N-functionalized graphene quantum dots: Charge transporting layer for high-rate and durable Li₄Ti₅O₁₂-based Li-ion battery

Highlights

• A novel strategy was presented to enhance the performance of LTO electrode.

• N-GQDs were acted as protecting layer on LTO surface from interfacial reactions.

• The Li-ion diffusion coefficient was enhanced by ∼19%.

• The capacity was enhanced by ∼23% at 50C via encapsulation of LTO by N-GQDs.

• N-GQDs can suppress the gassing during the cycling process.

Abstract

Spinel Li₄Ti₅O₁₂ can replace carbon in Li-ion battery anodes due to its high voltage, preventing decomposition of the electrolyte and formation of Li metal dendrites. However, Li₄Ti₅O₁₂ has a low electronic conductivity and Li-ion diffusion coefficient, limiting its charge/discharge properties at high rate capacities, and also suffers from gassing during cycling. Here, we used N-functionalized graphene quantum dots interfacial layer, which (1) protects Li₄Ti₅O₁₂ from ambient degradation, (2) forms a thin and smooth solid-electrolyte interphase layer on the Li₄Ti₅O₁₂ surface, (3) acts as a charge transfer layer, (4) protects the Li₄Ti₅O₁₂ electrode from reactions with the electrolyte, and (5) suppresses gassing during cycling. Consequently, the Li-ion diffusion coefficient increased by ∼19%. The effectiveness of the N-functionalized graphene quantum dots is manifested in the specific capacity of 161 mAh/g at 50C, which is improved by ∼23% compared to pure Li₄Ti₅O₁₂ electrode and maintained for over 500 cycles. Unlike graphene, N-functionalized graphene quantum dots themselves work as a stable charge transporting and protecting layer. Our strategy successfully obtained a good cycling performance and long cycling life of Li₄Ti₅O₁₂ at high C-rates.

Introduction

As modern transportation depends heavily on fossil fuels [1], one of the most effective ways to reduce CO₂ emissions and cope with the increasing scarcity of fossil fuels is to use electrical vehicles (EVs) or hybrid electrical vehicles (HEVs) [2]. Recently, considerable efforts have been devoted to the development of high-performance Li-ion batteries (LIBs) for new applications including plug-in HEVs and EVs [3], [4], [5], [6], [7]. Although LIBs have the advantage of intrinsically high energy density [8], the current LIB technologies have short cycling life, low power density, and safety hazards and therefore, cannot satisfy the requirements of efficient storage of renewable energy and/or powering EVs or HEVs [9]. Therefore, the development of electrode materials with high energy and power densities, and a long lifespan is urgently required [10].

Lithium titanate (Li₄Ti₅O₁₂; LTO) has received considerable attention as a LIB anode material owing to its outstanding high-rate capacity and cycling stability, and its improved safety compared to graphite [11], [12]. This material has a high and flat Li insertion/extraction voltage (∼1.55 V vs. Li/Li⁺) and good structural stability with negligible volume change during Li insertion/extraction; this prevents the growth of Li metal dendrites and the reduction decomposition of the electrolyte and provides long cycle life and high-rate capability [13]. Unfortunately, the high-rate performance of LTO cannot be achieved due to its low intrinsic electronic conductivity [14] and moderate lithium-ion diffusion coefficient [15]. Accordingly, various strategies have demonstrated significant improvement in the rate capability of LTO anodes, including the incorporation of conductive phases such as metal or carbonaceous materials [16], [17], control of the morphology and microstructure [18], [19], [20], and ion doping [21].

The large-scale application of LTO-based LIBs suffers from serious gassing and associated swelling during charge/discharge cycles, making storage challenging [22]. This is generally caused by electrolyte decomposition owing to interfacial reactions [23], [24]. Surface coating with a conductive carbon layer is widely used to enhance electronic and ionic transport within LTO, significantly improving the electrochemical performance [25], [26]. Such a barrier layer can effectively suppress gassing due to interfacial reactions [27], [28] because, during the cycling process, a thicker layer of solid-electrolyte interface (SEI) is formed on carbon-coated electrode [24], [28], which protect from the surface reaction of LTO with electrolyte [22]. Both the formation and decomposition of SEI layer causes gassing. With the increase in temperature, the SEI layer becomes unstable and started to decompose its carbonate species, which is accountable for gassing [22]. Also, the carbon materials have high reactivity with electrolyte solutions at elevated temperatures, which raises safety concerns [29]. Therefore, alternative surface coating layers of inorganic materials have been proposed, such as ZnO [30], AlF₃ [31], NiO_x [32], and TiN_x [33], that has negligible reactivity with the electrolyte. Surface modification of LTO resulted in a smoother and thinner SEI layer [30]. However, LTO electrode coated with these inorganic materials has the poor capacity [30], [31], [32], [33].

Here, we propose a facile method for coating LTO with an ultrathin protective and charge-transport layer of N-functionalized graphene quantum dots (N-GQDs) via a solution-based process (LTO-NGQ). The electrochemical performances of LTO-NGQ, including cycling stability (200 cycles at 20C followed by 100 cycles at 50C) and rate capability (0.2–50C), were extensively evaluated in half cells.