Infiltration-driven performance enhancement of poly-crystalline cathodes in all-solid-state batteries

Introduction

The development of next-generation batteries has recently gained prominence for the fabrication of energy-storage systems. Electronic devices such as the Internet of Things (IoT), wearable devices, and electric vehicles (EVs) require safe lithium-ion batteries (LIBs) with a high energy density that can be rapidly charged. Therefore, for the development of long-distance EVs, several recent studies have focused on alternative battery technologies, such as lithium–sulfur batteries^1,2,3,4^, with a high theoretical capacity and high-energy-density nickel-rich layered oxide cathodes^5,6,7. Commercial LIBs containing liquid electrolytes composed of flammable organic components are unsafe and often cause fires or explosions, as highlighted by numerous incidents involving fires in EVs and smartphones. Consequently, current research is focused on flame-retardant all-solid-state batteries (ASSBs) as next-generation batteries. Owing to several advantageous properties, such as high safety, durability, high energy density, and simple battery design, ASSBs exhibit high potential as alternatives to traditional LIBs^8,9,10.

The cathode material significantly influences several important aspects of ASSBs, such as their energy density, rate capability, and stability. As a result, several recent studies have investigated ASSBs comprising novel cathode materials, such as high-energy-density Ni-rich layered cathode materials, that outperform commercial LIBs. Cathode materials containing higher amounts of Ni enable the fabrication of batteries with a higher capacity and energy density. In particular, for high-Ni compositions, a cutoff voltage of approximately 4.3 V vs. Li⁺/Li leads to H2‒H3 phase transitions, causing internal strain and significant unit-cell shrinkage. The severe anisotropic volume changes and lattice mismatch observed between the adjacent primary particles in polycrystalline materials during charging and discharging result in the cracking of secondary particles. These issues of particle cracking and contact loss between the solid electrolyte and the active material negatively affect the electrochemical performance of ASSBs. Recent research has attributed the high stability of single-crystalline cathode materials to their crack-free nature; due to this nature, they can maintain excellent electrochemical performance with prolonged use. However, unlike the conventional powder-type dry composite electrode manufacturing approach, the novel infiltrated composite electrode fabrication method has shown differing results^{11,12,13,14,15,16,17,18}.

Furthermore, numerous papers have been published on solid electrolytes (SEs), which are essential components of all ASSBs. Among the different types of SEs developed to date, such as oxides, polymers, sulfides, and halides, sulfide SEs are particularly promising owing to their high ionic conductivity and excellent ductility^{19,20,21,22,23,24}. Several sulfide SEs show high conductivities; for example, Li₇P₃S₁₁²⁵, Li₁₀GeP₂S₁₂²⁶, Li_9.54Si_1.74P_1.44S_11.7C_l0.8²⁷, and Li_5.5PS_4.5Cl_1.5 show conductivities of 1.2 × 10⁻², 1.2 × 10⁻², 2.5 × 10⁻², and 9.4 × 10⁻³ S cm⁻¹, respectively, at 25 °C^{28,29,30,31,32}. Manufacturing large-scale electrodes involves several challenges; therefore, traditional dry methods exhibit limited applicability in the fabrication of bulk-type electrodes for commercialization. In commercial LIB production, a wet-slurry process is used for the large-scale manufacturing of sheet-type electrodes. However, sulfide SEs exhibit high reactivity toward common polar solvents during the wet-slurry fabrication of sheet-type cathodes; thus, this process is not suitable for the construction of ASSB cathodes. The infiltration method is well known for addressing these issues by fabricating composite electrodes for ASSBs^{33,34,35,36,37}.

