All-Solid-State Battery Cathode Mott-Schottky Plots

Abstract Electrochemical Impedance Spectroscopy (EIS) is a powerful technique to understand the electrode-electrolyte interaction and to evaluate degradation, resistive behavior and electrochemical activity of energy storage materials used in batteries, pseudocapacitors and supercapacitors among others. However, it can sometimes be misused or under-interpreted. To effectively acquire EIS results, the voltages imposed to the working electrode at which EIS spectra are obtained, shall be critically selected. This work follows a previous study on the EIS response of Nickel-Cobalt hydroxide, and highlights how the Mott-Schottky model can be used as a complementary tool to explain EIS results obtained at different potentials. The Mott-Schottky model is used to understand further the fundamental processes occurring at the electrode-electrolyte interface of nickel-cobalt hydroxide in alkali media and to explain the changes in conductivity of the material that ultimately determine the electrode electrochemical activity. The applicability of the model to assist in the potential selection for EIS studies on other important charge storage materials such as MnO x and MoO x is discussed too.

The space charge layer (SCL) effects were initially developed to explain the anomalous conductivity enhancement in composite ionic conductors. They were further extended to qualitatively as well as quantitatively understand the interfacial phenomena in many other ionic-conducting systems. Especially in nanometre-scale systems, the SCL effects could be used to manipulate the conductivity and construct artificial conductors. Recently, existence of such effects either at the electrolyte/cathode interface or at the interfaces inside the composite electrode in all solid state lithium batteries (ASSLB) has attracted attention. Therefore, in this article, the principle of SCL on basis of defect chemistry is first presented. The SCL effects on the carrier transport and storage in typical conducting systems are reviewed. For ASSLB, the relevant effects reported so far are also reviewed. Finally, the perspective of interface engineer related to SCL in ASSLB is addressed.

… electrolytes have been in the focus of researchers for the last decade thanks to the recent efforts to develop efficient all-solid-state … -ions inside the solid electrolyte is mobile and thus …

Abstract The roles of interfaces in either blocking or enhancing ionic conduction in various types of solid electrolytes, including lithium, sodium, oxygen and other types of ion conductors as well as proton conductors, are critically reviewed. Two important fundamental interfacial phenomena, namely the formation of space charges and two-dimensional interfacial phases (complexions), can markedly alter ion transport along or across various types of interfaces, including grain and phase boundaries as well as free surfaces. Since the experiments and models of space charges have been well documented in literature, a new focus of this short review and viewpoint article is to propose and discuss an emerging opportunity of utilizing the formation and transition of interfacial phases to either alleviate the blocking effects or further enhance ionic conduction in solid electrolytes.

Solid‐state lithium batteries have aroused wide interest with the probability to guarantee safety and high energy density at the same time. In the past decade, fruitful endeavors have been devoted to promoting each component of these batteries, including solid electrolyte with high conductivity, dendrite‐free lithium anode, and high‐capacity cathode. However, the currently achieved cell performances are still inconsistent with the original expectations, in which interfaces severely hamper the energy output and cycling stability for practical application. Herein, particular attentions are paid to the interface between cathode and solid electrolyte. The huge resistance caused at this interface can be found throughout the entire life of batteries from preparation to operation. Accordingly, these issues are divided into physical contact, thermal interdiffusion, space‐charge layer, electrochemomechanical breakdown, and undesirable side reactions and further elucidated in detail. Moreover, representative developments concerning the cathode/solid electrolyte interface in terms of compositional and morphological control in cathode, architecture and manufacture design in solid electrolyte, and artificial interphase building are summarized. With these efforts, the emphasis of the fundamental issues and perspectives of interface between cathode and solid electrolyte may eventually contribute to high‐energy long‐cycling solid‐state lithium batteries.

