Study On Preparation And Performance Of Polyimide Separator Containing Carboxyl Groups

With the growing oil crisis in the international community,the need to develop clean new energy sources is becoming more and more urgent.Lithium-ion batteries(LIBs)are widely used in electronic products and new energy vehicles as new and efficient energy storage devices.As an important component of LIBs,the performance of the separator is closely related to the performance of the LIBs.At present,LIBs mainly use polyolefin separators,but the low melting temperature and surface inertness of polyolefin separators lead to poor thermal stability and electrolyte wettability,while their lithium ion transport performance is poor(lithium ion transfer number in the range of0.2-0.3),limiting the application in high-safety high-performance lithium-ion batteries.Polyimide(PI)are polymers with excellent chemical stability and thermodynamic properties.The low thermal shrinkage of polyimide separators at high temperatures greatly improves the safety of LIBs and the abundant polar bonds in the polyimide molecular chain have good affinity for polar electrolytes.In addition,polyimides are structurally diverse and can be designed to introduce functional groups through the molecular structure,thereby improving the electrochemical performance of the separator.Non-solvent induced phase transition(NIPS)prepared porous PI separators feature adjustable pores,high porosity and high electrolyte absorption,and their zigzag through sponge-like pore structure facilitates long cycling in lithium-ion batteries at high current densities.Therefore,polyimide porous separators with designable structure and morphology have great potential for the manufacture of safe and high-performance lithium-ion batteries.Based on the above background,this paper firstly designs soluble polyimide containing carboxyl(-COOH)groups and prepares polyimide porous separators by a non-solvent induced desired transformation process.The effect of solidification bath process parameters on the separator morphology is investigated,and the effect of the presence of-COOH on the lithium ion transport performance of the separator and the safety of the polyimide separator at high temperatures is also investigated.The morphology,thermal stability and electrochemical properties of the separator during alkali etching were then analysed by increasing the-COOH content of PI separators based on carboxylated polyimide separators.Finally,nano-aramids(ANFs)were introduced into the PI porous membrane through a co-blending method,and the effects of the introduction of ANFs on the morphology,mechanical properties,electrochemical properties and lithium dendrite deposition of the PI separators were investigated.The main findings are as follows.(1)Soluble polyimides containing-COOH were synthesized by molecular structure design,and a series of PI separators with different morphologies were formed and the optimum solidification bath parameters were determined by modulating the solidification bath parameters in the NIPS process.The increase of the good solvent content in the solidification bath resulted in a transition from finger-like pores to a sponge-like pore structure.The PI separator with sponge-like porous structure has a porosity of 75.24% and electrolyte absorption of 343.99%.The presence of polar bonds in the PI molecules improves the electrolyte wettability of the separator.The PI separator also has excellent thermal stability and fire resistance,maintaining dimensional integrity even at 200°C.In addition,it was found that the-COOH in the PI separator plays a role in promoting the migration of lithium ions,and this role is enhanced with the increase of-COOH content.(2)In order to further improve the lithium ion transport performance of PI separators,the PI molecular chains were subjected to ring opening by alkali etching to allow more-COOH to appear on the surface of the separator thereby increasing the lithium ion transfer number.The alkali etching greatly improves the lithium ion transport performance of the separator without affecting the spongy cross-sectional pore structure of the separator,with lithium ion transfer number up to 0.69,ionic conductivity and electrolyte absorption also being effectively improved.(3)In order to further improve the mechanical strength of the separator,nano-aramid was introduced into the PI porous separator and the effect of the addition of ANFs on the mechanical strength,electrolyte wettability and electrochemical properties of the separator was investigated.The results show that the highest tensile strength(21.35 MPa)was achieved at 0.5% ANFs addition.At the same time,the higher polarity of ANFs improves the electrolyte wettability of the ANFs/PI composite separator.And the presence of the ANFs network promotes the uniform distribution of lithium ions and achieves the uniform deposition of lithium dendrites.