Novel polyimide binders integrated with soft and hard functional segments ensuring long-term high-voltage operating stability of high-energy NCM811 lithium-ion batteries up to 4.5 V

High-nickel layered oxides, like LiNi0.8Co0.1Mn0.1O2 (NCM811), are promising cathode material candidates for high-energy lithium-ion batteries (LIBs). However, the serious cathode-electrolyte interface reactions and cathode degradation usually lead to poor rate performance and pronounced capacity degradation, especially at high voltages (>4.3 V). Herein, two novel aromatic polyimide (PI) binders integrated with soft and hard functional segments, denoted as PI(OBO) and PI(BBP), were designed and used as NCM811 cathode binders, which possesses excellent thermal stability and provides abundant active sites to ensure excellent adhesion strength, and dramatic ion pathway. The NCM811 half-cells using PI(OBO), PI(BBP) and conventional PVDF binders exhibit rate capability of 138.0, 127.7 and 115.1 mA h g−1 at 5 C, respectively, within the voltage range of 2.5 to 4.3 V. Further test under higher cutoff voltages up to 4.5 V indicates that the NCM811 half-cells depending on PI(OBO) and PI(BBP) binders display remarkable cycling stability with 39% and 62% capacity retention after 100 cycles at 0.2 C, which are much higher than the 25% capacity retention of the PVDF analogues. CV, XPS, XRD, TEM data confirm that the synthesized PI binders form a uniform and continuous coating layer on the surface of NCM811, obviously stabilize the layered structure of NCM811 and inhibits the irreversible phase transformation of the NCM811 to its rock-salt form, consequently achieve superior long-term cycling stability at high-voltage operation. The DSC test indicates that the heat release of fully-charged PI(BBP)-NCM811 and PI(OBO)-NCM811 cathodes is 66.19 and 71.64 J g−1, which is much lower than PVDF-NCM811 cathode (108.01 J g−1) during heat treatment to 300 °C, demonstrating the superiority of the synthesized PI binders in improving the safety of LIBs. This study gives a valuable insight of advanced novel PI binder materials for high-energy and high-safety LIBs.