All-solid state batteries have the promise to increase the safety of Li-ion batteries. A prerequisite for high-performance all-solid-state batteries is a high Li-ion conductivity through the solid electrolyte. In recent decades, several solid electrolytes have been developed which have an ionic conductivity comparable to that of common liquid electrolytes. However, fast charging and discharging of all-solid-state batteries remains challenging. This is generally attributed to poor kinetics over the electrode-solid electrolyte interface because of poorly conducting decomposition products, small contact areas, or space-charge layers. To understand and quantify the role of space-charge layers in all-solid-state batteries a simple model is presented which allows to asses the interface capacitance and resistance caused by the space-charge layer. The model is applied to LCO (LiCoO2) and graphite electrodes in contact with an LLZO (Li7La3Zr2O12) and LATP (Li1.2Al0.2Ti1.8(PO4)3) solid electrolyte at several voltages. The predictions demonstrate that the space-charge layer for typical electrode–electrolyte combinations is about a nanometer in thickness, and the consequential resistance for Li-ion transport through the space-charge layer is negligible, except when layers completely depleted of Li-ions are formed in the solid electrolyte. This suggests that space-charge layers have a negligible impact on the performance of all-solid-state batteries.

… , and capacitance measurements at electrochemical interfaces in all-solid-state batteries. … In solid-state batteries, the interface of NMC111 cathode and Al-doped LLZO electrolyte is …

Introducing dielectric materials is a promising approach to mitigate space‐charge‐layer (SCL) formation, which negatively affects the electrochemical performance of sulfide‐based all‐solid‐state batteries (ASSBs). Most previous studies have focused on mitigating SCL formation by introducing dielectric materials, overlooking the fact that significant dielectric properties such as the dipole moment direction and the magnitude of the dielectric constant can influence SCL formation. To clarify the unclear mechanism of dielectric materials mitigating SCL formation, paraelectricity, ferroelectricity, and the magnitude of the dielectric constant are investigated to determine their effect on SCL formation. Paraelectric materials possessing no permanent dipole moment can effectively mitigate the SCL formation better than ferroelectric material with strong permanent dipole moment because of the intrinsic characteristics of the paraelectric material, in which the dipole moment can be aligned along the direction of the electric field applied inside of ASSB. Furthermore, paraelectric materials with a larger dielectric constant have a greater effect in mitigating SCL effect than paraelectric materials with a smaller dielectric constant. Thus, these properties should be considered in cathode‐solid‐electrolyte interface design. This study considers relevant dielectric material characteristics that had not been considered previously, suggesting a new paradigm for optimizing the interfacial resistance of sulfide‐based ASSBs originating from SCL formation.

… We theoretically elucidated the characteristics of the space–charge layer (SCL) at interfaces between oxide cathode and sulfide electrolyte in all-solid-state lithium-ion batteries (ASS-…

… the space-charge layer caused by the inherent chemical potential difference between the cathode … and theoretical calculations of the space-charge layer at the cathode/SE interface are …

Summary The influence of space-charge layers on the ionic charge transport over cathode-solid electrolyte interfaces in all-solid-state batteries remains unclear because of the difficulty to unravel it from other contributions to the ion transport over the interfaces. Here, we reveal the effect of the space-charge layers by systematically tuning the space-charge layer on and off between LixV2O5 and Li1.5Al0.5Ge1.5(PO3)4 (LAGP), by changing the LixV2O5 potential and selectively measuring the ion transport over the interface by two-dimensional (2D) NMR exchange. The activation energy is demonstrated to be 0.315 eV for lithium-ion exchange over the space-charge-free interface, which increases dramatically to 0.515 eV for the interface with a space-charge layer. Comparison with a space-charge model indicates that the charge distribution due to the space-charge layer is responsible for the increased interface resistance. Thereby, the present work provides selective and quantitative insight into the effect of space-charge layers over electrode-electrolyte interfaces on ionic transport.

All-solid-state batteries (ASSBs) have been considered next-generation energy storage. However, space charge layers (SCLs) at solid–solid interfaces due to Li chemical potential difference between electrode/electrolyte materials are essential to understanding the charge transfer of ASSBs. However, the influence of SCL on the Li-ion transport between interfaces is unclear because of the difficulty of distinguishing the impedance contribution of SCL towards the ionic transport from other effects. This review summarizes the various conclusions of the SCL effect and its impact on interface charge transfer resistance. We have proposed that further investigation of SCL in the full cell and bipolar stack cell is required for developing high-performance ASSBs. Moreover, we highlight the development and application of the electrochemical-based P2D model, which considered the SCL parameter for the simulation, and space charge limited current models for investigating Li-ion transport across the interface are needed to fully understand the SCL effect in ASSBs. Graphical abstract

