Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, USA.
Nano Lett. 2012 Oct 10;12(10):5186-91. doi: 10.1021/nl302249v. Epub 2012 Sep 19.
A variety of approaches are being made to enhance the performance of lithium ion batteries. Incorporating multivalence transition-metal ions into metal oxide cathodes has been identified as an essential approach to achieve the necessary high voltage and high capacity. However, the fundamental mechanism that limits their power rate and cycling stability remains unclear. The power rate strongly depends on the lithium ion drift speed in the cathode. Crystallographically, these transition-metal-based cathodes frequently have a layered structure. In the classic wisdom, it is accepted that lithium ion travels swiftly within the layers moving out/in of the cathode during the charge/discharge. Here, we report the unexpected discovery of a thermodynamically driven, yet kinetically controlled, surface modification in the widely explored lithium nickel manganese oxide cathode material, which may inhibit the battery charge/discharge rate. We found that during cathode synthesis and processing before electrochemical cycling in the cell nickel can preferentially move along the fast diffusion channels and selectively segregate at the surface facets terminated with a mix of anions and cations. This segregation essentially can lead to a higher lithium diffusion barrier near the surface region of the particle. Therefore, it appears that the transition-metal dopant may help to provide high capacity and/or high voltage but can be located in a "wrong" location that may slow down lithium diffusion, limiting battery performance. In this circumstance, limitations in the properties of lithium ion batteries using these cathode materials can be determined more by the materials synthesis issues than by the operation within the battery itself.
人们正在尝试各种方法来提高锂离子电池的性能。将多价过渡金属离子掺入金属氧化物阴极被认为是实现必要的高电压和高容量的重要方法。然而，限制其功率率和循环稳定性的基本机制仍不清楚。功率率强烈依赖于阴极中锂离子的漂移速度。从晶体学角度来看，这些基于过渡金属的阴极通常具有层状结构。在经典的认知中，锂离子在充电/放电过程中迅速地在层间进出阴极移动。在这里，我们报告了一个出乎意料的发现，即在广泛研究的锂镍锰氧化物阴极材料中，存在一种热力学驱动但动力学控制的表面修饰，这可能会抑制电池的充放电速率。我们发现，在电池电化学循环之前的阴极合成和处理过程中，镍可以优先沿着快速扩散通道移动，并选择性地在阴离子和阳离子混合终止的表面面择优分离。这种分离实质上会导致颗粒表面区域附近的锂离子扩散屏障升高。因此，似乎过渡金属掺杂剂可以帮助提供高容量和/或高电压，但可能位于“错误”的位置，这可能会减缓锂离子的扩散，从而限制电池性能。在这种情况下，使用这些阴极材料的锂离子电池的性能限制更多地取决于材料合成问题，而不是电池本身的运行。
