Electrochemical Strain Microscopy Research Articles

Correlation of the electronic structure and Li‐ion mobility with modulus and hardness in LiNi0.6Co0.2Mn0.2O2 cathodes by combined near edge X‐ray absorption finestructure spectroscopy, atomic force microscopy, and nanoindentation

AbstractThe electrochemical performance of cathode materials in Li‐ion batteries is reflected in macroscopic observables such as the capacity, the voltage, and the state of charge (SOC). However, the physical origin of performance parameters are atomistic processes that scale up to a macroscopic picture. Thus, revealing the function and failure of electrochemical devices requires a multiscale (and ‐time) approach using spectroscopic and microscopic techniques. In this work, we combine near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS) to determine the chemical binding state of transition metals in LiNi0.6Co0.2Mn0.2O2 (NCM622), electrochemical strain microscopy to understand the Li‐ion mobility in such materials, and nanoindentation to relate the mechanical properties exhibited by the material to the chemical state and ion mobility. Strikingly, a clear correlation between the chemical binding, the mechanical properties, and the Li‐ion mobility is found. Thereby, the significant relation of chemo‐mechanical properties of NCM622 on a local and global scale is clearly demonstrated.

In situ characterization of Ni-rich NMC811 thin films prepared by sol-gel method using different scanning probe microscopy techniques

In this paper, LiNi0.8Co0.1Mn0.1O2 (NMC811) thin films were prepared by the sol-gel method based on different parameter conditions, and the microstructure and electrochemical properties of the films were compared to obtain the film sample under the best preparation parameters. In order to explain the formation mechanism of cell battery performance based on the NMC811 cathode at the nano scale, electrochemical strain microscopy, conductive atomic force microscopy and Kelvin probe force microscopy are specifically utilized to study the changes of NMC811 thin film in electrochemical deformation, conductivity, and potential distribution under different electrical and thermal fields. The results indicate that the local amplitude, conductivity, and electrical potential of thin film in the scanning region increases with the voltage from 0 V to 3 V and the temperature from 25 °C to 150 °C. However, the electrochemical deformation, conductivity, and potential of the film decrease significantly when the voltage is applied to 4 V or the temperature rises to 200 °C.These results provide visual evidence for the influence of external field on the phase structure change and lithium-ion diffusion behavior of NMC811 thin film.

Correlating Nanoscale Structures with Electrochemical Properties of Solid Electrolyte Interphases in Solid-State Battery Electrodes.

Here, we investigate the nonlinear relationship between the content of solid electrolytes in composite electrodes and the irreversible capacity via the degree of nanoscale uniformity of the surface morphology and chemical composition of the solid electrolyte interphase (SEI) layer. Using electrochemical strain microscopy (ESM) and X-ray photoelectron spectroscopy (XPS), changes of the chemical composition and morphology (Li and F distribution) in SEI layers on the electrodes as a function of solid electrolyte contents are analyzed. As a result, we find that the solid electrolyte content affects the variation of the SEI layer thickness and chemical distributions of Li and F ions in the SEI layer, which, in turn, influence the Coulombic efficiency. This correlation determines the composition of the composite electrode surface that can maximize the physical and chemical uniformity of the solid electrolyte on the electrode, which is a key parameter to increase electrochemical performance in solid-state batteries.

In situ probing conductivity and deformation of single Ni-rich Li[Ni0.885Co0.1Al0.015]O2 and Li[Ni0.885Co0.1V0.015]O2 nanoparticles by scanning probe microscopy

Li[Ni0.885Co0.1V0.015]O2 (NCV), Li[Ni0.9Co0.1]O2 (NC), and Li[Ni0.885Co0.1Al0.015]O2 (NCA) nanoparticles are synthesized by means of oxalic acid co-precipitation with subsequent calcination. The evolution of Li-ion diffusion and deformation of both NCV and NCA under an external electric field are characterized by means of conductive atomic force microscopy and electrochemical strain microscopy. Macroscopic electrochemical characterization reveals that the Li-ion diffusivity in NCA is greater than that in NCV, and the undesirable irreversible H2–H3 phase transition occurs more readily in NCV than in NCA. The scanning probe microscopy results corroborate well with the macroscopic electrochemical measurements, which tell that vanadium and aluminum substitution can accelerate Li+ diffusion kinetics and enhance the reversibility of the H2–H3 phase transformation during the electrochemical process in varying degrees.

