Fabrication,Structure And Electrochemical Properties Of Self-Supported Array-Type Film Electrodes For Flexible Lithium-Ion Batteries

Achieving the flexible electrode with robust electrochemical performance and mechanical stability is crucial for flexible lithium-ion batteries,and is of great significance to promote the development of flexible film electronic devices.Although transition-metal chalcogen compound（e.g.Mn₃O₄、MoS₂ etc.）have the advantages of high theoretical specific capacity and low cost,they also suffer from defects of limited electronic conductivity and structural deterioration caused by volume variation during the lithiation/de-lithiation process when serve as anode.In this article,using the one-dimensional nanorod/nanotube arrays grown on flexible substrate（e.g.stainless steel foil,Ti foil and carbon cloth）as template precursor,the self-supported naontube arrays film electrode was fabricated based on a template-assisted self-assembly method,including Mn₃O₄@C and MoS₂-based nanotube array anode.As a result,the array-type tube-like composite electrode own robust structure stability and electrochemical property,as well as robust adhesive strength to flexible substrate.Meanwhile,a self-supported Li_xMn_yO₄ cathode with robust property is deposited on the carbon cloth substrate by hydrothermal lithiation and low temperature calcination,thus avoiding the high temperature damage the mechanical properties of carbon-based flexible substrates during the traditional solid state lithiation process.On these bases,the self-supported Mn₃O₄@C anode and self-supported Li_xMn_yO₄ cathode are assembled into coin-and pouch-type all-manganese battery,and the cyclic performance and energy density of them are studied,as well as the influence of bending condition tests on the electrochemical property of the pouch battery.First,an uniform and large-area ZnO nanorod arrays are deposited on metal foil and carbon cloth substrates by a low temperature chemical method,and further served as a self-sacrificial template.The resistance before and after the ultrasonic and bending tests reveal that the ZnO film own robust adhesive strength to the flexible substrate.Second,the self-supported Mn₃O₄@C nanotube array film could be synthesized by template-assisted self-assembly and CVD method by using ZnO nanorod array as self-sacrificial template.This tube-like array film not only own plenty voids,robust structural stability,high ion and electron conductivity,but also robustly adhere to the substrate.Thus,this composite film anode exhibits robust Li-ion storage performance.Although the increment on mass loading of Mn₃O₄@C film(from 0.25 to 0.48 mg cm^-2)will reduce the inner diameter of the nanotube and lithium ion diffusion rate,as well as increase the shell thickness of the nanotube,the electrochemical property and structural stability of the film still well maintain.The Mn₃O₄@C film anode and the commercial LiCoO₂ cathode were further assembled into coin-type full battery,its energy density(216.2 Wh kg^-1 under 46.0 W kg^-11 in first discharge process)is quite close to the highest level of LiCoO₂ and graphite commercial batteries.Third,in order to prepare higher performance flexible film anode,MoS₂ is selected with higher electronic and ionic conductivity.The self-supported CSTAs@MoS₂ NTAs and CSTAs@MoS₂@PPy SNTAs film are fabricated by the same template（CSTAs）assisted self-assembly method.Both two types films are uniformly arranged on stainless steel substrate and robustly adhere to the steel foil.In contrast,nanotube in SNTAs film have larger inner diameters but smaller shell thickness than that in NTAs.The MoS₂ nanocrystals in both two films are less than 20 nm and are well dispersed in the conductive CSTAs shell.Therefore,both of the two MoS₂-based tube array film anode own superior Li-ion storage performance.This is not only owe to the large voids and conductive skeleton,but also the small size of the MoS₂ nanocrystals,which improves the reversibility of the conversion reaction between Li₂S and Mo,and further reduces the formation of the S_n^2-with poor conductivity and easily dissolved in electrolyte.In addition,benefiting from the in-situ coating of PPy on the surface of MoS₂ nanosheet,the SNTAs anode has higher structural stability and electronic conductivity than NTAs anode.Therefore,the SNTAs anode has higher Li-ion and Na-ion storage capacity,better cycle stability and rate performance than the NTAs anode.Furthermore,the specific energy density of the SNTAs||LiCoO₂ Li-ion coin-type battery(197.5 Wh kg^-1)has reached the high level of the LiCoO₂ and graphite battery.And the energy density of SNTAs||Na₃V₂（PO₄）₃ Na-ion coin-type battery(97.6 Wh kg^-1)owns obvious advantages compared to general transition metal sulfides.Finally,in order to prepare a flexible all-manganese Li-ion battery,lithium manganese oxide film cathode(CC-Li_1.24Mn_1.72O₄-c450)with good crystallinity and electrochemcial performance is fabricated by hydrothermal lithiation and low temperature calcination method on condition that maintaining the mechanical properties of the carbon cloth substrate.The experiments show that the higher calcination temperature,the higher crystallinity of the lithium manganate,and the larger average grain size,but when the temperature exceeds450°C,the mechanical properties of the carbon cloth will be damaged.The performance comparison of lithium manganese oxide cathodes prepared by different calcination temperatures show that the higher calcination temperature,the faster of Li⁺diffusion,the smaller polarization,the lower impedance and the better cycleability.Further,the self-supported CC-Li_1.24Mn_1.72O₄-c450 film cathode and Mn₃O₄@C film anode were assembled into a coin-type battery,although the energy density(161.1 Wh kg^-1)is not dominant compared to LiCoO₂ and graphite battery,the cost is much lower and more environmentally friendly.The pouch all-manganese battery assembled by the same cathode and anode,owns lower reversible capacity value and capacity retention when compared to the corresponded coin-type battery,which is due to the continuous consumption of the cathode lithium by the not fully-prelithiated and high surface area anode during cycle process.However,the flexible pouch battery could operate normally when the flexible film electronic devices are under bending states.In addition,CSTAs@Li_1-xMn₂O₄ array film cathode is prepared by hydrothermal lithiation method,and its nano-array architecture design strategy could serve as a reference to the preparation of other one-dimensional film cathode.