All-solid-state lithium batteries, employing sulfide solid electrolytes and conventional layered or spinel lithium transition-metal oxides as cathodes, are extensively investigated due to the rapid development of sulfide electrolytes with high ionic conductivity of 10⁻² to 10⁻³ S cm⁻¹ and chemical stability. The energy density for the all-solid-state lithium battery using LiCoO₂ as a positive material has reached the level comparable to that of liquid one. However, it is still far from meeting the demand for the electric vehicle and hybrid electric vehicle applications due to its theoretical specific capacity limitation. Moreover, power density and cycling stability remain an obstacle for an all-solid-state lithium battery to be practically applied, owing to a large interfacial resistance between the cathode and sulfide electrolyte.

This issue can be somewhat alleviated by introducing an electron-insulating and ion-conducting material as a functional buffer layer at the active material and sulfide electrolyte interface. Consequently, favorable and stable solid−solid interfaces between electrodes and solid electrolytes are crucial to achieving excellent electrochemistry performances. … it is extremely urgent to develop a simple and efficient method for constructing intimate interface contact between active materials and sulfide electrolytes.

In this work, a novel interfacial architecture, i.e., ∼10 nm Li₇P₃S₁₁ electrolyte particles anchored on cobalt sulfide nanosheets, is achieved by an in situ liquid-phase approach. The unique structure endows an intimate contact interface and uniform volume changes of cobalt sulfide nanosheets, leading to an ultrastable all-solid-state lithium battery with excellent rate capability and cycling stability.

—Yao et al.

Schematic illustration of the synthesis strategy for (a) cobalt sulfide, (b) cobalt sulfide−Li7P3S11 nanocomposites, and (c) neat Li7P3S11 electrolyte. Credit: ACS, Yao et al. Click to enlarge.

In the study, the team compared the performance of cells using the cobalt sulfide−Li₇P₃S₁₁ nanocomposites with cells using just cobalt sulfide.

To create the cells, they used neat Li₇P₃S₁₁ and super P as electrolyte and electronic additive in the cathode layer to enable high ionic and electronic conduction, and a Li₁₀GeP₂S₁₂/70% Li₂S−29% P₂S₅−1% P₂O5 bilayer as the electrolyte in the solid-state cell to increase the ionic conduction and stability to lithium anode.

Schematic of the solid-state cell. Credit: ACS, Yao et al., Supplementary Information. Click to enlarge.

The cells with the nanocomposites showed excellent rate capability and cycling stability as well as very high energy and power densities. The team suggested the strong electrochemical performance could result from:

• The anchored Li₇P₃S₁₁ coating not only forms a stable solid−solid contact interface between cobalt sulfide and sulfide electrolyte but also efficiently prevents the cracking or crumbling of the electrode and electrolyte upon continuous cycling, thus maintaining high rate capability and cycling stability.

• The electrolyte in the cathode layer with reduced particle size and improved ionic conductivity could provide an intimate physical contact between active materials and electrolyte as well as fast lithium ions diffusion, leading to excellent power and cyclic performances.

• A solid electrolyte bilayer—Li₁₀GeP₂S₁₂ and 70% Li₂S−29% P₂S₅−1% P2O5, is used, avoiding the reaction between Li₁₀GeP₂S₁₂ and lithium metal and ensuring the compatibility between electrolyte and metallic lithium. The use of a lithium metal as an anode instead of its alloys could increase the energy density of the cell.

• The two-dimensional cobalt sulfide nanosheets could provide short pathways and high kinetics for lithium ion insertion and extraction due to their unique geometry with high surface-to-volume ratios, resulting in a higher specific capacity.

Discharge−charge curves of the cells using (a) cobalt sulfide−Li7P3S11 nanocomposite electrode and (b) cobalt sulfide nanosheet electrode at various current densities (mA cm−2). (c) Ragone plots. The plots were derived from the discharge curves in (a) and (b). (d) Cycling stability at current density of 1.27 mA cm−2. Credit: ACS, Yao et al. Click to enlarge.

Resources

• Xiayin Yao, Deng Liu, Chunsheng Wang, Peng Long, Gang Peng, Yong-Sheng Hu, Hong Li, Liquan Chen, and Xiaoxiong Xu (2016) “High-Energy All-Solid-State Lithium Batteries with Ultralong Cycle Life” Nano Letters doi: 10.1021/acs.nanolett.6b03448