Assessing the roles of mechanical cracks in Ni-rich layered cathodes in the capacity decay of liquid and solid-state batteries.

Cracks are ubiquitous in Ni-rich layered cathodes upon cycling in liquid electrolyte-lithium-ion batteries (LELIBs); however, their roles in the capacity decay are unclear. Furthermore, how cracks affect the performance of all solid-state batteries (ASSBs) has not been explored yet. Herein, cracks are created by mechanical compression in the pristine single crystal LiNi0.8Mn0.1Co0.1O2 (NMC811) and their roles in the capacity decay in solid-state batteries are asserted. These mechanically created fresh cracks are predominantly along the (003) planes with minor cracks along the planes slanted to the (003) planes, and both types of cracks contain little or no rock-salt phase, which is in sharp contrast to the chemomechanical cracks in NMC811 where rock-salt phase formation is ubiquitous. We reveal that mechanical cracks cause a significant initial capacity loss in ASSBs but little capacity decay during the subsequent cycling. In contrast, the capacity decay in LELIBs is principally governed by the rock salt phase and interfacial side reactions and thus does not result in an initial capacity loss, but a severe capacity decay during cycling.

Revealing the Electrochemistry of Solid-State Li-SeS2 Battery via In-Situ Transmission Electron Microscopy.

Sex Sy is considered as a promising cathode material as it can deliver higher energy density than selenium (Se) and offer improved conductivity and enhanced reaction kinetics compared with S. However, the electrochemistry of the Li-SeS2 all-solid-state battery (ASSB) has not been well understood to date. Herein the electrochemistry of Li-SeS2 battery was revealed by in-situ transmission electron microscopy. The charge products were phase-separated Se and S, rather than the widely believed SeS2 . Among the various Sex Sy cathodes, SeS2 achieved the best electrochemical performance. The Li-SeS2 ASSB delivered a high reversible capacity of 1052 mAh g-1 at 1 A g-1 over 350 cycles, and a high areal capacity of 4 mAh cm-2 was also achieved with a high cathode mass loading of 7.6 mg cm-2 . These results represent the best performance achieved to date in the Li-SeS2 ASSB and brings us one step closer toward its practical applications.

Cryo-EM Revealing the Origin of Excessive Capacity of the Se Cathode in Sulfide-Based All-Solid-State Li-Se Batteries.

Selenium (Se), whose electronic conductivity is nearly 25 orders higher than that of sulfur (S) and whose theoretical volumetric capacity is 3254 mAh cm-3, is considered as a potential alternative to S to overcome the poor electronic conductivity issue of the S cathode in the lithium (Li)-S battery. However, the study of the Li-Se battery, particularly a Li-Se all-solid-state battery (ASSB), is still in its infancy. Herein, we report the performance of Li-Se ASSBs at both room temperature (RT) and high temperature (HT, 50 °C), using a Li10Si0.3PS6.9Cl1.8 (LSPSCl) solid-state electrolyte and Li-In anode. With a Se loading of 7.6 mg cm-2, the Li-Se battery displayed a record high reversible capacity of 6.8 mAh cm-2 after 50 cycles at HT, which exceeds the theoretical areal capacity of 5.2 mAh cm-2 for Se. Moreover, the RT Li-Se ASSB delivered an initial areal capacity of about 2 mAh cm-2 at a current density of 1 A g-1 for 1200 cycles with a capacity retention of 67%. Cryo-electron microscopy revealed that the excessive capacity of Se at HT can be attributed to the formation of a previously unknown S5Se4 phase during charging, which participated reversibly in a subsequent redox reaction. The formation of the S5Se4 phase originated from the reaction of Se with S, which was generated by the decomposition of LSPSCl at HT. These results unlock the electrochemistry of a Li-Se ASSB, suggesting that a Li-Se ASSB is a viable alternative to a Li-S battery for energy storage applications.

Enabling Long Cycle Life and High Rate Iron Difluoride Based Lithium Batteries by In Situ Cathode Surface Modification.

Metals fluorides (MFs) are potential conversion cathodes to replace commercial intercalation cathodes. However, the application of MFs is impeded by their poor electronic/ionic conductivity and severe decomposition of electrolyte. Here, a composite cathode of FeF2 and polymer-derived carbon (FeF2 @PDC) with excellent cycling performance is reported. The composite cathode is composed of nanorod-shaped FeF2 embedded in PDC matrix with excellent mechanical strength and electronic/ionic conductivity. The FeF2 @PDC enables a reversible capacity of 500 mAh g-1 with a record long cycle lifetime of 1900 cycles. Remarkably, the FeF2 @PDC can be cycled at a record rate of 60 C with a reversible capacity of 107 mAh g-1 after 500 cycles. Advanced electron microscopy reveals that the in situ formation of stable Fe3 O4 layers on the surface of FeF2 prevents the electrolyte decomposition and leaching of iron (Fe), thus enhancing the cyclability. The results provide a new understanding to FeF2 electrochemistry, and a strategy to radically improve the electrochemical performance of FeF2 cathode for lithium-ion battery applications.

