Amorphous Li-Si structures found via a novel simulation method using a reactive force field

Cochabamba

Workshop; 8 th International Workshop on Lithium, Industrial Minerals and Energy (IWLiME); 2021

Silicon has been proposed as an anode material to replace graphite in Lithium-ion batteries due to its abundance, low cost, low discharge voltage and high theoretical capacity of 3579 mAhg -1, which is ten times higher than usual graphite. Despite these advantages, this material exhibits a large volume expansion, of about 300% at full lithiation and structural changes that lead to capacity degradation and short cycle life. The importance of studying amorphous LiSi structures comes from the fact that they are formed when Li reacts with Si during battery cycling [1] and the further knowledge of its structure allows researchers to optimize the different approaches proposed to overcome the named limitations.In this work, we studied the properties of amorphous structures of Li x Si for values of x ranging from 0.21 to 4.2. Crystalline initial structures were obtained from Materials Project [2] and replicated in different directions to obtain simulation boxes with 1000 to 2000 atoms, depending on x. For intermediate x values structures, a protocol of delithiation was followed in which an atom of Li was randomly selected and extracted from a given structure and the system was equilibrated in the NPT ensemble. This extraction process was repeated until a desired value of x was obtained. To obtain different amorphous structures for this system, we use a reactive force field [3] and a novel simulation method, accelerated exploration of local minima (AELM) [4]. This method consists in transforming the potential energy surface (PES) with a compression factor α to accelerate its exploration. Then, we apply the conjugate gradient method (CG) in the unbiased PES to eachstructure found.Figure 1 illustrates representative histograms of minimum energies obtained after applying AELM to different x and α values. It can be noticed that for different values of x there are different optimal values of α. We find that silicon tends to form an amorphous network that remains with its tetrahedral structure distorted, these bonds start to break as the concentration of lithium increases and the network reaches 1-d periodic chains for the most lithiated structures.The structures corresponding to the optimal value of α, at each composition, were characterized with different computational calculations. The volume variation and lithiation potentials represent the experimental electrochemical behavior well [4]. The radial distribution function, g(r), for Si-Si pairs, is shown in Figure 2, where the remaining first peak despite the lithiation is in concordance with experimental measurements reported by Key et al. [5].The other possible pairs in g(r) (Li-Li and Si-Li) were also characterized along with their respective coordination numbers, the formation of Si clusters and the short range order. These studies allowed a more complex description of the amorphous structures formed during the lithiation of the Si anodes.