solid-state diffusion at the nanoscale, enabling the fabrication of high-performance batteries. Figure 1

In all-solid-state batteries (ASSBs), (electro)chemo-mechanical aspects such as uneven interfaces, cathode-electrolyte interphase formation, delamination, fracture, defects, etc., are the major factors in capacity fade but remain largely unknown. Nonhomogeneous transfer of lithium ions can cause significant variations in strain and stress within battery electrodes, leading to degradation in battery performance. In all-solid-state batteries (ASSBs), the lithium pathway and the associated strain/stress field become more intricate due to the (electro)chemo-mechanical reaction at the electrode-electrolyte interfaces. The dynamic volume change in active particles are heavily influencing by strength of the solid electrolyte and interfacial conformality. This can continually alter the lithium pathway and the internal stress field, leading to recurrent redefinitions of (electro)chemo-mechanical environment. Here, we have developed an operando coherent X-ray imaging platform and associated analysis methodologies. This technology can track the nanoscale transport of lithium and the strain evolution of individual electrode particles in ASSBs. With this platform, we gained a comprehensive electrochemical and mechanical understanding of the cycling properties of single electrode particles in ASSBs.

Understanding the diffusion dynamics of lithium within solid-state electrodes is pivotal for developing high-performance batteries. In this context, layered oxides were utilized as a promising cathode material due to their high energy density and fast intraparticle lithium diffusivity. Despite advancements in material composition, coating, and doping, the understanding of intraparticle lithium diffusion has long been described by Fick's law. Conventionally, lithium diffusion is assumed to generate a monotonic lithium concentration gradient within solid-solution single-crystalline battery materials during cycling. This raises fundamental questions about diffusion in layered oxides; (1) Can the diffusion of Li in solids be interpreted as Fickian diffusion, similar to diffusion in gases or liquids, even though it involves structural and phase evolution throughout the battery cycle? and, (2) Does the fast diffusivity (10 ⁻¹¹ -10 ⁻⁹ cm ² /s) support the homogenization of Li? In this study, we address these questions surrounding lithium diffusion in layered oxide by utilizing operando scanning transmission X-ray microscopy. We revealed the formation of mobile Li-dense/-dilute nano-domains within individual single-crystalline LiNi 1/3 Mn 1/3 Co 1/3 O 2 (scNMC) during battery cycles. We term this phenomenon ‘multi-clustered lithium diffusion’, distinguishing our findings from the conventionally suggested Fickian diffusion model in solid-solution materials. These domains persist for at least 4 hours during relaxation, accompanied by locally residing strained domains, as confirmed by Bragg coherent diffraction imaging (BCDI), within a single particle. We believe these domains arise due to the compensation of localized chemical potential gradients that are generated by the sustained presence of strain within the battery particles during cycling. While maintaining integrity of Li-dense/-dilute domain at various C-rates, STXM result further show that Li-dilute domains maintain during the discharging. Given the lower concentration of Li at insertion boundaries, which could lower the surface charge transfer impedance of the system, Li-dilute domains facilitate lithium transport by functioning as low-resistance pathways. Through a comprehensive analysis of electrical impedance spectroscopy (EIS), STXM imaging and finite element analysis (FEA), we showed that controlling the local domain fraction is crucial for controlling the overpotential during subsequent charging. Our study introduces new insights into nanoscale solid-state diffusion, thereby enabling the fabrication of high-performance batteries. Figure 1

