Functional Scanning Force Microscopy for Energy Nanodevices

A transferable classical force field to describe glyme based lithium solvate ionic liquids

A non-polarizable force field for lithium (Li+) and bis(trifluoromethanesulfonyl)imide (TFSI-) ions solvated in diglyme at around 0.2 mol fraction salt concentration was developed based on ab initio molecular dynamics (AIMD) simulations and a modified polymer consistent force field model. A force-torque matching based scheme, in conjunction with a genetic algorithm, was used to determine the Lennard-Jones (LJ) parameters of the ion-ion and ion-solvent interactions. This force field includes a partial charge scaling factor and a scaling factor for the 1-4 interactions. The resulting force field successfully reproduces the radial distribution function of the AIMD simulations and shows better agreement compared to the unmodified force field. The new force field was then used to simulate salt solutions with glymes of increasing chain lengths and different salt concentrations. The comparison of the MD simulations, using the new force field, with experimental data at different salt concentrations and AIMD simulations on equimolar concentrations of the triglyme system demonstrates the transferability of the force field parameters to longer glymes and higher salt concentrations. Furthermore, the force field appears to reproduce the features of the experimental x-ray structure factors, suggesting accuracy beyond the first solvation shell, for equimolar salt solutions using both triglyme and tetraglyme as the solvent. Overall, the new force field was found to accurately reproduce the molecular descriptions of LiTFSI-glyme systems not only at various salt concentrations but also with glymes of different chain lengths. Thus, the new force field provides a useful and accurate tool to perform in silico studies of this family of systems at the atomistic level.

Identification of the conductivity type of single-walled carbon nanotubes via dual-modulation dielectric force microscopy

The conductivity type is one of the most fundamental transport properties of semiconductors, which is usually identified by fabricating the field-effect transistor, the Hall-effect device, etc. However, it is challenging to obtain an Ohmic contact if the sample is down to nanometer-scale because of the small size and intrinsic heterogeneity. Noncontact dielectric force microscopy (DFM) can identify the conductivity type of the sample by applying a DC gate voltage to the tip, which is effective in tuning the accumulation or depletion of charge carriers. Here, we further developed a dual-modulation DFM, which simplified the conductivity type identification from multiple scan times under different DC gate voltages to a single scan under an AC gate voltage. Taking single-walled carbon nanotubes as testing samples, the semiconducting-type sample exhibits a more significant charge carrier accumulation/depletion under each half-period of the AC gate voltage than the metallic-type sample due to the stronger rectification effect. The charge carrier accumulation or depletion of the p-type sample is opposite to that of the n-type sample at the same half-period of the AC gate voltage because of the reversed charge carrier type.

In Situ Electrochemical Atomic Force Microscopy: From Interfaces to Interphases

The electrochemical interface formed between an electrode and an electrolyte significantly affects the rate and mechanism of the electrode reaction through its structure and properties, which vary across the interface. The scope of the interface has been expanded, along with the development of energy electrochemistry, where a solid-electrolyte interphase may form on the electrode and the active materials change properties near the surface region. Developing a comprehensive understanding of electrochemical interfaces and interphases necessitates three-dimensional spatial resolution characterization. Atomic force microscopy (AFM) offers advantages of imaging and long-range force measurements. Here we assess the capabilities of AFM by comparing the force curves of different regimes and various imaging modes for in situ characterizing of electrochemical interfaces and interphases. Selected examples of progress on work related to the structures and processes of electrode surfaces, electrical double layers, and lithium battery systems are subsequently illustrated. Finally, this review provides perspectives on the future development of electrochemical AFM.

