If the charging rate of batteries shall be improved, it is worth taking a look at the mobility and intercalation processes of lithium ions within the battery components. After all, the way in which the ions can move within the materials (lithium mobility) and are inserted into the electrodes (intercalation) is the linchpin of optimization.

The operating principle of lithium-ion batteries (LIB) is based on the reversible “shuttling” of lithium ions between the electrodes, which are materials that can readily store lithium inside their crystal lattices. To achieve a fast charging process of the battery cell, a high rate capability, the lithium insertion rates into and mobility within the electrode materials as well as in the electrolyte should be high. Optimizing the lithium mobility hence plays a crucial role in battery performance.

The Quantistry Lab enables simulations of electrode materials, additives and electrolytes that offer atomic-level insights into processes such as lithium migration into and within electrode materials.

This allows users to tackle various R&D questions:

1. Which surface characteristics of an electrode material can facilitate the insertion of lithium ions?
2. Are there optimal diffusion pathways for lithium within an electrode material and how can they be improved?
3. Is there a risk of so-called plating, which means the deposition of metallic lithium on an electrode surface?

Starting Position

How can lithium insertion into an electrode be improved and the lithium mobility be increased? To address these questions, quantum chemical simulations were used to investigate different migration mechanisms. At the atomic level, lithium ions must overcome energy barriers as they migrate from one location to another – the lower these barriers, the higher the migration rate of the lithium ions. Intercalation of lithium should also be fast and easy to reduce the risk of metallic lithium being deposited on the electrode surface instead – an undesirable side reaction known as plating. Quantum chemical simulations allow us to compare different interaction scenarios of lithium with electrode surfaces, to offer insight into whether plating or intercalation is preferred at a given half-cell potential. The focus of our simulations is on identifying optimal electrode materials that favor lithium intercalation into the surfaces and reduce the risk of plating.

Simulation Approach

In this use case we selected two electrode materials for LIBs as examples – a graphite anode and a lithium manganese phosphate (LiMnPO₄) cathode. Graphite is a layered material consisting of graphene sheets stacked on top of each other, between which the lithium is inserted or “intercalated” upon charging of the battery cell. Cathode materials are typically transition metal compounds, into which lithium can likewise be incorporated (during discharge), like the lithium manganese phosphate we have selected here.

First, the intercalation of lithium ions into different types of surfaces that occur on graphite particles were simulated with the Quantistry Lab, namely “basal” planes and “edge” planes as illustrated in figure 1, and the barriers for each scenario were calculated.

Next, the Quantistry Lab was used to simulate the interaction of lithium with graphite surfaces, focusing on determining the conditions under which lithium intercalation is preferred over lithium plating.

And finally, the lithium ion mobility inside a Li_xMnPO₄ cathode material was investigated. Here, two migration pathways along different crystallographic directions inside the cathode material were considered, i.e., across and along channels in its crystal structure (a- and b-directions, respectively, see Fig. 2), and the respective migration barriers were determined.

Key Results

Lithium Intercalation Into Graphite

Figure 3 shows the simulated migration of lithium (in purple) into the edge plane of a graphite electrode surface (in gray). By looking at the energy profile it can be seen at first glance that lithium can be inserted unhindered into an edge plane of graphite, i.e., there is no energy barrier for intercalation.

Fig. 3: Lithium intercalation via edge surface | QUANTISTRY LAB 2022

In contrast, the intercalation of lithium via a basal plane exhibits a high energy barrier, as highlighted in figure 4 (see the energy profile along the migration path). This indicates that there is essentially no lithium intercalation across the basal planes, at least in the absence of defects; intercalation via an edge surface is clearly the preferred pathway.

Fig. 4: Lithium intercalation via basal surface | QUANTISTRY LAB 2022

In this way different intercalation paths can be compared quickly and easily by using the Quantistry Lab – be it in the comparison of different surfaces or different bulk electrode materials – and preferred pathways can be identified.

The focus in this use case was on one single aspect of the complex process of lithium intercalation – the comparison of two surfaces occurring on graphite particles. Such simulations can be used to optimize an electrode material, for example, by indicating a beneficial effect of increasing the fraction of edge planes on the particle surface for the lithium intercalation rate. In the search for efficient electrode materials, it is also of interest whether lithium can move rather freely in only one direction or, as in graphite, in two spatial dimensions (see the edge plane results), with the latter being preferable.

Lithium Plating on Graphite

To investigate plating, different interaction scenarios of lithium with graphite were compared depending on the half-cell potential, i.e., intercalation versus deposition of metallic lithium. Figure 5 summarizes the main results of these simulations showing the surface stability as a function of the potential for various graphite structures (lithium-plated and lithium-intercalated with an increasing lithium content of 25%, 50%, and 100%). It can be seen that the risk of lithium-plating increases at low potentials (-1 V vs. Li/Li⁺), whereas a fully charged graphite structure (100%) is favored at higher voltages.

Lithium Migration in Lithium Manganese Phosphate (Li_xMnPO₄)

Lithium ion mobility within the cathode material also plays an important role for the charge and discharge rate of a battery cell. Figures 6 and 7 compare the migration of lithium (highlighted in green) inside LiMnPO₄ along two directions, namely across and along the channels (a- and b-directions, respectively). In the results view on the right it can be seen that migration along the b-axis (i.e., along the channel) is clearly preferred over the a-axis, because the energy barrier for the former pathway is about 4 times lower (0.5 eV vs. 2.4 eV).

Fig. 6: Lithium migration to LiMnPO₄ - axis a | QUANTISTRY LAB 2022

Fig. 7: Lithium migration to LiMnPO₄ - axis b | QUANTISTRY LAB 2022

In addition to identifying preferred migration pathways, the simulations can be used to explore ways to further reduce the migration barriers (e.g., by substituting specific elements) and thereby optimize the lithium ion mobility. Beyond that, it is worth looking at the dimensionality of lithium diffusion: a material that allows for a two- or even three-dimensional migration mechanism could have advantages over Li_xMnPO₄, which has a low barrier and associated high lithium mobility in only one direction.

Conclusion

In summary, an important aspect that contributes to achieving fast (dis)charging rates is to optimize the migration of lithium ions into and within electrode materials. Reducing the migration barriers through a smart material design – for which atomistic simulations are ideally suited – can play an important role in improving the overall rates.

A major advantage is that these simulations can be easily transferred to other electrode materials or even solid electrolytes. You can use our workflows to study your new battery materials, to explore the impact of doping, defects, surface modifications etc. on the lithium mobility and intercalation barriers.

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