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• As electric vehicles and portable electronic devices developing, it is necessary to develop next-generation batteries with high safety and high energy density [1]. Lithium-ion batteries (LIBs) have the highest energy density among various commercially rechargeable chemical energy storage devices, and they have been widely used in various fields including large-scale energy storage, electric vehicles and portable devices [2]. However, conventional liquid electrolytes contain flammable organic solvents that pose a safety hazard [3]. Therefore, all-solid-state lithium batteries (ASSLBs) using non-flammable solid-state electrolytes (SSEs), which can simultaneously improve safety and energy density, have been considered as the most promising candidate for next-generation energy storage systems [4-6].

  The widely studied solid electrolytes can be divided into four types, including oxides [7], sulfides, polymers [8-10] and halides [11]. Among them, sulfide SSEs, with the highest ionic conductivity (up to 10^–2 S/cm at 25 ℃) and excellent ductility, are considered to be one of the most desirable components for all-solid-state batteries [12]. Nickel-rich layered oxide cathode materials (e.g., NCM811, NCM712, NCM622) with high capacity and high energy density are thought to be more suitable for constructing the high energy density sulfide-based ASSLBs [13]. Unfortunately, the combination of sulfide SSEs and nickel-rich oxide cathodes faces severe challenges [14]. First, the interfacial side reactions upon contact between sulfide SSEs and nickel-rich oxide cathodes lead to undesirable decomposition of the SSEs and structural degradation of the nickel-rich oxide cathodes [15]. Second, the interfacial reaction products are ionic isolated, which limits the transport kinetics of lithium ions, resulting in a sharp decrease in the electrochemical performance of ASSLBs, corresponding to low initial capacity and coulombic efficiency [16]. Last, the surface and grain boundary of the nickel-rich oxide cathode undergo structural degradation, resulting in reversible capacity reduction and voltage drop [17]. Thus, it is necessary to address the above issues simultaneously to realize a stable and reliable ASSLBs.

  In order to improve the stability of the sulfide SSEs/oxide cathodes interface, various electrochemically stable cathode coating materials have been extensively investigated. For instance, LiNbO₃ [18-20], Li₄Ti₅O₁₂ [21], Li₃PO₄ [22,23], Li₂O [24], Li₂ZrO₃ [25] and BaTiO₃ [26] have been shown to be effective in inhibiting the increase in interfacial resistance and improving the electrochemical performance of ASSLBs. Unfortunately, oxygen released from the high-voltage cathode materials under high state of charge (SOC) (> 80%) can also lead to severe interfacial degradation and chemically oxidize SSEs [27]. Therefore, in addition to introducing a buffer layer to inhibit the interfacial side reaction, the structural stability of cathode materials should also be considered [28].

  In this paper, we introduced titanium and phosphorus source onto the surface of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) to form a Ti₃(PO₄)₄ coating by wet chemistry approach. Subsequently, the coated material was further annealed to diffuse Ti⁴⁺ into the internal structure of NCM811 to achieve Ti⁴⁺ doped NCM811. The uniform Ti₃(PO₄)₄ (TiP) coating with thermodynamic/electrochemical stability avoids direct contact between the SSEs and the oxide cathode, which inhibits interfacial reactions. Meanwhile, Ti⁴⁺ doping can stabilize the internal structure of NCM811 and inhibit its oxygen release at high charge state, preventing further electrochemical oxidation of SSEs. As a result, the modified NCM811@TiP cathode exhibits excellent electrochemical performance when integrated with Li_5.3PS_4.3Cl_1.7 SSE, with a high reversible capacity of 174 mAh/g and a good capacity retention of 74.4% after 100 cycles at a current density of 0.044 mA/cm².

