Singapore

Electromechanical Failure Mechanism of All-solid-state Li-ion Batteries based on a NASICON-type Solid-state Electrolyte 
Linchun He, Stefan Adams
Department of Materials Science and Engineering, National University of Singapore

All-solid-state lithium batteries (ASSLiB) are promising candidates for next-generation energy storage. Beyond design requirements and concerns of conventional batteries, the undeformable nature of solid-state electrolytes (SSEs)1 raises new issues for ASSLiBs such as poor interface bonding, limited contact area and cathode utilization.2 Moreover, volume changes of electrodes during Li-ion (de)intercalation are restricted by the SSE, resulting in large internal stresses that may drive crack creation. Such cracks reduce the utilization of electrodes and the available cross-section of the SSE for ion transport. Increased currents in the remaining connected regions turn this into a self-accelerating aging process. Hence it is essential to synergize understanding of the ion transport with insight into mechanochemical processes in the SSE or at its interfaces to overcome these obstacles.
Here, we demonstrate and analyze a model bulk ASSLiB with MnO2-CNT nanocomposite anode, high voltage LiNi0.5Mn1.5O4 cathode and ambient-stable Li1.5Al0.5Ge(PO4)3 (LAGP) as SSE. This ASSLiB has a discharge capacity of 82 mAh g-1 at 0.15 C at 23.8°C. Electrochemical impedance studies reveal a decrease in impedance by solid interface layer formation, followed by an increase over extended cycling. Electrochemical evaluation and first-principle calculations are used to explore the aging processes. Li4P2O7, Li3PO4, Ge5P6O25, AlPO4 and GeO are identified as products of the decomposition of LAGP, which is assisted by Li-ions and free electrons from voids and grain boundaries. Finite element method (FEM) simulations reveal that the volume expansion on forming the decomposition products causes maximum internal stresses of 2.5 to 125 GPa (for Li excess ratios from 0 to 6), resulting in crack formation and growth within the SSE on multiple cycling. 
1. C. Chen, Y. Sun, L. He et al., ACS Appl. Mater. Interfaces (2020).
2. L. He, C. Chen, M. Kotobuki et al., J. Mater. Chem. A 7, 9748 (2019).



MATERIALS FOR HUMANITY (MH 21)

Electromechanical Failure Mechanism of All-solid-state Li-ion Batteries based on a NASICON-type Solid-state Electrolyte

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technical paper

Understanding the alkali interaction with the organic functional groups is significant for molecular-level design of potential anode materials.In this work,we investigate the chemical and electronic interactions between Li/Na and PTCDA/PTCDI during the deposition processes. The change of molecular electronic structures has been studied by X-ray photo electron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), synchrotron-based near edge X-ray absorption fine structure (NEXAFS), and corroborated by density functional theory (DFT) calculations.

Alkali Metal Storage Mechanism in Organic Semiconductor of PTCDA and PTCDI

Perovskite materials have attracted much attention due to their alternative structure and outstanding catalytic performance of OER and ORR. In this work, we synthesize a series of different ratio of Se-doped LCO, which greatly improved the conductivity of LCO. At the optimized Se-doping level(Se60%-LCO), the rechargeable lithium-oxygen battery has high specific capacity and long cycle life. With the good semiconductor characteristics of LaCoO3, the photo-electrochemical process is assisted by photoelectric double synergistic effect, which greatly improves the electrochemical performance of the battery. This work shows that the catalytic activity of oxide perovskite enhanced by Se doping combined with photo assistance, which make LaCoO3 a promising bifunctional catalyst.

