CR2032 button cell batteries

Research on CR2032 button cell batteries conductors focuses on oxides and sulfides

All-CR2032 button cell batteries batteries (ASBs) are considered to be the next generation of energy storage devices with broad prospects. The solid electrolyte (SE) in ASBs solves the safety issues caused by organic liquid electrolytes and can provide higher energy density. However, it is a huge challenge to develop CR2032 button cell batteries conductors with high Li+ conductivity at room temperature and good electrochemical stability at voltages greater than 4V. At present, research on CR2032 button cell batteries conductors focuses on oxides and sulfides. However, according to research findings, the ionic conductivity and electrochemical stability of oxides and sulfides cannot be reconciled. Sulfides such as Li10GeP2S12 and Li7P3S11 have good ionic conductivity, but narrow electrochemical windows and poor electrode stability, while oxides show wider electrochemical windows but low ionic conductivity. Developing anionic materials with low activation energy, good stability and other desired properties is a challenging but rewarding direction.

On April 11, 2019, Angewandte Chemie International Edition published online a paper titled "Lithium Chlorides and Bromides as Promising Solid-State Chemistry for Fast Ion Conductors with Good Electrochemical Stability". This work was completed by Professor Sun Qiang of Peking University and Professor Mo Yifei of the University of Maryland (co-corresponding author), and the first author of the paper is Shuo Wang of Peking University. CR2032 button cell batteries batteries can significantly improve the safety performance of lithium-ion batteries, and therefore have received widespread attention. However, it is a huge challenge to simultaneously obtain high Li ion conductivity and good electrochemical stability. Based on the recently discovered excellent solid electrolytes - chloride Li3YCl6 and bromide Li3YBr6, the researchers combined the first principles (DFT) to explore their lithium ion mobility, electrochemical stability and interface stability. It is proved that these new anion lithium compounds have the characteristics of high ionic conductivity, excellent electrochemical stability, low migration energy barrier and wide electrochemical window, and are expected to be widely used in solid-state lithium batteries.

The halide superionic conductors Li3YCl6 (LYC) and Li3YBr6 (LYB) have high ionic conductivity of about 1 mS/cm at room temperature, good mechanical deformability, stability in air, easy synthesis and large-scale processing, and good electrochemical stability, so they can work in 4V ASBs. LYC and LYB have different properties from oxides and sulfides. They are hexagonal close-packed (hcp) and face-centered cubic (fcc) lattices, respectively, and do not have high-doping components filled with Li. The researchers used molecular dynamics simulation (AIMD) to simulate the diffusion of Li+ in LYC and LYB. AIMD simulation confirmed the rapid diffusion of Li ions in these two materials. At 300K, the Li+ conductivity in LYB is 2.2mS/cm and the activation energy Ea is 0.28±0.02eV; in LYC, the upper limit is 14mS/cm, the lower limit is 4.5mS/cm and 0.19±0.03eV, respectively. The relevant results are listed in Table 1. This difference in ionic conductivity in LYC can be explained by its anisotropic conductive mechanism.

Different anion frameworks in fcc-type LYB and hcp-type LYC simulated by AIMD lead to different Li+ diffusion mechanisms and pathways. In LYB, Li+ diffuses isotropically through the 3D network and jumps from tetrahedral sites to other octahedral sites. In LYC, Li+ diffusion is anisotropic with fast one-dimensional (1D) diffusion channels, where Li+ jumps between sites sharing octahedrons on adjacent faces. Similar to LiFePO4 (LFP), the 1D diffusion channels in LYC are susceptible to channel-blocking defects such as dislocation defects, impurities, and grain boundaries, which cause errors between experimental and simulation results. The researchers used first-principles to confirm that the minimum formation energy for exchanging Li and Y in LYC is 0.80 eV. Since Y and Li+ have similar octahedral configurations and similar ionic radii, Li+ is easily exchanged with Y. Through further analysis, the channel-blocking defect mechanism proposed by the researchers was verified.