  NCM811 was selected as the nickel-rich oxide cathode material due to its comprehensive advantages of higher specific capacity, stability and lower cost compared to other nickel-rich oxide cathode materials with different transition metal ratios [29,30]. The preparation process of NCM811@TiP is shown in Fig. 1a. Tetrabutyl titanate (C₁₆H₃₆O₄Ti) and ammonium dihydrogen phosphate [(NH₄)H₂PO₄] were selected as titanium and phosphorus source, respectively. Ti-doped NCM811 with uniform TiP coating was obtained by wet chemical method and high temperature treatment. In order to confirm the existence of Ti₃(PO₄)₄ coating, the NCM811 and NCM811@TiP cathodes were characterized using Ti 2p and P 2p X-ray photoelectron spectroscopy (XPS). The Ti 2p_1/2 and Ti 2p_3/2 peaks at about 463.8 and 458.1 eV in Fig. S1a (Supporting information) confirm the exaction of Ti⁴⁺ in the NCM811@TiP sample [31]. The P 2p_1/2 and P 2p_3/2 peaks at about 135 and 134 eV in Fig. S1b (Supporting information) confirm the presence of PO₄^3- in the NCM811@TiP sample [22]. In contrast, the Ti 2p and P 2p XPS spectra of uncoated NCM811 show no signals of the Ti and P elements (Figs. S1c and d in Supporting information). The above results demonstrate the successful introduction of Ti₃(PO₄)₄ on the NCM811 cathode. It is worth mentioning that the phosphate material has a strong PO₄^3- tetrahedral framework and strong P═O covalent bonding, which is a structurally stable coating material, and it can avoid the direct contact between the electrolyte and the cathode to inhibit the interfacial side reactions to ensure the rapid Li⁺ transport [32,33]. In addition, (NH₄)H₂PO₄ consumes residual lithium on the cathode surface during the preparation process, thus stabilising the cathode surface structure and improving the ion transport properties of the cathode [34,35].

  Figure 1

  

  Figure 1.  Characterizations of the NCM811@TiP. (a) Schematic illustration of the synthesis process of NCM811@TiP (b) TEM/HRTEM images and corresponding FFT image. (c) HAADF-STEM image and elemental mapping of NCM811@TiP. (d, e) Enlarged images of XRD patterns. (f) An EDS line scan across the surface to internal of NCM811@TiP, where the inset shows TEM image and the areas where the EDS analyses were performed for NCM811@TiP.

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  As shown in Fig. S2 (Supporting information), the uncoated NCM811 particles are shown as secondary particles around 10 µm in size aggregated from primary particles, and the morphology is well maintained after surface coating of Ti₃(PO₄)₄ (Fig. S3 in Supporting information). Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) elemental mapping were used to characterize the morphology and chemical composition of the NCM811@TiP cathode. As shown in Fig. 1b, a uniform TiP coating layer on the surface of NCM811 particles with a thickness of ~11 nm can be clearly observed, and P and Ti elements are uniformly distributed on the surface of NCM811 (Fig. 1c), demonstrating that we successfully synthesised Ti₃(PO₄)₄-coated NCM811 cathode (NCM811@TiP). The TEM images and the corresponding EDS energy spectra at different regions are shown in Fig. S4 (Supporting information). The homogeneous TiP coating layer and the uniform distribution of P, Ti elements are observed in different regions of NCM811 particles, further proving the homogeneity of the coating layer. High-resolution transmission electron microscopy (HRTEM) image of selected region in Fig. 1b shows lattice stripes with a regular spacing of 0.474 nm, corresponding to the (003) plane of NCM811 [36]. The corresponding Fast Fourier Transform (FFT) in the inset of Fig. 1b further confirms that the internal structure of NCM811@TiP is consistent with layered NCM811 phase.

