The volume effects of electrode materials can cause local stress development, contact loss and particle cracking in the rigid environment of a solid-state battery.

In the near future, the targets for lithium-ion batteries concerning specific energy and cost can advantageously be met by introducing layered LiNi x Co y Mn z O2 (NCM) cathode materials with a high Ni content (x ≥ 0.6). Increasing the Ni content allows for the utilization of more lithium at a given cell voltage, thereby improving the specific capacity but at the expense of cycle life. Here, the capacity-fading mechanisms of both typical low-Ni NCM (x = 0.33, NCM111) and high-Ni NCM (x = 0.8, NCM811) cathodes are investigated and compared from crystallographic and microstructural viewpoints. In situ X-ray diffraction reveals that the unit cells undergo different volumetric changes of around 1.2 and 5.1% for NCM111 and NCM811, respectively, when cycled between 3.0 and 4.3 V vs Li/Li+. Volume changes for NCM811 are largest for x(Li) < 0.5 because of the severe decrease in interlayer lattice parameter c from 14.467(1) to 14.030(1) Å. In agreement, in situ light microscopy reveals that delithiation leads to different volume contractions of the secondary particles of (3.3 ± 2.4) and (7.8 ± 1.5)% for NCM111 and NCM811, respectively. And postmortem cross-sectional scanning electron microscopy analysis indicates more significant microcracking in the case of NCM811. Overall, the results establish that the accelerated aging of NCM811 is related to the disintegration of secondary particles caused by intergranular fracture, which is driven by mechanical stress at the interfaces between the primary crystallites.

Two major strategies are currently pursued to improve the energy density of lithium-ion batteries using LiNi x Co y Mn z O 2 (NCM) cathode materials. One is to increase the fraction of redox active Ni (≥80%), which allows larger amounts of Li to be extracted at a given cutoff voltage (U max ). The other is to increase U max , in particular for medium-Ni content NCM materials. However, the accompanying lattice changes ultimately lead to capacity fading in both cases. Here the structural changes occurring in Li 1.02 Ni x Co y Mn z O 2 (with x = 1 / 3 , 0.5, 0.6, 0.7, 0.8 and 0.85) during cycling operation in the voltage range between 3.0 and 4.6 V vs Li are quantified by means of operando X-ray diffraction combined with detailed Rietveld analysis. All samples show a large decrease in unit cell volume upon charging, ranging from 2.4% for NCM111 (33% Ni) to 8.0% for NCM851005 (85% Ni). To make a fair comparison of the structural stability of the different NCM materials, energy densities as a function of U max are estimated and correlated with X-ray diffraction results. It is shown that NCMs with a lower Ni content allow for specific energies similar to that of, e.g., Ni-rich NCM811 (80% Ni) when operated at sufficiently high U max , but still undergo less pronounced changes in structure. Nevertheless, as indicated by charge/discharge tests, the capacity retention of low-and medium-Ni content NCMs cycled to high U max is also strongly affected by factors other than stability of the layered crystal lattice (electrolyte decomposition etc.). Overall, it is demonstrated that the complexity of the degradation processes needs to be better understood to identify optimal cycling conditions for specific cathode compositions.

Ni-rich LiNi x Co y Mn z O2 (NCM) cathode materials have great potential for application in next-generation lithium-ion batteries owing to their high specific capacity. However, they are subjected to severe structural changes upon (de)­lithiation, which adversely affects the cycling stability. Herein, we investigate changes in crystal and electronic structure of NCM811 (80% Ni) at high states of charge by a combination of operando X-ray diffraction (XRD), operando hard X-ray absorption spectroscopy (hXAS), ex situ soft X-ray absorption spectroscopy (sXAS), and density functional theory (DFT) calculations and correlate the results with data from galvanostatic cycling in coin cells. XRD reveals a large decrease in unit cell volume from 101.38(1) to 94.26(2) Å3 due to collapse of the interlayer spacing when x(Li) < 0.5 (decrease in c-axis from 14.469(1) Å at x(Li) = 0.6 to 13.732(2) Å at x(Li) = 0.25). hXAS shows that the shrinkage of the transition metal–oxygen layer mainly originates from nickel oxidation. sXAS, together with DFT-based Bader charge analysis, indicates that the shrinkage of the interlayer, which is occupied by lithium, is induced by charge transfer between O 2p and partially filled Ni eg orbitals (resulting in decrease of oxygen–oxygen repulsion). Overall, the results demonstrate that high-voltage operation of NCM811 cathodes is inevitably accompanied by charge-transfer-induced lattice collapse.