Coupling Particle Ordering and Spherulitic Growth for Long-Term Performance of Nanocellulose/Poly(ethylene oxide) Electrolytes

Development of lithium-ion batteries with composite solid polymer electrolytes (CPSEs) has attracted attention due to their higher energy density and improved safety compared to systems utilizing liquid electrolytes. While it is well known that the microstructure of CPSEs affects the ionic conductivity, thermal stability, and mechanical integrity/long-term stability, the bridge between the microscopic and macroscopic scales is still unclear. Herein, we present a systematic investigation of the distribution of TEMPO-oxidized cellulose nanofibrils (t-CNFs) in two different molecular weights of poly(ethylene oxide) (PEO) and its effect on Li+ ion mobility, bulk conductivity, and long-term stability. For the first time, we link local Li-ion mobility at the nanoscale level to the morphology of CPSEs defined by PEO spherulitic growth in the presence of t-CNF. In a low-MW PEO system, spherulites occupy a whole volume of the derived CPSE with t-CNF being incorporated in between lamellas, while their nuclei remain particle-free. In a high-MW PEO system, spherulites are scarce and their growth is arrested in a non-equilibrium cubic shape due to the strong t-CNF network surrounding them. Electrochemical strain microscopy and solid-state 7Li nuclear magnetic resonance spectroscopy confirm that t-CNF does not partake in Li+ ion transport regardless of its distribution within the polymer matrix. Free-standing CSPE films with low-MW PEO have higher conductivity but lack long-term stability due to the existence of uniformly distributed, particle-free, spherulite nuclei, which have very little resistance to Li dendrite growth. On the other hand, high-MW PEO has lower conductivity but demonstrates a highly stable Li cycling response for more than 1000 h at 0.2 mA/cm2 and 65 °C and more than 100 h at 85 °C. The study provides a direct link between the microscopic dynamic, Li-ion transport, bulk mechanical properties and long-term stability of the derived CPSE and, and as such, offers a pathway towards design of robust all-solid-state Li-metal batteries.

Revealing Lithiation Kinetics and Battery Degradation Pathway in LiMn2O4-Based Commercial Cathodes via Electrochemical Strain Microscopy

The capacity fade during the cycling of lithium batteries is a key factor limiting further progress in the improvement of electric vehicles, wearable electronic devices, alternative energy sources, etc. One of the main reasons for capacity loss is battery cathode degradation, which significantly influences the battery lifetime. Despite in-depth knowledge of battery degradation at the chemical level, the kinetics of the degradation at the resolution of the individual elements of the cathode are not fully understood. Here, we studied lithiation kinetics in commercial cathodes based on lithium manganese spinel using the electrochemical strain microscopy local method. Supported by the experimental finding, the “viscous fingers” model of lithium ions intercalation–deintercalation in individual particles of the cathode was proposed. The non-linear dynamics of the lithiation front were suggested to be stimulated by the non-uniform stress field and gradient of the chemical potential. Irregularity of the lithiation front causes the formation of the residual lithiated pocket in the delithiated particles, which effectively reduces the volume available for chemical reaction. The obtained results shed further light on the degradation of the lithium battery cathodes and can be applicable for other cathode materials.

Ferroelectricity and Piezoelectricity in 2D Van der Waals CuInP2S6 Ferroelectric Tunnel Junctions.