  X-ray diffraction (XRD) was used to reveal the overall average crystal structure of materials. The XRD patterns in Fig. S5 (Supporting information) show that the peaks of both uncoated NCM811 and NCM811@TiP belong to the α-NaFeO₂ structure (space group R-3m) [37], and no obvious impurity peaks caused by wet chemistry method or annealing process are observed in the XRD pattern of NCM811@TiP. The distinct splitting peaks of (006)/(012) and (018)/(110) in the XRD patterns indicate a fully ordered layered structure [38], which is consistent with the TEM results. The enlarged images of XRD in Figs. 1d and e show that the diffraction peaks of (003) and (104) of NCM811@TiP are shifted to lower 2θ degrees compared to NCM811, which may be ascribed to the lattice expansion due to elemental doping [39]. It can be speculated that there may be some Ti⁴⁺ penetrates into the internal structure of NCM811, since its radius (Ti⁴⁺: 0.605 Å) is larger than that of Co³⁺ (0.545 Å), Mn⁴⁺ (0.53 Å) and Ni³⁺ (0.56 Å) [40]. To confirm the successful doping of NCM811 with Ti⁴⁺, an EDS line scan was performed from the surface to the interior of NCM811@TiP particles (Fig. 1f), where the inset shows the corresponding TEM image of NCM811@TiP. The trends of Mn and Co element content curves stabilize at 0.16–0.2 µm, indicating the internal structural region of NCM811 particles. It is noteworthy that the signal of Ti element is still present in this region, suggesting the successful diffusion of Ti element into the internal structure of NCM811 particles, which is in agreement with the XRD results.

  To compare the electrochemical performance of uncoated NCM811 and NCM811@TiP cathodes, ASSLBs were assembled with Li_5.3PS_4.3Cl_1.7 (LPSCl) as SSE and Li foils as anode. The cycling performance of uncoated NCM811 and NCM811@TiP cathodes tested at a cut-off voltage of 4.2 V at 0.1 C and 60 ℃ is shown in Fig. 2a. The capacity of uncoated NCM811 decreases significantly, with a capacity retention rate of only 52.1% after 100 cycles. On the contrary, the capacity retention rate of NCM811@TiP is as high as 74.4% after 100 cycles. Fig. 2b shows the initial charging/discharging curves of the ASSLBs in the voltage range of 2.8–4.2 V at 0.044 mA/cm² and 60 ℃. The initial charge and discharge specific capacities of the uncoated NCM811 are 166.9 and 125.1 mAh/g respectively, with an initial Coulombic efficiency of 74.9%. For NCM811@TiP, the initial charge and discharge specific capacities increase to 220.8 and 174 mAh/g respectively, with an initial Coulombic efficiency of 78.8%. The higher initial specific capacity and Coulombic efficiency of NCM811@TiP can be attributed to the electrochemically stabilized Ti₃(PO₄)₄ coating, which prevents direct contact between NCM811 and LPSCl SSE, thereby suppressing the interfacial side reactions.

  Figure 2

  

  Figure 2.  Electrochemical performance of all-solid-state Li-ion cells with an Li anode, Li_5.3PS_4.3Cl_1.7 solid electrolyte and various NCM811 cathodes at 60 ℃. (a) Cycling performance of the NCM811@TiP and the uncoated NCM811 at 0.1 C. (b) The first charge–discharge profiles of the ASSLBs at the range of 2.8–4.2 V. (c, d) Discharge curves of different cycles of the NCM811@TiP and the uncoated NCM811 at 0.1 C. (e) Discharge middle voltage of the NCM811@TiP and the uncoated NCM811 during 100 cycles. (f) Rate performance of the NCM811@TiP and the uncoated NCM811. Typical charge/discharge curves at various current densities of (g) the NCM811@TiP and (h) the uncoated NCM811.

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  Figs. 2c–e exhibit the discharging curves and mid voltage of the NCM811@TiP and the uncoated NCM811 in various cycles at 0.1 C. As the cycle increases, the discharge specific capacity and mid voltage of uncoated NCM811 decline rapidly. Compared with uncoated NCM811 (ΔV = 0.172 V), the mid voltage of NCM811@TiP (ΔV = 0.076 V) fades slightly after 100 cycles. The decline in the mid voltage during cycling can be ascribed to the irreversible phase transition and the improved resistivity [41], proving that the dual-functional Ti₃(PO₄)₄ coating on NCM811 effectively improves the interfacial stability and the structural stability of NCM811.

