Low-Temperature Densification of Nasicon-Type Electrolyte for Bulk-Type All-Solid-State Battery
Authors: Okumura Toyoki , Katoh Takashi , Nakajima Kousuke , Takeuchi Tomonari , Kobayashi Hironori 
Published: December 2020

Submicron particles of Na SuperIonic CONductor (NASICON)-type Li2O-Al2O3-TiO2-P2O5 (LATP) are possible to be densified at low temperature of 300 ºC by applying high pressure of 600 MPa during spark-plasma sintering (SPS) process. This process provides a novel route for assembling bulk-type all-solid-state lithium-ion battery (ASS-LIB), which promises high safety and high durability of energy storages since the origins of flammability and undesirable electrochemical events are prevented by using oxide as an electrolyte instead of the non-aqueous liquid.
The main challenge for assembling bulk-type ASS-LIB are how to reduce the sintering temperature of the oxide electrolyte since most of the electrode materials decompose readily at high temperatures. Previously, we have attempted to design LiCoO2-electrode/NASICON-type LiTi2(PO4)3-electrolyte interface by conventional SPS process since quick/low-temperature treatment by the SPS has a possibility for reducing the decomposition kinetically.1 However, the several-μm ion-blocking interphase was easily-produced at the interface. Therefore, further low-temperature technique is required for the design of impurity-free electrode/electrolyte heterointerfaces. Commonly, the low-temperature sinterability of ceramic (or metal) particles could be accomplished with the increase of the contact for active surfaces by reducing particle size and/or applying higher pressure under sintering. Kinemuchi et al. has suggested that the low-temperature densification (LTD) of the oxide was performed by combining the use of nano particles and external pressure of over several-hundred MPa during the SPS.2 The LTDs for lithium-ion conductive oxides have possibilities for producing impurity-free electrode/electrolyte interface, and therefore would be one of the promising techniques for assembling oxide-based bulk-type ASS-LIB.
In the present study, we used submicron size of NASICON-type Li2O-Al2O3-TiO2-P2O5 (LATP) powder as high bulk lithium-ion conductive oxides, which have been developed as Lithium Ion Conductive Glass Ceramics; LICGCTM, by OHARA INC..3,4 The densification of the LATP powder compact could be certified at low temperature of 200 - 250 °C under applying a pressure of 600 MPa during the SPS. Generally, the LTDs for oxides were achieved using not submicron particles but nano particles. However, the LTDs of the submicron-size particles could be confirmed in a phosphate group, which includes Li3PO4, AlPO4, and also LATP. Therefore, the bulk-type ASS-LIBs with phosphate-based electrodes such as LiFePO4 and Li2CoPO4F were successfully assembled by the LTD processes because the undesirable-thermal impurities at electrode/electrolyte interface were prevented during the low-temperature treatment. The characterization of LiFePO4/LATP interface and the electrochemical performance of ASS-LIB cell (Li/dry polymer/LATP/LiFePO4) will be presented in the conference.
References
[1] Y. Kobayashi et al., J. Power Sources, 1999, 81-82, 853.
[2] Y. Kinemuchi et al., RSC Adv., 2016, 6, 24661.
[3] J. Fu, Solid State Ionics, 1997,96, 195.
[4] J. Fu, J. Mater. Sci., 1998, 33, 1549.