Design And Controllable Preparation Of Anode Materials In Lithium-ion Batteries Towards Compact Energy Storage

With the continuing demand for the minimization of electrochemical energy storage devices,the volumetric performance has become equally important as the gravimetric metrics for rechargeable batteries used in limited spaces.High-capacity anode materials promise to significantly improve the volumetric performance of lithium-ion batteries,but the issues of mechanical instability induced by their large volume changes during cycling generally prevent their practical use.Nanostructured carbons have been widely used to buffer the volume change of such high-capacity noncarbon anodes.In this study,based on a precise control of the capillary shrinkage of carbon networks,strategies towards exact void space preservation,interface strengthening and framework toughening in carbon cages,are proposed.Consequently,the mechanical stability engineering enables high-density,high-mass-loading and large-size noncarbons to afford superior volumetric lithium storage together with an excellent structural and volumetric stability.The main research perspectives and related contributions are summarized as follows:(1)A flowable sulfur template is innovatively introduced to produce precisely-tunable pores in three-dimensional(3D)graphene assemblies,overcoming the limitations of rigid templates for porous carbon fabrication.Sulfur accompanied with the shrinking graphene sheets controls the production of pores with precise size and shape at nanoscale.The pores formed are open and interlinked due to the flowable characteristic of sulfur template in the graphene network.The precisely manipulated pore structure in the carbon-based electrode affords a high density and fast ion transport,which are highly required in the compact energy storage process.(2)A well-designed method to precisely introduce the void space in 3D graphene networks for noncarbons is demonstrated by developing sulfur as a novel Transformers-like template.In a typical synthesis using the capillary shrinkage of networked graphene hydrogels,soft and flowing sulfur was deformable with the hard noncarbon nanoparticles inside the shrinking hydrogels,and the void space around the noncarbon particles was precisely controlled by tuning the content of the surrounding and removable sulfur.As a typical example,an ultrahigh volumetric capacity of 2123 m Ah cm^–3 after a long cycle life(300 cycles)has been achieved with a graphene-caged tin oxide(Sn O₂)anode by packing the graphene nanosheets into a high density while providing just the required amount of void space.(3)A networked and interface strengthened carbon cage derived from shrinking graphene hydrogels for Si nanoparticles is developed by optimizing the area and interaction of Si/carbon interfaces,enabling robustly buffered volume expansion in a dense and thick anode(>6 m Ah cm^–2,>1000 m Ah cm^–3).Based on the in situ transmission electron microscope,mechanical-electrochemical coupling characterizations including compression,tearing and lithiation-induced Si expansion processes provide a solid evidence for the excellent mechanical properties and buffering ability of the interface-strengthened graphene-caged Si nanoparticles anode materials.(4)An“imperfection-tolerance”strategy by developing strong yet ductile carbon capsule cellular architectures is presented to stabilize the brittle Si microparticles from individual particle to electrode levels.During capillary drying of graphene hydrogels,carbon-caged Si microparticles with preserved voids are interweaved into a densely packed cellular graphene network to form a mechanical and electrical whole,enabling tough buffers and continuously fast electron transport.As a result,the high-tap-density and low-surface-area Si microparticle anode shows ultrahigh stability over 500 cycles in a lithium-ion battery and provides a volumetric energy density of 1048 Wh L^–1 in a pouch full cell.In summary,we focused on the mechanical stability designs of carbon cages to solve the high stress issues in compact noncarbon anodes,achieving much improved volumetric capacity and simultaneously cyclic stability.Especially for low-cost industrial Si microparticles,the carbon-based mechanically toughening design promotes their cycle stability substantially,representing a step forward toward the practical use in lithium-ion batteries.