  The rate performance of the NCM811@TiP and the uncoated NCM811 at a high loading of 7.5 mg/cm² are shown in Fig. 2f. Compared to uncoated NCM811, NCM811@TiP delivers higher capacity at 0.1, 0.2, 0.3, 0.5, 0.8 and 1 C (1 C means 180 mA/g based on the weight of cathodes). It is worth noting that the NCM811@TiP cathode exhibits a high capacity of 97.7 mAh/g at 1 C. In contrast, the uncoated NCM811 cathode exhibits an extremely low capacity of 23.8 mAh/g at 1 C. The average capacities of uncoated NCM811 and NCM811@TiP at 0.1 C are 104 and 147.8 mAh/g respectively. When recharged at 0.1 C after 1 C, the capacity of NCM811@TiP can be recovered to 138.4 mAh/g, while only 75.4 mAh/g of specific capacity can be restored in the battery with uncoated NCM811. Additionally, the charge-discharge curves of NCM811@TiP and uncoated NCM811 cathodes at various current densities in Figs. 2g and h verify the severe polarization of the cell with uncoated NCM811, while a stable charging and discharging platform and relatively low overpotential can be maintained when the cell was assembled with NCM811@TiP. The excellent rate performance of NCM811@TiP reveals its excellent electrochemical reversibility, which is attributed to the fact that the surface and internal structure of NCM811 particles were simultaneously stabilized by combining TiP coating and Ti⁴⁺ doping.

  The electrochemical reversibility of the NCM811@TiP and uncoated NCM811 cathodes was further evaluated by differential capacity analysis curves (Figs. 3a and b). For NCM811@TiP cathode, the peaks indicating the charge/discharge overpotentials show only minor shifts as the cycles increase. In contrast, the anode and cathode peaks of uncoated NCM811 exhibit significant high voltage shift and low voltage shift, respectively. The significant voltage fading for the uncoated NCM811 cathode indicates the presence of side reactions between uncoated NCM811 and sulfide SSE [42]. The polarization change of NCM811@TiP cathode is almost negligible, suggesting a stable cathode/SSE interface and rapid Li⁺ migration. Electrochemical impedance spectroscopy (EIS) was used to evaluate the interfacial stability and lithium ion transport kinetics. Both batteries were assembled with LPSCL as SSE and Li as anode, so the change of the resistance comes mainly from the cathode side [43]. The Nyquist plots of ASSLBs using two cathodes before and after 100 cycles are shown in Figs. 3c–f. The value of the intersection of the left side of semicircle and the x-axis corresponds to the bulk resistance (R_s) of the solid electrolyte. The semicircle indicates the charge transfer resistance (R_ct), which arises primarily from the interfacial resistance between cathode and SSE. Corresponding simulated results for the Nyquist plots are shown in Table S1 (Supporting information). Figs. 3c and d show the Nyquist plots of ASSLBs using the NCM811@TiP and uncoated NCM811 cathodes before cycling, and the low resistance in the impedance spectrum reflects the close contact between the SSEs and cathode materials. The Nyquist plots of ASSLBs after 100 cycles are shown in Figs. 3e and f. The impedance distribution of uncoated NCM811 changes significantly, and the interfacial resistance increases greatly due to the severe interfacial side reactions, which hinder the transfer of lithium ions. While the resistance of NCM811@TiP shows little change, which implies the enhanced ion transport kinetics and interfacial stability. The above results confirm that the dual-functional Ti₃(PO₄)₄ coating on NCM811 effectively inhibits the interfacial side reactions, and as a result contributes to lower impedance, which is essential for achieving excellent electrochemical performance.

  Figure 3

  

  Figure 3.  (a, b) The dQ/dV curves of the NCM811@TiP and the uncoated NCM811 cathodes for 20^th, 40^th, 60^th, 80^th and 100^th at 0.044 mA/cm². The Nyquist plots of the NCM811@TiP and the uncoated NCM811 (c, d) before cycle and (e, f) after 100^th cycles.

