English

• With the energy storage market booming [1-4], especially for electric vehicle industry in the past decade, the energy density and safety have been increasingly valued in next-generation Li-ion batteries [3,5,6]. Lithium metal battery can break through the energy density limit (300 Wh/kg) of current commercial lithium-ion batteries on account of their low redox potential (−3.04 V versus the standard hydrogen electrode) and high theoretical specific capacity (3860 mAh/g) [7-11]. Nevertheless, some core issues hinder the further development of lithium batteries [12], such as the safety hazards from traditional organic electrolytes and the uncontrolled growth of lithium dendrites [13-15]. To deal with the current dilemma, all-solid-state lithium batteries (SSBs) are considered the most competitive solution [16-18]. As a crucial part of solid state battery, solid electrolyte directly determines its mechanical properties and the conduction rate of lithium ions [19-23].

  Among various types of SSBs, NASICON type Li_1+xAl_xGe_2−x(PO₄)₃ (LAGP) is one of the most potential candidates on account of their high ionic conductivity [19,24,25], high oxidation resistance, and excellent environmental air stability [26,27]. Nevertheless, there are still many challenges with LAGP in practical applications [28,29]. Specifically, the intrinsic rigid property of LAGP result in a large interface resistance when in contact with lithium [30,31]. In addition, the high-valence Ge⁴⁺ of LAGP is easily reduced by Li metal during cycling process, which will produce an interphase that does not promote ion conduction and further lead to performance degradation [32,33]. To sum up, the large contact resistance and chemical instability on the interface between LAGP and Lithium metal are unfavorable to battery cycle stability and can lead to serious degradation of battery performance [34]. Adding an interface layer between the lithium anode and electrolyte is considered one of the most effective strategies to address the aforementioned issues [35]. Previous works have reported a series of potential interlayers as physical barriers to suppress adverse reactions on the LAGP interfaces, such as metal germanium coating, polymer gel layer [36], LiPON layer or conductive polyethylene oxide (PEO) layer [37,38]. However, the ionic conductivity of these interlayers is still unsatisfactory, thus limiting the cell performance. As we all know, carbon layer is widely used in lithium composite anode [39] and interface modification [37], LiC₆ is a fast ion conductor, which can improve the transfer ability of lithium ions [38]. For example, Ci et al. reported a flexible rGO/ZnO (GZO) membrane formed in situ to modify the LAGP/Li interface [40]. As a result, GZO film can effectively suppress the side-reactions on the Li/LAGP interface. Actually, the mechanical failure of SSBs has a more severe impact on battery performance during cycling. Cui and his workers revealed that C₃N₄ interlayers not only ameliorate the side reactions between LAGP and lithium anode, but also help to uniform lithium deposition during the cycling process [33]. In spite of these extraordinary exploratory works, obtaining a secure LAGP/Li interface through effective and feasible strategies remains a challenge.

  In this work, a multifunctional lithiophilic carbon buffer layer is grown in situ on the LAGP near the lithium electrode side, thereby avoiding direct contact between the LAGP and the lithium anode. It is found as a booster for the smooth operation of solid-state lithium metal batteries by superior lithium contact area and reduced interface side reactions debasing battery resistance and polarization to enhance cell kinetics. As a result, the electrochemical results show that the Li|LiC_x-LAGP|Li battery can endure stably cycle for 1000 h at 0.1 mA/cm² and the critical current density (CCD) up to 1.4 mA/cm². Whether assemble LiFePO₄ or NCM811 as cathode, the full cell exhibits improved electrochemical performance. All in all, this study provides new ideas for progressing the problem between LAGP and lithium anode, which will promote the research and application of SSBs.

