Interface Engineering on Solid-State Ceramic Electrolyte By Graphene-like Coating for Electrochemically Stable Sodium Metal Batteries
Authors: Matios Edward , Wang Huan , Li Weiyang 
Published: February 2020

Rechargeable sodium (Na) based battery can be a cost-effective successor of lithium (Li) ion battery owing to the natural abundance of Na resource and high capacity of metallic Na anode. However, liquid electrolytes pose critical safety issue that arises from their flammable organic solvent component and Na dendrite formation upon repeated electrochemical charge/discharge cycles. Solid electrolytes not only can eliminate the abovementioned safety concerns, they can also enhance electrochemical stability, broaden operating temperature range and prolong cycling life. One of the greatest challenges for solid electrolyte is the large interfacial resistance between electrolyte and electrode. Various optimization techniques for reducing interfacial resistance have been reported. Nevertheless, all-solid-state batteries still face stagnant advancements due to the limited effectiveness.
Na3Zr2Si2PO12 (NASICON) ceramic electrolyte remains to be a promising solid-state electrolyte for Na batteries because of its high ionic conductivity and excellent electrochemical and thermal stability. However, the pairing of pristine NASICON with high-capacity metallic Na anode gives rise to large NASICON/Na interfacial resistance and poor interface wettability that result in non-uniform Na+ flux across the interface. Specifically, Na+ ions tend to plate preferentially along the grain boundaries of NASICON where the Na+ flux is locally intensified, leading to detrimental dendrite-like nucleation on Na anode over repeated charge/discharge cycles. Theoretically, a homogeneous and stable interlayer with superior Na anode compatitability and ion conductivity can facilitate uniform Na+ flux across the interface, therefore effectively decrease the interfacial resistance and suppress unregulated dendrite-like Na formation during cycling. Past research efforts to implement such an interlayer on ceramic electrolyte include the employment of polymer layer, applying molten metal on ceramic electrolyte, and coating a layer of metal oxide using relatively expensive method of atomic layer deposition.
This motivated us to solve the challenges of large interfacial resistance and non-uniform plated Na for solid-state Na metal batteries through a facile and scalable approach. Graphene, with its unique properties of chemical inertness, mechanical flexibility and structural rigidness, has attracted great interests in various energy storage applications. Recent study demonstrated the use of ultrathin graphene film coating on Na metal as a stable artificial solid electrolyte interphase (SEI) layer in conventional liquid electrolyte to suppress the dendrite growth, in which the graphene defects network act as an efficient Na+ transport pathway. Therefore, graphene can be a promising interlayer for solid-state Na metal batteries to improve Na+ conductivity between the ceramic electrolyte and Na metal. The abundantly distributed defects network on graphene-like interlayer can enable uniform Na+ flux across the electrode/electrolyte interface, circumventing the preferential Na metal plating along the grain boundaries of NASICON and therefore suppress dendrite growth. However, direct growth of graphene on ceramic electrolyte has not yet been reported, largely due to the non-catalytic nature of ceramic electrolyte.
In this work, we report the direct growth of ultrathin graphene-like layer on NASICON ceramic electrolyte, hereafter referred as G-NASICON, to significantly decrease the interfacial resistance, improve Na plating/stripping cycling stability and enable uniform Na plating and minimize the unregulated dendrite-like Na formation on Na anode after repeated stripping/plating cycles. Graphene growth on NASICON was carried out in a chemical vapor deposition system. CVD growth of graphene materials towards energy-related applications is highly attractive due to the outstanding properties of graphene materials, as well as the scalability of CVD method. The formation mechanism of graphene-like layer on the ceramic substrate is based on the carbothermic reduction occurring at ceramic surface to initiate nucleation and growth. Optimized graphene-like layer on NASICON effectively decreases the NASICON/Na interfacial resistance more than ten-fold, from 524 Ω cm2 to 46 Ω cm2. The Na/G-NASICON/Na symmetric cells delivered remarkably stable Na stripping/plating cycling behavior at a current density of 1 mA/cm2 with 1 mAh/cm2 capacity over 1000 hours that, to the best of our knowledge, represents the best performance in solid-state Na symmetric cell systems. The surface of Na electrode after 1000 hours of cycling obtained from Na/G-NASICON/Na remained smooth because of uniform Na+ flux across graphene-coated-NASICON/Na interface enabled by the abundant graphene defects network for efficient Na+ transport. Solid-state room temperature battery consists of graphene-regulated NASICON electrolyte, Na3V2(PO4)3 cathode and Na anode delivered a reversible initial capacity of 108 mAh/g at 1C current density for 300 cycles with 85% capacity retention, far superior than the battery with pristine NASICON. This work can be a valuable contribution towards a safe and stable solid-state Na metal battery system, and provide insights for solid-state lithium metal batteries as well.