The instability of P─F bond-based electrolyte (PFE) under ambient conditions presents one of the biggest challenges for the production, usage, and recycling of lithium (Li) batteries. It increases the cost of battery production, decreases battery service life, and harms human health and environmental sustainability during the use of batteries. Here a stabilized P─F bond electrolyte (SPFE) is reported, which can effectively prevent the side-reactions in PFE at ambient conditions. The SPFE, which is pristine, containing ultra-high content of water (10 000 ppm or 10 g L⁻¹), can support the 2 Ah Li-ion pouch cell (200 Wh kg⁻¹) cycling 400 times with 90.2% of its capacity retained. The mostly dry room-free (DRF) production of commercial Li-ion (2Ah, 200 Wh kg⁻¹) and anode-free (AF) Li metal pouch cell (2 Ah, 410 Wh kg⁻¹) also demonstrated excellent cycling stability with the SPFE. Moreover, the SPFE enables AF Li-metal batteries (AFLMBs) to retain 54.1% of their charged capacity even after 180 days of open circuit storage. By intrinsically safeguarding PFE from hydrolysis, the present SPFE would have a broad impact on future battery technology, simplifying battery production, extending battery service life, and safeguarding battery recyclability.

One of the major challenges in advancing polymer-inorganic hybrid solid electrolytes (HSEs) lies in comprehending and controlling their internal structure. In addition, the intricate interplay between multiple phases further complicates efforts to establish the structure-property relationships. In this study, by introducing a multifunctional LiI additive to an HSE compromising of polyethylene oxide (PEO) polymeric electrolyte and the fast lithium-ion conductor Li₆PS₅Cl, the relationship between the bulk and interface structure and ascertaining their impact on lithium-ion dynamics within the HSE is disentangled. Using multidimensional solid-state nuclear magnetic resonance, we find that the addition of LiI stabilizes the internal interfaces and enhances lithium-ion mobility. A kinetically stable solid-electrolyte interphase is formed at the lithium-metal anode, increasing the critical current density to 1.3 mA cm⁻², and enabling long-term stable cycling of lithium symmetric cells (>1200 h). This work sheds light on tailoring the structure of HSEs to improve their conductivity and stability for enabling all-solid-state lithium-metal batteries.

Conspectus

Layered transition metal (TM) compounds are pivotal in the development of rechargeable battery technologies for efficient energy storage. The history of these materials dates back to the 1970s, when the concept of intercalation chemistry was introduced into the battery. This process involves the insertion of alkali-metal ions between the layers of a host material (e.g., TiS2) without causing significant structural disruption. This breakthrough laid the foundation for Li-ion batteries, with materials like LiCoO2 becoming key to their commercial success, thanks to their high energy density and good stability. However, despite these advantages, challenges remain in the broader application of these materials in batteries. Issues such as lattice strain, cation migration, and structural collapse result in rapid capacity degradation and a reduction in battery lifespan. Moreover, the performance of batteries is often constrained by the properties of the available materials, particularly in layered oxide materials. This has driven the exploration of materials with diverse compositions. The relationship between composition and structural chemistry is crucial for determining reversible capacity, redox activity, and phase transitions, yet predicting this remains a significant challenge, especially for complex compositions.

In this Account, we outline our efforts to explore rational principles for optimal battery materials that offer a higher performance. The core of this is the concept of ionic potential, a parameter that measures the strength of the electrostatic interaction between ions. It is defined as the ratio of an ion’s charge to its ionic radius, offering a quantitative way to evaluate interactions between cations and anions in crystal structures. By building on this concept, we introduce the cationic potential, which is emerging as a crystallographic tool that captures critical interactions within layered oxide materials. This approach provides insights into structural organization, enabling the prediction of P2- and O3-type stacking arrangements in layered oxides. A key advantage of using the cationic potential is its ability to guide the rational design of electrode materials with improved performance. For example, introducing P-type structural motifs into the material framework can significantly enhance ion mobility, mitigating detrimental phase transitions that often compromise battery efficiency and longevity. Furthermore, ionic potential serves as a representative parameter to quantitatively describe the properties of various TM compositions, providing a straightforward calculation method for designing multielement systems. We anticipate that this Account will provide fundamental insights and contribute to significant advancements in the design of layered materials, not only for battery applications but also for broader fields that require control of the material properties.

