All-Solid-State Lithium-Ion Battery Assembled By Low-Temperature / High-Pressure Spark Plasma Sintering
Authors: Okumura Toyoki , Takeuchi Tomonari , Kobayashi Hironori 
Published: February 2020

Oxides are generally non-flammable, durable and non-toxic materials; high safety and reliability in a battery system is assured by the use of oxide as an electrolyte instead of the highly-reactive non-aqueous liquid. To develop an oxide-based all-solid-state lithium-ion battery (ASS-LIB) for applications such as electric vehicles, the formation of strong interfacial contact between the electrolyte and the electrode powder is desirable, which can be achieved through powder technology. In oxide-based ASS-LIB, the good contact interfaces should be prepared by a simple powder sintering process, and also produces little ion-blocking impurities.[1] A Li2O-Al2O3-TiO2-P2O5 (LATP) solid electrolyte with NASICON-type structure possesses high bulk conductivity of over 10-4 S cm-1 at R.T. and high electrochemical stability (applicable potential range: 2.6−6 V vs Li/Li+)), which is a suitable candidate for assembling ASS-LIB with high safety and chemical stability.　However, the poor interfacial contact between electrode and solid electrolyte is one of the general problems for showing the electrochemical activity of ASS-LIB. Spark plasma sintering (SPS) would be a useful tool for designing the ASS-LIB since dense ceramics can be sintered at shorter time with suppressing the formation of by-products at the interface. Actually, Aboulaich et al. have successfully measured the charge-discharge profiles of Li3V2(PO4)3 and LiFePO4 electrodes at ASS-LIB assembled with Ge-based Li1.5Al0.5Ge1.5(PO4)3 electrolyte by this SPS technique.[2] However, most electrode materials produce impurities by contact with the solid electrolytes containing Ti4+ ions as LATP after sintering at high temperature (900oC).[3] In this presentation, the ASS-LIBs using LATP electrolyte were assembled by SPS with tungsten carbide die, instead of conventional carbon die, which could be applied high pressure over 200 MPa and processed at low temperature below 300 °C, resulting in suppressing the formation of by-products at the interface.
Samples for AC impedance measurement were prepared by SPS of Au/LATP powder (average particle size: 0.2 μm)/Au. The sintering temperature, applied pressure and time at SPS were 300 °C, 600 MPa and 1 min for the densification. During the SPS process, the shrink of the LATP pellet was observed at around 200 – 250 °C, which was obviously lower than the densification temperature of LATP at conventional sintering in furnace. The increase of neck region at the grain-boundary of LATP particles could be also observed by cross-sectional SEM image of LATP pellet. As a result, total Li-ion conductivities for LATP pellet was 2.2 x 10-5 S cm-1 at 30 °C.
Composite electrode powder was prepared from a mixture of 50 wt% carbon-coated LiFePO4 and 50 wt% LATP electrolyte. Au/composite electrode powder/ LATP powder was assembled by SPS process at the same condition for the sample of AC impedance measurement. Lithium foil was used as a reference/counter electrode. A poly(ethylene oxide)-based polymer electrolyte film was inserted between the lithium foil and the LATP electrolyte separator to prevent the reduction of Ti4+ ion in LATP by contacting with the lithium metal. Electrochemical charge-discharge test was performed at a constant current of 10 μA cm-2. The ASS-LIB shows initial charge-discharge profile which was similar to the liquid electrolyte case, and the discharge capacity was 75 mAh g-1 at 28 °C and 131 mAh g-1 at 60°C, respectively. No impurity peak was observed in the powder XRD pattern of the LiFePO4-LATP composite electrode after SPS process, due to the low sintering temperature. Therefore, the low-temperature and high-pressure SPS process with tungsten carbide die would be one of the superior techniques for assembling ASS-LIBs with suppressing the formation of by-products and reducing grain-boundary resistance at electrode/electrolyte and electrolyte/electrolyte interfaces.
References
[1] T. Okumura, Solid State Ionics, 2016, doi:10.1016/j.ssi.2016.01.045.
[2] A. Aboulaich et al., Adv. Energy Mater., 2011, 1, 179.
[3] Y. Kobayashi et al., J. Power Sources, 1999, 81-82, 853.