Kim et al. reported a pioneering strategy for the infiltration of an SE into an electrode by elevating the temperature of the infiltration process and enhancing the molecular motion of the SE. This strategy was used to fabricate a high-energy-density NCM622((Li[Ni_0.6Co_0.2Mn_0.2]O₂)/Li–In half cell with a capacity of 136 mAh g⁻¹ and a high loading level of 17 mg cm⁻². Electrochemical experimentation with cathode materials comprising small, large, and a mixture of small and large particles indicated that hybrid cathodes (containing a mixture of large and small particles) underwent efficient SE infiltration, minimizing the formation of spaces in the electrode material. Their proposed strategy enhanced the SE infiltration rate into the cathode through the capillary effect and facilitated intimate electrode–electrolyte contact during charge–discharge cycles. Among all the as-fabricated ASSBs, those containing SE–NCM electrodes composed of a mixture of active materials showed the best electrochemical performance, with a high capacity of 108.6 mA h g⁻¹³⁸.

Tron et al. focused on the infiltration of argyrodite Li₆PS₅Cl solid electrolyte into lithium-ion battery electrodes, aiming to optimize this process for commercialization. Although their work also involved infiltration, they were primarily focused on process optimization for solid electrolyte penetration into conventional lithium-ion battery electrodes. Despite these efforts, challenges still persist that need to be overcome for the scalable production of ASSBs³⁹.

In this study, an infiltration method for fabricating composite electrodes for ASSBs that can be scaled up to manufacture commercial sheet-type electrodes is proposed. The influence of the crystalline structure of the cathode material NCM811 (Li[Ni_0.8Co_0.1Mn_0.1]O₂), which exhibits a high theoretical capacity, was investigated with respect to its SE infiltration capacity. Subsequently, to demonstrate the effectiveness of the proposed SE infiltration method, half cells comprising poly- and single-NCM infiltration electrodes were fabricated and analyzed. The as-fabricated poly-NCM infiltration electrodes showed excellent electrochemical performance (with an initial discharge capacity of 196 mAh g⁻¹, a discharge capacity of 112 mAh g⁻¹ at a high rate of 2 C, and a retention of 81% after 30 charge–discharge cycles). The infiltration method proposed in this study highlights the potential of polycrystalline cathode materials in ASSBs, challenging conventional views and emphasizing the electrode-electrolyte contact for superior electrochemical performance. This study provides new alternatives for composite electrode fabrication methods and material selection. This progressive strategy could play a pivotal role in advancing ASSB technology in the development of high-performance, safe, and reliable solid-state batteries for a wide range of applications.

Materials and methods

Fabrication of the SE and SE-infiltrated electrodes

To synthesize the SE, Li₆PS₅Cl (LPSCl) powder, stoichiometric amounts of Li₂S (99.9%, Alfa Aesar), P₂S₅ (99%, Sigma‒Aldrich), and LiCl (99.99%, Sigma‒Aldrich) were mixed at a weight ratio of 42.8:41.4:15.8 at 600 rpm for 10 h at 25 °C with ZrO₂ balls using a Pulverizette 7 PL instrument (Fritsch GmbH). An SE solution consisting of LPSCl (0.37 M) was prepared by dissolving the ball-milled LPSCl powder in anhydrous ethanol (99.5%, Sigma‒Aldrich) at 25 °C with stirring for 6 h in an Ar-filled glove box. Single- and polycrystalline NCM811 powders were purchased from COSMO AM&T Co., Ltd. Conventional NCM-based composite electrodes that were infiltrated with the LPSCl solution were fabricated by casting a slurry consisting of NCM811 powder, a polymeric binder (polyvinylidene fluoride, PVDF), and the carbon additive Super P in a weight ratio of 96:2:2 in N-methyl pyrrolidinone on Al current collectors. The electrodes showed a mass loading of ~4 mg cm⁻². For SE infiltration, these electrodes were immersed in the SE solution, dried (for solvent removal) in an Ar-filled glovebox and subjected to heat treatment at 180 °C under high vacuum.

