基于改性聚酰亚胺的锂离子电池正极粘结剂制备与性能研究

锂离子电池具有高能量密度和低自放电率等特点，在可穿戴电子产品、新能
源汽车等领域具有广阔的应用前景。然而各种应用终端对轻量化和长续航的极致
追求使得开发高能量密度和长循环寿命的锂离子电池更为迫切。基于高电压平台
的镍钴锰酸锂（NCM）正极可显著提高锂离子电池的能量密度，然而 NCM 正极材料也存在缺陷，如循环过程中活性颗粒结构变化大、电极极片结构不稳定，严重影响电池的循环稳定性和循环寿命。开发适配于高电压 NCM 正极材料的新型粘结剂，对于进一步提高锂离子电池的电化学性能至关重要。本论文致力于开发基于改性聚酰亚胺共聚物的正极粘结剂，利用聚酰亚胺的高强度和高稳定性以提高正极极片的结构稳定性，从而提升电池的循环稳定性。本论文的具体研究内容如下：
（1）设计合成了聚酰亚胺-聚氨酯-聚二甲基硅氧烷的软硬段相结合的共聚物
（PI-PU），并将其作为粘结剂用于制备正极极片，显著提升了锂离子电池的电化学性能。以 PI-PU 为正极粘结剂制备的多晶 NCM811/Li 电池在 3.0 - 4.3 V 的电压区间内以 0.2 C 电流进行循环充放电，首圈放电比容量为 204 mAh/g，循环 100 周后的放电比容量为 168 mAh/g，其容量保持率为 83 %。使用扫描电子显微镜观察循环 100 周后的极片微观形貌，发现使用 PI-PU 粘结剂制备的极片裂纹很小，并且电极表面仍然光滑平整。相比之下，使用 PVDF 粘结剂的极片则显示出显著的大裂纹，且有明显的活性物质剥离脱落现象。这主要是因为 PI-PU 粘结剂的特殊软硬段相结合的分子结构特点，赋予其高机械强度和柔韧性，有利于保持极片在长期循环过程中的结构稳定性和完整性。
（2）为进一步提升基于聚酰亚胺类粘结剂的高电压稳定性和长循环性能，本
论文设计合成了聚砜-聚酰胺-聚酰亚胺共聚物（SPI），将其作为粘结剂应用于高电压正极的制备。该粘结剂的粘结性能与电化学性能均较为优异。以 SPI 作粘结剂制备的正极极片剥离强度达到 390 N/m。基于 SPI 正极粘结剂组装的单晶 NCM811/Li电池在 0.2 C 的电流密度下进行循环，首圈放电比容量为 176 mAh/g，循环 300 周后的电池容量保持率达到 80 %，且循环之后的放电比容量为 141 mAh/g。以 SPI为正极粘结剂组装的多晶 NCM811/Li 电池以 0.2 C 的电流密度进行充放电，初始放电比容量为 202 mAh/g，在经过 100 周循环后为 182 mAh/g，其容量保持率达 90%。使用透射电子显微镜观察正极极片，可见 SPI 粘结剂在 NCM 颗粒表面形成一层均匀、致密且连续的包覆层，这对于提高电池的电化学性能大有裨益。

Lithium-ion batteries are characterised by high energy density and low self-discharge rate, and have broad application prospects in wearable electronic products, new energy vehicles and other fields. However, the pursuit of lightweight and long-lasting endurance in various application terminals makes the development of lithium-ion batteries with high energy density and long cycle life more urgent. Nickel-cobalt-manganese lithium (NCM) cathodes based on high-voltage platforms can significantly increase the energy density of lithium-ion batteries. However, NCM cathode materials also have defects, such as large changes in the structure of active particles during cycling and unstable electrode structure, which seriously affect the cycling stability and cycle life of the battery. The development of new binders compatible with high-voltage NCM cathode materials is crucial for further improving the electrochemical performance of lithium-ion batteries. This paper is dedicated to the development of positive electrode binders based on modified polyimide copolymers, utilizing the high strength and stability of polyimide to enhance the structural stability of positive electrode sheets, thereby improving the cycling stability of batteries. The specific research contents of this paper are as follows:
(1)A copolymer of polyimide-polyurethane-polydimethylsiloxane (PI-PU) with combined soft and hard segments was designed and synthesized, and it was used as a binder for preparing cathode electrodes, significantly enhancing the electrochemical performance of lithium-ion batteries. Lithium-ion batteries based on NCM811/Li with cathode electrodes prepared using PI-PU as the binder were cycled at a current rate of 0.2 C in the voltage range of 3.0 to 4.3 V. The initial discharge specific capacity was 204 mAh/g, and after 100 cycles, it decreased to 168 mAh/g, with a capacity retention of 83 %. Scanning electron microscopy observations of the cathode electrodes after 100 cycles revealed minimal cracking and a smooth surface when using PI-PU as the binder, whereas electrodes with polyvinylidene fluoride (PVDF) binder showed significant cracking and noticeable detachment of active materials. This is mainly attributed to the unique molecular structure of PI-PU binder, with its combined soft and hard segments, providing high mechanical strength and flexibility, which helps maintain the structural stability and integrity of the electrodes during long-term cycling.
(2) To further improve the high voltage stability and long cycling performance basedon polyimide binder, this paper designs and synthesizes a copolymer of polyether sulfonepolyamide-polyimide (SPI) as a binder for the preparation of high voltage cathodes. The bonding and electrochemical properties of this binder are both excellent. The peel strength of the cathode electrode prepared with SPI as a binder reaches 390 N/m. Monocrystalline NCM811/Li batteries assembled with SPI as the cathode binder show a first-cycle discharge specific capacity of 176 mAh/g at a current density of 0.2 C, and after 300 cycles, the discharge specific capacity remains at 141 mAh/g, with a capacity retention rate of 80 %. Polycrystalline NCM811/Li batteries assembled with SPI as the cathode binder exhibit an initial discharge specific capacity of 202 mAh/g at a current density of 0.2 C, which decreases to 182 mAh/g after 100 cycles, with a capacity retention rate of 90 %. Transmission electron microscopy observation of the cathode electrodes reveals that the SPI binder forms a uniform, dense, and continuous coating layer on the surface of NCM particles, which greatly benefits the improvement of the battery’s electrochemical performance.

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