Modification Of Lithium-rich Manganese-based Cathode By Aqueous Binder And Study On Its Related Mechanism

In recent years,the vigorous development of lithium-ion batteries has been greatly promoted with the widespread use of new energy vehicles and various electronic products in people’s daily life.Lithium-rich manganese based cathode materials have quickly gained researcher’s interest because of their advantages such as high specific capacity,long life and low cost.However,during the charge-discharge process,these materials still have problems such as serious voltage attenuation,low first Coulomic effierency dissolution,and poor rate performance.Developing suitable binders is an effective strategy to modify lithium rich manganese based positive electrodes.Therefore,considering the important effect of the binders in the electrode components,this work develops single or composite aqueous binders to modify lithium rich manganese based cathode Li[Li_0.2Co_0.13Ni_0.13Mn_0.54]O₂（LRNCM）,and explores the relevant modification mechanisms.A new design concept and exploration ideas have been provided for the development of lithium-rich manganese based materials with high specific capacity and high stability.The main contents are as follows:（1）Carboxymethyl chitosan（CCTs）was investigated as aqueous binder to modify lithium rich manganese based cathode materials（LRNCM）.The effect of CCTs with different mass ratios（2.5%,5%,7.5%,10%）in the elctrode materials on the electrochemical performance of LRNCM and the modification mechanism have been explored.It is found that when the mass ratio of CCTs in the electrode is 5%,the prepared electrode shows a high initial discharge specific capacity of 241.20 m Ah g^–1 at 0.2C,and the capacity retention rate reaches 101.08%after 80 cycles.In addition,the discharge voltage of the Li-rich cathode modified by CCTs only decays by 0.32 V.The cycling performance and the voltage decay problem have been significantly improve due to the strong mechanical interlocking effect formed by CCTs with double helical structure on the surface of LRNCM,which increases the mechanical property of the particle surface and makes the electrode material stably attached to the current collector.（2）Humic acid（HA）was chosen to be combined with carboxymethyl chitosan（CCTs）to form a composite binder,and the effects of the mass ratios（1:2,1:1 and 2:1）of HA and CCTs on the performance of lithium-rich manganese-based cathode（LRNCM）and the modification mechanism were investigated.It is found that when the mass ratio of HA and CCTs in the composite binder is 1:1,the initial discharge specific capacity of the prepared electrode is 239.62 m Ah g^-1 at also 0.5C,and the capacity retention rate reached 82.79%after 200 cycles.The electrode exhibits excellent rate performance with a discharge specific capacity of 113.45 m Ah g^-1 at a high rate of 5 C.Compared with traditional PVDF binder,HA/CCTs composite binder effectively improves the cycling performance and rate performance of the electrode.HA is a mixture of concentrated nucleophiles,which are rich in conjugatedπ-bonds and contain a lot of polar functional groups.Therefore,the cross-linking reaction between with CCTs double helix structure and HA can form a tight three-dimensional conductive network between the active particles and conductive carbon particles,which enhances the conductivity and mechanical properties of the electrode system.（3）Carboxymethyl guar gum（CMGG）,which was produced by carboxymethylation reaction,was cross-linked with carboxymethyl chitosan（CCTs）to form a composite binder.The effects and of the the modification mechanism mass ratios（1:2,1:1 and 2:1）of CMGG and CCTs on the performance of lithium-rich manganese-based cathode（LRNCM）were investigated.It is found that when the mass ratio of CMGG and CCTs in the composite binder is 1:1,the initial discharge specific capacity of the prepared electrode is 240.95 m Ah g^-1 at 0.5 C,and the capacity retention reaches86.29%after 200 cycles.The electrode exhibits excellent rate performance with a discharge specific capacity of 113.45 m Ah g^-1 at a high rate of 5 C.Compared with HA/CCTs binder,the composite CMGG/CCTs binder can further improve the cycling performance and rate performance of the lithium-rich cathods.A large number of hydroxyl groups can be exposed on the surface of CMGG in the composite binder.Hydroxyl groups on the surface of CMGG in the composite binder result in the increase of hydrogen bonding sites.The composite binder formed by the cross-linking reaction between CMGG and CCTs can provide a great adhesive force between the active material,conductive agent and current collector,which maintains the structural stability of the electrode components during cycling.In addition,CMGG can accelerate the migration of lithium ions,which has a positive effect on improving the rate performance of electrodes.