Materials characterization

Images of the LPSCl SE-infiltrated NCM-based composite electrodes were acquired by field-emission scanning electron microscopy (FE–SEM) using a JSM-7619Plus (JEOL) instrument and cross-sectional energy-dispersive X-ray spectroscopy (EDS). A focused ion beam (FIB) (4 kV for 3 h, Ar ion beam) was used for sample preparation. X-ray diffraction (XRD) patterns were recorded using a MiniFlex 600 diffractometer (Rigaku Corp., Japan) after sealing the samples in holders with a Be window. Diffraction data (collected in the 2θ range of 10–50° at 15 mA and 40 kV) were used to validate the crystallinity and composition of the SE and recrystallization–SE (RC–SE) samples. An S3500 (Microtrac, USA) laser diffraction particle size analyzer was used to record the particle size data for the NCM and SE powders. Raman spectra were recorded on a Raman spectrometer (NTEGRA SPECTRA, NT-MDT Spectrum Instruments, Russia) with an excitation wavelength of 532 nm. X-ray photoelectron spectra (XPS) were collected on a Nexsa (Thermo Fisher Scientific) instrument with an X-ray spot size of 100 μm. An SDT Q600 (TA Instrument Corp.) instrument was used for thermogravimetric analysis (TGA) under Ar within the temperature range of 25–600 °C at a heating rate of 5 °C min⁻¹. The surface areas of the NCM powders used for experimentation were determined from N₂ adsorption/desorption isotherms using the Brunauer–Emmett–Teller (BET, 3 Flex, Micromeritics) method.

Electrochemical characterization

Pressed cells comprising a Li–Ag anode, a Ni-based cathode, and an LPSCl-pellet membrane (with a thickness of 800 μm) were constructed to investigate the electrochemical performance of the half cells containing SE-infiltrated NCM-based composite cathodes. All-solid-state NCM/Li–Ag half-cells at 55 °C were used for the electrochemical characterization of LPSCl SE-infiltrated NCM-based electrodes. A polyether ketone mold (13 mm in diameter) was assembled and then charged and discharged using a battery cycler (Won-A Tech) within the potential range of 3.0–4.3 V vs. Li/Li⁺. A pulse current of 0.5 C for 60 s followed by a resting period of 1 h was used for galvanostatic intermittent titration technique (GITT) measurements. Electrochemical impedance spectroscopy (EIS) measurements were conducted from 1 MHz to 10 MHz with an amplitude of 100 mV.

Results and discussion

In this study, the effect of the crystallinity of the cathode material on the SE infiltration process was investigated. SEM and particle size analysis (PSA) were used to characterize the features of the single- and polycrystalline NCM811 cathode materials. Both types of cathode materials had similar particle sizes within 4–5 µm and comparable surface areas (Supplementary Figs. S1 and S2, Supplementary Material). Sheet-type electrodes were fabricated by doctor-blade casting using two different electrodes of single- and polycrystalline materials. Subsequently, the LPSCl was dissolved in ethanol to prepare the SE solution, and this solution was then infiltrated into the electrode. After the infiltration process, the electrode was subjected to drying and heat treatment to fabricate the final SE-infiltrated-NCM composite electrode (Fig. 1). The establishment of effective ion pathways is important for the facile operation of ASSBs. The high resistance at the cathode–electrolyte interface, which primarily arises from poor electrode–SE contact, hinders the formation of an efficient ion-conducting network. To mitigate this issue, the SE particle size needs to be smaller than that for the cathode active material^40,41,42. Notably, composite electrodes produced through the infiltration process enabled close contact between the cathode and the electrolyte, independent of the size of the SE particles. Supplementary Fig. S3 shows that the SE-infiltrated NCM electrodes fabricated using LPSCl with small and large particles (D₅₀ = 3.11 and 8.45 μm, respectively) had comparable charge–discharge profiles. Therefore, the fabrication of composite electrodes through SE infiltration could facilitate the efficient fabrication of ASSBs by eliminating the process costs and time required for SE pulverization.