Excellent Polymerized Ionic-Liquid-Based Gel Polymer Electrolytes Enabled by Molecular Structure Design and Anion-Derived Interfacial Layer

Polymerized ionic liquid (PIL)-based gel polymer electrolytes (GPEs) are well known as highly safe and stable electrolytes but with low ambient ionic conductivity. Herein, we first designed and synthesized an IL monomer with a long and flexible side chain and then mixed it with LiTFSI and MEMPTFSI to construct a PIL-based GPE (denoted as GM-GPE). The special molecular structure of the monomer greatly improves the ionic transport through the PIL chain, and the introduction of MEMPTFSI plasticizer further improves the ionic conductivity, promoting a TFSI--anion-derived SEI formation to suppress Li dendrite growth and forming an electrostatic shielding effect of MEMP+ cations to promote the uniform deposition of Li+. Consequently, the as-prepared GM-GPE exhibits high ambient ionic conductivity (4.3 × 10-4 S cm-1, 30 °C), robust electrochemical stability, excellent thermal stability, nonflammability, and superior ability to inhibit Li dendrite growth. The resultant LiFePO4|GM-GPE|Li cell exhibits a high discharge capacity of 150 mA h g-1 at 0.2 C along with a good cycling stability and rate capability. This work brings about new guidance for the development of high-quality GPEs with high ionic conductivity, high stability, and safety for long cycling and dendrite-free lithium metal batteries.

Operando Imaging of Over-Discharge-Induced Surface Morphology Evolutions of LiMn2O4 Submicron-Sized Particles by Electrochemical High-Speed Atomic Force Microscopy

Spinel LiMn2O4 is a promising cathode material but suffers from severe capacity fading during battery operation. One of capacity fade mechanisms results from changes in its morphology and structure due to over-discharge. In this work, for the first time, we successfully tracked the morphologic evolution of LiMn2O4 submicron-sized particles during over-discharging by our home-made electrochemical high-speed atomic force microscopy (EC-HS-AFM). Seven hundred and sixty successive EC-HS-AFM images were stably captured at an imaging speed of ∼0.85 fps at corresponding potentials during over-discharging in ∼15 min, from which evolutions of nanoscale wrinkle-like and step-like structures on the particle surface were clearly observed. The phenomena could be resulted from the complex stresses due to structural distortion during the phase transformation from cubic (LiMn2O4) to tetragonal (Li2Mn2O4), and the formation of the Li2Mn2O4 phase was confirmed by ex situ XRD. Moreover, the particle surface area as a function of the potential was quantitatively extracted from the EC-HS-AFM images, revealing the irreversible expansion/contraction of the particles, and this finding obtained at the nanoscale was consistent with the macroscopic results tested by cyclic voltammetry and galvanostatic charge/discharge methods. These results demonstrate that the EC-HS-AFM is a powerful tool to establish the correlation between the over-discharge-induced surface morphology changes and irreversibility of the Li-ion insertion/extraction as well as capacity fading.

Polyfluorinated crosslinker-based solid polymer electrolytes for long-cycling 4.5 V lithium metal batteries

Solid polymer electrolytes (SPEs), which are favorable to form intimate interfacial contacts with electrodes, are promising electrolyte of choice for long-cycling lithium metal batteries (LMBs). However, typical SPEs with easily oxidized oxygen-bearing polar groups exhibit narrow electrochemical stability window (ESW), making it impractical to increase specific capacity and energy density of SPE based LMBs with charging cut-off voltage of 4.5 V or higher. Here, we apply a polyfluorinated crosslinker to enhance oxidation resistance of SPEs. The crosslinked network facilitates transmission of the inductive electron-withdrawing effect of polyfluorinated segments. As a result, polyfluorinated crosslinked SPE exhibits a wide ESW, and the Li|SPE|LiNi_0.5Co_0.2Mn_0.3O₂ cell with a cutoff voltage of 4.5 V delivers a high discharge specific capacity of ~164.19 mAh g^-1 at 0.5 C and capacity retention of ~90% after 200 cycles. This work opens a direction in developing SPEs for long-cycling high-voltage LMBs by using polyfluorinated crosslinking strategy.

