Phosphate-Based All-Solid-State Lithium-Ion Batteries Assembled By Spark Plasma Sintering: Application of High-Voltage LiCoPO4
Authors: Okumura Toyoki , Takeuchi Tomonari , Shikano Masahiro , Kobayashi Hironori 
Published: February 2020

Realization of all-solid-state lithium-ion batteries (ASS-LIB) consisting of non-flammable inorganic solid electrolytes is desired because of their high safety. To improve energy and power densities of the ASS-LIB, the decrease of the interfacial resistance between electrode and solid electrolyte is required as well as the synthesis of solid electrolyte with high lithium-ion conductivity and the development of high thermodynamically stable positive and negative electrode materials. Spark plasma sintering (SPS), by which sintered ceramics can be assembled at shorter time and lower temperature, would be one of the useful tools for designing the ASS-LIB since the by-products at the interface are inhibited as well as possible. Actually, Aboulaich et al. have first reported the charge-discharge profiles of Li3V2(PO4)3 or LiFePO4/Li1.5Al0.5Ge1.5(PO4)3/Li3V2(PO4)3ASS-LIB using the SPS technique [1].
LiCoPO4 has a high redox potential for lithium-ion (de-)intercalation of c.a. 4.8 V vs. Li/Li+ with a theoretical capacity of 167 mAh/g. However, this material shows the capacity fading during charge–discharge cycling in LiPF6 based electrolyte solutions as shown in inserted figure (b) because of a nucleophilic attack of F− anions on the P atoms of the electrode surface [2]. Therefore, we assembled high-voltage LiCoPO4 positive electrodes with high-electrochemically-stable Li2O-TiO2-Al2O3-P2O5(LATP) solid electrolytes, and then analyzed electrochemical properties as ASS-LIB in present study.
A LiCoPO4 powder was prepared by sucrose-aided combustion reaction. The XRD pattern of synthesized powder indicated the olivine-type structure (S.G.: Pnma). Li2O-TiO2-Al2O3-P2O5 (LATP) powder was sintered at 1100 oC as solid electrolyte pellet. Then, the mixed pellet of LiCoPO4, LATP, acetylene black and polytetrafluoroethylene (33 : 33 : 17 : 17 (wt%)) was put on the both faces of the solid electrolyte pellet, and sintered at 700 oC by using SPS. The carbon sheets were used as current collector of the ASS-LIB. The electrochemical test was carried out at a constant current density of 0.12 mA/cm2 at 250 oC using a battery test device (Solartron 1470). The specific two-step plateaus of LiCoPO4 during charging process can be observed around 2.2 and 2.7 V as shown in figure (a). This result means that lithium-ion removed from LiCoPO4 in one electrode side and extracted from LATP in the other side. (The redox plateau of LATP is around 2.45 V vs. Li/Li+). However, a large potential drop and low capacity could be observed during discharging. After charging, the large resistance of 2800 Ω/cm2 at the interfaces was measured by AC impedance spectroscopy. In the presentation, we also discuss the ASS-LIB test results of LiCoPO4 mixed with Li3PO4, which aided the contact between LCP electrode and LATP electrolyte during SPS.

References

[1] A. Aboulaich, R. Bouchet, G. Delaizir, V. Seznec, L. Tortet, M. Morcrette, P. Rozier, J. Tarascon, V. Viallet, M. Dolléb, Adv. Energy Mater., 1, 179–183 (2011).
[2] E. Markevich, R. Sharabi, H. Gottlieb, V. Borgel, K. Fridman, G. Salitra, D. Aurbach, G. Semrau, M.A. Schmidt, N. Schall, C. Bruenig, Electrochem. Commun.,15 (2012) 22–25.