Thermoplastic Polyurethane (TPU) Based All-Solid-State Lithium-Ion Batteries

The rapidly growing flexible/wearable electronic market drives the research and development of flexible power source systems, especially lithium-ion batteries with high specific energy/power, long cycle life and high safety1. Conventional lithium-ion batteries with fixed shapes and liquid electrolyte are not suitable for flexible applications as they can electrically short and/or cause fire when bended or stretched. However, the use of solid-state electrolytes can replace the liquid electrolytes, thereby leading to all-solid-state batteries that can eliminate the safety issues caused by traditional, flammable liquid electrolytes. Typical solid-state electrolytes for Li-based batteries fall into two categories:(1) inorganic glass/ceramic/crystalline electrolytes and (2) polymer electrolytes. Inorganic electrolytes including some oxides, sulfides, NASICON-type, perovskite-type, garnet-type materials, etc. have promising conductivity2. However, these inorganic electrolytes are typically fragile and not bendable, and therefore cannot be used for flexible electronic devices. In contrast, polymer electrolytes such as polyethylene oxide (PEO), polyvinylidene fluoride (PVdF), Poly(methyl methacrylate) (PMMA), thermoplastic polyurethane (TPU), etc. are elastic and represent good candidates for electrolytes in flexible batteries. In this work, TPU/Lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) was selected as the polymer electrolyte. The conductivity and electrochemical window of this electrolyte are shown in Fig. 1. The 10 mS/m conductivity of the TPU/LiTFSI should be adequate for low-power-consumption flexible electronic devices. The 0~6.2 V electrochemical window is beneficial for most cathode materials for lithium-ion batteries. A LiNi1/3Co1/3Mn1/3O2 (NCM, 4.0 V vs. Li/Li+, specific capacity: 278 mAh/g) and a Li4Ti5O12 (LTO, 1.5 V vs. Li/Li+, specific capacity: 175 mAh/g) was used as cathode and anode materials, respectively, for the all-solid-state battery (Fig. 2). The LTO was chosen to be the anode because the TPU is sensitive to lithium metal, and the LTO has a relative higher potential vs. Li/Li+so is safe for the TPU electrolyte. The all-solid-state cell could be a promising candidate power source for the flexible electronic devices. (1) Zhou, G.; Li, F.; Cheng, H.-M. Energy & Environmental Science 2014, 7, 1307. (2) Bachman, J. C.; Muy, S.; Grimaud, A.; Chang, H.-H.; Pour, N.; Lux, S. F.; Paschos, O.; Maglia, F.; Lupart, S.; Lamp, P.; Giordano, L.; Shao-Horn, Y. Chemical Reviews 2016, 116, 140. Figure 1