Soo Kim

Controlling the intercalation chemistry to design high-performance dual-salt hybrid rechargeable batteries

We have conducted extensive theoretical and experimental investigations to unravel the origin of the electrochemical properties of hybrid Mg(2+)/Li(+) rechargeable batteries at the atomistic and macroscopic levels. By revealing the thermodynamics of Mg(2+) and Li(+) co-insertion into the Mo6S8 cathode host using density functional theory calculations, we show that there is a threshold Li(+) activity for the pristine Mo6S8 cathode to prefer lithiation instead of magnesiation. By precisely controlling the insertion chemistry using a dual-salt electrolyte, we have enabled ultrafast discharge of our battery by achieving 93.6% capacity retention at 20 C and 87.5% at 30 C, respectively, at room temperature.

High-throughput computational design of cathode coatings for Li-ion batteries

Cathode degradation is a key factor that limits the lifetime of Li-ion batteries. To identify functional coatings that can suppress this degradation, we present a high-throughput density functional theory based framework which consists of reaction models that describe thermodynamic and electrochemical stabilities, and acid-scavenging capabilities of materials. Screening more than 130,000 oxygen-bearing materials, we suggest physical and hydrofluoric-acid barrier coatings such as WO3, LiAl5O8 and ZrP2O7 and hydrofluoric-acid scavengers such as Sc2O3, Li2CaGeO4, LiBO2, Li3NbO4, Mg3(BO3)2 and Li2MgSiO4. Using a design strategy to find the thermodynamically optimal coatings for a cathode, we further present optimal hydrofluoric-acid scavengers such as Li2SrSiO4, Li2CaSiO4 and CaIn2O4 for the layered LiCoO2, and Li2GeO3, Li4NiTeO6 and Li2MnO3 for the spinel LiMn2O4 cathodes. These coating materials have the potential to prolong the cycle-life of Li-ion batteries and surpass the performance of common coatings based on conventional materials such as Al2O3, ZnO, MgO or ZrO2.

Comprehensive Enhancement of Nanostructured Lithium-Ion Battery Cathode Materials via Conformal Graphene Dispersion

Efficient energy storage systems based on lithium-ion batteries represent a critical technology across many sectors including consumer electronics, electrified transportation, and a smart grid accommodating intermittent renewable energy sources. Nanostructured electrode materials present compelling opportunities for high-performance lithium-ion batteries, but inherent problems related to the high surface area to volume ratios at the nanometer-scale have impeded their adoption for commercial applications. Here, we demonstrate a materials and processing platform that realizes high-performance nanostructured lithium manganese oxide (nano-LMO) spinel cathodes with conformal graphene coatings as a conductive additive. The resulting nanostructured composite cathodes concurrently resolve multiple problems that have plagued nanoparticle-based lithium-ion battery electrodes including low packing density, high additive content, and poor cycling stability. Moreover, this strategy enhances the intrinsic advantages of nano-LMO, resulting in extraordinary rate capability and low temperature performance. With 75% capacity retention at a 20C cycling rate at room temperature and nearly full capacity retention at -20 °C, this work advances lithium-ion battery technology into unprecedented regimes of operation.

A green recycling process designed for LiFePO4 cathode materials for Li-ion batteries

A green process route to recycle LiFePO4/C electrode materials is proposed in this work. First, a robust strategy to synthesize LiFePO4/C cathode materials from a precursor of a crystalline FePO4·2H2O phase (metastrengite I) is presented. In order to prepare crystalline FePO4·2H2O, a solution precipitation route is adapted, where the reaction conditions such as temperature and pH are precisely controlled. Among various heat treatment temperatures to calcine our prepared FePO4·2H2O with lithium sources, we find that LiFePO4/C cathode materials synthesized at 700 °C deliver a maximum discharge capacity of 168.51 mA h g−1 at 0.1 C (1 C rate = 170 mA h g−1) with a capacity retention of 99.36% after the 25th cycle at 1 C. Furthermore, commercially available LiFePO4 powders and recovered LiFePO4 electrode materials from spent batteries are both tested with our developed recycling process, where we decompose LiFePO4 powders/electrodes to prepare crystalline FePO4·2H2O, and then re-synthesize LiFePO4/C cathode materials. In both cases, our recycled LiFePO4/C exhibits a very comparable discharge capacity of ∼140 mA h g−1 at 1 C with a capacity retention of ∼99%.

