Designing the perfect battery can be a challenging task, if you want to not only create a device which can store a lot of energy but is also safe. Liquid electrolytes, for example, work reliably over many cycles; however, chances are high that the battery might catch fire at some point. Therefore, scientists have turned their attention towards lithium batteries consisting entirely of solid components, so-called solid-state batteries, as they are safer and also offer a higher energy density.
Now (2022), scientists at Argonne National Laboratory have found a new solid electrolyte which has several important advantages. This electrolyte is made up of lithium, scandium, indium as well as chlorine and conducts lithium ions well but electrons poorly, which is an important trait if you want to create an all-solid-state battery that functions without losing a lot of capacity for over a hundred cycles at high voltage (above 4 volts) and thousands of cycles at intermediate voltage. The chloride nature of the electrolyte was responsible for its high stability at operating conditions which also makes it suitable for today’s commonly used lithium-ion cells. Current iterations of solid-state electrolytes rely heavily on sulphides, which can oxidise and degenerate above 2.5 volts and require an insulating coating to be applied around the cathode material that operates above 4 volts, which, in turn, hinders the ability of electrons and lithium ions to move from the electrolyte and into the cathode. Therefore, the scientists decided to replace half of the indium with scandium which afforded lower electronic and higher ionic conductivity. Also, the scientists had to find a solution to economically loading the spinel, the material’s crisscrossing 3D structure, with as many charge carrying ions as possible, but also leaving sites open for the ions to pass through. The ideal design, they found, would be to have half the sites occupied by lithium and leave the other half remaining open. In order to avoid electrolyte decomposition at high voltage, the scientists had to ensure a barrier existed so that the electrons could not get through. Even though the electronic conductivity in the new electrolyte was worse than in other chloride electrolytes, this design provided a clean interface between the cathode material and solid electrolyte, which was responsible for a stable performance of the solid-state lithium-ion electrolyte.
For a long time, scientists have concentrated their efforts on improving the performance of solid-state batteries. In 2019, a solid polymer electrolyte which enabled chloride ion transfer and was made up of poly(ethylene oxide) as the polymer matrix, tributylmethylammonium chloride as the chloride salt, and succinonitrile as the solid plasticiser was designed. The polymer electrolyte exhibited conductivities of 10−5–10−4 S (Siemens) cm−1 in the temperature range of 298–343 K. When incorporated within the iron oxychloride/lithium electrode system, reversible electrochemical redox reactions of FeOCl/FeO at the cathode side and Li/LiCl at the anode side were performed, which showed that this was a full all-solid-state rechargeable chloride ion battery.
Image: Schematic diagram of the ASS-RCIB with the PEO1-TBMACl1-SN3 SPE and the FeOCl/Li electrode couple. b) Optical photos of a soft-packing ASS-RCIB. c) Open circuit potential of flexible soft packet ASS-RCIB. d) Optical image of red LEDs powered by the soft-packing ASS-RCIB.
Source: Chao Chen, Tingting Yu, Meng Yang, Xiangyu Zhao, Xiaodong Shen/ An All-Solid-State Rechargeable Chloride Ion Battery/ Advanced Science Volume 6, Issue 6, March 20, 2019/ doi.org/10.1002/advs.201802130/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License.
In 2021, scientists designed a cost-effective chloride solid electrolyte, Li2ZrCl6, whose raw materials were quite inexpensive and ionic conductivity (0.81 mS cm–1 at room temperature) was high. Moreover, it showed good humidity tolerance without any moisture uptake or conductivity degradation after being exposed to an atmosphere with 5% relative humidity.
By combining the electrolyte with the Li-In anode and the single-crystal LiNi0.8Mn0.1Co0.1O2 cathode, a room-temperature all-solid-state cell which had a stable specific capacity of about 150 mAh g–1 for 200 cycles was achieved.
Image: BVSE analysis of Li-ion migration within the α-LZC and β-LZC structures.
Source: Kai Wang, Qingyong Ren, Zhenqi Gu, Chaomin Duan, Jinzhu Wang, Feng Zhu, Yuanyuan Fu, Jipeng Hao, Jinfeng Zhu, Lunhua He, Chin-Wei Wang, Yingying Lu, Jie Ma, Cheng Ma/ A cost-effective and humidity-tolerant chloride solid electrolyte for lithium batteries/ Nature Communications volume 12, Article number: 4410 (2021), 20 July 2021/ doi.org/10.1038/s41467-021-24697-2/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License.
There are many benefits to the new solid-state electrolyte design: one undoubtedly is that it does not easily catch fire and can be placed efficiently within the battery cell. This leads to stable high-voltage operation. Moreover, chloride electrolytes oxidise only at high voltages and are compatible with a wide range of cathodes.
Even though there are still a few minor obstacles to overcome, such as boosting the performance of the chloride electrolyte, the scientists are positive that their discoveries will have a lasting impact on the technology of solid-state lithium-ion batteries.