Design,Preparation And Application Studies Of Binders For Silicon-Based Anodes In Lithium-Ion Batteries

Lithium-ion batteries have been widely used in electronic products,electric vehicles,artificial intelligence,large-scale energy storage and other fields due to its portability,recyclability,longevity and environmental friendliness.However,given the rapid development of emerging technologies and the pressing demand for energy conservation and emission reduction,it has been extremely urgent to improve the energy density of lithium-ion batteries without safety concerns.Silicon has a very high specific capacity（3579 m Ah/g）and is one of the ideal anode materials,but it still faces many challenges in practical setting.In particular,the dramatic volume changes（up to300%）of silicon particles in lithiation-delithiation process can easily result in a series of problems such as self-crushing,repeated fracture and regeneration of SEI film on the surface,and collapse of electrode structure,which leads to rapid capacity fading and thus seriously hinders its commercial application.One of the most direct solutions is to use binder,since it can effectively connect the electrode components through physical entanglement,chemical bonding and other effects to ensure the structural integrity of the electrode.At present,commercial binders for anodes,such as sodium carboxymethyl cellulose（CMC）and styrene butadiene rubber（SBR）,do not form strong enough forces with active materials,to adapt the drastic volume change of Si.As such,they are not suitable for high specific capacity silicon anode.By gradually increasing the number of active sites between the binder and Si particles（linear-double helix）and modes of action（physical tangles and chemical bonding）,introducing conductive functions and other strategies,we are able to effectively retain the integrity of the charge transport network,suppress the volume changes of electrode in the delithiation-lithiation processes,thereby reducing the SEI rupture and regeneration and improving the electrochemical performance of silicon-based anode.We summarize the research findings as follows:（1）α-cyclodextrin（α-CD）and lithium oxalate were introduced into the long chain polysaccharide sodium alginate（Alg）to construct a hydrogen-bond network adjusting binder（Alg-α-Li-1）for Si/C anode.The addition of small moleculesα-CD or lithium oxalate to Alg would dissipate the hydrogen-bond and enhance the flexibility of the binder network,thereby improving its resilience in case of thevolume changes of silicon particles in the lithiation and delithiation processes.Meanwhile,the addition ofα-CD can effectively reduce the interface affinity between electrode and electrolyte,and lithium oxalate can provide extra Li⁺for electrode and improve the Li⁺diffusion coefficient of electrode.By adjusting the ratio ofα-CD and lithium oxalate,the Si/C@Alg-α-Li-1 electrode can achieve a balance of mechanical properties and Li⁺diffusion,and exhibit good long cycle performance and large rate performance.The capacity of Si/C@Alg-α-Li-1 electrode exhibits 826.4 m Ah/g even at 5 A/g,1.03m Ah/cm².（2）Double helix structure xanthan gum（XG）has more action sites with Si,so it has stronger adhesion to Si.At the same time,in order to ensure the integrity of the electronic channel in the electrode,the conductive polymer（3,4-ethylene dioxythiophene）:polystyrene sulfonate（PEDOT:PSS）was introduced into the binder.By thermal crosslinking between XG and PEDOT:PSS,the conductive binder c-XG-PEDOT:PSS was constructed.The study found that the-OH on the main chain of double helix XG effectively combined with the-SO₃H in PEDOT:PSS after crosslinked.A dense and strong three-dimensional crosslinking network structure was endowed to c-XG-PEDOT:PSS,which possesses the good electrical conductivity,the binding force of electrode between the coating and the current collector is also significantly improved and could better withstand the volume changes in lithiation-delithiation process.In addition,the conductive binder is beneficial to the Li⁺diffusion and improves the reaction kinetics of the electrode.Attributed to the better electronic conductivity of the binder,no conductive agent is required for the electrode.In electrochemical performance characterization,Si/C@c-XG-PEDOT:PSS electrode delivers a high initial coulombic efficiency of 92.27%,the capacity still remains at 719.1 m Ah/g at500 m A/g,2.1 m Ah/cm².At the same time,the binder can effectively inhibit the side reaction between Si/C and electrolyte under the condition of lithiation,and the battery has no significant self-discharge at room temperature and 55℃for about 1000 h.（3）Although XG and Si have abundant binding sites,the force between XG and Si is still weak.Therefore,guar gum（GG）was introduced into XG to obtain composite binder X/G,which improves the adhesion between the coating and current collector.Based on this,conductive binder for thick electrodes c-X/G-PEDOT:PSS was further constructed by combining the advantage of X/G and PEDOT:PSS.The binder is beneficial to the formation of a stable electrode/electrolyte interface.As a result,electrode surface is smooth and no obvious cracks after the long cycle,and the structure of binder is clear and continuous.When the proportion of Si/C was up to 95 wt%and the corresponding loading was up to 4.1 m Ah/cm²,the electrode shown a high reversible capacity of 594 m Ah/g,and there was no significant capacity loss after 50cycles.Moreover,freeze-drying method was used to fabricate a thick electrode（Si/C areal capactiy:6.67 m Ah/cm²）,the capacity of the thick electrode could maintain at566.7 m Ah/g after 100 cycles.（4）A composite binder is prepared with a soft component,guar gum（GG）,and a rigid polymer,anionic polyacrylamide（APAM）.Rich hydroxy,carboxyl and amide groups on the polymer chains not only enable intermolecular crosslinking to form a web-like binder,A2G1,but also realize strong chemical binding as well as physical encapsulating to Si particles.The resultant electrode shows limited thickness change of merely 9%on lithiation,and almost recover its original thickness on delithiation.As a result,the Si@A2G1 electrode exhibits 2104.3 m Ah/g at a high current density of 1.8A/g after 100 cycles at a high Si content（80 wt%）,low binder content（10 wt%）and areal capacity of 2.88 m Ah/cm².The Si@A2G1 electrode can also cycle stably for 392cycles at a constant capacity of 1000 m Ah/g,which is much higher than the Si@GG and Si@APAM electrodes.It also delivers excellent rate performance,with a reversible capacity of 966 m Ah/g at a very large current density of 10.8 A/g.In conclusion,in this paper,by regulating the molecular chain configuration（linear-double helix-three-dimensional network）and mechanical strength of binder,gradually increasing the number of the interaction sites between binder and Si-based anodes,and introducing conductive functions and other strategies,we are able to achieve the stable cycling performance of Si-based anodes with different areal capacities and different theoretical strain values.Notably,all binders are water-soluble and are mainly composed of environmentally friendly natural polymers.This should provide a foundation for future applications.