固态电池卤化物空间电荷层

Mixing electroactive materials, solid-state electrolytes, and conductive carbon to fabricate composite electrodes is the most practiced but least understood process in all-solid-state batteries, which strongly dictates interfacial stability and charge transport. We report on universal halide segregation at interfaces across various halogen-containing solid-state electrolytes and a family of high-energy chalcogen cathodes enabled by mechanochemical reaction during ultrahigh-speed mixing. Bulk and interface characterizations by multimodal synchrotron x-ray probes and cryo-transmission electron microscopy show that the in situ segregated lithium halide interfacial layers substantially boost effective ion transport and suppress the volume change of bulk chalcogen cathodes. Various all-solid-state lithium-chalcogen cells demonstrate utilization close to 100% and extraordinary cycling stability at commercial-level areal capacities.

Modulation of ion transport behavior and interfacial stability of halide SSEs by chemical substitution.

Over the past few years, halide solid‐state electrolytes (HSSEs) have attracted the attention of researchers, and many reports about HSSEs have been published. Their wide electrochemical window (ECW) and a quite good compatibility with the popular oxide cathode materials make them attractive for practical applications. As a result, HSSEs are exciting candidates for the future generation of all‐solid‐state Li batteries (ASSLBs). In recent years, noticeable efforts have been made to develop novel HSSEs and utilize them in ASSLBs. Herein, a comprehensive update on the progress of HSSEs development and their application in ASSLBs is provided. First, a brief summary of the conductivity of HSSEs and potential synthesis approaches is provided. Next, the moisture and phase stabilities of HSSEs are reviewed separately, and the techniques proposed in the recently published reports to achieve sufficient stabilities are summarized. In addition, the electrochemical stabilities of HSSEs with Li metal anode and oxide cathode materials, from experimental and theoretical points of view, are provided in parallel. Furthermore, the application and progress of HSSEs in high‐voltage ASSLBs are discussed. Finally, new research directions are suggested for the development of scalable HSSEs‐based ASSLBs.

The pursuit of all-solid-state batteries has motivated advancements in materials design. Here, the authors present a methodology demonstrating that ionic potential effectively captures crucial interactions within halide materials, guiding the design of the new materials with enhanced performance. All-solid-state lithium batteries have attracted widespread attention for next-generation energy storage, potentially providing enhanced safety and cycling stability. The performance of such batteries relies on solid electrolyte materials; hence many structures/phases are being investigated with increasing compositional complexity. Among the various solid electrolytes, lithium halides show promising ionic conductivity and cathode compatibility, however, there are no effective guidelines when moving toward complex compositions that go beyond ab -initio modeling. Here, we show that ionic potential, the ratio of charge number and ion radius, can effectively capture the key interactions within halide materials, making it possible to guide the design of the representative crystal structures. This is demonstrated by the preparation of a family of complex layered halides that combine an enhanced conductivity with a favorable isometric morphology, induced by the high configurational entropy. This work provides insights into the characteristics of complex halide phases and presents a methodology for designing solid materials.

ConspectusRechargeable all-solid-state Li batteries (ASSLBs) are considered to be the next generation of electrochemical energy storage systems. The development of solid-state electrolytes (SSEs), which are key materials for ASSLBs, is therefore one of the most important subjects in modern energy storage chemistry. Various types of electrolytes such as polymer-, oxide-, and sulfide-based SSEs have been developed to date and the discovery of new superionic conductors is still ongoing. Metal-halide SSEs (Li-M-X, where M is a metal element and X is a halogen) are emerging as new candidates with a number of attractive properties and advantages such as wide electrochemical stability windows (0.36-6.71 V vs Li/Li+) and better chemical stability toward cathode materials compared to other SSEs. Furthermore, some of the metal-halide SSEs (such as the Li3InCl6 developed by our group) can be directly synthesized at large scales in a water solvent, removing the need for special apparatus or handling in an inert atmosphere. Based on the recent advances, herein we focus on the topic of metal-halide SSEs, aiming to provide a guidance toward further development of novel halide SSEs and push them forward to meet the multiple requirements of energy storage devices.In this Account, we describe our recent progress in developing metal halide SSEs and focus on some newly reported findings based on state-of-the-art publications on this topic. A discussion on the structure of metal-halide SSEs will be first explored. Subsequently, we will illustrate the effective approaches to enhance the ionic conductivities of metal halide SSEs including the effect of anion sublattice framework, the regulation of site occupation and disorder, and defect engineering. Specifically, we demonstrated that proper structural framework, balanced Li+/vacancy concentration, and reduced blocking effect can promote fast Li+ migration for metal halide SSEs. Moreover, humidity stability and degradation chemistry of metal halide SSEs have been summarized for the first time. Some examples of the application of metal halide SSEs with stability toward humidity have been demonstrated. Direct synthesis of halide SSEs on cathode materials by the water-mediated route has been used to eliminate the interfacial challenges of ASSLBs and has been shown to act as an interfacial modifier for high-performance all-solid-state Li-O2 batteries. Taken together, this Account on metal halide SSEs will provide an insightful perspective over the recent development and future research directions that can lead to advanced electrolytes.