Unraveling the Conversion Evolution on Solid-State Na-SeS2 Battery via In Situ TEM.

All-solid-state (ASS) Na-S batteries are promising for a large-scale energy-storage system owing to numerous merits. However, the high conversion reaction barrier impedes their practical application. In this work, the basic mechanism on how Se catalyzes the conversion reaction in the Na-S batteries is unraveled. The sodiation/desodiation of Na-SeS2 nanobatteries are systematically evaluated via in situ transmission electron microscopy (in situ TEM) with a microheating device. The real-time analyses reveal an amorphous Na-Sex Sy intermediate phase appears during the direct conversion from SeS2 to Na2 S, and a reverse reaction succeeds at 100 °C with a prior formation of Se. The absence of polysulfides and a much lower desodiation temperature in contrast to Na-S nanobatteries demonstrate that the Se incorporation significantly lowers the conversion reaction barrier. According to these findings, the ASS SeS2 batteries using a Na3 SbS4 solid electrolyte (SE) are assembled using various SE:C ratios in the composite cathodes to investigate the effect of the ion and electron transport on the electrochemical properties, including the effective transport properties, MacMullin number, and the tortuosity factor. The obtained results in turn confirm the findings from the in situ TEM. These findings are applicable to optimize other S-based active materials and improve their utilization.

Size-Dependent Chemomechanical Failure of Sulfide Solid Electrolyte Particles during Electrochemical Reaction with Lithium.

The very high ionic conductivity of Li10GeP2S12 (LGPS) solid electrolyte (SE) makes it a promising candidate SE for solid-state batteries in electrical vehicles. However, chemomechanical failure, whose mechanism remains unclear, has plagued its widespread applications. Here, we report in situ imaging lithiation-induced failure of LGPS SE. We revealed a strong size effect in the chemomechanical failure of LGPS particles: namely, when the particle size is greater than 3 µm, fracture/pulverization occurred; when the particle size is between 1 and 3 µm, microcracks emerged; when the particle size is less than 1 µm, no chemomechanical failure was observed. This strong size effect is interpreted by the interplay between elastic energy storage and dissipation. Our finding has important implications for the design of high-performance LGPS SE, for example, by reducing the particle size to less than 1 µm the chemomechanical failure of LGPS SE can be mitigated.

In Situ Visualization of Lithium Penetration through Solid Electrolyte and Dead Lithium Dynamics in Solid-State Lithium Metal Batteries.

The two biggest promises of solid-state lithium (Li) metal batteries (SSLMBs) are the suppression of Li dendrites by solid-state electrolyte (SSE) and the realization of a high-energy-density Li anode. However, LMBs have not met their expectations due to Li dendrite growth causing short-circuiting. In fact, Li dendrites grow even more easily in SSE than in liquid electrolyte, but the reason for this remains unclear. Here we report in situ transmission electron microscopy observations of Li dendrite penetration through SSE and "dead" Li formation dynamics in SSLMBs. We show direct evidence that large electrochemomechanical stress generates cracks in the SSE and drives Li through the SSE directly. We revealed that fresh Li nucleation sites emerged in every discharge cycle, creating new "dead" Li in the following charging cycle and becoming the dominant Coulombic efficiency decay mechanism in SSLMBs. These results indicate that engineering flaw size and reducing electronic conductivity in SSEs are essential to improve the performance of SSLMBs.

In Situ Measurements of the Mechanical Properties of Electrochemically Deposited Li2CO3 and Li2O Nanorods.

Solid-electrolyte interface (SEI) is "the most important but least understood (component) in rechargeable Li-ion batteries". The ideal SEI requires high elastic strength and can resist the penetration of a Li dendrite mechanically, which is vital for inhibiting the dendrite growth in lithium batteries. Even though Li2CO3 and Li2O are identified as the major components of SEI, their mechanical properties are not well understood. Herein, SEI-related materials such as Li2CO3 and Li2O were electrochemically deposited using an environmental transmission electron microscopy (ETEM), and their mechanical properties were assessed by in situ atomic force microscopy (AFM) and inverse finite element simulations. Both Li2CO3 and Li2O exhibit nanocrystalline structures and good plasticity. The ultimate strength of Li2CO3 ranges from 192 to 330 MPa, while that of Li2O is less than 100 MPa. These results provide a new understanding of the SEI and its related dendritic problems in lithium batteries.

