The widespread applications of lithium-ion batteries (LIBs) generate tons of spent LIBs. Therefore, recycling LIBs is of paramount importance in protecting the environment and saving the resources. Current commercialized LIBs mostly adopt layered oxides such as LiCoO₂ (LCO) or LiNi_xCo_yMn_1−x−yO₂ (NMC) as the cathode materials. Converting the intercalation-type spent oxides into conversion-type cathodes (such as metal fluorides (MFs)) offers a valid recycling strategy and provides substantially improved energy densities for LIBs. Herein, two typical Co-based cathodes, LCO and LiNi_0.6Co_0.2Mn_0.2O₂ (NMC622), in spent LIBs were successfully converted to CoF₂ and (Ni_xCo_yMn_z)F₂ cathodes by a reduction and fluorination technique. The as converted CoF₂ and (Ni_xCo_yMn_z)F₂ delivered cell energy densities of 650 and 700 Wh/kg, respectively. Advanced atomic-level electron microscopy revealed that the used LCO and NMC622 were converted to highly phase pure Co metal and Ni_0.6Co_0.2Mn_0.2 alloys in the used graphite-assisted reduction roasting, simultaneously producing the important product of Li₂CO₃ using only environment friendly solvent. Our study provided a versatile strategy to convert the intercalation-type Co-based cathode in the spent LIBs into conversion-type MFs cathodes, which offers a new avenue to recycle the spent LIBs and substantially increase the energy densities of next generation LIBs.

Sulfide electrolyte-based all-solid-state batteries (ASSBs) are potential next generation energy storage technology due to the high ionic conductivity of sulfide electrolytes and potentially improved energy density and safety. However, the performance of ASSBs at/below subzero temperatures has not been explored systematically. Herein, low temperature (LT) performance of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811)|Li_9.54Si_1.74P_1.44S_11.7Cl_0.3 (LiSPSCl)|Li₄Ti₅O₁₂ (LTO) ASSBs was investigated. By charging the ASSB to 6 V at −40 °C, a capacity of 100.7 mAh∙g⁻¹ at 20 mA∙g⁻¹ was achieved, which is much higher than that charged to 4.3 V (4.6 mAh∙g⁻¹) at −40 °C. Moreover, atomic resolution microscopy revealed that the NCM811 remained almost intact even after being charged to 6 V. In contrast, NCM811 was entirely destructed when charged to 6 V at room temperature. The sharp difference arises from the large internal charge transfer resistance at LT which requires high voltage to overcome. Nevertheless, such high voltage is not harmful to the active material but beneficial to extracting most energy out of the ASSBs at LT. We also demonstrated that thinner electrolyte is favorable for LT operation of ASSBs due to the reduced ion transfer distance. This work provides new strategies to boost the capacity and energy density of sulfide-based ASSBs at LT for dedicated LT applications.

Sodium (Na) metal batteries (SMBs) using Na anode are potential “beyond lithium” electrochemical technology for future energy storage applications. However, uncontrollable Na dendrite growth has plagued the application of SMBs. Understanding Na deposition mechanisms, particularly the early stage of Na deposition kinetics, is critical to enable the SMBs. In this context, we conducted in situ observations of the early stage of electrochemical Na deposition. We revealed an important electrochemical Ostwald ripening (EOR) phenomenon which dictated the early stage of Na deposition. Namely, small Na nanocrystals were nucleated randomly, which then grew. During growth, smaller Na nanocrystals were contained by bigger ones via EOR. We observed two types of EOR with one involving only electrochemical reaction driven by electrochemical potential difference between bigger and smaller nanocrystals; while the other being dominated by mass transport governed by surface energy minimization. The results provide new understanding to the Na deposition mechanism, which may be useful for the development of SMB for energy storage applications.