Synthesis of layered–layered 0.5Li2MnO3·0.5LiCoO2 nanocomposite electrode materials by the mechanochemical process and first principles study

Li2MnO3-stabilized LiCoO2 electrode materials were synthesized using the method of mechanochemical process. Li2MnO3 was prepared and the mechanochemical process was carried out with LiCoO2, which yielded the layered–layered integrated structure nanocomposites. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy studies confirmed the structural integration of 0.5Li2MnO3·0.5LiCoO2 electrode materials. We also performed the high temperature heat treatment, where our 0.5Li2MnO3·0.5LiCoO2 electrode materials showed improvement in the discharge capacity (∼180 mA h g−1) with good cycleability. To obtain a physical insight into the performance of the nanocomposite structure, we carried out first principles calculations to obtain activation energy barriers of Li+ de-/intercalation, which suggested that utilizing both Li2MnO3 and LiCoO2 components can enhance the Li+ diffusion for the layered–layered integrated structure.

Material design of high-capacity Li-rich layered-oxide electrodes: Li2MnO3 and beyond

Lithium-ion batteries (LIBs) have been used widely in portable electronics, and hybrid-electric and all-electric vehicles for many years. However, there is a growing need to develop new cathode materials that will provide higher cell energy densities for advanced applications. Several candidates, including Li2MnO3-stabilized LiM′O2 (M′ = Mn/Ni/Co) structures, Li2Ru0.75Sn0.25O3 (i.e., 3Li2RuO3–Li2SnO3), and disordered Li2MoO3–LiCrO2 compounds can yield capacities exceeding 200 mA h g−1, alluding to the constructive role that Li2MO3 (M4+) end-member compounds play in the electrochemistry of these systems. Here, we catalog the family of Li2MO3 compounds as active cathodes or inactive stabilizing agents using high-throughput density functional theory (HT-DFT). With an exhaustive search based on design rules that include phase stability, cell potential, resistance to oxygen evolution, and metal migration, we predict a number of new Li2MIO3–Li2MIIO3 active/inactive electrode pairs, in which MI and MII are transition- or post-transition metal ions, that can be tested experimentally for high-energy-density LIBs.

Exploring Lithium-Cobalt-Nickel Oxide Spinel Electrodes for ≥3.5 V Li-Ion Cells

Recent reports have indicated that a manganese oxide spinel component, when embedded in a relatively small concentration in layered xLi2MnO3·(1-x)LiMO2 (M = Ni, Mn, or Co) electrode systems, can act as a stabilizer that increases their capacity, rate capability, cycle life, and first-cycle efficiency. These findings prompted us to explore the possibility of exploiting lithiated cobalt oxide spinel stabilizers by taking advantage of (1) the low mobility of cobalt ions relative to that of manganese and nickel ions in close-packed oxides and (2) their higher potential (∼3.6 V vs Li0) relative to manganese oxide spinels (∼2.9 V vs Li0) for the spinel-to-lithiated spinel electrochemical reaction. In particular, we revisited the structural and electrochemical properties of lithiated spinels in the LiCo1-xNixO2 (0 ≤ x ≤ 0.2) system, first reported almost 25 years ago, by means of high-resolution (synchrotron) X-ray diffraction, transmission electron microscopy, nuclear magnetic resonance spectroscopy, electrochemical cell tests, and theoretical calculations. The results provide a deeper understanding of the complexity of intergrown layered/lithiated spinel LiCo1-xNixO2 structures when prepared in air between 400 and 800 °C and the impact of structural variations on their electrochemical behavior. These structures, when used in low concentrations, offer the possibility of improving the cycling stability, energy, and power of high energy (≥3.5 V) lithium-ion cells.

