High magnesium mobility in ternary spinel chalcogenides.

Magnesium batteries appear a viable alternative to overcome the safety and energy density limitations faced by current lithium-ion technology. The development of a competitive magnesium battery is plagued by the existing notion of poor magnesium mobility in solids. Here we demonstrate by using ab initio calculations, nuclear magnetic resonance, and impedance spectroscopy measurements that substantial magnesium ion mobility can indeed be achieved in close-packed frameworks (~ 0.01-0.1 mS cm-1 at 298 K), specifically in the magnesium scandium selenide spinel. Our theoretical predictions also indicate that high magnesium ion mobility is possible in other chalcogenide spinels, opening the door for the realization of other magnesium solid ionic conductors and the eventual development of an all-solid-state magnesium battery.

Interfacial Stability of Li Metal-Solid Electrolyte Elucidated via in Situ Electron Microscopy.

Despite their different chemistries, novel energy-storage systems, e.g., Li-air, Li-S, all-solid-state Li batteries, etc., face one critical challenge of forming a conductive and stable interface between Li metal and a solid electrolyte. An accurate understanding of the formation mechanism and the exact structure and chemistry of the rarely existing benign interfaces, such as the Li-cubic-Li7-3xAlxLa3Zr2O12 (c-LLZO) interface, is crucial for enabling the use of Li metal anodes. Due to spatial confinement and structural and chemical complications, current investigations are largely limited to theoretical calculations. Here, through an in situ formation of Li-c-LLZO interfaces inside an aberration-corrected scanning transmission electron microscope, we successfully reveal the interfacial chemical and structural progression. Upon contact with Li metal, the LLZO surface is reduced, which is accompanied by the simultaneous implantation of Li+, resulting in a tetragonal-like LLZO interphase that stabilizes at an extremely small thickness of around five unit cells. This interphase effectively prevented further interfacial reactions without compromising the ionic conductivity. Although the cubic-to-tetragonal transition is typically undesired during LLZO synthesis, the similar structural change was found to be the likely key to the observed benign interface. These insights provide a new perspective for designing Li-solid electrolyte interfaces that can enable the use of Li metal anodes in next-generation batteries.

In Situ STEM-EELS Observation of Nanoscale Interfacial Phenomena in All-Solid-State Batteries.

Behaviors of functional interfaces are crucial factors in the performance and safety of energy storage and conversion devices. Indeed, solid electrode-solid electrolyte interfacial impedance is now considered the main limiting factor in all-solid-state batteries rather than low ionic conductivity of the solid electrolyte. Here, we present a new approach to conducting in situ scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) in order to uncover the unique interfacial phenomena related to lithium ion transport and its corresponding charge transfer. Our approach allowed quantitative spectroscopic characterization of a galvanostatically biased electrochemical system under in situ conditions. Using a LiCoO2/LiPON/Si thin film battery, an unexpected structurally disordered interfacial layer between LiCoO2 cathode and LiPON electrolyte was discovered to be inherent to this interface without cycling. During in situ charging, spectroscopic characterization revealed that this interfacial layer evolved to form highly oxidized Co ions species along with lithium oxide and lithium peroxide species. These findings suggest that the mechanism of interfacial impedance at the LiCoO2/LiPON interface is caused by chemical changes rather than space charge effects. Insights gained from this technique will shed light on important challenges of interfaces in all-solid-state energy storage and conversion systems and facilitate improved engineering of devices operated far from equilibrium.

Unravelling the Impact of Reaction Paths on Mechanical Degradation of Intercalation Cathodes for Lithium-Ion Batteries.

The intercalation compounds are generally considered as ideal electrode materials for lithium-ion batteries thanks to their minimum volume expansion and fast lithium ion diffusion. However, cracking still occurs in those compounds and has been identified as one of the critical issues responsible for their capacity decay and short cycle life, although the diffusion-induced stress and volume expansion are much smaller than those in alloying-type electrodes. Here, we designed a thin-film model system that enables us to tailor the cation ordering in LiNi(0.5)Mn(1.5)O4 spinels and correlate the stress patterns, phase evolution, and cycle performances. Surprisingly, we found that distinct reaction paths cause negligible difference in the overall stress patterns but significantly different cracking behaviors and cycling performances: 95% capacity retention for disordered LiNi(0.5)Mn(1.5)O4 and 48% capacity retention for ordered LiNi(0.5)Mn(1.5)O4 after 2000 cycles. We were able to pinpoint that the extended solid-solution region with suppressed phase transformation attributed to the superior electrochemical performance of disordered spinel. This work envisions a strategy for rationally designing stable cathodes for lithium-ion batteries through engineering the atomic structure that extends the solid-solution region and suppresses phase transformation.

