Beijing, Argonne researchers develop new solid-state Li-ion battery; glassy nanocomposite electrolyte with ILs
Researchers from the Beijing Institute of Technology and Argonne National Laboratory have developed a new solid-state Li-ion battery technology, consisting of a solid nanocomposite electrolyte using porous silica matrices with in situ immobilizing Li+-conducting ionic liquids; mesocarbon microbeads (MCMB) as anode material, and LiCoO2 (LCO), LiNi⅓Co⅓Mn⅓O2 (NCM), or LiFePO4 (LFP) as cathode material.
Solid-state full cells tested with the various cathodes exhibited high specific capacities, long cycling stability, and excellent high temperature performance. A paper on the work is published in the ACS journal Nano Letters.
In … solid-state LIBs [lithium-ion batteries], one important component that needs to be improved to make it more suitable for high performance applications is the electrolyte material. Generally, high Li+ ion mobility and a wide voltage window are required for high energy applications, efficient charge and discharge with a minimum of power loss to resistive heating, and good structural stability and electrode− electrolyte interface compatibility to guarantee battery safety. However, up to now, very few solid electrolytes have been developed with the above combination of performance parameters. For example, polymer solid electrolytes prevent leakage but do not solve the flammability issue due to their organic nature of thermal degradation; inorganic solid electrolytes, which are solvent free and thus keep batteries safe, suffer from low conductivities and insufficient electrode−electrolyte interfaces for high power applications.
Despite significant research in this area, there remains a need for improved electrolyte materials that can be easily incorporated into solid-state LIBs without expensive synthesis cost or a complex fabrication process.
Solid-state ion-conducting composite systems, which are based on in situ immobilizing ionic liquids (ILs) within organic, inorganic, or hybrid porous matrices, offer a wide choice of electrolyte materials for solid-state LIBs. In these composites, generally, the ILs always maintain their liquid dynamics, so they are responsible for ion conducting and other electrochemical properties; the porous matrices provide abundant channels to confine ILs while maintaining good mechanical properties, so the composites look like solid materials. Such solid composite electrolytes also have been previously shown to reduce lithium dendrite formation and proliferation in lithium metal batteries.—Tan et al.
The Li+-conducting IL the Beijing/Argonne team investigated was [BMI][TFSI]/ LiTFSI, where [BMI][TFSI] is 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonylimide), and LiTFSI is lithium trifluoromethanesulfonate. They incorporated this IL into mesoporous silica matrices via a nonaqueous sol−gel route. The resulting composite possessed a transparent glass shape, abundant internal cross-linking networks, hybrid solid/liquid nanocompositions, and fluid-like dynamics.
These properties result in excellent structural and electrochemical properties: stable mechanical strength; good thermal stability; high ionic conductivity; and wide voltage window.
The researchers constructed solid-state Li-ion full cells composed of an LCO, NCM, or LFP cathode, the solid electrolyte, and MCMB anode. They used two approaches:
The conventional method of stacking an as-prepared electrolyte thin film on a cathode, followed by a metal Li electrode to form a half cell;
The other by injection printing—using a syringe to print a layer of NE solvent precursor onto the as-prepared anodes and cathodes, then after gelation, drying them under vacuum at 100 °C. Several layers are required to obtain NEs with enough thickness about 30 μm to act as a solid electrolyte separator. Then, the NE-coated cathodes and anodes are stacked face to face in a coin cell to form a solid-state full cell.
Injection printing results in electrodes and electrolytes that are mixed uniformly, which improves the interface compatibility, reduces the interfacial resistance, and promotes charge transfer.
Among the findings from their testings:
The LCO cell delivered an initial discharge capacity of 138.8 mAh g−1; an initial Coulombic efficiency of 90.6%, and a capacity retention of 84.0% after 100 cycles.
The NCM cell showed an initial discharge capacity of 143.0 mAh g−1, an initial Coulombic efficiency of 89.2%, and a capacity retention of 92.4% after 100 cycles.
The LFP cell showed discharge capacity of 144.6 mAh g−1, Coulombic efficiency of 93.9%, and 98.9% capacity retention. The LFP full cell showed the best battery performance, including largest reversible capacity, highest Coulombic efficiency, and best cycling performance, which the team attributed to the LFP yielding the lowest charge/discharge voltage plateau and nanoscale effects of LFP particles.
The researchers attributed the performance to two characteristics of their solid-state design.
The fluid-like dynamics, which ensure fast Li+ ion conduction in the electrolyte and favorable charge transfer at electrode/electrolyte interfaces.
The accommodation of structural change over long-term cycling, which retains the structural integrity of the electrodes and electrolytes, and also stabilizes the solid−electrolyte interphase (SEI) on the electrode/electrolyte interface.
The volume change of electrodes in this solid-state battery is much less than that in conventional batteries containing organic electrolytes. Also, the accommodated structural change ensures that the SEI on the electrode is thin and stable. This mechanism not only decreases the loss of irreversible capacity but also improves the charge-transfer efficiency and cycling stability.
An additional advantage of such solid-state battery technology is that the NEs can be processed easily to give various shapes and sizes, allowing their application in electrical devices, and their synthesis does not involve any complex equipment or processes, resulting in the potential for low cost and easy reproducibility in commercial production and promotion.
… This approach allows fabrication of high-performance solid- state batteries with improved safety and cycle-life. This special solid-state design also provides new avenues for the rational engineering of battery configurations for lithium-ion batteries, beyond lithium-ion batteries, and other electrochemical devices.—Tan et al.
Guoqiang Tan, Feng Wu, Chun Zhan, Jing Wang, Daobin Mu, Jun Lu, and Khalil Amine (2016) “Solid-State Li-Ion Batteries Using Fast, Stable, Glassy Nanocomposite Electrolytes for Good Safety and Long Cycle-Life” Nano Letters doi: 10.1021/acs.nanolett.5b05234