Researchers from China and the US have developed a flexible slurryless nano-Li2S/reduced graphene oxide cathode paper (nano-Li2S/rGO paper) which can be directly used as a free-standing and binder-free cathode without metal substrate, which leads to significant weight savings in batteries. The cathode paper is intended to be paired with safer anode materials such as silicon, aluminum, tin, graphene, transition metal oxides, and so forth, rather than Li-metal anodes. (Earlier post.)
The flexible and conductive paper electrode shows excellent cycling life and rate capability with a reversible discharge capacity of 816.1 mAh g−1 after 150 cycles at 0.1 C, and 597 mAh g−1 even at 7 C. After cycling 200 times at 5 C, the capacity can still remain at 462.2 mAh g−1. A paper on the team’s work is published in the ACS journal Nano Letters.
Lithium sulfur (Li−S) battery is a promising candidate for next- generation energy storage devices. For traditional Li−S battery systems, Li metal is used as the anode material. However, lithium metal has safety and cyclability issues due to the formation of lithium dendrites; the pulverized and detached lithium metal formed during cycling can result in explosions when exposed to air. In view of this, lithium sulfide (Li2S) has attracted much attention because safer anodes such as nanostructured silicon, aluminum, tin, graphene, transition metal oxides, and so forth can be paired with Li2S, which do not have dendrite problem.
Li2S has a high theoretical capacity of 1166 mAh g−1 and a high melting point of 938 °C. However, two significant issues with the sulfur cathode, poor electrical conductivity and intrinsic polysulfide shuttle, also exist in Li2S. Furthermore, because Li2S is highly sensitive to moisture and oxygen its handling is often more complex than pure sulfur and should be put in a protected atmosphere all of he time. Also, even though Li2S should have better electrical conductivity than pure sulfur it is still essential to introduce additional electron conductors into Li2S.
Traditionally, this is done via the slurry approach, where the additional electron conductors (e.g., carbon), Li2S, and binder powders are mixed into a slurry, and then doctor-bladed on solid aluminum foil and baked. But the slurry has relatively poor electron conductivity and mechanical properties. Uniform dispersion of Li2S into a natively more conductive and flexible long-range 2D framework, like a macroscopic piece of paper, could be an effective way to utilize Li2S in Li−S batteries.—Wang et al.
The researchers developed a simple drop-coating process for the cathode paper. Graphene oxide was freeze-dried and then reduced in a heat treatment to produce reduced graphene oxide (rGO) paper. The nano-Li2S loading was then drop-coated using anhydrous ethanol as the solvent (Li2S is very soluble in ethanol).
The rGO paper has a strong solvent absorbency; the Li2S solution infiltrated throughout the rGO paper and the Li2S particle precipitates distributed very homogeneously inside the rGO paper. The researchers found that, after ethanol evaporation under a vacuum, the precipitated Li2S particles are nanosized spheres.
The flexible rGO paper prevents the agglomeration of Li2S particles and also can accommodate the stress caused by the large volumetric changes during lithiation and delithiation. Further, the long-range rGO network is conductive and mechanically stable during cycling.
The team suggested the strong electrochemical performance may be attributed toa number of factors:
the nanosized Li2S particles could decrease the energy barrier for Li ions transport;
the uniform distribution of Li2S nanoparticles in the rGO framework can improve the utilization of Li2S;
the good solvent absorbency of rGO paper can confine polysulfides between the graphene layers and alleviate the capacity loss during cycling; and
the interconnected conductive rGO paper can facilitate transportation for both electrons and Li ions.
By accommodating the stress generated from the large volume change during charge and discharge, the flexible nano-Li2S/rGO paper maintains the mechanical integrity of the electrode—important for cyclability.
Chao Wang, Xusheng Wang, Yuan Yang, Akihiro Kushima, Jitao Chen, Yunhui Huang, and Ju Li (2015) “Slurryless Li2S/Reduced Graphene Oxide Cathode Paper for High-Performance Lithium Sulfur Battery” Nano Letters doi: 10.1021/acs.nanolett.5b00112