Conformal Imidazolium 1D Perovskite Capping Layer Stabilized 3D Perovskite Films for Efficient Solar Modules

Although the perovskite solar cells have been developed rapidly, the industrialization of perovskite photovoltaics is still facing challenges, especially considering their stability issues. Here, the new type of benzimidazolium salt, N,N'-dialkylbenzimidazolium iodide, is proposed and functionalized to convert the three-dimensional (3D) FACs-perovskite films into one-dimensional (1D) capping layer topped 1D/3D structure either in individual device or module levels. This conformal interface modulation demonstrates that not only can effectively stabilize FACs-based perovskite films by inhibiting the lateral and vertical iodide diffusions in devices or modules, ensuring an excellent operation and environmental stability, but also provides an excellent charge transporting channel through the well-designed 1D crystal structure. Consequently, efficient device performance with power conversion efficiency up to 24.3% is readily achieved. And the large-area perovskite solar modules with high efficiency (19.6% for the active areas of 18 cm2 ) and long-term stability (about 500 h in AM 1.5G illumination or about 1000 h under double-85 conditions) are also successfully verified.

Formation sequence of solid electrolyte interphases and impacts on lithium deposition and dissolution on copper: an in situ atomic force microscopic study

Copper is the most widely used substrate for Li deposition and dissolution in lithium metal anodes, which is complicated by the formation of solid electrolyte interphases (SEIs), whose physical and chemical properties can affect Li deposition and dissolution significantly. However, initial Li nucleation and growth on bare Cu creates Li nuclei that only partially cover the Cu surface so that SEI formation could proceed not only on Li nuclei but also on the bare region of the Cu surface with different kinetics, which may affect the follow-up processes distinctively. In this paper, we employ in situ atomic force microscopy (AFM), together with X-ray photoelectron spectroscopy (XPS), to investigate how SEIs formed on a Cu surface, without Li participation, and on the surface of growing Li nuclei, with Li participation, affect the components and structures of the SEIs, and how the formation sequence of the two kinds of SEIs, along with Li deposition, affect subsequent dissolution and re-deposition processes in a pyrrolidinium-based ionic liquid electrolyte containing a small amount of water. Nanoscale in situ AFM observations show that sphere-like Li deposits may have differently conditioned SEI-shells, depending on whether Li nucleation is preceded by the formation of the SEI on Cu. Models of integrated-SEI shells and segmented-SEI shells are proposed to describe SEI shells formed on Li nuclei and SEI shells sequentially formed on Cu and then on Li nuclei, respectively. "Top-dissolution" is observed for both types of shelled Li deposits, but the integrated-SEI shells only show wrinkles, which can be recovered upon Li re-deposition, while the segmented-SEI shells are apparently top-opened due to mechanical stresses introduced at the junctions of the top regions and become "dead" SEIs, which forces subsequent Li nucleation and growth in the interstice of the dead SEIs. Our work provides insights into the impact mechanism of SEIs on the initial stage Li deposition and dissolution on foreign substrates, revealing that SEIs could be more influential on Li dissolution and that the spatial integration of SEI shells on Li deposits is important to improving the reversibility of deposition and dissolution cycling.

In Operando Visualization of Interfacial Band Bending in Photomultiplying Organic Photodetectors

Charge injection is a basic transport process that strongly affects performance of optoelectronic devices such as light-emitting diodes and photodetectors. In these devices, the charge injection barrier is related to the band bending at the active layer/electrode interface and exhibits sophisticated dependence on interface structure and device operating conditions, making it difficult to determine via either theoretical prediction or experimental measurements. Here, in operando cross-sectional scanning Kelvin probe microscopy (SKPM) has been applied in organic photodetectors to visualize the interfacial band bending. The photoinduced interfacial band bending becomes more significant with increasing reverse bias voltage, resulting in reduced charge injection barrier and facilitated charge injection. The photoinduced injection current is orders of magnitude higher than the photocurrent directly generated from light absorption and thus leads to significant photomultiplication. Furthermore, the interfacial structure is tuned to further enhance photoinduced interfacial band bending and the photomultiplication factor.

