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Nature Energy focuses on the key to large-scale solid electr

Time:2021/02/02 丨 source:未知 丨 visit count:

 Nature Energy focuses on the key to large-scale solid electrolytes: cost!

 

 
In the next few decades, to realize the true potential of lithium-ion batteries (LIBs), continuous innovation is required to build a safer, stronger and better battery system. In the use of higher energy density batteries, lithium metal batteries (LMBs) with lithium metal as the negative electrode stand out, but the safety of use needs to be further improved. One of the most promising ways to improve the safety of LMBs is to replace the "liquid" ion conductive electrolyte and polymer separator in traditional LIBs with "solid" conductive lithium electrolyte membranes in the structure of solid state batteries (SSBs).
 
Compared with polymer electrolytes, the conductivity of oxide and sulfide ceramic SSBs electrolytes can reach a level equivalent to that of "liquid" electrolytes. At the same time, only oxides can provide a relatively wide electrochemical stability window, and can be paired with high-voltage anodes to achieve high power density and high energy density, but the shortcomings are also prominent.
 
Firstly, oxides are brittle and have unfavorable mechanical properties. Once the electrolyte thickness is reduced, it may become more obvious. Secondly, the compatibility of oxide solid electrolytes with current cathode chemistries is limited, and it is mainly co-sintered with the components. The high temperature process involved is related. Reducing the processing temperature is a necessary prerequisite to ensure good chemical compatibility; third, oxides usually have a higher density than other types of electrolytes (sulfides and polymers), which is detrimental to the overall weight energy density, so it needs to be used Lithium metal negative electrode and high voltage positive electrode.
 
In view of this, Professor Jennifer LM Rupp from Massachusetts Institute of Technology (corresponding author) systematically discussed the research status of SSB preparation and its cost factors, and compared SSB oxide electrolyte materials and processing options from the performance parameters of thick ceramics and thin ceramics. . The author believes that for the future SSB design, it is very important that in addition to the classic diagram of Arrhenius lithium transport and electrochemical stability window, it is also necessary to pay attention to the heat treatment process and phase stability of the oxide solid electrolyte, including Lithium Phosphorus Oxide (LiPON), sodium superion conductor, perovskite and garnet structure. Finally, the thickness of the solid electrolyte membrane is close to the thickness of the lithium-ion battery separator, which provides ample opportunities for low-temperature ceramic preparation and potential cost reduction. Related research results "Processing thin but robust electrolytes for solid-state batteries" were published on Nature Energy.
 
1. Prospect of Li SSBs electrolyte manufacturing cost
 

 
Figure 1. The cost and design considerations of SSB based on lithium metal. (a) Typical Li metal-based SSB structure design; (b) Estimated cost of SSB and LIB based on LLZO estimates and material costs; (c) Typical thickness range of solid electrolytes reported.
 

Figure 2. SSB structure and actual processing temperature window for different composite cathode/electrolyte combinations. (a) Co-sintering and interface engineering, and the proposed half-cell structure for low-temperature processing without co-sintering SSB; (b) Processing temperature window of common oxide electrolyte and positive electrode.
 

 
2. Properties of thick and thin ceramic SSB electrolyte

Figure 3. The properties of different oxide solid electrolytes. (a) The structure, local bonding unit and network of oxide-based lithium ion conductors, including amorphous LiPON, NASICON-type LATP, perovskite-type LLTO and garnet-type LLZO; (b) Different processing methods are used to obtain solid state Electrolyte, and carried out relevant lithium ion conductivity analysis; (c) Compared with the most advanced liquid electrolyte EC:PC:LiPF6, lithium oxide-based solid electrolyte in the form of particles and film lithium ion conductivity; (d) Theoretical electrochemical stability window based on first-principles thermodynamic calculations; (e) the reported processing temperature.
 

Figure 4. The ionic conductivity of oxide solid electrolytes in different processing paths; (ad) Compared with their particle conductivity, LiPON film, NASICON type LATP film, perovskite type LLTO film and garnet type LLZO film Conductivity


Figure 5. Overview of the different available lithiation strategies. (a) Lithium loss mechanism during high-temperature film annealing; (b) Excessive lithiation during vacuum film deposition; (c) Co-deposition of vacuum film with lithium source; (d) Construction of lithiation inside vacuum film; (e) ) Excessive lithiation of wet chemical film precursor solution.


 
First of all, the author has collected a lot of evidence, showing that there are enough opportunities to manufacture ceramic membranes with the required size range of 1-20μm to replace the polymer separators in LIBs, and have the advantages of high electrochemical stability and compatibility with lithium.
 
Secondly, the preparation of electrolyte membranes based on LATP, LLTO or LLZO does not necessarily require the use of classic sintering methods, and wet-chemically scalable routes can also be developed. At the same time, many cost estimates for the preparation of SSBs are based on processing methods similar to SOFCs, but do not consider all alternative ceramic processing technology strategies that do not require sintering. If further progress can be made in film processing, so as to achieve cost targets on scale.
 
Third, low-temperature ceramic processing will make the new generation of amorphous solid lithium electrolyte ceramics more stable than LiPON, including LATP, LLTO and LLZO. At the same time, these materials are easier to obtain in the form of thin films or thick films without the need for high temperature sintering. They can be used as all-solid electrolytes or buffer layers to achieve long life cycles. In the SSB design, their use will achieve a higher lithium ion migration number and a wider electrochemical stability window, which may become more important when considering that the SSB electrolyte should be thinner.
 
Fourth, shift ceramic processing from high-temperature sintering to lower processing conditions to ensure the phase stability and high performance of SSB ceramics.
 
This article provides a strategy for how to control the intrinsic lithiation degree of SSB materials after ceramic processing, which is a key parameter to realize the continuous transition of most SSB electrolyte materials from thick to thin. The author encourages the preparation of SSB to use heat treatment maps to reflect the temperature that can be used to stabilize thin or thick films, and to establish a fusion strategy for the stability of the positive electrode and lithium.
 
Moran Balaish, Juan Carlos Gonzalez-Rosillo, Kun Joong Kim, Yuntong Zhu, Zachary D. Hood, Jennifer L. M. Rupp, Processing thin but robust electrolytes for solid-state batteries, 2021, DOI:10.1038/s41560-020-00759-5

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