Engineering Oxygen Vacancies of Oxide Perovskite by Selenium Doping for Electrocatalysis in Photo-assisted Li−O2 Batteries

Sodium-ion batteries (SIBs) have attracted growing interest as a promising energy storage technique due to their safety, low cost and excellent cycling performance. Among various electrode materials, phthalocyanine complexes (Pcs) is a kind of organometallic complexes (OMCs) with adjustable structure of organic moieties and metal active sites of inorganic moieties that possess promising redox activity in batteries. In this work, we investigate the interaction of Na atoms with CuPc and F16CuPc thin films by in-situ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) in an ultra-high vacuum (UHV) system, corroborated by the density functional theory (DFT) calculations. Na atoms are found to interact with inner pyrrole N ahead of outer aza N, for which process is accompanied by the reduction from Cu(II) to Cu(I). Na deposition leads to a charge transfer to CuPc that successfully fills up the lowest unoccupied molecular orbitals (LUMO) of CuPc. The role of fluorine groups is also investigated. F with high electronegativity can attract Na atoms to interact with outer aza N, and the proposed charge transfer from Na to F16CuPc as well as reduction of central Cu are also affected by the intense Na-F interaction. The microscopic-scale insights we obtained from the in-situ XPS and UPS characterization can be applied to other alkali ion-OMCs model systems, and will give comprehensive guidance for designing OMCs with required configurations and advanced features for next-generation energy storage devices.

Mechanism study of sodium storage in phthalocyanine based electrode materials

Stabilizing the active half-metallic LiO2 as discharge product is an effective approach to reduce the charge overpotential and enhance the stability of Li-O2 batteries. Here we developed a Rh nanoparticle decorated reduced graphene oxide (Rh/rGO) cathode to induce the formation of mixed discharge products of LiO2 and Li2O2. The as-assembled Li-O2 battery exhibited large discharge capacity (16157 mAh/g at 100 mA/g), long cycle life (136 cycles at 100 mA/g with capacity limited to 500 mAh/g) and reduced charge overpotential. The improved battery performance is ascribed to the intermetallic phase LiRh3 that stabilized crystalline LiO2 as substrate. Additionally, the high oxygen reduction reaction (ORR) catalytic activity of cathode material is determined to be crucial to stabilize crystalline LiO2 even with the presence of LiRh3. These results provide further insight into LiO2 based Li-O2 batteries and potential application of such template-assisted approach.

Rh/rGO stabilized LiO2 as Discharge Product in Lithium Oxygen Battery

In this work, in situ vertical growth of Sb2S3 on top of ultrathin TiO2 /CdS as the electron transport layer (ETL) by a solution method is demonstrated. The planar heterojunction solar cell using [hk1]-oriented Sb2S3 achieves a power conversion efficiency of 6.4%, performing at almost 20% higher than devices based on a [hk0]-oriented absorber. The main highlights of this work are summarized below: 1. In-situ growth of vertically oriented Sb2S3 on ETL by a facile solution method is realized 2. A convincing and enlightening crystal growth model is proposed. 3. Direct evidences are shown to reveal the relationship between crystal orientation of Sb2S3 and device performance 4. High efficient planar heterojunction Sb2S3 solar cell was obtained, performing almost 20% higher compared to device based on [hk0]-oriented absorber.

In Situ Growth of [hk1]-Oriented Sb2S3 for Solution Processed Planar Heterojunction Solar Cell with 6.4% Efficiency

Silver bismuth iodide has been studied comprehensively for its potential as light harvesters, owing to their low toxicity and environment conviviality. However, they often suffer with low efficiencies as compared to conventional light harvesters and hence there is a broad room for improvement. Here we propose doping the standard AgBiI4 composition with tin (Sn), to improve the relative absorbance and thereby potentially improving the efficiencies. It is observed that the addition of Sn did not alter the structural properties significantly, while it works on the orientation of the crystal growth during thin film synthesis, leading to ordered and uniform films. An improvement in absorption coefficient was observed, which can be considered to improve the performances of devices fabricated with AgBiI4 as light harvesters.

Tin Doped Silver Bismuth Iodide as Superior Energy Harvesters

All solution-processed semi-transparent perovskite solar cells (PSCs) with high efficiency were fabricated with Ag nanowire (AgNW) top electrode directly coated on the organic electron transport layer (ETL). A unique AgNW/ETL interface morphology was attained by the fabrication of a AgNW-C60 composite electrode through a sequential spin-coating method, resulting in a translucent PSC with PCE of 11.02% and remarkable stability improvement. The low-temperature solution-processed and semitransparent AgNW-C60 top electrode is also applicable to other PSC structures with various interfacial layers, thus showing a promising path towards fully-printable, flexible and translucent photovoltaic devices.

