Batteries

Overview

In the battery group, our research focuses on a variety of thermodynamic and kinetic phenomena observed in electrochemistry and searching for materials generally used in, yet not limited to, Li-ion batteries, including materials for anodes, cathodes, cathode coatings, and solid electrolytes. First-principles Density Functional Theory (DFT) Calculations, Monte Carlo Simulations, Non-equilibrium Phase Search Method, Nudged Elastic Band theory, and High-throughput/Machine Learning techniques are common tools used. Our work is funded by the Center for Electrical Energy Storage (CEES), an Energy Frontier Research Center (EFRC) based out of Argonne National Lab.

Anionic Redox Reactivity and High-energy-density Electrodes

Conventional cathode materials used are typically lithium-containing transition metal oxides and phosphides (e.g., LiCoO₂, LiFePO₄, LiMn₂O₄) which store (release) electrical energy via (de-)insertion of Li⁺ ions, accompanied by redox reactions of the transition metal cation. The specific capacity of the cathode is therefore limited by the number of electrons per transition metal cation that can participate in the redox reaction. This exclusive dependence on the transition metal cations as the redox center has been challenged by the recent discovery of oxygen redox reactivity in Li-excess cathode materials. Exploration of the novel combined cationic and anionic redox chemistries with the goals of high energy density, no O₂ loss, and low-cost are still ongoing and have drawn significant attention from the electrochemical energy storage field.

Unlocking the High Capacity of Super-Lithium-Rich Anti-Fluorite Li₅FeO₄ by Simultaneous Anionic and Cationic Redox

Anionic redox reactions that occur in cathode materials of lithium metal oxide are enabling new opportunities to boost the energy density of lithium-ion batteries by exceeding the specific capacity limit offered by redox reactions associated with the transition metal ions alone. Here, we present a comprehensive experimental and computational study of the combined iron and oxygen redox electrochemistry in a super-Li-rich anti-fluorite Li₅FeO₄ electrode. During the removal of the first two Li ions, the oxidation potential of O^2- is lowered to approximately 3.5 V vs. Li⁺/Li⁰, at which potential cationic oxidation (Fe³⁺ to Fe⁴⁺) occurs simultaneously. By carefully limiting the charging voltage, the anionic and cationic redox reactions show good reversibility without any obvious O₂ gas release. This is correlated with the stabilized O^-/O^2- redox in the Li₆-O configuration in the disordered rocksalt phase generated during the delithiation of Li₅FeO₄, as identified by the DFT calculation.

Further details:

1. C. Zhan^†, Z. Yao^†, J. Lu, L. Ma, V. Maroni, L. Li, E. Lee, E. E. Alp, T. Wu, J. Wen, Y. Ren, C. S. Johnson, M. M. Thackeray, M. Chan, C. Wolverton, K. Amine, Enabling the High Capacity of Lithium-rich Anti-fluorite Lithium Iron Oxide by Simultaneous Anionic and Cationic Redox, Nature Energy, 2, 963–971 (2017)

2. Z. Yao, S. Kim, J. He, V. I. Hegde, C. Wolverton, Interplay of Cation and Anion Redox in Li4Mn2O5 Material and Prediction of Improved Li₄(Mn,M)₂O₅ Education Cathodes for Li-ion Batteries, Science Advances, 4, eaao6754 (2018).

3. Z. Yao, C. Zhan, J. Lu, L. Li, M. K. Y. Chan, M. M. Thackeray, C. Wolverton, Exploring the Combined Anionic and Cationic Redox Reactivity in the Super Li-rich Li₅FeO₄ Based High-Energy-Density Cathode Materials, Under review.

