| As a clean energy storage technology,lithium ion batteries have dominated the market for portable electronic devices,and are being considered as the most promising technology for large-scale energy storage in electric vehicles and smart grid.With the increase of the energy density of lithium ion batteries,people are more concerned about the safety problem,which is mainly caused by the flammable liquid electrolytes used in current lithium ion batteries.Under abused conditions such as over-charging or over-heating,the batteries may catch fire or explosion.Replacing the liquid electrolyte with nonflammable,inorganic solid electrolyte to make all-solid-state lithium ion batteries could essentially improve the safety of the batteries.Another advantage of all-solid-state lithium ion batteries is that the formation of solid electrolyte interphase(SEI)is static,meaing that the SEI is not needed to be regenerated during charge/discharge cycles,which can dramatically suppress the side reactions and improve the cycling performance of the batteries.However,the limited amount of Li in the earth is another problem for large-scale development of lithium ion batteries.Sodium is more abundant than Li,and therefore developing all-solid-state sodium ion batteries could dramatically decrease the cost of batteries.However,the performances of all-solid-state lithium/sodium ion batteries are far worse than those of the liquid electrolyte batteries,which is mainly due to the huge interfacial resistances between solid electrode and solid electrolyte.Based on the previous study about the electrode design of high-capacity electrodes for liquid-electrolyte lithium ion batteries,we conclude that the key to addressing the interfacial problem in all-solid-state batteries is to maximize the triple-phase interfacial contact between active material,electronically conductive carbon,and ionically conductive solid electrolyte to form a continuous network for both electrons and ions.The interfacial problem in all-solid-state batteries is much more complicated than that in liquid-electrolyte batteries.The infiltrative nature of liquid electrolyte allows the perfect contacts between electrode and liquid electrolyte,and therefore we only need to focus on improving the electronic conductivity of electrodes.However,in all-solid-state batteries,the contact is point-to-point contact and therefore the contact area is much less than that in the liquid-electrolyte lithium ion batteries.Moreover,the electrodes in all-solid-state batteries involve the interfacial contact between three solids:active material,carbon,and solid electrolyte.The electrode can be effectively utilized only within the triple phase contact region.In addition,different solid electorlytes have different mechanical properties.For example,sulfide based solid electrolytes have a much lower Young's modulus than oxide based solid electrolytes,and therefore they will be much softer.The different mechanical properties of solid electrolytes will result in different mechanisms for the interfacial contact,and therefore different approaches should be used for different electrolyte-electrode combinations to improve the interficial contact.Based on these design principles,we proposed four approaches to improve the electrode/electrolyte interfaces in all-solid-state lithium/sodium ion batteries.(1)The interface between LiCoO2(LCO)cathode and Li7La3Zr2O12(LLZO)electrolyte was improved by introducing low-melting point,ionically conductive Li2.3C0.7B0.3O3 sintering additive at the interface.LCO has a good electronic conductivity,and therefore the main problem for this electrode is how to improve its interfacial contact with solid electrolyte.High temperature sintering was used in previous reports to achieve the contact between LCO and LLZO.However,LCO and LLZO will react during the sintering process,forming an electronic and ionic insulating passivating layer at the interface.By taking advantage of the spontaneously formed Li2CO3 layer on the surfaces of both LCO and LLZO,we introduced a low-melting point Li2.3C0.7B0.3O3 at the interface.Li2.3C0.7B0.3O3 is isostructural with Li2CO3 but has a higher ionic conductivity.During sintering.Li2.3C0.7B0.3O3 will react with L12CO3 on both surfaces of LCO and LLZO,and therefore will wet with both LCO and LLZO,and at the same time prevent the chemical reaction between LCO and LLZO.An all-ceramic cathode/electrolyte without adding any liquid/polymer electrolytes was achieved.Coupling with Li metal anode,such an all-ceramic cathode/electrolyte enabled the all-solid-state battery to cycle for 100 times at room temperature.(2)The interface between Mo6S8 and Na3PS4 solid electrolyte was improved by coating a thin layer of Na3PS4 solid electrolyte on the surface of Mo6Ss electrode.Here,we are transferring our understanding from the previous work to all-solid-state sodium ion batteries,as Mo6Ss is also highly electronically conductive.By soaking Mo6S8 into a Na3PS4 in THF solution,we were able to conformally coat a thin layer of Na3PS4 on the surface of Mo6Ss electrode,and therefore significantly improve the interfacial contact between Mo6S8 and Na3PS4 solid electrolyte.With Na3PS4 as the solid electrolyte and Na-Sn alloy as the anode,such a cathode is able to cycle for 500 times.representing the best cycling performance for all the reported all-solid-state sodium ion batteries based on sulfide electrolyte.(3)Mixed(electronic and ionic)conductive Na2S-Na3PS4-C nanocomposite was prepared by high-energy ball milling as the cathode for all-solid-state sodium sulfur batteries.All-solid-state sodium sulfur batteries could largely increase the energy density of all-solid-state sodium-ion batteries,but their performance is strongly limited by the low electronic conductivity and low ionic conductivity of Na2S.The key to addressing the interfacial problem in all-solid-state sodium sulfur batteries is how to realize sufficient contact of Na2S with both carbon and solid electrolyte.High-energy ball milling method is used to prepare Na2S nanoparticles and to achieve uniform mixing of Na2S,Na3PS4 and carbon in the Na2S-Na3PS4-C nanocomposite.Such a mixed conductive cathode delivered a high performance in all-solid-state sodium sulfur battery.(4)The interfacial contact in the Na2S-Na3PS4-C nanocomposite electrode was further improved by preparing the electrode using casting-annealing method.Since the mechanism of battery degradation for the previous work is mainly due to the high interfacial resistance in the cathode composite,we developed a novel approach to prepare the nanocomposite.The Na2S and P2S5 mixture with a corresponding molar ratio was heated until they melt,and the molten material was then infiltrated into mesoporous carbon(CMK-3).The sample was then quenched to room temperature and then annealed to in-situ form Na2S-Na3PS4-C nanocomposite.The in-situ formed Na2S/Na3PS4/CMK-3 enabled better interfacial contact between Na2S,Na3PS4 and CMK-3 in the electrode,and the electrochemical performance of this cathode composite is much better than that in the previous work.The reversible capacity is still 650 mAh g-1 after 50 cycles when cycled at 60 ?,representing the best performance for all the reported all-solid-state sodium sulfur batteries. |
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