Mechanistic Understanding of Oxidation of Tin-based Perovskite Solar Cells and Mitigation Strategies.

Tin (Sn)-based perovskites as the most promising absorber materials for lead-free perovskite solar cells (PSCs) have achieved the record efficiency of over 14 %. Although suppressing the oxidation of Sn-based perovskites is a frequently concerned topic for Sn-based PSCs, many studies have given vague explanations and the mechanisms are still under debate. This is in principal due to the lack of an in-depth understanding of various and complex intrinsic and extrinsic factors causing the oxidation process. In this context, we critically review the chemical mechanism of facile oxidation of Sn-based perovskites and differentiate its detrimental effects at material- and device-level. More importantly, we classify and introduce the intrinsic factors (raw materials and solvent of perovskite precursors) and extrinsic factors (exposure to neutral oxygen and superoxide) causing the oxidation with their corresponding anti-oxidation improvement methods. The presented comprehensive understanding and prospect of the oxidation provide insightful guidance for suppressing the oxidation in Sn-based PSCs "from the beginning to the end".

Substitution of Ethylammonium Halides Enabling Lead-Free Tin-Based Perovskite Solar Cells with Enhanced Efficiency and Stability.

Tin (Sn)-based perovskite solar cells (PSCs) have attracted extensive attention due to the irlow toxicity and excellent optoelectric properties. Nonetheless, the development of Sn-based PSCs is still hampered by poor film quality due to the fast crystallization and the oxidation from Sn2+ to Sn4+. In this work, we compare and employ three ethylammonium halides, EAX (X = Cl, Br, I) to explore their roles in Sn-based perovskites and solar cells. We find that crystallinity and crystallization orientation of perovskites are optimized with the regulation of EAI. EABr leads to reduced defect density and enhanced crystallinity but also the lowest absorption and the widest band gap owing to the substitution of Br-. Notably, perovskites with EACl exhibit the best crystallinity, lowest defect density, and excellent antioxidant capacity benefiting from the partial substitution of Cl-. Consequently, the EACl-modified device achieves a champion PCE of 12.50% with an improved Voc of 0.79 V. Meanwhile, an unencapsulated EACl device shows excellent shelf stability with negligible efficiency degradation after 5400 h of storage in a N2-filled glovebox, and the encapsulated device retains its initial efficiency after continuous light illumination at the maximum power point for 100 h in air.

A Better Zn-Ion Storage Device: Recent Progress for Zn-Ion Hybrid Supercapacitors.

As a new generation of Zn-ion storage systems, Zn-ion hybrid supercapacitors (ZHSCs) garner tremendous interests recently from researchers due to the perfect integration of batteries and supercapacitors. ZHSCs have excellent integration of high energy density and power density, which seamlessly bridges the gap between batteries and supercapacitors, becoming one of the most viable future options for large-scale equipment and portable electronic devices. However, the currently reported two configurations of ZHSCs and corresponding energy storage mechanisms still lack systematic analyses. Herein, this review will be prudently organized from the perspectives of design strategies, electrode configurations, energy storage mechanisms, recent advances in electrode materials, electrolyte behaviors and further applications (micro or flexible devices) of ZHSCs. The synthesis processes and electrochemical properties of well-designed Zn anodes, capacitor-type electrodes and novel Zn-ion battery-type cathodes are comprehensively discussed. Finally, a brief summary and outlook for the further development of ZHSCs are presented as well. This review will provide timely access for researchers to the recent works regarding ZHSCs.

High-Energy Aqueous Ammonium-Ion Hybrid Supercapacitors.

The development of novel electrochemical energy storage devices is a grand challenge. Here, an aqueous ammonium-ion hybrid supercapacitor (A-HSC), consisting of a layered δ-MnO2 based cathode, an activated carbon cloth anode, and an aqueous (NH4 )2 SO4 electrolyte is developed. The aqueous A-HSC demonstrates an ultrahigh areal capacitance of 1550 mF cm-2 with a wide voltage window of 2.0 V. An amenable peak areal energy density (861.2 µWh cm-2 ) and a decent capacitance retention (72.2% after 5000 cycles) are also achieved, surpassing traditional metal-ion hybrid supercapacitors. Ex situ characterizations reveal that NH4 + intercalation/deintercalation in the layered δ-MnO2 is accompanied by hydrogen bond formation/breaking. This work proposes a new paradigm for electrochemical energy storage.

Zn2+ Pre-Intercalation Stabilizes the Tunnel Structure of MnO2 Nanowires and Enables Zinc-Ion Hybrid Supercapacitor of Battery-Level Energy Density.

Although there has been tremendous progress in exploring new configurations of zinc-ion hybrid supercapacitors (Zn-HSCs) recently, the much lower energy density, especially the much lower areal energy density compared with that of the rechargeable battery, is still the bottleneck, which is impeding their wide applications in wearable devices. Herein, the pre-intercalation of Zn2+ which gives rise to a highly stable tunnel structure of Znx MnO2 in nanowire form that are grown on flexible carbon cloth with a disruptively large mass loading of 12 mg cm-2 is reported. More interestingly, the Znx MnO2 nanowires of tunnel structure enable an ultrahigh areal energy density and power density, when they are employed as the cathode in Zn-HSCs. The achieved areal capacitance of up to 1745.8 mF cm-2 at 2 mA cm-2 , and the remarkable areal energy density of 969.9 µWh cm-2 are comparable favorably with those of Zn-ion batteries. When integrated into a quasi-solid-state device, they also endow outstanding mechanical flexibility. The truly battery-level Zn-HSCs are timely in filling up of the battery-supercapacitor gap, and promise applications in the new generation flexible and wearable devices.

Synergy of Epoxy Chemical Tethers and Defect-Free Graphene in Enabling Stable Lithium Cycling of Silicon Nanoparticles.

We report a new approach for nanosilicon-graphene hybrids with uniquely stable solid electrolyte interphase. Expanded graphite is gently exfoliated creating "defect-free" graphene that is non-catalytic towards electrolyte decomposition, simultaneously introducing high mass loading (48 wt. %) Si nanoparticles. Silane surface treatment creates epoxy chemical tethers, mechanically binding nano-Si to CMC binder through epoxy ring-opening reaction while stabilizing the Si surface chemistry. Epoxy-tethered silicon pristine-graphene hybrid "E-Si-pG" exhibits state-of-the-art performance in full battery opposing commercial mass loading (12 mg cm-2 ) LiCoO2 (LCO) cathode. At 0.4 C, with areal capacity of 1.62 mAh cm-2 and energy of 437 Wh kg-1 , achieving 1.32 mAh cm-2 , 340.4 Wh kg-1 at 1 C. After 150 cycles, it retains 1.25 mAh cm-2 , 306.5 Wh kg-1 . Sputter-down XPS demonstrates survival of surface C-Si-O-Si groups in E-Si-pG after repeated cycling. The discovered synergy between support defects, chemical-mechanical stabilization of Si surfaces, and SEI-related failure may become key LIB anode design rule.