Juchuan Li

Crack Pattern Formation in Thin Film Lithium-Ion Battery Electrodes

Cracking of electrodes caused by large volume change and the associated lithium diffusion-induced stress during electrochemical cycling is one of the main reasons for the short cycle life of lithium-ion batteries using high capacity anode materials, such as Si and Sn. In this work, we study the fracture behavior and cracking patterns in amorphous Si thin film electrodes as a result of electrochemical cycling. A modified spring-block model is shown to capture the essential features of cracking patterns of electrode materials, including self-similarity. It is shown that cracks are straight in thick films, but show more wiggles in thin films. As the thickness of film decreases, the average size of islands separated by cracks decreases. A critical thickness bellow which material would not crack is found for amorphous Si films. The experimental and simulation results of this work provide guidelines for designing crack free thin-film lithium ion battery electrodes during cycling by patterning the electrode and reducing the film thickness.

Air-stable, high-conduction solid electrolytes of arsenic-substituted Li4SnS4

Lithium-ion-conducting solid electrolytes show promise for enabling high-energy secondary battery chemistries and solving safety issues associated with conventional lithium batteries. Achieving the combination of high ionic conductivity and outstanding chemical stability in solid electrolytes is a grand challenge for the synthesis of solid electrolytes. Herein we report the design of aliovalent substitution of Li4SnS4 to achieve high conduction and excellent air stability based on the hard and soft acids and bases theory. The solid electrolyte of composition Li3.833Sn0.833As0.166S4 has a high ionic conductivity of 1.39 mS cm−1 at 25 °C. Considering the high Li+ transference number, this phase conducts Li+ as well as carbonate-based liquid electrolytes. This research also addresses the compatibility of the sulfide-based solid electrolytes through chemical passivation.

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.

Using all energy in a battery

Controlled electrode structure improves energy utilization [Also see Report by Kirshenbaum et al.] It is not easy to pull all the energy from a battery. For a battery to discharge, electrons and ions have to reach the same place in the active electrode material at the same moment. To reach the entire volume of the battery and maximize energy use, internal pathways for both electrons and ions must be low-resistance and continuous, connecting all regions of the battery electrode. Traditional batteries consist of a randomly distributed mixture of conductive phases within the active battery material. In these materials, bottlenecks and poor contacts may impede effective access to parts of the battery. On page 149 of this issue, Kirshenbaum et al. (1) explore a different approach, in which silver electronic pathways form on internal surfaces as the battery is discharged. The electronic pathways are well distributed throughout the electrode, improving battery performance.

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.

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.

Influence of rare earth doping on thermoelectric properties of SrTiO3 ceramics

Thermoelectric properties of SrTiO3 ceramics, doped with different rare earth elements, were investigated in this work. Its found that the ionic radius of doping elements plays an important role on thermoelectric properties: SrTiO3 ceramics doped with large rare earth ions (such as La, Nd, and Sm) exhibit large power factors, and those doped with small ions (such as Gd, Dy, Er, and Y) exhibit low thermal conductivities. Therefore, a simple approach for enhancing the thermoelectric performance of SrTiO3 ceramics is proposed: mainly doped with large ions to obtain a large power factor and, simultaneously, slightly co-doped with small ions to obtain a low thermal conductivity. Based on this rule, Sr0.8La0.18Yb0.02TiO3 ceramics were prepared, whose ZT value at 1 023 K reaches 0.31, increasing by a factor of 19% compared with the single-doped counterpart Sr0.8La0.2TiO3 (ZT = 0.26).

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.

Pushing the Theoretical Limit of Li-CFx 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.

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.