Solid Polymer Electrolytes

Researchers Fernando Villafuerte (Ph.D. student in Materials Science), Yuchun Sun (Ph.D. student in Materials Science)

The ubiquitous lithium ion battery is hamstrung by the intercalation compounds that form its electrodes, which limit the total amount of energy it can store. Modern battery design makes use of thin films for the battery components, and increasing capacity requires either increasing the areal footprint or making them thicker. This storage limitation can be overcome by swapping the graphitic anode for metal lithium, which can increase the energy density by 50%. Switching to metal lithium anodes, however, presents particular challenges. Standard electrolytes in use for lithium-ion batteries are composed of organic solvents that are chemically unstable against lithium metal, and catastrophic failure of standard lithium-ion batteries typically begins with combustion of the electrolyte. This problem can be circumvented by using macroscopically solid materials like polymers as electrolytes, which unlike their liquid cousins, are more thermodynamically and electrochemically stable in both lithium-ion and metal lithium systems.

The focus of polymer electrolyte research has been polyethylene oxide (PEO), since its ability to thoroughly dissolve lithium salts and conduct lithium ions was discovered in the 1970’s. PEO is limited, however, by the relatively strong coordination of lithium cations by ether oxygen atoms in the polymer backbone, which results in low ionic conductivity at standard operating temperatures for lithium and lithium-ion batteries. Moreover, the corresponding anion is loosely coordinated by the PEO backbone, which leads to higher anionic mobility and thus a low transference number for lithium ions.

Research in the Greer Group is focused on polymer chemistries and polymer electrolyte designs that move beyond PEO. Particular emphasis is placed on the investigation of polymers with Lewis acidic moieties on the backbone, as opposed to the Lewis basic ether oxygen atoms in the PEO backbone; and gel polymer electrolytes, which swell PEO-based architectures with a cosolvent to aid lithium ion mobility while still preserving the advantages provided by a solid PEO structure. We fabricate our electrolytes and characterize their conductivity and lithium ion transference number through the use of potentiostatic techniques such as impedance spectroscopy, and investigate how these properties depend on factors such as salt concentration, molecular weight, and amount of cosolvent. We hope to understand the inherent structure-property relationships in order to optimize the design of our electrolytes for potential application.

On the left is an image of an electrolyte constructed from a polymer with Lewis-acidic moieties on the backbone. The image on the right is of a UV-cured gel polymer electrolyte.

3D Architected LiCoO₂ through Gel Infusion and Calcination

Researchers: Yuchun Sun (Ph.D. student in Materials Science)

LiCoO₂ is commonly used as the cathode material of lithium-ion batteries. It stores and releases lithium ions through a reversible intercalation mechanism, which gives great cycling stability to rechargeable batteries. Energy density improvement of conventional LiCoO₂ slurry electrodes has been challenging due to the limited electrode thickness, which is in general less than 50μm to optimize Li ion transport into electrodes. This constraint limits the mass ratio between active material and metal current collector, and low active material loading as a result limits the energy density of the whole battery system. To overcome this dilemma between active material loading and Li ion transport into electrodes, we developed a method for the fabrication of 3D architected LiCoO2 electrodes through gel infusion and calcination. A "blank" organogel is printed through vat polymerization into a designed architecture, where Li and Co ions are later swelled into the gel. After calcination, 3D architected LiCoO₂ with the same geometry as the printed gel can be obtained. By going from 2D slurry electrodes into 3D architected electrodes, we can achieve high active material loading while maintaining Li ion diffusion distance into the electrode material. This gel diffusion and calcination method developed in our group has been applied to fabricate a wide variety of materials. With large flexibility in the species of infused ions, this gel infusion and calcination method can also be a useful tool to fabricate other 3D architected electrode materials.