Irreducible solid electrolytes (SEs), characterized by non-Li framework ions in their lowest oxidation states, offer intrinsic compatibility with low-reduction-potential, high-capacity negative electrodes, such as lithium metal and silicon. In these SE materials, disorder engineering and vacancy formation reduce lithium-ion diffusion barriers, achieving room-temperature ionic conductivities exceeding 0.1 mS cm^–1. Experiments and atomistic simulations confirm that irreducible SEs form decomposition-free interfaces with Li metal. Their limited oxidative stability can be addressed by pairing them with an electrolyte layer stable with practical cathodes yet demanding interface compatibility between the two electrolyte layers. Here we highlight key research directions to accelerate irreducible SE transition from laboratory to practical application, including expanding compositional diversity, optimizing interfaces with cathode-facing electrolytes, developing scalable thin-film processing, and exploring compatibility with other low working potential anodes like silicon. Addressing these challenges is essential to unlock the full potential of irreducible SEs for high-energy, long-life, all-solid-state batteries.

Sulfide-based solid-state batteries (SSBs) are emerging as a top contender for next-generation rechargeable batteries with improved safety and higher energy densities. However, SSBs with Ni-rich cathode materials such as LiNi0.82Mn0.07Co0.11O2 (NMC82) exhibit several chemomechanical challenges at the cathode–electrolyte interface, such as contact loss and solid-electrolyte decomposition, resulting in poor interfacial Li+ ion transport. To overcome these challenges, we used polymerized ionic liquids (PIL) as coatings at the NMC82 cathode surface, with and without incorporating a lithium salt. The thin Li+ ion-conductive Li–PIL nanocoating shows excellent compatibility with sulfide solid electrolytes and enables efficient Li+ transfer over the cathode–solid electrolyte interface, as demonstrated by 2D solid-state exchange NMR. It also improves contact retention between the cathode–solid electrolyte particles and mitigates electrolyte oxidation-induced degradation. This is reflected in the electrochemical performance of coated NMC82 in sulfide SSBs, where both a higher rate performance (190 mA h g−1 vs. 163 mA h g−1 for uncoated at 0.1C) and a remarkable capacity retention of 82.7% after 500 cycles at 0.2C and ambient conditions (20 °C) are observed. These results emphasize the effectiveness of PILs with Li salts as multifunctional coatings that enable high-performance sulfide-based SSBs with Ni-rich cathode materials at ambient temperature.

Hybrid solid electrolytes (HSEs) leverage the benefits of their organic and inorganic components, yet optimizing ion transport and component compatibility requires a deeper understanding of their intricate ion transport mechanisms. Here, macroscopic charge transport is correlated with local lithium (Li)-ion diffusivity in HSEs, using poly(ethylene oxide) (PEO) as matrix and Li6PS5Cl as filler. Solvent- and dry-processing methods were evaluated for their morphological impact on Li-ion transport. Through multiscale solid-state nuclear magnetic resonance analysis, we reveal that the filler enhances local Li-ion diffusivity within the slow polymer segmental dynamics. Phase transitions indicate inhibited crystallization in HSEs, with reduced Li-ion diffusion barriers attributed to enhanced segmental motion and conductive polymer conformations. Relaxometry measurements identify a mobile component unique to the hybrid system at low temperatures, indicating Li-ion transport along polymer-filler interfaces. Comparative analysis shows solvent-processed HSEs exhibit better morphological uniformity and enhanced compatibility with Li-metal anodes via an inorganic-rich solid electrolyte interphase.