  The LAGP powders were first synthesized by solid state calcination strategy, and the XRD pattern is showed in Fig. S1 (Supporting information). Then LAGP powder was pressed and fired to form LAGP tablets. As shown in Fig. 1a, after brushing LAGP tablets with the glucose solution, LAGP coated with carbon (C-LAGP) can be prepared via sintering again in an argon atmosphere. Detailed synthesis processes of C-LAGP and LAGP are described in the Experimental section. Their XRD diffraction peaks are consistent with the standard card of LAGP (JCPDS No. 80–1924, Fig. S2 in Supporting information). Such a buffer layer can significantly increase solid-solid contact area and Li⁺ diffusion rate, substantially reducing the impedance of the Li metal battery due to its affinity for lithium. What's more, the carbon buffer layer can undergo in-situ reaction with metallic lithium to generate LiC_x (LiC_x include LiC₁₂ and LiC₆, which is proved in next tests). As shown in Figs. 1b and c, LiC_x not only guides the uniform deposition of lithium ions during the battery cycle, avoiding the large generation of lithium dendrites, but also helps to suppress interface side reactions. The Electrochemical impedance spectra (EIS) of the LiC_x-LAGP and LAGP symmetric cells is shown in Fig. S3 (Supporting information). The corresponding fitting results are shown in Table S1 (Supporting information). The bare LAGP symmetric cell presents a much larger interfacial resistance, probably due to poor contact and interfacial side reactions, whereas the LiC_x-LAGP symmetric cell has a much smaller interfacial impedance value than the bare LAGP due to the presence of the LiC_x layer that achieves a much larger contact area between the LAGP and the lithium metal, which results in a much more favourable interface with the transport of interfacial Li⁺. The i-t curve of the Li|LiC_x-LAGP|Li is presented in Fig. S4a (Supporting information). As we can see, the initial current is 10.3 µA and then stabilizes at 8.4 µA after polarization, and the corresponding interfacial impedance increases from 536.2 Ω to 573.5 Ω (inset of Fig. S4a). As a result, the LiC_x-LAGP presents a t_Li⁺ of 0.81, obviously higher than the value (~0.77) of the LAGP counterpart (Fig. S4b in Supporting information).

  Figure 1

  

  Figure 1.  (a) The preparation schematic of the LiC_x-LAGP. Schematic diagram of lithium deposition-stripping process of (b) pristine LAGP and (c) LAGP with lipophilic graphitization C-layer.

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  The surface morphologies of the as-obtained LiC_x-LAGP and LiC_x-LAGP plates are characterized by scanning electron microscopy (SEM). As shown in Figs. 2a and b, the surface of LAGP pellet is uneven composed by cubic micro/nanosized crystals, which could result in a "solid-to-solid" point contact with Li metal. For LiC_x-LAGP pellet, the gaps on the LAGP surface are fully filled by graphitization C-layer (Figs. 2c and d, Fig. S5 in Supporting information), increasing the contact area with electrode. Figs. 2e-h show the cross-sectional SEM and EDS mapping spectra of LiC_x-LAGP, there is a smooth graphitization C-layer 126.40 µm thick on LiC_x-LAGP surface. It is also worth noting that the elements C are distributed throughout the entire interface, while P and Ge can be seen below the dashed line. The modified carbon layer has the structure of both graphite and amorphous carbon as proved in Fig. S6 (Supporting information). The roughness of the C-LAGP and LAGP surfaces were further compared by AFM tests, and the results are shown in Figs. 2i and j, where the roughness of C-LAGP is only 38.6 nm, which is much smaller than that of LAGP at 123.0 nm. This is consistent with the SEM results. To demonstrate the in-situ formation of LiC_x, as-prepared LiC_x-LAGP and lithium sheet were pressed together for 24 h After that, an obvious LiC_x peak can be observed in XRD pattern of the lithium sheet (near the graphite layer, Fig. 2k) [41], in addition, we also characterized the interface products by SEM, and the results are shown in Fig. S7 (Supporting information), it further confirms the existence of LiC_x layer. The above results show that the graphitization C-layer can fill the gap on the LAGP surface well and achieve more intimate interfacial contacts.

  Figure 2

  

  Figure 2.  Surface SEM of (a, b) pristine LAGP and (c, d) LiC_x-LAGP. (e-h) Cross section SEM and EDS mapping images of LiC_x-LAGP. Surface AFM of (i) LiC_x-LAGP and (j) pristine LAGP. (k) XRD pattern of lithiated graphitization C-layer.

