Design And Performance Study Of Functional Current Collectors For Lithium-Sulfur Batteries

Lithium metal anodes and sulfur cathodes are of great interest due to their high specific capacities.However,they both undergo the transition between solid and liquid phases during charging and discharging,and the deposition process from the liquid phase to the solid phase is kinetically hindered.For lithium metal anodes,this problem is manifested as spontaneous deposition inhomogeneity,resulting in increased anode-electrolyte side reactions and lithium loss;for sulfur cathodes,this problem is manifested as incomplete transition of polysulfide intermediates during charging and discharging,and the diffusion of unreacted polysulfides into electrolytes results in active material loss and“shuttle effect”.In this thesis,from the concept of“guiding deposition”,various functional current collectors are designed and studied to induce the deposition processes of lithium metal anodes and sulfur cathodes towards better battery performance.The main research contents are as follows:（1）The lithiophilic metals were selected from a variety of metals based on the lithium affinity,the lithium plating overpotential,and the lithium plating/stripping efficiency.Afterwards these lithiophilic metals were made into films about 100 nm thick as functional current collectors to guide the lithium deposition.Even in the absence of the copper foil substrate,both the gold and the sliver thin films can still induce lithium ions to deposit into a complete,smooth lithium layer.The Li₂S anode-free cell using the gold thin film current collector achieves an initial specific capacity of 769 m Ah g^-1 and a capacity retention of99.6%per cycle for the first 100 cycles.The anode-free cell using the silver thin film current collector can also achieve a capacity retention of 99.4%per cycle.（2）The polymer-lithiophilic-metal composite film was prepared as a current collector by combining the lithiophilic metal thin film and the polymer base film through sputtering and bonding respectively.The polymer film has decent strength and ductility and the lithiophilic metal can induce lithium deposition.The flexible composite current collectors are lighter than copper foil and can improve the density of lithium deposition as well as the performance of anode-free batteries.The average capacity retention of the anode-free battery using the Au-sputtered polymer current collector exceeds 99.5%per cycle after 100cycles.This current collector can also prevent the short circuit of the cathode and anode current collectors during the puncture process.The anode-free cell using the polymer-bonded silver current collector can be cycled stably for 100 cycles with a capacity retention of 99.6%per cycle.（3）In analogy to the practice of pre-plating lithium on copper foil current collectors to reduce the lithium deposition overpotential,a trace amount of Li₂S was introduced into a three-dimensional carbon current collector as preloaded nucleus to guide the deposition of Li₂S during the discharge process.The trace Li₂S functions as preload nucleus to cross the nucleation barrier during Li₂S deposition,which enhances the specific capacity of the Li₂S₆liquid cathode.The Li₂S nucleus can also induce a uniform distribution of the Li₂S deposited during the discharge process.The distribution of these deposited products can still affect the electrode even after multiple cycles across the solid-liquid-solid transition.（4）The carbon interlayer and carbon-coated separator can effectively enhance the cycling performance of lithium-sulfur batteries.However,their mechanisms are not fully understood.The controlled experiments containing several cell configurations were designed to separate the“upper current collector”function of the carbon interlayer from the physical barrier function.It is demonstrated that the“upper current collector”function,which promotes the further charge/discharge and deposition of polysulfides,plays the most important role in enhancing the performance of lithium-sulfur batteries.Following experiments prove that a reasonable distribution of total carbon between the sulfur cathode and the carbon interlayer can further improve the battery performance.