Infiltration-driven performance enhancement of poly-crystalline cathodes in all-solid-state batteries — **Fig. 1: A schematic illustrating the fabrication process of an infiltrated-NCM composite electrode for solid-state batteries.**

To optimize the infiltration process, the effects of temperature, soaking time, and other conditions on the viscosity of the SE solution were investigated by a rheometer (Supplementary Fig. S4). High-temperature SE solutions facilitated smooth infiltration and showed excellent impregnation effects, possibly owing to increased molecular motion. Due to its low boiling point and high volatility, ethanol (EtOH) undergoes rapid evaporation at high temperatures, which increases the viscosity of the SE solution and hinders its dispersion into the electrode during the infiltration process. Therefore, a closed reactor system was developed to facilitate the inclusion of EtOH in the SE formulation under high-temperature conditions by addressing the viscosity surge of the SE and ensuring efficient infiltration. The results of this study could guide the optimization of similar infiltration processes for various applications.

To ensure successful infiltration of the solid electrolyte, EtOH was employed in the preparation of the LPSCl solution because of its compatibility with both the solid electrolyte and the components of the electrode, as well as the Al current collector^33,34,37,38. Following this preparation, XRD and Raman spectroscopy were then employed to investigate the optimal conditions for infiltrating LPSCl solutions into the NCM electrodes, further confirming the effectiveness of the selected solvent in the process. TGA was used to investigate the optimal recrystallization conditions for LPSCl. The TGA curves indicated rapid mass loss within 180–200 °C, possibly owing to the evaporation of the residual solvent that remained in the system after the solidification and drying of LPSCl (Supplementary Fig. S5). Porous NCM electrodes infiltrated with LPSCl were recrystallized via heat treatment. Considering the thermal stability of the PVDF polymer binder, which is used in the fabrication of conventional NCM electrodes, a heat treatment temperature of 180 °C was selected for the recrystallization process. The XRD pattern of the composite electrode contained the characteristic peak of argyrodite LPSCl (ICSD: 418490) without any impurity phases, confirming the infiltration and recrystallization of LPSCl into the electrode. The Raman spectrum of the composite electrodes contained a prominent peak at 423 cm⁻¹; this corresponded to an intrinsic peak of LPSCl that was observed in the spectra of both ball-milled and recrystallized LPSCl, and thus its recrystallization was confirmed.

The cross-sectional FE–SEM images of the composite electrodes fabricated by the infiltration of LPSCl into two types of NCM electrodes are shown in Fig. 2. EDS was used to investigate the elemental distribution of the sulfide-based SE and NCM active material in the composite electrodes. In the composite electrodes, Ni from the NCM electrodes was in close proximity to P, S, and Cl from the sulfide-based SE, indicating the uniform and deep penetration of the solution-based LPSCl into the NCM electrodes during the infiltration process. The NCM–LPSCl interface in the composite electrodes facilitated homogeneous SE infiltration into the interior of the porous electrodes. Furthermore, electrodes consisting of polycrystalline NCM particles exhibited more effective LPSCl infiltration than those consisting of single-crystalline particles (Supplementary Fig. S6).

