Synthesis and characterization of HE-DRXsOur study employed the UHS method to achieve a rapid synthesis of HE-DRXs with six cations, specifically Li1.3Mn2+0.1Co2+0.1Mn3+0.1Cr3+0.1Ti0.1Nb0.2O1.7F0.3 (TM6). A pressed green pellet of the TM6 precursor powders was placed between two Joule-heated carbon strips, and the rapid synthesizing reaction was achieved at 1100 °C for 3 s (Fig. 2a). The temperature rise curves are shown in Supplementary Fig. 1. X-ray Diffraction (XRD) analysis comparison of different synthesis temperatures shows that the precursor powders are completely converted to a single phase without any obvious impurity peaks28 when the reaction temperature is between 1100 °C and 1300 °C (Supplementary Fig. 2). Five peaks attributed to the (111), (200), (220), (311), and (222) planes were observed, confirming that the TM6 synthesized by UHS can be indexed to a disordered rock salt phase (Fm-3 m) (Fig. 2b)28,33. TM6 maintains a stable single-phase structure within the temperature range of 1100 to 1300 °C, indicative of excellent thermal stability. As the temperature rises to 1400 °C, the transition metal elements in the high oxidation states undergo reduction to the metallic elements due to the influence of the reducing atmosphere (Supplementary Figs. 2, 3).Fig. 2: Synthesis and characterization of HE-DRXs.a Rapid synthesis of TM6 with UHS at 1100 °C in 3 s. b XRD pattern of TM6 synthesized by UHS. c TEM-EDS mapping of the TM6 particle. d XPS spectra of Mn 2p. SEM images of TM2 (e), TM4 (f) and TM6 (g) sintered at 1150 °C for 10 s. h Comparison of synthesis temperature and sintering temperature of TM2, TM4, and TM6.The TM6 was characterized using transmission electron microscopy (TEM)/energy-dispersive spectroscopy (EDS) to analyze the distribution of various elements. The uniform elemental distribution suggests fast reaction kinetics and rapid mixing of the precursors at 1100 °C (Fig. 2c). X-ray photoelectron spectroscopy (XPS) was conducted to determine the valence states of the metal elements (i.e., Mn, Co, Cr, Ti, and Nb). Mn has valence states of both +2 and +3 (Fig. 2d). Co is in the +2 valence state, while Cr is in the +3 valence state (Supplementary Fig. 4). Ti and Nb, utilized to stabilize the positive electrode framework, exhibit oxidation states of +4 and +5, respectively (Supplementary Fig. 4). Notably, the transition metal elements are not reduced during the rapid synthesis process and form a stable high-entropy rock salt structure with the designed valence states due to the short sintering time of only 3 s. Elemental analysis confirms that the metal ratios in the as-synthesized materials closely align with the target compositions (Supplementary Table 1). Therefore, UHS technology is an effective way for the rapid synthesis of TM6, and its excellent thermal stability gives TM6 the potential to co-sinter with electrolytes at high temperatures.To verify the impact of the increasing number of cations on the synthesis and sintering temperature of rock salt positive electrodes (DRXs), precursors with varying cation compositions (TM2, TM4, TM6) were subjected to thermogravimetric analysis (TGA). The precursor mass of TM6 stabilized at 680 °C, whereas TM4 (Li1.3Mn2+0.2Mn3+0.2Ti0.1Nb0.2O1.7F0.3) and TM2 (Li1.3Mn3+0.4Ti0.3O1.7F0.3) reached stability only at 930 °C and 960 °C, respectively. This indicates that the synthesis temperatures of TM6 were 250 °C and 280 °C lower than those of TM4 and TM2 (Supplementary Fig. 5). Furthermore, we investigated the sintering behavior of positive electrode materials using UHS technology. TM2, TM4, and TM6 underwent one-step reaction sintering at 1150 °C for 10 s, resulting in successful phase formation (Supplementary Fig. 6). Scanning Electron Microscopy (SEM) analysis indicated that the grain size increased with an increasing number of cations during sintering at 1150 °C for 10 s. Specifically, TM6 exhibited a densification phenomenon in grain size, while the grain size of TM2 and TM4 appeared more dispersed (Fig. 2e–g). The densification of TM2 and TM4 occurred only when the temperature was raised to 1250 °C for 10 s (Supplementary Fig. 7), suggesting that the densification temperature of TM6 is 100 °C lower than that of TM4 and TM2 (Fig. 2h).Chemical stability of TM6 and LLZTOTo improve the interface contact between the positive electrode and electrolyte, a high co-sintering temperature is necessary, requiring a highly chemically stable temperature (Tstable). Therefore, we analyzed the chemical stability between TM6 and LLZTO by heating to 1000 °C at a rate of 10 °C/min. In situ XRD shows that no reaction between TM6 and LLZTO occurs when the temperature exceeds 800 °C (Fig. 3a). Compared to traditional positive electrode materials, TM6 exhibits superior chemical stability when co-sintered with LLZTO12,13,34,35,36. However, the impurity phase LaMnO3 appears from 800 °C (Supplementary Fig. 8), which is unfavorable for