Nanocomposite of N-doped TiO2 nanorods and graphene as an effective electrocatalyst for the oxygen reduction reaction.

Developing an effective electrocatalyst for the oxygen reduction reaction is a momentous issue in fuel cells. In this paper, we successfully synthesized the N-doped TiO2 nanorods/graphene (N-TiO2/NG) nanocomposite, which comprise the N-doped TiO2 (N-TiO2) nanorods (40-60 nm diameter and 90-300 nm length) and self-assembled nitrogen-doped graphene (NG) networks. We found that the nanocomposite exhibits great oxygen reduction reaction (ORR) electrocatalytic performance and also shows long durability and methanol tolerance than that of the commercial 20% Pt/C catalyst. This new nanocomposite may also have potential applications in other fields, which are related to energy storage, gas sensors, photocatalysis, and so on.

Supramolecular self-assembly of three-dimensional polyaniline and polypyrrole crystals.

Herein, we first report a novel approach for preparing 3D π-π stacked crystals of polyaniline and polypyrrole and show how they self-assemble in a suitable solution environment. 3D crystals of polyaniline and polypyrrole show high charge-transport properties of 130 S cm(-1) and 150 S cm(-1), respectively. Remarkably, the 3D crystals of polyaniline obtain excellent anisotropic conductivity.

Artificial solid electrolyte interphase to address the electrochemical degradation of silicon electrodes.

Electrochemical degradation on silicon (Si) anodes prevents them from being successfully used in lithium (Li)-ion battery full cells. Unlike the case of graphite anodes, the natural solid electrolyte interphase (SEI) films generated from carbonate electrolytes do not self-passivate on Si, causing continuous electrolyte decomposition and loss of Li ions. In this work, we aim at solving the issue of electrochemical degradation by fabricating artificial SEI films using a solid electrolyte material, lithium phosphorus oxynitride (Lipon), which conducts Li ions and blocks electrons. For Si anodes coated with Lipon of 50 nm or thicker, a significant effect is observed in suppressing electrolyte decomposition, while Lipon of thinner than 40 nm has a limited effect. Ionic and electronic conductivity measurements reveal that the artificial SEI is effective when it is a pure ionic conductor, but electrolyte decomposition is only partially suppressed when the artificial SEI is a mixed electronic-ionic conductor. The critical thickness for this transition in conducting behavior is found to be 40-50 nm. This work provides guidance for designing artificial SEI films for high-capacity Li-ion battery electrodes using solid electrolyte materials.

Pushing the theoretical limit of Li-CF(x) batteries: a tale of bifunctional electrolyte.

In a typical battery, the inert electrolyte functions solely as the ionic conductor without contribution to the cell capacity. Here we demonstrate that the most energy-dense Li-CF(x) battery delivers a capacity exceeding the theoretical maximum of CF(x) with a solid electrolyte of Li3PS4 (LPS) that has dual functions: as the inert electrolyte at the anode and the active component at the cathode. Such a bifunctional electrolyte reconciles both inert and active characteristics through a synergistic discharge mechanism of CF(x) and LPS. The synergy at the cathode is through LiF, the discharge product of CF(x), which activates the electrochemical discharge of LPS at a close electrochemical potential of CF(x). Therefore, the solid-state Li-CF(x) batteries output 126.6% energy beyond their theoretic limits without compromising the stability of the cell voltage. The additional energy comes from the electrochemical discharge of LPS, the inert electrolyte. This bifunctional electrolyte revolutionizes the concept of conventional batteries and opens a new avenue for the design of batteries with unprecedented energy density.

Novel TiO2/PEGDA hybrid hydrogel prepared in situ on tumor cells for effective photodynamic therapy.

A novel inorganic/organic hybrid hydrogel system containing titanium dioxide (TiO2)/poly(ethylene glycol) double acrylates (PEGDA) was prepared by in situ photopolymerization on tumor cells for photodynamic therapy (PDT). TiO2 nanorods with diameter of ∼5 nm and length of ∼25 nm in this system presented dual functions, as effective photosensitizers for PDT and initiators for causing the in situ formation of hydrogel, under near-infrared (NIR) irradiation. The hybrid hydrogel retained the TiO2 around tumor cell to form a drug-loaded hydrogel shell. This resulted in a high concentration of singlet oxygen ((1)O2) under NIR irradiation, which induced apoptosis of tumor cell. Also, the hydrogel could reduce the side effects by preventing TiO2 from migrating to normal tissue. Furthermore, the TiO2 nanorods in this hydrogel shell were photochemically recyclable and could be reused in regular treatment. The outcomes of this study provide a new way to exploit multifunction of inorganic semiconductor nanomaterials for a variety of biomedical applications.