3D Interdigitated Solid-State Lithium-Ion Batteries

Researchers: Yuchun Sun (Ph.D. student in Materials Science)

3D lattice electrodes enable high active material mass loadings and high gravimetric capacity of lithium-ion batteries. The idea of 3D batteries goes one step further than 3D lattice electrodes, which not only gives good gravimetric and areal capacities, but also shortens Li ion diffusion distance between two electrodes by making better use of the porous spaces within 3D lattices. Under current fabrication capabilities, 3D battery with interdigitated-plate geometry is one of the most promising approaches. With the gel infusion/calcination-based 3D LiCoO₂ fabrication method and the DLP printing/pyrolysis-based 3D carbon fabrication method previously developed in our group, we designed and fabricated LiCoO₂ and carbon electrodes with interdigitated-plate geometry, which can be inserted into each other with gel polymer electrolytes sandwiched in between. As a natural choice for 3D batteries with complex electrode surface geometries, photopolymerizable gel polymer electrolyte can be easily coated on electrode surface and cured in-situ, giving low electrode-electrolyte interfacial impedance than other solid electrolytes, while eliminating the safety concerns associated with liquid electrolytes.

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Developing Lithium Sulfur Batteries

Researchers: Max Saccone (Ph.D. student in Chemical Engineering),

Li-S batteries are poised to outcompete Li-ion batteries in key sectors such as transportation and grid storage due to their use of low cost and earth abundant materials. However, improving energy density and mitigating degradation in Li-S batteries is necessary before these applications can be realized. A significant and poorly understood degradation mechanism which causes irreversible capacity fade in these systems is mechanical failure and detachment of insulating Li2S from the conductive matrix in the cathode. To shed light on mechanical degradation in Li-S batteries, we investigate the material properties and deformation mechanisms of Li2S via in situ SEM mechanical experiments such as the compression of micron-sized Li2S particles.

We also explore the use of layer by layer additive manufacturing as a route towards producing mechanically strong and tough 3D architected electrodes with complex architectures, high active material loadings, and large areal capacities relative to conventional 2D film electrodes. We have developed a stereolithographic additive manufacturing process which enables fabrication of architected 3-dimensional lithium sulfide-carbon composite cathodes with feature sizes as small as 50 µm and a tunable hierarchal internal pore structure. This technique represents a 3-fold improvement in resolution over previously reported extrusion-based additive manufacturing processes for Li-S cathode materials, and can be used to study the effects of cathode architecture and microstructure on battery cycling performance and mechanical properties.

SEM micrograph of a 3D-architected Li2S-carbon lattice cathode

Past Projects

Electrochemical Characterization of Architected Electrodes

Researchers: Dr. Kai Narita (alumnus)

Developing safe batteries with long lifetimes is key to promoting the effective utilization and adoption of electric devices and vehicles. Understanding the aging mechanisms in battery systems facilitates this development, but this understanding is limited by multi-scale dynamics in the stochastic structure of slurry electrodes commonly used in commercial batteries. To address this challenge, we prescribe the design and fabrication of three-dimensionally architected battery electrodes to effectively characterize sources of aging such as solid electrolyte interface (SEI) formation and lithium dendrite growth via in situ visualization and electrochemical monitoring, in addition to post-characterization techniques.

Electrochemical phenomena in battery systems are coupled at various scales: namely, at the levels of interfacial reactions and the transport of carriers such as ions and electrons. These behaviors are governed by the distribution of current and potential, which are directly related to structural factors such as mean transport trajectories, electrode surface area, and the relative positions of electrode. By using prescribed 3D architected electrodes with fully controllable features such as characteristic length scale and fully flexible form-factors, we can systematically explore the relationship between these factors and undesired consequences of battery operation such as lithium dendrite formation.

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Past Research of Note: Design and in situ Lithiation of Mechanically Robust, Nano-architected Battery Electrodes

Researchers: Dr. Heng Yang (Former post-doc), Dr. Xiaoxing Xia (Former Ph.D. student in Materials Science)

Silicon anodes for Li-ion batteries have a 10-fold enhancement in theoretical capacity compared with intercalation-type graphite anodes. The alloying nature of Li insertion in Si allows each Si atom to accommodate up to four Li atoms, but it also causes up to ∼300% Si volume expansion/contraction during lithiation/delithiation, which leads to mechanical degradation and a reduced cycling life. Nano-structuring Si can alleviate this problem for each nanoscale element such as nanowires and nanoparticles but the traditional slurry fabrication method, suitable for intercalation materials with minimal volume expansion, does not provide efficient and reliable assembly of the nanoscale Si building blocks.

We design and create 3D nano-architected electrodes that could potentially resolve some of the key limitations of high-energy-density electrode materials that undergo alloying or conversion reactions upon lithiation: mechanical failure, sluggish kinetics, and low active material loading.