Electrode-Electrolyte Interface for Solid State Li-Ion Batteries: Point Defects and Mechanical Strain

Abstract

In this work, we present an ab-initio investigation of point defects in solid electrolyte γ-Li 3 PO 4 and in negative electrode-electrolyte interface (Li/γ-Li 3 PO 4 ). Our results on Li defects on γ-Li 3 PO 4 exhibit that Li interstitial defects dominate over vacancy defects, and that Li vacancy-interstitial pair defect formation energy in the interface is comparable to the sum of Li vacancy defect in the electrode and Li ion interstitial defects in the electrolyte region. Our study reveals that the high Li ion defect formation energy is the determining factor for the low ionic conductivity across Li metal/electrolyte interface. Moreover, in a realistic interface, the mechanical strain at the interface increases with the concentration of the impurities produced as a result of the cycling of the battery or due to surface impurities, also affecting the electrostatic potential and charge distribution. Thus, the study of the Li metal/electrolyte interface provides information on the defect formation and mechanical stability and, hence, it helps to understand the realistic modeling of the interface as a way to improve the ionic conductivity and stability of future solid state Li-ion batteries.There have been intense research activities focused on improving energy and power densities of Li-ion batteries for future applications in various micro devices. 1 A lot of research has been dedicated to understand the conduction phenomenon in all components of the Li-ion cell; cathode, electrolyte and anode. Among various electrolytes, solid electrolytes have several advantages over the currently used liquid and polymer electrolytes for microelectronics applications. 2-6 Recently, lithium phosphorous oxynitride (LiPON), which was developed and studied by Oak Ridge National Laboratory, has been adopted as an electrolyte in thin-film batteries 7-10 The Li 3 PO 4 is one of the natural and synthetic crystalline materials of the LiPON family. We know that, to be an ideal solid electrolyte, it must have high ionic and low electronic conductivities; it should be permeable to Li ions (Li + ) and impermeable to electrons from/to the electrodes. These electrolytes are important due to their stability in general and with the interfaces formed with cathode and anode materials in particular. LiPON are found to be suitable to use in thin films because of their chemical and physical stability. However, the main hindrance of solid electrolytes for the commercial use in new applications is their low ionic conductivity, which results in relatively low energy output of the Li-ion battery. [11][12][13][14] As previously mentioned in the literature, Li 3 PO 4 has three crystalline forms, labeled as α, β, and γ. 15,16 The crystal structure of α-Li 3 PO 4 has not been fully determined yet, while the crystal structures of β-and γ-Li 3 PO 4 are already well known. Both β-and γ-Li 3 PO 4 have been observed by several experimental groups, but the activated transport measurements have been limited to the γ-Li 3 PO 4 phase. 15,16 The β form is also stable ...

Computation-Accelerated Design of Materials and Interfaces for All-Solid-State Lithium-Ion Batteries

“…This formation energy of mobile carriers may be significant for pristine, non-doped materials with highly ordered Li sublattices, and should be carefully evaluated and considered as a part of the total activation energy. [49][50][51][52][53] As a final note, computational results are based on bulk single crystals, and experimentally measured values of diffusional properties are affected by impurity phases, grain boundaries and microstructure, and local variations in composition resulting from synthesis and processing conditions. All the caveats described above should be considered when drawing a quantitative comparison between computational and experimental results.…”

“…These calculations based on bulk thermodynamics may not fully reflect the actual atomic structure, stoichiometry, chemistry, defects, or microstructures at the interface. While computational studies were performed for the defects in SEs and interfaces, [49][50][51] the atomistic-scale diffusion mechanisms in amorphous SEs, grain boundaries, and SE-electrode interfaces are still largely unclear. To model the ion diffusion in amorphous materials and at the interfaces, large-scale atomistic modeling at the length scale of 1-100 nm is needed.…”

Structural and Mechanistic Insights into Fast Lithium-Ion Conduction in Li4SiO4–Li3PO4 Solid Electrolytes

“…The Li4SiO4-Li3PO4 solid solution system and the endmember parent phases have been identified as potential solid electrolytes, [15][16][17][18][19][20][21][22][23][24][25][26][27] but not all compositions have been fully characterized. It has been reported that the ionic conductivity can be increased by three orders of magnitude for Li4-xSi1-xPxO4 or -Li3+ySiyP1-yO4 compositions, compared with the two end members, Li4SiO4 and -Li3PO4.…”

Connecting the irreversible capacity loss in Li-ion batteries with the electronic insulating properties of solid electrolyte interphase (SEI) components

“…Many of these coatings materials are dense inorganic compounds, such as Al 2 O 3 , TiO 2 and TiN [27] [29]. Among all the coating materials, Li 3 PO 4 is an excellent choice due to its chemical stability, insulating property, ionic conductivity and also mechanical property [30][31] [32].…”