Solid Sodium Anodes Enabled by Low-Cost, Mesoporous Carbon Interlayer—Progress and Prospects for Scalable Energy Storage

Solid-state batteries (SSBs) based on a solid-state electrolyte (SSE) have the potential to enable higher energy density and promote categorically safer electrochemical energy storage compared to conventional high energy batteries such as Li-ion batteries (LIBs) or Na-ion batteries (SIBs). Ideally, SSBs utilize a pure alkali metal anode to maximize specific energy and energy density by replacing host-type anodes such as graphite for LIBs (372 vs 3860 mAh g^-1 for pure Li) or hard carbon for SIBs (~ 300 vs 1166 mAh g^-1 for pure Na). Theoretically, a pure alkali metal will unlock the highest performance SSBs. In practice, pure alkali metal anodes struggle at the alkali-metal-SSE interface due to reductive instability of the SSE (especially for sulfide or halide type SSEs), poor interfacial contact/wettability of the alkali metal with the SSE leading to high overpotential, failure of the SSE due to alkali metal dendrites on plating at practical current densities, or some combination of these. Where solid-state Li-metal batteries have the highest potential energy density among SSBs, the high cost, low abundance, and reactivity of Li motivate alternatives. Na-SSBs on the other hand have the potential to exceed the energy density of current state of the art LIBs with lower cost due to the natural abundance of Na.

While sulfide, chloride, closo-hydroborate, and other easily formable, high ionic conducting SSEs are currently of great interest for Na-SSBs, oxide ceramic SSEs still show the best intrinsic compatibility with pure alkali metals, motivating continued investigations to maximize the benefit of applying pure alkali anodes, as opposed to metal alloy type anodes (e.g., Li-In, Na-Sn), which show better stability with e.g. sulfide SSEs etc., at the cost of reduced specific capacity and cell voltage. To this end, our team at PNNL is currently leveraging our previous¹ efforts to improve the wettability of molten-Na toward oxide type Na-ß"-Al₂O₃ and NaSICON for molten-Na batteries, and applying this same concept to enable improved interfacial stability and performance of solid Na-metal anodes for Na-SSBs. While various wettability treatments for molten-Na have been demonstrated by our group, (e.g. application of Pb-particles), application of a mesoporous carbon interfacial layer with tuned porosity enables remarkable Na-wetting without toxic Pb or alloying elements. Initially, our carbon-based wetting layer also showed significantly better solid-state cycling performance than an alloy-based interfacial layer, nearly reaching current densities of 2 mA cm^-2 (vs. < 0.5 mA cm^-2 for a cell with Pb-based interfacial treatment) without applied external pressure.

In this presentation, continued development of this mesoporous carbon treatment as a host for pure Na-anodes for Na-SSBs will be discussed, with a focus on understanding the mechanism of improved cyclability and current density using a combination of electrochemical testing and high-resolution characterization methods such as Helium-ion microscopy, cryo-FIB analysis, and x-ray µCT imaging. Further, additives and microstructural/mesostructural design principles that can further improve achievable current densities in solid-state symmetric cells will be explored, showing that mesoporous carbon films optimized for solid-Na rather than molten-Na can enable current densities of at least 6 mA cm^-2 at room temperature and with little to no applied pressure – a factor which is crucial to enable practical SSBs at the pack level without excessive ancillary equipment to maintain pressure.

1 - J. Mark Weller, Henry H. Han, Evgueni Polikarpov, Keesung Han, Vaithiyalingam Shutthanandan, Yilin Wang, Mark H. Engelhard, Keeyoung Jung, David M. Reed, Vincent L. Sprenkle, Guosheng Li, Intrinsically sodiophilic, mesoporous metal-free wetting layers based on inexpensive carbon black for sodium-metal batteries, Nano Energy, 128, Part A, 2024, 109815, https://doi.org/10.1016/j.nanoen.2024.109815.