CuInP2S6 (CIPS) is a novel two-dimensional (2D) van der Waals (vdW) ferroelectric layered material with a Curie temperature of TC~315 K, making it promising for great potential applications in electronic and photoelectric devices. Herein, the ferroelectric and electric properties of CIPS at different thicknesses are carefully evaluated by scanning probe microscopy techniques. Some defects in some local regions due to Cu deficiency lead to a CuInP2S6–In4/3P2S6 (CIPS–IPS) paraelectric phase coexisting with the CIPS ferroelectric phase. An electrochemical strain microscopy (ESM) study reveals that the relaxation times corresponding to the Cu ions and the IPS ionospheres are not the same, with a significant difference in their response to DC voltage, related to the rectification effect of the ferroelectric tunnel junction (FTJ). The electric properties of the FTJ indicate Cu+ ion migration and propose that the current flow and device performance are dynamically controlled by an interfacial Schottky barrier. The addition of the ferroelectricity of CIPS opens up applications in memories and sensors, actuators, and even spin-orbit devices based on 2D vdW heterostructures.

Spatiotemporally Correlated Imaging of Interfacial Defects and Photocurrents in High Efficiency Triple-Cation Mixed-Halide Perovskites.

Triple-cation mixed-halide perovskites have attracted considerable attention due to their excellent photovoltaic properties and enhanced stability, though the power conversion efficiency (PCE) is still far below the theoretical expectation. In order to understand the microscopic mechanisms responsible for the gap, a Cs0.05 (FA0.85 MA0.15 )0.95 Pb(I0.85 Br0.15 )3 (CsFAMA)-based solar cell with respectful efficiency over 20% is examined, and distinct high- and low-current regions are observed in photoconductive atomic force microscopy (pc-AFM) mapping. Simulations attribute the difference in local photocurrents to interfacial donor defect densities at the NiO/CsFAMA interface, which is supported by electrochemical strain microscopy (ESM) mapping, revealing a negative correlation between ionic defects and photocurrents. The interfacial defects can be further manipulated by external bias upon relaxation study, resulting in reduced photocurrents accompanied by topography change when positive ions are driven toward the NiO/CsFAMA interface. It is also observed that both structure variation and photocurrent degradation upon accelerated aging test initiate at grain boundaries, which gradually expand at the expense of grain interior, suggesting that ionic defects are most active at grain boundaries. These findings render a direct correlation between interfacial defects and photocurrents while revealing degradation evolution, and if such interfacial defects heterogeneity can be mitigated, PCE toward the theoretical limit with enhanced stability can be envisioned.

Investigating the Interface between Ceramic Particles and Polymer Matrix in Hybrid Electrolytes by Electrochemical Strain Microscopy.

The interface between ceramic particles and a polymer matrix in a hybrid electrolyte is studied with high spatial resolution by means of Electrochemical Strain Microscopy (ESM), an Atomic Force Microscope (AFM)-based technique. The electrolyte consists of polyethylene oxide with lithium bis(trifluoromethanesulfonyl)imide (PEO6–LiTFSI) and Li6.5La3Zr1.5Ta0.5O12 (LLZO:Ta). The individual components are differentiated by their respective contact resonance, ESM amplitude and friction signals. The ESM signal shows increased amplitudes and higher contact resonance frequencies on the ceramic particles, while lower amplitudes and lower contact resonance frequencies are present on the bulk polymer phase. The amplitude distribution of the hybrid electrolyte shows a broader distribution in comparison to pure PEO6–LiTFSI. In the direct vicinity of the particles, an interfacial area with enhanced amplitude signals is found. These results are an important contribution to elucidate the influence of the ceramic–polymer interaction on the conductivity of hybrid electrolytes.

Unraveling the State of Charge-Dependent Electronic and Ionic Structure–Property Relationships in NCM622 Cells by Multiscale Characterization

LiNi0.6Co0.2Mn0.2O2 (NCM622) undergoes crystallographic and electronic changes when charging and discharging, which drive the cathode material close to or even beyond its stability window. To unravel the charge compensation mechanism of NCM622, spatially resolved atomic force microscopy (AFM) measurements in electrochemical strain microscopy (ESM) and conductive AFM (C-AFM) modes are obtained, and the spectroscopic information and crystallographic information are compared. All experiments are performed with two sets of samples: state-of-the-art samples that are composed of a binder, a conductive additive, and an active material and polished samples for single-particle analysis. Near-edge X-ray absorption fine structure spectroscopy shows that ionic Ni2+ reacts to give Ni3+ when charging and forms covalent bonds with its oxygen neighbors. A Ni2+/Ni3+ gradient across the particles balances out with the increasing state of charge, as verified by ESM. Therefore, the results also provide an important view that improves the mechanistic understanding of ESM in electrode materials. Finally, the interplay between the electronic and ionic conductivities and the crystallinities of NCM622 cathodes is elaborated and discussed.