Quantitative amplitude-modulation scanning Kelvin probe microscopy via the second eigenmode excitation

Amplitude modulation scanning Kelvin probe microscopy (AM-SKPM) is widely used to measure the contact potential difference (CPD) between probe and samples in ambient or dry inert atmosphere. However, AM-SKPM is generally considered quantitatively inaccurate due to crosstalk between the cantilever and the sample. Here we demonstrate that the accuracy of AM-SKPM-based CPD measurements is drastically improved by exciting the SKPM probe at its second eigenmode. In the second eigenmode of oscillation, there exists a stationary node at the cantilever towards its free end, across which the displacement bears opposite signs; therefore driving the SKPM probe at its second eigenmode helps to partially cancel the virtual work done by the cantilever and reduce the crosstalk effect. The improvement in accuracy is experimentally confirmed with interdigitating electrodes calibration samples as well as practical samples such as the cross-section of wafer-bonded GaAs/GaN heterojunction.

Sulfonate-Assisted Surface Iodide Management for High-Performance Perovskite Solar Cells and Modules

Owing to the ionic nature of lead halide perovskites, their halide-terminated surface is unstable under light-, thermal-, moisture-, or electric-field-driven stresses, resulting in the formation of unfavorable surface defects. As a result, nonradiative recombination generally occurs on perovskite films and deteriorates the efficiency, stability, and hysteresis performances of perovskite solar cells (PSCs). Here, a surface iodide management strategy was developed through the use of cesium sulfonate to stabilize the perovskite surface. It was found that the pristine surface of common perovskite was terminated with extra iodide, that is, with an I^-/Pb²⁺ ratio larger than 3, explaining the origination of surface-related problems. Through post-treatment of perovskite films by cesium sulfonate, the extra iodide on the surface was facilely removed and the as-exposed Pb²⁺ cations were chelated with sulfonate anions while maintaining the original 3D perovskite structure. Such iodide replacement and lead chelating coordination on perovskite could reduce the commonly existing surface defects and nonradiative recombination, enabling assembled PSCs with an efficiency of 22.06% in 0.12 cm² cells and 18.1% in 36 cm² modules with high stability.

Universal and versatile morphology engineering via hot fluorous solvent soaking for organic bulk heterojunction

After explosive growth of efficiency in organic solar cells (OSCs), achieving ideal morphology of bulk heterojunction remains crucial and challenging for advancing OSCs into consumer market. Herein, by utilizing the amphiphobic nature and temperature-dependent miscibility of fluorous solvent, hot fluorous solvent soaking method is developed to optimize the morphology with various donor/acceptor combinations including polymer/small-molecule, all-polymer and all-small-molecule systems. By immersing blend film into hot fluorous solvent which is utilized as liquid medium with better thermal conductivity, the molecular reorganization is accelerated. Furthermore, fluorous solvent can be miscible with the residue of chloroform and chloronaphthalene above upper critical solution temperature. This mixed solvent diffuses around inside the active layer and selectively promotes molecular reorganization, leading to optimized morphology. Compared to widely-used thermal annealing, this approach processed under mild conditions achieves superior photovoltaic performance, indicating the practicality and universality for morphological optimization in OSCs as well as other optoelectronic devices.

Reconfiguration of interfacial energy band structure for high-performance inverted structure perovskite solar cells

Charged defects at the surface of the organic–inorganic perovskite active layer are detrimental to solar cells due to exacerbated charge carrier recombination. Here we show that charged surface defects can be benign after passivation and further exploited for reconfiguration of interfacial energy band structure. Based on the electrostatic interaction between oppositely charged ions, Lewis-acid-featured fullerene skeleton after iodide ionization (PCBB-3N-3I) not only efficiently passivates positively charged surface defects but also assembles on top of the perovskite active layer with preferred orientation. Consequently, PCBB-3N-3I with a strong molecular electric dipole forms a dipole interlayer to reconfigure interfacial energy band structure, leading to enhanced built-in potential and charge collection. As a result, inverted structure planar heterojunction perovskite solar cells exhibit the promising power conversion efficiency of 21.1% and robust ambient stability. This work opens up a new window to boost perovskite solar cells via rational exploitation of charged defects beyond passivation.

Charged surface defects are expected to undermine the charge extraction in organic-inorganic perovskite solar cells. Here Zhang et al. design ionic fullerene derivatives to not only passivate the charged defects, but also optimize the interfacial energy due to aligned orientation of the fullerenes.