The researchers also found that regardless of the cation, this hcp and fcc framework based on the close packing of Cl and Br anions can generally provide good ionic conductivity. Using first-principles calculations, the researchers predicted several isomorphous structures to replace the Y3+ cations in LYC and LYB, such as Li3MX6 (M=Dy, Gd, Ho, La, Nd, Sc, Sm, Tb, Tm; X=Cl, Br). Through specific energy measurements (ΔEhull), it was found that these substances all have good phase stability. Compared with other cations based on the same fcc and hcp anion frameworks, the ionic conductivity of Li3ScCl6 and Li3HoCl6 can reach 10-4 to 10-3S/cm, which is comparable to superionic conductors.

The researchers also compared the effects of different anions and simulated the relationship between the energy and lattice volume of a single Li+ migration in the hcp and fcc sublattices of Cl, Br and S. As shown in Figure 1e-f, in LYC and LYB with the same lattice volume, when the crystal form of Cl and Br anions is hcp, the barrier of Li+ along the c-channel Oct-Oct pathway is 0.25eV, and the Oct-Tet-Oct pathway is 0.29eV; when the crystal form is fcc, the barrier of the OctTet-Oct pathway is 0.28eV, which is consistent with the activation energy results obtained by AIMD simulation. It can be found that the fcc and hcp anion lattices of chloride and bromide can exhibit migration barriers as low as 0.2-0.3eV, thus achieving a high ionic conductivity of 10-3S/cm. Because of these characteristics, chloride and bromide do not need to activate the cooperative migration of lithium ions to achieve rapid ion diffusion, and AIMD simulations also show no cooperative migration of multiple Li ions. Since cooperative migration requires Li filling in the main crystal structure, chloride and bromide are not subject to structural requirements similar to those encountered by oxides and sulfides, and can provide a wider range of structures for rapid ion migration.

The researchers found that these halides not only have excellent lithium ion conductivity, but also have a wide electrochemical window, poor electronic conductivity and good electrode interface compatibility. Through theoretical calculations, they found that LYC and LYB have wide band gaps of 6.02eV and 5.05eV, respectively, and are poor electronic conductors, as shown in Figure 2. Both LYB and LYC show wide electrochemical windows, with anode limits of 3.5V and 4.2V, respectively, and cathode limits of 0.6V, which are significantly wider than many sulfides and oxides, such as LGPS (1.72-2.29V), L3PS4 (1.71-2.31V), LISICON (1.44-3.39V) and Li0.33La0.56TiO3 (1.75-3.71V). The high oxidation stability of LYC at greater than 4V helps it to be applied in lithium-ion batteries; the oxidation stability of LYB at less than 4V is slightly poor, which is not enough to meet the requirements of positive electrode stability. High oxidation stability is an inherent characteristic of chloride and bromide anions. As shown in Figure 3, for the Li-M-X ternary compound (M is a cation, X is F, Cl, Br, I, O, S), calculations have confirmed that the electrochemical window is related to the anion characteristics. Although fluoride has the best oxidation stability, chloride can achieve a stability balance between reduction and oxidation. On the positive electrode side, chloride has higher stability than oxides and sulfides, fully meeting the 4V potential of the current lithium-ion battery positive electrode.

The interface compatibility of electrode materials plays a vital role in the performance of ASBs, such as affecting Coulomb efficiency, interface resistance and cycle life. The researchers also performed theoretical calculations on the possible interface reactions of LYC and LYB with ordinary positive electrode materials. The results showed that the reaction energy of LYC and LYB with the LCO positive electrode was as small as 45meV/atom, and LYC remained stable when L0.5CoO2 was delithiated, with a reaction energy as low as 24meV/atom, proving that it has good stability during cycling. The good interfacial stability of LYC and LCO is consistent with the high coulombic efficiency in the initial cycle of the ASB battery, which is a significant improvement over most sulfide SEs that will decompose into a lithium-depleting cathode when charged.

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