A Salt‐in‐Metal Anode: Stabilizing the Solid Electrolyte Interphase to Enable Prolonged Battery Cycling

Abstract

Metallic lithium (Li) is the ultimate anode candidate for high-energy-density rechargeable batteries. However, its practical application is hindered by serious problems, including uncontrolled dendritic Li growth and undesired side reactions. In this study a concept of "salt-in-metal" is proposed, and a Li/LiNO 3 composite foil is constructed such that a classic electrolyte additive, LiNO 3 , is embedded successfully into the bulk structure of metallic Li by a facile mechanical kneading approach. The LiNO 3 reacts with metallic Li to generate Li + conductive species (e.g., Li 3 N and LiN x O y ) over the entire electrode. These derivatives afford a stable solid electrolyte interphase (SEI) and effectively regulate the uniformity of the nucleation/growth of Li on initial plating, featuring a low nucleation energy barrier and large crystalline size without mossy morphology. Importantly, these derivatives combined with LiNO 3 can in-situ repair the damaged SEI from the large volume change during Li plating/stripping, enabling a stable electrode-electrolyte interface and suppressing side reactions between metallic Li and electrolyte. Stable cycling with a high capacity retention of 93.1% after 100 cycles is obtained for full cells consisting of high-loading LiCoO 2 cathode (≈20 mg cm −2 ) and composite metallic Li anode with 25 wt% LiNO 3 under a lean electrolyte condition (≈12 µL) at 0.5 C.

Insights on “nitrate salt” in lithium anode for stabilized solid electrolyte interphase

“…As a comparison, a fast increase of voltage polarization occurred for pure Li electrode. In our previous reports, we have revealed that the coexistence of Li 3 N, LiN x O y , and Li 2 O components can adjust the pristine Li nucleation and reduce the nucleation overpotential 29 . Same phenomenon was found in this LKNO composite electrode (0.1 V for LKNO compared with 0.38 V for metallic Li), further confirming the advancement of the “salt‐in‐metal” composite electrode structure (Figure 4D).…”

“…Exchange current density was also measured to compare the charge transfer kinetics for pure Li and LKNO composite electrodes. From the Tafel curves (derived using linear sweep voltammetry) in Figure 3I, exchange current densities were 0.1703 mA cm −2 for the LKNO electrode and 0.0482 mA cm −2 for the pure Li electrode, revealing the improved charge transfer kinetics for the LKNO composite electrode 29 …”

“…The LKNO composite electrode, featuring KNO 3 particles uniformly embedded within metallic Li bulk, was realized using a mechanical kneading approach according to our previous study 29 . The mechanism/effect of KNO 3 on the electrochemical behavior of metallic Li is illustrated in Figure 1.…”

“…In recent years, abundant studies have been devoted to addressing this issue, including loading nitrates between separator and electrode, in which nitrates can be released into electrolyte during cycling, and using solubilizers in the electrolyte to enhance the solubility of nitrates 16,17,21,27 . In our previous work, we proposed a “salt‐in‐metal” concept: we designed metal/nitrate composite foils (such as Li/LiNO 3 and Na/NaNO 3 ) using a facile and low‐cost mechanical kneading approach at ambient temperature, and nitrates were successfully implanted in the electrode 29,30 . Despite the significant improvement in electrochemical performance, the understanding of the mechanism/effect of nitrates on electrolytes and SEI is still lacking.…”

Lithium reduction reaction for interfacial regulation of lithium metal anode

“…7a). LiNO 3 can be reduced by metallic Li according to the following equations: 102 LiNO 3 + 2Li → LiNO 2 + Li 2 O Δ H calculated = −406 kJ mol −1 LiNO 2 + 6Li → Li 3 N + 2Li 2 O Δ H calculated = −857 kJ mol −1 …”