Revealing the role of the cathode-electrolyte interface on solid-state batteries.

Interfaces have crucial, but still poorly understood, roles in the performance of secondary solid-state batteries. Here, using crystallographically oriented and highly faceted thick cathodes, we directly assess the impact of cathode crystallography and morphology on the long-term performance of solid-state batteries. The controlled interface crystallography, area and microstructure of these cathodes enables an understanding of interface instabilities unknown (hidden) in conventional thin-film and composite solid-state electrodes. A generic and direct correlation between cell performance and interface stability is revealed for a variety of both lithium- and sodium-based cathodes and solid electrolytes. Our findings highlight that minimizing interfacial area, rather than its expansion as is the case in conventional composite cathodes, is key to both understanding the nature of interface instabilities and improving cell performance. Our findings also point to the use of dense and thick cathodes as a way of increasing the energy density and stability of solid-state batteries.

Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO3-x.

The short charging times and high power capabilities associated with capacitive energy storage make this approach an attractive alternative to batteries. One limitation of electrochemical capacitors is their low energy density and for this reason, there is widespread interest in pseudocapacitive materials that use Faradaic reactions to store charge. One candidate pseudocapacitive material is orthorhombic MoO3 (α-MoO3), a layered compound with a high theoretical capacity for lithium (279 mA h g-1 or 1,005 C g-1). Here, we report on the properties of reduced α-MoO3-x(R-MoO3-x) and compare it with fully oxidized α-MoO3 (F-MoO3). The introduction of oxygen vacancies leads to a larger interlayer spacing that promotes faster charge storage kinetics and enables the α-MoO3 structure to be retained during the insertion and removal of Li ions. The higher specific capacity of the R-MoO3-x is attributed to the reversible formation of a significant amount of Mo4+ following lithiation. This study underscores the potential importance of incorporating oxygen vacancies into transition metal oxides as a strategy for increasing the charge storage kinetics of redox-active materials.

Using X-ray Microscopy To Understand How Nanoporous Materials Can Be Used To Reduce the Large Volume Change in Alloy Anodes.

Tin metal is an attractive negative electrode material to replace graphite in Li-ion batteries due to its high energy density. However, tin undergoes a large volume change upon alloying with Li, which pulverizes the particles, and ultimately leads to short cycling lifetimes. Nevertheless, nanoporous materials have been shown to extend battery life well past what is observed in nonporous material. Despite the exciting potential of porous alloying anodes to significantly increase the energy density in Li-ion batteries, the fundamental physics of how nanoscale architectures accommodate the electrochemically induced volume changes are poorly understood. Here, operando transmission X-ray microscopy has been used to develop an understanding of the mechanisms that govern the enhanced cycling stability in nanoporous tin. We found that in comparison to dense tin, nanoporous tin undergoes a 6-fold smaller areal expansion after lithiation, as a result of the internal porosity and unique nanoscale architecture. The expansion is also more gradual in nanoporous tin compared to the dense material. The nanoscale resolution of the microscope used in this study is ∼30 nm, which allowed us to directly observe the pore structure during lithiation and delithiation. We found that nanoporous tin remains porous during the first insertion and desinsertion cycle. This observation is key, as fully closed pores could lead to mechanical instability, electrolyte inaccessibility, and short lifetimes. While tin was chosen for this study because of its high X-ray contrast, the results of this work should be general to other alloy-type systems, such as Si, that also suffer from volume change based cycling degradation.

High performance pseudocapacitor based on 2D layered metal chalcogenide nanocrystals.

Single-layer and few-layer transition metal dichalcogenides have been extensively studied for their electronic properties, but their energy-storage potential has not been well explored. This paper describes the structural and electrochemical properties of few-layer TiS2 nanocrystals. The two-dimensional morphology leads to very different behavior, compared to corresponding bulk materials. Only small structural changes occur during lithiation/delithiation and charge storage characteristics are consistent with intercalation pseudocapacitance, leading to materials that exhibit both high energy and power density.

A Nearly Packaging-Free Design Paradigm for Light, Powerful, and Energy-Dense Primary Microbatteries.

Billions of internet connected devices used for medicine, wearables, and robotics require microbattery power sources, but the conflicting scaling laws between electronics and energy storage have led to inadequate power sources that severely limit the performance of these physically small devices. Reported here is a new design paradigm for primary microbatteries that drastically improves energy and power density by eliminating the vast majority of the packaging and through the use of high-energy-density anode and cathode materials. These light (50-80 mg) and small (20-40 µL) microbatteries are enabled though the electroplating of 130 µm-thick 94% dense additive-free and crystallographically oriented LiCoO2 onto thin metal foils, which also act as the encapsulation layer. These devices have 430 Wh kg-1 and 1050 Wh L-1 energy densities, 4 times the energy density of previous similarly sized microbatteries, opening up the potential to power otherwise unpowerable microdevices.

Suppression of Electrochemically Driven Phase Transitions in Nanostructured MoS2 Pseudocapacitors Probed Using Operando X-ray Diffraction.

Pseudocapacitors with nondiffusion-limited charge storage mechanisms allow for fast kinetics that exceed conventional battery materials. It has been demonstrated that nanostructuring conventional battery materials can induce pseudocapacitive behavior. In our previous study, we found that assemblies of metallic 1T MoS2 nanocrystals show faster charge storage compared to the bulk material. Quantitative electrochemistry demonstrated that the current response is capacitive. In this work, we perform a series of operando X-ray diffraction studies upon electrochemical cycling to show that the high capacitive response of metallic 1T MoS2 nanocrystals is due to suppression of the standard first-order phase transition. In bulk MoS2, a phase transition between 1T and triclinic phases (Li xMoS2) is observed during lithiation and delithiation in both the galvanostatic traces (as distinctive plateaus) and the X-ray diffraction patterns with the appearance of the additional peaks. MoS2 nanocrystal assemblies, on the other hand, show none of these features. We hypothesize that the reduced MoS2 crystallite size suppresses the first-order phase transition and gives rise to solid solution-like behavior, potentially due to the unfavorable formation of nucleation sites in confined spaces. Overall, we find that nanostructuring MoS2 suppresses the 1T-triclinic phase transition and shortens Li-ion diffusion path lengths, allowing MoS2 nanocrystal assemblies to behave as nearly ideal pseudocapacitors.