Electrode–electrolyte interphases are critical determinants of the reversibility and longevity of lithium (Li)-metal batteries (LMBs). However, upon cycling, the inherently delicate interphases, formed from electrolyte decomposition, become vulnerable to chemomechanical degradation and corrosion, resulting in rapid capacity loss and thus short battery life. Here, we present a comprehensive analysis of the complex interplay between the thermodynamic and kinetic properties of interphases on Li-metal anodes, providing insights into interphase design to address these challenges. Direct measurements of ion-transport kinetics across various electrolyte chemistries reveal that interphases with high Li-ion mobility are essential for achieving dense Li deposits. Conversely, sluggish ion transport generates high-surface-area Li deposits that induce Li random stripping and the accumulation of isolated Li deposits. Surprisingly, interphases that support long cycle life do not necessarily require the formation of dense Li deposits but must avoid possible electrochemical/chemical reactions between the Li-metal deposits and electrolytes’ components. By that, in some specific electrolyte systems, isolated Li deposits can recover and electrically rejoin the active Li anodes’ mass. These findings challenge conventional understanding and establish new principles for designing durable LMBs, demonstrating that even with commercial carbonate-based electrolytes, LiNi0.8Co0.1Mn0.1O2||Cu cells can achieve high reversibility.

Lithium argyrodite thiophosphate superionic conductors are being explored as promising solid electrolytes for all-solid-state batteries, primarily due to their high ionic conductivity and ease of processing. Yet, these electrolytes present challenges such as chemical instability in humid conditions and incompatibility with cathode materials. Although some lithium argyrodites show improved air stability, their ionic conductivity deteriorates below the practically required value. Herein, based on hard soft acid base theory, a new family of lithium argyrodite, as solid solution Li6−xAsS5−xBr1+x (for 0.0 ≤ x ≤ 0.6), has been proposed to address these issues. Through a combination of neutron diffraction, NMR spectroscopy, and electrochemical impedance spectroscopy, it has been determined that the partial substitution of S2− by Br− weakens interactions within the Li+ “cage”, facilitating long lithium-ion movement throughout the structure. An additional T4 Li+ site is identified, offering a lower energy barrier for inter-cage jumps. Consequently, the Li5.5AsS4.5Br1.5 member of the composition series exhibits a higher Li-ion diffusivity resulting in a remarkable ionic conductivity of 15.4 mS cm−1. Compared with lithium thiophosphates, the Li5.5AsS4.5Br1.5 also shows excellent air stability. This research opens a new avenue for developing air-stable sulfide solid electrolytes with high ionic conductivity necessitated for practical application in solid-state batteries.

Phase separation, inducing a miscibility gap and non-monotonic open-circuit potential (OCP), is typical for widespread Li-ion battery electrodes such as LiFePO4, Li₄Ti₅OPhase separation, inducing a miscibility gap and non-monotonic open-circuit potential (OCP), is typical for widespread Li-ion battery electrodes such as LiFePO4, Li₄Ti₅O12 and Graphite. Although particle-scale effects of phase separation are well documented, its influence on transport-limited, porous electrodes remains largely overlooked. Here we embed physically consistent non-monotonic OCP profiles in a simplified Doyle–Fuller–Newman framework to compare their behavior against that of solid- solution materials with monotonic OCPs. Our findings provide deeper and general understanding of the different electrode ensemble behavior of solid solution (monotonic OCP) and phase separating (non-monotonic OCP) electrode materials, demonstrating why larger miscibility gaps are associated with decreasing rate capabilities and electrode utilization, amplifying local current heterogeneity and electrolyte depletion. By contrast, simulations employing conventional flat, fitted OCPs mask these effects and overpredict performance—particularly under dynamic cycling protocols such as galvanostatic intermittent titration (GITT). Our results reveal why accounting for realistic OCPs is essential for reliable modelling of high-loading electrodes, providing fundamental understanding and guidance for model-driven design and control of next-generation batteries and Graphite. Although particle-scale effects of phase separation are well documented, its influence on transport-limited, porous electrodes remains largely overlooked. Here we embed physically consistent non-monotonic OCP profiles in a simplified Doyle–Fuller–Newman framework to compare their behavior against that of solid- solution materials with monotonic OCPs. Our findings provide deeper and general understanding of the different electrode ensemble behavior of solid solution (monotonic OCP) and phase separating (non-monotonic OCP) electrode materials, demonstrating why larger miscibility gaps are associated with decreasing rate capabilities and electrode utilization, amplifying local current heterogeneity and electrolyte depletion. By contrast, simulations employing conventional flat, fitted OCPs mask these effects and overpredict performance—particularly under dynamic cycling protocols such as galvanostatic intermittent titration (GITT). Our results reveal why accounting for realistic OCPs is essential for reliable modelling of high-loading electrodes, providing fundamental understanding and guidance for model-driven design and control of next-generation batteries..

