Elastic Anode Binders Based On Modified Poly(acrylic Acid) For Lithium Ion Batteries

The need for electric vehicles and energy storage devices has spurred the continued development of high-energy density lithium-ion batteries.At present,lithium-ion batteries with graphite and lithium titanate oxide used as the anodes have been successfully commercialized.However,the low theoretical specific capacity of such anode materials restricts the further improvement of energy density.Silicon,as a novel kind of anode materials,has attracted much attention because of its extremely high theoretical specific capacity of 4200 m Ah g^-1,almost 10 times that of graphite.The major challenge associated with the development of silicon anode is the huge volume expansion during the charging/discharging cycles,as lithium ions are embedded/disembedded in the lattice of silicon crystals,which causes damage to the electrode structure,lowers the stability and long-term cycling performance.Although many strategies have been demonstrated to prepare silicon materials with various morphologies to improve the cycling stability,the development of an appropriate binder for silicon-based anodes is widely accepted as an effective approach to tackle the problem of volume expansion.Moreover,the development of high-performance binder materials also contributes to simplify the production process,controls the production cost,achieves large-scale production,and lays the foundation for the application of silicon anode in commercialization.In this dissertation,we aim to develop a series of novel binder materials for silicon anode via the modification of polyacrylic acid(PAA),a linear polymer of rich carboxylic acid groups that can interact with silicon materials.The design rationale is to alter the mechanical properties and elasticity of PAA to accommodate with the huge volume change of silicon anodes and thus improve the stability and long-term cycling performance.Our results showed that the physical and electrochemical properties of PAA-based binders can be achieved through both physical and chemical modificatons,which might provide insights for the design of new binders with low cost and high efficiency for large volume change of electrode materials.The major conclusions of this dissertation are briefed as follows:(1)In the first study,two commercially available polymers of PAA and PEO were simply blended in different proportions by the solvent mixing process to prepare a series of PAA-PEO complexes with hydrogen bonds as the physical crosslinking interactions.Various kinds of tests,including infrared spectroscopy and X-ray diffraction,were applied to prove that PAA and PEO formed a homogeneous and stable complex.The addition of flexible PEO improved the ionic conductivity and elasticity of the complex.Tensile tests showed that the resulting PAA-PEO complexes could withstand larger deformation without breaking than PAA.At the same time,the addition of PEO hindered the interaction between some carboxylic acid groups and the silicon nanoparticles,leading to decreased peeling strength with increasing PEO content.Specifically,silicon electrodes made with the PAA-PEO7.5 binder,which contained 7.5 wt%PEO,showed largely imporoved electrochemical cycling performance,which indicated that several key parameters,such as ionic conductivity,elasticity,and adhesion strength,reached an optimized balance for this particular binder material.After 100 cycles at 0.2 C and 0.5 C rates,the specific capacities of the silicon electrodes maintained at 2346 m Ah g^-1 and 1885 m Ah g^-1,respectively,which were much higher than that of silicon electrodes made with pure PAA.(2)We further prepared a series of random copolymers of P(AA-EG)by thermally initiated free radical polymerization of acrylic acid and a macromolecular monomer,namely,polyethylene glycol acrylate monomethyl ether(m OEG-AA),and evaluated their performance as the binder materials for silicon electrodes.It was reasoned that hydrogen bonding formed between the carboxylic acid groups and polyethylene glycol chains in the copolymers,which provided physical crosslinking sites for enhanced elasticity.Peeling tests showed that in sharp contrast to the PAA-PEO blends,the addition of EG units in P(AA-EG)did not significantly reduce the adhesion strength,which was favorable for maintaining the electrode integrity and improving the loading of active materials on the electrode.Moreover,tensile tests showed that P(AA-EG)could withstand up to 80%deformation without breaking,which helped to respond to the repeated expansion and contraction of silicon particles during the process of charge and discharge.Specifically,electrochemical tests showed that the specific capacity of the silicon electrode made with P(AA-EG7.5)remains 2697 m Ah g^-1 and2453 m Ah g^-1 after 100 cycles at 0.2 C and 0.5 C rates,respectively.The capacity declined slowly in the subsequent cycles until 200 cycles with a less than 4 m Ah g^-1 loss per cycle.In addition,electrodes made with P(AA-EG7.5)and a higher silicon loading of 0.8 mg/cm²could maintain a specific capacity up to 2388 m Ah g^-1 at 0.2 C rate after 100 cycles.The excellent performance of P(AA-EG7.5)might provide a promising binder material to achieve rapid charge and discharge of silicon anodes.(3)The excellent performance of P(AA-EG)binders inspired us to further explore the copolymerization strategy for the development of novel binder materials for silicon anodes.To tune the structure and properties of the PAA backbone,one possible method can be the introduction of a new comonomer.To elucidate the effects of the comonomer on the physical properties of the resulting copolymers,we studied a series of polymeric complexes of P(A6ACA-AA)-PEO,which were prepared by the photo-initiated polymerization of a mixed solution of linear polyethylene glycol(PEO),copolymer acrylic acid(AA)and N-acrylacyl-6-aminohexanoic acid(A6ACA)in water.The resulting complexes were optically transparent and highly elastic.Dynamic hydrogen bonds and chain entanglement formed between PEO and P(A6ACA-co-AA)in the complexes,resulting in a three-dimensional and highly elastic network structure.The resulting elastomers exhibited high strength up tp 1.73 MPa and high breaking strain up to 2400%.More importantly,the addition of A6ACA as a comonomer reduced the glass transition temperature of the PAA chain,promoted the chain mobility and the restructuring of the hydrogen bonding,thus enabling excellent self-healing properties.Under the umbient conditions(25?and 60%relative humidity),P(A6ACA-AA)-PEO elastomers could recover to their mechanical strength and elasticity of the original state within 24 h.Therefore,this strategy of combining flexible A6ACA comonomers into the PAA chain might provide binder materials with better elasticity and self-healing capability,which would be especially more attractive for silicon-based materials with large volume changes.(4)In the last section,we briefly studied the electrochemical cycling performance of silicon electrodes made with two binder materials based on the compolymers of AA and A6ACA.The P(AA-A6ACA)copolymers were prepared by random free radical polymerization,and were also used to blend with linear PEO to form ternary complexes.As the initial trials,we tested silicon electrode made with P(AA-A6ACA7.5).The reversible specific capacity of Si/P(A6ACA7.5-AA)electrode is 2392 m Ah g^-1 after 100 cycles,and the capacity decreases slowly in the subsequent cycle process until 200 cycles with a specific capacity of 2031 m Ah g^-1,which shows a robust electrochemical stability.Further,we introduced PEO as the third component to combine the flexible chains and ionic conductivity in order to obtain binder materials with better performance.PEO was added to the copolymer through solvent blending and then physically doped to prepare a new ternary compound of P(A6ACA7.5-AA)-PEO.The electrochemical performance of this kind of composite materials was studied as binders for silicon electrodes.The binder of ternary complex has multiple hydrogen bond binding sites,and the chain mobility of flexible A6ACA and PEO chains provides elasticity as well as a condition to facilitate the reconstruction of hydrogen bonds.Thus 5 w%PEO in the ternary compound was proved to be the most effective binder to maintain the specific capacity of Si/P(A6ACA7.5-AA)-PEO5 electrode,due to the balance of elasticity,hydrogen bonds,mobility of flexible chains,conductivity.These results inspire us to study polymer binders with flexible chains,rich functional groups,adhesiveness,conductivity and other functions,and this strategy of introducing new monomers with random copolymerization is expected to provide more space for the design and development of modified PAA-based binder materials.