Cantilever signature of tip detachment during contact resonance AFM.

Contact resonance atomic force microscopy, piezoresponse force microscopy, and electrochemical strain microscopy are atomic force microscopy modes in which the cantilever is held in contact with the sample at a constant average force while monitoring the cantilever motion under the influence of a small, superimposed vibrational signal. Though these modes depend on permanent contact, there is a lack of detailed analysis on how the cantilever motion evolves when this essential condition is violated. This is not an uncommon occurrence since higher operating amplitudes tend to yield better signal-to-noise ratio, so users may inadvertently reduce their experimental accuracy by inducing tip–sample detachment in an effort to improve their measurements. We shed light on this issue by deliberately pushing both our experimental equipment and numerical simulations to the point of tip–sample detachment to explore cantilever dynamics during a useful and observable threshold feature in the measured response. Numerical simulations of the analytical model allow for extended insight into cantilever dynamics such as full-length deflection and slope behavior, which can be challenging or unobtainable in a standard equipment configuration. With such tools, we are able to determine the cantilever motion during detachment and connect the qualitative and quantitative behavior to experimental features.

Characterization of Vegard strain related to exceptionally fast Cu-chemical diffusion in Cu_2Mo_6S_8 by an advanced electrochemical strain microscopy method

Electrochemical strain microscopy (ESM) has been developed with the aim of measuring Vegard strains in mixed ionic-electronic conductors (MIECs), such as electrode materials for Li-ion batteries, caused by local changes in the chemical composition. In this technique, a voltage-biased AFM tip is used in contact resonance mode. However, extracting quantitative strain information from ESM experiments is highly challenging due to the complexity of the signal generation process. In particular, electrostatic interactions between tip and sample contribute significantly to the measured ESM signals, and the separation of Vegard strain-induced signal contributions from electrostatically induced signal contributions is by no means a trivial task. Recently, we have published a compensation method for eliminating frequency-independent electrostatic contributions in ESM measurements. Here, we demonstrate the potential of this method for detecting Vegard strain in MIECs by choosing Cu_2Mo_6S_8 as a model-type MIEC with an exceptionally high Cu chemical diffusion coefficient. Even for this material, Vegard strains are only measurable around and above room-temperature and with proper elimination of electrostatics. The analyis of the measured Vegards strains gives strong indication that due to a high charge transfer resistance at the tip/interface, the local Cu concentration variations are much smaller than predicted by the local Nernst equation. This suggests that charge transfer resistances have to be analyzed in more detail in future ESM studies.

Ionically Mediated Mechanical Deformation Associated with Memristive Switching

AbstractIonically mediated phenomena underpin the functioning of various devices, including batteries, solid oxide fuel cells, memristors, and neuromorphic devices. The ionic behavior corresponding to ionically mediated phenomena causes not only variations in the electrical properties but also mechanical deformation, which is crucial for device reliability. However, the interrelation between ionically mediated electrical properties and mechanical deformation has not been elucidated yet. This study investigates ionically mediated mechanical deformation accompanied by memristive switching in a TiO2 single crystal through simultaneous conductive atomic force microscopy and electrochemical strain microscopy. A comprehensive analysis indicates the existence of a relationship between mechanical deformation and memristive switching based on the ionic behavior. Furthermore, an ionic state variable is used to simplify the interrelation between the electrochemical strain hysteresis and memristive switching associated with applied voltage. This study provides insights on the ionic behavior and can be extended to other systems for the general analysis of the relationship between mechanical deformation and electrical properties.

Nanowire Architectures Improve Ion Uptake Kinetics in Conjugated Polymer Electrochemical Transistors.