Rechargeable Li||I2 batteries based on liquid organic electrolytes suffer from pronounced polyiodides shuttling and safety concerns, which can be potentially tackled by the use of solid-state electrolytes. However, current all-solid-state Li||I2 batteries only demonstrate limited capacity based on a two-electron I−/I2 polyiodides chemistry at elevated temperatures, preventing them from rivaling state-of-the-art lithium-ion batteries. Herein, we report a fast, stable and high-capacity four-electron solid-conversion I−/I2/I+ chemistry in all-solid-state Li||I2 batteries at room temperature. Through the strategic use of a highly conductive, chlorine-rich solid electrolyte Li4.2InCl7.2 as the catholyte, we effectively activate the I2/I+ redox couple. This activation is achieved through a robust I-Cl interhalogen interaction between I2 and the catholyte, facilitated by an interface-mediated heterogeneous oxidation mechanism. Moreover, apart from serving as Li-ion conduction pathway, the Li4.2InCl7.2 catholyte is demonstrated to show a reversible redox behavior and contribute to the electrode capacity without compromising its conductivity. Based on the I−/I2/I+ four-electron chemistry, the as-designed all-solid-state Li||I2 batteries deliver a high specific capacity of 449 mAh g-1 at 44 mA g-1 based on I2 mass and an impressive cycling stability over 600 cycles with a capacity retention of 91% at 440 mA g-1 and at 25 °C.

Solid-state batteries currently receive extensive attention due to their potential to outperform lithium-ion batteries in terms of energy density when featuring next-generation anodes such as lithium metal or silicon. However, most highly conducting solid electrolytes decompose at the low operating voltages of next-generation anodes leading to irreversible lithium loss and increased cell resistance. Such performance losses may be prevented by designing electrolytes which are thermodynamically stable at low operating voltages (anolytes). Here, we report on the discovery of a new family of irreducible (i.e., fully reduced) electrolytes by mechanochemically dissolving lithium nitride into the Li₂S antifluorite structure, yielding highly conducting crystalline Li_2+xS_1-xN_x phases reaching >0.2 mS cm^-1 at ambient temperature. Combining impedance spectroscopy experiments and ab initio density functional theory calculations we clarify the mechanism by which the disordering of the sulfide and nitride ions in the anion sublattice boosts ionic conductivity in Li_2+xS_1-xN_x phases by a factor 10⁵ compared to the Li₂S host structure. This advance is achieved through a novel theoretical framework, leveraging percolation analysis with local-environment-specific activation energies and is widely applicable to disordered ion conductors. The same methodology allows us to rationalize how increasing nitrogen content in Li_2+xS_1-xN_x antifluorite-like samples leads to both increased ionic conductivity and lower conductivity-activation energy. These findings pave the way to understanding disordered solid electrolytes and eliminating decomposition-induced performance losses on the anode side in solid-state batteries.

