Electrochemical Reaction Mechanism of Li2S Active Material in All-Solid-State Lithium-Sulfur Batteries

Rechargeable lithium-ion batteries have been widely used in portable electronic devices such as cellular phones and personal computers.  Increasingly they are also being scaled up for using large applications in electric vehicles and smart grids. However, due to limited capacity in both cathodes and anodes, the specific energy density of lithium-ion batteries needs to improve remarkably to meet the requirements for practical applications.  Current cathode materials have an actual capacity less than half that of anode materials such as graphite and silicon.  Therefore, high-capacity cathode materials are required to realize lithium-ion batteries with higher-energy densities.  Lithium sulfide, Li₂S, is a high-capacity positive electrode material with the theoretical capacity of 1168 mAh g^-1.  However, there are two major issues to be solved.  One is that its low electronic conductivity restricts its actual capacity to much lower values than that theoretically predicted.  The other is capacity fading which arises from high dissolution of lithium polysulfide into the liquid electrolyte during the electrochemical reaction of Li₂S electrode.  The use of inorganic solid electrolyte (SE) is expected to inhibit the polysulfides dissolution.  We reported that a Li₂S–Cu composite electrode prepared by mechanical milling retained the reversible capacity of 350 mA h g^-1 for 20 cycles in all-solid-state cells using Li₂S-P₂S₅ SEs [1].  Recently it is found that Li₂S electrode prepared by milling with carbon materials exhibited excellent cycling stability with high capacity of 700 mAh g^-1 in the all-solid-state cells [2].  However, the origin of excellent reversibility has not been understood.  In this study, reversible structural and morphological changes at the nanoscale during the full-electrochemical cycles in all-solid-state lithium-sulfur batteries have been revealed by high-resolution TEM, electron diffraction, and energy-dispersed X-ray spectroscopy experiments.  The smooth and adhesive interface among Li₂S active material, Li₂S-P₂S₅ electrolyte, and acetylene black in Li₂S composite electrodes is realized at the nanoscale, which is fabricated by mechanical milling technique.  Reversible variations during cycles between crystallization and amorphization of sulfur-based active nanoparticles are responsible for the feasibility of the high capacity and excellent cycling stability.  

References

[1] A. Hayashi, R. Ohtsubo, T. Ohtomo, F. Mizuno, M. Tatsumisago, J. Power Sources, 183 (2008) 422.

[2] M. Nagao, A. Hayashi, M. Tatsumisago, J. Mater. Chem., 22 (2012) 10015.