Stabilizing Solid Electrolyte-Anode Interface in Li-Metal Batteries by Boron Nitride-Based Nanocomposite Coating

Abstract

•Sub-10-nm thin BN film deposited by CVD effectively protects LATP against Li•In situ TEM analyzes the failure mechanism of LATP and the protective effect of BN•Solid-state batteries with LATP/BN show capacity retention of 96.6% over 500 cycles Solid-state Li-metal batteries can improve safety and energy density compared to liquid-electrolyte-based Li-ion batteries; however, various ceramic electrolytes with high conductivities and low cost are not stable against Li metal, and the severe interfacial reaction devastates battery performance in several cycles. To stabilize the solid electrolyte-Li interface, we utilize boron-nitride-based nano-coating as the interfacial layer, which is not only electronically insulating and ionically conductive but also chemically and mechanically robust to preclude the reduction of solid electrolyte by the Li metal. With this strategy, LiFePO4/LATP/BN/PEO/Li solid-state batteries show capacity retention of 96.6% over 500 cycles in 70 days. The development of boron-nitride-based protective film holds great potential to extend the electrochemical window of unstable solid electrolytes and expands the family of applicable solid electrolytes stable against Li metal. Solid-state Li-metal batteries are promising to improve both safety and energy density compared to conventional Li-ion batteries. However, various high-performance and low-cost solid electrolytes are incompatible with Li, which is indispensable for enhancing energy density. Here, we utilize a chemically inert and mechanically robust boron nitride (BN) film as the interfacial protection to preclude the reduction of Li1.3Al0.3Ti1.7(PO4)3 (LATP) solid electrolyte by Li, which is validated by in situ transmission electron microscopy. When combined with ∼1–2 μm PEO polymer electrolyte at the Li/BN interface, Li/Li symmetric cells show a cycle life of over 500 h at 0.3 mA·cm−2. In contrast, the same configuration with bare LATP dies after 81 h. The LiFePO4/LATP/BN/PEO/Li solid-state batteries show high capacity retention of 96.6% after 500 cycles. This study offers a general strategy to protect solid electrolytes that are unstable against Li and opens possibilities for adopting them in solid-state Li-metal batteries. Solid-state Li-metal batteries are promising to improve both safety and energy density compared to conventional Li-ion batteries. However, various high-performance and low-cost solid electrolytes are incompatible with Li, which is indispensable for enhancing energy density. Here, we utilize a chemically inert and mechanically robust boron nitride (BN) film as the interfacial protection to preclude the reduction of Li1.3Al0.3Ti1.7(PO4)3 (LATP) solid electrolyte by Li, which is validated by in situ transmission electron microscopy. When combined with ∼1–2 μm PEO polymer electrolyte at the Li/BN interface, Li/Li symmetric cells show a cycle life of over 500 h at 0.3 mA·cm−2. 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Liu Z. et al.Ultrathin two-dimensional atomic crystals as stable interfacial layer for improvement of lithium metal anode.Nano Lett. 2014; 14: 6016-6022Crossref PubMed Scopus (573) Google Scholar Therefore, the BN layer can provide excellent protection to SSEs without sacrificing the energy density of batteries. Yet, the addition of trace amount of polyethylene oxide (PEO) or liquid electrolyte is necessary to infiltrate the interface and enhance interfacial ion transport across the BN layer, which will be discussed in this manuscript. We choose LATP as an example in this study, as it is inexpensive, highly conductive (∼1 mS cm−1),33Duluard S. Paillassa A. Puech L. Vinatier P. Turq V. Rozier P. Lenormand P. Taberna P.-L. Simon P. Ansart F. Lithium conducting solid electrolyte Li1.3Al0.3Ti1.7(PO4)3 obtained via solution chemistry.J. Eur. Ceram. Soc. 2013; 33: 1145-1153Crossref Scopus (113) Google Scholar and light (∼2.9 g cm−3 versus 5.1 g cm−3 for LLZO).31Jackman S.D. Cutler R.A. Effect of microcracking on ionic conductivity in LATP.J. Power Sources. 