Sören L. Dreyer

Improving interfacial stability between cathode active material (CAM) and solid electrolyte (SE) is vital for developing high-performance all-solid-state batteries (ASSBs), with compatibility issues among the cell components representing a major challenge. CAM surface coating with a chemically inert ion conductor is a promising approach to suppress side reactions occurring at the cathode interfaces. Another strategy to mitigate mechanical degradation involves utilizing single-crystalline particle morphologies. Their more robust bulk structure and lower tortuosity for charge transport, compared to polycrystalline (PC) CAMs, can significantly enhance cyclability in ASSBs. Herein, we coated a LiNbO₃ protective layer onto the free surface of quasi single-crystalline LiNi_0.83Co_0.12Mn_0.05O₂ (SC83) particles. Pellet-stack ASSB cells using the LiNbO₃@SC83 CAM and argyrodite Li₆PS₅Cl as SE showed a capacity retention of 88% after 1000 cycles at the 1 C rate, compared to only 71% for the uncoated counterpart and far superior to that of LiNbO₃@PC83 (30%). The effectiveness of LiNbO₃ coating and the SC-NCM nature in mitigating electro-chemo-mechanical degradation was studied by combining modeling and physical/electrochemical characterizations. We demonstrate that the capacity decay at fast charge is due primarily to the mechanical degradation of CAM particles, while it is strongly determined by CAM|SE interfacial reactions under slow-charging conditions.

The synergistic engineering of chemical complexity and crystal structures has been applied to Prussian blue analogue (PBA) cathodes in this work. More precisely, the high-entropy concept has been successfully introduced into two structure types of identical composition, namely, cubic and monoclinic. Through the utilization of a variety of complementary characterization techniques, a comprehensive investigation into the electrochemical behavior of the cubic and monoclinic PBAs has been conducted, providing nuanced insights. The implementation of the high-entropy concept exhibits crucial selectivity toward the intrinsic crystal structure. Specifically, while the overall cycling stability of both cathode systems is significantly improved, the synergistic interplay of crystal structure engineering and entropy proves particularly significant. After optimization, the cubic PBA demonstrates structural advantages, showcasing good reversibility, minimal capacity loss, high thermal stability, and unparalleled endurance even under harsh conditions (high specific current and temperature).

Lithium‐excess cation‐disordered rocksalt materials are a promising class of transition metal‐based cathodes that exhibit high specific capacity and energy density. The exceptional performance is achieved through participation of anionic redox in addition to cationic redox reactions in the electrochemistry. However, anionic redox reactions accompanied by oxygen evolution, accelerated electrolyte breakdown, and structural evolution lead to voltage hysteresis and low initial Coulombic efficiency. Herein, an Al₂O₃ layer with varying thickness has been coated onto a high‐entropy disordered rocksalt oxyfluoride cathode through atomic layer deposition to enhance battery performance. The results indicate that the utilization of a uniform Al₂O₃ coating improves the capacity retention and rate capability of the cathode, with the performance being strongly dependent on the layer thickness. Further investigation into cathode–electrolyte interfacial reactions reveals that the thin protecting Al₂O₃ coating can reduce the decomposition of electrolyte on the cathode surface but cannot prevent bulk phase degradation during prolonged cycling. These findings highlight the need for optimized coating design on the disordered rocksalt cathode to improve battery performance.

Invited for this month's cover picture is the group of Torsten Brezesinski at KIT. The cover picture shows gases evolving from a Prussian white cathode used in Na‐ion batteries in the presence of either NaPF₆‐ or NaClO₄‐containing electrolyte. H₂ and CO₂ are detected, as well as (CN)₂, for which a formation mechanism is proposed. Read the full text of the Research Article at 10.1002/batt.202300595.

As global energy storage demand increases, sodium‐ion batteries are often considered as an alternative to lithium‐ion batteries. Hexacyanoferrate cathodes, commonly referred to as Prussian blue analogues (PBAs), are of particular interest due their low‐cost synthesis and promising electrochemical response. However, because they consist of ~50 wt% cyanide anions, a possible release of highly toxic cyanide gases poses a significant safety risk. Previously, we observed the evolution of (CN)₂ during cycling via differential electrochemical mass spectrometry (DEMS), but were unable to determine a root cause or mechanism. In this work, we present a systematical investigation of the gas evolution of Prussian white (PW) with different water content via DEMS. While H₂ is the main gas detected, especially in hydrated PW and during overcharge (4.6 V vs. Na⁺/Na), the evolution of CO₂ and (CN)₂ depends on the electrolyte conductive salt. The use of oxidative NaClO₄ instead of NaPF₆ is the leading cause for the formation of (CN)₂. Mass spectrometric evidence of trace amounts of HCN is also found, but to a much lower extent than (CN)₂, which is the dominant safety risk when using NaClO₄‐containing electrolyte, which despite being a good model salt, is not a viable option for commercial applications.

