Lithium-ion batteries (LIBs), with relatively high energy density and power density, have been considered as a vital energy source in our daily life, especially in electric vehicles. However, energy density and safety related to thermal runaways are the main concerns for their further applications. In order to deeply understand the development of high energy density and safe LIBs, we comprehensively review the safety features of LIBs and the failure mechanisms of cathodes, anodes, separators and electrolyte. The corresponding solutions for designing safer components are systematically proposed. Additionally, the in situ or operando techniques, such as microscopy and spectrum analysis, the fiber Bragg grating sensor and the gas sensor, are summarized to monitor the internal conditions of LIBs in real time. The main purpose of this review is to provide some general guidelines for the design of safe and high energy density batteries from the views of both material and cell levels.

cathodes, [10][11][12] silicon-based anodes, [13][14][15][16] and optimizing organic liquid electrolytes. [17,18] However, the safety challenges related to the electrolyte are serious because operation of LIBs is exothermic and organic liquid electrolytes mostly with ester carbonates are highly flammable, generating massive heat. [19,20] Dendritic lithium in LIB represents a further challenge considering internal short circuit would occur if the dendrite punctures the separator. [21,22] Therefore, solutions for safety of LIBs are urgently required.Inorganic ceramic solid-state electrolyte (SSE) provides an ideal alternative to liquid flammable electrolytes for the design of safe ASSBs, since ceramic SSE is nonflammable and it has adequate fracture toughness to prevent internal short circuit from lithium dendrite. [23][24][25] Furthermore, lithium metal anode, the ultimate anode with the highest specific capacity and lowest electrochemical potential has been demonstrated in ASSBs, which exhibited intrinsic safety under rigorous conditions. [26][27][28][29][30] In the search for SSEs, while most of the superionic conductors with conductivity >1 mS cm −1 are based on sulfides, such as Li 10 GeP 2 S 12 , [31,32] Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 , [33] and Li 9.6 P 3 S 12 , [34] it has shown that garnet-type oxides are the most stable SSEs with lithium metal anode. [35][36][37][38][39] However, the lithium/garnet interface appeared to have a remarkably large impedance due to the poor interfacial contact. [40] This motivates a variety of studies to turn garnet from lithiophobic to lithiophilic by coating garnet with metal, [41][42][43][44] metal oxides, [45,46] semi-conductors, [47,48] polymer interlayers, [49,50] and graphite. [51] Although these approaches have shown great progress, they mainly addressed the interface issue from garnet side. As a result, ample opportunities remain on lithium metal side.Here we introduce a new strategy to synthesize a ceramic compatible lithium anode by using graphite additives. Our scheme to implement a lithium/garnet interface experiment is sketched in Figure 1. We find that pure lithium is not compatible with garnet, which is consistent with that expected for lithiophobic garnet surface and previous reports (Figure 1a). [52] On the other side, lithium-graphite (Li-C) composite presents lower fluidity and higher viscosity compared to pure Li. So the Li-C composite, like a paste, can be casted onto garnet and exhibits an intimate contact (Figure 1b). As expected, All-solid-state batteries (ASSBs) with ceramic-based solid-state electrolytes (SSEs) enable high safety that is inaccessible with conventional lithium-ion batteries. Lithium metal, the ultimate anode with the highest specific capacity, also becomes available with nonflammable SSEs in ASSBs, which offers promising energy density. The rapid development of ASSBs, however, is significantly hampered by the large interfacial resistance as a matched lithium/ ceramic interface that is not easy to pursue. Here, a lithium-graphite...

Solid‐state Li metal batteries (SSLMBs) have attracted considerable interests due to their promising energy density as well as high safety. However, the realization of a well‐matched Li metal/solid‐state electrolyte (SSE) interface remains challenging. Herein, we report g‐C3N4 as a new interface enabler. We discover that introducing g‐C3N4 into Li metal can not only convert the Li metal/garnet‐type SSE interface from point contact to intimate contact but also greatly enhance the capability to suppress the dendritic Li formation because of the greatly enhanced viscosity, decreased surface tension of molten Li, and the in situ formation of Li3N at the interface. Thus, the resulting Li‐C3N4|SSE|Li‐C3N4 symmetric cell gives a significantly low interfacial resistance of 11 Ω cm2 and a high critical current density (CCD) of 1500 μA cm−2. In contrast, the same symmetric cell configuration with pristine Li metal electrodes has a much larger interfacial resistance (428 Ω cm2) and a much lower CCD (50 μA cm−2).

Solid-state lithium metal batteries (SSLMBs) are promising energy storage devices by employing lithium metal anodes and solid-state electrolytes (SSEs) to offer high energy density and high safety. However, their efficiency is limited by Li metal/SSE interface barriers, including insufficient contact area and chemical/electrochemical incompatibility. Herein, a strategy to effectively improve the adhesiveness of Li metal to garnet-type SSE is proposed by adding only a few two-dimensional boron nitride nanosheets (BNNS) (5 wt %) into Li metal by triggering the transition from point contact to complete adhesion between Li metal and ceramic SSE. The interface between the Li-BNNS composite anode and the garnet exhibits a low interfacial resistance of 9 Ω cm2, which is significantly lower than that of bare Li/garnet interface (560 Ω cm2). Furthermore, the enhanced contact and the additional BNNS in the interface act synergistically to offer a high critical current density of 1.5 mA/cm2 and a stable electrochemical plating/striping over 380 h. Moreover, the full cell paired with the Li-BNNS composite anode and the LiFePO4 cathode shows stable cycling performance at room temperature. Our results introduce an appealing composite strategy with two-dimensional materials to overcome the interface challenges, which provide more opportunities for the development of SSLMBs.

Solid-state Li metal batteries (SSLMBs) have emerged as an important energy storage technology that offers the possibility of both high energy density and safety by combining a Li metal anode...