Commercial lithium-ion batteries are vulnerable to fire accidents, mainly due to volatile and flammable liquid electrolytes. Although solid polymer electrolytes (SPEs) are considered promising alternatives with antiflammability and processability for roll-to-roll mass production, several requirements have not yet been fulfilled for a viable lithium polymer battery. Such requirements include ionic conductivity, electrochemical stability, and interfacial resistance. In this work, the ionic conductivity of the SPEs is optimized by controlling the molecular weight and structural morphology of the plasticizers as well as introducing propylene oxide (PO) groups. Electrochemical stability is also enhanced using ethylene oxide (EO)/PO copolymer electrolytes, making the SPEs compatible with high-Ni LiNi x Co y Mn1–x–y O2 cathodes. The in situ cross-linking method, in which a liquid precursor first wets the electrode and is then solidified by a subsequent thermal treatment, enables the SPEs to soak into the 60 μm thick electrode with a high loading density of more than 8 mg cm–2. Thus, interfacial resistance between the SPE and the electrode is minimized. By using the in situ cross-linked EO/PO copolymer electrolytes, we successfully demonstrate a 4 V class lithium polymer battery, which performs stable cycling with a marginal capacity fading even over 100 cycles.

Recently, the development of silicon-based anodes for lithium-ion batteries has attracted tremendous attention for overcoming the disadvantages of commercial graphite-based anodes. In this work, we suggest a chemical methodology of synthesizing silicon–carbon composite anodes, with capacity values of 763 and 182 mAh/g at current densities of 0.1 and 5 A/g, respectively. An electrostatic assembly technique is designed to be triggered by a cationic polyelectrolyte, poly­(ethylenimine), for negatively charged silicon nanoparticles and graphene oxides. Amine-functionalized carbon nanotubes are synthesized in a nondestructive fashion and incorporated additionally to provide intraconnected conductive pathways between neighboring composite materials. It is revealed that the electrochemical performance of intraconnected composite materials is determined by the chemical/physical factors of constituent compartments. The applicability toward all-solid-state batteries is also suggested with usage of a solid polymer electrolyte synthesized from a mixture of bisphenol A ethoxylate diacrylate, polyethylene glycol dimethyl ether, tert-butyl peroxypivalate, and bis­(trifluoromethane)­sulfonimide lithium salt.

The volatility of liquid electrolytes is a major obstacle in the fabrication of efficient lithium-ion batteries that are safe. Solid-state electrolytes such as solid polymer electrolytes have been studied as a potential substitute for liquid electrolytes. However, their practical application is impeded owing to their low ionic conductivity and high interfacial resistance between the electrolyte and electrodes. Herein, we synthesize a novel ionic liquid crosslinker and use thermal crosslinking to prepare a gel polymer electrolyte (GPE), which shows a higher thermal stability than that of liquid electrolytes and a better ionic conductivity than that of solid electrolytes. The crosslinker, IL2, is designed to have a pyrrolidinium-bis­(trifluoromethyl sulfonyl)­amide structure and an acrylate terminal group with an ethylene oxide spacer connected between them. IL2-GPE, which is prepared by in situ thermal crosslinking, shows an ionic conductivity up to 5.37 mS cm–1 and high thermal and electrochemical stabilities. A cell with IL2-GPE sandwiched between a LiFePO4 cathode and lithium anode exhibits a capacity above 160 mA h g–1 and a high rate capability. By combining a crosslinker having four acrylate terminals with the IL2 crosslinker, we obtain HIL2-GPE, whose ionic conductivity is 20% higher than that of IL2-GPE. The HIL2-GPE cell exhibits capacities of 165 and 146 mA h g–1 at 0.1 and 1.0 C, respectively, thereby demonstrating better performance than that of the cell with IL2-GPE. We also prepared a cell using high-voltage cathode LiNi0.6Co0.2Mn0.2O2 (NCM622). The result suggested that the cell based on the GPEs maintained superior long-term stability even with high-voltage cathode materials over 100 cycles.