Layered-Layered-Spinel Cathode Materials Prepared by a High-Energy Ball-Milling Process for Lithium-ion Batteries

In this work, we report the electrochemical properties of 0.5Li2MnO3·0.25LiNi0.5Co0.2Mn0.3O2·0.25LiNi0.5Mn1.5O4 and 0.333Li2MnO3·0.333LiNi0.5Co0.2Mn0.3O2·0.333LiNi0.5Mn1.5O4 layered-layered-spinel (L*LS) cathode materials prepared by a high-energy ball-milling process. Our L*LS cathode materials can deliver a large and stable capacity of ∼200 mAh g(-1) at high voltages up to 4.9 V, and do not show the anomalous capacity increase upon cycling observed in previously reported three-component cathode materials synthesized with different routes. Furthermore, we have performed synchrotron-based in situ X-ray diffraction measurements and found that there are no significant structural distortions during charge/discharge runs. Lastly, we carry out (opt-type) van der Waals-corrected density functional theory (DFT) calculations to explain the enhanced cycle characteristics and reduced phase transformations in our ball-milled L*LS cathode materials. Our simple synthesis method brings a new perspective on the use of the high-power L*LS cathodes in practical devices.

Transportation properties in nanosized LiFePO4 positive electrodes and their effects on the cell performance

To achieve a high energy density for Li-ion batteries, it is important to optimize the electrode thickness and electrode density. It is common to design the electrodes to be thick and dense to achieve a high energy density. However, highly tortuous transport paths in thick and dense electrodes can lead to severe transport losses, which negatively affect the cell performance. In this work, we investigated the effects of varying the electrode thickness and density on lithium ion transport in the electrolytes by means of both experiments and simulations. Both results indicated that an additional capacity loss occurs from the electrode with low porosity because the effective diffusivity decreased in the electrolyte phase. Optimal ranges of the electrode thickness and density (porosity) that can be used to help design high-power LiFePO4/graphite batteries were also suggested in this work.

Interplay of cation and anion redox in Li4Mn2O5 cathode material and prediction of improved Li4(Mn,M)2O5 electrodes for Li-ion batteries

High–energy density cathode materials for Li-ion batteries leverage oxygen and transition metal redox activity with reduced cost. Significant research effort has focused on improving the specific energy of lithium-ion batteries for emerging applications, such as electric vehicles. Recently, a rock salt–type Li4Mn2O5 cathode material with a large discharge capacity (~350 mA·hour g−1) was discovered. However, a full structural model of Li4Mn2O5 and its corresponding phase transformations, as well as the atomistic origins of the high capacity, warrants further investigation. We use first-principles density functional theory (DFT) calculations to investigate both the disordered rock salt–type Li4Mn2O5 structure and the ordered ground-state structure. The ionic ordering in the ground-state structure is determined via a DFT-based enumeration method. We use both the ordered and disordered structures to interrogate the delithiation process and find that it occurs via a three-step reaction pathway involving the complex interplay of cation and anion redox reactions: (i) an initial metal oxidation, Mn3+→Mn4+ (LixMn2O5, 4 > x > 2); (ii) followed by anion oxidation, O2−→O1− (2 > x > 1); and (iii) finally, further metal oxidation, Mn4+→Mn5+ (1 > x > 0). This final step is concomitant with the Mn migration from the original octahedral site to the adjacent tetrahedral site, introducing a kinetic barrier to reversible charge/discharge cycles. Armed with this knowledge of the charging process, we use high-throughput DFT calculations to study metal mixing in this compound, screening potential new materials for stability and kinetic reversibility. We predict that mixing with M = V and Cr in Li4(Mn,M)2O5 will produce new stable compounds with substantially improved electrochemical properties.