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  The morphology and chemical composition of the SSEs/cathodes interface in composite cathodes after 100 cycles were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The SEM images of NCM811@TiP and uncoated NCM811 composite cathodes after cycling are shown in Fig. S6 (Supporting information). The NCM811@TiP composite cathode retains its mechanical integrity after 100 cycles, with close contact between the individual components. In contrast, a large number of cracks are distributed in uncoated NCM811 composite cathode, implying the severe degradation of the SSEs/cathodes interface caused by interfacial side reactions. Figs. 4a and b present the TEM images of NCM811@TiP and uncoated NCM811 composite cathodes after 100 cycles. There is an irregular decomposition product layer on the surface of uncoated NCM811 particles, indicating that the decomposition products cover the surface of the NCM811 particles unevenly, while the NCM811@TiP particles still retain a smooth surface. The surface of the NCM811 particles is evenly coated with TiP. The corresponding EDS mapping images in Fig. 4c show that P and Ti elements are uniformly dispersed on the surface of the NCM811 particles, further demonstrating the stability of the TiP coating during cycling. In contrast, strong P, S and Cl signal intensities are detected on the surface of uncoated NCM811 particles, suggesting that the observed irregular interfacial structure may be originated from the undesirable decomposition of LPSCl (Fig. S7 in Supporting information).

  Figure 4

  

  Figure 4.  Morphology and chemistry information of cycled NCM811 cathodes in the ASSLBs. TEM and HRTEM images of (a) the NCM811@TiP and (b) the uncoated NCM811 after 100th cycles. (c) HAADF-STEM image and elemental mapping of cycled NCM811@TiP. (d, e) S 2p XPS spectra of the composite NCM811@TiP electrode in the NCM811@TiP/LPSCl/Li cell and the composite NCM811 electrode in the NCM811/LPSCl/Li cell after 100 cycles. (f, g) P 2p XPS spectra of the composite NCM811@TiP electrode in the NCM811@TiP/LPSCl/Li cell and the composite NCM811 electrode in the NCM811/LPSCl/Li cell after 100 cycles.

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  In order to further understand the chemical composition of interfacial side reaction products, the NCM811@TiP and NCM811 composite cathodes were characterized using S 2p and P 2p X-ray photoelectron spectroscopy (XPS). The S 2p and P 2p signals split into two components due to spin-orbit coupling, with an area ratio of ~2/1. Each chemical environment of S and P corresponds to a 2p_3/2–2p_1/2 doublet. For the S 2p XPS spectra (Figs. 4d and e), the doublet with S 2p_3/2 peak at 161.7 eV belong to the S originating from LPSCl, while the doublet with S 2p_3/2 peak at the binding energies of 162.7, 163.5, 164.4 and 168.9 eV can be assigned to Li₂S_n, P₂S_x, S and SO₃^2-/SO₄^2-, respectively [31,44,45]. P 2p XPS spectra are shown in Figs. 4f and g, the doublet with P 2p_3/2 peak at 132 eV belong to the P⁵⁺ cation of LPSCl, while the doublet with P 2p_3/2 peak at the binding energies of 132.5 and 134 eV can be assigned to P₂S_x and PO₄^3-, respectively [31]. The XPS results further confirmed that the unstable interface between uncoated NCM811 and LPSCl, and the severe interfacial side reactions lead to undesired decomposition of LPSCl. The surface of uncoated NCM811 is covered with decomposition products (Li₂S_n, P₂S_x, S, SO₃^2-/SO₄^2- and PO₄^3-), and it is in agreement with the TEM results. Remarkably, the signal intensity of decomposition products in the XPS spectra of NCM811@TiP is significantly reduced, which is attributed to its stable surface and internal structure, and it may be achieved through double modification with surface coating and doping with transition metal elements. NCM811 is the only oxygen source in composite cathodes, and it is crucial to prevent further electrochemical oxidation of LPSCl by maintaining its structural stability [46].

  First principles calculations were used to reveal the substitution position of Ti⁴⁺ in NCM811 and the mechanism of stabilizing the internal structure of NCM811. Fig. 5a shows the structures and energies of NCM811 and Ti⁴⁺-doped NCM811 with different substitution sites. The green, gray, blue, purple, red and cyan spheres represent Li, Ni, Co, Mn, O and Ti atoms, respectively. The energies of NCM811, NCM811 (Ti-Mn), NCM811 (Ti-Co) and NCM811 (Ti-Ni) are −412.73, −414.39, −417.99 and −420.61 eV, respectively. It is notable that for all cases of Ti substitution, NCM811(Ti-Ni) has the lowest energy, indicating that Ti preferentially occupies the Ni site. The content of Ti from the TiP coating is very low, so all subsequent calculations are for the case of Ti substitution at the Ni site.