Designing highly conductive and (electro)chemical stable inorganic solid electrolytes using cost-effective materials is crucial for developing all-solid-state batteries. Here, we report halide nanocomposite solid electrolytes (HNSEs) ZrO_2(-ACl)-A_2ZrCl_6 (A = Li or Na) that demonstrate improved ionic conductivities at 30 °C, from 0.40 to 1.3 mS cm^−1 and from 0.011 to 0.11 mS cm^−1 for Li^+ and Na^+, respectively, compared to A_2ZrCl_6, and improved compatibility with sulfide solid electrolytes. The mechanochemical method employing Li_2O for the HNSEs synthesis enables the formation of nanostructured networks that promote interfacial superionic conduction. Via density functional theory calculations combined with synchrotron X-ray and ^6Li nuclear magnetic resonance measurements and analyses, we demonstrate that interfacial oxygen-substituted compounds are responsible for the boosted interfacial conduction mechanism. Compared to state-of-the-art Li_2ZrCl_6, the fluorinated ZrO_2−2Li_2ZrCl_5F HNSE shows improved high-voltage stability and interfacial compatibility with Li_6PS_5Cl and layered lithium transition metal oxide-based positive electrodes without detrimentally affecting Li^+ conductivity. We also report the assembly and testing of a Li-In||LiNi_0.88Co_0.11Mn_0.01O_2 all-solid-state lab-scale cell operating at 30 °C and 70 MPa and capable of delivering a specific discharge of 115 mAh g^−1 after almost 2000 cycles at 400 mA g^−1. Compositional tuning is a standard procedure to improve the ionic conductivity of inorganic superionic conductors. Here, the authors report (electro)chemical stable composite halide solid electrolytes applying a nanostructure approach that promotes interfacial superionic conductivity.

All-solid-state Li-ion batteries (ASSBs), considered to be potential next-generation energy storage devices, require solid electrolytes (SEs). Thiophosphate-based materials are popular, but these s...

Lithium‐metal‐chloride (Li‐M‐Cl) ionic conductors, with the high Li+ conductivity, excellent cathode compatibility, and favorable mechanical properties, have emerged as a promising solid electrolyte (SE) candidate for all‐solid‐state batteries (ASSBs). However, their poor compatibility with lithium anodes, due to the presence of high‐valence M species that are highly susceptible to reduction reaction, remains a major challenge. In this study, a strategic modification of the central metal's electronic structure is presented to enable controlled modulation of the orbital gap, thereby tuning its resistance to reduction. By incorporating lanthanide elements (Ho and Lu) at the M sites, which can effectively increase the orbital gap of the metal‐centered polyhedra, improving the reduction stability of the solid electrolytes. As proof of concept, a series of new Li‐M‐Cl halide superionic conductors is synthesized that exhibit excellent compatibility with Li anodes. ASSBs utilizing these newly developed SEs as a single electrolyte layer achieve outstanding cycling stability, retaining up to 80.1% of their capacity after 500 cycles at 1 C. This study offers an effective strategy to address the challenge of anode incompatibility in halide SEs, advancing the development of high‐energy‐density ASSBs.

Argyrodites, with fast lithium-ion conduction, are promising for applications in rechargeable solid-state lithium-ion batteries. We report a new compositional space of argyrodite superionic conduct...

