The battery management system is employed to monitor the external temperature of the lithium-ion battery in order to detect any potential overheating. However, this outside–in detection method often suffers from a lag and is therefore unable to accurately predict the battery’s real-time state. Herein, an inside–out frequency response approach is used to accurately monitor the battery’s state at various temperatures in real-time and correlate it with the solid electrolyte interphase (SEI) evolution of the graphite electrode. The SEI evolution at temperatures of −15, 25, 60, and 90 °C exhibits certain regular characteristics with temperature change. At a temperature of −15 °C, the Li⁺-solvent interaction of lithium-ion slowed down, resulting in a significant reduction in performance. At 25 °C, a LiF-rich inorganic SEI was identified as forming, which facilitated lithium-ion transportation. However, high temperatures would induce decomposition of lithium hexafluorophosphate (LiPF₆) and lithium-ion electrolyte. At the extreme temperature of 90 °C, the SEI would be organic-rich, and Li_xP_yF_z, a decomposition product of lithium salts, was further oxidized to Li_xPO_yF_z, which led to a surge in the charge-transfer resistance at SEI (R_sei) and a reduction in Coulombic efficiency (CE). This changing relationship can be recorded in real time from the inside out by electrochemical impedance spectroscopy (EIS) testing. This provides a new theoretical basis for the structural evolution of lithium-ion batteries and the regular characterization of EIS.

To meet the growing demand for wearable smart electronic devices, the development of flexible lithium-ion batteries (LIBs) is essential. Silicon is an ideal candidate for the anode material of flexible lithium-ion batteries due to its high specific capacity, low working potential, and earth abundance. The largest challenge in developing a flexible silicon anode is how to maintain structural integrity and ensure stable electrochemical reactions during external deformation. In this work, we propose a novel design for fabricating core–shell electrodes based on a copper nanowire (CuNW) array core and magnetron sputtered Si/C shell. The nanowire array structure has characteristics of bending under longitudinal stress and twisting under transverse stress, which helps to maintain the mechanical stability of the structure during electrode bending and cycling. The low-temperature annealing generates a small amount of Cu₃Si alloy, which enhances the connection strength between Si and the conductive network and solves the poor conductivity problem of Si, which is known as a semiconductor material. This unique configuration design of CuNW@Si@C-400 °C leads to stable long cycle performance of 1109 mAh∙g⁻¹ after 1000 cycles and excellent rate performance of 500 mAh∙g⁻¹ at a current density of 10 A∙g⁻¹. Furthermore, the CuNW@Si@C-400 °C||LiFePO₄ (LFP) full battery demonstrates excellent flexibility, with a capacity retention of more than 96% after 100 bends. This study provides a promising strategy for the development of flexible lithium-ion batteries.

An interlayer is usually employed to tackle the interfacial instability issue between solid electrolytes (SEs) and Li metal caused by the side reaction. However, the failure mechanism of the ionic conductor interlayers, especially the influence from electron penetration, remains largely unknown. Herein, using Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) as the model SE and LiF as the interlayer, we use metal semiconductor contact barrier theory to reveal the failure origin of Li/LiF@LATP interface based on the calculation results of density functional theory (DFT), in which electrons can easily tunnel through the LiF grain boundary with F vacancies due to its narrow barrier width against electron injection, followed by the reduction of LATP. Remarkably, an Al-LiF bilayer between Li/LATP is found to dramatically promote the interfacial stability, due to the highly increased barrier width and homogenized electric field at the interface. Consequently, the Li symmetric cells with Al-LiF bilayer can exhibit excellent cyclability of more than 2,000 h superior to that interlayered by LiF monolayer (~ 860 h). Moreover, the Li/Al-LiF@LATP/LiFePO₄ solid-state batteries deliver a capacity retention of 83.2% after 350 cycles at 0.5 C. Our findings emphasize the importance of tuning the electron transport behavior by optimizing the potential barrier for the interface design in high-performance solid-state batteries.

Solid-state batteries (SSBs) will potentially offer increased energy density and, more importantly, improved safety for next generation lithium-ion (Li-ion) batteries. One enabling technology is solid-state composite cathodes with high operating voltage and area capacity. Current composite cathode manufacturing technologies, however, suffer from large interfacial resistance and low active mass loading that with excessive amounts of polymer electrolytes and conductive additives. Here, we report a liquid-phase sintering technology that offers mixed ionic-electronic interphases and free-standing electrode architecture design, which eventually contribute to high area capacity. A small amount (~ 4 wt.%) of lithium hydroxide (LiOH) and boric acid (H₃BO₃) with low melting point are utilized as sintering additives that infiltrate into single-crystal Ni-rich LiNi_0.8Mn_0.1Co_0.1 (NMC811) particles at a moderately elevated temperature (~ 350 °C) in a liquid state, which not only enable intimate physical contact but also promote the densification process. In addition, the liquid-phase additives react and transform to ionic-conductive lithium boron oxide, together with the indium tin oxide (ITO) nanoparticle coating, mixed ionic-electronic interphases of composite cathode are successfully fabricated. Furthermore, the liquid-phase sintering performed at high-temperature (~ 800 °C) also enables the fabrication of highly dense and thick composite cathodes with a novel free-standing architecture. The promising performance characteristics of such composite cathodes, for example, delivering an area capacity above 8 mAh·cm⁻² within a wide voltage window up to 4.4 V, open new opportunities for SSBs with a high energy density of 500 Wh·kg⁻¹ for safer portable electronic and electrical transport.