Researchers at the University of Maryland have developed a novel, flexible, solid-state, ion-conducting membrane based on a 3D ion-conducting ceramic nanofiber network. The researchers said that their work, published in the Proceedings of the National Academy of Sciences (PNAS), represents a significant breakthrough to enable high performance lithium batteries. The all-solid ion-conducting membrane can be applied to flexible Li-ion batteries and other electrochemical energy storage systems, such as lithium–sulfur batteries.
The 3D ion-conducting network is based on percolative garnet-type Li6.4La3Zr2Al0.2O12 (LLZO) solid-state electrolyte nanofibers, which enhance the ionic conductivity of the solid-state electrolyte membrane at room temperature and improve the mechanical strength of the polymer electrolyte.
|Schematic structure of the 3D LLZO–polymer composite membrane. Fu et al. Click to enlarge.|
The flexible solid-state electrolyte composite membrane exhibited an ionic conductivity of 2.5 × 10−4 S/cm at room temperature. The membrane can effectively block dendrites in a symmetric Li | electrolyte | Li cell during repeated lithium stripping/plating at room temperature, with a current density of 0.2 mA/cm2 for around 500 h and a current density of 0.5 mA/cm2 for over 300 h.
… the success of beyond LIBs, such as lithium–sulfur and lithium–oxygen, will strongly rely on lithium metal anode designs with good stability to achieve their targeted goals of high energy density and long cycle life.
Using lithium metal in organic liquid electrolyte systems faces many challenges in terms of battery performance and safety. For example, lithium–sulfur batteries suffer from the dissolution of intermediate polysulfides in the organic electrolyte that causes severe parasitic reactions on lithium metal surfaces, leading to lithium metal degradation and low lithium cycling efficiency. Lithium–oxygen batteries have the challenge of chemically in-stable liquid electrolytes on the oxygen electrode that cause limited battery cycling. All of these challenges are associated with the use of lithium metal in liquid electrolyte battery systems.
Another major associated challenge is lithium dendrite growth on lithium metal anodes, which causes internal short circuits after lithium dendrites penetrate through the separator and touch the cathode. In addition, solid–electrolyte interphase (SEI) formation during the uneven lithium deposition will continuously consume Li metal and dry up the electrolyte, leading to an increase of cell resistance and decrease of cell Coulombic efficiency. … Li dendrite and SEI formation are inevitable and mainly caused by the intrinsic problems of the thermodynamically unstable Li with low-molecular weight organic solvents and the poor strength of formed SEI layers.
A fundamental strategy to address Li dendrite penetration and SEI formation is to develop a solid-state electrolyte to mechanically suppress the lithium dendrite and intrinsically eliminate SEI formation. … Based on our understanding, therefore, creating a continuous nanosized network with interconnected long-range ion transport and controlling a minimum/nonfiller agglomeration are the main directions to design high ionic-conductive polymer composite electrolytes.—Fu et al.
The researchers selected a garnet-type lithium-ion–conducting ceramic as the inorganic component for their 3D ceramic network for several desired physical and chemical properties:
high ionic conductivity approaching 10−3 S/cm at room temperature with optimized element substitution;
good chemical stability against lithium metal; and
good chemical stability against air and moisture.
The LLZO porous structure comprises of randomly distributed and interconnected nanofibers, creating a continuous lithium-ion–conducting network. The Li salt–PEO polymer is then filled into the porous 3D ceramic networks, forming the 3D garnet–polymer composite membrane.
Kun (Kelvin) Fu, Yunhui Gong, Jiaqi Dai, Amy Gong, Xiaogang Han, Yonggang Yao, Chengwei Wang, Yibo Wang, Yanan Chen, Chaoyi Yan, Yiju Li, Eric D. Wachsman, and Liangbing Hu (2016) “Flexible, solid-state, ion-conducting membrane with 3D garnet nanofiber networks for lithium batteries,” PNAS doi: 10.1073/pnas.1600422113