Poor rate capability is a significant obstacle for the practical application of inorganic all‐solid‐state lithium‐ion batteries (ASSLIBs). The charge transfer kinetics at the interface of current collectors is crucial for high‐rate capacity, but is typically neglected. In this paper, the interfacial evolution between Al foil current collectors and composite cathodes is studied in the LiCoO2/Li3InCl6 (LCO/LIC) ASSLIBs at both 25 and −10 °C. The results indicate that the side reactions between Al foil and LIC are the main challenges for the interfacial stability of current collector at 25 °C. The design of a graphene‐like carbon (GLC) coating for the modification of Al avoids side reactions at the interface of current collector, resulting in improved cycling stability and high‐rate capacity. GLC Al ASSLIB exhibits a high initial capacity of 102.9 mAh g–1 with a capacity retention of 89.1% after 150 cycles at 1 C. A high‐rate capacity of 69 mAh g–1 is also achieved at 5 C. At −10 °C, the low Li+/electron transfer kinetics along with side reactions is the key limitation for the rate capability. Thanks to the GLC coating, the improved electrochemical performance is achieved with the enhanced charge transfer kinetics at the interface of current collector.

… Emerging halide SSEs are currently the focuses of SSE … SSE/cathode interface system, the degradation driving force may be further self-limited by the decomposition interphase layer …

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.

Abstract When two different materials come into contact, mobile carriers redistribute at the interface according to their potential difference. Such a charge redistribution is also expected at the interface between electrodes and solid electrolytes. The redistributed ions significantly affect the ion conduction through the interface. Thus, it is essential to determine the actual distribution of the ionic carriers and their potential to improve ion conduction. We succeeded in visualizing the ionic and potential profiles in the charge redistribution layer, or space‐charge layer (SCL), formed at the interface between a Cu electrode and Li‐conductive solid electrolyte using phase‐shifting electron holography and spatially resolved electron energy‐loss spectroscopy. These electron microscopy techniques clearly showed the Li‐ionic SCL, which dropped by 1.3 V within a distance of 10 nm from the interface. These techniques could contribute to the development of next‐generation electrochemical devices.

… to the space charge region formed near the interface between the matrix electrolyte and the … et ai.% theory of space charge layer focused on the free surface of halides and the resulted …

Abstract All-inorganic solid-state batteries (AISSBs) have received considerable attention due to their excellent safety and high energy density. However, large interfacial challenges between oxide cathodes and inorganic solid electrolytes dramatically hinder AISSB development. Here we successfully eliminate the long-standing interfacial challenges by in-situ interfacial growth of a highly Li+-conductive halide electrolyte (Li3InCl6, LIC) on the cathode surface. Owing to strong interfacial interaction, high interfacial ionic conductivity (>1 mS cm−1), and excellent interfacial compatibility, LiCoO2 with 15 wt% LIC exhibits a high initial capacity of 131.7 mAh.g−1 at 0.1C (1C = 1.3 mA cm−2) and can be operated up to 4C at room temperature. The discharge capacity retains 90.3 mAh g−1 after 200 cycles. Moreover, a high areal capacity of 6 mAh cm−2 is demonstrated with a high loading of 48.7 mg cm−2. This work offers a versatile approach to eliminate interfacial challenges of AISSBs toward high-energy density and high-power density.

Inorganic solid-state electrolytes (ISSEs) have been extensively researched as the critical component in all-solid-state lithium-metal batteries (ASSLMBs). Many ISSEs exhibit high ionic conductivities up to 10-3 S cm-1. However, most of them suffer from poor interfacial compatibility with electrodes, especially lithium-metal anodes, limiting their application in high-performance ASSLMBs. To achieve good interfacial compatibility with a high-voltage cathode and a lithium-metal anode simultaneously, we propose Li3InCl6/Li2OHCl bilayer halide ISSEs with complementary advantages. In addition to the improved interfacial compatibility, the Li3InCl6/Li2OHCl bilayer halide ISSEs exhibit good thermal stability up to 160 °C. The Li-symmetric cells with sandwich electrolytes Li2OHCl/Li3InCl6/Li2OHCl exhibit long cycling life of over 300 h and a high critical current density of over 0.6 mA cm-2 at 80 °C. Moreover, the all-inorganic solid-state lithium-metal batteries (AISSLMBs) LiFePO4-Li3InCl6/Li3InCl6/Li2OHCl/Li fabricated by a facile cold-press method exhibit good rate performance and long-term cycling stability that stably cycle for about 3000 h at 80 °C. This work presents a facile and cost-effective method to construct bilayer halide ISSEs, enabling the development of high-performance AISSLMBs with good interfacial compatibility and thermal stability.