Ag-nanowires/C60 composite electrode for translucent perovskite solar cells

Mixed organic-inorganic halide perovskite solar cell’s (PSC) are an emerging photovoltaic technology that has the promise of significantly lower production costs coupled with the high absorption coefficient, tunable bandgap and long carrier lifetime. It also shows promising capability as semitransparent solar cell amid its strong absorption in very thin thickness. However, to maintain the morphology quality and maximize the solar cell performance, increasing the light absorbance is a necessity. Herein we study the effect of Au nanoparticles as plasmonic layers between the PSC and its transport layers in order to improve the light-harvesting capability through localized surface plasmon resonance (LSPR). varying thickness of the Au film were conducted through physical vapor deposition (PVD) and annealing under different temperatures to achieve thermal dewaetting of the Au film for the formation of separated Au nano islands. The photovoltaic performance of the fabricated cells was then evaluated and studied through morphology, crystallographic and optical correlations. Several architectures were studied, such as in between PSC and ETL as well as PSC and HTL. Although results demonstrated an overall decrease in the PCE as compared to the control, we elucidate the necessity of a capping layer surrounding the Au NPs in order to minimize surface recombination of the photo-generated excitons due to the formation of metal-semiconductor Schottky barriers. We found that the PSCs showed an improvement from an average PCE of 4.57% to 5.66% when an additional SnO2 layer was spin-coated between the Au and Cs0:05(MA0:17FA0:83)0:95Pb(I0:83Br0:17)3 layer. PCE of the best performing plasmonic-based device in this study was 11.54% by depositing Au between the TiO2 layers of the PSC. Further optimization in the capping structure, shape and the amount of Au as well as spacing between the Au can be performed to achieve the enhancement from the plasmon effect.

Thermal dewetting of Au film in between perovskite and its transport layers

A hybrid organic–inorganic perovskite in a diode structure can lead to multifunctional device phenomena exhibiting both a high-power conversion efficiency (PCE) of a solar cell and strong electroluminescence (EL) efficiency. Non radiative losses in such multifunctional devices lead to open circuit voltage (Voc) deficit which is a limiting factor to push the efficiency towards Shockley–Queisser (SQ) limit. In this work we analyse and quantify the radiative limit of Voc in perovskite solar cell as a function of its absorber thickness. We correlate PCE and EL efficiency at varying thicknesses to understand the limiting factors for high Voc. With certain increase in perovskite thickness, PCE improves but EL efficiency is compromised and, vice versa. Thus, correlating these two figures of merits of a solar cell guides towards light management strategy together with minimizing non-radiative losses. The results demonstrate that maximizing absorption and emission processes remains paramount for optimizing devices.

Effect of Perovskite Thickness on Electroluminescence and Solar Cell Conversion Efficiency

Our current work examines various solution-processed ternary zinc-tin oxide (ZTO) compositions as plausible compact ETL candidates in the fully planar perovskite configuration. Systematically optimized ETLs verified the superiority of ZTO ETL based devices compared to the devices made with pristine ZnO ETL. Furthermore, the use of SnO2 interlayer between the ZTO and perovskite to form a bilayer ETL structure enhanced the device performance significantly. Apart from prohibiting the unfavorable contact between the ZnO film with the perovskite layer, the bilayer ETL structure showed further improvement in reproducibility, charge transport, recombination resistance, device stability, and efficiency. Paramount PCE of 19.01 %, with an average PCE of 18.18 %, was demonstrated in the Zn1 Sn1O/SnO2 ETL based devices. In addition, exceptional stability was obtained in which 90% of its initial PCE was maintained after 90 days without encapsulation. The superior performance and device stability are attributed to the improved charge extraction, favourable energy alignment, and suppressed charge recombination derived from the compatible Zn1 Sn1O/SnO2 bilayer ETL. Our work highlighted the viability of ternary metal oxides as highly effective and reproducible ETL candidates for planar perovskite solar cell application. We also demonstrated the importance of a compatible SnO2 interlayer between the ternary metal/perovskite interface whereby nonpareil PCE and profound improvement in device stability were manifested.

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