Organic electrochemical transistors are believed to face an inherent material design tension between optimizing for ion mobility and for electronic mobility. These devices transduce ion uptake into electrical current, thereby requiring high ion mobility for efficient electrochemical doping and rapid turn-on kinetics and high electronic mobility for the maximum transconductance. Here, we explore a facile route to improve operational kinetics and volumetric capacitance in a high-mobility conjugated polymer (poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)], DPP-DTT) by employing a nanowire morphology. For equivalent thicknesses, the DPP-DTT nanowire films exhibit consistently faster kinetics (∼6-10× faster) compared to a neat DPP-DTT film. The nanowire architectures show ∼4× higher volumetric capacitance, increasing from 7.1 to 27.7 F/cm3, consistent with the porous structure better enabling ion uptake throughout the film. The nanowires also exhibit a small but energetically favorable shift in a threshold voltage of ∼17 mV, making the nanostructured system both faster and energetically easier to electrochemically dope compared to neat films. We explain the variation using two atomic force microscopy methods: in situ electrochemical strain microscopy and nanoinfrared imaging via photoinduced force microscopy. These data show that the nanowire film's structure allows greater swelling and ion uptake throughout the active layer, indicating that the nanowire architecture exhibits volumetric operation, whereas the neat film is largely operating via the field effect. We propose that for higher-mobility materials, casting the active layer in a nanowire form may offer faster kinetics, enhanced volumetric capacitance, and possibly lower threshold voltage while maintaining desirable device performance.

Mechanics of electrochemical strain microscopy: Computational simulations and experimental validations

In the last decade, electrochemical strain microscopy (ESM) has emerged as a powerful tool to study electrochemical processes at the nanoscale, yet its quantitative analysis is quite challenging, involving complex electrochemo-mechanical coupling under highly concentrated electromechanical fields. In this work, we develop a theoretical framework of thermodynamics and kinetics for mobile ions in electrochemically-active solids, wherein full electrochemo-mechanical coupling is considered via concentration-dependent Vegard strain and stress-dependent diffusivity. The theory is applied to model electrochemical processes underneath a charged scanning probe tip, and implemented numerically to solve for the highly inhomogeneous electrochemo-mechanical field via combined fast Fourier transform and finite difference analysis. The simulations reveal that the ESM amplitude correlates linearly with both ionic concentration and diffusivity, while relaxation time constant depends only on diffusivity, making it possible to decouple these two material parameters. In order to validate our model analysis, ESM mapping and point-wise relaxation studies are carried out on solid state electrolyte ceria, and the experimental data agree with model predictions well. The analysis thus provides a powerful technique to analyze ESM experiments, and sheds deep insight into the nanoscale electrochemical process under a tip.

Signal Origin of Electrochemical Strain Microscopy and Link to Local Chemical Distribution in Solid State Electrolytes.

Electrochemical strain microscopy (ESM) is a distinguished method to characterize Li-ion mobility in energy materials with extremely high spatial resolution. The exact origin of the cantilever deflection when the technique is applied on solid state electrolytes (SSEs) is currently discussed in the literature. Understanding local properties and influences on ion mobility in SSEs is of utmost importance to improve such materials for next generation batteries. Here, the exact signal formation process of ESM when applied on sodium super ionic conductor (NASICON)-type SSE containing Na- and Li-ions is investigated. Changes in the dielectric properties, which are linked to the local chemical composition, are found to be responsible for the observed contrast in the deflection of the cantilever instead of a physical volume change as a result of Vegard´s Law. The cantilever response is strongly reduced in areas of high sodium content which is attributed to a reduction of the tip-sample capacitance in comparison to areas with high lithium content. This is the first time a direct link between electrostatic forces in contact mode and local chemical information is demonstrated on SSEs. The results open up new possibilities in information gain since dielectric properties are sensitive to subtle changes in local chemical composition.