In Situ Imaging Polysulfides Electrochemistry of Li-S Batteries in a Hollow Carbon Nanotubule Wet Electrochemical Cell.

Understanding polysulfide electrochemistry is critical for mitigation of the polysulfide shuttle effect in Li-S batteries. However, in situ imaging polysulfides evolution in Li-S batteries has not been possible. Herein, we constructed a hollow carbon nanotubule (CNT) wet electrochemical cell that permits real-time imaging of polysulfide evolutions in Li-S batteries in a Cs-corrected environmental transmission electron microscope. Upon discharge, sulfur was electrochemically reduced to long-chain polysulfides, which dissolved into the electrolyte instantly and were stabilized by Py14+ cations solvation. Metastable polysulfides prove to be problematic for Li-S batteries, therefore, destabilizing the Py14+-solvated polysulfides by adding low polarized solvents into the electrolyte to weaken the interaction between Py14+ cation and long-chain polysulfides renders a rapid polysulfides-to-Li2S transition, thus efficiently mitigating polysulfide formation and improving the performance of Li-S batteries dramatically. Moreover, the CNT wet electrochemical cell proves to be a universal platform for in situ probing electrochemistry of various batteries.

In Situ TEM Observations of Discharging/Charging of Solid-State Lithium-Sulfur Batteries at High Temperatures.

Understanding the structural evolution of Li2 S upon operation of lithium-sulfur (Li-S) batteries is inadequate and a complete decomposition of Li2 S during charge is difficult. Whether it is the low electronic conductivity or the low ionic conductivity of Li2 S that inhibits its decomposition is under debate. Furthermore, the decomposition pathway of Li2 S is also unclear. Herein, an in situ transmission electron microscopy (TEM) technique implemented with a microelectromechanical systems (MEMS) heating device is used to study the precipitation and decomposition of Li2 S at high temperatures. It is revealed that Li2 S transformed from an amorphous/nanocrystalline to polycrystalline state with proceeding of the electrochemical lithiation at room temperature (RT), and the precipitation of Li2 S is more complete at elevated temperatures than at RT. Moreover, the decomposition of Li2 S that is difficult to achieve at RT becomes facile with increased Li+ ion conduction at high temperatures. These results indicate that Li+ ion diffusion in Li2 S dominates its reversibility in the solid-state Li-S batteries. This work not only demonstrates the powerful capabilities of combining in situ TEM with a MEMS heating device to explore the basic science in energy storage materials at high temperatures but also introduces the factor of temperature to boost battery performance.

Lithium whisker growth and stress generation in an in situ atomic force microscope-environmental transmission electron microscope set-up.

Lithium metal is considered the ultimate anode material for future rechargeable batteries1,2, but the development of Li metal-based rechargeable batteries has achieved only limited success due to uncontrollable Li dendrite growth3-7. In a broad class of all-solid-state Li batteries, one approach to suppress Li dendrite growth has been the use of mechanically stiff solid electrolytes8,9. However, Li dendrites still grow through them10,11. Resolving this issue requires a fundamental understanding of the growth and associated electro-chemo-mechanical behaviour of Li dendrites. Here, we report in situ growth observation and stress measurement of individual Li whiskers, the primary Li dendrite morphologies12. We combine an atomic force microscope with an environmental transmission electron microscope in a novel experimental set-up. At room temperature, a submicrometre whisker grows under an applied voltage (overpotential) against the atomic force microscope tip, generating a growth stress up to 130 MPa; this value is substantially higher than the stresses previously reported for bulk13 and micrometre-sized Li14. The measured yield strength of Li whiskers under pure mechanical loading reaches as high as 244 MPa. Our results provide quantitative benchmarks for the design of Li dendrite growth suppression strategies in all-solid-state batteries.

In situ imaging of electrocatalysis in a K-O2 battery with a hollandite α-MnO2 nanowire air cathode.

Using α-manganese dioxide (α-MnO2) nanowires as the air electrode, a K-O2 nanobattery is assembled in an aberration corrected environmental transmission electron microscope. It is found that the α-MnO2 nanowires are reduced into Mn3O4 and MnO during discharge; meanwhile, KO2 is formed on the surface of the α-MnO2 nanowires.