interface stability and Li+ transport. The differential scanning calorimetry (DSC) data revealed that as the temperature increased, TM2 reacted with LLZTO first at 598 °C, followed by TM4 at 605 °C (Fig. 3b). In contrast, a small peak only appeared at 835 °C for the TM6 and LLZTO system, indicating that the chemical stability of the DRX positive electrode and LLZTO gradually increases with an increasing number of cations.Fig. 3: Chemical stability of TM6 and LLZTO.a In situ XRD patterns of LLZTO and TM6. b DSC of TM2 and LLZTO, TM4 and LLZTO, and TM6 and LLZTO. EDS mapping (c) and Line profiles (d) of the TM6 | LLZTO interface. e TEM images of TM6 | LLZTO interface. f XRD patterns of LLZTO and TM6 heated by UHS. g Comparison of chemical stability between different positive electrodes and LLZTO. h Each individual component of TM6 vs LLZTO pseudobinary phase diagram. The most likely reaction products are shown in Supplementary Table 2.By employing UHS technology, the nonequilibrium rapid heat treatment conditions further extend the stable temperature range for LLZTO and TM6. Unprocessed TM6 and LLZTO particles exhibited a dispersed distribution, whereas a notable improvement in bonding was observed after heating treatment at 1100 °C for 3 s (Supplementary Fig. 9). Following heating at 1100 °C for 3 s, no detectable side reaction or cross-diffusion between LLZTO and TM6 (Fig. 3c, d, Supplementary Fig. 10). The TEM image (Fig. 3e) reveals that the TM6 | LLZTO interface exhibits a robust binding at the nanometer scale. On the right side, the lattice spacing measures 0.1635 nm, corresponding to the (222) plane of TM6 crystallization, while on the left side, the lattice spacing measures 0.5207 nm, consistent with the (211) plane of LLZTO crystallization. Similarly, the XRD patterns and Raman spectra also show that LLZTO and TM6 remain stable at 1100 °C for 3 s without any side reactions (Fig. 3f, Supplementary Fig. 11). The XPS results show that all transition metal elements maintain the designed valence states after co-sintering (Supplementary Fig. 12). However, after heating treatment at 1100 °C for 30 s, interdiffusion between the elements of TM6 and LLZTO is observed (Supplementary Fig. 9), with the emergence of the impurity phase LaMnO3 (Fig. 3f), highlighting the effectiveness of short sintering durations in inhibiting side reactions. Compared to conventional furnace treatment, UHS treatment generally increases the chemical stability temperature of the positive electrode materials and LLZTO (Fig. 3g and Supplementary Figs. 13–15). This further suggests that the rapid nonequilibrium heat treatment process effectively suppresses side reactions and cross-diffusion through kinetic control.TM6 and LLZTO exhibit higher chemical stability compared to other positive electrode materials, demonstrating the significance of the entropy stabilization effect. As a result, we study the chemical stability between each individual component of TM6 and LLZTO. First-principles calculations and XRD patterns reveal that rock salt oxides (MnO, CoO) exhibit higher reaction energy and chemical stability with LLZTO compared to non-rock salt oxides (Fig. 3h, Supplementary Figs. 16–21). Interestingly, XRD patterns of powders obtained by mixing LLZTO and the precursor powders of TM6 do not have secondary peaks after co-sintering at 1100 °C for 3 s (Supplementary Fig. 22). This result indicates that the large configurational entropy and rapid heating process facilitate fast reaction kinetics, enabling non-rock salt oxides to integrate into stable rock salt structured oxides, while the short reaction time ensures chemical stability between TM6 and LLZTO.TM6 | LLZTO interface characterizationThe in situ reactive sintering of the positive electrode on the SSE substrate is beneficial in achieving a robust interface. Leveraging the rapid reaction kinetics and favorable chemical stability between HE-DRXs and LLZTO provides an opportunity for in situ reactive sintering. Using UHS technology initiates a rapid reaction among precursor powders, resulting in the formation of pure HE-DRXs and a bilayer structure (Fig. 4a). The heating and cooling curve of this process is shown in Supplementary Fig. 1. The cross-section SEM of the bilayer structure reveals the uniform distribution of HE-DRXs on LLZTO (Fig. 4b), with HE-DRX particles robustly growing on LLZTO (Fig. 4c, d). The short reaction duration of merely 3 s effectively suppresses element diffusion and delays the occurrence of side reactions between HE-DRXs and LLZTO (Fig. 4e, Supplementary Fig. 23). XRD patterns further confirm the absence of second phases during the in situ synthesis (Fig. 4f). XPS analysis provides additional confirmation that the transition metal elements remain unaltered during the rapid synthesis process, forming a stable high-entropy rock salt structure (Supplementary Fig. 24).Fig. 4: Characterization of the TM6 | LLZTO interface.a