Lithium Alloying Nanoparticles Deposited 3D Carbon Architectures as Anodes for Li-Ion Batteries

Due to the increased demand and interest for mobile electronics, renewable energy production and electronic vehicles, more pressure is being placed on battery technology to deliver higher capacity, longer lasting devices. Researchers have focused on creating advanced materials to increase the energy storage and overall performance of lithium ion batteries.  While graphitic carbon is still the commercially preferred material for the negative electrode (anode) of Li-ion batteries, it fails to deliver a high enough capacity (372mAh/g) to meet the increasing energy and performance requirements. 

With a theoretical gravimetric capacity nearly 3 times greater than graphite (991mAh/g), tin has the potential to replace graphite as the next generation of lithium ion anode material [1].  However, pure Sn anodes suffer from poor cycle performance due to electrical detachment/isolation from the current collector due to particle fracturing.  Researches have tried alloying tin with transition metals [2], creating Sn-C composite nanoparticles [3], as well as using both pure SnO₂ and SnO₂deposited on hard carbon spheres [4,5] as anode materials.  A common result of research efforts is usually an increase in capacity but retention of the poor cycle performance.  This is due to aggregation of the tin particles which then leads to similar electronic detachment and fracturing characteristics of pure tin. It is believed that improving the mechanical stability of the anode material will, in turn, increase the electrochemical characteristics of the material [6]. 

Our group has created a material that can facilitate this mechanical stability required for a successful tin-based anode for lithium ion batteries.  This is done by using a one-step, solid state process to fabricate porous carbon particles in situ decorated with functional metal nanoparticles. By placing a mixture of powdered carbon and salt precursors in a high temperature furnace and heating these materials to 500-700 ^oC in an inert Ar atmosphere for an hour, unique metal supported carbon hybrid architectures are achieved. This material is essentially a porous carbon “host” with nanosized metallic “guests”. The carbon matrix helps to mechanically stabilize the tin nanoparticles which are chemically bound to the surface of this matrix. The material demonstrates a surface area of >700m²/g.  SEM images of the carbon-tin composite (Figure 1a-b) shows that the naosized Sn is uniformly deposited inside and outside of the supported 3D carbon architectures.

The previously mentioned materials were mixed with PVDF binder and carbon black in a ratio of 8:1:1 and coated onto copper foil to make electrodes for testing.  Electrochemical tests were performed on the materials against lithium metal in a half-cell configuration using 1M LiPF₆ electrolyte in a 1:1 EC:DMC solution and a polypropylene separator.  Figure 1 (c) shows the second discharge/charge curves for pure tin nanoparticle coated carbon matrices as well those of the pristine carbon matrix.  The pure tin nanoparticles coated carbon has a reversible capacity greater than 500 mAh/g, which shows a vast improvement over the uncoated carbon matrix (~220mAh/g).  In addition, the materials synthesized demonstrate stable cycling performance after the first discharge/charge is completed. Additional work will be done to decrease this first cycle inefficiency and to continue to increase the capacity of our new composite materials. A detail analysis of the potentiograms for Sn-carbon composite is demonstrated by their derivatization dQ/dV (Figure 1d), indicating reduction processes and corresponding oxidation processes [7] related to different Li-Sn alloying processes.

                       

Figure 1.a-b) Scanning electron micrographs of tin nanoparticles deposited on 3D carbon architecture product at various magnifications, c) Voltage profiles for pristine carbon and Sn-carbon composite cells, and d) dQ/dV plot of Sn-carbon composite cell

References

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