Lithium metal solid‐state batteries (LMSBs) have attracted extensive attention over the past decades, due to their fascinating advantages of safety and potential for high energy density. Solid‐state electrolytes (SEs) with fast ionic transport and excellent stability are indispensable components in LMSBs. Heretofore, a series of inorganic SEs have been extensively explored, such as sulfide‐ and oxide‐based electrolytes. Unfortunately, they both have difficulty in achieving a satisfactory balance of conductivity and stability, and oxides suffer from a high impedance of grain boundaries, while sulfides encounter poor stability. Halide‐based solid electrolytes are gradually emerging as one of the most promising candidates for LMSBs due to their advantages of decent room temperature ionic conductivity (>10−3 S cm−1), good compatibility with oxide cathode materials, good chemical stability, and scalability. Herein, research and development of the widely studied metal halide SEs including fluorides, chlorides, bromides, and iodides are reviewed, mainly focusing on the structures and ionic conductivities as well as preparation methods and electrochemical/chemical stabilities. And then, based on typical metal halide solid electrolytes, we emphasize the interface issues (grain boundaries, cathode−electrolyte and electrolyte–anode interfaces) that exist in the corresponding LMSBs and summarize the related work on understanding and engineering these interfaces. Furthermore, the typical (or in situ) characterization tools widely used for solid‐state interfaces are reviewed. Finally, a perspective on the future direction for developing high‐performance LMSBs based on the halide electrolyte family is put out.

Halide solid-state electrolytes (SSEs) with high ionic conductivity and high-voltage stability have attracted significant interest for application in all-solid-state batteries. However, they are not chemically stable against the lithium (Li) metal anode due to continuous side reduction reactions, hindering the application of halide SSEs in high-energy-density all-solid-state Li metal batteries (ASSLMBs). Here, we report a self-limiting layer (SLL) composed of InF3 and Li2ZrCl6 (LZC) to stabilize the halide SSEs and Li metal anode interface, where the in situ generated LiF-rich layer serves as a passivation layer to suppress ensuing reactions and kinetically stabilize the interface between LZC and Li metal anode. As a result, Li metal symmetric cells with LZC protected by the SLL exhibit excellent cycling performance for over 3000 h. The ASSLMBs with SLL achieve 99.2% capacity retention over 100 cycles at 0.5 C and 83.5% capacity retention after 250 cycles at 2 C. Density functional theory-computed thermodynamic data and postcycling experimental characterizations confirm the forming of a LiF-rich passivation layer between the SLL and the Li anode, which effectively prevents continuous side reactions. This self-limiting interface protection offers a feasible kinetical passivation strategy for halide SSEs and the Li metal anode toward high-performance ASSLMBs.

Halide solid electrolytes have recently emerged as a promising option for cathode‐compatible catholytes in solid‐state batteries (SSBs), owing to their superior oxidation stability at high voltage and their interfacial stability. However, their day‐ to month‐scale aging at the cathode interface has remained unexplored until now, while its elucidation is indispensable for practical deployment. Herein, the stability of halide solid electrolytes (e.g., Li3InCl6) when used with conventional layered oxide cathodes during extended calendar aging is investigated. It is found that, contrary to their well‐known oxidation stability, halide solid electrolytes can be vulnerable to reductive side reactions with oxide cathodes (e.g., LiNi0.8Co0.1Mn0.1O2) in the long term. More importantly, the calendar aging at a low state of charge or as‐fabricated state causes more significant degradation than at a high state of charge, in contrast to typical lithium‐ion batteries, which are more susceptible to high‐state‐of‐charge calendar aging. This unique characteristic of halide‐based SSBs is related to the reduction propensity of metal ions in halide solid electrolytes and correlated to the formation of an interphase due to the reductive decomposition triggered by the oxide cathode in a lithiated state. This understanding of the long‐term aging properties provides new guidelines for the development of cathode‐compatible halide solid electrolytes.