One way to obtain higher energy densities in lithium-ion batteries is to increase the nickel content in lithium nickel-cobalt-manganese oxide (NCM) cathode active materials (CAMs), ultimately approaching LiNiO₂ (LNO). At the same time, a higher nickel content and specific capacity comes with more severe degradation of the CAM during battery cycling. Doping of the CAM with additional elements is a common approach to increase structural stability. For any CAM, comparisons of different dopants with each other and with undoped references are therefore commonly found in the literature.^[1,2] In some cases, the effect of different dopants on the CAM particle morphology is also reported.^[3,4] However, the influence of the process route of dopant introduction is rarely discussed. The most common ways are either the co-calcination of dopant source, lithium source and CAM precursor (pCAM), or the co-precipitation of dopant ions during pCAM precipitation. Impregnating the pCAM surface with a suspension of smaller dopant particles or a dopant solution, followed by solvent removal, can provide better dopant distribution in case of low dopant ion mobility or phase separation tendencies during calcination. Building on the previous finding that the primary particle morphology is the determining factor of LNO performance,^[5,6] in this work, the primary particle morphology, but also dopant distribution in a series of 0.25 mol% Zr-doped LNO obtained from the three different process routes co-calcination, impregnation and co-precipitation, is investigated. This low dopant concentration is common in commercial CAM, as thus the content of redox-active transition metal content remains high to provide maximal energy density. Zr also tends to phase separate in the form of Li₂ZrO₃ at high concentrations, which is this way avoided as much as possible, since the chosen concentration is below the solubility limit of Zr in the LNO bulk. In this comparison, it is found that not the dopant itself, but the process route of its introduction affects the primary particle morphology of LNO, likely explained by the initial dopant localization early in the calcination. Differences in electrochemical performance, both in coin cells and long-term cycled pouch cells, as well as in specific capacitance and gas evolution are then also found to be due not to the dopant, as one would initially assume in comparisons of doped and undoped materials, but to the effect of the process route on LNO primary particle morphology. References [1] Z. Zui, X. Li, X. Bai, X. Ren, X. Ou, Energy Storage Mater. 2023, 57, 14–43. [2] Z. Ahaliabadeh, X. Kong, E. Fedorovskaya, T. Kallio, J. Power Sources 2022, 540, 231633. [3] G. T. Park, D. R. Yoon, U. H. Kim, B. Namkoong, J. Lee, M. M. Wang, A. C. Lee, X. W. Gu, W. C. Chueh, C. S. Yoon, Y.-K. Sun, Energy Environ. Sci. 2021, 14, 6616–6626. [4] H. H. Sun, U. H. Kim, J. H. Park, S. W. Park, D. H. Seo, A. Heller, C. B. Mullins, C. S. Yoon, Y. K. Sun, Nat. Commun. 2021, 12, 6552. [5] P. Kurzhals, F. Riewald, M. Bianchini, H. Sommer, H. A. Gasteiger, J. Janek, J. Electrochem. Soc. 2021, 168, 110518. [6] F. Riewald, P. Kurzhals, M. Bianchini, H. Sommer, J. Janek, H. A. Gasteiger, J. Electrochem. Soc. 2022, 169, 020529. Figure 1: Effect of doping process route on a,b) primary particle morphology, c) electrochemical performance and d) specific capacitance. Figure 1

Sodium-ion batteries (SIBs) see intensive research and commercialization efforts, aiming to establish them as an alternative to lithium-ion batteries. Among the reported cathode material families for SIBs, Na-deficient P2-type layered oxides are promising candidates, benefiting from fast sodium diffusion and therefore high charge/discharge rates. However, upon sodium extraction at high potentials, a transition from the P2 to O2 phase occurs, with the corresponding change in cell volume resulting in particle fracture and capacity degradation. A possible solution to this is to increase configurational entropy by introducing more elements into the transition-metal layer (so-called high-entropy concept), leading to some kind of structural stabilization. In this work, the acoustic emission (AE) of a series of P2-type layered oxide cathodes with increasing configurational entropy [Na_0.67(Mn_0.55Ni_0.21Co_0.24)O₂, Na_0.67(Mn_0.45Ni_0.18Co_0.24Ti_0.1Mg_0.03)O₂ and Na_0.67(Mn_0.45Ni_0.18Co_0.18Ti_0.1Mg_0.03Al_0.04Fe_0.02)O₂] is recorded during SIB operation and correlated to the materials properties, namely change in c lattice parameter and cracking behavior. A structure-property relationship between entropy, manifested in the extent of phase transition, and detected AE is derived, supported by the classification of signals by peak frequency. This classification in combination with microscopy imaging allows to distinguish between inter- and intragranular fracture. Relatively more intergranular and less intragranular crack formation is observed with increasing configurational entropy.