In the proposed research, functionalization of oxide-based solid electrolytes (SEs) by using ionic liquids (ILs) for all-solid-state lithium ion batteries (LIBs) is undertaken with the aim of reducing the interfacial resistances. SEs are promising materials which no doubt contribute to the improvements in the thermal stability and credibility of LIBs. However, oxide-based SEs generally have large grain boundary resistance and cannot exhibit a high ionic conductivity without high temperature sintering processes. As an alternative approach to reduce the grain boundary resistance, we propose the application of ILs on the SE surfaces so as to create amorphous interphases.
In the course of the last year, we have successfully obtained the composite electrolytes based on Li(7)La(3)Zr(2)O(12) (LLZO) and ILs. The ionic conductivity of pristine LLZO, specifically 2.0×10^-8 S/cm, was improved to 3.7×10^-4 S/cm merely by adding 15wt% of ILs. This result confirms the feasibility of our dual inorganic-organic concept. This year, our focus is placed on the improvement in interfacial properties between the composite electrolytes and electrodes. This is because the low conductivity across the electrolyte-electrode interphase severely limits the performance of the all-solid-state LIBs. Accordingly, the reduction of the interfacial resistance and the improvement in a long-term interfacial stability are the aim of this research, which will be done through the creation of high-performance solid electrolyte interphase (SEI) by suitably designed ILs.
The integration of inorganic-organic materials will improve the flexibility of inorganic SEs in terms of the multifariousness of ionic conductivity, the diversity of material design, and the variation in the structure of resulting batteries. We strongly deem that our composites will be a new class of electrolytes, which provide significant benefits for LIBs both in scientific- and industrial-level.
The design of LLZO-IL composites will be carried out based on the dual concept of organic and inorganic chemistry. This combination is not well discovered in the field of inorganic ceramic SE for LIBs. Accordingly, the originality of our LLZO-IL hybrid electrolytes is high. The LLZO-IL composites are expected to exhibit three unique advantages, all of which are associated to FLEXIBILITY.
1. FLEXIBILITY in the formation of ion conduction path
By applying ILs on LLZO surface, the ion conduction is allowed not only in the LLZO particle inside but also along the particle surfaces. Accordingly, the multifariousness of ion conduction pathways will be greatly enhanced. In addition, this will be beneficial also to reduce the charge transfer resistance between the electrode and electrolytes. Having a high ionic conductivity is the first foremost important property for the electrolytes. In general, to suppress the grain boundary resistance and to improve the ionic conductivity, SEs need to be sintered around 1000 oC. A cold sintering process has also been reported, but it still requires heating at around 300 oC. [J. Am. Ceram. Soc. (2017) 100, 2123] The ion conduction paths formed through these sintering procedures are not durable, and the conductivity drops if cracks are formed in the SE pellets. In contrast to this, the conduction paths formed by ILs are flexible and sustainable. Our strategy will be an alternative approach to control the unfavorable coarse interfaces so as to enhance the sustainable ion conduction.
2. FLEXIBILITY of shape processability
SEs are beneficial materials for eliminating the possibility of electrolyte leakage. On the other hand, SEs make difficult the creation of fine structures or thin films due to their brittleness. In the LLZO-IL composites, ILs work as a glue between SE particles. Therefore, the composite is expected to become malleable, and the processability of SEs is thus enhanced. This means thin film structures can be obtained without sensitive and complicated deposition techniques. Moreover, because the inherent flexibility of the materials is improved, the geometry of the resulting batteries will be shock resistive.
3. FLEXIBILITY in the functional design
The most significant advantages of the use of organic compounds are the diversity of their structures and functions. In the case of hybrid materials based on LLZO and ILs, further functions can be added through the modifications of ILs. As an example, the cations of ILs can be customized with allyl-group for polymerization of ILs to inhibit the IL bleed-out. Moreover, taking into account the fact that vinyl compounds are often used as an additive to artificially form SEI, also the allyl-functionalized ILs are expected to form suitable SEI in LLZO-IL composites. Also other functions, such as elasticity and self-healing properties, can be added by designing ILs. These functionalizations cannot be achieved only by using inorganic materials. The combination of inorganic-organic concept will exponentially widen the variety of the functionalization.
Overall, the composite materials promise the improvement of thermal stability of LIBs because they do not contain flammable component and will bring benefits especially for high temperature applications such as electric vehicles. We strongly deem that our electrolyte will be a new class of electrolytes, which have great impact in the field of all-solid-state LIBs.