Non-equilibrium Phenomenon in Electrochemistry

Lithium ion batteries (LIB) have become a widely-used technology since their commercialization in the 1990’s. However, the current materials are limited in terms of capacity and seeking new, high-capacity reactions has drawn significant attention from the electrochemical energy storage field. Conversion-type materials like transition metal oxides exhibit the possibility to take advantage of all the valence states of the TM (e.g. Co³⁺/²⁺₃O₄ + 8Li → 3 Co⁰ + 4Li₂O) and can thus achieve much higher capacity compared to the intercalation-type materials (e.g. LiCo³⁺O₂ → Li + Co⁴⁺O₂). However, the conversion-type electrodes, suffer from drawbacks such as huge volumetric expansion, inadequate reversibility, and large voltage hysteresis which have been hindering their practical applications. Thus, in order to push these materials and reactions towards real practical use, there is a large need to obtain a thorough understanding of the conversion reaction mechanisms. Different hypotheses have been suggested as discussed in the manuscript, but some critical issues like the origin of the large hysteresis during charge/discharge of these materials is still not well-understood. We design and utilize a novel computational mechanistic approach (NEPS) that provides, for the first time, a detailed explanation of the hysteresis and non-equilibrium reaction pathways associated with these conversion-type electrodes.

Origin of Fracture-resistance to Large Volume Change in Cu-substituted Co₃O₄ Electrode

The electrode materials conducive to conversion reaction have large volume change in cycles which restrict their further development. It has been demonstrated that incorporation of a third element into metal oxides can improve the cycling stability while the mechanism remains unknown. Here, we report an in-situ and ex-situ electron microscopy investigation of structural evolutions of Cu-doped Co₃O₄ supplemented by first-principles calculations to reveal the mechanism. An interconnected framework of ultrathin metallic copper formed provides a high conductivity backbone and cohesive support to accommodate the volume change and has a cube-on-cube orientation relationship with Li₂O. In charge a portion of Cu metal is oxidized to CuO, which maintains a cube-on-cube orientation relationship with Cu. The Co metal and oxides remain as nanoclusters (less than 5 nm) thus active in subsequent cycles. This adaptive architecture accommodates the formation of Li₂O in the discharge cycle, and underpins the catalytic activity of Li₂O decomposition in the charge cycle.

Further details:

1. H. Liu, Q. Li, Z. Yao, L. Li, Y. Li, C. Wolverton, M. C. Hersam, J. Wu, V. P. Dravid, Origin of Fracture-resistance to Large Volume Change in Cu-substituted Co₃O₄ Electrode, Advanced Materials, 2017.

2. Z. Yao, S. Kim, M. Aykol, Q. Li, J. Wu, J. He, C. Wolverton. Revealing the Conversion Mechanism of Transition Metal Oxide Electrodes during Lithiation from First Principles, Chemistry of Materials. 2017.

3. Q. Li^†, Z. Yao^†, J. Wu^†, S. Mitra, S. Hao, T. S. Sahu, Y. Li, C. Wolverton, and V. P. Dravid, Intermediate Phases in Sodium Intercalation into MoS₂ Nanosheets and Its Implications for Sodium-Ion Battery, Nano Energy 38, 342-349 (2017).

4. M. Amsler, Z. Yao, C. Wolverton. Cubine, A Quasi 2-dimensional Copper-bismuth Nano Sheet, Chemistry of Materials, 2017.

5. K. He, Z. Yao, S. Hwang, N. Li, K. Sun, H. Gan, Y. Du, H. Zhang, C. Wolverton, D Su. Kinetically-Driven Phase Transformation during Lithiation in Copper Sulfide Nanoflakes, Nano Letters 17(9), 5726-5733 (2017).

6. Q. Li^†, J. Wu^†, Z. Yao^†, M. M. Thackeray, C. Wolverton, V. P. Dravid, Dynamic Imaging of Metastable Reaction Pathways in Lithiated Metal Oxide Electrodes, Nano Energy, 2017.

7. S. Hwang, Z. Yao, L. Zhang, M. Fu, K. He, L. Mai, C. Wolverton, D. Su, Multi-step Lithiation of Tin Sulfide: An Investigation using In-Situ Electron Microscopy, ACS Nano, 2018, 12(4), 3638-3645 (2018).