2012; 218: 65-72Crossref Scopus (117) Google Scholar, 34Ohta S. Seki J. Yagi Y. Kihira Y. Tani T. Asaoka T. Co-sinterable lithium garnet-type oxide electrolyte with cathode for all-solid-state lithium ion battery.J. Power Sources. 2014; 265: 40-44Crossref Scopus (192) Google Scholar However, it can be easily reduced at a high reduction potential of 2.17 V versus Li/Li+.10Zhu Y. He X. Mo Y. Origin of outstanding stability in the lithium solid electrolyte materials: insights from thermodynamic analyses based on first-principles calculations.ACS Appl. Mater. Interfaces. 2015; 7: 23685-23693Crossref PubMed Scopus (978) Google Scholar The BN nano-coating layer can effectively protect LATP from reduction during Li plating and stripping, as validated by in situ transmission electron microscopy (TEM). Consequently, with the addition of 1–2 μm PEO at the interface, Li/BN/LATP/BN/Li symmetric cells show steady cycling over 500 h at 0.3 mA cm−2 and solid-state LiFePO4/LATP/BN/Li battery shows capacity retention of 96.6% for 500 cycles in 70 days. The BN layer can also be combined with liquid electrolyte for high performance at room temperature, and stable cycling is achieved in both Li/Li cells (700 h at 0.5 mA cm−2) and NMC/LATP/BN/Li cells (capacity retention of 93% in 100 cycles). These results demonstrate the effectiveness of BN coating to protect solid electrolytes against the high reducing power of Li metal. LATP powders were synthesized by a sol-gel method modified from previous literature.33Duluard S. Paillassa A. Puech L. Vinatier P. Turq V. Rozier P. Lenormand P. Taberna P.-L. Simon P. Ansart F. Lithium conducting solid electrolyte Li1.3Al0.3Ti1.7(PO4)3 obtained via solution chemistry.J. Eur. Ceram. Soc. 2013; 33: 1145-1153Crossref Scopus (113) Google Scholar, 35Zhai H. Xu P. Ning M. Cheng Q. Mandal J. Yang Y. A flexible solid composite electrolyte with vertically aligned and connected ion-conducting nanoparticles for lithium batteries.Nano Lett. 2017; 17: 3182-3187Crossref PubMed Scopus (317) Google Scholar Stoichiometric amounts of lithium acetate and ammonium phosphate monobasic were dissolved in water, and aluminum nitrate and titanium butoxide were dissolved in ethanol. These two solutions were thoroughly mixed and dried to produce the gel precursor, which was calcined at 850°C to synthesize LATP powders. Then, the powders were pressed into pellets with a thickness of 0.5–1 mm and further annealed at 950°C to form dense, uniform, and highly crystalline LATP pellets (Figure S1A). The pellet has a high density of 2.66 g cm−3, corresponding to 92% of its theoretical density. Such high density is crucial since the continuous deposition of BN can only be achieved on dense pellets to provide effective protection. Then, the boron nitride protective layer was deposited on the LATP pellet by CVD at 900°C with borane-ammonia complex (BH3-NH3) as the precursor in pure N2.27Shi Y. Hamsen C. Jia X. Kim K.K. Reina A. Hofmann M. Hsu A.L. Zhang K. Li H. Juang Z.-Y. et al.Synthesis of few-layer hexagonal boron nitride thin film by chemical vapor deposition.Nano Lett. 2010; 10: 4134-4139Crossref PubMed Scopus (982) Google Scholar Using 900°C, instead of a higher temperature, preserves the density of defective sites in the BN protective layer and thus yields better performance. After BN deposition, LATP pellets usually turn slightly blue (Figures S1D–S1F). Such a color change may originate from the formation of oxygen vacancies in the LATP during the BN deposition.36Liu L. Gao F. Zhao H. Li Y. Tailoring Cu valence and oxygen vacancy in Cu/TiO2 catalysts for enhanced CO2 photoreduction efficiency.Appl. Catal. B Environ. 2013; 134–135: 349-358Crossref Scopus (265) Google Scholar The scanning electron microscopy (SEM) image of LATP/BN pellet reveals a grainy and dense surface (Figure S2A), which is similar to that of bare LATP pellets (Figure S2B). The X-ray diffraction (XRD) patterns of both as-prepared LATP pellet and LATP/BN pellet match well with previous results (LiTi2[PO4]3, JCPDS No. 35-0754),37Morimoto H. Awano H. Terashima J. Shindo Y. Nakanishi S. Ito N. Ishikawa K. Tobishima S.