Understanding the interfacial impedance between the solid electrolyte and the electrode is a critical issue for the design of solid-state batteries. We propose a new equivalent circuit model that treats the interface not only as a capacitor but also includes the space charge layer resistance and the resultant polarization resistance. Moreover, the elements of the circuit model are quantified by the physical quantities based on the recently proposed modified Planck-Nernst-Poisson (MPNP) model, which includes the effect of the unoccupied regular lattice sites (vacancies) in the electro-diffusion problem and takes both the ion and electron contributions into the account. We provide a new analytical solution for the space charge layer capacitance. Comparative numerical results demonstrate that our proposed model with additional polarization resistance can explain well the real impedance tail at the low-frequency region, for which the pure capacitor interface model fails. The model is verified against the experimental impedance spectra of LiPON.

All-solid-state batteries (ASSBs) are poised to transform electrochemical energy storage, yet their performance remains critically limited by high interfacial impedance. A central origin of this bottleneck is the space charge...

The current controversies about the role of space charge layers hinder the development of better solid–solid interfaces and, thus, the improvement of solid-state batteries (ASSBs). To overcome this, we have combined high spatial resolution and nondestructive techniques, operando heterodyne Kelvin probe force microscopy (KPFM), and operando nuclear reaction analysis (NRA) to conduct a study of space charge layers in ASSBs. A model thin-film ASSB was fabricated from lithium (Li)|Li3PO4 (LPO)|LiCoO2 (LCO) for this study. This battery excels due to negligible interfacial defects and side reactions. For a working battery voltage range from 3.0 to 4.3 V vs Li/Li+, a space charge layer mainly exists at the LPO|LCO interface. This space charge layer with a width <50 nm arises from the redistribution of Li-ions at the interface. We clarified controversial views on the role of space charge layers in ASSBs by quantitatively determining the interfacial space charge layer resistance and found a maximum value between 18.4 and 19.1 Ω cm2 at 4.3 V vs Li/Li+. The absolute value of interfacial resistance from space charge layer formation is much smaller compared with the bulk solid electrolyte resistance in the fabricated thin-film ASSB. By employing KPFM and NRA techniques in ASSB research, our knowledge of space charge layer evolution at the solid electrolyte electrode interface is more comprehensive, even beyond the investigation of space charge layers.

The electrochemical performance of all‐solid‐state lithium batteries (ASSLBs) can be significantly improved by addressing the challenges posed by space charge layer (SCL) effect, which plays a crucial role in determining Li+ ions transport kinetic at cathodic interface. Therefore, it is critical to realize the in situ inspection and visualization of SCL behaviors for solving sluggish Li+ ions transport issues, despite remaining grant challenges. Therewith, the well‐defined model of LiNbO3‐coated NCM (NCM@LNO) cathode is constructed and assembled for the representative Li6PS5Cl‐based ASSLBs, which not only ensures excellent cathodic compatibility, but also preferably enables the better monitoring of Li+ ions transport kinetics. Combining ex situ analysis with DFT calculation, the formation and evolution mechanism of SCL are comprehensively understood, and the relationship between well‐controlled SCL configuration and Li+ electrochemical behavior has been also further illustrated and established through the operando Raman spectroscopy. On these grounds, the preferred NCM@LNO cathodes acquire the enhanced discharge capacity of 90.6% (144.8 mAh g−1) after 100 cycles and it can still deliver the exceptional capacity of 136.2 mAh g−1 after 800 cycles in ASSLBs. Hence, the research will pave up a new perspective for fundamental scientific insight of the SCL and reasonable tailoring of cathodic interface for high‐efficiency ASSLBs.

Contacts of two phases, which allow for synergistic dissociative storage of a component in two space charge zones ("job-sharing storage"), are considered from the viewpoint of point defect thermodynamics. The respective relations between charge and component activity (chemical potential of the component) are derived, or - for more complex cases - the recipes for their derivation are given. These relations describe - according to different experimental conditions - the connection between mass storage and outer partial pressure or between mass storage and the cell voltage in a coulometric titration cell. They also reflect theoretical charge/discharge curves in battery cells when job-sharing storage predominates, and thus are also significant for supercapacitors. In addition to explicitly worked-out cases, it is pointed out how more general situations, such as simultaneous storage in bulk and in boundaries, specific adsorption or size effects, are to be treated.

