Study On The Effect Of Conductive Additives In Sulfide All-Solid-State Batteries

In order to meet the growing demand for environmentally-friendly development,lithium-ion batteries serving as a clean technology have reduced the dependency on fossil fuel and become the go-to choice to power electrical equipment such as electric vehicles.However,the lifespan of conventional liquid lithium-ion batteries is limited by the formation of lithium dendrites during repeated cycles which could pierce the separator and finally result in inner short circuit.Another major weakness of lithium-ion batteries is safety hazards,due to the use of volatile and flammable organic electrolytes.To overcome these issues,“all-solid-state”batteries（ASSBs）applying solid electrolytes which qualify high mechanical strength and non-flammability have received significant attention and shown great potential.In recent years,Solid-state batteries demonstrate significant benefits in terms of safety and energy density,especially Sulfide All-Solid-State Batteries,have attracted widespread attention owing to Sulfide solid electrolyte have advantages of high ionic conductivity,which shows a promising application prospect in the field of grid-scale energy storage.Solid-solid contact between the cathode active materials and the solid-state electrolytes（SSEs）leads to sluggish interfacial ion transport because SSEs have poor fluidity and wettability.Therefore,SSEs are typically added into electrodes as the ion transport medium,whose electronic insulating nature would impede the electronic charge transfer in cathodes,especially for commercial transition metal layered oxides with low electronic conductivity of less than 10^-4 S cm^-1.To meet the demand for the commercialization of all-solid-state batteries with high capacity and high power density,it is vital to improve the electronic conductivity of cathodes for ASSBs.Carbon-based additives are preferred for constructing conductive networks in electrodes due to their high electron conductivity and large specific surface area.However,the carbon additives would accelerate the decomposition of SSEs during electrochemical reactions which shows narrow electrochemical stability window.In addition,oxygen-containing functional groups on the surface of the carbon additives tend to react with the SSEs inevitably.The by-products originating from the above side reactions such as Li₂S and P₂S_x exhibit extremely low ion conductivities,resulting in high charge transfer impedance and serious electrochemical performance degradation.Therefore,the rationalization of the conductive network construction in cathodes is a major challenge in terms of sulfide-based ASSBs.In order to realize rational construction of the conductive network in cathodes of sulfide-based ASSBs,it is necessary to reduce the decomposition of the sulfide-based SSEs while ensuring rapid electron transfer between active materials and the current collector.As mentioned,high electron density on the surface of carbon additives is one of the main causes of side reactions in cathodes.Therefore,regulating electron enrichment sites of the conductive additives through modulating morphology and specific surface area of the conductive additives has potential for improving performances of ASSBs.The effects of three conductive additives,Super P,carbon fiber,and graphene,on the performance of sulfide all-solid-state batteries were explored.According to their electrochemical performances,minimal side reactions were observed while employing graphene as the conductive additive,which possesses flake-shaped morphology and the lowest specific surface area.The corresponding coulomb efficiency,rate capability,and cycling stability of the ASSBs were significantly enhanced compared to cells applying Super P and carbon fiber:the 1^st discharge capacities at 0.1 C with 3.0 wt.%Super P,carbon fiber,and graphene as conductive additives are 143.1 m A h g^-1,170.2 m A h g^-1,and 173.6 m A h g^-1,respectively;their capacity retention after 30 cycles are 80.6%,80.4%,and 86.7%,respectively.Finally,the conformational relationships between the characteristics including morphology and surface area of carbon additives and the performance of the solid-state batteries were elucidated by d Q/d V analysis combining SEM characterization.The oxygen-containing functional groups on the surface of the conductive additives may undergo an uncontrolled redox reaction with the sulfide solid electrolyte.This necessitates modification of the conductive additives to prevent the redox reaction.However,the methods currently proposed,such as high-temperature sintering at2400°C,are not suitable for large-scale industrial production.Therefore,it is crucial to develop low-cost,simple,and easy-to-implement methods for modifying conductive additives to reduce the redox reaction between conductive additives and Li₆PS₅Cl.Therefore,a simple strategy to modify Super P through thermal reduction is proposed.All-solid-state batteries can be significantly enhanced in terms of specific capacity,multiplier performance and cycling performance by using low-cost Super P as a conductive additive.The batteries incorporating Super P and R-Super P as conductive additives exhibit an initial capacity of 131.8 m A h g^-1and 180.2 m A h g^-1 at 0.1 C,respectively;after 40 cycles,the capacity retention is 64.6%and 81.8%,respectively.Note that only 1.0 wt.%of cost-effective R-Super P in cathodes is sufficient.Based on the results,it can be concluded that the thermal reduction modification strategy eliminates the oxygen-containing functional groups on the surface of Super P and thus mitigate the interfacial side reactions between Super P and Li₆PS₅Cl.This approach is of practical importance in realizing high energy density and low-cost ASSBs.