8. Q. Li, Y. Xu, Z. Yao, J. Kang, X. Liu, C. Wolverton, M. Hersam, J. Wu, V. Dravid. Revealing the effects of electrode crystallographic orientation on battery electrochemistry via the anisotropic lithiation and sodiation of ReS2, ACS Nano, 2018, In press.

Kinetics and Mass Transport in LIB and Solid-state Electrolytes

Since the commercialization of LIBs, only organic liquid electrolytes have been used in commercial systems. However, their use poses a significant safety concern for emerging applications such as electric vehicles and grid storage because of the high risk of leakage and flammability. In addition, dendritic lithium growth over extended cell cycling can lead to short circuits in LIBs when a lithium metal anode and organic liquid electrolyte are used. Some cathode materials also have a tendency to dissolve in the electrolyte (e.g., Mn ions in LiMn2O4 spinel cathodes), which can further reduce the overall efficiency of LIBs. To circumvent these issues, the replacement of organic liquid electrolytes with inorganic solid-state electrolytes (SSEs) has been suggested. Meanwhile, the continuously increasing requirement of battery's rate capacity asks further understanding and improving of ionic kinetics in state-of-the-art electrode materials.

Cubine, A Quasi 2-dimensional Copper-bismuth Nano Sheet

We report on the discovery of a quasi 2-dimensional copper-bismuth nano sheet from ab initiocalculations, which we call cubine. According to our predictions, single layers of cubine can be isolated from the recently reported high-pressure CuBi bulk material at an energetic cost of merely ≈20 meV/Å2, comparable to values to separate single layers of graphene from graphite. Our calculations suggest that cubine has remarkable electronic and electrochemical properties: It is a superconductor with a moderate electron-phonon coupling λ=0.5, leading to a Tc of ≈1 K, and can be readily intercalated with lithium with a high diffusibility, rendering it a promising candidate material to boost the rate capacity of current electrodes in lithium-ion batteries.

1. M. Amsler, Z. Yao, C. Wolverton. Cubine, A Quasi 2-dimensional Copper-bismuth Nano Sheet, Chemistry of Materials, 2017.

2. Q. Li, H. Liu, Z. Yao, J. Cheng, T. Li, Y. Li, C. Wolverton, J. Wu, and V. P. Dravid. Electrochemistry of Selenium with Sodium and Lithium: Kinetics and Reaction Mechanism, ACS Nano 10(9), 8788-8795 (2016).

Layered Compounds

Layered cathodes such as LiCoO₂, LiNiO₂, LiNi_0.5Mn_0.5O₂ and LiNi_0.33Mn_0.33Co_0.33O₂ attract a lot of attention both experimentally and computationally. Recently, to achieve higher capacity and stability, new advanced layered cathode materials with improved electrochemical properties have been developed, such as Li-rich layered cathode and concentration gradient layered cathode. In our group, in addition to identifying structure and stability of phases at ground state, we study the further details of atomic arrangements in advanced cathode materials also at higher temperatures combining the first-principles calculations with methods such as cluster expansion, SQS, and Monte Carlo simulation.

Vacancy Order-Disorder and Intercalation Battery Voltages in Li_xCoO₂

We present first-principles technique, cluster expansion, and Monte Carlo simulations for predicting the ordered vacancy ground states, intercalation voltage profiles, and voltage-temperature phase diagrams of Li intercalation battery electrodes. Application to the LixCoO2 system yields correctly the observed ordered vacancy phases. We further predict the existence of additional ordered phases, their thermodynamic stability ranges, and their intercalation voltages in LixCoO2/Li battery cells.