Lithium Deposition in Single-Ion Conducting Polymer Electrolytes

Lithium metal is considered as potential anode material due to its superior energy density compared to conventional graphitic anodes.[1] However, so far its application is restricted due to inhomogeneous Li deposition during cycling, potentially causing contact losses between bulk and deposited Li or short circuit of the cell related to formation of ‘dendritic’ deposits. Theoretical models propose that single-ion conducting electrolytes afford safe and ‘dendrite-free’ cycling of lithium metal batteries (LMBs) due to immobilization of anions and thus, prevention of cell polarization.[2] In this contribution, a blend type quasi-solid single-ion conducting polymer electrolyte (SIPE), a polymer class that has attracted much attention recently, is consulted for analysis of the Li deposition behaviour. Combined scanning electron microscopy (SEM) and X-ray tomography are applied illustrating the occurrence of inhomogeneous, rather dense Li deposits agglomerating and eventually penetrating the polymer membrane independent of the applied current densities. Raman mapping, nano-mechanical (QNM) and electrochemical strain microscopy (ESM) mapping further reveal that membrane constituents and consequently membrane properties such as (mechanical) stiffness and ionic conductivity are non-homogeneously distributed within the membrane related to the phased separated morphology of the blend. This implies that the intrinsic nature of the polymer membrane is responsible for the formation of conduction channels, also governing the homogeneity of Li deposition. Beyond that, it is demonstrated how characteristics of the Li│polymer interface in salt- free SIPEs limit the achievable cell performance, particularly since limited Li+ ion diffusivity through the interfaces substantially impair efficient charge carrier transport within the cell system.

Spatially Resolved Electrochemical Strain of Solid‐State Electrolytes via High Resolution Sequential Excitation and Its Implication on Grain Boundary Impedance

AbstractSolid‐state electrolytes have great potential in solving the intrinsic safety issues of conventional lithium‐ion batteries utilizing liquid electrolytes, and there is a tremendous effort in developing solid‐state electrolytes with improved ionic conductivity via microstructure engineering spanning multiple length scales. Nevertheless, there still lacks an effective method to probe the local ionic conductivity at the nanoscale with sufficient resolution, and thus how the microstructure impacts macroscopic ionic conductivity of solid‐state electrolytes remains inadequately understood. Here, the newly developed sequential excitation (SE) electrochemical strain microscopy is applied to spatially resolve local electrochemical processes at the nanoscale, unraveling the ionic dynamics of grain boundary in Li1.3Al0.3Ti1.7(PO4)3 solid‐state electrolytes that correlate well with macroscopic impedance analysis. The high‐conductivity sample possesses comparable ionic dynamics at grain boundary and within grain interior, while low‐conductivity sample exhibits much higher resistance at the grain boundary, even though the conductivity of its grain interior is comparable to high‐conductivity sample. The study thus provides direct experimental evidence on the bottlenecking grain boundaries in ionic conduction, and offers a powerful tool to study local ionic dynamics at the nanoscale in one‐to‐one correspondence to the microstructure features.

Voltage- and Frequency-Based Separation of Nanoscale Electromechanical and Electrostatic Forces in Contact Resonance Force Microscopy: Implications for the Analysis of Battery Materials

In piezoresponse force microscopy and electrochemical strain microscopy (PFM, ESM), not only nanoscale electromechanical surface displacements (e.g., Vegard strain in case of ESM) are amplified in ...

Impact of dual-layer solid-electrolyte interphase inhomogeneities on early-stage defect formation in Si electrodes

While intensive efforts have been devoted to studying the nature of the solid-electrolyte interphase (SEI), little attention has been paid to understanding its role in the mechanical failures of electrodes. Here we unveil the impact of SEI inhomogeneities on early-stage defect formation in Si electrodes. Buried under the SEI, these early-stage defects are inaccessible by most surface-probing techniques. With operando full field diffraction X-ray microscopy, we observe the formation of these defects in real time and connect their origin to a heterogeneous degree of lithiation. This heterogeneous lithiation is further correlated to inhomogeneities in topography and lithium-ion mobility in both the inner- and outer-SEI, thanks to a combination of operando atomic force microscopy, electrochemical strain microscopy and sputter-etched X-ray photoelectron spectroscopy. Our multi-modal study bridges observations across the multi-level interfaces (Si/LixSi/inner-SEI/outer-SEI), thus offering novel insights into the impact of SEI homogeneities on the structural stability of Si-based lithium-ion batteries.