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  In order to evaluate the protection effect of in-situ generated interface layer on solid electrolyte LAGP, after cycling the lithium symmetric battery, the morphology of LiC_x-LAGP and LAGP was characterized by SEM. As illustrated in Figs. 3a and b, after cycling for 100 h, the surface of LiC_x-LAGP remains flat, while there is observable dead lithium on the surface of naked LAGP (Figs. 3c and d). This rough interface may be related to uneven lithium deposition, which further indicates that the formation of LiC_x layer can protect the interface of LAGP/Li and guide uniform lithium-ion deposition with effect. Subsequently, AFM tests were conducted on the surface of LiC_x-LAGP and LAGP after cycling. As shown in Figs. 3e and f, the LAGP interface protected by LiC_x layer is flat and smooth enough, while the bare LAGP surface become rougher after cycling due to the presence of a large amount of dead lithium, which is consistent with SEM results. As mentioned in the previous report, lithium metal is easy to LAGP reaction [29,42], so that the high price Ge in LAGP is often reduced to low price Ge, leading to a decrease in performance [43]. Herein, XPS is adopted to analyze the element composition of LAGP interface after cycling for 100 h In XPS spectrum of Ge 3d, the peaking area ratio of Ge⁴⁺ and Ge²⁺ after LiC_x-LAGP cycling is 1.57, which is greater than that of naked LAGP (0.48), indicating that LiC_x can act as an intermediate layer to protect Ge⁴⁺ from being reduced (Fig. 3g). Therefore, carbon coating of LAGP plates can inhibite the side reactions between LAGP and lithium anode with effect. In an effort to explore more deeply the protective effect of LiC_x layer on the LAGP interface, AIMD was adopted to simulate the evolution of the LAGP interface structure at the atomic level over time. As shown in Figs. 3h-k, there was no significant deformation in LiC_x-LAGP compared to the original Li LAGP interface, indicating that the introduction of a LiC_x layer can effectively suppress interface side reactions, which is in agreement with the XPS results mentioned earlier.

  Figure 3

  

  Figure 3.  Characterization of LiC_x-LAGP and LAGP solid electrolytes after 100 h of symmetrical cell cycling. SEM of (a, b) LiC_x-LAGP and (c, d) LACP. Atomic force microscopy images of (e) LiC_x-LAGP and (f) LAGP. (g) Ge 2p XPS spectra of LiC_x-LAGP and LAGP. AIMD simulation results of LAGP interface. LAGP-Li interface model (h) initial and (i) after simulation. LAGP-LiC₆ interface model (j) initial (k) after simulation.

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  The lithium half-cell electrochemical performances of the LiC_x-LAGP and LAGP are studied in Fig. 4. For the Li|LAGP|Li, critical current density (CCD) can only reach 0.8 mA/cm² because of the interface contact issues and terrible side reaction (Fig. 4a). After interface carbon treatment, the Li|LiC_x-LAGP|Li can withstand a current density of 1.4 mA/cm², benefiting from the introduction of carbon buffer layer and in situ generation of LiC_x. The plating/stripping performance at various current densities is shown in Fig. 4b. Evidently, the voltage profile varies linearly as the current density changes from 0.1 mA/cm² to 0.5 mA/cm². When it drops to 0.1 mA/cm², the voltage value also backs to 35 mV. However, Li|LAGP|Li symmetric battery suffers from the short circuit at 0.3 mA/cm² due to lithium dendrite growth in LAGP pellet. Fig. 4c displays the cycling stability at 0.1 mA/cm². In terms of Li|LiC_x-LAGP|Li cell, the phenomenon about obvious voltage polarization or lithium dendrite penetration have not been observed over 1000 h (Fig. 4d). Even at higher current density of 0.2 mA/cm² and 0.3 mA/cm², a stable Li plating/stripping can be detected over 200 h and 120 h, respectively (Fig. 4e and Fig. S8 in Supporting information). In comparison, the Li|LAGP|Li cell exhibits significant voltage fluctuations and even short circuits within a few cycles.

  Figure 4

  

  Figure 4.  Lithium half-cell test. (a) CCD of Li|LiC_x-LAGP|Li and Li|LAGP|Li symmetrical cells at stepped current densities; Lithium plating and stripping cycles of Li|LiC_x-LAGP|Li and Li|LAGP|Li symmetrical cells at (b) different current densities and (c) 0.1 mA/cm², (d) 0.2 mA/cm².

