Phosphate-Based All-Solid-State Lithium-Ion Batteries Assembled By Spark Plasma Sintering: Application of High-Voltage LiCoPO4

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/Li_1.5Al_0.5Ge_1.5(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.