Transitioning Li-Ion Batteries Past Their Thermal Limitations with a Multi-Functional Electrolyte

KEVIN WESTHOFF, TREVOR VERNON, DR. TODD BANDHAUER

DEPARTMENT OF MECHANICAL ENGINEERING

COLORADO STATE UNIVERSITY, FORT COLLINS, CO

PROJECT SUMMARY To address thermal limitations in lithium-ion batteries (LIB), a novel thermal management system (TMS) is proposed that utilizes the electrolyte as the working fluid for an internal phase change thermal management system (PCTMS). High vapor pressure co-solvents are investigated for both their electrochemical and heat transfer performance. Miscibility and solubility measurements are made with ethyl methyl carbonate (EMC) and LiTFSI salt. Conductivity testing is performed on candidate electrolytes using a Rosemount Analytical 400 series conductivity tester. Cyclic voltammetry, electrochemical impedance spectroscopy, half-cell, and full-cell cycling are performed with a Gamry Reference 3000 Potentiostat. Electrolyte boiling experiments are performed with a purpose-built test facility to simultaneously evaluate the thermal-hydraulic and electrochemical feasibility of the proposed PCTMS.

BACKGROUND AND RATIONALE LIBs are inherently thermally limited due to the current state-of-the-art electrolyte chemistry. The common organic solvents used in LIB electrolytes have low flash points and are the main source of fuel during a thermal runaway event. Current LIB thermal management for large battery packs entails significant cooling external to the individual cells to prevent thermal runaway from occurring. These systems, however, cannot cool without heat conduction through the cell. This leads to large thermal gradients in the center of a cell due to their low thermal conductivity. A more effective approach to cooling a LIB would be with an internal system, eliminating the thermal gradients. Phase-change thermal management systems (PCTMS) rely on the enthalpy of vaporization of the working fluid to provide the cooling effect. This cooling effect is produced when the working fluid boils at the desired operation temperature of the cooled device. A potential working fluid for a PCTMS, that is also native to a LIB, is the electrolyte.

OBJECTIVES Recent work on electrolyte modification to address the thermal issues of LIBs has sought to solely limit the electrolyte flammability through the use of fire-retardant co-solvents. Recent work on LIB TMSs has focused primarily on optimizing the current external cooling methods through improved electrochemical and thermal modeling of the battery as well as optimization of the external heat absorption medium. The current project will seek to design and implement a novel approach to TMSs for LIBS via a purpose-built electrolyte boiling test facility and highly sensitive electrochemical measurements.

EXPECTED OUTCOMES 1) Design novel electrolyte that includes high vapor pressure co-solvents

2) Test the thermal-hydraulic and electrochemical performance of the novel electrolyte in electrolyte boiling test facility

3) Demonstrate the operation of the PCTMS on a full cell

Advantages of the PCTMS: Continuous, passive temperature management of the battery; uniform battery temperature; uniform cycling of battery materials; minimal impact on overall battery system size (battery + TMS).

SYSTEM CONCEPT The proposed system utilizes thin, uncoated sections of the positive electrode as vapor channels for the evaporated electrolyte. The vapor collects at the top of the cell and exits to an external condenser where the internally generated heat is rejected. The condensed electrolyte is then returned to the cell.

The proposed system has a minimal impact on total pack energy density.

MULTI-FUNCTIONAL ELECTROLYTE DESIGN

TOOLS: HIGH PRESSURE ELECTROLYTE TEST FACILITY The high pressure electrolyte test facility is designed to test the miscibility, solubility, and electrochemical performance of electrolytes up to 150 psi. The test facility is purposefully designed to minimize losses of the high vapor pressure co-solvent while enabling significant sample manipulation inside the pressure vessel. Three sets of rotating leads allows for three different CV and EIS experiments to be performed with the candidate electrolyte solutions. The salt addition system allows for salt to be incrementally added to the high pressure solution without H2O and O2 through an atmosphere exchange system with argon and vacuum.

EXPERIMENTAL CAPABILITY: 1. Miscibility of high vapor pressure co-solvents and carbonate solvents

2. Solubility of LiTFSI salt in candidate solvent mixtures

3. Cyclic Voltammetry

4. Electrochemical Impedance Spectroscopy

5. Half-Cell Cycling

6. Full-Cell Cycling

PROJECT COLLABORATORS Dr. Amy Prieto Department of Chemistry, Colorado State University

ACKNOWLEDGEMENT

This project is funded through the Colorado Office of Economic Development and International Trade.

TOOLS: GAMRY REFERENCE 3000 POTENTIOSTAT The Gamry Reference 3000 Potentiostat is a single channel instrument with a maximum current capability of ± 3 A and a maximum voltage range of ± 32 V. The instrument has 11 current ranges enabling current control and sensing down to 300 pA. Additionally, the instrument is capable of making measurements over a frequency range of 10 µHz to 1 MHz. The experiments performed with this instrument include: cyclic voltammetry, potentiostatic electrochemical impedance spectroscopy, and galvanostatic half and full cell cycling.

CANDIDATE CO-SOLVENTS LOW PRESSURE HIGH PRESSURE

SALT ADDITION SYSTEM

SECTION VIEWS

ROTATING ELECTRODE LEAD SYSTEM

Conductivity Meter

Pressure Gauge

Sight Glass

TOOLS: LOW PRESSURE ELECTROLYTE BOILING TEST FACILITY The low pressure electrolyte boiling test facility is designed to test the thermal-hydraulic and electrochemical performance of electrolytes up to 50 psi. The electrode test pieces are designed to visualize the boiling electrolyte while simultaneously making sensitive electrochemical and temperature measurements.

EXPERIMENTAL CAPABILITY: 1. Qualitative boiling observations

2. Cyclic Voltammetry

3. Electrochemical Impedance Spectroscopy

4. Half-Cell Cycling

5. Full-Cell Cycling

Rotating Leads

Insulating Jar

Insulating Jar

Solvent

Electrode Leads O-Ring

Pressure Vessel Wall

Rotating Leads