-i. Preparation of lithium ion conducting solid electrolyte of NASICON-type Li1+xAlxTi2−x(PO4)3 (x=0.3) obtained by using the mechanochemical method and its application as surface modification materials of LiCoO2 cathode for lithium cell.J. Power Sources. 2013; 240: 636-643Crossref Scopus (93) Google Scholar validating that no change in crystal structure happens after BN deposition (Figure S2C). To verify the successful deposition of BN, X-ray photoelectron spectroscopy (XPS) was applied to examine the existence of BN on the surface of LATP pellets (Figure 1C). The N 1s peak and the B 1s peak are clearly observed at the binding energy of 398.1 eV and 190.7 eV, respectively, both corresponding to the B-N bond.38Ci L. Song L. Jin C. Jariwala D. Wu D. Li Y. Srivastava A. Wang Z.F. Storr K. Balicas L. et al.Atomic layers of hybridized boron nitride and graphene domains.Nat. Mater. 2010; 9: 430-435Crossref PubMed Scopus (1809) Google Scholar The shoulder peak at 192.6 eV of the B 1s spectrum indicates the existence of B2O3, which is probably formed by the reaction between borane-ammonia complex and LATP.39In S. Orlov A. Berg R. García F. Pedrosa-Jimenez S. Tikhov M.S. Wright D.S. Lambert R.M. Effective visible light-activated B-doped and B, N-codoped TiO2 photocatalysts.J. Am. Chem. Soc. 2007; 129: 13790-13791Crossref PubMed Scopus (563) Google Scholar Based on integrated peak area, the molar percentage of boron in B2O3 is 26%. The Raman spectrum (Figure 1D) shows that the BN film is mainly h-BN (1375 cm−1) with a small amount of cubic BN (1,060 cm−1).40Reich S. Ferrari A.C. Arenal R. Loiseau A. Bello I. Robertson J. Resonant Raman scattering in cubic and hexagonal boron nitride.Phys. Rev. B. 2005; 71: 205201Crossref Scopus (324) Google Scholar The TEM image displays that LATP particles are uniformly coated by BN with a thickness of 5–10 nm (Figure 1E). The magnified high-resolution transmission electron microscopy (HRTEM) image illustrates that the BN coating is polycrystalline and has a mosaic structure, where the lattice plane is clear but with different orientations (Figure 1F). The interlayer distance is ∼3.1 Å, similar to the previous report.41Kim K.K. Hsu A. Jia X. Kim S.M. Shi Y. Hofmann M. Nezich D. Rodriguez-Nieva J.F. Dresselhaus M. Palacios T. et al.Synthesis of monolayer hexagonal boron nitride on Cu foil using chemical vapor deposition.Nano Lett. 2012; 12: 161-166Crossref PubMed Scopus (962) Google Scholar Small amorphous regions are also observed. This suggests that the BN layer contains a high concentration of defects (e.g., grain boundaries), which could facilitate Li ions to better permeate the coating. The homogeneity of the BN coating is further supported by energy-dispersive X-ray spectroscopy (EDS) mapping in the scanning transmission electron microscope (STEM) mode (Figure S3), where N signal distributes uniformly over LATP particles. Electron energy loss spectroscopy (EELS) mapping also shows that B and N uniformly distribute on the surface of LATP particles, exhibiting a conformal coating (Figure S4). After confirming the conformal coating of BN, we used both cyclic voltammetry (CV) and in situ TEM to investigate whether the nanoscale BN coating can avoid the reduction of LATP particles by Li. In CV tests, Li is used as the counter electrode and Au is sputtered onto LATP pellets with and without BN (see Figure S5 for more details). In bare LATP, the reduction reaction is observed at 2.2 V versus Li/Li+, which is assigned to Ti3+/Ti4+. In contrast, BN-coated LATP is highly stable in the range of 0–4.5 V versus Li/Li+, demonstrating no activity toward reduction. To further study the BN protection at the nanoscale, LATP or LATP/BN particles were placed on a Cu grid and Li contacted with the grid with a voltage of 1 V applied. For a bare LATP particle (Figures 2A and 2B ), the diffraction pattern of Li3Al0.3Ti1.7(PO4)3 phase (JCPDS No. 40-0095) was observed after just 5 min (Figure S6), indicating the existence of the following intercalation reaction:Li1.3Al0.3Ti1.7(PO4)3 + 1.7Li+ + 1.7e− → Li3Al0.3Ti1.7(PO4)3(Equation 1) Then, the diffraction spots fainted and gradually disappeared in 20 min (Video S1). These results show that when LATP is reduced by Li metal, the intercalation of Li ions happens first and Ti4+ is reduced to Ti3+,42Lang B. Ziebarth B. Elsässer C. Lithium ion conduction in LiTi2(PO4)3 and related compounds based on the NASICON structure: a first-principles study.Chem. Mater. 