… alkali halides (or materials with a large band gap) are meagre in the literature [6,7]. Todheide-Haupt et al. [6] studied the SCL … of the mechanism of space charge injection into …

… of injecting charges we have been able to study SCL electron currents in potassium halides … The results of this paper enlarge the field of SCL currents to wide band gap type materials. It …

… Abstract Space charge layer (SCL) formation at cathode–… lithium superionic argyrodites by halide substitution. Angew … Elucidating and minimizing the space‐charge layer effect …

… It has been considered that the space charge layer (SCL) formed at the interface to balance … Extremely high silver ionic conductivity in composites of silver halide (AgBr, AgI) and meso…

Li3Y1-xInxCl6 undergoes a phase transition from trigonal to monoclinic via an intermediate orthorhombic phase. Although the trigonal yttrium containing the end member phase, Li3YCl6, synthesized by a mechanochemical route, is known to exhibit stacking fault disorder, not much is known about the monoclinic phases of the serial composition Li3Y1-xInxCl6. This work aims to shed light on the influence of the indium substitution on the phase evolution, along with the evolution of stacking fault disorder using X-ray and neutron powder diffraction together with solid-state nuclear magnetic resonance spectroscopy, studying the lithium-ion diffusion. Although Li3Y1-xInxCl6 with x ≤ 0.1 exhibits an ordered trigonal structure like Li3YCl6, a large degree of stacking fault disorder is observed in the monoclinic phases for the x ≥ 0.3 compositions. The stacking fault disorder materializes as a crystallographic intergrowth of faultless domains with staggered layers stacked in a uniform layer stacking, along with faulted domains with randomized staggered layer stacking. This work shows how structurally complex even the "simple" series of solid solutions can be in this class of halide-based lithium-ion conductors, as apparent from difficulties in finding a consistent structural descriptor for the ionic transport.

… , creating a repeating pattern that offsets each layer from the one below it. … layer is identical, resulting in a repeating two-layer pattern. Each type differs in its crystal structures and space …

Li 3 YCl 6 (LYC) has emerged as a highly promising halide solid electrolyte for next-generation all-solid-state lithium batteries (ASSLBs) due to its high ionic conductivity and good …

Halide-based solid electrolytes are promising candidates for all solid-state lithium-ion batteries (ASSLBs) due to their high ionic conductivity, wide electrochemical window, and excellent chemical stability with cathode materials. However, when tested in practice, their intrinsic electrochemical stability windows do not well match the conditions for stable operation of ASSBs. Existing literature reports halide-based ASSBs that still operate well outside the electrochemical stability window, while ASSBs that do not operate within the window are not well studied or the studies are based on the cathode material interface. In this study, we aim to elucidate the mechanism behind all-solid-state battery failure by investigating how the reduction potential of Li3YCl6 solid-state electrolyte itself changes under overcharging conditions. Our findings demonstrate that in Li-In|Li3YCl6|Li3YCl6-C half-cells during the first state of charge, Cl ions participate in charge compensation, resulting in a depletion of ligands. This phenomenon significantly affects the reduction potential of Y3+, causing it to be reduced to Y2Cl3 and ultimately to Y0 at conditions far exceeding its actual reduction potential. Furthermore, we analyze the interfacial impedance induced by this process and propose a novel perspective on battery failure.

All‐solid‐state lithium batteries (ASSLBs) are expected to revolutionize large‐scale energy storage and electric vehicles due to their exceptional safety and high energy density. Central to this technology is the development of solid electrolytes, with halide electrolytes emerging as highly promising candidates, offering high ionic conductivity, robust electrochemical stability and excellent mechanical properties. Since the breakthrough discovery of Li3YCl6 in 2018, research on halide electrolytes has entered a new era. However, despite continuous research efforts, practical challenges persist in their application, including the need to further enhance ionic conductivity, improve moisture resistance, and achieve better compatibility with electrode materials. This review provides a comprehensive overview of recent progress and ongoing challenges in halide electrolytes, examining crystal structures, conduction mechanisms, and scalable synthesis techniques. Various modification strategies are also explored aimed at boosting ionic conductivity and electrochemical stability, with a particular focus on improving interface stability. Finally, future perspectives are outlined for designing high‐performance halide electrolyte materials, offering guidance for ongoing research efforts toward their commercial application in ASSLBs.

