Tsinghua review of solid-state Li-metal batteries finds great promise, but much work still to be done
In a review published in the journal Chem of the technical issues generated by the mating of Li-metal anodes and solid-state electrolytes, a team from Tsinghua University concludes that such solid-state lithium metal batteries (SSLMBs) have “brilliant prospects, though practically there are still many barriers to be overcome.”
A solid-state li-metal anode with mixed ion and electron conducting framework in the electrode materials, as well as dense and flexible electrolyte in the separator layer, and the interfacial layer between them. Cheng et al.
Li metal is extensively focused because of its ultra-high theoretical capacity (3,860 mAh g-1, 10 times higher than that of graphite) and the most negative electrochemical potential (3.040 V versus standard hydrogen electrode). Relative to non-aqueous electrolyte, solid-state electrolyte exhibits enhanced safety. Therefore, the marriage of Li metal and solid-state electrolyte is expected to inherit the merits of both.
However, the competitive solid-state Li-metal batteries encounter formidable challenges, including dendrite issues and poor solid-solid contact. Investigating the interdisciplinary issues of solid-state electrolyte and Li metal in integrated cells is necessary to realize efficient and safe solid-state Li-metal batteries.—Cheng et al.
The review summarizes issues including the large interfacial resistance; uncontrolled dendrite growth; and low operation current or capacity. A section is dedicated primarily to understanding the ionic channels in the composite electrolyte and the space charge layers in the interfacial region.
Based on their review, the researchers outlined areas on which to concentrate in future research on SSLMBs:
Surface chemistry. A SSLMB is a complicated system, requiring comprehensive understanding of the roles of the SSE and Li-metal anode, especially operating at a large current and capacity. The surface and interface chemistry and materials evolution at the interface between SSE and Li-metal anode are still unknown. The current and capacity adopted so far are usually less than 1.0 mA cm-1 and 1.0 mAh cm-2—far from practical requirements.
Li-ion diffusion and reduction at the interfaces. Li ions from the bulk electrolyte have to diffuse through the interface and get reduced to Li metal. How do Li ions diffuse through the interfacial layer? Where do Li ions get electrons in the composite anode framework? These are still open questions.
Stable interface construction. Nanoscience provides more opportunities to regulate the component and structure of the artificial SEI. How should the component and structure of the interfacial film be accurately controlled? A fundamental understanding of chemical mechanisms is critically significant to construct a stable interface.
Reduction in the interfacial impedance. The solid-solid contact between SSE and Li metal can lead to a large interfacial impedance, while its origin and regulation rules are still not clear. The large impedance can result in fatal effects on the cycling performance of solid-state batteries. Is there any self-healing method to reduce the interfacial impedance efficiently and permanently?
Compatibility with cathodes. Under abuse, a metal oxide cathode can release oxygen, which can reach the SSE and Li-metal anode and lead to performance deterioration and safety concerns. Clearly understanding the materials evolution and chemical mechanism contributes to releasing controllably the energy and maintaining efficiently safety when solid-state Li-metal anodes are matched to different cathodes—especially the high-voltage intercalation cathodes.
Characterizations of the working batteries. The features of SSE, Li-metal anode, and their interfaces at different charge and discharge depths are largely unknown to researchers. In situ and in operando characterizations of the working batteries can help researchers to obtain the exact evolution process and chemical mechanism of different agents in the solid-state batteries and help to regulate them accurately and efficiently.
High-throughput screening. Artificial intelligence and machine learning can greatly improve the efficiency in the materials screening of SSE, interfacial layer, electron-conductive matrix, and other cell components. Combining high-throughput screening and theoretical chemistry, physical chemistry, and material chemistry can vastly improve the research progress of SSLMBs.
Cell system construction. A functional energy system includes not only a cathode, electrolyte, and anode but also the battery management system. Combining chemical information of the electrode and electrolyte materials with battery management and model prediction can possibly decouple the relationships between the real-time cell energy density and safety and the charge-discharge depth in a cell unit and package scale—the only way for a commercial battery.
Applying the research paradigm to other systems. The theory and mechanism of SSLMBs can be applied to other energy storage systems. Understanding the relations between solid-state batteries and energy chemistry can boost the development of next-generation energy storage systems.
It is the best of time for the development of energy storage systems, while it is also the dawn for SSLMBs if their practical applications could ever see the light of day. More efforts are required to boost the progress. Theoretical chemistry presents novel insights into the chemical reactions and a possibility to rapidly select the chemical systems; materials chemistry allows researchers to accurately synthesize the electrolyte and electrode materials; energy chemistry can clearly provide the interfacial features between SSE and Li-metal anode. The synergism from chemistry, engineering, energy, materials, mechanics, and battery management sheds new lights in the practical applications of SSLMBs. It is the spring of hope for SSLMBs, and we can have all the premises to overcome the difficulties with persistent researches from multidisciplinary fields.—Cheng et al.
Cheng et al. (2018) “Recent Advances in Energy Chemistry between Solid-State Electrolyte and Safe Lithium-Metal Anodes,” Chem doi: 10.1016/j.chempr.2018.12.002