Material characterization that informs research and development of batteries is generally based on well-established ex situ and in situ experimental methods that do not consider the band structure. This is because experimental extraction of structural information for liquid-electrolyte batteries is extremely challenging. However, this hole in the available experimental data negatively affects the development of new battery systems. Herein, we determined the entire band structure of a model thin-film solid-state battery with respect to an absolute potential using operando hard X-ray photoelectron spectroscopy by treating the battery as a semiconductor device. We confirmed drastic changes in the band structure during charging, such as interfacial band bending, and determined the electrolyte potential window and overpotential location at high voltage. This enabled us to identify possible interfacial side reactions, for example, the formation of the decomposition layer and the space charge layer. Notably, this information can only be obtained by evaluating the battery band structure during operation. The obtained insights deepen our understanding of battery reactions and provide a novel protocol for battery design. The electronic structure evolution within a battery during cycling can provide crucial cues for its optimization, but insights on operando band structures are extremely challenging to obtain. Here, the authors determine the overall band structure of a model thin-film solid-state lithium battery via operando hard X-ray photoelectron spectroscopy, considering the cathode and anode sides.

… flat-band state. However, the first defect will induce a local strain field as well as electrostatic potential modifications due to charge … space charge which is to convert Li chemical potential …

Density Functional Theory (DFT) calculations of electrode material properties in high energy density storage devices like lithium batteries have been standard practice for decades. In contrast, DFT modelling of explicit interfaces in batteries arguably lacks universally adopted methodology and needs further conceptual development. In this paper, we focus on solid-solid interfaces, which are ubiquitous not just in all-solid state batteries; liquid-electrolyte-based batteries often rely on thin, solid passivating films on electrode surfaces to function. We use metal anode calculations to illustrate that explicit interface models are critical for elucidating contact potentials, electric fields at interfaces, and kinetic stability with respect to parasitic reactions. The examples emphasize three key challenges: (1) the "dirty" nature of most battery electrode surfaces; (2) voltage calibration and control; and (3) the fact that interfacial structures are governed by kinetics, not thermodynamics. To meet these challenges, developing new computational techniques and importing insights from other electrochemical disciplines will be beneficial.

For solid-state batteries based on ceramic materials, the means of effective materials processing and co-processing presents a significant challenge. The high sintering temperature required for many solid electrolytes induces alkali...

… behavior of interfaces and bulk materials in an all-solid-state … with the electrode/electrolyte interfaces. Actually, this model is … for this family of commercial all-solid-state microbatteries. …

… 0 from impedance data, assuming a Mott–Schottky situation. Subsequently, we describe … Mott–Schottky and restricted-equilibrium data using various expressions from the Mott–Schottky …

… the Mott–Schottky approximation to obtain the electronic properties (flatband potential and charge carrier density… abruptly increases in cathodic polarization tests for several n-type films. …

The practical progress of lithium-sulfur batteries is hindered by the serious shuttle effect and the slow oxidation-reduction kinetics of polysulfides. Herein, the ZnFe2O4-Ni5P4 Mott-Schottky heterojunction material is prepared to address these issues. Benefitting from a self-generated built-in electric field, ZnFe2O4-Ni5P4 as an efficient bidirectional catalysis regulates the charge distribution at the interface and accelerates electron transfer. Meanwhile, the synergy of the strong adsorption capacity derived from metal oxides and the outstanding catalytic performance that comes from metal phosphides strengthens the adsorption of polysulfides, reduces the energy barrier during the reaction, accelerates the conversion between sulfur species, and further accelerates the reaction kinetics. Hence, the cell with ZnFe2O4-Ni5P4/S harvests a high discharge capacity of 1132.4 mAh g-1 at 0.5C and displays a high Coulombic efficiency of 99.3% after 700 cycles. The ZnFe2O4-Ni5P4/S battery still maintains a capacity of 610.1 mAh g-1 with 84.4% capacity retention after 150 cycles at 0.1C under a high sulfur loading of 3.2 mg cm-2. This work provides a favorable reference and advanced guidance for developing Mott-Schottky heterojunctions in lithium-sulfur batteries.