The research and development of advanced nanocoatings for high-capacity cathode materials is currently a hot topic in the field of solid-state batteries (SSBs). Protective surface coatings prevent direct contact between the cathode material and solid electrolyte, thereby inhibiting detrimental interfacial decomposition reactions. This is particularly important when using lithium thiophosphate superionic solid electrolytes, as these materials exhibit a narrow electrochemical stability window, and therefore, are prone to degradation during battery operation. Herein we show that the cycling performance of LiNbO 3 -coated Ni-rich LiNi x Co y Mn z O 2 cathode materials is strongly dependent on the sample history and (coating) synthesis conditions. We demonstrate that post-treatment in a pure oxygen atmosphere at 350 °C results in the formation of a surface layer with a unique microstructure, consisting of LiNbO 3 nanoparticles distributed in a carbonate matrix. If tested at 45 °C and C/5 rate in pellet-stack SSB full cells with Li 4 Ti 5 O 12 and Li 6 PS 5 Cl as anode material and solid electrolyte, respectively, around 80% of the initial specific discharge capacity is retained after 200 cycles (~ 160 mAh). Our results highlight the importance of tailoring the coating chemistry to the electrode material(s) for practical SSB applications.

The operation of combined mass spectrometry and electrochemistry setups has recently become a powerful approach for the in situ analysis of gas evolution in batteries. It allows for real-time insights and mechanistic understanding into different processes, including battery formation, operation, degradation, and behavior under stress conditions. Important information is gained on the safety and stability window as well as on the effect of protecting strategies, such as surface coatings, dopings, and electrolyte additives. This review primarily aims at summarizing recent findings on the gassing behavior in different kinds of liquid- and solid-electrolyte-based batteries, with emphasis placed on novel cathode-active materials and isotope labeling experiments, to highlight the relevance of in situ gas analysis for elucidation of reaction mechanisms. Various instrumental and experimental approaches are presented to encourage and inspire both novices and experienced scientists in the field. Graphical abstract

Bulk-type solid-state batteries (SSBs) constitute a promising next-generation technology for electrochemical energy storage. However, in order for SSBs to become competitive with mature battery technologies, (electro)chemically stable, superionic solid electrolytes are much needed. Multicomponent or high-entropy lithium argyrodites have recently attracted attention for their favorable material characteristics. In the present work, we report on increasing the configurational entropy of the Li 6+a P 1−x M x S 5 I solid electrolyte system and examine how this affects the structureconductivity/stability relationships. Using electrochemical impedance spectroscopy and 7 Li pulsed field gradient nuclear magnetic resonance (NMR) spectroscopy, multicationic substitution is demonstrated to result in a very low activation energy for Li diffusion of ∼0.2 eV and a high room-temperature ionic conductivity of ∼13 mS cm −1 (for Li 6.5 [P 0.25 Si 0.25 Ge 0.25 Sb 0.25 ]S 5 I). The transport properties are rationalized from a structural perspective by means of complementary neutron powder diffraction and magic-angle spinning NMR spectroscopy measurements. The Li 6.5 [P 0.25 Si 0.25 Ge 0.25 Sb 0.25 ]S 5 I solid electrolyte was also tested in high-loading SSB cells with a Ni-rich layered oxide cathode and found by X-ray photoelectron spectroscopy to suffer from interfacial side reactions during cycling. Overall, the results of this study indicate that optimization of conductivity is equally important to optimization of stability, and compositionally complex lithium argyrodites represent a new playground for a rational design of (potentially advanced) superionic solid electrolytes.

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