Transition-metal-based solid-state electrocatalysts undergo dynamic phase transformation governed by the local electrochemical environment during operation, e.g., oxygen evolution/reduction( 1,2 ), hydrogen evolution( 3 ), and carbon dioxide reduction( 4 ). Electrochemical active species, often hidden before operation, can become evolved due to the applied electrochemical potential and/or surrounding chemicals. These species are stabilized under the local environment dynamically generated by the reaction product, e.g., proton (H ⁺ ) or hydroxyl (OH ⁻ ) group( 5,6 ). Active species often account for a small fraction (i.e., minor motif) of the entire catalyst volume, and the nanoscale morphology and chemical composition of the electrocatalyst are inhomogeneous and continuously change during the electrochemical reaction. Thus, capturing the active species remains a major challenge. Spatiotemporal observation of the structural/chemical changes of the electrocatalyst and the correlation with the local electrochemical environment may reveal the active species and elucidate the governing step toward their formation. Furthermore, identification of the key intermediate step of catalyst phase transformation allows the redirection of low- to high-active catalysts; however, this remains challenging. During electrochemical CO 2 reduction (ECR), (hydr)oxide-derived Cu electrocatalysts experience significant phase transformation and show high activity and selectivity for carbon dimerization (C–C coupling)( 7,8,9 ). The location and chemical composition of the active species evolving during electrochemical phase transformation may be strongly heterogeneous; however, these species have not been clearly determined. Recent studies have employed operando characterization techniques, e.g., fluorescence hard/soft X-ray( 4,10 ), X-ray photoelectron( 11 ), and Raman spectroscopy( 6 ), to elucidate the active species responsible for high C–C coupling activity. However, although these techniques could track the changes in chemical composition of the electrocatalysts during operation, they did not reveal the spatiotemporal evolution of the active species or existence of minor motifs, owing to their inefficient chemical sensitivity. Thus, it is imperative to develop an operando analysis technique that can probe the nanoscopic chemical composition with high spatial/temporal resolution and sufficient detection limit. By observing the chemical and morphological evolution of highly efficient ECR catalysts during operation, we identified the key intermediate species toward highly active surfaces and significantly enhanced the C–C coupling activity. Operando transmission soft X-ray microscopy( 1,12,13 ), which visualizes the nanoscale chemical composition distribution of Cu-based catalysts during ECR, revealed that partially evolved Cu ⁺ phases and surface Cu ²⁺ phases are responsible for the dynamic dissolution–redeposition process( 4,8 ) and improvement of C–C coupling activity, respectively. We further demonstrated that the dissolution–redeposition process is electrochemically triggered by inducing Cu ⁺ phases, which are redirected to copper-carbonate-hydroxide species( 6,14,15 ) even under high cathodic potentials. DFT calculations suggest that these cationic Cu species potentially serve as active species and/or assistive sites for enhancing C–C coupling activity. (1) Mefford, J. T., et al. Nature 593(7857), 67-73 (2021) (2) Kreider, M. E., et al. ACS Applied Materials & Interfaces 11(30), 26863-26871 (2019) (3) Zhai, L., et al. ACS Energy Letters 5(8), 2483-2491 (2020) (4) De Luna, P., et al. Nature Catalysis 1(2), 103-110 (2018) (5) Wang, Y., et al. Nature Catalysis 3(2), 98-106 (2020) (6) Henckel, D. A., et al. ACS Catalysis 11(1), 255-263 (2021) (7) Lee, S. Y., et al. Journal of the American Chemical Society 140(28), 8681-8689 (2018) (8) Zhong, D., et al. Angewandte Chemie International Edition 60(9), 4879-4885 (2021) (9) Lei, Q., et al. Journal of the American Chemical Society 142(9), 4213-4222 (2020) (10) Eilert, A., et al. The Journal of Physical Chemistry Letters 7(8), 1466-1470 (2016) (11) Arán-Ais, R. M., et al. Nature Energy 5(4), 317-325 (2020) (12) Lim, J. et al. Science 353, 566–571 (2016) (13) de Smit, E., et al. Nature 456(7219), 222-225 (2008) (14) Spodaryk, M., et al. Electrochimica Acta 297, 55-60 (2019) (15) Jiang, S., et al. ChemSusChem 15(8), e202102506 (2022) Figure 1

Lithium-ion insertion kinetics fundamentally hinges upon phase transformation behavior during (dis)charging and understanding the rate-dependent kinetics is crucial for the development of high-power batteries. At high c-rates, kinetic hysteresis is amplified and phase evolution becomes heterogeneous and unpredictable. Specifically, discharge becomes more sluggish than charging of most battery electrodes including LiNi x Mn y Co z O 2 (NMC) and LiFePO 4 (LFP). Here, we developed an operando soft x-ray microscopy to simultaneously observe surface charge transfer and bulk lithium diffusion in facet-controlled individual battery particles over a wide range of cycling rates (0.01 – 10C). Our result unambiguously reveals that dynamic asymmetry between fast charging and discharging originates from auto-inhibitory Li-rich and autocatalytic Li-poor surface domains, respectively. In addition, we developed synchrotron-based operando fast XRD to track phase evolution during fast cycling. We directly observed that sluggish Li diffusion at high Li content induces different phase transformations during charging and discharging, with strong phase separation and homogeneous phase transformation during charging and discharging, respectively. Moreover, by electrochemically manipulating the lithium-ion concentration distribution within NCM particles, phase separation pathway could be redirected to solid-solution kinetics even at 7 C-rate. Our work lays the groundwork for developing high-power applications and ultrafast charging protocols Figure 1