**Fig. 2: Characterization of the SE-infiltrated NCM electrode.**

Figure 3a, b shows the cyclability and rate capability of cathodes with different crystalline structures, and Fig. 3c–f shows the charge–discharge voltage profiles of the electrodes for specific cycles. The poly- and single-crystalline NCM electrodes had loading levels of 2.64 and 2.52 mg cm⁻², respectively, with infiltrated LPSCl contents of 10.22% and 9.06%, respectively. Compared with the single-NCM electrode, the poly-NCM electrode demonstrated a greater initial discharge capacity (197 vs. 172 mA h g⁻¹) and superior initial Coulombic efficiency (80.02% vs. 69.49%). This enhanced performance was likely attributed to the polycrystalline nature of the active material, which consisted of aggregated primary particles; this potentially provided additional interfacial formation due to the penetration of the SE solution into the particles. Consequently, the increased discharge capacity and Coulombic efficiency of the poly-NCM electrode were likely a result of the extensive contact between the active material and the solid electrolyte. In contrast, the single-crystalline structure limited SE solution penetration, thereby hindering the formation of close contact with the solid electrolyte. Therefore, single-crystalline materials showed lower initial Coulombic efficiency than polycrystalline materials; thus, single-crystalline materials underwent significant side reactions compared to polycrystalline materials. Moreover, the performance of the cathodes consisting of single-crystalline particles decreased significantly with time, particularly at a high rate of 2 C (from 112 to 30 mAh g⁻¹). Single-crystalline cathode materials contain restricted lithium-ion diffusion pathways that result in the formation of a spatially uneven lithium concentration during charge–discharge cycles. This limitation, which is particularly pronounced at high rates and in cathodes with a high Ni content, leads to the formation of distinct coexisting phases within the cathode. The emergence of side reaction layers in the cathode induces structural irregularities, restricting the diffusion rate of lithium ions and thereby leading to a reduction in the capacity of the system. To elucidate the reasons behind the observed electrochemical performance differences attributed to the active material morphology, GITT analysis was conducted. Through GITT analysis, the lithium-ion diffusion ability within the active material, along with the polarization and coverage, was systematically evaluated and compared (Fig. 3g–h and Supplementary Fig. S7). Electrodes consisting of single-crystalline particles exhibited higher polarization than those consisting of polycrystalline particles. Moreover, the surface coverages of SE on the active materials for the LPSCl-infiltrated NCM electrode were 74.4% and 92.9% for single-crystalline and polycrystalline particles, respectively, via GITT analysis (Supplementary Fig. S7). These values were notably higher than the 61% coverage observed for the LPSCl-infiltrated LCO electrode reported in a previous study³³. This result could be attributed to the use of a closed reactor system, which facilitated effective infiltration. The greater coverage of the poly-crystalline particles indicated that SE penetrated into the interior of the particles. EIS analyses were conducted to examine the interfacial state between the electrode and electrolyte across various states of charge (SOCs). Measurements were carried out on single-crystalline and polycrystalline electrodes at 30%, 50%, 70%, and 100% SOC. Bode plots were generated to depict the frequencies associated with the semicircle formation. The frequency range of the semicircle extended from 10 Hz to 10⁴Hz (highlighted in the gray box in Supplementary Fig. S8). The findings revealed that poly-crystalline electrodes exhibited either similar or reduced interfacial resistance compared to single-crystalline electrodes at 30%, 50%, and 70% SOC, and this results indicated a reduced charge transfer resistance. However, at 100% SOC, the polycrystalline electrodes demonstrated slightly higher resistance, likely due to particle cracking and contact loss. Notably, volumetric changes resulting from H2‒H3 phase transitions at high voltages (approximately 4.3 V vs. Li⁺/Li) could contribute to the increased resistance. Nevertheless, owing to the reduced resistance observed in polycrystalline electrodes within the 30–70% SOC range, which represents the majority of the battery capacity, these electrodes demonstrate superior electrochemical performance (Supplementary Fig. S8).

**Fig. 3: Electrochemical performances of the NCM ASSBs using the SE-infiltrated electrodes.**

Microstructural modifications in the infiltrated electrodes were characterized using cross-sectional FE–SEM images; FE–SEM images of the samples after 30 charge–discharge cycles are shown in Fig. 4a, b. A comparison of Fig. 2c, d indicates internal cracking or voids within the electrode, possibly owing to volume expansion caused by the electrochemical–mechanical effects that occurred during battery charge–discharge. This phenomenon resulted in the detachment of the SE from the active material, followed by its disintegration. This observation was consistent with the cycling performance shown in Fig. 3a. Composite NCM electrodes consisting of single-crystalline particles underwent significant performance deterioration owing to volume expansion, SE disintegration, and the formation of voids within the electrode, resulting in a significant lifespan reduction. In contrast, despite some degradation due to SE loss at the particle surface, the composite polycrystalline NCM electrodes showed improved retention, mainly driven by the infiltration of the SE within the primary particles.