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  Subsequently, we assembled the SS/SS batteries and tested the impedance of the electrolyte within the range of 25–75 ℃ according to the formula σ = L/RS calculated its ionic conductivity, where S, R, and L represent the electrolyte area, SS/SS cells impedance, and electrolyte thickness, respectively. According to Fig. S9 and Table S2 (Supporting information), it can be seen that the ion conductivity of C-LAGP is significantly higher than that of LAGP in the temperature range of 25–75 ℃. In addition, the activation energies calculated from Arrhenius curves were compared, and the C-LAGP was significantly lower than the LAGP, suggesting that the addition of carbon coatings can significantly reduce the ion mobility energy barriers. Full solid-state lithium metal batteries (SSLMBs) were assembled by using LiFePO₄ (LFP) as cathode. Firstly, the carbon content and carbonization temperature are further optimized (Fig. S10 in Supporting information). Electrochemical impedance spectra show that the charge transfer resistance (R_ct) and the interface resistance (R_f) of Li|LiC_x-LAGP|LFP batteries are smaller than those of the Li|LAGP|LFP batteries (Fig. 5a). Fortunately, no self-discharge phenomenon derived from carbon layer was found in the Li|LiC_x-LAGP|LFP within 15 days (Table S1). The rate capabilities of both Li|LiC_x-LAGP|LFP and Li|LAGP|LFP batteries are recorded in Fig. 5b. As can be seen, the reversible capacities of Li|LiC_x-LAGP|LFP battery after 5 cycles are maintained at 151.7, 149.8, 145.5, 128.7 and 84.5 mAh/g with the current density increased from 0.1 C to 2 C, respectively. When back to 0.1 C, a reversible capacity of 143.6 mAh/g can be recovered (Fig. 5c). Obviously, Li|LiC_x-LAGP|LFP battery has better rate capability than Li|LiC_x-LAGP|LFP battery. In addition, a high reversible capacity of 119.1 mAh/g with a capacity retention of 85% is achieved over 200 cycles at 1 C (Fig. 5d). Even at 0.4 C, Li|LiC_x-LAGP|LFP battery can also operate for 200 cycles with capacity retention of 78% (Fig. 5e).

  Figure 5

  

  Figure 5.  Electrochemical performance of LAGP solid-state batteries using LFP cathodes at room temperature: (a) EIS of Li|LiC_x-LAGP|LFP and Li|LAGP|LFP solid-state batteries, and the insertion figure shows the equivalent circuit. (b) Rate performance of Li|LiC_x-LAGP|LFP and Li|LAGP|LFP batteries and (c) corresponding charge-discharge curves. Cycling performance of Li|LiC_x-LAGP|LFP and Li|LAGP|LFP batteries at (d) 1 C and (e) 0.4 C.

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  To investigate the high voltage tolerance of LiC_x-LAGP plates, Full SSLMBs using LiNi_0.8Mn_0.1Co_0.1O₂ (NCM811) cathode were configured, and their electrochemical performance are conducted in 3–4.3 V presented in Fig. 6. The rate capability of the Li|LiC_x-LAGP|NCM811 battery is presented in Fig. 6a, reaching a reversible capacity of 195.9, 177.3, 153.7, and 92.1 mAh/g at different rate from 0.1 C to 1.0 C. However, the reversible capacity of Li|LAGP|NCM811 battery is only 185.1, 159.6, 99.6, and 13.1 mAh/g on equal conditions. When the current rate drops to 0.1 C, the capacity can be restored to 170.5 mAh/g. Corresponding charge-discharge curves are illustrated in Fig. 6b. Furthermore, the Li|LiC_x-LAGP|NCM811 battery remains stable over 100 cycles at 0.2C with a capacity maintenance of 72%, which is superior to the Li|LAGP|NCM811 battery (Fig. 6c). Even at 0.5 C, Li|LiC_x-LAGP|NCM811 battery can also deliver an outstanding cyclic stability after 100 cycles with capacity retention of 75% (Fig. 6d).

  Figure 6

  

  Figure 6.  Electrochemical performance of NCM811 cathode battery at room temperature. (a) Rate performance and (b) corresponding charge-discharge curves of Li|LiC_x-LAGP|NCM811 and Li|LAGP|NCM811 batteries. Cycling performance of Li|LiC_x-LAGP|NCM811 and Li|LAGP|NCM811 batteries at (c) 0.2 C and (d) 0.5 C.