Schematic of the in situ synthesis of HE-DRXs on dense LLZTO. SEM images (b–d) and SEM/EDS mapping (e) of TM6 | LLZTO interface. f XRD of the in situ synthesized TM6. g Zoomed in EIS of TM6 symmetric cells. h EIS of LiCoO2 symmetric cells.The tight solid‒solid contact and excellent chemical stability are favorable to reduced interface impedance. We assembled TM6 and LiCoO2 (LCO) symmetric cells (Au|TM6 | LLZTO | TM6|Au and Au|LiCoO2 | LLZTO|LiCoO2 | Au) to quantify the interface impedance and performed electrochemical impedance spectroscopy (EIS) measurements. By analyzing the EIS fitting curves of symmetric cells (Supplementary Fig. 25), we calculated that the area-specific resistance (ASR) at the interface of TM6 | LLZTO is 31.6 Ω·cm2 (Fig. 4g, Supplementary Fig. 26 and Supplementary Table 3), whereas that of the LiCoO2 | LLZTO interface is 23,175.5 Ω·cm2 (Fig. 4h, Supplementary Table 4). Notably, the ASR of the TM6 | LLZTO interface is approximately 700 times lower than that of the LiCoO2 | LLZTO interface, indicating a substantial reduction in resistance at the TM6 | LLZTO interface compared to the LiCoO2 | LLZTO interface. As the temperature increases, the quasi-semicircle arc gradually disappears, and the total ASR of the TM6 | LLZTO interface and LLZTO drops to less than 10 Ω·cm2 at 150 °C (Supplementary Fig. 27), showing promising potential for enhancing the electrochemical performance of ASSLBs.Electrochemical performanceWe assembled all-solid-state Li |LLZTO | TM6 batteries (TM6-ASSLBs) featuring a highly stable HE-DRXs|LLZTO interface. To improve Li-ion conductivity, a small quantity of LLZTO was added to the positive electrode side at a mass ratio of 1:5 (LLZTO: HE-DRXs), while the load of active material was 1.3 mg/cm2. The electrochemical performance of the TM6-ASSLBs was evaluated via galvanostatic cycling. Voltage profiles exhibit characteristic plateaus consistent with TM6 behavior (Fig. 5a), further demonstrating the successful in situ synthesis of TM6 using the UHS technique. After two repeated experiments, cycling between 1.7 and 4.7 V at a specific current of 25 mA/g yielded a specific capacity of 239.7 ± 2 mAh/g at 150 °C. Even under a higher specific current of 100 mA/g, the battery retains a high capacity of 114.5 mAh/g, showing remarkable rate capability (Supplementary Fig. 28).Fig. 5: Electrochemical performance of TM6-ASSLBs.a Charge/Discharge profiles of the all-solid-state battery at different rates from 25 mA/g to 100 mA/g. XRD patterns (b) and SEM/ EDS images of the HE-DRX-ASSLBs c before and d after cycling. e Cycling performance and f Coulombic efficiency of the HE-DRX-ASSLBs at 25 mA/g. All electrochemical measurements were performed at 150 °C without organic electrolyte and external pressure.The XRD patterns of the TM6 | LLZTO interface exhibit no secondary peaks after cycling, indicating excellent electrochemical stability between TM6 and LLZTO (Fig. 5b). Despite slight volume expansion of the HE-DRXs following charge/discharge tests, the TM6 | LLZTO interface remains robust, showing no signs of interface cracking, and no migration of transition metals is observed (Fig. 5c, d). While the utilization of Mn-based DRX positive electrodes (including HE-DRX) in liquid batteries often results in poor cycle stability due to side reactions between the positive electrode and electrolyte (Supplementary Fig. 29), the construction of a highly stable HE-DRX | LLZTO interface effectively addresses these issues. TM6 demonstrates superior cycling performance in ASSLBs compared to liquid batteries (Supplementary Fig. 29)28,37,38,39,40,41,42. This stable interface also leads to the excellent cycle stability of ASSLBs. The HE-DRX-ASSLB was cycled 100 times at 25 mA/g, maintaining a high reversible capacity of 224.43 mAh/g with a capacity retention rate of 95% (Fig. 5e, f). We provide a summary of recent publications reporting on the performance of ASSLBs based on garnet (Supplementary Table 5). The results indicate that the HE-DRXs-ASSLBs significantly improve electrochemical performances in terms of the specific capacity, cycle life, and rate capability.We have demonstrated the efficacy of combining entropy stabilization effects with fast reaction kinetics to achieve a favorable balance of chemical stability and wettability. The high reaction temperature of the UHS technology enables the rapid synthesis of the TM6 positive electrode on the LLZTO surface, while the short sintering time and the excellent stability of the HE-DRXs ensure a conformal positive electrode interface without any side reactions. The stable TM6 | LLZTO solid-state interface also mitigates the issue of transition metal migration and interfacial side reactions typically observed with HE-DRXs in liquid electrolytes. Specifically, TM6-ASSLBs demonstrate an average specific capacity of 239.7 ± 2 mAh/g at 25 mA/g and exhibit a 95% capacity retention after 100 cycles at 150 °C.
