A Novel Organic Polyanionic Anode Material In Potassium Ion Batteries

Potassium-ion batteries（PIBs）are gradually attracting intensive attentions as an alternative energy storage system for lithium-ion batteries due to the abundance of potassium（K）element in earth.Great progress has been made by using inorganic anode and cathode materials for PIBs.However,inorganic electrodes usually contain toxic and expensive transition metals.More importantly,the large size of K⁺will lead to volume expansion and even structure crushing of inorganic electrodes during the intercalation-deintercalation process,resulting in reduced capacity and poor cycling performance.Delightfully,organic electrode materials have the advantages of low cost and environment friendly.Meanwhile,the void space between molecules of organic materials is conducive to the intercalation of K⁺ions.However,small organic molecules have serious dissolution problems in organic liquid electrolytes,resulting in fast capacity decay and short battery life.More importantly,the kinds of organic cathode and anode materials suitable for PIBs are very rare.In this thesis,a new polyanionic organic anode material namely potassium naphthalene-1,4,5,8-tetracarboxylate（K₄NTC）is designed and synthesized,and its electrochemical performance in PIBs is investigated.The multiple strong ionic bonds（O-K bond）in K₄NTC structure show great insolubility in ether-based electrolyte,thus realizing a stable cyclelife in PIBs.In the following sections,small organic molecule K₄NTC is prepared by one-step acid-base neutralization reaction.After purification,its characterization and solubility test are carried out.It is found that K₄NTC with polyanionic properties is almost insoluble in nonaqueous electrolyte of dimethoxyethane（DME）.Then,the influence of conductive carbon additives and binders on the electrochemical performance of K₄NTC is investigated.The reasons for the electrochemical performance of K₄NTC in different electrolyte systems are also discussed.For example,the possible reasons why the performance of K₄NTC in ether electrolyte is better than ester electrolyte in half cell are discussed from three aspects of dielectric constant,coordination capacity and coordination number.Afterwards,the electrochemical results of different electrolyte solutions show that the discharge capacity of K₄NTC is 123 m Ah/g at a small current density of 50 m A/g with 1M KPF₆ DME electrolyte,and the discharge capacity of K₄NTC is 97 m Ah/g at a high current density of 1 A/g.After 500 cycles,the capacity retention of K4NTC is 69%.Finally,an all-organic PIB is constructed by using perylene 3,4,9,10-tetracarboxylic dianhydride（PTCDA）as the organic cathode and the reduced state（K₆NTC）of K₄NTC as the starting organic anode.The full cell can achieve a discharge capacity of 121m Ah/g_anode at a low current density of 20 m A/g and a stable lifespan over 1500 cycles at a high current density of 500 m A/g in the voltage range of 0.5-3.0 V using 1M KPF₆ in DME.