  Figure 5

  

  Figure 5.  (a) Schematic structures and energies of NCM811 and NCM811 substituted by Ti⁴⁺ at Mn, Co and Ni sites respectively. (b) The oxygen release energies and corresponding structures of the uncoated NCM811 and the NCM811@TiP. (c) ELFs of the uncoated NCM811 and the NCM811@TiP. (d) CDDs of the uncoated NCM811 and the NCM811@TiP. (e) Schematic diagram of failure mechanism at the interface in the composite NCM811 electrode and functional mechanism of double modification at the interface in the composite NCM811@TiP electrode.

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  The oxygen release energies of NCM811 and NCM811@TiP are shown in Fig. 5b. Compared with NCM811 (6.64 eV), NCM811@TiP has a higher oxygen release energy (7.47 eV), which means that Ti doping can stabilize the lattice oxygen of NCM811, thus mitigating further oxidative decomposition of SSE during cycling. The charge distribution situation was investigated by electron localisation function (ELF) and charge density difference (CDD) using Bader charge analysis. The ELFs of the uncoated NCM811 and the NCM811@TiP are shown in Fig. 5c. An obvious electron distribution can be observed between the Ni atom and the adjacent O atoms. On the contrary, there is almost no electron distribution between the Ti atom and the adjacent O atoms, which indicates that the Ti-O chemical bonds is stronger than the Ni-O chemical bonds. Fig. 5d presents the CDDs of the uncoated NCM811 and the NCM811@TiP. The darker the colour, the higher the charge density. Compared with Ni atom, the green color between Ti atom and the adjacent O atoms is wider, indicating that the charge density between Ti and the adjacent O atoms is higher and the chemical bonds between Ti and the adjacent O atoms is more stable, which is consistent with the ELF results.

  In view of the experimental and calculation results, Ti⁴⁺-doped NCM811 with TiP coating (NCM811@TiP) prepared by wet chemistry approach and post-annealing process stabilized NCM811/LPSCl interface and suppressed the interfacial side reactions. To better illustrate the evolution of the interfacial structure between NCM811 and LPSCl, a schematic diagram is shown in Fig. 5e. The interface between uncoated NCM811 and LPSCl is unstable. During the electrochemical cycles at high voltage, the interface is severely decomposed due to the low Li⁺ chemical potential (µLi) vs. S^2-/S of NCM811 and the inherent narrow electrochemical window of sulfide SSEs [42]. The numerous undesirable decomposition products (Li₂S_n, P₂S_x, S, SO₃^2-/SO₄^2- and PO₄^3-) at NCM811/LPSCl interface hinder the lithium ion transport kinetics and lead to a sharp decrease in the cycling performance of the ASSLBs. In contrast, the interface between NCM811@TiP and LPSCl is highly stable. The electrochemically stable TiP coating avoids direct contact between NCM811 and LPSCl, thus suppressing interfacial side reactions. The layered structure of NCM811 is stabilized by Ti⁴⁺ doping, which avoids lattice oxygen loss and generation of oxygen vacancies [47], thus preventing further electrochemical oxidation of LPSCl. The highly stable NCM811@TiP/LPSCl interface ensures the rapid transfer of Li⁺ and greatly improves the electrochemical performance of ASSLBs.

