Modification And Performance Investigation Of Gelatin-based Hydrogel Electrolyte For Rechargeable Zinc-ion Batteries

MnO₂-based aqueous zinc-ion secondary batteries（Re AZMBs）have the advantages of environmental friendliness,low cost,high safety,and have broad application prospects in the field of energy storage and wearable electronic products.Conventional Re AZMBs using aqueous zinc salts as electrolytes,which cannot avoid unstable water-induced electrochemical side reactions and uncontrollable solid-liquid interface reactions.The low content of free water in hydrogel electrolytes is expected to solve the above problems.However,most hydrogel electrolytes have low ionic conductivity and poor water retention,resulting in rapid battery capacity decay and short cycle life.In this study,the gelatin hydrogel electrolyte was modified by doping,and the effects of different doping modification methods on its structure and properties and the effects on the electrochemical properties and mechanism of Re AZMBs constructed based on them were studied.The main research results are as follows:（1）Inorganic composite hydrogel electrolytes（ICHE）were prepared based on nano-Si O₂ doping,and Re AZMBs were constructed based on them.Scanning electron microscope（SEM）and electrochemical impedance spectroscopy（EIS）results show that nano-Si O₂ doping can increase gelatin hydrogel porosity,and when the addition amount of nano-Si O₂ was 10 wt%,the ionic conductivity of the prepared ICHE reached 10.3 m S·cm^-1,which was higher than the ionic conductivity of the unmodified hydrogel electrolyte(4.2 m S·cm^-1).The Re AZMBs constructed based on ICHE show better rate performance and cycling performance,especially at a current density of 1.5 A·g^-1,the initial specific capacity of the battery can reach 150 m Ah·g^-1,and the capacity retention rate after 1000 cycles as high as 67%,much higher than the battery using unmodified hydrogel electrolyte（33%）.This is mainly due to the high ionic conductivity of ICHE and their good compatibility with the electrode interface,which reduces the polarization effect of the battery during the charging and discharging process.In addition,the flexible battery assembled based on ICHE can still supply power normally under the condition of bending,showing the bending resistance.（2）Preparation of gelatin/γ-PGA hydrogel electrolytes（GPHE）based on linear polymerγ-polyglutamic acid（γ-PGA）doping and construction of Re AZMBs.SEM and EIS results show that that theγ-PGA doping stabilized the pore structure and enhanced the ion transport efficiency;the ionic conductivity of the gelatin hydrogel electrolyte（GHE）was 6.6 m S·cm^-1,which was lower than that of the GPHE prepared by adding 30 wt%ofγ-PGA(12.6 m S·cm^-1).Re AZMBs based on GPHE have lower self-discharge and better rate performance and cycling performance.Especially at a current density of 0.1 A·g^-1,the discharge specific capacity reaches 350 m Ah·g^-1,much higher than that of batteries using GHE.Moreover,the battery retains a discharge specific capacity of 105 m Ah·g^-1 after 1000 cycles at a current density of1.5 A·g^-1,which is about 2.33 times higher than of the battery using unmodified hydrogel electrolyte(45 m Ah·g^-1),far exceeding that of the battery using GHE.This is due to the high ionic conductivity of GPHE,its higher water retention and good compatibility with the electrode interface,which improves the electrochemical stability of Re AZMBs.（3）Water-soluble long-chain quaternized alkali lignin（LCQAL）was synthesized and doped to prepare gelatin/LCQAL composite hydrogel electrolytes（GLHE）,which in turn were used to construct Re AZMBs.SEM and EIS results show that GLHE has a highly porous pore structure,which can effectively promote ion migration;when LCQAL was added at 10 wt%,GLHE exhibited a high ionic conductivity(11.35 m S·cm^-1),which was higher than that of the unmodified hydrogel electrolyte(5.23 m S·cm^-1).Re AZMBs constructed on the basis of GLHE exhibit remarkable rate performance and cycling performance,with higher discharge specific capacity at full current density than the battery using unmodified hydrogel electrolytes.In particular,at a current density of 1.5 A·g^-1,the initial specific capacity of the battery using GLHE reaches 205 m Ah·g^-1 and retains a discharge specific capacity of 110 m Ah·g^-1 after 3000 cycles,which is much higher than the discharge specific capacity of the battery with unmodified hydrogel electrolyte(40 m Ah·g^-1).This is mainly because LCQAL optimizes zinc ion migration channels,enhances ionic conductivity,and reduces the polarization effect at the electrolyte/electrode interface,enabling the GLHE-based constructed Re AZMBs to exhibit excellent electrochemical performance.