Study Of Binders For Silicon-based Anodes And Lithium Compensation In Lithium-ion Batteries

Nowadays,lithium-ion batteries are widely used in the personal portable devices and electrical vehicles area,due to their advantages of high energy density,long cycle life and environmentally friendly characteristics.With the development of corresponding products and technology,the demand for high energy density lithium-ion batteries has been more and more urgent.So far,the anode materials in commercialized lithium-ion batteries are mainly graphite,with a capacity less than 400 mAh g^-1.Among various anode materials,silicon material has a far higher theoretical capacity(Li_4.4Si,4200 mAh g^-1)than graphite and moderate operating voltage?0.4 V vs Li/Li⁺?.Because of this,it has recently become a research hotspot for lithium ion batteries and is regarded as the anode material of the next generation lithium ion battery.However,silicon has an enormous volume effect during the charge/discharge process,giving rise to a series of problems including pulverization of electrodes and instability of solid electrolyte interface?SEI?,leading to deteriorated cycle performance.Therefore,developing a stable electrode structure during cycling is meaningful for the application of silicon material.As an important component of electrodes,the binder plays a key role in maintaining electrode structure stability.Moreover,in lithium-ion batteries,anodes will react with electrolytes to form SEI films during the first lithiation process,consuming a large amount of lithium and leading to a decrease of battery capacity and overall energy density.Especially for silicon-based electrodes,whose initial coulombic efficiency is lower than that of graphite,more lithium is consumed in the formation of SEI film,resulting in a more serious drop in energy density.Therefore,it is necessary to develop an efficient lithium compensation mechanism to improve the energy density of lithium ion batteries and to promote the development of silicon based electrodes.In terms of the above issues,this dissertation studied lithium ion batteries from two aspects of binders for silicon based anodes and lithium compensation.The detailed contents are as follows:1.A layer of polydopamine?PD?of about 2 nm was formed on the surface of silicon nanoparticles by in-situ polymerization of dopamine,and was used together with PAA binder to from a three-dimensional binding system with silicon particles as nodes in the electrode.On one hand,the O-diphenol hydroxyl groups in PD and the silicon hydroxyl groups on the surface of the silicon nanoparticles form hydrogen bonds.On the other hand,the imino groups in PD can react with carboxyl groups in PAA through condensation reactions to form chemical bonds,thus establishing solid connections.The binding system can effectively stabilize the structure of silicon electrodes during cycles and improve the cycle performance of the silicon electrodes.Compared with conventional Si/PAA,Si/CMC and Si/SA electrodes,the prepared Si@PD/PAA electrode exhibited excellent cycle performance under different silicon loadings,and thus guaranteed good rate performance.2 The silane coupling agent 3-aminopropyltriethoxysilane?APTES?was used as an additive to be added directly into the slurry during preparation of silicon-based electrodes,and used together with PAA to form a multi-dimensional network-like adhesive structure.Compared with PD coating,this method is simpler and not limited to the content of silicon in the electrodes and can be used for Si/Graphite hybrid electrodes.After addition of 0.1%APTES,the cycle performance of silicon electrodes was significantly improved with low silicon loadings(0.5-0.6 mg cm^-2)and high silicon loadings(2.0-2.1 mg cm^-2).In Si/Graphite hybrid electrodes,the electrode with 0.3%APTES maintained a reversible capacity of 2.0 mAh cm^-2 after 150 cycles,while the electrode without APTES was almost inactivated.In addition,APTES can enhance the mechanical strength of the electrode to improve the cycle performance when a small amount is added,while the mechanical strength of electrode after excess addition is too rigid to adapt to the change of volume and lead to the degradation of the electrochemical performance.3 Karaya gum?KG?and oxidized starch?OS?were used as binders for the Si-based electrodes and compared with conventional natural binders CMC and SA to study the relationship between their properties and electrochemical performance.Due to the multiple dimensional binding points from branched structure,OS and KG have higher binding strength than linear CMC and SA,and the corresponding Si electrodes exhibited better cycle performance.Due to its rigidity,KG had the best cycle performance in Si/Graphite electrodes with relatively small volume effect,and showed capacity of 870 mAh g^-1 after 100cycles while that of CMC and SA were 463 mAh g^-1 and 174 mAh g^-1,respectively.Due to its higher flexibility,OS had a significant advantage in high loading Si electrodes with remarkable volume effect,and retained the reversible capacity of 1909 mAh g^-1 after 120 cycles,while the capacities for CMC and SA were 888 mAh g^-1 and 658 mAh g^-1,respectively.Besides,since the dispersibilities of the conductive agent Super P in KG and OS solutions were superior to that of CMC and SA,the rate performance of corresponding silicon electrodes was also significantly improved.4 Li₂O₂ was studied as an additive in lithium nickel cobalt manganate?NCM?cathodes in lithium-ion batteries to offset lithium loss in anodes,and was used in NCM|graphite,NCM|SiO and NCM|Si/Graphite full cells.Under the catalysis of ball-milled NCM-6h,Li₂O₂ can be completely decomposed.This method allows one to realize highly efficient lithium compensation without residue and improve energy density of batteries.The improvement of lithium compensation in full batteries lies in two aspects.One is to increase the initial reversible capacity,and the other is to improve cycle performance.The lower the initial coulombic efficiency of the anode,the more significant improvement of initial capacity of the cathode.When the initial coulombic efficiencies are high,the lithium produced by the decomposition of 2%in the first cycle will improve the cycle performance and electrochemical reversibility in the subsequent cycles.In graphite full cells,addition of 2%Li₂O₂ increased the initial reversible capacity by 7.8%,and continued to stabilize the cycle performance in the next three cycles.The capacity of NCM cathodes during the steady cycles at 0.3C was increased from 137.2 mAh g^-1 to 166.0 mAh g^-1.