Advances in solid-state batteries have primarily been driven by the discovery of superionic conducting structural frameworks that function as solid electrolytes. We demonstrate the ability of high-entropy metal cation mixes to improve ionic conductivity in a compound, which leads to less reliance on specific chemistries and enhanced synthesizability. The local distortions introduced into high-entropy materials give rise to an overlapping distribution of site energies for the alkali ions so that they can percolate with low activation energy. Experiments verify that high entropy leads to orders-of-magnitude higher ionic conductivities in lithium (Li)–sodium (Na) superionic conductor (Li-NASICON), sodium NASICON (Na-NASICON), and Li-garnet structures, even at fixed alkali content. We provide insight into selecting the optimal distortion and designing high-entropy superionic conductors across the vast compositional space.
Despite their large theoretical storage capability, Na-Sn batteries exhibit poor round-trip energy efficiencies as compared to Li-Si batteries. Here, we report the results of a comprehensive study to elucidate how and why Na-Sn batteries exhibit such a low energy efficiency. As a convincing evidence for this behavior, we observed that the resistivity of the Sn anode increased by 8 orders of magnitude during in situ sodiation experiments, which is attributed to the formation of electrically resistive Zintl ions in the sodiated Sn. Continual sodiation induced the development of residual stresses at the Sn anode and caused the distortion of Zintl ions from their ideal configuration. This distortion caused a change in the electronic structure, resulting in the increased resistivity of the sodiated Sn. Our findings offer some solutions that can be used to improve the energy efficiency of Na-Sn batteries.
Sodiation was performed on crystalline Sn cylinders using an in situ electron microscope to evaluate the rate performance of the Sn anode by directly measuring the sodiation rate. We observed that the sodiation rate of the Sn anode is more than 2 orders of magnitude higher than the lithiation rate of the Si anode under the same conditions. This unprecedented rate displayed by the Na-Sn system is attributed to the bond characteristics and crystalline-to-amorphous transformation of the Sn crystal at the thin interface of the Na-Sn diffusion couple. Here, using atomic simulations, we explain how and why the Sn anode exhibits this high rate performance by resolving the diffusion process of Na ions in the Na-Sn interfacial region and the electron structure of the crystalline Sn. This work provides a useful insight into the use of Sn as an attractive anode material for realizing ultrafast-charging batteries for electric vehicles and mobile devices.
High-rate performance and mechanical stability of anode materials are the two important characteristics that are necessary to develop fast-charging batteries with longevity. In the present study, we demonstrate that both high rate performance and mechanical stability of the anode can be achieved with the Na–Sn battery system. Experiments show that the sodiation rate in crystalline Sn (c-Sn) is 2–3 orders of magnitude faster than that reported for the Li–Si system. Furthermore, this extraordinary rate is nearly the same regardless of the orientation of c-Sn, which can improve the cycle life by retarding the pulverization of c-Sn. Two main microstructural features responsible for the observed characteristics are identified: (1) a transformation from crystalline to amorphous phase occurring at thin layers of c-Sn near the interfacial front and (2) pipe diffusion of Na through sodiation-induced dislocations. In this study, the observed behaviors are explained by elucidating the diffusion kinetics, whereas the associated mechanistic origins are analyzed by resolving the diffusion process of Na+ near the Na/Sn interface using atomic simulations.
All-solid-state batteries have emerged as promising alternatives to conventional Li-ion batteries owing to their higher energy density and safety, which stem from their use of inorganic solid-state electrolytes instead of flammable organic liquid electrolytes. Among various candidates, sulfide solid-state electrolytes are particularly promising for the development of high-energy all-solid-state Li metal batteries because of their high ionic conductivity and deformability. However, a significant challenge remains as their inherent instability in contact with electrodes forms unstable interfaces and interphases, leading to degradation of the battery performance. In this review article, we provide an overview of the key issues for the interfaces and interphases of sulfide solid-state electrolyte systems as well as recent progress in understanding such interface and interphase formation and potential solutions to stabilize them. In addition, we provide perspectives on future research directions in this field.
The diffusion of carrier ions in alloying anodes often develops compressive stresses in front of the propagating interface, suppressing the carrier‐ion diffusion and limiting their full penetration into alloying anodes during battery cycles. This phenomenon, termed “self‐limiting diffusion (SLD)”, reduces the rate performance of batteries and hinders the full usage of anode materials. However, SLD is mitigated in some systems where tensile residual stresses develop at the interface, causing them to manifest significantly improved rate performance and energy capacity. Here, a comparative study of LiSi and NaSn systems to elucidate how the differing diffusion kinetics displayed by the two systems can influence SLD behaviors and the rate performance of batteries is performed. Experiments show that the Na diffusion into soft Sn crystals induces tensile stresses near the interface, promoting the nucleation of high‐density dislocations. Thus‐formed dislocations facilitate Na diffusion at ultrafast rates by providing pathways for dislocation pipe diffusion and alleviate SLD, making crystalline Sn suitable for fast‐charging anode material. The outcomes of this study, while filling the knowledge gaps on the reasons for SLD, offer some guidelines for the appropriate choice of potential anode materials with superior rate performance and energy capacity suitable for future applications.
Li‐excess disordered rocksalts (DRXs) are emerging as promising cathode materials for Li‐ion batteries due to their ability to use earth‐abundant transition metals. In this work, a new strategy based on partial Li deficiency engineering is introduced to optimize the overall electrochemical performance of DRX cathodes. Specifically, by using Mn‐based DRX as a proof‐of‐concept, it is demonstrated that the introduction of cation vacancies during synthesis (e.g., Li1.3‐xMn2+0.4‐xMn3+xNb0.3O1.6F0.4, x = 0, 0.2, and 0.4) improves both the discharge capacity and rate performance due to the more favored short‐range order in the presence of Mn3+. Density functional theory calculations and Monte Carlo simulations, in combination with spectroscopic tools, reveal that introducing 10% vacancies (Li1.1Mn2+0.2Mn3+0.2Nb0.3O1.6F0.4) enables both Mn2+/Mn3+ redox and excellent Li percolation. However, a more aggressive vacancy doping (e.g., 20% vacancies in Li0.9Mn3+0.4Nb0.3O1.6F0.4) impairs performance because it induces phase separation between an Mn‐rich and a Li‐rich phase.
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