All-solid-state batteries receive ample attention due to their promising safety characteristics and energy density. The latter holds true if they are compatible with next-generation high-capacity anodes, but most highly ion-conductive solid electrolytes decompose at low operating potentials, leading to lithium loss and increased cell resistances. Here the dynamic stability of solid electrolytes that can improve all-solid-state battery performance is demonstrated. Halide electrolytes Li₃YCl₃Br₃ and Li₂ZrCl₆, considered unstable at low potentials, are found to exhibit structurally reversible redox activity beyond their electrochemical stability windows, increasing compatibility with anodes and contributing to capacity without compromising ionic conductivity. The benefit of this dynamic stability window is demonstrated with cost-effective red phosphorus anodes, resulting in high reversible capacities (2,308 mAh g⁻¹), high rate capacity retention (1,024 mAh g⁻¹ at 7.75 mA cm⁻²) and extended cycle life (61% retention after 1,780 cycles). Furthermore, high areal capacity (7.65 mAh cm⁻²) and stability (70% retention after 1,000 cycles) are achieved for halide-based full cells with red phosphorous anodes. The beneficial redox activity of halide electrolytes greatly expands their application scenarios and suggests valuable battery design principles to enhance performance.

By varying the bromine content and cooling method, we are able to induce site disorder in the Li_6-xPS_5-xBr_1+x (x = 0, 0.3, 0.5) system via two routes, allowing us to disentangle the impact of site disorder and chemical composition on conductivity. Through solid-state nuclear magnetic resonance (NMR), we can explore the chemical environment as well as short-range lithium-ion dynamics and compare these to results obtained from neutron diffraction and electrochemical impedance spectroscopy (EIS). We find that the cooling method has a profound effect on the ⁷Li and ³¹P environment that cannot be explained through 4d site disorder alone. The configurational entropy (S_conf) is used as a more complete descriptor of structural disorder and linked to distortions in both the phosphorus and lithium environment. These distortions are correlated to increased intercage movement through ⁷Li T₁ spin-lattice relaxation (SLR) NMR. Further analysis of the prefactors obtained from SLR NMR and EIS allows us to obtain the migrational entropy (ΔS_m). For short-range SLR movement, the ΔS_m correlates well with S_conf, implying that increased intercage movement is related to distortion of the lithium cages as well as a decrease of the intercage distance. Comparison to EIS shows that an increase in short-range movement translates into increased long-range movement in a straightforward manner for slow-cooled samples. However, for quench-cooled samples, this correlation is lost. Lattice softness and phonon-ion interactions are suggested to play an important role in long-range conduction which only becomes apparent when chemical composition and disorder are disentangled. This work shows that by altering one synthesis step, the relationship between site-occupancy-based descriptors (site disorder or S_conf) and lithium dynamics is changed profoundly. Furthermore, it shows that chemical composition and descriptors of site disorder cannot be seen as one and the same, as both play a role that changes with the length scale probed. Finally, it challenges the implicit assumption that increased short-range diffusivity automatically results in increased long-range diffusivity.

Anode-free aqueous zinc metal batteries (AZMBs) offer significant potential for energy storage due to their low cost and environmental benefits. Ti₃C₂T_x MXene provides several advantages over traditional metallic current collectors like Cu and Ti, including better Zn plating affinity, lightweight, and flexibility. However, self-freestanding MXene current collectors in AZMBs remain underexplored, likely due to challenges with Zn deposition reversibility. This study investigates the combination of a Ti₃C₂T_x self-freestanding film with advanced electrolyte engineering, specifically examining the effects of Li-salt and propylene carbonate (PC) as additives on Zn plating reversibility. While using Li⁺ ions as an additive alone facilitates uniform Zn deposition on bulk metals through the electrostatic shielding effect, the addition of Li-salt negatively impacts Zn plating uniformity on Ti₃C₂T_x. Meanwhile, using PC additive alone forms an organic SEI layer on Ti₃C₂T_x and causes Zn agglomeration. The use of both additives together results in a ZnF₂-containing hybrid SEI layer with improved interfacial kinetics, promoting more uniform Zn deposition. This approach achieves an average Coulombic efficiency (CE) of 96.8% over 150 cycles (a maximum CE of 97.8%). The study highlights the strategic difference in electrolyte design, emphasizing the need for tailored approaches to optimize Zn deposition on MXenes, contrasting with traditional metallic current collectors.