2015; 27: 5040-5048Crossref Scopus (157) Google Scholar and then Li3Al0.3Ti1.7(PO4)3 is further reduced until it is fully amorphous (Figures 2C and 2D). If the reaction keeps going for 1 h, there are drastic volume changes that will finally lead to particle cracking (Figure S7). Control experiments show that the electron beam cannot amorphize the bare LATP particle (Figure S8), but the voltage bias can slightly amorphize the bare LATP particle after 1 h (Figure S9). This shows that the negative voltage is one of the driving forces for the amorphization but not enough to fully reduce the LATP particles without Li. https://www.cell.com/cms/asset/e9fb6bc1-1fef-4369-93dd-3d1a47c03ea2/mmc2.mp4Loading ... Download .mp4 (25.59 MB) Help with .mp4 files Video S1. The Temporal Evolution of Diffraction Pattern of a Pristine LATP Particle that reacted with Li under 1 V Applied VoltageThe reaction time is 20 min and the LATP particle eventually turned amorphous. To save space and time, the playing speed of the video is 3×. On the contrary, the LATP/BN particle (Figures 2E and 2F) shows no structural change when reacting with Li for 1 h (Video S2) since no Li3Al0.3Ti1.7(PO4)3 phase appears and the diffraction pattern (DP) matches well with that of LATP during the whole reaction (Figures 2G and 2H). Some new spots in Figure 2G, which appear with the double plane spacing of the original LATP crystal, are most likely caused by the secondary diffraction or as a result of small deviation from the simulation of a perfect LiTi2(PO4)3 crystal (Figure S10).43Boucher F. Gaubicher J. Cuisinier M. Guyomard D. Moreau P. Elucidation of the Na2/3FePO4 and Li2/3FePO4 intermediate superstructure revealing a pseudouniform ordering in 2D.J. Am. Chem. Soc. 2014; 136: 9144-9157Crossref PubMed Scopus (58) Google Scholar These results strongly support that ∼5–10 nm thin BN layer is enough to resist the reduction of LATP by Li. https://www.cell.com/cms/asset/e2221e90-fd65-4c6e-a325-e3cea306347c/mmc3.mp4Loading ... Download .mp4 (53.11 MB) Help with .mp4 files Video S2. The Temporal Evolution of Diffraction Pattern of a BN-Coated LATP Particle that Reacted with Li under 1 V Applied VoltageThe reaction time is 1 h, and the LATP/BN particle shows no structure change in the whole process. To save space and time, the playing speed of the video is 6×. The protection of BN is further validated at the macroscale by examining the color change of bare LATP and LATP/BN pellets in contact with Li. First, the bare LATP pellet turned black in 1 day after contacting dry Li metal (Figure S1B). This reduction process was dramatically accelerated with the addition of liquid electrolyte at the interface, as the LATP pellet turned black in just 5 min (Figure S1C). The reason is that the liquid electrolyte speeds up ion transport and hence the reduction reaction. In contrast, the LATP/BN pellet (Figure S1D) was stable with dry Li metal after 1 month (Figure S1E), even with the presence of the liquid electrolyte (Figure S1F), as the black spot rarely appeared on the LATP/BN pellet surface. Therefore, the BN coating allows the addition of liquid electrolyte to wet the electrode-electrolyte interface. Such benefit can efficiently resolve the long-standing challenge on Li-ion transport between the Li anode and the solid ceramic electrolyte and render cells with LATP or other unstable solid electrolytes operable at room temperature.44Han X. Gong Y. Fu K.K. He X. Hitz G.T. Dai J. Pearse A. Liu B. Wang H. Rubloff G. et al.Negating interfacial impedance in garnet-based solid-state Li metal batteries.Nat. Mater. 2017; 16: 572-579Crossref PubMed Scopus (1296) Google Scholar In situ TEM, CV, and the color change tests demonstrate the high stability of the BN layer. Next, we examine the performance of the BN coating in electrochemical cells. First, bare LATP and LATP/BN pellets are examined with Au as the blocking electrodes to study the bulk conductivity. The conductivity of as-prepared dense LATP pellets reaches 2.0 × 10