… Coating reagent including LiNbO 3 was necessary between the sulfide electrolyte and the oxide cathode to avoid the space-charge layer that causes a large interface resistance. …

… (19−21) The bilayer separator configuration is not commercially viable due to the extra cost of the additional SSE layer and processing step. Moreover, the bilayer itself introduces an …

… the interface between the crystal and the surrounding medium. The reason for this lies in the intrinsic space-charge formed at the interface … and electrical properties of space charge, we …

Halide solid‐state batteries (SSBs) show unparalleled application potential because of their outstanding advantages, such as high ionic conductivity and good compatibility with cathodes. However, operating halide SSBs under freezing temperatures faces big challenges, and the underlying degradation mechanisms are unclear. Herein, the impact of electronic conductivity in low‐temperature halide SSBs is investigated by designing different additives in the composite cathode. It is shown that the electrochemical stability of a halide electrolyte (Li3InCl6) with additives is significantly affected by the degree of electronic conductivity as well as the ambient operational temperature. When the ambient temperatures are below freezing point, the moderate electronic conductivity in the composite cathode is beneficial toward improving the charge transfer kinetics without inducing the decomposition of Li3InCl6. The electrode materials (LiCoO2 cathode and Li3InCl6 electrolytes) show excellent structural and interfacial stability during electrochemical reactions, resulting in a competitive performance at low temperatures. Stable long‐term cycling performance with a capacity retention of 89.2% after 300 cycles is achieved along with a C‐rate capacity of 77.6 mAh g–1 (0.6 C) at −10 °C. This in‐depth study investigates the role of electronic conductivity, which opens the door to future research on low‐temperature SSBs.

A hierarchically coated halide interface of composite cathodes in all-solid-state batteries improves material compatibility and electrochemical performance.

All-solid-state batteries have emerged as a promising technology for energy storage, offering improved safety and potential for higher energy density. Halide-based batteries have gained popularity due to the advantageous characteristics of electrolytes, including decent ion conductivity, good formability, high-voltage stability, and moisture resistivity. Despite the impressive cycle life observed in halide-based batteries under high stack pressures or at elevated temperatures, poor cathode–electrolyte stabilities still pose a significant challenge that results in rapid capacity decay under ambient temperature and low pressure. The poor stability at the halide–anode interface further limits the choice of electrode materials for high-energy applications. This article presents a review of interfacial instability in halide-based solid-state batteries, addressing both the chemical, electrochemical, and mechanical origins of these instabilities at the cathode–electrolyte and anode–electrolyte interfaces. We also discuss state-of-the-art approaches to mitigate interfacial instabilities and highlight their limitations. Finally, we propose perspectives and future directions for resolving interfacial instabilities in halide-based solid-state batteries.

Point defects in metal halide perovskites play a critical role in determining their properties and optoelectronic performance; however, many open questions remain unanswered. In this work, we apply impedance spectroscopy and deep-level transient spectroscopy to characterize the ionic defect landscape in methylammonium lead triiodide (MAPbI3) perovskites in which defects were purposely introduced by fractionally changing the precursor stoichiometry. Our results highlight the profound influence of defects on the electronic landscape, exemplified by their impact on the device built-in potential, and consequently, the open-circuit voltage. Even low ion densities can have an impact on the electronic landscape when both cations and anions are considered as mobile. Moreover, we find that all measured ionic defects fulfil the Meyer–Neldel rule with a characteristic energy connected to the underlying ion hopping process. These findings support a general categorization of defects in halide perovskite compounds. Defects in perovskite affect the properties and performance in optoelectronic devices, yet the nature of ionic defects remains elusive. Here, the authors investigate the ionic defect landscape in perovskite introduced by varying precursor stoichiometry, and find the defects fulfill the Meyer-Neldel rule.