Achieving both high redox activity and rapid ion transport is a critical and pervasive challenge in electrochemical energy storage applications. This challenge is significantly magnified when using large-sized charge carriers, such as the sustainable ammonium ion (NH₄⁺). A self-assembled MXene/n-type conjugated polyelectrolyte (CPE) superlattice-like heterostructure that enables redox-active, fast, and reversible ammonium storage is reported. The superlattice-like structure persists as the CPE:MXene ratio increases, accompanied by a linear increase in the interlayer spacing of MXene flakes and a greater overlap of CPEs. Concurrently, the redox activity per unit of CPE unexpectedly intensifies, a phenomenon that can be explained by the enhanced de-solvation of ammonium due to the increased volume of 3 Å-sized pores, as indicated by molecular dynamic simulations. At the maximum CPE mass loading (MXene:CPE ratio = 2:1), the heterostructure demonstrates the strongest polymeric redox activity with a high ammonium storage capacity of 126.1 C g⁻¹ and a superior rate capability at 10 A g⁻¹. This work unveils an effective strategy for designing tunable superlattice-like heterostructures to enhance redox activity and achieve rapid charge transfer for ions beyond lithium.

Solid-state batteries currently receive ample attention due to their potential to outperform lithium-ion batteries in terms of energy density when featuring next-generation anodes such as lithium metal or silicon. One key remaining challenge is identifying solid electrolytes that combine high ionic conductivity with stability in contact with the highly reducing potentials of next-generation anodes. Fully reduced electrolytes, based on irreducible anions, offer a promising solution by avoiding electrolyte decomposition altogether. In this study, we demonstrate the compositional flexibility of the disordered antifluorite framework accessible by mechanochemical synthesis and leverage it to discover irreducible electrolytes with high ionic conductivities. We show that the recently investigated Li ₉N ₂Cl ₃ and Li ₅NCl ₂ phases are part of the same solid solution of Li-deficient antifluorite phases existing on the LiCl-Li ₃N tie line with a general chemical formula of Li _1+2xCl _1−xN _x (0.33 < x < 0.5). Using density functional theory calculations, we identify the origin of the 5-order-of-magnitude conductivity increase of the Li _1+2xCl _1−xN _x phases compared to the structurally related rock-salt LiCl phase. Finally, we demonstrate that S _Cl- and Br _Cl-substituted analogues of the Li _1+2xCl _1−xN _x phases may be synthesized, enabling significant conductivity improvements by a factor of 10, reaching 0.2 mS cm ⁻¹ for Li _2.31S _0.41Br _0.14N _0.45. This investigation demonstrates for the first time that irreducible antifluorite-like phases are compositionally highly modifiable; this finding lays the ground for discovery of new compositions of irreducible antifluorite-like phases with even further increased conductivities, which could help eliminate solid-electrolyte decomposition and decomposition-induced Li losses on the anode side in high-performance next-generation batteries.

The interlaboratory comparability and reproducibility of all-solid-state battery cell cycling performance are poorly understood due to the lack of standardized set-ups and assembly parameters. This study quantifies the extent of this variability by providing commercially sourced battery materials—LiNi_0.6Mn_0.2Co_0.2O₂ for the positive electrode, Li₆PS₅Cl as the solid electrolyte and indium for the negative electrode—to 21 research groups. Each group was asked to use their own cell assembly protocol but follow a specific electrochemical protocol. The results show large variability in assembly and electrochemical performance, including differences in processing pressures, pressing durations and In-to-Li ratios. Despite this, an initial open circuit voltage of 2.5 and 2.7 V vs Li⁺/Li is a good predictor of successful cycling for cells using these electroactive materials. We suggest a set of parameters for reporting all-solid-state battery cycling results and advocate for reporting data in triplicate.