**Fig. 4: SEM and TEM images of the SE-infiltrated NCM electrode after 30 charge/discharge cycles.**

The interfacial electrochemistry of the composite poly- and single-crystalline NCM electrodes was investigated using ex situ transmission electron microscopy (TEM) analysis after 30 charge–discharge cycles. The TEM images and corresponding fast Fourier transform patterns are shown in Fig. 4c, d. The particles of the poly- and single-crystalline NCM electrodes exhibit a rock salt structure (similar to NiO) with the Fm3 ̅m space group consisting of a core region (A) and a distinct surface region in contact with LPSCl (B); redox reactions continuously occur between Ni⁴⁺ in NCM and sulfide in SE^11,16. Notably, the surface layer on the single-crystalline NCM particles (~12.9 nm) is thicker than that on the polycrystalline NCM particles (~3.0 nm), consistent with the electrochemical performances of both systems. These result highlight the limitations of single-crystalline NCM electrodes owing to the restricted lithium-ion diffusion paths in these systems. The GITT results indicate prolonged lithium-ion diffusion within single-crystalline NCM particles. This phenomenon causes high polarization and an uneven lithium concentration distribution in such systems, reducing their capacity and cycling stability.

The electrochemical performance of cathode materials is influenced by their particle morphology. Polycrystalline NCM structures with nanosized primary particles show better electrochemical performance than single-crystalline NCM structures owing to the homogeneous infiltration of the SE solution between the primary particles, which facilitates excellent electrode–SE contact. The strategy of using SE solutions, which enable excellent electrode–electrolyte interfacial contact, could facilitate the rapid advancement of ASSBs by enabling the optimal utilization of the unique properties of polycrystalline cathode materials, including their excellent initial Coulombic efficiency, retention, and electrochemical performance at high current densities. Overall, this study confirms the significant influence of the particle morphology on the design and development of high-performance ASSBs.

Conclusion

In this study, the relationship between the morphology of active materials formed through a scalable process called infiltration and their electrochemical performance was investigated. The polycrystalline active material exhibited excellent electrochemical performance and underwent effective interactions with the SE, as confirmed by the GITT and TEM analysis. TGA and XRD were utilized to optimize the drying and heat treatment conditions of the LPSCl solution used as the SE to enhance the initial discharge capacity of its systems. LPSCl-infiltrated polycrystalline NCM811 electrodes showed a capacity of ~196 mAh g⁻¹ at 55 °C. The cross-sectional morphology of the electrodes, investigated by FE–SEM and EDS, confirmed the infiltration of LPSCl into the as-fabricated electrodes. Despite the significant lifespan reduction attributed to the volume expansion of the active material, SE disintegration, and the formation of voids, which inherently leads to a decrease in capacity, the polycrystalline particles demonstrated superior retention due to the penetration of SE into the interior of the primary particles. Therefore, in this study, a promising and scalable approach for the fabrication of sheet-type electrodes for ASSBs using solution-based LPSCl SEs is reported. Supplementary Table S1 provides a comparison of the electrochemical performance with findings from a previous study that implemented the infiltration method. The proposed electrode-design strategy involving SE infiltration overcomes the challenges of large-scale ASSB fabrication by conventional methods involving SE pulverization, facilitating the construction of high-performance ASSBs with high potential for commercialization (Supplementary Fig. S3). The infiltration process induces the dissolution of SE particles, forming solution-state SEs, and enables the fabrication of composite electrodes with a high active material content; due to their low SE content, these electrodes exhibit a high energy density. Previous studies, such as that of Ceder et al.⁴³, indicated a reduction in the efficiency of cathode materials on active material loadings greater than 75%. However, the newly proposed infiltration method enables the production of composite electrodes with active material loadings exceeding 85%. Future research should focus on the design and development of electrodes based on the construction of high-energy-density high-safety ASSBs that exhibit high potential as next-generation high-performance batteries.