Modelling of mass transfer and electrode kinetics in liquid metal batteries

在德国能源转型的框架内，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质，届时大规模部署固定储能将成为强制性的。这样的存储设备将必须满足一些关键要求：低价格(10ct/kWh)、长寿命(7000个循环)、高效率(80%)以及短时间存储：快速反应时间。到目前为止，还没有一种技术满足所有上述标准；因此，开发新的或改进现有的存储设备将是必要的。一种全新的、非常有前途的存储技术是液态金属电池(LMB)--由两种液态金属稳定的密度分层建造，由同样的液态盐隔开。廉价的原材料，如钠或铅，加上简单的结构，就可以建造非常便宜的电池。完全液态的内部避免了微小的降解，并承诺非常长的使用寿命。在液体界面的最佳动力学允许潜在的非常高的电流密度。这使得可以提供高功率；然而，潜在的可能的电流密度在实践中并不总是能够实现的。该项目的目标是提高LMB的电流密度和效率。为此，应开发一个电化学电池模型，并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。在第一步中，应评估不同过电压(电阻如欧姆损耗、浓度极化、电荷转移)的影响。随后，应探讨它们对温度、电流密度、操作条件和电池几何形状的依赖关系。最后，计划分析如何通过阴极中的人工流动来提高效率和电流密度。为了改善传质，这种液体内部的混合被多次提出。文献中多次强调了上述过电压的重要性；然而，到目前为止，它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，该模型将成为详细模拟低分子膜的基础。将来，它可以很容易地得到补充，例如通过金属间化合物相的模型，以便研究电池中的其他过程。

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.