Deterministic Design of Chemistry and Mesostructure in Li-Ion Battery Electrodes.

All battery electrodes have complex internal three-dimensional architectures that have traditionally been formed through the random packing of the electrode components. What is now emerging is a new concept in battery electrode design, where the important electronic and ionic pathways, as well as the chemical interactions between the components of the electrode, are deterministically designed. Deterministic design enables far better properties than are possible through random packing, including dramatic improvements in both power and energy. Such a design approach is particularly attractive for emerging high-energy-density materials, which require available free volume as they swell during cycling. In addition to controlled structure, another important facet of the design of such systems is the stable chemical linkages between the active material and the conductive network that survive the lithiation and delithiation processes. In this Perspective, we discuss and provide our views on deterministically designed battery electrodes.

Nanoporous Tin with a Granular Hierarchical Ligament Morphology as a Highly Stable Li-Ion Battery Anode.

Next generation Li-ion batteries will require negative electrode materials with energy densities many-fold higher than that found in the graphitic carbon currently used in commercial Li-ion batteries. While various nanostructured alloying-type anode materials may satisfy that requirement, such materials do not always exhibit long cycle lifetimes and/or their processing routes are not always suitable for large-scale synthesis. Here, we report on a high-performance anode material for next generation Li-ion batteries made of nanoporous Sn powders with hierarchical ligament morphology. This material system combines both long cycle lifetimes (more than 72% capacity retention after 350 cycles), high capacity (693 mAh/g, nearly twice that of commercial graphitic carbon), good charging/discharging capabilities (545 mAh/g at 1 A/g, 1.5C), and a scalable processing route that involves selective alloy corrosion. The good cycling performance of this system is attributed to its nanoporous architecture and its unique hierarchical ligament morphology, which accommodates the large volume changes taking place during lithiation, as confirmed by synchrotron-based ex-situ X-ray 3D tomography analysis. Our findings are an important step for the development of high-performance Li-ion batteries.

Tuning Porosity and Surface Area in Mesoporous Silicon for Application in Li-Ion Battery Electrodes.

This work aims to improve the poor cycle lifetime of silicon-based anodes for Li-ion batteries by tuning microstructural parameters such as pore size, pore volume, and specific surface area in chemically synthesized mesoporous silicon. Here we have specifically produced two different mesoporous silicon samples from the magnesiothermic reduction of ordered mesoporous silica in either argon or forming gas. In situ X-ray diffraction studies indicate that samples made in Ar proceed through a Mg2Si intermediate, and this results in samples with larger pores (diameter ≈ 90 nm), modest total porosity (34%), and modest specific surface area (50 m2 g-1). Reduction in forming gas, by contrast, results in direct conversion of silica to silicon, and this produces samples with smaller pores (diameter ≈ 40 nm), higher porosity (41%), and a larger specific surface area (70 m2 g-1). The material with smaller pores outperforms the one with larger pores, delivering a capacity of 1121 mAh g-1 at 10 A g-1 and retains 1292 mAh g-1 at 5 A g-1 after 500 cycles. For comparison, the sample with larger pores delivers a capacity of 731 mAh g-1 at 10 A g-1 and retains 845 mAh g-1 at 5 A g-1 after 500 cycles. The dependence of capacity retention and charge storage kinetics on the nanoscale architecture clearly suggests that these microstructural parameters significantly impact the performance of mesoporous alloy type anodes. Our work is therefore expected to contribute to the design and synthesis of optimal mesoporous architectures for advanced Li-ion battery anodes.

Mesoporous Ni60Fe30Mn10-alloy based metal/metal oxide composite thick films as highly active and robust oxygen evolution catalysts †.

A major challenge in the field of water electrolysis is the scarcity of oxygen-evolving catalysts that are inexpensive, highly corrosion-resistant, suitable for large-scale applications and able to oxidize water at high current densities and low overpotentials. Most unsupported, non-precious metals oxygen-evolution catalysts require at least ~350 mV overpotential to oxidize water with a current density of 10 mA/cm2 in 1 M alkaline solution. Here we report on a robust nanostructured porous NiFe-based oxygen evolution catalyst made by selective alloy corrosion. In 1 M KOH, our material exhibits a catalytic activity towards water oxidation of 500 mA/cm2 at 360 mV overpotential and is stable for over eleven days. This exceptional performance is attributed to three factors. First, the small size of the ligaments and pores in our mesoporous catalyst (~10 nm) results in a high BET surface area (43 m2/g) and therefore a high density of oxygen-evolution catalytic sites per unit mass. Second, the open porosity facilitates effective mass transfer at the catalyst/electrolyte interface. Third and finally, the high bulk electrical conductivity of the mesoporous catalyst allows for effective current flow through the electrocatalyst, making it possible to use thick films with a high density of active sites and ~3×104 cm2 of catalytic area per cm2 of electrode area. Our mesoporous catalyst is thus attractive for alkaline electrolyzers where water-based solutions are decomposed into hydrogen and oxygen as the only products, driven either conventionally or by photovoltaics.