Study On The Limiting Factors Of The Cycle Life And Performance Improvement Of Zinc-air Batteries

Flexible zinc–air batteries（FZABs）are considered as one of the best candidates of energy storage systems for flexible wearable electronic devices,owing to their very high theoretical energy density,low cost,much high safety and good environmental benignity.Although primary ZABs have been successfully commercialized,large-scale commercial application of rechargeable FZABs is still greatly limited by their poor cycle life.In previous research,significant research efforts have been devoted to improving the cycle life of the FZABs by developing and optimizing materials of the electrode,electrolyte,and electrocatalyst,as well as the design of the battery configuration.However,very few studies have focused on the limiting factors of the cycle life of FZABs.To prolong cycle lifespan,it is of great importance to obtain a deeper exploration of the limiting factors of the cycle life of FZABs during discharge–charge cycles.Therefore,in this paper,we systematically investigate the limiting factors of the cycle life of FZABs by analyzing each component of the battery before and after cycling failure.In addition,by optimizing and designing the structure and composition of gel polymer electrolyte materials（GPE）,the bottleneck factor for improving the cycle performance of the FZAB is studied,thereby further prolonging the battery cycle lifespan.As the core component of the FZAB,the GPE plays a key role in battery electrochemistry and is crucial for battery performance such as discharge voltage,output power,and cycle life.Poly（vinyl alcohol）（PVA）is one of the widely used polymer matrices for zinc–air batteries due to its good chemical stability,mechanical properties,nontoxicity and easy fabrication process.Therefore,a PVA-based alkaline GPE（PVA–KOH）was prepared by a mechanically mixed solution casting method,and used for the FZAB assembly.Meanwhile,the limiting factors of the cycle life of the FZAB used the PVA–KOH electrolyte was investigated.The results show that the poor water retention performance of the PVA-KOH GPE is the bottleneck factor that restricts the development of long-life FZABs.In order to improve the water retention performance of the electrolyte,a polyacrylic acid（PAA）-based alkaline GPE（PAA–KOH）with high water retention was developed.The limiting factors of the cycle life of the FZAB used the PAA–KOH electrolyte at different current densities was explored.The results show that under a low current density of 2 m A cm^–2,the OH^-content in the GPE highly determines the cycling lifetime of the FZAB.During discharge–charge cycles of the battery,the formation of Zn O and carbonates as well as the carbon oxidation of the air electrode will inevitably reduce the content of the OH^-in the GPE,leading to early battery failure.Moreover,under a high current density of 10 m A cm^–2,due to the high charging potential,the carbon in the air electrode is easily oxidized in the alkaline GPE,causing the severe degradation of the electrochemical performance of the air electrode and thus shortening the battery cycling life.Furthermore,under a higher current density of 20m A cm^–2,the passivation of the Zn electrode,increased interface resistance between the Zn electrode and GPE,as well as the reduced electrochemical performance of the air electrode are the reasons for the battery cycle life.Owing to enhancing the electrolyte retention ability of GPEs and relieving the oxidation of carbon in the air electrode in alkaline electrolyte,we fabricated a PVA,PAA,and graphene oxide（GO）co-crosslinked alkaline GPE containing the KI additive（KI–PVAA–GO）.The multiple crosslinking effects among all the components and highly hydrophilic character of the resultant gel polymer matrix endow the PVAA–GO GPE with significantly improved water retention capability,ionic conductivity,and mechanical flexibility compared to the traditional PVA-based GPE.In addition,the conversion of I^-/IO₃^-in the GPE changed the OER pathway in a more thermodynamically favored direction,thereby lowering the charging potential down to1.69 V and improving the energy efficiency of the ZAB up to 73%.Due to the greatly decelerated water loss and alleviated air electrode degradation upon overcharging,the flexible ZABs delivered a long working life of 200 h.The cable-and sandwich-type ZABs based on PVAA–GO GPE exhibited high reliability under extreme working conditions and superior extensibility when integrated into various flexible and wearable electronic devices.In order to obtain GPEs with better stability and mechanical strength without sacrificing other properties,such as water absorption,water retention and ionic conductivity performance,a series of PAA–Al₂O₃ GPEs by adding different amounts of Al₂O₃ nanoparticles to PAA matrix were successfully prepared.The mechanical strength of PAA–Al₂O₃ gel polymer increases gradually with the increase of Al₂O₃nanoparticle content.When the addition of Al₂O₃ nanoparticles was 20 wt.%,the PAA–20 wt.%Al₂O₃ GPE achieves improved mechanical strength with enhancing its electrolyte absorption and retention capabilities,as well as ionic conductivity.Based on the above optimized properties,the FZAB assembled using PAA–20 wt.%Al₂O₃ GPE provided an ultra-long cycle life of 384 h.Furthermore,the above battery exhibited large power density,good rate capability and outstanding discharge performance.In addition,the above FZAB has excellent practical application value,which can be assembled in series and parallel to power different types of electronic devices.