Further details: C, Wolverton and A. Zunger, Phys. Rev. Lett. 1998 81, 606-609

Spinel Compounds

Relative stability of normal vs. inverse spinel for 3d transition metal oxides as lithium intercalation cathodes

Spinel oxides represent an important class of cathode materials for Li-ion batteries. Two major variants of the spinel crystal structure are normal and inverse. The relative stability of normal and inverse ordering at different stages of lithiation has important consequences in lithium diffusivity, voltage, capacity retention and battery life. We investigate the relative structural stability of normal and inverse structures of the 3d transition metal oxide spinels with first-principles DFT calculations. We find that for all lithiated spinels, the normal structure is preferred regardless of the metal. We observe that the normal structure for all these oxides has a lower size mismatch between octahedral cations compared to the inverse structure. With delithiation, many of the oxides undergo a change in stability with vanadium in particular, showing a tendency to occupy tetrahedral sites. We find that in the delithiated oxide, only vanadium ions can access a +5 oxidation state which prefers tetrahedral coordination.

Further details: J. Bhattacharya, C. Wolverton, Phys. Chem. Chem Phys. 2013, 15, 6486

Cathode Coatings

Cathode particle surfaces can be tailored with coating materials including oxides, phosphates, and fluorides. These cathode coatings range in thickness from <1 to >100 nanometers. These coatings are designed to improve lifespan, power, safety, and to increase voltage limits. A central challenge for most Li-ion cathode materials is stability during cycling and aging. Degradation often occurs at the electrode-electrolyte interface and is exacerbated by the high voltages, which can drive electrolyte oxidation and dioxygen evolution. This surface degradation reduces battery lifespan, slows Li diffusion, and increases the risk of thermal runaway. Today's electric vehicles use voltage limits and cell packaging to ensure safety, but this approach increases battery weight. To achieve a major breakthrough in battery performance, the cathode/electrolyte interface must be stabilized at the atomistic scale.

Designing cathode coating materials from first-principles by materials screening

The extent of protection a cathode coating can provide depends on many variables, both thermodynamic and kinetic. Using first-principles calculations, we have recently screened many prospective cathode coating materials based on their thermodynamic attributes, such as enthalpies of protective chemical reactions and equilibrium lithiation voltages. Besides the agreement with experimental observations, we were able to predict new promising coating materials.

Further details:

J.E. Saal, S. Kirklin, M. Aykol, B. Meredig and C. Wolverton, JOM, 2013, 65, 1501.

M. Aykol, S. Kirklin and C. Wolverton, Advanced Energy Materials, 2014, 1400690.

Cathode coatings can improve battery power by offering high Li diffusivity, stable surface chemistries and preventing undesirable passivation layers. Coatings can allow for charging to high potentials by protecting the electrolyte from oxidation; this feature is enabling for layered-layered Li₂MnO₃-LiMO₂ materials and high voltage LiNi_0.5Mn_1.5O₄ spinel materials. For LiMn₂O₄ spinel materials, Mn dissolution causes capacity fade, and cathode coatings must be selected to minimize Mn dissolution while maintaining fast Li diffusion. For all cathode coatings, high performance design is enabled by control of defects, Li+ and e- diffusivity, strain, phase diagrams and reactivity during processing, cycling, and aging.

Lithium diffusion in electrode coatings

As an example, we examine the Li diffusivity in a typical metal oxide (Al₂O₃) and metal fluoride (AlF₃). We use methods that combine first principles density functional theory calculations and statistical mechanics to investigate Li transport in amorphous Al₂O₃ and AlF₃. Because of unfavorable Li binding sites and relatively high diffusion barriers, the Li diffusivities are found to be very low. The diffusivities are also much lower than those in benchmark materials, Li-β-alumina and LiFePO₄, which have open channel structures. This work is one part of a framework for understanding the battery performance improvement associated with coatings and should aid in future discovering of coating materials.

Further details: S. Hao and C. Wolverton, J. Phys. Chem. C, 2013 117 (16), pp 8009–8013

Current Members:

Zhenpeng Yao (Zhenpeng's Research Website)

Shiqiang Hao

Shane Patel

Zhi Lu