One-Pot Spray-Dried Graphene Sheets-Encapsulated Nano-Li4Ti5O12 Microspheres for a Hybrid Batcap System

Global warming, oil dependency and environment pollution need to shift from an oil-based economy to an electricity-based civilization urgently. Li-ion batteries (LIBs) and electrochemical capacitors (ECs) are the most common types of energy storage systems. LIBs have high energy density of 120-200 Wh kg^-1. And ECs have the highest power density of 2-5 kW kg^-1, combined with the highest cycle-life. But their energy density is low, only from 2-5 Wh kg^-1.¹ A device of “BatCap” employs a hybrid system of a battery-like (faradic) electrode and a capacitor-like (non-faradic) electrode, producing high energy density and high power density at the same time. Among the possible battery materials, spinel-type Li₄Ti₅O₁₂ (LTO) is known to be the best choice because of its unique properties including high columbic efficiency, thermodynamically stable structure during cycling, minimal solid electrolyte interface and low cost. However LTO has low electronic conductivity (ca. 10^-13 S·cm^-1) ² and moderate Li⁺ diffusion coefficient (10^-9 --10^-13 cm² s^-1) ³, which limits its power capability -- the key characteristic for supercapacitors. Decreasing the particle size, improving the crystallinity, or surface coating with a more conductive material are the general ways to improve the rate performance of LTO anode.

In this study, we report a facile and suitable for large-scale process for preparing a three-dimension (3D) micro-spherical graphene sheets (GS) encapsulated LTO nanoparticles composites by one-pot spray-drying assisted solid-phase reaction method with anatase TiO₂ as starting material for BatCap system. The typical procedure for the synthesis was shown schematically in fig. 1. The mass ratio of raw graphene oxide (GO) was found to great effect on the phase formation, particle size and electrochemical performance of LTO@GS hybrid. The best electrochemical performance was obtained by GO content of 25 % corresponding to the final GS content of 1.93 % in LTO@GS production. The obtained material consists of a micron-size secondary sphere (1.5-2.0 mm) by GS-encapsulated nanosized LTO primary particles (20-40 nm). The coated GS layer has a nanothickness of about 2.2 nm from the TEM image. GS can effectively suppress the crystalline growth of spinel LTO phase, simultaneously, the dispersed LTO nano-particles could suppress the agglomeration and re-stacking of GS. As a consequence, the optimum sample delivers a reversible capacity of 174.4 mAh g^-1 at 1 C, and shows remarkable rate capability by maintaining 51.9 % of the capacity at 40 C (vs. 1 C). A full cell test with 50*50 mm monolithic soft packed hybrid battery-capacitor BatCap system using LTO@GS as the negative electrode and AC as the positive electrode was associated, which exhibited high rate performance and long cycle life.  The specific energy is 29.2 Wh kg^-1 at a power density of 58.4 W kg^-1; when the power output reaches 1782.7 W kg^-1, the specific energy retains at 13.4 Wh kg^-1. After 15,000 cycles at a high rate of 80 C (1600 mA g^-1), the specific capacitance retains 90 % of the initial value, and the coulombic efficiency is close to 100 %.

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