Oxides are generally non-flammable, durable and non-toxic materials; high safety and reliability in a battery system are 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 thicknesses of stacked electrode and electrolyte layers need to be curtailed to several ten to hundred μm order. This must 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. Well-known lithium-ion conductive oxides based on the garnet, perovskite, and NASICON-type structures possess high bulk conductivities of over 10^-4 S cm^-1; however, these oxides required sintering at high temperature beyond 1273 K to facilitate good contact between the electrode and electrolyte layers. In addition, most positive electrode materials produce impurities after sintering via, for example, their decomposition. To address these issues, the ASS-LIBs were assembled by the use of lithium-ion conductive oxides with low melting point and/or the use of spark plasma sintering (SPS), which could be processed at low temperature with relatively short time.[1]  

 Although the Li-ion conductivity of Li₂CO₃ was extremely low (about 10^-12 S cm^-1 at room temperature), the ion-blocking impurities were hardly formed at the electrode/Li₂CO₃ electrolyte interface since Li₂CO₃ could be sintered at lower temperatures below 973 K. Li_2+xC_1-xB_xO₃, which is isostructural with Li₂CO₃, is also a Li-ion conductive oxide. According to the literature, the Li-ion conductivity was enhanced dramatically from 10^-7 to 10^-3 S cm^-1 at 473 K as x in Li_2+xC_1-xB_xO₃ changes from 0 to 0.25. [2] We used Li_2.2C_0.8B_0.2O₃ as electrolyte material and assembled ASS-LIB by SPS method for reducing impurities under sintering.

 Li_2.2C_0.8B_0.2O₃ was synthesized using Li₂CO₃, LiOH·H₂O, and H₃BO₃ as the starting materials via a conventional solid-state reaction. The required amount of starting materials was ground and heated at 873 K for 10 h in air. The obtained powder was cold-pressed into a pellet and re-heated at 923 K for 10 h in air twice. A single phase of Li_2.2C_0.8B_0.2O₃ was identified by a powder XRD pattern of the synthesized product. For ion conductivity measurements, the prepared Li_2.2C_0.8B_0.2O₃ was firstly put in a 80 mL ZrO₂ cap with five of 5 mm ZrO₂ disks, and was crushed by disk mill. Samples for AC impedance measurement were prepared by two sintering methods; conventional furnace sintering (CFS) of uniaxial pressing powder and SPS of Au/ Li_2.2C_0.8B_0.2O₃ powder/Au. (Both sides were polished and coated with Au after the CFS method.) The sintering temperature and time at CFS and SPS were 923 K, 2h and 723 K, 1 min, respectively, for densification over 90%. Total Li-ion conductivities for Li_2.2C_0.8B_0.2O₃ SPS pellet was 2.1 x 10^-6 S cm^-1at 303 K, which was higher than that of CFS pellet: 6.7 x 10^-7 S cm^-1. The growth of Li_2.2C_0.8B_0.2O₃ grain was less observed at SEM image of the SPS pellet. The total conductivity was enhanced about three times by suppressing the grain growth via the SPS method, due probably to reducing the high-resistive layer near the grain boundary, which would be formed by long-term duration via the CFS method.

  Composite electrode powder was prepared from a mixture of 70 wt% LiCoO₂ and 30 wt% Li_2.2C_0.8B_0.2O₃ electrolyte. Au/composite electrode powder/Li_2.2C_0.8B_0.2O₃ powder was assembled by SPS method under 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 Li_2.2C_0.8B_0.2O₃ electrolyte as a separator to reduce the interfacial resistance with adhesion as possible. Electrochemical charge-discharge test was performed at a constant current of 10 μA cm^-2 at 333 K between 4.2 and 2.0 V vs. Li/Li⁺. The ASS-LIB showed an initial charge-discharge profile which is similar to the liquid electrolyte case, and the discharge capacity was 118 mAh g^-1. No impurity peak was observed in the powder XRD pattern of the LiCoO₂-Li_2.2C_0.8B_0.2O₃ composite electrode after SPS method. Thus, we have successfully prepared the oxide-based ASS-LIB, working with demonstrating the comparable discharge capacity to the LIB using liquid electrolyte, by the combination of the low-melting Li_2.2C_0.8B_0.2O₃ and the rapid sintering method.

References

[1] A. Aboulaich et al., Adv. Energy Mater., 2011, 1, 179.

[2] R. D. Shannon et al., Electrochem. Acta, 22 (1977) 783.