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  In summary, we constructed a carbon buffer layer on the surface of the LAGP near the Li electrode side to alleviate the interface issues between LAGP and Li metal such as large contact resistance and chemical instability. Benefiting from the carbon buffer layer LiC_x-LAGP showed excellent ionic conductivity, high interfacial stability and lithiophilic. During cycling process, Li react with carbon to form LiC_x, which can act as an intermediate layer to protect Ge⁴⁺ from being reduced, thereby inhibiting the occurrence of side effects. Finally, assembled symmetric cell with LiC_x-LAGP plate can be stably worked for 1000 h with a lower over potential of merely 73 mV at 0.1 mA/cm². Besides, the assembled Li|LiC_x-LAGP|LFP battery full battery can cycle for 200 cycles at a high rate of 1 C with an 85% capacity retention and average coulombic efficiency of 99.41%. When using NCM811 as cathode, the full cell also delivers a high capacity retention ratio of 75% after operating 100 cycles at 0.5 C.

  CRediT authorship contribution statement

  Bao Li: Formal analysis, Conceptualization. Pengyao Yan: Data curation. Mengmin Jia: Investigation. Liang Wang: Investigation. Yaru Qiao: Formal analysis. Haowen Li: Conceptualization. Canhui Wu: Funding acquisition. Zhuangzhuang Zhang: Investigation. Dongmei Dai: Funding acquisition. Dai-Huo Liu: Investigation.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (Nos. 52372188, 51902090), Henan Key Research Project Plan for Higher Education Institutions (Nos. 24A150019, 23A150038), Key Scientific Research Project of Education Department of Henan Province (No. 22A150042), the National students' platform for innovation and entrepreneurship training program (No. 201910476010), CAS Henan Industrial Technology Innovation & Incubation Center (No. 2024121), 2023 Introduction of studying abroad talent program, the China Postdoctoral Science Foundation (No. 2019 M652546), and the Henan Province Postdoctoral Start-Up Foundation (No. 1901017).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110145.

  
  
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  Figure 1  (a) The preparation schematic of the LiC_x-LAGP. Schematic diagram of lithium deposition-stripping process of (b) pristine LAGP and (c) LAGP with lipophilic graphitization C-layer.

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  Figure 2  Surface SEM of (a, b) pristine LAGP and (c, d) LiC_x-LAGP. (e-h) Cross section SEM and EDS mapping images of LiC_x-LAGP. Surface AFM of (i) LiC_x-LAGP and (j) pristine LAGP. (k) XRD pattern of lithiated graphitization C-layer.

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  Figure 3  Characterization of LiC_x-LAGP and LAGP solid electrolytes after 100 h of symmetrical cell cycling. SEM of (a, b) LiC_x-LAGP and (c, d) LACP. Atomic force microscopy images of (e) LiC_x-LAGP and (f) LAGP. (g) Ge 2p XPS spectra of LiC_x-LAGP and LAGP. AIMD simulation results of LAGP interface. LAGP-Li interface model (h) initial and (i) after simulation. LAGP-LiC₆ interface model (j) initial (k) after simulation.

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  Figure 4  Lithium half-cell test. (a) CCD of Li|LiC_x-LAGP|Li and Li|LAGP|Li symmetrical cells at stepped current densities; Lithium plating and stripping cycles of Li|LiC_x-LAGP|Li and Li|LAGP|Li symmetrical cells at (b) different current densities and (c) 0.1 mA/cm², (d) 0.2 mA/cm².

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  Figure 5  Electrochemical performance of LAGP solid-state batteries using LFP cathodes at room temperature: (a) EIS of Li|LiC_x-LAGP|LFP and Li|LAGP|LFP solid-state batteries, and the insertion figure shows the equivalent circuit. (b) Rate performance of Li|LiC_x-LAGP|LFP and Li|LAGP|LFP batteries and (c) corresponding charge-discharge curves. Cycling performance of Li|LiC_x-LAGP|LFP and Li|LAGP|LFP batteries at (d) 1 C and (e) 0.4 C.

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  Figure 6  Electrochemical performance of NCM811 cathode battery at room temperature. (a) Rate performance and (b) corresponding charge-discharge curves of Li|LiC_x-LAGP|NCM811 and Li|LAGP|NCM811 batteries. Cycling performance of Li|LiC_x-LAGP|NCM811 and Li|LAGP|NCM811 batteries at (c) 0.2 C and (d) 0.5 C.

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