Calculation of the intercalation behaviour in cathode materials of modern accumulator batteries
Abstract
This white paper should discover the possibilities of atomic scale calculations at an actual meaningful example of energy management. Due to the world wide high energy demand, shrinking resources and an increasing ecological awareness of the people, the interest to clean and renewable energies increases strongly. Many of these new energies i.e. gotten from wind- or solar power are in contrast to fossil fuels not permanently available. Therefore, a buffering in modern accumulation batteries is necessary. Particularly the automobile industry needs powerful accumulators to introduce electrically powered cars. Additionally, there is an increasing market for accumulators in the area of mobile electronic hardware like mobile phones and laptops.
Introduction
According to their application, there are several criteria for accumulators like energy-, power density or thermal stability, which must be satisfied depending on their priority. In the area of automobile industry, primarily a high energy density (to achieve a long range) and a high power density (high power and low charge time) are necessary. But also the manufacturing costs and the environmental compatibility plays an important role for the development of new accumulators.
One of the most promising accumulation systems is the lithium ion accumulator, due to the high specific charge of lithium. Related to the performance of the accumulator, one of the most important processes is the intercalation of lithium in anode and cathode materials. In general intercalation describes the storage of chemical species into a host material, without changing the structure of the host material. A key role in this process plays the intercalation potential, which is the electrochemical potential to storage the lithium ion into the electrode. This potential is referred to the potential of metallic lithium.
Cathode materials in lithium ion accumulators
Crystalline[1] cathode materials can roughly divided into one dimensional (1D), two dimensional (2D) and three dimensional (3D) structures. Typical 3D structures are spinells i.e. LiMn2O4 or LiMn1.5Me0.5O4 [WBS98]. The 2D structures are mainly arranged in layers. Representatives are LiCO2, LiCO1/3Ni1/3Mn1/3O2 [Nag90,Oza94] and LiV2O5 [CDP91]. At last the olivine structure should be mentioned, with the representatives LiFePO4 [PNG97] and LiMePO4. The specified examples are only a small selection of the actual existing cathode materials. In addition to the mentioned crystalline structures, there exist a lot of efforts to structure these materials at the nanometer scale. For example, it is possible to reduce the effective surface area by the usage of nano wires, nano ribbons or amorphous[2] and porous[3] materials [WKT06]. In many cases, a grasp of such processes can only be achieved with computational calculations at the atomic scale.
Intercalation of lithium in vanadium pentoxide (vanadia)
- Figure 1: Different views of the 2x3x3 V2O5 super cell. One can see the layered structure.
In the present ab initio calculation the intercalation behavior of vanadium pentoxid was studied. Therefore, several structures and intercalation potentials for different lithium concentrations were calculated. Additionally, to study the thermal stability of the material, a molecular dynamic (MD) simulation was performed.
Simulation details
All calculations were done by the usage of density functional theory (DFT). By the reason of vanadium pentoxid being a layer like structure, additional corrections describing the van der Waals forces were used [Grim06]. To realize structures with small lithium concentrations, a 2x3x3 V2O5 super cell[4] was constructed (see figure 1). Thus, the smallest reachable lithium concentration is determined by x=1/36 (Li1/36V2O5). The arrangement of the lithium ions were choose randomly.
Results
Structural changes based on the intercalation process
- Figure 2: Depiction of different LixV2O5 super cells with different lithium concentration x. The concentrations are 0.028 (1), 0.22 (2), 0.44 (3), 0.66 (5) und 0.88 (6). One can see the increasing puckering with increasing lithium concentration.
First of all, it should be mentioned that the symmetry of the host lattice remains unchanged for concentrations in the range of x=0 to 1. The main structural change is observed by an increase of the cell vector c (3.4%) and a decrease of cell vector a (6.7%) with increasing lithium concentration from x=0 to 1. See figure 2.
Additionally a tilting of the VO5 pyramids occurs, which means that the structure gets more puckered with increasing lithium concentration. This is the explanation for the length changes of the cell vectors a and c. Due to the increase of the puckering, different structural phases appears. Such structural changes must be taken into account for the industrial production process, because electrodes and electrolyte must be packed into a leak-proof shell.
Calculation of intercalation potentials
- Figure 3: Perspective side view of the discovered LiV2O5 phase. One can clearly see the angle of slope between the cell vectors b and c.
Beside the determination of structural properties, it is possible to calculate intercalation potentials with the usage of ground state energies. This was done for the corresponding lithium concentrations. Hence, the technical important average cell potential was determined in the concentration range x=0 to 1. Thus the average cell potential was calculated to 3.7V, which identifies V2O5 as a technical usable cathode material.
Molecular dynamics of LiV2O5
To determine the thermal properties of the cathode material, a MD calculation was performed. During the simulation the LiV2O5 super cell was heated from -263°C up to 670°C. A result of this simulation is the observation of an interesting phase transition. This phase posses a strong tilting of the cell vector c, which can be seen in the figure 3.
The stability of the phase can be explained by the reason that the tilting is a result of shifting the weakly van der Waals bonded layers. In particular the lower layer was shifted by b/2. Therefore, the volume of this phase is 3.6% larger. With supplying more energy (increase of temperature) a further shift by b/2 was observed. This temperature depending effect must also be taken into account during the industrial manufacturing process. The following video shows impressively the temperature depending structural change.
Summary
The example of the lithium intercalation in V2O5 has shown that ab initio calculations are convenient methods to determine intercalation based structural changes. Additionally it is possible to calculate average cell potentials as well as thermal based structural changes and phase transitions with ab initio MD simulations. Thus it is possible to make first conclusions about the principle technical usage as an electrode material
Finally, the following advantages of ab initio calculations for determining properties of electrode materials can be summarized:
- massively reduction of costs due to the conclusion about the principle technical usage before the material is synthesized first time
- shorter development time based on the efficient determination of material parameters
- simultaneously determination of multiple material parameters with a few ab initio calculations -> saving expansive experiments
- getting a fine grasp of the processes -> prediction of new material systems
Footnotes
1 Materials with this property possess near and far ordering at the atomic level.
2 Materials with this property possess no exact ordering at the atomic level.
3 Such materials have a certain fraction of cavities. This quantity can be determined by the ratio of cavities and material.
4 Describes the construction of a material by its smallest periodical repeatable units (unit cells).
boehm(at)matcalc.deFurther reading ...
[CDP91] J. M. Cocciantelli, J. P. Doumerc, M. Pouchard, M. Broussely and J. Labat, J. Power Sources, 34, 103, 1991
[Gri06] S. Grimme, J. of Comp. Chem, 27, 1787-1799 (2006)
[Nag90] T. Naguara and K. Tozawa, Prog. Batteries Solar Cells, 9, 209, 1990
[Oza94] K. Ozawa, Solid State Ionics, 69, 212, 1994
[PNG97] A. K. Padhi, K. S. Nanjundaswamy and J. B. Goodenough, J. Electrochem. Soc., 144, 1188 , 1997
[WBS98] M. Winter, J. O. Besenhard, M. E. Spahr und P. Novàk, Adv. Mater., 10, 725, 1998
[WKT06] Y. Wang, K. Takahashi, K. Lee and G Cao, Adv. Func. Mater., 16, 1133, 2006