Modelling of mass transfer and electrode kinetics in liquid metal batteries

在德国能源转型的框架下，到2050年，80%的电力将由可再生能源产生。由于风能和太阳能的波动性，固定式储能的部署将成为大规模的强制性部署。这种存储设备必须满足许多关键要求：低价格（10ct/kWh），长寿命（7000次循环），高效率（>80%）和短时间存储：快速反应时间。到目前为止，还没有一种技术满足上述所有标准；因此，开发新的或改进现有的存储设备将是必要的。液态金属电池（LMB）是一种全新的、非常有前途的存储技术，它是由两种液态金属组成的密度稳定的分层，由同样的液态盐分开。廉价的原材料，如钠或铅和简单的结构允许制造非常便宜的电池。完全液体的内部避免了微降解，并保证了非常长的使用寿命。液体界面的最佳动力学允许潜在的非常高的电流密度。这些允许提供高功率；然而，在实际应用中，潜在的电流密度并不总能达到。本项目的目的是提高lmb的电流密度和效率。为此，应开发电化学电池模型，并将其与现有的流体动力学模型相结合。它将在开源CFD库OpenFOAM中实现，也是为了简化其他研究人员的使用。随后，计划进行分析和实验验证。第一步，应评估不同过电压的影响（电阻如欧姆损耗、浓度极化、电荷转移）。随后，将探讨它们与温度、电流密度、操作条件和电池几何形状的关系。最后，计划分析如何通过在阴极中人工流动来提高效率和电流密度。为了改善传质，多次提出了这种内部液体混合的方法。上述过电压的重要性在文献中多次强调；然而，到目前为止，他们的影响从未被详细或定量地研究过。通过电动力学、流体动力学和电化学的耦合，该模型将成为lmb详细仿真的基础。在未来，它可以很容易地得到补充，例如通过金属间相模型，以便研究细胞中的其他过程。

In frame of the German energy transition, 80% of all electricity shall be generated by renewable sources by the year 2050. Due to the fluctuating nature of wind and solar energy, the deployment of stationary energy storage will then become mandatory on a large scale. Such storage devices will have to fulfil a number of crucial requirements: low price (10ct/kWh), long life-time (7000 cycles), high efficiency (>80%) and for short-time storage: fast reaction times. Up to now, no technology fulfils all mentioned criteria; therefore, a development of new, or an improvement of existing storage devices will be necessary.A totally new, and very promising storage technology is the liquid metal battery (LMB) - built as a stable density stratification of two liquid metals, separated by a likewise liquid salt. Cheap raw materials, as Na or Pb and a simple construction allow for building very cheap cells. The completely liquid interior avoids micro-degradation and promises very long life-time. The optimal kinetics at the liquid interfaces allow for potentially very high current densities. These allow providing high power; however, the potentially possible current densities are not always achieved in practice.Objective of this project is an increase of current density and efficiency of LMBs. For this purpose, an electrochemical cell model shall be developed and coupled with an existing fluid dynamics model. It will be implemented in the open source CFD library OpenFOAM, also to simplify its use by other researchers. Later, an analytical and experimental validation is planned. In a first step the influence of different overvoltages (resistances as Ohmic losses, concentration polarisation, charge transfer) shall be evaluated. Subsequently, their dependence on temperature, current density, operation condition and geometry of the cell shall be explored. Finally, it is planned to analyse, how efficiency and current density can be increased by an artificial flow in the cathode. Such a mixing of the liquid interior was proposed several times for an improvement of mass transfer.The significance of the mentioned overvoltages was emphasised several times in the literature; however, their influence was never studied detailed or quantitatively, up to now. By coupling electrodynamics, fluid dynamics and electrochemistry, the model shall be the basis for a detailed simulation of LMBs. In the future it can be easily complemented, e.g. by a model for intermetallic phases, in order to study other processes in the cells.