Organic Electrode Materials Based On Dihydroxybenzoquinone

Conventional Li-ion batteries based on inorganic cathode materials have been successfully commercialized for decades of years.However,there are growing concerns on not only the electrochemical performance breakthrough,but also the resource sustainability of transition metals including cobalt and nickel.As an alternative,organic electrode materials are expected to play an important role in the next generation of effective and sustainable rechargeable battery technologies,due to the various advantages including promising electrochemical performance,high resource sustainability,wide applications,and low environmental footprint.Among all kinds of organic electrode materials,those based on conjugated carbonyl groups,especially quinone groups,have been intensively investigated as cathode for rechargeable Li Na and K batteries.However,most of conjugated carbonyl-based electrode materials have high solubility in organic electrolytes,which limits their practical applications.It is crucial to enhance the inherent insolubility of organic electrode materials for reliable cycling stability.Two strategies have been employed to modify the molecule structure for this goal:polymerization for higher molecular weight and salinization for stronger intermolecular interaction by O···M···O（M=Li,Na,K）bonding.However,it is a huge challenge to synthesize high-molecular-weight polymers with high quinone density for desirable reversible capacity and cycling stability,especially when taking the accessibility and cost into account.For instance,benzoquinone（BQ）is an attractive building block for high-energy polymeric cathode materials,but unfortunately very difficult to be polymerized due to the relatively high oxidizability.As an alternative,dihydroxybenzoquinone（DHBQ）is a more feasible electroactive unit to construct polymers although with reduced theoretical capacity(383 m Ah g^–1)and redox potential.Moreover,it can be even produced from the decomposition of celluloses,satisfying the desire to synthesize electrode materials from biomass toward sustainable and green batteries.Therefore,in this paper,we modified dihydroxybenzoquinone to improve its electrochemical performance by polymerization and salification.The main research contents and results are as follows:（1）A high-performance polymeric lithium salt electrode material based on dihydroxybenzoquinone.A novel polymeric lithium salt,namely dilithium salt of poly（2,5-dihydroxy-1,4-benzoquinone-3,6-methylene）（Li₂PDBM）,which is easily synthesized by phenol-formaldehyde condensation,followed by lithiation in Li OH aqueous solution to eliminate the negative effect of phenol groups in PDBM.Benefiting from the high theoretical capacity(327 m Ah g^–1),structure stability,insolubility,and redox reversibility,Li₂PDBM exhibits superior electrochemical performance as a cathode for rechargeable lithium batteries,including high reversible capacity(256 m Ah g^–1),high rate capability(79%@2000 m A g^–1),and high cycling stability(77%@2000^th cycle).Beside the cost-effective electrode material,this work also provides important mechanism understanding on the structure-performance relationship of carbonyl-based electrode materials,especially those with—OH or—OM（M=Li,Na,K）substituents.（2）The influence of crystal water on the electrochemical performance of dilithium salt of dihydroxybenzoquinone named Li₂DHBQ（Li₂C₆H₂O₄·2H₂O）.We find that the small-molecule lithium salt can also show high cycling stability,and the crystal water may have important influence on its electrochemical performance.The two kinds of dilithium salt of 2,5-dihydroxy-1,4-benzoquinone（Li₂DHBQ and Li₂DHBQ＿dehydrated）are synthesized by salification to be applied in rechargeable lithium batteries.Compared with DHBQ and Li₂DHBQ＿dehydrated,Li₂DHBQ exhibits excellent electrochemical performance,including a higher reversible specific capacity of 256 m Ah g^–1,high rate capability(68%@1000 m A g^–1),and high cycling stability(84%@100^th cycle,50 m A g^–1),which shows that appropriate crystal water can promote the reaction kinetics and cycling stability of this kind of organic lithium salts.Besides,this work provides important insights for understanding the influence of crystal water on cycling stability.