Jiang Cui

Enhanced conversion reaction kinetics in low crystallinity SnO2/CNT anodes for Na-ion batteries

The specific capacities of SnO2 anodes in sodium ion batteries (SIBs) are far below the values expected from theory. Herein, we propose that the kinetically-controlled, reversible ‘conversion reaction’ between Na ions and SnO2 is responsible for Na ion storage in SnO2 anodes where the ion diffusion rate is the limiting factor. This revelation is contrary to the current understanding of the ‘alloying reaction’ as the major reaction process. Aiming to fully utilize the theoretical capacity from the conversion reaction, a composite electrode consisting of carbon nanotubes coated with a mainly amorphous SnO2 phase together with crystalline nanoparticles is synthesized. The SnO2/CNT anodes deliver a superior specific capacity of 630.4 mA h g−1 at 0.1 A g−1 and 324.1 mA h g−1 at a high rate of 1.6 A g−1 due to the enhanced kinetics. The volume expansion of the composite is accommodated by the CNT substrate, giving rise to an excellent 69% capacity retention after 300 cycles. The aforementioned findings give new insight into the fundamental understanding of the electrochemical kinetics of SnO2 electrodes and offer a potential solution to the low capacity and poor cyclic stability of other metal oxide anodes based on conversion reactions.

In situ TEM study of lithiation into a PPy coated α-MnO2/graphene foam freestanding electrode

Even with the many desirable properties, natural abundance and low cost of α-MnO2, its application as the anode in lithium-ion batteries has been limited because of its low intrinsic electrical conductivity and large volume expansion occurring during charge/discharge cycles. In this work, a ternary composite electrode consisting of MnO2-polypyrrole (PPy) core–shell arrays is grown on graphene foam (GF) to address the above critical issues. The freestanding MnO2–PPy/GF electrode exhibits a high reversible capacity of 945 mA h g−1 at 0.1 A g−1 after 150 cycles with a coulombic efficiency of over 98%, far better than 550 mA h g−1 for the uncoated counterpart. An in situ TEM examination reveals several functional features of the PPy coating that ameliorate the MnO2 conversion reaction kinetics, and thus the electrochemical performance of the electrode. The PPy coated MnO2 nanowires have a lithiation speed three times faster than that of the uncoated MnO2 along with improved electronic conduction and a stable structure against volume expansion. Such a rational design of an electroactive core and a highly conductive polymer shell on a GF conductive substrate offers a potential solution to developing novel MnO2-based electrodes with enhanced electrochemical performance.

Highly conductive porous graphene/sulfur composite ribbon electrodes for flexible lithium sulfur batteries

Flexible batteries have become an indispensable component of emerging devices, such as wearable, foldable electronics and sensors. Although various flexible batteries have been explored based on one-dimensional and two-dimensional platforms, developing a high energy density electrode with high structural integrity remains challenging. Herein, a scalable, one-pot wet spinning strategy is used to synthesize a flexible porous cathode for lithium-sulfur batteries (LSBs) for the first time, which consists of reduced graphene oxide (rGO), graphene crumples (GCs) and sulfur powders. The electrode structures are tailored using GCs with different dimensions and functional features that are critical to its robustness under mechanical deformation and electrolyte penetration into the battery components. The optimized rGO/GC/S composite ribbon cathodes deliver a high capacity of 524 mA h g-1 after 100 cycles at a current rate of 0.2 C. A shape-conformable battery prototype comprising an rGO/GC/S cathode and a lithium anode demonstrates a stable discharge characteristic under repeated bending/flattening cycles. The LSB prototype supported by an elastomer presents stable discharge behavior with high mechanical robustness against an extension of up to 50%. The above-mentioned findings shed new light on developing sulfur cathodes for flexible, high performance LSBs based on the rational design of graphene structures.

Chemical interactions between red P and functional groups in NiP3/CNT composite anodes for enhanced sodium storage

Red phosphorus has thus far the highest theoretical capacity among all known anode materials for sodium ion batteries (SIBs). However, its low electronic conductivity and large volume expansion during cycles cause rapid capacity fading, leading to poor electrochemical stability. Herein, we report a facile and scalable ball milling approach to synthesize NiP3/carbon nanotube (CNT) composites consisting of NiP3 particles chemically bonded with functionalized CNTs. The conductive CNTs play an important role in stabilizing the composite electrode through an enhanced Na+ diffusion coefficient by two orders of magnitude and six-fold reduction in its charge transfer resistance. The NiP3/CNT composite anode delivers a high initial reversible capacity of 853 mA h g−1 with more than 80% capacity retention after 120 cycles at 200 mA g−1 and an excellent high-rate capacity of 363.8 mA h g−1 after 200 cycles at 1600 mA g−1. The density functional theory (DFT) calculations combined with ab initio molecular dynamics (AIMD) simulations elucidate strong chemical interactions between the red P in NiP3 and the functional groups on CNTs to form P–C and P–O–C bonds by ball milling for the first time. The facile synthesis strategy devised in this study can be applied to other alloy-based composites with relatively low carbon content for use as high performance anodes for SIBs.