Fast Li-Ion Conduction in Spinel-Structured Solids

Abstract

Spinel-structured solids were studied to understand if fast Li⁺ ion conduction can be achieved with Li occupying multiple crystallographic sites of the structure to form a "Li-stuffed" spinel, and if the concept is applicable to prepare a high mixed electronic-ionic conductive, electrochemically active solid solution of the Li⁺ stuffed spinel with spinel-structured Li-ion battery electrodes. This could enable a single-phase fully solid electrode eliminating multi-phase interface incompatibility and impedance commonly observed in multi-phase solid electrolyte-cathode composites. Materials of composition Li_1.25M(III)_0.25TiO₄, M(III) = Cr or Al were prepared through solid-state methods. The room-temperature bulk Li⁺-ion conductivity is 1.63 × 10^-4 S cm^-1 for the composition Li_1.25Cr_0.25Ti_1.5O₄. Addition of Li₃BO₃ (LBO) increases ionic and electronic conductivity reaching a bulk Li⁺ ion conductivity averaging 6.8 × 10^-4 S cm^-1, a total Li-ion conductivity averaging 4.2 × 10^-4 S cm^-1, and electronic conductivity averaging 3.8 × 10^-4 S cm^-1 for the composition Li_1.25Cr_0.25Ti_1.5O₄ with 1 wt. % LBO. An electrochemically active solid solution of Li_1.25Cr_0.25Mn_1.5O₄ and LiNi_0.5Mn_1.5O₄ was prepared. This work proves that Li-stuffed spinels can achieve fast Li-ion conduction and that the concept is potentially useful to enable a single-phase fully solid electrode without interphase impedance.

Keywords: Li-ion battery; cathode-electrolyte interface; fast Li+ ion conductor; solid electrolyte; solid-state battery; spinel.

Figure 1

XRD of Li_1.25Cr_0.25Ti_1.5O₄ solid electrolyte powder (bottom) and Li_1.25Al_0.25Ti_1.5O₄ solid electrolyte powder (top). XRD peaks are indexed to the Fd$\overline{3}$m spinel structure.

Figure 2

XRD of hot-pressed pellets of Li_1.25Cr_0.25Ti_1.5O₄ (LCTO) and Li_1.25Al_0.25Ti_1.5O₄ (LATO) with variable amounts of Li₃BO₃ (LBO). Pellets were ground to a powder prior to XRD data collection.

Figure 3

Representative SEM image of fracture surface images of hot-pressed Li_1.25Cr_0.25Ti_1.5O₄ without LBO, left, and with 1% LBO, right.

Figure 4

Room temperature impedance plot of hot-pressed Li_1.25Cr_0.25Ti_1.5O₄ and the equivalent circuit used to interpret the data.

Figure 5

Room temperature impedance plot of hot-pressed Li_1.25Cr_0.25Ti_1.5O₄/3% LBO and the equivalent circuit used to interpret the data where R₁ = R_eR_b/(R_e + R_b), R₂ =R_e(R_b + R_gb)/(R_e + R_b + R_gb), and R₃ =R_e.

Figure 6

Bulk ionic, total ionic and electronic conductivities and (1/R_gb) grain boundary resistance plots of Li_1.25CrTi_1.5O₄ (LCTO) and Li_1.25Al_0.25Ti_1.5O₄ (LATO) with varied weight percent Li₃BO₃ (LBO) as a function of temperature. E_A is the activation energy. Plotting (1/R_gb) was done since the grain boundary conductivity cannot be calculated because the grain boundary volume is unknown [35].

Figure 7

XRD pattern of 0.3 [Li_1.25Cr_0.25Mn_1.5O₄]: 0.7 [LiNi_0.5Mn_1.5O₄] solid solution formed at 850 °C (nominal composition: Li_1.025Cr_0.025Ni_0.45Mn_1.5O₄). XRD peaks are indexed to the Fd$\overline{3}$m spinel structure.

Figure 8

(a) Electrochemical charge and discharge curve of 30% Li_1.25Cr_0.25Mn_1.5O₄ and 70% LiNi_0.5Mn_1.5O₄ solid solution formed at 850 °C (nominal composition: Li_1.025Cr_0.025Ni_0.45Mn_1.5O₄) (b) Discharge capacity of Li_1.075Cr_0.075Ni_0.35Mn_1.5O₄ as function of charge and discharge rate at a loading of about 6 mg cm⁻¹ (~ 0.7 mAh cm⁻¹). Symmetrical charge and discharge rate indicated on the figure were varied each five cycles (cycles 1–30) and fixed at 1C for cycles 31–60.