  In summary, Ti₃(PO₄)₄ coating and Ti⁴⁺ doping on the LiNi_0.8Co_0.1Mn_0.1O₂ have been constructed simultaneously by wet chemistry method and post-annealing process. The interfacial side reactions are suppressed by introducing an electrochemically stabilized Ti₃(PO₄)₄ buffer layer between NCM811 and LPSCl. Meanwhile, the internal structure of NCM811 is stabilized by Ti⁴⁺ doping, which prevents further oxidation of LPSCl by the oxygen released from the oxide cathode under high state of charge. As a result, the NCM811@TiP/LPSCl interface exhibits lower interfacial resistance and fewer undesirable degradation products compared to the interface between uncoated NCM811 and LPSCl. The modified NCM811@TiP cathode exhibits a high initial discharge capacity of 174 mAh/g with 74.4% capacity retention after 100 cycles at a cut-off voltage of 4.2 V. This work simultaneously stabilizes the surface and internal structure of NCM811 through a dual modification strategy, which is instructive for the practical application of high-energy nickel-rich layered oxide cathodes in sulfide-based all-solid-state lithium batteries.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Xiaodong Wang: Writing – original draft, Investigation, Data curation, Conceptualization. Miaomiao Zhou: Software, Investigation, Formal analysis. Yirui Deng: Investigation, Formal analysis. Zijun Liu: Visualization, Methodology. Huiyou Dong: Software, Methodology. Peng Yan: Formal analysis. Ruiping Liu: Writing – review & editing, Supervision, Funding acquisition, Conceptualization.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (No. 52272258), the Beijing Nova Program (No. 20220484214), and Key R&D and transformation projects in Qinghai Province (No. 2023-HZ-801), the Fundamental Research Funds for the Central Universities (No. 2023ZKPYJD07).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110307.

  
  
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  Figure 1  Characterizations of the NCM811@TiP. (a) Schematic illustration of the synthesis process of NCM811@TiP (b) TEM/HRTEM images and corresponding FFT image. (c) HAADF-STEM image and elemental mapping of NCM811@TiP. (d, e) Enlarged images of XRD patterns. (f) An EDS line scan across the surface to internal of NCM811@TiP, where the inset shows TEM image and the areas where the EDS analyses were performed for NCM811@TiP.

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  Figure 2  Electrochemical performance of all-solid-state Li-ion cells with an Li anode, Li_5.3PS_4.3Cl_1.7 solid electrolyte and various NCM811 cathodes at 60 ℃. (a) Cycling performance of the NCM811@TiP and the uncoated NCM811 at 0.1 C. (b) The first charge–discharge profiles of the ASSLBs at the range of 2.8–4.2 V. (c, d) Discharge curves of different cycles of the NCM811@TiP and the uncoated NCM811 at 0.1 C. (e) Discharge middle voltage of the NCM811@TiP and the uncoated NCM811 during 100 cycles. (f) Rate performance of the NCM811@TiP and the uncoated NCM811. Typical charge/discharge curves at various current densities of (g) the NCM811@TiP and (h) the uncoated NCM811.

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  Figure 3  (a, b) The dQ/dV curves of the NCM811@TiP and the uncoated NCM811 cathodes for 20^th, 40^th, 60^th, 80^th and 100^th at 0.044 mA/cm². The Nyquist plots of the NCM811@TiP and the uncoated NCM811 (c, d) before cycle and (e, f) after 100^th cycles.

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  Figure 4  Morphology and chemistry information of cycled NCM811 cathodes in the ASSLBs. TEM and HRTEM images of (a) the NCM811@TiP and (b) the uncoated NCM811 after 100th cycles. (c) HAADF-STEM image and elemental mapping of cycled NCM811@TiP. (d, e) S 2p XPS spectra of the composite NCM811@TiP electrode in the NCM811@TiP/LPSCl/Li cell and the composite NCM811 electrode in the NCM811/LPSCl/Li cell after 100 cycles. (f, g) P 2p XPS spectra of the composite NCM811@TiP electrode in the NCM811@TiP/LPSCl/Li cell and the composite NCM811 electrode in the NCM811/LPSCl/Li cell after 100 cycles.

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  Figure 5  (a) Schematic structures and energies of NCM811 and NCM811 substituted by Ti⁴⁺ at Mn, Co and Ni sites respectively. (b) The oxygen release energies and corresponding structures of the uncoated NCM811 and the NCM811@TiP. (c) ELFs of the uncoated NCM811 and the NCM811@TiP. (d) CDDs of the uncoated NCM811 and the NCM811@TiP. (e) Schematic diagram of failure mechanism at the interface in the composite NCM811 electrode and functional mechanism of double modification at the interface in the composite NCM811@TiP electrode.

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