The Investigation Of Energy Storage Mechanism And Performance Optimization For Aqueous Mn-based Zinc Batteries

The global energy tightening and environmental crisis have stimulated the development of renewable energy technologies.In recent years,there has been a lot of research enthusiasm for finding alternatives to non-renewable fossil energy.As a key technology of energy conversion,lithium-ion batteries（LIBs）with high reversibility and energy density are widely used in various fields such as portable devices and electric vehicles.However,the limited lithium resources and the low safety of LIBs seriously limit their further large-scale applications.Therefore,aqueous zinc batteries（AZBs）are expected to become one of the next generation of new energy storage technologies due to low operating cost,considerable performance,and unparalleled safety.Although AZBs have the intrinsic advantages that LIBs cannot make up,the research on AZBs has just started,such as the selection of energy storage system,energy conversion mechanism,energy density and other aspects have greatly improved and explored space.As one of the important energy conversion systems of AZBs,Mn-based AZBs system has the advantages of high specific capacity,high voltage plateau,abundant element reserves and low toxicity.However,due to the multivalent and polycrystalline phase of manganese,manganese will undergo various phase combinations and transitions during charging/discharging process.In addition,although Mn²⁺additives can suppress the disproportionation reaction of manganese oxide in the electrochemical process,the resulting abnormal capacity fluctuations will make the disordered charging/discharging mechanism even worse.In addition,the cause of unsatisfactory battery performance of the full Mn-based AZBs system is due to the poor conductivity of manganese oxide and uncontrollable side reactions on the zinc anode.In the process of designing full Mn-based AZBs,the cathode material,electrolyte,and anode should be considered and optimized at the same time.In order to solve the above concerns,this paper has achieved the following innovative results:（1）In virtue of manganese oxide based on cation pre-insertion engineering,we elaborate the origin of capacity fluctuation,which is found to be correlated with the unique Mn²⁺behavior.For the first time,we figure out new metrics such as effective cycling percentage（η）and maximum Mn²⁺contribution ratio（ε）to reappraise the electrochemical performance of the current Mn-based AZBs via a new capacity evaluation protocol,i.e.,Mn-based competitive capacity evolution（Mn-CCE）.The universality of the protocol and its metrics were further verified via quantitative analyses of the reported Mn-based AZBs.More significantly,the failure of Mn-based electrodes is demonstrated to be rescuable via facile acid treatment,which is expected to quintuple the lifespan of batteries.The findings can provide new insights to understand the electrochemical behaviors,serve as the assessment criteria,and further guide the development of Zn/Mn-related devices for practical applications.（2）Although sulfate-and sulfonate-based electrolytes have been widely used in the study on aqueous zinc-ion batteries（AZBs）,the discrepancies in the Faradaic reaction kinetics of cations interfacial chemistry including Mn²⁺redox reaction and Zn deposition are observed in these two systems,mechanism of which is still unclear.Herein,through focusing on the electrolyte solvation structure,we constructed a specific adsorption model involving the coexistence of anions,cations,and water molecules in the electrode/electrolyte interface.Distinguished from the traditional investigation of isolated adsorbed particles,we demonstrate that the specific adsorption model enables a rational explanation on reaction difference of cations with different solvation structures on the electrode/electrolyte interfaces（EEI）.Specifically,owing to the more intense adsorption of active solvated Mn²⁺near the inner Helmholtz plane,the deposition reaction of Mn species is enhanced in sulfate-based electrolyte,resulting in stronger capacity increase and fluctuation in comparison with sulfonate electrolyte.Similarly,a more rapid Zn²⁺deposition kinetic in sulfate-based electrolyte can be attributed to its strong adsorption behavior at EEI.Furthermore,as a validation of the as-proposed model,a sulfate/sulfonate hybrid electrolyte system is proposed,in which the optimized adsorption behavior at EEI endows it with synergistic improvement both in the capacity and cycling stability for aqueous Zn-Mn cells.This work provides a rationale for understanding the interfacial electrochemistry in various aqueous electrolyte systems from the view of the adsorption behavior of the solvation structure.（3）A finger-paint method is proposed to enable quick physical modification of glass-fiber separator without complicated chemical technology to modulate EEI of bilateral electrodes for aqueous zinc-ion batteries（AZBs）.An elaborate biochar derived from Aspergillus Niger is exploited as the modification agent of EEI,in which the multi-functional groups assist to accelerate Zn²⁺desolvation and create a hydrophobic environment to homogenize the deposition behavior of Zn anode.Importantly,the finger-paint interface on separator can effectively protect cathodes from abnormal capacity fluctuation and/or rapid attenuation induced by H₂O molecular on the interface,which is demonstrated in modified Mn O₂,V₂O₅,and Mn HCF-based cells.The as-proposed finger-paint method opens a new idea of bilateral interface engineering to facilitate the access to practical application of the stable zinc electrochemistry.（4）We developed a dynamic electrolyte interphase strategy to prevent hydrogen evolution corrosion on Zn anode in an acidic environment（p H=2.2）.This dynamic process is completed by phosphate,sulfuate precipitation and phytic acid modification layer.During plating/stripping,hydrogen ions are stored in phytic acid modification layer and are buffered by sulfate and sulfate precipitation,thus forming a reversible dynamic interphase.This strategy ensures that the Zn anode provides a stable stripping/plating in acid electrolyte more than 3600 h and 400 h at a current density of 1 m A cm^-2 and 20 m A cm^-2,respectively.Due to the proton enrichment in the electrolyte,the high voltage platform in electrolytic Zn-Mn battery can be unlocked at 1.85 V with a long-term cycling over 1600 cycles.More interesting is that Zn-Mn battery can operate in the seawater-based electrolyte with a capacity of 40 m Ah at 1.7 V.This strategy is expected to provide a new idea for expanding the working p H range of zinc metal.（5）In literature,Zn-Mn aqueous batteries（ZMABs）confront abnormal capacity behavior,such as capacity fluctuation and diverse“unprecedented performances.”Because of the electrolyte additive induced complexes,various charge/discharge behaviors associated with different mechanisms are being reported.However,the current performance assessment remains unregulated,and only the electrode or the electrolyte is considered.The lack of a comprehensive and impartial performance evaluation protocol for ZMABs hinders forward research and commercialization.In this perspective,we first propose a p H clue（proton-coupled reaction,PCR）to understand different mechanisms and normalize the capacity contribution.Then,a series of performance metrics,including rated capacity（C_r）and electrolyte contribution ratio from Mn²⁺（Cf M）,are systematically discussed based on diverse energy storage mechanisms.Finally,we propose the concrete design concepts of a tunable H⁺/Zn²⁺/Mn²⁺storage system for customized application scenarios,opening the door for the next-generation high-safety and reliable energy storage system.In conclusion,the mechanism of the abnormal reaction in the Zn-Mn system was first discovered（innovation point 1）,and the regulation of the abnormal deposition reaction was realized according to the microscopic zinc/manganese carrier behavior（innovation point 2）.Furthermore,on the basis of the discovery of the reason for the abnormal reaction mechanism,the interface design strategy was used to suppress the side reaction of the bilateral electrodes,and the high stability of the zinc-manganese battery was realized（innovation point 3）.As a commercialization orientation,we designed zinc metal batteries that can run stably under acidic environment,unlocking a high-energy density electrolytic zinc-manganese battery system（innovation point 4）.Finally,based on the above research results,we critically summarized the charging/discharging mechanism of zinc-manganese system,and proposed the development direction of future design of zinc-manganese system（innovation point 5）.