Preparations And Electrochemical Performances Of The Anode Materials For Aqueous Rechargeable Batteries

In order to solve the current energy and environmental crisis,the development and utilization of the renewable energy,such as,wind farms,solar arrays and biomass energy,has become the important research issues.Due to the inherently intermittence,and scattered distribution properties of these new energy,it is extremely essential to develop safe and affordable large-scale energy storage systems to integrate these intermittent renewable energy sources to meet simple life and production needs.Aqueous rechargeable batteries（ARBs）,which is employing aqueous electrolyte and possessing the advantages of low cost,safety,easy to assemble,is considered as an ideal choice for the new large-scale energy storage technology.With the limitation of the narrow electrochemical window of water（～1.23V）,there are few high-performing anode materials,that can be used in ARBs.Therefore,the development of the high-performing anode materials is very important for preparing the ARBs with high capacity and long cycle life.In this contribution,we sythesized three advanced anode materials,i.e.,a core-shell Li Ti₂（PO₄）₃/C（PDA-c LTP）,a poly（2,5-dihydroxy-1,4-benzoquinone-3,6-methylene）（PDBM）,and phenazine materials,for ARBs.Firstly,with the help of the adhesion effect of polydopamine,a core-shell Li Ti₂（PO₄）₃/C（PDA-CLTP）was prepared by using the simple solid-state reaction route incorporating a poly-dopamine polymerization-coating method.The uniform carbon layer with a thickness of～7 nm was formed on the surface of Li Ti₂（PO₄）₃particles could improve the electrochemical stability of Li Ti₂（PO₄）₃in the aqueous solutions by inhibited the corrosion of H₂O and dissolved oxygen in water effectively.The PDA-c LTP anode delivered the high specific capacity of 117 m Ah g^-1at 1 C.In addition,even at 10 C,the PDA-c LTP anode still delivered the initial discharge specific capacity of 80 m Ah g^-1with the capacity retention of 76%after the long 1,000cycles,demonstrating the excellent rate capability and cycling stability.Secondly,in order to break through the limitation of the low theoretical specific capacity of inorganic anode materials,the poly（2,5-dihydroxy-1,4-benzoquinone-3,6-methylene）（PDBM）polymer anode material,which was also synthesized by the polymerization reaction,not only delivered the higher specific capacity than that of PDA-c LTP but also exhibited the wide applicability to various metal cations.In 1 M Na CH₃COO,1 M Mg（CH₃COO）₂and 0.5 M Al（CH₃COO）₂electrolyte,PDBM anode delivered the specific capacity of 140,227 and 201 m Ah g^-1,respectively.Additionally,in the Mg（CH₃COO）₂,Ca（CH₃COO）₂,Sr（CH₃COO）₂,Ba（CH₃COO）₂electrolyte,the specific capacity of PDBM anode was 227,242,220 and 251m Ah g^-1,respectively.What’s more,the redox potentials of the PDBM anode in these electrolytes delivered the trend of gradual decresment of with the increasing of the radius of the metal cations.Meanwhile,the PDBM anode exhibited excellent electrochemical performance with the specific capacity of 233 and 195 m Ah g^-1in 1 M Zn（CH₃COO）₂and 1 M Mn（CH₃COO）₂,respectively.The comprehensive DFT analysis showed that the monovalent Li⁺or Na⁺would prefer to occupy the in-plane sites respect to the benzene rings while the divalent or trivalent cations were prone to occupy the the out-of-plane sites during the electrochemical coordinating reactions.It was the novel electrochemical mechanism of the divalent or trivalent cations coordinating with PDBM that was favorable for using more active site in PDBM molecule due to their relatively large charge density.Meanwhile,it also demonstrated that the redox potential of PDBM anode decreased with the decreasing of the electrochemical reduced PDBM with the metal cations.Considering the existing different electrochemical mechanisms of metal cations with different valence coordinating with PDBM anode,we further explored the electrochemical coordinating mechanism when the PDBM anode was charging/discharging in the mixed electrolyte containing both monovalent Li⁺and divalent Zn²⁺.A superior higher specific capacity and longer cycling life(255 m A h g^-1and 83%capacity retention for 700 cycles at 1 C)were obtained in a mixed 1 M Li₂SO₄//2 M Zn SO₄electrolyte,which was better than that in a 1 M Li₂SO₄(150 m A h g^-1and 76%for 700 cycles)or in a 2 M Zn SO₄electrolyte(206 m A h g^-1and 76%for 700 cycles).It demonstrated a defined synergistic mechanism of Li⁺and Zn²⁺during redox reactions.When coupled with Li Mn₂O₄cathode,the PDBM‖1 M Li₂SO₄//2 M Zn SO₄‖Li Mn₂O₄full battery delivered an average discharge voltage of 1.15 V,,a long lifespan of over 700 cycles（i.e.,capacity retention,81%at 1 C）and a high energy density of 94.6 Wh kg^-1.Thirdly,due to the exremely low electronic conductivity and its poor adhesion effect with conductive carbon,it leads to the low utilization rate of active mass of PDBM.In the paper,a nano‐phenazine@Ketjen black（n PZ/KB）composite was achieved by the simple in‐situ dissolution‐precipitation method.Ketjen black（KB）possessed the highest specific surface area(1386 m²g^-1),total pore volume(2.54 cm³g^-1)and electron conductivity(458 S cm^-1).It enabled the high specific surface area(277.6 m²g^-1)and total pore volume(0.71 g cm^-3)and electron conductivity(87.0 S cm^-1)of the n PZ/KB composite after the nano-phenazine precipitating on the highly conductive and hierarchical porous KB surface.Hence,the inherent properties of the n PZ/KB anode could satisfy the cations and electrons transferring during the electrochemical redox reaction to ensure the high specific capacity and rate capability of the n PZ/KB anode.Meanwhile,the insolubilities of both phenazine and its reduction products in aqueous electrolytes and the excellent structure stability of phenazine crystal ensured the ultralong cycling stability of the n PZ/KB anode.In the 6 M KOH electrolyte,the n PZ/KB anode exhibited the high specific capacity of 286 m Ah g^-1,of which the utilization rate of the active mass was close to 100%.Even under the ultrahigh rate of 100 C,its specific capacity also reached up to 102 m Ah g^-1and achieved an ultralong cycle life of 100,000 times with the high-capacity retention of76%.Additionally,according to the XRD results,each of the interplanar spacing in phenazine crystal was large enough to the diffusion of the Mg²⁺,Ca²⁺and Zn²⁺during the electrochemical redox reactions.So these divalent cations(Mg²⁺,Ca²⁺and Zn²⁺)could diffuse into the phenazine crystal to coordinate with the C=N active site.The n PZ/KB anode also delivered the good electrochemical performance in the neutral 1 M Mg（CH₃COO）₂,1 M Ca（CH₃COO）₂and 2 M Zn SO₄,respectively.Namely,at 0.2 C,the specific capacity of n PZ/KB anode was 225,224 and 230 m Ah g^-1respectively.Furthermore,when the current density increased to 10 C,the n PZ/KB still delivered the high cycling stability（the capacity retention 70%,51%and 86%after 10,000cycles,respectively）.