Ordered layered structures serve as essential components in lithium (Li)-ion cathodes^1–3. However, on charging, the inherently delicate Li-deficient frameworks become vulnerable to lattice strain and structural and/or chemo-mechanical degradation, resulting in rapid capacity deterioration and thus short battery life^2,4. Here we report an approach that addresses these issues using the integration of chemical short-range disorder (CSRD) into oxide cathodes, which involves the localized distribution of elements in a crystalline lattice over spatial dimensions, spanning a few nearest-neighbour spacings. This is guided by fundamental principles of structural chemistry and achieved through an improved ceramic synthesis process. To demonstrate its viability, we showcase how the introduction of CSRD substantially affects the crystal structure of layered Li cobalt oxide cathodes. This is manifested in the transition metal environment and its interactions with oxygen, effectively preventing detrimental sliding of crystal slabs and structural deterioration during Li removal. Meanwhile, it affects the electronic structure, leading to improved electronic conductivity. These attributes are highly beneficial for Li-ion storage capabilities, markedly improving cycle life and rate capability. Moreover, we find that CSRD can be introduced in additional layered oxide materials through improved chemical co-doping, further illustrating its potential to enhance structural and electrochemical stability. These findings open up new avenues for the design of oxide cathodes, offering insights into the effects of CSRD on the crystal and electronic structure of advanced functional materials.

Due to their high ionic conductivity, lithium-ion conducting argyrodites show promise as solid electrolytes for solid-state batteries. Aliovalent substitution is an effective technique to enhance the transport properties of Li₆PS₅Br, where aliovalent Si substitution triples ionic conductivity. However, the origin of this experimentally observed increase is not fully understood. Our density functional theory (DFT) study reveals that Si⁴⁺ substitution increases Li diffusion by activating Li occupancy in the T4 sites. Redistribution of Li-ions within the lattice results in a more uniform distribution of Li around the T4 and neighboring T5 sites, flattening the energy landscape for diffusion. Since the T4 site is positioned in the intercage jump pathway, an increase in the intercage jump rate is found, which is directly related to the macroscopic diffusion and bulk conductivity. Analysis of neutron diffraction experiments confirms partial T4 site occupancy, in agreement with the computational findings. Understanding the aliovalent substitution effect on interstitials is crucial for improving solid electrolyte ionic conductivity and advancing solid-state battery performance.

Ceramic membranes made of garnet Li7Zr3La2O12 (LLZO) are promising separators for lithium metal batteries because they are chemically stable to lithium metal and can resist the growth of lithium dendrites. Free-standing garnet separators can be produced on a large scale using tape casting and sintering slurries containing LLZO powder, but the quality of the separators is severely compromized by the protonation of the moisture-sensitive LLZO during processing and the irreversible loss of lithium during sintering. In this work, an approach is presented to mitigate the degradation of the LLZO and produce high quality separators using Li2CO3 as a source of excess lithium. By systematically investigating the effects of Li2CO3 addition during the different steps of the tape casting process and the intricate relationship between the protonation and relithiation of LLZO phase, the formation of highly protonated LLZO during ball milling was identified as the most critical step. It was shown that the addition of minimal amounts of Li2CO3 during wet milling effectively suppresses LLZO protonation and ensure the effectiveness of relithiation during subsequent sintering. Using this modified method, flat LLZO separators with a relative density of 95.3 % were prepared in a simplified process with a significantly reduced excess lithium of only 5 mol % with respect to the stoichiometric LLZO, exhibiting an ionic conductivity of 0.18 mS cm−1 at room temperature and a critical current density of 1 mA cm−2 at 60 °C for lithium stripping/plating.