Pui-Kit Lee

Copper substituted P2-type Na0.67CuxMn1−xO2: a stable high-power sodium-ion battery cathode

While sodium-ion batteries (SIBs) are considered as a next-generation energy storage device because of the higher abundance and lower cost of sodium compared to those of lithium, developing high-power and stable cathode materials remains a great challenge. Here, micron-sized plate-like copper-substituted layered P2-type Na0.67CuxMn1−xO2 is demonstrated to rapidly charge and discharge within 5 minutes while giving a capacity of more than 90 mA h g−1, corresponding to a half-cell energy density of 260 W h (kg cathode)−1 at a power density of 3000 W (kg cathode)−1, which is comparable to that of high-power lithium-ion cathodes. The materials show excellent stability, retaining more than 70% of the initial capacity after 500 cycles at 1000 mA g−1. The good cycle and rate performances of the materials are attributed to copper in the lattice, which stabilizes the crystal structure, increases the average discharge potential and improves sodium transport. This makes Na0.67CuxMn1−xO2 an ideal choice as a cathode for high-power sodium-ion batteries.

Leveraging Titanium to Enable Silicon Anodes in Lithium-Ion Batteries

Silicon is a promising anode material for lithium-ion batteries because of its high gravimetric/volumetric capacities and low lithiation/delithiation voltages. However, it suffers from poor cycling stability due to drastic volume expansion (>300%) when it alloys with lithium, leading to structural disintegration upon lithium removal. Here, it is demonstrated that titanium atoms inside the silicon matrix can act as an atomic binding agent to hold the silicon atoms together during lithiation and mend the structure after delithiation. Direct evidence from in situ dilatometry of cosputtered silicon-titanium thin films reveals significantly smaller electrode thickness change during lithiation, compared to a pure silicon thin film. In addition, the thickness change is fully reversible with lithium extraction, and ex situ post-mortem microscopy shows that film cracking is suppressed. Furthermore, Raman spectroscopy measurements indicate that the Si-Ti interaction remains intact after cycling. Optimized Si-Ti thin films can deliver a stable capacity of 1000 mAh g-1 at a current of 2000 mA g-1 for more than 300 cycles, demonstrating the effectiveness of titanium in stabilizing the material structure. A full cell with a Si-Ti anode and LiFePO4 cathode is demonstrated, which further validates the readiness of the technology.

Robust Micron-Sized Silicon Secondary Particles Anchored by Polyimide as High-Capacity, High-Stability Li-Ion Battery Anode

Silicon is an attractive high-capacity anode material for lithium-ion battery. With the help of nanostructures, cycling performance of silicon anode has improved significantly in the past couple of years. However, three major shortcomings associated with nanostructures still need to be addressed, namely, their high surface area, low tap density, and poor scalability. Herein, we present a facile and practical method to produce micron-sized Si secondary particle cluster (SiSPC) with a high tap density and a low surface area from bulk Si by high-energy ball-milling. By coupling SiSPC with a mechanically robust polyimide binder, more than 95% of the initial capacity is retained after 500 cycles at 3500 mA g-1 (1C rate). Reversibility of electrode thickness change is confirmed by in situ dilatometry. In addition, the polyimide binder suppresses the surface reaction of the particles with electrolyte, resulting in a high Coulombic efficiency of 99.7%. Excellent cycling performance is obtained even for thick electrodes with an areal capacity of 3.57 mAh cm-2, similar to those in commercial lithium-ion batteries. The presented Si electrode system has a high volumetric capacity of 598 mAh cm-3, which is higher than that of the commercial graphite anode materials.